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Author Nielsen, Robert J
Title Computational strategy in catalyst design
book jacket
Descript 109 p
Note Source: Dissertation Abstracts International, Volume: 66-08, Section: B, page: 4219
Adviser: William A. Goddard, III
Thesis (Ph.D.)--California Institute of Technology, 2005
The strategy and efficacy of applying computational tools to the development of new catalytic cycles is discussed using the enantioselective palladium-catalyzed aerobic oxidation of secondary alcohols as a model case. The key interactions responsible for the reactivity of ((-)-sparteine)PdX2 complexes (X = chloride, acetate) in kinetic resolutions of secondary alcohols are elucidated using density functional theory with a polarizable continuum solvent model. Enantioselectivities in these reactions are found to follow directly from calculated energies of diastereomeric beta-hydride elimination transition states incorporating (R) and (S) substrates. This reveals an important role of the anion, namely to communicate the steric interaction of the ligand on one side of the PdII square plane and the substrate on the other side. When no anion is included, no enantioselectivity is predicted. Locating these transition states in different solvents shows that higher dielectrics stabilize charge separation between the anion and metal and draw the anion farther into solution. Thus the solvent influences the barrier height (rate) and selectivity of the oxidation
Based on this understanding, computational assays for selectivity, reaction rate and stability are developed and used to screen possible synthetic mimics of (-)-sparteine. Bispidine derivatives are predicted to have high selectivity but poor stability on palladium. Experimental results verify that catalytically active (bispidine)PdX2 complexes do not form
Mechanisms by which palladium diacetate complexes of N-heterocyclic carbenes may oxidize alcohols are examined computationally. No traditional beta-hydride elimination is predicted to be capable of generating the high deuterium kinetic isotope effect measured using this catalyst. Instead, the low-energy pathway consistent with experimental observations (KIE, activation parameters, kinetics) is a "reductive" beta-hydride elimination, in which the beta-hydrogen of the alcohol is transferred directly to an acetate ligand. Assuming that relative energies of transition states of this type will determine enantioselectivity, new carbenes are hypothesized and screened. Careful placement of stereocenters and steric bulk has led to ligands with high predicted enantioselectivity and stability
Recurring factors in the induction of selectivity by asymmetric ligands are observed. Strengths and weaknesses of quantum chemistry as applied to catalytic cycles are discussed, along with the synergy of theory and experiment
School code: 0037
Host Item Dissertation Abstracts International 66-08B
Subject Chemistry, Inorganic
Alt Author California Institute of Technology
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