Acta Chim. Sinica ›› 2018, Vol. 76 ›› Issue (11): 850-856.DOI: 10.6023/A18060244 Previous Articles     Next Articles

Special Issue: 有机小分子-金属协同催化



王乐明, 王骞, 陈杰安, 黄湧   

  1. 北京大学深圳研究生院 省部共建肿瘤化学基因组学国家重点实验室 深圳 518055
  • 投稿日期:2018-06-23 发布日期:2018-08-17
  • 通讯作者: 陈杰安, 黄湧;
  • 基金资助:


Switching Reaction Pathways by Cooperative Catalysis of N-Heterocyclic Carbene and Lewis Acids

Wang Leming, Wang Qian, Chen Jiean, Huang Yong   

  1. State Key Laboratory of Chemical Oncogenomics, Key Laboratory of Chemical Genomics, Peking University, Shenzhen Graduate School, Shenzhen 518055
  • Received:2018-06-23 Published:2018-08-17
  • Contact: 10.6023/A18060244;
  • Supported by:

    Project supported by the National Natural Science Foundation of China (Nos. 21602007, 21572004), Guangdong Science and Technology Program (No. 2013B090500009) and Shenzhen Basic Research Program (Nos. JCYJ20170818085510474, JCYJ20170818085438996).

The combination of N-heterocyclic carbenes (NHCs) and Lewis acids (LA) have been occasionally employed in asymmetric annulation reactions. However, synergistic effect of LA on NHC-mediated reactions remains scarce. Herein, we demonstrate that by switching LA co-catalysts, two distinct active species, homoenolate and acyl azolium, can be accessed from the same set of substrates. NHC-catalyzed enantioselective hydroesterification is one of the most straightforward strategies to prepare β-chiral esters. Despite recent advances for this redox-neutral transformation, obtaining high enantioselectivity and yield remains challenging. We recently reported synergistic catalysis, combining an achiral NHC and a chiral phosphoric acid, enables highly enantioselective hydrothioesterification and hydroesterification of enals. However, both stereoselectivity and yield for hydroesterification are far from ideal. Specifically, sluggish reactions, accompanied with ee's in mid-80% are often obtained. Additionally, competing pathways for E/Z isomerization and oxidative esterification of enal are serious for a number of substrates. In order to address this issue, we propose a new cooperative catalytic system, consisting of a NHC, a LA and a proton-shuttling agent, might accelerate the pivotal asymmetric β-protonation process. We suspect that the choice of LA might provide complementary reaction pathways from the same enal substrates. Starting from β-alkyl cinnamaldehydes, highly enantioselective hydroesterification is accomplished via asymmetric β-protonation enabled by a magnesium co-catalyst. In sharp contrast, the same homoenolate intermediate can undergo aerobic oxidation, via single electron transfer (SET), in the presence of a ruthenium co-catalyst. Control experiments show distinct rate difference between the E-and Z-isomers of enal. Substrates with Z-configuration react significantly slower under the standard reactions. E/Z isomerization is also slow. Photoirradiation was applied to address the challenging issue of isomeric enals and both high yield and ee are obtained using start materials as E/Z mixtures. General procedure for the asymmetric β-protonation is as following:NHC pre-catalyst (0.01 mmol), MgCl2 (0.01 mmol), DABCO (0.12 mmol), 4 ? MS (100 mg), alcohol (0.6 mmol) and enal substrate (0.1 mmol) were dissolved in toluene (1.0 mL). The resulted mixture was stirred at room temperature under Ar atmosphere for 15 h. Upon complete consumption of the enal, the mixture was concentrated and purified by flash column chromatography. For the aerobic oxidation, the reaction proceeded with RuCl3 (0.01 mmol) under O2 atmosphere.

Key words: N-heterocyclic carbenes, Lewis acid, synergistic catalysis, asymmetric protonation, aerobic oxidation