Photolytic cleavage of 1-(2-nitrophenyl)ethyl ethers involves two parallel pathways and product release is rate-limited by decomposition of a common hemiacetal intermediate

Time-resolved FTIR spectroscopic studies of the flash photolysis of several 1-(2-nitrophenyl)-ethyl ethers derived from aliphatic alcohols showed that a long-lived hemiacetal intermediate was formed during the reaction. Breakdown of this intermediate was rate-limiting for product release. One of these compounds (methyl 2-[1-(2-nitrophenyl)ethoxy]ethyl phosphate, 9) was studied in detail by a combination of time-resolved FTIR and UV-vis spectroscopy. In addition, product studies confirmed clean photolytic decomposition to the expected alcohol, 2-hydroxyethyl methyl phosphate, and the 2-nitrosoacetophenone byproduct. At pH 7.0, 1 °C, the rate constant for product release was 0.11 s -1 , very much slower than the 5020 s -1 rate constant for decay of the photochemically generated aci-nitro intermediate (pH 7.0, 2 °C). Time-resolved UV-vis measurements showed that the hemiacetal intermediate is formed by two competing pathways, with fast (∼80% of the reaction flux) and slow (∼20% of the flux) components. Only the minor, slower path is responsible for the observed aci-nitro decay process. These competing reactions are interpreted with the aid of semiempirical PM3 calculations of reaction barriers. Furthermore, AMSOL calculations indicate that the pK a of the nitronic acid isomer formed by photolysis is likely to determine partition into the alternate paths. These unusual results appear to be general for 1-(2-nitrophenyl)ethyl ethers and contrast with a related 2-nitrobenzyl ether that photolyzed without involvement of a long-lived hemiacetal.