Impact of added ligand oxides in the sulfoxidation of phenylmercaptoacetic acids with oxovanadium(IV)‐salen complexes

Abstract

The mechanistic aspects covering the effect of added ligand oxides, picoline N-oxide (PicNO), pyridine N-oxide (PyNO), and triphenyl phosphine oxide (TPPO) on the selective sulfoxidation of phenylmercaptoacetic acids to PSAA by H 2 O 2 catalyzed by oxovanadium(IV)-salen complexes have been studied spectrophotometrically in 100% acetonitrile medium. Hydroperoxovanadium(V)-salen is the reactive species in this sulfoxidation.The rate of sulfoxidation is found to be enhanced significantly by increasing the concentrations of PMAA, salen, and H 2 O 2 . The ascertained order of reactivity among the oxovanadium(IV)-salen complexes is 5,5 0dimethyl > unsubstituted salen > 5,5 0 -dichloro. The reaction is found to be impeded by the addition of ligand oxide and introduction of substituents in PMAA. The observed order of rate retardation among ligand oxides is TPPO < PyNO < PicNO. Downward Hammett plot obtained has been rationalized by change in rate-determining step from coordination of PMAA to active species for electron-withdrawing substituents to transfer of oxygen to PMAA for electron-donating substituents within the same mechanism.

Highly Selective Thioether Oxidation with H2O2 Catalyzed by Zr‐Substituted Keggin Phosphotungstate: Mechanistic Insights

“…Although it is not typical for nucleophilic sulfides, at least, two examples of such kind of Hammett dependence have been documented, namely, thioether oxidation with manganese(V)‐oxo corrolazine complex [Mn V (O)(TBP 8 Cz)(X)] − (X=CN − or F − ) [23] and Nb‐substituted Lindqvist tungstate (Bu 4 N) 3 [Nb(O 2 )W 5 O 18 ] [5k] . Besides, oppositely curved Hammett plots were obtained for the oxidation of aryl methyl sulfides with dimethyldioxirane [24] and H 2 O 2 ‐based oxidation of phenylmercaptoacetic acids in the presence of oxovanadium(IV)‐salen complexes [25] . Such non‐linear, curved dependences are usually explained either by a biphilic nature of the oxidant [13,23] or a change of the rate‐determined step on going from electron‐donating to electron‐withdrawing substituents [21,24,25] .…”

“…[5k] Besides, oppositely curved Hammett plots were obtained for the oxidation of aryl methyl sulfides with dimethyldioxirane [24] and H 2 O 2 -based oxidation of phenylmercaptoacetic acids in the presence of oxovanadium(IV)-salen complexes. [25] Such non-linear, curved dependences are usually explained either by a biphilic nature of the oxidant [13,23] or a change of the rate-determined step on going from electron-donating to electron-withdrawing substituents. [21,24,25] Recently, Licini and Zonta revisited the Hammett ρ parameter for the determination of philicity and suggested considering nucleophilic substitution with inverse charge interaction, [9] where an "electrostatically positive nucleophile" (the sulfur atom of sulfoxide) reacts with "electrostatically negative electrophile" (peroxy acid anion).…”