Intramolecular Hydrogen Abstraction from Triplet States of 2,4,6-Triisopropylbenzophenones: Importance of Hindered Rotation in Excited States

Article abstract of DOI:10.1021/ja00344a029

Photochemically initiated benzocyclobutenol formation from a variety of 4'-substituted (4'-X) 2,4,6-triisopropylbenzophenones 1a-f (a, X = OMe; b, X = Me; c, X = H; d, X = CO2Me; e, X = CF3; f, X = CN) as well as from 2,4,6-trimethylbenzophenone (3a) and 2,4,6-triethylbenzophenone (3b) was studied.The quantum yields of the benzocyclobutenols 2a-f ranged from 0.60 for 2c to 0.06 for 2f in benzene.By usual Stern-Volmer quenching and sensitization methods using diene as quencher or sensitizer, various photokinetic data for these ketones, i.e., triplet lifetime (τ_T) and its temperature dependence (E_a and log A), rate constant for intramolecular hydrogen abstraction from o-isopropyl methine hydrogens (k_r) and its isotope effect (k_H/k_D), rate constant for bimolecular triplet quenching (k₂) with hydrogen donors (Bu3SnH, mesitylene, and cyclooctane), and intersystem crossing yield (Θ_T), were estimated.The effect of 4'-substituents (4'-X) on k_r (or k_T) was unusual for a series of compounds 1a-c and 1e in that k_r decreased in going from 1a (X = OMe) to 1e (X = CF3).This novel substituent effect was interpreted on the basis of hindered rotation in the excited state around the bond linking the 2,4,6-triisopropylphenyl and carbonyl groups.This interpretation was nicely supported by the results obtained for E_a (unusually large, e.g., E_a = 90 kcal/mol for 1c), k_H/k_D (1.5 for 1c), and k₂ (increased in going from 1a to 1e).It is deduced that an increased n?^* character of aromatic ketone triplets results in an increased barrier to rotation (viz., an increased double-bond character) about c_Ar-C(=O) single bond in triplet excited state.