The first highly enantioselective catalytic protocol for the reductive coupling of ketones and hydrazones is reported. These reactions proceed through neutral ketyl radical intermediates generated via a concerted proton-coupled electron transfer (PCET) event jointly mediated by a chiral phosphoric acid catalyst and the photoredox catalyst Ir(ppy)2(dtbpy)PF6. Remarkably, these neutral ketyl radicals appear to remain H-bonded to the chiral conjugate base of the Brønsted acid during the course of a subsequent C-C bond-forming step, furnishing syn 1,2-amino alcohol derivatives with excellent levels of diastereo- and enantioselectivity. This work provides the first demonstration of the feasibility and potential benefits of concerted PCET activation in asymmetric catalysis.

Enantioselective photoredox catalysis enabled by proton-coupled electron transfer

Carbon dioxide (CO2) is an important C1 source for the generation of important carboxylic acids and derivatives. Stoichiometric and catalytic carboxylation of nucleophiles, including organometallic...

Visible-Light-Driven Catalytic Reductive Carboxylation with CO2

The past decade has seen unprecedented growth in the development of new chemical methods that proceed by mechanisms involving radical intermediates. This new attention has served to highlight a long-standing challenge in the field of radical chemistry - that of controlling absolute stereochemistry. This Review will examine developments using a strategy that offers enormous potential, in which attractive non-covalent interactions between a chiral catalyst and the substrate are leveraged to exert enantiocontrol. In a simplistic sense, such an approach mimics the modes of activation and control in enzyme catalysis and the realization that this can be achieved in the context of small-molecule catalysts has had sizable impact on the field of asymmetric catalysis in recent years. This strategy is now starting to quickly gather pace as a powerful approach for control of enantioselectivity in radical reactions and we hope that this focused survey of progress so far will inspire future developments in the area.

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Exploiting attractive non-covalent interactions for the enantioselective catalysis of reactions involving radical intermediates.

Seleno-containing heterocycles exist widely in pharmaceutical molecules and the skeletons of natural products. The addition of organoselenium to alkenes and alkynes via intramolecular tandem selenocyclization is an efficient method for preparing selenofunctionalized heterocycles. In this protocol, multiple bonds are formed in a single reaction without the need to isolate intermediates. This review highlights recent progress in this rapidly growing area with an emphasis on the scopes, limitations and the mechanisms of these different reactions. Besides, tandem tellurocyclization with alkenes and alkynes is also briefly discussed.

https://pubs.rsc.org/en/content/articlepdf/2020/qo/d0qo00849d

Recent advances in tandem selenocyclization and tellurocyclization with alkenes and alkynes

Benefiting from the impressive increase in fundamental knowledge, the last 20 years have shown a continuous burst of new ideas and consequently a plethora of new catalytic methods for enantioselective radical reactions. This review aims to provide a complete survey of progress achieved over this latter period. The first part of this review focuses on the use of chiral organocatalysts, and these include catalysts covalently linked to the substrate and those that interact with the substrate by weaker interactions like hydrogen bonds. The second part of the review is devoted to transition-metal redox catalysis which is organized according to increasing atomic number for the first-row transition metals (Ti, Cr, Fe, Mn, Co, Ni, Cu). Bioinspired manganese- and iron-mediated hydroxylations and oxidations are also discussed. A specific section is dedicated to the reactivity of Ru, Rh, and Ir complexes as Lewis acids with a special focus on complexes chiral at metal. Absorption of photons result in different events such as energy transfer, single-electron transfer, and hydrogen-atom transfer facilitating the formation of radicals. Organocatalysis has been successfully combined with photocatalysts, a reactivity which has opened new pathways enlarging the number of radical precursors available. The merger of photocatalysis with organo- or metalla-photocatalysis has brought novelty and allowed for the discovery of a large number of original transformations. The use of enzyme-catalyzed reactions involving radical intermediates which also largely benefit from visible-light irradiation are included in the review. This review provides a comprehensive inventory of progress in enantioselective radical reactions with a goal of detailing the reaction mechanisms involved in these transformations such that any nonspecialist could find their own creativity to invent yet unknown applications.

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Enantioselective Radical Reactions Using Chiral Catalysts.

Chiral hydrogen-bonding catalysis is a classic strategy in asymmetric organocatalysis, and it has been extensively used in a variety of fundamental chemical transformations. At the same time, visible light-driven photoredox catalysis is a powerful and sustainable tool commonly used in radical chemistry. The intriguing combination of these two catalysis platforms would open a new avenue for the direct and highly efficient synthesis of enantioenriched compounds. Inspired by the conceptual breakthrough of T. Bach, in recent years, significant progress has been made in cooperative photoredox and chiral hydrogen-bonding catalysis. By developing a variety of important types of reactions, a wide range of valuable chiral compounds have been successfully synthesized. In this review, the advances in this key area are systematically described, and the examples are organized according to the distinct bond-forming patterns in the construction of the stereocentres.

https://pubs.rsc.org/en/content/articlepdf/2020/qo/d0qo00276c

Cooperative photoredox and chiral hydrogen-bonding catalysis

Abundant and structurally diverse nitrogen‐containing aromatic heterocycles (azaarenes) are of great importance in many scientific and industrial fields. The development of methods for accessing azaarene derivatives has thus attracted substantial attention from chemists. In particular, directly exploiting the electronic properties of azaarenes to trigger transformations of azaarene‐containing substrates has long been appreciated as a powerful tool because it avoids the use of extra activating groups and allows the straightforward functionalization of azaarenes. In recent years, numerous advances have been made in the synthesis of imine‐containing azaarene derivatives under photoredox catalysis; the unique electronic and structural properties of these azaarenes as well as highly reactive and non‐traditional radical‐based process have inspired a number of targeted strategies and the corresponding efficient methodologies. In view of the currently available reviews, here, we will discuss the advances in the strategies of using these azaarenes as analogues of carbonyls and as single‐electron oxidants from the first example to the present.

Advances in the Synthesis of Imine‐Containing Azaarene Derivatives via Photoredox Catalysis

An unprecedented catalytic asymmetric reaction between azlactones and N -itaconimides has been developed. In the presence of an l - tert -leucine-based urea–tertiary amine Bronsted base catalyst, the tandem conjugate addition–protonation products could be attained in moderate yields with excellent enantio- and diastereoselectivities (up to 99% ee and >20:1 dr). The method provides an efficient approach to access valuable chiral γ-tertiary amine substituted succinimides containing nonadjacent stereocenters.

Yanli Yin

Papers

Cooperative photoredox and chiral hydrogen-bonding catalysis

Advances in the Synthesis of Imine‐Containing Azaarene Derivatives via Photoredox Catalysis

Organocatalytic Asymmetric Tandem Conjugate Addition–Protonation of Azlactones to N-Itaconimides

Organocatalytic Enantioselective and Diastereodivergent Mannich Reactions of 5‐Substituted Oxazolidin‐2,4‐Diones