Patterned after synthetic model systems for dioxomolybdenum enzymes, our 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 oxo-transfer reaction. Here, our substrate, PMe(3), approaches [Mo(VI)O(2)](2+) at an O-Mo-O-P dihedral angle of 90 degrees, i.e. perpendicular to the MoO(2) 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, [Mo(IV)O(OPMe(3))](2+), which is 69 kcal/mol more stable than the reactants. The direction of the substrate's attack leaves the two d electrons of this Mo(IV) system in an orbital which is delta with respect to the remaining spectator Mo-O bond, a configuration which allows this O to form a formal triple Mo-O bond. The displacement of the product, OPR(3), by water, H(2)O, proceeds via an associative mechanism with a barrier of only 19 kcal/mol. In our model, [Mo(IV)O(OH(2))](2+) then reacts with [Mo(VI)O(2)](2+) to form [Mo(V)O(OH)](2+), a process which is exothermic by 14 kcal/mol. The addition of O(2) then oxidizes [Mo(V)O(OH)](2+) to [Mo(VI)O(2)](2+) to complete our model catalytic cycle.

The relative reaction thermodynamics for the probable pathways leading to alkene epoxidation, cis-dihydroxylation, and C−C cleavage as well as alkenylation of aldehydes have been investigated for the reaction of ethylene with LReO3, L = CH3, Cl, OH, OCH3, O-, C5H5(Cp), and C5(CH3)5 (Cp*). We find the reaction pathway exothermicities to be quite sensitive to the ligand. For ClReO3, CH3ReO3, HOReO3, CH3OReO3, and O-ReO3, reaction with ethylene is endothermic with metallaoxetane preferred over dioxylate formation. For CpReO3 and Cp*ReO3, the reaction with ethylene is exothermic but nearly thermoneutral with dioxylate formation preferred. The π-binding attributes of the ligand are found to correlate with thermodyanmic preference.

Properties such as the rate of crystallization and degree of crystallinity of polymers formed from vinyl olefins such as propylene can be affected by the tacticity of the material. 1 The tacticity in turn can be controlled by the nature of the active site of the polymerization catalyst. 2 A catalyst active site shape that favors reaction by the same prochiral olefin face will produce isotactic polymer while a catalyst site shape that does not favor one olefin face over the other will produce atactic polymer. With this active site shape knowledge in hand, an interesting intellectual challenge remains: If one can, at will, design a catalyst that will form atactic material or isotacitc material, or even syndiotactic material, can one design a catalyst that will alternate between active site shapes and make welldefined blocks with differing tacticities and hence differing physical properties? Catalysts have recently been discovered which produce polymeric materials with alternating atactic and isotactic blocks. Chien reported the first metallocene catalyst that produced blocks of atactic and isotactic polypropylene. 3 More recently, Coates and Waymouth invented a class of elastomeric polypropylene catalysts that operate through a welldefined, well-constructed interconversion between active site shapes that favor isotactic and atactic polymerization. 4 The crystal structure of the Coates-Waymouth catalyst precursor, bis(2-phenylindenyl)zirconium dichloride ((2-PhInd)-ZrCl 2 , 1) contained two distinct conformers in the unit cell. In one conformer the indenyl ligands were syn to one another (a meso stereochemistry), while in the other conformer the indenyl ligands were anti to one another (a rac stereochemistry). Since both conformations appeared in the crystal structure, they must be of similar energy; otherwise, only the thermodynamically favored conformer would be present, barring differential crystal packing. When methylaluminoxane (MAO) was added to the catalyst precursor, polypropylene with atactic and isotactic blocks was formed. Atactic blocks can be ascribed to polymerization when the catalyst is in the meso conformation and isotactic blocks to polymerization when the catalyst is in the rac conformation. All in all a well-described system.An important question that still remains unanswered with the Coates-Waymouth system is the precise source of the barrier between the meso and rac conformers. This conformational barrier could not be frozen out at low temperatures 4 for the catalyst precursor 1, but must be of modest but finite size to be competitive with the barrier for olefin insertion since block polymer is formed. Observation of a modest but finite-sized barrier is intriguing because the individual barriers for the conformational degrees of freedom in the catalyst precursor 1 are intrinsically quite small. For example, the barrier to rotation is roughly 1-2 kcal/mol for biphenyls 5 and nearly 0 kcal/mol for cyclopentadienyl rings. 6 The barrier between the meso and rac conformers must, however, be betw...