6 - C5H6−C5H10

Abstract

This chapter presents hydrocarbon isomerizations, which have been at or near the center of experimental and theoretical efforts by physical organic chemists. The intramolecular vicinal hydrogen shift undergone by cyclopentadiene (CPD) was first recognized when a mixture of double bond isomers was obtained when specific alkyl-substituted CPDs were heated above room temperature. The possibility that the reaction could proceed by acid dissociation was dismissed by the observation that the reaction occurred only 1.4 times faster in acetone than in the gas phase. The isomerization of bicycle[2.1.0]pentene involved bridgehead bond cleavage in a disrotatory sense. Vinylcyclopropane rearranged to cyclopentene on heating. Minor amounts of hydrogen-shifted, ring-opened isomers are also observed. Spiropentane gives methylenecyclobutane upon heating at high pressures. Studies on the intermolecular cycloaddition indicate that a biradical intermediate is involved although the reaction often occurs with high stereospecificity about the ethylenic moiety. Thus, dimethyl maleate gives substantial stereospecificity in the formation of the two possible regio-isomers.