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

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.