Photolytic and radical induced decompositions of O-alkyl aldoxime ethers

Direct photolytic and radical induced homolyses of O-alkyl arylaldoxime ethers (ArCH=NOR) 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 C=N double bond to give adduct oxyaminyl radicals ((ArCHXNOR)-O-.) 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=OCHR21), t-BuO. radicals also abstracted this hydrogen to yield oxyalkyl radicals that underwent rapid beta-scission to afford iminyl radicals (ArCHN.) and an aldehyde or ketone ((R2CO)-C-1). 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 C=N 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.