Thermal rearrangements of some hydroxamic acid derivatives

An historical survey of the thermolyses of hydroxamic acids and O-acylated hydroxamic acids is presented, and thermal rearrangements which proceed by [l,3 ]-sigmatropic and free radical shifts are reviewed, together with the phenomenon of chemically induced dynamic nuclear polarisation (CIDNP). N-Methyl hydroxamic acids are found to undergo, on distillation, a novel thermal rearrangement to the isomeric N-methyl-O-acylhydroxylamines, the thermodynamically less stable product. The rearrangement is shown to occur, at least in part, via an intermolecular mechanism involving formation of N-methylhydroxylamine and N-methyl-N,0-diacylhydroxylamine. Various O-thiocarbamoylated N-methyl hydroxamic acids are found to undergo, in solution, a thermal [l,3 ] rearrangement to the isomeric hydrosulphamine derivatives. This rearrangement, which is accompanied by appreciable fragmentation to the corres­ponding N-methyl amide, is shown to proceed largely via a pathway involving caged free radical pairs.