title: Demonstration of promoted zinc Schlenk equilibria, their equilibrium values and derived reactivity.
abstract: 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).
authors.affiliation: School of Chemistry, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK.
authors.forename: Alexander J
authors.initials: AJ
authors.lastname: Blake
authors.forename: Jonathan
authors.initials: J
authors.lastname: Shannon
authors.forename: John C
authors.initials: JC
authors.lastname: Stephens
authors.forename: Simon
authors.initials: S
authors.lastname: Woodward