Halogen- and chalcogen-based σ-hole interactions have recently received increased interest in non-covalent organocatalysis. However, the closely related pnictogen bonds have been neglected. In this study, we introduce conceptually simple, neutral, and monodentate pnictogen-bonding catalysts. Solution and in silico binding studies, together with high catalytic activity in chloride abstraction reactions, yield compelling evidence for operational pnictogen bonds. The depth of the σ holes is easily varied with different substituents. Comparison with homologous halogen- and chalcogen-bonding catalysts shows an increase in activity from main group VII to V and from row 3 to 5 in the periodic table. Pnictogen bonds from antimony thus emerged as by far the best among the elements covered, a finding that provides most intriguing perspectives for future applications in catalysis and beyond.

/pdf/catalysis-with-pnictogen-chalcogen-and-halogen-bonds-4vnygqtlvj.pdf

Catalysis with Pnictogen, Chalcogen, and Halogen Bonds

Since the trailblazing discoveries of Lehn, Cram and Pedersen, supramolecular chemistry has established itself as a cornerstone of organic chemistry. Supramolecular hosts offer defined microenvironments that mimic the active sites of enzymes, utilizing specific host–guest interactions to enable remarkable rate enhancements and product selectivity. The development of a diverse array of self-assembled hosts, coupled with the increased demand for shorter and greener synthetic routes, have spurred significant progress in the field of supramolecular catalysis. This Review Article covers recent advances in the field, ranging from novel organic reactivity aided by supramolecular hosts to catalytic cooperation between hosts and organometallic compounds or metal nanoparticles. Strides have also been made in the synthetic application of these hosts in site-selective substrate modifications and challenging photochemical reactions. These efforts have enabled the incorporation of non-covalent macromolecular catalysis in natural product syntheses, evidencing their unique advantages as a synthetic tool, and their powerful potential for practical applications. Supramolecular hosts can readily self-assemble from simple components and allow remarkable chemical rate enhancements and product selectivity utilising defined microenvironments and specific host–guest interactions. This Review Article describes recent conceptual and catalytic advances made in this field.

Advances in supramolecular host-mediated reactivity

Chiral iminium ions-generated upon condensation of α,β-unsaturated aldehydes and amine catalysts-are used extensively by chemists to make chiral molecules in enantioenriched form. In contrast, their potential to absorb light and promote stereocontrolled photochemical processes remains unexplored. This is despite the fact that visible-light absorption by iminium ions is a naturally occurring event that triggers the mechanism of vision in higher organisms. Herein we demonstrate that the direct excitation of chiral iminium ions can unlock unconventional reaction pathways, enabling enantioselective catalytic photochemical β-alkylations of enals that cannot be realized via thermal activation. The chemistry uses readily available alkyl silanes, which are recalcitrant to classical conjugate additions, and occurs under illumination by visible-light-emitting diodes. Crucial to success was the design of a chiral amine catalyst with well-tailored electronic properties that can generate a photo-active iminium ion while providing the source of stereochemical induction. This strategy is expected to offer new opportunities for reaction design in the field of enantioselective catalytic photochemistry.

/pdf/visible-light-excitation-of-iminium-ions-enables-the-18onqvm32d.pdf

Visible-light excitation of iminium ions enables the enantioselective catalytic β-alkylation of enals

ConspectusIn this Account, we outline our investigation into the supramolecular resorcinarene capsule as a catalyst. Molecular capsules not only are of interest due to the similarities of their binding pockets with those of natural enzymes but also feature potential advantages for catalysis. Due to the restricted internal volume of the binding pockets, substrate selectivities are commonly observed. Substrates that are encapsulated more efficiently will be converted selectively in the presence of less suitable substrates. This size selectivity cannot be obtained in a regular solution experiment. In addition, because of the distinct chemical environment inside the capsule, different product selectivities may be observed. Furthermore, the encapsulation of reactive catalysts inside confined environments may improve catalyst compatibility for multicatalyst tandem reactions.Although the potential advantages of performing catalysis inside closed microenvironments are generally recognized, the number of known cat...

/pdf/catalysis-inside-the-hexameric-resorcinarene-capsule-4obap8lrq8.pdf

Catalysis inside the Hexameric Resorcinarene Capsule.

The employment of weak intermolecular interactions in supramolecular chemistry offers an alternative approach to project artificial chemical environments like the active sites of enzymes. Discrete molecular architectures with defined shapes and geometries have become a revolutionary field of research in recent years because of their intrinsic porosity and ease of synthesis using dynamic non-covalent/covalent interactions. Several porous molecular cages have been constructed from simple building blocks by self-assembly, which undergoes many self-correction processes to form the final architecture. These supramolecular systems have been developed to demonstrate numerous applications, such as guest stabilization, drug delivery, catalysis, smart materials, and many other related fields. In this respect, catalysis in confined nanospaces using such supramolecular cages has seen significant growth over the years. These porous discrete cages contain suitable apertures for easy intake of substrates and smooth release of products to exhibit exceptional catalytic efficacy. This review highlights recent advancements in catalytic activity influenced by the nanocavities of hydrogen-bonded cages, metal-ligand coordination cages, and dynamic or reversible covalently bonded organic cages in different solvent media. Synthetic strategies for these three types of supramolecular systems are discussed briefly and follow similar and simplistic approaches manifested by simple starting materials and benign conditions. These examples demonstrate the progress of various functionalized molecular cages for specific chemical transformations in aqueous and nonaqueous media. Finally, we discuss the enduring challenges related to porous cage compounds that need to be overcome for further developments in this field of work.

Molecular Cavity for Catalysis and Formation of Metal Nanoparticles for Use in Catalysis.

Asymmetric Organocatalytic Benzylation of α,β‐Unsaturated Aldehydes with Toluenes

Our group is interested in applying supramolecular host systems as catalysts for challenging transformations. Although a variety of supramolecular containers has been described in literature, their use as reaction chambers is still underexplored. We herein describe our first steps in this exciting interdisciplinary field of research.

Exploring Catalysis inside Self-assembled Supramolecular Containers.

In the past decades, supramolecular structures have emerged as promising tools for catalysis. In this thesis, the first iminium-catalyzed reaction performed in a supramolecular assembly has been accomplished. The host was shown to have a modulation effect on the stereochemical outcome of the reaction occurring inside its cavity. The research demonstrates that the non-covalent combination of supramolecular containers with iminium catalysis is offering new opportunities to bias stereoselectivity in challenging reactions.

Iminium Catalysis Inside a Self-Assembled Supramolecular Capsule

Iminiumkatalyse in einer selbstorganisierten supramolekularen Kapsel: Modulation des Enantiomerenüberschusses

The noncovalent combination of a supramolecular host with iminium organocatalysis is described. Due to cation-pi interactions the reactive iminium species is held inside the host and reacts in this confined environment. The products formed differ up to 92% ee from the control experiments without added host. A model rationalizing the observed difference is presented.

Iminium Catalysis inside a Self-Assembled Supramolecular Capsule: Modulation of Enantiomeric Excess.

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