Metal-free transfer hydrogenation of olefins via dehydrocoupling catalysis

Air- and water-stable Lewis acids: synthesis and reactivity of P-trifluoromethyl electrophilic phosphonium cations

Electrophilic phosphonium cations (EPCs) containing a –CF₃ group are stable to air, water, alcohol and strong Brønsted acid and function as Lewis acid catalysts without requiring anhydrous reaction conditions.

Non-Metal-Catalyzed Heterodehydrocoupling of Phosphines and Hydrosilanes: Mechanistic Studies of B(C6F5)3-Mediated Formation of P-Si Bonds

Non-metal-catalyzed heterodehydrocoupling of primary and secondary phosphines (R¹R²PH, R² = H or R¹) with hydrosilanes (R³R⁴R⁵SiH, R⁴, R⁵ = H or R³) to produce synthetically useful silylphosphines (R¹R²P-SiR³R⁴R⁵) has been achieved using B(C₆F₅)₃ as the catalyst (10 mol %, 100 °C). Kinetic studies demonstrated that the reaction is first-order in hydrosilane and B(C₆F₅)₃ but zero-order in phosphine. Control experiments, DFT calculations, and DOSY NMR studies suggest that a R¹R²HP·B(C₆F₅)₃ adduct is initially formed and undergoes partial dissociation to form an "encounter complex". The latter mediates frustrated Lewis pair type Si-H bond activation of the silane substrates. We also found that B(C₆F₅)₃ catalyzes the homodehydrocoupling of primary phosphines to form cyclic phosphine rings and the first example of a non-metal-catalyzed hydrosilylation of P-P bonds to produce silylphosphines (R¹R²P-SiR³R⁴R⁵). Moreover, the introduction of PhCN to the reactions involving secondary phosphines with hydrosilanes allowed the heterodehydrocoupling reaction to proceed efficiently under much milder conditions (1.0 mol % B(C₆F₅)₃ at 25 °C). Mechanistic studies, as well as DFT calculations, revealed that PhCN plays a key mechanistic role in facilitating the dehydrocoupling reactions rather than simply functioning as H₂-acceptor.

Lewis Acidic Ionic Liquids

Until very recently, the term Lewis acidic ionic liquids (ILs) was nearly synonymous with halometallate ILs, with a strong focus on chloroaluminate(III) systems. The first part of this review covers the historical context in which these were developed, speciation of a range of halometallate ionic liquids, attempts to quantify their Lewis acidity, and selected recent applications: in industrial alkylation processes, in supported systems (SILPs/SCILLs) and in inorganic synthesis. In the last decade, interesting alternatives to halometallate ILs have emerged, which can be divided into two sub-sections: (1) liquid coordination complexes (LCCs), still based on halometallate species, but less expensive and more diverse than halometallate ionic liquids, and (2) ILs with main-group Lewis acidic cations. The two following sections cover these new liquid Lewis acids, also highlighting speciation studies, Lewis acidity measurements, and applications.

Borane-catalysed postpolymerisation modification of the Si–H bonds in poly(phenylsilane)

An unusually wide variety of new sidechains are introduced at a polysilane through chemoselective catalytic hydrosilation, heterodehydrocoupling, and demethanative coupling.

Organomagnesium amide catalyzed cross-dehydrocoupling of organosilanes with amines

Heteroleptic organomagnesium complex pre-catalyzed cross-dehydrocoupling of silanes with amines.

Metal-free pincer ligand chemistry polycationic phosphonium Lewis acids

Oxidation with or without subsequent methylation of the pyridine of 2,6-bis(diphenylphosphine)methyl pyridine affords di- and tricationic phosphonium salts. These species are used as Lewis acid catalysts for the dimerization of 1,1-diphenylethylene, the hydrodefluorination of 1-fluoroadamantane, and the dehydrocoupling of phenol and silane.

Dicationic phosphonium salts: Lewis acid initiators for the Mukaiyama-aldol reaction

Two strategies were used to prepare dicationic phosphonium cations.

Catalytic double hydroarylation of alkynes to 9,9-disubstituted 9,10-dihydroacridine derivatives by an electrophilic phenoxyphosphonium dication

An electrophilic pyridinium-substituted phenoxyphosphonium dicationic salt catalyzes the double hydroarylation of alkynes with diarylamines to give dihydroacridine derivatives.

A model for C–F activation by electrophilic phosphonium cations

The electrophilic phosphonium cation (EPC) salt [C₁₀H₆(CF₃)PF(C₆F₅)₂][B(C₆F₅)₄] 4 exhibited structural and spectroscopic features evidencing an interaction between the CF₃ and fluorophosphonium units. It thus models a key step in the proposed mechanism of main group C–F activation.

Synthesis and oxidation of phosphine cations

Cationic phosphines of the form [(L)PPh₂]⁺ are prepared from Ph₂PCl and carbenes (L), including a chiral bis(oxazoline)-based carbene, a cyclic(alkyl)(amino) carbene, and a 1,2,3-triazolium-derived carbene. A related dication was prepared from PhPCl₂ and a bis-carbene. The monocations, but not the dication, can be oxidized with XeF₂.

t-BuOK-mediated reductive addition of P(O)–H compounds to terminal alkynes forming β-arylphosphine oxides

A novel and efficient t-BuOK-mediated reductive addition of P(O)–H compounds to terminal alkynes forming β-arylphosphine oxides was developed. This reaction may proceed via a tandem process involving regio-selective addition and subsequent transfer hydrogenation.

Frustrated Lewis Pair-Catalyzed Cycloisomerization of 1,5-Enynes via a 5-endo-dig Cyclization/Protodeborylation Sequence

The first frustrated Lewis pair-catalyzed cycloisomerization of a series of 1,5-enynes was developed. The reaction proceeds via the π-activation of the alkyne and subsequent 5-endo-dig cyclization with the adjacent alkene. The presence of PPh3 was of utmost importance on the one hand to prevent side reactions (for example, 1,1-carboboration) and on the other hand for the efficient protodeborylation to achieve the catalytic turnover. The mechanism is explained on the basis of quantum-chemical calculations, which are in full agreement with the experimental observations.

Synthesis of quaternary aryl phosphonium salts: photoredox-mediated phosphine arylation

We report a synthesis method for the construction of quaternary aryl phoshonium salts at ambient temperature. The regiospecific reaction involves the coupling of phosphines with aryl radicals derived from diaryliodonium salts under photoredox conditions.

Catalytic reduction of amides to amines by electrophilic phosphonium cations via FLP hydrosilylation

A catalytic methodology for the conversion of amides to amines is reported. Among the 25 examples, 14 reduce N-trifluoroacetamides to give trifluoroethylamines.

Dehydrocoupling routes to element–element bonds catalysed by main group compounds

This Tutorial Review focuses on recent applications of main group compounds in the catalytic synthesis of heteronuclear element–element bonds within the p-block.

A pendant phosphorus Lewis acid: route to a palladium-benzoyl derived phosphorane

The traditional use of phosphorus compounds is as ligands for transition metals, however herein we synthesize Lewis acidic fluorophosphonium cation appended to a palladium complex and effect CO insertion into a Pd–C bond to obtain an unprecedented metal-benzoyl derived phosphorane.

Cooperative Lewis acidity in borane-substituted fluorophosphonium cations

Fluorophosphonium salts C₆X₄(BR₂)Ph₂PF[B(C₆F₅)₄] (X = H, F; R = Cy, Mes) exhibit enhanced Lewis acid catalytic reactivity as a results of the proximity of the fluorophosphonium and borane moieties.

Electrophilic phosphonium cations (EPCs) with perchlorinated-aryl substituents: towards air-stable phosphorus-based Lewis acid catalysts

A series of fluorophosphonium cations incorporating (C₆Cl₅) substituents, [Ph₂PF(C₆Cl₅)][B(C₆F₅)₄] 8, [PhPF(C₆Cl₅)₂][B(C₆F₅)₄] 9, and [(C₆F₅)PF(C₆Cl₅)₂][B(C₆F₅)₄] 10, were prepared and tested for air stability and catalytic activity.

Lewis acid catalysis: catalytic hydroboration of alkynes initiated by Piers' borane

Terminal and internal alkynes are efficiently hydroborated to (E)-alkenyl pinacol boronic esters with excellent yields and selectivities using Piers' borane, (HB(C₆F₅)₂) as a pre-catalyst for the dissymmetrically gem-diborylated catalyst of the form RCH₂CR′(Bpin)(B(C₆F₅)₂).

Electrophilic phenoxy-substituted phosphonium cations

A family of electrophilic phenoxy-substituted phosphonium salts [(RO)P(C₆F₅)₃][B(C₆F₅)₄] (R = C₆H₅, 4-FC₆H₄, 2,4-F₂C₆H₃, C₆F₅) have been synthesized, used as Lewis acid catalysts and their air stability was evaluated.

Pyridinium–phosphonium dications: highly electrophilic phosphorus-based Lewis acid catalysts

Using commercially available 2-pyridyldiphenylphosphine (o-NC₅H₄)PPh₂, a family of electrophilic phosphonium cations [(o-NC₅H₄)PFPh₂]⁺ (2) and dications [(o-MeNC₅H₄)PRPh₂]²⁺ (R = F (4); Me (5)) were prepared.

Phosphorus Lewis acids: emerging reactivity and applications in catalysis

Part of the renaissance in main group chemistry has been a result of the focus on reactivity and catalysis. In this tutorial review, we focus attention on the role of phosphorus-based Lewis acids in such advances.

Ferrocenyl-derived electrophilic phosphonium cations (EPCs) as Lewis acid catalysts

Ferrocenyl-derived electrophilic phosphonium cations are catalysts for a series of reactions including the deoxygenation of a series of ketones.

Phosphine and carbene azido-cations: [(L)N3]+ and [(L)2N3]. Chem Sci 2015;6:6367-6372. [PMID: 30090255 PMCID: PMC6054069 DOI: 10.1039/c5sc02336j]

The cationic N₃-species [(p-HC₆F₄)₃PN₃]⁺ (1) featuring a perfluoro-arene phosphonium group serves as a N₃⁺-source in stoichiometric reactions with several Lewis bases (L) allowing for the stepwise formation of [(L)N₃]⁺ and [(L)₂N₃]⁺ cations (L = phosphine, carbene) with liberation of (p-HC₆F₄)₃P.

The cationic N₃-species [(p-HC₆F₄)₃PN₃]⁺ (1) featuring a perfluoro-arene phosphonium group serves as a N₃⁺-source in stoichiometric reactions with several Lewis bases (L) allowing for the stepwise formation of [(L)N₃]⁺ and [(L)₂N₃]⁺ cations (L = phosphine, carbene) with liberation of (p-HC₆F₄)₃P. X-Ray diffraction analysis and computational studies provide insight into the bonding in these remarkably stable azido-cations.

Electrophilic Fluorophosphonium Cations in Frustrated Lewis Pair Hydrogen Activation and Catalytic Hydrogenation of Olefins

The combination of phosphorus(V)-based Lewis acids with diaryl amines and diaryl silylamines promotes reversible activation of dihydrogen and can be further exploited in metal-free catalytic olefin hydrogenation. Combined experimental and density functional theory (DFT) studies suggest a frustrated Lewis pair type activation mechanism.