Light-Induced Chemistry of Oximes and Derivatives

Photoreactions of oximes have been known for 120 years. The earliest recognized reaction was geometric isomerization, which is still the most prominent oxime photoreaction; it proceeds via a ππ* triplet state. More than sixty years later the intriguing photo-Beckmann rearrangement came to light, a C–C=N–O to O=C–N–C conversion, generating, for example, ɛ-caprolactam from cyclohexanone ξιμɛ. The rearrangement proceeds via a ππ* singlet state forming an oxazirane intermediate; the oxazirane ring usually opens in stereospecific fashion. Detailed insight into the stereo- and regiochemistry of the rearrangement was gained from steroidal oximes, the most thoroughly investigated class of oximes. Additional photoreactions of oximes or of their ethers and esters include: net loss of hydroxylamine generating carbonyl compounds; loss of water from aldoximes forming nitriles; conversion of ketoximes to nitriles by more complex mechanisms; photochemical O–H cleavage generating iminoxyl radicals, an interesting family of free radicals; N–O cleavage giving rise to iminyl radials, which undergo radical cyclizations. β,γ-Unsaturated oximes undergo aza-di-π-methane rearrangements, cyclize to oxazolines, and undergo further reorganizations. The palette of oxime reactions has been further enriched by triplet energy transfer or electron transfer sensitization; oximes serve as electron donors or acceptors. Keywords: Light-induced chemistry of oximes and derivatives; HEINZ D. ROTH; E, Z isomerization; photo-Beckmann rearrangement; oxazirane formation; O–H cleavage; N–O cleavage; aza-di-π-methane rearrangement; isoxazoline formation; iminoxyl radical; iminyl radial; steroidal oximes; oxo-oximes