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

• Published in: Journal of the American Chemical Society 2003-07, Vol.125 (28), p.8546-8554
• Main Authors: Corrie, John E. T, Barth, Andreas, Munasinghe, V. Ranjit N, Trentham, David R, Hutter, Michael C
• Format: Article
• Language: English
• Subjects: Acetals - chemistry; Chemistry; Ethyl Ethers - chemistry; Exact sciences and technology; General and physical chemistry; Kinetics; Nitrobenzenes - chemistry; Photochemistry; Photolysis; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Spectroscopy, Fourier Transform Infrared
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja034354c

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.