Cyclopropenes Vinylcyclopropenes

A transition metal mediates rearrangement of a substituted (noncyclo) C5 species. This includes ring expansion reactions of, for example, substituted 3-vinyl- 1-cyclopropene (cf. Scheme 22) or cyclobutene, both bonded to a metal to give supracyclopentadiene Rh and Ru complexes (92- 94). The rearrangement of vinylcyclopropenes to cyclopentadienes can also occur photolytically (66). 

The photochemical rearrangement of 3-vinylcyclopropenes exhibits some regioselec-tivity. Thus, contrary to the selectivity observed with the thermal rearrangement of cyclopropenecarboxylates (see equation 149), the major product obtained from the irradiation of unsymmetrical substituted cyclopropenes was derived from the preferential cleavage of the cyclopropene single C-C bond which is methyl rather than phenyl substituted (equation 154). ... 

Vinylcyclopropenes, e.g. 245a-c, undergo rearrangements analogous to those of their 3-aryl counterparts Direct photolyses of the cyclopropenes 127 afford preparatively... 

The stereoselectivity of the cycloaddition of arylcarbonitrile (V-oxides to 3,3-dimethyl-, 3-aryl-3-methyl-, 3-methyl-3-vinyl- and 3-cyano-3-methylcyclopropenes has been studied. In the case of the 3-vinylcyclopropene, addition to the external double bond can occur in competition with addition to the cyclopropene. 

The reaction apparently proceeds by an initial monodehydrochlorination followed by cleavage of the derived cyclopropene to a diradical which is converted into the product by loss of Cl and addition of a hydrogen atom. The vinylcyclopropene may also be obtained from 2-chloro-l-vinylcyclopropane by dehydrochlorination under the same conditions. The NMR signals for the product persisted at — 100 C in tetrahydrofuran, but diminished at temperatures above — 70 C, and at room temperature a [2 - - 2] dimer was observed. 

Disubstituted cyclopropenes undergo addition to tetraarylcyclopentadienones to produce mixtures of exo- and emfo-adducts in which the endo-isomer predominates (Table 12).187188 The predominance of enr/o-product has been explained in terms of secondary orbital interactions, involving the methylene group of the cyclopropene.147 3-Methyl-3-vi-nylcyclopropene reacted with 2,3,4,5-tetraphenylcyclopenta-2,4-dien-l-one at the cyclopropene double bond and a [4+2] cycloadduct of unspecified stereochemistry was isolated (41% mp 231-234 °C) together with decarbonylated products reaction of 2-oxo-4,5-diphenyl-l,3-bis(methoxycarbonyl)cyclopentadiene with 3-methyl-3-vinylcyclopropene was more complicated and apparently attack occurred at both 71-bonds.1... 

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