Co(III)-Catalyzed Regioselective Functionalization of Isoquinolones with Naphthoquinones.

A strategy for Co(III)-catalyzed C(sp2)-H alkenylation of N-protected isoquinolones with 1,4-naphthoquinones has been disclosed. The developed protocol was efficiently applied for diversely substituted isoquinolones. Preliminary mechanistic experiments revealed the involvement of a five-membered cobaltacycle as an intermediate. Deuterium labeling experiments suggested the reversible nature of the C-H activation step. The scale-up reaction was also carried out, and the product was utilized as a chemosensor to detect Fe3+ ions.

Inherent directing group-enabled Co(III)-catalyzed C-H allylation/vinylation of isoquinolones.

Co(III)-catalysed site-selective C8-allylation and vinylation of isoquinolones with allyl acetate and vinyl acetates has been accomplished. The oxo group of isoquinolone has been utilised as an inherent directing group. Based on preliminary mechanistic studies, a plausible mechanism for the developed reaction has also been delineated. Broad substrate scope with good to excellent yields and post-synthetic transformations of allylated and vinylated isoquinolines highlight the importance of the reaction.

Chiral phosphoric acids: a chiral lobby for enantioselective C-H activation.

Enantioselective C-H activation is one of the promising methods to obtain enantiopure molecules in high enantiomeric excess with excellent control of regio and chemo-selectivity. The chiral phosphoric acids have evolved as the leading ligands in the enantioselective C-H activation. Chiral phosphoric acids can interact in different ways with the substrate to induce chirality into the system. This review summarizes the use of chiral phosphoric acids in the exciting field of enantioselective C-H activation.

Depolymerization of lignin using laccase from Bacillus sp. PCH94 for production of valuable chemicals: A sustainable approach for lignin valorization.

Lignin is the most abundant aromatic polymer in nature, and its depolymerization offers excellent opportunities to develop renewable aromatic chemicals. In the present study, Bacillus sp. PCH94 was investigated for laccase production and lignin depolymerization. Maximum production of laccase enzyme was achieved within 6.0 h at 50 °C on a natural lignocellulosic substrate. Furthermore, Bacillus sp. PCH94 was used to bioconvert lignin dimeric and polymeric substrates, validated using FT-IR, NMR (1H, 13C), and LCMS. Genome mining of Bacillus sp. PCH94 revealed laccase gene (lacBl) as multicopper oxidase (spore coat CotA). Further, lacBl from Bacillus sp. PCH94 was cloned, expressed, and kinetically characterized. LacBl enzyme showed activity for substrates ABTS (40.64 IU/mg), guaiacol (5.43 IU/mg), and DMP (11.93 IU/mg). The LacBl was active in higher temperatures (10 to 100 °C) and showed a half-life of 36 and 27 h at 50 and 60 °C, respectively. The purified LacBl was able to depolymerize kraft lignin into valuable products (ferulic acid and acetovanillone), which have applications in the pharmaceutical and food industries. Overall, the current study demonstrated the role of bacterial laccase in the depolymerization of lignin and opened a promising prospect for the green production of valuable compounds from recalcitrant lignin.

Anisole hydrodeoxygenation over Ni-Co bimetallic catalyst: a combination of experimental, kinetic and DFT study.

Catalytic hydrodeoxygenation (HDO) of anisole was performed with a series of Ni and Co containing catalysts with different weight ratios on activated carbon (AC) for cyclohexanol production. The catalytic activities of various catalysts revealed that Ni5Co5-AC was the best catalytic system. Structural analysis obtained from XRD, TPR, XPS, and TEM evidently demonstrates that Ni5Co5-AC sample consists of a distorted metal alloy spinel structure and optimum particle size, enhancing its catalytic performance. Kinetics were investigated to identify cyclohexanol production rate, activation energy, and reaction pathway. Structural, experimental, kinetics and density functional simulations suggested that high amount of distorted metallic alloy in Ni5Co5-AC, presence of water, high adsorption efficiency of anisole, and low adsorption tendency of cyclohexanol on metallic alloy surface were the critical factors for HDO of anisole to cyclohexanol.

Regioselective C(sp3)-H amidation of 8-methylquinolines with N-hydroxyphthalimides.

Herein, the Rh(III)-catalysed C(sp3)-H bond amidation of 8-methylquinolines using N-hydroxyphthalimides as the amidation source is explored. Diversely substituted 8-methylquinolines were well tolerated and furnished the amidated products in excellent yields with high regioselectivity. The developed reaction conditions were also applied successfully for the secondary C(sp3)-H amidation of 8-ethylquinolines. Besides that, the reaction is also applicable for the gram-scale synthesis of the amidated product. In addition, the late-stage amidation of santonin oxime as well as carvone oxime and the diversification of the amidated product was also carried out to illustrate the relevance of the developed methodology. Mechanistic studies revealed that the current reaction proceeds through a five-membered rhodacycle intermediate and does not involve the radical pathway.

Recent Advances in the High-Valent Cobalt-Catalyzed C-H Functionalization of N-Heterocycles.

Direct functionalization of heterocycles using C-H activation widely relies on the precious metal complexes. In past decade, the use of earth abundant and inexpensive transition metal to functionalize heterocycles has become an attractive alternate strategy. This concept is also interesting due to the unique reactivity pattern of these inexpensive metals. In this context we and other research groups have utilized the high-valent cobalt complexes as an inexpensive and readily available catalyst for the functionalization of heterocycles. In this review, we intend to brief recent progress made in the area of high-valent cobalt complexes catalyzed C-H functionalization of N-containing heterocycles.

Co(III)-catalysed regioselective linear C(8)-H olefination of isoquinolone with terminal aromatic and aliphatic alkynes.

A regioselective C8 linear olefination of isoquinoline-1H-2-one with terminal (aromatic and aliphatic) alkynes is reported under Co(III) catalysis. This is an exclusive report on the C8 functionalization of isoquinolone using non-noble transition metal complexes. Experimental and computational mechanistic studies have also been performed to depict the reaction pathway.

Cp*Rh(III)-Catalyzed Regioselective C(sp3)-H Electrophilic Trifluoromethylthiolation of 8-Methylquinolines.

Rh(III)-catalyzed regioselective trifluoromethylthiolation of the unactivated C(sp3)-H bond of 8-methylquinolines with bench-stable electrophilic trifluoromethylthiolating reagent via C(sp3)-H activation is explored. Various substituted 8-methylquinolines provided the products in good yields with high regioselectivity. Current reaction conditions are also applicable for the late-stage functionalization of natural molecule santonin and caffeine-substituted 8-methylquinoline.

Merging kinetic resolution with C-H activation: an efficient approach for enantioselective synthesis.

In the last two decades tremendous progress has been made in transition-metal (TM)-catalyzed C-H bond functionalization, paving the way to design complex molecules. Despite significant advances, enantioselective C-H activation is still in the age of infancy. For the enantioselective synthesis, several TM catalyst based approaches are well known, including kinetic resolution (KR) and its advanced versions [dynamic kinetic resolution (DKR) and parallel kinetic resolution (PKR)]. These strategies have recently been successfully applied synergetically with the TM catalyzed C-H activation to achieve enantioselective synthesis in a more economical and sustainable way. This review will summarize the recent advancements made towards merging KR with TM-catalysed C-H activation for enantioselective synthesis.

Cp*RhIII -Catalyzed Sterically Controlled C(sp3 )-H Selective Mono- and Diarylation of 8-Methylquinolines with Organoborons.

Herein, the RhIII -catalyzed selective monoarylation and diarylation (symmetrical and unsymmetrical) of 8-methylquinolines with organoboron reagents are disclosed. The selective monoarylation of primary C(sp3 )-H bonds is achieved by using 7-substituted 8-methylquinolines or by changing the quantity of the aryl boronic acids. The method is also applicable for the arylation of 2-ethylpyridines, and the heteroarylation with thiophene-2-ylboronic acids. Symmetrical and unsymmetrical diarylation of 8-methylquinolines have been carried out in one-pot and sequential manner, respectively. Late-stage monoarylation of oxime derivatives and gram-scale synthesis of monoarylated products has also been carried out. A mechanistic study revealed that the current reaction is first order with respect to both reactants and a five-membered rhodacycle intermediate may be involved in the catalytic cycle.

Catalyst-Free Synthesis of 2-Anilinoquinolines and 3-Hydroxyquinolines via Three-Component Reaction of Quinoline N-Oxides, Aryldiazonium Salts, and Acetonitrile.

A rapid microwave-assisted, catalyst-free, three-component synthesis of various 2-anilinoquinolines from quinoline N-oxides and aryldiazonium salts in acetonitrile under microwave irradiation is reported. This reaction utilizes acetonitrile as a single nitrogen source and involves the formation of two new C-N bonds via the formal [3 + 2] cycloaddition reaction. In the case of 2-substituted quinolines, 3-hydroxyquinoline was observed as the main product via a 1,3 shift of the oxygen atom from N-oxide to the C3 position of quinolines.

Cp*CoIII-Catalyzed Alkylation of Primary and Secondary C(sp3)-H Bonds of 8-Alkylquinolines with Maleimides.

The cobalt(III)-catalyzed C(sp3)-H bond alkylation of 8-methyl quinoline with maleimides is reported. In contrast to the rhodium-catalyzed method, in the current cobalt-catalyzed method, a catalytic amount of acid is used, and importantly, it is also applicable to secondary C(sp3)-H bond alkylation. The developed methodology is applicable for N-alkyl- and N-aryl-substituted maleimides and unsubstituted maleimides, and it also tolerates the variety of functional groups on the 8-methyl quinoline moiety. Atom-economy and high regioselectivity with good to excellent yields of the alkylated products under mild reaction conditions are important features of this method.