Activation parameters for three reactions interconverting isomeric 4-and 6-deuteriobicyclo[3.1.0]hex-2-enes

Classic investigations of thermal equilibrations between 4,4-d2- and 6,6-d2-bicyclo[3.1.0]hex-2-enes (Doering, W. von E.; Roth, W. R. Angew. Chem., Int. Ed. Engl. 1963, 2, 115-122) and among deuterium-labeled and non-racemic 2-methyl-5-isopropylbicyclo[3.1.0]hex-2-enes (Δ3-thujenes) (Doering, W. von E.; Lambert, J. B. Tetrahedron 1963, 19, 1989-1994 and Doering, W. von E.; Schmidt, E. K. G. Tetrahedron 1971, 27, 2005-2030) and bicyclo[3.1.0]hex-2-enes (Cooke, R. S.; Andrews, U. H. J. Am. Chem. Soc. 1974, 96, 2974-2980) identified three different paths leading to distinct degenerate products. The equilibrations take place through [1,3]-carbon shifts with retention,suprafacial and inversion,antarafacial stereochemistry and a two-centered epimerization resulting in enantiomerization of the bicyclic skeleton through a "ring-flip" process. Activation parameters for the rate constants kr, kf, and ki associated with these paths have now been secured: Ea 43.8, 44.3, 44.8 kcal/mol and log A 14.1, 14.2, 14.1, respectively, nearly identical values, differing by less than probable error limits, consistent with a rate-determining formation followed by a rapid partitioning of a common diradical intermediate under dynamic control. Copyright © 2002 American Chemical Society.