PHOTOCYCLIZATION OF ALPHA-(ORTHO-TOLYL)ACETOPHENONES - TRIPLET AND 1,5-BIRADICAL REACTIVITY

Several ring-substituted alpha-(o-tolyl)acetophenones undergo photocyclization to 2-indanol derivatives in high quantum efficiency in solution and in high chemical yield as solids. The mechanism for reaction involves triplet state delta-hydrogen atom ion that generates 1,5-biradicals. Quenching studies indicate that the n,pi* excited triplets of these ketones react, with rate constants > 10(8) s-1. Variations in triplet reactivity are ascribed to conformational equilibria that populate reactive and unreactive geometries to different extents. The alpha-aryl ring eclipses the carbonyl in the lowest energy geometry, from which the most favorable geometry for reaction can be reached by small bond rotations. Alpha-(2,4,6-Triisopropylphenyl)acetophenone forms the relatively long lived enol as well as indanol in solvent-dependent ratios; deuterium labeling indicates that the 1,5-biradical disproportionates to form enol. This does not happen with alpha-mesitylacetophenone, so its 54% cyclization quantum efficiency is ascribed to an internal triplet quenching that competes with hydrogen abstraction. This internal quenching is presumed to be of the charge-transfer type and does not appear to lead directly to 1,5-biradicals. 1-Methyl-2-phenyl-2-indanol is formed from alpha-(o-ethylphenyl)acetophenone with a Z/E ratio of 20:1 in benzene and 2:1 in methanol. The 1,5-biradical intermediates were characterized by flash spectroscopy; they have lifetimes between 15 and 45 ns, with those derived from alpha-(o-isopropylphenyl) ketones being twice as long-lived as those derived from alpha-(o-methylphenyl) ketones, and show only a small solvent dependence. Biradical lifetimes and the diastereoselectivity of cyclization are interpreted in terms of biradical intersystem crossing occurring preferentially along the reaction coordinate for cyclization, such that the two processes effectively occur concurrently. The applicability of this concept to other biradicals is discussed.