Formation and decay of nitronic acid in the photorearrangement of o-nitrobenzyl esters

Abstract

o-Nitrobenzyl benzoate (oNBB), α-methyl-o-nitrobenzyl benzoate (αMoNBB), α-phenyl-o-nitrobenzyl benzoate (αPoNBB) and copolymers of methyl methacrylate and of o-nitrobenzyl, methyl-o-nitrobenzyl and phenyl-o-nitrobenzyl acrylates were flash photolysed or continuously irradiated with UV light in dilute solution. The quantum yield φ(PR) of bond cleavage in the photorearrangement of the o-nitrobenzyl esters does not depend on the chemical nature of the ester group but is significantly influenced by the substituent in the α position φ(PR) = 0.22 ± 0.03 (αMoNBB, αPoNBB) and φ = 0.10 ± 0.02 (oNBB). This was revealed from the yields of carboxylic acid and o-nitrobenzyl groups. The differences in φ(PR) correspond to the differences in the yields of nitronic acid formed as an intermediate. In aqueous systems the kinetics of the decay of the nitronic acid were found to be determined by the formation of rather stable nitronate ions according to the equilibrium

The relatively long lifetime of the nitronate ion formed in the case of oNBB (longer than 100 ms) is assumed to be caused by a rather strong intramolecular interaction of the hydrogen at the α-carbon with the nitronate group and the carbonyl group. In acetonitrile solution, where nitronate ions were not formed, the non-substituted compound (oNBB) also behaved differently from the substituted compounds (αMoNBB and αPoNBB): two transients differing in lifetime (6 × 10⁻⁴ and 5.8 × 10⁻³ s) were observed with oNBB which were ascribed to different isomers of the nitronic acid. Addition of H₂SO₄ to acetonitrile solutions accelerated the rearrangement of the nitronic acid. The acceleration is assumed to be caused by protonation of the oxygen of the carbonyl group. The carbocation formed this way strongly interacts intramolecularly with oxygens of the nitronic acid group, thus inducing the rearrangement.