Details of the Cu-catalyzed enantioselective allyl–allyl coupling reaction between allylboronates and (Z)-allylic phosphates using a new chiral N-heterocyclic carbene (NHC) ligand containing a phenolic hydroxy group are presented. The copper catalysis delivers enantioenriched chiral 1,5-dienes with a tertiary stereogenic center. Compatibility with various functional groups and the use of earth-abundant and relatively low-toxicity copper as a metal are attractive features of this protocol. The utility of the chiral phenol–NHC ligand for enantioselective copper catalysis with organoboron compounds is demonstrated and enantiodiscrimination models are discussed.
AbstractAsymmetric synthesis of mechanically planar chiral rotaxanes and topologically chiral catenanes has been a long-standing challenge in organic synthesis. Recently, an excellent strategy was developed based on diastereomeric synthesis of rotaxanes and catenanes with mechanical chirality followed by removal of the chiral auxiliary. On the other hand, its enantioselective approach has been quite limited. Here, we report enantioselective preparation of mechanically planar chiral rotaxanes by kinetic resolution of the racemates via remote asymmetric acylation of a hydroxy group in the axis component, which provides an unreacted enantiomer in up to >99.9% ee in 29% yield (the theoretical maximum yield of kinetic resolution of racemate is 50%). While the rotaxane molecules are expected to have conformational complexity, our original catalysts enabled to discriminate the mechanical chirality of the rotaxanes efficiently with the selectivity factors in up to 16.
In the presence of a suitable acid or base, α-hydroxyaldehydes, ketones, and imines can undergo isomerization that features the 1,2-shift of an alkyl or aryl group. In the process, the hydroxy group is converted to a carbonyl and the aldehyde/ketone or imine is converted to an alcohol or amine. Such α-ketol/α-iminol rearrangements are used in a wide variety of synthetic applications including asymmetric synthesis, tandem reactions, and the total synthesis and biosynthesis of natural products. This review explores the use of α-ketol rearrangements in these contexts over the past two decades.
Copper-Catalyzed Enantioselective Coupling between Allylboronates and Phosphates Using a Phenol–Carbene Chiral Ligand: Asymmetric Synthesis of Chiral Branched 1,5-Dienes
