C8H6 Thermal Chemistry. 7-Methylenecyclohepta-1,3,5-dienyne (Heptafulvyne) by Flash Vacuum Thermolysis–Matrix Isolation. Chemical Activation in the Rearrangements of Phenylenedicarbenes and of Benzocyclobutadiene to Phenylacetylene

Abstract

Methylenecycloheptadienyne 11 (heptafulvyne) is obtained very cleanly by flash vacuum thermolysis (FVT) of the diazobenzocyclobutene precursor 8 at 400°C followed by isolation as a neat solid at 77 K or in an Ar matrix at 7–10 K. Compound 11 is a yellow solid, stable till ~–100°C in the neat state. The diazo compound itself (2) is observable by IR spectroscopy following mild decomposition of the tosylhydrazone salt 1 at 115°C. FVT of 8 at 200°C also generates diazo compound 2 as observed by IR spectroscopy and on-line mass spectrometry. FVT of 8 at 600–800°C causes rearrangement of 11 to phenylacetylene 12 and benzocyclobutadiene 13. Mechanisms for the rearrangements are proposed. Facile rearrangement of benzocyclobutadiene to phenylacetylene is ascribed to chemical activation, which is also seen to be involved in the rearrangement of p-, m-, and o-phenylenebiscarbenes 25–27 to phenylacetylene 12.