Substrate selectivity is an important output function for the validation of different enzyme models, catalytic cavity compounds, and reaction mechanisms as demonstrated in this review. In contrast to stereo-, regio-, and chemoselective catalysis, the field of substrate-selective catalysis is under-researched and has to date generated only a few, but important, industrial applications. This review points out the broad spectrum of different reaction types that have been investigated in substrate-selective catalysis. The present review is the first one covering substrate-selective catalysis and deals with reactions in which the substrates involved have the same reacting functionality and the catalysts is used in catalytic or in stoichiometric amounts. The review covers real substrate-selective catalysis, thus only including cases in which substrate-selective catalysis has been observed in competition between substrates.

Taking advantage of the unconventional reactivity of twisted mono- and bis-amides of Tröger's base (TB), rac-6 and rac-7, respectively, the first synthesis of a 6-endo-monosubstituted TB analogue, rac-9, and the first rational synthesis of a 6,12-endo,endo-disubstituted TB analogue, rac-11, have been achieved. The bis-TB crown ether, meso-13, was prepared starting from rac-7. Meso-13 constitutes a rare example of a crown ether with an inverted methylene bridge-to-bridge bis-TB conformation both in solution and in the solid state, resulting in a reluctance to act as a receptor for cations.

The synthesis of two conformationally restricted Cr(III) salen complexes, 2 and 3, is described. Together, they constitute a supramolecular hydrogen‐bonding catalytic system for the recently reported asymmetric ring‐opening reactions of epoxides by a dynamic supramolecular catalyst. The synthesis involves state‐of‐the art transformations in frontline synthetic chemistry applied to heterocyclic chemistry. Hence, palladium‐catalyzed reactions were employed, including carbonylative annelation and Suzuki cross‐coupling reactions, for the formation of one of the heterocyclic rings (quinolone) and the functionalization of the formed rings. For the construction of the second heterocyclic ring (isoquinolone), a Curtius rearrangement was employed. The corresponding salen ligands were then prepared by Schiff‐base reactions, yielding the final complexes after metal insertion. For reference purposes the less conformationally restricted Cr(III) complexes 4 and 5 were also synthesized.

An efficient protocol has been developed for the exo-mono and exo,exo-bis functionalization of Tröger's base in the benzylic 6 and 12 positions of the diazocine ring. The lithiation of Tröger's base using s-BuLi/TMEDA followed by electrophilic quench affords exo-mono- and exo,exo-bis-substituted derivatives of Tröger's base in good to excellent yields. The variation of the number of equivalents of s-BuLi/TMEDA and the order of addition of the electrophile strongly govern the outcome of the reaction for each electrophile.

A bis(18-crown-6) Tröger's base receptor and 4substituted hepta-1,7-diyl bisammonium salt ligands have been used as a model system to study the interactions between non-polar side chains of peptides and an aromatic cavity of a protein. NMR titrations and NOESY/ROESY NMR spectroscopy were used to analyze the discrimination of the ligands by the receptor based on the substituent of the ligand, both quantitatively (free binding energies) and qualitatively (conformations). The analysis showed that an allanti conformation of the heptane chain was preferred for most of the ligands, both free and when bound to the receptor, and that for all of the receptor-ligand complexes, the substituent was located inside or partly inside of the aromatic cavity of the receptor. We estimated the free binding energy of a methyl-and a phenyl group to an aromatic cavity, via CH-π, and combined aromatic CH-π and π-π interactions to be À 1.7 and À 3.3 kJ mol À 1 , respectively. The experimental results were used to assess the accuracy of different computational methods, including molecular mechanics (MM) and density functional theory (DFT) methods, showing that MM was superior. Results and Discussion Description of the model systemReceptor 1 consists of a Tröger's base motif with 18-crown-6 moieties fused to each end of the aromatic cavity, and is synthesized in one step by the condensation of commercially [a] A

A double conformationally restricted kinetically labile supramolecular catalytic system, the third generation, was designed and synthesized. We investigated the substrate selectivity of this system by performing competitive pairwise epoxidations of pyridyl‐ and phenyl‐appended olefins. We compared the obtained substrate selectivities to previous less preorganized generations of this system. Five different substrate pairs were investigated, and the present double conformationally restricted system showed higher normalized substrate selectivities (pyridyl versus phenyl) for two of the substrate pairs than the previous less conformationally restricted generations. As for the preorganization of the components of the system, the catalyst, and the receptor part, it was shown that for each substrate pair there was one generation that was better than the other to generate substrate‐selective catalysis.

The first examples of enantiopure catalysts that are chiral merely due to coordination of different metal ions at enantiotopic positions of an achiral meso‐ligand are reported. These catalysts exhibit a pseudo‐Cs symmetry and are able to catalyze reactions demanding simultaneous involvement of two catalytic sites. The latter was demonstrated by application in the asymmetric ring‐opening of meso‐epoxides.

A model system to study interactions between aromatic cavities and non‐polar side chains was developed and studied by different NMR methods, where a weak but evident side‐chain discrimination was observed. The experimental quantitative and qualitative data was used to evaluate different computational methods, with the conclusion that, for this system, molecular mechanics gave more accurate results than density functional theory calculations. More information can be found in the Full Paper by P.‐O. Norrby, K. Wärnmark, et al. (DOI: 10.1002/chem.202100890).