Pushing the Lewis Acidity Boundaries of Boron Compounds With Non‐Planar Triarylboranes Derived from Triptycenes

Bending the planar trigonal boron center of triphenylborane by connecting its aryl rings with carbon or phosphorus linkers gave access to a series of 9‐boratriptycene derivatives with unprecedented structures and reactivities. NMR spectroscopy and X‐ray diffraction of the Lewis adducts of these non‐planar boron Lewis acids with weak Lewis base revealed particularly strong covalent bond formation. The first Lewis adduct of a trivalent boron compounds with the Tf₂N^? anion illustrates the unrivaled Lewis acidity of these species. Increasing the pyramidalization of the boron center and using a cationic phosphonium linker resulted in an exceptional enhancement of Lewis acidity. Introduction of a phosphorus and a boron atom at each edge of a triptycene framework, allowed access to new bifunctional Lewis acid‐base 9‐phospha‐10‐boratriptycenes featuring promising reactivity for the activation of carbon‐halogen bonds.     

Asymmetric Lewis Acid Catalyzed Electrochemical Alkylation

Lewis‐acid catalysis and electrochemistry represent two powerful fields that have found widespread application in organic chemistry. Reported herein is an asymmetric electrosynthesis in combination with a chiral Ni catalyst leading to an intermolecular alkylation reaction in good yields with excellent enantioselectivities (up to 97 % ee). Mechanistic studies suggest that the Lewis‐acid‐bound radical intermediate from a single‐electron anodic oxidation selectively reacts with the benzylic radical species to generate the desired adducts.     

Formation Mechanism of NF4+ Salts and Extraordinary Enhancement of the Oxidizing Power of Fluorine by Strong Lewis Acids

Although the existence of the NF₄⁺ cation has been known for 51 years, and its formation mechanism from NF₃ , F₂ , and a strong Lewis acid in the presence of an activation energy source had been studied extensively, the mechanism had not been established. Experimental evidence had shown that the first step involves the generation of F atoms from F₂ , and also that the NF₃⁺ cation is a key intermediate. However, it was not possible to establish whether the second step involved the reaction of a F atom with either NF₃ or the Lewis acid (LA). To distinguish between these two alternatives, a computational study of the NF₄ , SbF₆ , AsF₆ , and BF₄ radicals was carried out. Whereas the heats of reaction are small and similar for the NF₄ and LAF radicals, at the reaction temperatures, only the LAF radicals possess sufficient thermal stability to be viable species. Most importantly, the ability of the LAF radicals to oxidize NF₃ to NF₃⁺ demonstrates that they are extraordinary oxidizers. This extraordinary enhancement of the oxidizing power of fluorine with strong Lewis acids had previously not been fully recognized.     

Enantioselective Direct Mannich‐Type Reactions Catalyzed by Frustrated Lewis Acid/Brønsted Base Complexes

An enantioselective direct Mannich‐type reaction catalyzed by a sterically frustrated Lewis acid/Brønsted base complex is disclosed. Cooperative functioning of the chiral Lewis acid and achiral Brønsted base components gives rise to in situ enolate generation from monocarbonyl compounds. Subsequent reaction with hydrogen‐bond‐activated aldimines delivers β‐aminocarbonyl compounds with high enantiomeric purity.     

Oxidative‐Insertion Reactivity Across a Geometrically Constrained Metal→Borane Interaction

While interest in cooperative reactivity of transition metals and Lewis acids is receiving significant attention, the scope of known reactions that directly exploit the polarized reverse‐dative σ‐bond of metal‐borane complexes (i.e., M → BR₃) remains limited. Described herein is that the platinum (boryl)iminomethane (BIM) complex [Pt(κ²‐N,B‐^Cy2BIM)(CNAr^Dipp2)] can effect the oxidative insertion of a range of unsaturated organic substrates, including azides, isocyantes, and nitriles, as well as CO₂ and elemental sulfur (S₈). In addition, alkyl migration processes available to the BIM framework allow for post‐insertion reaction sequences resulting in product release from the metal center.     

Selective Metal‐free HB(C6F5)2 Catalyzed Allene Cyclotrimerization: Formation of 1,3,5‐Trimethylenecyclohexane and Its Tris‐hydroboration Product

Allene is cyclotrimerized under metal‐free conditions with the borane HB(C₆F₅)₂ catalyst to selectively give 1,3,5‐trimethylenecyclohexane ( 3 a ). Three‐fold hydroboration of the 1,3,5‐cyclotrimer with Piers’ borane gives the all‐cis 1,3,5‐CH₂B(C₆F₅)₂ substituted cyclohexane product 14 .     

Visible‐Light‐Mediated Metal‐Free Hydrosilylation of Alkenes through Selective Hydrogen Atom Transfer for Si−H Activation

Although there has been significant progress in the development of transition‐metal‐catalyzed hydrosilylations of alkenes over the past several decades, metal‐free hydrosilylation is still rare and highly desirable. Herein, we report a convenient visible‐light‐driven metal‐free hydrosilylation of both electron‐deficient and electron‐rich alkenes that proceeds through selective hydrogen atom transfer for Si−H activation. The synergistic combination of the organophotoredox catalyst 4CzIPN with quinuclidin‐3‐yl acetate enabled the hydrosilylation of electron‐deficient alkenes by selective Si−H activation while the hydrosilylation of electron‐rich alkenes was achieved by merging photoredox and polarity‐reversal catalysis.