An extended Hückel molecular orbital study has been made on diazoethanone, 3-diazo-2-propanone, 2-diazo-3-propanone, 3-diazo-2-butanone, their decomposition products, the oxirenes, ketenes, and related intermediates. From computed MO and state energies together with symmetry correlations it is concluded that the thermal decomposition of diazo ketones follows the reaction path. The oxirene state lies above the transition state of the ketene forming step and can only be reached in photochemical excitation where rearrangement of the zwitterion to oxirene can effectively compete with ketene rearrangement. Therefore, the intervention of oxirene in the photochemical and its absence in the thermal Wolff rearrangement of diazo ketones is attributed to the difference in excess vibrational energy of the ketocarbene or its tautomeric zwitterion, from the two modes of excitation. Oxirene and its simple methyl derivatives are inherently unstable with respect to C-O bond cleavage and they are predicted to exist only as short-lived transients. The decomposition sequences of thermally, photolytically, and chemically activated ketenes are also discussed in relation to the role of oxirene intermediates.
ASJC Scopus subject areas
- Colloid and Surface Chemistry