Preparation and Photosensitized Oxidation of Isopropylidenecyclobutanes and -cyclobutenes

Isopropylidenecyclobutanes 2-5 underwent facile ene reaction with singlet dioxygen, yielding (upon Ph₃P reduction) the corresponding pairs of epimeric allylic alcohols 9 and 10, 11 and 12, 13 and 14, and 15 and 16, respectively. A combination of spectral evidence and molecular modeling studies were utilized in the structural assignment of the epimers. The data clearly indicate that steric considerations play an important role in determining the face of the ring which ¹O₂ approaches. Isopropylidenecyclobutenes 6 and 7 reacted with singlet oxygen more slowly than their monoolefinic analogs, yielding upon reduction allylic alcohols 21b and 22, respectively. Benzo analog 7 also generated a small and solvent-dependent amount of isomeric aldehydes 23 and 24, presumably via a free-radical mechanism. n-Butyl diene 8 underwent rapid photosensitized oxygenation producing allylic alcohol 35 (as the ¹O₂ ene product) and dione 37 (the Hock-cleavage product of allylic hydroperoxide 39, formed in turn via a free-radical route) in a 1:9 ratio. Ab initio (STO-3G) calculations confirm that, in their lowest energy conformations, compounds 2-8 are planar with the methylene ring hydrogens displaced ca. 36° from the perpendicular. As a result, only exocyclic ene product is formed, since ¹O₂ strongly prefers axial or pseudoaxial allylic hydrogens. These calculations combined with the relative rate data suggest that the initial interaction between the electrophilic ¹O₂ and alkylidenecyclobutenes involves both ends of the singlet dioxygen molecule, in which the “front” end attacks the reactive exocyclic double bond while the “back” end obtains stabilization by interacting with the more electron rich but unreactive endocyclic olefin linkage. Because of this added, and presumably substantial, stabilization, the relative rates within this system are determined in part by the orbital coefficients at the latter olefinic center.