Origin of the regiochemistry in the photochemical cycloaddition reaction of 2-cyclopentenone with allene: Trapping of triplet 1,4-biradical intermediates with hydrogen selenide

Abstract

The photochemical cycloaddition reaction between 2-cyclopentenone and allene in toluene at −78°C yields cyclobutane adducts with head-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-biradical intermediates involved in the photocycloaddition reaction are quantitatively trapped in the ratio 68:29:3. The dominant biradical intermediate is derived from bonding of the enone 2-position to one of the allene terminal carbons and is the precursor of the minor, head-to-tail cycloadduct; in the absence of hydrogen selenide this biradical therefore 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 third biradical is derived from bonding of the enone 2-position to the central carbon of allene; both of these biradicals close to yield the head-to-head cycloadduct regiosomer.