Catalytic System for Aerobic Oxidation That Simultaneously Functions as Its Own Redox Buffer.

The control of the solution electrochemical potential as well as pH impacts products in redox reactions, but the former gets far less attention. Redox buffers facilitate the maintenance of potentials and have been noted in diverse cases, but they have not been a component of catalytic systems. We report a catalytic system that contains its own built-in redox buffer. Two highly synergistic components (a) the tetrabutylammonium salt of hexavanadopolymolybdate TBA 4 H 5 [PMo 6 V 6 O 40 ] ( PV 6 Mo 6 ) and (b) Cu(ClO 4 ) 2 in acetonitrile catalyze the aerobic oxidative deodorization of thiols by conversion to the corresponding nonodorous disulfides at 23 °C (each catalyst alone is far less active). For example, the reaction of 2-mercaptoethanol with ambient air gives a turnover number (TON) = 3 × 10 2 in less than one hour with a turnover frequency (TOF) of 6 × 10 -2 s -1 with respect to PV 6 Mo 6 . Multiple electrochemical, spectroscopic, and other methods establish that (1) PV 6 Mo 6 , a multistep and multielectron redox buffering catalyst, controls the speciation and the ratio of Cu(II)/Cu(I) complexes and thus keeps the solution potential in different narrow ranges by involving multiple POM redox couples and simultaneously functions as an oxidation catalyst that receives electrons from the substrate; (2) Cu catalyzes two processes simultaneously, oxidation of the RSH by PV 6 Mo 6 and reoxidation of reduced PV 6 Mo 6 by O 2 ; and (3) the analogous polytungstate-based system, TBA 4 H 5 [PW 6 V 6 O 40 ] ( PV 6 W 6 ), has nearly identical cyclic voltammograms (CV) as PV 6 Mo 6 but has almost no catalytic activity: it does not exhibit self-redox buffering.