Thermal Rearrangements, 28. Competing Reaction Pathways in the Thermal Cycloisomerization of 1,3‐Hexadien‐5‐ynes

The thermal cycloisomerization of the 1,3‐hexadiene‐5‐ynes 1a–e has been studied at temperatures above 500°C and reaction times of about 0.3 s ( 1a–d ) and at a pressure of 1.5 · 10 −2 Torr (FVP, 1e ), respectively, at low partial pressures in quartz flow systems. While 1a and 1b cycloisomerize exclusively to the corresponding benzenes and pentafulvenes, substrates 1c–e cyclize primarily to 4‐substituted 1,2‐dihydronaphthalenes and biphenyls. The experimental results suggest a predominance of unimolecular reactions such as electrocyclic ring closures and 1,6‐C,H‐insertion processes of alkenylidene carbenes initially formed by an acetylene→vinylidene rearrangement.