Homochiral Metal–Organic Frameworks for Asymmetric Heterogeneous Catalysis

In the investigation of efficient chemical transformations, the carbon-carbon bond-forming reactions play an outstanding role. In this context, organocatalytic processes have achieved considerable attention. 1 Beside their facile reaction course, selectivity, and environmental friendliness, new synthetic strategies are made possible. Particularly, the inversion of the classical reactivity (Umpolung) opens up new synthetic pathways. 2 In nature, the coenzyme thiamine (vitamin B1), a natural thiazolium salt, utilizes a catalytic variant of this concept in biochemical processes as nucleophilic acylations. 3 The catalytically active species is a nucleophilic carbene. 4

Organocatalysis by N-Heterocyclic Carbenes

Asymmetric enamine catalysis.

The design and implementation of cascade reactions is a challenging facet of organic chemistry, yet one that can impart striking novelty, elegance, and efficiency to synthetic strategies. The application of cascade reactions to natural products synthesis represents a particularly demanding task, but the results can be both stunning and instructive. This Review highlights selected examples of cascade reactions in total synthesis, with particular emphasis on recent applications therein. The examples discussed herein illustrate the power of these processes in the construction of complex molecules and underscore their future potential in chemical synthesis.

Cascade reactions in total synthesis.

Hydrogen bonding is responsible for the structure of much of the world around us. The unusual and complex properties of bulk water, the ability of proteins to fold into stable three-dimensional structures, the fidelity of DNA base pairing, and the binding of ligands to receptors are among the manifestations of this ubiquitous noncovalent interaction. In addition to its primacy as a structural determinant, hydrogen bonding plays a crucial functional role in catalysis. Hydrogen bonding to an electrophile serves to decrease the electron density of this species, activating it toward nucleophilic attack. This principle is employed frequently by Nature's catalysts, enzymes, for the acceleration of a wide range of chemical processes. Recently, organic chemists have begun to appreciate the tremendous potential offered by hydrogen bonding as a mechanism for electrophile activation in small-molecule, synthetic catalyst systems. In particular, chiral hydrogen-bond donors have emerged as a broadly applicable class of catalysts for enantioselective synthesis. This review documents these advances, emphasizing the structural and mechanistic features that contribute to high enantioselectivity in hydrogen-bond-mediated catalytic processes.

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Asymmetric catalysis by chiral hydrogen-bond donors.

The term "organocatalysis" describes the acceleration of chemical reactions through the addition of a substoichiometric quantity of an organic compound. The interest in this field has increased spectacularly in the last few years as result of both the novelty of the concept and, more importantly, the fact that the efficiency and selectivity of many organocatalytic reactions meet the standards of established organic reactions. Organocatalytic reactions are becoming powerful tools in the construction of complex molecular skeletons. The diverse examples show that in recent years organocatalysis has developed within organic chemistry into its own subdiscipline, whose "Golden Age" has already dawned.

In the Golden Age of Organocatalysis

Unter dem Begriff “Organokatalyse” werden chemische Reaktionen zusammengefasst, die durch Zusatz substochiometrischer Mengen von organischen Verbindungen beschleunigt werden. Das Interesse an diesem Forschungsgebiet und den damit verbundenen neuen Konzepten hat in den vergangenen Jahren stark zugenommen. Viele der bekannten organokatalytischen Reaktionen sind bezuglich ihrer Effizienz und Selektivitat mit etablierten organischen Reaktionen vergleichbar, und die Organokatalyse hat sich beim Aufbau komplexer Molekulgeruste bewahrt. Die verschiedenartigen Beispiele zeigen, dass sich die Organokatalyse in den letzten Jahren zu einem selbstandigen Teilgebiet der Organischen Chemie entwickelt hat, dessen “Goldenes Zeitalter” bereits angebrochen ist.

Im Goldenen Zeitalter der Organokatalyse

Molecular systems that can be remotely controlled by light are gaining increasing importance in cell biology, physiology, and neurosciences because of the spatial and temporal precision that is achievable with laser microscopy. Two-photon excitation has significant advantages deep in biological tissues, but raises problems in the design of "smart" probes compatible with cell physiology. This Review discusses the chemical challenges in generating suitable two-photon probes.

From one-photon to two-photon probes: "caged" compounds, actuators, and photoswitches.

Olefin isomerization by a ruthenium carbenoid complex. Cleavage of allyl and homoallyl groups

Replacing the oxygen atom of the sugar ring with carbon is easy on paper, but it is more difficult to carry out this transformation and form five- and six-membered carbon rings (see scheme) in the flask. Nature has found a way to perform this tandem fragmentation/cyclization reaction in a stereospecific way at neutral pH values. Human creativity and good luck have uncovered synthetic strategies which can compete efficiently with nature.

Peter I. Dalko

Papers

In the Golden Age of Organocatalysis

Im Goldenen Zeitalter der Organokatalyse

From one-photon to two-photon probes: "caged" compounds, actuators, and photoswitches.

Olefin isomerization by a ruthenium carbenoid complex. Cleavage of allyl and homoallyl groups

Recent Advances in the Conversion of Carbohydrate Furanosides and Pyranosides into Carbocycles.