Ruthenium-Catalyzed α-Alkylation of Ketones Using Secondary Alcohols to β-Disubstituted Ketones

Acid-Promoted Multicomponent Reaction To Synthesize 4-Phosphorylated 4H-Chromenes

An effective multicomponent reaction for the synthesis of 4-phosphorylated 4H-chromenes via a tandem phosphorylation/alkylation/cyclization/dehydration sequence with water as the only byproduct was developed. Extensive mechanistic investigations involving in situ NMR experiments, time control experiments, and in situ HRMS experiment allowed us to elucidate the order of each subreaction to arrive at a complete understanding of the underlying mechanism of this multicomponent reaction. Mechanistic data confirm that the reaction begins with a phospha-aldol-elimination, followed by addition of a ketone enolate, intermolecular alkylation, intramolecular cyclization, and dehydration under acidic conditions.

Borrowing hydrogen C-alkylation with secondary saturated heterocyclic alcohols

The borrowing hydrogen methodology (BH) has emerged as a powerful tool for the rapid construction of C-C bonds, offering a greener alternative to traditional multi-step syntheses. This methodology involves the activation of inactivated alcohols followed by condensation or aldolization, ultimately leading to the regeneration of the saturated product. Herein, we report the C-alkylation of a hindered ketone with challenging secondary saturated heterocyclic alcohols. Our study encompasses the optimization of reaction conditions using either an iridium or a ruthenium catalyst and exploration of substrate scope. We demonstrate the efficient synthesis of substituted pyrrolidines and piperidines directly from a triol precursor, showcasing the versatility of this methodology. Moreover, we illustrate the post-functionalization of BH products, significantly broadening their chemical utility.

Nickel-Catalyzed α-Alkylation of Arylacetonitriles with Challenging Secondary Alcohols

Nickel(II) complex 1 was utilized as a sustainable catalyst for α-alkylation of arylacetonitriles with challenging secondary alcohols. Arylacetonitriles with a wide range of functional groups were tolerated, and various cyclic and acyclic secondary alcohols were utilized to yield a large number of α-alkylated products. The plausible mechanism involves the base-promoted activation of precatalyst 1 to an active catalyst 2 (dehydrochlorinated product) which activates the O-H and C-H bonds of the secondary alcohol in a dehydrogenative pathway.

Cu(II) Promoted C(sp3 )-H Activation in Unactivated Cycloalkanes: Oxo-Alkylation of Styrenes to Synthesize β-Disubstituted Ketones

We report the Cu(II) catalyzed synthesis of β-disubstituted ketones from styrene via oxo-alkylation with unactivated cycloalkanes as the alkylating agent in presence of tert-butylhydroperoxide (TBHP) and 1-methylimidazole as oxidant and base respectively. β-disubstituted ketones are known to be synthesized by using either expensive Ru/Ir complexes, or low-cost metal complexes (e. g., Fe, Mn) with activated species like aldehyde, acid, alcohol, or phthalimide derivatives as the alkylating agent, however, use of unactivated cycloalkanes directly as the alkylating agent remains challenging. A wide range of aliphatic C-H substrates as well as various olefinic arenes and heteroarene (35 substrates including 14 new substrates) are well-tolerated in this method. Hammett analysis shed more light on the substitution effect in the olefinic part on the overall mechanism. Furthermore, the controlled experiments, kinetic isotope effect study, and theoretical calculations (DFT) enable us to gain deeper insight of mechanistic intricacies of this new simple and atom-economic methodology.

Chemo- and Regioselective Catalytic Cross-Coupling Reaction of Ketones for the Synthesis of β, γ-Disubstituted β, γ-Unsaturated Ketones

C-C bond forming reaction of ketone with aldehyde is well-studied for the synthesis of α, β-unsaturated ketones, however, the reaction with two different ketones to unsaturated carbonyl compound has not yet been systematically studied. Probably due to the relatively low reactivity of ketones as electrophiles (aldol acceptors), its propensity for retro-aldol reaction. The reactions often suffer from unsatisfactory chemoselectivity (self- vs. crossed aldol products) and regioselectivity (thermodynamic vs. kinetic enolate). In this quest, we report here for the first time selective cross-coupling reaction of ketones to β-branched β, γ-unsaturated ketones by using ruthenium catalysis. Interestingly, single crossed aldol condensation products are formed even in reactions where a mixture of products is possible. Reaction is highly chemoselective, regioselective and produces H2 O as the only byproducts making the protocol environmentally benign. Method is compatible with a wide variety of sensitive functional group and applicable for even problematic aliphatic ketones as substrates. Notably, acetone was found as a three-carbon feedstock for the syntheses of simple β, γ-unsaturated ketone compounds. The process can further be extended to the gram-scale reaction and late-stage functionalization of natural products. With the help of DFT calculations, several control experiments, and deuterium-labeling experiments, the mechanistic finding demonstrated that initial aldol-condensation of ketones to a β, β-disubstituted α, β-unsaturated ketone, which further isomerizes to a β, γ- unsaturated ketone via η3 -allyl ruthenium complex.

Ruthenium-Catalyzed Self-Coupling of Secondary Alcohols

A simple catalytic method for self-coupling of secondary alcohols leading to the synthesis of β-branched ketones under mild conditions is reported. Well-defined ruthenium pincer complex catalyzed the reactions. Optimization studies revealed that sodium tert-butoxide is an appropriate base for this transformation. Functionalized aryl methanols, heteroaryl methanols, and linear and branched aliphatic secondary alcohols underwent facile catalytic self-coupling reactions. Mechanistic studies revealed that both catalyst and base are crucial to achieve dehydrogenation of secondary alcohols to ketones, their subsequent controlled aldol condensation, and further hydrogenation of α,β-unsaturated intermediates, leading to the selective formation of β-branched ketone products. Notably, the noninnocent PNP ligand which displays amine-amide metal-ligand cooperation operative in a catalyst played a key role in facilitating this catalytic self-coupling of secondary alcohols. Liberated molecular hydrogen and water are the only byproducts.

Ruthenium-Catalyzed Selective α-Alkylation of β-Naphthols using Primary Alcohols: Elucidating the Influence of Base and Water

Functionalized arenes and arenols have diverse applications in chemical synthesis and material chemistry. Selective functionalization of arenols is a topic of prime interest. In particular, direct alkylation of arenols using alcohols is a challenging task. In this report, a ruthenium pincer catalyzed direct α-alkylation of β-naphthol using primary alcohols as alkylating reagents is reported. Notably, aryl and heteroaryl methanols and linear and branched aliphatic alcohols underwent selective alkylation reactions, in which water is the only byproduct. Notably, catalytically derived α-alkyl-β-naphthol products displayed high absorbance, emissive properties, and quantum yields (up to 93.2 %). Dearomative bromination on α-alkyl-β-naphthol is demonstrated as a synthetic application. Mechanistic studies indicate that the reaction involves an aldehyde intermediate. DFT studies support this finding and further reveal that a stoichiometric amount of base is required to make the aldol condensation as well as elementary steps required for regeneration of catalytically active species. In situ-generated water molecule from the aldol condensation reaction plays an important role in the regeneration of an active catalyst.

Direct Synthesis of Aldoximes: Ruthenium-Catalyzed Coupling of Alcohols and Hydroxylamine Hydrochloride

A catalytic method for the direct synthesis of oximes from alcohols and hydroxyl amine hydrochloride salt is reported. The reaction is catalyzed by a ruthenium pincer catalyst, which oxidizes alcohols involving amine-amide metal-ligand cooperation, and the in situ formed aldehydes condense with hydroxyl amine to deliver the oximes. Notably, the reaction requires only a catalyst and base; water and liberated hydrogen are the only byproducts, making this protocol attractive and environmentally benign.

Multicomponent Reaction: Pd/Cu-Catalyzed Coupling and Boration of Acyl Chlorides and Alkynes to β-Boryl Ketones

An unprecedented and challenging multicomponent reaction has been developed that allows for the direct transformation of acyl chlorides with alkynes into the corresponding saturated β-boryl ketones via Pd/Cu-catalyzed coupling and boration with ethyl acetate as the hydrogen sources. Various β-boryl ketones were synthesized in good to excellent yields with broad functional group tolerance. In addition, the introduction of boron groups into the products provides substantial opportunities for further conversions.

Ruthenium-Catalyzed Reductive Coupling of Epoxides with Primary Alcohols via Hydrogen Transfer Catalysis

Herein, we report the ruthenium-catalyzed synthesis of β-alkylated secondary alcohols via the regioselective ring-opening of epoxides with feedstock primary alcohols. The reaction utilized alcohol as the carbon source and the terminal reductant. Kinetic and labeling experiments elucidate the hydrogen transfer catalysis that operates via tandem Markovnikov selective transfer hydrogenation of terminal epoxides and hydrogen transfer-mediated cross-coupling of the resulting alcohol with primary alcohol substrates. A broad scope (40 examples including drugs/natural product derivatives) and excellent regioselectivity for a variety of substrates were shown.

Ruthenium-catalysed α-prenylation of ketones using prenol

Prenol and isoprenoids are common structural motifs in biological systems and possess diverse applications. An unprecedented direct catalytic prenylation of ketones using prenol is attained. This C-C bond formation reaction requires only a ruthenium pincer catalyst and a base, and H₂O is the only byproduct.

Recent advances in C/N-alkylation with alcohols through hydride transfer strategies

This review highlights the most recent reports in three powerful and ever-growing fields of borrowing hydrogen, acceptorless dehydrogenative coupling, and base-mediated hydride transfer strategies; which pave the way for generating reactive intermediates via shuttling hydrogen (or hydride) between starting materials without any need for an external hydrogen source to easily construct more complex structures. There is a thorough focus on diversifying the utility of alcohols for C/N-alkylation leading to the synthesis of branched ketones, alcohols, amines, indols, and 6-membered nitrogen-containing heterocycles such as pyridines and pyrimidines, various transformations with the focus on C-C and C-N bond-forming reactions via metal-based catalysis or metal-free approaches in this context to give a global overview in this area.

Interception of Nickel Hydride Species and Its Application in Multicomponent Reactions

The hydrogen borrowing strategy is an economical method for the α-functionalization of ketones. While this strategy is extremely advantageous, it does not lend itself to the synthesis of β,β-disubstituted ketones. This can be achieved, if the in situ generated metal hydride can be intercepted with a nucleophilic coupling partner. We present a multicomponent strategy for the coupling of alcohols, ketones, and boronic acids using only 1 mol % nickel catalyst and without the need for added ligands.

1,2,3-Triazole based ligands with phosphine and pyridine functionalities: synthesis, PdII and PtII chemistry and catalytic studies

This manuscript describes the syntheses of pyridine appended triazole-based mono- and bisphosphines, [o-Ph₂P(C₆H₄){1,2,3-N₃C(Py)C(H)}] (2), [o-Br(C₆H₄){1,2,3-N₃C(Py)C(PPh₂)}] (3), [C₆H₅{1,2,3-N₃C(Py)C(PPh₂)}] (4), [Ph₂P(C₆H₄){1,2,3-N₃C(Py)C(PPh₂)}] (5) and [3-Ph₂P-2-{1,2,3-N₃C(Ph)C(PPh₂)}C₅H₃N] (6), their palladium and platinum chemistry and catalytic applications. These ligands upon treatment with [M(COD)Cl₂] (M = Pd or Pt) yielded complexes with different coordination modes, depending on the reaction conditions. Both κ²-P,N and κ²-P,P coordination modes were observed in many of the complexes indicating the ambidentate nature of these ligands. Monophosphine 2 in the presence of a base afforded rare fused-5,6-membered PCN pincer complexes [MCl{o-Ph₂P(C₆H₄){1,2,3-N₃C(Py)C(H)}}-κ³-P,C,N] (7, M = Pd; 8, M = Pt), whereas the reactions of 4 with [M(COD)Cl₂] (M = Pd, Pt) produced κ²-P,N chelate complexes [MCl₂{C₆H₅{1,2,3-N₃C(Py)C(PPh₂)}-κ²-P,N}] (9, M = Pd; 10, M = Pt). Similar reactions of 5 and 6 resulted in κ²-P,P chelate complexes [MCl₂{{3-Ph₂P-2-{1,2,3-N₃C(Ph)C(PPh₂)}C₅H₃N}-κ²-P,P}] (11, M = Pd; 12, M = Pt) and [MCl₂{3-Ph₂P-2-{1,2,3-N₃C(Ph)C(PPh₂)}C₅H₃N}-κ²-P,P}] (13, M = Pd; 14, M = Pt), respectively. The palladium(II) complexes have shown excellent catalytic activity in the α-alkylation reaction of acetophenone derivatives.

Cleavage via Selective Catalytic Oxidation of Lignin or Lignin Model Compounds into Functional Chemicals

Lignin, a complex aromatic polymer with different types of methoxylated phenylpropanoid connections, enables the sustainable supply of value-added chemicals and biofuels through its use as a feedstock. Despite the development of numerous methodologies that upgrade lignin to high-value chemicals such as drugs and organic synthesis intermediates, the variety of valuable products obtained from lignin is still very limited, mainly delivering hydrocarbons and oxygenates. Using selective oxidation and activation cleavage of lignin, we can obtain value-added aromatics, including phenols, aldehydes, ketones, and carboxylic acid. However, biorefineries will demand a broad spectrum of fine chemicals in the future, not just simple chemicals like aldehydes and ketones containing simple C = O groups. In particular, most n-containing aromatics, which have found important applications in materials science, agro-chemistry, and medicinal chemistry, such as amide, aniline, and nitrogen heterocyclic compounds, are obtained through n-containing reagents mediating the oxidation cleavage in lignin. This tutorial review provides updates on recent advances in different classes of chemicals from the catalytic oxidation system in lignin depolymerization, which also introduces those functionalized products through a conventional synthesis method. A comparison with traditional synthetic strategies reveals the feasibility of the lignin model and real lignin utilization. Promising applications of functionalized compounds in synthetic transformation, drugs, dyes, and textiles are also discussed.

Ru-MACHO-Catalyzed Direct Inter/Intramolecular Macrocyclization of Alcohols and Ketones

Herein we describe a new approach for end-to-end cyclization to construct macrocycles through the inter/intramolecular dehydrogenative coupling of alcohols and ketones in the presence of a Ru-MACHO catalyst. This method is highly atom economical and sustainable and can be used for many substrates. Additionally, this method results in the generation of only water as the byproduct. Moreover, in this approach, high dilution of the reactants is crucial for cyclization and high-yield macrocycle synthesis.

A Proton-Responsive Pyridyl(benzamide)-Functionalized NHC Ligand on Ir Complex for Alkylation of Ketones and Secondary Alcohols

A Cp*Ir(III) complex (1) of a newly designed ligand L¹ featuring a proton-responsive pyridyl(benzamide) appended on N-heterocyclic carbene (NHC) has been synthesized. The molecular structure of 1 reveals a dearomatized form of the ligand. The protonation of 1 with HBF₄ in tetrahydrofuran gives the corresponding aromatized complex [Cp*Ir(L¹ H)Cl]BF₄ (2). Both compounds are characterized spectroscopically and by X-ray crystallography. The protonation of 1 with acid is examined by ¹ H NMR and UV-vis spectra. The proton-responsive character of 1 is exploited for catalyzing α-alkylation of ketones and β-alkylation of secondary alcohols using primary alcohols as alkylating agents through hydrogen-borrowing methodology. Compound 1 is an effective catalyst for these reactions and exhibits a superior activity in comparison to a structurally similar iridium complex [Cp*Ir(L² )Cl]PF₆ (3) lacking a proton-responsive pendant amide moiety. The catalytic alkylation is characterized by a wide substrate scope, low catalyst and base loadings, and a short reaction time. The catalytic efficacy of 1 is also demonstrated for the syntheses of quinoline and lactone derivatives via acceptorless dehydrogenation, and selective alkylation of two steroids, pregnenolone and testosterone. Detailed mechanistic investigations and DFT calculations substantiate the role of the proton-responsive ligand in the hydrogen-borrowing process.

Borrowing Hydrogen for Organic Synthesis

Borrowing hydrogen is a process that is used to diversify the synthetic utility of commodity alcohols. A catalyst first oxidizes an alcohol by removing hydrogen to form a reactive carbonyl compound. This intermediate can undergo a diverse range of subsequent transformations before the catalyst returns the "borrowed" hydrogen to liberate the product and regenerate the catalyst. In this way, alcohols may be used as alkylating agents whereby the sole byproduct of this one-pot reaction is water. In recent decades, significant advances have been made in this area, demonstrating many effective methods to access valuable products. This outlook highlights the diversity of metal and biocatalysts that are available for this approach, as well as the various transformations that can be performed, focusing on a selection of the most significant and recent advances. By succinctly describing and conveying the versatility of borrowing hydrogen chemistry, we anticipate its uptake will increase across a wider scientific audience, expanding opportunities for further development.