With this award from the Organic and Macromolecular Chemistry Program, Prof. Michael Doyle will focus on two fundamental discoveries in the chemistry of metal compounds that have exciting potential for revolutionary developments in chemistry. The first is chemical oxidations of organic compounds by inexpensive tert-butyl hydroperoxide (70% in water) using these metal compounds. The second is based on their discovery of a new class of metal-containing organic compounds that show high potential as new and unexplored materials. Both are related to the unique ability of rhodium to bind to a second rhodium when connected to amide structures that bridge the two rhodium atoms. The compounds that result have two rhodium atoms connected to four amide structures in the form of a paddlewheel, and their unique properties include an ability to be oxidized with low input of energy. Encouraged by results in catalytic oxidations by tert-butyl hydroperoxide that reveal significant advantages for their strategy over alternative catalytic strategies, among which are (1) low catalyst loading (down to 0.1 mol %) to achieve high product yields, (2) formation of ketones selectively in a variety of organic compounds, and (3) reactions occur in water without destruction of catalyst, they will develop a broad understanding of these oxidative processes. They will determine the specific pathway or pathways for oxidation, as well as methods to direct the reaction to a specific pathway, so that applications can be broadened to oxidations of complex organic compounds that include steroids and unsaturated fatty acids. The discovery of rhodium-containing organic compounds that have structural rigidity, stability, and design flexibility, and a general methodology for their preparation, has made possible the construction of previously unknown organometallic materials. Because these materials do not have a metal-metal bond, they are potentially good insulators; the group will focus on the production of molecular wires with these compounds with efforts to achieve conductance through electron transfer so that, as insulators and conductors, they may be able to develop molecular switches. Broader Impacts. The implications of this research promise new insights into the interception of radical intermediates and control of reaction pathways for product formation, many of which are related to biological processes. The compounds that they prepare by mild methods provide a broader selection of effective catalysts in carbon-carbon bond forming reactions. Rhodium-containing organic compounds are a new class of easily accessed organometallic materials whose function could be of benefit in electronics or optics, as well in the development of new polymeric substances. The variety of education and training afforded by this research benefits postdoctoral, graduate student and undergraduate student participants.
The major focus of our research program is the development of highly selective and efficient catalytic processes for organic synthesis. Many of these investigations are built upon unique, highly efficient and selective, catalytic uses of dirhodium carboxamidates. The fixed stereodefined geometry of these catalysts provides access to highly enantioenriched products in metal carbene reactions of diazoacetates and, together with their low oxidation potentials, also provides capabilities for highly selective Lewis acid catalyzed reactions and efficient chemical oxidations with high turnover numbers and high selectivities. We are also developing diazo chemistry for catalytic stereoselective transformations to further enhance applicability of catalytic metal carbene chemistry in organic synthesis. New catalytic syntheses of multi-functional β-keto-α-diazoesters and enoldiazoacetates with subsequent catalytic transformations provide highly efficient access to more complex carbon frameworks than previously possible through reactions with traditionally-used diazocarbonyl compounds. Lewis acidic chiral dirhodium(II,III) carboxamidate catalysts are being used to broaden the range of asymmetric Lewis acid catalyzed carbon-carbon bond forming transformations. Reactivity and selectivity enhancement through chiral Rh25+ catalysts expands their utilization to Lewis acid catalyzed reactions for which chiral Rh24+ catalysts are ineffective. The selectivities achiened with these catalysts are mainly due to configurational and conformational constraints from association of Lewis basic reactants at the rhodium center of the catalyst. Catalytic oxidative methodologies to prepare compounds that are of biological significance are being developed. Newly discovered tert-butyl hydroperoxide oxidations catalyzed by dirhodium caprolactamate, based in large part on its low oxidation potential and solubility in water and organic solvents, offer a unique opportunity to develop a spectrum of oxidative transformations, compatible with water as a solvent, that are not easily achieved by other methods (especially allylic and benzylic oxidations). Applications encompass reactions with steroids, phenolic compounds, unsaturated fatty acid derivatives, terpenes, amines, and other biologically relevant substrates. Mechanistic implications of this work have removed many misconceptions regarding chemical oxidations with hydroperoxides. PhRh(cap)4RhPh. Our discovery of bis(phenyl)dirhodium(III) compounds has opened an entirely new area in dirhodium chemistry. Their structural rigidity, stability, and design flexibility are ideal for the generation of new materials. Opportunities now exist for the creation of novel organometallic structures and for examination of their electronic, physical and spectroscopic properties. Thermally and chemically stable paddlewheel dirhodium(III,III) compounds with phenyl substituents in the axial positions and lactamates as the bridging ligands have been prepared. When the lactamate ligand is caprolactam, two non-interconvertable conformational isomers are formed, one having a biplanar orientation of the caprolactamate ligands and the other with a propeller orientation . These conformational isomers differ in the orientation of the bridging caprolactamate ligands, and possess unique spectral, chromatographic, and chemical properties.
