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The process resembles the creation of a work of art, which is firmly guided by the final whole even though the whole can be definitely conceived only in terms of its yet undiscovered particulars".1There is no reason why a reaction originating in an excited state cannot be catalyzed. Catalysis of ground-state reactions, both homogeneous and heterogeneous, is of course a well-developed concept; it enables us to explain many natural phenomena2 and has fostered chemical changes of commercial value.3 Perhaps owing to the lack of a clear definition, catalysis of excited-state reactions has not emerged as an identifiable field of study, despite significant activity in this and related areas and conspicuous progress in understanding of photochemical reaction mechanisms.4 In this Account we propose a definition of a catalyst of a photochemical reaction and examine and classify according to the definition a variety of reported acceleratory Gene G. Wubbels was born In Preston, MN, In 1942. He received his B.S. from Hemline University and his Ph.D. from Northwestern University. In 1968 he joined the faculty of Grinnell College and In 1979 became Professor of Chemistry. He was a Visiting Professor at the

Photoreactions of 4-nitroanisole and the 2-halo-4-nitroanisoles (halogen ) F, Cl, Br, and I) with the nucleophiles hydroxide ion and pyridine have been investigated quantitatively to extend the findings recently communicated for cyanide ion. The halonitroanisoles on excitation form triplet π,π* states, which undergo substitution of the halogen by nucleophiles. Chemical yields of photoproducts, Stern-Volmer kinetic plots, triplet lifetimes, and triplet yields are reported for the five compounds with the three nucleophiles. Following a standard kinetic treatment, 73 rate constants are determined for elementary reactions of the triplets including quenching and various nucleophilic addition processes. The photoadditions are roughly 14 orders of magnitude faster than thermal counterparts. Rate constants for attack at the fluorine-bearing carbon of triplet 2-fluoro-4-nitroanisole are 2.9 × 10 9 , 1.3 × 10 9 , and 6.3 × 10 8 M -1 s -1 for cyanide ion, hydroxide ion, and pyridine, respectively. The relative rates for attack at the halogenbearing carbons for F/Cl/Br/I are 27:1.9:1.9:1 (cyanide ion), 29:2.6:2.4:1 (hydroxide ion), and 39:3.9: 3.5:1 (pyridine), respectively. The relative nucleophilicities vary somewhat with the attack site; they are about 5:2:1 for cyanide ion, hydroxide ion, and pyridine for attack at the halogen-bearing carbons. The trend of the element effect opposes that of aliphatic substitution and elimination but is similar in size and parallel to that of thermal nucleophilic aromatic substitution. Relative nucleophilicities in the photoreactions are also similar to those of comparable but vastly slower thermal reactions. The findings imply that the efficiency-determining step of the halogen photosubstitution is simple formation of a σ-complex through electron-paired bonding within the triplet manifold.

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added over an hour to the cold solution and stirred for another 0.5 h. Phenylselenyl bromide (PhSeBr) in 7 mL of dry THF was added rapidly to the cold stirred enolate solution (immediate discoloration), and the cold reaction mixture was diluted with EtjO. The ether solution was washed with 10% HC1, saturated NaHC03, and saturated NaCl. The organic layer was dried (MgS04) and concentrated in vacuo. The crude material (1.7 g) was dissolved in 20 mL of THF and 0.5 mL of AcOH at 0 °C. Hydrogen peroxide (30 mL, 30%) was added to the cold solution over 0.5 h and then stirred for an additional 0.5 h. The reaction mixture was poured into cold saturated NaHC03 and extracted with Et^O. The organic layer was washed with H20 and saturated NaCl and dried (MgS04), and the solvent was removed in vacuo.Vacuum distillation afforded a colorless liquid (0.56 g, 25%): bp 54-66 °C (0.4 mm); NMR (CC14) 1.85 (s, 3 H, CH3), 2.25-2.50 (m, 2 ), 4.30 (t, 2 H), 6.53 (m, 1 H); IR (film) v 1703 (C=0) cm'1; MS (70 eV), m/e (relative abundance) 112 (67), 94 (11), 82 (62),