The aim of the present study is to develop readily available and stable pre‐catalysts that could be easily prepared on large scale from simple starting materials. Based on the hypothesis that ...substitution of classical PCy3 with phosphanes of varying electron‐donating properties could be a straightforward manner to improve catalytic activity, a methodical study dealing with the effect of phosphane fine‐tuning in ruthenium–indenylidene catalysts was performed. Challenged to establish how the electronic properties of para‐substituted phosphane ligands translate into catalyst activity, the versatile behaviour of these new ruthenium–indenylidene complexes was investigated for a number of metathesis reactions.
Ringmaster: A simple synthetic route leads to a series of RuCl2(NHC)(P(p‐RC6H4)3)(Ind) (Ind=indenylidene) complexes that has been evaluated in a number of olefin metathesis reactions. The optimised catalysts have been identified for ring‐closing metathesis, ring‐rearrangement metathesis, ring‐opening metathesis polymerisation and cross‐metathesis reactions.
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Olefin metathesis is a powerful tool for the formation of carbon–carbon double bonds. Several families of well-defined ruthenium (Ru) catalysts have been developed during the past 20 years; however, ...the reaction mechanism for all such complexes was assumed to be the same. In the present study, the initiation mechanism of Ru–indenylidene complexes was examined and compared with that of benzylidene counterparts. It was discovered that not all indenylidene complexes followed the same mechanism, highlighting the importance of steric and electronic properties of so-called spectator ligands, and that there is no single mechanism for the Ru-based olefin metathesis reaction. The experimental findings are supported quantitatively by DFT calculations.
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5.
A Diverse View of Science to Catalyse Change Urbina‐Blanco, César A.; Jilani, Safia Z.; Speight, Isaiah R. ...
Angewandte Chemie,
October 12, 2020, Volume:
59, Issue:
42
Journal Article
Peer reviewed
Open access
Valuing diversity leads to scientific excellence, the progress of science and most importantly, it is simply the right thing to do. We can value diversity not only in words, but also in actions.
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Easy as π: An unusual indenylidene to η5‐indenyl rearrangement of a ruthenium catalyst has been observed. The resulting complex is the formal decomposition product of an olefin metathesis precatalyst ...that displays exceptional activity in the racemization of chiral alcohols at very low catalyst loadings.
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While the fundamental series of 2+2cycloadditions and retro2+2cycloadditions that make up the pathways of ruthenium-catalysed metathesis reactions is well-established, the exploration of mechanistic ...aspects of alkene metathesis continues. In this Feature Article, modern mechanistic studies of the alkene metathesis reaction, catalysed by well-defined ruthenium complexes, are discussed. Broadly, these concern the processes of pre-catalyst initiation, propagation and decomposition, which all have a considerable impact on the overall efficiency of metathesis reactions.
While the fundamental series of 2+2cycloadditions and retro2+2cycloadditions that make up the pathways of ruthenium-catalysed metathesis reactions is well-established, the exploration of mechanistic aspects of alkene metathesis continues.
Catalytic transfer hydrogenation of ketones and aldehydes is generally accepted to follow a dehydrogenation–hydrogenation mechanism on copper, which makes the increased hydrogenation rate and ...selectivity rather puzzling. Using first-principles microkinetics on a Cu(111) surface, we show that, rather than a dehydrogenation–hydrogenation mechanism, there is also direct proton transfer between the sacrificial alcohol and the reacting ketone. The ketone is hydrogenated to a stable alkoxy intermediate using surface hydrogen. This alkoxy intermediate is subsequently hydrogenated to the alcohol product via direct proton transfer from the sacrificial alcohol, also forming a sacrificial alkoxy intermediate. To close the catalytic cycle, the sacrificial alkoxy species dehydrogenates, forming its corresponding ketone. We also observed a surprising catalytic effect of molecular hydrogen, which can be explained by the rate-controlling step in transfer hydrogenation: the direct hydrogenation of the ketone to its alkoxy intermediate by surface hydrogen. Under all realistic reaction conditions, this step has the highest degree of rate control.
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Density functional theory (DFT) calculations were used to predict and rationalize the effect of the modification of the structure of the prototype 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) ...N-heterocyclic carbene (NHC) ligand. The modification consists in the substitution of the methyl groups of ortho isopropyl substituent with phenyl groups, and here we plan to describe how such significant changes affect the metal environment and therefore the related catalytic behaviour. Bearing in mind that there is a significant structural difference between both ligands in different olefin metathesis reactions, here by means of DFT we characterize where the NHC ligand plays a more active role and where it is a simple spectator, or at least its modification does not significantly change its catalytic role/performance.
The synthesis and characterization of a novel indenylidene-containing ruthenium catalyst bearing the N-heterocyclic carbene (NHC) ligand 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene ...(SIPr) are reported. Comparative reactivity studies with other indenylidene complexes using various substrates show the importance of the sterically demanding SIPr ligand on catalyst reaction profile. The investigation of the reaction scope for ring-closing metathesis transformations establishes the influence of the NHC on catalyst activity especially as a function of substrate steric bulk. The novel catalyst was found very efficient at room temperature for nonsterically hindered substrates. Since the new catalyst was found to be soluble in a variety of solvents, a number of these were examined to gauge the importance of solvent effects.
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