Photoaddition Reactions of 1,2-Diketones with Silyl Ketene Acetals. Formation of β-Hydroxy-γ-ketoesters

Abstract

Photochemical reactions taking place between 1,2-diketones and silyl ketene acetals and their excited state reaction mechanisms have been explored. Irradiation of benzene, acetone, or acetonitrile solutions containing 1,2-diketones and silyl ketene acetals is observed to promote formation of 1,4-dioxenes, resulting from [4 + 2]-cycloaddition, oxetanes, arising by Paterno-Buchi processes, and beta-hydroxy-gamma-ketoesters, generated by SET-promoted Claisen-type condensation. These competitive pathways leading from the excited states of the 1,2-diketones to these products are influenced by solvent polarity and the nature of the silyl ketene acetal and 1,2-diketone. The Claisen-type condensation process, following an SET desilylation pathway and predominating when the photoreactions are carried out in the polar solvent acetonitrile, represents an efficient method to prepare a variety of diversely substituted beta-hydoxy-gamma-ketoesters.

Photoaddition Reactions of 1,2-Diketone

“…Second, solvent polarity is not significant factor that governs the reaction pathways, which are found in previous studies. 1,4,13 Even though more studies are necessary to investigate the substituent' s effect of silyl enol ether and reaction pathways in detail about the photoreaction of 1,2-diketones and silyl enol ethers, observations made in this study show that SET promoted pathways take place highly efficiently and regioselectively.…”

“…In recent studies, we explored photochemical reactions of 1,2-diketone and silyl ketene acetals and showed that versatile C-C or C-O bond forming processes take place. On the basis of an analysis of the protoproducts presented previous study 1 , ground state/ excited state redox properties of 1,2-diketone electron acceptor [2][3] and silyl ketene acetal electron donor 4,5 , and results of photoreactions of monoketone analogs 4,[6][7][8] give plausible mechanistic pathways for 1,2-diketone with silyl ketene acetals (Scheme 1). Single electron transfer (SET)-promoted or direct [4+2] cycloaddition 1,9 , classical Paterno-Buchi [2+2] cycloaddition 1,4,10 , and sequential SET-desilylation processescompete for deactivation of the 1,2-dicarbonyl excited state by the silyl ketene acetals.…”

Photo-Induced Cycloaddition Reactions of α-Diketones and Transformations of the Photocycloadducts

“…While phenanthrenequinone preferentially adds with both carbonyl oxygens to give the [4 + 2] cycloadduct, acenaphthenequinone reacted with only one carbonyl group to exclusively yield the [2 + 2] addition product [64]. In the photoreactions of acenaphthenequinone with silyl ketene acetals in benzene, the [4 + 2] photocycloadducts could be obtained with modest yield [78].…”

“…Photocycloadditions of acyclic o -diones including biacetyl, benzil and 1,2-diketones conjugated with ene-yne have been reported. The photocycloaddition of biacetyl with olefins such as indene, furan, and enol ether gave oxetanes with higher orientational selectivity than in the case of monoketones [78,79]. The photoreaction of biacetyl with electron-deficient olefins did not give cycloadducts, indicating that the excited biacetyl was electrophilic in its reaction with olefins.…”

“…In our work using benzil as the α -diketone to react with oxazoles, [2 + 2] photocycloadducts were obtained as the exclusive photocycloadducts [24]. However, in a recent work using benzil as the 1,2-diketone to react with silyl ketene acetals in benzene solution, production of the [4 + 2] cycloadduct 1,4-dioxene was observed [78].…”