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 13C NMR studies. Thus, addition of methylaluminoxane (MeAlO)n, MAO, to RZnX (R=Et, Bn, ArCH2, (CH2)3CO2Et; X=Cl, Br) leads to the formation of ZnR2 and ZnX2⋅MAO. For EtZnCl, equilibration of ZnEt2 and ZnX2⋅MAO is rapid at −35 °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 13C NMR spectroscopy (at equilibrium, K≈1). Asymmetric conjugate addition in this system can be realised (up to 92 % ee for additions to 2‐cyclohexenone). Beware cascade reactivity! NMR studies confirm that the formation of the ZnR2 species from RZnX is strongly promoted by the use of ZnMe2 or methylaluminoxane [(MeAlO)n, MAO; see graphic]. The resulting diorganozincs can be used in asymmetric 1,4‐addition (up to 92 % ee) or in an unprecedented triple conjugate‐addition/aldol cascade.