ORIGIN OF THE REGIOCHEMISTRY IN THE PHOTOCHEMICAL CYCLOADDITION REACTION OF 2-CYCLOPENTENONE WITH ALLENE - TRAPPING OF TRIPLET 1,4-BIRADICAL INTERMEDIATES WITH HYDROGEN SELENIDE

The photochemical cycloaddition reaction between 2-cyclopentenone and allene in toluene at -78-degrees-C yields cyclobutane adducts with hea d-to-head and head-to-tail regiochemistry in a ratio of 88:12. In the presence of high concentrations of hydrogen selenide the three 1,4-bir adical intermediates involved in the photocycloaddition reaction are q uantitatively trapped in the ratio 68:29:3. The dominant biradical int ermediate is derived from bending of the enone 2-position to one of th e allene terminal carbons and is the precursor of the minor, head-to-t ail cycloadduct; in the absence of hydrogen selenide this biradical th erefore preferentially reverts to ground state starting materials. The biradical formed in least amount is derived from bonding of the enone 3-position to one of the terminal carbons of the allene, while the th ird biradical is derived from bonding of the enone 2-position to the c entral carbon of allene; both of these biradicals close to yield the h ead-to-head cycloadduct regiosomer.