Lewis acid assisted metal-ligand bifunctional hydrogenation of amides
Synnott, Stephanee Lynn.
Thesis (M.S.)--Georgetown University, 2009.; Includes bibliographical references. In the past decade, metal-ligand bifuctional catalysis has been a growing interest in the field of hydration of polar double bonds. In particular, these catalysts have provided a new mechanistic approach for the direct reduction of carbonyl compounds.1 The most well studied example of the metal-ligand bifuctional catalysis is Noyori's Ru catalyst shown in figure 1. The hydrogens for the reduction are located on the ruthenium metal and the amine ligand. The hydridic Ru-H and the protic N-H are simultaneously inserted to the carbonyl, which has a partial negative charge on the oxygen and a partial positive charge on the carbon, to form a six member transition state, resulting in an S alcohol.6 This type of catalysis however, has not been effective for amide reductions.; Because of the growing interest in developing mild and catalytic methods for reduction of amides, we propose a modification to the typical Noyori system which is designed to expand metal-ligand bifuctional activity to amide hydrogenations. Amides have a substantial contribution from a polar resonance form where a partial double bond between the carbon of the carbonyl and the nitrogen reduces the carbonyl-like character of this group and prevents normal hydrogenation.5 It is therefore proposed that a tethered Lewis acid, such as a borane, will interact with the amide nitrogen and minimize the polar resonance contribution to the carbonyl double bond. In this manner the amide will have greater "carbonyl" characteristics, and the reduction could proceed via a concerted transition state resembling the Noyori hydrogenation of ketones. Presented will be the synthesis and future goals of amide reduction activity of catalysts that contain a tethered borabicyclo[3.3.1]nonane fragment.
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