Pergamon TetrahedronLetters, Vol. 38, No. 11, pp. 1877-1880, 1997 © 1997 ElsevierScienceLtd All rights reserved.Printedin Great Britain PII: S0040-4039(97)00225-6 0040-4039/97 $17.00 + 0.00 Thermal Migration of an Ethynyl Group From One Benzene Ring to Another by Reversible Vinylidene C-H Insertion l Lawrence T. Scott* and Atena Necula Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, MA 02167-3860 USA Abstract. Evidence is presented for the high temperature opening of a 5-membered ring by extrusion of a carbene (the reverse of a C-H bond insertion), which results in the net thermal migration of an ethynyl group from one benzene ring to another. © 1997 Elsevier Science Ltd. All rights reserved. Thermal isomerization of a terminal alkyne to a vinylidene under flash vacuum pyrolysis (FVP) conditions and trapping of the transient carbene by intramolecular C-H insertion to form an etheno-bridged aromatic compound was first demonstrated by R. F. C. Brown et al. in the 1970s. 2 This high-temperature cyclization reaction is estimated to be exothermic by 15-20 kcal/mol overall, 3 and only the endothermic hydrogen shift has generally been regarded as reversible. We have now encountered a case, however, in which the vinylidene insertion step also appears to be reversible, and this reversibility leads to the net migration of an ethynyl group from one benzene ring to another along the edge of a polycyclic aromatic hydrocarbon ring system (eq 1). H H Z netet n rou 0raon & Extrusion of a carbene by de-insertion from a remote C-H bond is enormously endothermic, 3 of course, and reported examples are quite rare, 4 but every unimolecular reaction, in principle, must be reversible at sufficiently high temperatures. Accordingly, it seems reasonable that such ethynyl group migrations could play a significant role in the chemistry of hydrocarbons in flames, in the mechanism of carcinogen formation in smoke, and in the production of fullerenes and carbon nanotubes under high energy conditions inter alia. Our discovery of the title reaction occurred during the course of experiments designed to prepare the benzo- pyracylene 1, a molecule we had hoped could be used to test the feasibility of the "Stone-Wales rearrangement 5'' (eq 2). 1 2 1877