Mechanistic aspects of bond activation with perfluoroarylboranes

Zintl Clusters as a Platform for Lewis Acid Catalysis

Clusters of the main group elements phosphorus and arsenic, commonly categorized as Zintl clusters, have been known for over a century. And, only now is the application of these systems as catalysts for organic synthesis being investigated. In this work, boranes are tethered via an aliphatic linker to Zintl-based clusters and their Lewis acidity is examined experimentally, by the Gutmann-Beckett test and competency in the hydroborative reduction of six organic substrates, as well as computationally, by fluoride ion affinity and hydride ion affinity methods. The effects of tuning the aliphatic linker length, substituents at the boron, and changing the cluster from a seven-atom phosphorus system to a seven-atom arsenic system on reactivity are studied.

Ring Opening Reduction of Cyclic Anhydrides Catalyzed by Tris(pentafluorophenyl)borane Using Hydrosilanes as a Hydride Source

Hydrosilanes are widely used as reducing agents in the reduction of carbonyl groups, and various catalysts have been developed for the activation of hydrosilanes, the majority of them being transition metal-based. A main-group-based Lewis acid tris(pentafluorophenyl)borane (BCF) has gained increasing attention due to its Lewis acidity and versatility, along with being nonmetal. Herein, we describe the BCF-catalyzed ring opening reduction of cyclic anhydrides using hydrosilanes as a source of hydrides. The reduction affords unsymmetrical bis(silyl) protected hydroxy acids, leading to an efficient way for the synthesis of silyl ester functionalities. The capability of forming protected hydroxy acids under mild conditions with high yields in one step is also advantageous. A range of hydrosilanes and cyclic anhydrides can be employed with quantitative conversion, high yields, relatively fast reaction time, and mild reaction conditions. NMR spectroscopy is used in the characterization of the products, along with gaining insight into the potential mechanism.

Single electron reduction of NHC-CO2-borane compounds

The carbon dioxide radical anion [CO2˙-] is a highly reactive species of fundamental and synthetic interest. However, the direct one-electron reduction of CO2 to generate [CO2˙-] occurs at very negative reduction potentials, which is often a limiting factor for applications. Here, we show that NHC-CO2-BR3 species - generated from the Frustrated Lewis Pair (FLP)-type activation of CO2 by N-heterocyclic carbenes (NHCs) and boranes (BR3) - undergo single electron reduction at a less negative potential than free CO2. A net gain of more than one volt was notably measured with a CAAC-CO2-B(C6F5)3 adduct, which was chemically reduced to afford [CAAC-CO2-B(C6F5)3˙-]. This room temperature stable radical anion was characterized by EPR spectroscopy and by single-crystal X-ray diffraction analysis. Of particular interest, DFT calculations showed that, thanks to the electron withdrawing properties of the Lewis acid, significant unpaired spin density is localised on the carbon atom of the CO2 moiety. Finally, these species were shown to exhibit analogous reactivity to the carbon dioxide radical anion [CO2˙-] toward DMPO. This work demonstrates the advantage provided by FLP systems in the generation and stabilization of [CO2˙-]-like species.

Synthesis and Properties of Boron-Containing Heteromerous Bistricyclic Aromatic Enes: Structural Effects on Thermodynamic Stability and Photoreactivity

Overcrowded bistricyclic aromatic enes (BAEs) have several conformations such as twisted and anti-folded conformers, and their stereochemistry and chromism have been studied in earnest. In this study, boron-containing heteromerous BAEs having various tricyclic structures were synthesized and their photophysical properties investigated. Single-crystal X-ray analysis revealed that the introduction of a rigid fluorene unit resulted in a twisted conformer, whereas the introduction of flexible units such as thioxanthene and 9,9-dimethyl-9,10-dihydroanthracene units resulted in an anti-folded conformer. The absorption spectra of the heteromerous BAEs were dependent on the introduced tricyclic structures, suggesting the immense impact of the tricyclic structures on the electronic structures of BAEs. DFT calculations revealed the large effect of the flexibility of the tricyclic structures on the thermodynamic stability of the conformers. In addition, the boron-containing heteromerous BAEs underwent photocyclization reactions, indicating their potential application as precursors of polyaromatic hydrocarbons and helical aromatic materials.

"Activated Borane": A Porous Borane Cluster Polymer as an Efficient Lewis Acid-Based Catalyst

Borane cluster-based porous covalent networks, named activated borane (ActB), were prepared by cothermolysis of decaborane(14) (nido-B10H14) and selected hydrocarbons (toluene, ActB-Tol; cyclohexane, ActB-cyHx; and n-hexane, ActB-nHx) under anaerobic conditions. These amorphous solid powders exhibit different textural and Lewis acid (LA) properties that vary depending on the nature of the constituent organic linker. For ActB-Tol, its LA strength even approaches that of the commonly used molecular LA, B(C6F5)3. Most notably, ActBs can act as heterogeneous LA catalysts in hydrosilylation/deoxygenation reactions with various carbonyl substrates as well as in the gas-phase dehydration of ethanol. These studies reveal the potential of ActBs in catalytic applications, showing (a) the possibility for tuning catalytic reaction outcomes (selectivity) in hydrosilylation/deoxygenation reactions by changing the material's composition and (b) the very high activity toward ethanol dehydration that exceeds the commonly used γ-Al2O3 by achieving a stable conversion of ∼93% with a selectivity for ethylene production of ∼78% during a 17 h continuous period on stream at 240 °C.

Quest for Active Species in Al/B-Catalyzed CO2 Hydrosilylation

We demonstrate the catalytic role of aluminum and boron centers in aluminum borohydride [(2-Me2CH2C6H4)(C6H5)Al(μ-H)2B(C6H5)2] (6) during carbon dioxide (CO2) hydrosilylation. Preliminary investigations into CO2 reduction using [(2-Me2NCH2C6H4)(H)Al(μ-H)]2 (1) and [Ph3C][B(3,5-C6H3Cl2)4] (2) in the presence of Et3SiH and PhSiH3 resulted in CH2(OSiR3)2 and CH3OSiR3, which serve as formaldehyde and methanol surrogates, respectively. In pursuit of identifying the active catalytic species, three compounds, B(3,5-C6H3Cl2)3 (3), [(2-Me2NCH2C6H4)(3,5-C6H3Cl2)Al(μ-H)2B(3,5-C6H3Cl2)2] (4), and [(2-Me2NCH2C6H4)2Al(THF)][B(3,5-C6H3Cl2)4] (5), were isolated. Among compounds 2-5, the highest catalytic conversion was achieved by 4. Further, 4 and 6 were prepared in a straightforward method by treating 1 with 3 and BPh3, respectively. 6 was found to be in equilibrium with 1 and BPh3, thus making the catalytic process of 6 more efficient than that of 4. Computational investigations inferred that CO2 reduction occurs across the Al-H bond, while Si-H activation occurs through a concerted mechanism involving an in situ generated aluminum formate species and BPh3.

Binding, Sensing, And Transporting Anions with Pnictogen Bonds: The Case of Organoantimony Lewis Acids

Motivated by the discovery of main group Lewis acids that could compete or possibly outperform the ubiquitous organoboranes, several groups, including ours, have engaged in the chemistry of Lewis acidic organoantimony compounds as new platforms for anion capture, sensing, and transport. Principal to this approach are the intrinsically elevated Lewis acidic properties of antimony, which greatly favor the addition of halide anions to this group 15 element. The introduction of organic substituents to the antimony center and its oxidation from the + III to the + V state provide for tunable Lewis acidity and a breadth of applications in supramolecular chemistry and catalysis. The performances of these antimony-based Lewis acids in the domain of anion sensing in aqueous media illustrate the favorable attributes of antimony as a central element. At the same time, recent advances in anion binding catalysis and anion transport across phospholipid membranes speak to the numerous opportunities that lie ahead in the chemistry of these unique main group compounds.

Frustrated Radical Pairs in Organic Synthesis

Frustrated radical pairs (FRPs) describe the phenomenon that two distinct radicals─which would otherwise annihilate each other to form a closed-shell covalent adduct─can coexist in solution, owing to steric repulsion or weak bonding association. FRPs are typically formed via spontaneous single-electron transfer between two sterically encumbered precursors─an oxidant and a reductant─under ambient conditions. The two components of a FRP exhibit orthogonal chemical properties and can often act in cooperativity to achieve interesting radical reactivities. Initially observed in the study of traditional frustrated Lewis pairs, FRPs have recently been shown to be capable of homolytically activating various chemical bonds. In this Perspective, we will discuss the discovery of FRPs, their fundamental reactivity in chemical bond activation, and recent developments of their use in synthetic organic chemistry, including in C-H bond functionalization. We anticipate that FRPs will provide new reaction strategies for solving challenging problems in modern organic synthesis.

Insights into Single-Electron-Transfer Processes in Frustrated Lewis Pair Chemistry and Related Donor-Acceptor Systems in Main Group Chemistry

The activation and utilization of substrates mediated by Frustrated Lewis Pairs (FLPs) was initially believed to occur solely via a two-electron, cooperative mechanism. More recently, the occurrence of a single-electron transfer (SET) from the Lewis base to the Lewis acid was observed, indicating that mechanisms that proceed via one-electron-transfer processes are also feasible. As such, SET in FLP systems leads to the formation of radical ion pairs, which have recently been more frequently observed. In this review, we aim to discuss the seminal findings regarding the recently established insights into the SET processes in FLP chemistry as well as highlight examples of this radical formation process. In addition, applications of reported main group radicals will also be reviewed and discussed in the context of the understanding of SET processes in FLP systems.

Regioselective aliphatic C-H functionalization using frustrated radical pairs

Frustrated Lewis pairs (FLPs) are well documented for the activation of small molecules such as dihydrogen and carbon dioxide1-4. Although canonical FLP chemistry is heterolytic in nature, recent work has shown that certain FLPs can undergo single-electron transfer to afford radical pairs5. Owing to steric encumbrance and/or weak bonding association, these radicals do not annihilate one another, and they have thus been named frustrated radical pairs (FRPs). Notable preliminary results suggest that FRPs may be useful reagents in chemical synthesis6-8, although their applications remain limited. Here we demonstrate that the functionalization of C(sp3)-H bonds can be accomplished using a class of FRPs generated from disilazide donors and an N-oxoammonium acceptor. Together, these species undergo single-electron transfer to generate a transient and persistent radical pair capable of cleaving unactivated C-H bonds to furnish aminoxylated products. By tuning the structure of the donor, it is possible to control regioselectivity and tailor reactivity towards tertiary, secondary or primary C-H bonds. Mechanistic studies lend strong support for the formation and involvement of radical pairs in the target reaction.

Examining the reactivity of tris(ortho-carboranyl)borane with Lewis bases and application in frustrated Lewis pair Si-H bond cleavage

Reactions of tris(ortho-carboranyl)borane with Lewis bases reveals only small bases bind. The tremendous bulk and Lewis acidity is leveraged in frustrated Lewis pair Si-H cleavage with a wider range of Lewis bases and greater efficacy than B(C₆F₅)₃.

Amides as modifiable directing groups in electrophilic borylation

Amide directed C-H borylation using ≥two equiv. of BBr3 forms borenium cations containing a R2N(R')C[double bond, length as m-dash]O→B(Ar)Br unit which has significant Lewis acidity at the carbonyl carbon. This enables reduction of the amide unit to an amine using hydrosilanes. This approach can be applied sequentially in a one-pot electrophilic borylation-reduction process, which for phenyl-acetylamides generates ortho borylated compounds that can be directly oxidised to the 2-(2-aminoethyl)-phenol. Other substrates amenable to the C-H borylation-reduction sequence include mono and diamino-arenes and carbazoles. This represents a simple method to make borylated molecules that would be convoluted to access otherwise (e.g. N-octyl-1-BPin-carbazole). Substituent variation is tolerated at boron as well as in the amide unit, with diarylborenium cations also amenable to reduction. This enables a double C-H borylation-reduction-hydrolysis sequence to access B,N-polycyclic aromatic hydrocarbons (PAHs), including an example where both the boron and nitrogen centres contain functionalisable handles (N-H and B-OH). This method is therefore a useful addition to the metal-free borylation toolbox for accessing useful intermediates (ArylBPin) and novel B,N-PAHs.

Structure-Reactivity Relationships in Borane-Based FLP-Catalyzed Hydrogenations, Dehydrogenations, and Cycloisomerizations

ConspectusThe activation of molecular hydrogen by main-group element catalysts is an extremely important approach to metal-free hydrogenations. These so-called frustrated Lewis pairs advanced within a short period of time to become an alternative to transition metal catalysis. However, deep understanding of the structure-reactivity relationship is far less developed compared to that of transition metal complexes, although it is paramount for advancing frustrated Lewis pair chemistry.In this Account, we provide detailed insight into how Lewis acidity and Lewis basicity correlate to reactivity. The reactivity of frustrated Lewis pairs will be systematically discussed in context with selected reactions. The influence of major electronic modifications of the Lewis pairs is correlated with the ability to activate molecular hydrogen, to channel reaction kinetics and reaction pathways, or to achieve C(sp³)-H activations.First, we will describe how we entered this emerging field of research after quickly realizing that information was lacking on how the reactivity changes with modification of the frustrated Lewis pair. This led us to the development of a qualitative and quantitative structure-reactivity relationship in metal-free imine hydrogenations. The imine hydrogenation was utilized as the model reaction to experimentally determine the activation parameters of the FLP-mediated hydrogen activation for the first time. This kinetic study revealed autoinduced catalytic profiles when Lewis acids weaker than tris(pentafluorophenyl)borane were applied, opening up to study the Lewis base dependency within one system. With this knowledge of the interplay between Lewis acid strength and Lewis basicity, we developed methods for the hydrogenation of densely functionalized nitroolefins, acrylates, and malonates. Here, the reduced Lewis acidity needed to be counterbalanced by a suitable Lewis base to ensure efficient hydrogen activation. The opposite measure was necessary for the hydrogenation of unactivated olefins. For these, comparably less electron-releasing phosphanes were required to generate strong Brønsted acids by hydrogen activation. These systems displayed highly reversible hydrogen activation even at temperatures as low as -60 °C. A systematic study of these systems enabled the development of acceptorless dehydrocouplings of amines with silanes and dehydrogenations of aza-heterocycles by C(sp³)-H activations. Furthermore, the C(sp³)-H and π-activation was utilized to achieve cycloisomerizations by carbon-carbon and carbon-nitrogen bond formations. Lastly, new frustrated Lewis pair systems featuring weak Lewis bases as active components in the hydrogen activation were developed for the reductive deoxygenation of phosphane oxides and carboxylic acid amides.

Calcium-catalysed synthesis of amines through imine hydrosilylation: an experimental and theoretical study

A method to reduce aldimines through hydrosilylation is reported. The catalytic system involves calcium triflimide (Ca(NTf₂)₂) and potassium hexafluorophosphate (KPF₆) which have been shown to act in a synergistic manner. The expected amines are obtained in fair to very high yields (40-99%) under mild conditions (room temperature in most cases). To illustrate the potential of this method, a bioactive molecule with antifungal properties was prepared on the gram scale and in high yield in environmentally friendly 2-methyltetrahydrofuran. Moreover, it is shown in this example that the imine can be prepared in situ from the aldehyde and the amine without isolating the imine. The mechanism involved has been explored experimentally and through DFT calculations, and the results are in accordance with an electrophilic activation of the silane by the calcium catalyst.

Hydrogen Activation by Frustrated and Not So Frustrated Lewis Pairs Based on Pyramidal Lewis Acid 9-Boratriptycene: A Computational Study

Structural features and reactivity of frustrated Lewis pairs (FLPs) formed by pyramidal group 13 Lewis acids based on 9-bora and 9-alatriptycene and bulky phosphines P ^t Bu₃, PPh₃, and PCy₃ are considered at the M06-2X/def2-TZVP level of theory. Classic FLP is formed only in the B(C₆Me₄)₃CH/P ^t Bu₃ system, while both FLP and donor-acceptor (DA) complex are observed in the B(C₆F₄)₃CF/P ^t Bu₃ system. Formation of DA complexes was observed in other systems; the B(C₆H₄)₃CH·P ^t Bu₃ complex features an elongated DA bond and can be considered a "latent" FLP. Transition states and reaction pathways for molecular hydrogen activation have been obtained. Processes of heterolytic hydrogen splitting are energetically more favored in solution compared to the gas phase, while activation energies in the gas phase and in solution are close. The alternative processes of hydrogenation of B-C or Al-C bonds in the source pyramidal Lewis acids in the absence of a Lewis base are exergonic but have larger activation energies than those for heterolytic hydrogen splitting. The tuning of Lewis acidity of 9-boratriptycene by changing the substituents allows one to control its reactivity with respect to hydrogen activation. Interestingly, the most promising system from the practical point of view is the DA complex B(C₆H₄)₃CH·P ^t Bu₃, which is predicted to provide both low activation energy and thermodynamic reversibility of the heterolytic hydrogen splitting process. It appears that such "not so frustrated" or "latent" FLPs are the best candidates for reversible heterolytic hydrogen splitting.

Evidence for the encounter complex in frustrated Lewis pair chemistry

Frustrated Lewis Pairs (FLPs) are combinations of bulky Lewis acids and bases that can carry out small-molecule activation and catalysis. Mechanistically, the reaction of the acid, base and substrate involves the collision of three distinct molecules, and so the pre-association of the acid and base to form an encounter complex has been proposed. This article will examine the evidence for the formation of this encounter complex, focusing on the archetypal main-group combinations P(^tBu)₃/B(C₆F₅)₃ and PMes₃/B(C₆F₅)₃ (Mes = mesityl), and includes quantum chemical calculations, molecular dynamics simulations, NMR spectroscopic measurements and neutron scattering. Furthermore, the recent discovery that the associated acid and base can absorb a photon to promote single-electron transfer has enabled the encounter complex to also be studied by UV-Vis spectroscopy, EPR spectroscopy, transient absorption spectroscopy, and resonance Raman spectroscopy. These data all support the notion that the encounter complex is only weakly held together and in low concentration in solution. The insights that these studies provide underpin the exciting transformations that can be promoted by FLPs. Finally, some observations and unanswered questions are provided to prompt further study in this field.

Ga+-catalyzed hydrosilylation? About the surprising system Ga+/HSiR3/olefin, proof of oxidation with subvalent Ga+ and silylium catalysis with perfluoroalkoxyaluminate anions

Already 1 mol% of subvalent [Ga(PhF)2]+[pf]- ([pf]- = [Al(ORF)4]-, RF = C(CF3)3) initiates the hydrosilylation of olefinic double bonds under mild conditions. Reactions with HSiMe3 and HSiEt3 as substrates efficiently yield anti-Markovnikov and anti-addition products, while bulkier substrates such as HSiiPr3 are less reactive. Investigating the underlying mechanism by gas chromatography and STEM analysis, we unexpectedly found that H2 and metallic Ga0 formed. Without the addition of olefins, the formation of R3Si-F-Al(ORF)3 (R = alkyl), a typical degradation product of the [pf]- anion in the presence of a small silylium ion, was observed. Electrochemical analysis revealed a surprisingly high oxidation potential of univalent [Ga(PhF)2]+[pf]- in weakly coordinating, but polar ortho-difluorobenzene of E 1/2(Ga+/Ga0; oDFB) = +0.26-0.37 V vs. Fc+/Fc (depending on the scan rate). Apparently, subvalent Ga+, mainly known as a reductant, initially oxidizes the silane and generates a highly electrophilic, silane-supported, silylium ion representing the actual catalyst. Consequently, the [Ga(PhF)2]+[pf]-/HSiEt3 system also hydrodefluorinates C(sp3)-F bonds in 1-fluoroadamantane, 1-fluorobutane and PhCF3 at room temperature. In addition, both catalytic reactions may be initiated using only 0.2 mol% of [Ph3C]+[pf]- as a silylium ion-generating initiator. These results indicate that silylium ion catalysis is possible with the straightforward accessible weakly coordinating [pf]- anion. Apparently, the kinetics of hydrosilylation and hydrodefluorination are faster than that of anion degradation under ambient conditions. These findings open up new windows for main group catalysis.

Hydrogen-Bridged Oligosilanylsilyl Mono- and Oligosilanylsilyl Dications

Hydrogen‐bridged oligosilanylsilyl borates 8 [B(C₆F₅)₄], 9[B(C₆F₅)₄] and diborates 10 [B(C₆F₅)₄]₂ have been prepared by hydride transfer between α‐ω‐dihydrido‐ (11) and branched tetrahydrido‐oligosilanes (13) and trityl cation. The obtained cyclic intramolecularly stabilized silylium ions 8, 9 and bissilylium ion 10 were characterized by low temperature NMR spectroscopy supported by the results of density functional calculations. The branched Si−H−Si monocation 9 undergoes at low temperatures a fast degenerate rearrangement, which exchanges the Si−H groups with a barrier of 31 kJ mol⁻¹ via an antarafacial transition state. Reaction of the branched monocation 9 with a second equivalent of trityl cation or of the branched oligosilane 13 with two equivalents of trityl cation, gives at −80 °C the corresponding bissilylium ion 10, an example for a new class of highly reactive poly‐Lewis acids.

Hydrosilylative reduction of carbon dioxide by a homoleptic lanthanum aryloxide catalyst with high activity and selectivity

An efficient tandem hydrosilylation of CO2, which uses a combination of a simple, homoleptic lanthanum aryloxide and B(C6F5)3, was performed. Use of a less sterically hindered silane led to an exclusive reduction of CO2 to CH4, with a turnover frequency of up to 6000 h-1 at room temperature. The catalytic system is robust, and 19 400 turnovers could be achieved with 0.005 mol% loading of lanthanum. The reaction outcome depended highly on the nature of the silane reductant used. Selective production of the formaldehyde equivalent, i.e., bis(silyl)acetal, without over-reduction, was observed when a sterically bulky silane was used. The reaction mechanism was elucidated by stoichiometric reactions and DFT calculations.

Systematic Design of a Frustrated Lewis Pair Containing Methyleneborane and Carbene for Dinitrogen Activation

Activation of atmospherically abundant dinitrogen (N₂) by metal-free species under mild reaction conditions has been one of the most challenging areas in chemistry for decades. Very recent but limited progress in N₂ activation by boron species, including two-coordinated borylene and methyleneborane and three-coordinated borole and borane, has been made toward metal-free N₂ activation. Here, we systematically probe an experimentally viable frustrated Lewis pair (FLP) containing two moieties (methyleneborane and carbene) for N₂ activation via density functional theory (DFT) calculations, which has proven to be an efficient approach for N₂ activation in a thermodynamically and kinetically favorable manner. Aromaticity is found to play a crucial role in stabilization of the product. This study could be a valuable alternative for the development of metal-free N₂ activation chemistry, highlighting great potential of FLP for N₂ activation and functionalization.

Site-Selective Csp3-Csp/Csp3-Csp2 Cross-Coupling Reactions Using Frustrated Lewis Pairs

The donor-acceptor ability of frustrated Lewis pairs (FLPs) has led to widespread applications in organic synthesis. Single electron transfer from a donor Lewis base to an acceptor Lewis acid can generate a frustrated radical pair (FRP) depending on the substrate and energy required (thermal or photochemical) to promote an FLP into an FRP system. Herein, we report the Csp3-Csp cross-coupling reaction of aryl esters with terminal alkynes using the B(C6F5)3/Mes3P FLP. Significantly, when the 1-ethynyl-4-vinylbenzene substrate was employed, the exclusive formation of Csp3-Csp cross-coupled products was observed. However, when 1-ethynyl-2-vinylbenzene was employed, solvent-dependent site-selective Csp3-Csp or Csp3-Csp2 cross-coupling resulted. The nature of these reaction pathways and their selectivity has been investigated by extensive electron paramagnetic resonance (EPR) studies, kinetic studies, and density functional theory (DFT) calculations both to elucidate the mechanism of these coupling reactions and to explain the solvent-dependent site selectivity.

Bulky Polyfluorinated Terphenyldiphenylboranes: Water Tolerant Lewis Acids

Protocols for the synthesis of the bulky polyfluorinated triarylboranes 2,6‐(C₆F₅)₂C₆F₃B(C₆F₅)₂ (1), 2,6‐(C₆F₅)₂C₆F₃B[3,5‐(CF₃)₂C₆H₃] (2), 2,4,6‐(C₆F₅)₃C₆H₂B(C₆F₅)₂ (3), 2,4,6‐(C₆F₅)₃C₆H₂B[3,5‐(CF₃)₂C₆H₃] (4) were developed. All boranes are water tolerant and according to the Gutmann‐Beckett method, 1–3 display Lewis acidities larger than that of the prominent B(C₆F₅)₃.

Reversible Silylium Transfer between P-H and Si-H Donors

The Mo=PR₂ π* orbital in a Mo phosphenium complex acts as acceptor in a new P^III -based Lewis superacid. This Lewis acid (LA) participates in electrophilic Si-H abstraction from E₃ SiH to give a Mo-bound secondary phosphine ligand, Mo-PR₂ H. The resulting Et₃ Si⁺ ion remains associated with the Mo complex, stabilized by η¹ -P-H donation, yet undergoes rapid exchange with an η¹ -Si-H adduct of free silane in solution. The equilibrium between these two adducts presents an opportunity to assess the role of this new LA in catalytic reactions of silanes: is the LA acting as a catalyst or as an initiator? Preliminary results suggest that a cycle including the Mo-bound phosphine-silylium adduct dominates in the catalytic hydrosilylation of acetophenone, relative to a putative cycle involving the silane-silylium adduct or "free" silylium.

Partnering a Three-Coordinate Gallium Cation with a Hydroborate Counter-Ion for the Catalytic Hydrosilylation of CO2

A novel β-diketiminate stabilized gallium hydride, (^Dipp L)Ga(Ad)H (where (^Dipp L)={HC(MeCDippN)₂ }, Dipp=2,6-diisopropylphenyl and Ad=1-adamantyl), has been synthesized and shown to undergo insertion of carbon dioxide into the Ga-H bond under mild conditions. In this case, treatment of the resulting κ¹ -formate complex with triethylsilane does not lead to regeneration of the hydride precursor. However, when combined with B(C₆ F₅ )₃ , (^Dipp L)Ga(Ad)H catalyses the reductive hydrosilylation of CO₂ . Under stoichiometric conditions, the addition of one equivalent of B(C₆ F₅ )₃ to (^Dipp L)Ga(Ad)H leads to the formation of a 3-coordinate cationic gallane complex, partnered with a hydroborate anion, [(^Dipp L)Ga(Ad)][HB(C₆ F₅ )₃ ]. This complex rapidly hydrometallates carbon dioxide and catalyses the selective reduction of CO₂ to the formaldehyde oxidation level at 60 °C in the presence of Et₃ SiH (yielding H₂ C(OSiEt₃ )₂ ). When catalysis is undertaken in the presence of excess B(C₆ F₅ )₃ , appreciable enhancement of activity is observed, with a corresponding reduction in selectivity: the product distribution includes H₂ C(OSiEt₃ )₂ , CH₄ and O(SiEt₃ )₂ . While this system represents proof-of-concept in CO₂ hydrosilylation by a gallium hydride system, the TOF values obtained are relatively modest (max. 10 h^-1 ). This is attributed to the strength of binding of the formatoborate anion to the gallium centre in the catalytic intermediate (^Dipp L)Ga(Ad){OC(H)OB(C₆ F₅ )₃ }, and the correspondingly slow rate of the turnover-limiting hydrosilylation step. In turn, this strength of binding can be related to the relatively high Lewis acidity measured for the [(^Dipp L)Ga(Ad)]⁺ cation (AN=69.8).

Frustrated Radical Pairs: Insights from EPR Spectroscopy

Progress in frustrated Lewis pair (FLP) chemistry has revealed the importance of the main group elements in catalysis, opening new avenues in synthetic chemistry. Recently, new reactivities of frustrated Lewis pairs have been uncovered that disclose that certain combinations of Lewis acids and bases undergo single-electron transfer (SET) processes. Here an electron can be transferred from the Lewis basic donor to a Lewis acidic acceptor to generate a reactive frustrated radical pair (FRP). This minireview aims to showcase the recent advancements in this emerging field covering the synthesis and reactivities of frustrated radical pairs, with extensive highlights of the results from Electron Paramagnetic Resonance (EPR) spectroscopy to explain the nature and stability of the different radical species observed.

(C6F5)3B·(HF)n-catalyzed glycosylation of disarmed glycosyl fluorides and reverse glycosyl fluorides

(C₆F₅)₃B·(HF)_n-catalyzed glycosylation of disarmed glycosyl fluorides and reverse glycosyl fluorides with structurally diverse nucleophiles has been achieved.

Influence of the tetraalkoxysilane crosslinker on the properties of polysiloxane-based elastomers prepared by the Lewis acid-catalysed Piers-Rubinsztajn reaction

We investigate the influence of the tetrafunctional alkoxysilan R-groups, with a range of sterics and electronics. This is through a solvent free polysiloxane network formation under ambient conditions using Lewis acid catalysed Piers-Rubinsztajn (PR) reaction.

Mechanistic insight into B(C6F5)3 catalyzed imine reduction with PhSiH3 under stoichiometric water conditions

A DFT and experimental study on the mechanism of B(C₆F₅)₃ catalyzed imine reduction is performed using PhSiH₃ as reductant under stoichiometric water conditions. Ingleson’s path B is reconfirmed here. And four novel (C₆F₅)₃B–OH₂ induced pathways (paths C2, C3, D2 and D3) entirely different from all the previous mechanisms were determined for the first time. They are all B(C₆F₅)₃ and water/amine catalyzed cycles, in which the nucleophilic water or amine catalyzed addition step between PhSiH₃ and the N-silicon amine cation is the rate-determining step of paths C2/D2 and C3/D3 with activation Gibbs free energy barriers of 23.9 and 18.3 kcal mol⁻¹ in chloroform, respectively, while the final desilylation of the N-silicon amine cation depends on an important intermediate, (C₆F₅)₃B–OH⁻. The competitive behavior of the 5 paths can explain the experimental facts perfectly; if all the reactants and catalysts are added into the system simultaneously, water amount and nucleophiles (excess water and produced/added amine) provide on–off selectivity of the pathways and products. 1 eq. water leads to quick formation of (C₆F₅)₃B–OH⁻, leading to B-II being turned off, and nucleophiles like excess water and produced/added amine switch on CD-II, leading to production of the amine. B-I′ of Ingleson’s path B is the only mechanism for anhydrous systems, giving N-silicon amine production only; B-I and C-I are competitive paths for systems with no more than 1 eq. water, producing the N-silicon amine and the [PhHC Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> NHPh]⁺[(C₆F₅)₃B–OH]⁻ ion pair; and paths C2, C3, D2 and D3 are competitive for systems with 1 eq. water and nucleophiles like excess water or added/produced amine, directly giving amination products.

Hydrosilylation or amination products? It depends on water amount and nucleophiles like excess water or produced/added amines.

Single-Electron Transfer in Frustrated Lewis Pair Chemistry

Frustrated Lewis pairs (FLPs) are well known for their ability to activate small molecules. Recent reports of radical formation within such systems indicate single-electron transfer (SET) could play an important role in their chemistry. Herein, we investigate radical formation upon reacting FLP systems with dihydrogen, triphenyltin hydride, or tetrachloro-1,4-benzoquinone (TCQ) both experimentally and computationally to determine the nature of the single-electron transfer (SET) events; that is, being direct SET to B(C6 F5 )3 or not. The reactions of H2 and Ph3 SnH with archetypal P/B FLP systems do not proceed via a radical mechanism. In contrast, reaction with TCQ proceeds via SET, which is only feasible by Lewis acid coordination to the substrate. Furthermore, SET from the Lewis base to the Lewis acid-substrate adduct may be prevalent in other reported examples of radical FLP chemistry, which provides important design principles for radical main-group chemistry.

Linkage Isomerism Leading to Contrasting Carboboration Chemistry: Access to Three Constitutional Isomers of a Borylated Phosphaalkene

We describe the reactivity of two linkage isomers of a boryl-phosphaethynolate, [B]OCP and [B]PCO (where [B]=N,N'-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl), towards tris- (pentafluorophenyl)borane (BCF). These reactions afforded three constitutional isomers all of which contain a phosphaalkene core. [B]OCP reacts with BCF through a 1,2 carboboration reaction to afford a novel phosphaalkene, E-[B]O{(C6 F5 )2 B}C=P(C6 F5 ), which subsequently undergoes a rearrangement process involving migration of both the boryloxy and pentafluorophenyl substituents to afford Z-{(C6 F5 )2 B}(C6 F5 )C=PO[B]. By contrast, [B]PCO undergoes a 1,3-carboboration process accompanied by migration of the N,N'-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl to the carbon centre.