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 13 C NMR studies. Thus, addition of methylaluminoxane (MeAlO) n , MAO, to RZnX (R=Et, Bn, ArCH 2 , (CH 2 ) 3 CO 2 Et; X=Cl, Br) leads to the formation of ZnR 2 and ZnX 2 ⋅ MAO. For EtZnCl, equilibration of ZnEt 2 and ZnX 2 ⋅ MAO is rapid at −35 °C; a K value of 0.19  M −1 indicates the equilibrium favours ZnEt 2 (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 ZnMe 2 addition and the presence of intermediate EtZnMe was observed by 13 C NMR spectroscopy (at equilibrium, K ≈1). Asymmetric conjugate addition in this system can be realised (up to 92 %  ee for additions to 2‐cyclohexenone).