摘要/Abstract
该研究基于氮杂环卡宾（N-heterocyclic carbene，NHC）新颖的协同催化策略，通过Lewis酸共催化剂调控反应具体途径.从α，β-不饱和醛类化合物出发，立足于多反应位点的高烯醇中间体，与氯化镁协作实现高对映选择性的质子转移历程，构建β-手性酯类产物；在相似的反应体系中与氯化钌协作实现高效的空气氧化，构建α，β-不饱和酯类化合物.这两个迥异的反应途径对底物均有较好的官能团容忍性，以高转化率得到目标产物.
关键词: 氮杂环卡宾, Lewis酸, 协同催化, 不对称质子化, 空气氧化
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
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