The thermal rearrangement of 2,2'-bismethylenebicyclopropane

[摘要] 2,2$spprime$-Bismethylenebicyclopropane rearranges to 6-methylenespiro (2.4) hept-4-ene and 3,6-bismethylenecyclohexene in solution at 195$spcirc$C. The formation of six dimers is also observed. 2,2$spprime$-Bismethylenebicyclopropane equilibrates between meso and dl diastereomers. 6-Methylenespiro (2.4) hept-4-ene undergoes a facile surface catalyzed rearrangement to 5-methylspiro (2.4) hepta-4,6-diene. The surface rearrangement occurred both in solution and during preparative gas chromatography. The equilibrium of 2,2$spprime$-bismethylenebicyclopropane favors the meso diastereomer. The activation parameters for the equilbrium process were found to be: k$sb{m forward}$= 10$sp{m 14.1}$ exp($sp{-}$38,500cal/RT) and k$sb{m reverse}$ = 10$sp{15.1}$ exp($sp{-}$36,700cal/RT); where the forward reaction is defined as the dl racemate isomerizing to the meso diastereomer. The exact structural identity of the six dimers was not achieved due to experimental difficulty. Flash Vacuum Pyrolysis of 2,2$spprime$-bismethylenebicyclopropane also resulted in the formation of 3,6-$spprime$-bismethylenecyclohexene and 6-methylenespiro (2.4) hept-4-ene. No bimolecular reactions were observed. The major product in the gas phase is 3,6-bismethylenecyclohexene. Both concerted or diradical mechanisms could describe the rearrangements of 2,2$spprime$-bismethylenebicyclopropane. Either type of mechanism could explain the interconversion of meso and dl 2,2$spprime$-bismethylenebicyclopropane and the formation of 1-methylene-2-(cyclopropylidenemethyl)cyclopropane. 3,6-Bismethylenecyclohexene must result from either a concerted rearrangement of the trimethylenemethane diradical from 2,2$spprime$-bismethylenebicyclopropane or directly from a concerted rearrangement of 2,2$spprime$-bismethylenebicyclopropane. Overall, a diradical mechanism is implicated due to the observation of dimerization between 2,2$spprime$-bismethylenebicyclopropane and 3,6-bismethylenecyclohexene. No concerted mechanism could explain this result. The formation of 6-methylenespiro (2.4) hept-4-ene is thought to occur through the intermediacy of 1-methylene-2-(cyclopropylidenemethyl)cyclopropane. 1-Methylene-2-(cyclopropylidenemethyl)cyclopropane is not observed, as it rapidly undergoes a vinylmethylenecyclopropane rearrangement to 6-methylenespiro (2.4) hept-4-ene. Dimerization could result through the combination of monomers with the diradical formed from 2,2$spprime$-bismethylenebicyclopropane. Other mechanistic possibilities include (4+2) cycloadditions between 5-methylspiro (2.4) hepta-4,6-diene and 3,6-bismethylenecyclohexane or 5-methylenespiro (2.4) hepta-4,6-diene with itself. (Abstract shortened by UMI.)

[发布机构] Rice University