Photochemistry of phenyl alkyl ketones adsorbed on zeolite molecular sieves. Observation of pronounced effects on Type I/Type II photochemistry

The photolysis of phenyl alkyl ketones adsorbed on a number of commonly available zeolites (molecular sieves) can result in dramatic changes in Type I/Type II photochemistry.The photochemistry of ketones in ordered environments is a topic of current interest^2–4. Environmental effects can have important influence on the conformational flexibility of organic molecules, which in turn can affect the outcome of photochemical reactions⁵. Two recent reports^2,3 on the Norrish Type II reaction in ordered media prompts us to report our initial studies of the photochemistry of a number of phenyl alkyl ketones adsorbed in zeolites.Zeolites are crystalline aluminosilicates of usually well-defined structure⁶. Within the zeolite framework are a system channels and cavities of varying dimensions(2 – 13Å)⁶, some of which are capable of adsorption of large organic molecules (e.g., substituted benzenes). Thus the possibility that the internal spaces (or void volumes) of zeolites can exert topological control on organic photochemical reactions warrants investigation, since it is well-known that zeolites display shape-selective catalytic and adsorptive properties in important industrial chemical processes¹⁰. However, only a handful of reports of photochemical reactions on zeolites are known^4,7–9; the majority of these concern the catalytic splitting of water⁹. In this study, several commonly available zeolites were studied, and the results are compared to those obtained in homogeneous solution.Phenyl alkyl ketones $\text{1}$ - $\text{3}$ were employed in this study. The photochemistry of valerophenone $\text{1}$, octanophenone $\text{2}$, and α,α-dimethylvalerophenone $\text{3}$ is relatively well-understood in solution^11–13. For $\text{1}$ and $\text{2}$, the photobehaviour is characterized by Type II reaction, to give a triplet 1,4-biradical, which can either fragment, to give acetophenone, or cyclize, to give $\text{cis}$ and $\text{trans}$ cyclobutanols (eqn.1). Type I reaction in not observed. In solution, the ratio of fragmentation of cyclization (F/C) products is ～ 4 for $\text{1}$ and $\text{2}$. In general, the $\text{trans}$ isomer dominates, with t/c = 3 – 5 in benzene, and decreasing to a limit of 1 in more polar solvents (MeOH or micelles)^11-–13. For $\text{3}$. Type I reaction is observed in addition to Type II. The ratio of Type I/Type II product has been reported to be 0.3 in benzene without added thiol, and 0.6 with added thiol¹¹.Ketones $\text{1}$ and $\text{3}$ were deposited on Zeolites Na⁺-A, Na⁺-X, Na⁺-Y, Na⁺-Mordenite and resembling $\text{C}$ is also possible, from which cyclization is prohibited. For $\text{3}$, Type I reaction is known to compete with the Type II process¹¹. Adsorption of this ketone on Silicalite results in reaction via the least-motion pathway, namely Type I reaction, to give benzaldehyde as the aromatic product. Thus the behaviour of this ketone on Silicalite is quite consistent with the explanation offered for $\text{1}$ and $\text{2}$.Na⁺-Y is a large pore zeolite, with a pore diameter of ～8 Å and an internal cavity (supercage) of ～13 Å⁶. Additionally, we have found from related studies¹⁶ that of the zeolites capable of adsorbing benzene-type molecules studied in this work, Na⁺-Y allows the greatest degree of molecular mobility for photogenerated benzyl radical. Thus the observed F/C ratios of less than unity for this zeolite probably reflects the increased mobility of the photogenerated 1,4-biradical, allowing it to undergo ring closure readily. Interestingly, this zeolite also gave an $\text{inverse}$ t/c ratio for cyclobutanols of $\text{2}$.The results for the other zeolites are not readily distinguishable from those observed in solution, although two of these (Na⁺-X and Na⁺-Mordenite) are capable of adsorbing $\text{1}$ - $\text{3}$⁶. In any event, these two zeolites offer a medium for Type I/Type II reaction which essentially duplicate the behaviour in solution without the presence of solvent. Additional studies are in progress to further study the use of zeolites as a medium for photochemical reactions.