Benzyne, Cycloalkyne, and Complexes of Other Reactive Alkynes

Benzyne, Cycloalkyne, and Complexes of Other Reactive Alkynes 

An important use of transition metal complexation has been in the stabilization of reactive molecules including cyclobutadiene, trimethylene-methane, o-xylylene, carbenes, carbynes, and others. In many cases, release of the ligand by chemical means has proven possible, making the complexed precursors valuable as shelf-stable sources of the reactive species for structural study or synthetic deployment. There also has been considerable interest in analogous complexes of benzyne, other strained cycloalkynes, and highly reactive acetylenes. 

Both methods [Eqs. (1) and (2)] involve direct reaction with the alkyne to be complexed. However, when the alkyne itself is unstable or highly reactive, e.g., a strained cycloalkyne or benzyne, in situ generation and trapping can be employed. Since such complexes are interesting as potential sources of these theoretically significant molecules, we shall discuss their preparation in more detail. 

The synthetic approach most commonly employed in attempts to prepare such complexes has involved generation of the reactive species from a suitable precursor in the presence of a transition metal complex which serves as the trapping agent. As an example, Bennett and co-workers reductively deha-logenated 1,2-dibromocyclohex- and -heptene in the presence of Pt(PPhj)3 to produce the corresponding cyclohexyne and cycloheptyne complexes, respectively (Bennett et aU 1971) [Eq. (3)]. The generally observed nonlin- 

These complexes are extremely stable, resisting displacement of the cyclohexyne and cycloheptyne by several donor ligands with the exception 

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