The thermal ring opening of 3,3-disubstituted cyclobutenes

Abstract

The rates of the thermal unimolecular isomerisation of 3-t-butyl-3-methylcyclobutene and 3-methyl-3-phenylcyclobutene have been measured in the gas phase over the temperature ranges 158–190° and 120–180°, respectively. They are correlated by the Arrhenius expressions k= 10^{14.08 ± 0.30}exp(–150.8 ± 2.5/RT) s^–1 and k= 10^{12.50 ± 0.12}exp(–126.15 ± 0.95/RT) s^–1, respectively. The ratio of Z- to E-isomer of the butadienes formed has been measured both for these two and for the 3-methyl-3-R-cyclobutenes, where R = isopropyl, n-propyl, cyclopropyl, pentadeuterioethyl, 4-methoxyphenyl, 3-methoxyphenyl, and 4-cyanophenyl. In each case the proportion of Z-isomer formed is much larger than would be predicted on steric grounds. For the aryl compounds an attractive interaction during the formation of the Z-diene is proposed to account for its excess formation. For the alkyl substituents, a repulsive electronic interaction between the group and the developing remote double bond should be larger for methyl than for the other alkyl groups, accounting for the observed product ratios. The rates of isomerization of the 3-aryl-3-methylcyclobutenes at 161° correlate with the LUMO of the aromatic ring. The conclusions are shown to apply to results which have been reported by other authors.