Frustrated Lewis pair induced boroauration of terminal alkynes

Bis(1-Methyl-ortho-Carboranyl)Borane

The Lewis superacid, bis(1-methyl-ortho-carboranyl)borane, is rapidly accessed in two steps. It is a very effective hydroboration reagent capable of B-H addition to alkenes, alkynes, and cyclopropanes. To date, this is the first identified Lewis superacidic secondary borane and most reactive neutral hydroboration reagent.

Mechanistic Study of Alkyne Insertion into Cu-Al and Au-Al Bonds: A Paradigm Shift for Coinage Metal Chemistry

In this work, the mechanism of the insertion reaction of 3-hexyne into Cu-Al and Au-Al bonds in M-aluminyl (M = Cu, Au) complexes is computationally elucidated. The mechanism is found to be radical-like, with the Cu-Al and Au-Al bonds acting as nucleophiles toward the alkyne, and predicts a less efficient reactivity for the gold-aluminyl complex. The proposed mechanism well rationalizes the kinetic (or thermodynamic) control on the formation of the syn (or anti) insertion product into the Cu-Al bond (i.e., dimetallated alkene) which has been recently reported. A comparative analysis of the electronic structure reveals that the reduced reactivity at the gold site─usually showing higher efficiency than copper as a "standard" electrophile in alkyne activation─arises from a common feature, i.e., the highly stable 6s Au orbital. The relativistic lowering of the 6s orbital, making it more suitable for accepting electron density and thus enhancing the electrophilicity of gold complexes, in the gold-aluminyl system is responsible for a less nucleophilic Au-Al bond and, consequently, a less efficient alkyne insertion. These findings demonstrate that the unconventional electronic structure and the electron-sharing nature of the M-Al bond induce a paradigm shift in the properties of coinage metal complexes. In particular, the peculiar radical-like reactivity, previously shown also with carbon dioxide, suggests that these complexes might efficiently insert/activate other small molecules, opening new and unexplored paths for their reactivity.

1,1,2-Trifunctionalization of terminal alkynes by imine/borane frustrated Lewis pairs

The first metal-free 1,1,2-trifunctionalization of terminal alkynes is reported, providing a straightforward means to synthesize trisubstituted-alkenyl boranes.

Isomerization of a cis-(2-Borylalkenyl)Gold Complex via a Retro-1,2-Metalate Shift: Cleavage of a C-C/C-Si Bond trans to a C-Au Bond

This manuscript describes the first example of an alkyne insertion to the Au-B bond of a di(o-tolyl)borylgold complex to afford a cis-2-borylalkenylgold complex, and its isomerization to result in interchanging substituents on the alkenyl carbon atom and the boron atom. The former reaction is the first example of an alkyne insertion to a Au-B bond. In the latter reaction, the regiochemistry of the isomerized alkenylgold products varied depending on the substituents. DFT calculations revealed the formation of gold alkynylborates as a common intermediate via a "retro-1,2-metalate shift", which can be considered as an anti-β-carbon/silicon elimination, and identified a subsequent 1,2-metalate shift as the regiochemistry-determining step. Relative energies of the transition states to each isomer and natural-bond-orbital (NBO) analyses were used to clearly rationalize the regiochemistry of the products.

Formation of amidino-borate derivatives by a multi-component reaction

Cyclohexene reacts with the (Fmes)BH2·SMe2 borane reagent and three molar equivalents of the isonitrile CN-Xyl to give the five membered 1,3-BN heterocyclic product 7 that contains a zwitterionic borata-amidinium moiety and a cyclohexenyl substituent. The analogous five-component coupling between cyclopentene, (Fmes)BH2·SMe2 and CN-Xyl in a 1 : 1 : 3 molar ratio gives the related cyclic amidino-borate derivative 10. The reaction of the (Fmes)BH2 derived frustrated Lewis pair 12, in situ generated or employed as the isolated dimer, reacts with 3 CN-Xyl equivs. at elevated temperature (60 °C) to yield the analogous coupling product 13.

Synthesis, Characterization, and Density Functional Theory Studies of Three-Dimensional Inorganic Analogues of 9,10-Diboraanthracene-A New Class of Lewis Superacids

The three-dimensional inorganic analogues of 9,10-diboraanthracene, B₂X₂(C₂B₁₀H₁₀)₂ (X = Cl, 1; X = Br, 2), were attained by salt elimination of Li₂C₂B₁₀H₁₀ and trihaloboranes. The methyl- and phenyl-substituted compounds B₂Me₂(C₂B₁₀H₁₀)₂ (3) and B₂Ph₂(C₂B₁₀H₁₀)₂ (4) were obtained by treating 1 or 2 with the corresponding Grignard reagents. These compounds were fully characterized by NMR, cyclic voltammetry (CV), IR, and single-crystal X-ray diffraction analyses. Experimental (CV and Gutmann-Beckett method) and computational (fluoride ion affinity, hydride ion affinity and LUMO energy) results suggest that the order of Lewis acidity is 2 > 1 > 4 > 3 > SbF₅. Treatment of 1 or 2 with HSiEt₃ gave a rare neutral borane-silane adduct, (Et₃SiH)₂B₂H₂(C₂B₁₀H₁₀)₂ (5). The equilibrium of 5 in solution was thoroughly investigated by spectroscopy and quantum calculations.

Electron-deficient boron-based catalysts for C-H bond functionalisation

In contrast to transition metal-catalysed C-H functionalisation, highly efficient construction of C-C and C-X (X = N, O, S, B, Si, etc.) bonds through metal-free catalytic C-H functionalisation remains one of the most challenging tasks for synthetic chemists. In recent years, electron-deficient boron-based catalyst systems have exhibited great potential for C-H bond transformations. Such emerging systems may greatly enrich the chemistry of C-H functionalisation and main-group element catalysis, and will also provide enormous opportunities in synthetic chemistry, materials chemistry, and chemical biology. This article aims to give a timely comprehensive overview to recognise the current status of electron-deficient boron-based catalysis in C-H functionalisation and stimulate the development of more efficient catalytic systems.

Using the FpXylBH2•SMe2 reagent for the regioselective synthesis of cyclic bis(alkenyl)boranes

The reactive borane reagent FpXylBH2•SMe2 was prepared from 1,4-bis(trifluoromethyl)benzene by treatment with n-BuLi, followed by H3B·SMe2 and subsequent removal of hydride. It undergoes a regioselective hydroboration reaction with 1,2-bis(trimethylsilylethynyl)benzene to give the "dimeric" product 13a featuring a conjugated 14-membered core heterocyclic structure that contains a pair of FpXylB units.

Computationally Guided Molecular Design to Minimize the LE/CT Gap in D-π-A Fluorinated Triarylboranes for Efficient TADF via D and π-Bridge Tuning

In this combined experimental and theoretical study, a computational protocol is reported to predict the excited states in D-π-A compounds containing the B(FXyl)2 (FXyl = 2,6-bis(trifluoromethyl)phenyl) acceptor group for the design of new thermally activated delayed fluorescence (TADF) emitters. To this end, the effect of different donor and π-bridge moieties on the energy gaps between local and charge-transfer singlet and triplet states is examined. To prove this computationally aided design concept, the D-π-B(FXyl)2 compounds 1-5 were synthesized and fully characterized. The photophysical properties of these compounds in various solvents, polymeric film, and in a frozen matrix were investigated in detail and show excellent agreement with the computationally obtained data. Furthermore, a simple structure-property relationship is presented on the basis of the molecular fragment orbitals of the donor and the π-bridge, which minimize the relevant singlet-triplet gaps to achieve efficient TADF emitters.

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.

Preparation of the Borane (Fmes)BH2 and its Utilization in the FLP Reduction of Carbon Monoxide and Carbon Dioxide

Treatment of 1,3,5-tris(trifluoromethyl)benzene with n-BuLi, followed by H₃ B⋅SMe₂ and subsequent hydride removal gave the (Fmes)BH₂ reagent, which was isolated as a SMe₂ stabilized monomer or a ligand free (μ-H)₂ -bridged dimer. Reaction with Mes₂ P(vinyl) gave the respective ethylene-bridged P/B(Fmes)H FLP. It reduced carbon monoxide to the formyl stage and carbon dioxide to the formaldehyde oxidation state. Most new compounds were characterized by X-ray diffraction.

Reversible C-H Activation, Facile C-B/B-H Metathesis and Apparent Hydroboration Catalysis by a Dimethylxanthene-Based Frustrated Lewis Pair

A dimethylxanthene-based phosphine/borane frustrated Lewis pair (FLP) is shown to effect reversible C-H activation, cleaving phenylacetylene, PhCCH, to give an equilibrium mixture of the free FLP and phosphonium acetylide in CD₂ Cl₂ solution at room temperature. This system also reacts with B-H bonds although in a different fashion: reactions with HBpin and HBcat proceed via C-B/B-H metathesis, leading to replacement of the -B(C₆ F₅ )₂ Lewis acid component by -Bpin/-Bcat, and transfer of HB(C₆ F₅ )₂ to the phosphine Lewis base. This transformation underpins the ability of the FLP to catalyze the hydroboration of alkynes by HBpin: the active species is derived from the HB(C₆ F₅ )₂ fragment generated in this exchange process.

Phosphirenium borate betaines from alkynylphosphanes and the halogeno-B(C6F5)2 reagents

A series of phosphanyl substituted alkynes R¹₂P–CC–R² [R¹₂P: Mes₂P or ^tBu₂P; R²: SiMe₃, Ph, ^tBu] reacts with the X–B(C₆F₅)₂ reagents [X: Cl, Br] to give thermally rather stable neutral zwitterionic phosphirenium borate products.

Frustrated Lewis Pair Mediated 1,2-Hydrocarbation of Alkynes

Frustrated Lewis pair (FLP) chemistry enables a rare example of alkyne 1,2-hydrocarbation with N-methylacridinium salts as the carbon Lewis acid. This 1,2-hydrocarbation process does not proceed through a concerted mechanism as in alkyne syn-hydroboration, or through an intramolecular 1,3-hydride migration as operates in the only other reported alkyne 1,2-hydrocarbation reaction. Instead, in this study, alkyne 1,2-hydrocarbation proceeds by a novel mechanism involving alkyne dehydrocarbation with a carbon Lewis acid based FLP to form the new C-C bond. Subsequently, intermolecular hydride transfer occurs, with the Lewis acid component of the FLP acting as a hydride shuttle that enables alkyne 1,2-hydrocarbation.

Theoretical design and mechanistic study of the metal-free reduction of CO2 to CO

The metal-free silylborane Me₂BSi(CH₂F)₃, screened based on the rules of thumb coming from the summary of a hierarchy of silylboranes, can be used to catalyze the reduction of CO₂ to CO.

Optical and electronic properties of air-stable organoboron compounds with strongly electron-accepting bis(fluoromesityl)boryl groups

Three compounds with phenyl (1), 4-tert-butylphenyl (2) and 4-N,N-diphenylaminophenyl (3) groups attached to bis(fluoromesityl)boryl ((FMes)2B) through B-C bonds have been prepared. The restricted rotation about the B-C bonds of boron-bonded aryl rings in solution has been studied by variable-temperature 19F NMR spectroscopy, and through-space F-F coupling has been observed for 3 at low temperature. Steric congestion inhibits binding of 1 by Lewis bases DABCO and tBu3P and the activation of H2 in their presence. Photophysical and electrochemical studies have been carried out on 2, 3, and an analogue of 3 containing a bis(mesityl)boryl ((Mes)2B) group, namely 4. Both 2 and 3 show bright emission in nonpolar solvents and in the solid-state, very strong electron-accepting ability as measured by cyclic voltammetry, and good air-stability. In addition, 2 displayed unusually long-lived emission (τ = 2.47 s) in 2-MeTHF at 77 K. The much stronger acceptor strength of (FMes)2B than (Mes)2B leads to significantly red-shifted emission in solution and the solid state, stronger emission solvatochromism, and significantly lower reduction potentials. Theoretical calculations confirm that 2 and 3 tend to form highly twisted excited states with good conjugation between one FMes group and the boron atom, which correlate well with their blue-shifted solid-state emissions and low kr values in solution.

Metal-free HB(C6F5)2-catalyzed hydrogenation of unfunctionalized olefins and mechanism study of borane-mediated σ-bond metathesis

Out with the metal: Metal-free hydrogenation of unfunctionalized olefins can be achieved by employing HB(C6F5)2 as the catalyst. The key step in the catalytic reaction is believed to involve a novel borane-mediated σ-bond metathesis, which has been investigated both experimentally and theoretically.