Photolysis of cyclopropyl ketones in the vapor phase and in solution

Abstract

The vapor-phase (at 120°) and solution photochemistry of dicyclopropyl ketone (I), methyl 2,2-dimethylcyclopropyl ketone (II), cyclopropyl 2,2-dimethylcyclopropyl ketone (III), propyl cyclopropyl ketone (IV), 3butenyl cyclopropyl ketone (V; in the vapor phase), and 3-methyl-3-butenyl cyclopropyl ketone (VI; in solution) has been investigated. Dicyclopropyl ketone (I), when irradiated in the vapor phase in its -* * band at 2537-2654 A, yields cyclopropyl c/j-propenyl ketone, cyclopropyl irans-propenyl ketone, allyl cyclopropyl ketone, and carbon monoxide as major products with quantum yields of 0.125, 0.025, 0.01, and 0.01, respectively. These quantum yields are temperature and pressure independent. Carbon monoxide is formed via a Norrish type I elimination, whereas the photoisomers result from a ir*-assisted cyclopropane fission reaction. On irradiation of ketone I at 2537 and at 3130 Á in isooctane and benzene solutions, no photochemical reaction is observed. In isopropyl alcohol, however, irradiation leads to reductive cleavage of one ring to yield propyl cyclopropyl ketone (IV). The reaction is presumably due to hydrogen abstraction from the solvent by the excited carbonyl in the primary photochemical process. Ketones II and III, when photolyzed in their ntr* bands at 2537-2654 Á in the vapor phase and at 2537 or 3130 A in isooctane, benzene, or ethanol solutions, undergo Norrish type II processes to yield 5-methyl-5-hexen-2-one (VII) and 3-methyl-3-butenyl cyclopropyl ketone (VI), respectively. The quantum yields in the vapor phase are 0.30 and 0.25, respectively, and they are independent of pressure. On the basis of experiments with nitric oxide and 2,3-dimethyl-2-butene in the vapor phase, the Norrish type II process for ketones II and III presumably proceeds through their first excited singlet state. Infrared spectroscopy evidence for an enol intermediate as the primary photoproduct of II in the vapor phase is presented. Ketones IV and V proved to be quite stable to irradiation in the vapor phase at 2537-2654 A. Similar photostabilities were observed for ketone IV in isopropyl alcohol and for ketone VI in ethanol solution using 3130-Á light.