Demonstration of promoted zinc schlenk equilibria, their equilibrium values and derived reactivity

The presence of promoted Schlenk equilibria for organozinc halide species has been explicitly demonstrated by (CNMR)-C-13 studies. Thus, addition of methylaluminoxane (MeAlO)(n), MAO, to RZnX (R=Et, Bn. ArCH2, (CH2)(3)CO2Et; X=Cl, Br) leads to the formation of ZnR2 and ZnX2 center dot MAO. For EtZnCl, equilibration of ZnEt2 and ZnX2 center dot MAO is rapid at -35 degrees C; a K value of 0.19 M-1 indicates the equilibrium favours ZnEt2 (0.75-3.0 equiv MAO). Use of RZnX/MAO mixtures allows copper-catalysed 1,4-addition to 2-cyclohexenone to be achieved, but a competing cascade reaction (two subsequent Michael additions and an intramolecular aldol reaction) leads to novel tetracyclic by-products (characterised crystallographically in one case). Activation of EtZnCl is also achieved by ZnMe2 addition and the presence of intermediate EtZnMe was observed by (CNMR)-C-13 spectroscopy (at equilibrium, K 1). Asymmetric conjugate addition in this system can be realised (up to 92 % ee for additions to 2-cyclohexenone).