Lutz Greb

Light-Driven Molecular Motors: Imines as Four-Step or Two-Step Unidirectional Rotors

Chiral N-alkyl imines undergo unidirectional rotation induced by light and heat, thus providing a new class of molecular motors. Depending on the conformational flexibility of the stator part (the carbonyl residue) and the nitrogen inversion barrier of the rotor part (the amine residue) in the molecule, the operation mode of the motor can be controlled as either a four- or a two-step cycling motion of the rotor part.

Functional‐Group Tolerance in Frustrated Lewis Pairs: Hydrogenation of Nitroolefins and Acrylates

Apart from molecular hydrogen (H2), [1] other small molecules, such as CO2, [2] N2O, [3] NO, and SO2, [5] were targeted by frustrated Lewis pairs (FLPs) and resulted in the fixation or activation of these small molecules by Lewis acid interaction. Consequently, functionalized molecules, such as a,b-unsaturated esters, sulfoxides, or nitro compounds, are challenging substrates for FLP-catalyzed hydrogenations, and strategies toward higher functional-group tolerance are topic of current research. So s et al. demonstrated that bulky mesityl-substituted boranes could attain a level of functional-group tolerance according to the size exclusion principle. Other modifications of boranes to achieve stronger and weaker Lewis acidity in conjunction with FLP chemistry were reported. In particular, weaker Lewis acids appear to be favorable for the reduction of electron-deficient double bonds, such as a,b-unsaturated ketones, owing to the pronounced nucleophilicity of the corresponding hydridoborate anion. However, reduced Lewis acidity might also suppress the H2 activation, and a careful balance must be met. To date, there has been no report on the FLP-catalyzed hydrogenation of nitroolefins and acrylates. Only recently, B(C6F5)3 (1) was applied for the reduction of strongly electrophilic malonates at elevated temperatures and pressure (80 8C, 60 bar H2). [10a] Herein we report the reactivity of B(2,6-F2-C6F3)3 (2) as Lewis acid in FLP-catalyzed hydrogenations and its unique structural features in solid state and solution. The unprecedented FLP-mediated hydrogenation of nitroolefins and a,b-unsaturated esters under mild conditions (RT or 40 8C; 4 bar H2) was established. In contrast to the size-exclusion concept, in this case functional-group tolerance was solely achieved by modification of the electronic nature of the Lewis acid and the Lewis base. We initiated our studies with the synthesis of the borane B(2,6-F2-C6H3)3–THF adduct (2·THF) according to Naumann. Surprisingly, 2 has not been studied in FLP chemistry to date, even though it resembles an electronically modified B(C6F5)3 with identical steric shielding. The solidstate structure of 2 was established and displays similar structural features to BPh3 (Figure 1a). [14]

Electronic effects of triarylphosphines in metal-free hydrogen activation: a kinetic and computational study

The frustrated Lewis pair-mediated reversible hydrogen activation is studied as a function of the electron-donor quality of a series of phosphines. The increasing acidity of the generated phosphonium species leads to a stepwise lowering of the temperature for the highly reversible H2-activation and permits concrete classification for the first time. The influence of the acid strength on the metal-free hydrogenation of selected olefins is investigated by kinetic experiments and quantum chemical calculations. Detailed information for the rate-determining steps fully support our mechanistic model of a protonation step prior to hydride transfer. The rate of hydrogenation is strongly dependent on the electronic nature of the phosphine and of the acidity of the corresponding phosphonium cation. A careful balance of these two factors provides highly efficient metal-free hydrogenation catalysts. The provided findings are used to revise the reactivity of Lewis bases in the hydrogenation of imines, one of the most recognized applications of FLPs.

[2.2]Paracyclophane derived bisphosphines for the activation of hydrogen by FLPs: application in domino hydrosilylation/hydrogenation of enones

The heterolytic splitting of hydrogen by two types of [2.2]paracyclophane derived bisphosphines (1, 2a and 2b) in combination with tris(pentafluorophenyl)borane (3) at room temperature is described. The corresponding frustrated Lewis pairs (FLPs) exhibit different behavior in the activation of hydrogen. This results from diverse steric and electronic properties of the bisphosphines. The reactivity of the frustrated Lewis pairs was exploited in the first diastereoselective domino hydrosilylation/hydrogenation reaction catalyzed by FLPs.

Synthetic molecular motors: Thermal N inversion and directional photoinduced C=N Bond Rotation of Camphorquinone Imines

The thermal and photochemical E/Z isomerization of camphorquinone-derived imines was studied by a combination of kinetic, structural, and computational methods. The thermal isomerization proceeds by linear N inversion, whereas the photoinduced process occurs through C=N bond rotation with preferred directionality as a result of diastereoisomerism. Thereby, these imines are arguably the simplest example of synthetic molecular motors. The generality of the orthogonal trajectories of the thermal and photochemical pathways allows for the postulation that every suitable chiral imine qualifies, in principle, as a molecular motor driven by light or heat.

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.

Photo- and Metallo-responsive N-Alkyl α-Bisimines as Orthogonally Addressable Main-Chain Functional Groups in Metathesis Polymers

N-alkyl α-bisimines were employed as main-chain functional groups in acyclic diene metathesis (ADMET)-polymers, conferring dual responsiveness for the controlled switching of the polymeric particle shape with light and metal ions. Photochemical Z/E-isomerization leads to a significant and reversible change in hydrodynamic volume, thus introducing simple imines as novel photoswitches for light-responsive materials. Mild imine-directed CH activation by Pd(OAc)2 is demonstrated as a new single-chain nanoparticle (SCNP) folding process, enabling a controlled atom- and step-economic SCNP synthesis. The combination of light- and metallo-responsiveness in the same polymer provides the ability for orthogonal switching, a valuable tool for advanced functional material design.

Switchable fluorescence by click reaction of a novel azidocarbazole dye

Imaging is – even these days – still restricted to of a few classes of robust dyes. A demand for switchable tags led us to the design of a new class of pre-fluorophores. We achieved this by using a non-fluorescent N-(4-azidophenyl)-carbazole tag which turns fluorescent by click reaction with alkynes and cyclooctynes. The spectral properties of the labelled dyes were investigated. Our results suggest that a twisted internal charge transfer (TICT) transition is responsible for the emission. DFT calculations and single-crystal X-ray diffraction of selected examples support this explanation. The feasibility of the new dyes for biological application has also been tested via confocal microscopy.

Bis(perchlorocatecholato)silane—A Neutral Silicon Lewis Super Acid

No neutral silicon Lewis super acids are known to date. We report on the synthesis of bis(perchlorocatecholato)silane and verify its Lewis super acidity by computation (DLPNO-CCSD(T)) and experiment (fluoride abstraction from SbF6- ). The exceptional affinity towards donors is further demonstrated by, for example, the characterization of an unprecedented SiO4 F2 dianion and applied in the first hydrodefluorination reaction catalyzed by a neutral silicon Lewis acid. Given the strength and convenient access to this new Lewis acid, versatile applications might be foreseen.

Paracyclophane Derivatives in Frustrated Lewis Pair Chemistry

The metal-free activation of hydrogen was achieved using [2.2]paracyclophane-derived bisphosphines as Lewis base in frustrated Lewis pair chemistry. The rigid scaffold allows the orientation of functional groups so that steric aspects can be studied without altering the electronic nature. Depending on the geometry, structurally different phosphonium hydridoborates were generated when the frustrated Lewis pairs were exposed to hydrogen. The bisphosphines were applied in the 1,4-hydrosilylation-hydrogenation domino reaction providing access to secondary silyl-protected alcohols from enones in one step. Additionally, the planar-chiral scaffold was applied for the synthesis of novel enantiopure Lewis acids and Lewis bases.