Methyleneketenes and methylenecarbenes. VII. Evidence for the pyrolytic generation of methyleneketene (propadienone)

Abstract

Oxidation of 2,2,5-trimethyl-5-phenylseleno-1,3-dioxan-4,6-dione with m-chloroperbenzoic acid in methylene chloride gives a solution containing 2,2-dimethylspiro[1,3-dioxan-5,2'-oxiran]-4,6-dione and the unstable 2,2-dimethyl-5-methylene-1,3-dioxan-4,6-dione, which forms stable adducts with cyclohexadiene and cyclopentadiene. Flash vacuum pyrolysis of the cyclopentadiene adduct over the temperature range 460- 570° and detection of the components present in the pyrolysate by mass spectrometry showed that cyclopentadiene, acetone, carbon dioxide and methyleneketene (CH₂=C=C=O) are formed; the last breaks down into acetylene and carbon monoxide at higher temperatures (520-570°). Infrared measurements on the pyrolysis products kept at liquid nitrogen temperature showed absorption near 2100 cm^-1 which is attributed to methyleneketene. Reaction of the pyrolysate with aniline vapour or methanol vapour yielded acrylanilide and methyl acrylate respectively. Pyrolysis in the absence of trapping agents gave a glassy solid on an uncooled glass surface. This solid is considered to be formed by addition of methyleneketene to 2,2-dimethyl-5-methylene-1,3-dioxan-4,6- dione. Methanolysis of the solid and esterification with diazomethane gave dimethyl 2-methoxycarbonyl-4-methylenepentanedioate.