PREPARATION AND PHOTOSENSITIZED OXIDATION OF ISOPROPYLIDENECYCLOBUTANES AND ISOPROPYLIDENECYCLOBUTENES

Isopropylidenecyclobutanes 2-5 underwent facile ene reaction with sing let dioxygen, yielding (upon Ph(3)P reduction) the corresponding pairs of epimeric allylic alcohols 9 and 10, 11 and 12, 13 and 14, and 15 a nd 16, respectively. A combination of spectral evidence and molecular modeling studies were utilized in the structural assignment of the epi mers. The data clearly indicate that steric considerations play an imp ortant role in determining the face of the ring which O-1(2) approache s. Isopropylidenecyclobutenes 6 and 7 reacted with singlet oxygen more slowly than their monoolefinic analogs, yielding upon reduction allyl ic alcohols 21b and 22, respectively. Benzo analog 7 also generated a small and solvent-dependent amount of isomeric aldehydes 23 and 24, pr esumably via a free-radical mechanism. n-Butyl diene 8 underwent rapid photosensitized oxygenation producing allylic alcohol 35 (as the O-1( 2) ene product) and dione 37 (the Hock-cleavage product of allylic hyd roperoxide 39, formed in turn via a free-radical route) in a 1:9 ratio . Ah initio (STO-3G) calculations confirm that, in their lowest energy conformations, compounds 2-8 are planar with the methylene ring hydro gens displaced ca. 36 degrees from the perpendicular. As a result, onl y exocyclic ene product is formed, since O-1(2) strongly prefers axial or pseudoaxial allylic hydrogens. These calculations combined with th e relative rate data suggest that the initial interaction between the electrophilic O-1(2) 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 stabiliz ation by interacting with the more electron rich but unreactive endocy clic 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.