Exploitation of aldoxime esters as radical precursors in preparative and EPR spectroscopic roles

Abstract

Photolyses of aldoxime esters, containing a considerable range of alkyl groups, lead to cleavage of their N-O bonds and formation of aryliminyl and alkyl radicals. The process was found to be favoured by 4-methoxyacetophenone as a photosensitiser and by methoxy substituents in the aryl rings. 4-Nitro-and pentafluoro-substitutions of the aryl rings were, on the other hand, deleterious. The intermediate iminyl radicals, together with primary, secondary and tertiary alkyl radicals were characterised by 9 GHz EPR spectroscopy. Cyclopropyl, CF 3 , and CCl 3 radicals were probably also formed, but were too reactive for direct EPR spectroscopic detection. Photosensitised reaction of benzophenone oxime O-nonanoyl ester produced the diphenylmethaniminoxyl, as well as the expected n-octyl and iminyl radicals. This indicated that O-C bond scission accompanied O-N scission for this ketoxime ester.At higher temperatures the C-centred radicals added to the starting oxime esters to produce alkoxyaminyl radicals that were also spectroscopically detected in some cases. No evidence for abstraction of the iminyl hydrogen by tertbutoxyl radicals was obtained. Instead, the t-BuO ؒ radicals added to the C᎐ ᎐ N double bonds of the oxime esters. Similarly, chlorine abstraction from alkylbenzohydroximoyl chlorides by trimethyltin radicals did not take place. Preparative scale experiments with oxime esters containing suitably unsaturated alkyl groups showed that good yields of cyclised products could be obtained in the presence of the photosensitiser. This process constitutes a general method by which carboxylic acids or acid chlorides can be converted into alkyl radicals and hence to cyclised derivatives.

Pentafluorobenzaldehyde and its utilizing in organic synthesis

“…The PFBA oxime derivatives were prepared by the reaction with substituted phthalimide (Scheme 48 [522,523]). The PFBA with O,N-bis(trifluoroacetyl)hydroxylamine in benzene yields 76% pentafluorobenzonitrile (Scheme 48 [524]).…”

Synthesis and Characterization of Novel Methacrylate Monomers Having Pendant Oxime Esters and Their Copolymerization with Styrene

“…Dissociation of oxime leads to iminyl radical and acyloxyl radical (RCO 2 ). If the decarboxylation is not fast enough, these two radicals can be recombined to give the starting oxime, and therefore the global efficiency of the photodissociation is decreased (36). On the contrary, the occurrence of the decarboxylation process leads to the formation of carbon dioxide and a new radical (R).…”

Quinoline synthesis: scope and regiochemistry of photocyclisation of substituted benzylidenecyclopentanone O-alkyl and O-acetyloximes

“…However iminyl radicals 36 have been proposed as intermediates in the formation of 35, generated by nitrogen-oxygen bond photocleavage. Such homolysis, yielding acyloxy and aryliminyl radicals, occurs in the photochemistry of O-acyloxime derivatives of simple aromatic carbonyl compounds such as benzaldehyde, acetophenone, benzophenone and 9-fluorenone and has been 310 used as a convenient source of carbon-centred radicals for synthetic investigations 26,27 and as a photochemical source of amines for polymer cross-linking, the amines resulting from hydrolysis of the imines formed following nitrogen-oxygen bond cleavage.…”