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 approximate to 10/5/1. For all reactions rate constants for both steps (thiophene derivative to sulfoxide, k(1); sulfoxide to sulfone, k(2)) were measured. While k(1) approximate to k2 for DBT and DMDBT, k(1) < < k(2) for BT, independent of catalyst. Activation parameters for the stepwise oxidations of thiophene derivative to sulfoxide (BT to BTO, Delta H-# = 11.4(5) kcal mol(-1) and Delta S-# = -26.1(1.6) eu; DBT to DBTO Delta H-# = 7.7(6) kcal mol(-1) and Delta S-# = -33(2) eu) and sulfoxide to sulfone (BTO to BTO2, Delta H-# = 10.8(5) kcal mol(-1) and Delta S-# = -21.8(1.6) eu; DBTO to DBTO2, Delta H-# = 10.3(9) kcal mol-1 and Delta S-# = -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.