Mechanistic study of the reactions of 1,1-dihalo-2-methyl-2-phenylpropanes with LDA. Evidence for radical and carbene pathways

An attempt was made to determine the mechanisms involved in the reactions of the model systems 1,1-dichloro-2-methyl-2-phenylpropane (1) and 1,1-diiodo-2-methyl-2-phenylpropane (2) with LDA. These systems were chosen as ones capable of providing evidence for the formation of radical as well as carbene products. The techniques employed in investigating the mechanistic features of these reactions involved studying the effect of the leaving group, the effect of radical and carbene trapping agents on the product distribution, and isotopic tracer experiments using labeled solvent (THF-d(8)) and nucleophile (LDA-d(2)). The major product of the reaction of the geminal dichloride (1) is thought to be derived from a chlorocarbene, whereas the geminal diiodide (2) appears to form products derived from both carbene and radical intermediates. On the basis of the results of radical trapping experiments and those of deuterium-labeling experiments, evidence is presented to support the notion that products A, E, and H are derived from a radical precursor. In addition, products A and H are also believed to be formed from the vinylic halide D (or B) and the monoiodide E, respectively. Reasonable mechanisms for the formation of the other products formed in these reactions have been proposed on the basis of the available data.