Electrophilic and nucleophilic pathways in ligand oxide mediated reactions of phenylsulfinylacetic acids with oxo(salen)chromium(V) complexes

Abstract

a b s t r a c tThe mechanism of oxidative decarboxylation of phenylsulfinylacetic acids (PSAA) by oxo(salen)Cr(V) + ion in the presence of ligand oxides has been studied spectrophotometrically in acetonitrile medium. Addition of ligand oxides (LO) causes a red shift in the k max values of oxo(salen) complexes and an increase in absorbance with the concentration of LO along with a clear isobestic point. The reaction shows first-order dependence on oxo(salen)-chromium(V) + ion and fractional-order dependence on PSAA and ligand oxide. Michaelis-Menten kinetics without kinetic saturation was observed for the reaction. The order of reactivity among the ligand oxides is picoline N-oxide > pyridine N-oxide > triphenylphosphine oxide. The low catalytic activity of TPPO was rationalized. Both electron-withdrawing and electrondonating substituents in the phenyl ring of PSAA facilitate the reaction rate. The Hammett plots are non-linear upward type with negative q value for electron-donating substituents, (q À = À0.740 to À4.10) and positive q value for electron-withdrawing substituents (q + = +0.057 to +0.886). Non-linear Hammett plot is explained by two possible mechanistic scenarios, electrophilic and nucleophilic attack of oxo(salen)chromium(V) + -LO adduct on PSAA as the substituent in PSAA is changed from electrondonating to electron-withdrawing. The linearity in the log k vs. E ox plot confirms single-electron transfer (SET) mechanism for PSAAs with electron-donating substituents.

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

“…Similar results are observed during the oxidations of PMAAs by oxo(salen) Mn(V) complexes [64] and oxovanadium(IV)-salen. [37,39] In contradiction, addition of ligand oxide such as TPPO, PicNO, and PyO to the solution of [oxo(salen) chromium(V)] + ion [13,14] during sulfoxidation gives rise to an immediate color change from dark greenish black to emerald green, a significant red shift in the λ max and increase in absorbance in the absorption maxima. Shift in λ max value is observed when LO is added to oxochromium(V) cations.…”

“…Similar nonlinear concave downward Hammett plots were reported during the oxidation of anils by isonicotinium dichromate, [96] conversion of rhenium azides to imido complexes, [97] nucleophilic substitution reactions between N-methyl-α-bromoacetanilides and benzylamines, [98] reaction of β-benzoylamines with pseudomonas fluorescens kynureninase, [99] pyridinolysis of methyl phenyl phosphinic chloride, [100] oxidation of sulfides with pyridinium dichromate, [101] and vanadium catalyzed sulfoxidation. [102] In contradiction, nonlinear concave upward Hammett plots were observed in the oxidative decarboxylation of PSAA [13] and sulfoxidation of PMAA [14] by [oxo(salen) chromium(V) + ] complexes in the presence of ligand oxides; oxidative decarboxylation of PSAA by iron (III) polypyridyl complexes [38] ; oxidation of substituted transcinnamic acids by pyridinium chlorochromate, [103] chloramines, [104] and acid bromate [105] ; oxygen atom transfer reaction of axially ligated Mn(V)oxo complexes [106] ; and stereoselective catalytic sulfoxidations mediated by titanium and zirconium trialkylanolamine complexes. [107]…”

“…Among them, salen complexes constitute a major share that have been exploited since 1933 [5] as catalysts for a wide variety of reactions. [6][7][8][9] In recent years, metal salen complexes such as oxovanadium, [10][11][12] oxochromium, [13][14][15][16] oxo-molybdenum, [11,17] and iron salen [18,19] have been established as efficient catalyst in oxygen transfer to heteroatoms in particular to sulfur atom. The advantages of working with VO(salen) are its high stability and simple method of synthesis from commercially available VOSO 4.…”

“…[35] Vanadium compounds can be used as potential therapeutic agent for COVID-19. [36] With the continuing work from our laboratory on the sulfoxidation of PMAA [37] and PSAA [15,19,22,38] and their influence with nitrogen bases, [39] trichloroacetic acid, [40] and ligand oxides, [13,14] it has been planned to extent the work on the sulfoxidation of PMAA in the presence of ligand oxides. The chemistry and applications of ligand oxides have received much attention due to their usefulness as conformers, [41] protecting groups, [42] auxiliary agents, [43] oxidants, [44] ligands, [45] catalysts, [46] and biological importance.…”

“…The chemistry and applications of ligand oxides have received much attention due to their usefulness as conformers, [41] protecting groups, [42] auxiliary agents, [43] oxidants, [44] ligands, [45] catalysts, [46] and biological importance. [47,48] Ligand oxides have wide range of effects on Mn-salen complexes, [49][50][51] oxo(salen)chromium(V) complexes [13,14,[52][53][54][55] and vanadium complexes. [56] Ligand oxides have been employed in porphyrin and salen catalyzed reactions to enhance the turnover rates and product selectivity.…”