N-heterocyclic carbene catalysis: expansion of
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Asymmetric hydrogenation as a general route to polypropionates has been explored for allylic alcohols, acids, and derivatives, which has led to the generation of 2,4-dimethylated hexane derivatives. Quantitative yields in most cases and enantioselectivities greater than 98% were obtained. A remarkable stereofacial inversion was observed when an ester or acid was present in the allylic position instead of an alcohol or alcohol derivative. This led to the construction of all four diastereomers of the hexanol series from a single enantiomer of hydrogenation catalyst. Also described are an attempted synthesis of (-)-lardolure, a formal synthesis of the methyl ester portion of the preen gland extract from the domestic goose Anser anser, and a total synthesis of an extract from the fungi A. cruciatus. The synthesis of these compounds demonstrated shortcomings of the known catalyst system showing enantioselectivities for polymethylated compounds was high, while diastereoselectivity was low. Methodology to develop new N-heterocyclic carbene catalysts was developed using the cyanide coupling of aldimines to generate electronically tunable 1,3,4,5-tetraaryl complexes, and X-ray, IR, and calculations were used to elucidate their electronic characteristics. These studies indicate that the 4,5-positions have as great an influence on the metal-ligand bond as the 1,3-positions. In addition, they are among the most electron-donating 2- metalated N-heterocyclic carbenes found thus far. An intrinsic relationship between catalytic activity and electron donating ability was found in transfer hydrogenations.
Ogle, James William (2008). N-heterocyclic carbene catalysis: expansion of. Doctoral dissertation, Texas A&M University. Available electronically from