3,3′-Anisyl-Substituted BINOL, H4BINOL, and H8BINOL Ligands: Asymmetric Synthesis of Diverse Propargylic Alcohols and Their Ring-Closing Metathesis to Chiral Cycloalkenes
journal contributionposted on 20.11.2009 by Yang Yue, Mark Turlington, Xiao-Qi Yu, Lin Pu
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A series of optically active BINOL, H4BINOL, and H8BINOL derivatives were prepared. These compounds in combination with ZnEt2 and Ti(OiPr)4 were used to catalyze the asymmetric reaction of alkynes with aldehydes to generate chiral propargylic alcohols at room temperature. Through this comparative study, a 3,3′-bisanisyl-substituted H8BINOL (S)-7 was found to be a generally enantioselective catalyst for the reaction of structurally diverse terminal alkynes with a variety of aldehydes. It catalyzed the reactions of alkyl propiolates with 88−99% ee; the reactions of phenylacetylene with 81−87% ee; the reactions of 4-phenyl-1-butyne, an alkyl alkyne, with 77−89% ee; and the reactions of trimethylsilylacetylene with 92−97% ee. The optically active propargylic alcohols generated from this catalytic asymmetric alkyne addition were observed to undergo efficient ring-closing-metathesis (RCM) reaction in the presence of the Grubbs II catalyst to produce chiral cycloalkenes. It was further found that some of the chiral propargylic alcohols underwent a highly chemoselective tandem RCM hydrogenation reaction with retention of the enantiomeric purity.