Department of Chemistry, University of Minnesota , 207 Pleasant Street, SE, Minneapolis, Minnesota 55455, United States.
J Org Chem. 2017 Dec 15;82(24):13288-13296. doi: 10.1021/acs.joc.7b02411. Epub 2017 Dec 5.
A series of methylated and octylated pyridinium and quinolinium containing thiourea salts with a chiral 2-indanol substituent are reported. These organocatalysts are positively charged analogues of privileged bis(3,5-trifluoromethyl)phenyl substituted thioureas, and are found to be much more active catalysts despite the absence of an additional hydrogen bond donor or acceptor site (i.e., the presence of a heteroatom-hydrogen or heteroatom). Friedel-Crafts reactions of trans-β-nitorostyrenes with indoles are examined, and good yields and enantioselectivities are obtained. Mechanistic studies indicate that this is a second-order transformation under the employed conditions, and is consistent with the dimer of the thiourea being the active catalyst. Charged organocatalysts, consequently, represent an attractive design strategy for catalyst development.
本文报道了一系列含有手性 2-吲哚醇取代基的甲基化和辛基化的吡啶鎓和喹啉鎓硫脲盐。这些有机催化剂是受保护的双（3,5-三氟甲基）苯基取代硫脲的正电荷类似物，尽管没有额外的氢键供体或受体位点（即杂原子-氢或杂原子），但它们被发现是活性更高的催化剂。本文还考察了反式-β-硝基肉桂烯与吲哚的傅克反应，得到了良好的收率和对映选择性。机理研究表明，在所用条件下这是一个二级转化，与硫脲二聚体是活性催化剂一致。因此，带电荷的有机催化剂代表了一种有吸引力的催化剂设计策略。
