Synthesis of alternating metallocopolymers by chiral recognition

International audienceWe report the synthesis of chiral enantiopure polytopic bridging ligands, which may lead to the formation of metallosupramolecular polymers with zinc (II) as metal linker. We show that chiral C2‐symmetric bisoxazoline ligands are useful moieties to efficiently generate heterochiral complexes and thus polymeric entities. The corresponding metallopolymers were further characterized by powder X‐ray diffraction (PXRD) to obtain information on the level of crystallinity of our different metallopolymers

Easy ruthenium-catalysed oxidation of primary amines to nitriles under oxidant free conditions

International audienceA dehydrogenation of primary amine to give the corresponding nitrile under oxidant-and base-free conditions catalysed by simple [Ru(p-cym)Cl2]2 with no extra ligand is reported. The system is highly selective for alkyl amine whereas benzylamine derivatives gave the nitrile product together with the imine in ratio ranging from 14:1 to 4:1 depending on the substrate. Preliminary mechanistic investigations have been performed to identify the key factors that govern the selectivity

Observation of Hyperpositive Non-Linear Effects in Asymmetric Catalysis

Asymmetric amplification is a phenomenon that plays a key role in the emergence of homochirality in life. In asymmetric catalysis, theoretical and experimental models have been investigated for understanding how chiral amplification is possible, in particular through non-linear effect. In the most remarkable cases, the degree of enantio-induction from a non-enantiopure catalyst can be as high as with an enantiopure reference system. Interestingly, it has been proposed a quarter century ago that chiral catalysts, when not enantiopure may be more enantioselective than their enantiopure counterparts, though such a case has never been observed to date. We show here than such hyperpositive non-linear effect in asymmetric catalysis is absolutely possible. A depth study into the underlying mechanism was carried out and our conclusion differs from the proposed models.<br /

Enantiodivergent Non-Linear Effects in Asymmetric Catalysis

In this paper, we theoretically discuss the enantiodivergent product formation in asymmetric catalysis, a process in which the sign of the overall product enantiomer switches upon a change of catalyst concentration. The presented model is based on a catalytic system that consists of both discrete and dimeric aggregated metal complexes, in competition and in equilibrium. These concepts were then expanded to a non-enantiopure catalyst, giving rise to enantiodivergent non-linear effects – a special case of a hyperpositive non-linear effects where the product enantiomer’s sign switches upon a change of the catalyst enantiomeric excess. Different cases are considered allowing a discussion of the influence of the parameters governing both models. Finally, we present experimental results that support the enantiodivergence while varying the concentration of enantiopure catalyst or while varying the enantiomeric excess of the catalyst, using chiral N-methylephedrine as a ligand for the enantioselective addition of dimethylzinc to benzaldehyde

Absence of Non-Linear Effects Despite Evidence for Catalyst Aggregation

An in-depth study of the catalytic system, consisting of the enantioselective addition of ZnEt2 to benzaldehyde with (1R,2S)-(-)-N-Methylephedrine (NME) as chiral ligand, suggests the presence of dimeric and trimeric aggregates, as deduced from product ee vs catalyst loading and NMR investigations (1H, DOSY). Formation of catalyst aggregate was excluded in earlier studies as this system displays a linear product ee vs ligand ee-correlation, which is usually taken as an indication for the absence of catalyst aggregation. A subsequent theoretical study, using the monomer-dimer competition model we recently have developed, highlights the possible parameter configurations leading to linear product ee vs ligand ee plots – despite the presence of catalyst dimers. It shows that, while the Kagan and Noyori models allow linearity in very specific cases only, a multitude of scenarios may lead to linearity here, especially if heterochiral dimers are catalytically active.</p

Chiral Self-Sorting Process with Ditopic Ligands: Alternate or Block Metallopolymer Assembly as a Function of the Metal Ion

We report an extensive study on the coordination behavior of chiral ditopic bridging ligands, which lead to metallosupramolecular polymers in the presence of Zn(II) and Cu(II) in solution. With the help of UV–vis and circular dichroism spectroscopies, we show that the metallopolymer sequence can be controlled by chirality and by the choice of the metal ion. Although the formation of a block metallopolymer proceeds through the assembly of homoleptic complexes, an alternate metallopolymer may be obtained only when heteroleptic complexes are formed. This demonstrates how the prevalent coordination geometries at metal centers may be used to control the sequences of the metallopolymers

Structural diversity and versatility for organoaluminum complexes supported by mono- and di-anionic aminophenolate bidentate ligands

The present contribution describes the synthesis and structural characterization of structurally diverse organoaluminum species supported by variously substituted aminophenolate-type ligands: these Al complexes are all derived from the reaction of AlMe3 with aminophenols 2-CH2NH(R)-C6H3OH (1a, R = mesityl (Mes); 1b, R = 2,6-di-isopropylphenyl (Diip)) and 2-CH2NH(R)-4,6-Bu-t(2)-C6H2OH (1c, R = Mes; 1d, R = Diip). The low temperature reaction of AlMe3 with 1a-b readily affords the corresponding Al dimeric species [mu-eta(1), eta(1)-N,O-{2-CH2NH(R)-C6H4O}](2)Al2Me4 (2a-b), consisting of twelve-membered ring aluminacycles with two mu-eta(1), eta(1)-N,O-aminophenolate units, as determined by X-ray crystallographic studies. Heating a toluene solution of 2a (80 degrees C, 3 h) affords the quantitative and direct formation of the dinuclear aluminium complex Al[eta(2)-N; mu, eta(2)-O-{2-CH2N(Mes)-C6H4O}](AlMe2) (4a) while species 2b, under the aforementioned conditions, affords the formation of the Al dimeric species [eta(2)-N,O-{2-CH2N(Dipp)-C6H4O}AlMe](2) (3b), as deduced from X-ray crystallography for both 3b and 4a. In contrast, the reaction of bulky aminophenol pro-ligands 1c-d with AlMe3 afford the corresponding monomeric Al aminophenolate chelate complexes eta(2)-N,O-{2-CH2NH(R)-4,6-Bu-t(2)-C6H2O}AlMe2 (5c-d; R = Mes, Diip; Scheme 3) as confirmed by X-ray crystallographic analysis in the case of 5d. Subsequent heating of species 5c-d yields, via a methane elimination route, the corresponding Al-THF amido species eta(2)-N,O-{2-CH2N(R)-4,6-Bu-t(2)-C6H2O}Al(Me)(THF) (6c-d; R = Mes, Diip). Compounds 6c-6d, which are of the type {X-2}Al(R)(L) (L labile), may well be useful as novel well-defined Lewis acid species of potential use for various chemical transformations. Overall, the sterics of the aminophenol backbone and, to a lesser extent, the reaction conditions that are used for a given ligand/AlMe3 set essentially govern the rather diverse “structural” outcome in these reactions, with a preference toward the formation of mononuclear Al species (i.e. species 5c-d and 6c-d) as the steric demand of the chelating N,O-ligand increases. (C) 2011 Elsevier B.V. All rights reserved