Abstract
New C2-symmetric six-membered carbene ligands for asymmetric diethylzinc addition of arylaldehydes
Highlights
The same group designed and synthesized a series of new bidentate N-heterocyclic carbene (NHC) precursors 3 based on commercial available amino alcohols,2123 which were successfully applied in Cu II-catalyzed asymmetric addition of diethylzinc to cyclohexanone, as well as asymmetric allylic substitution of allyl phosphates with Grignard reagents
24 With similar starting materials, Wilhelm’s group prepared several new tridentate NHC precursors 4 as ionic liquids,[25,26] and these salts were used as catalyst in asymmetric diethylzinc addition to arylaldehydes, giving the corresponding secondary alcohols in good yields and moderate ees
We reported the synthesis of several enantiopure 3,4,5,6-tetrahydropyrimidinium salts 5 incorporating two hydroxyl groups as effective ligands in Pd-catalyzed deprotonative cross-coupling process (DCCP).[27]
Summary
Since the first isolation of free N-heterocyclic carbene (NHC) by Arduengo’s group in 1991,1 these types of ligands have gathered considerable interests because of their attractive properties such as higher stability to air, thermal and moisture than phosphane ligands, and NHCs are ubiquitous in modern synthetic chemistry.[2,3,4,5,6,7,8,9,10,11,12] As an extension, the development of new chiral NHC precursors for catalysis has become an important issue and an enormous number of new chiral carbenes emerged, some of which have shown excellent enantioselectivity in asymmetric catalysis.[13,14,15,16,17,18] among the chiral NHCs synthesized, examples of chiral hydroxyalkyl NHC ligands are still rare.
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