Computational investigations of organic reaction mechanisms and stereoselectivities

This dissertation describes investigations of reaction mechanisms and stereoselectivities observed in organic and organometallic reactions investigated computationally. Radical rearrangements, selectivities of bromonium and iodonium ion cyclizations, and the mechanisms and selectivities of molybdenum-catalyzed asymmetric allylations were investigated.;In Chapter 1, rearrangements of cyclohexyl radicals to cyclopentylmethyl radicals are discussed. The rearrangement occurs when highly radical stabilizing groups are present at the 2- and 3-positions of the cyclohexyl radical. Such substituents at the 3-position enable ring-cleavage of the cyclohexyl radical. Radical stabilizing substituents at the 2-position stabilize the cyclopentylmethyl radical, enabling the overall rearrangement and reversing the normal thermodynamic preference for a cyclohexyl radical.;Conformational analyses of substituted oxan-2-yl cations formed after bromonium ion cleavage is the subject of Chapter 2. Individual substituents were investigated, as well as the conformational preferences of combined substituents. Conformational preferences of siloxy and bromomethyl groups correspond with the selectivities observed experimentally.;Selectivities of fluoro substituted iodocyclizations were investigated in Chapter 3. Conformational analysis of iodonium ions and iodine-pi complexes were investigated using density functional theory. Transition states for intermolecular cleavage of the iodine-pi complexes were located and results from these studies agree with experimental findings.;Chapter 4 is a study of the stability of the eta3-pi-allyl molybdenum crystal structure that was isolated during molybdenum-catalyzed asymmetric allylation. Calculations show that the observed pi-allyl molybdenum intermediate is the most stable because of two factors: the intermediate maximizes the bonding and back-bonding interactions between molybdenum and the pi-allyl ligand and it minimizes steric interactions between the chiral ligand and the pi-allyl group.;Possible molybdenum-catalyzed asymmetric allylation mechanisms are discussed in Chapter 5. The role of the pi-allyl intermediate in the catalytic cycle was determined, and the experimental observation that additional CO is required for the reaction to proceed is explained. A potential energy surface is given, and the enantioselectivity and regioselectivity of the reaction are explained.