No abstract

A comprehensive study of the symmetry forbidden fragmentation of short-lived radical anions (RAs) has been undertaken for the complete set of polyfluorinated benzoates (C 6 F n H 5-n CO 2 -, n ) 1-5). The decay rate constants (k c ) of RAs have been determined in aqueous alkaline solution (pH 13.4) by electron photoinjection (EPI) from mercury electrodes and were found to increase dramatically from e3 × 10 3 s -1 (3-F-C 6 H 4 CO 2 -) to (1.2 ( 0.8) × 10 9 s -1 (C 6 F 5 CO 2 -). The regioselectivity of C-F bond cleavage in the RA fragmentation has been revealed by structure assignment of reduction products of the polyfluorinated benzoic acids by Na, K, and Zn in liquid NH 3 , as well as by Zn in aqueous NH 3 and aqueous alkaline solutions. The k c values depend on the position of the cleaved fluorine to the CO 2group generally in the order para > ortho > meta, and to sharply increase if adjacent fluorine atoms are present. The observed trends reveal that the kinetics of the RA fragmentation reaction is not controlled by the reaction thermodynamics. Semiempirical UHF/INDO calculations, the validity of which has been confirmed by ab initio ROHF/6-31+G calculations, were done to rationalize the observed trends. The reaction transition state (TS) was considered to arise from the RA's Π and Σ* states crossing avoided due to out-of-plane deviation of the cleaving C-F bond. The satisfactory linear correlation (R ) 0.96) between the model reaction energy barrier E a and log k c has been achieved with modeling the local solvation of the CO 2group by its protonation.

Keywords: Amides / Hydrodefluorination / Ab initio calculations / Nitrogen heterocycles / Zinc N-Acetylation of polyfluoroarylamines is proposed as a means to remove the amino group blocking effect of their hydrodefluorination by zinc in aqueous ammonia. With pentafluoroacetanilide, the Zn ion specific effect has been demonstrated to be responsible for ortho hydrodefluorination. This regiochemistry is accompanied by the removal of a fluorine atom from the para position, which occurs predominantly in the initial phase of the process in the absence of deliberately added zinc salt. The CuCl 2 additive has been found to accelerate the reaction and to propel it to double defluorination. Quantum chemical calculations suggest a diminished electron affinity of pentafluoroaniline, which is responsible for its inertness in relation to the hydrodefluorination reaction. The pentafluoroaniline radical anion, which essentially has a nonplanar structure, is prone to easy fragmentation to give an aminotetrafluorophenyl radical. For pentafluoroacetani-