1,721,288 research outputs found
(NHC)copper(I)-catalyzed [3+2] cycloaddition of azides and mono- or disubstituted alkynes
No full textA versatile and highly efficient catalyst for the Huisgen cycloaddition reaction has been developed. Previously isolated or in situ generated azides yielded 1,2,3-triazoles with differently substituted alkynes in the presence of a [(NHC)CuBr] complex (NHC=N-heterocyclic carbene). Extremely high reaction rates and excellent yields were obtained in all cases. This catalytic system fulfils the requirements of "click chemistry" with its mild and convenient conditions, nota-bly in water or solvent free reactions and simple isolation with no purification step. Furthermore, for the first time, an internal alkyne was successfully used in this copper-catalyzed cycloaddition reaction. DFT calculations on this particular system allowed for the proposition of a new mechanistic pathway for disubstituted alkynes
Gold-mediated synthesis of alpha-ionone
Full text linkA simple and convenient synthesis of a-ionone, an important component of flowers and fragrances. The key step in the formation of the a,b-unsaturated ketone moiety involves an NHC-AuI cat- alyzed Meyer–Schuster-like rearrangement of readily prepared propargylic esters
Mechanism of CO2 Fixation by IrI–X Bonds (X = OH, OR, N, C)
No full textDensity functional theory calculations have been used to investigate the CO2 fixation mechanism proposed by Nolan et al. for the IrI complex [Ir(cod)(IiPr)(OH)] (1; cod = 1,5-cyclooctadiene;
IiPr = 1,3-diisopropylimidazol-2-ylidene) and its derivatives. For 1, our results suggest that CO2 insertion is the rate-limiting step rather than the dimerization step. Additionally, in agreement with the experimental results, our results show that CO2 insertion into the Ir–OR1 (R1 = H, methyl, and phenyl) and Ir–N bonds is kinetically facile, and the calculated activation energies span a range of only 12.0–23.0 kcal/mol. Substantially higher values (35.0–50.0 kcal/mol) are reported for analogous Ir–C bond
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Steric and Electronic Properties of N-Heterocyclic Carbenes (NHC): A Detailed Study on Their Interaction with Ni(CO)4.
No full textN-heterocyclic carbene ligands IMes (1), SIMes (2), IPr (3), SIPr (4), and ICy (5) react with Ni(CO)4 to give the saturated tricarbonyl complexes Ni(CO)3(IMes) (8), Ni(CO)3(SIMes) (9), Ni(CO)3(IPr) (10), Ni(CO)3(SIPr) (11), and Ni(CO)3(ICy) (12), respectively. The electronic properties of these complexes have been compared to their phosphine analogues of general formula Ni(CO)3(PR3) by recording their îCO stretching frequencies. While all of these NHCs are better donors than tertiary phosphines, the differences in donor properties between ligands 1-5 are surprisingly small. Novel, unsaturated Ni(CO)2(IAd) (13) and Ni(CO)2(ItBu) (14) compounds are obtained from the reaction of Ni(CO)4 with IAd (6) and ItBu (7). Complexes 13 and 14 are highly active toward substitution of the NHC as well as the carbonyl ligands. This has allowed the determination of Ni-C(NHC) bond dissociation energies and the synthesis of various unsaturated Ni- (0) and Ni(II) complexes. Computational studies on compounds 8-14 are in line with the experimental findings and show that IAd (6) and ItBu (7) are more bulky than IMes (1), SIMes (2), IPr (3), SIPr (4), and ICy (5). Furthermore, a method based on %Vbur values has been developed for the direct comparison of steric requirements of NHCs and tertiary phosphines. Complexes 8-14, as well as NiCl(C3H5)(ItBu) (16) and NiBr(C3H5)(ItBu) (17), have been characterized by X-ray crystallography
Machine learning directed discovery and optimisation of a platinum-catalysed amide reduction
No full textThe discovery and optimisation of reaction conditions leading to the reduction of amides, a fundamental large-scale industrial reaction, is achieved using a machine learning (ML) platform and a platinum catalyst. The optimisation leads to the discovery of a new platinum-based catalytic system that displays unexpectedly high performance. The approach enables rapid and high conversions at ppm-level catalyst loadings
N-heterocyclic carbene ligands in palladium and iridium organometallic chemistry
Full text linkThe use of ligand in transition-metal catalysed reactions has had a considerable impact. The
present manuscript aims at showing the influence of ligands in the palladium catalysed Suzuki-Miyaura cross-coupling reaction. In chapter one, the mechanism of this reaction will be described
based on the numerous contribution published in the literature. It will be shown that the electronic
and steric properties of the ligands have a huge repercussion on the catalytic activity of the metal.
In the second chapter, the electronic and steric properties of the widely used Buchwald-phosphine ligand will be investigated. For this purpose, bis-carbonyl iridium(I) complexes were
synthesized and their characterization allowed determining their TEP (Tolman electronic parameter)
and their buried volume %V[subscript(bur)].
Then three next chapters of this thesis will focus on the syntheses and characterizations of
new palladium complexes bearing N-heterocyclic carbenes (NHC). Their design was made in a
view to obtain high activity in cross coupling reaction, particularly in the Suzuki-Miyaura cross
coupling.
Their activation under the catalytic conditions leads to the formation of palladium(0) species
that can be mono- or bis-ligated. The influence of the ligand on the catalyst activity will be
discussed. A palladium(II) precatalyst leading to mono-ligated active species will be described. Its
activity in cross-coupling is very good, since activated and non-activated aryl chlorides could be
coupled with aryl boronic acids at room temperature using low catalyst loadings. Unfortunately, the
catalyst activity decreased with temperature. This result showed the fragility of the mono-ligated
active species. In a view to obtain more robust catalysts that can perform high turnover numbers,
new palladium(II) precatalysts bearing two ancillary ligands were developed. Mixed systems NHC-
phosphites and NHC-phosphines are described. NHC-phosphites precatalysts displayed very good
activity, but the phosphites are unfortunately sensitive to reaction conditions, limiting their role of
active species shield. NHC-phosphine bearing complexes were highly active and could perform up
to 10,000 turnovers with unactivated aryl chlorides. Very interestingly, these catalysts were also
able to couple benzylchlorides and allyl chlorides with a wide range of boronic acids at very low
catalyst loadings. These reactions had also the great advantage to proceed in aqueous solvents at
very high substrate concentration.
The activation mechanism of these complexes was investigated. Dichloropalladium(II)
complexes were reduced under the catalytic conditions to lead palladium(0) species. Therein, it is
shown that this reduction step was rate-determining in catalysis. Some palladium(0) intermediates
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were synthesized and showed good to excellent activities at low temperature under the exact same
conditions. They displayed high reactivity towards oxygen and moisture and have to be handled
under inert atmosphere. A particular complex had the ability to react with molecular dioxygen to
form a stable peroxo-complex. Interestingly, this complex displayed excellent activity in water
under aerobic conditions. This system was of great advantage since the reaction could be set up
under air using cheap and user-friendly reagents displaying low toxicity. Moreover, the readily
available distilled water used as solvent did not require prior degassing.
Symmetrical and unsymmetrical bis-NHC palladium(0) complexes were successfully
synthesized. They display excellent activity in the Suzuki-Miyaura cross coupling and turnover
frequencies as high as 300 could be obtained at room temperature with unactivated arylchlorides
and arylboronic acids. These complexes were also found excellent catalysts for the coupling of
benzylchlorides with arylboronic acids. Mechanistic studies showed that no ligand dissociation
occurred during the coupling suggesting as bis-ligated active species
N-heterocyclic carbene gold hydroxide complexes as bond activation reagents
No full textAlthough known since the 1930s, organogold chemistry has been dormant until recently, primarily due to preconceptions about the inertness of gold in transformations. However, this last decade has witnessed the emergence of a Golden Age with the development of a wealth of reports on gold in a plethora of reactions. In recent years, the drive for more atom- and step-economical and environmentally friendly reactions has become a field of intense research. In our on-going research on well-defined transition metal complexes bearing NHC ligands, our group recently discovered a new gold(I) hydroxide complex [Au(OH)(IPr)] (1a) that can be easily synthesised from the chloride precursor [AuCl(IPr)] (1b). A preliminary survey of the reactivity of this gold synthon has demonstrated interesting reactivity that holds great potential in bond activation reactions and the development of useful synthetic methods. Simplistically, this gold hydroxide complex can be seen as a strong Brønsted base. This thesis is dedicated to an in-depth examination of the reactivity of this complex in base-free bond activation reactions. Two themes predominate in the following chapters: the first part demonstrates the activity of gold(I) hydroxide as a bond activation agent to readily and efficiently access organogold complexes while the second part studies the reactivity of this compound in decarboxylation processes with carboxylic acids. Chapter 2 and 3 were dedicated to the development of new synthetic routes to access organogold complexes via base-free transmetalation reactions with organoborons and silanes using 1a. The combination of experimental and computational studies allowed identification and isolation of key intermediates in these reactions. Chapter 4 can be seen as a transition between the development of novel methodologies to synthesise aryl and heteroarylgold complexes and the first steps of gold hydroxide 1a as mediator in decarboxylation reaction. As a result, a novel mode of reactivity for gold was discovered and the synthetic route developed constitutes one of the greenest procedures to prepare organogold complexes with the generation of water and CO₂ as only side products. Chapter 5 and 6 venture further into the exploration of 1a in decarboxylation reactions and detail the development of a catalytic process for the protodecarboxylation reaction and subsequent mechanistic investigations of this reaction through stoichiometric experiments and kinetic and computational studies
From olefin metathesis to organoruthenium homogeneous catalysis : synthesis, applications and mechanistic understanding
No full textOlefin metathesis is a valuable synthetic tool, widely used in several fields of science. Due to the importance of this transformation several contributions have been made in this field in order to understand mechanistic aspects, reactivity and applicability of this process.
In this topic, ruthenium indenylidene complexes have shown great activity and stability in metathesis, making them very valuable pre-catalysts. However, several aspects of these pre-catalysts have not been evaluated yet. For example, even though reports of active second generation ruthenium indenylidene complexes bearing bulky N-heterocyclic carbenes are present in the literature, no studies have been done to understand how steric hindrance affects the process. For these reasons, [RuCl₂(IPr*)(PPh₃)(3-phenylindenylidene)] (IPr*-PPh₃) and [RuCl₂(IPr*)(Py)(3-phenylindenylidene)] (IPr*-Py), bearing the very bulky ligand, IPr* have been synthesised and compared with [RuCl₂(IPr)(PPh₃)(3-phenylindenylidene)] (IPr-PPh₃) and the new [RuCl₂(IPr)(Py)(3-phenylindenylidene)] (IPr-Py).
Another important aspect, presented in this thesis, is the investigation of the stability of indenylidene pre-catalysts in alcohol solvents. Surprisingly, several different decomposition processes occur depending on the starting complex and the alcohol used. Mechanistic investigation into this decomposition, allowed us to develop a better understanding of this process, and to predict the decomposition product based on the environment. In particular, this study revealed that [RuCl(η⁵-3-phenylindenyl)(PPh₃)₂] (Eta-5) is accessed from [RuCl₂(3-phenylindenylidene)(PPh₃)₂] (M₁₀) via a novel indenylidene to η⁵-indenyl rearrangement. This formal decomposition product has been found to be active in at least 20 different catalytic transformations, rendering it a versatile catalytic tool
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