CROSS-CONJUGATED BIRADICALS. KINETICS AND KINETIC ISOTOPE EFFECTS IN THE THERMAL ISOMERIZATIONS OF CIS- AND TRANS-2,3-DIMETHYLMETHYLENECYCLOPROPANE, CIS- AND TRANS-3,4-DIMETHYL-1,2-DIMETHYLENECYCLOBUTANE, AND OF 1,2,8,9-DECATETRAENE

Article abstract of DOI:10.1016/0040-4020(82)80166-X

cis- and trans-2,3-Dimethylmethylenecyclopropane (C and T) interconvert at 160.0^o 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^o rotation of the exo-methylene carbon.The kinetics of the thermal 1,3- and 3,3-shifts of cis- and trans-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^o 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 formed from the minor product, cis-7,8-dimethylenebicyclo<4.2.0>octane (CO), at higher temperatures.The trans isomer, TO, also gives DC at about the same rate as CO.