Methyleneketenes and methylenecarbenes. IX. Thermal rearrangements of Alkyl-and Aryl-acetylenes involving Alkyl- and Aryl-methylenecarbenes

Abstract

Flash vacuum pyrolysis of 1-(1?-adamantyl)acetylene[2-¹⁴C] (780°) gives recovered alkyne which contains 25% of the rearranged product, 1-(1?- adamantyl)acetylene[1-¹⁴C]. 1-Ethynyl-1-methylcyclohexane(740°) is converted into toluene and benzene, while evidence for rearrangement of 3,3-dimethylbut-1-yne[2-¹³C] (790°) and 3-methyl-3-phenylbut-1-yne[2- ¹³C] (680°) was not obtained before the onset of alternative reactions. These reactions of alkyl-substituted alkynes are discussed in terms of possible methylene carbene intermediates. Similar intermediates may possibly be involved in reactions of arylalkynes. Thus, pyrolysis of 1- ethynylnaphthalene (750°) yields acenaphthylene and of 1-ethynyl-8- methylnaphthalene (750°) yields phenalene. The pathways for the formation of 2-phenyl- and 3-phenyl-indene from (2- methylphenyl)phenylethyne (790°) and of indene from (2- methylphenyl)ethyne (740°) are obscured by facile thermal rearrangements.