Reactions Which Occur at the Coordinated Triple Bond

Reactions Which Occur at the Coordinated Triple Bond 

Alkyne complexes of low oxidation state metals react with electrophiles generally to produce alkene derivatives resulting from addition of the electrophilic species to the coordinated triple bond. These reactions often proceed by way of (x-vinyl-metal derivatives, and in several cases such intermediates have been isolated. The vinyl complexes, in turn, may be cleaved in a subsequent reaction to afford substituted alkenes [Eq. (29)]. 

The interaction of alkyne complexes with protic acids has been the most thoroughly studied of electrophilic reactions. When these reactions proceed to organic products, alkenes are formed, often stereospecifically. Schwartz and co-workers have studied the reactions of Cp2MH(alkyne) complexes of the early transition metals, Ta (Labinger et al, 1974) and Nb (Labinger and Schwartz, 1975). These complexes gave cis alkenes exclusively in nearly quantitative yield when treated with strong acids, as exemplified by the formation of cis-2-octene from the tantalum-octyne complex. Since the 

The related cyclo entadienylmolybdenum complexes appear to have similar reactivity. The 2-butyne complex thus gave ds-2-butene as the only organic product upon treatment with hydrogen chloride at low temperature (Thomas, 1973). The generality of this reaction has not been demonstrated. 

The reactions of zerovalent platinum-alkyne complexes with adds have been studied by several groups (Barlex et ai, 1969 Mann et aU 1971 Tripathy and Roundhill, 1970 Kemmitt et al, 1973). When one equivalent of acid is employed, it is possible to isolate the r-vinyl derivatives. Nmr studies have generally indicated a cis arrangement about the double bond (Mann et ai, 1971), presumably resulting from initial protonation of the metal, followed by cis M—H addition. The yields for this first step were uniformly good. Shaw and co-workers have reported that addition to ter- 

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B. Reactions Which Occur at the Coordinated Triple Bond... 

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