Phosphenium Ion Research Articles

Towards Catalytic C–H Activation Using Main Group Elements

AbstractCatalytic C–H activation reactions are now established as a means to directly transform organic molecules and are commonly associated with metals such as palladium, rhodium, ruthenium and iridium. This Account will describe a short number of reports demonstrating that structures containing main group elements can facilitate C–H activation processes. In particular, boron-based catalysts can promote catalytic arene C–H borylation reactions, and an emerging approach using phosphenium ions can also cleave sp2 C–H bonds. These processes use a Lewis acidic main group atom combined with a pendant base to cleave C–H bonds, which compares with metal-catalyzed reactions that proceed via concerted metalation deprotonation mechanisms.1 Introduction2 Metal-Catalyzed C–H Activation via CMD/AMLA Mechanisms3 C–H Borylation via Boron-Based Catalysts4 C–H Activation Using Phosphenium Ions5 Conclusions

Electrophilic Functionalization of a Hexaphosphabenzene Ligand in [(Cp*Mo)2(μ,η6 : 6-P6)

The electrophilic functionalization of the triple-decker sandwich complex [{Cp*Mo}2(μ,η6:6-P6)] (A) and its mono-oxidized counterpart [{Cp*Mo}2(μ,η6:6-P6)][SbF6] (B) with reactive main-group electrophiles as well as radical scavengers is shown to be a reliable method for the selective functionalization of the hexaphosphabenzene ligand. Depending on the electrophile used, the regioselectivity of the functionalization can be adjusted. Using group 16 electrophiles, the trisubstituted compounds [{Cp*Mo}2{(μ,η3 : 3-P3)(μ,η1 : 1 : 1 : 1-1,3-(SePh)2-2-Br-P3)}][TEF] (1), [{Cp*Mo}2(μ,η3 : 3-P3)(μ,η1 : 1 : 1 : 1-1,2,3-(EPh)3-P3)][SbF6] (E=S (2), Se (3)) as well as the side product [{Cp*Mo}2(μ,η4:4-P4)(μ,η1 : 1-P(SPh)2)][SbF6] (4) are obtained. By switching to phosphenium ions as group 15 electrophiles, the ring-inserted products [{Cp*Mo}2(μ,η3 : 3 : 2 : 2-P7R2)][TEF] (R=Cy (5), iPr (6)) are isolated, showing an unprecedented P7R2 structural motif. Furthermore, the reaction with MeOTf yields the dimeric [{Cp*Mo}4(1,4-Me2-μ4,η1 : 1 : 1 : 1 : 1 : 1-P6)(μ,η3 : 3-P3)2][TEF]2 (7) as the first example of a complex featuring two interconnected cyclo-P6 middle deck ligands. Finally, by combination of the methylation step with Ph2Se2, the mixed group 14/16 complex [{Cp*Mo}2{(μ,η3 : 3-P3)(μ,η1 : 1 : 1 : 11,2-(SePh)2-3-Me-P3)}][OTf] (8) is obtained.

Reversible Oxidative Addition of Nonactivated C-H Bonds to Structurally Constrained Phosphenium Ions.

A series of structurally constrained phosphenium ions based on pyridinylmethylamidophenolate scaffolds are shown to undergo P(III)/P(V) oxidative addition with C-H bonds of alkynes, alkenes, and arenes. Nonactivated substrates such as benzene, toluene, and deactivated chlorobenzene are phosphorylated in quantitative yields. Computational and spectroscopic studies suggest a low-barrier isomerization from a bent to a T-shaped isomer that initiates a phosphorus-ligand-cooperative pathway and subsequent ring-chain tautomerism. Remarkably, C-H bond activations occur reversibly, allowing for reductive elimination back to P(III) at elevated temperatures or the exchange with other substrates.

New Insight into the Reactivity of S,S-Bis-ylide.

The present work focuses on the reactivity of S,S-bis-ylide 2, which presents a strong nucleophilic character, as evidenced by reactions with methyl iodide and CO2, affording C-methylated salts 3 and betaine 4, respectively. The derivatization of betaine 4 affords the corresponding ester derivative 6, which is fully characterized by using NMR spectroscopy and X-ray diffraction analysis. Furthermore, an original reaction with phosphenium ions leads to the formation of a transient push-pull phosphino(sulfonio)carbene 8, which rearranges to give stabilized sulfonium ylide derivative 7.

Protonation of P-Stereogenic Phosphiranes: Phospholane Formation via Ring Opening and C-H Activation.

Protonation of cyclopropanes and aziridines is well-studied, but reactions of phosphiranes with acids are rare and have not been reported to result in ring opening. Treatment of syn-Mes*PCH2CHR (Mes* = 2,4,6-(t-Bu)3C6H2, R = Me or Ph, syn-1-2) or anti-Mes*PCH2CHPh (anti-2) with triflic acid resulted in regiospecific anti-Markovnikov C-protonation with ring opening and cyclophosphination of a Mes* ortho-t-Bu group to yield the phospholanium cations [PH(CH2CH2R)(4,6-(t-Bu)2-2-CMe2CH2C6H2)][OTf] (R = Me or Ph, 3-4), which were deprotonated with NEt3 to give phospholanes 5-6. Enantioenriched or racemic syn-1 both gave racemic 3. The byproduct [Mes*PH(CH2CH2Me)(OH)][OTf] (7) was formed from syn-1 and HOTf in the presence of water. Density functional theory calculations suggested that P-protonation followed by ring opening and hydride migration to C yields the phosphenium ion, [Mes*P(CH2CH2Me)][OTf], which undergoes C-H oxidative addition of an o-t-Bu methyl group. This work established a new reactivity pattern for phosphiranes.

Ring expansion vs. addition - reactivity of a cyclo-P4 complex towards pnictogenium cations.

A systematic study on the reactivity of the cyclo-P4 complexes [CpRTa(CO)2(η4-P4)] towards pnictogenium cations results in the formation of functionalised interpnictogen cations. Phosphenium ions insert into one of the P-P bonds to give ring-expanded cyclo-P5R2 products. In contrast, an arsenium-functionalised P4AsCy2 ligand displays an interesting borderline case between ring expansion and coordination, while stibenium cations afford addition products. Tuning of the steric and electronic properties of the stibenium ion shows a drastic influence on the reaction outcome.

Phosphorus-Involved Wagner-Meerwein Rearrangement of Phosphiranes: An Entry to Four-Membered Phosphacycles.

Phosphenium ions [R2P]+ are important and highly reactive dicoordinate phosphorus species. Herein, we report a rearrangement of the carbocation into the phosphenium cation driven by ring strain. This phosphorus-involved Wagner-Meerwein rearrangement pathway converted the 1-acylphosphirane complex into phosphetane and 1,2-dihydrophosphete derivatives depending on the reaction temperature. The generation of the intermediate phosphenium cation was identified by the intramolecular reaction with ether, which also disclosed its strong Lewis acidity. This work expands the boundary of the phosphorus-carbon analogy.

Alan Herbert Cowley. 29 January 1934—2 August 2020

Alan Cowley was one of the most creative main group chemists of his generation, and had a central role in what was often described as the renaissance of main group chemistry. Throughout his career Alan always had an eye for what was new. In his early years as an independent researcher, Alan made many fundamental contributions to the chemistry of phosphorus, not only in terms of the synthesis of new compounds but also in their study by employing novel analytical and computational methods. Starting in the 1980s he was at the forefront of emerging research into low-coordinate phosphorus chemistry and made seminal contributions in the areas of multiply bonded species, such as phosphenium ions and diphosphenes, as well as in the transition metal coordination chemistry of phosphinidenes. In the second half of his career, Alan also turned his attention to the study of single source precursors for important solid-state electronic materials, many of which were far superior to known examples. In all of the many areas in which Alan worked, he was a great collaborator with colleagues and researchers across the world, both in chemistry and in other disciplines. This was made all the easier by Alan's charm and easy-going nature, which was also manifest in the interactions he had with his many group members over a period of almost half a century. Alan was a gentleman in every sense and is much missed by friends, colleagues, collaborators and family.

Ferrocene‐Based Phosphenium Ion with Intramolecular Phosphine Coordination

AbstractA bis(ferrocenyl)phosphenium ion with intramolecular phosphine‐coordination, incorporated within [2]ferrocenophane framework, was synthesized as an air‐stable compound. Although it can be thought as a phosphino‐phosphonium derivative, considering the potential rotation of the cyclopentadienyl group of the ferrocene moiety, the isolated compound should be a masked phosphenium ion with intramolecular phosphine coordination. The redox behavior of the isolated phosphine‐coordinated phosphenium ion has been revealed by CV measurements and the theoretical calculations, suggesting its oxidation at the ferrocenyl moieties and reduction at the phosphorus center consecutively giving the corresponding phosphinyl radical and phosphide ion.

Cationic Functionalisation by Phosphenium Ion Insertion.

The reaction of [Cp′′′Ni(η3‐P3)] (1) with in situ generated phosphenium ions [RR′P]+ yields the unprecedented polyphosphorus cations of the type [Cp′′′Ni(η3‐P4R2)][X] (R=Ph (2 a), Mes (2 b), Cy (2 c), 2,2′‐biphen (2 d), Me (2 e); [X]−=[OTf]−, [SbF6]−, [GaCl4]−, [BArF]−, [TEF]−) and [Cp′′′Ni(η3‐P4RCl)][TEF] (R=Ph (2 f), tBu (2 g)). In the reaction of 1 with [Br2P]+, an analogous compound is observed only as an intermediate and the final product is an unexpected dinuclear complex [{Cp′′′Ni}2(μ,η3:η1:η1‐P4Br3)][TEF] (3 a). A similar product [{Cp′′′Ni}2(μ,η3:η1:η1‐P4(2,2′‐biphen)Cl)][GaCl4] (3 b) is obtained, when 2 d[GaCl4] is kept in solution for prolonged times. Although the central structural motif of 2 a–g consists of a “butterfly‐like” folded P4 ring attached to a {Cp′′′Ni} fragment, the structures of 3 a and 3 b exhibit a unique asymmetrically substituted and distorted P4 chain stabilised by two {Cp′′′Ni} fragments. Additional DFT calculations shed light on the reaction pathway for the formation of 2 a–2 g and the bonding situation in 3 a.

The Bis(ferrocenyl)phosphenium Ion Revisited.

The bis(ferrocenyl)phosphenium ion, [Fc2P]+, reported by Cowley et al. (J. Am. Chem. Soc. 1981, 103, 714–715), was the only claimed donor‐free divalent phosphenium ion. Our examination of the molecular and electronic structure reveals that [Fc2P]+ possesses significant intramolecular Fe⋅⋅⋅P contacts, which are predominantly electrostatic and moderate the Lewis acidity. Nonetheless, [Fc2P]+ undergoes complex formation with the Lewis bases PPh3 and IPr to give the donor–acceptor complexes [Fc2P(PPh3)]+ and [Fc2P(IPr)]+ (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazole‐2‐ylidene).

Transient Phosphenium and Arsenium Ions versus Stable Stibenium and Bismuthenium Ions.

Fluoride abstraction from bis‐m‐terphenylelement fluorides (2,6‐Mes2C6H3)2EF (E=P, As) generated the highly reactive phosphenium ion [(2,6‐Mes2C6H3)2P]+ and the arsenium ion [(2,6‐Mes2C6H3)2As]+, which immediately underwent intramolecular electrophilic substitution and formation of an 1,2,4‐trimethyl‐6‐mesityl‐5‐m‐terphenyl‐benzo[b]phospholium ion and an 1,2,4‐trimethyl‐6‐mesityl‐5‐m‐terphenyl‐benzo[b]arsolium ion, respectively. The formation of the latter involved a methyl group migration from the ortho‐position of a flanking mesityl group to the meta‐position. This reactivity of [(2,6‐Mes2C6H3)2E]+ (E=P, As) is in sharp contrast to the related stibenium ion [(2,6‐Mes2C6H3)2Sb]+ and bismuthenium ion [(2,6‐Mes2C6H3)2Bi]+, which have been recently isolated and fully characterized (Angew. Chem. Int. Ed. 2018, 57, 10080–10084). On the basis of DFT calculations, a mechanism for the rearrangement of the phosphenium and arsenium ions into the phospholium and arsolium ions is proposed, which is not feasible for the stibenium and bismuthenium ions.

Phosphenium Ion Catalyzed Enantioselective Reduction of Cyclic Imines

Phosphenium Ion Catalyzed Enantioselective Reduction of Cyclic Imines

Enantioselective Imine Reduction Catalyzed by Phosphenium Ions.

The first use of phosphenium cations in asymmetric catalysis is reported. A diazaphosphenium triflate, prepared in two or three steps on a multigram scale from commercially available materials, catalyzes the hydroboration or hydrosilylation of cyclic imines with enantiomeric ratios of up to 97:3. Catalyst loadings are as low as 0.2 mol %. Twenty-two aryl/heteroaryl pyrrolidines and piperidines were prepared using this method. Imines containing functional groups such as thiophenes or pyridyl rings that can challenge transition-metal catalysts were reduced employing these systems.

Functionalized Fluorophosphonium Ions.

Efforts to prepare an elusive donor-free phosphenium ion, [R2 P]+ , led us to synthesize functionalized fluorophosphonium cations of the type [R2 P(F)X]+ (X=SiEt3 , H, F), which were obtained from the related neutral fluorophosphines R2 PF and R2 PF3 upon protonation and reaction with solvated [Et3 Si]+ ions (R=2,6-Mes2 C6 H3 ). The hypothetical reductive elimination of [R2 P(F)SiEt3 ]+ and [R2 P(F)H]+ affording [R2 P]+ , Et3 SiF and HF, respectively, was calculated to be endothermic by 40.1 and 190.6 kJ mol-1 .

Alkyne Insertion into P–C(sp2) Bonds as a Route to Fused Phospholes: Transition-Metal-Like Reactivity at Phosphorus

While alkyne insertion into metal–carbon(sp2) bonds is common in transition metal chemistry, direct alkyne insertions into E–C(sp2) bonds are very rare in main group chemistry. Herein, we report a direct alkyne insertion reaction into the P–C(aryl) bonds of tungsten-coordinated phenyl-aryl phosphenium ions, which lead to phenyl-vinyl phosphenium ions. The insertion reaction is followed by electrophilic substitution of the phosphenium ion onto the aryl group, leading to a net annulation reaction and formation of arene and heteroarene-fused phospholes. The regioselectivity of the products and computational chemistry have been used to elucidate the insertion-electrophilic substitution mechanism. This methodology has been used to synthesize known benzophosphole and phospholo[3,2-b]thiophene and new phospholo[3,2-b]pyrrole, phospholo[2,3-b]pyrrole, and phospholo[2,3-b]indole ring systems.

Phosphorus-carbon bond forming reactions of iron tetracarbonyl-coordinated phosphenium ions

Abstraction of chloride from [Fe(CO)4(PPh2Cl)] (1) in the presence of PPh3 leads to [Fe(CO)4(PPh2(PPh3))][AlCl4] (2), an iron complex of a phosphine-coordinated phosphenium ion. The PPh3 is readily displaced by ferrocene, leading to an electrophilic aromatic substitution reaction, and formation of [Fe(CO)4{PPh2Fc}] (3) (Fc = ferrocenyl). Alternately, chloride abstraction from 1 in the presence of ferrocene leads directly to 3, via a transient phosphenium ion complex. The transient phosphenium ion complex also reacts with N,N-diethylaniline, indole, and pyrrole to form the respective p-anilinyl, 3-indolyl, and 2-pyrryl phosphine complexes via electrophilic aromatic substitution. Chloride abstraction from [Fe(CO)4(PPhCl2)] in the presence of ferrocene leads to a double substitution reaction, forming [Fe(CO)4{PPhFc2}] (13).

E-H (E = B, Si, C) Bond Activation by Tuning Structural and Electronic Properties of Phosphenium Cations.

In this work, strategic enhancement of electrophilicity of phosphenium cations for the purpose of small-molecule activation was described. Our synthetic methodology for generation of novel two-coordinate phosphorus(III)-based compounds [{C6H4(MeN)2C}2C·PR]2+ ([2a]2+, R = NiPr2; [2b]2+, R = Ph) was based on the exceptional electron-donating properties of the carbodicarbene ligand (CDC). The effects of P-centered substituent exchange and increase in the overall positive charge on small substrate activation were comparatively determined by incorporating the bis(amino)phosphenium ion [(iPr2N)2P]+ ([1]+) in this study. Implemented structural and electronic modifications of phosphenium salts were computationally verified and subsequently confirmed by isolation and characterization of the corresponding E-H (E = B, Si, C) bond activation products. While both phosphenium mono- and dications oxidatively inserted/cleaved the B-H bond of Lewis base stabilized boranes, the increased electrophilicity of doubly charged species also afforded the activation of significantly less hydridic Si-H and C-H bonds. The preference of [2a]2+ and [2b]2+ to abstract the hydride rather than to insert into the corresponding bond of silanes, as well as the formation of the carbodicarbene-stabilized parent phosphenium ion [{C6H4(MeN)2C}2C·PH2]+ ([2·PH2]+) were experimentally validated.

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. ion 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.

Formation of the spirocyclic, Si-centered cage cations [ClP(NSiMe3)2Si(NSiMe3)2P5](+) and [P5(NSiMe3)2Si(NSiMe3)2P5](2).

On account of our interest in P4 activation by phosphenium ion insertion into P-P bonds we have developed synthetic routes to bicyclic N-P-Si-heterocycle 7 and probed its reactivity towards GaCl3 and P4. A GaCl3-induced rearrangement of 7 leads to the in situ formation of spirocyclic, Si-centered phosphenium ions. Their insertion into P-P bonds of one or two P4 tetrahedra yields polyphosphorus cages [ClP(NSiMe3)2Si(NSiMe3)2P5](+) (19(+)) and [P5(NSiMe3)2Si(NSiMe3)2P5](2+) (13(2+)).