CHEMISTRY OF DIHALOCYCLOPROPANES .25. THE INFLUENCE OF HALOGEN SUBSTITUENTS ON THE VINYLCYCLOPROPYLIDENE CYCLOPENTENYLIDENE REARRANGEMENT

Starting from 2,2-dibromocyclopropanecarbaldehyde and its 1-methyl ana logue, some chloro- and bromo-vinyl-substituted derivatives have been prepared, and their reactions with methyllithium studied at 0 and -78- degrees-C with emphasis on the formation of cyclopentadienes and allen es. Most reactions produced allenes, monobromides and bicyclopropylide nes. A rearrangement product, viz. 1-chloro-4-methylcyclopentadiene, w as only encountered from the reaction of (E)-1,1-dibromo-2-(2-chloroet henyl)cyclopropane. The electronic effect of a chlorine substituent do es not influence significantly the allene to cyclopentadiene ratio, wh ich seems to be governed mainly by steric interactions. Reactions of 1 ,1-dibromo-2-(2,2-dibromoethenyl)cyclopropane and its 2-methyl analogu e furnished the corresponding ethynyl derivatives as sole products; li thium-bromine exchange occurred preferentially at the vinylic carbon.