Photolytic and radical induced decompositions of O-alkyl aldoxime ethers

Abstract

Direct photolytic and radical induced homolyses of O-alkyl arylaldoxime ethers (ArCHNOR) were studied by EPR spectroscopy and by end product analyses. Initiating radicals (X˙), including t-BuO˙, t-BuS˙, alkyl and Me₃Sn˙, added rapidly to the CN double bond to give adduct oxyaminyl radicals (ArCHXN˙OR) that could be observed and characterised by EPR spectroscopy. For O-alkyl arylaldoxime ethers containing H-atoms attached to the carbon adjacent to the ether oxygen (OR = OCHR¹₂), t-BuO˙ radicals also abstracted this hydrogen to yield oxyalkyl radicals that underwent rapid β-scission to afford iminyl radicals (ArCHN˙) and an aldehyde or ketone (R¹₂CO). As judged by the relative importance of ROH and ArCN amongst the products, abstraction of the iminyl hydrogen atom also took place to yield oximidoyl radicals (ArC˙NOR), although this could not be confirmed by EPR spectroscopic observation of these radicals. Thus, homolysis induced by t-BuO˙ radicals took place comparatively unselectively. Addition of the t-BuO˙ radical to the CN double bond of oxime ethers was very fast, the rate constant being comparable to that for addition of the same radical to nitrones. Direct and photosensitised UV photolysis of O-alkyl arylaldoxime ethers gave alkoxyl and aryliminyl radicals in very low yields. Although traces of 2-methyltetrahydrofuran were detected from cyclisation of the pent-4-enyloxyl radical generated by direct photolysis of O-pent-4-enyl benzaldoxime, yields were too low for preparative purposes.