PHOTOSOLVOLYSIS OF 5-SUBSTITUTED DIBENZOSUBEROL DERIVATIVES - A STEREOELECTRONIC REQUIREMENT FOR CARBOCATION PHOTOGENERATION

The possible photosolvolysis of 5-substituted 5H-dibenzosuberols (3-6) has been studied in aqueous methanol. Only the parent system, 5H-dibenzosuberol (3) reacts via photosolvolysis. On extended photolysis, compounds 3-6 undergo a fragmentation reaction (PHI = 0.02-0.08) to give 9,10-dihydrophenanthrene (10) and a carbene-derived product. Evidence gathered from conformational analyses of compounds 3-6 using PCMODEL and MMX, an X-ray crystal structure of 6 and proton nuclear magnetic resonance (H-1 NMR) studies using a lanthanide shift reagent indicates that only in alcohol 3 is the hydroxyl group of the substrate oriented in a pseudo-axial position, whereas for the compounds 4-6 the hydroxyl group is in a pseudo-equatorial position. It appears that an axial hydroxyl group is a necessary requirement for photosolvolysis in these systems since it allows for correct stereoelectronic alignment of the pi-orbitals of the benzene rings with the developing empty p(z) orbital as the Ar2C-OH bond dissociates heterolytically.