Kinetics of Thermal Rearrangements in the Δ2-Thujene System: A Full Quadrisection of a Perturbed Bicyclo[3.1.0]hex-2-ene

Abstract

Further insight into the behavior of suppositional diradicals in a caldera is sought in the thermal rearrangements among the four "Delta2-thujenes", two 1-isopropyl-4-methylbicyclo[3.1.0]hex-2-enes [(-)-cis-1 and (+)-trans-2] and two isomers, exo- and endo-3-isopropyl-6-methylbicyclo[3.1.0]hex-2-ene [(+)-exo-3 and (-)-endo-4]. Optically pure trans-3-isopropyl-5-vinylcyclopentene (5) is the final, strongly thermochemically favored product, the result of an intramolecular homodienyl shift of a methyl hydrogen atom in (-)-endo-4. The set of twelve specific rate constants, four sets of three each, that define the interrelations among the four isomers has been extracted from data acquired starting from each isomer. An attractive mechanistic hypothesis involving an intermediate diradical of iso conformation, common, for example, to both (-)-cis-1 and (+)-exo-3 (as educts), that proceeds to an anticonformer common to both (+)-trans-2 and (-)-endo-4 does not lead to a satisfactory rationalization of the product distribution. Addition of a second mechanistic conceptual scheme, that of a diradical-in-transit behaving as if there were a measure of continuous bonding (for example, (+)-trans-2 proceeding directly to (-)-cis-1), improves agreement with experiment. Over a 30 degrees C range of temperature, there is no credible change in product distribution.