Structures and photoreactivities of 2,4,6-triisopropylbenzophenones

Crystal structures of 2,4,6-triisopropylbenzophenone (I) and its ten derivatives have been determined: (II) 2,4,6-triisopropyl-3'-methoxybenzophenone, (III) 3'-chloroformyl-2,4,6-triisopropylbenzophenone, (IV) methyl-3-(2,4,6-triisopropylbenzoyl)benzoate, (V) 2,4,6-triisopropyl-4'-methylbenzophenone, (VI) 4'-tert-butyl-2,4,6-triisopropylbenzophenone, (VII) 2,4,6-triisopropyl-4'- methoxybenzophenone, (VIII) 4-(2,4,6-triisopropylbenzoyl)benzoic acid, (IX) 4'-chloroformyl-2,4,6-triisopropylbenzophenone, (X) methyl 4-(2,4,6-triisopropylbenzoyl)benzoate and (XI) ethyl 4-(2,4,6-triisopropy-benzoyl)benzoate. The conformations of these molecules are essentially the same. The central carbonyl moiety is approximately coplanar with the 3'- or 4'-substituted phenyl group and perpendicular to the triisopropylphenyl ring. The photostability of (IX) and (X) in the solid state is attributed to the narrow reaction cavity of the triisopropylphenylcarbonyl moiety, i.e. the intramolecular movements of isopropyl groups in the triisopropylphenyl plane toward the carbonyl C atom are suppressed by intermolecular close contacts. One of the barriers for the photocyclization of (IX) and (X) is caused by a dimeric structure in crystals through the pi-pi interaction, which is the result of having a polar methoxycarbonyl or chloroformyl group at position 4' of the phenyl ring.