Quo Vadis CO2 Activation: Catalytic Reduction of CO2 to Methanol Using Aluminum and Gallium/Carbon-based Ambiphiles

We report on so-called "hidden FLPs" (FLP: frustrated Lewis pair) consisting of a phosphorus ylide featuring a group 13 fragment in the ortho position of a phenyl ring scaffold to form five-membered ring structures. Although the formation of the Lewis acid/base adducts was observed in the solid state, most of the title compounds readily react with carbon dioxide to provide stable insertion products. Strikingly, 0.3-3.0 mol% of the reported aluminum and gallium/carbon-based ambiphiles catalyze the reduction of CO2 to methanol with satisfactory high selectivity and yields using pinacol borane as stoichiometric reduction equivalent. Comprehensive computational studies provided valuable mechanistic insights and shed more light on activity differences.

A crystalline aluminium-carbon-based ambiphile capable of activation and catalytic transfer of ammonia in non-aqueous media

Despite recent achievements in the field of frustrated Lewis pairs (FLPs) for small molecule activations, the reversible activation and catalytic transformations of N-H-activated ammonia remain a challenge. Here we report on a rare combination of an aluminium Lewis acid and a carbon Lewis base. A so-called hidden FLP consisting of a phosphorus ylide featuring an aluminium fragment in the ortho position of a phenyl ring scaffold is introduced. Although the formation of the Lewis acid/base adduct is observed in the solid state, which at first glance leads to formally quenched FLP reactivity, we show that the title compound readily reacts with non-aqueous ammonia thermoneutrally and splits the N-H bond reversibly at ambient temperature. In addition, NH3 transfer reactions mediated by a main-group catalyst are presented. This proof-of-principle study is expected to initiate further activities in utilizing N-H-activated ammonia as a readily available, atom-economical nitrogen source.

Imidazolyl-Substituted Benzo- and Naphthodithiophenes as Precursors for the Synthesis of Transient Open-Shell Quinoids

The synthesis of three novel imidazolyl-substituted sulfur-containing heteroacenes is reported. These heteroacenes consisting of annelated benzo- and naphthothiophenes serve as precursors for the generation of open-shell quinoid heteroacenes by oxidation with alkaline ferric cyanide. Spectroscopic and computational experiments support the formation of reactive open-shell quinoids, which, however, quickly produce paramagnetic polymeric material.

Dispersion Energy-Stabilized Boron and Phosphorus Lewis Pairs

An isostructural series of boron/phosphorus Lewis pairs was systematically investigated. The association constants of the Lewis pairs were determined at variable temperatures, enabling the extraction of thermodynamic parameters. The stabilization of the Lewis adduct increased with increasing size of the dispersion energy donor groups, although the donor and acceptor properties of the Lewis pairs remained largely unchanged. This data was utilized to challenge state-of-the-art quantum chemical methods, which finally led to an enhanced workflow for the determination of thermochemical properties of weakly bound Lewis pairs within an accuracy of 0.6 to 1.0 kcal mol-1 for computed association free energies.

Boron-Centered Lewis Superacid through Redox-Active Ligands: Application in C-F and S-F Bond Activation

A series of redox-responsive ferrocenyl-substituted boranes and boronic esters were synthesized. Oxidation of the ferrocenyl ligand to the ferrocenium resulted in a drastic increase in the Lewis acidity beyond the strength of SbF₅ , which was investigated experimentally and computationally. The resulting highly Lewis acidic boron compounds were used for catalytic C-F and S-F bond activation.

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.

Frustrated Lewis Pair Catalysed Reactions

In recent years, frustrated Lewis pairs have been widely used in small molecules activation and catalytic transformations. This graphic review is aimed to provide the fundamental understanding of frustrated Lewis pair reactivity and the exploitation thereof in catalytic reactions.

Sigmatropic [1,5] Carbon Shift of Transient C3 Ammonium Enolates

The stereospecific sigmatropic [1,5] carbon shift of C3 ammonium enolates is discovered. According to mechanistic, kinetic and computational experiments, this new rearrangement proceeds via the catalytic generation of a transient C3 ammonium enolate by intramolecular aza‐Michael addition. This intermediate rapidly undergoes [1,5] sigmatropic carbon migration to furnish the respective tetrahydroquinoline‐4‐ones with excellent diastereoselectivities of d.r. >99 : 1 and in 61–98 % yield.

Impact of Heterocycle Annulation on NIR Absorbance in Quinoid Thioacene Derivatives

The synthesis and characterisation of a homologous series of quinoid sulfur‐containing imidazolyl‐substituted heteroacenes is described. The optoelectronic and magnetic properties were investigated by UV/vis, fluorescence and EPR spectroscopy as well as quantum‐chemical calculations, and were compared to those of the corresponding benzo congener. The room‐temperature and atmospherically stable quinoids display strong absorption in the NIR region between 678 and 819 nm. The dithieno[3,2‐b:2′,3′‐d]thiophene and the thieno[2′,3′:4,5]thieno[3,2‐b]thieno[2,3‐d]thiophene derivatives were EPR active at room temperature. For the latter, variable‐temperature EPR spectroscopy revealed the presence of a thermally accessible triplet state, with a singlet‐triplet separation of 14.1 kJ mol⁻¹.

Towards the Development of Frustrated Lewis Pair (FLP) Catalyzed Hydrogenations of Tertiary and Secondary Carboxylic Amides

The development of the frustrated Lewis pair catalyzed hydrogenation of tertiary and secondary amides is reviewed. Detailed insight into our strategies in order to overcome challenges during the reaction development process is provided. Furthermore, the developed chemistry is extended to the hydrogenation of polyamides and of trifluoroacetamides for the convenient introduction of trifluoroethyl groups into organic molecules.

Coupling of CO2 and epoxides catalysed by novel N-fused mesoionic carbene complexes of nickel(II)

We report the syntheses of two rigid mesoionic carbene (MIC) ligands with a carbazole backbone via an intramolecular Finkelstein-cyclisation cascade and investigate their coordination behavior towards nickel(II) acetate. Despite the nickel(II) carbene complexes 4a,b showing only minor differences in their chemical composition, they display curious differences in their chemical properties, e.g. solubility. Furthermore, the potential of these novel MIC complexes in the coupling of carbon dioxide and epoxides as well as the differences in reactivity compared to classical NHC-derived complexes are evaluated.

Hydrogenation of Secondary Amides using Phosphane Oxide and Frustrated Lewis Pair Catalysis

The metal‐free catalytic hydrogenation of secondary carboxylic acid amides is developed. The reduction is realized by two new catalytic reactions. First, the amide is converted into the imidoyl chloride by triphosgene (CO(OCCl₃)₂) using novel phosphorus(V) catalysts. Second, the in situ generated imidoyl chlorides are hydrogenated in high yields by an FLP‐catalyst. Mechanistic and quantum mechanical calculations support an autoinduced catalytic cycle for the hydrogenation with chloride acting as unusual Lewis base for FLP‐mediated H₂‐activation.

Mixed-Valence Compounds as Polarizing Agents for Overhauser Dynamic Nuclear Polarization in Solids*

Herein, we investigate a novel set of polarizing agents—mixed‐valence compounds—by theoretical and experimental methods and demonstrate their performance in high‐field dynamic nuclear polarization (DNP) NMR experiments in the solid state. Mixed‐valence compounds constitute a group of molecules in which molecular mobility persists even in solids. Consequently, such polarizing agents can be used to perform Overhauser‐DNP experiments in the solid state, with favorable conditions for dynamic nuclear polarization formation at ultra‐high magnetic fields.

Diastereoselective Synthesis of Dihydro-quinolin-4-ones by a Borane-Catalyzed Redox-Neutral endo-1,7-Hydride Shift

The borane-catalyzed synthesis of dihydroquinoline-4-ones is developed. The amino-substituted chalcones undergo a 1,7-hydride shift upon Lewis acid activation to form a zwitterionic iminium enolate, which collapses to the dihydroquinoline-4-one scaffold. The reaction proceeds in high yields (75-99%) with an excellent diastereoselectivity of up to >99:1 (cis:trans). The reaction mechanism is investigated by kinetic, isotope labeling, and computational experiments.

Dually Crosslinked Supramolecular Hydrogel for Cancer Biomarker Sensing

Lysophosphatidic acid (LPA) as the biomarker of early stage ovarian cancer is essentially difficult to detect due to lack of target spots. A dually crosslinked supramolecular hydrogel (DCSH) was developed to achieve sensing of LPA, which acts as a competitive guest molecule triggering the responsive crosslinking of the DCSH. Through this strategy, the surface plasmon resonance combined with optical waveguide spectroscopy could be used to quantitatively detect LPA with a responsive range covering physiological conditions (in pure form as well as mimicking LPA plasma solution) with high selectivity and sensitivity. LPA efficiently immerses into the host molecule β-cyclodextrin (β-CD) up to a 1:2 ratio by the competitive interaction mechanism, confirmed by one-dimensional nuclear overhauser effect spectroscopy (1D NOESY), high-resolution mass spectrometry (HRMS), isothermal titration calorimetry (ITC), and computational simulation. Our method opens a new strategy to detect biomarkers without target spots and provides a platform for surface plasmon resonance (SPR)-based sensors measuring small molecules.

FLP-catalysis meets hydrogen-bond activation

The potential of two chiral amidines and three non-chiral boranes in the metal-free hydrogen activation was explored.

Redox-responsive phosphonite gold complexes in hydroamination catalysis

Very high activities were observed in the redox-induced hydroamination of alkynes by employing a redox-active gold(i) complex featuring an electron-deficient, terphenyl-substituted phosphonite-based ligand. The hydroamination proceeds roughly two-fold faster with the in situ oxidized catalysts than with their reduced form.

Borane-catalyzed indole synthesis through intramolecular hydroamination

Catalytic metal-free intramolecular hydroamination for the synthesis of indoles and tetrahydroisoquinolines was achieved.

Frustrated Lewis Pair Catalyzed Dehydrogenative Oxidation of Indolines and Other Heterocycles

An acceptorless dehydrogenation of heterocycles catalyzed by frustrated Lewis pairs (FLPs) was developed. Oxidation with concomitant liberation of molecular hydrogen proceeded in high to excellent yields for N-protected indolines as well as four other substrate classes. The mechanism of this unprecedented FLP-catalyzed reaction was investigated by mechanistic studies, characterization of reaction intermediates by NMR spectroscopy and X-ray crystal analysis, and by quantum-mechanical calculations. Hydrogen liberation from the ammonium hydridoborate intermediate is the rate-determining step of the oxidation. The addition of a weaker Lewis acid as a hydride shuttle increased the reaction rate by a factor of 2.28 through a second catalytic cycle.

Structure-Reactivity Relationship in the Frustrated Lewis Pair (FLP)-Catalyzed Hydrogenation of Imines

The autoinduced, frustrated Lewis pair (FLP)-catalyzed hydrogenation of 16-benzene-ring substituted N-benzylidene-tert-butylamines with B(2,6-F2 C6 H3 )3 and molecular hydrogen was investigated by kinetic analysis. The pKa values for imines and for the corresponding amines were determined by quantum-mechanical methods and provided a direct proportional relationship. The correlation of the two rate constants k1 (simple catalytic cycle) and k2 (autoinduced catalytic cycle) with pKa difference between imine and amine pairs (ΔpKa ) or Hammett's σ parameter served as useful parameters to establish a structure-reactivity relationship for the FLP-catalyzed hydrogenation of imines.

Frustrated Lewis Pair-Catalyzed Cycloisomerization of 1,5-Enynes via a 5-endo-dig Cyclization/Protodeborylation Sequence

The first frustrated Lewis pair-catalyzed cycloisomerization of a series of 1,5-enynes was developed. The reaction proceeds via the π-activation of the alkyne and subsequent 5-endo-dig cyclization with the adjacent alkene. The presence of PPh3 was of utmost importance on the one hand to prevent side reactions (for example, 1,1-carboboration) and on the other hand for the efficient protodeborylation to achieve the catalytic turnover. The mechanism is explained on the basis of quantum-chemical calculations, which are in full agreement with the experimental observations.

Microwave-assisted FLP-catalyzed hydrogenations

Microwave-irradiation accelerates FLP-catalyzed hydrogenations.

Autoinduced catalysis and inverse equilibrium isotope effect in the frustrated Lewis pair catalyzed hydrogenation of imines

The frustrated Lewis pair (FLP)-catalyzed hydrogenation and deuteration of N-benzylidene-tert-butylamine (2) was kinetically investigated by using the three boranes B(C6F5)3 (1), B(2,4,6-F3-C6H2)3 (4), and B(2,6-F2-C6H3)3 (5) and the free activation energies for the H2 activation by FLP were determined. Reactions catalyzed by the weaker Lewis acids 4 and 5 displayed autoinductive catalysis arising from a higher free activation energy (2 kcal mol(-1)) for the H2 activation by the imine compared to the amine. Surprisingly, the imine reduction using D2 proceeded with higher rates. This phenomenon is unprecedented for FLP and resulted from a primary inverse equilibrium isotope effect.