Catalysis orbital considerations

The present volume continues our effort to provide diverse exposure. We include two articles devoted to stereochemical aspects of catalytic reactions (J. K. A. Clarke and J. J. Rooney R. L. Augustine), and one (J. D. Morrison, W. F. Masler, and M. K. Neuberg) devoted to the control of a yet more subtle level of chemical structure asymmetry (or optical activity) a comprehensive review of liquid phase organic oxidation catalysis (R. A. Sheldon and J. K. Kochi) a review of specific adsorption and poisoning action as a means to learn more about active sites (H. Knozinger) and some of the latest considerations to catalysis of molecular orbital theory (R. C. Baetzold). 

A more detailed consideration of the Woodward-Hofimann postu-ulates for olefinic systems in the presence of a transition metal indicates that the thermally forbidden dimerization of two ethylene molecules to cyclobutane becomes allowed if the orbitals of the olefins can interact symmetrically with the dxt and dyz orbitals of the transition metal catalyst (53). One would consequently also expect transition metal complexes to catalyze the conversion of quadricyclene (IV) back to norbornadiene. This has been reported to be the case (54). The reactions leading to the formation of VI, XXX, and XXXI are examples of processes in which thermally allowed sigmatropic reactions become subject to catalysis by transition metal complexes. The catalysts thus display the dual role of removing symmetry restrictions and of generally lowering activation energies. 

In certain cases, the reduction can take place without electrophilic catalysis (n-BU4NBH4 or phase-transfer conditions), but most frequently it requires the coordination of the carbonyl group by a Lewis acid before nucleophilic attack [S2]. The Lewis acid may be the cation associated with the reagent, an added acid, or even the boron or aluminum atom of tricoordinate reagents (AIH3, DIBAH, boranes). The importance of this phenomenon has been shown by the introduction of coordinating macrocyclic molecules into solutions of LAH and LiBH4. This considerably retards the reduction of carbonyl compounds in an ether medium [DCl, HPl], Electrophilic catalysis is more important when the lowest unoccupied molecular orbital (LUMO)... 

For fundamental understanding of catalytic sites, this crude treatment is not enough. Some consideration of surface geometry and orbital availabiL ity, in much the same way as in metal catalysis, will be necessary for greater understanding. This is one area of scientific catalysis awaiting exploitation. 

Using rate constants derived from reaction of H2A and HA" with [Co(ox)3] and [Fe(phen)3], comparisons of the Marcus-derived one-electron potentials for H2A /H2A and HA /HA" with molecular orbital calculations for the homo energy confirm the greater reactivity of HA" over H2A. It is pointed out that the Marcus-derived potential for HA /HA", 0.85-1.0 V, is greater than the best available measurement for this parameter, 0.68 V. The self-exchange rate for ascorbate radical is lO -lO" M s" and indicates a considerable barrier to electron transfer. The ascorbate radical A also has a high intrinsic barrier to electron transfer, and detection of second-order kinetics in the decomposition of A" suggests a dimerization step with subsequent acid catalysis. 

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