α-Diazophosphonic Acids as Potential Photoaffinity Labeling Reagents: Synthesis, Stability, and Photochemistry

Article abstract of DOI:10.1021/jo00346a025

A series of α-diazophosphonic acid salts < Et2NSO2CN2PO3^2-, i-Pr2O3PCN2PO3^2-, and RR'NCOCN2PO3^2- > have been synthesized by diazo transfer to a diester precursor followed by ester cleavage with trimethylsilyl bromide.These compounds show disparate stabilities: the sulfamoyl- and phosphono-substituted derivatives decompose slowly even at pH 6.0 (21 deg C, 0.2 M phosphate buffer; t_1/2 ca. 5 h), whereas the N,N-dimethylcarbamoyl-substituted analogue decomposes rapidly even at pH 9.0 (t_1/2 ca. 40 min).For all three derivatives the decomposition reaction involves initial loss of the PO3^2- group to give the neutral diazo compound, ZCHN2.The photochemical behavior of the α-diazophosphonic diester precursors ZCN2PO3Me2 and the dianions ZCN2PO3^2- (Z=Et2NSO2 and i-Pr2O3P) was also investigated by using light of λ > 300 nm.In alcohol or water as solvent, the neutral esters undergo the expected hydroxyl insertion and photoreduction reactions.In contrast, on photolysis in methanol, the anions undergo neither of these reactions; the major products appear to be those of 1,2-migration of one of the phosphate oxygens, leading to the α-hydroxy monoesters ZCHOHPO3Me^-.This represents the first report of a photochemical study of a diazo compound with an anionic substituent.Although it is also the first example of formal Wolff rearrangement of an oxygen substituent from phosphorus to an adjacent carbene, we suggest that the migration proceeds via an oxaphosphirane intermediate instead of the classical Wolff-type mechanism.Unfortunately, the intervention of this transformation means that α-diazophosphonate dianions are unlikely to be useful as photoaffinity labeling reagents.