Tris(pentafluorophenyl)borane and Beyond: Modern Advances in Borylation Chemistry

Ring-Opening Polymerization of 1,3-Benzoxazines via Borane Catalyst

Tris(pentafluorophenyl)borane was used as Lewis acid catalyst to lower the ring opening polymerization temperature (ROP) of 1,3-benzoxazines. Dynamic scanning calorimeter studies revealed that on-set ROP temperatures were decreased as much as 98 °C for model benzoxazine compounds. Catalytic polymerization was traced by both FTIR and ¹H NMR, and revealed that tris(pentafluorophenyl)borane acted rapidly and fast curing achieved. Moreover, thermal properties of resulting polybenzoxazines were investigated by thermogravimetric analysis (TGA) and found out that the catalyst has high impact on char yield and even 3 mol % catalyst augmented char yields up to 13%.

Introduction of Boron Functionalities into Silsesquioxanes: Novel Independent Methodologies

Owing to their versatile application possibilities, silsesquioxanes (SQs) are of considerable interest for creating hybrid inorganic-organic materials. In this report, two novel and independent strategies for the direct attachment of boron functionalities to silsesquioxanes are presented. Encouraged by our previous work concerning the synthesis of borasiloxanes through the catalyst-free dehydrogenative coupling of silanols and boranes, we decided to apply our method to a synthesis of various boron-functionalized silsesquioxanes. During our tests, we also investigated the activity of scandium(III) triflate, which we have previously used as an excellent catalyst for the obtaining of Si-O-Si and Si-O-Ge moieties. As a result, we also discovered a novel approach for the O-borylation of Si-OH groups in silsesquioxanes with allylborane. Both routes are highly chemoselective and efficiently lead to the obtaining of Si-O-B moiety under air atmosphere and at room temperature.

An efficient strategy for achieving controlled ring-opening polymerization of O-carboxyanhydrides via amine initiation in collaboration with metal-alkoxide catalysis

Amines and Zn(C₆F₅)₂ for polymerization initiation and functionality transfer from Zn(C₆F₅)₂ to a metal-alkoxide species for promoting chain propagation to achieve well controlled ring-opening polymerization of O-carboxyanhydrides.

Push and pull: the potential role of boron in N2 activation

Recent developments in main group chemistry towards the activation and conversion of N₂ have lead to the revelation that boron can greatly affect these processes.

N-Heterocycle-Ligated Borocations as Highly Tunable Carbon Lewis Acids

The relative (to BEt₃) hydride ion affinity (HIA) of a series of acridine borenium salts has been calculated, with some HIAs found to be similar to that for [Ph₃C]⁺. The HIA at the acridine C9 position is controlled by both acridine and the boron substituents, the latter presumably affecting the strength of the B=N bond in the acridane-BY₂ products from hydride transfer. Through a range of hydride abstraction benchmarking reactions against organic hydride donors the experimental HIA of [F₅acr-BCat]⁺ (cat = catechol, F₅acr = 1,2,3,4,7-pentafluoroacridine) has been confirmed to be extremely high and closely comparable to that of [Ph₃C]⁺. The high HIA of [F₅acr-BCat]⁺ enables H₂ and alkene activation in a FLP with 2,6-di-tert-butylpyridine. Finally, the HIA of pyridine and quinoline borenium cations has been determined, with the HIA at boron in [PinB(amine)]⁺ (pin = pinacol, amine = pyridine or quinoline) found to be relatively low. This enabled the hydroboration of pyridine and quinoline by HBPin to be achieved through the addition of 5–10 mol % of bench-stable cationic carbon Lewis acids such as 2-phenyl-N,N-dimethylimidazolium salts.

Non-Metal-Catalyzed Heterodehydrocoupling of Phosphines and Hydrosilanes: Mechanistic Studies of B(C6F5)3-Mediated Formation of P-Si Bonds

Non-metal-catalyzed heterodehydrocoupling of primary and secondary phosphines (R¹R²PH, R² = H or R¹) with hydrosilanes (R³R⁴R⁵SiH, R⁴, R⁵ = H or R³) to produce synthetically useful silylphosphines (R¹R²P-SiR³R⁴R⁵) has been achieved using B(C₆F₅)₃ as the catalyst (10 mol %, 100 °C). Kinetic studies demonstrated that the reaction is first-order in hydrosilane and B(C₆F₅)₃ but zero-order in phosphine. Control experiments, DFT calculations, and DOSY NMR studies suggest that a R¹R²HP·B(C₆F₅)₃ adduct is initially formed and undergoes partial dissociation to form an "encounter complex". The latter mediates frustrated Lewis pair type Si-H bond activation of the silane substrates. We also found that B(C₆F₅)₃ catalyzes the homodehydrocoupling of primary phosphines to form cyclic phosphine rings and the first example of a non-metal-catalyzed hydrosilylation of P-P bonds to produce silylphosphines (R¹R²P-SiR³R⁴R⁵). Moreover, the introduction of PhCN to the reactions involving secondary phosphines with hydrosilanes allowed the heterodehydrocoupling reaction to proceed efficiently under much milder conditions (1.0 mol % B(C₆F₅)₃ at 25 °C). Mechanistic studies, as well as DFT calculations, revealed that PhCN plays a key mechanistic role in facilitating the dehydrocoupling reactions rather than simply functioning as H₂-acceptor.

Stoichiometric and catalytic isomerization of alkenylboranes using bulky Lewis bases

1,1-Carboboration of alkynes with boranes R'B(C₆F₅)₂ (R' = (CH₂)₃Ph, C₆F₅) affords mixtures of the E and Z isomers of RCH = CR'B(C₆F₅)₂ (R = (CH₂)₂Me, (CH₂)₃Me, (CH₂)₂Ph, CH₂CHMe₂, CHMe₂, CMe₃, CH₂OMe, CH₂N(CO)₂C₆H₄), treatment of these alkenylboranes with stoichiometric or catalytic amounts of tBu₂PH or other donors are shown to induce isomerization. The proposed mechanism involving a zwitterionic borataalkene intermediate is supported by computations.

Mechanistic Studies of Formal Thioboration Reactions of Alkynes

Several formal heteroborylative cyclization reactions have been recently reported, but little physical-organic and mechanistic data are known. We now investigate the catalyst-free formal thioboration reaction of alkynes to gain mechanistic insight into B-chlorocatecholborane (ClBcat) in its new role as an alkynophilic Lewis acid in electrophilic cyclization/dealkylation reactions. In kinetic studies, the reaction is second-order globally and first-order with respect to both the 2-alkynylthioanisole substrate and the ClBcat electrophile, with activation parameters of ΔG^‡ = 27.1 ± 0.1 kcal mol^-1 at 90 °C, ΔH^‡ = 13.8 ± 1.0 kcal mol^-1, and ΔS^‡ = -37 ± 3 cal mol^-1 K^-1, measured over the range 70-90 °C. Carbon kinetic isotope effects supported a rate-determining Ad_E3 mechanism wherein alkyne activation by neutral ClBcat is concerted with cyclative attack by nucleophilic sulfur. A Hammett study found a ρ⁺ of -1.7, suggesting cationic charge buildup during the cyclization and supporting rate-determining concerted cyclization. Studies of the reaction with tris(pentafluorophenyl)borane (B(C₆F₅)₃), an activating agent capable of cyclization but not dealkylation, resulted in the isolation of a postcyclization zwitterionic intermediate. Kinetic studies via UV-vis spectroscopy with this boron reagent found second-order kinetics, supporting the likely relevancy of intermediates in this system to the ClBcat system. Computational studies comparing ClBcat with BCl₃ as an activating agent showed why BCl₃, in contrast to ClBcat, failed to mediate the complete the cyclization/demethylation reaction sequence by itself. Overall, the results support a mechanism in which the ClBcat reagent serves a bifunctional role by sequentially activating the alkyne, despite being less electrophilic than other known alkyne-activating reagents and then providing chloride for post-rate-determining demethylation/neutralization of the resulting zwitterionic intermediate.