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42667-B4
Mechanisms of Nucleophilic Aromatic Photosubstitution Reactions

Gene G. Wubbels, University of Nebraska at Kearney

Investigation of the element effect, a named kinetic effect, of halogens in nucleophilic aromatic photosubstitution was completed through use of the 2-halo-4-nitroanisole series with the nucleophiles cyanide ion, hydroxide ion, and pyridine. This followed the precedents of classical studies of the element effect in aliphatic substitution and elimination reactions for which I>Br>Cl>>F (widely called leaving group effects), and thermal aromatic substitution for which F>> Cl > Br > I (owing to σ-bond polarization and local steric effects). In the process, a new photoreaction of 4-nitroanisole with cyanide ion giving a stable nitronate ion product was discovered. According to quantitative NMR and UV/vis analyses, the major reaction of the three nucleophiles with the four 2-halo-4-nitroanisoles is the displacement of the halogen. We measured product distributions in each case, Stern-Volmer plots for each, and triplet lifetimes and triplet yields for each 2-halo-4-nitroanisole with a nanosecond transient spectrometer. These numbers when used in a standard kinetics analysis afforded elementary rate constants for reactions from the triplet states. Rate constants for attack at the fluorine-bearing carbon are 2.9 × 10⁹, 1.3 × 10⁹, and 6.3 × 10⁸ M-1sec-1 for cyanide ion, hydroxide ion, and pyridine, respectively. The relative rates for attack at the halogen-bearing carbons for F : Cl : Br : I are: cyanide ion, 27 : 1.9 : 1.9 : 1; hydroxide ion, 29 : 2.6 : 2.4 : 1; and pyridine, 39 : 3.9 : 3.5 : 1, respectively. The trend of the element effect opposes that of aliphatic substitution and elimination reactions, but parallels that of thermal nucleophilic aromatic substitution. The sizes of the variation in element effects, and of the relative nucleophilicities of cyanide ion, hydroxide ion, and pyridine are quite close to those of the analogous thermal reactions despite being about 14 powers of ten faster. These results evidence conclusively that nucleophilic aromatic photosubstitutions of the S_N2Ar* type proceed from triplet states through a σ-complex that is also a triplet state, and that the elementary reaction step is electron-paired formation of a bond by attack of a nucleophile, just as it is in the vastly slower thermal reaction.

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