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Molecular solar thermal systems (MOST) represent an auspicious solution for the storage of solar energy. We report silver salts as a unique class of catalysts, capable of releasing the stored energy from the promising 1,2-dihydro-1,2-azaborinine based MOST system. Mechanistic investigations provided insights into the silver catalyzed thermal backreaction, concurrently unveiling the first crystal structure of a 2-aza-3-borabicyclo[2.2.0]hex-5-ene, the Dewar isomer of 1,2-dihydro-1,2-azaborinine. Quantification of activation energies by kinetic experiments has elucidated the advantageous energy change associated with Lewis acid catalysts, a phenomenon corroborated through computational analysis. By means of low temperature NMR spectroscopy, mechanistic insights into the coordination of Ag+ to the 1,2-dihydro-1,2-azaborinine were gained.
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We report a Fukuyama-type coupling of thioesters with aliphatic organomanganese reagents utilizing a cheap and easily available iron(III) precatalyst. The reactions exhibit a wide tolerance of solvents and functional groups, allowing for the conversion of thioesters derived from natural products and pharmaceutical compounds. A strong steric impact from each reaction component (carboxylic moiety, thiol substituent and manganese reagent) was displayed, which enabled regioselective transformation of dithioesters. Mechanistic investigations showed that the released thiolate does not act as a mere spectator ligand, but rather positively influences the stability of intermediate alkyl(II)ferrates.
Assuntos
Compostos Férricos , Ferro , Indicadores e Reagentes , Catálise , Compostos de SulfidrilaRESUMO
Herein, we describe the acid/Pd-tandem-catalyzed transformation of glycol derivatives into terminal formic esters. Mechanistic investigations show that the substrate undergoes rearrangement to an aldehyde under [1,2] hydrogen migration and cleavage of an oxygen-based leaving group. The leaving group is trapped as its formic ester, and the aldehyde is reduced and subsequently esterified to a formate. Whereas the rearrangement to the aldehyde is catalyzed by sulfonic acids, the reduction step requires a unique catalyst system comprising a PdII or Pd0 precursor in loadings as low as 0.75â mol % and α,α'-bis(di-tert-butylphosphino)-o-xylene as ligand. The reduction step makes use of formic acid as an easy-to-handle transfer reductant. The substrate scope of the transformation encompasses both aromatic and aliphatic substrates and a variety of leaving groups.
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We disclose a nickel/Brønsted acid-catalyzed tandem process consisting of double bond isomerization of allyl ethers and amines and subsequent intramolecular reaction with nucleophiles. The process is accomplished by [(Me3P)4NiH]N(SO2CF3)2 in the presence of triflic acid. The methodology provides rapid access to tetrahydropyran-fused indoles and other oxacyclic scaffolds under very low catalyst loadings.
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We report the first homogeneous palladium-based transfer hydrogenolysis of benzylic alcohols using an in situ formed palladium-phosphine complex and formic acid as reducing agent. The reaction requires a catalyst loading as low as only 1â mol % of palladium and just a slight excess of reductant to obtain the deoxygenated alkylarenes in good to excellent yields. Besides demonstrating the broad applicability for primary, secondary and tertiary benzylic alcohols, a reaction intermediate could be identified. Additionally, it could be shown that partial oxidation of the applied phosphine ligand was beneficial for the course of the reaction, presumably by stabilizing the active catalyst. Reaction profiles and catalyst poisoning experiments were used to characterize the catalyst, the results of which indicate a homogeneous metal complex as the active species.
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Herein, a new catalytic system to synthesize benzofuranones is reported. A palladium-catalyzed intramolecular alkoxycarbonylation is employed to generate 3-substituted-benzofuran-2(3H)-ones from alkenylphenols under mild reaction conditions, linked to an ex situ formation of CO from N-formylsaccharin. The carefully chosen catalytic system enables an efficient reaction with a novel functional group tolerance, despite the high polymerization tendency of the starting material.
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While thioesters are common intermediates in biochemical processes, they are much less appreciated in organic synthesis, also compared to other carboxylic acid derivatives. However, their chemistry and reactivity is intriguing and diversified, reaching much further than the acyl substitution and aldol chemistry. Herein, we focus on metal-catalyzed reactions for the synthesis of thioesters as well as their transformations. Reactions such as thiocarbonylation, cross-coupling, decarbonylation, allylic substitution or dual photoredox/metal catalysis are discussed. On one hand, new atom economic methods allow for convenient synthesis of thioesters from well available starting materials. On the other hand, various synthetically important compounds can by synthesized due to the multifaceted reactivity of thioesters that we aimed to depict.
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The Pd-catalyzed Fukuyama reaction of thioesters with organozinc reagents is a mild, functional-group-tolerant method for acylation chemistry. Its Ni-catalyzed variant might be a sustainable alternative to expensive catalytic Pd sources. We investigated the reaction of S-ethyl thioesters with aryl zinc halides with hetero- and homotopic Ni precatalysts and several ligands. The results show that both homo- and heterotopic species may contribute to catalysis. The substrate scope using an operationally homogeneous defined Ni complex was established. Acyl radicals are postulated as short-lived intermediates.
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A palladium-catalyzed thiocarbonylation of styrene derivatives is reported for the first time. The combination of thiols as nucleophiles and a bidentate ligand ensures a unique reaction outcome with high regioselectivity toward the more valuable branched isomer and new reactivity. The ambient reaction conditions (temperature, catalyst loading) and the use of a CO surrogate render this transformation a useful method for the synthesis of thioesters from available feedstock. Various functional groups on arene and thiol substituents are tolerated by the system. Notably, challenging ortho-substituted styrenes are converted with unprecedentedly high regioselectivity.
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Homogeneous ruthenium complexes modified by imidazole-substituted monophosphines as catalysts for various highly efficient hydroformylation reactions were characterized by in situ IR spectroscopy under reaction conditions and NMR spectroscopy. A proper protocol for the preformation reaction from [Ru3 (CO)12] is decisive to prevent the formation of inactive ligand-modified polynuclear complexes. During catalysis, ligand-modified mononuclear ruthenium(0) carbonyls were detected as resting states. Changes in the ligand structure have a crucial impact on the coordination behavior of the ligand and consequently on the catalytic performance. The substitution of CO by a nitrogen atom of the imidazolyl moiety in the ligand is not a general feature, but it takes place when structural prerequisites of the ligand are fulfilled.
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The combination of -I and -M effects and the aromaticity of the conjugate base renders the chiral pentacarboxy-substituted cyclopentadienes (PCCPs) acidic. Their convenient synthesis together with substrates derived from the chiral pool and their performance in the tested reactions renders them prodigious candidates to become a new and highly competitive class of chiral Brønsted acid catalysts.
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Tradition and innovation: The 51st Bürgenstock Conference on Stereochemistry took place from May 1-6, 2016 and offered its usual mixture of impressive science and superb discussions in a wonderful atmosphere. In the Meeting Review, Ivana Fleischer outlines the program.
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An efficient domino ruthenium-catalyzed reverse water-gas-shift (RWGS)-hydroformylation-reduction reaction of olefins to alcohols is reported. Key to success is the use of specific bulky phosphite ligands and triruthenium dodecacarbonyl as the catalyst. Compared to the known ruthenium/chloride system, the new catalyst allows for a more efficient hydrohydroxymethylation of terminal and internal olefins with carbon dioxide at lower temperature. Unwanted hydrogenation of the substrate is prevented. Preliminary mechanism investigations uncovered the homogeneous nature of the active catalyst and the influence of the ligand and additive in individual steps of the reaction sequence.
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An alternative ruthenium-based catalyst for the hydroesterification of olefins with formates is reported. The good activity of our system is ensured by the use of a bidentate P,N-ligand and ruthenium dodecacarbonyl. A range of formates can be used for selective alkoxycarbonylation of aromatic olefins. In addition, the synthesis of selected aliphatic esters is realized. The proposed active ruthenium complex has been isolated and characterized.
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An efficient and regioselective ruthenium-catalyzed hydroaminomethlyation of olefins is reported. Key to success is the use of specific 2-phosphino-substituted imidazole ligands and triruthenium dodecacarbonyl as catalyst. Both industrially important aliphatic as well as various functionalized olefins react with primary and secondary amines to give the corresponding secondary and tertiary amines generally in high yields (up to 96%) and excellent regioselectivities (n/iso up to 99:1).
Assuntos
Alcenos/química , Aminas/síntese química , Compostos Organometálicos/química , Rutênio/química , Aminas/química , Catálise , Metilação , Estrutura Molecular , EstereoisomerismoRESUMO
In the presence of 2-phosphino-substituted imidazole ligands and Ru3(CO)12 or Ru(methylallyl)2(COD) direct hydroformylation and hydrogenation of alkenes to alcohols takes place. In addition to terminal alkenes, also more challenging internal olefins are converted preferentially to industrially important linear alcohols in high yield (up to 88%) and regioselectivity (n:iso up to 99:1).
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The ruthenium-catalyzed hydroformylation of 1- and 2-octene to give preferentially the corresponding linear aldehyde is reported. The catalyst system comprising of Ru3 (CO)12 and an imidazole-substituted monophosphine ligand allows for high chemo- and regioselectivity. The hydroformylation proceeds with unprecedented rates for a ruthenium-based catalyst.
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Transition-metal-catalyzed hydroformylation reactions constitute one of the most powerful tools for C-C bond formation in organic synthesis and represent an outstanding example of the application of homogeneous catalysis on an industrial scale. This process allows for the straightforward conversion of inexpensive chemical feedstock into broadly applicable aldehydes, which serve as major building blocks for numerous chemical products. These products are highly valuable for the chemical industry and used as plasticizers, detergents, and surfactants on a million ton scale. Moreover, aldehydes serve as versatile chemical intermediates for the production of fine chemicals and pharmaceuticals. Currently, most of the bulk hydroformylation processes rely on rhodium-based catalysts. The increasing demand and resulting high cost of this precious metal has resulted in alternative transition-metal catalysts becoming highly desirable. The following Review summarizes the progress achieved utilizing Ru, Ir, Pd, Pt, and Fe catalysts in hydroformylation reactions.
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We report a nickel catalyzed C-S cross-coupling of aryl and alkenyl triflates with alkyl thiols. A variety of the corresponding thioethers were synthesized using an air-stable nickel precatalyst under mild reaction conditions with short reaction times. A broad substrate scope, including pharmaceutically relevant compounds, could be demonstrated.
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Herein we report a tandem olefin migration/hydrothiolation of allyl benzenes facilitated by an in situ generated palladium hydride. A catalyst system composed of palladium acetate and bidentate ligand dtbpx (1,2-bis(di- tert-butylphosphinomethyl)benzene in the presence of catalytic amounts of triflic acid led to the tandem transformation, which furnished benzylic thioethers. The reaction exhibits high regioselectivity and can be conducted under mild conditions. The robustness of the catalyst is displayed through reactions with coordinating thiols.