Asymmetric Oxidative Cyclization of Enynes via Pd(II/IV) Catalysis

摘要

Asymmetric catalysis is unambiguously an efficient and economically feasible protocol for the synthesis of optically active organic compounds both in the laboratory and on an industrial scale. Palladium is probably one of the most widely used metals in such processes. Compared to the impressive development of the enantioselective reactions via Pd(0)/Pd(II) catalytic cycles, only scattered attention has been devoted to exploring the asymmetric Pd(II)/Pd(IV) catalysis. Recently, catalytic reactions by way of Pd(IV) intermediates, which is generated from a Pd(II) precursor by function of a powerful oxidant such as hypervalent iodine reagents, have been developed. Tse and Sanford groups independently reported cyclization of enynes via Pd(II)/Pd(IV) catalysis to afford lactons, lactams, and ethers bearing a bicyclo[3.1.0]hexane skeleton. We have found that our original spiro bis(isoxazoline) compounds, abbreviated as SPRIXs, serve as an effective chiral ligand in Pd-catalyzed enantioselective transformations.4 The high affinity of SPRIXs for Pd(II) and the remarkable stability of SPRIXs under oxidative conditions promoted us to utilize them in enantioselective reactions involving key Pd(IV) intermediates. Herein we report the first example of asymmetric cyclization of enyne derivatives via a Pd(H)/Pd(IV) catalytic cycle using (P,R,R)-i-Pr-SPRIX 1.