3,3′-Anisyl-Substituted BINOL, H<sub>4</sub>BINOL, and H<sub>8</sub>BINOL Ligands: Asymmetric Synthesis of Diverse Propargylic Alcohols and Their Ring-Closing Metathesis to Chiral Cycloalkenes
2009-11-20T00:00:00Z (GMT) by
A series of optically active BINOL, H<sub>4</sub>BINOL, and H<sub>8</sub>BINOL derivatives were prepared. These compounds in combination with ZnEt<sub>2</sub> and Ti(O<sup><i>i</i></sup>Pr)<sub>4</sub> 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 H<sub>8</sub>BINOL (<i>S</i>)-<b>7</b> 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.
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