Transition Metal-Free Regioselective Phosphonation of Pyridines: Scope and Mechanism.

Regioselective phosphonation of pyridines is an interesting transformation in synthesis and medicinal chemistry. We report herein a metal-free approach enabling access to various 4-phosphonated pyridines. The method consists of simply activating the pyridine ring with a Lewis acid (BF3·OEt2) to facilitate the nucleophilic addition of a phosphine oxide anion. The formed sigma complex is subsequently oxidized with an organic oxidant (chloranil) to yield the desired adducts in good to excellent yields. We furthermore showed that access to C2-phosphoinated pyridines can be achieved in certain cases with strong Lewis basic phosphorus nucleophiles or with strong Lewis acidic pyridines. Both experimental and computational mechanistic investigations were undertaken and allowed us to understand the factors controlling the reactivity and selectivity of this reaction.

Novel Ionic Conducting Composite Membrane Based on Polymerizable Ionic Liquids.

In this work, the design and characterization of new supported ionic liquid membranes, as medium-temperature polymer electrolyte membranes for fuel-cell application, are described. These membranes were elaborated by the impregnation of porous polyimide Matrimid® with different synthesized protic ionic liquids containing polymerizable vinyl, allyl, or methacrylate groups. The ionic liquid polymerization was optimized in terms of the nature of the used (photo)initiator, its quantity, and reaction duration. The mechanical and thermal properties, as well as the proton conductivities of the supported ionic liquid membranes were analyzed in dynamic and static modes, as a function of the chemical structure of the protic ionic liquid. The obtained membranes were found to be flexible with Young's modulus and elongation at break values were equal to 1371 MPa and 271%, respectively. Besides, these membranes exhibited high thermal stability with initial decomposition temperatures > 300 °C. In addition, the resulting supported membranes possessed good proton conductivity over a wide temperature range (from 30 to 150 °C). For example, the three-component Matrimid®/vinylimidazolium/polyvinylimidazolium trifluoromethane sulfonate membrane showed the highest proton conductivity-~5 × 10-2 mS/cm and ~0.1 mS/cm at 100 °C and 150 °C, respectively. This result makes the obtained membranes attractive for medium-temperature fuel-cell application.

Metal-Free Deoxygenation of Amine N-Oxides: Synthetic and Mechanistic Studies.

We report herein an unprecedented combination of light and P(III)/P(V) redox cycling for the efficient deoxygenation of aromatic amine N-oxides. Moreover, we discovered that a large variety of aliphatic amine N-oxides can easily be deoxygenated by using only phenylsilane. These practically simple approaches proceed well under metal-free conditions, tolerate many functionalities and are highly chemoselective. Combined experimental and computational studies enabled a deep understanding of factors controlling the reactivity of both aromatic and aliphatic amine N-oxides.

Functionalization of [2.2]Paracyclophanes via a Reductive Sulfanylation Reaction.

An expeditious route to planar chiral sulfur-based scaffolds has been achieved in two operational steps from cheap and commercial [2.2]paracyclophane hydrocarbon. The sulfur atom was introduced in a specific benzylic position of the [2.2]paracyclophane according to a reductive sulfanylation reaction, which proceeds under two complementary reaction conditions with either the BF3·OEt2/Et3SiH or TFA/BH3·THF combinations. The study was completed by the development of a highly efficient resolution approach by HPLC.

Visible-Light-Mediated C-H Alkylation of Pyridine Derivatives.

We report herein a visible-light-mediated C-H alkylation of pyridine derivatives that proceeds by simple combination of a large variety of N-alkoxypyridinium ions with alkanes in the presence of 2 mol % of fac-Ir(ppy)3 under blue illumination. The mild reaction conditions together with the high group functional tolerance make of this process a useful synthetic platform for the construction of structurally strained heterocycles. Detailed mechanistic investigations, including density functional theory calculations and quantum yield measurement, allowed us to understand factors controlling the reactivity and the selectivity of the reaction.

Visible-Light-Mediated Access to Phosphate Esters.

We report herein a practically simple visible-light photocatalytic approach for the synthesis of a large variety of phosphate esters through the combination of N-alkoxypyridinium salts and phosphites under mild conditions. The scope of the reaction is broad and the protocol was successfully applied to the synthesis of biologically relevant structures. Quantum yield measurements, as well as EPR experiments, allowed the suggestion of a reasonable reaction mechanism.

Taming the Reactivity of Phosphiranium Salts: Site-Selective C-Centered Ring Opening for Direct Synthesis of Phosphinoethylamines.

Advances in the field of phosphorus chemistry are documented, by revealing the synthetic utility of previously underutilized quaternary phosphiranium salts (QPrS) as three-chain-atom electrophilic building blocks. Notably, control of their challenging C-centered electrophilicity is disclosed with an expedient synthesis of tertiary ß-anilino phosphines as a proof-of-concept.

Diphenyliodonium Ion/Et3N Promoted Csp2-H Radical Phosphorylation of Enamides.

This work reports a simple and efficient method for the direct phosphorylation of enamide under metal-free conditions. The P-centered radicals, derived from secondary phosphine oxides, are generated under mild reaction conditions in the presence of diphenyliodonium salt and Et3N and are introduced onto a range of enamides in good isolated yields. The method features broad substrate scope, good functional group tolerance, and efficient scale-up.

Evidence of Phosphonium-Carbenium Dication Formation in a Superacid: Precursor to Fluorinated Phosphine Oxides.

Unambiguously confirmed by low-temperature in situ NMR experiments, X-ray diffraction and vibrational spectroscopy, phosphonium-carbenium superelectrophiles are shown to be generated in strong acidic conditions. Representing crucial intermediates, their exploitation allows for the synthesis of unprecedented fluorinated (cyclic) phosphine oxides.

Visible-Light-Mediated Metal-Free Synthesis of Aryl Phosphonates: Synthetic and Mechanistic Investigations.

This work describes a straightforward access to a large variety of aryl phosphonates by the simple combination of diaryliodonium salts with phosphites in the presence of a base and under visible-light illumination. The reaction proceeds smoothly, tolerates various functionalities, and was applied for the synthesis of pharmaceutically relevant compounds. Mechanistic investigations, including EPR, NMR, and DFT calculations, support the postulated reaction mechanism.

Synthesis and Identification of Aryl and Alkyl Gem-Dilithium Phosphido-Boranes: A Boost to the Chemistry of Phosphandiides.

The synthesis and identification of unprecedented gem-dianionic phosphorus compounds, that is, gem-dilithium phosphido-boranes Li2 [RP⋅BH3 ], with R=Ph or Cy, are reported in THF solution. These were obtained by double deprotonation of the corresponding primary phosphine-borane precursors RPH2 ⋅BH3 . Their in-depth structural study, based on multinuclear (1 H, 6 Li, 7 Li, 11 B, 13 C, 31 P) mono- and bi-dimensional NMR analyses, indicates a strong influence of the phosphorus substituent on the structure of the gem-dianionic phosphorus structure; a monomeric arrangement was obtained when R=phenyl, whereas a cyclic oligomer was observed for R=cyclohexyl. These compounds represent a new type of useful reagent, and their access paves the way for the concept of "RP synthons" (i.e., RP2- phosphandiides), likely to be the most flexible precursors of a variety of phosphorus targets.

Direct Syn Addition of Two Silicon Atoms to a C≡C Triple Bond by Si-Si Bond Activation: Access to Reactive Disilylated Olefins.

A catalytic intramolecular silapalladation of alkynes affords, in good yields and stereoselectively, syn-disilylated heterocycles of different chemical structure and size. When applied to silylethers, this reaction leads to vinylic silanols that undergo a rhodium-catalyzed addition to activated olefins, providing the oxa-Heck or oxa-Michael products, depending on the reaction conditions.

Transition metal-free stereospecific access to (E)-(1-fluoro-2-arylvinyl)phosphine borane complexes.

This work describes the first transition metal-free stereospecific synthesis of (E)-(1-fluoro-2-arylvinyl)phosphine boranes through the addition of diarylphosphine-boranes to gem-bromofluoroalkenes in the presence of a base at room temperature. The reaction proceeds well under very mild conditions and tolerates a variety of functionalities. Scope and limitations of the reaction are discussed. Mechanistic investigations have been undertaken and revealed that the reaction takes place through an SRN1 mechanism. The formation of the fluorinated vinyl radical has been evidenced by electron paramagnetic resonance (EPR) experiment.

Mechanistic investigation of the NH-sulfoximination of sulfide. Evidence for λ6-sulfanenitrile intermediates.

We report a new procedure for the preparation of NH-sulfoximines from sulfides using PIDA as an oxidant and ammonium carbamate as the ammonia source. Excellent yields were obtained with a wide range of sulfides. The formation of acetoxy- and methoxy-λ6-sulfanenitrile as intermediates was proposed, both of which were converted to the NH-sulfoximine by the action of the solvent. The structure of these intermediates was confirmed by 1H, 13C and 15N NMR and HRMS analysis.

Mechanistic Investigations of Reactions of the Frustrated Lewis Pairs (Triarylphosphines/B(C6F5)3) with Michael Acceptors.

Frustrated Lewis pair (FLP)-catalyzed reduction of Michael acceptors is a challenging reaction that proceeds with specific FLP structures. Kinetics and equilibrium of the reactions of two phosphines (Ar3P), namely tri(1-naphthyl)phosphine and tri(o-tolyl)phosphine, are reported with reference electrophiles. The reason for the failure of the FLPs (Ar3P/B(C6F5)3) to reduce activated alkenes under H2 pressure is shown to be a hydrophosphination process that inhibits the reduction reaction. Kinetic and thermodynamic factors controlling both pathways are discussed in light of Mayr's free linear energy relationships.

How Do Phosphinates React with Unactivated Alkenes Under Organic Photocatalyzed Conditions? Substrate Scope and Mechanistic Insights.

The first visible-light-mediated, metal-free hydrophosphinylation of unactivated alkenes with ethyl and butyl phosphinates is described. The reaction works with a low catalyst loading of 9-mesityl-10-methylacridinium perchlorate (Fukuzumi photocatalyst, 0.5 mol %) in the presence of diphenyliodonium triflate as oxidant. The reaction proceeds smoothly and covers a broad scope of substrates. Detailed mechanistic investigations, including EPR spectroscopy, fluorescence and steady-state photolysis, allowed the mechanism of this photoreaction to be rationalized.

Metal-Free Synthesis of 6-Phosphorylated Phenanthridines: Synthetic and Mechanistic Insights.

A novel and efficient method for the generation of phosphinoyl radicals from the combination of diphenyliodonium salt (Ph2I+,-OTf) with triethylamine (Et3N) in the presence of secondary phosphine oxides is reported. By employing this practical and simple approach, a large variety of 6-phosphorylated phenanthridines have been synthesized through the addition of phosphinoyl radicals to isonitriles as radical acceptors. The reaction works smoothly in the absence of any transition metal or photocatalyst. On the basis of electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations, the mechanism of this reaction is discussed.

Access to Constrained Fluoropseudopeptides via Ring-Closing Metathesis of Fluoroalkenes.

Bis-alkene substrates, containing one fluoroalkene and linked by an amide moiety, have been designed and synthesized to be subjected to ring-closing metathesis reactions. The substitution of fluoroalkene by a phenyl group enhanced the reactivity, and the resulting fluorinated lactams were obtained in high yields except when a hindered alkyl group was present in the molecule. The cycles were then subjected to ring opening in order to develop a new route to constrained fluoropseudopeptides bearing a fluoroalkene as a peptide bond mimic.

Metal-Free, Visible Light-Photocatalyzed Synthesis of Benzo[b]phosphole Oxides: Synthetic and Mechanistic Investigations.

Highly functionalized benzo[b]phosphole oxides were synthesized from reactions of arylphosphine oxides with alkynes under photocatalytic conditions by using eosin Y as the catalyst and N-ethoxy-2-methylpyridinium tetrafluoroborate as the oxidant. The reaction works under mild conditions and has a broad substrate scope. Mechanistic investigations have been undertaken and revealed the formation of a ground state electron donor-acceptor complex (EDA) between eosin (the photocatalyst) and the pyridinium salt (the oxidation agent). This complex, which has been fully characterized both in the solid state and in solution, turned out to exhibit a dual role, i.e., the oxidation of the photocatalyst and the formation of the initiating radicals, which undergoes an intramolecular reaction avoiding the classical diffusion between the two reactants. The involvement of ethoxy and phosphinoyl radicals in the photoreaction has unequivocally been evidenced by EPR spectroscopy.

Enantiomerically Pure [2.2]Paracyclophane-4-thiol: A Planar Chiral Sulfur-Based Building Block Readily Available by Resolution with an Amino Acid Chiral Auxiliary.

Acyl chloride of N-phthaloyl-(S)-isoleucine is an efficient chiral auxiliary for the resolution of (±)-[2.2]paracyclophane-4-thiol. A preparative protocol, based on the conversion into diastereoisomeric thiolesters and separation by two fractional crystallizations and column chromatography, was developed. Deprotection with LiAlH4 allowed isolation of the individual thiol enantiomers in good yield (∼40%) and high enantiomeric purity (ee >93%). The absolute configurations were determined by comparison of the optical rotation value of the products with literature data and were confirmed by X-ray crystallography.