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Elisabete Maria Saraiva Sanchez - One of the best experts on this subject based on the ideXlab platform.
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Obtenção de novos catalisadores para a transesterificação de oleos vegetais
2017Co-Authors: Elisabete Maria Saraiva SanchezAbstract:Resumo: Os óleos vegetais têm sido considerados candidatos potenciais a substituir o óleo diesel. Estudos mostram que ésteres metílicos obtidos a partir desses óleos são os compostos que mais semelhanças apresentam em relação ao óleo diesel, requerendo mínimas modificações nos motores. Ésteres metílicos podem ser facilmente obtidos pela transesterificação de óleos vegetais em presença de catalisadores fortemente básicos, como as guanidinas. Esponjas de prótons são possíveis catalisadores para essa reação porque apresentam elevado pKa. Dois compostos pertencentes a essa classe foram preparados e testados como catalisadores. O 1,8-bis(dimetilamino)naftaleno foi obtido por metilação do 1,8-diaminonaftaleno, com 89% de rendimento. Para a preparação do 4,5-bis(dimetilamino)fluoreno fez-se necessária a reelaboração de sua síntese a partir do ácido difênico, em etapas que envolveram ciclização, redução de Wolff-Kishner, nitração, rearranjo de Schmidt, hidrogenação catalítica e metilação. O rendimento total de todas essas etapas foi da ordem de 2,4%. Esse baixo rendimento se deve ao fato da reação de nitração produzir dois isômeros, sendo que o isômero necessário à continuação da síntese foi obtido em menor quantidade. Outra etapa que causou um abaixamento no rendimento foi o rearranjo de Schmidt, devido a necessidade de purificação em coluna cromatográfica. Os testes catalíticos foram realizados nas mesmas condições utilizadas com a N,N,N',N'-tetrametilguanidina, de comprovada eficiência na transesterificação de óleo de soja com metanol. Verificou-se que os dois compostos testados não catalisam essa reação, o que é explicado pela inércia cinética dessas esponjas de prótons.Abstract: Vegetable oils are potential candidates to substitute diesel fuel. Studies have shown that. the methyl esters obtained from these oils behave very similar to diesel fuel, allowing their use in diesel engines with minimal adjustments. Methyl esters can be easily preparad by transesterification of vegetable oils in the presence of strongly basic catalysts, such as guanidines. Proton sponges are possible catalysts for this reaction as they have a high pKa value. Two compounds of this class were prepared and tested as catalysts. 1,8-bis(dimethylamino)naphthalene was obtained by methylation of 1,8-diaminonaphthalene in 89% yield. The preparation of 4,5-bis(dimethylamino)fuorene required the reelaboration of its synthesis, starting from diphenic acid and using cyclization, Wolff-Kishner Reduction, niration, Schmidt rearrangement, catalytic hydrogenation and methylation. The overall yield was only 2.4%, due to the nitration which produces two isomeres, whose separation is difficult. Furthermore, the desired isomer is only formed in small yield and its Smith rearrangement further decreases the quantity of product obtained, as it needs to be purified by column chromatography. The catalytic tests were performed under the same conditions used for N,N,N',N'-tetramethylguanidine, which was shown to be very effective in the transesterification of soybean oil with methanol. It was shown that the two proton sponges obtained do not catalyse this reaction, which we believe is due to the fact that they are cinetically inert
Frederick Sweet - One of the best experts on this subject based on the ideXlab platform.
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1 5 hydride shift in wolff kishner Reduction of 20r 3β 20 26 trihydroxy 27 norcholest 5 en 22 one synthetic quantum chemical and nmr studies
Steroids, 2002Co-Authors: Zsuzsanna Szendi, Peter Forgo, Gyula Tasi, Zsolt Bocskei, Levente Nyerges, Frederick SweetAbstract:Abstract Heating (20R)-3β,20,26-trihydroxy-27-norcholest-5-en-22-one (1) with hydrazine and KOH at 160°C completely converted the steroid to a diastereoisomeric mixture of the new (20R,22RS)-27-norcholest-5-ene-3β,20,22-triols (2). Exclusive formation of 2 suggests that the expected Wolff-Kishner Reduction to a methylene group at the C-22 ketone in 1 was diverted to the C-26 position by a 1,5-hydride shift. All attempts under acid conditions failed to produce a C-22 phenyl hydrazone from 1. However, reaction of 1 was reacted with phenylhydrazine in hot KOH, gave the C-26 phenylhydrazone 4 as the sole product. Evidently, under alkaline conditions, first a hydride ion undergoes an intramolecular transfer from the C-26 CH2OH group to the C-22 ketone in 1, and then the phenylhydrazine traps the newly formed aldehyde. To examine this hypothesis, we constructed computer-simulated transition state models from quantum chemical calculations and then compared data from these models with NMR measurements of the reaction mixtures containing 2. The NMR data showed that the C-22 diastereoisomers of 2 are formed in a nearly 1:1 ratio exactly as predicted from the energy-optimized transition states, which were calculated for intramolecular 1,5-hydride shifts that produced each of the two C-22 diastereoisomers. Accordingly, these results support the hypothesis that an intramolecular 1,5-hydride shift mechanism promotes complete conversion of 1 to 2 under classical Wolff-Kishner Reduction conditions.
Zsuzsanna Szendi - One of the best experts on this subject based on the ideXlab platform.
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1 5 hydride shift in wolff kishner Reduction of 20r 3β 20 26 trihydroxy 27 norcholest 5 en 22 one synthetic quantum chemical and nmr studies
Steroids, 2002Co-Authors: Zsuzsanna Szendi, Peter Forgo, Gyula Tasi, Zsolt Bocskei, Levente Nyerges, Frederick SweetAbstract:Abstract Heating (20R)-3β,20,26-trihydroxy-27-norcholest-5-en-22-one (1) with hydrazine and KOH at 160°C completely converted the steroid to a diastereoisomeric mixture of the new (20R,22RS)-27-norcholest-5-ene-3β,20,22-triols (2). Exclusive formation of 2 suggests that the expected Wolff-Kishner Reduction to a methylene group at the C-22 ketone in 1 was diverted to the C-26 position by a 1,5-hydride shift. All attempts under acid conditions failed to produce a C-22 phenyl hydrazone from 1. However, reaction of 1 was reacted with phenylhydrazine in hot KOH, gave the C-26 phenylhydrazone 4 as the sole product. Evidently, under alkaline conditions, first a hydride ion undergoes an intramolecular transfer from the C-26 CH2OH group to the C-22 ketone in 1, and then the phenylhydrazine traps the newly formed aldehyde. To examine this hypothesis, we constructed computer-simulated transition state models from quantum chemical calculations and then compared data from these models with NMR measurements of the reaction mixtures containing 2. The NMR data showed that the C-22 diastereoisomers of 2 are formed in a nearly 1:1 ratio exactly as predicted from the energy-optimized transition states, which were calculated for intramolecular 1,5-hydride shifts that produced each of the two C-22 diastereoisomers. Accordingly, these results support the hypothesis that an intramolecular 1,5-hydride shift mechanism promotes complete conversion of 1 to 2 under classical Wolff-Kishner Reduction conditions.
Dai Xi-jie - One of the best experts on this subject based on the ideXlab platform.
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Exploring oxygenated compounds for efficient transition metal-catalyzed molecular transformations
McGill University, 2017Co-Authors: Dai Xi-jieAbstract:This thesis advances the knowledge in two fundamentally important organic chemical transformations: (1) cleavage of carbon-oxygen bonds and (2) formation of carbon-carbon bonds. Such advancement consists of four late transition metal-catalyzed reactions based on the oxygenated chemical feedstock, which will be discussed on a chapter-by-chapter basis. Chapter 2 introduces our initial attempts to address a 40-year-old scientific challenge in the field of alcohol deoxygenation: how to selectively and efficiently remove hydroxyl groups in organic molecules without affecting other existing functional groups. We hypothesize a single-step, redox process to solve this problem, whereby the dehydrogenative oxidation of alcohols and the Wolff-Kishner Reduction are combined. As a proof-of-concept discovery, the early development of this reaction is catalyzed by iridium complexes and mediated by hydrazine under forcing reaction conditions. This deoxygenation protocol proves effective for many simple activated substrates such as benzylic and allylic alcohols. The major limitation, however, is the poor reactivity and selectivity seen in aliphatic alcohol substrates. Chapter 3 describes the adaptation of ruthenium(II) catalysis for the direct deoxygenation of primary aliphatic alcohols in a completely chemo- and regio-selective manner. Such a robust catalytic system, comprising [Ru(p-cymene)Cl2]2 and 1,2-bis(dimethylphosphino)ethane, is vital to lower the activation energy barriers to the dehydrogenative oxidation of aliphatic alcohols, and makes this step more kinetically favorable. Equally important is the combination of KOt-Bu, DMSO and t-BuOH, which promotes the subsequent Wolff-Kishner Reduction at low temperature. This method is thus more practical compared with the iridium-based protocol, proceeding under milder thermal conditions. Its synthetic utility is demonstrated by the selective cleavage of carbon-oxygen bonds in both simple and complex organic molecules such as steroids and alkaloids. Chapter 4 presents a synthetic approach to utilize naturally occurring carbonyl compounds (i.e. aldehydes and ketones) as more sustainable alkyl carbanion equivalents for formation of carbon-carbon bonds via carbonyl addition reactions. Traditionally, such transformations depend on organometallic reagents which are made from petroleum-derived chemical feedstocks and a stoichiometric quantity of metal. Accessing this new chemical reactivity of carbonyl compounds attributes to the ruthenium(II) catalytic system discovered in the early deoxygenation chemistry. By controlling the basicity, preformed carbonyl-derived hydrazones can intercept another carbonyl compounds to form new carbon-carbon bonds via a Zimmerman-Traxler chair-like transition state. This chemical transformation delivers a wide range of synthetically valuable secondary and tertiary alcohols. Additional highlights include excellent functional group compatibility and good stereochemical control governed by chiral amido and phosphine ligands. Chapter 5 focuses on the further exploration of such unique 'umpolung' reactivity for formation of carbon-carbon bonds via conjugate addition reactions. Inspired by the softness of ruthenium(II) pre-catalyst, which bears a resemblance to that of 'soft' transition metals in the classical 1,4-conjugate addition, we presume that this ruthenium(II)-based catalytic system may be more effective for conducting nucleophilic conjugate additions. Indeed, a variety of highly functionalized aromatic carbonyl compounds are used as latent benzyl carbanions, to couple with electron-deficient α,β-unsaturated compounds including esters, ketones, sulfones, phosphonates, and amides. Two bidentate phosphine ligands (dppp and dmpe) are found to facilitate this process in a complementary manner, largely depending on electronic profiles of the carbonyl compounds. Chapter 6 summarizes all research present in this thesis and contributions to knowledge advancement.Cette thèse fait progresser la connaissance de deux transformations fondamentalement importantes en chimie organique : (1) la rupture des liaisons carbone-oxygène et (2) la formation de liaisons carbone-carbone. Une telle avancée repose sur quatre réactions hautement originales de réaction catalysées par des métaux de transition, à partir de matière première et de dérivés chimiques composés d'atome d'oxygène. Le développement de toutes ces méthodes de synthèse sera discuté chapitre par chapitre. Le chapitre 2 présente nos premières tentatives pour aborder un défi scientifique datant de 40 ans dans le domaine de la désoxygénation d'alcools: comment éliminer sélectivement et efficacement les groupes hydroxyles dans les molécules organiques. Pour résoudre ce problème, nous proposons un processus redox en une seule étape combinant l'oxydation déshydrogénante d'alcools suivie de la réduction de Wolff-Kishner. Le développement de la réaction catalysée par un complexe d'iridium et assistée par l'intermédiaire d'hydrazine dans de fortes conditions oxydantes, démontre la faisabilité de notre hypothèse. Ce protocole de désoxygénation s'avère efficace pour une large gamme de fonctions alcools activées, telles que les fonctions alcools benzyliques et allyliques. Le chapitre 3 décrit l'adaptation de la catalyse au ruthénium(II) pour la désoxygénation directe d'alcools primaires et aliphatiques. Il a été vital de développer un système catalytique robuste comprenant du [Ru(p-cymene)Cl2]2 et du dmpe. La combinaison de t-BuOK, DMSO et t-BuOH, est tout aussi importante. Par conséquent, cette méthode est nettement plus fonctionnelle, vis-à-vis du précédent protocole reposant sur la catalyse à l'iridium, en opérant dans des conditions thermiques nettement plus douces. Son utilité synthétique est démontrée avec élégance par la scission sélective des liaisons carbone-oxygène dans des molécules organiques simples et complexes telles que les stéroïdes et les alcaloïdes. Le chapitre 4 présente une nouvelle approche pour utiliser les composés carbonylés naturels en tant que source masquée de carbanion pour la formation de liaisons carbone-carbone, via des réactions d'addition sur des dérivés carbonylés. Traditionnellement, de telles transformations ne sont possibles qu'avec des réactifs organométalliques, composés de matières premières dérivées du pétrole et d'une quantité stœchiométrique de métal. En ajustant la basicité dans le système réactionnel, les dérivés d'hydrazones, préformés, s'additionnent à d'autres composés carbonylés pour former de nouvelles liaisons carbone-carbone. Cette addition est accomplie par l'intermédiaire d'un état de transition Zimmerman-Traxler, en forme chaise. Une large gamme d'alcools secondaires et tertiaires, possédant une grande valeur synthétique, est fournie dans des conditions réactionnelles très douces grâce à cette transformation unique. Parmi les autres points saillants, figurent une excellente compatibilité des groupes fonctionnels et un bon contrôle stéréochimique, contrôlé par des ligands chiraux amide et phosphine.Le chapitre 5 se concentre sur l'exploration plus approfondie de la réactivité 'umpolung' pour la formation de liaisons carbone-carbone, via des réactions d'addition conjuguées. Inspiré par la nature molle du pré-catalyseur au ruthénium(II), on suppose que le système catalytique à base de ruthénium(II) peut être plus efficace afin de mener des additions nucléophiles conjuguées. En effet, pour la première fois, des composés carbonylés aromatiques hautement fonctionnalisés sont utilisés en tant que carbanions benzyliques masqués pour s'additionner à des composés α, β-insaturés possédants des groupes déficients en électrons, comme des esters, des cétones, des sulfones, des phosphonates et des amides. Le chapitre 6 résume toutes les recherches présentées dans cette thèse et les contributions à l'avancement des connaissances
Shigeyoshi Sakaki - One of the best experts on this subject based on the ideXlab platform.
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Substrate dependent reaction channels of the Wolff–Kishner Reduction reaction: A theoretical study
Beilstein-Institut, 2014Co-Authors: Shinichi Yamabe, Guixiang Zeng, Wei Guan, Shigeyoshi SakakiAbstract:Wolff–Kishner Reduction reactions were investigated by DFT calculations for the first time. B3LYP/6-311+G(d,p) SCRF=(PCM, solvent = 1,2-ethanediol) optimizations were carried out. To investigate the role of the base catalyst, the base-free reaction was examined by the use of acetone, hydrazine (H2N–NH2) and (H2O)8. A ready reaction channel of acetone → acetone hydrazine (Me2C=N–NH2) was obtained. The channel involves two likely proton-transfer routes. However, it was found that the base-free reaction was unlikely at the N2 extrusion step from the isopropyl diimine intermediate (Me2C(H)–N=N–H). Two base-catalyzed reactions were investigated by models of the ketone, H2N–NH2 and OH−(H2O)7. Here, ketones are acetone and acetophenone. While routes of the ketone → hydrazone → diimine are similar, those from the diimines are different. From the isopropyl diimine, the N2 extrusion and the C–H bond formation takes place concomitantly. The concomitance leads to the propane product concertedly. From the (1-phenyl)ethyl substituted diimine, a carbanion intermediate is formed. The para carbon of the phenyl ring of the anion is subject to the protonation, which leads to a 3-ethylidene-1,4-cyclohexadiene intermediate. Its [1,5]-hydrogen migration gives the ethylbenzene product. For both ketone substrates, the diimines undergoing E2 reactions were found to be key intermediates
