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Abstract
The action of lithium amalgam on various poly-halogenomethanes and their derivatives has been investigated. Useful yields of gem-dichlorocyclopropanes were obtained from reaction of lithium amalgam with carbon tetrarchloride in the presence of cyclohexene, 2,3-dimethylbut-ene or styrene, and the reaction is believed to proceed via the product of dichlorocarbene:
CCl4 + 2Li -------> : CCl2 + 2LiCl
Lower yields of gem-dibromocyclopropanes were obtained when carbon tetrabromined replaced the techloride; this appears to be due to attack on the dibromocyclopropane by the amalgam.
The pyrolysis of sodium tribromoacetate was found to be a convenient source of dibromocarbene, but the yield of dibromocyclopropane obtained by reaction with 2,3-dimethylbut-2-ene depended on the temperature of the pyrolysis. The adduct, 1,1-dibromo-2,2,3,3,-tetramethylcyclopropane, decomposes rapidly at 150°C to give a monobrominated-diene, probably CMe2:CBr.CMe:Ch2.
Diflurocarbene was produced when lithium amalgam reacted with dibromodifluoromethane; diflourocarbene prepared in this way reacted with 2,3-dimethylbut-2-ene to give a new cyclopropane, also prepared by the use of sodium chlorodiflouroacetate as a source of difluorocarbene.
Chloroform reacted with lithium amalgam to give dichlorocarbene and methylene chloride; the stoichiometry approximates to:
2CHCl3 + 2Li ------> :CCl2 + CH2Cl2 + 2LiCl
And possible routes to the methylene chloride are considered. Treatment of 1,1,1-trichloroethane with lithium amalgam in the presence of 2,3-dimethylbut-2-ene gave the new compound 1-chloro-1,2,2,3,3,-pentamethylcyclopropane, thus providing evidence that methylchlorocarbene had been produced. This is a convenient route for the preparation of this carbine.
Methylene chloride did not react readily with the amalgam, and gave only low yields of products which could have arisen via attack of chlorocarbene on cyclohexene.
Attempts to prepare di-iodocarbene by the reaction of carbon tetraiodide or iodoform with the amalgam, were unsuccessful. An attempt to prepare diflourocarbene and/or dichlorocarbene in good yield by the photolysis of 1,1-dichloro-2,2-difluoroethylene was also unsuccessful.
Dichlorocarbene, generated either from carbon tetrachloride or chloroform, reacts with tetrahydrofuran to give 2-dichloromethyltetrahydrofuran; no dimerization of the carbine to tetrachloroethylene was observed.
