TRIPLET 1,4-BIRADICAL INTERMEDIATES IN THE PHOTOCYCLOADDITION REACTIONS OF ENONES AND N-ACYLINDOLES WITH ALKENES

The structures, lifetimes, pathways for decay, and rate constants for formation and reaction of the triplet 1,4-biradicals produced as intermediates in the reaction of the triplet excited states of 2-cyclopentenone and of N-acylindoles with alkenes to produce cyclobutane adducts have been investigated. For the photochemical reaction of cyclopentenone with methoxy-and alkyl-substituted alkenes it is concluded that the biradicals are formed by bonding of both the alpha and the beta-positions of the enone to the less substituted end of the alkene. It is also concluded that the regiochemistry and stereochemistry of the products of the reaction are determined by the outcome of the competition between reversion to starting material and closure to product for each of the various intermediate biradicals which lead to each of the reaction products. This implies that the long held triplet exciplex theory used to rationalise the reaction regiochemistry is incorrect. Evidence is also presented which suggests that while the same may be true for the photocycloaddition of acrylonitrile with cyclopentenone, this reaction may also be proceeding from the singlet excited state of the enone. For the photochemical reaction of N-acylindoles with alkenes it is concluded that the reaction regiochemistry is determined by preferential formation of the biradical derived from bonding between the 2-position of the indole and the less substituted end of the alkene.