Evaluation of chiral oxazolines for the highly enantioselective diethylzinc addition to N-(diphenylphosphinoyl) imines.
On the basis of the principle that the incorporation of the structurally rigid and conformationally restricted skeleton in beta-amino alcohols is beneficial to the enantioselective diethylzinc addition to imines, a series of chiral oxazolines, which had been designed and conveniently prepared from commercially available (1S,2S)-2-amino-1-phenylpropane-1,3-diol, were applied in the diethylzinc addition to diphenylphosphinoyl imines to give high yields of 68-84% and excellent ee values of 90-95%. The configuration of the product was controlled by the chirality of the carbon bonded to the hydroxyl group in the oxazoline. Oxazolines bearing a para- or meta-substituted phenyl group generally offered higher enantioselectivity than those containing an ortho-substituted phenyl. The X-ray structures of 4f and 4j, in combination with the proposed transition state, preliminarily explained why oxazolines with a para- or meta-substituent on the phenyl group gave higher enantioselectivities than those bearing an ortho-substituent. This successful example using chiral oxazolines to promote the titled reaction implies that a large family of chiral compounds containing an oxazoline ring moiety have the potential to be developed for promoting the highly enantioselective dialkylzinc addition to N-(diphenylphosphinoyl) imines.
Zhang
Xiao-Mei
XM
Union Laboratory of Asymmetric Synthesis, Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, China.
Zhang
Hai-Le
HL
Lin
Wen-Qing
WQ
Gong
Liu-Zhu
LZ
Mi
Ai-Qiao
AQ
Cui
Xin
X
Jiang
Yao-Zhong
YZ
Yu
Kai-Bei
KB