ReI-Catalyzed highly regio- and stereoselective C–H addition to terminal and internal alkynes

Yu-Che Chang , Sekar Prakash and Chien-Hong Cheng *
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan. E-mail: chcheng@mx.nthu.edu.tw

Received 5th October 2018 , Accepted 31st October 2018

First published on 1st November 2018


We have developed an effective ortho C–H functionalization of arylpyridines and detachable N-pyrimidyl indoles by terminal and internal alkynes using a Re(I) catalyst providing an efficient access to various E-selective alkenylation products. The catalytic reaction is compatible with various aliphatic alkynes, aromatic terminal alkynes and internal alkynes, and structurally different nitrogen heterocycles. Deuterium-labeling experiments indicate that significant deuterium scrambling occurs with the directing groups and acetylenic sp C–H bonds before the migratory insertion.


Hydroarylation of carbon–carbon multiple bonds by metal-catalyzed directed C–H activation reactions is an atom- and step-economical approach for the synthesis of alkenylated or alkylated hetero-aromatics without wasting any components.1a–d Intramolecular hydroarylation of heteroarenes with tethered alkenes or alkynes also efficiently forms cyclic heterocycles.1e,f This environmentally benign approach increasingly replaces the traditional cross-coupling reactions such as Heck reactions, and Suzuki and Stille reactions which require the use of pre-functionalized substrates and generate substantial amounts of wastes.

Transition-metals such as Ru,2 Rh,3 and Ir4 are known to convert C–C triple bonds to double bonds through the directed hydroarylation strategy. Recently, the use of first-row transition metals such as Co,5 Mn,6 and Fe7 as the catalysts for hydroarylation reactions via C–H activation has been realized. Transition metals other than 3d and 4d elements also have been studied, mainly a 5d transition metal, rhenium, on C–H functionalizations. Noticeably, due to its π-acidic property and high polarity of carbon–rhenium bonds, rhenium has displayed a distinctive catalytic reactivity in C–H activation reactions.8a,b Besides, very little is known about the toxicity of rhenium complexes.8c

Kuninobu/Takai,8d–h Wang,8i–k and others8l–q have extensively tackled rhenium-catalyzed C–H functionalization reactions. Inspired by these reports, we investigated the hydroarylation reaction of terminal and internal alkynes under ReI-catalysis through directed C–H functionalization reactions. Although certain transition metals can convert terminal alkynes into o-alkenylated products via the commonly proposed 2,1-insertion pathway, few hydroarylations of the terminal and internal alkynes with the aid of rhenium catalysis via the C–H bond activation strategy have been reported until now.8p

Zhang et al. in 2008 established a directed ortho alkenylation of phenyl pyridine derivatives with terminal alkynes using Ru as a catalyst.2a Later on, RhIII-catalyzed hydroarylation of internal alkynes for the synthesis of C-2 alkenylated indoles was reported. However, terminal alkynes afforded only a trace amount of the alkenylation product under the reaction conditions.3a In 2010, Yoshikai and co-workers published an in situ CoI-catalyzed alkenylation of phenyl pyridine derivatives with internal alkynes.5a Notably, Chen, Wang, and coworkers reported a MnI-catalyzed ortho C–H alkenylation of phenyl pyridines with the challenging terminal alkynes. In addition, the isolation of the manganacycle intermediate and the performed DFT calculation supported the proposed 2,1-insertion pathway. However, the hydroarylation failed with internal alkynes.6a Later, a number of metal catalytic systems were developed for the directing group-assisted ortho C–H alkenylations.2–7

During the preparation of the manuscript, Rueping reported a Mn- and Re-catalyzed alkenylation of indole derivatives with internal and terminal alkynes under an Ar atmosphere with higher catalyst loadings.8p Herein, we wish to report a ReI-catalyzed highly regio- and stereoselective directed C–H addition to terminal and internal alkynes. It is noteworthy that the reaction selectively affords the alkenylation products with terminal alkynes and internal alkynes with complete (E)-selectivity. The reaction even works with more vulnerable aliphatic internal and terminal alkynes in air to give the insertion products in excellent yields.

To begin optimization for this hydroarylation reaction, we chose 2-phenylpyridine (1a) and phenylacetylene (2a) as the coupling partners. When 1a was treated with 2a in the presence of ReBr(CO)5 (5 mol%) and NaOAc (30 mol%) in 1,4-dioxane at 130 °C in air for 12 h, alkenylated product 3aa was obtained in 88% yield (Table 1, entry 1). With this promising result in hand, we then screened several solvents for this conversion. Chlorinated solvents such as DCE and PhCl yielded low to moderate yields of 3aa (entries 2 and 3). In o-xylene, the reaction gave the desired product 3aa in good yield (entry 4). To our delight, toluene afforded the expected product 3aa in 96% yield (entry 5). Later, the efficacy of additives such as NaOMe, NEt3, and Na2CO3 was tested; notably, Na2CO3 gave 3aa in high 91% yield (entries 6–8). The quantity of additives was also screened: reducing the NaOAc loading from 30 mol% to 20 or 10 mol% did not alter the reaction efficacy and gave similar yields (entries 9 and 10). Indeed, no alkenylation product detected in the absence of NaOAc seems to indicate that the acetate ion plays an important role in achieving the desired conversion (entry 11). Lowering the reaction temperature to 120 °C consistently gave a similar yield of 3aa (entry 12). However, when the amount of NaOAc was reduced to 5 mol%, the yield dropped to 75% (entry 13). Notably, the reaction produced 3aa in 90% yield when we reduced the catalyst loadings to 2.5 mol% (entry 14) and gave 3aa in 90% yield, when we also decreased the reaction time to 4 h (entry 15).

Table 1 Optimization studiesa,b

image file: c8qo01068d-u1.tif

Entry Additive (mol%) Solvent Temp. (°C)/time (h) Yield (%)
a Unless otherwise mentioned, all reactions were performed using 2-phenylpyridine 1a (0.2 mmol), phenylacetylene 2a (0.3 mmol), ReBr(CO)5 (5 mol%), additives (5–30 mol%) and solvent (1 ml). b Yields were determined by the 1H NMR integration method using mesitylene as the internal standard. c 2.5 mol% of ReBr(CO)5 used. An isolated yield is given in parentheses.
1 NaOAc (30) 1,4-Dioxane 130/12 88
2 NaOAc (30) DCE 130/12 63
3 NaOAc (30) PhCl 130/12 42
4 NaOAc (30) o-Xylene 130/12 84
5 NaOAc (30) Toluene 130/12 96
6 NaOMe (30) Toluene 130/12 N.R.
7 Et3N (30) Toluene 130/12 13
8 Na2CO3 (30) Toluene 130/12 91
9 NaOAc (20) Toluene 130/12 96
10 NaOAc (10) Toluene 130/12 91
11 Toluene 130/12 Trace
12 NaOAc (10) Toluene 120/12 93
13 NaOAc (5) Toluene 120/12 75
14c NaOAc (10) Toluene 120/12 90
15c NaOAc (10) Toluene 120/4 90 (87)


Next, we found out the scope of this alkenylation reaction with various directing groups under our optimized reaction conditions (Scheme 1). Phenyl pyridines substituted with electron-donating or -withdrawing functional groups at the para position of the phenyl ring (1a–1h) yielded the corresponding alkenylated products (3aa–3ha) in moderate to excellent yields. Next, substituents attached to the pyridine ring of the phenyl pyridine (1i and 1j) offered the respective alkenylation products in good yields. Several m-substituted arylpyridines were tested under this condition, and m-methyl arylpyridine (1k) resulted in a mixture of regioisomers 3ka and 3ka′ in a 5[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio, where the major isomer 3ka arises from the C–H activation at the least hindered site. Similarly, 2-pyridylnaphthalene (1l) showed a moderate site selectivity and yielded a mixture of regioisomers, 3la and 3la′, in a 3[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio with an overall yield of 82%.


image file: c8qo01068d-s1.tif
Scheme 1 Substrate scope of arylpyridines. (Unless otherwise mentioned, all reactions were performed using 2-arylpyridine, 1a–1s, (0.2 mmol), phenylacetylene 2a (0.3 mmol), ReBr(CO)5 (2.5 mol%), NaOAc (10 mol%), toluene (1 ml), 120 °C, and 4 h. Isolated yields. The ratio of regioisomers is given in parentheses.)

Only when we used m-fluoroarylpyridine (1m) as a substrate, did we observe a site-selective C–H alkenylation at the hindered position of the phenyl ring. This is due to the strong ortho effect of the fluorine atom, which increases the acidity of the C–H bond next to it.6a We also considered the C–H activation of a 2-(thien-2-yl)pyridine (1n) and then the reaction with 2a to provide 3na in 85% yield. The reaction between benzo(h)quinoline (1o) and 2a proceeded efficiently to give 3oa in 89% yield. The activation of the very challenging vinylic sp2 C–H bond (1p) also worked well and afforded 58% of (E,E)-1,3-diene product 3pa. Later, the directing group ability of cleavable N-pyrimidyl indole derivatives, 1q–1s, was tested under the conditions to give the corresponding desired C–H alkenylation products 3qa–3sa in 84–94% yields.

This alkenylation reaction was also tested with various terminal alkynes (Scheme 2). Terminal alkynes bearing electron-donating and halogen substituents at para and meta to alkynyl groups (2b–2h) underwent efficient alkenylation and produced the products (3ab–3ah) in 70–93% yields. However, terminal alkynes with the electron withdrawing –CF3 group afforded a slightly smaller yield of alkenylation product 3ah. Sterically hindered mesityl acetylene (2i) and electron-deficient 3,5-difluoro phenylacetylene (2j) gave alkenylation products (3ai and 3aj) in moderate to excellent yields. The efficacy of aliphatic terminal alkynes was examined, and when the reaction mixture was treated with homo benzylic terminal alkyne (2k), the reaction mixture afforded 3ak in 79% yield with no considerable double bond migration. Cyclohexyl terminal alkyne (2l) and alkynyl alcohol (2m) would seem to be suitable substrates for this hydroarylation and provided the products 3al and 3am in 91% and 41% yields, respectively. It is possible that the –OH group in 2m might coordinate with the active rhenium metal that partially deactivates the catalyst efficiency and justifies the lower yield of 3am. Eventually, N-pyrimidyl indole (1q) would also be capable of producing the desired alkenylated products (3qb and 3qe) in good yields, when treated with substituted terminal alkynes (2b and 2e).


image file: c8qo01068d-s2.tif
Scheme 2 Substrate scope of terminal alkynes. (Unless otherwise mentioned, all reactions were performed using 2-phenyl pyridine 1a (0.2 mmol), terminal alkyne, 2b–2m, (0.3 mmol), ReBr(CO)5 (2.5 mol%), NaOAc (10 mol%), toluene (1 ml), 120 °C, and 4 h. Isolated yields.)

We set out to explore the reactivity of internal alkynes for this hydroarylation strategy (Scheme 3). In general, internal alkynes are electron-rich pi donors and, therefore, tend to react faster with metallacycles in C–H functionalization reactions. However, detailed optimization studies revealed that for internal alkynes, the reaction required a quite high temperature and longer reaction time (140 °C and 12 h) than the terminal alkynes. We assume that the steric repulsion between the rhenium metallacycle and internal alkyne would increase the activation energy barrier for the effective insertion, thus demanding slightly harsh reaction conditions. Aliphatic and aromatic internal alkynes (2n–2p) were also well tolerated and afforded the corresponding hydroarylated products (3an–3ap) in 79–86% yields. For unsymmetrical alkyne (2q), a mixture of inseparable regioisomers with moderate selectivity (3aq + 3aq′) has been achieved. Pyrimidyl group assisted C-2 hydroarylation of indole derivatives with aliphatic and aromatic internal alkynes also led to the desired alkenylated products (3qn and 3qp). In contrast to the reactivity of 1a with 2q, N-pyrimidyl indole (1q) reacts with the unsymmetrical alkyne 2q, affording the single regioisomeric compound 3qq in 92% yield. The present reaction conditions also show compatibility with diynes. Treatment of diyne 2r with 1a offered a mixture of inseparable regioisomeric C–H addition products (3ar + 3ar′) in 56% yield. The observed regioselectivity was further confirmed by NOE experiments.


image file: c8qo01068d-s3.tif
Scheme 3 Substrate scope of internal alkynes. (Unless otherwise mentioned, all reactions were performed using 2-phenylpyridine 1a (0.2 mmol), internal alkyne, 2n–2r, (0.3 mmol), ReBr(CO)5 (2.5 mol%), NaOAc (10 mol%), toluene (1 ml), 140 °C, 12 h, and N2 atm. Isolated yields. a[thin space (1/6-em)]The ratio of the regioisomers was determined by 1H NMR analysis and is given in the parentheses.

Deuterium labeling and kinetic isotopic experiments were probed for the mechanistic understanding of this hydroarylation reaction (Scheme 4). Treating 1a in the presence of 10 equiv. of CD3OD under this reaction condition gives 71% of H/D exchanged 1a. This result shows the reversibility of the reaction in the absence of alkyne. Later, the reaction between D5–1a and 2i for 2 h at 120 °C recovered 34% of D5–1a with 20% of H/D scrambling at the ortho positions and afforded product D4–3ai in 62% yield with significant H/D exchange at the alkenyl protons. Furthermore, 47% of deuterium incorporation at the alkenyl proton suggests that protonation occurs at the last step of the catalytic cycle. Finally, parallel experiments of 1a and D5–1a with 2a gave a kinetic isotopic effect kH/kD of 1.59. However, the reversible formation of the metallacycle and observed lower KIE values seem to indicate that the C–H metallation step may not involve in the rate-limiting step.


image file: c8qo01068d-s4.tif
Scheme 4 Preliminary mechanistic studies.

Based on these mechanistic studies and earlier reports, we postulate the following mechanism for this hydroarylation reaction (Fig. 1). The reaction of ReBr(CO)5 with NaOAc gives NaBr and Re(CO)5OAc, I. The latter starts the catalytic cycle by a CO dissociation to give Re(CO)4OAc, II. Coordination of arylpyridine to II, followed by ortho C–H activation, affords the reversible 5-membered rhenacycle III. Substitution of a coordinated CO group by the alkyne gives rhenacycle IV, which then undergoes regioselective 2,1-insertion of the coordinated alkyne to form 7-membered metallacycle V with the phenyl group and rhenium atoms both attached to the same carbon atom. Eventually, protonolysis of intermediate V affords the alkenylated product 3aa and regenerates the active rhenium complex II for the next catalytic cycle.8q


image file: c8qo01068d-f1.tif
Fig. 1 Proposed reaction mechanism.

A gram-scale reaction between 1a and 2a under the conditions gave product 3aa in 84% yield (Scheme 5). The result demonstrates that this protocol can be applied to larger scale synthesis of alkenylated heterocycles under an air atmosphere. Moreover, successful de-protection of the pyrimidyl group from the alkenylated product 3ra afforded the unprotected indole 4 in 86% yield. Finally, the Diels Alder reaction between alkene (3qb) and maleimide (5) gave 90% of the cyclization product 6. Deprotection and aromatization of the derivatives of this structural core have been identified as inhibitors of checkpoint kinase Wee1 that have potential application in the cancer chemotherapeutics.9


image file: c8qo01068d-s5.tif
Scheme 5 Applications of the hydroarylation strategy.

In summary, the reaction of various arylpyridines and N-pyrimidyl indoles with terminal and internal alkynes under ReI-catalysis offers an operationally simple and convenient protocol for the synthesis of alkenylated heterocycles in high yield with excellent stereo- and regio-selectivity. This protocol can be extended to the hydroarylation of internal alkynes with a slightly higher reaction temperature and longer reaction time. It is rare that a metal catalyst is capable of catalyzing both the insertion of terminal and internal alkynes via C–H activation efficiently.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the Ministry of Science and Technology of the Republic of China (MOST 106-2119-M-007-020) for support of this research.

Notes and references

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Footnote

Electronic supplementary information (ESI) available. CCDC 1825212, 1831607 and 1841353. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8qo01068d

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