Cyclohexyne, transition metal complexes

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-... 

Generally, cyclohexyne is an unstable molecule because of its ring strain. However, it can be stabilized by coordination to transition metals.35 The reduction of 1,2-dibromocyclohexene by sodium/mercury in the presence of a nickel-bromide complex afforded the Ni-alkyne complex 66 as a thermally stable and isolable compound (Scheme 22).36 Complex 66 smoothly reacted with C02 under atmospheric pressure to give nickelacycle 67 in good yield. Dimethyl acetylenedicarboxylate was inserted into the vinyl-nickel bond in 67 to give the seven-membered oxanickelacycle 68. 

Benzynes are highly strained molecules, which are recognized as useful intermediates in organic synthesis.44 They can be isolated by coordination to transition metals.45 Similar to the reaction of the cyclohexyne species 66, Ni-benzyne complex such as 85 reacted with C02 to give the corresponding five-membered oxanickelacyle complex 86 (Scheme 31).46... 

Synthesis, reactions, and physical properties of stable mononuclear platinum and zirconium complexes of cyclohexyne reported prior to the late 1980s have been comprehensively covered in earlier reviews.2-8 More recently, reaction of the zirconocene complex of cyclohexyne with trimethyla-luminum and trimethylgallium has been reported to give 247 and 248, respectively [Eq. (36)].57-58 These products are novel because four atoms (carbons Cl and C3, the transition metal, and the main group metal) are covalently bonded to an sp2 hybridized carbon (C2) in a planar tetracoordi-nate fashion. Synthesis of this type of complex, which Erker describes as anti-van t Hoff/LeBel, does not require the strained cycloalkyne ring zirconocene complexes of acyclic alkynes react similarly.57-58... 

Like strained olefins, strained alkynes, some of which are unstable when free in solution, form stable complexes with transition metals. Cycloheptyne, cyclohexyne, cyclopentyne, and benzyne complexes have aU been prepared. The lengths of the coordinated and uncoordinated C-C bonds of most benz)me complexes are equivalent. Thus, the structures and reactions of the benzyne complexes are more like those of arene dianions than those of a fragment containing a strained C-C triple bond. In other words, these complexes are better described by resonance structure B in Figure 2.27 than by resonance structure A. 

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