Christophe Bour

Gold(I) Complexes with Hydrogen-Bond Supported Heterocyclic Carbenes as Active Catalysts in Reactions of 1,6-Enynes

Complexes [AuCl{C(NHR)(NHPy-2)}] (Py-2 ) 2-pyridyl; R ) Me, tBu, nBu, iPr, nheptyl) have been prepared in amodular way from [AuCl(CNPy-2)]. The carbene moiety has a hydrogen-bond supported heterocyclic structure similar to the nitrogen heterocyclic carbenes in the solid state, and in CH2Cl2 or acetone solution, which is open in the presence of MeOH. The compounds are good catalysts for the skeletal rearrangement of enynes, and for the methoxycyclization of enynes. In contrast, the complexes [AuCl{C(NHR)(NHPy-4)}] are scarcely active due to the blocking effect of the coordination position required for the catalysis by the nitrogen of the NHPy-4 group.

Cationic Gallium(III) Halide Complexes: A New Generation of π-Lewis Acids

Shun Tang, Julien Monot, Ahmad El-Hellani, Bastien Michelet, R gis Guillot, Christophe Bour, and Vincent Gandon* Gallium ACHTUNGTRENNUNG(III) halides are powerful Lewis acids that are used in a variety of catalytic transformations. These salts are strong oxo-, thioor azaphilic s-Lewis acids, yet they are also able to activate simple olefins and alkynes [Eq. 1]. For instance, as s-Lewis acids, they catalyze nucleophilic additions to epoxides, 1,4-addition to a, b-unsaturated carbonyl compounds, Nazarov reactions, skeletal rearrangement of aldehydes, allylation of imines, sulfidation of alkynes or alkenes, as well as C C bond formation involving C H, N H, and O H bond activation. As p-Lewis acids, GaX3 salts catalyze interand intramolecular nucleophilic additions to alkynes and alkenes. In spite of their capabilities, galliumACHTUNGTRENNUNG(III) halides are not widely used and keep giving rise to mistrust from the synthetic chemists community. The main reasons are: 1) the difficulty in handling these species due to their highly hygroscopic nature, and 2) the presupposed ill-defined nature of the intermediates, which makes the anticipation and the rationalization of the results quite difficult. The issue of hygroscopy is indeed a critical one; even after taking precautions, hydration products can be observed, sometimes in large amounts. Ga(OH)Cl2, which would arise from adventitious hydrolysis, has even been proposed as the active species of a coupling reaction catalyzed by GaCl3. [14] Thus, high loadings are usually required, typically between 10 and 100 mol %, to ensure a sufficient concentration of intact catalyst. In this context, the use of well-defined airand moisture-stable L·GaX3 adducts seems highly desirable to ensure further development of Ga-catalyzed transformations. Among these, a few examples of N-heterocyclic carbene– GaX3 adducts have been reported. [15] However, the stabilization of GaX3 by a strongly coordinating ligand will certainly be detrimental to its reactivity. To generate an active species, one could think of opening a vacant site on gallium by halide abstraction, that is, by using a silver salt [Eq. 2].

A new pd-catalyzed cascade reaction for the synthesis of strained aromatic polycycles.

Two new palladium catalyzed cascade reactions involving a 4-exo-dig cyclocarbopalladation are described. These processes are shown to convert bromoenediynes and bromodienynes into strained aromatic compounds in a single step.

Silver‐Free Two‐Component Approach in Gold Catalysis: Activation of [LAuCl] Complexes with Derivatives of Copper, Zinc, Indium, Bismuth, and Other Lewis Acids

Complexes of type [LAuCl] (L = phosphine, phosphite, NHC and others) are widely employed in homogeneous catalysis, however, they are usually inactive as such and must be used jointly with a halide scavenger. To date, this role has mostly been entrusted to silver salts (AgSbF6 , AgPF6, AgBF4, AgOTf, etc.). However, silver salts can be the source of deactivation processes or side reactions, so it is sometimes advisable to use silver-free cationic gold complexes, which can be difficult to synthesize and to handle compared with the more robust chloride. We show in this study that various Lewis acids of the transition and main group metal families are expedient substitutes to silver salts. We have tested Cu(I), Cu(II), Zn(II), In(III), Si(IV), Bi(III), and other salts in a variety of typical Au(I)-catalyzed transformations, and the results have revealed that [LAuCl] can form active species in their presence.

Relationship between Gallium Pyramidalization in L·GaCl3 Complexes and the Electronic Ligand Properties

Six new molecular GaCl3 adducts of electron rich compounds of the carbone (carbodiphosphorane, tetraaminoallene) and cyclic alkyl amino carbene (CAAC) families have been synthesized and characterized by X-ray crystallography. The sum of their Cl-Ga-Cl angles has been compared to those of 20 other complexes exhibiting various oxygen-, nitrogen-, phosphorus-, and carbon-donor ligands for which good quality X-ray analyses have been reported. The pyramidalization of the GaCl3 moiety in L·GaCl3 complexes has been checked against the computed antisymmetric stretching of the Ga-Cl bonds. It has also been compared to the symmetric stretching of the C-O bonds of the corresponding L·Ni(CO)3 complexes (Tolman Electronic Parameter). On this basis, a relationship between the pyramidalization observed in the gallium complexes and the electronic ligand properties has been established.

Synthesis of fluoranthenes by hydroarylation of alkynes catalyzed by gold(I) or gallium trichloride

Summary Electrophilic gold(I) catalyst 6 competes with GaCl3 as the catalyst of choice in the synthesis of fluoranthenes by intramolecular hydroarylation of alkynes. The potential of this catalyst for the preparation of polyarenes is illustrated by a synthesis of two functionalized decacyclenes in a one-pot transformation in which three C–C bonds are formed with high efficiency.

Gallium-assisted transfer hydrogenation of alkenes.

We report a rare case of alkene transfer hydrogenation using a main-group compound instead of a transition-metal complex as catalyst. We disclosed that 1,4-cyclohexadiene can be used as H2 surrogate towards olefin reduction in the presence of [IPrGaCl2 ][SbF6 ]. Hydrogenative cyclizations have also been carried out because this cationic gallium complex is also a potent hydroarylation catalyst.

Molecular versus Ionic Structures in Adducts of GaX3 with Monodentate Carbon-Based Ligands

A molecular donor-acceptor adduct has been isolated by the reaction of the N-heterocyclic carbene 1,3-dimethyl imidazol-2-ylidene (diMe-IMD) with GaCl(3). In contrast, the structurally related, yet much more nucleophilic, 1,3-dimethyl-2-methylene-2,3-dihydro-1H-imidazole (diMe-MDI) gave rise to ion pairs of type [L(2)GaX(2)][GaX(4)], where X = Cl, Br, or I. With IBioxMe(4), a N-heterocyclic carbene that is more nucleophilic than diMe-IMD, the outcome of the reaction was dependent on the nature of the halide. Ionic 1:1 adducts between monodentate ligands and GaX(3) salts have only one precedent in the literature. The peculiar behavior of carbon-based ligands was explained on the basis of their electronic properties and reaction kinetics.

CaII‐Catalyzed Alkenylation of Alcohols with Vinylboronic Acids

Direct alkenylation of a variety of alcohols with vinylboronic acids has been accomplished using the air-stable calcium(II) complex Ca(NTf2 )2 under mild conditions with short reaction times. For reluctant transformations, an ammonium salt was used as an additive to circumvent the reactivity issue.

First Evidence for the Existence of Hexafluoroantimonic(V) Acid

By reacting IPr⋅InBr3 with AgSbF6 in dichloroethane at room temperature, we could obtain single crystals comprising [{(IPr⋅InBr3 )(Ag⋅(CH2 Cl)2 )}2 ][SbF6 ]2 (1) and two identical HSbF6 units. It is presumed that the reaction gave rise to [IPr⋅InBr3-x ][SbF6 ]x (x=1, 2, or 3) which reacted with adventitious water to give HSbF6 and [IPr⋅InBr3-x (OH)][SbF6 ]x-1 . The experiment was reproduced at 60u2009°C, eventually leading to the indium hydroxide [{IPr⋅In(OH)0.5 (H2 O)4.5 }2 ][SbF6 ]5 (2).