Oxidation of L-Thiols in the Presence of Iron(III) Complex Ions Anchored to Asymmetric Polymers. A Kinetic and Conformational Investigation

The H202 oxidation of L-cysteine in the presence of enantiomeric systems formed by [Fe(tetpy)(OH),]+ (FeT) ions anchored to ordered poly(o-glutamate) (FeTD) or poly(1-glutamate) (FeTL) matrices has been studied at pH 7 (tetpy = 2,2’,2’’,2’’’-tetrapyridyl). The reaction follows a total third-order kinetics (kox = 6.1 X lo4 M-2 s-l, 26 “C) and does not exhibit any stereoselectivity. These results markedly differ, in terms of both rate law and chiral discrimination, from those obtained under similar conditions by using optically active o-dihydroxy substrates, such as L-ascorbic acid, L-adrenaline, and L-dopa. They also differ from those obtained with reduced L-glutathione, which follows a total second-order kinetics (k = 6.4 M-’ s-’, 26 “C). In the absence of hydrogen peroxide, evidence is produced for the formation of a stable, polymer-supported, Fe”’T-cysteinate complex, to which no stereoselectivity is associated because of the high conformational mobility of the axially bound substrate. This complex is characterized by electron paramagnetic resonance (EPR) signals (10 K) at g = 2.20, 2.15, and 1.92 and by a rather slow, [H+]-dependent kinetics of formation (k = 12.7 M-’ SKI, 26 “C) that are suggestive of a ligand-exchange process at the polymer-shielded active sites, within an outer-sphere compound involving the entering substrate molecule. Conformational energy calculations on the noncovalent diastereomeric adducts show the absence of a sterically preferred pathway for the closest approach of cysteine to the central metal ion and support the hypothesis of the ligand-interchange reaction on steric and geometric grounds. Implications of these effects on the absence of stereoselectivity in the reaction investigated are briefly discussed