Controlling Stereoselectivity with Noncovalent Interactions in Chiral Phosphoric Acid Organocatalysis

Chiral phosphoric acids (CPAs) have emerged as highly effective Brønsted acid catalysts in an expanding range of asymmetric transformations, often through novel multifunctional substrate activation modes. Versatile and broadly appealing, these catalysts benefit from modular and tunable structures, and compatibility with additives. Given the unique types of noncovalent interactions (NCIs) that can be established between CPAs and various reactants─such as hydrogen bonding, aromatic interactions, and van der Waals forces─it is unsurprising that these catalyst systems have become a promising approach for accessing diverse chiral product outcomes. This review aims to provide an in-depth exploration of the mechanisms by which CPAs impart stereoselectivity, positioning NCIs as the central feature that connects a broad spectrum of catalytic reactions. Spanning literature from 2004 to 2024, it covers nucleophilic additions, radical transformations, and atroposelective bond formations, highlighting the applicability of CPA organocatalysis. Special emphasis is placed on the structural and mechanistic features that govern CPA-substrate interactions, as well as the tools and techniques developed to enhance our understanding of their catalytic behavior. In addition to emphasizing mechanistic details and stereocontrolling elements in individual reactions, we have carefully structured this review to provide a natural progression from these specifics to a broader, class-level perspective. Overall, these findings underscore the critical role of NCIs in CPA catalysis and their significant contributions to advancing asymmetric synthesis.

Determination of the pKa Value of a Brønsted Acid by 19F NMR Spectroscopy

Brønsted acids, such as phosphoric acids derived from chiral 1,1'-bi-2-naphthol (BINOL), are important catalysts in the formation of carbon-carbon and carbon-heteroatom bonds, for example. The catalytic activity of these Brønsted acids is strongly linked to their acidity, and as such, the evaluation of compounds to determine pKa values provides insight into their catalytic activity. Herein, a 19F{1H} NMR methodology is detailed to determine the pKa of a fluorinated binaphthyl-derived phosphinic acid, rac-1, in acetonitrile and in the presence of a fluorinated sulfonamide reference compound (2-4). The approach was tested initially using 2 and 3, with the ΔpKa (0.08) in strong agreement with previously reported values (6.6 for 2 and 6.68/6.73 for 3). Sigmoidal curves of normalised chemical shift change (Δδ) against equivalents of the base phosphazene P1-tBu added overlapped for 2 and 3, but in the case of rac-1 and either 2, 3 or 4, there was significant separation. A variety of different approaches for determining the ΔpKa were compared. Values of pKa determined when the normalised Δδ was 90% were optimal for 2 and 3, whereas a normalised Δδ of 75% was optimal for 4, resulting in the pKa of rac-1 being determined to be 8.47-8.71.

Multifunctional Organocatalysts - Singly-Linked and Macrocyclic Bisphosphoric Acids for Asymmetric Phase-Transfer and Brønsted-Acid Catalysis

The linking of phosphoric acids via covalent or mechanical bonds has proven to be a successful strategy for the design of novel organocatalysts. Here, we present the first systematic investigation of singly-linked and macrocyclic bisphosphoric acids, including their synthesis and their application in phase-transfer and Brønsted acid catalysis. We found that the novel bisphosphoric acids show dramatically increased enantioselectivities in comparison to their monophosphoric acid analogues. However, the nature, length and number of linkers has a profound influence on the enantioselectivities. In the asymmetric dearomative fluorination via phase-transfer catalysis, bisphosphoric acids with a single, rigid bisalkyne-linker give the best results with moderate to good enantiomeric excesses. In contrast, bisphosphoric acids with flexible linkers give excellent enantioselectivities in the transfer-hydrogenation of quinolines via cooperative Brønsted acid catalysis. In the latter case, sufficiently long linkers are needed for high stereoselectivities, as found experimentally and supported by DFT calculations.

Kinetic Resolution of Racemic Tertiary Allylic Alcohols through SN2’ Reaction Using a Chiral Bisphosphoric Acid/Silver(I) Salt Co-catalyst System

A highly efficient kinetic resolution (KR) of racemic tertiary allylic alcohols was achieved through an intramolecular allylic substitution reaction using a co-catalyst system composed of chiral bisphosphoric acid and silver carbonate. This reaction afforded enantioenriched diene monoepoxides along with the recovery of tertiary allylic alcohols in a highly enantioselective manner, realizing an extremely high s-factor in most cases. The present method provides a new access to enantioenriched tertiary allylic alcohols, multifunctional compounds that are applicable for further synthetic manipulations.

A highly efficient KR of racemic tertiary allylic alcohols was developed through the intramolecular S_N2′ reaction using the chiral bisphosphoric acid/silver carbonate co-catalyst system, affording cis-epoxides and recovered alcohols in a high s-factor.

Electrochemical Organic Synthesis of Electron-Rich Biaryl Scaffolds: An Update

Biaryl scaffolds are widely spread in biologically important natural products, in numerous therapeutic agents, but they are also considered a privileged class of ligands and (organo)catalysts; therefore, the development of efficient alternative methodologies to prepare such compounds is always attracting much attention. The present review discusses the organic electrosynthesis of biaryls starting from phenols, anilines, naphthols, and naphthylamines. The most significant examples of the works reported in the last decade are presented and classified according to the single class of molecules: after the introduction, the first three sections relate to the reactions of phenols, naphthols, and anilines, respectively; the other two sections refer to cross-coupling and miscellaneous reactions.

Enantioconvergent Substitution Reactions of Racemic Electrophiles by Organocatalysis

Over the past decades, the development of enantioselective catalysis using organocatalysts has evolved into an active research field and a number of enantioselective transformations have been established. However, despite their being a highly desirable process for the synthesis of organic molecules in an enantioenriched form, the enantioconvergent substitution reactions of racemic electrophiles using organocatalysts still present several challenges. Although intrinsic difficulties in the catalytic stereocontrol abound due to the initial chiral information of racemic electrophiles, in recent years, mechanistically diverse enantioconvergent processes have been intensively investigated in organocatalysis. This Minireview focuses on recent achievements in the development of enantioconvergent substitution reactions of racemic electrophiles using organocatalysts. The contents are classified on the basis of the mechanistic types of enantioconvergent processes.

Chiral Brønsted Acid Catalyzed Enantioconvergent Propargylic Substitution Reaction of Racemic Secondary Propargylic Alcohols with Thiols

Despite the significant progress of the enantioselective reaction using chiral catalysts, the enantioselective nucleophilic substitution reaction at the chiral sp³ -hybridized carbon atom of a racemic electrophile has not been largely explored. Herein, we report the enantioconvergent propargylic substitution reaction of racemic propargylic alcohols with thiols using chiral bis-phosphoric acid as the chiral Brønsted acid catalyst. The substitution products were formed in high yields with high enantioselectivities in most cases. The cation-stabilizing effect of the sulfur functional group introduced at the alkynyl terminus is the key to achieving the efficient enantioconvergent process, in which chiral information originating from not only the racemic stereogenic center but also the formed contact ion pair is completely eliminated from the present system.