Methylenecyclopropanes, ring opening intermediates

In the hydrocarbonation of methylenecyclopropanes 8 with nitriles, the hydro-palladation of 8 with 16 gives the alkylpalladium complexes 23 and/or 24 (Scheme 5). The complex 23 would undergo rearrangement by distal bond cleavage to give the Jt-allylpalladium 25 (route A). The reductive elimination of Pd(0) from 25 produces 9. The palladium complex 24 would isomerize to the Jt-allylpalladium complex 27 via proximal bond cleaved ring-opened intermediate 26 (route B). The reductive elimination of Pd(0) from 27 gives 10. 

Reaction of the cyclopropyl-substituted pivalate (25) with dimethyl benzylidenema-lonate in the presence of a palladium catalyst gave a mixture of alkylidenecyclo-propane (26) and vinylcyclopropane (27). The ratio of these two adducts is found to be quite sensitive to the choice of ligand and solvent. While triisopropyl phosphite favors the formation of the methylenecyclopropane (26), this selectivity is completely reversed with the use of the bidentate phosphite ligand dptp (12). Interestingly there was no evidence for any products that would have derived from the ring opening of the cyclopropyl-TMM intermediate (Scheme 2.8) [18]. 

Hydroxymelhyl)cyc opropan-l-ol (11), readily obtained by hydrogen peroxide oxidation of methylenecyclopropane, was rearranged to cyclobutanone (12) either directly, or via an intermediate l-(tosyloxymethyl)cyclopropan-l-ol.114 115 Competitive ring opening to 1-hydroxy-butan-2-one (13) was observed in the boron trifluoridc catalyzed process. 

The reaction mechanism involved the cyclisation of ketyl radical anion 3 on to the methylenecyclopropane moiety in a 5-exo-trig manner. Ring opening of cyclopropane intermediate 4 gave rise to the cyclohexyl radical 5, which then cyclised in a 5-exo-dig fashion to form the second ring (Scheme 6.2).5,6... 

Trimethylenemethane has been postulated to be the reactive intermediate in a number of reactions involving the ring-opening and rearrangement of methylenecyclopropanes In fact, methylenecyclopropanes are known to undergo degenerate thermal rearrangements. An early example, the ethylidenecyclopropane-methyl methylenecyclopropane interconversion, is shown in equation 20. ... 

In many cases, the elimination of hydrogen halide from a 1,1-dihalocyclopropanc has been reported to lead to an intermediate 1-halocyclopropene, which again reacts further under the reaction conditions, leading either to a cyclopropane, a methylenecyclopropane, a vinylcyclo-propane or to ring-opened products. Such reactions are discussed elsewhere. However, in other cases, as shown below, the I-halocyclopropcnes 3 were isolated. 

Reaction with Methylenecyclopropanes. Methylenecyclo-propanes can serve as alkene substrates in Mn(OAc)3 processes. The reaction proceeds first by regioselective addition of the mal-onic ester radical on the alkene, followed by rapid ring opening of the cyclopropyl radical intermediate. The latter leads to free radical cyclization on the aromatic ring, followed by rearomatiza-tion to furnish the 3,4-dihydronaphthalene product in acceptable yield (eq 27). The method accepts different radical precursors, such as -ketoesters and /3-cyanoesters. When -diketones and more sterically hindered methylenecyclopropanes are used, it is possible to block the tandem reaction, and classical cyclization occurs (eq 28). ... 

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