Photolytic and radical induced decompositions of O-alkyl aldoxime ethers

摘要

Direct photlytic and radical induced homolyses of O-alkyl arylaldoxime ethers (ArCH-NORO) were studied by EPR spectrosopy and by end product analyses. Initiating racdicals (X') including t-BuO', t-BuS', alkyl and Me_3Sn',added rapidly to the C=N 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 = OCHR_2~1), t-BuO~* radicals also abstrated this hydrogen to yield oxygalkyl radicals that underwent rapid #beta#-scission to afford iminyl radicals (ArCHN~*) and an aldehyde or ketone (R_2~1CO). 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 comapratively unslectively. Addiiton 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 photoloysis of O-alkyl arylaldoxime ethers gave alkoxyl and aryliminyl radicals in very low yields. Although traces of 2-methyl-tetrahydrofuran 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.