Asymmetric catalysis with chiral organocopper-copper arenethiolates

Abstract

Novel copper arenethiolates, ⌈CuSAr⌉n, which have as a special feature the presence of a potentially coordinating ortho amino group, CH(R’)NMe2, have been developed as catalysts for the conjugate 1,4-addition of Grignards to Michael acceptors. The arenethiolate anion appears to behave as an excellent non-transferable group. By selecting a chiral ortho amino group (CuSAr*; c.g.,R’ = Me)achiral copper arene thiolate catalyst is obtained. The reaction of methylmagnesium iodide with benzylidene acetone and 3 mol% CuSAr* in toluene takes place with complete conversion to the 1,4-addition product and an e.e. of 76%. The preferred mode of addition is the simultaneous addition of RlMgX (Rt is a transferable group) and benzylidene acetone to a toluene solution of [CuSAr-(R)]3 in benzene/toluene. Model studies revealed that the [CuSAr-(R)]3 aggregate (X-ray and NMR) reacts with RlMgX to a new aggregate [CuRl]4[Mg(SAr-(R)2]2 (X-ray and NMR) from which the kinetic intermediate is produced through interaction with the substrate. The selective formation of both the heterocuprate [CuRl][Mg(SAr-(R))2] and the kinetic intermediate, which most probably has the seemingly simple dinuclear structure I (see Scheme 8), are the first examples of selective self-assembly of mononuclear units RCu, CuSAr and RlMgX to one larger aggregate rather than to a number of different mixed aggregates with almost the same thermodynamic stability. It is proposed that coordination of the ortho amino group (of SAr-) to the MgX-unit stabilizes the kinetic intermediate. The Michael acceptor anchors via Mg-O=C coordination which brings the C=C part of the enone-substrate in the right position for alkene-copper coordination and subsequent transfer of the transferable group R1. © 1994 IUPAC.