Isotopic labeling, alkynes mechanism

The intermediacy of a platinum vinylidene in Yamamoto s reaction was supported by the results of isotopic labeling studies. D FT calculations were used to further probe the proposed reaction mechanism. In contrast to the prevailing model of alkyne/ vinylidene interconversion for Rh(I)-catalysts, direct Ca Cp 1,2-H-migration is implicated in the formation of vinylidene 130. Direct C—H insertion via a single... 

Since the first reports of Pd(II)-catalyzed hydroselenation and hydrothiolation 1992, considerable investigations have accumulated experimental evidence for the mechanism, in particular for Type I mechanism. Each step of Type I mechanism, structures of active catalysts, the reaction of alkynes with the active catalysts, and the protonolysis of the resulting vinyl metal complexes, has been verified for Pd, Ni, Zr, Ln, and An-catalyzed hydrochalcogenations by isolation of intermediates, isotope-labeled experiments, and kinetic studies. With regard to Type II mechanism, while the initial oxidative addition of REH (E = S, Se) to a low-valent transition metal catalyst (metal = Pd and Pt) has been verified by direct (for Pt) or indirect (for Pd) experimental evidence, the following steps of alkyne insertion to chalcogenolate-hydrido complex and reductive elimination of resultant vinyl metal complexes leave room for further mechanistic investigations to obtain direct evidence. On the other hand, a hybrid mechanism of Type I and Type II has been clarified for the hydrothiolation with Rh(I) complexes. 

Possibly the measurement of the chlorine isotope effect for a series of suitably substituted substrates may help to decide whether or not greater stereospecificity is associated with decreased carbanion character of the transition state for elimination from alkenes. Of course, such studies must be accompanied by labelling experiments to exclude the possibility that alkynes arise by an apparent alpha-elimination (Section 5.1) involving formation of a carbanion at the alpha carbon, which subsequently eliminates alpha halogen simultaneously with migration of a substituent from the beta carbon. Results with alcoholic media, however, tend to indicate this alternative mechanism is unimportant if a normal beta-elimination is possible. 

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