Véronique Dufaud

Azaphosphatranes as Structurally Tunable Organocatalysts for Carbonate Synthesis from CO2 and Epoxides

Three azaphosphatranes were used as organocatalysts for the synthesis of cyclic carbonates from CO2 and epoxides. They proved to be efficient single-component, metal-free catalysts for the reaction of simple or activated epoxides (styrene oxide, epichlorohydrin, glycidyl methyl ether) with CO2 under mild reaction conditions, displaying high stability and productivity over several days of reaction. Substitution patterns on the catalyst were shown to affect activity and stability. Kinetic analysis allowed investigation of the reaction mechanism.

Nanoscale organization of thiol and arylsulfonic acid on silica leads to a highly active and selective bifunctional, heterogeneous catalyst.

Ordered mesoporous silicas functionalized with alkylsulfonic acid and thiol group pairs have been shown to catalyze the synthesis of bisphenols from the condensation of phenol and various ketones, with activity and selectivity highly dependent on the distance between the acid and thiol. Here, a new route to thiol/sulfonic acid paired catalysts is reported. A bis-silane precursor molecule containing both a disulfide and a sulfonate ester bond is grafted onto the surface of ordered mesoporous silica, SBA-15, followed by simultaneous disulfide reduction and sulfonate ester hydrolysis. The resulting catalyst, containing organized pairs of arylsulfonic acid and thiol groups, is significantly more active than the alkylsulfonic acid/thiol paired catalyst in the synthesis of bisphenol A and Z, and this increase in activity does not lead to a loss of regioselectivity. The paired catalyst has activity similar to that of a randomly bifunctionalized arylsulfonic acid/thiol catalyst in the bisphenol A reaction but exhibits greater activity and selectivity than the randomly bifunctionalized catalyst in the bisphenol Z reaction.

Oxide supported surface organometallic complexes as a new generation of catalysts for carbon–carbon bond activation

Abstract This paper reviews recent applications of well-defined silica-supported hydrides of the group 4 and 5 transition metals in the field of carbon–carbon and carbon–hydrogen bonds activation of alkanes. The synthesis and characterization of the zirconium and tantalum hydrides is presented. (ue606SiO) 3 M–H complexes (M=Ti 1 , Zr 2 , Hf 3 ) are obtained by hydrogen treatment at c.a. 150°C of ue606Si–O–MNp 3 (Np=CH 2 C(CH 3 ) 3 ). These three surface complexes are formally 8 electrons species and are consequently very electrophilic. (ue606SiO) 2 Ta–H 4 is obtained by hydrogen treatment at c.a. 150°C of (ue606Si–O) x Ta(ue605CHC(CH 3 ) 3 )(CH 2 C(CH 3 ) 3 ) 3− x ( x =1,2) and is also highly electrophilic. Examples of applications of these hydrides in the field of the activation of alkanes at moderate temperatures are then given. The complexes 1 – 3 can achieve the hydrogenolysis of alkanes at low temperature. With 1 a simultaneous reaction of skeletal isomerization occurs. With 2 and 3 the mechanism of C–C bond cleavage passes through an elementary step of β-alkyl transfer. The mechanism of hydroisomerization observed with 1 passes also by an elementary step of β-alkyl transfer but in this case the β-H elimination–olefin reinsertion occurs quite rapidly so that a skeletal isomerization also occurs. Complexes of type 2 or 3 were found to catalyze under olefin pressure the olefin polymerization, and under hydrogen pressure, the polyolefin hydrogenolysis. Here the equilibrium between the olefin insertion into a metal–alkyl and the β-alkyl transfer is shown to occur with the same catalyst in agreement with the concept of microreversibility. A new catalytic reaction, called “alkane metathesis” has been discovered with complex 4 . By this reaction alkanes are catalytically transformed into higher and lower alkanes. The mechanism by which this reaction occurs is not fully understood. The products distribution, especially with labeled alkanes is explained by a concerted mechanism by which a Ta–C bond and a C–C bond of the alkane can be cleaved and reformed simultaneously via a kind of four centered σ-bond metathesis which has no precedent in classical organometallic chemistry.

Palladium-catalyzed carbon—carbon bond formation from (η6-chloroarene)Cr(CO)3 complexes an example of bimetallic activation in homogeneous catalysis

Abstract The effect of the coordination of a tricarbonylchromium moiety to the aromatic ring of aryl chlorides in the activation toward oxidative addition of the Cue5f8Cl bond to zerovalent palladium complexes is described. It presents an overview of catalytic reactions and fundamental investigations with the aim to rationalizing the role of the Cr(CO) 3 moiety on some of the elementary steps of these reactions. This topic is divided according to the nature of the catalytic reactions involving the tricarbonyl (chloroarene) chromium complexes. Firstly, the results obtained during the cross-coupling reaction with nucleophilic reagents (copper acetylides, organostannanes) and the catalytic Heck olefination reaction are presented. Then, several carbonylation processes, such as hydrocarbonylation into aldehydes, mono- and double carbonylation into amides and α-oxo-amides, and finally alkoxycarbonylation into esters, are discussed. Particular attention is given to the catalytic and stoichiometric aspects of this last reaction.

Superbases in confined space: control of the basicity and reactivity of the proton transfer.

Endohedral functionalization of the molecular cavity of host molecules is in high demand in many areas of supramolecular chemistry. When highly reactive species are incarcerated in the confined space of a molecular cavity, deep changes of their chemical properties are expected. Here, we show that the superbasic properties of proazaphosphatranes can be improved in the confined space of the molecular cavity of hemicryptophane hosts. A general and modular procedure is described to prepare supramolecular superbases with various cavity sizes. The rate of proton transfer is strongly dependent on the shape and size of the inner cavity of the designed superbasic structure. Kinetic and thermodynamic data are strongly correlated to the space available around the basic center as revealed by the X-ray molecular structures analyses.

Bi-metallic activation in homogeneous catalysis: palladium-catalysed carbonylation of tricarbonyl(chloroarene)chromium complexes to the corresponding aldehydes, esters, amides, and α-oxo amides

Tricarbonyl(chloroarene)chromium complexes undergo palladium-catalysed carbonylation to give aromatic aldehydes, esters, amides, and α-oxo amides.

Catalytic activity of an encaged Verkade's superbase in a base-catalyzed Diels-Alder reaction.

Organocatalysis in a confined space has been performed through encapsulation of a proazaphosphatrane superbase in a hemicryptophane host. The resulting catalyst displays good to high catalytic activity in the base-catalyzed Diels-Alder reactions investigated. A comparison with the model superbase, which lacks a cavity, shows much higher diastereomeric excess with the encaged proazaphosphatrane for the reaction of 3-hydroxy-2-pyrone with N-methylmaleimide. The use of an encaged superbase as organocatalyst is unprecedented and highlights how the confinement may impact the stereoselectivity.

Bimetallic activation in homogeneous catalysis: Part II. Formation of α-oxo amide by palladium-catalyzed double carbonylation of the tricarbonyl (chlorobenzene) chromium complex

We now wish to report that tricarbonyl (chlorobenzene) chromium can undergo a double carbonylation in the presence of diethylamine to give the corresponding α-oxo amide, which is the first report of an easy access to the double carbonylation of chloroarene derivatives

Surface organometallic chemistry of zirconium: Chemical reactivity of the SiOZrNp3 surface complex synthesized on dehydroxylated silica and application to the modification of mordenite

Abstract The organometallic complex ue606Siue5f8Oue5f8Zr(CH2C(CH3)3)3 was synthesized on the surface of silica dehydroxylated at 500xa0°C and its reactivity towards oxygenated molecules was studied. The materials were characterized by IR, 13 C CP-MAS NMR, analysis of evolved gases and elemental analysis. Reaction with neopentyl alcohol ((CH3)3CCH2OH) leads to the quantitative formation of ue606Siue5f8Oue5f8Zr(OCH2C(CH3)3)3. Similarly, reaction with pivalic acid ((CH3)3CCOOH) leads to the quantitative formation of ue606Siue5f8Oue5f8Zr(OCOC(CH3)3)3. Addition of dry oxygen at 25xa0°C to ue606Siue5f8Oue5f8Zr(CH2C(CH3)3)3 leads to the formation of ue606Siue5f8Oue5f8Zr(OCH2C(CH3)3)3. When heated at 200xa0°C under oxygen, the alkoxy ligands transform into carboxylate species ((CH3)3CCOO)− probably coordinated to zirconium. These results, together with molecular modelling studies of the adsorption of hydrocarbons on mordenite modified by such complexes, explain the different behaviours observed in presence of air or inert gases.

Palladium catalysed carbonylation of aryl chlorides to the corresponding methyl esters

A series of aryl chlorides have been carbonylated at 200 °C to their corresponding methyl esters, in the presence of a 5% Pd/C catalyst; the addition of K2Cr2O7 enhances the catalytic activity.