Photochemistry of α-oxo-oximes. Part 7. Photolysis of some α-oxo-oxime esters

Abstract

The photochemistry of six α-oxo-oxime esters has been investigated in benzene as solvent. The photoformation of the various products has been explained in terms of initial N–O cleavage, followed by β-scission of the resulting α-oxo-iminyl and acyloxyl radicals to yield a carbonitrile and an acyl radical, and carbon dioxide and an alkyl radical, respectively. The presence of the acyl and alkyl radicals has been demonstrated by e.s.r. by the spin trapping technique. The structures of the secondary amides formed upon photolysis of the various α-oxo-oxime esters, e.g. 3-acetamidobutan-2-one (14) from 3-acetoxyiminobutan-2-one (1a), suggest the intermediacy of the N-acyl α-oxoimine (37), which is then further photoconverted. The formation of 4-oxopentanenitrile (19) in the photolysis of 3-acetoxyiminopentan-2-one (1b) has been explained in terms of an initial intramolecular γ-hydrogen abstraction by the excited ketone carbonyl [¹S(n–π*)], followed by cyclobutanol formation and a subsequent thermal reaction. Pyrolysis of 3-acetoxyiminobutan-2-one (1a) at 200° leads to a rapid conversion into mainly acetonitrile (4) and acetic acid (5).