Asymmetric C(sp(3))-H functionalization of unactivated alkylarenes such as toluene enabled by chiral Brønsted base catalysts

C-H functionalization through benzylic deprotonation with π-coordination or cation-π-interactions

Benzylic C-H functionalization is a valuable tool to make complex aromatic molecules from simple, readily available alkylbenzenes. While methods that involve benzylic radicals or cations generated by hydrogen atom transfer or oxidation have been well demonstrated, they often require oxidative conditions. In contrast, deprotonation methods offer a complementary approach to transform benzylic C-H bonds through a benzylic carbanion generated by deprotonation. Electrophilic transition metal complexes acidify benzylic protons upon π-coordination to the phenyl ring of substrates, facilitating deprotonation by stabilizing the corresponding benzylic carbanion. Cation-complexes with group(I) metals also acidify benzylic C-H bonds. These approaches enable a significant expansion of the scope and diversity of alkylarenes with various electrophilic reagents. In this review, we discuss the development of benzylic functionalization through deprotonation of η6-arene complexes of transition-metals and cation-π interactions with group(I) metals, as well as progress made in catalysis through reversible arene-metal interactions.

Synthesis of Indolines via Base-Mediated C-H Activation and Defluorinative C-N Coupling, with no Need for Transition-Metals

Syntheses of (partially) aromatic nitrogen heterocycles increasingly rely on transition-metal catalyzed C-C- and C-N-cross-coupling reactions. Here we describe a different approach to the synthesis of indolines by a domino C(sp3)-H activation, 1,2-addition, and defluorinative SNAr-cyclization sequence to provide the target 1,2-diarylindolines (1,2-diaryl-2,3-dihydroindoles) from ortho-fluorinated methyl-arenes and N-aryl imines (benzylidene anilines) in a cyclocondensation that is mediated by potassium hexamethyldisilazide (KHMDS) as base exclusively. This transition-metal-free process via C-H and C-F bond activation provides a one-step entry into a wide array of indoline scaffolds (43 examples, up to 96 % yield). This privileged substructure is common in natural products and pharmaceuticals alike, and cannot be accessed by traditional condensation reactions.

Masters of Mediation: MN(SiMe3)2 in Functionalization of C(sp3)-H Latent Nucleophiles

Organoalkali compounds have undergone a far-reaching transformation being a coupling partner to a mediator in unusual organic conversions which finds its spot in the field of sustainable synthesis. Transition-metal catalysis has always been the priority in C(sp3)-H bond functionalization, however alternatively, in recent times this has been seriously challenged by earth-abundant alkali metals and their complexes arriving at new sustainable organometallic reagents. In this line, the importance of MN(SiMe3)2 (M=Li, Na, K & Cs) reagent revived in C(sp3)-H bond functionalization over recent years in organic synthesis is showcased in this minireview. MN(SiMe3)2 reagent with higher reactivity, enhanced stability, and bespoke cation-π interaction have shown eye-opening mediated processes such as C(sp3)-C(sp3) cross-coupling, radical-radical cross-coupling, aminobenzylation, annulation, aroylation, and other transformations to utilize readily available petrochemical feedstocks. This article also emphasizes the unusual reactivity of MN(SiMe3)2 reagent in unreactive and robust C-X (X=O, N, F, C) bond cleavage reactions that occurred alongside the C(sp3)-H bond functionalization. Overall, this review encourages the community to exploit the untapped potential of MN(SiMe3)2 reagent and also inspires them to take up this subject to even greater heights.

Base-Promoted Synthesis of Isoquinolines through a Tandem Reaction of 2-Methyl-arylaldehydes and Nitriles

A convenient method for preparing 3-aryl isoquinolines via a base-promoted tandem reaction is presented. Simply combining commercially available 2-methyl-arylaldehydes, benzonitriles, NaN(SiMe3)2, and Cs2CO3 enabled the synthesis of a variety of isoquinolines (23 examples, ≤90% yield). Among the syntheses of isoquinolines, the transition metal-free method described here is straightforward, practical, and operationally simple.

Dual Nickel/Photoredox-Catalyzed Asymmetric Carbamoylation of Benzylic C(sp3 )-H Bonds

Radical-mediated Hydrogen Atom Abstraction of Csp3 -H bonds has become a powerful tool for the asymmetric functionalization of organic feedstocks. Here, we present an asymmetric synthesis of α-aryl amides via carbamoylation of alkylarenes with isocyanates as electrophiles. The synergistic combination of a photoredox and a chiral nickel-catalyst, enables the use of readily available and neutral reagents under mild reaction conditions and provides straightforward access to pharmacologically relevant motifs in enantiomerically pure form.

A Tandem Madelung Indole Synthesis Mediated by a LiN(SiMe3)2/CsF System

A tandem Madelung indole synthesis by the reaction of methyl benzoate and N-methyl-o-toluidine has been discovered. The combination of LiN(SiMe3)2 with CsF is the key factor, which secures the high efficiency of such tandem transformations. Simply combining methyl benzoate, N-methyl-o-toluidine LiN(SiMe3)2, and CsF generated a diverse array of N-methyl-2-phenylindoles (31 examples, 50-90% yields). Furthermore, the scalability and the poststructural modifications of this indole synthesis were demonstrated.

One-pot synthesis of 3-functionalized (Z)-silyl enol ethers from 1-arylallylic alcohols by C,O-difunctionalization of dipotassio α,β-dianion intermediates

Previously reported syntheses of 3-functionalized silyl enol ethers using allyloxysilanes are hindered by undesirable reactions owing to retro Brook rearrangements. In this study, various 3-functionalized (Z)-silyl enol ethers were synthesized from readily available 1-arylallylic alcohols using (trimethylsilyl)methylpotassium as a base. C,O-Difunctionalization of the in situ-generated dipotassio α,β-dianion with electrophiles and silyl chlorides is key to the success of this transformation. Control experiments confirmed that the dianion has higher nucleophilicity and thermal stability than related siloxyallylpotassiums.

Rapid C-H Transformation: Addition of Diarylmethanes to Imines in Seconds by Catalytic Use of Base

The addition of diarylmethanes or methylarenes via activation of benzylic C(sp³)-H bonds to N-aryl imines proceeds under catalysis by alkali hexamethyldisilazide (HMDS) base to give N-(1,2,2-triarylethyl)anilines or N-(1,2-diarylethyl)anilines, respectively. In the presence of 10 mol % of LiHMDS at room temperature, the diarylmethane addition equilibrates within 20-30 s and is driven to near completion by cooling the reaction mixture to -25 °C, providing N-(1,2,2-triarylethyl)aniline in a >90% yield.

LiHMDS-Mediated Deprotonative Coupling of Toluenes with Ketones

We demonstrate that lithium hexamethyldisilazide (LiHMDS) acts as an effective base for deprotonative coupling reactions of toluenes with ketones to afford stilbenes. Various functionalities (halogen, OCF₃ , amide, Me, aryl, alkenyl, alkynyl, SMe, and SPh) are allowed on the toluenes. Notably, this system proved successful with low-reactive toluenes bearing a large pKa value compared to that of the conjugate acid of LiHMDS (hexamethyldisilazane, 25.8, THF), as demonstrated by 4-phenyltoluene (38.57, THF) and toluene itself (∼43, DMSO).

Regio- and diastereoselective synthesis of unsymmetrical 1,4-diketone-derived (Z)-monosilyl enol ethers via siloxyallylpotassium intermediates

This paper describes the regio- and diastereoselective synthesis of unsymmetrical 1,4-diketone-derived (Z)-monosilyl enol ethers from 1-arylallyloxysilanes and Weinreb amides using (trimethylsilyl)methylpotassium as a base. The metalation of 1-arylallyloxysilanes to generate siloxyallylpotassiums is the key step in this transformation. The products can be transformed into diverse α-monofunctionalized unsymmetrical 1,4-diketones.

Asymmetric Formal Nucleophilic o-Cresolylation with Morita-Baylis-Hillman Carbonates of 2-Cyclohexenones via Palladium Catalysis

Here we report an asymmetric formal nucleophilic o-cresolylation reaction with the Morita-Baylis-Hillman (MBH) carbonates from 2-cyclohexanones and diverse aldehydes under palladium catalysis, by in situ generation of electron-neutral and HOMO-raised η²-Pd(0)-dienone complexes via an oxidative insertion/π-σ-isomerization/β-H elimination activation sequence. The subsequent umpolung vinylogous addition to a variety of imines is realized upon Pd(0)-mediated π-Lewis base catalysis, finally furnishing o-cresolylated products followed by another cascade of a π-σ-isomerization/β-H elimination/aromatization process. Moderate to excellent diastereo- and enantioselectivity are achieved for substantial substrate assemblies by employing a newly designed bulky chiral phosphonamidite ligand, and the resultant multifunctional products can be facilely elaborated to access diverse enantioenriched architectures. In addition, the catalytic reaction pathway is finely illuminated by control experiments.

Identification of a Self-Photosensitizing Hydrogen Atom Transfer Organocatalyst System

We developed organocatalyst systems to promote the cleavage of stable C-H bonds, such as formyl, α-hydroxy, and benzylic C-H bonds, through a hydrogen atom transfer (HAT) process without the use of exogenous photosensitizers. An electronically tuned thiophosphoric acid, 7,7'-OMe-TPA, was assembled with substrate or co-catalyst N-heteroaromatics through hydrogen bonding and π-π interactions to form electron donor-acceptor (EDA) complexes. Photoirradiation of the EDA complex induced stepwise, sequential single-electron transfer (SET) processes to generate a HAT-active thiyl radical. The first SET was from the electron-rich naphthyl group of 7,7'-OMe-TPA to the protonated N-heteroaromatics and the second proton-coupled SET (PCET) from the thiophosphoric acid moiety of 7,7'-OMe-TPA to the resulting naphthyl radical cation. Spectroscopic studies and theoretical calculations characterized the stepwise SET process mediated by short-lived intermediates. This organocatalytic HAT system was applied to four different carbon-hydrogen (C-H) functionalization reactions, hydroxyalkylation and alkylation of N-heteroaromatics, acceptorless dehydrogenation of alcohols, and benzylation of imines, with high functional group tolerance.

Brønsted base-catalyzed imino-ene-type allylation reactions of simple alkenes as unactivated allyl compounds

Catalytic imino-ene-type allylation reactions of unactivated allyl compounds were achieved. In the presence of a catalytic amount of a strongly basic KO^tBu-LiTMP or NaO^tBu-LiTMP mixed system, the desired reactions proceeded smoothly at low temperature. Notably, a gaseous alkene, propylene, could also be used in this reaction system.

Development of chiral potassium strong Brønsted base catalysts for enantioselective carbon-carbon bond-forming reactions

Chiral alkaline metal Brønsted bases are traditional and reliable promoters in enantioselective catalysis. Here, new chiral potassium strong base catalysts were developed for enantioselective carbon-carbon bond-forming reactions of weakly acidic carbon pronucleophiles. Chiral potassium amide or alkyl potassium catalyzed enantioselective addition reactions to imines or α,β-unsaturated amides with good to high enantioselectivities. The good potential of chiral potassium Brønsted bases to act as proton transfer catalysts has been shown.

Benzylic Aroylation of Toluenes Mediated by a LiN(SiMe3)2/Cs+ System

Chemoselective deprotonative functionalization of benzylic C-H bonds is challenging, because the arene ring contains multiple aromatic C(sp²)-H bonds, which can be competitively deprotonated and lead to selectivity issues. Recently it was found that bimetallic [MN(SiMe₃)₂ M = Li, Na]/Cs⁺ combinations exhibit excellent benzylic selectivity. Herein, is reported the first deprotonative addition of toluenes to Weinreb amides mediated by LiN(SiMe₃)₂/CsF for the synthesis of a diverse array of 2-arylacetophenones. Surprisingly, simple methyl benzoates also react with toluenes under similar conditions to form 2-arylacetophenones without double addition to give tertiary alcohol products. This finding greatly increases the practicality and impact of this chemistry. Some challenging substrates with respect to benzylic deprotonations, such as fluoro and methoxy substituted toluenes, are selectively transformed to 2-aryl acetophenones. The value of benzylic deprotonation of 3-fluorotoluene is demonstrated by the synthesis of a key intermediate in the preparation of Polmacoxib.