Spontaneous Reduction of a Hydroborane To Generate a B−B Single Bond by the Use of a Lewis Pair

The ansa‐aminohydroborane 1‐NMe₂‐2‐(BH₂)C₆H₄ crystallizes in an unprecedented type of dimer containing a B?H bond activated by one FLP moiety. Upon mild heating and without the use of any catalyst, this molecule liberates one equivalent of hydrogen to generate a diborane molecule. The synthesis and structural characterization of these new compounds, as well as the kinetic monitoring of the reaction and the DFT investigation of its mechanism, are reported.     

A Highly Reactive Geminal P/B Frustrated Lewis Pair: Expanding the Scope to C−X (X=Cl,Br) Bond Activation

The geminal frustrated Lewis pair tBu₂PCH₂B(Fxyl)₂ ( 1 ; Fxyl=3,5‐(CF₃)₂C₆H₃) is accessible in 65 % yield from tBu₂PCH₂Li and (Fxyl)₂BF. According to NMR spectroscopy and X‐ray crystallography, 1 is monomeric both in solution and in the solid state. The intramolecular P ??? B distance of 2.900(5) Å and the full planarity of the borane site exclude any significant P/B interaction. Compound 1 readily activates a broad variety of substrates including H₂, EtMe₂SiH, CO₂/CS₂, Ph₂CO, and H₃CCN. Terminal alkynes react with heterolysis of the C?H bond. Haloboranes give cyclic adducts with strong P?BX₃ and weak R₃B?X bonds. Unprecedented transformations leading to zwitterionic XP/BCX₃ adducts occur on treatment of 1 with CCl₄ or CBr₄ in Et₂O. In less polar solvents (C₆H₆, n‐pentane), XP/BCX₃ adduct formation is accompanied by the generation of significant amounts of XP/BX adducts. FLP 1 catalyzes the hydrogenation of PhCH=NtBu and the hydrosilylation of Ph₂CO with EtMe₂SiH.     

N‐Methylacridinium Salts: Carbon Lewis Acids in Frustrated Lewis Pairs for σ‐Bond Activation and Catalytic Reductions

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.     

Functionalization of Intramolecular Frustrated Lewis Pairs by 1,1‐Carboboration with Conjugated Enynes

The vicinal P/B frustrated Lewis pair (FLP) Mes₂PCH₂CH₂B(C₆F₅)₂ undergoes 1,1‐carboboration reactions with the Me₃Si‐substituted enynes to give ring‐enlarged functionalized C₃‐bridged P/B FLPs. These serve as active FLPs in the activation of dihydrogen to give the respective zwitterionic [P]H⁺/[B]H^? products. One such product shows activity as a metal‐free catalyst for the hydrogenation of enamines or a bulky imine. The ring‐enlarged FLPs contain dienylborane functionalities that undergo “bora‐Nazarov”‐type ring‐closing rearrangements upon photolysis. A DFT study had shown that the dienylborane cyclization of such systems itself is endothermic, but a subsequent C₆F₅ migration is very favorable. Furthermore, substituted 2,5‐dihydroborole products are derived from cyclization and C₆F₅ migration from the photolysis reaction. In the case of the six‐membered annulation product, a subsequent stereoisomerization reaction takes place and the resultant compound undergoes a P/B FLP 1,2‐addition reaction with a terminal alkyne with rearrangement.     

Cooperative Lewis Pairs Based on Late Transition Metals: Activation of Small Molecules by Platinum(0) and B(C6F5)3

A Lewis basic platinum(0)–CO complex supported by a diphosphine ligand and B(C₆F₅)₃ act cooperatively, in a manner reminiscent of a frustrated Lewis pair, to activate small molecules such as hydrogen, CO₂, and ethene. This cooperative Lewis pair facilitates the coupling of CO and ethene in a new way.     

Activation of Carbon Dioxide by Silyl Triflate‐Based Frustrated Lewis Pairs

Silyl triflates of the form R_4?nSi(OTf)_n (n=1, 2; OTf=OSO₃CF₃) are shown to activate carbon dioxide when paired with bulky alkyl‐substituted Group 15 bases. Combinations of silyl triflates and 2,2,6,6‐tetramethylpiperidine react with CO₂ to afford silyl carbamates via a frustrated Lewis pair‐type mechanism. With trialkylphosphines, the silyl triflates R₃Si(OTf) reversibly bind CO₂ affording [R′₃P(CO₂)SiR₃][OTf] whereas when Ph₂Si(OTf)₂ is used one or two molecules of CO₂ can be sequestered. The latter bis‐CO₂ product is favoured at low temperatures and by excess phosphine.     

Exploring the Reactivity of a Frustrated Sn/P Lewis Pair: The Highly Selective Complexation of the cis-Azobenzene Photoisomer

The reactivity of the geminal frustrated Lewis pair (FLP) (F₅C₂)₃SnCH₂P(tBu)₂ ( 1 ) was explored by reacting it with a variety of small molecules (PhOCN, PhNCS, PhCCH, tBuCCH, H₃CC(O)CH=CH₂, Ph[C(O)]₂Ph, PhN=NPh and Me₃SiCHN₂), featuring polar or non-polar multiple bonds and/or represent α,β-unsaturated systems. While most adducts are formed readily, the binding of azobenzene requires UV-induced photoisomerization, which results in the highly selective complexation of cis-azobenzene. In the case of benzil, the reaction does not lead to the expected 1,2- or 1,4-addition products, but to the non-stereoselective (tBu)₂PCH₂-transfer to a prochiral keto function of benzil. All adducts of 1 were characterised by means of multinuclear NMR spectroscopy, elemental analyses and X-ray diffraction experiments.     

Homolytic Cleavage of a B−B Bond by the Cooperative Catalysis of Two Lewis Bases: Computational Design and Experimental Verification

Density functional theory (DFT) investigations revealed that 4‐cyanopyridine was capable of homolytically cleaving the B?B σ bond of diborane via the cooperative coordination to the two boron atoms of the diborane to generate pyridine boryl radicals. Our experimental verification provides supportive evidence for this new B?B activation mode. With this novel activation strategy, we have experimentally realized the catalytic reduction of azo‐compounds to hydrazine derivatives, deoxygenation of sulfoxides to sulfides, and reduction of quinones with B₂(pin)₂ at mild conditions.     

An Adduct of Sulfur Monoxide to a Frustrated Sn/P Lewis Pair

The geminal frustrated Lewis pair (F₅C₂)₃SnCH₂P(tBu)₂ ( 1 ) reacted with N‐sulfinylaniline PhNSO to afford the first sulfur monoxide adduct of a main group metal, (F₅C₂)₃SnCH₂P(tBu)₂?SO ( 2 ), which contains a SnCPSO ring. The second product is a phenylnitrene adduct of 1 . The surprising stability of 2 was compared with the stabilities of the so far inaccessible O₂ and S₂ adducts of 1 . Attempts to prepare these from 1 and the elemental chalcogens (O₂, S₈, Se_∞, Te_∞) led to four‐membered SnCPE ring systems. Quantum‐chemical investigations of 2 demonstrate the bond polarity of the SO unit to stabilize 2 .     

A P−H Functionalized Al/P Frustrated Lewis Pair: Substrate Activation and Selective Hydrogen Transfer

Hydroalumination of an alkynylphosphine gave an unprecedented P?H functionalized frustrated Lewis pair (FLP). The reactive P?H group does not influence the typical FLP properties, but the activation of substrates follows a new reaction pattern involving hydrogen transfer to yield unusual compounds with phosphaurea, iminophosphine, or phosphanyltriazene structural motifs.     

Dihydrogen Activation with a Neutral,Intermolecular Silicon(IV)-Amine Frustrated Lewis Pair

The heterolytic cleavage of dihydrogen constitutes the hallmark reaction of frustrated Lewis pairs (FLP). While being well-established for planar Lewis acids, such as boranes or silylium ions, the observation of the primary H₂ splitting products with non-planar Lewis acid FLPs remained elusive. In the present work, we report bis(perfluoro-N-phenyl-ortho-amidophenolato)silane and its application in dihydrogen activation to a fully characterized hydridosilicate. The strict design of the Lewis acid, the limited selection of the Lewis base, and the distinct reaction conditions emphasize the narrow tolerance to achieve this fascinating process with a tetrahedral Lewis acid.     

Accessing Frustrated Lewis Pair Chemistry from a Spectroscopically Stable and Classical Lewis Acid‐Base Adduct

B(C₆F₅)₃ and P(MeNCH₂CH₂)₃N form a classical Lewis adduct, (C₆F₅)₃BP(MeNCH₂CH₂)₃N. Although (C₆F₅)₃BP(MeNCH₂CH₂)₃N does not exhibit spectroscopic evidence of dissociation into its constituent acid and base, products of frustrated Lewis pair (FLP) addition reactions are seen with PhNCO, PhCH₂N₃, PhNSO, and CO₂. Computational studies show that thermal access to the dissociated acid and base permits FLP reactivity to proceed. These results demonstrate that FLP reactivity extends across the entire continuum of equilibria governing Lewis acid‐base adducts.     

Rational Design of a Carbon‐Boron Frustrated Lewis Pair for Metal‐free Dinitrogen Activation

Molecular nitrogen (N₂) is abundant in the atmosphere and, found in many biomolecules, an essential element of life. The Haber–Bosch process, developed over 100 years ago, requires relatively harsh conditions to activate N₂ on the iron surface and generate ammonia for use as fertilizer or to produce other chemicals, leading to consumption of more than 2 % of the world's annual energy supply. Thus, developing “green” approaches for N₂ activation under mild conditions is particularly important and urgent. Here we demonstrate that a metal‐free N₂ activation could be favorable both thermodynamically and kinetically (with an activation energy as low as 9.1 kcal mol^?1) by using a carbon‐boron formal frustrated Lewis pair, which is supported by high‐level coupled cluster calculations. Mechanistic studies reveal that aromaticity plays a crucial role in stabilizing both the transition state and the product. Our findings highlight the importance of a combination of an N‐heterocyclic carbene with a methyleneborane unit in metal‐free N₂ activation, providing conceptual guidance for experimental realization.     

Exploring the Reactivity of B-Connected Carboranylphosphines in Frustrated Lewis Pair Chemistry: A New Frame for a Classic System

The primary phosphines MesPH₂ and tBuPH₂ react with 9-iodo-m-carborane yielding B9-connected secondary carboranylphosphines 1,7-H₂C₂B₁₀H₉-9-PHR (R=2,4,6-Me₃C₆H₂ (Mes; 1 a ), tBu ( 1 b )). Addition of tris(pentafluorophenyl)borane (BCF) to 1 a , b resulted in the zwitterionic compounds 1,7-H₂C₂B₁₀H₉-9-PHR(p-C₆F₄)BF(C₆F₅)₂ ( 2 a , b ) through nucleophilic para substitution of a C₆F₅ ring followed by fluoride transfer to boron. Further reaction with Me₂SiHCl prompted a H−F exchange yielding the zwitterionic compounds 1,7-H₂C₂B₁₀H₉-9-PHR(p-C₆F₄)BH(C₆F₅)₂ ( 3 a , b ). The reaction of 2 a , b with one equivalent of R'MgBr (R’=Me, Ph) gave the extremely water-sensitive frustrated Lewis pairs 1,7-H₂C₂B₁₀H₉-9-PR(p-C₆F₄)B(C₆F₅)₂ ( 4 a , b ). Hydrolysis of the B−C₆F₄ bond in 4 a , b gave the first tertiary B-carboranyl phosphines with three distinct substituents, 1,7-H₂C₂B₁₀H₉-9-PR(p-C₆F₄H) ( 5 a , b ). Deprotonation of the zwitterionic compounds 2 a , b and 3 a , b formed anionic phosphines [1,7-H₂C₂B₁₀H₉-9-PR(p-C₆F₄)BX(C₆F₅)₂]⁻[DMSOH]⁺ (R=Mes, X=F ( 6 a ), R=tBu, X=F ( 6 b ); R=Mes, X=H ( 7 a ), R=tBu, X=H ( 7 b )). Reaction of 2 a , b with an excess of Grignard reagents resulted in the addition of R’ at the boron atom yielding the anions [1,7-H₂C₂B₁₀H₉-9-PR(p-C₆F₄)BR’(C₆F₅)₂]⁻ (R=Mes, R’=Me ( 8 a ), R=tBu, R’=Me ( 8 b ); R=Mes, R’=Ph ( 9 a ), R=tBu, R’=Ph ( 9 b )) with [MgBr(Et₂O)_n]⁺ as counterion. The ability of the zwitterionic compounds 3 a , b to hydrogenate imines as well as the Brønsted acidity of 3 a were investigated.     

空间受阻型Lewis酸碱对在小分子活化中的应用

作为一种新型的非金属有机催化剂,空间受阻型路易斯酸碱对由于具有的独特化学反应性,近年来在有机小分子活化和催化加氢领域的研究非常活跃。本文总结了近年来国外发表的有关空间受阻型路易斯酸碱对的研究报道,从其结构特征、有机小分子的活化、H₂活化机理的探讨及在催化领域应用的研究成果4个方面进行了详细的归纳整理。空间受阻型路易斯酸碱对对有机小分子的活化作用来源于其“分子间静电相互作用力”。这个在有机化学中早已熟知的概念却在2006年才被发现可活化有机小分子,并在之后受到广泛的关注。空间受阻型路易斯酸碱对化学的发展也扩展了有机化学反应研究的范畴,继续深入研究这种“分子间静电相互作用力”在其他未知领域的应用必将带来更大的发现。然而目前国内从事相关领域研究的报道很少,因此希望本文能够引起国内更多科研工作者对这个新兴领域产生研究兴趣。