Abstract
The triplet 1,4-biradical intermediates formed in the photochemical cycloaddition reaction between 2-cyclohexenone and the alkenes cyclopentene and ethoxyethene have been trapped quantitatively with hydrogen selenide. The structures of the biradicals reveal that they are formed by bonding of both the 2-position and the 3-position of the enone to one terminus of the alkene; in the case of ethoxyethene it is exclusively the less substituted terminus of the alkene that becomes bonded to the enone in the biradical intermediates. The yields of the trapped biradicals indicate that the regiochemistry of the photocycloaddition reaction is determined by the manner in which the biradicals partition between fragmentation to give ground state starting materials and closure to give product, and not by their relative rates of formation. The results suggest that the mechanistic factors governing the photocycloaddition chemistry of cyclohexenone are similar to those controlling the photochemistry of cyclopentenones.
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