Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
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Nickel-Catalyzed 1,2-Arylboration of Vinylarenes Wang Wang, Chao Ding, Hailiang Pang, and Guoyin Yin* The Institute for Advanced Studies, Wuhan University, Wuhan, Hubei 430072, China
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ABSTRACT: A novel nickel-catalyzed 1,2-arylboration of vinylarenes is reported. A variety of 2-boryl-1,1-diarylalkanes, which constitute a class of significant pharmacophores, are efficiently prepared from readily available olefins and aryl halides in the presence of bis(pinacolato)diboron under mild reaction conditions. The success of this three-component cascade is mainly attributed to the redox-active nitrogen-based ligand. Moreover, this method exhibits good functional group tolerance and excellent chemo- and stereoselectivity.
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ransition-metal catalyzed difunctionalization of alkenes has been recognized as a significant strategy to rapidly access complex molecules.1 In this context, the carboboration of alkenes serves as an efficient approach to prepare aliphatic boron containing compounds.2 The introduction of a boron group is particularly compelling because it provides substantial opportunities for downstream diversifications.3 Therefore, in the past few decades, many efforts have been devoted to the study of transient organocopper intermediates generated from the borylcupration of alkenes, owing to their stability with respect to β-hydride elimination (Figure 1a, left side).4 Recently, stimulated by the special reactivity of nickel,5 the transient alkylNi species generating from olefin migratory insertion into a Ni− B bond have also been investigated by Brown6 and Yin,7 but only limited examples with a restricted olefin scope have been
reported to date (Figure 1a, right side). Although great achievements have been made in the 1,2-arylboration of vinylarenes by an elegant dual metal synergistic catalysis8 based on the transient organocopper intermediates (Figure 1b, top line),9 nickel-catalyzed arylboration of alkenylarenes has not been reported.10 Moreover, the chemoselectivity of different classes of olefins is rarely discussed in one system. Herein, we communicate the results of our exploration of the nickelcatalyzed 1,2-arylboration of vinylarenes (Figure 1b, bottom line). First, substrates with heterocycles are well tolerated. Second, chemoselectivity between different types of olefins is studied. Finally, preliminary mechanistic studies are discussed. As an extension of our interest in manipulating nickel catalyst,11 particularly after the achievement of the nickelcatalyzed migratory arylboration of unactivated terminal alkenes,7 we wondered if the arylboration of vinylarenes could be realized. Additionally we were curious as to the differences from migratory arylboration. Guided by these questions, our exploration was initiated by choosing styrene (1a), aryl bromide (2a), and bis(pinacolato)diboron (B2pin2, 3) as model substrates and conducted a ligand surveying for this threecomponent reaction. As illustrated in Scheme 1, we have found that bipyridine-type ligands without substituents at the ortho position, for example, L1, led to no arylboration product formation, while either methyl or dimethyl substitution on this type of ligand (L2−L5) resulted in the formation of the desired product 4a: Neocuproine (L5) delivered the best result, 4a, in 95% isolated yield on a 1.0 mmol scale in the presence of only 3 mol % catalyst. These results demonstrated that the steric hindrance of the ligand is critical to the success of this nickel-catalyzed reaction, but the reason is still unclear at this stage. Furthermore, the PyrOx12 type ligands (L6−L8) were also investigated. It is surprising to find that the ligand L6, which was ineffective in the migratory arylboration reaction, could also deliver the
Figure 1. Metal-catalyzed 1,2-arylboration of vinylarenes.
Received: March 29, 2019
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.9b01120 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 1. Ligand Effectsa
Scheme 2. Scope of Aryl Halidesa
General conditions: NiCl2·DME (3 mol %), ligand (3 mol %), 1a (0.5 mmol), 2a (0.6 mmol, 1.2 equiv), 3 (0.75 mmol, 1.5 equiv), LiOMe (0.75 mmol, 1.5 equiv), dioxane (2 mL), 30 °C, 20 h. Yields were determined by GC using naphthalene as the internal standard. b Isolated yield on a 1.0 mmol scale. DME = dimethoxyl ethane. a
difunctionalized product 4a in a moderate yield with styrene. In addition, L8, the best ligand for the migratory reaction, could also provide a competitive yield as L5. Finally, considering the cost and commercial availability, we chose neocuproine13 (L5) as the optimal ligand. With the optimal reaction conditions in hand, we sought to explore the generality of this three-component reaction. First, aryl bromides bearing a diverse range of substituents were tested. As shown in Scheme 2, all electron-deficient aryl bromides afforded the difunctionalized products in excellent yields, while the electron-rich ones provided slightly lower yields. In addition, styrenyl bromide (4n) was also able to furnish the corresponding arylboration product in a moderate yield. Importantly, a broad set of heterocyclic aryl bromides, including carbazole, benzothiophene, pyridine, naphthalene, pyrimidine, indole, and thiophene, could yield the corresponding products in good yields. It should be pointed that the arylboration of heterocylic substrates by Pd/Cu dual catalysis was only achieved very recently by the Brown group.9h Notably, no Markovnikov arylboration products were detected in all examples, under the limit of our instruments. In addition, a broad set of functional groups, such as chlorides, amines, esters, ketones, internal olefins, and amides, were all well-tolerated in this robust earthabundant metal-catalyzed reaction. We next shifted our attention to the scope of the alkene partner (Scheme 3). It was found that the electronic properties of alkenylarenes did not show an obvious effect on the efficiency of this transformation. A set of 2-boryl-1,1-diarylalkanes, which are significant pharmaceutically interested scaffolds, are efficiently prepared from the readily available materials via this approach.14 It is worth noting that styrenes even with a strong electron-withdrawing group, such as cyano and ester groups, still were able to provide satisfying results (4ao−4ar, 4az, 4bb).15,16 An o-methyl substituent on styrene only resulted in a slightly lower yield (4ae). An internal alkene, such as indene, could also give rise to the cis-products in good yields with an excellent diastereoselectivity (>20:1) (4as and 4aw). In addition, both α-
Standard conditions: NiCl2·DME (3 mol %), L5 (3 mol %), 1a (1.0 mmol), 2 (1.2 mmol, 1.2 equiv), 3 (1.5 mmol, 1.5 equiv), LiOMe (1.5 mmol, 1.5 equiv), dioxane (4 mL), 30 °C, 20 h. Isolated yield. b Isolated yield on a 10 mmol scale. a
and β-methylstyrene were also examined in this reaction but failed to offer the desired products. Notably, 1,1-bisheteroarylalkanes (4at and 4au) could be prepared by this method as well. Remarkably, this reaction showed excellent chemoselectivity toward the activated double bonds in more complex molecules (4az−4bb). To further investigate the chemoselectivity of this reaction, several competition experiments were conducted in the following studies. In Brown’s ligand-free, nickel-catalyzed system, styrene was not able to yield the arylboration product.6 However, in the first competition experiment with cyclohexene, only the product of 4f was isolated in 78% yield (Scheme 4A). In an independent reaction, cyclohexene showed no reactivity under our standard conditions either. These results indicated that the nitrogen-based ligand enabled a different reactivity from the prior ligand-free system. Furthermore, only the product 4f from styrene was observed in the competition reaction with an unactivated monosubstituted olefin (Scheme 4B). Intriguingly, a good selectivity was also displayed in the competition reaction with allylbenzene 5c (Scheme 4C). Notably, the addition of both 5a and 5b slightly inhibited the styrene arylboration. However, the addition of a strong electron-deficient olefin, acrylate ester 5d, highly influenced the reactivity (Scheme 4D). In order to get insight into the details on the olefin insertion step, a terminal D-labeled olefin 1b-d2 was synthesized and examined in the arylboration reaction. The product 4ad-d2 was isolated in 95% yield, with no deuterium lost at the terminal B
DOI: 10.1021/acs.orglett.9b01120 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 3. Scope of Vinylarenesa
Scheme 4. Competition Experiments of Different Olefins
Scheme 5. Mechanistic Studies
olefin migratory insertion in an anti-Markovnikov fashion leads to the formation of a stable benzyl-Ni(II) species (III),17 which subsequently reacts with an aryl halide to deliver the 1,2arylboration product 4. The reaction of intermediate III with 2 probably involves a key step of transmetalation between two nickel species18 or a radical chain mechanism.19 In conclusion, based on the studies toward transient benzyl nickel species generating from alkenes migratory insertion into the Ni−B bond, we have developed a ligand-enabled, nickelcatalyzed anti-Markovnikov arylboration of vinylarenes. Accordingly, a series of pharmaceutically important diarylalkane derivatives can be efficiently synthesized from the simple vinylarenes and aryl halide in the presence of a diboron reagent under mild conditions. Significantly, this robust Ni-catalyzed reaction shows good functional group tolerance and excellent chemoselectivity toward the activated vinylarenes. The preliminary mechanistic study indicates that a reversible β-H elimination is less likely to occur after the formation of a stable benzylic nickel species. Further mechanistic investigations are currently ongoing in our laboratory.
Standard conditions: NiCl2·DME (3 mol %), L5 (3 mol %), 1 (1.0 mmol), 2 (1.2 mmol, 1.2 equiv), 3 (1.5 mmol, 1.5 equiv), LiOMe (1.5 mmol, 1.5 equiv), dioxane (4 mL), 30 °C, 20 h. Isolated yield. bYield of the corresponding alcohol after oxidation. a
position (Scheme 5A). These results combined with the exclusive cis-diastereoselectivity reveal that reversible β-H elimination likely does not occur in the transformation. No signal was detected when the reaction was monitored by electron paramagnetic resonance (EPR) spectroscopy, which suggested that the catalyst resting state was not Ni(I) or Ni(III) species. Finally, a catalytic cycle is proposed to rationalize this transformation. As illustrated in Scheme 5B, the reaction is initiated by a Ni(II) species (I), which undergoes transmetalation with B2pin2 to generate the Ni(II)-Bpin (II). Then,
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01120. C
DOI: 10.1021/acs.orglett.9b01120 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
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(10) Examples on arylboration of vinylarenes by single Pd catalysis: (a) Yang, K.; Song, Q. J. Org. Chem. 2016, 81, 1000−1005. (b) Yang, K.; Song, Q. Org. Lett. 2016, 18, 5460−5463. (11) (a) Peng, L.; Li, Y.; Li, Y.; Wang, W.; Pang, H.; Yin, G. ACS Catal. 2018, 8, 310−313. (b) Peng, L.; Li, Z. Org. Lett. 2018, 20, 1880−1883. (12) A review on PyrOx ligand: Yang, G.; Zhang, W. Chem. Soc. Rev. 2018, 47, 1783−1810. (13) Selected examples on metal catalysis using neocuproine as ligand: (a) O’Brien, E. M.; Bercot, E. A.; Rovis, T. J. Am. Chem. Soc. 2003, 125, 10498−10499. (b) Shrestha, R.; Dorn Stephanie, C. M.; Weix, D. J. J. Am. Chem. Soc. 2013, 135, 751−762. (c) Zheng, C.-W.; Wang, D.; Stahl, S. S. J. Am. Chem. Soc. 2012, 134, 16496−16499. (d) Davis, D. C.; Walker, K. L.; Hu, C.-H.; Zare, R. N.; Waymouth, R. M.; Dai, M.-J. J. Am. Chem. Soc. 2016, 138, 10693−10699. (e) Meng, Q.-Y.; Wang, S.; Huff, G. S.; König, B. J. Am. Chem. Soc. 2018, 140, 3198−3201. (f) Zhang, X.-J.; Xie, X.; Liu, Y.-H. J. Am. Chem. Soc. 2018, 140, 7385− 7389. (14) A review on bioactivity of diarylalkanes: (14) Vanjari, R.; Singh, K. N. Chem. Soc. Rev. 2015, 44, 8062. (15) Examples on the preparation of diarylethane derivatives by difunctionalization of vinylarenes: (a) Wang, F.; Wang, D.; Mu, X.; Chen, P.; Liu, G. J. Am. Chem. Soc. 2014, 136, 10202−10205. (b) Yang, Y.; Buchwald, S. L. Angew. Chem., Int. Ed. 2014, 53, 8677−8681. (c) Ouyang, X.-H.; Song, R.-J.; Hu, M.; Yang, Y.; Li, J.-H. Angew. Chem., Int. Ed. 2016, 55, 3187−3191. (d) Yang, Y.; Song, R.-J.; Ouyang, X.-H.; Wang, C.-Y.; Li, J.-H.; Luo, S. Angew. Chem., Int. Ed. 2017, 56, 7916− 7919. (e) Wang, D.; Wu, L.; Wang, F.; Wan, X.; Chen, P.; Lin, Z.; Liu, G. J. Am. Chem. Soc. 2017, 139, 6811−6814. (f) Chen, B.; Cao, P.; Yin, X.; Liao, Y.; Jiang, L.; Ye, J.; Wang, M.; Liao, J. ACS Catal. 2017, 7, 2425−2429. (g) Xiao, L.-J.; Cheng, L.; Feng, W.-M.; Li, M.-L.; Xie, J.H.; Zhou, Q.-L. Angew. Chem., Int. Ed. 2018, 57, 461−464. (h) KC, S.; Dhungana, R. K.; Shrestha, B.; Thapa, S.; Khanal, N.; Basnet, P.; Lebrun, R. W.; Giri, R. J. Am. Chem. Soc. 2018, 140, 9801−9805. (i) Yu, X.-Y.; Zhao, Q.-Q.; Chen, J.; Chen, J.-R.; Xiao, W.-J. Angew. Chem., Int. Ed. 2018, 57, 15505−15509. (j) Chen, Y.-G.; Shuai, B.; Xu, X.-T.; Li, Y.-Q.; Yang, Q.-L.; Qiu, H.; Zhang, K.; Fang, P.; Mei, T.-S. J. Am. Chem. Soc 2019, 141, 3395−3399. (k) Anthony, D.; Lin, Q.; Baudet, J.; Diao, T.-N. Angew. Chem. Int. Ed. 2019, 58, 3198−3202. (16) Examples on the preparation of diarylethane derivatives by direct cross-coupling: (a) Taylor, B. L. H.; Swift, E. C.; Waetzig, J. D.; Jarvo, E. R. J. Am. Chem. Soc. 2011, 133, 389−391. (b) Zhou, Q.; Srinivas, H. D.; Dasgupta, S.; Watson, M. P. J. Am. Chem. Soc. 2013, 135, 3307−3310. (c) Liang, Y.; Fu, G. C. Angew. Chem., Int. Ed. 2015, 54, 9047−9051. (d) Poremba, K. E.; Kadunce, N. T.; Suzuki, N.; Cherney, A. H.; Reisman, S. E. J. Am. Chem. Soc. 2017, 139, 5684−5687. (17) Lin, B.-L.; Liu, L.; Fu, Y.; Luo, S.-W.; Chen, Q.; Guo, Q.-X. Organometallics 2004, 23, 2114−2123. (18) It is similar to the dual metal catalysis reaction, please see refs 8 and 9. (19) Benzyl-Ni(II) species probably has a similar reactivity as allylNi(II) species. References on allyl-Ni(II) complexes reacting with electrophiles: (a) Hegedus, L. S.; Thompson, D. H. P. J. Am. Chem. Soc. 1985, 107, 5663−5669. (b) Shrestha, R.; Dorn, S. C. M.; Weix, D. J. J. Am. Chem. Soc. 2013, 135, 751−762.
Details on the condition investigation, extended data about the reaction, NMR data, and characterization (PDF)
AUTHOR INFORMATION
Corresponding Author
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[email protected]. ORCID
Guoyin Yin: 0000-0002-2179-4634 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Profs. Qianghui Zhou, Wen-Bo Liu, and Aiwen Lei at Wuhan University for the lending of lab space and sharing of basic instruments. We thank Dr. Shanshan Liu at the Institute for advanced Studies for the assistance on NMR measurement. We are grateful for the financial support from National Natural Science Foundation of China (21702151, 21871211) and the Fundamental Research Funds for Central Universities (2042019kf0208).
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DOI: 10.1021/acs.orglett.9b01120 Org. Lett. XXXX, XXX, XXX−XXX
