B(C6F5)3‐Catalyzed sp3 C—Si Bond Forming Consecutive Reactions

Transition metal‐catalyzed hydrosilylation is one of the most widely utilized reduction methods as an alternative to hydrogenation in academia and industry. One feature distinct from hydrogenation would be able to install sp³ C—Si bond(s) onto substrates skeleton via hydrosilylation of alkenes. Recently, B(C₆F₅)₃ with hydrosilanes has been demonstrated to be an efficient, metal‐free catalyst system for the consecutive transformation of heteroatom‐containing substrates accompanied by the formation of sp³ C—Si bond(s), which has not been realized thus far under the transition metal‐catalyzed hydrosilylative conditions. In this review, I outline the B(C₆F₅)₃‐mediated consecutive hydrosilylations of heteroarenes containing quinolines, pyridines, and furans, and of conjugated nitriles/imines to provide a new family of compounds having sp³ C—Si bond(s) with high chemo‐, regio‐ and/or stereoselectivities. The silylative cascade conversion of unactivated N‐aryl piperidines to sila‐N‐heterocycles catalyzed by B(C₆F₅)₃ involving consecutive dehydrogenation, hydrosilylation, and intramolecular C(sp²)—H silylation, is presented in another section. Chemical selectivity and mechanism of the boron catalysis focused on the sp³ C—Si bond formation are highlighted.     

N,P‐Heterocyclic Germylene/B(C6F5)3 Adducts: A Lewis Pair with Multi‐reactive Sites

An N,P‐heterocyclic germylene/B(C₆F₅)₃ Lewis adduct 2 presenting multi‐reactive sites (P/B Lewis pair, germylene, Ge=P π‐bond) is reported. In contrast to classical frustrated Lewis pairs or divalent Group 14 element species, 2 is able to activate two small molecules simultaneously. Of particular interest, 2 reacts with silanes leading to the formation of original cationic germylenes 3 , and can be used as a metal‐free catalyst for selective CO₂‐hydrosilylation to H₂C(OSiEt₃)₂.     

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.     

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.     

Effects of tris(pentafluorophenyl)borane on the activation of a metal alkyl‐free Ni‐based catalyst in the polymerization of 1,3‐butadiene

The activation of a metal alkyl‐free Ni‐based catalyst with B(C₆F₅)₃ was investigated in the polymerization of 1,3‐butadiene. A catalyst of bis(1,5‐cyclooctadiene)nickel (Ni(COD)₂)/B(C₆F₅)₃ was found to have high catalytic activity and 1,4‐cis stereoregularity. The catalyst was also found to provide polybutadiene having a molecular weight (M_w) of up to 117,000, even in the absence of AlR₃ and MAO. Variations in the mol ratio of B(C₆F₅)₃ to Ni affected catalytic activity, 1,4‐cis stereoregularity, and the M_w of polybutadiene, while the molecular weight distribution (MWD) of polybutadiene showed little correlation with the mol ratio of B(C₆F₅)₃ to Ni. The use of other borane compounds such as B(C₆H₅)₃, BEt₃, and BF₃ etherate in place of B(C₆F₅)₃ clearly showed the two main functions of B(C₆F₅)₃ in the present catalyst. The high Lewis acidity of B(C₆F₅)₃ enabled it to activate catalytic complexes, thus inducing the polymerization. The steric bulkiness of B(C₆F₅)₃ suppressed chain transfer reactions, contributing to the production of polybutadiene with a high M_w. Kinetic studies showed that the catalyst had an induction period, possibly due to the time needed for the formation of catalytic complexes starting from Ni(COD)₂. A plot of ?ln (1?X), where X is the fractional conversion, as a function of time resulted in a linear relationship, showing that the present catalyst system followed first‐order kinetics with respect to monomer concentration. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1164–1173, 2004     

To Coordinate or not to Coordinate: The Special Role of Chalcogen Ether Functionalities in the Design of Twofold Functionalized Cyclopentadienyl Ligands [Cp,O,Ch (Ch = S,Se)]

The solvent‐ and catalyst free synthesis of two β‐thio ketones L1a and L1b is reported. L1a , L1b , and a β‐seleno ketone L1c were successfully employed as ligand precursors in the synthesis of a novel series of cationic titanium complexes 4a – 4c via a well‐established reaction sequence: insertion of the carbonyl functional group into the polarized Ti–C_q,exo bond of the monopentafulvene complex Cp*Ti(Cl)(π‐η⁵:σ–η¹‐C₅H₄=CR₂) ( 1 ) (CR₂ = adamantylidene), subsequent methylation, and final activation with B(C₆F₅)₃. The cationic titanium complexes 4a – 4c bear twofold functionalized cyclopentadienyl [Cp,O,Ch (Ch = S, Se)] ligand frameworks built directly in the coordination sphere of the metal, in which the chalcogen ether functionalities do not coordinate to the central metal atoms as demonstrated by NMR experiments. Consequently, Cp,O σ,π chelating ligand systems are formed with free coordination sites at the central titanium atoms and pendant chalcogen ether moieties.     

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.     

Frustrated Lewis Pair Chemistry Derived from Bulky Allenyl and Propargyl Phosphanes

The dimesitylpropargylphosphanes mes₂P?CH₂?C≡C?R 6 a (R=H), 6 b (R=CH₃), 6 c (R=SiMe₃) and the allene mes₂P?C(CH₃)=C=CH₂ ( 8 ) were reacted with Piers’ borane, HB(C₆F₅)₂. Compound 6 a gave mes₂PCH₂CH=CH(B(C₆F₅)₂] ( 9 a ). In contrast, addition of HB(C₆F₅)₂ to 6 b and 6 c gave mixtures of 9 b (R=CH₃) and 9 c (R=SiMe₃) with the regioisomers mes₂P?CH₂?C[B(C₆F₅)₂]=CRH 2 b (R=CH₃) and 2 c (R=SiMe₃), respectively. Compounds 2 b , c underwent rapid phosphane/borane (P/B) frustrated Lewis pair (FLP) reactions under mild conditions. Compound 2 c reacted with nitric oxide (NO) to give the persistent FLP NO radical 11 . The systems 2 b , c cleaved dihydrogen at room temperature to give the respective phosphonium/hydridoborate products 13 b , c . Compound 13 c transferred the H⁺/H^? pair to a small series of enamines. Compound 13 c was also a metal‐free catalyst (5 mol %) for the hydrogenation of the enamines. The allene 8 reacted with B(C₆F₅)₃ to give the zwitterionic phosphonium/borate 17 . The ‐PPh₂‐substituted mes₂P‐propargyl system 6 d underwent a typical 1,2‐P/B‐addition reaction to the C≡C triple bond to form the phosphetium/borate zwitterion 20 . Several products were characterized by X‐ray diffraction.     

Boron Lewis Acid‐Catalyzed Hydroboration of Alkenes with Pinacolborane: BArF3 Does What B(C6F5)3 Cannot Do!

The transition‐metal‐free hydroboration of various alkenes with pinacolborane (HBpin) initiated by tris[3,5‐bis(trifluoromethyl)phenyl]borane (BAr^F₃) is reported. The choice of the boron Lewis acid is crucial as the more prominent boron Lewis acid tris(pentafluorophenyl)borane (B(C₆F₅)₃) is reluctant to react. Unlike B(C₆F₅)₃, BAr^F₃ is found to engage in substituent redistribution with HBpin, resulting in the formation of Ar^FBpin and the electron‐deficient diboranes [H₂BAr^F]₂ and [(Ar^F)(H)B(μ‐H)₂BAr^F₂]. These in situ‐generated hydroboranes undergo regioselective hydroboration of styrene derivatives as well as aliphatic alkenes with cis diastereoselectivity. Another ligand metathesis of these adducts with HBpin subsequently affords the corresponding HBpin‐derived anti‐Markovnikov adducts. The reactive hydroboranes are regenerated in this step, thereby closing the catalytic cycle.     

Cooperative Lewis Acid/Cp*CoIII Catalyzed C−H Bond Activation for the Synthesis of Isoquinolin‐3‐ones

A facile route toward the synthesis of isoquinolin‐3‐ones through a cooperative B(C₆F₅)₃‐ and Cp*Co^III‐catalyzed C?H bond activation of imines with diazo compounds is presented. The inclusion of a catalytic amount of B(C₆F₅)₃ results in a highly efficient reaction, thus enabling unstable NH imines to serve as substrates.     

Step‐Growth Coordination Polymerization of 5‐Hydroxymethyl Furfural with Dihydrosilanes: Synergistic Catalysis Using Heteroscopionate Zinc Hydride and B(C6F5)3

The utilization of 5‐hydroxymethyl furfural (HMF) as a renewable feedstock for polymer synthesis has not yet been achieved as it is structurally asymmetric and contains three active functional groups. Reported here is the unprecedented step‐growth copolymerization of HMF and dihydrosilanes, through a coordination mechanism, to afford linear poly(silyl ether)s in the presence of B(C₆F₅)₃ and the heteroscorpionate zinc hydride complex LZnH [L=(^MePz)₂CP(Ph)₂NPh, ^MePz=3,5‐dimethylpyrazolyl]. The adduct B(C₆F₅)₃???H???Zn, confirmed by NMR spectroscopy and DFT calculations, plays a key role in the synergistic catalysis, where B(C₆F₅)₃ activates ZnH and stabilizes the Zn⁺ active species, and the sterically bulky ZnH effectively inhibits (C₆F₅)₃B from reacting with dihydrosilane to form (C₆F₅)₃B‐H‐Si, which facilely initiates ring opening of furan. The mechanism was studied by DFT simulations.     

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 .     

Pd(II) complexes bearing di‐ and monochelate fluorinated β‐ketonaphthyliminato ligand and their catalytic properties towards vinyl‐addition polymerization and copolymerization of norbornene and ester‐functionalized norbornene derivative

Fluorinated β‐ketonaphthyliminate ligand CF₃C(O)CHC[HN(naphthyl)]CH₃ ( L1 ) and Pd(II) complexes with dichelate fluorinated β‐ketonaphthyliminato ligand, {CF₃C(O)CHC[N(naphthyl)]CH₃}₂Pd ( C1 ), as well as with monochelate fluorinated β‐ketonaphthyliminato ligand, {CF₃C(O)CHC[N(naphthyl)]CH₃}Pd(CH₃)(PPh₃) ( C2 ), were synthesized and their solid‐state structures were confirmed using X‐ray crystallographic analysis. The Pd(II) complexes were employed as precursors to catalyze norbornene (NB) homo‐ and copolymerization with ester‐functionalized NB derivative using B(C₆F₅)₃ as a co‐catalyst. High activity up to 2.3 × 10⁵ g_polymer mol_Pd^?1 h^?1 for the C1 /B(C₆F₅)₃ system and 3.4 × 10⁶ g_polymer mol_Pd^?1 h^?1 for the C2 /B(C₆F₅)₃ system was exhibited in NB homopolymerization. Moreover, the Pd(II) complexes exhibited a high level of tolerance towards the ester‐functionalized MB monomer. In comparison with the C1 /B(C₆F₅)₃ system, the C2 /B(C₆F₅)₃ system exhibited better catalytic property towards the copolymerization of NB with 5‐norbornene‐2‐carboxylic acid methyl ester (NB‐COOCH₃), and soluble vinyl‐addition‐type copolymers were obtained with relatively high molecular weights (3.6 × 10⁴–7.5 × 10⁴ g mol^?1) as well as narrow molecular weight distributions (1.49–2.15) depending on the variation of monomer feed ratios. The NB‐COOCH₃ insertion ratio in all copolymers could be controlled in the range 2.8–21.0 mol% by tuning a content of 10–50 mol% NB‐COOCH₃ in the monomer feed ratios. Copolymerization kinetics were expressed by the NB and NB‐COOCH₃ monomer reactivity ratios: r_NB‐COOCH3 = 0.18, r_NB = 1.28 were determined for the C1 /B(C₆F₅)₃ system and r_NB‐COOCH3 = 0.19, r_NB = 3.57 for the C2 /B(C₆F₅)₃ system using the Kelen–Tüdõs method. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

Regioselective C?F Bond Activation of Hexafluoropropylene on Palladium(0): Formation of a Cationic η2‐Perfluoroallylpalladium Complex

A chemoselective C(sp²) F or C(sp³) F bond activation of hexafluoropropylene (HFP) was achieved by adopting the proper combination of a Lewis acid co‐additive with a ligand which coordinates Pd⁰. The treatment of [(η²‐HFP)Pd(PCy₃)₂] with B(C₆F₅)₃ allowed a chemoselective C(sp³) F bond cleavage of HFP to give a unique cationic perfluoroallypalladium complex. In this complex, the coordination mode of the perfluoroallyl ligand was considered to be of the unique η²‐fashion.     

Regioselective CF Bond Activation of Hexafluoropropylene on Palladium(0): Formation of a Cationic η2‐Perfluoroallylpalladium Complex

A chemoselective C(sp²)? F or C(sp³)? F bond activation of hexafluoropropylene (HFP) was achieved by adopting the proper combination of a Lewis acid co‐additive with a ligand which coordinates Pd⁰. The treatment of [(η²‐HFP)Pd(PCy₃)₂] with B(C₆F₅)₃ allowed a chemoselective C(sp³)? F bond cleavage of HFP to give a unique cationic perfluoroallypalladium complex. In this complex, the coordination mode of the perfluoroallyl ligand was considered to be of the unique η²‐fashion.     

Selective Metal‐free HB(C6F5)2 Catalyzed Allene Cyclotrimerization: Formation of 1,3,5‐Trimethylenecyclohexane and Its Tris‐hydroboration Product

Allene is cyclotrimerized under metal‐free conditions with the borane HB(C₆F₅)₂ catalyst to selectively give 1,3,5‐trimethylenecyclohexane ( 3 a ). Three‐fold hydroboration of the 1,3,5‐cyclotrimer with Piers’ borane gives the all‐cis 1,3,5‐CH₂B(C₆F₅)₂ substituted cyclohexane product 14 .     

Synthesis,Structure, and Reactivity of Diazene Adducts: Isolation of iso‐Diazene Stabilized as a Borane Adduct

This work describes the synthesis and full characterization of a series of GaCl₃ and B(C₆F₅)₃ adducts of diazenes R¹?N?N?R² (R¹=R²=Me₃Si, Ph; R¹=Me₃Si, R²=Ph). Trans‐Ph?N?N?Ph forms a stable adduct with GaCl₃, whereas no adduct, but instead a frustrated Lewis acid–base pair is formed with B(C₆F₅)₃. The cis‐Ph?N?N?Ph ? B(C₆F₅)₃ adduct could only be isolated when UV light was used, which triggers the isomerization from trans‐ to cis‐Ph?N?N?Ph, which provides more space for the bulky borane. Treatment of trans‐Ph?N?N?SiMe₃ with GaCl₃ led to the expected trans‐Ph?N?N?SiMe₃ ? GaCl₃ adduct but the reaction with B(C₆F₅)₃ triggered a 1,2‐Me₃Si shift, which resulted in the formation of a highly labile iso‐diazene, Me₃Si(Ph)N?N; stabilized as a B(C₆F₅)₃ adduct. Trans‐Me₃Si?N?N?SiMe₃ forms a labile cis‐Me₃Si?N?N?SiMe₃ ? B(C₆F₅)₃ adduct, which isomerizes to give the transient iso‐diazene species (Me₃Si)₂N?N ? B(C₆F₅)₃ upon heating. Both iso‐diazene species insert easily into one B?C bond of B(C₆F₅)₃ to afford hydrazinoboranes. All new compounds were fully characterized by means of X‐ray crystallography, vibrational spectroscopy, CHN analysis, and NMR spectroscopy. All compounds were further investigated by DFT and the bonding situation was assessed by natural bond orbital (NBO) analysis.     

Synthesis of Branched Polyethylene by Tandem Catalysis

Linear low‐density polyethylene (LLDPE) can be prepared by addition of ethylene to a mixture of two catalysts. In this “tandem catalysis” scheme one catalyst dimerizes or oligomerizes ethylene to α‐olefins while the second site incorporates these α‐olefins into a growing polyethylene chain. A variety of classical catalyst combinations are available for this purpose. Better control over the polymerization process, and therefore product properties, is attained by the use of homogenous “single site” catalysts. The best‐behaved tandem processes take advantage of well‐defined catalysts that require stoichiometric quantities of activators. One such system employs [(C₆H₅)₂PC₆H₄C(OB(C₆F₅)₃)O‐κ²P,O]Ni(η³‐CH₂CMeCH₂) and {[(η⁵‐C₅Me₄)SiMe₂(η¹‐NCMe₃)]TiMe}{MeB(C₆F₅)₃}. The nickel sites are responsible for converting ethylene to 1‐butene or mixtures of 1‐butene with 1‐hexene. These olefins are copolymerized with ethylene at the titanium sites. It is possible to obtain a linear correlation between the branching content in the polymer product and the Ni/Ti ratio. The effect of ligand substitution at nickel has also been investigated. When the benzyl derivative [(C₆H₅)₂PC₆H₄C(O‐B(C₆F₅)₃)O‐κ²P,O]Ni(η³‐CH₂C₆H₅) is used instead of the methallyl counterpart [(C₆H₅)₂PC₆H₄C(OB(C₆F₅)₃)O‐κ²P,O]Ni(η³‐CH₂CMeCH₂), one obtains, at a constant Ni/Ti ratio, considerably more branching in the final polymer structure. These results are rationalized in terms of a more efficient initiation when the more labile benzyl ligand is used.     

Addition polymerization of norbornene using bis(β‐ketoamino)nickel(II)/tris(pentafluorophenyl)borane catalytic systems

Norbornene polymerizations proceeded in toluene with bis(β‐ketoamino)nickel(II) {Ni[CH₃C(O)CHC(NR)CH₃]₂ [R = phenyl ( 1 ) or naphthyl ( 2 )]} complexes as the catalyst precursors and the organo‐Lewis compound tris(pentafluorophenyl)borane [B(C₆F₅)₃] as a unique cocatalyst. The polymerization conditions, such as the cocatalyst/catalyst ratio (B/Ni), catalyst concentration, monomer/catalyst ratio (norbornene/Ni), polymerization temperature, and polymerization time, were studied in detail. Both bis(β‐ketoamino)nickel(II)/B(C₆F₅)₃ catalytic systems showed noticeably high conversions and activities. The polymerization activities were up to 3.64 × 10⁷ g of polymer/mol of Ni h for complex 1 /(B(C₆F₅)₃ and 3.80 × 10⁷ g of polymer/mol of Ni h for complex 2 /B(C₆F₅)₃, and very high conversions of 90–95% were maintained; both polymerizations provided high‐molecular‐weight polynorbornenes with molecular weight distributions (weight‐average molecular weight/number‐average molecular weight) of 2.5–3.0. The achieved polynorbornenes were confirmed to be vinyl‐addition and atactic polymers through the analysis of Fourier transform infrared, ¹H NMR, and ¹³C NMR spectra, and the thermogravimetric analysis results showed that the polynorbornenes exhibited good thermal stability (decomposition temperature > 410 °C). © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4733–4743, 2007