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Synthesis and Application of New Chiral Ligands for Enantioselectivity Tuning in Transition Metal Catalysis

Description: A set of five new C3-symmetric phosphites were synthesized and tested in palladium-catalyzed asymmetric Suzuki coupling. The observed reactivity and selectivity were dependent upon several factors. One of the phosphites was able to achieve some of the highest levels of enantioselectivity in asymmetric Suzuki couplings with specific substrates. Different hypotheses have been made for understanding the ligand effects and reaction selectivities, and those hypotheses were tested via various methods including DOSY NMR experiments, X-ray crystallography, and correlation of catalyst selectivity with Tolman cone angles. Although only modest enantioselectivities were observed in most reactions, the ability to synthesis these phosphites in only three steps on gram scales and to readily tune their properties by simple modification of the binaphthyl 2´-substituents makes them promising candidates for determining structure-selectivity relationships in asymmetric transition metal catalysis, in which phosphites have been previously shown to be successful. A series of novel chiral oxazoline-based carbodicarbene ligands was targeted for synthesis. Unfortunately, the chosen synthetic route could not be completed due to unwanted reactivity of the oxazoline ring. However, a new and efficient route for Pd-catalyzed direct amination of aryl halides with oxazoline amine was developed and optimized during these studies. Chiral binaphthyl based Pd(II) ADC complexes with different substituent groups have been synthesized and tested in asymmetric Suzuki coupling reactions. Although only low enantioselectivities were observed in Suzuki coupling, this represents a new class of chiral metal-ADC catalysts that could be tested in further catalytic.
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Date: August 2017
Creator: Kong, Fanji
Partner: UNT Libraries

Synthesis, Characterization and Catalytic Studies of Chiral Gold Acyclic Diaminocarbene Complexes

Description: Chiral gold complexes have been applied in homogeneous catalytic reactions since 1986, in some cases with high enantioselectivity. Acyclic diaminocarbene (ADC) ligands are acyclic analogues of N-heterocyclic carbenes (NHCs) that have larger N-CCarbene-N angles and stronger donating ability. ADCs have been developed as alternatives to phosphine and NHC ligands in homogeneous gold catalysis. In 2012, a new series of chiral gold(I) ADCs were first developed by Slaughter's group and were shown to give remarkable enantioselectivities in some reactions. Because of the hindered rotation of the N-CCarbene bonds of ADC, chiral ADC substituents can easily get close to the metal center in some conformations, although two rotameric structures are formed if the chiral amine is nonsymmetric. The selective of specific ADC conformations was the initial focus of this study. Formational selectivity of one diastereomer of an ADC ligand during synthesis was examines by measuring the relative rates of diastereomer formation in a 1H NMR kinetic study. The potential for converting multiple conformational isomers of ADCs into a single conformation, or at least a simpler mixture, was examined. This study used the analogy that anti- isomer has electronic and structural similarity with urea/thiourea, raising the possibility that 1,8-naphthyridine can be used to favor certain conformations through a self-assembled hydrogen-bonding complex. Gold(I) is a soft carbophilic Lewis acid able to active C-C π bonds to nucleophilic attack, and ADC-gold complexes are potentially useful in this regard. Therefore, biaryl gold(I) ADC complexes were examine with silver salt additives in catalytic 1,6-enyne cyclization reaction. A detailed study found that the counteranion affects the regioselectivities of these reactions more than substituents on the ancillary ADC ligands.
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Date: August 2016
Creator: Zhang, Xiaofan
Partner: UNT Libraries

Carbon Nanostructure Based Donor-acceptor Systems for Solar Energy Harvesting

Description: Carbon nanostructure based functional hybrid molecules hold promise in solarenergy harvesting. Research presented in this dissertation systematically investigates building of various donor-acceptor nanohybrid systems utilizing enriched single walled carbon nanotube and graphene with redox and photoactive molecules such as fullerene, porphyrin, and phthalocyanine. Design, synthesis, and characterization of the donor-acceptor hybrid systems have been carefully performed via supramolecular binding strategies. Various spectroscopic studies have provided ample information in terms of establishment of the formation of donor-acceptor hybrids and their extent of interaction in solution and eventual rate of photoinduced electron and/or energy transfer. Electrochemical studies enabled construction of energy level diagram revealing energetic details of the possible different photochemical events supported by computational studies carried out to establish the HOMO-LUMO levels in the donor acceptor systems. Transient absorption studies confirmed formation of charge separated species in the donor-acceptor systems which have been supported by electron mediation experiments. Based on the photoelectrochemical studies, IPCE of 8% was reported for enriched SWCNT(7,6)-ZnP donor-acceptor systems. In summary, the present investigation on the various nanocarbon sensitized donor-acceptor hybrids substantiates tremendous prospect, that could very well become the next generation of materials in building efficient solar energy harvesting devices andphotocatalyst.
Date: December 2013
Creator: Das, Sushanta Kumar
Partner: UNT Libraries

Design of New Monodentate Ligands for Regioselectivity and Enantioselectivity Tuning in Late Transition Metal Catalysis

Description: The ability of gold(I) to activate many types of unsaturated bonds toward nucleophilic attack was not widely recognized until the early 2000s. One major challenge in gold catalysis is the control over regioselectivity when there are two or more possible products as a result of complicated mechanistic pathways. It is well know that the choice of ligand can have dramatic effects on which pathway is being followed but very rarely are the reasons for this selectivity understood. The synthesis of new acyclic diaminocarbenes was developed and a study of the ligand effects on the regioselectivity of a gold-catalyzed domino enyne cyclization hydroarylation reaction and a Nazarov cyclization was undertaken. New chiral acyclic diaminocarbenes were also developed and tested along side new C3-symmetric phosphite ligands in an asymmetric intramolecular hydroamination of allenes. Structure activity correlations were developed for the potential use in further rational ligand design. The synthesis of 6a,7-dihydro-5-amino-dibenzo[c,g]chromene derivatives via a gold-catalyzed domino reaction of alkynylbenzaldehydes in the presence of secondary amines was developed. These were sent to be screened for biological activity.
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Date: May 2016
Creator: Ruch, Aaron Anthony
Partner: UNT Libraries