Reactions of carbene-stabilized borenium cations

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.

Oxathiaborolium-catalyzed enantioselective [2 + 2] cycloadditions

The one-pot-prepared oxathiaborolium pentachlorostannate is an excellent Lewis acid and is successfully used to catalyze the [2 + 2] cycloadditions of N-substituted maleimides and silyl enol ethers with excellent enantioselectivities.

Highly sensitive detection of polyborosiloxane (PBS) hydrolysis with mannitol using electrochemical methodology

The change in current in the solution was detected with high sensitivity by applying an electrochemical methodology that showed the hydrolysis of PBSs.

Oxathiaborolium-Catalyzed Enantioselective [4 + 2] Cycloaddition and Its Application in Lewis Acid Coordinated and Chiral Lewis Acid Catalyzed [4 + 2] Cycloaddition

The nascency of second-generation sulfur-stabilized borenium cations by halophilic Lewis acid SnCl₄ leads to highly active chiral Lewis acids that are very effective catalysts for [4 + 2] cycloaddition. Oxathiaborolium pentachlorostannate (5-10 mol %) successfully catalyzed cycloaddition of various dienes and dienophiles to afford cycloadducts with excellent enantioselectivity (20 examples, up to 99% ee). This super Lewis acid also exhibited good enantioselectivity for the first Lewis acid coordinated and chiral Lewis acid catalyzed [4 + 2] cycloaddition to α,β-unsaturated mixed ester amide.

The [(NHC)B(H)C6 F5 ]+ Cations and Their [B](H)-CO Borane Carbonyls

Hydride abstraction from the heterocyclic carbene borane adducts (NHC)BH₂ C₆ F₅ (NHC: IMes or IMe₄ ) gave the B-H containing [(NHC)B(H)C₆ F₅ ]⁺ borenium cations. They added carbon monoxide to give the respective [(NHC)B(H)(C₆ F₅ )CO]⁺ boron carbonyl cations. Carbon nucleophiles add to the boron carbonyl to give [B](H) acyls. Hydride reduced the [B]CO cation to hydroxymethylborane derivatives.

A Cationic NHC-Supported Borole

This work describes the synthesis and characterization of a highly reactive cationic borole. Halide abstraction with Li{Al[OC(CF3 )3 ]4 } from the NHC-chloroborole adduct yields the first stable NHC-supported 1-(Me NHC)-2,5-(SiMe3 )2 -3,4-(Ph*)2 -borole cation. Electronically, it features both a five-membered cyclic conjugated 4 π-electron system and a cationic charge and thus resembles the yet elusive cyclopentadienyl cation. The borole cation was characterized crystallographically, spectroscopically (NMR, UV/Vis), by cyclovoltammetry, microanalysis and mass-spectrometry and its electronic structure was probed computationally. The cation reacts with tolane and reversibly binds carbon monoxide. Direct comparison with the structurally related, yet neutral, 1-mesityl borole reveals strong Lewis acidity, reduced HOMO-LUMO gaps, and increased anti-aromatic character.