Oligomerization of alkynes by the RhCl 3-aliquat 336 catalyst system : Part 2. formation of 2,3-disubstituted 1-phenylnaphthalenes by cyclodimerization of phenylalkynes

Abstract

The RhCl 3-Aliquat® 336 ion pair in 1,1,2,2-tetrachloroethane was shown to catalyze both cyclodimerization and trimerization of internal phenyl-alkynes in a highly regioselective manner. Thus, 1-phenyl-1-propyne, 1-phenyl-1-butyne and 4-phenyl-3-butyn-2-one result in the corresponding 2,3-disubstituted 1-phenylnaphthalenes, in addition to the respective 3,5,6-trisubstituted 1,2,4-triphenylbenzenes as the only cyclotrimerization products. Diphenylacetylene yields 1,2,3-triphenylnaphthalene and hexaphenyl-benzene. Formation of small amounts of 1-chloro-2,3-dimethyl-4-phenylnaphthalene and 1-(2-chlorophenyl)-2,3-dimethylnaphthalene, in the cyclo-oligomerization of 1-phenyl-1-propyne, supports a mechanism in which initial oxidative coupling of the alkyne functions produces a 2,5-diphenylrhodacyclopentadiene capable of subsequent ortho-metallation of the phenyl substituents, followed by metal hydride transfer and reductive elimination of the resultant benzometallacycloheptatriene intermediate.