Cross-conjugated biradicals: Kinetics and kinetic isotope effects in the thermal isomerizations of cis- and trans-2,3-dimethylemethylnecyclopropane, cis- and trans-3,4-dimethyl-1,2-dimethylenecyclobutane, and of 1,2,8,9-decatetraene

Abstract

cis- and trans - 2,3 - Dimethylenemethylenecyclopropane ( C and T ) interconvert at 160.0° with a small normal kinetic isotope effect (KIE) when the exo-methylene is deuterated, but the 1,3-shift products, 2-methylethylidenecyclopropane, show a large normal KIE, 1.35 and 1.31, when formed from C and T , respectively. This data can be interpreted in terms of either parallel reactions or a common trimethylenemethane diradical intermediate formed with a normal KIE of 1.11 and closing to 1,3-shift product with a normal KIE of 1.29 due to the effect of deuterium in the required 90° rotation of the exo-methylene carbon. The kinetics of the thermal 1,3- and 3,3-shifts of cis- and rans-3,4-dimethyl-1,2-dimethylenecyclobutane ( CB and TB ) were determined in a flow reactor. The first order rate constants are log k CB (sec −1) = 13.7 − 42,200/2.3 RT and log k TB (sec −1) = 13.6 − 41,900/2.3 RT (E a in kcal/m) which compare favorably to that from the parent hydrocarbon. 1,2-dimethylenecyclobutane, after reasonable correction for dimethyl substitution. Rearrangement of TB and its bis(dideuteriomethylene) derivative at 230.0° revealed a normal KIE of 1.08. This KIE could be interpreted in terms of either a methylene rotational isotope effect in a concerted reaction or formation of a bisallyl diradical with the expected normal rotational IE on closure to the 1,3-shift product of 1.12 with no IE in the ring opening when the result is corrected for return of the biradical to starting material. The kinetics of intramolecular 2 + 2 cycloaddition of 1,2,8,9-decatetraene were determined in a flow reactor. The first order rate constant is log k(sec −1) = 9.4 − 30,800/2.3 RT (E a in kcal/m). These energetics are compared with those of other 2 + 2 cycloadditions. The major product is 3,4-dimethylenecyclooctene ( DC ) which is also found from the minor product, cis-7,8-dimethylenebicvyclo[4.2.0]octane ( CO ), at higher temperatures. The trans isomer, TO , also gives DC at about the same rate as CO .