Ultrafast Vibrational Dynamics of Biomimetic Catalysts

Ultrafast two-dimensional infrared (2D-IR) spectroscopy is used in this work to study the vibrational dynamics of a series of biomimetic catalysts. We set out to investigate the vibrational dynamics of catalytic compounds in systems directly relevant to molecular reactivity, specifically reactive oxidation states, catalytically relevant self-isomerizations, dendritically-induced nano-confinement, and excitonic coherence transfer. For most of the work performed for this thesis we used diiron hexacarbonyl small-molecule mimics of the [FeFe]-hydrogenase enzyme’s active site. The vibrational dynamics of [(1,1’-bis(diphenylphosphino)ferrocene)chromium-(CO)4] (DPPFCr) in its neutral, closed-shell state were compared to the vibrational dynamics of DPPFCr as a cation radical. This comparison is possible because molecular oxidation does not significantly change the vibrational displacements of the carbonyl modes, which are studied here. Molecular oxidation induces an acceleration of the vibrational relaxation of the carbonyl modes but does not significantly affect the spectral diffusion dynamics of the carbonyl groups. We attribute this to an idiosyncrasy of the non-interacting solvent used for the experiment, CH2Cl2, which was chosen specifically for the weak nature of its solvent-solute interactions. Unexpectedly pronounced and slow spectral diffusion in the carbonyl modes of (µ-pdt)[Fe(CO)3]2 (pdt = 1,3-propanedithiolate) was observed in alkane solvents. The contribution of solvent-solute interactions in alkane solvents to spectral diffusion is expected to be minimal, and we related the spectral diffusion to fluctuations of the carbonyl potential induced by a catalytically-relevant mode of molecular fluxionality in (µ-pdt)[Fe(CO)3]2. Comparison with a different diiron hexacarbonyl compound, (µ-edt)[Fe(CO)3]2 (edt = 1,2-ethanedithiolate), effectively ruled out isomerization of the bridging organic disulfide group, and a Boltzmann distribution of states derived from electronic structure calculations supported our hypothesis by suggesting that a significant distribution of molecular conformations were present in at room temperature. Other fluxional organometallic complexes M3(CO)12 (M=Ru, Os) displayed similar spectral diffusion. This is the first use of spectral diffusion to study molecular conformational flexibility. We also observed an unexpected dependence of the rate of intracarbonyl IVR on the chain length of the alkane solvent. Nano-confinement has been reported on several occasions to favorably modulate the reactivity of diiron hexacarbonyl compounds, and dendritic assemblies with diiron hexacarbonyl cores were synthesized and the vibrational dynamics of the carbonyl groups were compared to the vibrational dynamics of carbonyls on similar diiron hexacarbonyl compounds without dendritic groups. Slower IVR and an additional timescale of spectral diffusion were observed in dendritic assemblies, which are hypothesized to reflect nano-modulation of the carbonyl group’s first solvation shell by the dendritic groups. Three diiron hexacarbonyl compounds with differing bridging disulfide groups (edt, pdt, and o-xylyldithiolate) are found to display unusual modulations of cross peak intensity which have previously been identified as spectral signatures of vibrational coherence transfer. Specific modulations of cross peak amplitude are observed in all three compounds, suggesting that certain coherence transfer events are common in diiron hexacarbonyl compounds, and an oscillatory frequency resulting from coherence transfer between bright and dark vibrational modes is identified.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/145788/1/zpeckert_1.pd

Ethylene or propylene based copolymers with higher 1-olefins by metallocene catalysts: correlation between microstrusture and properties.

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Importance of Electrostatically Driven Non-Covalent Interactions in Asymmetric Catalysis

Computational chemistry has become a powerful tool for understanding the principles of physical organic chemistry and rationalizing and even predicting the outcome of catalytic and non-catalytic organic reactions. Non-covalent interactions are prevalent in organic systems and accurately capturing their impact is vital for the reliable description of myriad chemical phenomena. These interactions impact everything from molecular conformations and stability to the outcome of stereoselective organic reactions and the function of biological macromolecules. Driven by the emergence of density functional theory (DFT) methods that can account for dispersion-driven noncovalent interactions, there has been a renaissance in terms of computational chemistry shaping modern organic chemistry. DFT Studies of the origins of stereoselectivity in asymmetric organocatalytic reactions can not only provide key information on the mode of asymmetric induction, but can also guide future rational catalyst design. We start with an overview of weak intermolecular interactions and aromatic interactions. Special emphasis is given to the methods that one can use to study these ephemeral interactions. We next provide a brief account how computational chemistry has aided our understanding of chiral phosphoric acid (CPA) catalyzed reactions. Thereafter, three case studies showcasing the importance of non-covalent interactions in chiral NHC catalysis, CPA catalysis, and chiral nucleophilic catalysis has been elaborated. Each of these studies highlights the importance of electrostatically-driven non-covalent interactions in controlling reactivity and selectivity. Moreover, unprecedented activation modes are identified and new predictive selectivity models developed that can be used to rationalize the outcome of future reactions. Studying these reactions using state of art DFT methods, we aimed not only to contribute to the understanding of their selectivity and the importance of noncovalent interactions in catalysis, but also to bring a sound understanding that will enable the design of new reactions and better catalysts. Overall, this dissertation highlights the underappreciated role of electrostatic interactions in controlling reactivity and selectivity in asymmetric catalysis

Development and application of metal-catalyzed diamination reactions, The

2014 Fall.Includes bibliographical references.Nitrogen-rich molecules are of great interest in chemistry and incorporation of nitrogen into molecules is an on-going active field of study. In particular, vicinal diamines are important functional moieties that are found throughout biologically active molecules and natural products as well as highly effective chiral control agents in organic synthesis. There has been much effort directed toward the efficient synthesis of vicinal diamines; however the development of a direct route has proven to be challenging. This dissertation discusses the application of diamination products from existing methods to synthesize biologically active motifs, as well as the development of new metal-catalyzed diamination methods for the synthesis of biologically interesting motifs from readily available starting materials. The β, γ-diamino acid motif is an area of active research because of its prevalence in biologically active molecules and its use in peptide library syntheses. Cyclization of β, γ-diamino acids give the closely related 4-aminopyrrolidinones. These five-membered amino lactams have been reported to potentiate insulin activity when incorporated into hypoglycemic peptide analogues and made the analogues more stable towards physiological degradation. Current methods for the synthesis of these compounds require multi-step procedures and rely heavily on commercially available amino acids as starting materials, thus limiting the structural variability for biological studies. Using a diamination method discovered in our lab, 4-aminopyrrolidinones were efficiently synthesized in 40% overall yield, over five steps from readily available terminal olefins or conjugated dienes, providing a comparable process in the synthesis of these compounds. As part of our ongoing efforts to study the mechanism of metal-catalyzed diaminations using diaziridinone as nitrogen source, it was found that regioselectivity in the diamination of conjugated dienes could be controlled using Cu(I) as catalyst and varying reaction conditions. An alternative nitrogen source, thiadiaziridine 1,1-dioxide, which has shown to display interesting reactivity, was chosen to further investigate the Cu(I)-catalyzed regioselective diamination. Upon varying reaction conditions with Cu(I) catalysts, regioselective diamination occurred for various conjugated dienes and allowed direct access to a range of diverse cyclic sulfamides which have interesting biological potential. With the racemic synthesis of cyclic sulfamides, it was of interest to obtain these compounds asymmetrically, as their biological properties are of value and current methods for their asymmetric synthesis do not allow much variation in substitution patterns. Using Pd2(dba)3 and a chiral phosphoramidite ligand, a variety of chiral cyclic sulfamides were synthesized in moderate to high yields and with ee's greater than 90%, providing direct access to these valuable compounds in one step from readily available conjugated diene substrates. Lastly, N,N-Di-tert-butyl thiadiaziridine 1,1-dioxide has been found to be a versatile reagent for interesting reactivity. Other uses of this reagent include the Pd(II)-catalyzed terminal diamination of conjugated dienes, diamination of allenes, and the Pd-catalyzed oxidation of alcohols to form α, β-unsaturated compounds