Cyclopropene oxidative cleavage

Cyclopropenes undergo an oxidative cleavage to yidd substituted enones, as shown in equation (27). The reaction is believed to proceed through the unstable qwxide. The regioselectivity is genoaOy low if R 9 R, but if one of the substituents is trimethylsilyl a hi y selective conversion to the a-silyl enone takes place. There is one report of a similar oxidative cleavage that takes place with thallium(ni). ... 

Similar to the addition of halogens, cycloproparenes react under hydrogenation conditions either by initial attack at the cyclopropene bond or via cleavage of one of the lateral cyclopropane bonds. In the former case, the intermediate norcaradiene opens to a cycloheptatriene which is further reduced to cycloheptane. In the latter, the reduction leads to toluene or methyl-cyclohexane. For example, benzocyclopropene, on reaction with platinum oxide in acetic acid, was hydrogenated to cycloheptane (49%) and methylcyclohexane (51%). 1,1-Difluoroben-zocyclopropene, under the same conditions, gave cycloheptane exclusively (85%). Hydrogenation of benzocyclopropene with Raney nickel gave toluene in 90% yield. 

C-C cleavage of strained rings and ketones has been used to develop useful catalytic reactions. For example, vinylcyclopropanes and vinylcyclobutanes react with alkynes (Equation 6.66) to generate products from 5+2 and 6+2 addition processes that form seven- and eight-membered ring products by overall transformations that are homologs of the Diels-Alder reaction. " The mechanism of these catalytic reactions continues to be studied, but these reactions most likely occur by coordination of the olefin to rhodium and insertion of the metal into the cyclopropene or cyclobutane. Decarbonylation of dialkyl ketones, including relatively unstrained cyclic ketones, has been reported and most likely occurs by oxidative addition into the acyl-alkyl C-C bond, subsequent de-insertion of CO, and C-C reductive elimination. 

A new carbometallation/oxidation/C-C bond cleavage sequence for cyclopropenes allows the preparation of aldehyde enantiomers bearing a-quaternary stereocentres using different materials in two pathways as shown in Scheme 1." ... 

The oxidative ring cleavage of cyclopropenes and of cyclobutenes by thallium(m), mercury(n), palladium(n), or lead(iv) bears some resemblance to the isomerizations discussed above. In particular, the first step in the oxidations is probably insertion of the metal into a carbon-carbon bond, as in isomerization. 

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