Lewis酸对氮杂环卡宾协同催化体系中反应途径的调控  

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

作　　者：王乐明 王骞 陈杰安 黄湧[1] WangLeming;Wang Qian;Chen Jiean;HuangYong(State Key Laboratory of Chemical Oncogenomics,Key Laboratory of Chemical Genomics,Peking University,Shenzhen Graduate School,Shenzhen 518055)

机构地区：[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

出　　处：《化学学报》2018年第11期850-856,共7页Acta Chimica Sinica

基　　金：国家自然科学基金(Nos.21602007;21572004);广东省科技计划(No.2013B090500009);深圳市基础研究计划(Nos.JCYJ20170818085510474;JCYJ20170818085438996)资助~~

摘　　要：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.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 y5-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 isomer

关 键 词：氮杂环卡宾 LEWIS酸 协同催化 不对称质子化 空气氧化 

分 类 号：O621.251[理学—有机化学]