Three new photoproducts, ethyl O-benzoyl mandelate (5a), ethyl O-acetylmandelate (6a), and biphenyl triketone (7a) are isolated and identified in the reactions of ethyl phenylglyoxylate (1a) in benzene. Quantum yields and initial rate constants of product formation are shown to be concentration dependent. For the formation of carbonyl product 3 at lower starting material concentrations (<0.01 M), quantum yields greater than 1 are observed. Variations in the quantum yields as a function of reaction time are due to the accumulation of alpha-hydroxyphenyl ketene (D). The relative reactivities of triplet excited states of phenylglyoxylates 1 and phenyl ketones are compared. A mechanism involving both intramolecular gamma-H abstraction and intermolecular H abstraction, which leads to radical chain reaction, is proposed. Rate constants for intramolecular gamma-H abstraction (k(N)) and intermolecular H abstraction (k(I)) of methyl phenylglyoxylate (1d) are measured.

A new method for determining the relative efficiencies of free-radical photoinitiators is reported. Called the fluorescence probe technique (FPT), the method has been used to determine the relative initiation efficiencies of several commercial photoinitiators and new phenyl glyoxylate derivatives in the photopolymerization of a model monomer, triethylene glycol diacrylate. The reactivity of each initiator was measured in reference to that of a standard commercial initiator, 2,2-dimethoxy-2-phenylacetophenone, Irgacure 651. Efficiencies differ widely among the acetophenone derivatives studied, and the phenylglyoxylate initiators are somewhat less reactive than the standard.

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The photochemical reactions of alkyl phenylglyoxylate esters whose ester function contains double bonds of differing substitution, situated at varying distances from the carbonyl group, have been studied. The intramolecular Paterno `-Bu ¨chi reaction is the dominating photoreaction when an electron rich alkenyl group is situated at the proper distance as in 1′,5′-dimethylhex-4′-enyl phenylglyoxylate (6). When the distance between the excited carbonyl group and the same alkenyl function is increased, as it is in 3′,7′-dimethyl-6′-octenyl phenylglyoxylate (7), intramolecular γ-hydrogen abstraction (Norrish type II) products, as well as the products of intramolecular photocycloaddition, are observed. We suggest this to be the result of a competitive distant dependent electron transfer reaction between the excited carbonyl group and the alkene. A longer distance of separation between these two reactive functions decreases the efficiency of formation of the cycloaddition product. When the separation of the same alkene from the glyoxylate is shortened as in 4′-methylpent-3′-enyl phenylglyoxylate (5), electron transfer-induced remote proton abstraction leading to cyclol formation occurs, in addition to the Paterno `-Bu ¨chi reaction. Further reducing the distance as in 3′-methylbut-2′-enyl phenylglyoxylate (3) results in only the normal intramolecular (Norrish type II) and intermolecular hydrogen abstraction products. At a distance where both oxetane and cyclol formation are possible as in 5, decreasing the electron richness of the alkenyl group as in cis-3′-hexenyl phenylglyoxylate (cis-4) eliminates the oxetane product. Formation of cyclol is shown to be conformationally controlled in that a similar product is not observed to be formed from trans-3′-hexenyl phenylglyoxylate (trans-4). If both the electron richness of the alkenyl group and the distance of separation are decreased as in 2′-hexenyl phenylglyoxylate (2), only γ-hydrogen abstraction products result. Cis-trans isomerization of the olefin is not observed in either 2 or 4.

Triplet excited states of alkyl phenylglyoxylates react rapidly (k = 9.4 x 10(9) M(-)(1) s(-)(1)) with electron rich alkenes forming oxetanes with high regio and stereoselectivity. The well-known intramolecular gamma-hydrogen abstraction (Norrish type II) cannot compete. When less electron-rich alkenes are used, the Norrish type II reaction becomes competitive. Intramolecular Paternò-Büchi reactions predominate in those alkyl phenylglyoxylates containing properly situated electron-rich alkene groups. The regioselectivity of this reaction is explained by the stability of the intermediate 1,4-biradical. The appropriate conformation at the instant of intersystem crossing determines the stereoselectivity of the products. The priority of the Paternò-Büchi over the Norrish type II reaction is understood by considering the conformation of phenylglyoxylate esters.