Nucleophilic Carbonyl Addition of Organometallics: Reaction Pathway, Transition State Structure and Stereoselectivity

羰基化合物的亲核加成反应有两条可能的途径：极性加成反应和电子转移-自由基偶联反应。我们证明了一种新的利用卤代二苯甲酮脱卤化的自由基阴离子探针实验可以有效地检测反应过程中电子转移的发生。进行了三个反应：Grignard反应，它的电子转移机制被高度推荐；加入烯丙基三丁基锡，被认为是通过极性机制进行的反应；以及Wittig反应，其反应路径目前尚不清楚。在Omicron-BR或I-二苯甲酮与MeMgl和PhMgBr的反应中，观察到大量的脱卤化产物，这清楚地表明反应过程中发生了电子转移。相反，与烯丙基三丁基锡的类似探针反应则为阴性。这两组反应证实了新的探针实验在检测所述反应期间发生的电子转移方面的有效性。将这一新技术应用到Wittig反应中，表明反应机理高度依赖于叶立德的结构；不稳定的叶立德的反应通过电子转移途径进行，而半稳定的叶立德的反应通过极性机理进行。这些结果与去年在这些Wittig反应中获得的同位素效应以及取代基效应数据完全一致。反应路线的差异被认为是由于观察到的Wittig反应在立体化学上的多样性所致。

Tow reaction routes are possible for nucleophilic additions to carbonyl compounds, polar addition and electron transfer-radical coupling sequence. We have demonstrated that a new radical anion probe experiment utilizing dehalogenation of halo-substituted benzophenone is effective to detect the occurrence of electron transfer during the reaction. Three reactions were carried out: the Grignard reactions, for which the electron transfer mechanism is highly recommended, the addition of allyltributyltin, the reaction considered to proceed through the polar mechanism, and the Wittig reaction for which the reaction pathway is not known at present. In the reaction of omicron-Br or I-benzophenone with MeMgl and PhMgBr considerable amount of dehalogenation products was observed, which clearly indicate the occurrence of electron transfer during the reaction. In contrast, a similar probe reaction with allyltributyltin was found negative. These two sets of reactions confirmed the usefulness of the new probe experiment to detect the occurrence of electron transfer during the reaction in question. Application of this new technique to the Wittig reaction then indicated that the reaction mechanism if highly dependent on the structure of ylide; the reactions of nonstabilized ylides proceed via electron transfer route while those of a semistabilized ylide go through the polar mechanism. These results are fully consistent with the isotope effect as well as the substituent effect data obtained last year in these Wittig reactions. The difference in the reaction route is considered to by responsible to the observed diversity in stereochemistry of the Wittig reaction.