Stoichiometric and Catalytic C−C and C−H Bond Formation with B(C6F5)3 via Cationic Intermediates

This work showcases a new catalytic cyclization reaction using a highly Lewis acidic borane with concomitant C−H or C−C bond formation. The activation of alkyne‐containing substrates with B(C₆F₅)₃ enabled the first catalytic intramolecular cyclizations of carboxylic acid substrates using this Lewis acid. In addition, intramolecular cyclizations of esters enable C−C bond formation as catalytic B(C₆F₅)₃ can be used to effect formal 1,5‐alkyl migrations from the ester functional groups to unsaturated carbon–carbon frameworks. This metal‐free method was used for the catalytic formation of complex dihydropyrones and isocoumarins in very good yields under relatively mild conditions with excellent atom efficiency.     

Homolytic C−H Bond Activation by Phosphine−Quinone-Based Radical Ion Pairs

Herein, we present the formation of transient radical ion pairs (RIPs) by single-electron transfer (SET) in phosphine−quinone systems and explore their potential for the activation of C−H bonds. PMes₃ (Mes=2,4,6-Me₃C₆H₂) reacts with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) with formation of the P−O bonded zwitterionic adduct Mes₃P−DDQ ( 1 ), while the reaction with the sterically more crowded PTip₃ (Tip=2,4,6-iPr₃C₆H₂) afforded C−H bond activation product Tip₂P(H)(2-[CMe₂(DDQ)]-4,6-iPr₂-C₆H₂) ( 2 ). UV/Vis and EPR spectroscopic studies showed that the latter reaction proceeds via initial SET, forming RIP [PTip₃]⋅⁺[DDQ]⋅⁻, and subsequent homolytic C−H bond activation, which was supported by DFT calculations. The isolation of analogous products, Tip₂P(H)(2-[CMe₂{TCQ−B(C₆F₅)₃}]-4,6-iPr₂-C₆H₂) ( 4 , TCQ=tetrachloro-1,4-benzoquinone) and Tip₂P(H)(2-[CMe₂{oQ^tBu−B(C₆F₅)₃}]-4,6-iPr₂-C₆H₂) ( 8 , oQ^tBu=3,5-di-tert-butyl-1,2-benzoquinone), from reactions of PTip₃ with Lewis-acid activated quinones, TCQ−B(C₆F₅)₃ and oQ^tBu−B(C₆F₅)₃, respectively, further supports the proposed radical mechanism. As such, this study presents key mechanistic insights into the homolytic C−H bond activation by the synergistic action of radical ion pairs.     

Frustrated Lewis Pair Chemistry Enables N2 Borylation by Formal 1,3-Addition of a B−H Bond in the Coordination Sphere of Tungsten

The first example of a formal 1,3-B−H bond addition across the M−N≡N unit of an end-on dinitrogen complex has been achieved. The use of Piers’ borane HB(C₆F₅)₂ was essential to observe this reactivity and it plays a triple role in this transformation: 1) electrophilic N₂-borylation agent, 2) Lewis acid in a frustrated Lewis pair-type B−H bond activation, and 3) hydride shuttle to the metal center. This chemistry is supported by NMR spectroscopy and solid-state characterization of products and intermediates. The combination of chelate effect and strong σ donation in the diphosphine ligand 1,2-bis(diethylphosphino)ethane was mandatory to avoid phosphine dissociation that otherwise led to complexes where borylation of N₂ occurred without hydride transfer.     

B(C6F5)3-Enabled Synthesis of a Cyclic cis-Arsaphosphene

The synthesis and characterization of an (arsino)phosphaketene, As(PCO){[N(Dipp)](CH₂)}₂ (Dipp=2,6-diisopropylphenyl) is reported along with its subsequent reactivity with B(C₆F₅)₃. When reacted in a stoichiometric ratio, B(C₆F₅)₃ drove the insertion of the P=C bond of the phosphaketene into one of the As−N bonds of the arsino functionality, affording an acid-stabilized, seven-membered, cyclic arsaphosphene. In contrast, when catalytic amounts of B(C₆F₅)₃ were employed, dimeric species, which formed through a formal [2+2] cycloaddition of the cyclic arsaphosphene, were generated. The cyclic arsaphosphene product represents the first example of such a compound in which the two substituents are arranged in a cis-configuration.     

Front Cover: Reactivity of a Free Aluminylene towards Boron Lewis Acids: Accessing Aluminum-Boron-Bonded Species (Eur. J. Inorg. Chem. 25/2023)

The reactivity of the free aluminylene [N]-Al ( 1 ) ([N]=1,8-bis(3,5-di-tert-butylphenyl)-3,6-di-tert-butylcarbazolyl) towards boron Lewis acids is investigated. A facile oxidative addition reaction of 1 with Ph₂BOBPh₂ furnishes an exceedingly scarce example of the free alumaborane [N]-Al(BPh₂)(OBPh₂) ( 2 ) with an Al−B electron-sharing bond. By contrast, complexation of 1 with B(C₆F₅)₃ and HB(C₆F₅)₂ gives rise to the corresponding Lewis adducts [N]-Al→B(C₆F₅)₃ ( 3 ) and [N]-Al→BH(C₆F₅)₂ ( 4 ), respectively, with an Al→B dative bond. Crystallization of 4 in Et₂O produces the adduct [N]-Al(Et₂O)→BH(C₆F₅)₂ ( 5 ). Quantum chemical calculations are carried out to understand the formation of 2 as well as the bonding situation of 3 and 5 .     

Lewis Acid Coordination Redirects S-Nitrosothiol Signaling Output

S-Nitrosothiols (RSNOs) serve as air-stable reservoirs for nitric oxide in biology. While copper enzymes promote NO release from RSNOs by serving as Lewis acids for intramolecular electron-transfer, redox-innocent Lewis acids separate these two functions to reveal the effect of coordination on structure and reactivity. The synthetic Lewis acid B(C₆F₅)₃ coordinates to the RSNO oxygen atom, leading to profound changes in the RSNO electronic structure and reactivity. Although RSNOs possess relatively negative reduction potentials, B(C₆F₅)₃ coordination increases their reduction potential by over 1 V into the physiologically accessible +0.1 V vs. NHE. Outer-sphere chemical reduction gives the Lewis acid stabilized hyponitrite dianion trans-[LA-O-N=N-O-LA]²⁻ [LA=B(C₆F₅)₃], which releases N₂O upon acidification. Mechanistic and computational studies support initial reduction to the [RSNO-B(C₆F₅)₃] radical anion, which is susceptible to N−N coupling prior to loss of RSSR.     

Selective C−O Bond Cleavage of Sugars with Hydrosilanes Catalyzed by Piers’ Borane Generated In Situ

Described herein is the selective reduction of sugars with hydrosilanes catalyzed by using Piers’ borane [(C₆F₅)₂BH] generated in situ. The hydrosilylative C−O bond cleavage of silyl‐protected mono‐ and disaccharides in the presence of a (C₆F₅)₂BH catalyst, generated in situ from (C₆F₅)₂BOH, takes place with excellent chemo‐ and regioselectivities to provide a range of polyols. A study of the substituent effects of sugars on the catalytic activity and selectivity revealed that the steric environment around the anomeric carbon (C1) is crucial.     

N-Heterocyclic Carbene Derived 3-Azabutadiene as a π-Base in Classic and Frustrated Lewis Pair Chemistry

N-Heterocyclic carbene (NHC) derived 3-azabutadienes 1 and 2 have been prepared by a single-step reaction of the corresponding NHC with cyclohexyl isocyanide. Compound 1 features π-basic, delocalized nucleophilic sites over the 3-azabutadiene moiety, therefore allowing for coordinating with small Lewis acids, such as AlCl₃, GaCl₃, and Me₂SAuCl, to form diverse classic Lewis adducts 3 – 5 . Combination of 1 with B(C₆F₅)₃ or [Ph₃C][B(C₆F₅)₄] resulted in single-electron transfer and the obtained radical cation was detected by EPR. In addition, a frustrated Lewis pair comprised of the π-basic 1 and BPh₃ effects the splitting of the O−H bond of phenol and the N−H bond of imidazole to give 7 and 8 , respectively. An intrinsic bond orbital (IBO) analysis of the pathway leading to 8 showcases the transformation of the delocalized π-electrons of 1 to a newly formed C−H localized σ-bond.     

New Reactivity Patterns in 3H-Phosphaallene Chemistry [Aryl-P=C=C(H)-tBu]: Hydroboration of the C=C Bond,Deprotonation and Trimerisation

3H-Phosphaallenes, R−P=C=C(H)C−R’ ( 3 ), are accessible in a multigram scale on a new and facile route and show a fascinating chemical reactivity. BH₃(SMe₂) and 3 a (R=Mes*, R’=tBu) afforded by hydroboration of the C=C bonds of two phosphaallene molecules an unprecedented borane ( 7 ) with the B atom bound to two P=C double bonds. This compound represents a new FLP based on a B and two P atoms. The increased Lewis acidity of the B atom led to a different reaction course upon treatment of 3 a with H₂B-C₆F₅(SMe₂). Hydroboration of a C=C bond of a first phosphaallene is followed in a typical FLP reaction by the coordination of a second phosphaallene molecule via B−C and P−B bond formation to yield a BP₂C₂ heterocycle ( 8 ). Its B−P bond is short and the B-bound P atom has a planar surrounding. Treatment of 3 a with tBuLi resulted in deprotonation of the β-C atom of the phosphaallene ( 9 ). The Li atom is bound to the P atom as demonstrated by crystal structure determination, quantum chemical calculations and reactions with HCl, Cl-SiMe₃ or Cl-PtBu₂. The thermally unstable phosphaallene Ph−P=C=C(H)-tBu gave a unique trimeric secondary product by P−P, P−C and C−C bond formation. It contains a P₂C₄ heterocycle and was isolated as a W(CO)₄ complex with two P atoms coordinated to W ( 15 ).     

Borane‐Catalyzed Cross‐Metathesis Strategy for Facile Transformation of Cyclic (Alkyl)(Amino)Germylenes

A borane B(C₆F₅)₃‐catalyzed metathesis reaction between the Si?C bond in the cyclic (alkyl)(amino)germylene (CAAGe) 1 and the Si?H bond in a silane (R₃SiH; 2 ) is reported. Mechanistic studies propose that the initial step of the reaction involves Si?H bond activation to furnish an ionic species [ 1 ‐SiR₃]⁺[HB(C₆F₅)₃]^?, from which [Me₃Si]⁺[HB(C₆F₅)₃]^? and an azagermole intermediate are generated. The former yields Me₃SiH concomitant with the regeneration of B(C₆F₅)₃ whereas the latter undergoes isomerization to afford CAAGes bearing various silyl groups on the carbon atom next to the germylene center. This strategy allows the straightforward synthesis of eight new CAAGes starting from 1 .     

Stereochemical Behavior of Pairs of P-stereogenic Phosphanyl Groups at the Dimethylxanthene Backbone

The P-stereogenic bis(phosphanes) 7 and 9 , featuring pairs of P(Mes)-ethynyl or vinyl substituents at the dimethyl xanthene backbone show rather low barriers of stereochemical inversion at phosphorus. π-Conjugative effects are probably causing these low inversion barriers. Compound 7 reacted with B(C₆F₅)₃ to form the nine-membered heterocyclic product 10 , featuring a [P]−C≡C−B(C₆F₅)₃ substituent. Compound 7 was converted to the bis[P(Mes)vinyl] xanthene derivative 9 , which gave the zwitterionic P(H)(Mes)−CH=CH−B(C₆F₅)₃ containing product 16 upon treatment with B(C₆F₅)₃. Thermally induced epimerization barriers at phosphorus of ca. 20 to 27 kcal mol⁻¹ were calculated by DFT for the alkenyl- and alkynyl-P derived systems 6 to 9 , 15 and 16 and experimentally determined for the examples 7 and 16 .     

Coordinatively Unsaturated Amidotitanocene Cations with Inverted σ and π Bond Strengths: Controlled Release of Aminyl Radicals and Hydrogenation/Dehydrogenation Catalysis

Cationic amidotitanocene complexes [Cp₂Ti(NPhAr)][B(C₆F₅)₄] (Cp=η⁵-C₅H₅; Ar=phenyl ( 1 a ), p-tolyl ( 1 b ), p-anisyl ( 1 c )) were isolated. The bonding situation was studied by DFT (Density Functional Theory) using EDA-NOCV (Energy Decomposition Analysis with Natural Orbitals for Chemical Valence). The polar Ti−N bond in 1 a–c features an unusual inversion of σ and π bond strengths responsible for the balance between stability and reactivity in these coordinatively unsaturated species. In solution, 1 a–c undergo photolytic Ti−N cleavage to release Ti(III) species and aminyl radicals ⋅ NPhAr. Reaction of 1 b with H₃BNHMe₂ results in fast homolytic Ti−N cleavage to give [Cp₂Ti(H₃BNHMe₂)][B(C₆F₅)₄] ( 3 ). 1 a–c are highly active precatalysts in olefin hydrogenation and silanes/amines cross-dehydrogenative coupling, whilst 3 efficiently catalyzes amine-borane dehydrogenation. The mechanism of olefin hydrogenation was studied by DFT and the cooperative H₂ activation key step was disclosed using the Activation Strain Model (ASM).     

Molybdenum(VI) Bis(imido) Complexes: From Frustrated Lewis Pairs to Weakly Coordinating Cations

Molybdenum(VI) bis(imido) complexes [Mo(NtBu)₂(L^R)₂] (R=H 1 a ; R=CF₃ 1 b ) combined with B(C₆F₅)₃ ( 1 a /B(C₆F₅)₃, 1 b /B(C₆F₅)₃) exhibit a frustrated Lewis pair (FLP) character that can heterolytically split H−H, Si−H and O−H bonds. Cleavage of H₂ and Et₃SiH affords ion pairs [Mo(NtBu)(NHtBu)(L^R)₂][HB(C₆F₅)₃] (R=H 2 a ; R=CF₃ 2 b ) composed of a Mo(VI) amido imido cation and a hydridoborate anion, while reaction with H₂O leads to [Mo(NtBu)(NHtBu)(L^R)₂][(HO)B(C₆F₅)₃] (R=H 3 a ; R=CF₃ 3 b ). Ion pairs 2 a and 2 b are catalysts for the hydrosilylation of aldehydes with triethylsilane, with 2 b being more active than 2 a . Mechanistic elucidation revealed insertion of the aldehyde into the B−H bond of [HB(C₆F₅)₃]⁻. We were able to isolate and fully characterize, including by single-crystal X-ray diffraction analysis, the inserted products Mo(NtBu)(NHtBu)(L^R)₂][{PhCH₂O}B(C₆F₅)₃] (R=H 4 a ; R=CF₃ 4 b ). Catalysis occurs at [HB(C₆F₅)₃]⁻ while [Mo(NtBu)(NHtBu)(L^R)₂]⁺ (R=H or CF₃) act as the cationic counterions. However, the striking difference in reactivity gives ample evidence that molybdenum cations behave as weakly coordinating cations (WCC).     

Linkage Isomerism Leading to Contrasting Carboboration Chemistry: Access to Three Constitutional Isomers of a Borylated Phosphaalkene

We describe the reactivity of two linkage isomers of a boryl-phosphaethynolate, [B]OCP and [B]PCO (where [B]=N,N’-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl), towards tris- (pentafluorophenyl)borane (BCF). These reactions afforded three constitutional isomers all of which contain a phosphaalkene core. [B]OCP reacts with BCF through a 1,2 carboboration reaction to afford a novel phosphaalkene, E-[B]O{(C₆F₅)₂B}C=P(C₆F₅), which subsequently undergoes a rearrangement process involving migration of both the boryloxy and pentafluorophenyl substituents to afford Z-{(C₆F₅)₂B}(C₆F₅)C=PO[B]. By contrast, [B]PCO undergoes a 1,3-carboboration process accompanied by migration of the N,N’-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl to the carbon centre.     

A Germacalicene: Synthesis,Structure, and Reactivity

The synthesis of the germacalicene 7 from the reaction of the dipotassium germole dianion K₂[ 6 ] with 1,2-bis-diisopropylamino-3-chlorocyclopropenyl perchlorate is reported. Based on the crystal structure analysis and the results of DFT calculations, the germacalicene 7 can be viewed as a cyclopropenium germacyclopentadienide ylide that is isoelectronic to α-cationic phosphanes. First reactivity studies revealed its nucleophilic character and resulted in the isolation of the air- and moisture-stable carbonyl iron complex 15 and the cationic silver complex 20 . One-electron oxidation of the germacalicene 7 was achieved by its reaction with [Ph₃C][B(C₆F₅)₄] and the bis-cationic Ge−Ge-bonded dimer 22 was isolated.     

Enantioselective Nazarov Cyclizations Catalyzed by an Axial Chiral C6F5‐Substituted Boron Lewis Acid

A chiral variant of B(C₆F₅)₃ with a 3,3′‐disubstituted binaphthyl backbone is shown to catalyze Nazarov cyclizations with high levels of enantio‐ and diastereocontrol. The parent B(C₆F₅)₃ also promotes these ring closures efficiently. This electrocyclization is another example of the still small family of C?C bond formations mediated by B(C₆F₅)₃ as the catalyst.     

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.     

The B(C6F5)3 Boron Lewis Acid Route to Arene‐Annulated Pentalenes

4,5‐Dimethyl‐1,2‐bis(1‐naphthylethynyl)benzene ( 12 ) undergoes a rapid multiple ring‐closure reaction upon treatment with the strong boron Lewis acid B(C₆F₅)₃ to yield the multiply annulated, planar conjugated π‐system 13 (50 % yield). In the course of this reaction, a C₆F₅ group was transferred from boron to carbon. Treatment of 12 with CH₃B(C₆F₅)₂ proceeded similarly, giving a mixture of 13 (C₆F₅‐transfer) and the product 15 , which was formed by CH₃‐group transfer. 1,2‐Bis(phenylethynyl)benzene ( 8 a ) reacts similarly with CH₃B(C₆F₅)₂ to yield a mixture of the respective C₆F₅‐ and CH₃‐substituted dibenzopentalenes 10 a and 16 . The reaction is thought to proceed through zwitterionic intermediates that exhibit vinyl cation reactivities. Some B(C₆F₅)₃‐substituted species ( 26 , 27 ) consequently formed by in situ deprotonation upon treatment of the respective 1,2‐bis(alkynyl)benzene starting materials ( 24 , 8 ) with the frustrated Lewis pair B(C₆F₅)₃/P(o‐tolyl)₃. The overall formation of the C₆F₅‐substituted products formally require HB(C₆F₅)₂ cleavage in an intermediate dehydroboration step. This was confirmed in the reaction of a thienylethynyl‐containing starting material 21 with B(C₆F₅)₃, which gave the respective annulated pentalene product 23 that had the HB(C₆F₅)₂ moiety 1,4‐added to its thiophene ring. Compounds 12 – 14 , 23 , and 26 were characterized by X‐ray diffraction.     

B(C6F5)3/Chiral Phosphoric Acid Catalyzed Ketimine–Ene Reaction of 2-Aryl-3H-indol-3-ones and α-Methylstyrenes

The enantioselective ketimine–ene reaction is one of the most challenging stereocontrolled reaction types in organic synthesis. In this work, catalytic enantioselective ketimine–ene reactions of 2-aryl-3H-indol-3-ones with α-methylstyrenes were achieved by utilizing a B(C₆F₅)₃/chiral phosphoric acid (CPA) catalyst. These ketimine–ene reactions proceed well with low catalyst loading (B(C₆F₅)₃/CPA=2 mol %/2 mol %) under mild conditions, providing rapid and facile access to a series of functionalized 2-allyl-indolin-3-ones with very good reactivity (up to 99 % yield) and excellent enantioselectivity (up to 99 % ee). Theoretical calculations reveal that enhancement of the acidity of the chiral phosphoric acid by B(C₆F₅)₃ significantly reduces the activation free energy barrier. Furthermore, collective favorable hydrogen-bonding interactions, especially the enhanced N−H⋅⋅⋅O hydrogen-bonding interaction, differentiates the free energy of the transition states of CPA and B(C₆F₅)₃/CPA, thereby inducing the improvement of stereoselectivity.     

Cover Feature: Mechanism of Oxidative Activation of Fluorinated Aromatic Compounds by N-Bridged Diiron-Phthalocyanine: What Determines the Reactivity? (Chem. Eur. J. 63/2019)

The biodegradation of compounds with C−F bonds is challenging due to the fact that these bonds are stronger than the C−H bond in methane. In this work, results on the unprecedented reactivity of a biomimetic model complex that contains an N-bridged diiron-phthalocyanine are presented; this model complex is shown to react with perfluorinated arenes under addition of H₂O₂ effectively. To get mechanistic insight into this unusual reactivity, detailed density functional theory calculations on the mechanism of C₆F₆ activation by an iron(IV)-oxo active species of the N-bridged diiron phthalocyanine system were performed. Our studies show that the reaction proceeds through a rate-determining electrophilic C−O addition reaction followed by a 1,2-fluoride shift to give the ketone product, which can further rearrange to the phenol. A thermochemical analysis shows that the weakest C−F bond is the aliphatic C−F bond in the ketone intermediate. The oxidative defluorination of perfluoroaromatics is demonstrated to proceed through a completely different mechanism compared to that of aromatic C−H hydroxylation by iron(IV)-oxo intermediates such as cytochrome P450 Compound I.