Heterolysis vs Homolysis and ortho - meta -Effects in the Photohydrolysis of Biphenylamine Photocages

High Resolution Image Download MS PowerPoint Slide Methyl ethers and acetyl esters of N,N -dimethylamino-hydroxymethylbiphenyls were synthesized with varying positions of the amino activating group and the methoxy or acetyl leaving group. Their photochemical reactivity was studied through photohydrolysis reactions to evaluate their potential as photocleavable protecting groups (PPGs) for the release of alcohols and carboxylic acids, respectively. The compounds were designed to investigate the ortho-meta- effects in biphenyl photochemistry. Photoreactivity was examined via irradiations in aqueous media, followed by isolation of photoproducts. The quantum yields of photoelimination (Φ R ) showed correlation with the ortho-meta- effects when both functional groups were located on the same aromatic ring, but not when distributed across different rings. To rationalize differences, potential energy surfaces for the CH 2 –O bond cleavage were analyzed. Mechanistic insights were obtained using computational studies, fluorescence spectroscopy, and laser flash photolysis. The reaction proceeds via a heterolytic cleavage in the excited state, followed by the formation of benzyl radicals (τ = 25–50 μs) upon relaxation to the ground state. These radicals either recombine to form oligomers or undergo further electron transfer in aqueous solution, producing carbocations and acetate. The synthetic applicability was demonstrated through benzoyl and palmitoyl esters, which underwent photochemical cleavage with yields exceeding 70%.