Biomass is increasingly used as a feedstock in global energy production. This may present operational challenges in energy conversion processes which are related to the inorganic content of these biomasses. As a larger variety of biomass is used the need for a basic understanding of ash transformation reactions becomes increasingly important. This is not only to reduce operational problems but also to facilitate the use of ash as a nutrient source for new biomass production. Ash transformation reactions were examined in the present work using the Lewis acid-base concept. The model presented in Paper I was further extended and discussed, including the definition of tertiary ash transformation reactions as reaction steps where negatively charged molecular ions, Lewis bases, other than hydroxides are present in the reactants. The effect of such reactions for bonding of various metal ions, Lewis acids, were discussed. It was found that the formation of various phosphates through secondary and tertiary ash transformation reactions is important for the behaviour of biomass ash in combustion. The suggested model was supported by findings in Papers II-VIII. The experimental findings in Papers II-VIII were discussed in terms of ash transformation reactions. The fuel design choices made to investigate the effect of phosphorus in particular on ash transformation reactions were high-lighted. Addition of phosphoric acid to woody-type and agricultural biomasses showed that phosphate formation has a large influence on the speciation of Si, S, and Cl. Co-combustion of a problematic agricultural residue with other biomasses showed that the relation between phosphorus, alkali and alkaline earth metal content is important. Co-combustion of biosolids with wheat straw was shown to greatly improve the combustion properties of wheat straw. It was suggested that fuel analyses should be presented using molar concentration (mole/kg) in diagrams based on ash transformation reactions and elements forming Lewis acids or bases. This may facilitate the assessment of the combustion behaviour of a fuel. Some comments were made on fuel design and additives, specifically pointing out that phosphorus content should always be carefully considered in relation to alkali and alkaline earth metals in fuels and fuel blends.

Submitted by Noeme Timbo (noeme.timbo@metodista.br) on 2017-01-25T13:38:43Z No. of bitstreams: 1 Paulo Cruz.pdf: 1065817 bytes, checksum: c89d4c886e6d9f4f04095b666ec92c44 (MD5) / Made available in DSpace on 2017-01-25T13:38:43Z (GMT). No. of bitstreams: 1 Paulo Cruz.pdf: 1065817 bytes, checksum: c89d4c886e6d9f4f04095b666ec92c44 (MD5) Previous issue date: 2016-03-18 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / This paper presents a study on C. S. Lewis’ Space Trilogy — that embraces Out of the Silent Planet, Perelandra, and That Hideous Strength. We analize the theological concepts used by the author, specially the Original Sin doctrine and its relations with the concept of moral imagination, developed by the american thinker Russell Kirk. / O presente trabalho apresenta um estudo da obra Triologia Cósmica de C. S. Lewis — composta pelas obras Além do Planeta Silencioso, Perelandra e Essa força medonha —, analisando os conceitos teológicos utilizados pelo autor, sobretudo a doutrina do Pecado Original, e suas relações com o conceito de Imaginação Moral, desenvolvido pelo filósofo americano Russell Kirk.

Ce travail a essentiellement porté sur la description et la rationalisation de certaines propriétés des complexes du Zinc contenant des ligands de base de Schiff par le biais d’approches mixtes théorique et expérimentale. La première partie de ce travail concerne la synthèse et la caractérisation d’un nouveau complexe dinucléaire de Zn(II) à ligand base de Schiff tétra-azoté N4. Ce complexe s’auto-assemble en hélice à double brin avec un environnement tétraédrique rarement observé pour l’ion Zn2+ avec ce type de ligand. Ce complexe s’est avéré être thermodynamiquement instable en présence de traces d’eau, subissant ainsi une démétallation rapide. De ce fait, sa réactivité vis-à-vis de l’eau a été étudiée par des mesures spectroscopiques et des calculs DFT. Puis, le mécanisme réactionnel de démétallation a été élucidé pour la première fois et pourrait facilement être généralisé à l’hydrolyse des complexes Zn-sal(ph)en apparentés. Dans la seconde partie, le caractère acide de Lewis, principal facteur de la réactivité/coordination des complexes Zn-sal(ph)en a été étudié au moyen des descripteurs de la DFT-conceptuelle. Une analyse préliminaire a été réalisée sur un prototype modèle à savoir [ZnCl4]2-, et les résultats ont été généralisés aux complexes de Zn-sal(ph)en comportant différents types d’espaceurs diamine (flexible, semi-rigide, et rigide) afin de rationaliser l’acidité de Lewis de ces complexes avec la nature du pont diamine. L’inclusion des densités des états excités par le biais du descripteur dual dit « state specific dual descriptor» a permis de décrire avec succès la réactivité appropriée du complexe choisi. Enfin, la troisième partie de ce travail, est consacré à l’effet dit de « self-interaction » survenue dans la modélisation de la réaction de cyclo-addition du CO2 sur les époxydes catalysée par un complexe Zn-N4 base de Schiff en présence de NBu4X (X=I). Cette erreur menant à des profils énergétiques irréalistes au niveau DFT, a été identifiée et une nouvelle approche théorique a été proposée et développée afin de contourner cette erreur / In this work, we focus our interest on the description and rationalization of some properties of Zinc complexes of Schiff bases schemes through theoretical and experimental approaches. The first study deals with the syntheses and characterization of a novel Zn-N4-Schiff base L= ((±) -trans-N, N’-Bis(2-aminobenzylidene)-1,2-diaminocyclohexane) showing an unexpected self-assembled double-stranded helicate structure , in which zinc atoms are in distorted tetrahedral environments, revealing an M- (R, R) left-handed helicity in its asymmetric unit. This dimer is thermodynamically unstable in presence of water traces and undergoes a rapid demetallation process that is studied by both experimental and theoretical approaches. The reaction is monitored through DOSY NMR analysis, and the theoretical mechanism of the demetallation process is elucidated for the first time using DFT method and which should be easily generalized to the demetallation of N2O2 sal(ph)en Zn complexes. In the second part of this study, the Lewis acidic behavior, the main factor of the reactivity/coordination of Zn-sal(ph)en complexes, is studied using conceptual density functional theory descriptors. This Lewis acidic character is addressed in a first stage to a prototype complex, namely [ZnCl4]2- and the results have been generalized to Zn-sal(ph)en complexes with different diamine bridges through flexible to semi-rigid then to rigid ranges, in order to characterize and rationalize this Lewis acid effect in relation with the nature of the diamine bridge. The inclusion of the electronic excited state densities by the so-called state specific dual descriptor allows us to recover successfully the appropriate reactivity of these chosen complexes. Finally, in the third part, we consider the effect of the « self-interaction » occurring when modeling the cycloaddition of CO2 to epoxide reactions catalyzed via the binary system Zn-N4 Schiff base /NBu4X (X=I). This error leading to unrealistic energy profiles at the DFT level has been identified and a new theoretical approach is proposed and developed to correct this error

Une nouvelle tentative pour faire face au défi sceptique est menée depuis une quarantaine d’années. Cette tentative repose sur une théorie de la connaissance centrée sur la notion d’alternatives pertinentes. La thèse se propose de montrer les faiblesses de cette théorie, même lorsqu’elle s’appuie sur l’appareillage des mondes possibles, et suggère que le scepticisme demeure la meilleure position épistémologique. Dans le premier chapitre on passe en revue une série de difficultés liées au sujet des alternatives pertinentes et l'on essaye d'argumenter en faveur de l'idée qu'il n'y a peut-être pas, finalement, de moyens en quelque sorte objectifs pour établir quelles sont les alternatives pertinentes relativement à une situation quelconque. À partir du deuxième chapitre, il est procédé à une analyse critique des tentatives de certains auteurs qui se sont servis, pour élaborer leurs propres conceptions bien précises, du langage des mondes possibles. Dans le deuxième chapitre, l'on se concentre surtout sur la question du degré de proximité qu'un monde possible donné doit exhiber pour être considéré comme suffisamment proche du monde actuel et l'on essaye de montrer qu'il n’est probablement pas possible de tracer d'une manière non arbitraire une ligne de démarcation entre ces mondes possibles qu’on peut ignorer et ceux qu’on ne peut ignorer dans nos attributions de connaissance. Dans le troisième chapitre, l'on se concentre surtout sur la question des critères qui devraient guider nos évaluations de proximité et l'on essaye de montrer le caractère discutable de n'importe quelle règle visant à établir quels seraient ces critères-là. / Over the last forty years, a new attempt to answer to the skeptic challenge has been proposed. This attempt is based on a theory of knowledge, which is grounded on the notion of relevant alternative. My dissertation aims to show the problems of such a theory, even when formulated in terms of possible worlds, and suggests that in the end skepticism remains the best epistemological option. In the first chapter, I will offer a discussion of the issue of relevant alternatives, and I will argue in favor of the idea that perhaps there are no objective criteria to establish which are the relevant alternatives with respect to a certain given situation. In the second chapter, I will propose a critical analysis of the attempts of some philosophers to formulate their own proposals in the language of possible worlds. In particular, I will focus on the issue of the proximity degree that a certain possible world must have in order to be considered as sufficiently closed to the real world, and I will try to show that perhaps it is not possible to draw a sharp line of demarcation between those possible worlds that we can ignore and those that we must take into account in our attribution of knowledge. In the third chapter, I will critically discuss the criteria that should guide our evaluations about proximity, and I will show the problematic aspects of any rule aimed to establish which these criteria in effect should be.

Le concept de “paires frustrées de Lewis” (plus communément désignés par l’acronyme anglais FLPs) a suscité un vif intérêt depuis sa formulation en 2006. Initialement décrit à partir d’une phosphine encombrée comme base de Lewis et d’un borane comme acide de Lewis pour l’activation coopérative d’hydrogène sans métal, ce concept a été ensuite très largement développé en utilisant divers éléments du groupe principal (N/B, P/Al, N/Al …). Le concept a ensuite été étendu aux métaux de transitions pour pallier cette faiblesse: sont ainsi apparues les premières paires frustrées de Lewis organométalliques (OmFLPs). Dans le but de développer de telles OmFLPs, nous nous sommes intéressés à la synthèse de complexes cationiques titanocèniques et zirconocèniques en présence d’une amine. La première partie de cette thèse présente les travaux précédemment décrits sur les ligands azotés, en vue de synthétiser des complexes du groupe 4 N-fonctionnalisés. Une description plus détaillée du concept de FLP est ensuite réalisée, et un parallèle est fait avec des concepts connexes (coopérativité métal-ligand, systèmes ambiphiles). La seconde partie de ce manuscrit développe la synthèse de nouveaux ligands (aminomethyl)cyclopentadiènylure de potassium ainsi qu’une étude de leur coordination aux métaux du groupe 4. Cette étude a permis d’accéder à toute une série de nouveaux complexes dichlorotitanocènes et zirconocènes porteurs d’une fonction amine tertiaire encombrée (diisopropylaminyl et 2,2,6,6-tétraméthylpipéridine) à proximité du centre métallique. Ces travaux ont montré que l’amine ne se coordine pas audit centre métallique. Les métallocènes ainsi formés ont ensuite été transformés en cation afin de renforcer le caractère acidité de Lewis du centre métallique. Ces espèces ont montré une réactivité inattendue donnant lieu à des réarrangements par activation CH au voisinage de l’atome d’azote. Ces réarrangements ainsi que des études mécanistiques font l’objet du troisième chapitre. La quatrième partie de ce mémoire porte sur la synthèse de complexes phosphido- et amidotitanocènes cationiques. Ces complexes montrent une très bonne activité en catalyse d’hydrogénation de petites molécules dans des conditions relativement douces, vraisemblablement pour des raisons d’effets coopératifs entre le métal et le ligand. Dans une dernière partie, la synthèse de complexes titanocéniques cationiques portant une fonction iminophosphorane est développée, suivie d’une étude de réactivité de ces complexes en tant que paires frustrées de Lewis organométalliques. / In 2006, the concept of “frustrated Lewis pairs” (called FLPs) was introduced. The main characteristic of these compounds is their ability to activate cooperatively small molecules without the use of a metal (H2, CO2, alkene alkyne…). Initially based on P/B combination, the concept has been extended to several other main group elements (N/B, P/Al, N/Al …). Recently, FLPs have been extended to the transition metal realm. These organometallic FLPs (OmFLPs) are obviously non-metal free systems but they extend significantly the scope of FLP applications. Seeking to develop such systems, a research toward new omFLP combinations (N/Ti+, N/Zr+) has been initiated in our group, based on the synthesis of N-based titanocene and zirconocene complexes. The first part of this manuscript deals with a survey of the literature of such compounds, and a more detailed presentation of FLPs and related concepts (metal-ligand cooperativity, ambiphilic ligands) are also developed. In a second chapter, the synthesis of new N-based cyclopentadienyl ligands and their coordination to group 4 metals is presented. The formation of a cationic complex is then developed in a third part on selected titanocenes. In these complexes, the amine function undergoes CH activation by the cationic metal centre, leading to unexpected rearrangements. Investigations on their plausible mechanism are also presented. In a fourth part, the synthesis of new cationic phosphido- and amidotitanocenes, discovered in the course of our study on OmFLPs, is developed. The cationic amidotitanocenes are shown to be catalytically active towards hydrogenation of small molecules. Lastly, the potential of cationic titanocenyl iminophosphoranes as OmFLPs, was developed.

Dans un contexte de diminution des émissions polluantes émises par les moteurs à combustion interne, le secteur des transports assiste à une amélioration des motorisations mais également à une diversification des carburants pour l’automobile. L’utilisation de ces différents carburants entraîne souvent un impact sur les performances de la combustion. Dans le cas du moteur à allumage commandé, la performance dépend du dégagement d’énergie, image de la vitesse de la combustion, soit du front de flamme consommant le mélange air-carburant. Or toute flamme en expansion est théoriquement soumise à des effets de courbure et de cisaillement, toutes deux contributions de l’étirement. La réponse à l’étirement étant propre à chaque type de mélange air-carburant (lié au carburant proprement dit, à la richesse du mélange, à la dilution …), ce travail de thèse est centré sur la compréhension de l’impact de l’étirement sur les performances des carburants dans les moteurs à allumage commandé. Pour cela, différents mélanges air-carburant similaires du point de vue des propriétés thermodynamiques et des vitesses fondamentales de combustion laminaire mais avec des sensibilités à l’étirement différentes ont été sélectionnés. Ces mélanges ont ensuite été étudiés dans différentes configurations expérimentales et à l’aide de différentes techniques de mesure: moteur monocylindre opaque et à accès optiques, chambre sphérique de combustion turbulente. Les résultats montrent que les propriétés de sensibilités à l’étirement déterminées en régime laminaire comme la longueur de Markstein et le nombre de Lewis sont indicatrices du comportement des mélanges en combustion turbulente, comme dans la chambre de combustion caractéristique des moteurs à allumage commandé, et sont des paramètres à prendre en considération afin de prédire les performances plus globales de ces carburants que ce soit expérimentalement qu’en simulation. / In a context of decreasing pollutant emissions, the transport sector is facing an improvement of engine concept as well as a fuel diversification. The use of these different fuels often involves an impact on the combustion performance itself. In the case of Spark ignition engine, the efficiency is a function of the released heat, image of the combustion speed, i.e. the flame front speed consuming the air-fuel mixture. It is well known that every expanding flame is submitted to flame curvature and strain rate which are both contributors to flame stretch. As the answer of each air-fuel mixture (i.e. the fuel itself, the equivalence ratio, the dilution …) is different to flame stretch, the objective of this work is to understand flame stretch impact on fuel performance in Spark-Ignition engines. To achieve this goal, different fuel-air mixtures with similar unstretched laminar burning speed and thermodynamic properties but different responses to stretch were selected. Those mixtures were then studied with different experimental devices with different measurement techniques: single-cylinder metallic and optical engines, turbulent combustion spherical vessel. Results show that flame stretch sensitivity properties such as Markstein length and Lewis number, determined in laminar combustion regime, are relevant parameters to describe the flame propagation in turbulent combustion as in the combustion chamber of the Spark-Ignition engine and need to be taken into consideration to evaluate global performance of these fuels, experimentally and also in modeling simulation.

Dans la problématique de l'utilisation de nouvelles énergies non polluantes, l'hydrogène est l'un des principaux carburants verts du futur. Les dérivés d'hydrazine et de borane sont potentiellement intéressants pour le stockage chimique de l'hydrogène. Les complexes entre hydrazines ou nitriles avec des boranes ou des alanes sont à la base de cette étude. Ces composés ont été synthétisés afin d'étudier leur structure électronique avant et après la création de la liaison entre les acides et les bases de Lewis. La spectroscopie photoélectronique à rayonnement UV (UV-SPE) est utilisée comme outil principal de caractérisation fournissant des énergies d'ionisation (IE). L’interprétation des résultats expérimentaux est supportée par des calculs quantiques comme ΔSCF + TD-DFT, OVGF, P3 et SAC-CI. Des simulations et des expériences par Flash Vacuum Thermolysis (FVT) ont été effectuées, portant sur l’élimination d'hydrogène à partir de dérivés d'hydrazine borane. / Within the problematic of the use of new non-polluting energies, hydrogen is one of the main green fuels of the future. Hydrazine borane derivatives are potentially interesting chemical hydrogen storage materials. Complexes between hydrazines or nitriles with boranes or alanes are the basis of this study. These compounds were synthesized in order to study their electronic structure before and after creation of the bond between the Lewis acids and bases. Ultraviolet Photoelectron Spectroscopy (UV-PES) is used as a main characterization tool, providing Ionization Energies (IE). The interpretation of the experimental results is supported by Quantum Chemical Calculations as ΔSCF+TD-DFT, OVGF, P3 and SAC-CI methods. Simulations and experiments by Flash Vacuum Thermolysis (FVT) were carried out on hydrogen release from hydrazine borane derivatives.

nÃo hà / Esta pesquisa busca analisar os elementos utilizados por Tim Burton para representar a Era Vitoriana atravÃs do figurino, meio de transporte e decoraÃÃo. Trabalharemos tambÃm com a traduÃÃo em imagens do nonsense em sua adaptaÃÃo fÃlmica Alice in Wonderland (2010). Nessa perspectiva, o presente trabalho resulta de um estudo comparativo entre o filme e a obra na qual se baseia, o cÃnone da literatura infantil Aliceâs Adventures in Wonderland de Lewis Carroll (1865). Para tanto, faremos um cotejo entre as duas obras à luz das teorias de Elizabeth Sewell (2015), Wim Tigges, (1988) e Susan Stewart (1980) que alicerÃam o gÃnero literÃrio nonsense. LanÃaremos mÃo da versÃo literÃria Lewis Carroll: The Complete Fully Illustrated Works de 1995 da editora Gramercy, contendo toda a obra de Lewis Carroll com ilustraÃÃes originais. Isso, a fim de viabilizar e facilitar a comparaÃÃo com as figuras presentes no texto literÃrio, quando necessÃrio, para investigar se houve aproximaÃÃo na representaÃÃo visual feita pelo diretor.

Made available in DSpace on 2014-07-29T15:06:24Z (GMT). No. of bitstreams: 1 RENATO MENDES ROCHA.pdf: 735741 bytes, checksum: 6559707c78ce13d85086e58b691dd7f3 (MD5) Previous issue date: 2010-08-30 / In this dissertation we will defend Lewis Modal Realism (LMR), i.e., the metaphysical hypothesis about the real existence of a plurality of worlds. We will try to show the pragmatic character of Lewis arguments in support of this metaphysics of possible worlds. In this sense, we approximate Lewis (1986) and Quine (1960) and we aim to show that Lewis uses criteria for taking ontological decisions similar to those defended by Quine. These criteria are: simplicity of formulation, theoretical economy and distrust in purely intuitive criteria as only guide for Philosophy. To accomplish our intention, we divided the text in three chapters. In the first we present the philosophical benefits of LMR that demonstrate the theoretical utility of possible-worlds talk. These benefits are related to important concepts in Philosophy of Language and Epistemology, such as, Modality, Counterfactuals and a uniform treatment to Properties and Propositions. The second chapter is divided into two parts. At first we present the philosophical background we believe is related to Lewis philosophy. A neo-humean scenario and the resumption of metaphysics discussion in contemporary analytic philosophy compose this background. In the second part we present three fundamental thesis formulate by Lewis the consistence of his Modal Realism: concreteness, isolation and plenitude. In the third and latter chapter we discuss two criticism of LMR: (a) David Armstrong & Peter Forrest (1984) and (b) Susan Haack (1977). Each of these papers present criticism from distinct points of view. The first aims to identify a paradox in the metaphysics of possible worlds, and the latter focuses on semantics aspects of LMR. Finally, we show Lewis reply to objection (a) and that argument (b) could be inserted as an incredulous stare on LMR, and that it consists on a petition principi. / Nessa dissertação defenderemos o Realismo Modal de David Lewis (RML), ou seja, a hipótese metafísica acerca da existência real de uma pluralidade de mundos. A defesa que apresentaremos procura evidenciar o caráter pragmático dos argumentos a favor dessa metafísica dos mundos possíveis. Nesse sentido, aproximaremos a filosofia de David Lewis (1986) a de W. V. O. Quine (1960) e procuraremos mostrar que Lewis utiliza critérios para tomada de decisões ontológicas semelhantes aos defendidos por Quine. Esses critérios são: a simplicidade de formulação, a economia teórica e a desconfiança de critérios meramente intuitivos como guias para a Filosofia. Para cumprir nosso objetivo, estruturamos a dissertação em três capítulos. No primeiro apresentamos benefícios filosóficos do RML que demonstram a utilidade teórica do idioma dos mundos possíveis. Esses benefícios estão relacionados a noções importantes para a Filosofia da Linguagem e a Teoria do Conhecimento tais como: Modalidades, Contrafatuais, e um tratamento uniforme para Propriedades e Proposições. O segundo capítulo divide-se em duas partes. Na primeira traçamos um pano de fundo filosófico que acreditamos estar relacionado à filosofia de Lewis. Compõem esse pano de fundo: um cenário que identificamos como neo-humeano e a retomada de discussões metafísicas na Filosofia Analítica Contemporânea. Na segunda parte apresentamos três teses formuladas por Lewis para garantir a consistência e a coerência de seu Realismo Modal: a Concretude, o Isolamento e a Plenitude. No terceiro capítulo discutimos duas críticas ao RML: (a) David Armstrong & Peter Forrest (1984) e (b) Susan Haack (1977). Cada um desses artigos apresenta uma crítica diferente. A primeira procura identificar um paradoxo na metafísica dos mundos possíveis e a segunda concentra-se em aspectos semânticos do RML. Por fim, mostramos como a objeção em (a) é refutada por Lewis e como o argumento em (b) poderia se inserir no grupo de críticas que Lewis classifica como um olhar incrédulo sobre o RML. Por isso, concluímos que esse olhar consiste em uma petição de princípio.

O objetivo deste trabalho foi investigar o uso de NbCl5 como ácido de Lewis em reações de cicloadição. Foram estudadas algumas reações de cicloadição [2 + 2], Reações de Diels-Alder entre enonas e ciclopentadieno e Reações da aza-Diels-Alder com bases de Schiff, avaliando o efeito da temperatura e da concentração molar do NbCl5. A comparação dos rendimentos, dos produtos formados e do tempo de reação com NbCl5, também foi um ponto fundamental da pesquisa. As reações de cicloadição [2 + 2] foram realizadas entre ésteres propiólicos e diferentes tipos de alcenos (éteres enólicos de silício e alcenos alifáticos). Nas reações com os éteres enólicos de silício não foi verificada a formação dos respectivos adutos de ciclobuteno, pois o NbCl5 promove a quebra da ligação oxigênio-silício, não levando à formação do produto desejado. Nas reações com os alcenos alifáticos foi possível obter o respectivo aduto, porém com rendimentos menores que com outros ácidos de Lewis. Quanto às reações de Diels-Alder, foram investigadas as reações entre diferentes ciclo-enonas (dienófilos de baixa reatividade) com ciclopentadieno (dieno) na presença de NbCl5. Os resultados obtidos indicam que o NbCl5 é um bom ácido de Lewis para ativar reações de Diels-Alder, apresentando tempos reacionais menores e alta diastereosseletividade a temperaturas mais baixas, quando comparado com outros ácidos de Lewis. A possibilidade de efetuar reações do Diels-Alder a -78 0C é um dos aspectos de destaque neste trabalho, pois, além de demonstrar a forte ativação do sistema enona exercido pelo NbCl5, possibilita obter produtos com alta seletividade. Paralelamente aos estudos de sistemática reacional foram realizados estudos de elucidação estrutural completa de alguns dos compostos sintetizados, utilizando-se diversas técnicas de RMN (uni e bidimensionais), e o auxílio de cálculos teóricos. Nas reações de aza-Diels-Alder entre bases de Schiff e di-hidropirano, o NbCl5 se mostrou um ótimo catalisador para a síntese de derivados de piranoquinolinas. Estes derivados formam uma importante classe de produtos naturais que apresentam ampla atividade biológica. As reações foram conduzidas com baixas concentrações de nióbio e em tempos relativamente curtos, obtendo-se rendimentos variando de 72 a 96 %. Outro fator a se destacar é a alta diastereosseletividade encontrada nestas reações. Também foram realizados estudos da atividade tripanocida dos derivados de piranoquinolinas preparados através das reações de aza-Diels-Alder catalisadas por NbCl5. / The aim of this work was to investigate the use of NbCl5 as Lewis acid in cycloadittion reactions. We have studied [2 + 2] cycloaddition reactions, Diels-Alder reactions between enonas and cyclopentadiene and aza-Diels-Alder reactions with Schiff bases. The effects of the temperature and of the molar concentration of NbCl5 were also evaluated. Comparasion of reaction yields, obtained products as well as reation time with NbCl5 were also a key point on this work. The [2 + 2] cycloaddition reactions were performed using propiolic ester and different types of alkenes (silyl enol ethers and aliphatic alkenes). In the reactions with silyl enol ethers, formation of the corresponding cyclobutene aductts was not verified, since the NbCl5 promotes the rupture of oxygen-silicon bonds, and the desired product is not obtained. In the reactions with aliphatic alkenes, it was possible to obtain the adduct, however, in lower yields as compared to those obtained with others Lewis acids. Regarding the Diels-Alder reactions, we have investigated reactions using different cycloenones (dienophiles of low reactivity) with cyclopentadiene (diene) with NbCl5. The obtained results indicate that NbCl5 is a good Lewis acid to activate these Diels-Alder reactions, resulting in shorter reaction times and higher diastereoselectivity at lower temperatures than other Lewis acids. The possibility of carrying out Diels-Alder reaction at -78 0C is another remarkable aspect of this work. Besides demonstrating the strong activation of the enone system by NbCl5, it opens the possibility of obtaining high stereoselectivity. We have also performed studies of complete structural elucidation of some compounds by using different NMR techniques (uni and bidimensional), with the help of theoretical calculations. In the aza-Diels-Alder reactions between Schiff bases and dihydropyran, the NbCl5 was an excellent catalyst for the synthesis of pyranoquinoline derivatives. These derivatives are an importante class of natural products that exhibit wide range of biological activity. The reactions were carried out at low concentration of niobium and in relatively short times, resulting in yields varying from 72 to 96 %. Another aspect that should be remarked is the high diastereoselectivity found in these reactions. We have also carried out studies of tripanocydal activity of pyranoquinoline derivatives prepared through aza-Diels-Alder reactions catalyzed by NbCl5.