Thermal Rearrangements, XIX. The Kinetics of the Thermal Isomerization of 1‐Ethynyl‐2,2,3,3‐tetramethylcyclopropane

Abstract 1‐Ethynyl‐2,2,3,3‐tetramethylcyclopropane (9) has been prepared and subjected to gas phase pyrolysis. Above 200°C the hydrocarbon rearranges to 4,4,5‐trimethyl‐1,2,5‐hexatriene (10) and 5,6‐dimethyl‐5‐hepten‐1‐yne (11). Whereas the allene is produced directly from 9 by a 1,5‐hydrogen shift, the alkyne is a secondary product formed from 10 by a [3,3] sigmatropic rearrangement. The activation parameters (Ea, lg A) for both processes have been determined for the 210 to 250°C temperature range. The data are discussed and compared with those for other concerted reactions of alkynes and alkenes.