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

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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.