Dimeric Potassium Amide-Catalyzed α-Alkylation of Benzyl Sulfides and 1,3-Dithianes

Potassium tert-butoxide mediated stereoselective/direct Mannich reaction of α-substituted-γ-lactams with in situ generated aryl N-silyl imines

A potassium tert-butoxide (KOt-Bu)-mediated Mannich reaction between α-substituted-γ-lactams and N-silyl imines is reported. N-silyl imines are generated in situ from readily available aryl nitriles and directly combined with the lactams, without preformation of the lactam enolate, to afford the α-quaternary center-bearing Mannich bases in high yield and with high diastereoselectivity (24 examples). This reaction is shown to be catalytic with respect to KOt-Bu and the catalytic mechanism has been investigated using density functional theory calculations. The computational investigations suggest that the diastereoselectivity is controlled by explicit interactions between a binuclear potassium complex and both the imine nitrogen and the enolate oxygen atoms in the selectivity-determining transition states. The Mannich products are shown to be useful in accessing novel spirocyclic pyrrolidines.

Alkali Metal Amide-Catalyzed α-Deuteration of Sulfides

The catalytic α-deuteration of sulfides was developed under mild conditions by using alkali metal amides [KN(SiMe3)2 and CsN(SiMe3)2] as the catalyst. This approach successfully achieves a selective and efficient H/D exchange reaction of sulfides with D2 without using transition metal catalysts. A series of deuterium-labeled thioanisoles and alkyl methyl sulfides were obtained in good to high levels of deuterium incorporation.

Potassium Hexamethyldisilazide (KHMDS): Solvent-Dependent Solution Structures

Solution structures of potassium hexamethyldisilazide [KHMDS] and labeled [15N]KHMDS were examined using a number of analytical methods including 29Si NMR spectroscopy and density functional theory computations. A combination of 15N-29Si couplings, 29Si chemical shifts, and the method of continuous variations reveals dimers, monomers, and ion pairs. Weakly coordinating monofunctional ligands such as toluene, N,N-dimethylethylamine, and Et3N afford exclusively dimers. 1,3-Dioxolane, THF, dimethoxyethane, hexamethylphosphoramide, and diglyme provide dimers at low ligand concentrations and monomers at high ligand concentrations. N,N,N',N'-Tetramethylethylenediamine and N,N,N',N'-tetramethylcyclohexanediamine provide exclusively dimers at all ligand concentrations at ambient temperatures and significant monomer at -80 °C. Studies of 12-crown-4 ran into technical problems. Equimolar 15-crown-5 forms a dimer, whereas excess 15-crown-5 affords a putative ion pair. Whereas equimolar 18-crown-6 also affords a dimer, an excess provides a monomer rather than a solvent-separated ion pair. [2.2.2]cryptand affords what is believed to be a contact-ion-paired cryptate. Solvation was probed using largely density functional theory (DFT) computations. Thermally corrected energies are consistent with lower aggregates and higher solvates at low temperatures, but the magnitudes of the computed temperature dependencies were substantially larger than the experimentally derived data.

KOtBu/DMF-Mediated Hydroalkylation of Alkenes via Benzylic C-H Bond Activation

Catalytic hydroalkylation reaction of alkenes with benzylic hydrocarbons involving t-BuOK/DMF-mediated benzylic C-H bond activation is demonstrated. This direct and operational simple protocol affords a rapid and reliable access to a wide scope of benzylic compounds in good-to-excellent yields. The benzylic C-H's of either activated diarylmethanes (pKa ∼ 32.2) and benzyl thioethers (pKa ∼ 30.8) or inert alkylbenzenes could all act as useful synthetic platforms to be conveniently alkylated under mild reaction conditions.

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).

Mechanistic insights into rare-earth-catalysed C-H alkylation of sulfides: sulfide facilitating alkene insertion and beyond

The catalytic C-H alkylation with alkenes is of much interest and importance, as it offers a 100% atom efficient route for C-C bond construction. In the past decade, great progress in rare-earth catalysed C-H alkylation of various heteroatom-containing substrates with alkenes has been made. However, whether or how a heteroatom-containing substrate would influence the coordination or insertion of an alkene at the catalyst metal center remained elusive. In this work, the mechanism of Sc-catalysed C-H alkylation of sulfides with alkenes and dienes has been carefully examined by DFT calculations, which revealed that the alkene insertion could proceed via a sulfide-facilitated mechanism. It has been found that a similar mechanism may also work for the C-H alkylation of other heteroatom-containing substrates such as pyridine and anisole. Moreover, the substrate-facilitated alkene insertion mechanism and a substrate-free one could be switched by fine-tuning the sterics of catalysts and substrates. This work provides new insights into the role of heteroatom-containing substrates in alkene-insertion-involved reactions, and may help guide designing new catalysis systems.

Direct alkylation of N,N-dialkyl benzamides with methyl sulfides under transition metal-free conditions

Amides are a fundamental and widespread functional group, and are usually considered as poor electrophiles owing to resonance stabilization of the amide bond. Various approaches have been developed to address challenges in amide transformations. Nonetheless, most methods use activated amides, organometallic reagents or transition metal catalysts. Here, we report the direct alkylation of N,N-dialkyl benzamides with methyl sulfides promoted by the readily available base LDA (lithium diisopropylamide). This approach successfully achieves an efficient and selective synthesis of α-sulfenylated ketones without using transition-metal catalysts or organometallic reagents. Preliminary mechanism studies reveal that the deprotonative aroylation of methyl sulfides is promoted by the directed ortho-lithiation of the tertiary benzamide with LDA.

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.

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.

C-Alkylation of alkali metal carbanions with olefins

C-Alkylations of alkali metal carbanions with olefins, first reported five decades ago, is a class of reaction undergoing a resurgence in organic synthesis in recent years. As opposed to expectations from classical chemistry and transition metal-catalysis, here olefins behave as closed-shell electrophiles. Reactions range from highly reactive alkyllithiums giving rise to anionic polymerization, to moderately reactive alkylpotassium or alkylsodium compounds that give rise to defined, controlled and bimolecular chemistry. This review presents a brief historical overview on C-alkylation of alkali metal carbanions with olefins (typically mediated by KOtBu and KHMDS), highlights contemporary applications and features developing mechanistic understanding, thereby serving as a platform for future studies and the widespread use of this class of reaction in organic synthesis.

Photocatalytic Reductive Radical-Polar Crossover for a Base-Free Corey-Seebach Reaction

A metal-free generation of carbanion nucleophiles is of prime importance in organic synthesis. Herein we report a photocatalytic approach to the Corey-Seebach reaction. The presented method operates under mild redox-neutral and base-free conditions giving the desired product with high functional group tolerance. The reaction is enabled by the combination of photo- and hydrogen atom transfer (HAT) catalysis. This catalytic merger allows a C-H to carbanion activation by the abstraction of a hydrogen atom followed by radical reduction. The generated nucleophilic intermediate is then capable of adding to carbonyl electrophiles. The obtained dithiane can be easily converted to the valuable α-hydroxy carbonyl in a subsequent step. The proposed reaction mechanism is supported by emission quenching, radical-radical homocoupling and deuterium labeling studies as well as by calculated redox-potentials and bond strengths.

Synthesis of diarylalkanes through an intramolecular/intermolecular addition sequence by auto-tandem catalysis with strong Brønsted base

An auto-tandem catalysis with a strong Brønsted base enabled the synthesis of diarylalkanes containing a benzofuran moiety. Potassium tert-butoxide efficiently catalyzed both the intramolecular cyclization of less acidic ortho-alkynylaryl benzyl ethers and the following intermolecular addition of diarylmethanes to styrenes, demonstrating the high potential of the catalysis in organic synthesis.

Base-catalyzed C-alkylation of potassium enolates with styrenes via a metal-ene reaction: a mechanistic study

Base-catalyzed, C-alkylation of potassium (K) enolates with styrenes (CAKES) has recently emerged as a highly practical and convenient method for elaboration or synthesis of pharmaceutically-relevant cores. K enolate-type precursors such as alkyl-substituted heterocycles (pyridines, pyrazines and thiophenes), ketones, imines, nitriles and amides undergo C-alkylation reactions with styrene in the presence of KOtBu or KHMDS. Surprisingly, no studies have probed the reaction mechanism beyond the likely initial formation of a K enolate. Herein, a synergistic approach of computational (DFT), kinetic and deuterium labelling studies rationalizes various experimental observations and supports a metal-ene-type reaction for amide CAKES. Moreover, our approach explains experimental observations in other reported C-alkylation reactions of other enolate-type precursors, thus implicating a general mechanism for CAKES.

CuSO4-Catalyzed Tandem C(sp3)-H Insertion Cyclization of Toluenes with Isonitriles to Form Indoles

A CuSO₄-catalyzed tandem benzylic C-H insertion cyclization of toluene derivatives and isonitriles is described. The naturally abundant salt CuSO₄ serves as a low-cost ligand-free redox catalyst. This reaction provides a practical modular synthesis of N-aryl indoles from isonitriles.