1,1,2-Trihalocyclopropanes (halogen= chlorine or bromine) undergo 1,2dehalogenation on reaction with methyl lithium, and in a number of cases the product is a 1-halocyclopropene. In the reactionsof (20, X = Br, Cl) and ( 25) a rearrangement occurs even at low temperatures and propargylic halides are isolated, while (16) is converted to ,?-I' illorocyclohex-2-enylidene which may be trapped by furan.
The dehydrohalogenation of 2-alkyl-l,l-dihalocyclopropanes with potassium alkoxide in alcohol or dimethylsulphoxide has been widely postulated to lead initially to l-halocyclopropenes.' In general, these compounds have not been isolated, but undergo very rapid prototropic shifts to 2-methylene-1-halocyclopropanes followed by further dehydrohalogenation.
In particular cases where the prototropic shift is blocked the halocyclopropene has been isolated, but can be accompanied by products in which the ring has opened: 2 M e But 4 Cl , An alternative route to 1-halocyclopropenes involves the addition of an alkyl lithium (RLi) to a l,l-dihalocycloprop-2-ene to give (11, followed by elimination of lithium halidea The ready availability of 1,1,2-trihalocyclopropanes, 4 and the known fast lithium-halogen exchange in l,l-dihalocyclopropanes 5 suggests an alternative process involving 1,2-elimination of LiX