Diradicals, cyclopropene thermal

The strain energy of cyclopropene is about 53 kcal/mol. The energy of a normal C—C bond is about 83 kcal/mol. If all the cyclopropene strain were concentrated in one C—C bond, then only ca. 30 kcal/mol extra energy would be required to break the bond. Indeed, cyclopropenes thermally open to vinylcarbenes with activation energies of 30—40 kcal/mol i9.38-40) (Table 3). The carbene-diradical resonance hybride 19 can also be formed from appropriately substituted vinyl-diazoalkanes (20) 41,42). 

A detailed, extensive review of cyclopropene has appeared. Cyclopropene is the last of the small strained ring hydrocarbons to have its thermal decomposition subjected to intensive investigation.158 Cyclopropenes can decompose by a variety of mechanisms involving diradicals, vinylcarbenes, and vinylidenes. Cyclopropene itself has been shown to be an intermediate in the allene-propyne rearrangement. 

The importance of cyclopropene as an intermediate in the thermal isomerization of allene to propyne is matched in the photorearrangements of 1,1-diarylallenes, e.g. 260, which give cyclopropenes, e.g. 261, alkynes, e.g. 262, and indenes, e.g. 263, as the primary photoproducts. The proposed mechanism (equation 81) requires two consecutive H shifts to give 262, whilst bond rotation after one such shift provides a less congested diradical which has cyclization paths available to it to give 261 and 263. 

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