Date of Award

January 2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Guodong Du

Abstract

This current work primarily focuses on the fundamental study of zinc complexes of amido-oxazolinate ligands that have shown a great deal of promise in their ability to catalyze various polymerization reactions. A family of new C1-symmetric, monoanionic, amido-oxazolinate ligands containing oxazoline moiety has been synthesized in moderate to high yields (typically 30-95%) via a Pd-catalyzed amination reaction of chiral oxazolines with primary amines and amides. The obtained ligands were treated with Zn[N(SiMe3)2]2 to generate the corresponding C1-symmetric, monometallic zinc complexes. Single-crystal X-ray crystallographic studies confirm that most of the complexes are three-coordinate and mononuclear in nature. These complexes are viable initiators for alternating copolymerization of carbon dioxide (CO2) and cyclohexene oxide (CHO), yielding moderate isotactic poly(cyclohexene carbonate) (PCHC) with good to high carbonate linkage and moderate molecular weights and PDI values, depending on the substituents. The asymmetric induction is generally low, with up to 71% SS unit in the main chain of the produced PCHCs.

A family of new chiral C2-symmetric dinucleating amido-oxazolinate ligands that are bridged by three different linkers (m-phenylenediamine, 4-(4'-aminobenzyl)benzenamine, and 1,8-diaminoanthracene) has been synthesized in excellent yields (85-90%). Two-fold deprotonation of ligands with Zn[N(SiMe3)2]2 in dry toluene generated a series of homoleptic bimetallic zinc complexes while the reaction of ligands with one equiv. of Zn[N(SiMe3)2]2 leads to formation of bischelating zinc amido-oxazolinate complexes in decent yield. All complexes were characterized by NMR spectroscopy (1H, 13C, COSY, and HETCOR). Dinucleating zinc complexes were also found to be efficient catalysts for asymmetric alternating copolymerization of CO2 and CHO, generated highly pure isotactic poly(cyclohexene carbonate), irrespective of transformation of dinuclear complexes into mononuclear bis(amido-oxazolinate) complexes. The cooperative mechanism between the metal centers was evident by the enhanced catalytic activity of the dinucleating zinc

complexes.

These results intrigued us to investigate the efficiency and stereo selectivity of our catalytic systems for ROP of rac-lactides. We have also explored the efficiency of zinc complexes of amido-oxazolinate ligands for the ring opening polymerization of rac-lactides at mild reaction conditions. These catalysts deliver a viable synthetic route to produce highly selective steroblock PLA (Pm = 0.90), an unprecedented selectivity in the case of zinc based catalysts. The same series of zinc complexes have also shown excellent activity for ring opening copolymerization of styrene oxide and cyclic anhydrides, including succinic anhydride, maleic anhydride, and phthalic anhydride. These catalysts exhibited unprecedented TOFs 8000 h-1 for coupling of styrene oxide and maleic anhydride. The ligand geometrical parameters strongly influence the stereochemistry of the obtained poly(styrene oxide-co-anhydride)s.

In addition to this, hydrosilylation of a variety of unsaturated organic substrates, including aldehydes, ketones, and imines are effectively reduced to alcohols and amines, respectively, using a high-valent nitridoruthenium(VI) compound as a catalyst and phenylsilane as a reductant. Mechanistic studies indicate that the catalysis proceeds via silane activation rather than carbonyl activation, and the silane is likely activated via multiple pathways, including a radical-based pathway. The mechanistic investigation was carried out using a variety of spectroscopic and analytical techniques, including NMR spectroscopy, FT-IR spectrophotometry, gas chromatography (GC/FID and GC/MS), and high-resolution mass spectroscopy.

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