Computational Study on the Inhibition Mechanisms of the Ziegler-Natta Catalyst in the Propylene Polymerization Process: Part 1 Effects of Acetylene and Methylacetylene.

Acetylene and methylacetylene are impurities commonly found in the raw materials used for the production of polymers such as polypropylene and polyethylene. Experimental evidence indicates that both acetylene and methylacetylene can decrease the productivity of the Ziegler-Natta catalyst and alter the properties of the resulting polymer. However, there is still a lack of understanding regarding the mechanisms through which these substances affect this process. Therefore, elucidating these mechanisms is crucial to develop effective solutions to this problem. In this study, the inhibition mechanisms of the Ziegler-Natta catalyst by acetylene and methylacetylene are presented and compared with the incorporation of the first propylene monomer (chain initiation) to elucidate experimental effects. The Density Functional Theory (DFT) method was used, along with the B3LYP-D3 functional and the 6-311++G(d,p) basis set. The recorded adsorption energies were -11.10, -13.99, and -0.31 kcal mol-1, while the activation energies were 1.53, 2.83, and 28.36 kcal mol-1 for acetylene, methylacetylene, and propylene, respectively. The determined rate constants were 4.68 × 1011, 5.29 × 1011, and 2.3 × 10-8 M-1 s-1 for acetylene, methylacetylene, and propylene, respectively. Based on these values, it is concluded that inhibition reactions are more feasible than propylene insertion only if an ethylene molecule has not been previously adsorbed, as such an event reinforces propylene adsorption.

In Situ DRIFTS Analysis during Hydrogenation of 1-Pentyne and Olefin Purification with Ag Nanoparticles.

The catalytic performance of nanoparticles (NPs) of Ag anchored on different supports was evaluated during the selective hydrogenation of 1-pentyne and the purification of a mixture of 1-pentene/1-pentyne (70/30 vol %). The catalysts were identified: Ag/Al (Ag supported on É£-Al2 O3 ), Ag/Al-Mg (Ag supported on É£-Al2 O3 modified with Mg), Ag/Ca (Ag supported on CaCO3 ) and Ag/RX3 (Ag supported on activated carbon-type: RX3). In addition, in situ DRIFTS analysis of 1-pentyne adsorption on each support, catalyst, and 1-pentyne hydrogenation were investigated. The results showed that the synthesized catalysts were active and very selective (≥85 %) for obtaining the desired product (1-pentene). Different adsorbed species (-C≡C- and -C=C-) were observed on the supports and catalysts surface using in situ DRIFT analysis, which can be correlated to the activity and high selectivity reached. The role of the supports and electronic properties over Ag improve the H2 dissociative chemisorption during the hydrogenation reactions; promoting the selectivity and the high catalytic performance. Ag/Al and Ag/Al-Mg were the most active catalysts. This was due to the synergism between the active Ag/Ag+ species and the supports (electronic effects). The results show that Ag/Al and Ag/Al-Mg catalysts have favorable properties and are promising for the alkyne hydrogenation and olefin purification reactions.

Electrophile-Promoted Nucleophilic Cyclization of 2-Alkynylindoles to Give 4-Substituted Oxazinoindolones.

A method for the synthesis of 4-organoselanyl oxazinoindolone derivatives by the cascade cyclization of N-(alkoxycarbonyl)-2-alkynylindoles using iron(III) chloride and diorganyl diselenides as promoters was developed. This protocol was applied to a series of N-(alkoxycarbonyl)-2-alkynylindoles containing different substituents. The reaction conditions also tolerated a variety of diorganyl diselenides having both electron donating and electron withdrawing groups. However, the reaction did not work for diorganyl disulfides and ditellurides. The reaction mechanism seems to proceed via an ionic pathway and the cooperative action between iron(III) chloride and diorganyl diselenides is crucial for successful cyclization. We also found that using the same starting materials, by simply changing the electrophilic source to iodine, led to the formation of 4-iodo-oxazinoindolones. The high reactivity of Csp2 -Se and Csp2 -I bonds were tested under cross-coupling conditions leading to the preparation of a new class of functionalized indole derivatives. In addition, the absorption, emission and electrochemical properties of 4-organoselanyl oxazinoindolones showed an important relationship with the substituents of the aromatic rings. The advantages of the methodology include the use of electrophilic to promote the cyclization reaction and functionalization of the indole ring, and the electronic properties presented by the prepared compounds can be exploited as probes, analyte detectors and optical materials.

Rose Bengal-photocatalyzed perfluorohexylation reactions of organic substrates in water. Applications to late-stage syntheses.

The Rose Bengal-photocatalyzed perfluorohexylation of olefins, alkynes, and electron-rich aromatic compounds in water was achieved employing perfluorohexyl iodide as fluoroalkyl source and TMEDA as sacrificial donor under green LED irradiation. Alkenes and alkynes rendered products derived from the atom transfer radical addition (ATRA) pathway, and in the case of alkynes, exclusively as E-stereoisomers. These are the first examples of photocatalyzed ATRA reactions carried out excursively in water alone. The reactions of aromatic compounds under the current protocol in water present the advantage of employing a perfluoroalkyl iodide (C6F13-I) as source of perfluorohexyl radicals. Examples of photocatalytic late-stage incorporations of fluoroalkyl moieties into two commercial drugs of widespread use are reported.

Catalysis at gold-ligand interfaces: a frustrated Lewis pair mechanism for selective reduction reaction / Catalise na interface ouro-ligante: pares de Lewis frustrados aplicados em reações de redução seletivas

The interaction of the organic ligands with metal nanoparticle has a very important role for applications in catalysis, as well as other processes involving ligands that can activate or poison the surface of metal nanoparticles. Very little has been studied so far on the role of organic ligands used either in the preparation of nanoparticles for applications in catalysis or addition in the reaction to activate the catalyst. In this thesis, we have studied strategies for the synthesis of metal nanoparticles, their use as components for the preparation of supported catalysts and activation and deactivation processes involving the ligands used as stabilizers or purposely added to the reaction medium or support for stimulate new reactivity and selectivity in reactions of industrial interest, such as hydrogenation. Here, the concept of frustrated Lewis pairs (FLPs) has been expanded to surface-FLP analogous formed by combining gold nanoparticles (NPs) and Lewis bases, such as amines or phosphines, creating a new channel for the heterolytic cleavage of H2, and thereby performing selective hydrogenation reactions with gold. A first approach to improve the catalytic activity of gold nanoparticles was to analyze the effect of nitrogen-containing bases. The starting inactive gold nanoparticles became highly active for the selective hydrogenation of alkyne into cis-alkenes. The hydrogenation proceeded smoothly and fully selective using H2 as the hydrogen source and under relatively mild conditions (80 °C, 6 bar H2). Our studies also have revealed that the presence of capping ligands blocks the adsorption of the amine to the gold surface, avoiding the FLPs interface and thereby leading to low catalytic activity. When the capping ligands were removed from the catalyst surface and an amine ligand was added, the FLPs interface is recovered and an enhanced catalytic activity was observed. Furthermore, we have demonstrated the successful use of simple organophosphorus ligands to boost the catalytic activity of Au NPs for a range of important reduction reactions, namely, epoxides, N-oxides, sulfoxides, and alkynes. Furthermore, the choice of phosphorus-containing ligands resulted in a decrease in the amount necessary to reach high conversion and selectivity in comparison with our previous study with N-containing ligands. The ligand-to-metal ratio decreased from 100 (amine/Au) to 1 (phosphite/Au). The synthesis of gold nanoparticles supported on N-doped carbon supports was used as an alternative method for the synthesis of a heterogeneous active gold catalyst for selective hydrogenations. The main advantage with respect to previous studies was to avoid the addition of external ligands, in large excess, for the activation of gold surfaces via FLP, making the whole process environmentally and economically attractive

A interação dos ligantes orgânicos com nanopartículas de metal certamente tem um papel muito importante para aplicações em catálise, bem como outros processos envolvendo ligantes que podem ativar ou envenenar a superfície de nanopartículas metálicas. Até agora, muito pouco foi estudado sobre o papel dos ligantes orgânicos utilizados na preparação de nanopartículas para aplicações em catálise ou adição na reação para ativar o catalisador. Nesta tese, foram estudadas estratégias para a síntese de nanopartículas metálicas, seu uso como componentes para a preparação de catalisadores suportados e processos de ativação e desativação envolvendo ligantes empregados como estabilizantes ou propositalmente adicionados ao meio de reação ou suporte para estimular novas reatividades e seletividade em reações de interesse industrial, como reações de hidrogenação. Aqui, o conceito de pares de Lewis frustrados (FLPs) foi expandido para o seu análogo de superfície formado pela combinação de nanopartículas (NPs) de ouro e bases de Lewis, como aminas ou fosfinas, criando um novo canal para a clivagem heterolítica de H2 e, assim, realizando reações seletivas de hidrogenação com ouro. Uma primeira abordagem para melhorar a atividade catalítica das nanopartículas de ouro foi analisar o efeito de bases contendo nitrogênio. As nanopartículas de ouro inicialmente inativas tornaram-se altamente ativas para a hidrogenação seletiva de alquino em cis-alquenos. A hidrogenação prosseguiu foi factível e totalmente seletiva usando H2 como fonte de hidrogênio e sob condições relativamente amenas (80 °C, 6 bar de H2). Nossos estudos também revelaram que a presença de estabilizantes pode bloquear a adsorção da base na superfície do ouro, impedindo a formação da interface FLPs e, portanto, levando a baixa atividade catalítica. Quando os estabilizantes foram removidos da superfície do catalisador e um ligante foi adicionado, o FLPs é formado sendo a atividade catalítica aprimorada. Além disso, demonstramos o uso bem-sucedido de ligantes organofosforados atuando como ativadores de Au NPs em uma série de importantes reações de redução, como, epóxidos, N-óxidos, sulfóxidos e alquinos. Além disso, a escolha do ligante fosforado resultou em uma diminuição na quantidade necessária para alcançar alta conversão mantendo a seletividade inalterada. A relação ligante/metal diminuiu de 100/1 (amina/Au) para 1/1 (fosfito/Au). A síntese de nanopartículas de ouro suportadas em carbono dopado com nitrogênio foi utilizada como método alternativo para a síntese de um catalisador heterogêneo de ouro ativo para hidrogenações seletivas. A principal vantagem em relação aos estudos anteriores foi evitar a adição de ligantes externos, em grande excesso, para a ativação de superfícies de ouro via FLP, tornando todo o processo ambiental e economicamente atraente

Carbon-11 carboxylation of terminal alkynes with [11 C]CO2.

A copper-catalysed radiosynthesis of carbon-11 radiolabelled carboxylic acids was developed by reacting terminal alkynes and cyclotron-produced carbon-11 carbon dioxide ([11 C]CO2 ) in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). A small library of 11 C-labelled propiolic acid derivatives were obtained with a total synthesis time of 15 min from end of bombardment (EOB) with a (non-isolated) radiochemical yield ranging from 7% to 28%.

Alkynes and Nitrogen Compounds: Useful Substrates for the Synthesis of Pyrazoles.

The easy preparation and functionalization of pyrazoles associated with their innumerable biological properties have made this class of N-heterocycles very attractive for the development of new synthetic routes and applications. The cyclization reactions of alkynes and nitrogen compounds represent a powerful tool for the preparation of pyrazoles. This Review covers the recent advances in the preparation of pyrazoles by reacting alkynes and nitrogen compounds under transition-metal-catalyzed or metal-free conditions.

Synthesis and Reactivity of an Early-Transition-Metal Alkynyl Cubane Mn4 C4 Cluster.

While the coordination chemistry of monometallic complexes and the surface properties of extended metal particles are well understood, the control of metal nanocluster formation has remained challenging. The isolation of discrete metal clusters provides an especially rare snapshot at the nanoscale of cluster growth. The synthesis and full characterization of the first early-transition-metal alkynyl cubane and the first µ3 -alkynyl Mn3 motif are reported.

Ytterbium-catalyzed formal [4+2] cycloaddition: synthesis of chalcogen-quinolines 3-unsubstituted

A regioselective ytterbium-catalyzed annulation reaction between ethylglyoxalate, anilines and silyl-alkynes bearing selenyl- and telluryl-moieties for the formation of poly-substituted quinolones is described. A series of examples formed under mild conditions are presented, including a scaled-up reaction and a study on catalyst recyclability.

Synthesis of Terminal Ethynyl Aryl Selenides and Sulfides Based on the Retro-Favorskii Reaction of Hydroxypropargyl Precursors.

The retro-Favorskii reaction is an excellent way to achieve terminal alkynes. Methodologies that connect the synthesis of terminal alkynes and organochalcogen motifs are important for the construction of novel compounds. Fourteen new terminal alkynes containing either Csp -S or Csp -Se bonds were selectively prepared through the retro-Favorskii reaction from the respective carbinol precursors. It was discovered that terminal chalcogen alkynes were stable for weeks if stored as a solution in hexanes.

Transition-metal-free one-pot synthesis of alkynyl selenides from terminal alkynes under aerobic and sustainable conditions.

Alkynyl selenides were synthesized by a straightforward one-pot and three-step methodology, without the need of diselenides as starting reagents, under an oxygen atmosphere and using PEG 200 as the solvent. This procedure involves the in situ generation of dialkyl diselenides through a K3PO4-assisted reaction of an alkyl selenocyanate obtained by a nucleophilic substitution reaction between KSeCN and alkyl halides. Successive reaction with terminal alkynes in the presence of t-BuOK affords the corresponding alkyl alkynyl selenide in moderate to good yields. Finally, this methodology allowed the synthesis of 2-alkylselanyl-substituted benzofuran and indole derivatives starting from convenient 2-substituted acetylenes.

Double Gold Activation of 1-Ethynyl-2-(Phenylethynyl)Benzene Toward 5-exo-dig and 6-endo-dig Cyclization Reactions.

In this work, a detailed characterization was carried out of the ring-closure mechanism of EPB (1-ethynyl-2-(phenylethynyl)benzene) toward the 5-exo-dig and 6-endo-dig cyclization reactions, catalyzed by two Au-N-heterocyclic carbene (NHC) moieties. It was found that the 5-exo-dig cyclization takes place with a slightly lower activation barrier and larger exothermicity compared to that of the 6-endo-dig cyclization, in agreement with the available experimental data. A phenomenological partition (structural and electronic) for rate constants computed using transition-state theory and the reaction force analysis was used to shed light into the nature of the activation rate constant. It was found that rate constants are influenced by a strong structural component, which is larger for the 5-exo-dig cyclization due to the strain to form the five-membered ring. On the other hand, the gold activation mechanism is evidenced by a σ- and π-coordination of the Au-NHC moieties to the EPB substrate. It was found that differences in the σ-coordination arise on the reaction path for the 5-exo-dig and 6-endo-dig cyclizations. Thus, in the 6-endo-dig cyclization the σ gold-EPB interaction is weakened as a consequence of the formation of the cationic aryl intermediate, while for the 5-exo-dig cyclization this interaction was found to be favored. Furthermore, although minor changes in the Au-EPB coordination occur on the reaction path, these bonds are formally established in the TS vicinity. Results support the concerted nature of the dual gold activation mechanism.

Glycerol as Precursor of Organoselanyl and Organotellanyl Alkynes.

Herein we describe the synthesis of organoselanyl and organotellanyl alkynes by the addition of lithium alkynylchalcogenolate (Se and Te) to tosyl solketal, easily obtained from glycerol. The alkynylchalcogenolate anions were generated in situ and added to tosyl solketal in short reaction times, furnishing in all cases the respective products of substitution in good yields. Some of the prepared compounds were deprotected using an acidic resin to afford new water-soluble 3-organotellanylpropane-1,2-diols. The synthetic versatility of the new chalcogenyl alkynes was demonstrated in the iodocyclization of 2,2-dimethyl-1,3-dioxolanylmethyl(2-methoxyphenylethynyl)selane 3f, which afforded 3-iodo-2-(2,2-dimethyl-1,3-dioxolanylmethyl) selenanylbenzo[b]furan in 85% yield, opening a new way to access water-soluble Se-functionalized benzo[b]furanes.