As part of a worldwide effort to develop efficient catalysts for use in organic chemistry and in the synthesis of highly valuable molecules, work performed during the course of my stay in St Andrews has focused on the design and synthesis of new group 11 metal complexes for their applications in catalysis. The aim of this work was to develop new, active and stable, easy to synthesise group 11 complexes and investigate their catalytic activity as well as to try to understand their mode of action. Two different types of complexes were explored in order to develop more active catalysts: the neutral N-Heterocyclic carbene metal complexes and the cationic derivatives. More than 20 new catalysts were developed and their reactivity studied in different catalytic reactions. New hydroxide and tert-butoxide copper(I) or silver(I) complexes were developed and compared to the common NHC metal systems. Overall, the neutral NHC-metal catalysts showed to be highly active in a broad range of applications: in the methylation of amines using CO₂ as a C1 source, in a multicomponent reaction (A³ coupling) and in dual catalysis (hydrophenoxylation). Additionally, mechanistic studies were undertaken to obtain a greater understanding of these transformations and to possibly lead to the design of new generations of catalyst. Regarding the cationic NHC metal complexes, a straightforward methodology was developed leading to a library of highly stable catalysts. Bis-NHC, mixed NHC/ phosphine as well as NHC/pyridine species were efficiently synthesised using thermal or microwave heating, in high purity and yields. In addition, the effect of the presence of two different or identical ligands on catalytic reactivity was investigated in the 3+2 cycloaddition and in the alkynylation of ketones. Insight into the catalytic cycle was obtained via mechanistic studies. These showcased the release of one ligand during the catalytic cycle and the crucial role of this ligand displacement in generating the catalytically relevant active species. The results highlight the importance of understanding the reactivity of catalyst in order to develop new and improved ones.

No description available.

No description available.

SERS (surface-enhanced Raman spectroscopy) has the potential to be used in a variety of commercial and basic applications, which often rely on molecules that are bound to a nanostructured metal surface. Thiols are usually used as the intermediate to modify the substrate surface for SERS. In recent years, N-heterocyclic carbene (NHC) has been introduced as an alternative approach for metal surface modification. Nanostructured gold surfaces suitable for SERS had been modified by NHC species. Those studies showed the promising of the NHC modification route for the fabrication of a robust platform for SERS. The objective of this work is to explore the SERS characteristics of NHC species on silver surfaces. The interactions between two different NHC molecules and a nanostructured silver surface, instead of a gold surface, were studied for the first time. The experiments were realized in electrochemical conditions, using a three-electrodes system, to fully test the stability of the NHC-modified surfaces. The SERS spectra were compared to theoretical calculations and normal Raman in order to identify the vibrational characteristics of the NHC molecules. The effects of different NHC molecule substituents on the electrochemical stability of the surface were also discussed. The results showed that NHC molecules can be decomposed on the silver surface easily under electrochemical conditions. This contrast with the observations in gold, where the NHC monolayers showed a high level of stability. This work also discusses potential side products which may be derived from the decomposition of the NHC molecules. Raman spectra of potential side products were collected and compared to the NHC SERS collected under electrochemical control at different potentials. This study provides insights into the influence of the substituents at the NHC on their stability under the electrochemical condition, which should guide the development of future applications. / Graduate

This thesis describes the development of nickel(I) complexes incorporating ring expanded N-heterocyclic carbene (RE NHC) ligands and examines their electronic characterisation, activation of O2, reactivity and catalytic applications. A series of three coordinate, paramagnetic Ni(I) complexes of the form Ni(RE NHC)(PPh3)Br (1 – 10) were prepared by comproportionation of Ni(COD)2 and Ni(PPh3)2Br2 in the presence of RE NHCs. The RE NHCs employed varied in the degree of ring expansion (6-, 7- and 8-membered), extent of N-substituent steric bulk (Mes, oTol, oAnis) and the donor/acceptor properties of the carbene (diamino vs. diamido). EPR and DFT electronic characterisation of 1 – 10 confirmed that the unpaired electron was located on the nickel ion in a mixed orbital of predominantly 3dz2 character. Yellow solutions of 1 or 6 (RE NHC = 6Mes and 7Mes respectively) immediately became purple in the presence of O2 due to O2 activation and incorporation of the oxygen atoms as bridging ligands resulting from C-H activation/oxygenation of an RE NHC N-substituent. This generated the dimeric Ni(II) complexes Ni(6/7Mes)Br(µ-OH)(µ-O-6/7Mes)’NiBr (6Mes = 13; 7Mes = 14). Mass spectrometry demonstrated that the doubly activated complexes [NiBr(µ-O-6/7Mes)’]2 (15 and 16 respectively) were also formed in the reactions. UV-vis spectroscopy revealed the reactions took place rapidly, even at 190 K. Contrasting reactivity was observed when 2 or 7, bearing the less sterically encumbered N-oTol substituents 6oTol and 7oTol respectively, were exposed to O2, which led to the ligand redistribution products NiII(6/7oTol)(PPh3)Br2 (17 and 18 respectively). The less electron rich diamido analogue containing 6MesDAC (5), underwent dissociation and oxidation of the RE NHC and PPh3 ligands. Attempts to abstract the bromide from 1 generated novel two and three coordinate Ni(I) products. Reaction with additional 6Mes produced the two coordinate cation [Ni(6Mes)2]+ (19), which could be reduced with KC8 to Ni(6Mes)2 (20). SQUID analysis of 19 revealed it to be the first example of a nickel containing mononuclear single molecular magnet (SMM). Addition of [Et3Si]+ to 1 followed by work up in toluene led to the isolation of the Ni(I)-(η2-toluene) complex [Ni(6Mes)(η2-C6H5CH3)]+ (21). Mesitylene generated the analogous [Ni(6Mes)(η2-C6H3(CH3)3)]+ (23), but neither 1,4-xylene nor naphthalene gave isolable products. In all cases, cocrystallisation of [6MesH]+…arene was observed in variable amounts, which compromised reaction studies of the Ni-arene complexes. Removal of bromide from 1 with TlPF6 in THF generated the solvent coordinated cationic species [Ni(6Mes)(PPh3)(THF)]+ (24). Attempts to remove the ligated THF molecule were unsuccessful, however, it could be directly substituted by CO to form [Ni(6Mes)(PPh3)(CO)]+ (26). Similarly to 1, complex 24 activated O2, generating a dimer analogous to the singly activated complex 13 (Ni(6Mes)(PPh3)(µ-OH)(µ-O-6Mes)’NiBr (25)). Reactivity of 1 with NaBH4 produced [Ni(6Mes)(κ2-BH4)]2 (28), a Ni(I) dimer bridged by two BH4 ligands. The catalytic efficiency of neutral 1 in Kumada cross-coupling of aryl halides and PhMgCl or MesMgBr was probed. Of note was the high activity towards both aryl chlorides and aryl fluorides. Comparisons with cationic 24, larger 7- (7) and 8-membered ring (8 and 9) variants and the Ni(II) complexes Ni(6Mes/6oTol/7oTol)(PPh3)Br2 (29, 17 and 18 respectively) revealed that 1 exhibited the highest reactivity of all the precursors.

Methodologies for expanding the architectural diversity of -N-heterocyclic carbene (NHC) pincer ligand precursors and transition metal complexes have been developed for the production of more efficient catalyst, which will be employed in the synthesis of pharmaceuticals. An efficient route for the synthesis and isolation of bis-1,3-(3'-aryl-N-heterocycl-1'-yl)arenes has been established, and preliminary data for metalation and transmetalation of a N, N'-diaryl imidazolium salt has been acquired. Additionally, a proficient methodology for the synthesis of mixed, unsymmetrical -NHC pincer ligand precursors has been discovered, and preliminary data for mixed, unsymmetrical transition metal complexes is also included. These methodological expansions will lead to more efficient catalyst that decrease the expenditure of energy required for the synthesis of pharmaceuticals, thus making their synthesis more favorable for the environment and their price more affordable for consumers.

The recent development of materials with autonomous repair capabilities has opened an exciting new field of polymer science expected to impact nearly every facet of modern society. Similar to natural systems, these "self-healing" materials sense when their structural integrity has been compromised (e.g., due to wear or damage) and respond with a viable repair mechanism. Despite the extraordinary number of successes and advances in this area, a means to ascertain instantaneous knowledge of a material's structural integrity, and more importantly, when it has been compromised, remains a considerable challenge in current systems and materials. To address this challenge, we report recent efforts toward the development of an electronically conductive material that is structurally dynamic and responds to various types of external stimuli. In particular, we have developed new synthetic methodology to prepare a variety of organometallic polymers containing a novel benzobisimidazolylidene or bis(benzoimidazolylidene) ligand, which is comprised of two linearly opposed N-heterocyclic carbenes (NHCs) annulated to a common linker, and various types of transition metals in the polymer's main-chain. Using this approach, polymers with molecular weights up to 10⁶ Da were prepared and cast into robust thin films. Using four-point probe technique, the inherent conductivities of these materials were found to be on the order of 10⁻³ S/cm. Secondly, the dynamics of these polymers were probed in solution using gel permeation chromatography. At specific cross-linker loadings, thermally-responsive gels were obtained. Collectively, these experiments suggested that the essential features for a thermally-responsive, structurally dynamic, conjugated organometallic polymer were developed. Efforts toward probing their ability to display self-healing characteristics in the solid-state are described. The inherent conductivity of the polymers permitted the healing behavior of thin films to be observed by scanning electron microscopy in the absence of a dopant. Long range goals of implementing and utilizing these materials in electronic circuits and other advanced devices are also described. An additional approach towards a dynamic material utilized functional imidazolium-based ionic liquids. A series of functional ionic liquids were produced by appending N-substituents containing pendant halides, alkynes, azides, furans and maleimides. These functional groups allowed for polymerization and crosslinking. The physical properties of the imidazolium monomers, as well as the resulting polymers, could be tuned by altering the anion. When a trifunctional monomer is used in conjunction with the polymerization of difunctional ionic liquids an insoluble crosslinked material forms. This behavior, combined with NHCs ability to bind transition metals as ligands and catalyze various organic transformations, provides potential for this system to be used as a method for catalyst recovery and ultimately catalyst recycling. / text

Chemistry / The efforts described in this thesis were bifurcated along two distinct projects, but generally were directed toward the development of new materials to solve outstanding issues in contemporary electrochemical applications. The first project involved the synthesis and application of redox-switchable olefin metathesis catalysts. N-heterocyclic carbenes (NHCs) bearing ferrocene and other redox-active groups were designed, synthesized, and incorporated into model iridium complexes to evaluate their intrinsic electrochemical and steric parameters. Using these complexes, the ability to switch the electron donating ability of the ligands via redox processes was quantified using a variety of electrochemical and spectroscopic techniques. The donicity was either enhanced or attenutated upon reduction or oxidation of the redox-active group, respectively. The magnitude of the change in donicity upon reduction or oxidation did not vary significantly as a function of the proximity of the redox-active group from the metal center. Thus, other factors, including synthetic considerations, sterics, and redox potential requirements, were determined to guide ligand design. Regardless, redox-active NHCs were adapted into ruthenium-based olefin metathesis catalysts and used to gain control control over various ring-opening metathesis polymerizations and ring-closing metathesis reactions. The second project was focused on the development of new basic polymers for acid/base crosslinked proton exchange membranes intended for applications in direct methanol fuel cells. Polymers containing pendant pyridinyl and pyrimidinyl groups were obtained via the post polymerization functionalization of UDEL® poly(sulfone) and then blended with sulfonated poly(ether ether ketone) (SPEEK). Fuel cells containing these blends were found to exhibit reduced methanol crossover, higher open circuit voltages, and higher maximum power densities compared to plain SPEEK. The differences in fuel cell performance were attributed to the basicity and sterics of the pendant N-heterocycles. / text

Thesis advisor: James P. Morken / Sesquiterpene quinones are a prolific class of marine natural products that are particularly interesting due to their antibacterial, antiviral, and anti-inhibitory properties. Hundreds of these biologically active molecules are based on decalin frameworks, both cis- as well as trans-fused, however, significantly less synthetic work has focused on targeting the cis-fused series of compounds. In this chapter, progress towards an asymmetric, general route to various sesquiterpene quinones in the cleordane family of natural products will be described. The key steps of the synthesis include a highly convergent and diastereoselective reductive alkylation to forge both the requisite cis-ring fusion well as the all carbon quaternary center, as well as a scandium-catalyzed ring expansion of a 6,5-ring system to deliver the decalin core of the molecule. Additionally, the chapter includes the development and substrate scope of both methodologies utilized in the key complexity building reactions. Iron complexes ligated by bis(imino)pyridine ligands are remarkably active catalysts for a vast range of organic transformations including polymerization, hydrogenation, hydrosilylation, and hydroboration. Whereas much work has been done to probe the importance of the imine-substituents on catalysis, significantly less information is known about the nature of the central pyridine donor. To study the effects of a more donating ligand in which the pyridine is replaced with an N-heterocyclic carbene, a series of novel ligands and their corresponding iron complexes were synthesized and characterized. Whereas imidazole-derived complexes exhibited exclusively bidentate binding modes, 4,5,6-trihydropyrimidylidene-based ligands adopted a tridentate pincer conformation analogous to complexes of bis(imino)pyridines. Bonding in the five-coordinate bis(imino)-N-heterocyclic carbene complex displayed considerably contracted iron-ligand bond distances compared to the analogous bis(imino)pyridine iron complex. The study of physical and electronic structure and bonding in organometallic compounds is a critical for understanding and predicting complex behavior and reactivity. Having synthesized a completely new type of N-heterocyclic carbene (NHC) ligand and the corresponding iron complex, a rigorous study of metal-NHC bonding, magnetism, and redox activity in bis(imino)-NHC (or carbenodiimine, CDI) complexes of iron was carried out. A series of oxidation and reduction reactions on CDI complexes of iron were performed, enabling access to complexes spanning from formally iron(0) to iron (III) oxidation states. A battery of spectroscopic and computational methods, including X-ray crystallography, Mössbauer spectroscopy, SQUID magnetometry, and EPR spectroscopy established the CDI ligand as a redox active chelator. Additionally, a unique iron-carbene interaction was discovered, in which the metal center antiferromagnetically couples with the carbon of the NHC. Intent on developing CDI complexes of iron into practical catalysts for both synthetic organic transformations and polymerization, a series of stoichiometric as well as catalytic reactions were carried out to evaluate the reactivity profile of the novel complexes. Halide atom abstraction generated a new cationic species, which demonstrated different coordination chemistry compared to the bis(imino)pyridine analogue. Furthermore, the addition of a hydride or alkyl lithium reagent to the parent (CDI)FeCl2 species resulted in interesting and unexpected reactivity involving the carbene ligand. Preliminary catalytic hydrogenation experiments established (CDI)FeCl2 as a competent catalyst for the reduction of simple alkenes in the presence of Na(Hg) as a reductant under 80 psi of hydrogen. Additionally, the dichloride species could be readily converted into bis(aryloxide) complexes that were active for the polymerization of lactide to produce poly(lactic acid). The polymerization is very controlled (PDI values are <1.3), and polymers with molecular weights of around 35 kDa can be obtained after 3 hours at room temperature. / Thesis (PhD) — Boston College, 2014. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Thesis advisor: Amir H. Hoveyda / Chapter 1. A Review of Catalytic Enantioselective Allylic Substitution (EAS) with Chiral Sulfonate Containing N-heterocyclic Carbenes (NHC). A comprehensive review of enantioselective allylic substitution reactions, which are promoted by a chiral N-heterocyclic carbene metal complex that features a unique sulfonate motif, is provided in this chapter. Reactions are classified into two categories. One class of transformations is catalyzed by a series of easily modifiable sulfonate bearing NHC-Cu complexes, with which a range of nucleophilic organometallic reagents (i.e., organozinc-, aluminum-, magnesium- and boron-based) that carry different carbon-based units are readily utilized in efficient and highly selective C-C bond forming processes. Another set of reactions exclude the use of a copper salt; catalytic amount of a sulfonate containing imidazolinium salt is capable of promoting additions of alkyl Grignard, zinc and aluminum species to easily available allylic electrophiles in a site- and enantioselective fashion. The mechanistic scenarios of both catalytic systems that account for the observed experimental data are discussed in detail. Chapter 2. Cu-Catalyzed Enantioselective Allylic Substitutions with Aryl- and Heteroarylaluminum Reagents. In this chapter, the first examples of EAS reactions of aryl- and heteroaryl-substituted dialkylaluminum reagents to a wide range of trisubstituted allylic phosphates are demonstrated through a facile and selective catalysis rendered possible by an in situ generated sulfonate containing NHC-Cu complex, delivering enantiomerically enriched olefin products that bear an all carbon quaternary stereogenic center. The requisite organometallic species are easily prepared from either the corresponding aryl- and heteroaryl halides, or through efficient and site selective deprotonation at the C-2 position of furan and thiophene; such aluminum entities are readily used in situ without the requirement of purification. Application to small molecule natural product synthesis is also carried out to illustrate the utility of the present protocol. Chapter 3. Cu-Catalyzed Enantioselective Allylic Substitutions with Alkenylaluminum Reagents. This chapter focuses on our research towards construction of enantioenriched tertiary and quaternary stereogenic centers that are substituted with two further functionalizable alkenes. The first combination of the study involves the addition of stereochemically well-defined trisubstituted alkenylaluminum reagents to disubstituted allylic phosphates; the transformation commences with a silyl-directed stereoselective hydroalumination and finishes with an enantioselective Cu-catalyzed EAS promoted by a sulfonate bearing NHC. Such reactions deliver molecules that feature silicon containing trisubstituted olefin adjacent to the tertiary stereogenic center; subsequent conversion of the versatile silicon group to a proton reveals the first set of examples that incorporate pure Z alkene in Cu-catalyzed EAS. The stereoselective and concise synthesis of naturally occurring small molecule nyasol demonstrates the utility of the above method. On a different front, Ni-catalyzed site-selective hydroalumination of terminal alkynes has opened new possibility of introducing 1,1-disubstituted olefins in Cu-catalyzed EAS in the formation of tertiary stereogenic center containing enantioenriched organic building blocks. Such catalytic hydrometallation procedure also allows efficient access to alkenylaluminums that are derived from the conventionally problematic aromatic alkynes. The importance of efficient and selective synthesis of terminal aryl-substituted alkenylaluminum species is showcased in NHC-Cu-catalyzed EAS reactions that construct all-carbon quaternary stereogenic centers; a three-step convergent synthesis of natural product bakuchiol in enantiomerically enriched form highlights the potential of the current protocol in chemical synthesis. Chapter 4 Cu-Catalyzed Enantioselective Allylic Substitutions with Alkenylboronic Acid Pinacol Ester Reagents and Applications in Natural Product Synthesis. Within this chapter, we disclose the efficient utilization of alkenylboron reagents in Cu-catalyzed EAS reactions, which lead to highly site and enantioselective formations of molecules that contain both tertiary and quaternary carbon stereogenic centers. Unlike their aluminum-based counterparts, the use of boron-based reagents allows effective delivery of sensitive organic function groups, such as a carbonyl, which would be incompatible in the hydrometallation process with dibal-H. Our efforts accumulate to the first report of incorporation of all carbon quaternary centers that are substituted with unsaturated ester and aldehyde units in the EAS products; such a method facilitates the concise diastereo- and enantioselective synthesis of Pummerer's ketone and it's trans isomer. Further development of the above protocol towards the construction of tertiary stereogenic centers requires the design of new chiral sulfonate-containing imidazolinium salts as the ligand precursors and has lead to the employment of a broader range of alkenylboron species, which feature readily functionalizable motifs. Subsequent demonstrations in enantioselective synthesis of a variety of small molecule natural products showcase the utility. / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.