Photodecarboxylation of nitrophenylacetate ions

Abstract

These studies characterize the primary process and products in the photodecarboxylation of nitrophenylacetate ions in aqueous solutions. 3-Nitrophenylacetate, 4-nitrophenylacetate, and 4-nitrohomophthalate ions decarboxylate with a quantum yield of about 0.6 at 367 nm, while that of o-nitro structures (2-nitrophenylacetate and 2,4-dinitrophenylacetate ions) is only 0.04. acz'-Nitro intermediates are identified spectrally by flash photolyses from the o-and p-nitro structures, but not from the m-nitro structure. Results are consistent with a mechanism of photodecomposition into carbon dioxide and a nitrobenzyl anion which is stabilized as an ««-nitro anion in o-and p-nitro structures. The nitrophenylacetic acids are more stable photochemically than their ions; neither C02 nor a«'-nitro intermediates are observed from photolysis of the undissociated acids. The ««-nitro yield varies with pH according to the pA-of the acid being photolyzed; thus a pK of 4.8 is calculated for the acarboxyl group of 4-nitrohomophthalic acid. Three of the ««'-nitro anions occur via photochemical hydrogentransfer reactions from other compounds so that by comparison with the photodecarboxylations the quantum yields of H-transfer photochromism in 2-nitrotoluene, 2,4-dinitrotoluene, and 5-nitro-otoluate ions are estimated to be about 0.01, 0.003, and 0.07, respectively.In the course of studies on the photochromism of compounds with nitrobenzyl structures,1 we noted that an irreversible photochemical decarboxylation occurs in aqueous solutions of compounds with nitrophenylacetate ion structures.2 In both types of reaction '-nitro intermediates are postulated to occur from photochemical acid-base processes. While the photochromic process appears to be a reversible proton-transfer reaction, the photodecarboxylation mechanism is based upon the formation of carbon dioxide and a carbanion residue, as in eq 1. The carboxyl forms of these nitrophenylacetic acids photodecompose less readily and do not yield the same intermediates or products as their ions.

A Zinc(II) Photocage Based on a Decarboxylation Metal Ion Release Mechanism for Investigating Homeostasis and Biological Signaling

“…[21] Nitrophenylacetates (NPAs) utilize a similar photodecarboxylation mechanism, but require UV light to initiate the uncaging chemistry. [22][23][24] Although both oNB and NPAs contain nitrobenzyl groups, NPA decarboxylation does not involve the hydrogen atom abstraction utilized in oNB uncaging. Despite the potential advantages, this type of decarboxylation process has not been applied to metal ion photocages.…”

Molecular Mechanism of Uncaging CO2 from Nitrophenylacetate Provides General Guidelines for Improved ortho‐Nitrobenzyl Cages

“…[4] ortho-NPA (o-NPA) exhibits a quantum yield of only 4 % with respect to CO 2 release, whereas meta-and para-NPA have quantum yields of 63 % and 59 %, respectively. [5] For p-NPA, the first reaction intermediate has been assigned to a triplet species, being responsible for CO 2 release. [6] This is supported by efficient intersystem crossing (ISC) of nitrobenzylic compounds in general, with triplet rise times of 5-10 ps and yields of 60-80 %.…”

Caged CO2 for the Direct Observation of CO2‐Consuming Reactions

“…Mass spectra and 1 H and 13 C NMR spectra confirmed the structures of the products. The mass spectral analysis of 18 O-mNPAA showed that 18 O-labeling was not complete, most likely due to exposure of the substance to acidic aqueous conditions during the final extraction. It is known that in aqueous solutions carboxylic acids exchange carbonyl oxygen atoms with the solvent.…”

“…was then added, and the now brownish suspension was filtered. The filtrate was washed with water (2 50 cm 3 ), dried (MgSO 4 ), and concentrated under reduced pressure to yield a brownish oil, which was fractionally distilled in vacuo to afford product 3 (4.4 g, 48 %) as a yellowish oil, containing [18]crown-6 (6 %). ) in a screw-cap vial was exposed to a stream of gaseous HCl for 2 h until the solution solidified.…”

“…[7][8][9][10][11][12][13] Many of these reagents release CO 2 in addition to their intended effector molecule, either as a result of their mechanism of effector release (e.g., in the amine-yielding photolysis of carbamates [14,15] ) or in undesired side-reactions that limit the cage's efficiency. [16,17] Nitrophenylacetic acids and their anions have been shown to decarboxylate quickly and efficiently upon irradiation with UV light, [18] and their good solubilities and stabilities have made them attractive candidates for use as caged CO 2 com-CO 2 -consuming reactions, in particular carboxylations, play important roles in technical processes and in nature. Their kinetic behavior and the reaction mechanisms of carboxylating enzymes are difficult to study because CO 2 is inconvenient to handle as a gas, exists in equilibrium with bicarbonate in aqueous solution, and typically yields products that show no significant spectroscopic differences from the reactants in the UV/Vis range.…”