Over the past decade the focus of our group has been production of polycarbonates through environmentally friendly routes. Continuing with this tradition, one such route is the ring opening polymerization of cyclic carbonates. The aliphatic polycarbonate derived from trimethylene carbonate, (TMC, 1, 3-dioxan-2-one), has been studied extensively for its potential use as a biodegradable polymer in biomedical and pharmaceutical systems. Its important applications include sutures, drug delivery systems and tissue engineering. To date, majority of the literature concerning catalysts for polymerization of TMC has been restricted to the use of simple Lewis acids with a marked absence of well defined and characterized catalysts. Metal salen complexes have been effective in the ring opening of cyclohexene oxide and the copolymerization of epoxide and carbon dioxide. The ability of this system as a catalyst for the polymerization of cyclic carbonates to polycarbonates is reported in this dissertation. The salen ligand is among the most versatile ligands in chemistry. Our attempts to optimize the catalytic activity by manipulating the salen structure and reaction conditions are also discussed. Our initial efforts were concentrated in understanding the efficacy of Lewis acidic metal salen complexes (Al & Sn), as catalysts for this process. This was followed by the utilization of metal salen complexes of biometals as catalysts for the synthesis of these biodegradable polymers, as well as for the copolymerization of cyclic carbonates with cyclic esters. These copolymers are presently in great demand for their applications as sutures in the medical industry. During the course of our investigations, a novel method of synthesizing polytrimethylene carbonate, by the copolymerization of CO2 and trimethylene oxide, has come to our attention. Surprisingly this reaction has received very little scientific exposure. We observed that metal salen derivatives, along with n-alkyl ammonium salts, were effective catalysts for the selective coupling of CO2 and oxetane (trimethylene oxide) to provide the corresponding polycarbonate with only trace quantities of ether linkages. A section is also dedicated to our investigations in this area of research.

No description available.

Doctor of Philosophy / Department of Chemistry / Christopher J. Levy / Stepped helical salen complexes with vanadium as the central metal were synthesized and characterized. The helicity in these complexes arise from the fused phenyl rings (phenanthryl and benz[a]anthryl) as sidearms, whereas the chirality arises from the chiral cyclohexyl diamine or binaphthyl diamine backbones. These complexes showed good yields and moderate enantioselectivity in asymmetric sulfoxidation reactions with methylphenyl sulfide as the substrate and H2O2 or cumene hydroperoxide as the oxidants. To further improve the closed nature of these complexes with a tetradentate salen ligand, we synthesized and characterized vanadium complexes with tridentate (S)-NOBIN backbone Schiff base ligands with phenanthryl and benz[a]anthryl as the sidearms. After initial catalytic study, we concluded that these catalysts are too open in nature to impose face selection during asymmetric induction. We also synthesized and characterized vanadium and titanium salan complexes. These complexes can adopt β-cis geometry, thereby making the complex “chiral at metal” and they are known for better catalysts in terms of asymmetric induction than their unreduced counterparts. However, these complexes showed better catalytic activity than their unreduced counterparts in sulfoxidation reactions with methylphenyl sulfide as the substrate and H2O2 or cumene hydroperoxide as the oxidants. We also put an effort to synthesize mixed salen complexes with vanadium as the central metal. These complexes have two different sidearms attached to one backbone unit. However, our method did not work well to produce pure mixed salen ligands. The catalysis results for mixed salen vanadium complexes are also comparable to the unreduced vanadyl salen complexes. Lastly, we synthesized and characterized new helical titanium Schiff base complexes with (S)-NOBIN backbone and phenanthryl and benz[a]anthryl sidearms. Single crystal studies showed that these complexes exist in the M helical conformation in the solid state. These complexes showed moderate activity in asymmetric aldol addition reactions between 2-methoxy propene and different aldehydes.

Biodegradable aliphatic polycarbonates are important components of non-toxic thermoplastic elastomers, which have a variety of medical applications. Industrially, aliphatic polycarbonates derived from six-membered cyclic carbonates such as trimethylene carbonate (TMC or 1,3-dioxan-2-one) are produced via ring-opening polymerization (ROP) processes in the presence of a tin catalyst. It is worth mentioning that TMC is readily obtained by transesterification of 1,3-propanediol with various reagents including phosgene and its derivatives. Therefore, it has been of great interest to investigate greener routes for the production of this important class of polymers. Toward this goal, the synthesis of aliphatic polycarbonates via the metal catalyzed alternative coupling of oxetanes and carbon dioxide represents an attractive alternative. The use of an abundant, inexpensive, non-toxic, and biorenewable resource, carbon dioxide, makes this method very valuable. Furthermore, in this reaction, the sixmembered cyclic carbonate byproduct, TMC, can also be ring-opened and transformed into the same polycarbonate. For over a decade, the Darensbourg research group has successfully utilized metal salen complexes as catalysts for the epoxide/CO2 copolymerization process. Hence, this dissertation focuses on the examination of these complexes as catalysts for the oxetane/CO2 copolymerization reaction and the further elucidation of its mechanism. Chromium(III) salen derivatives in the presence of an azide ion initiator were determined to be very effective catalysts for the coupling of oxetanes and carbon dioxide providing polycarbonates with minimal amounts of ether linkages. Kinetic and mechanistic investigations performed on this process suggested that copolymer formation proceeded by two routes. These are the direct enchainment of oxetane and CO2, and the intermediacy of trimethylene carbonate, which was observed as a minor product of the coupling reaction. Anion initiators which are good leaving groups, e.g. bromide and iodide, are effective at affording TMC, and hence, more polycarbonate can be formed by the ROP of preformed trimethylene carbonate. Research efforts at tuning the selectivity of the oxetane/CO2 coupling process for TMC and/or polycarbonate produced from the homopolymerization of preformed TMC have been performed using cobalt(II) salen derivatives along with anion initiators. Lastly, investigations of this process involving 3-methoxy-methyl-3-methyloxetane will be presented.

Computational chemists are concerned about two aspects when choosing between the myriad of theoretical methodologies: the accuracy (the "truth") and the computational cost (the tractability). Among the least expensive methods are the Hartree-Fock (HF), density functional theory (DFT), and second-order Moller-Plesset perturbation theory (MP2) methods. While each of these methods yield excellent results in many cases, the inadequate inclusion of certain types of electron correlation (either high-orders or nondynamical) can produce erroneous results. The compromise for the computation of noncovalent interactions often comes from empirically scaling DFT and/or MP2 methods to fit benchmark data sets. The DFT method with an empirically fit dispersion term (DFT-D) often yields semi-quantitative results. The spin-component scaled MP2 (SCS-MP2) method parameterizes the same- and opposite-spin correlation energies and often yields less than 20% error for prototype noncovalent systems compared to chemically accurate CCSD(T) results. There is no simple fix for cases with a large degree of nondynamical correlation (such as transition metal-salen complexes). While testing standard and new DFT functionals on the spin-state energy gaps of transition metal-salen complexes, no DFT method produced reliable results compared to very robust CASPT3 results. Therefore each metal-salen complex must be evaluated on a case-by-case basis to determine which methods are the most reliable. Utilizing a combination of DFT-D and SCS-MP2 methods, the reaction mechanism for the addition of cyanide to unsaturated imides catalyzed by the Al(Cl)-salen complex was performed. Various experimental observations are rationalized through this mechanism.