Asymmetric Allylboration of Ketones Catalyzed by Chiral Diols

摘要

The asymmetric allylation reaction is a powerful method for the construction of chiral building blocks for use in synthesis.Many useful and innovative methods exist for the allylation of aldehydes;however,the enantioselective allylation of ketones to yield chiral tertiary homoallylic alcohols remains a challenge for asymmetric methods.Notable advances in this area include the use of chiral allylsilanes,chiral allylboranes,and boronates,the asymmetric allylation of ketones using allyl stannanes,the asymmetric Cu(I)-catalyzed allylboration reaction,and the Ag(I)-catalyzed asymmetric Sakurai-Hosomi reaction of ketones with allyl silanes.In developing an asymmetric allylation reaction of ketones,we considered two key observations.First,recent reports illustrate that the addition of allylboronates to aldehydes is accelerated by the use of Lewis acids or strong Br0nsted acids.The observed rate acceleration is presumably due to Lewis acid activation of the boron atom via coordination to the boronate alkoxy ligand.Second,we sought to capitalize on the facility with which acyclic dialkoxybo-ranes undergo ligand exchange.We postulated that chiral diols could act as catalytic promoters of asymmetric allylboration reactions (eq 1):exchangeable chiral ligands with Bronsted acidic characteristics.Herein,we report the first example of a highly enantioselective asymmetric allylboration of ketones using chiral BINOL-derived catalysts and allyldiisopropoxyborane.