Photochemical acceleration of the hydrolysis of nitrophenyl phosphates and nitrophenyl sulphates

Abstract

Neutral and alkaline solutions in water of the nitrophenyl phosphates are very stable in the dark. These compounds. however, and the meta derivative in particular were found to undergo a remarkable rapid hydrolysis when exposed to visible or ultra-violet light. The reaction velocity is independent of the hydrogen ion concentration at pH's between 3 and 12 but increases strongly at higher pH's. Nitrophenol and phosphoric acid could be traced as the products of this photochemical reaction. The reaction proved to be zero order; the rate did not change upon varying of the temperature from 25" to 40" C. The nitrophenyl sulphates s h m an analogous behaviour but for the fact that the increase of reaction rate at higher pH's does not occur and that the difference in the rate of photochemical reaction between the meta compound on the one hand and the ortho and para derivatives on the other hand are even greater than with the phosphates. The experimental evidence points to a purely photochemical (non sensitized) reaction, the nature of which is tentatively discussed. W e wish to report a remarkable example of the promotion of a very simple (ionic) reaction by light. During investigations into the reactivity of nitrophenyl phosphates towards water, alcohols, amines and other nucleophilic reagents it was observed that hydrolysis of such esters was very strongly promoted by light.In the dark hydrolysis could hardly be demonstrated even after Iong periods of time in an alkaline medium at room temperature. This was to be expected on the strength of the presence of the negative charge due to the ionized phosphate group. In diffuse daylight, however, hydrolysis becomes noticeable in a period of one or two hours, the solutions turning yellow as nitrophenolate ions are formed. The presence of free inorganic phosphate could be demonstrated. The three isomers -ortho, rneta-and para-nitrophenyl phosphate -show the phenomenon which is most pronounced with the mefa derivative.Closer investigations with solutions of the latter compound revealed that the rate of this light-accelerated hydrolysis decreases at pH values

Substituent effects in the photohydration of styrenes and phenylacetylenes. An attempt to establish a σhν scale for excited‐state reactions

“…Before attempting correlations of these dl" values with other photochemical reaction series, it is worth noting that in each case (F, Me, Et, OMe) the value suggests that not only are direct conjugative effects of the meta substituents greater than they are in ground states, but also that they are more electrondonating than the corresponding parasubstituents. This is generally consistent with many previous suggestions of a so-called meta effect in photochemical reactions, originally noted by Zimmerman and coworkers 19*20 and Havinga et al 21 For example, Wehry and Rogers' have attempted to estimate this effect semiquantitatively by using the Taft equation:…”

Radical Anion Intermediates in Organic Chemistry

“…This route, however, seems unlikely in view of the results of other photonucleophilic aromatic substitution reactions. Such reactions include hydroxylation [49,50], a m i n a t i~n [~~.~' ] , c y a n a t i~n [~~I , halogenat i~n [~~I , and alkoxylati~n[~~I. These reactions apparently proceed by aromatic excitation to the S, state followed by coupling with the nucleophile rather than an electron transfer Another process shown to have a radical anion pathway is the reaction of sodium naphthalenide with alkyl tolu- Chem.…”

Photosensitive Protecting Groups — A Review

“…Photosolvolysis reactions are closely related to a relatively new class of reactions, l.e., aromatic photosubstitution. Interest'in this area was initiated by the observation of Havinga et al [2] that isomeric nitrophenyl phosphate and sulfate esters, stable in aqueous solutions over a wide pH range, undergo a smooth hydrolysis to the corresponding nitrophenol and inorganic acid upon irradiation with light that is absorbed by the aromatic system. It was particularly interesting that the m-nitrophenyl esters underwent the most efficient photohydrolysis.…”

“…Thus for example, in photoreactions, the meta position in nitrobenzene is the site of attack by nucleophiles, and, in anisole, the meta position is the preferred site for an The photohydrolysis of m-nitrophenyl esters (Eq. (2» is the first example of the use of a light-sensitive protecting group that appeared in the literature [2]. Later on, this reaction was used by other workers to protect phosphoric acid derivatives.…”