Enecyclopropanes methylenecyclopropanes

Despite its high reactivity, the a-chloro ester 1-Me does not polymerize like simple acrylates. However, in close analogy to other 1,1-disubstituted methyl-enecyclopropanes [29], 1-Me slowly dimerizes in a head-to-head fashion even at room temperature to give the two diastereomeric dimethyl dispiro[2.0.2.2]oc-tanedicarboxylates (E)- and (Z)-16 (ratio 1 1) (Scheme 5), and at 120 °C the dimerization proceeds almost quantitatively (89% isolated yield) [15]. Apparently the dimerization of 1-Me occurs considerably more readily than that of methylenecyclopropane [4a,b] and of bicyclopropylidene [4 d]. Surprisingly, under high pressure (10 kbar) this dimerization did not proceed more efficiently (Scheme 5) [30], but more cleanly, which facilitated the separation of isomers. 

More recently, the parent mono- and di-methylenecyclopropane cyclopentadienyl-cobalt -complexes were prepared by ligand exchange reactions583. Reaction of methyl-enecyclopropane (and its tetramethyl derivative) with CpCo(ethylene)2 in pentane affords... 

As with the chloropalladation reaction (vide supra)m the rearrangement of >/2-methyl-enecyclopropane to >/4-TMM was shown experimentally to proceed stereoselectively by disrotatory ring cleavage of the distal frontier molecular orbital considerations, which predict that the out-of-phase interaction between the metal orbital in the distal ring-opening of -methylenecyclopropane complexes can be minimized by bending the bond up away from the metal (equation 348)410 ... 

For the transformation of more highly substituted methylenecyclopropanes, tri-alkylphosphane modified Ni(0) catalysts are more effective172). Representative examples are the cyclodimerizations of 2,2-dimethyl- and 2,2,3,3-tetramethylmethyl-enecyclopropane (Eqs. 67 and 68). 

It is also possible to obtain methylenecydopropanes by monodehydrochlorination of compounds other than 1-chloro-l-methylcyclopropanes. For example, inverse addition of potassium terf-butoxide in dimethyl sulfoxide at 25°C to l,l-dichloro-2,2,3-trimethylcyclopropane (20) gave a 33% yield of 2-chloro-l,l-dimethyl-3-methylenecyclopropane (21) along with a 30% yield of ring-opened product 22. This is one case in which the usual double dehydrochlorination of 1,1-dichlorocyclopropanes (see Section 5.2.2.1.2.3.) is blocked and a single elimination occurs instead. Another example is the reaction of l-chloro-2,3-dimethylcyclopropane (23) with potassium tcr/-butoxide in dimethyl sulfoxide at 80-90 to give l-methyl-2-methyl-enecyclopropane (24) in 84% yield (see also below and Sections 5.2.2.1.1.1.2. and 5.2.2.1.1.2.). 

The elimination of hydrogen chloride from an allylic chloride provides one of the most effective routes to simple alkylcyclopropenes, and to cyclopropene itself The choice of base can be critical, particularly when the product cyclopropene has a 2-alkyl substituent and may undergo a base-induced rearrangement to a methylenecyclopropane, e.g. compare formation of methyl-enecyclopropane and 3-methylcyclopropene. ... 

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