Lewis acids play a major role in all areas of chemistry. For a long time, toxic, corrosive and oxidizing SbF was considered as the strongest Lewis acid known. Lately, species significantly exceeding the Lewis acidity of SbF have been realized and were termed Lewis superacids (LSA). Prospective new candidates emerge steadily, which not only outperform SbF by their strength, but also in terms of their accessibility and ease of handling. In principle, Lewis superacids allow us to combine the outstanding activity of Brønsted superacids with the excellent selectivity of a common Lewis acid. However, the broad application of Lewis superacids in synthesis is all but popular. The present review deals with strong Lewis acids. First, it critically discusses Lewis acidity scaling methods and suggests an extended definition for Lewis superacidity. It then summarizes the properties and applications of the strongest currently known Lewis acids, indexed by the fluoride ion affinity (FIA). The supporting information contains a comprehensive list of experimentally and theoretically derived FIA data as a guide for the choice of Lewis acidic reagents/catalyst. This contribution shall encourage the search for new Lewis superacids and promote their application in non-specialized laboratories.

The computed fluoride ion affinity (FIA) is a valuable descriptor to assess the Lewis acidity of a compound. Despite its widespread use, the varying accuracy of applied computational models hampers the broad comparability of literature data. Herein, we evaluate the performance of selected methods (like DLPNO-CCSD(T)) in FIA computations against CCSD(T)/CBS data and guide for the choice of suitable density functionals that allow the treatment of larger Lewis acids. Based on the benchmarked methods, we computed an extensive set of gas-phase and solvation corrected FIA, that is covering group 13-16 elements featuring moderate to strong electron-withdrawing substituents (190 entries). It permits an unbiased comparison of FIA over a significant fraction of the periodic table, serves as a source of reference for future synthetic or theoretical studies, and allows to derive some simple design principles for strong fluoride ion acceptors. Finally, the manuscript includes a tutorial section for the computation of FIA with and without the consideration of solvation.

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.

The hydrogenation of double bonds is one of the most fundamental transformations [1] in organic chemistry, and has numerous applications in the commodity chemical, agrochemical, pharmaceutical, polymer, and food industries. [2] Despite significant advances in the last 100 years, efforts to improve metal-based technologies for hydrogenation are still the focus of current research. [3] In parallel to these continuing efforts, metal-free strategies for effecting reductions have also been pursued. While organic reagents such as Hantschs esters [4] and silanes [5] have been used as stoichiometric reducing agents, it was not until 2006 [6] that the first metalfree systems, the so-called frustrated Lewis pairs (FLPs), [7] were shown to reversibly activate dihydrogen. This discovery allowed the development of FLP-based catalysts for the reduction of polar unsaturated bonds such as imines, [8] nitriles, [8a,c] aziridines, [8a,c] enamines, [8b] silylenolethers, [9] and aromatic reductions of anilines.[10] Herein, we report the discovery of FLP systems which, while appearing unreactive at room temperature, in fact are capable of dihydrogen activation at temperatures as low as À80 8C. This finding was then exploited for the catalytic hydrogenation of olefins at temperatures between 25 and 70 8C. Experimental and computational data support a plausible mechanism involving protonation of the olefin with subsequent hydride transfer.These FLPs represent the first metal-free hydrogenation catalysts for the reduction of olefins bearing carbocationstabilizing moieties.It is well known that the reactions of olefins with Brønsted acids in the presence of a nucleophilic halide, leads to addition products according to a protonation/addition mechanism. In considering the potential of such a mechanism for FLP hydrogenation of C=C double bonds, it was recognized that while the generated borohydride would act as the nucleophile, this pathway would require the generation of a countercation which was sufficiently acidic to effect protonation of the olefin. While the majority of FLP activations of dihydrogen have been demonstrated for phosphine/borane combinations, [7b] a variety of other donors including amines, [8a, 11] pyridines, [12] carbenes, [13] and phosphinimines [14] have been shown to be effective when paired with boron or aluminum Lewis acids. However, in all of these cases, the generated cations are only weak Brønsted acids and thus are incapable of protonation of olefinic double bonds.Seeking to enhance the Brønsted acidity of the cation generated by the FLP activation of dihydrogen, we initiated investigations employing (C 6 F 5 ) 3 B (1) in combination with the weakly basic phosphine (C 6 F 5 )Ph 2 P (2). An NMR spectroscopic examination of a 1:1 mixture of 1 and 2 at 25 8C resulted in spectra that did not differ from those of the individual components. Exposure of this FLP to hydrogen (5 bar) did not lead to significant changes in the NMR spectra at room temperature. However, the situation altered when the temperatu...