Asymmetric catalysis with chiral organocopper-copper arenethiolates
Pure and Applied Chemistry
Abslracl --Novel copper nrcnetliiol:ites, [CuSAr]", which have ;is a special feature the presence of a potentially coordinating orflu) amino group, CI-I(II')NMe2, have been developed as catalysts for the conjugnte 1,4-addition of Grignards to Miclioel acceptors. The arenctliiolate anion appears to behave as an cxccllcnt iioii-tr;iiisfcr;iblc group. By selecting ii cliirol orflu) amino group (CuSAr*; c.g.,R' = Me) a chiral copper arene thiolate catalyst is obtained. The reaction of
... m iodide with benzylidene ;icetone and 3 mol% CuSAr* in toluene tokes place with complete conversion to the 1 ,Caddition product and an e.e. of 76%. The preferred mode of addition is the simultaneous addition of RtMgX (Rt is a triinsfcrnblc group) and benzylidene ncetoiie to ii toluene solution of [CuSAr-(R)I3 in benzene/toluene. Model studies revealed that the [CuSAr-(R)j3 aggregate (X-ray and NMR) reacts with RLMgX to a new aggregate [CUR~],[M~(SA~-(R)~]~ (X-ray and NMR) from which the kinetic intermediate is produced through interaction with the substrate. The selective forniation of both the heterwuprate [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 IIlMgX to one larger aggregate rather thnn to a number of different mixed aggregates with almost the same tlieimcdyniimic stability. I t is proposed that coordination of the orrlw amino group (of SAr-) to the MgXunit stabilizes the kinctic intermediate. Thc Michael ;icceptor anchors viu Mg-O=C coordination which brings the C=C part of the enone-substrate i n the right position for alkene-copper coordination and subsequent transfer of the transferable group RI.