DIRECT OBSERVATION OF THE CYCLOPROPENE VINYLCARBENE REARRANGEMENT - MATRIX-ISOLATION OF BICYCLO[3.1.0]HEXA-3,5-DIEN-2-ONES

4-Oxocyclohexa-2,5-dienylidenes 4b-g have been generated in argon matrices at 10 K by visible-light irradiation of the corresponding quinone diazides 5. Carbenes 4 have been characterized by IR and UV-vis spectroscopy and by their characteristic thermal reaction with molecular oxygen. On irradiation into the longest-wavelength absorption (420-700 nm), carbenes 4c-g rearrange to give the highly strained bicyclo[3.1.0]hexa-3,5-dien-2-ones 3c-g, while 4b is photostable under the same conditions. The photochemical 4 --> 3 rearrangement is completely reversible: infrared irradiation or visible light irradiation (lambda > 470 nm) of the cyclopropenes 3 lead back to triplet carbenes 4 quantitatively. In addition, several of the 1,3-bridged cyclopropenes 3 undergo a thermal rearrangement to give carbenes 4. This indicates that the highly strained cyclopropenes 3 are thermodynamically less stable than the corresponding carbenes 4 and kinetically only stabilized by a shallow energy barrier. Even under the conditions of matrix isolation at cryogenic temperatures, cyclopropenes 3 are metastable compounds.