Catalytic air and amine N-oxide oxidation of p-substituted benzoin by molybdenum(VI) complexes. Identification of the deactivation process by dioxygen

Abstract

The catalytic oxidation of benzoin and t w o p-substituted benzoins, e.g. 4,4'-dimethoxy-and 4,4'dichloro derivatives, by dioxygen or pyridine N-oxide, in the presence of [ MoO,(cysS-OMe),] (cysS-OMe = S-deprotonated cysteinate methyl ester) and [ MoO,(S,CNEt,),] was studied kinetically. The catalytic oxidation rates indicate a trend, M e 0 > H > CI, which is related to the ease in apparent hydride release from the C-H group of benzoin derivatives. The stoicheiometric oxidation indicates a similar 0x0-transfer reactivity of the above complexes towards the three p-substituted benzoins. Only one l80 atom from [180] dioxygen was found in one of the benzil carbonyl groups. The catalytic dioxygen oxidation by [ MoO,(S,CN Et , ), ] suffers from deactivation through the formation of [MoVO(S,CN Et , ), ] formed by a one-electron-transfer reaction from [MoIVO(S,CN Et , ), ]to dioxygen.

Trans Influence of Oxo and Dithiolene Coordination in Oxidized Models of Molybdenum Oxidoreductase: Synthesis, Structures, and Properties of Q2[MoVIO2(1,2-benzenedithiolato)2] (Q = NEt4, PPh4) and Related Complexes

“…The results are in the similar range of the electrochemical reduction potentials of the related complexes, e.g. E pc ) -0.87 21 in DMF.…”

“…Numerous model complexes for the oxidized state of these molybdenum enzymes have been synthesized using a (thiolato, N), (thiolato, thioketone), or (thiolato, thioether) ligand, e.g. [Mo VI O 2 (S,N-cys-OR) 2 ] (R = Me, Et), Mo VI O 2 (S 2 CNEt 2 ) 2 , [Mo VI O 2 (L) 2 ] (L = N,N ‘-dimethyl- N,N’ -bis(2-mercaptophenyl)ethylenediamine), Mo VI O 2 (L-NS 2 ) 2 (L-NS 2 = 2,6-bis(2,2-diphenyl-2-thioethyl)pyridinato), and Mo VI O 2 (dttd) (dttd = 2,3:8,9-dibenzo-1,4,7,10-tetrathiadecane), Many model reactions in the presence of dioxomolybdenum(VI) and monooxomolybdenum(IV) complexes have also been examined for the redox reaction of nitrate, triphenylphosphine, − benzoin, , amine N -oxide, , and dimethyl sulfoxide …”

Oxo-molybdenum and oxo-tungsten complexes of Schiff bases relevant to molybdoenzymes

“…One of the problems of using O 2 seems to be the intermediate formation of a mononuclear Mo(V) species and the superoxide radical anion by electron transfer prior to oxygen atom transfer. 47 Such difficulties in the formation of dioxomolybdenum from mononuclear Mo(V) compounds have been reported previously and polynuclear molybdenum oxo compounds were isolated, 47 a fact also pointed to by the often observed occurrence of a purple color; in contrast, with the present systems, no color change to purple was ever observed. An older report suggests the formation of [MoO 2 (S 2 CNEt 2 ) 2 ] from the reaction of [MoO(S 2 CNEt 2 ) 2 ] with O 2 via the intermediate purple [Mo 2 O 3 (S 2 CNEt 2 ) 4 ], however no experimental details are given.…”

Reactivity of Mo−Ot Terminal Bonds toward Substrates Having Simultaneous Proton- and Electron-Donor Properties: A Rudimentary Functional Model for Oxotransferase Molybdenum Enzymes

“…This information has prompted several recent reports on oxomolybdenum complexes of biomimetic ligands. 7d,− Numerous reactions that replicate the primary oxygen atom transfer step, both forward and reverse (eq 1), have been achieved with many of these complexes using nitrate, sulfite, benzoin, and a variety of phosphines, amine N -oxides, and sulfoxides as substrates. In contrast, examples of model complexes that regenerate cis -MoO 2 2+ species via a correlated electron−proton transfer step (eq 2) have been few. , …”