Enantioselective synthesis of trans-aryl- and -heteroaryl-substituted cyclopropylboronates by copper(I)-catalyzed reactions of allylic phosphates with a diboron derivative.

A new asymmetric route for the synthesis of trans-2-aryl- and -heteroaryl-substituted cyclopropylboronates has been developed. (Z)-3-arylallylic phosphates were converted to the optically active products with high yield and diastereo- and enantioselectivity through a copper(I)-catalyzed reaction with a diboron derivative. Under mild reaction conditions, the reaction affords the arylcyclopropane products with a functional group and a heteroaromatic group in a highly enantioselective manner. When (E)-allylic phosphates were used as substrates, a ligand-controlled product switch between the trans and cis configurations was observed.

Direct enantio-convergent transformation of racemic substrates without racemization or symmetrization.

Asymmetric reactions that transform racemic mixtures into enantio-enriched products are in high demand, but classical kinetic resolution produces enantiopure compounds in <50% yield even in an ideal case. Many deracemization processes have thus been developed including dynamic kinetic resolution and dynamic kinetic asymmetric transformation, which can provide enantio-enriched products even after complete conversion of the racemic starting materials. However, these dynamic processes require racemization or symmetrization of the substrates or intermediates. We demonstrate a direct chemical enantio-convergent transformation without a racemization or symmetrization process. Copper(I)-catalysed asymmetric allylic substitution of a racemic allylic ether afforded a single enantiomer of an α-chiral allylboronate with complete conversion and high enantioselectivity (up to 98% enantiomeric excess). One enantiomer of the substrate undergoes an anti-S(N)2'-type reaction whereas the other enantiomer reacts via a syn-S(N)2' pathway. The products, which cannot be prepared by dynamic procedures, have been used to construct all-carbon quaternary stereocentres.