STRUCTURES AND LIFETIMES OF 1,4-BIRADICAL INTERMEDIATES IN THE PHOTOCHEMICAL CYCLOADDITION REACTIONS OF N-BENZOYLINDOLE WITH ALKENES

The photochemical cycloaddition reaction of N-benzoylindole with 1,6-heptadiene, tetramethylethylene, and vinylcyclopropane has been examined. The structures of the products suggest that the reaction proceeds via a 1,4-biradical intermediate that is formed by bonding between one terminus of the alkene and the 2-position of the indole derivative. This result is used to explain the origin of the previously observed regioselectivity of the photochemical cycloaddition reaction. The biradical intermediates obtained in the photochemical cycloaddition reaction of N-benzoylindole with vinylcyclopropane and 1,6-heptadiene can undergo rearrangement reactions whose rate constants can be estimated. Using these rates as clocks, the lifetimes of the intermediate biradicals in the photochemical cycloaddition reaction of N-benzoylindole with alkenes are estimated to be of the order of 100 ns. The consequences of this for the potential success of synthetically useful trapping of the intermediate biradicals is discussed.