Functionalization of Intramolecular Frustrated Lewis Pairs by 1,1‐Carboboration with Conjugated Enynes

The vicinal P/B frustrated Lewis pair (FLP) Mes₂PCH₂CH₂B(C₆F₅)₂ undergoes 1,1‐carboboration reactions with the Me₃Si‐substituted enynes to give ring‐enlarged functionalized C₃‐bridged P/B FLPs. These serve as active FLPs in the activation of dihydrogen to give the respective zwitterionic [P]H⁺/[B]H^? products. One such product shows activity as a metal‐free catalyst for the hydrogenation of enamines or a bulky imine. The ring‐enlarged FLPs contain dienylborane functionalities that undergo “bora‐Nazarov”‐type ring‐closing rearrangements upon photolysis. A DFT study had shown that the dienylborane cyclization of such systems itself is endothermic, but a subsequent C₆F₅ migration is very favorable. Furthermore, substituted 2,5‐dihydroborole products are derived from cyclization and C₆F₅ migration from the photolysis reaction. In the case of the six‐membered annulation product, a subsequent stereoisomerization reaction takes place and the resultant compound undergoes a P/B FLP 1,2‐addition reaction with a terminal alkyne with rearrangement.     

Characterization of the Zwitterionic Intermediate in 1,1‐Carboboration of Alkynes

The reaction of a Lewis acidic borane with an alkyne is a key step in a diverse range of main group transformations. Alkyne 1,1‐carboboration, the Wrackmeyer reaction, is an archetypal transformation of this kind. 1,1‐Carboboration has been proposed to proceed through a zwitterionic intermediate. We report the isolation and spectroscopic, structural and computational characterization of the zwitterionic intermediates generated by reaction of B(C₆F₅)₃ with alkynes. The stepwise reactivity of the zwitterion provides new mechanistic insight for 1,1‐carboboration and wider B(C₆F₅)₃ catalysis. Making use of intramolecular stabilization by a ferrocene substituent, we have characterized the zwitterionic intermediate in the solid state and diverted reactivity towards alkyne cyclotrimerization.     

The Arene‐Stabilized η5‐Pentamethylcyclopentadienyl Arsenic Dication [(η5‐Cp*)As(toluene)]2+

Double chloride abstraction of Cp*AsCl₂ gives the dicationic arsenic species [(η⁵‐Cp*)As(tol)][B(C₆F₅)₄]₂ ( 2 ) (tol=toluene). This species is shown to exhibit Lewis super acidity by the Gutmann–Beckett test and by fluoride abstraction from [NBu₄][SbF₆]. Species 2 participates in the FLP activation of THF affording [(η²‐Cp*)AsO(CH₂)₄(THF)][B(C₆F₅)₄]₂ ( 5 ). The reaction of 2 with PMe₃ or dppe generates [(Me₃P)₂As][B(C₆F₅)₄] ( 6 ) and [(σ‐Cp*)PMe₃][B(C₆F₅)₄] ( 7 ), or [(dppe)As][B(C₆F₅)₄] ( 8 ) and [(dppe)(σ‐Cp*)₂][B(C₆F₅)₄]₂ ( 9 ), respectively, through a facile cleavage of C?As bonds, thus showcasing unusual reactivity of this unique As‐containing compound.     

Reaction of Frustrated Lewis Pairs with Ketones and Esters

The frustrated Lewis pair (FLP) Mes₂PCH₂CH₂B(C₆F₅)₂ ( 1 ) reacts with an enolizable conjugated ynone by 1,4‐addition involving enolate tautomerization to give an eight‐membered zwitterionic heterocycle. The conjugated endione PhCO‐CH?CH‐COPh reacts with the intermolecular FLP tBu₃P/B(C₆F₅)₃ by a simple 1,4‐addition to an enone subunit. The same substrate undergoes a more complex reaction with the FLP 1 that involves internal acetal formation to give a heterobicyclic zwitterionic product. FLP 1 reacts with dimethyl maleate by selective overall addition to the C?C double bond to give a six‐membered heterocycle. It adds analogously to the triple bond of an acetylenic ester to give a similarly structured six‐membered heterocycle. The intermolecular FLP P(o‐tolyl)₃/B(C₆F₅)₃ reacts analogously with acetylenic ester by trans‐addition to the carbon–carbon triple bond. An excess of the intermolecular FLP tBu₃P/B(C₆F₅)₃, which contains a more nucleophilic phosphane, reacts differently with acetylenic ester examples, namely by O? C(alkyl) bond cleavage to give the {R‐CO₂[B(C₆F₅)₃]₂^?}[alkyl‐PtBu₃⁺] salts. Simple aryl or alkyl esters react analogously by using the borane‐stabilized carboxylates as good leaving groups. All essential products were characterized by X‐ray diffraction.     

Ru–η6‐Arene Cations [{(Ph2PC6H4)2B(η6‐Ph)}RuX]+ (X=Cl,H) as Lewis Acids

The reactivity of [{(Ph₂PC₆H₄)₂B(η⁶‐Ph)}RuCl][B(C₆F₅)₄] ( 1 ) as a Lewis acid was investigated. Treatment of 1 with mono and multidentate phosphorus Lewis bases afforded the Lewis acid–base adducts with the ortho‐carbon atom of the coordinated arene ring. Similar reactivity was observed upon treatment with N‐heterocyclic carbenes; however, adduct formation occurred at both ortho‐ and para‐carbon atoms of the bound arene with the para‐position being favoured by increased steric demands. Interestingly treatment with isocyanides resulted in adduct formation with the B‐centre of the ligand framework. The hydride‐cation [{(Ph₂PC₆H₄)₂B(η⁶‐Ph)}RuH] [B(C₆F₅)₄] was prepared via reaction of 1 with silane. This species in the presence of a bulky phosphine behaves as a frustrated Lewis pair (FLP) to activate H₂ between the phosphorus centre and the ortho‐carbon atom of the η⁶‐arene ring.     

Bifunctional Behavior of Unsaturated Intramolecular Phosphane–Borane Frustrated Lewis Pairs Derived from Uncatalyzed 1,4‐Hydrophosphination of a Dienylborane

Three unsaturated C₄‐bridged phospane/borane frustrated Lewis pairs (P/B FLPs) are prepared by uncatalyzed hydrophosphination of a dienylborane. The systems are bifunctional. Consequently, two examples undergo clean hydroboration reactions with HB(C₆F₅)₂ to yield B/B/P systems. The 1,4‐P/B system (C₆F₅)₂B?CH₂CH?CMeCH₂PMes₂ reacts with benzaldehyde initially by allylborane addition, followed by internal P/B FLP addition to the pendant C?C double bond, to yield a bicyclic product. The corresponding reaction of (C₆F₅)₂B?CH₂CH?CMeCH₂PtBu₂ stops at the allylborane/benzaldehyde addition product. The related system (C₆F₅)₂B?CH₂CH?CMeCH₂PPh₂ shows a similar bifunctional reaction pattern, whereby allylborane addition to benzaldehyde is combined with P/B addition to a second aldehyde equivalent to form the eight‐membered heterocyclic 1:2 addition product.     

Cyclisation versus 1,1‐Carboboration: Reactions of B(C6F5)3 with Propargyl Amides

A series of propargyl amides were prepared and their reactions with the Lewis acidic compound B(C₆F₅)₃ were investigated. These reactions were shown to afford novel heterocycles under mild conditions. The reaction of a variety of N‐substituted propargyl amides with B(C₆F₅)₃ led to an intramolecular oxo‐boration cyclisation reaction, which afforded the 5‐alkylidene‐4,5‐dihydrooxazolium borate species. Secondary propargyl amides gave oxazoles in B(C₆F₅)₃ mediated (catalytic) cyclisation reactions. In the special case of disubstitution adjacent to the nitrogen atom, 1,1‐carboboration is favoured as a result of the increased steric hindrance (1,3‐allylic strain) in the 5‐alkylidene‐4,5‐dihydrooxazolium borate species.     

An Air‐Stable Heterobimetallic Si2M2 Tetrahedral Cluster

Air‐ and moisture‐stable heterobimetallic tetrahedral clusters [Cp(CO)₂MSiR]₂ (M=Mo or W; R=SitBu₃) were isolated from the reaction of N‐heterocyclic carbene (NHC) stabilized silyl(silylidene) metal complexes Cp(CO)₂M=Si(SitBu₃)NHC with a mild Lewis acid (BPh₃). Alternatively, treatment of the NHC‐stabilized silylidene complex Cp(CO)₂W=Si(SitBu₃)NHC with stronger Lewis acids such as AlCl₃ or B(C₆F₅)₃ resulted in the reversible coordination of the Lewis acid to one of the carbonyl ligands. Computational investigations revealed that the dimerization of the intermediate metal silylidyne (M≡Si) complex to a tetrahedral cluster instead of a planar four‐membered ring is due to steric bulk.     

B(C6F5)3‐Catalyzed Transfer Hydrogenation of Imines and Related Heteroarenes Using Cyclohexa‐1,4‐dienes as a Dihydrogen Source

The strong boron Lewis acid tris(pentafluorophenyl)borane, B(C₆F₅)₃, is shown to abstract a hydride from suitably donor‐substituted cyclohexa‐1,4‐dienes, eventually releasing dihydrogen. This process is coupled with the FLP‐type (FLP=frustrated Lewis pair) hydrogenation of imines and nitrogen‐containing heteroarenes that are catalyzed by the same Lewis acid. The net reaction is a B(C₆F₅)₃‐catalyzed, i.e., transition‐metal‐free, transfer hydrogenation using easy‐to‐access cyclohexa‐1,4‐dienes as reducing agents. Competing reaction pathways with or without the involvement of free dihydrogen are discussed.     

Preparation of Dithienylphospholes by 1,1‐Carboboration

In this study the scope of the 1,1‐carboboration reaction was extended to the preparation of mixed heterole‐based conjugated π‐systems. Two arylbis(alkynyl)phosphane starting materials 2 were synthesized bearing two thiophene isomers at the alkyne units and the bulky tipp‐substituent (tipp=2,4,6‐triisopropylphenyl) at the phosphorous atom. The bis(thienylethynyl)phosphanes 2 were converted into the corresponding 2,5‐thienyl‐substituted 3‐borylphospholes 4 in a double 1,1‐carboboration reaction sequence employing the strongly electrophilic B(C₆F₅)₃ reagent under mild reaction conditions. Subsequent Suzuki–Miyaura type cross‐coupling yielded the corresponding 3‐phenylphospholes 7 in a one‐pot procedure from phosphanes 2 in high yields. Phospholes 7 were converted into the respective phosphole oxides 8 . A photophysical characterization of derivatives 7 and 8 was carried out. The results presented here demonstrate the suitability of the 1,1‐carboboration reaction for the preparation of phosphole‐/thiophene‐based, light‐emitting systems.     

Carboboration Reactions of 1,2‐Bis[(diarylphosphino)ethynyl]benzenes with Tris(pentafluorophenyl)borane

The 1,2‐bis[(diarylphosphino)ethynyl]benzene derivatives 1a (R=Ph) and 1b (R=o‐tolyl) undergo 1,1‐carboboration at one of their acetylene units upon treatment with (C₆F₅)₃B at elevated temperature to give the products 5a and 5b , respectively. At room temperature, we observed the formation of the corresponding phosphireniumborate zwitterions, 7a and 7b , respectively, which may be intermediates of the 1,1‐carboboration reactions. The reaction of the more bulky 1,2‐bis[(dimesitylphosphino)ethynyl]benzene 1c with (C₆F₅)₃B takes a different course. At 110°, we observed the complete conversion to the benzopentafulvene derivative 8 which is probably formed in a typical carbocation rearrangement sequence after the initial (C₆F₅)₃B Lewis acid‐addition step. The compounds 5a, 5b, 7b , and 8 were characterized by X‐ray crystal‐structure analyses.     

A 1,1‐Carboboration Route to Bora‐Nazarov Systems

Hydroboration of the conjugated enynes 1 a and 1 b with Piers’ borane [HB(C₆F₅)₂] gave the respective dienylboranes trans‐ 2 c and trans‐ 2 d . Their photolysis resulted in the formation of the dihydroborole products 3 c and 3 d . Both were converted to their pyridine adducts 5 c and 5 d , respectively. Compounds 3 c and 5 c,d were characterized by X‐ray diffraction. The reaction of the bis(enynyl)boranes 6 a and 6 b with B(C₆F₅)₃ resulted in the formation of the dihydroboroles 7 a and 7 b , respectively. This reaction is thought to proceed by 1,1‐carboboration of one of the enynyl substituents at boron to generate the dienylborane intermediates 8 a / 8 b , followed by thermally induced bora‐Nazarov ring‐closure and subsequent stabilizing 1,2‐pentafluorophenyl group migration from boron to carbon. Compound 7 a was characterized by X‐ray diffraction and solid‐state ¹¹B NMR spectroscopy.     

A Neutral “Aluminocene” Sandwich Complex: η1‐ versus η5‐Coordination Modes of a Pentaarylborole with ECp* (E=Al,Ga; Cp*=C5Me5)

The pentaaryl borole (Ph*C)₄BXyl^F [Ph*=3,5‐tBu₂(C₆H₃); Xyl^F=3,5‐(CF₃)₂(C₆H₃)] reacts with low‐valent Group 13 precursors AlCp* and GaCp* by two divergent routes. In the case of [AlCp*]₄, the borole reacts as an oxidising agent and accepts two electrons. Structural, spectroscopic, and computational analysis of the resulting unprecedented neutral η⁵‐Cp*,η⁵‐[(Ph*C)₄BXyl^F] complex of Al^III revealed a strong, ionic bonding interaction. The formation of the heteroleptic borole‐cyclopentadienyl “aluminocene” leads to significant changes in the ¹³C NMR chemical shifts within the borole unit. In the case of the less‐reductive GaCp*, borole (Ph*C)₄BXyl^F reacts as a Lewis acid to form a dynamic adduct with a dative 2‐center‐2‐electron Ga?B bond. The Lewis adduct was also studied structurally, spectroscopically, and computationally.     

Synthesis and reactivity of alkynyl-linked phosphonium borates

The phosphine tBu₂PC?CH ( 1 ) was reacted with B(C₆F₅) to give the zwitterionic species tBu₂P(H)C?CB(C₆F₅)₃ ( 2 ). The analogous species tBu₂P(Me)C?CB(C₆F₅)₃ ( 3 ), tBu₂P(H)C?CB(Cl)(C₆F₅)₂ ( 4 ), tBu₂P(H)C?CB(H)(C₆F₅)₂ ( 5 ), and tBu₂P(Me)C?CB(H)(C₆F₅) ₂ ( 6 ) were also prepared. The salt [tBu₂P(H)C?CB(C₆F₅)₂(THF)][B(C₆F₅)₄] ( 7 ) was prepared through abstraction of hydride by [Ph₃C][B(C₆F₅)₄]. Species 5 reacted with the imine tBuN?CHPh to give the borane–amine adduct tBu₂PC?CB[tBuN(H)CH₂Ph](C₆F₅)₂ ( 8 ). The related phosphine Mes₂PC?CH ( 9 ; Mes=C₆H₂Me₃) was used to prepare [tBu₃PH][Mes₂PC?CB(C₆F₅)₃] ( 10 ) and generate Mes₂PC?CB(C₆F₅)₂. The adduct Mes₂PC?CB(NCMe)(C₆F₅)₂ ( 11 ) was isolated. Reaction of Mes₂PC?CB(C₆F₅)₂ with H₂ gave the zwitterionic product (C₆F₅)₂(H)BC(H)?C[P(H)Mes₂][(C₆F₅)₂BC?CP(H)Mes₂] ( 12 ). Reaction of tBu₂PC?CB(C₆F₅)₂, a phosphine–borane generated in situ from 5 , with 1‐hexene gave the species [tBu₂PC?CB(C₆F₅)₂](CH₂CHnBu)[tBu₂PC?CB(C₆F₅)₂] ( 13 ) and subsequent reaction with methanol or hexene resulted in the formation of [tBu₂P(H)C?CB(C₆F₅)₂](CH₂CHnBu)[tBu₂PC?CB(C₆F₅)₂](OMe) ( 14 ) or the macrocycle {[tBu₂PC?CB(C₆F₅)₂](CH₂CH₂nBu)}₂ ( 15 ), respectively. In a related fashion, the reaction of 13 with THF afforded the macrocycle [tBu₂PC?CB(C₆F₅)₂](CH₂CHnBu)[tBu₂PC?CB(C₆F₅)₂][O(CH₂)₄] ( 16 ), although treatment of tBu₂PC?CB(C₆F₅)₂ with THF lead to the formation of {[tBu₂PC?CB(C₆F₅)₂][O(CH₂)₄]}₂ ( 17 ). In a related example, the reaction of Mes₂PC?CB(C₆F₅)₂ with PhC?CH gave {[Mes₂PC?CB(C₆F₅)₂](CH?CPh)}₂ ( 18 ). Compound 5 reacted with AlX₃ (X=Cl, Br) to give addition to the alkynyl unit, affording (C₆F₅)₂BC(H)?C[P(H)tBu₂](AlX₃) (X=Cl 19 , Br 20 ). In a similar fashion, 5 reacted with [Zn(C₆F₅)₂] ? C₇H₈, [Al(C₆F₅)₃] ? C₇H₈, or HB(C₆F₅)₂ to give (C₆F₅)₃BC(H)?C[P(H)tBu₂][Zn(C₆F₅)] ( 21 ), (C₆F₅)₃BC(H)?C[P(H)tBu₂][Al(C₆F₅)₂] ( 22 ), or [(C₆F₅)₂B]₂HC?CH[P(H)tBu₂] ( 23 ), respectively. The implications of this reactivity are discussed.     

An Air-Stable Heterobimetallic Si2M2 Tetrahedral Cluster

Air- and moisture-stable heterobimetallic tetrahedral clusters [Cp(CO)₂MSiR]₂ (M=Mo or W; R=SitBu₃) were isolated from the reaction of N-heterocyclic carbene (NHC) stabilized silyl(silylidene) metal complexes Cp(CO)₂M=Si(SitBu₃)NHC with a mild Lewis acid (BPh₃). Alternatively, treatment of the NHC-stabilized silylidene complex Cp(CO)₂W=Si(SitBu₃)NHC with stronger Lewis acids such as AlCl₃ or B(C₆F₅)₃ resulted in the reversible coordination of the Lewis acid to one of the carbonyl ligands. Computational investigations revealed that the dimerization of the intermediate metal silylidyne (M≡Si) complex to a tetrahedral cluster instead of a planar four-membered ring is due to steric bulk.     

Accessing Frustrated Lewis Pair Chemistry from a Spectroscopically Stable and Classical Lewis Acid‐Base Adduct

B(C₆F₅)₃ and P(MeNCH₂CH₂)₃N form a classical Lewis adduct, (C₆F₅)₃BP(MeNCH₂CH₂)₃N. Although (C₆F₅)₃BP(MeNCH₂CH₂)₃N does not exhibit spectroscopic evidence of dissociation into its constituent acid and base, products of frustrated Lewis pair (FLP) addition reactions are seen with PhNCO, PhCH₂N₃, PhNSO, and CO₂. Computational studies show that thermal access to the dissociated acid and base permits FLP reactivity to proceed. These results demonstrate that FLP reactivity extends across the entire continuum of equilibria governing Lewis acid‐base adducts.     

1,1‐Hydroboration and a Borane Adduct of Diphenyldiazomethane: A Potential Prelude to FLP‐N2 Chemistry

Diphenyldiazomethane reacts with HB(C₆F₅)₂ and B(C₆F₅)₃, resulting in 1,1‐hydroboration and adduct formation, respectively. The hydroboration proceeds via a concerted reaction involving initial formation of the Lewis adduct Ph₂CN₂BH(C₆F₅)₂. The highly sensitive adduct Ph₂CN₂(B(C₆F₅)₃) liberates N₂ and generates Ph₂CB(C₆F₅)₃. DFT computations reveal that formation of Ph₂CN₂B(C₆F₅)₃ from carbene, N₂, and borane is thermodynamically favourable, suggesting steric frustration could preclude carbene–borane adduct formation and affect FLP‐N₂ capture.     

Metal‐Free Hydrogenation Catalyzed by an Air‐Stable Borane: Use of Solvent as a Frustrated Lewis Base

In recent years ‘frustrated Lewis pairs’ (FLPs) have been shown to be effective metal‐free catalysts for the hydrogenation of many unsaturated substrates. Even so, limited functional‐group tolerance restricts the range of solvents in which FLP‐mediated reactions can be performed, with all FLP‐mediated hydrogenations reported to date carried out in non‐donor hydrocarbon or chlorinated solvents. Herein we report that the bulky Lewis acids B(C₆Cl₅)_x(C₆F₅)_3?x (x=0–3) are capable of heterolytic H₂ activation in the strong‐donor solvent THF, in the absence of any additional Lewis base. This allows metal‐free catalytic hydrogenations to be performed in donor solvent media under mild conditions; these systems are particularly effective for the hydrogenation of weakly basic substrates, including the first examples of metal‐free catalytic hydrogenation of furan heterocycles. The air‐stability of the most effective borane, B(C₆Cl₅)(C₆F₅)₂, makes this a practically simple reaction method.     

A Stable but Highly Reactive Phosphine‐Coordinated Borenium: Metal‐free Dihydrogen Activation and Alkyne 1,2‐Carboboration

Borenium cations have been found to be valuable analogues of boranes as a result of their cationic character which imparts high electrophilicity. Herein, we report the synthesis, characterization, and reactivity of a new type of borenium cation employing a naphthyl bridge and a strong intramolecular P→B interaction. The cation reacts with H₂ in the presence of PtBu₃ (frustrated Lewis pair (FLP) approach) but also on its own. The mechanism of the reaction between the borenium cation and H₂ in the absence of PtBu₃ has been investigated using deuterium‐labeling experiments and DFT calculations. Both experiments and calculations imply the side‐on coordination of H₂ to the B center, followed by heterolytic splitting and B? C bond cleavage. An uncommon syn 1,2‐carboboration has also been observed upon reaction of the borenium ion with 3‐hexyne.     

Cationic,Methylene‐Bridged,Intramolecular Donor‐Acceptor Systems Based on Zirconium and Hafnium and Phosphino‐methanides

A new access to cationic zirconium and hafnium compounds [L₂MCH₂PR₂][MeB(C₆F₅)₃] (L = Cp, Ind; R = iso‐Pr, tert‐Bu; M = Zr, Hf) exhibiting an intramolecular donor‐acceptor system was established by treating the precursors L₂M(Me)CH₂PR₂ with B(C₆F₅)₃ (BCF). Precursors 1 – 6 [L₂M(Me)CH₂PR₂ with L = Cp, Ind; R = iso‐Pr, tert‐Bu; M = Zr, Hf] were fully characterized. The crystal structures of these compounds revealed large M–CH₂–P bond angles with values of about 134° indicating the absence of interactions between the Lewis‐acid and Lewis‐base. The cationic compounds [L₂MCH₂PR₂][MeB(C₆F₅)₃] ( 7 – 12 ) were obtained by treatment of 1 – 6 with BCF. They were characterized by NMR spectroscopy, mass spectrometry, and elemental analyses; in H/D‐scrambling experiments with H₂/D₂ mixtures 7 – 12 disclosed their reactivity towards cleavage of hydrogen.