Copper-catalyzed remote double functionalization of allenynes

Abstract

Addition reactions of molecules with conjugated or non-conjugated multiple unsaturated C–C bonds are very attractive yet challenging due to the versatile issues of chemo-, regio-, and stereo-selectivities. Especially for the readily available conjugated allenyne compounds, the reactivities have not been explored. The first example of copper-catalyzed 2,5-hydrofunctionalization and 2,5-difunctionalization of allenynes, which provides a facile access to versatile conjugated vinylic allenes with a C–B or C–Si bond, has been developed. This mild protocol has a broad substrate scope tolerating many synthetically useful functional groups. Due to the highly functionalized nature of the products, they have been demonstrated as platform molecules for the efficient syntheses of monocyclic products including poly-substituted benzenes, bicyclic compounds, and highly functionalized allene molecules.

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Unsaturated hydrocarbons are a class of very important compounds due to their ubiquity in organic synthesis, natural products, materials, and pharmaceutics.^1–4 Of particular interest, studies on versatile reactivities of C–C multiple bonds have been drawing more and more attention from organic, medicinal, and materials chemists.^5–7 The reactions of the C–C double bond and C–C triple bond, including hydrofunctionalization and difunctionalization reactions, have been extensively developed with attractive regio- and stereo-selectivity (Schemes 1A and B).^8–42 Recently, the addition reactions of allenes, which deliver stereodefined olefins with decent regioselectivities, have also been established (Scheme 1C).^43–60 Such reactions of molecules with multiple unsaturated C–C bonds are very attractive yet challenging due to the versatile issues of chemo-, regio-, and stereo-selectivities (Scheme 1D).^61–63 Although conjugated enynes and dienes have been studied,^64–66 the reaction of allenynes merging an alkyne unit and an allene unit in a conjugated manner has not been studied.^67–69 It would be challenging to control the related selectivities forming different 1,2-, 2,3-, 2,5-, 4,5-addition products with different unsaturated C–C bonds (Scheme 1E). Herein, we wish to disclose our recent observation on copper-catalyzed highly regioselective 2,5-boryl- and silyl-cupration of readily available conjugated allene-ynes for efficient synthesis of conjugated vinylic allenes with a versatile C–B or C–Si bond (Scheme 1F).

Scheme 1 Double functionalization of alkynes, alkenes, allenes, and conjugated allene-ynes.

We initiated the study with the reaction of allenyne 1a^70–89 and B₂(pin)₂2a in the presence of MeOH with NaO^tBu as the base and CuCl (5 mol%) and Biphep (L₁) (6 mol%) as the catalyst at 30 °C in THF for 16 h. To our delight, the 2,5-addition product, 1,3,4-trienyl boronate 3aa, was formed in 33% yield with 3% yield of the 2,3-addition product, 1,4-enyne 3aa′ (entry 1, Table 1). Upon adjusting the ratio of 1a/2a to 1/1, a much higher yield of the desired product 3aa was achieved (entries 2–5, Table 1). When the reaction was conducted at 15 °C, the yield of 3aa was improved to 85% with 5% yield of 3aa′ (entry 6, Table 1). Interestingly, when MeOH was replaced with ^tBuOH, the generation of 3aa′ was completely suppressed with 43% yield of 2,5-double functionalization product 3aa (entry 9, Table 1). Further screening of the ligand and [Cu] with ^tBuOH led to the observation that the combination of Binap (L₂) and Cu(CH₃CN)₄PF₆ was better than that with Biphep and CuCl as the catalyst with the yield of 86% with 4% recovery of 1a (entries 10–11, Table 1). Upon adjusting the ratio of 1a/2a to 1/1.05, the reaction delivered the best results affording 3aa in 87% yield with no recovery of 1a and no formation of 3aa′ (entry 12, Table 1).

Table 1 Optimization of the conditions^a

Entry	ROH	X	T (^oC)	t (h)	1a (%)^b	3aa/3aa′ (%)^b
a Reaction conditions: 1a (0.2 mmol), 2a (X equiv.), CuCl (5 mol%), Biphep (L1) (6 mol%), NaO^tBu (20 mol%), and ROH (2 equiv.) in THF (1 mL). b Determined by ^1H NMR analysis of the crude product using mesitylene as the internal standard. c Binap (L2) was applied instead of Biphep (L1). d Cu(CH3CN)4PF6 was applied instead of CuCl.
1	MeOH	2.0	30	16	0	33/3
2	MeOH	1.5	30	16	0	51/6
3	MeOH	1.2	30	15.5	0	62/7
4	MeOH	1.1	30	15.5	0	73/6
5	MeOH	1.0	30	15.5	0	80/6
6	MeOH	1.0	15	5.5	5	85/5
7	MeOH	1.0	40	5.5	4	80/4
8	MeOH	1.0	50	5	4	71/3
9	^tBuOH	1.0	15	16	35	43/0
10^c	^tBuOH	1.0	15	13	10	83/0
11^c^,^d	^tBuOH	1.0	15	1	4	86/0
12^c^,^d	^tBuOH	1.05	15	1	0	87/0

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With the optimized reaction conditions in hand (entry 12, Table 1), we next investigated the reactivity of various allenynes (Table 2). A series of allenynes bearing different R¹ substituents incorporating functional groups, such as ester (3aa), highly strained cyclopropyl (3ca), benzyl (3da), halide (3ea), and nitrile (3ga) were all tolerated with the yields of 77–97%. In addition, the substrate with the OH group being protected as TBS ether also could undergo the reaction smoothly with the yield of 80% (3fa). Even the glucose-derived allenyne also afforded the boryl-substituted 1,3,4-triene 3ha with the yield of 65%. R¹ may also be aryl groups; a variety of different substituents on the aryl units including electron-donating (3ja and 3ka) and electron-withdrawing groups as well as biologically or synthetically useful groups such as F, Cl, Br, CF₃, CN, and COOMe at the para-, ortho-, or meta-position (3la–3sa) were accommodated to afford the 1,3,4-trien-2-yl boronates in moderate to good yields (64–87%). p-NO₂ on the benzene ring also survived with 48% yield of 3ta. Different R² groups bearing halide (3ua, 76%) and benzyl (3va, 53%) were tolerated.

Table 2 Scope of 2,5-hydrofunctionalization

a Reaction conditions: 1, 2a (1.05 equiv.), ^tBuOH (2.0 equiv.), Cu(CH₃CN)₄PF₆ (5 mol%), Binap (L₂) (6 mol%), and NaO^tBu in THF (5 mL) at 15 °C on a 1.0 mmol scale. b 1.1 equiv. 2a was used. c Reaction conditions: 1 (1 equiv.), 4a (1.5 equiv.), ^tBuOH (2.0 equiv.), Cu(CH₃CN)₄PF₆ (5 mol%), Binap (6 mol%) and NaO^tBu in THF (5 mL) at 15 °C on a 1.0 mmol scale. d 3ua′ is 2-(3-(3-chloropropyl)-9-methoxy-9-oxonon-1-en-4-yn-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. e 3va′ is 2-(3-benzyl-9-methoxy-9-oxonon-1-en-4-yn-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. f On a 0.5 mmol scale.

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Furthermore, B₂(pin)₂2a may be replaced with Me₂PhSiBpin 4a. The corresponding 1,3,4-trien-2-yl silanes bearing different R¹ functional groups, such as ester (5aa), halide (5ea), and nitrile (5ga), were obtained in yields of 70–78%. In addition, synthetically versatile p-OMe (5ka), -Cl (5ma), -Br (5pa), and -CN (5ra) on the benzene ring also survived with the yields of 66–79%. As for different substitutions at the R² position, 1u (R² = ClC₃H₆–) and 1y (R² = PhC₂H₄–) were tested to afford the corresponding silane products 5ua in 61% yield and 5ya in 71% yield, respectively.

In addition, trisubstituted allenyne (1w) and tetrasubstituted allenyne (1x) are also suitable for affording the corresponding Z- and E-isomers of 1,3,4-trienyl boronates 3wa in 68% yield and 3xa in 68% yield. Furthermore, the reaction of 1w also works with Me₂PhSiBpin 4a affording 1,3,4-trien-2-yl silanes E-5wa in 75% yield (Scheme 2). When we tried iodobenzene as the electrophile under standard conditions, the target arylborylation product 7aa was not produced. Interestingly, 7aa was formed when an extra catalytic amount of Pd(dba)₂ was introduced (Table 3). Further screening of the temperature and ligand led to the formation of 7aa in 53% yield. Aryl iodides with different electron-donating or -withdrawing groups afforded 7ab, 7ac, 7ad, 7ae, and 7af in 42–51% yields. Allenynes with different alkyl substituents (R² = n-C₆H₁₃–, AcOC₄H₈–) also afforded the desired products 7ya and 7za (Table 3). 2,5-Hydrofunctionalization byproducts 3 were observed in all the cases, indicating that the Cu-catalyzed borylation reaction occurred before the Pd cross-coupling reaction with ArI.

Scheme 2 Scope of trisubstituted and tetrasubstituted allenynes

Table 3 Scope of 2,5-arylboration^a

a Reaction conditions: 1 (0.1 mmol), 2a (1.4 equiv.), 6 (1.4 equiv.), Cu(CH₃CN)₄PF₆ (7.5 mol%), Pd(dba)₂ (5 mol%), dppf (12.5 mol%), and NaO^tBu (2 equiv.) in THF (1 mL) at 70 °C on a 0.1 mmol scale. b ¹H NMR yield of 2,5-hydroboration by-products 3aa, 3ya, and 3za in parentheses.

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To gain insight into the mechanism, a deuterium-labelling experiment was conducted. When the reaction was performed with a stoichiometric amount of ^tBuOD, 70% of [D]-3aa with 75% deuterium incorporation at the allenic position was observed (Scheme 3A). When the R¹ group in 1z′ is a highly sterically hindered TMS group, the reaction failed to afford the 1,3,4-trienes. Instead, 1,4-enyne [D]-3za′ with 66% deuterium incorporation was formed, indicating that the steric hindrance of TMS plays a critical role in determining the regioselectivity (Scheme 3B). On the basis of these experimental data, we proposed a possible mechanism (Scheme 3C). Initially, the copper–boryl complex I is generated in situ via the reaction of B₂(pin)₂ with the Cu precursor. Then, the C=C bond in the allene next to the C–C triple bond highly regioselectively inserted into the copper–boryl bond in I with the boron connected to the middle carbon atom to afford the propargylic copper species II, which would rapidly isomerize to allenyl copper species III and subsequently get trapped with ^tBuOD to produce the final product [D]-3, regenerating LCuO^tBu to complete this catalytic cycle. Meanwhile, protonation of another isomeric form II at the γ position would also afford product [D]-3. When the R group is the sterically bulky TMS group, II would directly undergo protonolysis to form 1,4-enyne [D]-3za′. In the first step, the allene was inserted into the copper–boryl bond in intermediate I since the allene unit is more reactive than the alkyne unit. The regioselectivity for the protonolysis depended on the steric hindrance of the R¹ group in copper species II or III.

Scheme 3 Deuteration experiment and the proposed mechanism.

In order to showcase the synthetic utility of the products, further transformations of the 1,3,4-trien-2-yl boronates/silanes were demonstrated (Scheme 4): The one-pot sequential 2,5-borylcurpration of allenyne 8 and a Diels–Alder reaction with maleic anhydride afforded stereodefined bicyclic lactonyl borate 9 (Scheme 4A).⁹⁰ Similarly, allenene 3aa could also be readily converted to corresponding bicyclic lactone product 10, 1,4-cyclohexadiene with a stereodefined exo-cyclic C=C double bond 12, and penta-substituted benzene 13via a Diels–Alder reaction with dimethyl but-2-ynedioate under different conditions. Such differently polysubstituted benzenes are difficult to prepare yet very useful.⁹¹ In order to show the reactivity of the C–B bond, the Diels–Alder product 13 was employed for further transformation. Firstly, 13 was successfully coupled with methyl 4-iodobenzoate to afford 14 in 89% yield.⁹² Secondly, the iodination of 13 with KI afforded the iodination product 15 in 49% yield catalyzed by copper iodide under air.⁹³ Thirdly, bromination of the C–B bond in 13 has been realized to afford 16 in an excellent yield by its treatment with copper bromide.⁹³ The intramolecular cycloisomerization of 3aa also proceeded smoothly to give the corresponding cyclopentadienyl boronate 11 (Scheme 4B).⁹⁴ Moreover, 3aa could be readily converted into 2-alkynyl-1,3,4-triene 17via alkynylation with alkynyl bromide⁷⁹ and allenyl ketone 18via oxidation with H₂O₂.⁹⁵ In addition, the C–B bond in 3aa was successfully coupled with methyl 4-iodobenzoate to afford 19 in 38% yield even in the presence of a highly reactive allene unit (Scheme 4C). Meanwhile, other dienophiles, such as 1,4-naphthoquinone, maleic anhydride, and N-phenylmaleimide, could all undergo the Diels–Alder reaction with silane 5aa to produce the corresponding products 20, 21, and 22 in decent yields (Scheme 4D).⁹⁰

Scheme 4 Synthetic applications.

Conclusions

In summary, we have developed the first example of copper-catalyzed 2,5-boryl- or silyl-cupration of allenynes, providing an efficient protocol for substituted 1,3,4-trien-2-yl boronates or silanes with excellent regioselectivity. This method has advantages of a readily available catalyst and starting materials, high catalytic activity, mild conditions, a broad substrate scope tolerating many synthetically useful functional groups, and synthetically useful functionalities. The synthetic utility of this protocol has been demonstrated via the versatile transformations of the resulting products to furnish valuable functionalized molecules. Further studies are being actively pursued in our laboratory.

Data availability

The data supporting this article have been uploaded as the ESI.†

Author contributions

S. M. and J. Z. conceived and designed the experiments. Y. S., J. Z. performed the experiments. Y. S., C. F., J. Z., and S. M. wrote the manuscript. S. M. and J. Z. directed the research.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the National Key R&D Program of China (2022YFA1503200), the National Natural Science Foundation of China (21988101), and the Fundamental Research Funds for the Central Universities (226-2022-00224) is greatly appreciated. We thank Mr Wenxiang He in this group for reproducing the preparation of 3ga, 3ta and 5ka. Shengming Ma is a Qiu Shi Adjunct Professor at Zhejiang University.

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Footnote

† Electronic supplementary information (ESI) available. As well as CCDC 2085439 and 2119437 available. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4sc00034j

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