Phosphinoborenium cations stabilized by N-heterocyclic carbenes: synthesis, structure, and reactivity

Phosphinoborenium cations stabilized by N-heterocyclic carbenes (NHCs) were synthesized via the reaction of bromo(phosphino)boranes with NHCs. Their structures were investigated by heteronuclear magnetic resonance spectroscopy, X-ray diffraction, and density functional theory calculations. They possess a planar trigonal boron center directly bonded with the pyramidal phosphanyl group (PR2) and can be treated as cationic phosphinoboranes. The reactivity of the selected NHC-phosphinoborenium cation was tested toward AuCl·SMe2 and Ph2PCl. In both reactions, the titled compound acted as a phosphido group donor under heterolytic cleavage of the P-B bond. Control experiments with parent phosphinoborane emphasized differences between the reactivity of low-coordinate neutral and cationic species with P-B functionality.

Boron Polycation Supported by Cyclic Bis(carbodiphosphorane)

A novel cyclic bis(carbodiphosphorane) ligand was prepared and investigated in coordination with group 13 elements, B, Al, and Ga. Al and Ga afforded dinuclear adduct where two metal centers were bridged by the bis(carbodiphosphorane) ligand. In contrast, the reaction with boron trichloride afforded a monomeric dicationic three-coordinate boron species composed of one boron moieties and one ligand. The structures of these products were determined by X-ray crystallography. In the dicationic boron compound, the sterically constrained cyclic structure enforced the boron center to acquire strained trigonal geometry with wide C-B-C angle of 140°. Furthermore, theoretical investigation with DFT and NBO suggested a significant contribution of tricationic two-coordinate boron resonance structure supported by two CDP ligands.

Synthesis of Electrophiles Derived from Dimeric Aminoboranes and Assessing Their Utility in the Borylation of π Nucleophiles

Dimeric aminoboranes, [H₂BNR₂]₂ (R = Me or CH₂CH₂) containing B₂N₂ cores, can be activated by I₂, HNTf₂ (NTf₂ = [N(SO₂CF₃)₂]), or [Ph₃C][B(C₆F₅)₄] to form isolable H₂B(μ-NR₂)₂BHX (for X = I or NTf₂). For X = [B(C₆F₅)₄]⁻ further reactivity, presumably between [H₂B(μ-NMe₂)₂BH][B(C₆F₅)₄] and aminoborane, forms a B₃N₃-based monocation containing a three-center two electron B-(μ-H)-B moiety. The structures of H₂B(μ-NMe₂)₂BH(I) and [(μ-NMe₂)BH(NTf₂)]₂ indicated a sterically crowded environment around boron, and this leads to the less common O-bound mode of NTf₂ binding. While the iodide congener reacted very slowly with alkynes, the NTf₂ analogues were more reactive, with hydroboration of internal alkynes forming (vinyl)₂BNR₂ species and R₂NBH(NTf₂) as the major products. Further studies indicated that the B₂N₂ core is maintained during the first hydroboration, and that it is during subsequent steps that B₂N₂ dissociation occurs. In the mono-boron systems, for example, ⁱPr₂NBH(NTf₂), NTf₂ is N-bound; thus, they have less steric crowding around boron relative to the B₂N₂ systems. Notably, the monoboron systems are much less reactive in alkyne hydroboration than the B₂N₂-based bis-boranes, despite the former being three coordinate at boron while the latter are four coordinate at boron. Finally, these B₂N₂ electrophiles are much more prone to dissociate into mono-borane species than pyrazabole [H₂B(μ-N₂C₃H₃)]₂ analogues, making them less useful for the directed diborylation of a single substrate.

Borylation Directed Borylation of Indoles Using Pyrazabole Electrophiles: A One-Pot Route to C7-Borylated-Indolines

Pyrazabole (1) is a readily accessible diboron compound that can be transformed into ditopic electrophiles. In 1 (and derivatives), the B⋅⋅⋅B separation is ca. 3 Å, appropriate for one boron centre bonding to N and one to the C7 of indoles and indolines. This suitable B⋅⋅⋅B separation enables double E-H (E=N/C) functionalisation of indoles and indolines. Specifically, the activation of 1 with HNTf₂ generates an electrophile that transforms N-H indoles and indolines into N/C7-diborylated indolines, with N-H borylation directing subsequent C7-H borylation. Indole reduction to indoline occurs before C-H borylation and our studies indicate this proceeds via hydroboration-C3-protodeboronation to produce an intermediate that then undergoes C7 borylation. The borylated products can be converted in situ into C7-BPin-N-H-indolines. Overall, this represents a transient directed C-H borylation to form useful C7-BPin-indolines.

Metal-Free Phosphorus-Directed Borylation of C(sp2 )-H Bonds

Spectacular progress has recently been achieved in transition metal-catalyzed C-H borylation of phosphines as well as directed electrophilic C-H borylation. As shown here, P-directed electrophilic borylation provides a new, straightforward, and efficient access to phosphine-boranes. It operates under metal-free conditions and leverages simple, readily available substrates. It is applicable to a broad range of backbones (naphthyl, biphenyl, N-phenylpyrrole, binaphthyl, benzyl, naphthylmethyl) and gives facile access to various substitution patterns at boron (by varying the boron electrophile or post-derivatizing the borane moiety). NMR monitoring supports the involvement of P-stabilized borenium cations as key intermediates. DFT calculations reveal the existence and stabilizing effect of π-arene/boron interactions in the (biphenyl)(i-Pr)₂ P→BBr₂ ⁺ species.

Borenium-Catalyzed Reduction of Pyridines through the Combined Action of Hydrogen and Hydrosilane

Mesoionic carbene-stabilized borenium ions efficiently reduce substituted pyridines to piperidines in the presence of a hydrosilane and a hydrogen atmosphere. Control experiments and deuterium labeling studies demonstrate reversible hydrosilylation of the pyridine, enabling full reduction of the N-heterocycle under milder conditions. The silane is a critical reaction component to prevent adduct formation between the piperidine product and the borenium catalyst.

Lewis pairing and frustration of group 13/15 elements geometrically enforced by (ace)naphthalene, biphenylene and (thio)xanthene backbones

The synthesis, structure, and reactivity of mixed group 13/group 15 compounds (E¹³ = B, Al, Ga, In, Tl; E¹⁵ = N, P, Sb, Bi) featuring a rigid (ace)naphthalene or (thio)xanthene backbone are discussed in this review. The backbone may either enforce or prevent E¹⁵→E¹³ interactions, resulting in Lewis pairing or frustration. The formation of strong E¹⁵→E¹³ interactions is possible upon peri-substitution of (ace)naphthalenes. This gives the opportunity to access and study highly reactive species, as exemplified by P-stabilised borenium salts and boryl radicals. In turn, rigid expanded spacers such as biphenylenes, (thio)xanthenes and dibenzofurans impose long distances and geometrically prevent E¹⁵→E¹³ interactions. Such P-B derivatives display ambiphilic coordination properties and frustrated Lewis pair behaviour towards small molecules, their preorganised structure favouring reversible interaction/activation. Throughout the review, the importance of the scaffold in enforcing or preventing E¹⁵→E¹³ interactions is highlighted and discussed based on experimental data and theoretical calculations.

Cationic Boron Formazanate Dyes*

Incorporation of cationic boron atoms into molecular frameworks is an established strategy for creating chemical species with unusual bonding and reactivity but is rarely thought of as a way of enhancing molecular optoelectronic properties. Using boron formazanate dyes as examples, we demonstrate that the wavelengths, intensities, and type of the first electronic transitions in BN heterocycles can be modulated by varying the charge, coordination number, and supporting ligands at the cationic boron atom. UV-vis absorption spectroscopy measurements and density-functional (DFT) calculations show that these modulations are caused by changes in the geometry and extent of π-conjugation of the boron formazanate ring. These findings suggest a new strategy for designing optoelectronic materials based on π-conjugated heterocycles containing boron and other main-group elements.

Boronic, diboronic and boric acid esters of 1,8-naphthalenediol - synthesis, structure and formation of boronium salts

The 1,8-naphthalenediolate [1,8-O2C10H8] supported boronic and boric acid esters of general formula X-B(1,8-O2C10H8), where X = C6H5 (1a), C6F5 (2a), 3,4,5-F3-C6H2 (3a), 2,4,6-F3-C6H2 (4a), 2,6-F2-C6H3 (5a), 2,6-Cl2-C6H3 (6a), 2,4,6-Me3-C6H2 (7a), 2,6-(MeO)3-C6H3 (8a), Bun (9a), MeO (10a), OH (11a) and Cl (13a), were synthesized, NMR spectroscopically characterized, and the solid-state structures of 1a-5a, 8a and 10a determined by X-ray crystallography. The acceptor numbers of 1a-7a and 13a were determined and found to be similar to their catecholate analogues, R-Bcat, indicating similar Lewis acidities of these two classes of boronic acid esters. The reaction of B2(NMe2)4 with 1,8-naphthalenediol, followed by addition of HCl furnished the diboronic acid ester B2(1,8-O2C10H8)4 (16a) in ca. 70% yield. Cl-B(1,8-O2C10H8) (13a) was shown to react with O[double bond, length as m-dash]PEt3, DMAP, 1,10-phenanthroline and 2,2'-bipyridine, resp., to give the boronium salts [(Et3P[double bond, length as m-dash]O)2B(1,8-O2C10H8)]Cl (18a), [(DMAP)2B(1,8-O2C10H8)]Cl (22a), [(2,2'-bipyridine)B(1,8-O2C10H8)]Cl (23a) and [(1,10-phenanthroline)B(1,8-O2C10H8)]Cl (24a), which were characterized by NMR spectroscopy and X-ray crystallography.

Stable Borepinium and Borafluorenium Heterocycles: A Reversible Thermochromic "Switch" Based on Boron-Oxygen Interactions

The first examples of N-heterocyclic carbene (NHC) and cyclic(alkyl)(amino) carbene (CAAC) stabilized borepinium and borafluorenium heterocycles are reported herein. The optical properties of the heterocyclic borenium cations were tuned by varying the Lewis base and by changing the number of atoms in the ring. More importantly, functionalizing the cationic boron ring system in the NHC-borafluorenium cation affords a temperature-sensitive molecule with reversible colorimetric "turn off/turn on" properties in solution. Notably, this is the first report of thermochromism in these cationic species. This property, which is mediated by an intermolecular boron-oxygen bond equilibrium, was examined in detail by X-ray crystallography, variable temperature-UV/Vis absorption spectroscopy (VT-UV/Vis), and density functional theory (DFT).

Addition of dihydrogen to a borylborenium center

The activation of a H-H bond, the simplest covalent bond, is a fundamentally important process. Addition of H2 to an elemental center typically occurs on low valent transition metal or main group complexes through oxidative addition to afford metal dihydride complexes. In contrast, activation of H2 on a high valent center generally results in heterolytic cleavage of the H-H bond to a proton and hydride. Here, we report experimental and computational evidence for the addition of H2 to a borenium center in an N-heterocyclic carbene (NHC) coordinated borylborenium cation, which leads to the formation of a dihydroborenium complex accompanied by the elimination of two σ-bonded substituents, namely mesityl (Mes) and pinacolboryl (Bpin) groups, as mesitylboronic ester.

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.

Using Ylide Functionalization to Stabilize Boron Cations

The metalated ylide YNa [Y=(Ph3 PCSO2 Tol)- ] was employed as X,L-donor ligand for the preparation of a series of boron cations. Treatment of the bis-ylide functionalized borane Y2 BH with different trityl salts or B(C6 F5 )3 for hydride abstraction readily results in the formation of the bis-ylide functionalized boron cation [Y-B-Y]+ (2). The high donor capacity of the ylide ligands allowed the isolation of the cationic species and its characterization in solution as well as in solid state. DFT calculations demonstrate that the cation is efficiently stabilized through electrostatic effects as well as π-donation from the ylide ligands, which results in its high stability. Despite the high stability of 2 [Y-B-Y]+ serves as viable source for the preparation of further borenium cations of type Y2 B+ ←LB by addition of Lewis bases such as amines and amides. Primary and secondary amines react to tris(amino)boranes via N-H activation across the B-C bond.