In recent years, N-hetereocyclic carbenes (NHCs) have received considerable attention as organocatalysts due to their unusual ability to induce a reversal of reactivity (Umpolung) in aldehydes. Indeed, NHCs unique properties have been applied to the efficient and metal-free synthesis of organic compounds that have proven elusive using traditional approaches.<p> My Masters research program has been focused on the use of NHCs as organocatalysts in ring expansion reactions and their applications to cascade reactions.<p> During my Masters studies, an NHC-catalyzed efficient ring expansion of 4-, 5-, and 6-membered oxacycloalkane-2-carboxaldehydes to generate the corresponding lactone derivatives was developed. This reaction provides access to a variety of lactones using readily available NHCs under mild conditions.<p> Then, the ring-expansion lactonization has been successfully extended to an efficient lactamization using azacycloalkane-carboxaldehydes, which could provide functionalized lactams in moderate yields under mild conditions.<p> In addition, intrigued by the possibility of effecting the Umpolung of electron-poor dienes using NHC catalysts, the ring-expansion lactonization was applied to an attempted Diels-Alder-ring expansion cascade reaction. Though no cascade reactions were observed, some very interesting results were obtained, and those results will guide future investigations in this area.

1. Generation of nitrenes by means of photolysis of arylmethylazides and its derivatives have been studies. Pyrolysis of 2-azidomethylthiophene¡]44a¡^ gave 2-thiophenecarboxaldehyde¡]77a¡^and (2-thienylmethylidene)-2-thienylamine¡]45a¡^, and pyrolysis of 2-azidomethylbenzo[b]thiophene¡]44b¡^gave the corresponding products. Pyrolysis of 2-azido-1-(2-thienyl)ethanone¡]52a¡^gave 2-thiophenecarboxaldehyde¡]77a¡^, 2-acetylthiophene¡]80a¡^and 2-(thiophene-2-carbonyl)amino-1-(2-thienyl)ethanone¡]83a¡^, and pyrolysis of 2-azido-1-(2-benzo[b]thienyl)ethanone¡]52b¡^gave the corresponding products. 2. Pyrolysis of 3-cyclohexeno[b]furylmethyl benzoate¡]35¡^gave cyclohexeno-4-methylenecyclobuten-3-one¡]25¡^via highly reactive carbene intermediate. At high temperature, compound 25 can continue the reaction of elimination and ring opening to give benzene¡]43¡^, fulvene¡]46¡^, 2-ethylnylcyclohex-1-ene carbaldehyde¡]44¡^ and 4,5-dimethylenecyclopent-2-enone¡]45¡^.

The use of N-heterocyclic carbenes (NHCs) as ligands in catalysis is one of the most significant developments in modern catalysis and organometallic chemistry. One way to extend the scope of NHC ligand tuning is by means of annulation of carbocyclic and heterocyclic rings to the NHC backbone. The bis(imino)acenaphthene-supported N-heterocyclic carbene [IPr(BIAN)] has been synthesized and can be regarded as originating from the fusion of a naphthalene ring to an NHC. Several metal complexes of IPr(BIAN), including those incorporating copper(I), silver(I), gold(I), or iridium(I) have been synthesized and characterized, including single-crystal X-ray diffraction studies. The doncity of IPr(BIAN) was investigated using the Tolmen Electronic Parameter (TEP) method. A TEP value of 2042 cm-1 was calculated for the IPr(BIAN) ligand using the Ir(CO)2Cl complex which indicates that IPr(BIAN) is a relatively strong electron donating NHC ligand. The well-behaved redox chemistry of the BIAN ligand class rendered IPr(BIAN) an excellent candidate for exploration of the relationship between ligand charge and carbene donicity. The electrochemical reduction of IPr(BIAN) was studied by cyclic voltammetry (CV) in a THF solution and a reversible reduction wave was detected at - 1.79 V vs SCE. Spectroelectrochemical IR studies were also undertaken to further characterize the nature of the reduced state. IPr(BIAN) was found to be a stronger electron donating ligand in the reduced state in comparison with the neutral state of the ligand. IPr(BIAN) was also chemically reduced using potassium graphite and the resulting radical anion was studied by electron paramagnetic resonance (EPR) techniques. An isotropic EPR signal was observed at a g value of 2.0112. Due to the known antimicrobial activities of silver and gold NHCs, the activities of the silver and gold complexes of IPr(BIAN) and the imidazolium salts of several BIAN ligands were investigated using the minimum inhibitory concentration test. The silver(I) and gold(I) complexes of IPr(BIAN) were found to be moderately active. The most active compounds were found to be the imidazolium salts, with MIC values ranging between < 0.6 μg/mL and 78 μg/mL for the diisopropylphenyl(BIAN) and the mesityl(BIAN) imidazolium chlorides against S. aureas, B. subtilis, E. coli, and P. aeruginosa. The preparation of nanofibers impregnated with IPr(BIAN)AuCl by the process of electrospinning was also explored. The antimicrobial activities of the resulting nanofiber mats were determined on the basis of the inhibition zone test, and a localized antimicrobial activity was observed for the Gram-positive bacteria M. leuteus. / text

In an effort to develop broadly applicable photoswitchable catalysts, we have reported a method for modulating N-heterocyclic carbene (NHC) donicity using light by incorporating a photochromic diarylethene (DAE) into the backbone of a NHC scaffold. UV irradiation of 4,5-dithienylimidazolone or an analogous NHC-Ir(CO)₂Cl complex effected a photocyclization between the two thiophene rings, which led to a change in the electron donating ability of the NHC scaffold. Subsequent exposure to visible light reversed the photocyclization reaction. The concept of photo-modulating NHC donicity in this manner enabled photoswitchable NHC organocatalysis. The catalytic activity of a DAE-annulated imidazolium pre-catalyst in transesterification and amidation reactions was successfully switched between the active and nearly inactive states ([kappa]vis/[kappa]UV = 100) upon alternate UV ([lambda]irr = 313 nm) or visible ([lambda]irr > 500 nm) irradiation. The photoswitchable NHC organocatalysis was later extended to facilitating ring-opening polymerizations of cyclic esters, the rates of which were controlled via external light stimuli. Additionally, a photochromic dithienylethene-annulated N-heterocyclic carbene (NHC)-Rh(I) complex was synthesized and enabled photoswitching of the catalytic activity in series of hydroboration reactions. All of the examples demonstrate extremely rare instances of photomodulating a catalyst's activity by tuning its electronic properties. Furthermore, by taking advantage of the versatility of NHCs in both organo- and organometallic catalysis, we have developed novel photoswitchable catalysts for a variety of applicable transformations. Nanoparticles that can be transported in subsurface reservoirs at high salinities and temperatures are expected to have a major impact on enhanced oil recovery and electromagnetic imaging. We have developed an approach that will facilitate nanopaticle transport through porous media at high salinity by adsorbing or grafting rationally designed co-polymers on platform nanoparticles. Notably, co-polymers of acrylic acid with either 2-acrylamido-2-methylpropanesulfonate or styrenesulfonate have been electrostatically adsorbed or covalently grafted onto iron oxide nanoclusters. The presence of sulfonate groups on the iron oxide surface enabled long-term colloidal stability of the particles in extremely concentrated brine (8% wt. NaCl + 2% wt. CaCl₂) at elevated temperatures (90 °C) and minimized their adsorption on model mineral surfaces. / text

No description available.

Despite previous extensive study of the widely-employed ruthenium-catalysed olefin metathesis reaction, the finer mechanistic details have not been elucidated. An area that is noticeably lacking is spectroscopic exploration of the relevant complexes. In this work, organometallic ruthenium complexes of importance to olefin metathesis have been investigated using Ru K-edge X-ray absorption spectroscopy. The lowest energy feature in the Ru K-edge spectrum has been unambiguously assigned as due to Ru 4d←1s transitions. These electric-dipole-forbidden transitions are extremely sensitive to geometry. For centrosymmetric complexes, the pre-edge feature has very low intensity because it is limited by the weak electric quadrupole mechanism. By contrast, non-centrosymmetric complexes exhibit a substantial increase in pre-edge intensity because Ru 5p-4d mixing introduces electric-dipole-allowed character to the Ru 4d←1s transitions. The energy of the edge feature in the Ru K-edge spectrum corresponds to ionisation of 1s electrons and is a good indicator of the charge on the metal centre. Unexpectedly, we found that the first-generation (L = PCy₃) Grubbs precatalyst (1) has a higher 1s ionisation energy than the second-generation (L = H₂IMes) complex (2). This effect provides a compelling rationale for the unexplained differences in phosphine dissociation kinetics for complexes 1 and 2: the phosphine dissociation rate of 2 is slower than 1 because the metal centre is more electron-deficient in 2. Density functional theory calculations confirm the charge differences and offer some insight into the nature of bonding in these complexes, particularly with regard to the N-heterocyclic carbene and trialkylphosphine ligands. On the basis of these results, we propose that, for this system, the NHC ligand is a weaker σ-charge donor than the phosphine ligand, and that the NHC accepts significant π-electron density from the metal; both interactions function to reduce the electron density on the ruthenium centre. An ultimate goal is to investigate reactive species in the olefin metathesis mechanism; accordingly, we have made considerable progress toward collecting XAS data for a metallacyclobutane species, and we are pursuing methods to trap the four-coordinate intermediate in the metathesis cycle.

Els compostos organoborats són intermedis de reacció molt valuosos en síntesi orgànica, degut a que l'enllaç carboni-bor pot ser derivatitzat de múltiples maneres. La diboració catalitzada d'alquins i alquens ha estat ampliament estudiada en els darrers quinze anys, essent la diboració catalitzada d'alquins un procés en el que s'han obtingut elevats rendiments i activitats. No obstant això, en la diboració catalitzada d'alquens la presència de la reacció secundaria de b-eliminació de H sempre s'ha presentat com un seriós inconvenient, impedint un bon compromís entre activitat catalítica i quimioselectivitat. En el primer capítol de la present tesi es recull l'evolució a través de la bibliografia de les reaccions de diboració d'alquens i alquins, els diferents metalls i lligands utilitzats en la reacció catalítica de diboració i els estudis mecanístics que s'han realitzat fins a data d'avui. Tanmateix, s'han posat de manifest les diferents derivatitzacions que s'han dut a terme a partir d'intermitjos organoborats. També es descriu breument la tècnica de les microones. Com a punt final del capítol, s'introdueixen els objectius de la tesi, incloent-hi el desenvolupament de nous sistemes catalítics que millorin l'activitat, quimioselectivitat i enantioselectivitat dels catalitzadors reportats prèviament en la reacció de diboració catalítica, l'estudi del mecanisme de la reacció de diboració d'alquens catalitzada per rodi, i la busqueda de nous mètodes de fluorofuncionalització d'esters vinic(bisboronics). En el segon capítol es descriu l'activitat, quimioselectivitat i enantioselectivitat de diferents precursors catalítics en la diboració catalitzada d'alquens i alquins. En el primer apartat es porta a terme un estudi en profunditat de la diboració d'alquens catalitzada per compostos de Rh(I), observant-se que en aquest cas els efectes estèrics en el diborà tenen un efecte dramàtic en la quimioselectivitat de la reacció. També s'observa que el lligand que ofereix una major quimioselectivitat es el DPPM (bis(difenilfosfino)metà), mostrant una influència del bite angle del lligand, mentre que el lligand QUINAP (1-(2-difenilfosfino-1-naftil)isoquinolina) és el que ofereix una major enantioselectivitat. En els següents apartats es descriu la utilització de diferents complexos d’or, argent, coure i platí modificats amb lligands carbens, els quals augmenten la quimioselectivitat de la reacció, reduïnt la producció de subproductes de b-eliminació de H. Malgrat que s’han utilitzat diferents carbens quirals, només en un cas s’ha aconseguit induir asimetria, utilitzant un complex de Cu(I) modificat amb un lligand carbé quiral, però comprometent la quimioselectivitat. També es descriu l’aplicació de carbens de platí i coure a la diboració catalitzada d’alquins, obtenint-se bons resultats d’activitat i quimioselectivitat. La utilització d’unes noves condicions de reacció, en les que es requereix un excés de diborà (2 eq.) i l’addició d’una base (NaOAc), fa que precursors catalítics en principi inactius, com complexos d’Au(I) modificats amb lligands difosfina tipus BINAP (2,2'-bis(difenilfosfino)-1,1'-binaftil), donin bons resultats d’activitat i quimioselectivitat, encara que malauradament no indueixen asimetria. Per últim, es descriu l’aplicació de les tècniques de microones com a mitjà d’acceleració en la diboració d’alquins catalitzada per Pt(0), disminuint-se espectacularment els temps de reacció. En el tercer capítol de la tesi es porta a terme un estudi mecanístic de la reacció de diboració d’alquens catalitzada per Rh(I)-QUINAP. En primer lloc es porta a terme un estudi de RMN (Ressonància Magnètica Nuclear) per a detectar les possibles espècies metàl·liques implicades. A partir d’aquí, es realitza un estudi computacional DFT (Density Functional Theory) del mecanisme de reacció, observant-se que després de l’addició oxidant es produeix la inserció de l’alqué en un enllaç Rh-B, seguida d’un reordenament per a ocupar la posició vacant creada, finalitzant amb la eliminació reductora del producte, essent el camí més favorable aquell en el qual l’alquè queda coordinat trans al nitrogen del lligand QUINAP. L’estudi de la reacció secundària de b-eliminació de H demostra que la utilització de BINAP com a lligand l’afavoreix, en comparació amb la utilització del lligand QUINAP. En el quart capítol es descriu la flurofuncionalització d’ésters vinil (bisborònics), la qual dóna lloc a la formació de cetones a,a-difluorades a través d’un procés de fluoració electròfila. Primer de tot, es fa una petita introducció als processos de fluoració electròfila, amb especial interès en la fluoració electròfila de compostos organosilats, els quals estan força relacionats amb els compostos organoborats. La reacció es duu a terme a partir dels alquins, a través d’un procés tandem de diboració catalítica/fluoració electròfila. Només els esters vinil(bisborònics) derivats del bis(pinacolato)diborà són susceptibles d’ésser derivatitzats d’aquesta manera. Els alquins interns són més actius que els alquins terminals. També es descriu la síntesi d’a,a-difluoroimines directament a partir d’alquins mitjançant un procés tandem de diboració catalítica/fluoració electròfila/iminació, l’eficiència del qual depen de les propietats electròniques del sustrat. Per últim, en el capítol 5 es descriu la fluoració electròfila asimètrica d’a- nitroésters, la qual es porta a terme mitjançant la utilització d’auxiliars quirals derivats d’alcaloids de cincona, obtenint-se excessos enantiomèrics de fins a un 40%. / Organoboron compounds are very useful intermediates in organicsynthesis, because the carbon-boron bond can be cleaved in a variety of ways leading to the formation of useful functional groups. The catalyzed diboration ofalkenes and alkynes has been widely studied in the last 15 years, obtaining high yields and activities in the alkyne catalyzed diboration reaction. However, when alkenes are used as substrates in the catalyzed diboration reaction, the problem of b-hydride elimination could arise, preventing a good agreement between catalytic activity and chemoselectivity. In the first chapter of this thesys an overview of the precedents of the diboration reactions of alkenes and alkynes is presented, including the different metals and ligands used in this reactions and the mechanistic studies published to date. Moreover, there has been collected the different derivatizations of organoboron intermediates carried out. The microwave technique is also described briefly. Finally, the scope of this thesys is explained, including the development of new catalytic systems which improve the activity, chemoselectivity and enantioselectivity of the catalytic systems previously reported, the study of the mechanism of the rhodium catalyzed alkene diboration reaction, and the search of new routes for the fluorofunctionalization of organoboron compounds. In the second chapter, the activity, chemoselectivity and enantioselectivity of different catalytic precursors in the alkene and alkyne catalytic diboration reaction is described. In the first part, a deep study on the rhodium catalyzed alkene diboration reaction is carried out, finding in this case that the steric effects on the diborating reagent have a dramatic effect on the chemoselectivity of the reaction. It is also observed that the DPPM (bis(diphenylphosphino)methane) is the ligand which provide a better chemoselectivity, showing an important bite angle influence on the ligand, while QUINAP (1-(2-diphenilphosphino-1-naphthyl)isoquinoline) is the ligand which offers a higher enantioselectivity. In the next parts it is described the utilization of different gold, silver, copper and platinum complexes as catalyst precursors, which improve the chemoselectivity of the reaction, reducing the b-hydride elimination side reaction. Despite several chiral carbene modified complexes have been used, only in one case some enantioselectivity was induced, using a carbene modified copper complex, but reducing chemoselectivity. It is also described the application of arbene modified copper and platinum complexes as catalyst precursors in the alkyne diboration reaction, obtaining good results in activity and chemoselectivity. The utilization of new reaction conditions, in which an excess of the diborating reagent (2 eq.) and the addition of a base (NaOAc) is required, improve the activity of catalytic systems like BINAP 2,2’-bis(difenilfosfino)-1,1’-binaphthyl) modified gold complexes, whose activity was very low under the typical conditions; unfortunately, no enantioselectivity was obtained in this case. Finally, it is described the application of microwave techniques to the platinum catalyzed alkyne diboration reaction, in order to reduce the reaction times.In the third chapter, an in-depth study of the mechanism of the Rh(I)-catalyzed alkene diboration reaction is described. First of all, an NMR (Nuclear Magnetic Ressonance) study was carried out in order to identify plausible intermediates. Next, a DFT (Density Functional Theory) study of the reaction mechanism was carried out, finding that after the oxidative addition of the diborane, an insertion of the alkene into a Rh-B bond is produced, followed by an internal rearrangement in order to ocupy the vacant position created, and,finally, reductive elimination of the product is produced, being the most favourable path that in which the alkene is placed trans to the nitrogen of the QUINAP ligand. The study of the b-hydride elimination side reaction shows that the utilization of BINAP as ligand favours it, with respect to the utilization of QUINAP. In the fourth chapter, the fluorofunctionalization of cis-1,2-bis(boryl)alkenes is described, leading to the formation of a,a-difluorinated ketones through an eletrophilic fluorination process. First of all, a little introduction to the electrophilic fluorination processes is made, with special interest to the electrophilic fluorination of organosilanes, which are quite similar to the organoboron compounds. The reaction is carried out starting from alkynes, through a tandem catalytic diboration/electrophilic fluorination process process. Only the cis-1,2-bis(boryl)alkenes derived from bis(pinacolato)diboron are susceptible to the fluorination process. Internal alkynes are more reactive than terminal ones. It is also described the synthesis of a,a-difluoroimines directly from alkynes through a tandem catalytic diboration/electrophilic fluorination/imination process, the efficiency of which depends on the electronic properties of the substrate.Finally, in the fifth chapter, the asymmetric electrophilic fluorination of a-nitro esters is described. This process was carried out using cinchona derivatives chiral auxiliaries, obtaining enantiomeric excesses up to 40%.

No description available.

Ruthenium - catalyzed olefin metathesis is now an invaluable tool in organic synthesis. However, routes to the dominant metathesis catalysts, the second - generation Grubbs and Hoveyda catalysts (RuCl 2 (PCy 3 )(H 2 IMes)(=CHPh) and RuCl 2 (H 2 IMes)[= CH( o - O i Pr)C 6 H 4 ], respectively) are plagued with problems. The common reliance on in situ methods to generate the N - heterocyclic carbene H 2 IMes severely limits stoichiometric control, and results in contamination by byproducts, some of which are readily overlooked, and some of which are difficult to remove. Both can affect batch - to - batch reproducibility in catalysis. This thesis work demonstrated that widespread perceptions of the instability of free H 2 IMes are erroneous, and that the free carbene is readily handled under water - free conditions. Clean, convenient, near - quantitative routes were developed to these second - generation catalysts by ligand exchange of their first - gen eration counterparts RuCl 2 (PCy 3 ) 2 (=CHPh), RuCl 2 (PCy 3 )[= CH( o - O i Pr)C 6 H 4 ] with free H 2 IMes, with sequestration of the liberated phosphine by an ion - exchange resin. A second focus was examination of a much - debated hypothesis in olefin metathesis: that is, the extent to which the high productivity of the Hoveyda catalysts reflects re - uptake of the styrenyl ether functionality released in the initial cycle of metathesis. Current evidence for and against this "boomerang" hypothesis is critically examined, and new approaches to examining its operation are described. Specifically, the rate of decomposition, vs. re - uptake, is examined for the active species RuCl 2 (PCy 3 )(=CH 2 ), and background exchange of the parent catalyst with free styrenyl ether is measured by use of a 13 C - labelled styrenyl ether. These studies confirm the relevance of the boomerang mechanism for first - generation Hoveyda catalysts.

Several new late transition metal complexes containing P-O and P-N ligands derived from 2-dicyclohexylphosphinoacetaldehyde were synthesized. A facile one-pot template method is used for the synthesis of P-N complexes, where the phosphine aldehyde and amine can undergo a condensation reaction to form a phosphine-imine metal complex in the presence of a metal precursor. Metal complexes with phosphino-enolate, imine, and oxime ligands are synthesized. Ni(II), Pt(II), Rh(I) and Ir(I) metal centres were investigated. The Rh(I) and Ir(I) complexes contain a 1,5-cyclooctadiene ligand, thus resembling Crabtree’s hydrogenation catalyst [Ir(COD)(py)(PCy3)][PF6]. These complexes are also active catalysts for olefin hydrogenation. Furthermore, the synthesis of a new chiral amine functionalized NHC ligand is explored, which has potential applications as a ligand in the metal-catalyzed enantioselective hydrogenation of polar bonds. This ligand is inspired by previous achiral hydrogenation catalysts reported by Morris et al. that displayed high activity for a variety of unsaturated substrates.