Catalytic Carbon-Carbon Bond-Forming Reactions of Weakly Acidic Carbon Pronucleophiles Using Strong Brønsted Bases as Catalysts

Proton Transfer Anionic Polymerization of Methyl Methacrylate with Ligands for Dual Control of Molecular Weight and Tacticity

Dual control of the molecular weight and tacticity in proton transfer anionic polymerization (PTAP) of methyl methacrylate (MMA) was investigated by using various ligands in the presence of a bulky potassium base catalyst and an organic compound with a weakly acidic C-H bond as dormant species in toluene at 0 °C. The tacticity of the resulting poly(MMA) (PMMA) produced without ligands was nearly atactic (rr/mr/mm = 22/54/24). However, the use of 18-crown-6 as a ligand afforded predominantly syndiotactic PMMA (rr ≈ 58%), whereas the use of chiral bis(oxazoline) ligands gave slightly isotactic-rich PMMA (mm ≈ 32%). Molecular weight control of PMMA was achieved (Đ = 1.1-1.2) by adding 1,1-diphenylethanol as a reversible terminator while maintaining control of the tacticity with the above ligands. Stereoblock PMMA consisting of atactic and syndiotactic segments was successfully synthesized via sequential PTAP using macroinitiator/macro-CTA methods.

Proton transfer anionic polymerization with C-H bond as the dormant species

Living anionic polymerization-the most common living polymerization and the one with the longest history-generally requires stringent, water-free conditions and one metal initiator per polymer chain. Here we present the proton transfer anionic polymerization of methacrylates using acidic C-H bonds as the dormant species that are activated by base catalysts. The polymerization mechanism involves reversible chain transfer or termination of the growing enolate species. A weakly acidic compound, such as an alkyl isobutyrate, serves as the initiator or chain-transfer agent in the presence of a bulky potassium base catalyst to produce a polymer chain and, thereby, diminishes the metal compound per chain ratio. An added alcohol serves as a reversible terminator to tame the propagation. End-functionalized, star, block and graft polymers are easily accessible from compounds with C-H bonds.

Dual Basicity and Nucleophilicity of Organosodium Reagents in Benzylic C-H Additions of Toluenes to Diarylethenes and Ketones

Profiting from the dual high basicity and nucleophilicity of organosodium complexes, here we report the stepwise lateral metalation of a wide range of alkyl arenes (MeAr), mediated by hydrocarbon-soluble NaCH2SiMe3 ⋅ PMDETA (PMDETA=N,N,N',N'',N''-pentamethyldiethylenetriamine), followed by nucleophilic addition to diarylethenes of the newly generated NaCH2Ar ⋅ PMDETA complexes. This method grants access to a range of functionalised hydrocarbons in excellent yields and can be upgraded to catalytic regimes when using trans-stilbene, a 10 mol% of the alkyl sodium base and toluene as a solvent. Extending this approach to aromatic ketones leads to the formation of stilbenes under mild reaction conditions, resulting from the deprotonative coupling of toluenes with ketones. Combining spectroscopic studies with the trapping and characterisation of key reaction intermediates, mechanistic insights have been gained, advancing the understanding of coordination effects in organosodium chemistry, and shedding light on their special reactivity profiles.

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.

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.

KOtBu-catalysed α-homoallylic alkylation of acyclic amides with 1-aryl-1,3-dienes

Herein, we report a KOtBu-catalysed α-homoallylic alkylation of acyclic amides with 1-aryl-1,3-dienes. With this transition metal-free and atom-economic protocol, a series of α-homoallylic alkylated acyclic amides were synthesized in good to excellent yields. This transformation is proposed to proceed through a cation-π interaction-based C-C bond formation from the in situ-generated potassium enolate with the diene unit.

Diastereoselective Direct Catalytic Asymmetric Mannich-Type Reactions of Alkylnitriles with a Ni(II)-Carbene Complex

Despite recent advances in reactions using alkylnitriles as carbon nucleophiles, diastereoselective direct catalytic asymmetric reactions, in which two consecutive chiral centers could be controlled, remain largely unexplored. Herein, we report the addition of alkylnitriles (such as propionitrile) to imines in the presence of a catalytic amount of a chiral pincer-type Ni-carbene complex and potassium 2,6-di-tert-butyl-4-methylphenoxide (KBHT). BHT and molecular sieves were used as additives to improve the yields, diastereoselectivity, and enantiomeric ratio up to >99%, 90:10 anti/syn, and 97.5:2.5 er, respectively. The Mannich adducts can be readily converted to the corresponding β-amino acids.

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.

Lewis Basic Salt-Promoted Organosilane Coupling Reactions with Aromatic Electrophiles

Lewis basic salts promote benzyltrimethylsilane coupling with (hetero)aryl nitriles, sulfones, and chlorides as a new route to 1,1-diarylalkanes. This method combines the substrate modularity and selectivity characteristic of cross-coupling with the practicality of a base-promoted protocol. In addition, a Lewis base strategy enables a complementary scope to existing methods, employs stable and easily prepared organosilanes, and achieves selective arylation in the presence of acidic functional groups. The utility of this method is demonstrated by the synthesis of pharmaceutical analogues and its use in multicomponent reactions.

Chiral Metal Salts as Ligands for Catalytic Asymmetric Mannich Reactions with Simple Amides

Catalytic asymmetric Mannich reactions of imines with weakly acidic simple amides were developed using a chiral potassium hexamethyldisilazide (KHMDS)-bis(oxazoline) potassium salt (K-Box) catalyst system. The desired reactions proceeded to afford the target compounds in high yields with high diastereo- and enantioselectivities. It was suggested that a K enolate interacted with K-Box to form a chiral K enolate that reacted with imines efficiently. In this system, K-Box (potassium salt of Box) worked as a chiral ligand of the active potassium species.

Alkali-Metal Mediation: Diversity of Applications in Main-Group Organometallic Chemistry

Organolithium compounds have been at the forefront of synthetic chemistry for over a century, as they mediate the synthesis of myriads of compounds that are utilised worldwide in academic and industrial settings. For that reason, lithium has always been the most important alkali metal in organometallic chemistry. Today, that importance is being seriously challenged by sodium and potassium, as the alkali-metal mediation of organic reactions in general has started branching off in several new directions. Recent examples covering main-group homogeneous catalysis, stoichiometric organic synthesis, low-valent main-group metal chemistry, polymerization, and green chemistry are showcased in this Review. Since alkali-metal compounds are often not the end products of these applications, their roles are rarely given top billing. Thus, this Review has been written to alert the community to this rising unifying phenomenon of "alkali-metal mediation".

Asymmetric C(sp3)-H functionalization of unactivated alkylarenes such as toluene enabled by chiral Brønsted base catalysts

Benzylic functionalisation of unactivated alkylarenes remains as a significant challenge in asymmetric catalysis due to their less reactive nature. Here, we show development of catalytic asymmetric C(sp³)-H functionalization of unactivated alkylarenes such as toluene with imines. The reactions proceeded smoothly under proton-transfer conditions using a chiral, strong Brønsted base catalyst system. A chiral Brønsted base prepared from an alkylpotassium and a chiral amine ligand was found to effectively form a promising asymmetric environment around a benzyl anion. Optimization of the reaction conditions revealed that the use of the alkaline metal amide, potassium hexamethyldisilazide (KHMDS), as an additive was most effective, and enantioselective and atom economical carbon-carbon bond-forming reactions at the benzylic positions of unactivated alkylarenes was achieved without using any transition-metal catalyst.

Catalytic amide base system generated in situ for 1,3-diene formation from allylbenzenes and carbonyls

The amide base generated in situ from tetramethylammonium fluoride and N(TMS)₃ catalyzes the synthesis of 1,3-diene from an allylbenzene and carbonyl compound. The system is applicable to the transformations of a variety of allylbenzenes with functional groups (halogen, methyl, phenyl, methoxy, dimethylamino, ester, and amide moieties). Acyclic and cyclic diaryl ketones, pivalophenone, pivalaldehyde, and isobutyrophenone are used as coupling partners. The role of transβ-methyl stilbenes in product formation is also elucidated.

Base-promoted addition of DMA with 1,1-diarylethylenes: application to a total synthesis of (-)-sacidumlignan B

A base-promoted addition of DMA (N,N-dimethylacetamide) to 1,1-diarylethylenes has been developed, and it provides a new strategy for the synthesis of N,N-dimethyl-4,4-diarylbutanamides from 1,1-diarylethylenes at room temperature. This method allows us to achieve the goal of synthesizing (-)-sacidumlignan B, and provides simple operation and broad substrate scope by avoiding the use of transition metal catalysts.

V. K, Hari K, Kaliyamoorthy A. Base-promoted 1,6-conjugate addition of alkylazaarenes to para-quinone methides

1,1,2-Triarylethanes embedded with an azaarene unit were prepared in a single step at ambient temperature via the sodium hexamethyldisilazide mediated 1,6-conjugate addition of unactivated alkylazaarenes on para-quinone methides (p-QMs).

Alkyl lithium-catalyzed benzylic C–H bond addition of alkyl pyridines to α-alkenes

The alkyl lithium catalyst successfully achieved the benzylic C–H bond addition of alkyl pyridines to α-alkenes, and displayed distinct selectivity from those of transition metal catalysts.

Electrochemical, Manganese-Assisted Carbon-Carbon Bond Formation between β-Keto Esters and Silyl Enol Ethers

The electrochemical carbon-carbon bond formation process between β-keto esters and silyl enol ethers was investigated utilizing manganese salts. The tricarbonyl compounds were generated in moderate to good yields under neutral conditions. Control experiments revealed that an electro-generated base at the cathode is important. Electroanalytical measurements with a Mn(TPA) complex suggested that the oxidation of the silyl enol ether is the first step in the oxidation process initiated by a corresponding Mn(IV) species.

One-pot aminobenzylation of aldehydes with toluenes

Amines are fundamental motifs in bioactive natural products and pharmaceuticals. Using simple toluene derivatives, a one-pot aminobenzylation of aldehydes is introduced that provides rapid access to amines. Simply combining benzaldehydes, toluenes, NaN(SiMe3)2, and additive Cs(O2CCF3) (0.35 equiv.) generates a diverse array of 1,2-diarylethylamine derivatives (36 examples, 56-98% yield). Furthermore, suitably functionalized 1,2-diarylethylamines were transformed into 2-aryl-substituted indoline derivatives via Buchwald-Hartwig amination. It is proposed that the successful deprotonation of toluene by MN(SiMe3)2 is facilitated by cation-π interactions between the arene and the group(I) cation that acidify the benzylic C-Hs.

Catalytic alkylation reactions of weakly acidic carbonyl and related compounds using alkenes as electrophiles

Catalytic alkylation reactions of weakly acidic carbonyl and related pronucleophiles such as amides, esters, and sulfonamides with substituted alkenes have been reported.