In methylenecyclopropane

On the other hand, as much as the increase in SE on going from methylenecyclopropane (2) to spiropentane (96) is substantial (23.4 kcal/mol), ASE is 21.1 kcal/mol on going from bicyclopropylidene (1) to [3] triangulane (97) (Table 4). As an expression of its increased SE, the double bond in 1 demonstrates a uniquely enhanced reactivity compared to the one in methylenecyclopropane (2) (see below). 

The central carbon atom in methylenecyclopropane is almost sp-hybridized, like that in aUene the carbon atoms in cyclopropane are almost sp -hybridized, cf de Meijere A (1979) Angew Chem 91 867 Angew Chem Int Ed Engl 18 809... 

The highly strained double bond in methylenecyclopropane displays enhanced reactivity in cycloaddition reactions. In addition to normal [4+2] cycloaddition to 1,3-dienes (e.g. equation 13)32, methylenecyclopropane and its derivatives have a pronounced tendency to undergo thermal [2+2] cycloaddition reactions. For example, thermal dimerization of methylenecyclopropane in the gas phase results in formation of isomeric dispirooctanes 16 and 17 (equation 14)33. This unusual cyclization is considered to proceed via a stepwise radical mechanism involving the intermediacy of biradical 18 (equation 15)34. Equation 15 demonstrates that methylenecyclopropanes possessing substituents capable of stabilizing intermediate radicals undergo efficient [2+2] dimerization even... 

In the base-induced cyclization of a number of y-chloro-y-nitro carboxylic esters and derivatives, it was found that use of excess base led to elimination of nitrous acid resulting in methylenecyclopropanes. Thus, reaction of ethyl 4-chloro-4-nitropentanoate (1) with one equivalent of sodium hydride in dimethylformamide gave, in addition to a 53% yield of ethyl 2-methyl-2-nitrocyclopropanecarboxylate(2), 12% of ethyl 2-methylenecyclopropanecarboxylate (3). The latter could be prepared in 70% yield from the former, or in 55% yield from the starting ester by the use of two equivalents of sodium hydride. 

The increased strain in methylenecyclopropane, which is relieved on ring opening, makes the cyclic bonds more liable to cleavage than methylcyclopropane. Photochlorination of methylenecyclopropane (3) in the liquid phase produced a mixture of several addition products including 3-chloro-2-chloromethylprop-2-ene (42%), 2,4-dichlorobut-l-ene (27%), 1-chloro-l-chloromethylcyclopropane (18 /o), 1,2,3,4-tetrachlorobutane (11 %), and 1,3-dichloro-2-chloro-methylprop-l-ene (2%). ... 

We have carried out the procedure for 1-methylcyclopropene several times with freshly prepared sodamide (see Chap. I, Exp. 7). In most cases the ratio of 1-methylcyclopropene to methylenecyclopropane was greater than 3, in one experiment even 10. The original paper reports the formation of almost pure 1-methylcyclopropene. The authors used commercial sodamide, which might be less reactive than the freshly prepared base, and therefore unable to cause isomerization. Since the protons in methylenecyclopropane are less acidic than the vinylic proton in 1-methylcyclopropene, the exocyclic isomer is not likely to interfere in metallation reactions with alkali amides in liquid ammonia or with LD A... 

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