Studies of some copper hydride catalyzed reductive rearrangements

Abstract

Several rearrangement reactions based on reductive formation of enolates have been studied. A copper hydride-catalyzed reductive Ireland-Claisen rearrangements of propargylic acrylates was developed, and various allenoic acids were obtained in excellent yields and diastereoselectivities. The copper hydride, in-situ generated from a catalytic amount of Cu(OAc)2, P(OEt)3 and stoichiometric PhSiH3, reduced propargylic acrylates stereoselectively to form (E)-silyl enolates, which proceeded the rearrangement via chair-like transition state to afford corresponding allenoic acids in excellent diastereoselectivities. More importantly, we employed the silanes instead of boranes to generate silyl enolates as the rearrangement precursors that avoided the E,Z-isomerization of enolates, and successfully improve the diastereoselectivity of this Ireland-Claisen rearrangement. A copper hydride-catalyzed reductive Ireland-Claisen rearrangement of allylic allenoates was developed, and various 1,5-dienes were synthesized in good yield. Using copper hydride in-situ generated from a catalytic amount of Cu(OAc)2, PPh3 and stoichiometric PinBH, allylic allenoates were easily reduced to generate O-boron enolates, and underwent Ireland-Claisen rearrangement to afford 1,5-dienes in excellent yields. We found that the rearrangement of allenoates derived from primary and secondary alcohols was poorly diastereoselective, however, the Ireland-Claisen rearrangements of allylic allenoates derived from tertiary alcohols or tri-substituted allene substrates were highly diastereoselective due to the stereoselective reductive enolizations. A copper hydride-catalyzed reductive divinylcyclopropane Cope rearrangement of vinyl acryloyl cyclopropanes was developed, and various cycloheptenones were obtained. Using a catalytic amount of Stryker’s reagent and stoichiometric PinBH, the reductive enolizations of vinyl acryloyl cyclopropanes were highly (E)-selective to generate (E)-enolates, and consequent Cope rearrangements proceed smoothly at room temperature to afford cycloheptenones in excellent yields and dr. Particularly, we found that by employing the electronic effect of boron and steric hindrance of pinacolboryl group, the rate acceleration of these Cope rearrangements was significant compared to the rearrangements of other silyl or boryl enolates.