Recent Advances in Group 14 and 15 Lewis Acids for Frustrated Lewis Pair Chemistry

Frustrated Lewis pairs (FLP) which rely on the cooperative action of Lewis acids and Lewis bases, played a prominent role in the advancement of main-group catalysis. While the early days of FLP chemistry witnessed the dominance of boranes, there is a growing body of reports on alternative Lewis acids derived from groups 14 and 15. This short review focuses on the discovery of such non-boron candidates reported since 2015.

Boron-Catalyzed Hydroarylation of 1,3-Dienes with Arylamines

Catalytic hydroarylation reactions of conjugated dienes are achieved using tris(pentafluorophenyl)borane as a Lewis acid catalyst under mild reaction conditions. This new protocol shows a broad substrate scope for the highly regioselective functionalization of sterically hindered aniline derivatives. Experimental and extensive density functional theory mechanistic studies show that the complex of residual water and B(C₆F₅)₃ plays a crucial role in the aryl-assisted protonation of conjugated dienes, forming allyl cation intermediates that induce the facile electrophilic aromatic substitution of aniline substrates.

Modular Synthesis of 9,10-Dihydroacridines through an ortho-C Alkenylation/Hydroarylation Sequence between Anilines and Aryl Alkynes in Hexafluoroisopropanol

9,10-Dihydroacridines are frequently encountered as key scaffolds in OLEDs. However, accessing those compounds from feedstock precursors typically requires multiple steps. Herein, a modular one-pot synthesis of 9,10-dihydroacridine frameworks is achieved through a reaction sequence featuring a selective ortho-C alkenylation of diarylamines with aryl alkynes followed by an intramolecular hydroarylation of the olefin formed as an intermediate. This transformation was accomplished by virtue of the combination of hexafluoroisopropanol and triflimide as a catalyst that triggers the whole process.

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.

Frustrated Lewis pair-catalyzed double hydroarylation of alkynes with N-substituted pyrroles

Metal-free hydroarylation of alkynes with N-substituted pyrroles is shown to be efficiently mediated by B(C₆F₅)₃ to yield variants of dipyrrole methanes. The mechanism is shown to proceed via an FLP addition pathway.

Carbonyl and olefin hydrosilylation mediated by an air-stable phosphorus(iii) dication under mild conditions

The readily-accessible, air-stable Lewis acid [(terpy)PPh][B(C6F5)4]21 is shown to mediate the hydrosilylation of aldehydes, ketones, and olefins. The utility and mechanism of these hydrosilylations are considered.

Phosphorous(v) Lewis acids: water/base tolerant P3-trimethylated trications

The water/base intolerance of the previously reported electrophilic phosphonium cations has been overcome by replacing the labile electron-withdrawing groups generally attached to phosphorus (e.g. -F, -OAr, -CF3) with methyl groups. Tri-phosphorus(v) tricationic species, accessible in one-pot from commercially available materials, are air and water/base tolerant, yet are sufficiently Lewis acidic for catalysis.

Nitrogen-Based Lewis Acids Derived from Phosphonium Diazo Cations

Reaction of PPh₃ and [(p-ClC₆ H₄ )N₂ ][BF₄ ] affords [(p-ClC₆ H₄ )N(PPh₃ )N(PPh₃ )][BF₄ ] 1, while reaction with (Ph₂ PCH₂ )₂ gave [(p-ClC₆ H₄ )(NPh₂ PCH₂ )₂ )][BF₄ ] 2. These species confirm the Lewis acidity of [(p-ClC₆ H₄ )N₂ (PR₃ )][BF₄ ] cations at N. In contrast, use of bulky phosphines afford the species [ArN₂ (PR₃ )][BF₄ ] (R=tBu 3, Mes 4). Compound 3 undergoes one electron reduction to give the stable radical [(p-ClC₆ H₄ )N₂ (PtBu₃ )]^. 5. Combination of 3 and PtBu₃ acts as an FLP to effect (SPh)₂ cleavage, generating [PhSPtBu₃ ]⁺ and the radical [ArN₂ (PR₃ )]^. . Collectively, these data affirm the ability of the cations [ArN₂ (PR₃ )]⁺ to behave as one or two electron acceptors.

The global electrophilicity index as a metric for Lewis acidity

The global electrophilicity index, GEI, is used as a general, quantitative and base-independent metric of Lewis acidity. This parameter has been benchmarked against the established fluoride ion affinity, FIA, for a range of 22 neutral and cationic Lewis acids from across the p-block, including boranes, trityl derivatives, phosphonium cations and sulfoxonium cations. As a demonstration of utility, the GEI is used to catalogue a library of fluoroaryl boranes as it provides a rapid tool for the comparison of a series of Lewis acids featuring peripheral substituent variation.

Probing steric influences on electrophilic phosphonium cations: a comparison of [(3,5-(CF3)2C6H3)3PF]+ and [(C6F5)3PF]+

The electrophilic phosphonium cation (EPC) salt [(3,5-(CF₃)₂C₆H₃)₃PF][B(C₆F₅)₄] (2) can display catalytic activity greater than its thermodynamic acidity would suggest. The role of steric factors is explored.

Air- and water-stable Lewis acids: synthesis and reactivity of P-trifluoromethyl electrophilic phosphonium cations

Electrophilic phosphonium cations (EPCs) containing a –CF₃ group are stable to air, water, alcohol and strong Brønsted acid and function as Lewis acid catalysts without requiring anhydrous reaction conditions.