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The BH3‐mediated reduction of P‐stereogenic hydroxyalkylphosphine oxides was investigated by Density Functional Theory calculations. By combining theoretical approaches with experimental observations, we explored two mechanisms that account for its stereospecificity, implying an inversion of the configuration at the phosphorus atom. One mechanism proceeds through a cyclic intermediate in which the coordination of borane to the oxygen atoms lowers the BH3.THF‐attack barrier. However, the free energy barrier for this combined process is too high for a reaction feasible at room temperature. The calculations rather support a second mechanism that proceeds through from the same cyclic intermediate and the in‐situ formation of BH4− as reducing agent. [ABSTRACT FROM AUTHOR]

A rhodium(III)‐based catalytic system has been used to develop a C−H bond activation of benzaldehyde derivatives and subsequent functionalization with dioxazolones in order to afford imides. The importance of the nature of the directing group to perform selectively the aldehydic C−H bond activation has been highlighted. The scope investigation showed that this transformation could be applied to various dioxazolones and many benzaldehyde derivatives as well as an acrolein derivative. Derivatization reactions of the imide products demonstrated the synthetic utility of this rhodium‐catalyzed aldehydic C−H amidation. [ABSTRACT FROM AUTHOR]

The racemization mechanism of tert‐butylphenylphosphido‐borane is investigated experimentally and theoretically. Based on this converging approach, it is shown, first, that several phosphido‐borane molecular species coexist at the time of the reaction and, second, that one particular of both initially assumed reactive routes most significantly contribute to the overall racemization process. From our converging modeling and experimental measurement, it comes out that the most probable species to be here encountered is a phosphido‐borane‐Li (THF)2 neutral solvate, whose P‐stereogenic center monomolecular inversion through a Y‐shaped transition structure (ΔrG°≠: 81 kJ mol−1) brings the largest contribution to the racemization process. [ABSTRACT FROM AUTHOR]

From achiral imidazolinium salts, chiral transition metal complexes containing an N‐heterocyclic carbene (NHC) ligand were prepared (metal = palladium, copper, silver, gold, rhodium). Axial chirality in these complexes results from the formation of the metal‐carbene bond leading to the restriction of rotation of dissymmetric N‐aryl substituents about the C–N bond. When these complexes exhibited a sufficient configurational stability, a resolution by chiral high‐performance liquid chromatography (HPLC) on preparative scale enabled isolation of enantiomers with excellent enantiopurities (>99% ee) and good yields. A study of the enantiomerization barriers revealed the effect of the backbone nature as well as the type of transition metal on its values. Nevertheless, the evaluation of palladium‐based complexes in asymmetric intramolecular α‐arylation of amides demonstrated that the ability to induce an enantioselectivity cannot be correlated to the configurational stability of the precatalysts. [ABSTRACT FROM AUTHOR]

Azaphosphatranes were found to act as efficient organocatalysts in the presence of water for the cyanation of differently substituted imines. A relatively safer source of cyanide, trimethylsilyl cyanide (TMSCN), was used, and excellent yields could be obtained with only 0.05 mol % of catalyst. The influence of acidity of azaphosphatranes on catalytic activity as well as the role of azaphosphatranes in the mechanism of the reaction were also investigated. [ABSTRACT FROM AUTHOR]

The synthesis of phosphorus molecules presenting a chiral center on the P‐atom, also known as P‐stereogenic compounds, has long attracted the curiosity of the scientific community. Indeed, these chemical compounds feature many peculiar properties, allowing their use in various fields of applications, ranging from medicine to enantioselective catalysis. However, their synthesis, and more particularly the introduction and retention of the chiral information on the phosphorus center, remains a very challenging task. That is why this review article focuses on the recent advances in the enantioselective synthesis of P‐stereogenic molecules, with a particular focus on the introduction of the chiral center on the phosphorus atom. This article summarizes the main synthetic approaches directed towards the enantioselective synthesis of such chemical entities with a historical perspective. Thus, approaches based on the use of chiral auxiliaries attached to the phosphorus atom and the use of chiral stoichiometric reagents will be discussed first, as they were historically the first to be developed. Then, the recent discoveries in the catalytic and enantioselective synthesis and the direct optical resolution of P‐chiral compounds will be discussed. [ABSTRACT FROM AUTHOR]

Verkade's superbases, entrapped in the cavity of enantiopure hemicryptophane cages, have been synthesized with enantiomeric excess (ee) superior to 98%. Their absolute configuration has been determined by using electronic circular dichroism (ECD) spectroscopy. These enantiopure encaged superbases turned out to be efficient chiral derivatizing agents for chiral azides, underlining that the chirality of the cycloveratrylene (CTV) macrocycle induces different magnetic and chemical environments around the phosphazide functions. [ABSTRACT FROM AUTHOR]

Proazaphosphatranes ‐Verkade's superbases‐ proved to be efficient organocatalysts for the Strecker reaction between protected imines and trimethylsilyl cyanide (TMSCN). Excellent to quantitative yields were reached and, compared to other systems, only low catalyst loading and short reaction times were required for the reaction to proceed efficiently. A remarkable initial turnover frequency (TOF), close to 105 h–1, was achieved, associated with an excellent selectivity since no side reactions were observed. A reaction mechanism was proposed and the key role played by the apical nitrogen in the proazaphosphatrane structure was demonstrated. Verkade's superbases were found to be efficient organocatalysts for the Strecker reaction, leading to high yield, in short reaction time and requiring low catalyst loading. [ABSTRACT FROM AUTHOR]

The synthesis of P,N heterobidentate phosphine / palladium complexes has been realized from P‐stereogenic enantiopure ligands. Five, six or seven membered ring complexes have been fully characterized, notably by X‐ray diffraction, allowing the study of the chelation to a "palladium (II) dichloride" unit. The nature of nitrogen coordination site as well as the size of the ring modify the bite‐angle at the solid state. Five, six or seven membered ring P,N heterobidentate phosphine / palladium complexes has been synthesized from P‐stereogenic ligands. X‐ray diffraction allowed the effect study of the nature of nitrogen coordination site as well as the size of the ring on the bite‐angle at the solid state. [ABSTRACT FROM AUTHOR]

Covalent cages with a well‐defined cavity located close to a reactive center are of increasing interest because of their outstanding ability to mimic the catalytic properties of enzymatic systems. The size and shape of such synthetic nanosized reactors strongly affect the behavior of the trapped reaction partners, which can adopt specific conformations and orientations. In particular, the use of molecular cages with confined reactive functions strongly modifies the outcome of catalytic transformations that are carried out therein. This review article describes covalent molecular cages presenting an endohedral functionalization of their inner space and reports on their catalytic activities compared with those of model systems that lack cavity. The confinement of endohedral‐functionalized cavities affects their stability as catalysts and the rate and selectivity of reactions they mediate. [ABSTRACT FROM AUTHOR]

A frustrated Lewis pair (FLP) system was obtained by confinement of the Lewis base partner, a Verkade's superbase, in a molecular cavity. Whereas the model superbase lacking cavity displayed no catalytic activity in Morita–Baylis–Hillman (MBH) reactions, when associated to titanium (IV) chloride, the encaged superbase turns out to be an efficient catalyst under the same conditions. The crucial role of the endohedral functionalized cage on catalytic performance was further demonstrated by the fact that model superbases with bulky substituents were much less efficient to produce active catalysts, as well as by inhibition and substrate selection experiments. 31P NMR spectroscopy and mass spectrometry experiments evidenced that no interaction between the Lewis acidic and basic partners occurred when the superbase was capped by a cycloveratrylene (CTV) unit, thus creating a true FLP active system. Switching on the catalytic activity: A frustrated Lewis pair (FLP) system has been designed, where the Lewis base partner is encapsulated in a cage structure. Endohedral functionalization of a well‐defined cavity allows for the formation of a FLP, restoring the catalytic activity of the Verkade's superbase. [ABSTRACT FROM AUTHOR]

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We present recent advances in the understanding of the reduction of optically pure hydroxyalkylphosphinates and phosphine oxides, which represent key intermediates for the preparation of P-stereogenic ligands. Their reduction leads to P-chiral phosphinites and phosphines, respectively, and occurs stereospecifically with inversion of configuration using BH3·THF, which plays three roles: activating, reducing and protecting agent. The formation of by-products as hydroxyalkyl secondary phosphine-boranes has also been studied. [ABSTRACT FROM AUTHOR]

Access to hydroxy-functionalized P-chiral phosphine-boranes has become an important field in the synthesis of P-stereogenic compounds used as ligands in asymmetric catalysis. A family of optically pure α and β-hydroxyalkyl tertiary phosphine-boranes has been prepared by using a three-step procedure from readily accessible enantiopure adamantylphosphinate, obtained by semi-preparative HPLC on multigram scale. Firstly, a two-step one-pot transformation affords the enantiopure hydroxyalkyl tertiary phosphine oxides in good yields and enantioselectivities. The third step, BH3-mediated reduction, allows the formation of the desired phosphine-boranes with excellent stereospecifity. The mechanistic study of this reduction provides new evidence to elucidate the crucial role of the pendant hydroxy group and the subsequent activation of the PO bond by the boron atom. [ABSTRACT FROM AUTHOR]

Achiral or chiral phosphines are widely used in two main domains: ligands in organometallic catalysis and organocatalysis. For this reason, the obtention of optically pure phosphine has always been challenging in the development of asymmetric catalysis. The simplest method to obtain phosphines is the reduction of phosphine oxides. The essential difficulty is the strength of the P=O bond which involves new procedures to maintain a high chemio- and stereoselectivity. The reduction can occur with retention or inversion of the stereogenic phosphorus atom depending on the nature of the reducing agent and the presence of additives. In fact, the reactivity of the phosphine oxides and the mechanism of the reduction are not always well understood. Since the first work in the 1950's, numerous studies have been realised in order to develop methodologies with different reagents or to understand the mechanism of the reaction. In the last decade, efficient stereospecific methodologies have been developed to obtain optically pure tertiary phosphines from P-stereogenic phosphine oxides. In this review, we intend to provide a comprehensive and critical overview of these methodologies. [ABSTRACT FROM AUTHOR]

We compare the influence of using either molecular or colloidal precursors on the synthesis of a ceramic material containing SiC and porous carbon. Remarkably, the temperature of synthesis for crystalline SiC is independent of the route chosen. The excess carbon in the initial mixture is the source of the excess porous carbon that binds to the crystalline domains of SiC in the final products. Interestingly, increasing the initial area of surface contact between carbon and silicon in the ceramic precursor results in different porosities in the 'meso' range. Simultaneous control of the size and the relative amounts of Si and C in the precursors allows control to be exerted over the nature and texture of the final powders. A simple and general mechanism is herein proposed to explain the evolution of the surface area as a function of the volume fraction of residual carbon in the synthesised ceramic. [ABSTRACT FROM AUTHOR]

In this paper we describe the synthesis of functionalised mesoporous organosilicas containing a mannitol derivative in the framework. For this purpose, a bis-silylated precursor 3,4-Di-O-[3-(triethoxysilylpropyl)carbamate]-1,2:5,6-di-O-isopropylidene-D-mannitol was prepared by coupling of 1,2:5,6-di-O-isopropylidene-D-mannitol with 3-(triethoxysilylpropyl)isocyanate. The framework-functionalised materials were obtained in one step by the “direct synthesis” method which consists of a co-hydrolysis and polycondensation of a bis-silylated mannitol precursor with tetraethylorthosilicate (TEOS) in the presence of a non-ionic triblock co-polymer (P123) as structure-directing agent. Interestingly, deprotection of the 1,2,5,6 OH functional groups occurred during the material synthesis. The obtained solids were characterized by 13C and 29Si CP-MAS NMR, N2adsorption-desorption, powder X-ray diffraction, TEM and elemental analysis. We have shown that, the OH functional groups, which are released during the synthesis of the mesoporous silica, can be used for chelation of ions and stabilisation of nanoparticles. The subsequent growth of gold (0) nanoparticles in the wall has been investigated and evidenced. [ABSTRACT FROM AUTHOR]

Boronsäureester, Homogene Katalyse, Kinetische Racematspaltung, Selektivität Keywords: Boronsäureester; Homogene Katalyse; Kinetische Racematspaltung; Selektivität EN Boronsäureester Homogene Katalyse Kinetische Racematspaltung Selektivität 14337 14337 1 06/16/21 20210621 NES 210621 In this Communication, structure B 1 b was incorrectly drawn with boron atom, rather than nitrogen atom, in the benzylic position, and should have had the revised structure shown below. [Extracted from the article]

Boronates, homogeneous catalysis, kinetic resolution, selectivity Keywords: boronates; homogeneous catalysis; kinetic resolution; selectivity EN boronates homogeneous catalysis kinetic resolution selectivity 14217 14217 1 06/18/21 20210621 NES 210621 In this Communication, structure B 1 b was incorrectly drawn with boron atom, rather than nitrogen atom, in the benzylic position, and should have had the revised structure shown below. [Extracted from the article]