Applications of pentafluorophenyl boron reagents in the synthesis of heterocyclic and aromatic compounds

Frustrated Lewis Pair-Mediated Cycloisomerization of Propargylaniline and Aryl Propargyl Ether Derivatives via a 6-endo-dig Cyclization/Dehydrogenation Sequence

An efficient FLP-mediated cycloisomerization is described, providing easy access to quinolinium and chromenylium derivatives by treatment of readily available propargylanilines and aryl propargyl ethers with Lewis acidic boranes, respectively. The reaction proceeds via a 6-endo-dig cyclization/dehydrogenation sequence. The heteroatom functions serve as Lewis bases in combination with Lewis acidic boranes to effect synergistic activation of an alkynyl triple bond and a C-H bond.

A Highly Sterically Encumbered Boron Lewis Acid Enabled by an Organotellurium-Based Ligand

Lewis acidic boron compounds are ubiquitous in chemistry due to their numerous applications, yet tuning and optimizing their properties towards different purposes is still a challenging field of research. In this work, the boron-based Lewis acid B[OTeF3(C6F5)2]3 was synthesized by reaction of the teflate derivative HOTeF3(C6F5)2 with BCl3 or BCl3 ⋅ SMe2. This new compound presents a remarkably high thermal stability up to 300 °C, as well as one of the most sterically encumbered boron centres known in the literature. Theoretical and experimental methods revealed that B[OTeF3(C6F5)2]3 exhibits a comparable Lewis acidity to that of the well-known B(C6F5)3. The affinity of B[OTeF3(C6F5)2]3 towards pyridine was accessed by Isothermal Titration Calorimetry (ITC) and compared to that of B(OTeF5)3 and B(C6F5)3. The ligand-transfer reactivity of this new boron compound towards different fluorides was demonstrated by the formation of an anionic Au(III) complex and a hypervalent iodine(III) species.

Radical reactions enabled by polyfluoroaryl fragments: photocatalysis and beyond

Polyfluoroarenes have been known for a long time, but they are most often used as fluorinated building blocks for the synthesis of aromatic compounds. At the same time, due to peculiar fluorine effect, they have unique properties that provide applications in various fields ranging from synthesis to materials science. This review summarizes advances in the radical chemistry of polyfluoroarenes, which have become possible mainly with the advent of photocatalysis. Transformations of the fluorinated ring via the C-F bond activation, as well as use of fluoroaryl fragments as activating groups and hydrogen atom transfer agents are discussed. The ability of fluoroarenes to serve as catalysts is also considred.

The Effect of Carborane Substituents on the Lewis Acidity of Boranes

The Lewis acidity of primary, secondary, and tertiary boranes with phenyl, pentafluorophenyl, and all three isomers of the C-substituted icosahedral carboranes (ortho, meta, and para) was investigated by computing their fluoride, hydride, and ammonia affinities as well as their global electrophilicity indices and LUMO energies. From these calculations, it was determined that the substituent effects on the Lewis acidity of these boranes follow the trend of ortho-carborane > meta-carborane > para-carborane > C6F5 > C6H5.

Boron Lewis Acid Catalysis Enables the Direct Cyanation of Benzyl Alcohols by Means of Isonitrile as Cyanide Source

The development of an efficient and straightforward method for cyanation of alcohols is of great value. However, the cyanation of alcohols always requires toxic cyanide sources. Herein, an unprecedented synthetic application of an isonitrile as a safer cyanide source in B(C₆F₅)₃-catalyzed direct cyanation of alcohols is reported. With this approach, a wide range of valuable α-aryl nitriles was synthesized in good to excellent yields (up to 98%). The reaction can be scaled up and the practicability of this approach is further manifested in the synthesis of an anti-inflammatory drug, naproxen. Moreover, experimental studies were performed to illustrate the reaction mechanism.

Recent Advances in Asymmetric Catalysis Associated with B(C6F5)3

The prevalence and significance of asymmetric catalysis in the modern medicinal industry has been witnessed in recent years, which have already been used to manufacture the (S)-Naproxen and the (S)-Propranolol. With matched specificities such as the Lewis acidity and steric bulk, B(C₆F₅)₃ has gained accelerating attention on its application in asymmetric catalysis of Diels-Alder cycloaddition reactions, carbonyl-ene cyclization, and other various reactions, which have been demonstrated by the elegant examples from the most recent literature. Some significant progress in the reaction of indirect activation of substrates through in situ generation of numerous supramolecular catalysts from B(C₆F₅)₃ based on Lewis-acid-assisted Lewis acid (LLA) or Lewis acid assisted Brønsted acid (LBA) strategies or the reaction promoted by cooperative actions of chiral co-catalysts and B(C₆F₅)₃ which played a direct role on the activation of substrates have been demonstrated in this review.

Tris(ortho-carboranyl)borane: An Isolable, Halogen-Free, Lewis Superacid

The synthesis of tris(ortho-carboranyl)borane (BoCb3 ), a single site neutral Lewis superacid, in one pot from commercially available materials is achieved. The high fluoride ion affinity (FIA) confirms its classification as a Lewis superacid and the Gutmann-Beckett method as well as adducts with Lewis bases indicate stronger Lewis acidity over the widely used fluorinated aryl boranes. The electron withdrawing effect of ortho-carborane and lack of pi-delocalization of the LUMO rationalize the unusually high Lewis acidity. Catalytic studies indicate that BoCb3 is a superior catalyst for promoting C-F bond functionalization reactions than tris(pentafluorophenyl)borane [B(C6 F5 )3 ].

Bistriazoles Connected Through a B-B Bridge, Synthesized by Highly Selective Dipolar Cycloaddition Reactions of a Diazido-diborane(4)

In this work we report the first cycloaddition reactions between a diazido diborane(4) and terminal alkynes, providing unique access to bis-1,2,3-triazoles connected by a B-B bridge. The catalyst-free reactions are highly selective, yielding exclusively the thermodynamically disfavored bis-1,4-triazoles. The reactions are enabled by the high thermal stability of the diazido-diborane [B(hpp)(N3 )]2 (hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-α]pyrimidinate). Due to the tetra-coordinate boron atoms in this reagent, the reactions are tolerant with respect to the introduction of Lewis-basic groups at the alkyne. The scope and limitations of the new reactions are discussed.

Borane-catalyzed arylation of aryldiazoacetates with N,N-dialkylanilines

A selective arylation of donor-acceptor diazo compounds with aniline derivatives catalyzed by Lewis acidic boranes is developed. This simple reaction protocol provides an efficient method for the synthesis of diarylacetates under metal-free conditions.

Borane-catalyzed cascade Friedel–Crafts alkylation/[1,5]-hydride transfer/Mannich cyclization to afford tetrahydroquinolines

We report a redox-neutral annulation reaction of tertiary amines with electron-deficient alkynes under metal-free and oxidant-free conditions.

Carboboration-Driven Generation of a Silylium Ion for Vinylic C-F Bond Functionalization by B(C6 F5 )3 Catalysis

Strong main-group Lewis acids such as silylium ions are known to effectively promote heterolytic C(sp³ )-F bond cleavage. However, carrying out the C(sp² )-F bond transformation of vinylic C-F bonds has remained an unmet challenge. Herein, we describe our development of a new and simple strategy for vinylic C-F bond transformation of α-fluorostyrenes with silyl ketene acetals catalyzed by B(C₆ F₅ )₃ under mild conditions. Our theoretical calculations revealed that a stabilized silylium ion, which is generated from silyl ketene acetals by carboboration, cleaves the C-F bond of α-fluorostyrenes. A comparative study of α-chloro or bromostyrenes demonstrated that our reaction can be applied only to α-fluorostyrenes because the strong silicon-fluorine affinity facilitates an intramolecular interaction of silylium ions with fluorine atom to cleave the C-F bond. A broad range of α-fluorostyrenes as well as a range of silyl ketene acetals underwent this C-F bond transformation.

Systematic Design of a Frustrated Lewis Pair Containing Methyleneborane and Carbene for Dinitrogen Activation

Activation of atmospherically abundant dinitrogen (N₂) by metal-free species under mild reaction conditions has been one of the most challenging areas in chemistry for decades. Very recent but limited progress in N₂ activation by boron species, including two-coordinated borylene and methyleneborane and three-coordinated borole and borane, has been made toward metal-free N₂ activation. Here, we systematically probe an experimentally viable frustrated Lewis pair (FLP) containing two moieties (methyleneborane and carbene) for N₂ activation via density functional theory (DFT) calculations, which has proven to be an efficient approach for N₂ activation in a thermodynamically and kinetically favorable manner. Aromaticity is found to play a crucial role in stabilization of the product. This study could be a valuable alternative for the development of metal-free N₂ activation chemistry, highlighting great potential of FLP for N₂ activation and functionalization.

Bulky Polyfluorinated Terphenyldiphenylboranes: Water Tolerant Lewis Acids

Protocols for the synthesis of the bulky polyfluorinated triarylboranes 2,6‐(C₆F₅)₂C₆F₃B(C₆F₅)₂ (1), 2,6‐(C₆F₅)₂C₆F₃B[3,5‐(CF₃)₂C₆H₃] (2), 2,4,6‐(C₆F₅)₃C₆H₂B(C₆F₅)₂ (3), 2,4,6‐(C₆F₅)₃C₆H₂B[3,5‐(CF₃)₂C₆H₃] (4) were developed. All boranes are water tolerant and according to the Gutmann‐Beckett method, 1–3 display Lewis acidities larger than that of the prominent B(C₆F₅)₃.

Tris(pentafluorophenyl)borane catalyzed C-C and C-heteroatom bond formation

A series of boron based Lewis acids have been reported to date, but among them, tris(pentafluorophenyl)borane (BCF) has gained the most significant attention in the synthetic chemistry community. The viability of BCF as a potential Lewis acid catalyst has been vastly explored in organic and materials chemistry due to its thermal stability and commercial availability. Most explorations of BCF chemistry in organic synthesis has occurred in the last two decades and many new catalytic reactivities are currently under investigation. This review mainly focuses on recent reports from 2018 onwards and provides a concise knowledge to the readers about the role of BCF in metal-free catalysis. The review has mainly been categorized by different types of organic transformation mediated through BCF catalysis for the C-C and C-heteroatom bond formation.

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{(C6 F5 )2 B}C=P(C6 F5 ), which subsequently undergoes a rearrangement process involving migration of both the boryloxy and pentafluorophenyl substituents to afford Z-{(C6 F5 )2 B}(C6 F5 )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.

Triarylborane-Catalyzed Alkenylation Reactions of Aryl Esters with Diazo Compounds

Herein we report a facile, mild reaction protocol to form carbon-carbon bonds in the absence of transition metal catalysts. We demonstrate the metal-free alkenylation reactions of aryl esters with α-diazoesters to give highly functionalized enyne products. Catalytic amounts of tris(pentafluorophenyl)borane (10-20 mol %) are employed to afford the C=C coupled products (31 examples) in good to excellent yields (36-87 %). DFT studies were used to elucidate the mechanism for this alkenylation reaction.

Chemo- and Diastereoselective Hydrosilylation of Amorphadiene toward the Synthesis of Artemisinin

A formal synthesis of artemisinin starting from amorphadiene is described. This new route relies on the development of a catalytic chemo- and diastereoselective hydrosilylation. The practicability of this method is demonstrated by converting amorphadiene to dihydroartemisinic aldehyde using a one-pot hydrosilylation/oxidation sequence, minimizing the number of purifications and maximizing the productivity through a practical one-pot procedure. In addition, this approach can be coupled with a crystallization-induced diastereoselective transformation (CIDT) to enhance the optical purity of the key target intermediate, dihydroartemisinic aldehyde.

Halogenated triarylboranes: synthesis, properties and applications in catalysis

Halogenated triarylboranes (BAr₃) have been known for decades, however it has only been since the surge of interest in main group catalysis that their application as strong Lewis acid catalysts has been recognised. This review aims to look past the popular tris(pentafluorophenyl)borane [B(C₆F₅)₃] to the other halogenated triarylboranes, to give a greater breadth of understanding as to how tuning the Lewis acidity of BAr₃ by modifications of the aryl rings can lead to improved reactivity. In this review, a discussion on Lewis acidity determination of boranes is given, the synthesis of these boranes is discussed, and examples of how they are being used for catalysis and frustrated Lewis pair (FLP) chemistry are explained.

Synthesis of Stereodefined Trisubstituted Alkenyl Silanes Enabled by Borane Catalysis and Nickel Catalysis

Regioselective and stereoselective synthesis of trisubstituted alkenyl silanes via hydrosilylation is challenging. Herein, we report the first β-anti-selective addition of silanes to thioalkynes with B(C₆F₅)₃ as the catalyst. The reaction shows broad substrate scope. The products were proven to be useful intermediates to other trisubstituted alkenyl silanes by Ni-catalyzed stereoretentive cross-coupling reactions of the C-S bond. A mechanism study suggests that nucleophilic attack of thioalkyne to an activated silylium intermediate might be the rate-determining step.

Twenty-five years of bis-pentafluorophenyl borane: a versatile reagent for catalyst and materials synthesis

Highlights of the extensive chemistry and applications of bis-pentafluorophenyl borane (“Piers’ borane”) from the 25 years since its first appearance are featured.

B(C6F5)3- and HB(C6F5)2-mediated transformations of isothiocyanates

This contribution reports on the reactivity of isothiocyanates towards the boranes B(C6F5)3 and HB(C6F5)2. The reactions of alkyl-substituted isothiocyanates with B(C6F5)3 were found to result in rearrangement reactions to yield stable thiocyanate-B(C6F5)3 adducts. Treatment of isothiocyanates with HB(C6F5)2 leads to 1,2-hydroboration and thus, B,N,C,S heterocycles are formed, which react further under non-inert conditions. Hydrolysis of the hydroboration products leads to a new access to thioformamides.

Boron-Catalyzed N-Alkylation of Arylamines and Arylamides with Benzylic Alcohols

A sustainable boron-based catalytic approach for chemoselective N-alkylation of primary and secondary aromatic amines and amides with primary, secondary, and tertiary benzylic alcohols has been presented. The metal-free protocol operates at low catalyst loading, tolerates several functional groups, and generates H₂O as the sole byproduct. Preliminary mechanistic studies were performed to demonstrate the crucial role of boron catalyst for the activation of the intermediate dibenzyl ether and to identify the rate-determining step.

Superhalogens as Building Blocks of Super Lewis Acids

Lewis acids play an important role in synthetic chemistry. Using first-principle calculations on some newly designed molecules containing boron and organic heterocyclic superhalogen ligands, we show that the acid strength depends on the charge of the central atom as well as on the ligands attached to it. In particular, the strength of the Lewis acid increases with increasing electron withdrawing power of the ligand. With this insight, we highlight the importance of superhalogen-based ligands in the design of strong Lewis acids. Calculated fluoride ion affinity (FIA) values of B[C₂ BNO(CN)₃ ]₃ and B[C₂ BNS(CN)₃ ]₃ show that these are super Lewis acids.

Synthesis of sterically hindered 4,5-diarylphenanthrenes via acid-catalyzed bisannulation of benzenediacetaldehydes with alkynes

The synthesis of sterically hindered phenanthrenes via acid-catalyzed bisannulation reaction is described. Treatment of 1,4-benzenediacetaldehyde with terminal aryl alkynes in the presence of B(C6F5)3 provides 4,5-diarylphenanthrenes in good yields with excellent regioselectivity. The use of internal alkyne substrates enabled the synthesis of sterically hindered 3,4,5,6-tetrasubstituted phenanthrenes displaying augmented backbone helicity. Furthermore, 1,5-disubstituted, 1,8-disubstituted, 1,2,5,6-tetrasubstituted, and 1,2,7,8-tetrasubstituted phenanthrenes can be obtained through the reaction of alkynes with 1,3-benzenediacetaldehyde or 1,2-benzenediacetaldehyde disilyl acetal.

Tris(pentafluorophenyl)borane-Catalyzed Reactions Using Silanes

The utility of an electron-deficient, air stable, and commercially available Lewis acid tris(pentafluorophenyl)borane has recently been comprehensively explored. While being as reactive as its distant cousin boron trichloride, it has been shown to be much more stable and capable of catalyzing a variety of powerful transformations, even in the presence of water. The focus of this review will be to highlight those catalytic reactions that utilize a silane as a stoichiometric reductant in conjunction with tris(pentafluorophenyl) borane in the reduction of alcohols, carbonyls, or carbonyl-like derivatives.

Carboboration of isocyanates with tris(pentafluorophenyl)borane and evidence for dissociative FLP chemistry of an acid-base pair

We report on the reactivity of isocyanates towards tris(pentafluorophenyl)borane which was found to result in the 1,2-carboboration of the isocyanate C[double bond, length as m-dash]O bond. The resulting six-membered heterocyclic compounds can be rationalized as adducts of imino-boranes with isocyanates and react accordingly, allowing for the dissociative displacement of the trapped isocyanate moiety in a series of exchange reactions. Related carboelementation reactions with the heavier group 13 analogues, Al(C6F5)3 and Ga(C6F5)3, result in a dimerization of the carboboration products to afford eight-membered heterocycles.

Borylative cyclisation of diynes using BCl3 and borocations

Products from the borylative cyclisation of diynes using BCl₃ is dependent on substituent effects, however, some control of product outcome is achieved using borocations or BCl₃ in the presence of [BCl₄]⁻.

Computational and experimental investigation on the BCl3 promoted intramolecular amination of alkenes and alkynes

The BCl₃ promoted aminoboration of alkenes and alkynes was investigated both computationally and experimentally, leading to the discovery of a metal-free hydroamination of alkynes.

B(C6F5)3-catalyzed Markovnikov addition of indoles to aryl alkynes: an approach toward bis(indolyl)alkanes

The first example of the metal- and solvent-free B(C6F5)3-catalyzed Markovnikov addition of indoles to aryl alkynes was disclosed. Both N-H and N-protected indoles were tolerated, leading to a wide spectrum of versatile bis(indolyl)alkanes in moderate to good yields with high regioselectivities.

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.

Tunable Cu(i)-catalyzed site-selective dehydrogenative amination of β,γ-unsaturated hydrazones for divergent synthesis of pyrazol-4-ones and 1,6-dihydropyradazines

A new tunable Cu(i)-catalyzed site-selective dehydrogenative amination of β,γ-unsaturated hydrazones has been established using readily accessible dioxygen as a green oxidant.

Reactions of biologically inspired hydride sources with B(C6F5)3

The combination of 1-benzyl-1,4-dihydropyridines with the strong Lewis acid, B(C6F5)3, generates a stable pyridinium borohydride species in high yields (94%) in as little as 10 min. This use of biologically inspired hydride sources further builds on the recent work of new hydride donors in the formation of borohydrides. When functionalizing the dihydropyridine with an amide or carboxylic acid moiety, a disproportionation reaction composed of a series of protonation/reduction steps is observed upon the addition of B(C6F5)3 As a result, one equivalent of dihydropyridine undergoes net hydrogenation, whereas the other is dehydrogenated yielding the pyridinium counterpart in a transfer hydrogenation-type mechanism.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

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.