The photolysis of benzil in cyclohexane solution

Abstract

Benzil has been irradiated in cyclohexane solution. The principal products are: benzoic acid; benzal-dehyde; phenyl cyclohexyl ketone; benzoin; benzoin benzoate; a compound tentatively identified as desyl benzoyloxydesyl ether; another compound not yet identified; and approximately one-third of the reacted benzil that was not accounted for. A small amount of unsaturation is also found. As long as there is enough benzil present (presumably to absorb all of the light) the benzil disappearance is linear with time. All products appear at the shortest times checked and so cannot be secondary products. If one neglects the benzoic acid approximately 19% of the benzoyl groups that react do so in the form of benzoyl radicals. No evidence for phenyl radical formation was found. The upper limit for the quantum yield of benzil disappearance at 366 mμ is 0.25. The benzoic acid does not arise directly during the photolysis but only when the solution is exposed to oxygen during the work-up. Preliminary experiments in cumene solution have shown that the amount of benzil lost for the same length of irradiation increases by a factor of about 1.5. In 2-propanol solution the increase is by a factor of 2.5. When 2-propanol is the solvent, a new product appears which is most probably the dimer of the ketyl radical which is formed when an excited benzil abstracts a hydrogen atom. This product is very unstable.

Catalyst-free [2 + 2] photocycloadditions between benzils and olefins under visible light

“…The main product observed upon UV light irradiation was compound (±)-S1. Bunbury reported that one of the major decomposition pathways of benzil in cyclohexane under UV light irradiation is the carbonyl-carbonyl homolysis to afford benzoyl radicals [25]; thus, we speculate that the stepwise, double addition of such intermediates onto the C=C double bond of 2a leads to the formation of (±)-S1. Importantly, we safely excluded that the latter compound is formed upon UV lightpromoted photolysis of the oxetane product (see Sect.…”

“…Quite interestingly, when the irradiation was carried out with shorter wavelengths (e.g. 390 nm) uncontrolled photodecomposition of benzil was observed [25]; on the other hand, light is essential for the reaction to occur and benzil 1a is photostable when irradiated in neat cyclohexane at 456 nm (see Scheme S2 in the SI). It is important to mention here that 2a does not absorb in the investigated window (> 310 nm) and it is not accountable for the observed photoreactivity [26].…”

“…As far as compound S3 is concerned, it is formed only in traces (never exceeding 10% yield) and its formation can be explained by taking into consideration a hydrogen atom transfer (HAT) reactivity [30,31]. Indeed, the excited state of benzil has been reported to activate hydrocarbons by homolytic cleavage of a C(sp 3 )−H bond [25]. In the case of olefins 2c and 2f, the HAT step would afford a resonance-stabilized allyl radical through the cleavage of a rather labile C-H bond.…”

Photolysis of benzil in 2-propanol and in cumene

“…The photo-reaction of benzil in cyclohexane has already been reported (1). This study has been extended using 2-propanol and cumene as solvents.…”

“…produced either abstract hydrogen from the solvent or react with benzil as proposed by Bunbury and Wang (1). Then the proportion of benzaldehyde should increase with the ease of hydrogen abstraction from the solvent but no such correlation is found; cyclohexane 6.2%, cumene 7.4 %, 2-propano16 %.…”