Demonstration of Promoted Zinc Schlenk Equilibria, their Equilibrium Values and Derived Reactivity
作者:Alexander J. Blake、Jonathan Shannon、John C. Stephens、Simon Woodward
DOI:10.1002/chem.200601739
日期:2007.3.16
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
通过13 C NMR研究已明确证明了有机卤化锌物种存在促进的Schlenk平衡。因此,向RZnX(R = Et,Bn,ArCH 2,(CH 2)3 CO 2 Et; X = Cl,Br)中添加甲基铝氧烷(MeAlO)n,MAO导致形成ZnR 2和ZnX 2 MAO。对于EtZnCl,在-35摄氏度时ZnEt2和ZnX2MAO的平衡迅速;K值为0.19 M-1表示平衡有利于ZnEt2(0.75-3.0当量MAO)。使用RZnX / MAO混合物可实现将铜催化的1,4-加成至2-环己烯酮,但竞争的级联反应(随后的两次Michael加成和分子内的醇醛反应)导致产生新的四环副产物(在结晶学上表征为一种情况)。EtZnCl的活化也可以通过添加ZnMe2来实现,并且通过13C NMR光谱法观察到中间EtZnMe的存在(平衡时,K约为1)。可以实现该系统中的不对称共轭物加成(2-环己烯酮的加成最多可达到ee的92%)。