THEORETICAL-STUDIES ON MODELS FOR THE OXO-TRANSFER REACTION OF DIOXOMOLYBDENUM ENZYMES

Patterned after synthetic model systems for dioxomolybdenum enzymes, o ur theoretical model system produces an energy profile and structures for the various species and oxidation states in the catalytic cycle. A key step in this cycle is the ore-transfer reaction. Here, our substr ate, PMe(3), approaches [(MoO2)-O-VI](2+) at an O-Mo-O-P dihedral angl e of 90 degrees, i.e. perpendicular to the MoO2 plane, crosses over a barrier of 14 kcal/mol, and rotates to an O-Mo-O-P dihedral angle of 0 degrees to form an intermediate, [(MoO)-O-IV(OPMe(3))](2+), which is 69 kcal/mol more stable than the reactants. The direction of the subst rate's attack leaves the two d electrons of this Mo(IV) system in an o rbital which is delta with respect to the remaining spectator Mo-O bon d, a configuration which allows this O to form a formal triple Mo-O bo nd. The displacement of the product, OPR(3), by water, H2O, proceeds v ia an associative mechanism with a barrier of only 19 kcal/mol. In our model, [(MoO)-O-IV(OH2)](2+) then reacts with [(MoO2)-O-VI](2+) to fo rm [Mo-VI(OH)](2+), a process which is exothermic by 14 kcal/mol. The addition of O-2 then oxidizes [(MoO)-O-V(OH)](2+) to [(MoO2)-O-VI](2+) to complete our model catalytic cycle.