Elucidating dramatic ligand effects on SET processes: iron hydride versus Iron borohydride catalyzed reductive radical cyclization of unsaturated organic halides

An iron(II) borohydride complex ([(η1-H3BH)FeCl(NCCH3)4]) is employed as the precatalyst in iron-catalyzed radical cyclizations of unsaturated organic halides in the presence of NaBH4. Mechanistic investigations have established that the ligand bound to the metal center (acetonitrile versus ethylenebis(diphenylphosphine) (dppe)) plays a crucial role in the structure and reactivity of the active anionic iron(I) hydride ([HFeCl(dppe)2]−) and borohydride ([(η1-H3BH)FeCl(NCCH3)4]−) with unsaturated haloacetals. This work provides new insights into iron(I) hydride and borohydride species and their potential implication in single-electron processes

Alkenyl boost for Catellani

The Catellani reaction is a multi-component cascade sequence, catalysed by palladium and norbornene, which typically uses aromatic starting materials. Now, through the use of a modified norbornene co-catalyst, the scope of this reaction has been extended to alkenyl reagents, enabling the preparation of all-carbon tetrasubstituted olefin

Etude théorique de la formation et de la réactivité de complexes organométalliques à base de zirconium (thèse pour le doctorat en sciences spécialité Chimie Théorique)

Dans ce mémoire, nous abordons dans un premier temps le mécanisme de formation des espèces du type Cl2Zr(alcène) à partir de Cl2Zr(alkyl)2. Nous montrons, par des calculs utilisant la DFT, que le mécanisme est une b-élimination concertée et associative. L'hypothèse de g-élimination est réfutée ainsi que pour les espèces de types Cp2Zr(alkyl)2. Pour ces dernières, il n'est pas nécessaire de faire intervenir spécifiquement un solvant polaire (éther) ou une phosphine pour générer Cp2Zr(alcène), alors que dans le cas des ligands chlores, cela est indispensable. Cet effet de solvant spécifique est expliqué par une décomposition énergétique. Nous comparons ensuite le comportement des espèces L2M(liaison p) où L est un chlore, un cyclopentadiènyle ou un méthoxy, M un zirconium, un titane et où la liaison p est constituée par une famille d'alcènes et une famille de carbonyles. La stabilité relative de ces différents composés est analysée ainsi que la complexation d'une molécule de solvant polaire. Nous étudions par la suite l'insertion d'une seconde molécule comportant une double liaison sur ces molécules, ce qui conduit à des métallacyclopentanes. Nous montrons en particulier que la stabilité relative de ces cycles peut directement être reliée à la stabilité relative des métallacycles à trois chaînons pour l'insertion d'un même substrat. La dernière partie est consacrée à l'étude de la réactivité de Cl2ZrEt2 en présence de différents substrats (formaldéhyde, benzaldéhyde, diméthylfulvène) soit en milieu polaire, soit en milieu apolaire. Nous démontrons ainsi, qu'en milieu apolaire, Cl2ZrEt2 réagit avec ces substrats insaturés par un mécanisme concerté à six centres de transfert d'hydrogèneIn this thesis, we initially start on the mechanism of formation of the species of the type Cl2Zr(alkene) starting from Cl2Zr(alkyl)2. We show, by calculations using DFT, that the mechanism is a concerted and associative b hydride abstraction. The hypothesis of g hydride abstraction is refuted, and also for the species of the Cp2Zr(alkyl)2 types. But whereas for these last, it was not necessary to utilize specifically a polar solvent (ether) or a phosphine to generate Cp2Zr(alkene), we show that in the case of the chlorine ligands, that is essential. This effect of specific solvent is explained by an energetic decomposition. We compare then the behavior of L2M(p bond) species where L is a chlorine, a cyclopentadienyl or a methoxy, M a zirconium, a titanium and where the p bond is constituted by an alkene family or a family of carbonyls. The relative stability of these various compounds is analyzed as well as complexation of one molecule of polar solvent. We study subsequently the insertion of a second molecule comprising a double bond on these molecules, which leads to metallacyclopentanes. We show in particular that the relative stability of these cycles can directly be connected to the stability of the metallacyclopropanes for the insertion of the same substrate. The last part is devoted to the reactivity study of Cl2ZrEt2 in the presence of various substrates (formaldehyde, benzaldehyde, dimethylfulvene) in polar and non-polar medium. We demonstrate thus, that in non-polar medium, Cl2ZrEt2 reacts with these unsaturated substrates by a concerted six-centers mechanism of hydrogen transferREIMS-BU Sciences (514542101) / SudocSudocFranceF

Massively parallel implementation of Steered Molecular Dynamics in Tinker-HP: comparisons of polarizable and non-polarizable simulations of realistic systems

International audienceSteered Molecular Dynamic (SMD) is a powerful technique able to accelerate rare events sampling in Molecular Dynamics (MD) simulations by applying an external force to a set of chosen atoms. Despite generating non-equilibrium simulations, SMD remains capable of reconstructing equilibrium properties such as the Potential of Mean Force (PMF). Of course, one would like to use all types of force fields (FF) ranging from classical ones to more advanced polarizable models using point induced dipoles and dis- tributed multipoles such as AMOEBA. To enable such studies, the SMD methodology has been implemented in the framework of the massively parallel Tinker–HP softwareallowing for both long polarizable and non-polarizable MD simulations of large proteins. To validate this new implementation, we first compared the Tinker–HP SMD results to the Literature. Tests have been performed on three different benchmarks: the M–A deca–alanine (112 atoms), the Ubiquitin (9737 atoms) and the CD2CD58 com- plex (97594 atoms). Non–polarizable (AMBER99, AMBER99SB, CHARMM22CMAP and OPLSAAL) and polarizable (AMOEBABIO09, AMOEBAPRO13 and AMOEBABIO18) force fields have been used. For each one of them, PMFs have been re-constructed and compared in terms of free energy barrier results and hydrogen bonding behavior fluctuations over time. Using a SMD velocity of 0.01 Å/ps applied to a set of 20 trajectories, we show that polarizable and non–polarizable force fields do notalways agree. As it could be anticipated, strong differences are noticed between polarizable and non-polarizable models when considered in vacuum, whereas results are morecomparable when a water environment is added. However, for the largest system, i.e. the CD2CD58 complex, strong differences related to the modeling of a salt bridge are noticed exhibiting some potential issues with classical FFs. Overall, such simulations highlight the importance of the inclusion of polarization effects as PMFs free energy barriers computed with AMOEBA always decrease compared to non–polarizable force fields

Hydrido-Cobalt Catalyst as a Selective Tool for the Dimerisation of Arylacetylenes: Scope and Theoretical Studies

International audienceA simple hydrido-cobalt complex efficiently catalyses the highly regio- and stereoselective dimerisation of various terminal arylacetylenes under mild conditions. The corresponding (E)-1,4-enynes are obtained as sole isomers with good to excellent yields. DFT calculations revealed that the reaction proceeds via a CH activation/hydrocobaltation pathway

How valence bond theory can help you understand your (bio)chemical reaction

Almost a century has passed since valence bond (VB) theory was originally introduced to explain covalent bonding in the H-2 molecule within a quantum mechanical framework. The past century has seen constant improvements in this theory, with no less than two distinct Nobel prizes based on work that is essentially developments in VB theory. Additionally, ongoing advances in both methodology and computational power have greatly expanded the scope of problems that VB theory can address. In this Tutorial Review, we aim to give the reader a solid understanding of the foundations of modern VB theory, using a didactic example of a model S(N)2 reaction to illustrate its immediate applications. This will be complemented by examples of challenging problems that at present can only be efficiently addressed by VB-based approaches. Finally, the ongoing importance of VB theory is demonstrated. It is concluded that VB will continue to be a major driving force for chemistry in the century to come

The Valence Bond Workshop in Paris: The Phoenix Rises from the Ashes or, Has a Love Story with MO-Based Theories Begun?

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