Direct Catalytic Asymmetric Aldol Reaction of α‐Alkoxyamides to α‐Fluorinated Ketones

Chiral bisphosphine Ph-BPE ligand: a rising star in asymmetric synthesis

Chiral 1,2-bis(2,5-diphenylphospholano)ethane (Ph-BPE) is a class of optimal organic bisphosphine ligands with C2-symmetry. Ph-BPE with its excellent catalytic performance in asymmetric synthesis has attracted much attention of chemists with increasing popularity and is growing into one of the most commonly used organophosphorus ligands, especially in asymmetric catalysis. Over two hundred examples have been reported since 2012. This review presents how Ph-BPE is utilized in asymmetric synthesis and how powerful it is as a chiral ligand or even a catalyst in a wide range of reactions including applications in the total synthesis of bioactive molecules.

Recent advances in catalytic asymmetric synthesis

Asymmetric catalysis stands at the forefront of modern chemistry, serving as a cornerstone for the efficient creation of enantiopure chiral molecules characterized by their high selectivity. In this review, we delve into the realm of asymmetric catalytic reactions, which spans various methodologies, each contributing to the broader landscape of the enantioselective synthesis of chiral molecules. Transition metals play a central role as catalysts for a wide range of transformations with chiral ligands such as phosphines, N-heterocyclic carbenes (NHCs), etc., facilitating the formation of chiral C-C and C-X bonds, enabling precise control over stereochemistry. Enantioselective photocatalytic reactions leverage the power of light as a driving force for the synthesis of chiral molecules. Asymmetric electrocatalysis has emerged as a sustainable approach, being both atom-efficient and environmentally friendly, while offering a versatile toolkit for enantioselective reductions and oxidations. Biocatalysis relies on nature's most efficient catalysts, i.e., enzymes, to provide exquisite selectivity, as well as a high tolerance for diverse functional groups under mild conditions. Thus, enzymatic optical resolution, kinetic resolution and dynamic kinetic resolution have revolutionized the production of enantiopure compounds. Enantioselective organocatalysis uses metal-free organocatalysts, consisting of modular chiral phosphorus, sulfur and nitrogen components, facilitating remarkably efficient and diverse enantioselective transformations. Additionally, unlocking traditionally unreactive C-H bonds through selective functionalization has expanded the arsenal of catalytic asymmetric synthesis, enabling the efficient and atom-economical construction of enantiopure chiral molecules. Incorporating flow chemistry into asymmetric catalysis has been transformative, as continuous flow systems provide precise control over reaction conditions, enhancing the efficiency and facilitating optimization. Researchers are increasingly adopting hybrid approaches that combine multiple strategies synergistically to tackle complex synthetic challenges. This convergence holds great promise, propelling the field of asymmetric catalysis forward and facilitating the efficient construction of complex molecules in enantiopure form. As these methodologies evolve and complement one another, they push the boundaries of what can be accomplished in catalytic asymmetric synthesis, leading to the discovery of novel, highly selective transformations which may lead to groundbreaking applications across various industries.

Development of Catalytic Enantioselective Mannich Reactions Using Esters

Catalytic enantioselective Mannich reactions of simple nonactivated esters proceeded using a chiral potassium strong base catalyst prepared from a chiral bisoxazoline and potassium hexamethyldisilazide. Alkyl acetates, alkyl propionates, and an alkyl butyrate were employed as the simple esters, and the desired reactions proceeded smoothly to afford Mannich products in good to high yields with high enantioselectivities. One of the products was successfully employed in the asymmetric total synthesis of Maraviroc.

Synthesis of Difluoromethylated Carbinols via a HFIP-Promoted Hydroxydifluoromethylation of Aniline, Indole, and Pyrrole Derivatives with Difluoroacetaldehyde Ethyl Hemiacetal

A hexafluoroisopropanol (HFIP)-promoted hydroxydifluoromethylation of aniline, indole, and pyrrole derivatives with difluoroacetaldehyde ethyl hemiacetal has been developed. This protocol provides a facile and straightforward approach to access diverse difluoromethylated carbinols in good to excellent yields under mild conditions. Furthermore, gram-scale and synthetic derivatization experiments have also been demonstrated.

Visible-Light-Induced Diradical-Mediated ipso-Cyclization towards Double Dearomative [2+2]-Cycloaddition or Smiles-Type Rearrangement

When mono-radical ipso-cyclization of aryl sulfonamides tend to undergo Smiles-type rearrangement through aromatization-driven C-S bond cleavage, diradical-mediated cyclization must perform in a distinct reaction pathway. It is interesting meanwhile challenging to tune the rate of C-S bond cleavage to achieve a chemically divergent reaction of (hetero) aryl sulfonamides in a visible-light induced energy transfer (EnT) reaction pathway involving diradical species. Herein a chemically divergent reaction based on the designed indole-tethered (hetero)arylsulfonamides is reported which involves a diradical-mediated ipso-cyclization and a controllable cleavage of an inherent C-S bond. The combined experimental and computational results have revealed that the cleavage of the C-S bond in these substrates can be controlled by tuning the heteroaryl moieties: a) If the (hetero)aryl is thienyl, furyl, phenanthryl, etc., the radical coupling of double dearomative diradicals (DDDR) precedes over C-S bond cleavage to afford cyclobutene fused indolines by double dearomative [2+2]-cycloaddition; b) if the (hetero)aryl is phenyl, naphthyl, pyridyl, indolyl etc., the cleavage of C-S bond in DDDR is favored over radical coupling to afford biaryl products.

Diastereodivergent Aldol-Type Coupling of Alkoxyallenes with Pentafluorophenyl Esters Enabled by Synergistic Palladium/Chiral Lewis Base Catalysis

As a fundamental and synthetically useful C-C bond formation reaction, the aldol reaction is one of the most versatile transformations in organic synthesis. However, despite extensive research on asymmetric versions of the reaction, a unified method for stereoselective access to the complementary syn and anti diastereomeric products remains to be developed. In this study, we developed a synergistic palladium/chiral Lewis base system that overcomes the inherent diastereoselectivity bias of aldol reactions and, as a result, allowed us to achieve the first diastereodivergent coupling reactions of alkoxyallenes with pentafluorophenol esters. Computational studies suggest a mechanism involving an intermolecular protonative hydropalladation pathway rather than a palladium-hydride migratory insertion pathway. The origin of the stereochemistry for this synergistic catalysis system is rationalized by DFT calculations.

Hydrogen-Bond-Donor-Directed Switching of Enantioselectivity in the Vinylogous Aldol-Cyclization Cascade Reaction of Prostereogenic 3-Alkylidene Oxindoles with Isatins and o-Quinones

In this study, we reported a hydrogen-bond-donor-directed enantiodivergent vinylogous aldol-cyclization cascade reaction of 3-alkylidene oxindoles with isatins and o-quinones. Both enantiomers can be prepared by thiourea or squaramide cinchona alkaloid bifunctional organocatalysts with the same quinine scaffold. Kinetic study data provided the possible reaction mechanism for the vinylogous aldol-cyclization cascade reaction. The DFT calculation data showed the geometry of the generated dienolates from pronucleophiles dominated the observed switch of enantioselectivity.

Catalytic Asymmetric Construction of Tertiary Carbon Centers Featuring an α-Difluoromethyl Group with CF2H-CH2-NH2 as the "Building Block"

We report here for the first time a novel difluoromethylated ketimine building block condensed by thioisatin and difluoroethylamine, offering efficient access to a broad range of enantioenriched products bearing difluoroethylamine units (27 examples, ≤98% yield, >99% ee) in the presence of quinine-derived squaramide. Further transformation of the intermediate would generate a variety of versatile functional blocks like α-difluoromethyl amines, β-amino acid, and β-diamine with retention of the enantiomeric excess at the difluoromethyl-bound carbon.

Fluorinated Ketones as Trapping Reagents for Visible-Light-Induced Singlet Nucleophilic Carbenes

Singlet nucleophilic carbenes (SNCs) containing only one heteroatom donor remain underutilized in chemical synthesis. We recently discovered that visible-light-induced SNC intermediates can be trapped by fluorinated ketones via 1,2-carbonyl addition to afford benzoin-type products. This discovery represents a rare example of nucleophilic carbenes reacting with ketones and delivers an efficient, user-friendly, and scalable process for accessing fluorinated tertiary alcohol derivatives driven only by light circumventing the use of exogenous catalysts or additives.

Stereoselective Copper-Catalyzed Direct Aldol Reaction of β, γ-Unsaturated α-Ketoesters with Coumaran-3-Ones

An efficient direct aldol reaction between coumaran-3-ones and β, γ-unsaturated α-ketoesters by virtue of a chiral copper complex is developed. A series of coumaran-3-one derivatives containing chiral tertiary alcohol structures are obtained in excellent yields and stereoselectivities.

Stereoselective synthetic strategies of stereogenic carbon centers featuring a difluoromethyl group

The scope of this review is to summarize routine asymmetric synthetic methods which enable the effective and selective introduction of difluoromethyl groups into the desired compounds, providing a general introduction to this important research area.

H-bond donor-directed switching of diastereoselectivity in the Michael addition of α-azido ketones to nitroolefins

The development of catalyst-controlled stereodivergent asymmetric catalysis is important for providing facile access to all stereoisomers of chiral products with multiple stereocenters from the same starting materials. Despite progress, new design strategies for diastereodivergent asymmetric catalysis are still highly desirable. Here we report the potency of H-bond donors as the governing factor to tune diastereoselectivity in a highly diastereoselective switchable enantioselective Michael addition of α-azido ketones to nitroolefins. While a newly developed bifunctional tertiary amine, phosphoramide, preferentially afforded syn-adducts, an analogous squaramide catalyst selectively gave anti-adducts. The resulting multifunctional tertiary azides can be converted to spiro-pyrrolidines with four continuous stereocenters in a one-pot operation. Mechanistic studies cast light on the control of diastereoselectivity by H-bond donors. While the squaramide-catalyzed reaction proceeded with a transition state with both squaramide N–H bonds binding to an enolate intermediate, an unprecedented model was proposed for the phosphoramide-mediated reaction wherein an amide N–H bond and an alkylammonium ion formed in situ interact with nitroolefins, with the enolate stabilized by nonclassical C–H⋯O hydrogen-bonding interactions.

We report the successful reversal of the diastereoselectivity in an unprecedented Michael addition of α-azido ketones to nitroolefins catalyzed by bifunctional tertiary amines, simply by varying the H-bond donor from phosphoramide to squaramide.

Catalytic Enantioselective Addition of an Allyl Group to Ketones Containing a Tri-, a Di-, or a Monohalomethyl Moiety. Stereochemical Control Based on Distinctive Electronic and Steric Attributes of C-Cl, C-Br, and C-F Bonds

We disclose the results of an investigation designed to generate insight regarding the differences in the electronic and steric attributes of C-F, C-Cl, and C-Br bonds. Mechanistic insight has been gleaned by analysis of variations in enantioselectivity, regarding the ability of electrostatic contact between a halomethyl moiety and a catalyst's ammonium group as opposed to factors lowering steric repulsion and/or dipole minimization. In the process, catalytic and enantioselective methods have been developed for transforming a wide range of trihalomethyl (halogen = Cl or Br), dihalomethyl, or monohalomethyl (halogen = F, Cl, or Br) ketones to the corresponding tertiary homoallylic alcohols. By exploiting electrostatic attraction between a halomethyl moiety and the catalyst's ammonium moiety and steric factors, high enantioselectivity was attained in many instances. Reactions can be performed with 0.5-5.0 mol % of an in situ generated boryl-ammonium catalyst, affording products in 42-99% yield and up to >99:1 enantiomeric ratio. Not only are there no existing protocols for accessing the great majority of the resulting products enantioselectively but also in some cases there are hardly any instances of a catalytic enantioselective addition of a carbon-based nucleophile (e.g., one enzyme-catalyzed aldol addition involving trichloromethyl ketones, and none with dichloromethyl, tribromomethyl, or dibromomethyl ketones). The approach is scalable and offers an expeditious route to the enantioselective synthesis of versatile and otherwise difficult to access aldehydes that bear an α-halo-substituted quaternary carbon stereogenic center as well as an assortment of 2,2-disubstituted epoxides that contain an easily modifiable alkene. Tertiary homoallylic alcohols containing a triazole and a halomethyl moiety, structural units relevant to drug development, may also be accessed efficiently with exceptional enantioselectivity.

Catalytic Asymmetric Synthesis of α,α-Difluoromethylated and α-Fluoromethylated Tertiary Alcohols

The catalytic asymmetric synthesis of α,α-difluoromethylated tertiary alcohols is described, using an asymmetric dihydroxylation reaction. This protocol using either the AD-mix-α or AD-mix-β allowed an easy access to these valuable fluorinated chiral building blocks, which have been obtained with excellent yields and er. In addition, the reaction was extended to the α-fluoromethylated analogues.

A AgOAc/quinine-derived aminophosphine complex as an efficient catalyst for diastereo- and enantioselective 1,3-dipolar cycloaddition of α,β-unsaturated 7-azaindoline amides and azomethine ylides

The Dixon’s catalyst system is effective for the asymmetric 1,3-dipolar cycloaddition.