Demonstration of promoted zinc schlenk equilibria, their equilibrium values and derived reactivity
Demonstration of promoted zinc schlenk equilibria, their equilibrium values and derived reactivity
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DOI：
10.1002/chem.200601739
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发表时间：
2007-01-01
影响因子：
4.3
通讯作者：
Woodward, Simon
中科院分区：
文献类型：
--
作者：
Blake, Alexander J.;Shannon, Jonathan;Woodward, Simon
促进Schlenk平衡的存在下，有机锌卤化物物种已明确证明（CNMR）-C-13的研究。因此，将甲基铝氧烷（MeAlO）（n），MAO，添加到RZnX（R=Et，Bn. ArCH_2，（CH_2）（3）CO_2 Et; X=Cl，Br）导致形成ZnR_2和ZnX_2中心点微弧氧化。对于EtZnCl，ZnEt 2和ZnX 2中心点MAO的平衡在-35 ° C下是快速的; 0.19 M-1的K值表明平衡有利于ZnEt 2（0.75-3.0当量MAO）。使用RZnX/MAO混合物允许实现铜催化的2-环己烯酮的1，4-加成，但是竞争级联反应（两个随后的迈克尔加成和分子内羟醛缩合反应）导致新的四环副产物（在一种情况下以晶体学表征）。EtZnCl的活化也通过ZnMe 2的添加来实现，并且通过（CNMR）-C-13光谱（在平衡时，K1）观察到中间体EtZnMe的存在。在该体系中可以实现不对称共轭加成（加成到2-环己烯酮时高达92%ee）。
The presence of promoted Schlenk equilibria for organozinc halide species has been explicitly demonstrated by (CNMR)-C-13 studies. Thus, addition of methylaluminoxane (MeAlO)(n), MAO, to RZnX (R=Et, Bn. ArCH2, (CH2)(3)CO2Et; X=Cl, Br) leads to the formation of ZnR2 and ZnX2 center dot MAO. For EtZnCl, equilibration of ZnEt2 and ZnX2 center dot MAO 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 (CNMR)-C-13 spectroscopy (at equilibrium, K 1). Asymmetric conjugate addition in this system can be realised (up to 92 % ee for additions to 2-cyclohexenone).