BCl₃ is an inexpensive electrophile which induces the borylative cyclization of a wide range of substituted alkynes to regioselectively form polycycles containing synthetically versatile C(sp²)boronate esters. It proceeds rapidly, with good yields and is compatible with a range of functional groups and substitution patterns. Intermolecular 1,2-carboboration of alkynes is also achieved using BCl₃ to generate trisubstituted vinyl boronate esters.

The direct C(sp²)C(sp³) cross-coupling of diaryl zinc reagents with benzylic, primary, secondary, and tertiary alkyl halides proceeded in the absence of coordinating ethereal solvents at ambient temperature without the addition of a catalyst. The C(sp²)C(sp³) cross-coupling showed excellent functional-group tolerance, and products were isolated in high yields, generally without the requirement for purification by chromatography. This process represents an expedient, operationally simple method for the construction of new C(sp²)C(sp³) bonds.

BCl₃ is an inexpensive electrophile which induces the borylative cyclization of a wide range of substituted alkynes to regioselectively form polycycles containing synthetically versatile C(sp²)boronate esters. It proceeds rapidly, with good yields and is compatible with a range of functional groups and substitution patterns. Intermolecular 1,2-carboboration of alkynes is also achieved using BCl₃ to generate trisubstituted vinyl boronate esters.

We report the isolation and detailed structural characterization, by solid-state and solution NMR spectroscopy, of the neutral mono- and bis-NHC adducts of bis(catecholato)diboron (B₂cat₂). The bis-NHC adduct undergoes thermally induced rearrangement, forming a six-membered -BCNCC-N-heterocyclic ring via CN bond cleavage and ring expansion of the NHC, whereas the mono-NHC adduct is stable. Bis(neopentylglycolato)diboron (B₂neop₂) is much more reactive than B₂cat₂ giving a ring expanded product at room temperature, demonstrating that ring expansion of NHCs can be a very facile process with significant implications for their use in catalysis.

The direct C(sp²)C(sp³) cross-coupling of diaryl zinc reagents with benzylic, primary, secondary, and tertiary alkyl halides proceeded in the absence of coordinating ethereal solvents at ambient temperature without the addition of a catalyst. The C(sp²)C(sp³) cross-coupling showed excellent functional-group tolerance, and products were isolated in high yields, generally without the requirement for purification by chromatography. This process represents an expedient, operationally simple method for the construction of new C(sp²)C(sp³) bonds.

N-methylacridinium salts are Lewis acids with high hydride ion affinity but low oxophilicity. The cation forms a Lewis adduct with 4-(N,N-dimethylamino)pyridine but a frustrated Lewis pair (FLP) with the weaker base 2,6-lutidine which activates H₂, even in the presence of H₂O. Anion effects dominate reactivity, with both solubility and rate of H₂ cleavage showing marked anion dependency. With the optimal anion, a N-methylacridinium salt catalyzes the reductive transfer hydrogenation and hydrosilylation of aldimines through amine–boranes and silanes, respectively. Furthermore, the same salt is active for the catalytic dehydrosilylation of alcohols (primary, secondary, tertiary, and ArOH) by silanes with no observable over-reduction to the alkanes.

N-methylacridinium salts are Lewis acids with high hydride ion affinity but low oxophilicity. The cation forms a Lewis adduct with 4-(N,N-dimethylamino)pyridine but a frustrated Lewis pair (FLP) with the weaker base 2,6-lutidine which activates H₂, even in the presence of H₂O. Anion effects dominate reactivity, with both solubility and rate of H₂ cleavage showing marked anion dependency. With the optimal anion, a N-methylacridinium salt catalyzes the reductive transfer hydrogenation and hydrosilylation of aldimines through amine–boranes and silanes, respectively. Furthermore, the same salt is active for the catalytic dehydrosilylation of alcohols (primary, secondary, tertiary, and ArOH) by silanes with no observable over-reduction to the alkanes.

The reaction of 8-(trimethylsiloxy)quinoline (QOTMS) with BCl₃ and (aryl)BCl₂ forms QOBCl₂ and QOBCl(aryl). The subsequent addition of stoichiometric AlCl₃ follows one of two paths, dependent on the steric demands of the QO ligand and the electrophilicity of the resulting borenium cation. The phenyl- and 5-hexylthienylborenium cations, QOBPh⁺ and QOBTh⁺, are formed, whereas QOBCl⁺ is not. Instead, AlCl₃ preferentially binds with QOBCl₂ at oxygen, forming QOBCl₂⋅AlCl₃, rather than abstracting chloride. A modest increase in the steric demands around oxygen, by installing a methyl group at the 7-position of the quinolato ligand, switches the reactivity with AlCl₃ back to chloride abstraction, allowing formation of QOBCl⁺. All the prepared borenium cations are highly chlorophilic and exhibit significant interaction with AlCl₄⁻ resulting in an equilibrium concentration of Lewis acidic “AlCl₃” species. The presence of “AlCl₃^” species limits the alkyne substrates compatible with these borenium systems, with reaction of [QOBPh][AlCl₄] with 1-pentyne exclusively yielding the cyclotrimerised product, 1,3,5-tripropylbenzene. In contrast, QOBPh⁺ and QOBTh⁺ systems effect the syn-1,2-carboboration of 3-hexyne. DFT calculations at the M06-2X/6-311G(d,p)/PCM(DCM) level confirm that the higher migratory aptitude of Ph versus Me leads to a lower barrier to 1,2-carboboration relative to 1,1-carboboration.

A range of frustrated Lewis pairs (FLPs) containing borenium cations have been synthesised. The catechol (Cat)-ligated borenium cation [CatB(PtBu₃)]⁺ has a lower hydride-ion affinity (HIA) than B(C₆F₅)₃. This resulted in H₂ activation being energetically unfavourable in a FLP with the strong base PtBu₃. However, ligand disproportionation of CatBH(PtBu₃) at 100 °C enabled trapping of H₂ activation products. DFT calculations at the M06-2X/6-311G(d,p)/PCM (CH₂Cl₂) level revealed that replacing catechol with chlorides significantly increases the chloride-ion affinity (CIA) and HIA. Dichloro–borenium cations, [Cl₂B(amine)]⁺, were calculated to have considerably greater HIA than B(C₆F₅)₃. Control reactions confirmed that the HIA calculations can be used to successfully predict hydride-transfer reactivity between borenium cations and neutral boranes. The borenium cations [Y(Cl)B(2,6-lutidine)]⁺ (Y=Cl or Ph) form FLPs with P(mesityl)₃ that undergo slow deprotonation of an ortho-methyl of lutidine at 20 °C to form the four-membered boracycles [(CH₂{NC₅H₃Me})B(Cl)Y] and [HPMes₃]⁺. When equimolar [Y(Cl)B(2,6-lutidine)]⁺/P(mesityl)₃ was heated under H₂ (4 atm), heterolytic cleavage of dihydrogen was competitive with boracycle formation.