Steady-state photochemistry (Pschorr cyclization) and nanosecond transient absorption spectroscopy of twisted 2-bromoaryl ketones

Abstract

The steady-state as well as transient absorption spectroscopy of a series of 2-bromo-aryl ketones have been comprehensively examined to gain insights concerning (i) the transient phenomena (absorption spectral attributes as well as lifetimes), (ii) rates of C–Br homolysis, and (iii) the behavior of 2-aroylaryl radicals thus generated. The X-ray crystal structure analyses of selected ketones in which the mesomeric effects operate differently reveal that the two aryl rings are drastically twisted about the C=O bond. The twisting manifests itself in the spectral features of the transients, attributed to triplet–triplet (T–T) absorptions, such that they are not readily comparable in some cases to the transients of parent diaryl ketones that lack the 2-bromo group. By associating triplet decays with C–Br cleavage rates, the absolute rate data have been determined for diverse 2-bromoaryl ketones. With the exception of 2-bromo ketones containing meta-methoxy substituents, all other ketones are found to undergo efficient C–Br bond cleavage with rates of ca. 0.1–1.0 × 108 s–1. For m-methoxy-substituted ketones, intriguingly slower deactivation of the triplets was observed. Based on solvent-dependent variation of the lifetimes (longer lifetimes in polar solvents), intramolecular charge transfer has been proposed. The preparative photochemistry and transient phenomena permit invaluable inferences as to the reactivity of 2-aroylaryl radicals in general. Quantum yield determinations and product analyses reveal that highly electrophilic aryl radicals undergo radical recombination, in a poor hydrogen-donating solvent, almost exclusively (>90 %) in the absence of incentive for stabilization via conversion to π-conjugated hydrofluorenyl radicals. Of course, when the latter is feasible, Pschorr cyclization leads to productive photochemical outcome. Moderately electrophilic radicals that lack stabilization via conversion to hydrofluorenyl radicals lend themselves to intramolecular 1,5-hydrogen shifts in conjunction with the formation of dehalogenated diaryl ketones and cyclized fluorenones (Fls) or its analogs.