Nathalie Saffon-Merceron

Stable phosphonium sila-ylide with reactivity as a sila-Wittig reagent.

The silicon congeners of well-known phosphonium ylides have been considered only as short-lived reactive intermediates. We successfully synthesized a remarkably stable phosphonium sila-ylide. Its X-ray structure reveals a long Si-P bond and a strongly pyramidalized silicon center, indicating a very weak P-Si pi interaction. In addition, it exhibits an alpha,beta-ambiphilic character with a nucleophilic silicon center, similar to its carbon-congener phosphonium ylides. This property of the phosphonium sila-ylide allows its use as a sila-Wittig reagent with carbonyl derivatives.

Synthesis and Characterization of an Isolable Base‐Stabilized Silacycloprop‐1‐ylidene

Strained but stable: An isolable silacycloprop-1-ylidene stabilized by intramolecular complexation with an iminophosphorus ylide fragment was successfully synthesized and fully characterized. The formation of this small highly strained cyclic silylene involves an unprecedented Si(IV)→Si(II) rearrangement under very mild conditions.

A Significant but Constrained Geometry Pt→Al Interaction: Fixation of CO2 and CS2, Activation of H2 and PhCONH2.

Reaction of the geminal PAl ligand [Mes2PC(═CHPh)AltBu2] (1) with [Pt(PPh3)2(ethylene)] affords the T-shape Pt complex [(1)Pt(PPh3)] (2). X-ray diffraction analysis and DFT calculations reveal the presence of a significant Pt→Al interaction in 2, despite the strain associated with the four-membered cyclic structure. The Pt···Al distance is short [2.561(1) Å], the Al center is in a pyramidal environment [Σ(C-Al-C) = 346.6°], and the PCAl framework is strongly bent (98.3°). Release of the ring strain and formation of X→Al interactions (X = O, S, H) impart rich reactivity. Complex 2 reacts with CO2 to give the T-shape adduct 3 stabilized by an O→Al interaction, which is a rare example of a CO2 adduct of a group 10 metal and actually the first with η(1)-CO2 coordination. Reaction of 2 with CS2 affords the crystalline complex 4, in which the PPtP framework is bent, the CS2 molecule is η(2)-coordinated to Pt, and one S atom interacts with Al. The Pt complex 2 also smoothly reacts with H2 and benzamide PhCONH2 via oxidative addition of H-H and H-N bonds, respectively. The ensuing complexes 5 and 7 are stabilized by Pt-H→Al and Pt-NH-C(Ph) = O→Al bridging interactions, resulting in 5- and 7-membered metallacycles, respectively. DFT calculations have been performed in parallel with the experimental work. In particular, the mechanism of reaction of 2 with H2 has been thoroughly analyzed, and the role of the Lewis acid moiety has been delineated. These results generalize the concept of constrained geometry TM→LA interactions and demonstrate the ability of Al-based ambiphilic ligands to participate in TM/LA cooperative reactivity. They extend the scope of small molecule substrates prone to such cooperative activation and contribute to improve our knowledge of the underlying factors.

Catalytic Dinitrogen Reduction at the Molybdenum Center Promoted by a Bulky Tetradentate Phosphine Ligand

Stoichiometric reduction of N2 at a Mo center stabilized by a bulky tetradentate phosphine ligand (PP3(Cy)) allowed isolation of Mo-imidoamine and Mo-imido complexes. Both complexes as well as the Mo(II) precursor are equally suitable catalysts for the synthesis of NTMS3 (TMS = trimethylsilyl) from N2, TMSCl, and electron sources. Mechanistic studies prove the involvement of a TMS radical at least in one of the catalytic steps.

From hexaoxy-[6]pericyclynes to carbo-cyclohexadienes, carbo-benzenes, and dihydro-carbo-benzenes: synthesis, structure, and chromophoric and redox properties.

When targeting the quadrupolar p-dianisyltetraphenyl-carbo-benzene by reductive treatment of a hexaoxy-[6]pericyclyne precursor 3 with SnCl(2)/HCl, a strict control of the conditions allowed for the isolation of three C(18)-macrocyclic products: the targeted aromatic carbo-benzene 1, a sub-reduced non-aromatic carbo-cyclohexadiene 4A, and an over-reduced aromatic dihydro-carbo-benzene 5A. Each of them was fully characterized by its absorption and NMR spectra, which were interpreted by comparison with calculated spectra from static structures optimized at the DFT level. According to the nucleus-independent chemical shift (NICS) value (NICS≈-13 ppm), the macrocyclic aromaticity of 5A is indicated to be equivalent to that of 1. This is confirmed by the strong NMR spectroscopic deshielding of the ortho-CH protons of the aryl substituents, but also by the strong shielding of the internal proton of the endocyclic trans-CH=CH double bond that results from the hydrogenation of one of the C≡C bonds of 3. Both the aromatics 1 and 5A exhibit a high crystallinity, revealed by SEM and TEM images, which allowed for a structural determination by using an X-ray microsource. A good agreement with calculated molecular structures was found, and columnar assemblies of the C(18) macrocycles were evidenced in the crystal packing. The non-aromatic carbo-cyclohexadiene 4A is shown to be an intermediate in the formation of 1 from 3. It exhibits a remarkable dichromism in solution, which is related to the occurrence of two intense bands in the visible region of its UV/Vis spectrum. These properties could be attributed to the dibutatrienylacetylene (DBA) unit that occurs in the three chromophores, but which is not involved in a macrocyclic π-delocalization in 4A only. A versatile redox behavior of the carbo-chromophores is evidenced by cyclic voltammetry and was analyzed by calculation of the ionization potential, electron affinity, and frontier molecular orbitals.

Activation of CO2 and SO2 by Boryl(phosphino)carbenes

The stable boryl(phosphino)carbene 1 can cleave small organic dioxide molecules. With CO2 and SO2, 1 gives, respectively, the phosphacumulene ylide [Mes(i Pr2N)B‐O‐P(CCO)(Ni Pr2)Mes] (see scheme and structure) and boryl(phosphoryl)sulfine [Mes(i Pr2N)B‐C(SO)‐P(O)(Ni Pr2)Mes] which have been structurally and spectroscopically characterized.

Borylated methylenephosphonium salts: precursors of elusive boryl(phosphino)carbenes.

The synthesis of a new family of boryl-substituted methylenephosphonium derivatives, the phosphorus analogues of iminium salts, has been developed. They were used in the preparation of the first stable boryl(phosphino)carbene, which has been fully characterized by NMR spectroscopy and X-ray crystallography. Density functional theory calculations indicate that these carbenes can be classified as push-pull carbenes with a relatively small singlet-triplet energy gap.

An Isolable Mixed P,S‐Bis(ylide) as an Asymmetric Carbon Atom Source

The transformation of molecules by introduction of atomic elements is one of the most fundamental reactions in chemistry. Indeed, the reactions of atomic hydrogen (reduction, hydrogenation) and oxygen (oxidation, epoxidation) are exceedingly important in organic synthesis, and numerous reagents and catalysts for this purpose are available. In marked contrast, the manipulation of atomic elements with a higher valence number, such as nitrogen and carbon, remains a more difficult task. Although several C1 sources have been intensely investigated, only a few atomic carbon synthetic equivalents, such as unsaturated diazo derivatives I and II, have been

Synthesis and characterization of rhodium complexes with phosphine-stabilized germylenes.

The reaction of phosphine-stabilized germylenes (1a,b) with dimer complex [Rh(2)(μ-Cl)(2)(COD)(2)] leads to the corresponding phosphine-germylene-Rh(I) complexes (2a,b). Interestingly, the stability of these complexes depends strongly on the nature of the substituent of the germylene fragment. Indeed, the complex (2a) with the chloro-germylene ligand isomerizes into a metallacycle rhodium complex (3a) via germylene insertion into the Rh-Cl bond, while the complex with the phenyl-substituted germylene (2b) was isolated and represents the first stable Rh(I)-germylene complex with a Rh-Cl bond.

Synthesis and Ligand Properties of a Stable Five‐Membered‐Ring Vinylidenephosphorane

Over the years, the success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of the various systems. Recently, N-heterocyclic carbenes (NHCs) A (Scheme 1) have emerged as powerful ligands, largely because of their strong donor properties, which are due to the presence of a lone pair and a partially filled vacant orbital. In our search for even stronger donor ligands, we became interested in carbodiphosphoranes, especially their cyclic versions B, because, as we have learned from carbene chemistry, they lead to more robust transition-metal complexes than their acyclic congeners. We have demonstrated using the nav(CO) infrared frequency of [(L)Rh(CO)2Cl] complexes that these species, which feature a carbon formally bearing two lone pairs, were indeed stronger donors than NHCs A. Along this line, we have also reported the synthesis of acyclic and cyclic push–push bent allenes (carbodicarbenes) C, which are electronically similar to carbodiphosphoranes. These results led us to consider a mixed system D with a phosphorus–carbon ylidic bond and a carbon–carbon double bond. Interestingly, such heteroallenes have a resonance form D directly related to the previously reported push–pull allenes E and push–pull carbenes F, as shown by their resonance forms. In contrast to the numerous examples of acyclic derivatives, only one cyclic vinylidenephosphorane, stable at low temperature for a short period of time, has been described, and no crystallographic data or coordination behavior has been reported. Herein we present the synthesis, single crystal X-ray diffraction study, and ligand properties of a cyclic vinylidenephosphorane 5 that is stable at room temperature both in solution and in the solid state. The cyclic phospholium salt 4, without undesired acidic protons, was chosen as the precursor for the target vinylidenephosphorane 5. Starting from the readily available diene 1, a formal [4+1] cycloaddition with bis(diisopropyl)aminophosphenium triflate gives rise to the phospholenium salt 2 (Scheme 2). Deprotonation of 2 with NaNH2 in ammonia Scheme 1. Similarities and differences between NHCs A, carbodiphosphoranes B, push–push allenes C, vinylidenephosphoranes D, push– pull allenes E, and push–pull carbenes F.