Chloride-Mediated Alkene Activation Drives Enantioselective Thiourea and Hydrogen Chloride Co-Catalyzed Prins Cyclizations

Selective Noncovalent Catalysis with Small Molecules

In catalysis, selectivity reflects the energetic preference for the formation of a single product out of multiple possible reaction outcomes. The classic steric biasing approach in small-molecule catalysis employs steric destabilization of the undesired competing transition states to achieve energetic differentiation. In contrast, enzymes achieve high levels of rate acceleration and selectivity by accelerating the pathway leading to the major product, often through networks of attractive, stabilizing noncovalent interactions. This Review showcases selective noncovalent catalysis (NCC) with small organic molecules and transition-metal complexes. We collect and highlight examples whereby selectivity was documented experimentally to arise from selective stabilization of the transition state leading to the major product. We also showcase how synergistic experimental and computational investigations have enabled the elucidation of specific noncovalent interactions responsible for selective stabilization.

Catalytic Enantioconvergent Alkylation Reactions That Construct Chiral Quaternary Carbon Centers and Tune C(sp3)-C(sp2) Bond Rotation

In molecules with central chiral centers functionalized with aryl groups, the rotation of the single bond between the aryl group and the central chiral center may be hindered. Enantioconvergent alkylation reactions of racemic diastereomers to construct all-carbon quaternary stereocenters resulting in easing of the rotation around the axes are described. We demonstrated that the rotation of the bond between a central chiral carbon center and a dihydrobenzofuranone can be tuned by selection of either a tertiary carbon bearing a hydrogen or a quaternary carbon at the central chiral center. The products have the benefits of a central chiral center and a flexibly rotatable bond, which can facilitate interactions with other molecules.

Catalytic Enantioselective α-Fluorination of Ketones with CsF

Disclosed here is a catalytic enantioselective nucleophilic α-fluorination of simple ketones. A new hydrogen bonding donor catalyst was designed to not only overcome the competing catalyst deactivation but also enable efficient enantiocontrol in C-F bond formation between racemic α-keto sulfoniums and CsF. Careful condition optimization resulted in a general and mild protocol applicable for the configurational flexible acyclic α-fluoro ketones bearing a tertiary stereogenic center, thus complementary to the previous electrophilic fluorination methods that were only effective to cyclic ketones and/or tetrasubstituted stereogenic centers. Preliminary mechanistic studies support a phase transfer and dynamic kinetic resolution pathway operated by HBD-enabled anion-binding.

Advancements in hydrochlorination of alkenes

The hydrochlorination of alkenes has been extensively studied in research and is commonly featured in organic chemistry textbooks as an exemplification of the Markovnikov rule. However, the application of this reaction is typically limited to specific alkenes, such as highly substituted ones, styrenes, or strained systems. Conversely, monosubstituted or 1,2-disubstituted alkenes do not readily react with HCl gas or solutions of HCl gas at practical rates. The challenges associated with hydrochlorination reactions for these "non-activated" alkenes have spurred considerable research efforts over the past 30 years, which constitute the primary focus of this review. The discussion begins with classical polar hydrochlorinations, followed by metal-promoted radical hydrochlorinations, and concludes with a brief overview of recent anti-Markovnikov hydrochlorinations.

Catalytic Enantioselective Nucleophilic α-Chlorination of Ketones with NaCl

Catalytic enantioselective α-chlorination of ketones is a highly desirable process. Different from the conventional approaches that employ corrosive electrophilic chlorination reagents, the process disclosed here employs nucleophilic chloride, aqueous NaCl solution, and even seawater, as green inexpensive chlorine sources. This mechanistically distinct and electronically opposite approach provides facile access to diverse highly enantioenriched acyclic α-chloro ketones that are less straightforward by conventional approaches. With a chiral thiourea catalyst, a range of racemic α-keto sulfonium salts underwent enantioconvergent carbon-chlorine bond formation with high efficiency and excellent enantioselectivity under mild conditions. The sulfonium motif plays a crucial triple role by permitting smooth dynamic kinetic resolution to take place via a chiral anion binding mechanism in a well-designed phase-transfer system. This protocol represents a new general platform for the asymmetric nucleophilic α-functionalization of carbonyl compounds.

Catalytic Enantioselective [4+2] Cycloadditions of Salicylaldehyde Acetals with Enol Ethers

A readily accessible conjugate-base-stabilized carboxylic acid (CBSCA) catalyst facilitates highly enantioselective [4+2] cycloaddition reactions of salicylaldehyde-derived acetals and cyclic enol ethers, resulting in the formation of polycyclic chromanes with oxygenation in the 2- and 4-positions. Stereochemically more complex products can be obtained from racemic enol ethers. Spirocyclic products are also accessible.

Turning on Asymmetric Catalysis of Achiral Metal-Organic Frameworks by Imparting Chiral Microenvironment

The development of heterogeneous asymmetric catalysts has attracted increasing interest in synthetic chemistry but mostly relies on the immobilization of homogeneous chiral catalysts. Herein, a series of chiral metal-organic frameworks (MOFs) have been fabricated by anchoring similar chiral hydroxylated molecules (catalytically inactive) with different lengths onto Zr-oxo clusters in achiral PCN-222(Cu). The resulting chiral MOFs exhibit regulated enantioselectivity up to 83 % ee in the asymmetric ring-opening of cyclohexene oxide. The chiral molecules furnished onto the catalytic Lewis sites in the MOF create multilevel microenvironment, including the hydrogen interaction between the substrate and the chiral -OH group, the steric hindrance endowed by the benzene ring on the chiral molecules, and the proximity between the catalytic sites and chiral molecules confined in the MOF pores, which play crucial roles and synergistically promote chiral catalysis. This work nicely achieves heterogeneous enantioselective catalysis by chiral microenvironment modulation around Lewis acid sites.

Dual-Hydrogen-Bond Donor and Brønsted Acid Cocatalysis Enables Highly Enantioselective Protio-Semipinacol Rearrangement Reactions

A catalytic protio-semipinacol ring-expansion reaction has been developed for the highly enantioselective conversion of tertiary vinylic cyclopropyl alcohols into cyclobutanone products bearing α-quaternary stereogenic centers. The method relies on the cocatalytic effect of a chiral dual-hydrogen-bond donor (HBD) with hydrogen chloride. Experimental evidence is provided for a stepwise mechanism where protonation of the alkene generates a short-lived, high-energy carbocation, which is followed by C-C bond migration to deliver the enantioenriched product. This research applies strong acid/chiral HBD cocatalysis to weakly basic olefinic substrates and lays the foundation for further investigations of enantioselective reactions involving high-energy cationic intermediates.

Exploring Structure-Function Relationships of Aryl Pyrrolidine-Based Hydrogen-Bond Donors in Asymmetric Catalysis Using Data-Driven Techniques

Hydrogen bond-based organocatalysts rely on networks of attractive noncovalent interactions (NCIs) to impart enantioselectivity. As a specific example, aryl pyrrolidine substituted urea, thiourea, and squaramide organocatalysts function cooperatively through hydrogen bonding and difficult-to-predict NCIs as a function of the reaction partners. To uncover the synergistic effect of the structural components of this catalyst class, we applied data science tools to study various model reactions using a derivatized, aryl pyrrolidine-based, hydrogen-bond donor (HBD) catalyst library. Through a combination of experimentally collected data and data mined from previous reports, statistical models were constructed, illuminating the general features necessary for high enantioselectivity. A distinct dependence on the identity of the electrophilic reaction partner and HBD catalyst is observed, suggesting that a general interaction is conserved throughout the reactions analyzed. The resulting models also demonstrate predictive capability by the successful improvement of a previously reported reaction using out-of-sample reaction components. Overall, this study highlights the power of data science in exploring mechanistic hypotheses in asymmetric HBD catalysis and provides a prediction platform applicable in future reaction optimization.