Oxidation of thiophene derivatives with H2O2 in acetonitrile catalyzed by [Cp*2M2O5] (M = Mo, W): a kinetic study

The oxidation of benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (DMDBT) by H2O2 to the corresponding sulfoxides and sulfones has been studied under homogeneous conditions in MeCN with the 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, ?Hq ) 11.4(5) kcal mol-1 and ?Sq ) -26.1(1.6) eu; DBT to DBTO, ?Hq ) 7.7(6) kcal mol-1 and ?Sq ) -33(2) eu) and sulfoxide to sulfone (BTO to BTO2, ?Hq ) 10.8(5) kcal mol-1 and ?Sq ) -21.8(1.6) eu; DBTO to DBTO2, ?Hq ) 10.3(9) kcal mol-1 and ?Sq ) -25(3) eu) 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.