Phosphorus as a Lewis Acid: CO 2 Sequestration with Amidophosphoranes

Poison to Promise: The Resurgence of Organophosphorus Fluoride Chemistry

Organophosphorus(V) fluorides have a long and tumultuous history, with early applications as toxins and nerve agents reflecting their poisonous past. Behind these very real safety considerations, there is also growing potential in a wide range of fields, from chemical biology to drug development. The recent inclusion of organophosphorus(V) fluorides in click chemistry exemplifies the promise these compounds possess and brings these molecules to the brink of a resurgence. In this Perspective, we delve into the history of P(V)-F compounds, discuss the precautions needed to work with them safely, and explore recent advancements in their synthesis and application. We conclude by discussing how this field can continue on a path toward innovation.

Aluminum in Frustrated Lewis Pair Chemistry

This review article describes the development of the use of aluminum compounds in the chemistry of frustrated Lewis pairs (FLPs) over the last 14 years. It also discusses the synthesis, reactivity and catalytic applications of intermolecular, intramolecular and so-called hidden FLPs with phosphorus, nitrogen and carbon Lewis bases. The intrinsically higher acidity of aluminum compounds compared to their boron analogs opens up different reaction pathways. The results are presented in a more or less chronological order. It is shown that Al FLPs react with a variety of polar and non-polar substrates and form both stable adducts and reversibly activate bonds. Consequently, some catalytic applications of the title compounds were presented such as dimerization of alkynes, hydrogenation of tert-butyl ethylene and imines, C-F bond activation, reduction of CO2, dehydrogenation of amine borane and transfer of ammonia. In addition, various Al FLPs were used as initiators in polymerization reactions.

Pnictogen-Rich Heterocycles Derived from a Phosphadiazonium Cation

Heterocycles that pair main group elements and nitrogen are extremely important within the π-conjugated heterocycles research community. Compared to the vast number of boron-nitrogen heterocycles, those that include phosphorus are less common. Furthermore, the use of phosphorus-nitrogen triple bonds of any type to prepare such compounds is unprecedented. Here, we pair pyridyl hydrazonide ligands with phosphadiazonium cations and demonstrate that the chelated Mes*NP group is directly implicated in the photophysical and redox properties observed for the resulting heterocycles. In doing so, we introduce a novel building block for the production of phosphorus-containing heterocycles that could find use in small molecule activation and catalysis or as the functional component of emerging organic electronics.

Fluorophosphoniums as Lewis acids in organometallic catalysis: application to the carbonylation of β-lactones

We describe the synthesis and characterisation of four organic Lewis acids based on fluorophosphoniums, with tetracarbonyl cobaltate as the counter-anion: [R3PF]+[Co(CO)4]- (with R = o-Tol, Cy, iPr, and tBu). Their catalytic activity was investigated for the carbonylation of β-lactones to succinic anhydrides. In the presence of [tBu3PF]+[Co(CO)4]- IV (3 mol%), 90% of succinic anhydride was afforded from β-propiolactone after 16 h at 80 °C, at a very mild pressure of 2 bar of carbon monoxide. Our study sets the first example of the use of a main-group cation as a Lewis acidic partner in the cobalt-catalyzed carbonylation of β-lactones.

Rapid Generation of P(V)-F Bonds Through the Use of Sulfone Iminium Fluoride Reagents

Phosphorus-fluorine bonds have become increasingly relevant in the pharmaceutical industry. To continue their exploration, more efficient synthetic methods are needed. Here, we report the application of sulfone iminium fluoride (SIF) reagents to the synthesis of P(V)-F bonds. The SIF reagents promote the deoxyfluorination of phosphinic acids in just 60 s with excellent yields and scope. The same P(V)-F products can also be synthesized from secondary phosphine oxides using an SIF reagent.

ortho-Phenylene-bridged phosphorus/silicon Lewis pairs

A series of five ortho-phenylene-bridged phosphorus-silicon Lewis pairs was synthesized, with phosphorus bearing isopropyl groups while the substituents at the silicon atom vary (-CH₃, -Cl, -F). Possible interactions between Lewis acid and base were investigated both experimentally (NMR, XRD) and theoretically to determine the influence of the different substituents. Calculated ortho-interaction energies (OIEs) show a stabilizing interactions between the acidic and basic units which were also found for the meta- and para-interaction energies (MIEs and PIEs, respectively), indicating stabilization resulting not from direct acid-base interaction but from electronic interactions through the ring. Further spectroscopic (NMR, XRD) and theoretical (NBO, QTAIM, SAPT) investigations confirmed the absence of direct interactions between silicon and phoshorus.

Four-membered C^N chelation in main-group organometallic chemistry

Main-group organometallic compounds containing four-membered C^N chelating rings are being studied because of the interest in harnessing the enhanced reactivity of such compounds which arises as a result of the release of steric strain. In this article, we have reviewed the literature on these systems. This review is organised in terms of the types of ligand systems that allow the assembly of such compounds, viz., compounds containing aliphatic amine motifs, pyridine motifs and aniline motifs. In addition to a discussion on the synthesis and structure, we also examine the reactivity and applications of the main-group element compounds involved. In particular, applications involving H₂ activation, carbonyl activation, olefin reduction, C-H activation, hydroalumination, cyanamide oligomerisation, borylation of olefins and heteroarenes, isocyanate activation and C-C bond activation are discussed.

Phosphinophosphoranes: Mixed-Valent Phosphorus Compounds with Ambiphilic Properties

Herein, we present a simple synthesis of mixed-valent phosphinophosphoranes bearing three- and five-coordinate phosphorus centers. Compounds with phosphorus-phosphorus bonds were synthesized via a reaction of lithium phosphides RR'PLi with cat₂PCl (cat = catecholate), whereas derivatives with methylene-linked phosphorus centers were obtained via a reaction of phosphanylmethanides RR'CH₂Li with cat₂PCl. The presence of accessible lone-pair electrons on the P-phosphanyl atom of phosphinophosphoranes during the reaction of the title compounds with H₃B·SMe₂, where phosphinophosphorane-borane adducts were formed quantitatively, was confirmed. Furthermore, the Lewis basic and Lewis acidic properties of the phosphinophosphoranes in reactions with phenyl isothiocyanate were tested. Depending on the structure of the starting phosphinophosphorane, phosphinophosphorylation of PhNCS or formation of a five-membered zwitterionic adduct was observed. The structures of the isolated compounds were unambiguously determined by heteronuclear nuclear magnetic resonance spectroscopy and single-crystal X-ray diffraction. Moreover, by applying density functional theory calculations, we compared the Lewis basicity and nucleophilicity of diversified trivalent P-centers.

Chelating Phosphorus-An O, C, O-Coordinating Pincer-Type Ligand Coordinating PIII and PV Centres

The sequence of reactions of the phosphorus-containing aryllithium compound 5-t-Bu-1,3-[(P(O)(O-i-Pr)₂ ]₂ C₆ H₂ Li (ArLi) with Ph₂ PCl, KMnO₄ , elemental sulfur and elemental selenium, respectively, gave the aryldiphenylphosphane chalcogenides 5-t-Bu-1,3-[(P(O)(O-i-Pr)₂ ]₂ C₆ H₂ P(E)Ph₂ (1, E=O; 2, E=S; 3, E=Se). Compound 1 partially hydrolysed giving [5-t-Bu-1-{(P(O)(O-i-Pr)₂ }-3-{(P(O)(OH)₂ }C₆ H₂ ]P(O)Ph₂ (4). The reaction of ArLi with PhPCl₂ provided the benzoxaphosphaphosphole [1(P), 3(P)-P(O)(O-i-Pr)OPPh-6-t-Bu-4-P(O)(O-i-Pr)₂ ]C₆ H₂ P (5i) as a mixture of the two diastereomers. The oxidation of 5i with elemental sulfur gave the benzoxaphosphaphosphole sulfide [1(P), 3(P)-P(O)(O-i-Pr)OP(S)Ph-6-t-Bu-4-P(O)(O-i-Pr)₂ ]C₆ H₂ (5) as pair of enantiomers P1(R), P3(S)/P1(S), P3(R) of the diastereomer (RS/SR)-5 (5b). The aryldiphenylphosphane 5-t-Bu-1,3-[(P(O)(O-i-Pr)₂ ]₂ C₆ H₂ PPh₂ (6) was obtained from the reaction of the corresponding aryldiphenylphosphane sulfide 2 with either sodium hydride, NaH, or disodium iron tetracarbonyl, Na₂ Fe(CO)₄ . The oxidation of the aryldiphenylphosphane 6 with elemental iodine and subsequent hydrolysis yielded the aryldiphenyldioxaphosphorane 9-t-Bu-2,6-(OH)-4,4-Ph₂ -3,5-O₂ -2,6-P₂ -4λ⁵ -P-[5.3.1.0]-undeca-1(10),7(11),8-triene (7). Both of its diastereomers, (RR/SS)-7 (7a) and (RS/SR)-7 (7b), were separated as their chloroform and i-propanol solvates, 7a⋅2CHCl₃ and 7b⋅i-PrOH, respectively. DFT calculations accompanied the experimental work.

Aromaticity-promoted CS2 activation by heterocycle-bridged P/N-FLPs: a comparative DFT study with CO2 capture

Carbon dioxide (CO₂) capture has attracted considerable attention from both experimental and theoretical chemists. In comparison, carbon disulfide (CS₂) activation is less developed. Here, we carry out a thorough comparative density functional theory study to examine the reaction mechanisms of CS₂ activation by five-membered heterocycle-bridged P/N frustrated Lewis pairs (FLPs). Calculations suggest that despite a weaker carbon-sulfur bond, all the CS₂ activations have higher reaction barriers than the CO₂ capture, which could be attributed to electrostatic repulsion between FLPs and CS₂ caused by the reversed polarity of CS in CS₂ rather than the electrostatic attraction in CO₂ capture. In addition, aromaticity is found to play an important role in CS₂ capture as it stabilizes both the transition states and products in heterocycle-bridged FLPs. All these findings could be useful for experimentalists to realize small molecule activations by FLPs.

Calix[4]pyrrolato gallate: square planar-coordinated gallium(iii) and its metal–ligand cooperative reactivity with CO2 and alcohols

Forcing a priori tetracoordinate atoms into planar configuration represents a promising concept for enhanced reactivity of p-block element-based systems. Herein, the synthesis, characterization, and reactivity of calix[4]pyrrolato gallates, constituting square planar-coordinated gallium(iii) atoms, are reported. Unusual structural constraint-induced Lewis acidity against neutral and anionic donors is disclosed by experiment and rationalized by computations. An energetically balanced dearomatization/rearomatization of a pyrrole unit enables fully reversible metal–ligand cooperative capture of CO₂. While alcohols are found unreactive against the gallates, a rapid and selective OH-bond activation can be triggered upon protonation of the ligand. Secondary ligand–sphere modification adds a new avenue to structurally-constrained complexes that unites functional group tolerance with unconventional reactivity.

Ideally square-planar coordinated gallium(iii) species is isolated and fully characterized. Spontaneous metal–ligand cooperative reactivity towards CO₂ is observed, while OH-bond activation of alcohols can be triggered by protonation of the ligand.

Sustainable catalyst-free N-formylation using CO2 as a carbon source

The development of new sustainable catalytic conversion methods of carbon dioxide (CO2) is of great interest in the synthesis of valuable chemicals. N-formylation of CO2 with amine nucleophiles as substrates has been studied in depth. The key to benign formylation is to select a suitable reducing agent to activate CO2. This paper showcases the activation modes of CO2 and the construction strategies of sustainable and catalyst-free N-formylation systems. The research progress of catalyst-free N-formylation of amines and CO2 is reviewed. There are two broad prominent categories, namely reductive amidation of CO2 facilitated by organic solvents and ionic liquids in the presence of hydrosilane. Attention is also paid to discussing the involved reaction mechanism with practical applications and identifying the remaining challenges in this field.

Bifunctional Fluorophosphonium Triflates as Intramolecular Frustrated Lewis Pairs: Reversible CO2 Sequestration and Binding of Carbonyls, Nitriles and Acetylenes

Electrophilic fluorophosphonium triflates bearing pyridyl (3[OTf]) or imidazolyl (4[OTf])-substituents act as intramolecular frustrated Lewis pairs (FLPs) and reversibly form 1 : 1 adducts with CO₂ (5⁺ and 6⁺ ). An unusual and labile spirocyclic tetrahedral intermediate (7²⁺ ) is observed in CO₂ -pressurized (0.5-2.0 bar) solutions of cation 4⁺ at low temperatures, as demonstrated by variable-temperature NMR studies, which were confirmed crystallographically. In addition, cations 3⁺ and 4⁺ actively bind carbonyls, nitriles and acetylenes by 1,3-dipolar cycloaddition, as shown by selected examples.

Electrophilic fluorosulfoxonium cations as hidden Brønsted acid catalysts in (n + 2) annulations of strained cycloalkanes

A fluorosulfoxonium cation and triflic acid were shown to promote (n + 2) annulations of donor–acceptor cyclopropanes or -butanes with 1,2-dipoles, with different activity and selectivity but a presumed similar role as (hidden) Brønsted acids.

Deoxygenative Fluorination of Phosphine Oxides: A General Route to Fluorinated Organophosphorus(V) Compounds and Beyond

Fluorinated organophosphorus(V) compounds are a very versatile class of compounds, but the synthetic methods available to make them bear the disadvantages of 1) occasional handling of toxic or pyrophoric P^III starting materials and 2) a dependence on hazardous fluorinating reagents such as XeF₂ . Herein, we present a simple solution and introduce a deoxygenative fluorination (DOF) approach that utilizes easy-to-handle phosphine oxides as starting materials and effectively replaces harsh fluorinating reagents by a combination of oxalyl chloride and potassium fluoride. The reaction has proven to be general, as R₃ PF₂ , R₂ PF₃ , and RPF₄ compounds (as well as various cations and anions derived from these) are accessible in good yields and on up to a multi-gram scale. DFT calculations were used to bolster our observations. Notably, the discovery of this new method led to a convenient synthesis of 1) new difluorophosphonium ions, 2) hexafluorophosphate salts, and 3) fluorinated antimony- and arsenic- compounds.

A synthetic equivalent for unknown 1,3-zwitterions? – A K/OR phosphinidenoid complex with an additional Si–Cl function

Although the chemistry of frustrated Lewis pairs (FLPs) has seen tremendous developments, investigations on anionic, mono-molecular FLPs are still scarce and 1,3-zwitterions are unknown, but a functional K/OR phosphinindenoid complex can be used instead.

Recent advances in the synthesis of organophosphorus compounds via Kobayashi's aryne precursor: a review

This review systematically summarizes the progress in aryne chemistry for the synthesis of organophosphorus compounds via aryne insertion into the C–P, P–N, P–P, P–O, PP, PN and PS bonds.

A Tautomeric λ3/λ5-Phosphane Pair and Its Ambiphilic Reactivity

The central phosphorus atom of a novel hydroxyl-functionalized triarylphosphane was shown to reversibly insert into one of the molecule's O-H bonds, which forms the basis for a tautomeric λ³/λ⁵-phosphane equilibrium. For the first time, this equilibrium was detected for a λ³-triarylphosphane and the underlying dynamic process was elucidated by NMR spectroscopy. On the basis of reactivity studies, a nucleophilic character was assigned to the minor species present in solution, the λ³-phosphane. Upon methylation, for example, the λ³-form was selectively removed from the equilibrium and converted to the corresponding phosphonium salt. However, upon generation of an alkoxide via proton abstraction, the electrophilic character of the λ⁵-phosphane in the equilibrium became evident since the alkoxide was found to attack the molecule's phosphorus atom. This intramolecular reaction led to the selective formation of a new anionic λ⁶-hydridospirophosphane.

Aryne insertion into the PO bond: one-pot synthesis of quaternary phosphonium triflates

A novel transition-metal free synthetic strategy for the direct synthesis of quaternary phosphonium triflates via insertion of aryne into phosphine oxide.

An umpolung of Lewis acidity/basicity at nitrogen by deprotonation of a cyclic (amino)(aryl)nitrenium cation

A cyclic (amino)(aryl)nitrenium cation 2 has been achieved by treatment of spiro[fluorene-9,3'-indazole] (1) with Ph2CHCl and AgBF4. This cation 2 is Lewis acidic at nitrenium N1, reacting with PMe3 affording a Lewis acid/base adduct 3. In contrast, deprotonation of 2 with other bases provides a neutral compound 4 which is Lewis basic at N1, reacting with electrophiles including GaCl3, MeOTf and PhNCO.

A strategy for developing metal-free hydrogenation catalysts: a DFT proof-of-principle study

Using DFT computations, a metal-free strategy has been formulated to activate hydrogen reversibly and to construct hydrogenation catalysts, calling for experimental realizations.

Sequential Electrophilic Substitution Reactions of Tungsten-Coordinated Phosphenium Ions and Phosphine Triflates

ion of chloride from [W(CO)₅{PPhCl₂}] with AgOSO₂CF₃ leads to the phosphine triflate complex [W(CO)₅{PPhCl(OSO₂CF₃)}] which undergoes electrophilic substitution reactions with N,N-diethylaniline, anisole, N,N-dimethyl-p-toluidine, toluene, biphenyl, naphthalene, 2,7,9,9-tetramethyl xanthene, and allyltrimethylsilane to form the chlorophosphine complexes [W(CO)₅{PPhClR}], where R = p-diethylanilinyl, p-anisyl, 2-(N,N-dimethyl-4-methylphenyl), p-tolyl, p-phenylphenyl, 1-naphthyl, 4-(2,7,9,9-tetramethylxanthyl), and allyl. Abstraction of the second chloride with AgOSO₂CF₃ leads, in most cases, to the respective phosphine triflates [W(CO)₅{PPhR(OSO₂CF₃)}], which react with ferrocene to form the ferrocenyl phosphine complexes [W(CO)₅{PPhR(C₁₀H₉Fe)}]. The W(CO)₅ unit can be removed via photolysis in the presence of bis(diphenylphosphino)ethane to form metal-free phosphines.

Carbodiphosphorane mediated synthesis of a triflyloxyphosphonium dication and its reactivity towards nucleophiles

A carbodiphosphorane ((Ph₃P)₂C) mediated synthesis of the first triflyloxyphosphonium dication (12+) bearing two electrophilic sites is presented. Depending on the nucleophile, 12+ reacts selectively either at the sulfur atom of the triflyloxy moiety or at the directly attached phosphorus atom. In substitution reactions at the phosphorus atom the triflyloxy moiety serves as a leaving group and enables the synthesis of rare examples of pseudo-halophosphonium dications.

Synthesis of a monomolecular anionic FLP complex

Despite intense research in FLP chemistry, nothing is known about monomolecular anionic FLPs and/or complexes thereof. Herein, synthesis and reaction of the first anionic FLP complex is described using [(OC)₅W{(Me₃Si)₂HCP(H)OLi(12-crown-4)}], Cy₂BCl and subsequent deprotonation by KHMDS. The obtained anionic FLP complex reacts readily with CO₂ in a concerted manner.