From Building Blocks to Catalysts: The Underinvestigated Potential of Boronic Acid Esters

Lewis acids are crucial in chemistry, with applications in pharmaceuticals, agrochemicals, and materials science. In main-group chemistry, they offer alternatives to transition metals, prompting our study of halogenated boronic acid esters (BAEs). Although BAEs are well-known, their catalytic potential has been overlooked. Our investigation found their Lewis acidity superior to that of boron trifluoride and comparable to that of tris(pentafluorophenyl)borane. Additionally, their catalysis of the Sakurai allylation of aldehydes has been documented, paving the way for future advancements.

Pd-Catalyzed Asymmetric Allylation Reaction of 2-Aryl-3H-indol-3-ones with Allyltrimethylsilane

An efficient method for the first ene-reaction of 2-aryl-3H-indol-3-ones with allyltrimethylsilane has been developed for the first time. The reaction proceeded under the catalysis of Pd(OAc)2 and chiral phosphoric ligand L11 in the presence of Cu(CF3COO)2·XH2O, PivOH, and 5 Å molecular sieves in DMSO at 60 °C. The present methodology can avoid the impact of amine products generated by the reaction on the catalyst, and at the same time, the high catalytic activity of classical palladium catalysts still has catalytic ability for low electrophilic keto-imines. The desired products were furnished in excellent yields with good enantioselectivity.

Chiral Phosphoric Acid-Catalyzed Enantio- and Diastereoselective Allylboration of Aldehydes with β,γ-Substituted Allylboronates

The catalytic asymmetric addition of β,γ-substituted allylboronates to aldehydes has been described. Promoted by 5 mol % chiral phosphoric acid, the reactions were broadly applicable, scalable, and efficient, allowing for the formation of 3,4-anti/syn-homoallylic alcohols bearing adjacent tertiary or quaternary stereogenic centers in a highly enantio- and diastereoselective manner (≤99% ee and dr >20:1). The rigid chairlike transition state involving the chiral phosphoric acid contributed to the highly controlled reaction.

Stereoselective Synthesis of Atropisomeric Acridinium Salts by the Catalyst‐Controlled Cyclization of ortho ‐Quinone Methide Iminiums

Quinone methides are fundamental intermediates for a wide range of reactions in which catalyst stereocontrol is often achieved by hydrogen bonding. Herein, we describe the feasibility of an intramolecular Friedel–Crafts 6π electrocyclization through ortho‐quinone methide iminiums stereocontrolled by a contact ion pair. A disulfonimide catalyst activates racemic trichloroacetimidate substrates and imparts stereocontrol in the cyclization step, providing a new avenue for selective ortho‐quinone methide iminium functionalization. A highly stereospecific oxidation readily transforms the enantioenriched acridanes into rotationally restricted acridiniums. Upon ion exchange, the method selectively affords atropisomeric acridinium tetrafluoroborate salts in high yields and an enantioenrichment of up to 93 : 7 e.r. We envision that ion‐pairing catalysis over ortho‐quinone methide iminiums enables the selective synthesis of a diversity of heterocycles and aniline derivatives with distinct stereogenic units.

Unified Synthesis of Polycyclic Alkaloids by Complementary Carbonyl Activation*

A complementary dual carbonyl activation strategy for the synthesis of polycyclic alkaloids has been developed. Successful applications include the synthesis of tetracyclic alkaloids harmalanine and harmalacinine, pentacyclic indoloquinolizidine alkaloid nortetoyobyrine, and octacyclic β-carboline alkaloid peganumine A. The latter synthesis features a protecting-group-free assembly and an asymmetric disulfonimide-catalyzed cyclization. Furthermore, formal syntheses of hirsutine, deplancheine, 10-desbromoarborescidine A, and oxindole alkaloids rhynchophylline and isorhynchophylline have been achieved. Finally, a concise synthesis of berberine alkaloid ilicifoline B was completed.

Silylium Ions: From Elusive Reactive Intermediates to Potent Catalysts

The history of silyl cations has all the makings of a drama but with a happy ending. Being considered reactive intermediates impossible to isolate in the condensed phase for decades, their actual characterization in solution and later in solid state did only fuel the discussion about their existence and initially created a lot of controversy. This perception has completely changed today, and silyl cations and their donor-stabilized congeners are now widely accepted compounds with promising use in synthetic chemistry. This review provides a comprehensive summary of the fundamental facts and principles of the chemistry of silyl cations, including reliable ways of their preparation as well as their physical and chemical properties. The striking features of silyl cations are their enormous electrophilicity and as such reactivity as super Lewis acids as well as fluorophilicity. Known applications rely on silyl cations as reactants, stoichiometric reagents, and promoters where the reaction success is based on their steady regeneration over the course of the reaction. Silyl cations can even be discrete catalysts, thereby opening the next chapter of their way into the toolbox of synthetic methodology.

Chiral disulfonimides: a versatile template for asymmetric catalysis

Since the emergence of pseudo-C2-symmetric chiral phosphoric acids (CPA), much work has been done to utilize these systems in stereoselective, organocatalytic processes. Despite the success in this field, reasonably basic substrates such as imines are often required to achieve appreciable activation. In order to access a wider variety of potential reaction partners, many related organocatalysts with enhanced Brønsted acidity have since been developed. Chiral disulfonimides (DSIs) have materialized as one such powerful class of organocatalysts and have been shown to expand the list of potential substrates to include aldehydes and ketones via Brønsted, Lewis, or bifunctional acid activation. This versatility renders DSIs amenable to an impressive scope of reaction types, typically with remarkable stereoselectivity induced by asymmetric counteranion-directed catalysis (ACDC). This review serves to provide a complete analysis of the successful applications, mechanistic insights, and unmet challenges exhibited to date in DSI-catalyzed and -assisted processes.

Allylsilane Reagent-Controlled Divergent Asymmetric Catalytic Reactions of 2-Naphthoquinone-1-methide

Herein, the first allylsilane reagent-controlled divergent asymmetric Hosomi-Sakurai conjugate allylation and hetero-Diels-Alder (HDA) with 2-naphthoquinone-1-methide (2-Nap-Q-1-M) under the catalysis of Sc^III /Feng ligand complex is reported. With these methods, a variety of uniquely substituted chiral allyl-functionalized diaryl compounds and naphthopyran products were obtained in a straightforward and highly stereoselective (up to 96.5:3.5 e.r.) manner under mild conditions. Moreover, it is demonstrated that 2-Nap-Q-1-M can serve as an efficient diene for a side asymmetric Diels-Alder (D-A) reaction. This principle can provide a straightforward access to hydrophenalene in an optically active form, which represents a structural core of various natural products and bioactive molecules.

IDPi Catalysis

High acidity and structural confinement are pivotal elements in asymmetric acid catalysis. The recently introduced imidodiphosphorimidate (IDPi) Brønsted acids have met with remarkable success in combining those features, acting as powerful Brønsted acid catalysts and "silylium" Lewis acid precatalysts in numerous thus far inaccessible transformations. Substrates as challenging to activate as simple olefins were readily transformed, ketones were employed as acceptors in aldolizations allowing sub-ppm level catalysis, whereas enolates of the smallest donor aldehyde, acetaldehyde, did not polymerize but selectively added a single time to a variety of acceptor aldehydes.

Catalytic enantioselective Hosomi–Sakurai reaction of α-ketoesters promoted by chiral copper(ii) complexes

The first catalytic enantioselective Hosomi−Sakurai reaction of α-ketoesters.

Can a Ketone Be More Reactive than an Aldehyde? Catalytic Asymmetric Synthesis of Substituted Tetrahydrofurans

O-heterocycles bearing tetrasubstituted stereogenic centers are prepared via catalytic chemo- and enantioselective nucleophilic additions to ketoaldehydes, in which the ketone reacts preferentially over the aldehyde. Five- and six-membered rings with both aromatic and aliphatic substituents, as well as an alkynyl substituent, are obtained. Moreover, 2,2,5-trisubstituted and 2,2,5,5-tetrasubstituted tetrahydrofurans are synthesized with excellent stereoselectivities. Additionally, the synthetic utility of the described method is demonstrated with a three-step synthesis of the side chain of anhydroharringtonine.

Dicationic phosphonium salts: Lewis acid initiators for the Mukaiyama-aldol reaction

Two strategies were used to prepare dicationic phosphonium cations.

A Disulfonimide Catalyst for Highly Enantioselective Mukaiyama-Mannich Reaction

A new BINOL-derived chiral disulfonimide has been developed by introducing 4-methyl-3,5-dinitrophenyl substituents at its 3- and 3'-positions. This chiral disulfonimide catalyst displays high catalytic efficacy toward the asymmetric Mukaiyama-Mannich reaction of imines with ketene silyl acetals leading to β-amino acid esters in good yields (up to 99%) with high diastereoselectivities (syn/anti up to 97:3) and enantioselectivities (up to 98% ee). The long-standing problem of the chiral phosphoric acid-catalyzed asymmetric Mukaiyama-Mannich reaction that requires a 2-hydroxyphenyl moiety was solved by this disulfonimide catalyst.

Asymmetric counteranion-directed Lewis acid organocatalysis for the scalable cyanosilylation of aldehydes

Due to the high versatility of chiral cyanohydrins, the catalytic asymmetric cyanation reaction of carbonyl compounds has attracted widespread interest. However, efficient protocols that function at a preparative scale with low catalyst loading are still rare. Here, asymmetric counteranion-directed Lewis acid organocatalysis proves to be remarkably successful in addressing this problem and enabled a molar-scale cyanosilylation in quantitative yield and with excellent enantioselectivity. Also, the catalyst loading could be lowered to a part-per-million level (50 ppm: 0.005 mol%). A readily accessible chiral disulfonimide was used, which in combination with trimethylsilyl cyanide, turned into the active silylium Lewis acid organocatalyst. The nature of a peculiar phenomenon referred to as a "dormant period", which is mainly induced by water, was systematically investigated by means of in situ Fourier transform infrared analysis.

Asymmetric Latent Carbocation Catalysis with Chiral Trityl Phosphate

Stable carbocations such as tritylium ions have been widely explored as organic Lewis acid catalysts and reagents in organic synthesis. However, achieving asymmetric carbocation catalysis remains elusive ever since they were first identified over one century ago. The challenges mainly come from their limited compatibility, scarcity of chiral carbocations, as well as the extremely low barrier to racemization of chiral carbenium ions. We reported here a latent concept for asymmetric carbocation catalysis. In this strategy, chiral trityl phosphate is employed as the carbocation precursor, which undergoes facile ionic dissociation upon mild external stimulation (e.g., acid, H-bonding, polar substrates) to form a catalytically active chiral ion pair for substrate activation and chiral induction. The latent strategy provides a solution for the long sought-after asymmetric carbocation catalysis as illustrated in three different enantioselective transformations.

Chiral derivatives of 1,2-benzenedisulfonimide as efficient Brønsted acid catalysts in the Strecker reaction

Two chiral derivatives of 1,2-benzenedisulfonimide, namely 4-methyl-3,6-bis(o-tolyl)-1,2-benzenedisulfonimide and 4,5-dimethyl-3,6-bis(o-tolyl)-1,2-benzenedisulfonimide, have been easily synthesized in good overall yields (respectively 34% and 41%) by means of an eleven-step synthetic protocol from commercially available 2-methyl-6-nitroaniline or 2,3-dimethyl-6-nitroaniline. 4,5-Dimethyl-3,6-bis(1-naphthyl)-1,2-benzenedisulfonimide was also synthesized, but the overall yield from 2,3-dimethyl-6-nitroaniline was lower (9%). The atropisomers of these compounds have been resolved and (-)atropisomers have been demonstrated to be efficient chiral catalysts in the Strecker reaction.