In this review, we summarize the origin and advancements of iridium-catalyzed asymmetric allylic substitution reactions during the past two decades. Since the first report in 1997, Ir-catalyzed asymmetric allylic substitution reactions have attracted intense attention due to their exceptionally high regio- and enantioselectivities. Ir-catalyzed asymmetric allylic substitution reactions have been significantly developed in recent years in many respects, including ligand development, mechanistic understanding, substrate scope, and application in the synthesis of complex functional molecules. In this review, an explicit outline of ligands, mechanism, scope of nucleophiles, and applications is presented.

Iridium-Catalyzed Asymmetric Allylic Substitution Reactions

The transition metal-catalyzed allylic substitution of unactivated allylic substrates (allylic alcohols, allylic ethers and allylic amines) is rapidly becoming an important area of research. There are several advantages to using these substrates in allylic substitution reactions: the use of unactivated alcohols minimizes the production of waste by-products and reaction steps; and allylic ethers and allylic amines are useful substrates because of their stability and their presence in numerous biologically active compounds. Research in this field has therefore gained widespread attention for promoting the development of efficient and environmentally benign procedures for the formation of C–C, C–N and C–O bonds.

Transition metal-catalyzed allylic substitution reactions with unactivated allylic substrates

The cooperation and interplay between organic and metal catalyst systems is of utmost importance in nature and chemical synthesis. Here innovative and selective cooperative catalyst systems can be designed by combining two catalysts that complement rather than inhibit one another. This refined strategy can permit chemical transformations unmanageable by either of the catalysts alone. This review summarizes innovations and developments in selective organic synthesis that have used cooperative dual catalysis by combining simple aminocatalysts with metal catalysts. Considerable efforts have been devoted to this fruitful field. This emerging area employs the different activation modes of amine and metal catalysts as a platform to address challenging reactions. Here, aminocatalysis (e.g., enamine activation catalysis, iminium activation catalysis, single occupied molecular orbital (SOMO) activation catalysis, and photoredox activation catalysis) is employed to activate unreactive carbonyl substrates. The trans...

Combinations of Aminocatalysts and Metal Catalysts: A Powerful Cooperative Approach in Selective Organic Synthesis.

This Perspective presents an overview of catalytic enantioselective transformations that allow convenient access to all stereoisomers of a given product with multiple stereogenic centers. Particular focus is placed on discussion of the concept of stereodivergent dual catalysis and its application in target-oriented synthesis. The potential of this concept in the development of new transformations as well as implications for achieving stereochemical diversity in library design and diversity-oriented synthesis are also discussed.

Stereodivergence in Asymmetric Catalysis

An Ir/Zn dual catalysis has been developed for the enantio- and diastereodivergent α-allylation of unprotected α-hydroxyketones under mild conditions, in the absence of any additional base. The cooperative action of a chiral iridium complex derived from phosphoramidites and a chiral Zn-ProPhenol complex is most likely responsible for its high reactivity, excellent enantioselectivity (up to >99% ee), and good diastereoselectivity (up to >20:1 dr). All four product stereoisomers could be prepared from the same set of starting materials and under identical conditions by simple selection of appropriate catalyst combinations.

An Ir/Zn Dual Catalysis for Enantio- and Diastereodivergent α-Allylation of α-Hydroxyketones.

We describe a fully stereodivergent synthesis of a range of α,α-disubstituted α-amino acids via an Ir/Cu-catalyzed α-allylation of readily available imine esters The introduction of a Cu-Phox complex-activated imine ester into the chiral iridium-catalyzed allylic allylation process is crucial for its high reactivity and excellent enantio- and diastereoselectivity (up to >99% ee and >20:1 dr) Importantly, the two chiral catalysts allow for full control over the configuration of the stereocenters, affording all stereoisomers of the desired products The utility of this methodology was demonstrated by synthesizing dipeptides and analogues of bioactive molecules in a stereodivergent manner

Ir/Cu Dual Catalysis: Enantio- and Diastereodivergent Access to α,α-Disubstituted α-Amino Acids Bearing Vicinal Stereocenters

Hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes are widely utilized in the synthesis of valuable allylic compounds. In the past decades, asymmetric catalysis has emerged as one of the most attractive fields in organic synthesis. Recently, the asymmetric versions of hydrofunctionalisation and difunctionalisation reactions have become powerful and compelling tools to afford enantiopure allylic compounds, appealing to a large range of chemists. Various metal complexes modified with a large number of chiral ligands and several chiral organocatalysts have been developed to promote the hydrofunctionalisation and difunctionalisation reactions and expand substrate scope. This review provides an overview of this field, and aims at summarizing the chiral ligand used in this area of research. A detailed discussion of the development of these reactions and the general reaction mechanisms is provided.

Asymmetric synthesis of allylic compounds via hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes.

We report a stereoselective and site-specific allylic alkylation of Schiff base activated amino acids and small peptides via a Pd/Cu dual catalysis. A range of noncoded α,α-dialkyl α-amino acids were easily synthesized in high yields and with excellent enantioselectivities (up to >99% ee). Furthermore, a direct and highly stereoselective synthesis of small peptides with enantiopure α-alkyl or α,α-dialkyl α-amino acids residues incorporated at specific sites was accomplished using this dual catalyst system.

Stereoselective and Site-Specific Allylic Alkylation of Amino Acids and Small Peptides via a Pd/Cu Dual Catalysis

Allylic alcohols were directly used in Pd-catalyzed allylic alkylations of simple ketones under mild reaction conditions. The reaction proceeded smoothly at 20 °C by the concerted action of a Pd catalyst, a pyrrolidine co-catalyst, and a hydrogen-bonding solvent, and does not require any additional reagents. A computational study suggested that methanol plays a crucial role in the formation of the π-allylpalladium complex by lowering the activation barrier.

Xiaohong Huo

Papers

An Ir/Zn Dual Catalysis for Enantio- and Diastereodivergent α-Allylation of α-Hydroxyketones.

Ir/Cu Dual Catalysis: Enantio- and Diastereodivergent Access to α,α-Disubstituted α-Amino Acids Bearing Vicinal Stereocenters

Asymmetric synthesis of allylic compounds via hydrofunctionalisation and difunctionalisation of dienes, allenes, and alkynes.

Stereoselective and Site-Specific Allylic Alkylation of Amino Acids and Small Peptides via a Pd/Cu Dual Catalysis

Palladium‐Catalyzed Allylic Alkylation of Simple Ketones with Allylic Alcohols and Its Mechanistic Study