A Stereochemical Study of the Thermolysis of cis-anti- and trans-1,2-Dimethyl-cis-3,4-dideuteriocyclobutane

Article abstract of DOI:10.1021/jo00230a023

The stereochemistry of the fragmentation and isomerization of cis-anti- and trans-1,2-dimethyl-cis-3,4-dideuteriocyclobutane at 510 deg C is reported.The cis-anti-cis isomer undergoes fragmentation to yield cis/trans-propene-d1 (1.5/1, major pathway), cis/trans-2-butene (1.4/1), and cis/trans-ethylene-d2 (1/1, minor pathway).Recovered cis-1,2-dimethylcyclobutane-d2 containing approximately 40percent of the double rotation product relative to the product of single methyl rotation, trans-1,2-dimethylcyclobutane-d2.The trans isomer behaves similarly, yielding cis/trans-propene-d1 (1/1, major pathway), cis/trans-2-butene (1/5), and cis/trans-ethylene-d2 (1/1, minor pathway).Recovered cis-1,2-dimethylcyclobutane-d2 from thermolysis of the trans isomer consists mainly of equal amounts of cis-anti-cis- and cis-syn-cis-1m2-dimethylcyclobutane-d2 as analyzed by NMR.On the basis of product composition, the thermal chemistry of this system can be explained as proceeding through 2,5-hexanediyl (major pathway) and 3-methyl-1,4-pentanediyl (minor pathway).On the basis of the observed stereochemistry, it can be concluded that the lifetimes of both 2,5-hexanediyl and 3-methyl-1,4-pentanediyl are similar and of the same order as bond rotations at a radical center.This suggests that the gauche to trans conformational changes involving carbon-carbon bond rotation at carbon 2 and 3 of 1,4-diyls may not be competitive with fragmentation.