Catalytic and catalyst-free diboration of alkynes

Fei Zhao a, Xiuwen Jia a, Pinyi Li a, Jingwei Zhao a, Yu Zhou bcd, Jiang Wang bcd and Hong Liu *bcd
aAntibiotics Research and Re-evaluation Key Laboratory of Sichuan Province, Sichuan Industrial Institute of Antibiotics, Chengdu University, 168 Hua Guan Road, Chengdu 610052, P. R. China
bState Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zu Chong Zhi Road, Shanghai 201203, P. R. China. E-mail:
cCAS Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zu Chong Zhi Road, Shanghai 201203, P. R. China
dUniversity of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, P. R. China

Received 21st July 2017 , Accepted 28th August 2017

First published on 5th September 2017

The diboration of alkynes has become the most straightforward and powerful method for the synthesis of functionalized 1,2-diborylalkenes, which are widely used as building blocks in organic synthesis. The present review provides a comprehensive summary of the diboration of alkynes, including platinum-, palladium-, cobalt-, copper-, iridium-, gold-, iron-, base- and small organic molecule-catalyzed diboration, direct and metal-free diboration, and base-promoted diboration of alkynes up to early 2017. The reaction conditions, regio- and stereoselectivities, and mechanisms are summarized in detail. Moreover, synthetic applications and perspectives on the diboration of alkynes are also discussed.

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Fei Zhao

Fei Zhao received his Ph.D. degree in Drug Design under the supervision of Prof. Hong Liu at the Shanghai Institute of Materia Medica in 2015; now he works as a group leader in Chengdu University. His research interests are focused on the discovery of anti-BPH drugs and the synthesis of bioactive heterocyclic molecules through metal catalysis.

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Hong Liu

Hong Liu received her M.S. and Ph.D. degrees in Medicinal Chemistry from the China Pharmaceutical University under the supervision of Professor Weiyi Hua. After a postdoctoral stay with Professor Ruyun Ji and Professor Kaixian Chen at the Shanghai Institute of Materia Medica, she joined the Shanghai Institute of Materia Medica. As a visiting scientist, she worked with Professor James Halpert at the University of Texas Medical Branch at Galveston. Her research interests include medicinal chemistry, organic chemistry, computer modeling, and pharmacologically active molecules, especially those targeting tumor and diabetes.

1. Introduction

Element–element addition to alkynes has become an important strategy for the functionalization of carbon–carbon triple bonds. These intermolecular and intramolecular addition reactions afforded the highly functionalized alkenes and cyclic compounds, respectively, with high efficiency and high atom-economy.1 In particular, the addition of a B–B bond to carbon–carbon triple bonds, namely the diboration of alkynes, has become the most straightforward and powerful method for the preparation of functionalized 1,2-diborylalkenes, which are widely used as building blocks in organic synthesis to construct a diverse array of complex molecules via Suzuki–Miyaura coupling, Petasis–Mannich reaction, etc.2 Other reported strategies for 1,2-diborylalkene synthesis mainly included the hydroboration of monoborylacetylenes (Scheme 1a),3 Miyaura-borylation of premonoborylated vinyl bromines (Scheme 1b),4 oxidative C–H borylation of premonoborylated alkenes (Scheme 1c),5 dehydrogenative diboration of alkenes (Scheme 1d),6 Diels–Alder reaction of 2,3-bisboryl-1,3-butadienes (Scheme 1e),7 1,4-addition reaction of 2,3-bisboryl-1,3-butadienes (Scheme 1f),8 and the rearrangement of diborated allenylcyclopropanes (Scheme 1g).9 However, these methods suffered from major or minor drawbacks, such as inaccessible starting materials, tedious procedures, generation of large amounts of unwanted byproducts, and unsatisfactory chemo- and stereoselectivity. Compared with these abovementioned methods, the diboration of diverse commercially available alkynes with various diboron reagents (Scheme 1h), which can construct two C–B bonds in a single step with excellent regio- and stereoselectivity, provides a much more straightforward and efficient access to 1,2-diborylalkenes. In addition, the diboration of alkynes with diborons, which is characterized by high atom- and step-economy, is more in accordance with the concept of “green and sustainable chemistry”.
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Scheme 1 Strategies for the preparation of 1,2-diborylalkenes.

It should be noticed that the diboration of alkynes was seldom systematically reviewed before.10 Moreover, the diboration of alkynes has achieved lots of important developments in the recent ten years, especially in organocatalysis and catalyst-free processes. Therefore, an updated review focused on the recent achievements would enrich the knowledge of synthetic chemists who are interested in 1,2-diborylalkene synthesis through the diboration of alkynes. The aim of the present review is to provide a comprehensive summary of the diboration of alkynes, including platinum-, palladium-, cobalt-, copper-, iridium-, gold-, iron-, base- and small organic molecule-catalyzed diboration, direct and metal-free diboration, and base-promoted diboration of alkynes up to early 2017. We hope that this review will serve as a handy reference for chemists interested in 1,2-diborylalkene synthesis, and will encourage further developments in this field to overcome the remaining challenges.

2. Diboration reagents

Simple diboron compounds carrying a B–B σ bond are the common diboration reagents and are used widely in the diboration of alkynes. Fig. 1 shows the most common diborons, including symmetrical diborons, such as bis(pinacolato)diboron [B2pin2 (1)], tetramethoxydiborane [B2(OMe)4 (2)], bis(catecholato)diboron [B2cat2 (3)], bis(neopentylglycolato)diboron [B2neop2 (4)], bis(hexyleneglycolato)diboron [B2hex2 (5)], 1,2-Cl2-1,2-(NMe2)2B2 (6), tetrakis(dimethylamido)diboron [B2(NMe2)4 (7)] and tetrahalodiborane [B2X4 (8)], and unsymmetrical diborons, such as pinBBdan 9 (dan = naphthalene-1,8-diaminato) and pinBBmes210 (mes = mesityl). Among them, B2pin2 and B2cat2 are the most frequently used diborons because they are commercially available, easy to handle and conveniently storable. Due to the relatively high strength of the B–B σ bond, in most cases, the activation of the B–B σ bond and addition of the B–B σ bond to carbon–carbon triple bonds are usually aided by transition metal catalysts, which can lower the reaction energy barrier.
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Fig. 1 Common diboration reagents.

3 Catalytic diboration of alkynes

3.1 Platinum catalysis

3.1.1 Phosphine based Pt(0) catalysts. Platinum complexes have been studied extensively in the diboration of alkynes after Miyaura and Suzuki's pioneering work on the addition of the B–B bond to alkynes through platinum(0)/platinum(II) catalysis in 1993 (Scheme 2).11 They presented the first example of metal-catalyzed diboration of alkynes using Pt(PPh3)4 as the catalyst. The diboration reactions of B2pin2 with alkynes 11 proceeded in a highly cis-selective manner and afforded the desired products 12 in high yields. The platinum catalyst Pt(PPh3)4 played a crucial and indispensable role, while other metal complexes, such as Pd(PPh3)4, RhCl(PPh3)3 and CoCl(PPh3)3, were ineffective. It is worth noting that the solvent used didn't alter the outcome of the reaction. However, the reaction could be apparently accelerated in polar solvents such as N,N-dimethylformamide (DMF). Interestingly, steric hindrance didn't have much influence on the reaction; no obvious differences in reaction yields and rates between internal and terminal alkynes were observed. In addition, this process could also be applied to other diboron derivatives such as B2(OMe)4 and B2(NMe2)4.12 The proposed mechanism (Scheme 3) involved the oxidative addition of Pt across the B–B bond, subsequent cis-insertion of alkynes to the B–Pt bond, and the following reductive elimination which produced the products and regenerated the catalyst. The single crystal structure of the proposed intermediate 13 and its high reactivity towards alkynes further demonstrated the mechanism.13
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Scheme 2 Pt(PPh3)4-catalyzed diboration of alkynes.

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Scheme 3 Proposed mechanism of Pt(PPh3)4-catalyzed diboration of alkynes.

Encouraged by the excellent catalytic performance of Pt(PPh3)4 towards the diboration of alkynes,11 Miyaura and Suzuki further screened a series of other platinum complexes and found that the bisphosphine Pt(0)-based catalyst Pt(CO)2(PPh3)2 also showed to be an effective catalyst in the diboration of 1-octyne 15 with B2pin2 in DMF at 80 °C, affording the desired cis-product 16 in 94% yield (eqn (1)).12

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Soon afterwards, Siebert and co-workers represented a Pt(PPh3)22-C2H4)-catalyzed diboration of diborylacetylene 17 with B2cat2 (eqn (2)),14 producing the tetraborylethene compound 18 in moderate yield, which could be used for the synthesis of B-containing heterocycles such as tetraborafulvalenes. Later, Lesley and Norman successfully used the same catalyst to achieve the diboration of alkynes 11 employing 1,2-Cl2-1,2-(NMe2)2B2 as the diboration reagent in 1999 (eqn (3)).15 The chemistry of 1,2-Cl2-1,2-(NMe2)2B2 was quite different from that of tetraalkoxydiborons; the redistribution of B–Cl and B–N bonds of the initial cis-addition adducts gave the rearrangement products 19 in excellent yields, which could be converted into bis(catecholatoboryl) derivatives by reacting with catechol for synthetic utility.

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In 2001, Marder and co-workers developed efficient monophosphine platinum catalysts for alkyne cis-diboration (eqn (4)).16 The catalyst could be generated in situ by mixing equimolar Pt(NBE)3 (NBE = norbornene) and phosphines. Optimization studies disclosed that the nature of the phosphines had an important influence on the catalytic activity; PCy3 and PPh2(o-Tol) turned out to be the best phosphine ligands. Particularly, the authors revealed that the isolable and stable complex Pt(PCy3)(η2-C2H4)2 was also an effective catalyst. These monophosphine platinum catalysts exhibited high catalytic performances, allowing the diboration of a range of alkynes 11 even at room temperature.

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Kappe's group further studied the Pt(PPh3)4-catalyzed diboration of alkynes developed by Miyaura and Suzuki;11 their research revealed that the use of microwave irradiation17 could reduce the reaction time and the catalytic amount of Pt(PPh3)4 to increase the catalyst turnover frequency (TOF). The diboration reactions proceeded efficiently in dioxane at 180 °C under microwave heating (Scheme 4).18 It is worth noting that a silicon carbide-passive (SiC) heating element was required to achieve the high reaction temperature.19 More importantly, the cis-products 12 could be subjected to a subsequent microwave-assisted Suzuki–Miyaura coupling without isolation, thus affording a high-speed one-pot diboration/Suzuki–Miyaura coupling protocol which allowed the rapid synthesis of tri- and tetrasubstituted ethylenes 20.

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Scheme 4 Microwave-assisted diboration of alkynes catalyzed by Pt(PPh3)4.

Particularly interesting is the Pt(PEt3)3 or Pt sponge-catalyzed addition of strained [2]borametalloarenophanes 21 to alkynes 22, reported by Braunschweig and co-workers (eqn (5)).20 These reactions proceeded well with an excessive amount of alkynes and a long reaction time, providing the novel ansa-bis(boryl)alkenes 23 in good to high yields. Moreover, their study showed that the desired products 23

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could also be achieved with a stoichiometric amount of Pt(PEt3)3via a sequential reaction, in which a diboryl Pt(II) intermediate 24 was involved (Scheme 5). Similarly, the group of Wagner reported the insertion of 3,3-dimethyl-1-butyne into the B–B bond of barrelene-type 1,2-diaminodiborane 25 (Scheme 6).21 The diboration product 26 could be obtained with a catalytic amount of Pt(PEt3)3, albeit with low yield, or through a two-step process in which a diboryl Pt(II) intermediate 27 was involved. These findings not only revealed that strained, but thermally stable diborons could also serve as facile diboron reagents for the diboration of alkynes and further enriched the species of diboron reagents, but also provided complex and interesting diboron frameworks for organic chemistry and material chemistry.

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Scheme 5 Pt(PEt3)3-catalyzed diboration of alkynes with strained [2]borametalloarenophanes.

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Scheme 6 Pt(PEt3)3-catalyzed diboration of 3,3-dimethyl-1-butyne with barrelene-type 1,2-diaminodiborane.
3.1.2 Phosphine-free based Pt(II) and Pt(0) catalysts. Pt(COD)Cl2 (COD = 1,5-cyclooctadiene) was found to be an efficient catalyst for the diboration of alkynes 11, as was reported by Baker's group in 2000 (Scheme 7).22 Both terminal and internal alkynes reacted smoothly with B2cat2 in benzene at 55 °C under the catalysis of 5 mol% Pt(COD)Cl2, yielding the cis-diborylated alkenes 28 in high yields. This process demonstrated that phosphine-free based Pt(II) catalysts were also effective for the diboration of alkynes to prepare 1,2-diborylalkenes.
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Scheme 7 Pt(COD)Cl2-catalyzed diboration of alkynes.

The NHC (N-heterocyclic carbene)–platinum(0) complex developed by Lillo et al. in 2006 also acted as an efficient catalyst for the cis-diboration of alkynes (eqn (6)).23 The reactions of a variety of aryl-substituted alkynes 11 with B2cat2 proceeded efficiently in the presence of 5 mol% catalyst in tetrahydrofuran (THF) even at room temperature. In contrast, lower yields were observed when less reactive B2pin2 was used as the diboron reagent.

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Symmetrical diborons, such as B2pin2 and B2cat2, which shift the problem of regiochemical control, have been utilized extensively in catalytic diboration reactions for decades since Miyaura and Suzuki's first report.11 However, diboration with unsymmetrical diborons was seldom involved, even though reactivity enhancement and synthetic merits would be provided by unsymmetrical diborons. Until 2010, Suginome's group realized the highly regioselective cis-diboration of unsymmetrical alkynes employing the unsymmetrical diboron pinBBdan 9 as the diboration reagent (Scheme 8).24 Optimization of the metal sources revealed that Pt(dba)2 was the most effective catalyst as compared with Pd, Ni and Rh complexes, and the addition of phosphine ligands could dramatically change the regioselectivity. As a matter of fact, the combination of Pt(dba)2 and electron-deficient tris[3,5-bis(trifluoromethyl)phenyl]phosphine effectively transformed a variety of differently substituted alkynes 11 into 1,2-diborylated alkenes with high cis-stereoselectivity and excellent regioselectivity, in which the less reactive Bdan group was dominantly located at the less hindered position. Notably, the sufficiently differentiable reactivities of the two boryl groups allowed selective Suzuki–Miyaura coupling at the more reactive Bpin group to afford the monoborylated alkenes 32 with excellent chemoselectivity (eqn (7)). This internal-selective Bpin coupling is a complete contrast to the terminal-selective Bpin coupling of B2pin2-based diboration.25 More importantly, the monoborylated alkenes obtained could undergo a second coupling reaction to synthesize unsymmetrical multisubstituted olefins.

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Scheme 8 Pt(dba)2-catalyzed diboration of alkynes with pinBBdan.

In 2010, Yoshida and co-workers showed that Pt-catalyzed diboration could also be applicable to arynes (Scheme 9),26 which were generated in situ using Kobayashi's method27 by the treatment of 2-(trimethylsilyl)phenyltriflate derivatives 33 with KF and 18-crown-6. The combination of Pt(dba)2 and an isocyanide ligand was crucial for this diboration, and the isocyanide ligand played a decisive role. Among the isocyanides tested, 1-adamantyl isocyanide (1-AdNC) proved to be the most effective ligand, with which various vic-diborylarenes 34[thin space (1/6-em)]28 could be afforded in good to high yields. The synthesis of symmetrical ortho-terphenyls 36via straightforward Suzuki–Miyaura coupling (eqn (8)) and unsymmetrical ortho-terphenyls 37via stepwise Suzuki–Miyaura coupling (eqn (9)) using the obtained vic-diborylarene 35 was also demonstrated by the authors, further highlighting the synthetic utility of this method. Interestingly, Oestreich's group employed this platinum/isocyanide catalytic system to achieve the diboration of indolynes, furnishing the interesting diborated indole building blocks for organic synthesis.29

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Scheme 9 Pt(dba)2-catalyzed diboration of arynes.
3.1.3 Supported platinum nanoparticles. Catalysis by supported metal nanoparticles (MNPs) is an important component of metal catalysis and has been used widely in organic synthesis.30 The Garcia group used MNPs to conduct the diboration of alkynes in 2011. They disclosed that platinum nanoparticles supported on magnesia (Pt/MgO) could catalyze the diboration of terminal and internal alkynes with or without PPh3 at a high temperature, providing the cis-diborylated alkenes exclusively in high yields (Scheme 10).31 Notably, the power of this methodology was further reinforced by a one-pot diboration/hydrogenation sequence, which allowed the synthesis of vic-diboronated alkanes from alkynes directly. Later, they also demonstrated that platinum nanoparticles supported on active carbon (Pt/C) could efficiently promote the stereoselective diboration of alkynes 38 in the absence of triphenylphosphine at a lower temperature to render the cis-products 39 in high yields (Scheme 11).32 It is worth mentioning that high reaction temperature and the addition of PPh3 had an adverse effect on yield and selectivity in this Pt/C catalytic system.
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Scheme 10 Pt/MgO-catalyzed diboration of alkynes.

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Scheme 11 Pt/C-catalyzed diboration of alkynes.

In 2014, Alonso and co-workers further extended the diboration processes catalyzed by platinum nanoparticles and developed a Pt/TiO2 catalyzed cis-diboration of alkynes under solvent- and ligand-free conditions in air (Scheme 12).33 A variety of aromatic terminal alkynes were proved to be suitable substrates in these reactions and gave the desired products in high yields. Electron-neutral, electron-withdrawing, or electron-releasing groups on the aryl ring didn't significantly alter the outcome of the transformations. Similarly, a range of aliphatic terminal alkynes could undergo the reaction to give rise to the corresponding products in excellent yields. In addition, internal alkynes were also successfully subjected to the reaction to furnish the expected products in good to high yields. Remarkably, exclusive cis-diboration products 12 were obtained in all cases. This environmentally benign protocol is attractive for the synthesis of cis-1,2-diborylalkenes because of the low catalyst loading, broad substrate scope, high yields and excellent stereoselectivity.

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Scheme 12 Pt/TiO2-catalyzed diboration of alkynes.

In summary, 1,2-diborylalkene synthesis via platinum catalysis has been deeply researched ever since the Pt(PPh3)4-catalyzed diboration of alkynes reported by Miyaura and Suzuki.11 Phosphine based Pt(0) catalysts, phosphine-free based Pt(II) and Pt(0) catalysts, and supported platinum nanoparticles all proved to be efficient in the diboration of alkynes. However, with regard to commercial availability, substrate scope and reaction yields, Pt(0) catalysts such as Pt(PPh3)4 and Pt(dba)2 will be a better choice.

3.2 Palladium catalysis

The first example of the Pd-catalyzed diboration of alkynes was reported by Braunschweig and co-workers in 2006, which used Pd/C as the catalyst and strained [2]borametalloarenophanes 21 as the diboration reagents to construct the structurally interesting ansa-bis(boryl)alkenes 23 (eqn (10)).20a However, this process suffered from limited substrate scope and long reaction time, which restricted its synthetic applications.
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In contrast, Ansell et al. reported very recently a more general and practical diboration of alkynes catalyzed by Pd(0)(NHC)2(PhC[triple bond, length as m-dash]CPh) (Scheme 13).34 This process was applicable to various terminal and internal alkynes and furnished the cis-diboration products as the single stereoisomer. For alkyl and aryl terminal alkynes, the diboration reactions proceeded well in the presence of 0.5 mol% Pd(0)(NHC)2(PhC[triple bond, length as m-dash]CPh) at room temperature. For more sterically hindered aryl internal alkynes, the diboration transformations also proceeded well with the same catalyst loading or an increased, but still low catalyst loading of 2 mol% at a higher temperature. However, the reaction of dialkyl substituted 4-octyne was sluggish. This may be attributed to the electron-rich nature of 4-octyne, which resulted in a low binding affinity to the electron-rich catalyst and therefore discouraged diboration. Density functional theory (DFT) studies suggested that a reaction pathway similar to that proposed for Pt(PPh3)4-catalyzed diboration may be involved. Pd(0)(NHC)2(PhC[triple bond, length as m-dash]CPh) acted as a highly reactive pre-catalyst which was activated by diphenylacetylene dissociation to yield the active catalyst 40; subsequent oxidative addition of Pd(0) across the B–B bond gave 41, followed by NHC ligand dissociation and alkyne coordination. Then, insertion of the alkynes into the Pd–B bond and cistrans isomerization on Pd afforded the key intermediate 45, the following recoordination of the NHC ligand to 45, and the final reductive elimination gave the products and regenerated the catalyst (Scheme 14).

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Scheme 13 Pd(0)(NHC)2(PhC[triple bond, length as m-dash]CPh)-catalyzed diboration of alkynes.

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Scheme 14 Proposed catalytic cycle of the Pd(0)(NHC)2(PhC[triple bond, length as m-dash]CPh)-catalyzed diboration of alkynes.

3.3 Cobalt catalysis

In 2006, Adams et al. reported the preparation of 1,2-diborylalkenes through cobalt catalysis (eqn (11)).35 The reaction proceeded smoothly by the treatment of alkyne 47 and B2cat2 with a catalytic amount of Co(PMe3)4, affording the cis-isomer 48 as the major product in good yield. Although the reaction pathway remains unclear, this method provides an alternative strategy and additional complementarity to the preexisting methods for the diboration of alkynes.
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3.4 Copper catalysis

Pérez and Fernández reported the first example of the Cu-catalyzed cis-diboration of alkynes in 2007, in which a copper–NHC complex acted as the catalyst (eqn (12)).36 However, this transformation was limited to reactive B2cat2, while B2pin2 couldn't be tolerated. Besides, the catalyst used was uneconomical and not easily accessible.
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In contrast, a much more practical, economical and efficient Cu-catalyzed diboration of alkynes was developed by Yoshida's group in 2012. They found that B2pin2 smoothly added to alkynes 11 in a cis fashion to produce the cis-diborated products 12 in the presence of Cu(OAc)2 and PCy3 (Scheme 15).37 A broad range of aliphatic alkynes, diarylalkynes, and aryl(alkyl)alkynes could be converted by this procedure to deliver the corresponding products in

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moderate to high yields. Quite surprisingly, rather than the expected diboration, several alkyl-substituted propargyl ethers 51 tended to undergo triborylation when excessive B2pin2 was used, in which the MeO groups were replaced by boryl groups, and the triborylated compounds 52 were obtained as the major products (eqn (13)). Equally surprisingly, propargyl ether 54 likewise underwent tetraborylation to afford the tetraborylated product 55 exclusively (eqn (14)). It is worth mentioning that the triborylation and tetraborylation results were completely different from those obtained by the well-established Pt(PPh3)4-catalyzed diboration, in which the MeO groups remained intact throughout the reaction. The formation of three or four C–B bonds in a single one step demonstrated the high efficiency and unique access of this method in synthesizing polyborylated compounds. Remarkably, this efficient Cu-catalyzed diboration could also be applied to transient arynes, which were generated in situ from 2-(trimethylsilyl)phenyltriflate derivatives 33 with KF/18-crown-6. Various monosubstituted and disubstituted arynes were diborated by B2pin2 under the catalysis of (PPh3)3CuOAc, producing the vic-diborylarenes 34 in good yields (Scheme 16). Finally, a plausible reaction mechanism was outlined in Scheme 17. The interaction of copper acetate, ligand and the diboron reagent generated the key boryl-copper intermediate 56; subsequent addition of the B–Cu bond of 56 to alkynes formed the organocopper species 57. Then, C–Cu σ bond metathesis with the diboron reagent provided the diboration products and regenerated 56.

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Scheme 15 Cu(OAc)2-catalyzed diboration of alkynes.

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Scheme 16 (PPh3)3CuOAc-catalyzed diboration of arynes.

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Scheme 17 Proposed catalytic cycle for the Cu(OAc)2-catalyzed diboration of alkynes.

In 2014, Szabó and co-workers achieved the Cu-catalyzed diboration of propargyl epoxides 58, providing the unexpected ring-opening 1,2-diborylated butadiene derivatives 59 (Scheme 18).38 This diboration could be carried out successfully under a bimetallic CuI/Pd(PPh3)4 or CuCl/PCy3/KOt-Bu catalytic system, but the latter generally gave substantially improved yields and comparable or higher stereoselectivities than the former. Notably, the reactions occurred with high trans-selectivity under both catalytic systems, affording mainly or exclusively the E stereoisomer, which is not common and quite different from the cis-selectivity observed in other metal-catalyzed diborations. It should be emphasized that this is also the first example of the trans-selective diboration of alkynes. Although the reaction mechanism remains unclear, the involvement of an allenyl-Bpin species was hypothesized in the present diboration.

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Scheme 18 CuI/Pd(PPh3)4- or CuCl-catalyzed diboration of propargylic epoxides.

It is particularly worth noting that the Cu-catalyzed diboration of terminal alkynes developed by Hoveyda's group allowed the enantioselective synthesis of 1,2-diborylalkanes (Scheme 19).39 This is very different from other reported diboration of alkynes, in which 1,2-diborylalkenes were obtained as the products. In this reaction, the combination of CuCl and chiral NHC ligand 60[thin space (1/6-em)]40 converted diverse alkyl- and aryl-substituted terminal alkynes into the corresponding 1,2-diborylalkanes 61 with high enantiomeric purity. The authors conceived that the double site-selective B–Cu addition to the C–C triple bond and protonation of the C–Cu bond by MeOH gave the double hydroboration products 61.

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Scheme 19 Cu-catalyzed enantioselective diboration of terminal alkynes.

3.5 Iridium catalysis

In line with the Pt-catalyzed diboration of alkynes utilizing the unsymmetrical pinBBdan, Suginome's group also developed an Ir-catalyzed protocol employing the same batch of substrates 11 (Scheme 20).24 They disclosed that [IrCl(COD)]2 showed comparative catalytic activity to that of Pt(dba)2, generally affording the cis-adducts in good yields with excellent regioselectivity. Aryl terminal alkynes bearing electron-donating or electron-withdrawing groups, alkyl terminal alkynes, and internal alkynes were all suitable substrates in this iridium catalysis system. A variety of functional groups, such as ester, ketone and bromine groups, were well tolerated. In contrast to Pt(dba)2-catalyzed diboration, [IrCl(COD)]2-catalyzed diboration showed excellent regioselectivity without the assistance of any phosphine ligands, and generally displayed comparable or even higher regioselectivity except for electron-rich arylalkynes.
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Scheme 20 [IrCl(COD)]2-catalyzed diboration of alkynes with pinBBdan.

Particularly interesting is the Ir-catalyzed one-pot, two-step triborylation of terminal alkynes with excessive HBpin, as was reported by Ozerov's group in 2015 (Scheme 21).41 This process involved the dehydrogenative borylation of terminal alkynes catalyzed by [(SiNN)Ir(COE)] 64[thin space (1/6-em)]42 and subsequent dehydrogenative diboration of the formed alkynylboronate intermediates catalyzed by [(SiNN)Ir(CO)] 65, a new catalyst which was produced by the treatment of [(SiNN)Ir(COE)] in the reaction mixture with CO (eqn (15)). Aryl- and alkyl-substituted terminal alkynes were smoothly converted into the corresponding triborylalkenes 63 in moderate to high yields by this tandem C–H borylation and diboration reaction, thus generating a convenient method for the synthesis of triborylalkenes.43 Moreover, the synthesis of the trans-diaryldiborylalkenes through the terminal-selective Bpin coupling of triborylalkenes with iodobenzenes was also demonstrated by the authors, thus affording a potential synthetic strategy for

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trans-diaryldiborylalkenes since the reported trans-selective alkyne diboration generally required specific alkynes, such as propargyl epoxides,38 as the substrates. Although the mechanism of dehydrogenative diboration remains unclear, the authors confirmed based on their mechanistic studies that the diboration didn't proceed via 1,1-diborylalkenes or B2pin2 species. They speculated that the insertion of alkynylboronate into the Ir–B bond, the subsequent conversion of the Ir–H bond into the Ir–B bond, and the final elimination of the catalyst were likely involved.

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Scheme 21 Ir-catalyzed one-pot, two-step triborylation of terminal alkynes.

3.6 Gold catalysis

In 2013, Chen et al. demonstrated that nanoporous gold (AuNPore) exhibited a remarkable catalytic property towards the activation of diborons (Scheme 22).44 They found that AuNPore could cleave the B–B bond of diborons such as B2pin2 and B2hex2 directly without any ligands or additives, which allowed the efficient diboration of alkynes 11 in a heterogeneous process. Optimization studies showed that the choice of solvent played a key role in the reaction yield. Among the tested solvents, toluene was found to be more appropriate. Notably, various terminal and internal alkynes bearing diverse electron-withdrawing, electron-donating, and functional groups were compatible under the catalytic conditions to deliver the corresponding 1,2-diborylalkenes in good to high yields with high cis-selectivity (cis[thin space (1/6-em)]:[thin space (1/6-em)]trans = 75[thin space (1/6-em)]:[thin space (1/6-em)]25–100[thin space (1/6-em)]:[thin space (1/6-em)]0). More importantly, the AuNPore catalyst could be easily removed out by taking out the skeleton catalyst from the reaction mixture after the reaction without the change of nanoporous structure, the loss of catalytic activity and the metal contamination of the products. This is in sharp contrast with homogeneous catalysis, in which the catalysts are usually difficult to recycle for reuse. Furthermore, the synthetic utility of this approach was demonstrated elegantly by the authors in the one-pot synthesis of 1,1,2,2-tetraphenylethene 69 and 9,10-diphenylphenanthrene 70 without isolation or purification of the 1,2-diborylalkene intermediate 68 (Scheme 23). Mechanistic studies were also carried out with the radical inhibition experiment and the cross-addition experiment. The addition of the radical inhibitor 2,6-di-tert-butyl-4-methylphenol (BHT) didn't decrease the reaction rate and yield, indicating that radical species were not related. In addition, the reaction of phenylacetylene (1 mmol) with B2pin2 (1.5 mmol) and B2hex2 (1.5 mmol) in the presence of AuNPore resulted in a mixture of self-addition and cross-addition products in comparable yields, suggesting that the oxidative addition of Au(0) to diborons was not involved, which is quite different from Pt(0)-catalyzed diboration reactions. According to these results, a conceivable reaction pathway was proposed, as shown in Scheme 24. The absorption of B2pin2 onto the surface of the AuNPore catalyst and the following cleavage of the B–B bond produced the Au-Bpin species; subsequent simultaneous or stepwise addition of two Au-Bpin species to alkynes resulted in the desired products.
image file: c7qo00614d-s22.tif
Scheme 22 AuNPore catalyzed diboration of alkynes.

image file: c7qo00614d-s23.tif
Scheme 23 Synthetic utility of AuNPore catalyzed diboration of alkynes.

image file: c7qo00614d-s24.tif
Scheme 24 Possible reaction mechanism of AuNPore catalyzed diboration of alkynes.

Very recently, Stratakis and co-workers reported the diboration of alkynes catalyzed by Au nanoparticles supported on TiO2 (Au/TiO2) (Scheme 25).45 This catalytic system efficiently promoted the diboration of terminal and internal alkynes with B2pin2 in benzene or toluene to produce the cis-products 12 as the single isomer in most cases. Functional groups such as halides, nitriles, esters and silanes were all well tolerated. In order to achieve high yields, it should be noted that a dry solvent was required to avoid the Au-catalyzed hydrolysis of B2pin2. Compared with Jin's method,44 this approach showed milder conditions, shorter reaction time, and lower catalyst loading. Similarly, the catalyst could also be recovered without the profile change of the catalyst and the loss of catalytic activity. The possible reaction mechanism (Scheme 26) may involve the insertion of an Au nanoparticle into the B–B σ bond of B2pin2, the delivery of the boron moieties to the carbon–carbon triple bond, and the final elimination of the Au nanoparticle.

image file: c7qo00614d-s25.tif
Scheme 25 Au/TiO2 catalyzed diboration of alkynes.

image file: c7qo00614d-s26.tif
Scheme 26 Proposed mechanism of the Au/TiO2 catalyzed diboration of alkynes.

3.7 Iron catalysis

In 2015, Nakamura and co-workers disclosed a method for the preparation of diverse cis-1,2-diborylalkenes 12 through the Fe-catalyzed diboration of alkynes 11 with B2pin2 and an external borating agent MeOBpin (Scheme 27).46 Based on the optimization studies, the combination of FeBr2, LiOMe and MeOBpin turned out to be crucial and efficient for this transformation, and the lack of any of the three resulted in significantly reduced yields. As for the substrate scope, various internal alkynes were well tolerated and provided the diboration products with good to high yields and exclusive cis-selectivity. However, this reaction cannot be applicable to terminal alkynes because of the trimerization of alkynes triggered by the deprotonation of terminal alkynes.47 It should be noted that the solvent used also played a key role on the reaction yields. The reactions of alkynes with alkyl substituents proceeded smoothly in THF, while alkynes with aryl or bulky alkyl substituents didn't react in THF, and Bu2O which could drastically improve the yields was found to be more appropriate for these sterically hindered substrates. This may be because THF hampered the coordination of sterically hindered alkynes to the iron catalyst, whereas the more weakly coordinating solvent Bu2O didn't.
image file: c7qo00614d-s27.tif
Scheme 27 FeBr2-catalyzed diboration of alkynes.

In addition, unsymmetrical cis-1,2-diborylalkenes could also be obtained by the reaction of internal alkynes, B2pin2, and another borating agent such as MeOBneop (eqn (16)). This is of great significance because the resulting unsymmetrical cis-1,2-diborylalkenes could be transformed in a stepwise manner with excellent chemoselectivity due to the distinguishable reactivity of the two different B groups, allowing the use of these unsymmetrical cis-1,2-diborylalkenes as a platform for the synthesis of unsymmetrical multisubstituted alkenes.

image file: c7qo00614d-u16.tif(16)
image file: c7qo00614d-u17.tif(17)

To understand the reaction mechanism, the diboration of oct-4-yne 73 with B2pin2 and MeOBneop was carried out (eqn (17)). As a result, nuclear magnetic resonance (NMR) analysis showed that unsymmetrical diborylalkene 74 was produced as the major product (82% NMR yield), while the symmetrical diborylalkene 75 was only detected in an extremely low yield (9% NMR yield). This result indicated that the first incorporation of the boryl unit (Bpin) of B2pin2 and the second subsequent incorporation of the boryl unit (Bneop) of MeOBneop furnished the final products. It should be emphasized that this Fe-catalyzed diboration of alkynes is unique as compared with all the other reported diboration reactions, because the two B atoms of the products came from two different kinds of borating agents. Based on mechanistic studies, the authors proposed that the catalytic cycle (Scheme 28) commenced with the transmetalation of FeBr2 with LiOMe to produce the methoxyiron(II) intermediate 76, which reacted with B2pin2 to give the boryliron(II) intermediate 77. Then, coordination of alkynes to 77 and subsequent B–Fe bond addition to alkynes produced the alkenyliron(II) intermediate 78, which underwent an electrophilic substitution reaction with another borating reagent such as MeOBpin or MeOBneop, providing the diboration products and regenerating the methoxyiron(II) intermediate 76. Detailed mechanistic studies based on DFT calculations further supported the hypothetical mechanism.

image file: c7qo00614d-s28.tif
Scheme 28 Proposed mechanism of the FeBr2-catalyzed diboration of alkynes.

3.8 Base catalysis

Quite recently, Lin and Yamashita studied the base-catalyzed diboration of alkynes using the more reactive unsymmetrical pinB-Bmes210 as the diboron reagent (eqn (18)).48 Optimization studies revealed that a system consisting of n-BuLi and 1,2-dimethoxyethane in toluene gave optimal results. In general, this base-catalyzed diboration of terminal alkynes 38 produced a mixture of two cis-isomers (79 and 80) and one trans-isomer (81). In contrast, the less reactive
image file: c7qo00614d-u18.tif(18)
diboron reagents, such as B2pin2, didn't react under identical conditions. Finally, the authors speculated that two possible reaction pathways might be responsible for the regioselectivity and stereoselectivity observed. The direct diboration pathway (see Scheme 38 for details) led to the formation of 79 and 80, and the base-catalyzed pathway (Scheme 29), which involved a borataallene intermediate 84, furnished 80 and 81.

image file: c7qo00614d-s29.tif
Scheme 29 Possible reaction mechanism of the n-BuLi-catalyzed diboration of alkynes.

3.9 Organocatalysis

In addition to the metal-catalyzed diboration of alkynes, processes utilizing organocatalysis, which feature advantages such as low cost, environmental economy and the avoidance of metal contamination as compared with metal catalysis, have also been developed in recent years. Very recently, Ohmiya and Sawamura reported the first example of the diboration of alkynes through phosphine organocatalysis (Scheme 30).49 They found that alkynoates 85 were suitable substrates for diboration; various aryl- and alkyl-substituted alkynoates carrying diverse functional groups reacted with B2pin2 successfully to deliver the α,β-diboryl acrylates 86 in moderate to high yields. Remarkably, this metal-free PBu3 catalyzed diboration exhibited excellent stereoselectivity, producing the trans-adducts exclusively irrespective of the R substituents. This contrasts sharply with most of metal-catalyzed diborations, in which cis-stereoselectivity is favored. More importantly, the electron-withdrawing resonance effect of the ester group made β-C less nucleophilic than α-C. As a result, the two boron sites of the trans-α,β-diboryl acrylates could be differentiated and transformed in a selectively stepwise manner, allowing the synthesis of a variety of unsymmetrical tetrasubstituted olefins (Scheme 31). As depicted in Scheme 32, generation of the zwitterionic allenolate intermediate 90 from the conjugate addition of PBu3 to alkynoates and subsequent migration of the boryl groups to the sp-hybridized central carbon of the allene moiety followed by intramolecular cyclization and elimination of PBu3 were proposed as the reaction mechanism.
image file: c7qo00614d-s30.tif
Scheme 30 PBu3-catalyzed diboration of alkynes.

image file: c7qo00614d-s31.tif
Scheme 31 Selective stepwise C–B functionalization of trans-α,β-diboryl acrylates (DtBPF = 1,1′-bis(di-tert-butylphosphino)ferrocene).

image file: c7qo00614d-s32.tif
Scheme 32 Possible catalytic cycle of PBu3-catalyzed diboration of alkynes.

In 2015, Ogawa's group demonstrated that a catalytic amount of organosulfides under light could significantly promote the addition of B2pin2 to the terminal alkynes 38 (Scheme 33).50 Organosulfides, such as (PhS)2, (p-ClC6H4S)2, (p-CH3C6H4S)2, (p-NO2C6H4S)2, (p-CH3OC6H4S)2, (n-BuS)2, (MeS)2, PhSH and Ph2S, worked in different degrees, and (PhS)2 turned out to be the most effective catalyst which could enable the diboration of terminal alkynes 38 in the presence of light to offer the desired products 94 in good yields. However, this method suffered from unsatisfactory stereoselectivity and a mixture of cis/trans-diboration products was obtained (the ratio of cis[thin space (1/6-em)]:[thin space (1/6-em)]trans was around 3[thin space (1/6-em)]:[thin space (1/6-em)]7). Although the reaction mechanism was not yet completely understood, electron spin resonance (ESR) spectroscopy analysis of the reaction mixture and control experiments with radical initiators in the absence of light indicated that a boryl-centered radical 97, which was generated with the aid of light and (PhS)2, was likely involved in the mechanism (Scheme 34). It should be emphasized that B2pin2 couldn't add to alkynes under light but only with the assistance of (PhS)2, highlighting the important role of (PhS)2 in this radical reaction.

image file: c7qo00614d-s33.tif
Scheme 33 (PhS)2-catalyzed diboration of alkynes under light.

image file: c7qo00614d-s34.tif
Scheme 34 Possible pathway for the photoinduced diboration of alkynes catalyzed by (PhS)2.

Soon afterwards, the same group revealed that photoinduced diboration of terminal alkynes could also be carried out in the presence of a catalytic amount of PPh3 (Scheme 35).51 Various terminal alkynes 38 bearing ester, chloro, cyano, hydroxyl, and ether groups underwent diboration successfully, affording the trans-adducts as the major products, albeit in low yields because of the incomplete consumption of the substrates. Generally, increasing the amount of PPh3 could improve the reaction yields, but lower the trans/cis ratios of diborated products. Compared with (PhS)2-catalyzed diboration, the PPh3-catalyzed procedure displayed much higher trans-selectivity, but lower yields. Similarly, the diboration didn't occur in the dark or in the absence of PPh3, suggesting that boron-centered radicals 97 and 99 may be involved in the reaction pathway (Scheme 36), and the trans-selectivity may be ascribed to the steric hindrance between the boryl radical 100 and B2pin2.

image file: c7qo00614d-s35.tif
Scheme 35 Photoinduced diboration of alkynes catalyzed by PPh3.

image file: c7qo00614d-s36.tif
Scheme 36 Plausible catalytic cycle for the PPh3-catalyzed diboration of alkynes under light.

4 Catalyst-free diboration of alkynes

4.1 Direct and metal-free diboration

Although the first direct diboration of acetylene 101 with B2X4 (X = F, Cl) 8 in the absence of a catalyst had already been reported by Schlesinger in 1959 (eqn (19)), the diboron tetrahalides used in this reaction are thermally unstable and
image file: c7qo00614d-u19.tif(19)
cannot be easily handled.52 Therefore, the synthetic applications of this method were limited to some extent by these disadvantages.

Very recently, Yamashita and co-workers employed the more reactive unsymmetrical pinB-Bmes210 as the diboron reagent for the direct diboration of alkynes (eqn (20)).48 As a result, the diboration of aromatic and aliphatic terminal alkynes with pinB-Bmes2 could take place smoothly in toluene at 100 °C without any additives, producing two cis-isomers 79 (major) and 80 (minor). This direct and metal-free diboration was also applicable to the internal alkynes 73 and 104, even though lower reaction rates and longer reaction time were observed (Scheme 37). Besides, the authors also applied this reaction to synthesize fluorescent molecules via a diboration/Suzuki–Miyaura coupling sequence. As shown in Scheme 38, the proposed catalytic cycle of this reaction was illustrated to

image file: c7qo00614d-u20.tif(20)
commence by the coordination of the triple bond to the vacant p-orbital of the Bmes2 moiety; then pinB-Bmes2 directly added to the triple bond in a cis fashion via two possible transition states (TS1 and TS2). TS1 was more favored than TS2 because of the steric repulsion between the pinB moiety and the alkyne substituents in TS2, thus leading to the formation of 79 as major products.

image file: c7qo00614d-s37.tif
Scheme 37 Direct diboration of internal alkynes with pinB-Bmes2.

image file: c7qo00614d-s38.tif
Scheme 38 Proposed mechanism of the direct diboration of alkynes with pinB-Bmes2.

4.2 Base-promoted diboration

In 2014, Hirano and Uchiyama reported a stoichiometric amount of base promoted trans-diboration of propargylic alcohols 107 by adopting a pseudo-intramolecular strategy (Scheme 39),53 which could significantly lower the activation barrier of the addition reaction. They revealed that the reaction temperature and countercation of the base greatly influenced the reaction yields. Lithium bases such as n-BuLi and MeLi could promote this transformation significantly and allow the diboration of propargylic alcohols 107 to proceed smoothly in a two-step, one-pot manner. Interestingly, the present diboration gave the unexpected ring-closing oxaboroles 108 as the final products after work up with aqueous NH4Cl. The pseudo-intramolecular reaction facilitated B–B bond activation and C–B bond formation with high efficiency and high functional group compatibility. Various propargylic alcohols 107 carrying diverse substituents at R1, R2, and R3 underwent diboration
image file: c7qo00614d-u21.tif(21)
with different diborons such as B2pin2, B2neop2, and pinBBdan, providing the densely functionalized 4-borylated 1,2-oxaborol-2(5H)-oles 108 in moderate to high yields. Remarkably, oxaboroles 109 could also be obtained from terminal alkynes 38 and acetone through a three-step, one-pot sequential reaction (Scheme 40). This offered a much more convenient access to oxaboroles. It should be noted that this example was also one of the few earliest reported trans-selective diboration of alkynes.54 The perfect trans-stereoselectivity is in sharp contrast to that observed in most metal-catalyzed diboration reactions, in which cis-stereoselectivity is preferred. In addition, the tetrasubstituted olefin 111 was successfully prepared by the authors through a sequential diboration/Suzuki–Miyaura coupling process in one pot (eqn (21)), indicating the synthetic applications of this approach in the preparation of functionalized alkenes. The proposed reaction mechanism is shown in Scheme 41: deprotonation of propargylic alcohols formed the intermediate 112, then complexation of 112 with diborons generated the complex 113, which allowed the reaction to proceed in a pseudo-intramolecular way. Subsequent stepwise borylation gave the intermediate 115, which furnished the diboration products after a workup process.

image file: c7qo00614d-s39.tif
Scheme 39 n-BuLi or MeLi promoted diboration of propargylic alcohols.

image file: c7qo00614d-s40.tif
Scheme 40 n-BuLi promoted one-pot trans-diboration of terminal alkynes.

image file: c7qo00614d-s41.tif
Scheme 41 Proposed mechanism of n-BuLi or MeLi promoted diboration of propargylic alcohols.

5. Applications in organic synthesis

The diboration of alkynes can construct various functionalized 1,2-diborylalkenes, which are very common and important building blocks in organic synthesis, from readily available and simple materials, featuring advantages such as atom- and step-economy, excellent regio- and stereoselectivity, and high yields. With the rapid development of metal catalysis, organocatalysis and catalyst-free processes in the past two decades, the diboration of alkynes has become an important and powerful tool in organic synthesis. Herein, we selected representative examples on the synthesis of chiral 1,2-diols, functionalized heterocycles and pharmaceutical agents to demonstrate the significance of the diboration of alkynes.

The 1,2-diborylalkenes, which were prepared through the Pt-catalyzed diboration of alkynes, were used for the synthesis of chiral 1,2-diols via an enantioselective hydrogenation/oxidation sequence by Morken and co-workers in 2004 (Scheme 42).55 The combination of Rh(NBD)2BF4 (NBD = 2,5-norbornadiene) and ligand 117 turned out to be efficient in the enantioselective hydrogenation of 1,2-diborylalkenes 116, and the following basic peroxide oxidation delivered the desired 1,2-diols 118 with high enantioselectivity and yields. This process provides an alternative synthetic strategy to Sharpless asymmetric dihydroxylation.56

image file: c7qo00614d-s42.tif
Scheme 42 Synthesis of chiral 1,2-diols from 1,2-diborylalkenes via an enantioselective hydrogenation/oxidation sequence.

In 2008, Carson's group reported a diboration, inter-/intramolecular Suzuki–Miyaura coupling sequence which enabled the synthesis of pyridylbenzoxepines 120.57 As shown in Scheme 43, the Pt(PPh3)4-catalyzed diboration of 119 proceeded efficiently in a cis-selective fashion, followed by highly regioselective sequential intra-/intermolecular cross couplings to produce diverse pyridylbenzoxepines 120 carrying a geometrically pure exocyclic tetrasubstituted alkene moiety. Notably, as pyridine analogues of dibenzoxepines, these pyridylbenzoxepine derivatives may also be potential nuclear hormone receptor modulators.58

image file: c7qo00614d-s43.tif
Scheme 43 Synthesis of pyridylbenzoxepines via a diboration, inter-/intramolecular Suzuki–Miyaura coupling sequence.

Shimizu and co-workers developed a diboration/annulation sequence to synthesize functionalized indenes in 2009 (Scheme 44).2l The 1,2-diborylalkenes obtained with Miyaura and Suzuki's method11 were subjected to a Pd-catalyzed double cross-coupling reaction with vic-bromo(bromomethyl)arenes 121, and the annulation products 122 were produced in good to high yields. In addition, heterocycle-fused cyclopentenes, fluorenes, dihydroindacenes and indeno[2,1-b]fluorines could also be prepared via a similar procedure.

image file: c7qo00614d-s44.tif
Scheme 44 Synthesis of indenes via a diboration/annulation sequence.

Very recently, Harrity and co-workers employed an alkyne diboration/6π-electrocyclization strategy for the synthesis of pyridine-based boronic acid derivatives (Scheme 45).59 They initiated their study with the Pt(PPh3)4-catalyzed cis-diboration of 2-alkynyl aryloximes 123 with B2pin2, which produced the diboration products 124 in good to high yields. The following 6π-electrocyclization of 124 in 1,2-dichlorobenzene at an elevated temperature delivered a variety of isoquinoline boronic acid derivatives 125 in good to excellent yields after the elimination of MeOBpin. Moreover, other bicyclic pyridine-based heterocycles, such as 5,6,7,8-tetrahydroisoquinolines and 3,4-dihydro-1H-pyrano[3,4-c]pyridines, were also synthesized through a similar procedure. Considering the ubiquitous presence of pyridine-based heterocyclic compounds in a large number of natural products and pharmaceutical agents, these abovementioned pyridine-based boronic acid derivatives are regarded as valuable building blocks to assemble pharmacologically active compounds by C–B bond functionalization.

image file: c7qo00614d-s45.tif
Scheme 45 Synthesis of isoquinoline boronic acid derivatives through a diboration/6π-electrocyclization strategy.

The diboration of alkynes and subsequent Suzuki–Miyaura coupling provide a versatile access to tetrasubstituted olefins, and numerous examples have been reported.2a–u,60 It is particularly worth noting that the process developed by Ohmiya and Sawamura allowed the preparation of tamoxifen-type tetrasubstituted olefins (Scheme 46).49 This process was initiated with the PBu3-catalyzed diboration of 126, followed by a selectively stepwise Suzuki–Miyaura coupling to provide 128. Specifically, compound 128a was reduced to yield 129, which was then transformed to Z-tamoxifen 130[thin space (1/6-em)]61 according to a known procedure.62 Similarly, other tamoxifen-type tetrasubstituted olefins could also be synthesized through this procedure.

image file: c7qo00614d-s46.tif
Scheme 46 Synthesis of tamoxifen-type compounds via a diboration/Suzuki–Miyaura coupling sequence.

6 Conclusions and perspectives

Over the past two decades, catalytic and catalyst-free diboration of alkynes has contributed tremendously to 1,2-diborylalkene synthesis; it has doubtless become the most straightforward and efficient method for the preparation of functionalized 1,2-diborylalkenes. Among the reported methods, except for the Cu-catalyzed diboration of propargyl epoxides, metal catalysis has been the most efficient strategy for the synthesis of symmetrical and unsymmetrical cis-1,2-diborylalkenes because of its high catalytic activity, general applicability, and excellent cis-selectivity. In contrast, the efficiency and applications of base catalysis and direct diboration are limited by the requirement of particular diboron reagents and unsatisfactory regio- and stereoselectivity. In addition, although organocatalysis and base-promoted diboration need specific alkynes such as alkynoates and propargylic alcohols as substrates, they do offer a facile access to trans-1,2-diborylalkenes, which are usually difficult to synthesize via metal catalysis.

Despite the remarkable achievements made, there are at least four areas where some critical advances are necessary to make the diboration of alkynes more powerful: (a) the exploration of novel catalytic systems, especially cheap metal catalysis and organocatalysis, will continue to drive this field; (b) the development of trans-selective diboration and reusable catalysts will also be a good direction to take; (c) as all the reported diboration reactions proceeded in an intermolecular manner, the development of intramolecular processes employing novel diboron substrates linked with an alkyne moiety is highly demanding to construct B-containing heterocycles; and (d) the development of cascade reactions involving diboration is also highly desirable considering their efficiency and step economy in constructing complex heterocyclic compounds.

Conflicts of interest

There are no conflicts to declare.


We gratefully acknowledge financial support from the National Natural Science Foundation of China (21602022, 81620108027 and 21632008), the Major Project of Chinese National Programs for Fundamental Research and Development (2015CB910304), and the Open Project Program of Key Laboratory of Medicinal and Edible Plants Resources Development of Sichuan Education Department (10Y201711).

Notes and references

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