Reaction of Thiols with N-Bonded Sulfenamide Complexes of Cobalt(III): Steric Effect and Reaction Pathway

The products and kinetics of the reaction of several thiols (RSH = 2-aminoethanethiol, cysteine, penicillamine,
cysteine ethyl ester) with N-bonded sulfenamide complexes ([Co(en)2(NH2S(CH2)2NH2]3+ (IA), [Co(en)2(NH2SCH2CH(CO2H)NH2]3+ (IC), [Co(en)2(NH2SC(CH3)2CH(CO2H)NH2]3+ (IP)) have been studied. The reaction proceeds by
nucleophilic attack at sulfur with cleavage of the N−S bond to form a disulfide and leave a coordinated NH3 ligand.
The kinetics (pH 4−10) reveal that the deprotonated thiol, RS-, is the reactive nucleophile and that the N-deprotonated
sulfenamide complex is unreactive. The reactions of IP are ∼104 times slower than those of IA or IC, and the
reasons and consequences of this large steric effect are discussed. It is concluded, on the basis of these and
other observations from the literature, that there will be substantial steric retardation to nucleophilic attack at two-coordinate sulfur in a R−C(CH3)2−S−X−R‘ unit because of the regiospecificity of the reaction. The acid dissociation
constants of IP and the X-ray structure of its bromide salt also are reported.