Oxidation of Thiophene Derivatives with H₂O₂ in Acetonitrile Catalyzed by [Cp*₂M₂O₅] (M = Mo, W): A Kinetic Study

Abstract

The oxidation of benzothiophene (BT), dibenzothiophene (DBT) and 4,6dimethyldibenzothiophene (DMDBT) by H2O2 to the corresponding sulfoxides and sulfones has been studied under homogeneous conditions in MeCN with compounds [Cp*2M2O5] (M = Mo, 1; W, 2) as precatalysts. The W system is ca. 100 times more efficient than the Mo analogue, while the relative reactivity of the thiophene substrates is approximately DBT:DMDBT:BT  10:5:1. For all reactions rate constants for both steps (thiophene derivative to sulfoxide, k1; sulfoxide to sulfone, k2) were measured. While k1  k2 for DBT and DMDBT, k1 << k2 for BT, independent of catalyst. Activation parameters for the stepwise oxidations of thiophene derivative to sulfoxide (BT to BTO: H ‡ = 11.4(5) kcal mol -1 and S ‡ = -26.1(1.6) e.u.; DBT to DBTO: H ‡ = 7.7(6) kcal mol -1 and S ‡ = -33(2) e.u.) and sulfoxide to sulfone (BTO to BTO2: H ‡ = 10.8(5) kcal mol -1 and S ‡ = -21.8(1.6) e.u.; DBTO to DBTO2: H ‡ = 10.3(9) kcal mol -1 and S ‡ = -25(3) e.u.) were calculated from variable temperature studies using [Cp*2W2O5]. DFT calculations suggest that the greater reactivity of DBT relative to BT is not caused by ground state effects but rather by a transition state effect associated with the greater thermodynamic gain in DBT oxidation.

Oxodiperoxomolybdenum complex immobilized onto ionic liquid modified SBA-15 as an effective catalysis for sulfide oxidation to sulfoxides using hydrogen peroxide

“…Mass diffusion, substrate solubility and solvent nature were effects that influence the catalytic performance of the SILP catalyst for these substrates. Conversely to others [63], benzothiophene was found to be more easily oxidized with hydrogen peroxide in methanol (entry 6) than dibenzothiophene (entry 2). The performance of SILP catalyst in benzothiophene oxidation was similar to related metal catalyst on silica [64], while for dibenzothiophene oxidation was worse than other Mo-silica systems [65].…”

Olefin Epoxidation by H2O2/MeCN Catalysed by Cyclopentadienyloxidotungsten(VI) and Molybdenum(VI) Complexes: Experiments and Computations

“…These conditions are similar to those used in a recent contribution dealing with the catalytic action of the same compounds in the oxidation of thiophene derivatives by H 2 O 2 . [21] The only difference relative to the previous report is the need for toluene as a co-solvent (25 % v/v) to insure homogeneous conditions (cyclooctene is not miscible with the H 2 O 2 -H 2 O-acetonitrile phase). Reaction monitoring by gas chromatography revealed the total consumption of the substrate and the final formation of the corresponding epoxide, as the sole terminal product, to indicate that the reactions are selective and quantitative.…”

“…This activity ratio is similar to that found for the related oxidation of thiophene derivatives under analogous catalytic conditions (k W /k Mo % 100). [21] A remarkable observation is that the Mo system, albeit less active than the W system, still affords selective epoxidation under these conditions. This observation is in stark contrast with previous reports, where cyclooctene oxidation by Cp*Mo VI catalysts was described as efficient only if using tBuOOH as an oxidant in a non aqueous solvent, whereas the addition of water or the use of H 2 O 2 as an oxidant was reported to give rise to catalyst deactivation.…”

“…A higher catalytic activity of [Cp* 2 W 2 O 5 ] compared to the related Mo system has been recently underlined for the oxidation of thiophene derivatives. [21] By using H 2 O 2 as oxidant in acetonitrile solution, the W compound was found to be more active than the Mo analogue by a factor of approximately 100.…”

Reaction of [Cp∗2W2O5] with mercaptocarboxylic acids: Addition rather than reduction. Isolation and characterization of Cp∗WO2(SCH2CH2COOH)

“…We have explored for some time the chemistry of halfsandwich complexes of the heavier Group 6 metals Mo and W in high-oxidation states [5e13] and recently reported on the activity of the dinuclear oxido complexes Cp * 2 M 2 O 5 (M ¼ Mo, W) as precatalysts in the oxidation of thiophene derivatives to sulfoxides and sulfones [14], and in cyclooctene epoxidation [15,16]. Most of the studies have been centered on the molybdenum system because of its more straightforward preparation, but we have recently developed an improved and convenient synthesis of the tungsten compound [17] and have thus renewed our attention to this system, since its catalytic performance in oxidation processes appears to be far superior to that of the Mo congener, at least under certain conditions [14,16].…”