Oxidative cleavage of carbon-carbon triple bond - a-review

Abstract

Oxidation of phenylacetylene with molecular oxygen led to the formation of 10 oxygen containing products along with dimerisation, oligomerisation and polymerisation products. Benzaldehyde, benzoic acid and benzoic anhydride are the main oxidation products. The autoxidation of diphenylacetylene gives benzil as the main product and its yield increases upon use of ditertiary-butylperoxide as initiator. Examination of the crude oxidation mixtures of 1-phenylalk-1-ynes revealed the presence of corresponding α-dicarbonyl compounds in small quantities along with typical products like benzil, benzoic anhydride and phenyl benzoate. Similar oxidation of 1-phenylalkan-1,2-diones also yielded benzoic acid and the lower carboxylic acids as in the case of the corresponding 1-phenyl-1-alkynes and the oxidation is faster than the corresponding acetylenes. α-Dicarbonyl compounds undergo photochemical dissociation to free radicals in diffuse daylight but a thermal dissociation under the experimental conditions is not noted. Phenylglyoxal is regarded as one of the intermediates of phenylacetylene autoxidation. The formation of benzoic acid in all cases and considerable amounts of benzil from 1-phenyl-propane-1,2-dione and 1-phenyl-butane-1,2-dione in the thermal decomposition at 130° in presence of dibenzoyl peroxide, diacetyl peroxide or di-tert-butyl peroxide using chlorohenzene as solvent show that the reactions of the type Ph-CO-CO-R + X_ → Ph-CO_ + R-CO-X (with X_ = R'-O_ or R'-CO-O_) take place during the course of autoxidation of α-diketones. The products expected from the reaction of the short lived intermediate ketocarbenes with oxygen i.e. α-dicarbonyl compounds and lower carboxylic acids, especially benzoic acid were not detected in the thermal and photochemical decomposition of α-diazoacetophcnone, α-diazopropiophenone, α-diazobutyrophenone and a-diazodesoxyhenzoin. This shows that the oxidative scission of the C≡C bond proceeds via the corresponding α-dicarbonyl compounds as intermediates and not via the corresponding oxirenes or the valence isomeric ketocarbenes.