Tuning Reactivity of Zr-Substituted Keggin Phosphotungstate in Alkene Epoxidation through Balancing H₂O₂ Activation Pathways: Unusual Effect of Base

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

“…Recently, a V-shaped Hammett dependence was acquired for epoxidation of p-substituted styrenes in the presence of 1. [13] The negative ρ value of À 1.53 (Figure 1) is associated with an electron-demanding transition state, typical of electrophilic oxidation processes and is consistent with either oxygen-or electron-transfer mechanism, while positive ρ of 0.80 specifies a nucleophilic mechanism with a partial negative charge build-up on S atom in the transition state for sulfides or sulfoxides with electron-withdrawing substituents. [20] Earlier, similar curved Hammett plots were reported for the oxidation of substituted sulfoxides with cumyl hydroperoxide in the presence of Ti (IV) / N(CH 2 -CHPhO) 3 (O i Pr) [21] and with H 2 O 2 over methyltrioxorhenium.…”

“…This can be rationalized if we remember that the increase in the catalyst amount relative to H 2 O 2 is unfavorable for the formation of monomeric Zr peroxo species, which are known to enhance H 2 O 2 decomposition. [13] In turn, the decrease in temperature also retards this side reaction because the activation energy for H 2 O 2 decomposition over 1 (46 kJ/mol) [13] is higher than that of the 1-catalyzed MPS oxidation (33 kJ/mol, see below).…”

“…Some 31 P NMR signal shifting and broadening observed for 1 (δ À 12.26 vs À 12.22 ppm; Figure S2) is, most likely, caused by the formation of Zr-POM peroxo species and/ or coordination of the sulfoxide product (see spectra in Figure S3). [13]…”

“…[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).…”

“…Recently we have found that the Zr-monosubstituted dimeric phosphotungstate (Bu 4 N) 8 [{PW 11 O 39 Zr(μ-OH)(H 2 O)} 2 ] (1) with the Keggin structure is an effective catalyst for the epoxidation of C=C bonds in various alkenes with aqueous H 2 O 2. [13] Spectroscopic studies implicated that 1 interacts with H 2 O 2 to form dimeric mono-and diperoxo complexes, [Zr 2 (μ- 10À (II), along with monomeric Zr (hydro)peroxo species, depending on the amount of H 2 O 2 added. While both peroxo dimers were inert toward alkenes, monomeric hydroperoxo and peroxo species could accomplish C=C bond epoxidation.…”