Solid state and solution phase photocyclization of a 2-(2′-hydroxyphenyl)benzyl alcohol: twisting motion to planarity occurs efficiently in the crystal

Abstract

The photocyclization of α,α-diphenyl-2-(2′-hydroxphenyl)benzyl alcohol ( 5), a highly twisted biphenylmethanol from the X-ray crystal structure (dihedral angle between of the biphenyl ring system ≈ 80°), photocyclizes efficiently in CH 3CN solution as well as in the crystalline state, to give the much more planar α,α-diphenyldibenzo[b,f]pyran ( 7). The mechanism of reaction in neat CH 3CN solution is believed to involve initial excited state intramolecular proton transfer (from the phenol OH to the benzylic CPh 2OH) whereas the reaction in the crystal is proposed to involve excited state intermolecular proton transfer as the compound crystallizes as H-bonded dimers in a unit cell consisting of eight molecules (absence of included water) in which the phenol OH is H-bonded to the oxygen atom of the benzylic CPh 2OH group.