ZnTPPS-Sensitized Photodebromination of 2,3-Dibromo-3-phenylpropionic Acids. Electron Transfer Initiated Chain Debromination

Article abstract of DOI:10.1021/jo00243a019

Photodebromination of 2,3-dibromo-3-phenylpropionic acids (1) sensitized with zinc tetrasodium tetrakis(sulfonatophenyl)porphyrin (ZnTPPS) has been studied in the presence of triethanolamine (TEOA) as a sacrificial reductant.An efficient debromination took place with ZnTPPS, giving cinnamic acids (or their sodium salts) in aqueous or CH3CN solution.Electron-withdrawing substituents on the phenyl ring of the acids promote the debromination, log Φ being correlated with their reduction potentials.A similar free-energy relationship between 1 and excited metallo tetrasodium tetrakis(sulfonatophenyl)porphyrin (MTPPS; M=Sn, Cd, Zn, and H2) implies that the reaction is initiated by a one-electron transfer from the excited ZnTPPS to dibromide.A laser flash photolysis study shows that the excited triplet porphyrin is reductively quenched by 1 with quenching rate constant k_q = (6.5 +/- 0.2) * 10⁵ M^-1 s^-1, giving ZnTPPS⁺ and 1 -radical anion.A radical chain propagation is involved in the debromination, thus leading to a quantum efficiency higher than unity with higher concentrations of TEOA or 1.