426209-91-8Relevant academic research and scientific papers
Oxo transfer reactions mediated by bis(dithiolene)tungsten analogues of the active sites of molybdoenzymes in the DMSO reductase family: Comparative reactivity of tungsten and molybdenum
Sung,Holm
, p. 1931 - 1943 (2001)
The discovery of tungsten enzymes and molybdenum/tungsten isoenzymes, in which the mononuclear catalytic sites contain a metal chelated by one or two pterin - dithiolene cofactor ligands, has lent new significance to tungsten - dithiolene chemistry. Reaction of [W(CO)2(S2C2Me)2′] with RO- affords a series of square pyramidal desoxo complexes [WIV(OR′)(S2C2Me2) 2]1-, including R′ = Ph (1) and Pri (3). Reaction of 1 and 3 with Me3NO gives the cis-octahedral complexes [WVIO(OR′)(S2C2Me2) 2]1-, including R′ = Ph (6) and Pri (8). These W(IV.VI) complexes are considered unconstrained versions of protein-bound sites of DMSOR and TMAOR (DMSOR = dimethylsulfoxide reductase, TMAOR = trimethylamine N-oxide reductase) members of the title enzyme family. The structure of 6 and the catalytic center of one DMSO reductase isoenzyme have similar overall stereochemistry and comparable bond lengths. The minimal oxo transfer reaction paradigm thought to apply to enzymes, WIV + XO → WVIO + X, has been investigated. Direct oxo transfer was demonstrated by isotope transfer from Ph2Se18O. Complex 1 reacts cleanly and completely with various substrates XO to afford 6 and product X in second-order reactions with associative transition states. The substrate reactivity order with 1 is Me3NO > Ph3AsO > pyO (pyridine N-oxide) > R2SO ? Ph3PO. For reaction of 3 with Me3NO, k2 = 0.93 M-1 s-1 and for 1 with Me2SO, k2 = 3.9 × 10-5 M-1 s-1: other rate constants and activation parameters are reported. These results demonstrate that bis(dithiolene)W(IV) complexes are competent to reduce both N-oxides and S-oxides: DMSORs reduce both substrate types, but TMAORs are reported to reduce only N-oxides. Comparison of kcat/KM data for isoenzymes and k2 values for isostructural analogue complexes reveals that catalytic and stoichiometric oxo transfer, respectively, from substrate to metal is faster with tungsten and from metal to substrate is faster with molybdenum. These results constitute a kinetic metal effect in direct oxo transfer reactions for analogue complexes and for isoenzymes provided the catalytic sites are isostructural. The nature of the transition state in oxo transfer reactions of analogues is tentatively considered. This research presents the first kinetics study of substrate reduction via oxo transfer mediated by bis(dithiolene)tungsten complexes.
