Photochemical Reaction of Aromatic Compounds. XLVI. Stereochemical Reactions Courses in the Photolytic Ring-Cleavage Reactions of Diarylcyclobutanes:Implications of Conformation-Controlled Orbital Interactions in the Excited-State Chemistry

Article abstract of DOI:10.1246/bcsj.62.1951

The photolyses of cis-transoid-cis-cyclobutadiindene (1), cis-2-phenylcyclobutindene (2), cis-1,2-diphenylcyclobutane (3), the trans isomer (4), and its dimethyl (5) and tetramethyl (6) derivatives have been investigated to explore stereoelectronic factors controlling the excited-state reaction courses for the ring cleavages.The symmetric ring cleavage occurs regiospecifically in the cases of 1-3 and with 98percent specificity in the case of 4.The photolysis of 5 results in both the symmetric and the asymmetric cleavages in a ca. 1:3 ratio accompanied by substantial losses of stereointegrity in the olefin formation, and the regiospecific asymmetric cleavage occurs in the case of sterically more congested 6.The ring-cleavage efficiencies systematically decrease in the order 1>2>3>4>5>6 and reveal negative temperature dependences, particularly in the cases of 1, 5, and 6.The features of the reactions are in parallel with stable conformations and conformational mobilities of the aryl groups associated with different ?-? orbital interactions, while the spectroscopic properties of the cyclobutanes are essentially identical with those of benzene chromophores with little electronic perturbation.The photolitic ring-cleavage reactions are discussed in terms of crossing from the localized ¹?,?* state to a ?* hypersurface, in wich conformation-controlled orbital interactions play important roles.The final products are formed via a pericyclic minimum which might be biradicaloid in nature.