Quinone-sensitized Steady-state Photolysis of Acetophenone Oximes Under Aerobic Conditions: Kinetics and Product Studies[dagger]

Photochemistry and Photobiology, Jan/Feb 2006 by Park, Adriana, Kosareff, Nicole M, Kim, Jason S, de Lijser, H J Peter

The results show that the anti isomers are of lower energy than the corresponding syn isomers. A plot of the measured peak potentials against the calculated IP (anti isomers) also gives an excellent correlation (r^sup 2^ = 0.93; Fig. 3B). These results are most consistent with an initial electron transfer step rather than hydrogen atom transfer.

Further analysis of the electrochemical data revealed that they also correlate well with several σ values such as σ^sub pol^ (r^sup 2^ = 0.896) and σ^sub mb^ (r^sup 2^ = 0.898), but not with σ^sub JJ^ (r^sup 2^ = 0.010), or σ^sub rad^ (r^sup 2^ = 0.001). This indicates that the electrochemical process results in the formation of cationic species rather than radicals, which is consistent with a study by Benchariff, Tallec and Tardivel, who have proposed a sequence of reactions at the electrode very similar to those proposed for the photoinduced electron transfer reactions (39).

Photosensitized reactions of meta- and para-substituted acetophenone oximes

The kinetic studies involved steady-state (SS) photolysis experiments in which a mixture of the substituted oxime and the unsubstituted oxime (0.015 M each) as well as the sensitizer (CA, 0.005 M) in acetonitrile was photolyzed for 1 h. The experiments were carried out several times, and the relative conversions of the substituted oximes (%X/%H; average of three or four separate experiments) were correlated against different sets of substituent constants in order to determine the importance of radical and polar effects on these reactions. The results of these photolysis experiments are listed in Table 1. At first glance the reactivity seems to follow roughly that which would be expected on the basis of electron-accepting and donating properties. The p-methoxyacetophenone oxime is the most reactive and reacts almost six times as fast as the unsubstituted oxime. The slowest reaction is observed for p-(trifluoromethyl)acetophenone oxime; overall, the difference between the fastest and the slowest substrate is approximately 13. The correlation between the reactivity (kinetic) data and the measured peak potentials (r^sup 2^ = 0.51), the calculated IPs (r^sup 2^ = 0.64), or the measured quenching rates (r^sup 2^ = 0.63) are all poor. This suggests that the initial electron transfer step and the experimental data representing it (E^sub P^, IP, k^sub q^) cannot be used to draw conclusions on the overall outcome of the reaction. To look for trends and patterns of the overall reaction, the kinetic data (relative conversion; log[k^sub X^/k^sub H^]) was correlated with various Hammett parameters (Table 1). For these studies we have focused on two polar (σ^sub pol^ and σ^sub mb^) (40,41) and two radical (σ^sub rad^ and σ^sup *^^sub JJ^) (41-43) substituent constants. Analysis of the complete data set (both meta- and para-substituted oximes are included) gave only moderate correlations when using any of the single substituent constants. Some improvement was observed when a dual-parameter set was used; however, the best results only gave a correlation (r^sup 2^) of 0.63.