Computational Organometallic Catalysis : Where We Are, Where We Are Going

Altres ajuts: Acord transformatiu CRUE-CSICThis essay gives my personal perspective of the current stage of computational methods applied to modeling organometallic catalysis, as well as the new directions the field is taking. The first part of the essay deals with what I consider the state-of-the-art to build up energy profiles, regarding both chemical and computational models. With a proper choice of the chemical model and computational methods, quantum mechanical calculations are nowadays able to provide accurate energy profiles of organometallic reactions in solution involving closed-shell species. However, in most cases they are still used to "predict the past", providing after-the-fact explanations and missing out the full potential of contemporary simulation techniques. Simulations are mature enough to be incorporated at the design stage and to guide the experimental exploration. The new directions the field is taking, incorporating automated exploration methods and combined with extensive data analysis and machine learning algorithms, approach the holy grail of catalyst discovering

On acabarà el protó?

Les reaccions de transferència protònica són reaccions importants en processos catalítics i es creu que poden també participar en el mecanisme d'acció de sistemes biològics com les hidrogenases. La protonació dels hidrurs de metalls de transició és un procés formalment simple entre un donador i un acceptor de protons. En canvi, resulta ser un procés més complex del que podria semblar, doncs varia segons quin és el centre metàl·lic i els lligands que l'acompanyen, del donador de protons i del solvent on té lloc la reacció, que poden afavorir la protonació en un centre bàsic o en un altre, i inclús fer que aquesta no es produeixi.Las reacciones de transferencia electrónica son reacciones importantes en procesos catalíticos y se cree que también pueden participar en el mecanismo de acción de sistemas biológicos, como las hidrogenasas. La protonación de los hidruros de metales de transición es un proceso formalmente sencillo entre un donador y un aceptor de protones. En cambio, resulta ser un proceso más complejo de lo que podría parecer, pues varía según cuál es el centro metálico y los ligandos que lo acompañan, del donador de protones y del solvente donde tiene lugar la reacción, que pueden favorecer la protonación en un centro básico o en otro, e incluso hacer que no se produzca la reacción

C-H Oxidative Addition of Bisimidazolium Salts to Iridium and Rhodium Complexes, and N-Heterocyclic Carbene Generation. A Combined Experimental and Theoretical Study

Premi a l'excel·lència investigadora. Àmbit de les Ciències Experimentals. 2008A series of bis-N-heterocyclic carbenes of rhodium and iridium have been obtained and characterized. The formation of the M-C bond has been studied according to experimental and theoretical data, showing that two different mechanisms are operative for the first (single proton deprotonation of the bisimidazolium salt, or oxidative addition followed by deprotonation of the metal hydride) and second (oxidative addition of the second bisimidazolium C-H bond, yielding a NHC-IrIII-H species) metalation processes. In the case of complexes with long linkers between the imidazolium rings, reductive elimination of HCl affords bisimidazolylidene complexes of IrI. According to the theoretical studies we concluded that thermodynamic parameters would determine the formation of the NHC-IrIII-H species, while IrI-NHC species would be kinetically favored in the case of complexes with long linkers between the azole rings. The crystal structures of a series of Ir-bis(NHC) complexes are described

Beyond Continuum Solvent Models in Computational Homogeneous Catalysis

Altres ajuts: Acord transformatiu CRUE-CSICIn homogeneous catalysis solvent is an inherent part of the catalytic system. As such, it must be considered in the computational modeling. The most common approach to include solvent effects in quantum mechanical calculations is by means of continuum solvent models. When they are properly used, average solvent effects are efficiently captured, mainly those related with solvent polarity. However, neglecting atomistic description of solvent molecules has its limitations, and continuum solvent models all alone cannot be applied to whatever situation. In many cases, inclusion of explicit solvent molecules in the quantum mechanical description of the system is mandatory. The purpose of this article is to highlight through selected examples what are the reasons that urge to go beyond the continuum models to the employment of micro-solvated (cluster-continuum) of fully explicit solvent models, in this way setting the limits of continuum solvent models in computational homogeneous catalysis. These examples showcase that inclusion of solvent molecules in the calculation not only can improve the description of already known mechanisms but can yield new mechanistic views of a reaction. With the aim of systematizing the use of explicit solvent models, after discussing the success and limitations of continuum solvent models, issues related with solvent coordination and solvent dynamics, solvent effects in reactions involving small, charged species, as well as reactions in protic solvents and the role of solvent as reagent itself are successively considered

Anti-Markovnikov Intermolecular Hydroamination of Alkenes and Alkynes : A Mechanistic View

Altres ajuts: acords transformatius de la UABHydroamination, the addition of an N-H bond across a C-C multiple bond, is a reaction with a great synthetic potential. Important advances have been made in the last decades concerning catalysis of these reactions. However, controlling the regioselectivity in the amine addition toward the formation of anti-Markovnikov products (addition to the less substituted carbon) still remains a challenge, particularly in intermolecular hydroaminations of alkenes and alkynes. The goal of this review is to collect the systems in which intermolecular hydroamination of terminal alkynes and alkenes with anti-Markovnikov regioselectivity has been achieved. The focus will be placed on the mechanistic aspects of such reactions, to discern the step at which regioselectivity is decided and to unravel the factors that favor the anti-Markovnikov regioselectivity. In addition to the processes entailing direct addition of the amine to the C-C multiple bond, alternative pathways, involving several reactions to accomplish anti-Markovnikov regioselectivity (formal hydroamination processes), will also be discussed in this review. The catalysts gathered embrace most of the metal groups of the Periodic Table. Finally, a section discussing radical-mediated and metal-free approaches, as well as heterogeneous catalyzed processes, is also included

Rationalization of a Streptavidin Based Enantioselective Artificial Suzukiase : An Integrative Computational Approach

Altres ajuts: acords transformatius de la UABAn Artificial Metalloenzyme (ArM) built employing the streptavidin-biotin technology has been used for the enantioselective synthesis of binaphthyls by means of asymmetric Suzuki-Miyaura cross-coupling reactions. Despite its success, it remains a challenge to understand how the length of the biotin cofactors or the introduction of mutations to streptavidin leads the preferential synthesis of one atropisomer over the other. In this study, we apply an integrated computational modeling approach, including DFT calculations, protein-ligand dockings and molecular dynamics to rationalize the impact of mutations and length of the biotion cofactor on the enantioselectivities of the biaryl product. The results unravel that the enantiomeric differences found experimentally can be rationalized by the disposition of the first intermediate, coming from the oxidative addition step, and the entrance of the second substrate. The work also showcases the difficulties facing to control the enantioselection when engineering ArM to catalyze enantioselective Suzuki-Miyaura couplings and how the combination of DFT calculations, molecular dockings and MD simulations can be used to rationalize artificial metalloenzymes

True and masked three-coordinate T-shaped platinum(II) intermediates

Although four-coordinate square-planar geometries, with a formally 16-electron counting, are absolutely dominant in isolated Pt(II) complexes, three-coordinate, 14-electron Pt(II) complexes are believed to be key intermediates in a number of platinum-mediated organometallic transformations. Although very few authenticated three-coordinate Pt(II) complexes have been characterized, a much larger number of complexes can be described as operationally three-coordinate in a kinetic sense. In these compounds, which we have called masked T-shaped complexes, the fourth position is occupied by a very weak ligand (agostic bond, solvent molecule or counteranion), which can be easily displaced. This review summarizes the structural features of the true and masked T-shaped Pt(II) complexes reported so far and describes synthetic strategies employed for their formation. Moreover, recent experimental and theoretical reports are analyzed, which suggest the involvement of such intermediates in reaction mechanisms, particularly C–H bond-activation processes

Computational analysis on the Pd-catalyzed C-N coupling of ammonia with aryl bromides using a chelate phosphine ligand

The Buchwald-Hartwig amination of arylhalides with the Pd-Josiphos complex is a very useful process for the generation of primary amines using ammonia as a reactant. Density-functional theory (DFT) calculations are carried out to examine the reaction mechanism for this process. Although the general mechanism for the C-N cross-coupling reaction is known, there are still some open questions regarding the effect of a chelate phosphine ligand and the role of the base in the process. Reaction pathways involving the release of one of the arms of the phosphine ligand are compared with those where the chelate phosphine remains fully coordinated. Conformational analysis for the complex with the open chelate phosphine is required to properly evaluate the proposed pathways. The role played by the added base (t-BuO-) as a possible ligand or just as a base was also evaluated. The understanding of all of these aspects allowed us to propose a complete reaction mechanism for the Pd-catalyzed C-N coupling of arylhalides with ammonia using the chelate Josiphos ligand

Challenges in modelling homogeneous catalysis : new answers from ab initio molecular dynamics to the controversy over the Wacker process

The controversial reaction mechanism considering experimental results and theoretical treatment from static to ab initio molecular dynamic simulations is reviewed.</p

Ketone Hydrogenation with Iridium Complexes with “non N–H” Ligands: The Key Role of the Strong Base

Ferrocenyl phosphine thioether ligands (PS), not containing deprotonatable functions, efficiently support the iridium catalyzed ketone hydrogenation in combination with a strong base co-catalyst. Use of an internal base ([Ir(OMe)(COD)]2 in place of [IrCl(COD)]2) is not sufficient to insure activity and a strong base is still necessary, suggesting that the active catalyst is an anionic hydride complex. Computational investigations that include solvent effects demonstrate the thermodynamically accessible generation of the tetrahydrido complex [IrH4(PS)]-and suggest an operating cycle via a [Na+(MeOH)3∙∙∙Ir-H4(PS)] contact ion pair with an energy span of 18.2 kcal/mol. The cycle involves an outer sphere stepwise H-/H+ transfer, the proton originating from H2 after coordination and heterolytic activation. The base plays the dual role of generating the anionic complex and providing the Lewis acid co-catalyst for ketone activation. The best cycle for the neutral system, on the other hand, requires an energy span of 26.3 kcal/mol. This work highlights, for the first time, the possibility of outer sphere hydrogenation in the presence of non deprotonatable ligands and the role of the strong base in the activation of catalytic systems with such type of ligands