Cyclohexyne cycloaddition

Bottini s group has investigated the reactions of 1-halogenocyclohexenes with potassium t-butoxide. From 1-chloro-, 1-bromo-, or 1-iodo-cyclohexene and Bu OK in either Bu OH or DMSO a yield of 4—12% of 1-t-butoxy-cyclohexene is obtained this product was rapidly converted into cyclohexanone on treatment with acid. Labelling experiments lead to the conclusion that 1-t-butoxycyclohexene is formed with concurrent operation of elimination addition mechanisms involving cyclohexa-1,2-diene and cyclohexyne. A number of C 2 hydrocarbons are also formed, arising from cyclohexa-1,2-diene via dimerization and 1,2- and 1,4-cycloaddition of cyclohexa-1,3-diene. 

An elimination-addition mechanism has also been invoked for the nucleophilic substitution of cyclohexenyliodonium salts with acetate ion. For example, the reaction of either the 4- or 5-substituted cyclohex-l-enyl(phenyl)iodonium tetra-fluoroborate (31 or 32 respectively) with tetrabutylainmonium acetate in aprotic solvents gives the ipso and cine acetate substitution products (33 and 34 and vice-versa, respectively) in almost the same ratio (Scheme 23). These results were consistent with an elimination-addition mechanism involving 4-substituted cyclohexyne 35 as a common intermediate. The presence of a cyclohexyne intermediate was confirmed by deuterium labelling and trapping studies leading to [4-l-2]-cycloaddition products. 

Reaction of cyclohexyne with cyclic ketones has been found theoretically to involve 2-b 2-cycloaddition to the enolate followed by ring opening of the intermediate cyclobutene alkoxide for which alternative mechanisms have been addressed." ... 

A computational study of the mechanism of cyclohexyne insertion into a C(0)-Ca bond of cyclic ketones has been reported to proceed through a stepwise 2 + 2-cycloaddition of cyclohexyne to the enolate, followed by thermally allowed conrotatory electrocyclic ring opening, thermally forbidden disrotatory electrocyclic ring opening, or nonpericyclic C-C bond cleavage (Scheme 34)... 

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