Thermal Isomerizations of 2-d-1-(E)-Propenylcyclobutanes to 4-d-3-Methylcyclohexenes

Abstract
Racemic trans-2-d-1-(E)-propenylcyclobutane at 276 °C in the gas phase fragments to give ethylenes and pentadienes, equilibrates with its cis isomer, and rearranges to mixtures of 4-d- and 6-d-3-methylcyclohexenes through [1,3] carbon shifts. The time-dependent distributions of deuterium-labeled isomers of propenylcyclobutanes and 3-methylcyclohexenes reveal a significant secondary deuterium kinetic isotope effect favoring C1−C4 over C1−C2 bond breaking (kH/kD = 1.16 ± 0.02) and a 72:28 preference for structural isomerizations giving (si + ar) rather than (sr + ai) products through conformationally flexible short-lived diradical intermediates.

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