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 ZnX2MAO. For EtZnCl, equilibration of ZnEt2 and ZnX2MAO 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 13C NMR spectroscopy (at equilibrium, K approximately 1). Asymmetric conjugate addition in this system can be realised (up to 92% ee for additions to 2-cyclohexenone).
Blake
Alexander J
AJ
School of Chemistry, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK.
Shannon
Jonathan
J
Stephens
John C
JC
Woodward
Simon
S