Cyclohexyne stability

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. 

A stabilization of the unstable cyclohexyne (20) and cycloheptyne (16) has been achieved through zerovalent platinum complexes46,47) X-ray analyses of these complexes have been published 231). 

The relative stabilities of cycloheptyne and cyclohexyne generated from 1-amino-4,5-cycloalkeno-l,2,3-triazoles (21) and lead tetraacetate were investigated. After the nitrogen evolution, which occurred instantaneously on addition of lead tetraacetate, had ceased, the trapping reagent was added after different time intervals to the reaction mixture, and yields of adduct (23) were determined. The results... 

Competitive addition of phenyllithium and lithium piperidide to cyclooctyne, cycloheptyne, cyclohexyne and cyclopentyne has been studied in order to gain information on the relative stabilities of cyclic acetylenes . 

Other extremely strained cycloalkynes such as cyclohexyne and even cyclo-pentyne have been stabilized by the complexation with 4d and 5d transition... 

Alternative indirect methods have been used in the synthesis of (3,3-dimethylcyclopen-tyne)Cp2Zr(PMe3> [52] and (cyclohexyne)[CpMo(CO)2]2 [53]. The smallest cycloalkyne stabilized by complexation is cyclobutyne, incorporated by Adams into tri- and tetrametallic ruthenium and osmium clusters as a P3-ligand 30 using 1-Br and -SPh substituted cyclobutene precursors (Scheme 4-11) [54]. Liberation of these strained alkynes from their metallic bondage has not been reported but some have been shown to be reactive towards insertion of unsaturated substrates, as illustrated in Scheme 4-12 with the Zr-cyclohexyne derivative 31 [55]. 

Highly strained cyclic alkynes can be stabilized by complexing them with metal compounds [1 b, c 75, 76]. Thus, even reactions with elusive species such as cyclohexyne are possible, as exemplified in Scheme 8-26 [78]. 

Many other species are stabilized in 18-electron organometallic complexes car-benes and carbynes, enyls and polyenyls (XL ligands), o-xylylene (o-quinodime-thane), trimethylenemethane, benzyne, norbornadiene-7-one, cyclohexyne, 1,2-di-hydropyridines (intermediates in biological processes), thermodynamically unfavorable organic tautomers such as vinyl alcohols [less stable by 14 kcafrmol (58.5 kJ mol ) than their aldehyde tautomers], aromatic anions resulting from deprotonation in juxta-cyclic position such as tautomers of phenolates and benzylic carbanions. All these species have a specific reactivity that can lead to synthetic applications in the same way as cyclobutadiene above. 

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