The regioselectivity of internal hydrogen abstraction by triplet o-tert-amylbenzophenone

The title ketone undergoes triplet-state δ-hydrogen abstraction to produce two diastereomeric sets of 1-phenyl-1-indanols: the 2,3,3-trimethyl isomers 2Z and 2E by abstraction of a methylene hydrogen and the 3-methyl 3-ethyl isomers 1Z and 1E by abstraction of a methyl hydrogen. The 2/1 ratio varies only modestly with changes in solvent, temperature, and phase, ranging from 0.7 in hydrocarbons to 1.6 in methanol, 2.4 in the crystal, and 3.5 on silica. Total quantum yields are 0.034 in benzene and 0.43 in methanol. After correction for reversion of the intermediate 1,5-biradicals to the ketone, the product ratios indicate a 1/1.6 ratio for abstraction of secondary versus primary hydrogens in solution and a ≤7/1 ratio in the solid. X-ray analysis and conformational modeling indicate a high steric energy for the compound, with only a few conformations populated even in solution. The favored orientations of the methyl and methylene hydrogens with respect to the carbonyl oxygen are significantly different, with the former lying at positions that are predicted to be of low reactivity. Molecular flexibility might alter these geometries, but the low (2.5 kcal/mol) activation energy for hydrogen abstraction suggests that the geometries of the transition states for hydrogen abstraction cannot deviate significantly from those of the conformational minima of the triplet reactant. Whereas there is no Z/E diastereoselectivity in solution, both Z isomers are favored by 3-4/1 in the solid. Least-motion cyclization of the biradicals from their initial geometries explains the preference for Z products. These results suggest a lack of stringent stereoelectronic requirements for hydrogen abstraction.