Streitwieser Andrew, Husemann Marc, Kim Yeong-Joon
Department of Chemistry, University of California, Berkeley, CA 94720-1460, USA.
J Org Chem. 2003 Oct 17;68(21):7937-42. doi: 10.1021/jo034377+.
UV-vis spectra of the dilithium, 1-Li, and dicesium, 1-Cs, enediolates of alpha-naphthylacetic acid show no systematic change with concentration in dilute THF solution, but addition of small amounts of HMPA causes a bathochromic shift in the spectrum of 1-Li. These results indicate that these salts are aggregated and that HMPA breaks up the aggregates of 1-Li. The quantitative effect of small increments of HMPA indicates that 1-Li is a dimer. Alkylation reactions of 1-Cs show half-order kinetics in enediolate indicating that this salt is also dimeric but that the small amount of monomer in equilibrium is the actual reactant. Alkylation of 1-Li, however, is much slower and shows first-order kinetics interpreted as a direct reaction of the dimer; the amount of monomer in this case is too small to compete. A solution of 1-Li in THF containing 10% HMPA is much more reactive in alkylation than 1-Li alone and the first-order dependence in 1-Li is now interpreted as reaction of the monomer. Compound 1-Li is found to form a mixed aggregate with LDA, a finding that has possible synthetic significance since enediolates used in syntheses are frequently prepared using LDA. Structures of these compounds are suggested based on model ab initio computations.
α-萘乙酸的二锂盐（1-Li）和二铯盐（1-Cs）烯二醇盐在稀四氢呋喃溶液中的紫外-可见光谱随浓度无系统变化，但加入少量六甲基磷酰三胺（HMPA）会使1-Li的光谱发生红移。这些结果表明这些盐是聚集的，且HMPA会破坏1-Li的聚集体。少量HMPA增加的定量效应表明1-Li是二聚体。1-Cs的烷基化反应在烯二醇盐中显示出半级动力学，表明该盐也是二聚体，但平衡态下少量的单体是实际反应物。然而，1-Li的烷基化要慢得多，显示出一级动力学，可解释为二聚体的直接反应；在这种情况下单体的量太少无法参与竞争。在含10% HMPA的四氢呋喃中1-Li的溶液在烷基化反应中比单独的1-Li更具反应性，现在将对1-Li的一级依赖性解释为单体的反应。发现化合物1-Li与二异丙基氨基锂（LDA）形成混合聚集体，这一发现具有可能的合成意义，因为合成中使用的烯二醇盐经常是用LDA制备的。基于模型从头算计算提出了这些化合物的结构。
