Counterion-Enhanced Pd/Enamine Catalysis: Direct Asymmetric α-Allylation of Aldehydes with Allylic Alcohols by Chiral Amines and Achiral or Racemic Phosphoric Acids

Recent Advances on the Catalytic Asymmetric Allylic α-Alkylation of Carbonyl Derivatives Using Free Allylic Alcohols

During the last years, the development of more sustainable and straightforward methodologies to minimize the generation of waste organic substances has acquired high importance within synthetic organic chemistry. Therefore, it is not surprising that many efforts are devoted to ameliorating already well-known successful methodologies, that is, the case of the asymmetric allylic allylation reaction of carbonyl compounds. The use of free alcohols as alkylating agents in this transformation represents a step forward in this sense since it minimizes waste production and the substrate manipulation. In this review, we aim to gather the most recent methodologies describing this strategy by paying special attention to the reaction mechanisms, as well as their synthetic applications.

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

The Catalytic Asymmetric Allylic Alkylation of Acyclic Enolates for the Construction of Quaternary and Tetrasubstituted Stereogenic Centres

To facilitate the discovery and development of new pharmaceuticals, the demand for novel stereofunctionalised building blocks has never been greater. Whilst molecules bearing quaternary and tetrasubstituted stereogenic centres are ideally suited to explore untapped areas of chemical space, the asymmetric construction ofsterically congested carbon centres remains a longstanding challenge in organic synthesis. The enantioselective assembly of acyclic stereogenic centres is even more demanding due to the need to restrict a much wider range of geometries and conformations of the intermediates involved. In this context, the catalytic asymmetric allylicalkylation (AAA) of acyclic prochiral nucleophiles, namely enolates, has become an indispensable tool to access a range of linearα-quaternary andα-tetrasubstituted carbonyl compounds. However, unlike the AAA of cyclic enolates with a fixed enolate geometry, to achieve high levels of stereocontrol in the AAA of acyclic enolates, the stereoselectivity of enolisation must be considered. The aim of this review is to offer acomprehensivediscussion of catalytic AAA reactions of acyclic prochiral enolates and their analogues to generate congested quaternary and tetrasubstituted chiral centres using metal, non-metal and dual catalysis, with particular focus given to the control of enolate geometry and its impact on the stereochemical outcome of the reaction.

Sugar-Assisted Kinetic Resolutions in Metal/Chiral Amine Co-Catalyzed α-Allylations and [4+2] Cycloadditions: Highly Enantioselective Synthesis of Sugar and Chromane Derivatives

Functionalized triose-, furanose and chromane-derivatives were synthesized by the titled reactions. The sugar-assisted kinetic resolution/C-C bond-forming cascade processes generate a functionalized sugar derivative with a quaternary stereocenter in a highly enantioselective fashion (up to >99 % ee) by using a simple combination of metal and chiral amine co-catalysts. Notably, the interplay between the chiral sugar substrate and the chiral amino acid derivative allowed for the construction of a functionalized sugar product with high enantioselectivity (up to 99 %) also when using a combination of racemic amine catalyst (0 % ee) and metal catalyst.

Multicomponent Assembly of Complex Oxindoles by Enantioselective Cooperative Catalysis

Chiral oxindoles are important chemical scaffolds found in many natural products, and their enantioselective synthesis thus attracts considerable attention. Highly diastereo- and enantioselective synthetic methods for constructing C3 quaternary oxindoles have been well-developed. However, the efficient synthesis of chiral 3-substituted tertiary oxindoles has been rarely reported due to the ease of racemization of the tertiary stereocenter via enolization. Therefore, we herein report on the multicomponent assembly (from N-aryl diazoamides, aldehydes, and enamines/indoles) of complex oxindoles by enantioselective cooperative catalysis. These reactions proceed under mild conditions and show broad substrate scope, affording the desired coupling products (>90 examples) with good to excellent stereocontrol. Additionally, this research also demonstrates the synthetic potential of this annulation by constructing the 6,6,5-tricyclic lactone core structure of Speradine A.

Synergistic Strategies in Aminocatalysis

Synergistic catalysis offers the unique possibility of simultaneous activation of both the nucleophile and the electrophile in a reaction. A requirement for this strategy is the stability of the active species towards the reaction conditions and the two concerted catalytic cycles. Since the beginning of the century, aminocatalysis has been established as a platform for the stereoselective activation of carbonyl compounds through HOMO-raising or LUMO-lowering. The burgeoning era of aminocatalysis has been driven by a deep understanding of these activation and stereoinduction modes, thanks to the introduction of versatile and privileged chiral amines. The aim of this review is to cover recent developments in synergistic strategies involving aminocatalysis in combination with organo-, metal-, photo-, and electro-catalysis, focusing on the evolution of privileged aminocatalysts architectures.

Sterically Demanding Flexible Phosphoric Acids for Constructing Efficient and Multi-Purpose Asymmetric Organocatalysts

Herein, we present a novel approach for various asymmetric transformations of cyclic enones. The combination of readily accessible chiral diamines and sterically demanding flexible phosphoric acids resulted in a simple and highly tunable catalyst framework. The careful optimization of the catalyst components led to the identification of a particularly powerful and multi-purpose organocatalyst, which was successfully applied for asymmetric epoxidations, aziridinations, aza-Michael-initiated cyclizations, as well as for a novel Robinson-like Michael-initiated ring closure/aldol cyclization. High catalytic activities and excellent stereocontrol was observed for all four reaction types, indicating the excellent versatility of our catalytic system. Furthermore, a simple change in the diamine's configuration provided easy access to both product antipodes in all cases.

Sterically Demanding Flexible Phosphoric Acids for Constructing Efficient and Multi‐Purpose Asymmetric Organocatalysts

Herein, we present a novel approach for various asymmetric transformations of cyclic enones. The combination of readily accessible chiral diamines and sterically demanding flexible phosphoric acids resulted in a simple and highly tunable catalyst framework. The careful optimization of the catalyst components led to the identification of a particularly powerful and multi‐purpose organocatalyst, which was successfully applied for asymmetric epoxidations, aziridinations, aza‐Michael‐initiated cyclizations, as well as for a novel Robinson‐like Michael‐initiated ring closure/aldol cyclization. High catalytic activities and excellent stereocontrol was observed for all four reaction types, indicating the excellent versatility of our catalytic system. Furthermore, a simple change in the diamine's configuration provided easy access to both product antipodes in all cases.

Combining Palladium and Chiral Organocatalysis for the Enantioselective Deconjugative Allylation of Enals via Dienamine Intermediates

A catalytic enantioselective deconjugative allylation of enals is reported. A variety of enals underwent this transformation in high yield and ee, and products can be readily transformed into γ-allyl enals via a Cope rearrangement without erosion of ee. This transformation was used to install the quaternary stereocenter in (S)-bakuchiol, enabling completion of a concise formal synthesis.

Practical allylation with unactivated allylic alcohols under mild conditions

A practical palladium/calcium catalytic system was developed for dehydrative allylation with unactivated allylic alcohols.