9,563 research outputs found
Estudos cenéticos da reação de heck intramolecular catalisada por nanopartículas bimetálicas estabilizadas por polietilenoimina linear funcionalizada
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas, Programa de Pós-graduação em Química, Florianópolis, 2013Reações de acoplamento C-C tem grande destaque na síntese de novos compostos orgânicos, em especial a reação de Heck. Porém, o desenvolvimento de novos sistemas catalíticos, com metodologias livres de ligantes fosfina, ausência de atmosfera inerte e realizadas em meio aquoso, tem sido um grande desafio nos últimos anos. Ainda, dos poucos trabalhos relatados nesta área, raros são os que se dedicam ao estudo sistemático dos mecanismos de reação. Neste sentido, este trabalho pretende contribuir no desenvolvimento de novos catalisadores para reações de acoplamento C-C, nas condições anteriormente citadas, baseando-se no preparo e caracterização de nanopartículas bimetálicas de prata e paládio, estabilizadas por polietilenoimina linear, e sua aplicação em uma reação do tipo Heck intramolecular. Análise Cinética do Progresso da Reação foi utilizada para a determinação da ordem da reação em relação aos reagentes e obtenção de evidências mecanísticas. Também foi verificada a influência da base utilizada (trietilamina) na reação e sobre a morfologia do catalisador. Por fim, experimentos de inibição do catalisador e análise por microscopia eletrônica de transmissão das nanopartículas, após a reação, sugerem a participação da superfície das nanopartículas na catálise de Heck sob as condições estudadas. Abstract: Carbon-carbon coupling is a prominent reaction in the synthesis of new organic compounds, specially the Heck reaction. However, the development of new catalytic systems, with methods free of phosphine ligands, without inert atmosphere and held in an aqueous medium, has been of great challenge in the last years. Moreover, from the few studies reported in this area, very little are dedicated to the systematic study of reaction mechanisms. Thus, this work aims to contribute in the development of new catalysts for C-C coupling reactions, under the conditions previously mentioned, based on the preparation and characterization of bimetallic nanoparticles of silver and palladium, stabilized by linear polyethyleneimine, and its application in an intramolecular Heck type reaction. Reaction Progress Kinetic Analysis was used to determine the reaction order with respect to reactants and obtention of mechanistic evidences. It was also verified the influence of the base (triethylamine) on the reaction and on the catalyst morphology. Finally, catalyst inhibi-tion experiments and analysis by transmission electron microscopy of the nanoparticles, after the reaction, suggest the involvement of the nanoparticles surface on catalysis under the studied conditions
Oxidative Heck Reactions with Terminal Olefins
No full textSequential transformations in a single reaction have the potential to dramatically increase efficiency with respect to resources, time, and number of steps to access key intermediates. When sequential C-H bonds are activated a bifunctional handle arises from seemingly inert functionality. This work describes a one-pot sequential allylic C-H
esterification, vinylic C-H arylation. A previously reported Pd(II)/sulfoxide system is used to generate branched allylic esters from α-olefins with only the addition of an aryl boronic acid to the reaction mixture. Styrenyl allylic esters are generated in good overall yield and excellent selectivities. The wide functional group tolerance and mild conditions of this three-component coupling reaction provide an attractive manifold for
the rapid build-up of dense functionality around terminal olefins with minimal protecting
group strategies or undesirable oxidation/reduction reactions.The synthetic utility of this
reaction has been demonstrated through the synthesis of several intermediates to biologically active molecules.
The ready availability and inertness of α-olefins relative to the oxidized precursors required for other C—C bond forming methods means that fewer steps are required for their installation and maintenance throughout a synthetic sequence. Previously, allylic esters served as non-resonance directing groups on terminal olefin for the vinylic C—H arylation (Heck reaction). This manifold is unique because previous intermolecular Heck reaction conditions require an excess of resonance activated olefin coupling partner. Further exploration of directing groups to determine the underlying directing factors led to the discovery of several terminal olefin classes with diversedirecting elements. A general and highly selective intermolecular Heck arylation of non-resonance stabilized α-olefins with aryl and stryenyl boronic reagents has been developed. The Pd(II)/sulfoxide catalyzed Heck reaction is performed under oxidative, acidic conditions and proceeds with good yields and excellent regio- and stereoselectivities to generate linear E-arylated olefins.
Polyenes are prevalent motiefs in natural products and pharmaceuticals. Polyene
functionality often requires mild and selective synthetic methods. The Heck vinylation uses orthogonally reactive C-H bonds and these bonds are often easy to carry through synthetic sequences. Despite this advantage, the synthetic potential of the intermolecular Heck reaction has not been realized in complex molecule formation. A method has been developed which overcomes the previous intermolecular Heck-vinylation shortcomings of excess terminal olefin and required resonance activation for regio- and stereoselectivities. The Pd(II)/sulfoxide catalyzed oxidative Heck vinylation proceeds
under mild conditions to give polyene products with a variety of substitution patterns amid diverse functionality. The polyenes are formed in synthetically relevant yields with excellent stereoselectivities. Overall, the oxidative Heck vinylation compares favorably with many of the methods commonly used to synthesize polyunsaturated hydrocarbon
segments. This method increases the synthetic potential of the intermolecular Heck reaction closer to that of other classic palladium cross-couplings.Item withdrawn by Mark Zulauf ([email protected]) on 2010-08-23T18:07:02Z
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Nanocatalisadores metálicos de paládio e prata estabilizados com novos líquidos iônicos piridínicos para aplicação em reações de Heck e redução
No full textTese (doutorado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas, Programa de Pós-Graduação em Química, Florianópolis, 2015.Este trabalho foi dividido em: (i) síntese de novos líquidos iônicos (LIs) piridínicos para utilização na estabilização de nanopartículas de prata (Ag-NPs) para aplicação na reação de redução do p-nitrofenol (NIP) e (ii) a introdução da ciclização do N-(o-iodobenzoil)-3-metilindol (MIAc), como modelo de reação de Heck para avaliação in situ, por espectroscopia UV-visível (UV-vis), da atividade de catalisadores a base de paládio molecular ou coloidal. Nove LIs foram sintetizados variando-se a cadeia alquílica ligada ao nitrogênio piridínico (metil, butil e octil) e contra-íons (haletos, tetrafluoroborato (BF4) e bis(trifluorometilsulfonil)imidato (NTf2)) e utilizados na estabilização de Ag-NPs. Para comparar os nove sistemas Ag-NPs/LI, verificou-se sua atividade catalítica na reação de redução do NIP, cujo progresso é facilmente monitorado por UV-vis. Todos os nanocatalisadores apresentaram atividade na redução do NIP sendo que, em geral, as maiores constantes de velocidade foram obtidas quando LIs metilados foram utilizados como estabilizante. Na reação de ciclização do MIAc, três sistemas catalíticos foram testados: nanopartículas de paládio (Pd-NPs/LI-Me(I)) pré-formadas e estabilizadas pelo LI N-metilpiridínico (LI-Me(I)); Pd(OAc)2 e Pd(OAc)2 com adição de LI-Me(I) (Pd(OAc)2/LI-Me(I)). Diversos experimentos foram realizados como variação da quantidade do catalisador, da concentração e natureza da base, do solvente e do LI, além de análises por microscopia eletrônica de transmissão (TEM) em diferentes tempos de reação e envenenamento com Hg. Após essa série de experimentos, pode-se dizer que a ciclização do MIAc foi introduzida com êxito como modelo de reação de Heck uma vez que seu progresso foi acompanhado in situ por UV-vis facilitando a realização de diversos experimentos para obtenção dos parâmetros cinéticos TON (mol de catalisador/mol de produto) e TOF (TON/tempo), com os quais comparou-se a atividade e aplicabilidade de sistemas catalíticos a base de paládio. Além disso, demonstrou ser uma reação relativamente branda ocorrendo em meio aquoso, na ausência de atmosfera inerte e a 80 ºC. O sistema Pd(OAc)2/LI-Me(I), com 1x10-3 mmol L-1 de LI-Me(I), resultou nos maiores parâmetros TON e TOF e maior conversão ao produto de ciclização.Abstract : The present work is divided in: (i) synthesis of new pyridinium ionic liquids (LIs) to use as stabilizing agents of silver nanoparticles (Ag-NPs), whose application was the reduction reaction of p-nitrophenol (NIP) and (ii) the introdution of (2-iodophenyl)(3-methyl-1H-indol-1-yl)methanone (MIAc) ciclization as a model reaction, for the in situ evaluation by UV-vis Spectroscopy (UV-vis) of Pd catalysts based on molecular or colloidal palladium. Nine LIs were synthesized modifying the alkyl chain bounded to the pyridine nitrogen (methyl, butyl and octyl) and the counteranion (halides, tetrafluoroborate (BF4-) and bis(trifluoromethylsulfonyl)imidate (NTf2-)). To compare the nine Ag-NPs/LI systems, their catalytic activity was evaluated in the NIP reduction reaction, whose progress can be easily monitored by UV-vis. All Ag-NPs/LI catalyst showed a good catalytic activity for the NIP reduction but, in general, the highest rate constants were obtained with methylic LIs. For the MIAc ciclization, three catalytic systems were evaluated: as-formed palladium nanoparticles (Pd-NPs/LI-Me(I)) stabilized by N-methylpyridinic LI, Pd(OAc)2, and Pd(OAc)2 with LI-Me(I) addition (Pd(OAc)2/LI-Me(I)). A series of experiments was carried out including varying the amount of catalyst, the concentration and nature of the base, solvent, and LI, analysis by transmission electronic microscopy (TEM) at different times of the reaction and poisoning tests with Hg. After this series of experiments, it can be said that the cyclization of MIAc was successfully introduced as a model for Heck reaction since its progress was easily monitored in situ by UV-vis, facilitating the implementation of several experiments and obtaining the kinetic parameters TON (mol of product/mol of catalyst) and TOF (TON/time), in which the activity and applicability of the catalytic systems could be compared. Furthermore, the reaction was carried out at relatively mild conditions once it occurs in aqueous medium, in the absence of inert atmosphere, and at 80 ° C. The Pd(OAc)2/LI-Me(I) system with 1x10-3 mmol L-1 of LI-Me(I) resulted in both, highest kinetic parameters TON and TOF and ciclization product conversion
Estudos cinéticos de uma reação de Heck intramolecular em meio aquoso catalisadas por nanopartículas de paládio suportadas em polietilenoimina ramificada
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas, Programa de Pós-Graduação em Química, Florianópolis, 2013Reações de acoplamento Carbono-Carbono do tipo Heck, em meio aquoso, tem se destacado como uma nova forma de realizar síntese orgânica, com uma metodologia mais barata, segura e confiável para a construção de diferentes moléculas. Até o momento poucos trabalhos foram dedicados ao estudo mecanístico destas reações. Neste sentido, este trabalho envolve a preparação, caracterização e o emprego de colóides de paládio, estabilizados pelo polímero comercial polietilenoimina, como catalisador em uma Reação de Heck Intramolecular, na presença de base, usando água como solvente e sem controle da atmosfera. O estudo dos diversos aspectos que influenciaram a velocidade da reação, assim como também a conversão ao produto, foi realizado por uma nova metodologia cinética. Análise Cinética do Progresso da Reação foi aplicada e pelos resultados obtidos foi observado que a reação é de primeira-ordem em relação ao catalisador e ordens em relação aos outros reagentes. Além disso, identificou-se uma forte relação, tanto na velocidade da reação quanto na conversão ao produto, com as concentrações iniciais da base. A análise destes resultados, aliados à estudos morfológicos das nanopartículas de paládio por microscopia eletrônica de transmissão, indicam a participação da superfície da nanopartícula e a alteração de seu tamanho durante o progresso da reação. Ainda, nas condições reacionais estudadas foi possível obter até 90% de conversão a produto após 24 horas, sem uso de atmosfera inerte e em condições brandas de temperatura.Abstract : Carbon-Carbon Heck coupling reactions, in aqueous media, have become a new way to make organic synthesis, as a cheaper, safer and more reliable methodology to build different molecules. So far only a few works have been dedicated to the mechanistic studies of these reactions. Through this way, the present work involves the preparation, characterization and the use of palladium colloids, stabilized by the commercial polymer polyethyleneimine, in an Intramolecular Heck Reaction, in the presence of a base, using water as the reaction solvent and without control of the atmosphere. The study of the different aspects that influenced the reaction rate, as well as the product conversion, was performed by a new kinetic methodology. Reaction Progress Kinetic Analysis was applied and from the results it was observed that the reaction is first-order in respect to the catalyst and have complex orders in respect to other reactants. Besides, it has been identified a strong relationship of the reaction rate, and product conversion, with the initial concentrations of the base. Analysis of these results, allied to the palladium nanoparticles morphological studies by transmission electron microscopy, indicated a participation of the nanoparticle#s surface and its size shifting during the reaction progress. Moreover, in the reaction conditions studied it was possible to obtain up to 90% of conversion to product in 24 hours without inert atmosphere and under mild conditions of temperature
Study of the Heck coupling reactions of 2-iodo-1-C-subtituated glycals
No full textKutatásunk során a 2-jód-1-C-szubsztituált glikálok Pd-katalizált Heck kapcsolási reakcióját tanulmányoztuk. Munkám során előálítottuk a kapcsoláshoz szükséges jódglikálokat a megfelelő telítetlen származékból kiindulva. A Heck reakció optimalizálása közben kísérleteket végeztünk a katalizátor, a ligandum, a bázis és az oldószer cseréjével.hbkKémiaBSc/B
Alternation of chemoselective control in Stille-Heck and Heck-Stille reaction sequences
No full textSequential and one-pot Stille–Heck and Heck–Stille reaction processes have been invoked to give divergent access to polycyclic ring systems. Both reaction conditions and substrate structure are important in determining the nature of the reaction products formed. The Heck–Stille reactions have involved a reversal of the usual Heck regioselectivity and both cine- and ipso-substitutions have been observed in the Stille reaction
Aliphatic Radical Relay Heck Reaction at Unactivated C(sp3)–H Sites of Alcohols
No full textThe Mizoroki−Heck reaction is one of the most efficient methods for alkenylation ofaryl, vinyl, and alkyl halides. Due to its innate nature, this protocol requires the employment of compounds possessing a halogen atom at the site of functionalization. However, the accessibility of organic molecules possessing a halogen atom at a particular site in aliphatic systems is extremely limited. Thus, a protocol that would allow a Heck reaction to occur at a specific non-functionalized C(sp3)−H site would be highly desirable.Here, we report a radical relay Heck reaction which allows for a selective remotealkenylation of aliphatic alcohols at unactivated β-, γ- and δ-C(sp3 20 )–H sites. The use of easily installable/removable Si-based auxiliary enables selective I-atom/radical translocation events at remote C−H sites followed by the Heck reaction. Notably, the reaction proceeds smoothly under mild visible light-mediated conditions at room temperature, producing highly modifiable and valuable alkenol products from readily available alcohols feedstocks. <br /
Cyclopropane-Fused <i>N</i>‑Heterocycles via Aza-Heck-Triggered C(sp<sup>3</sup>)–H Functionalization Cascades
No full textUnique examples of aza-Heck-based C(sp3)–H
functionalization
cascades are described. Under Pd(0)-catalyzed conditions, the aza-Heck-type
cyclization of N-(pentafluorobenzoyloxy)carbamates
generates alkyl–Pd(II) intermediates that effect C(sp3)–H palladation en route to cyclopropanes. Key factors that
control the site selectivity of the cyclopropanation process have
been elucidated such that selective access to a wide range of ring-
or spiro-fused systems can be achieved
Heck-Type Coupling of Fused Bicyclic Vinylcyclopropanes: Synthesis of 1,2-Dihydropyridines, 2,3-Dihydro1Hazepines, 1,4-Cyclohexadienes, and 2HPyrans
No full textHerein, we report a versatile approach
for the endocyclic ring
opening of bicyclic vinylcyclopropanes triggered by Heck arylations.
The key step for this transformation is a β-C-elimination allowing
the ring expansion of cyclopropanated pyrroles, piperidines, furans,
as well as cyclopentadienes to grant access to the corresponding 1,2-dihydropyridines,
2H-pyrans, 2,3-dihydro-1H-azepines,
and 1,4-cyclohexadienes, respectively. Additionally, gem-disubstituted cyclopropanated furans showed unexpected behavior
by giving diastereoselectively asymmetrically substituted dienes.
Mechanistic studies and theoretical calculations point toward a facile
β-C-elimination with a concomitant shift of Pd along the cyclopropane
moiety, which can successfully compete with the usual termination
step of a Heck reaction via a syn-β-hydride elimination
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