Chiral amino acids: evolution in atroposelective C-H activation.

This review covers the journey of chiral amino acids as ligands in atroposelective C-H bond activation/functionalization via transition metal catalysis. Herein, we intend to demonstrate how these chiral amino acids have evolved and flourished in this stimulating field. Unprotected amino acids, mono-N-protected amino acids, and di-N-protected amino acids have been devised for atroposelective C-H activation. In each section, we have briefly discuss the key successes of amino acids in the atroposelective synthesis of biaryls, heterobiaryls, and non-biaryl atropisomers and their advantages in atroposelective C-H activation.

Construction of unsymmetrical heterobiaryls via the Cp*Rh(III)-catalysed C-H/C-H coupling of heteroarenes.

Herein, a concise method for the Rh(III)-catalyzed, directing-group-assisted C-H/C-H cross-coupling of N-heterocycles (quinolines, indolines, indoles, pyridines, pyrimidines, pyrazoles) with other heteroarenes (benzoxazoles, benzofurans, and thiophenes) is disclosed for the synthesis of unsymmetrical heterobiaryl compounds in good to excellent yields. A plausible catalytic cycle has been delineated based on experimental and computational mechanistic studies.

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.

Cp*Co(III)-Catalyzed Selective C8-Olefination and Oxyarylation of Quinoline N-Oxides with Terminal Alkynes.

Herein we report Cp*Co(III)-catalyzed site-selective (C8)-H olefination and oxyarylation of quinoline N-oxides with terminal alkynes. The selectivity for C8-olefination and oxyarylation is sterically and electronically controlled. In the case of quinoline N-oxides (unsubstituted at the C2 position), only the olefination product was obtained irrespective of the nature of the alkynes. In contrast, oxyarylation was observed exclusively when 2-substituted quinoline N-oxides were reacted with 9-ethynylphenanthrene. However, alkynes with electron-withdrawing groups provided only olefination products with 2-substituted quinoline N-oxides. The developed strategy allowed a facile functionalization of quinoline N-oxides bearing natural molecules and an estrone-derived terminal alkyne to deliver the corresponding olefinated and oxyarylated products. To understand the reaction mechanism, control experiments, deuterium-labeling experiments, and kinetic isotope effect (KIE) studies were performed.

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.

Ru(II)-Catalyzed Chemoselective C(sp3)-H Monoarylation of 8-Methyl Quinolines with Arylboronic Acids.

The transition-metal-promoted C-H activation has become an efficient as well as atom-economic methodology for the synthesis of a wide array of organic molecules, but the cost of the metal catalyst and selectivity remain the major challenges. Herein, the first [Cl2Ru(p-cymene)]2-catalyzed direct monoarylation of unactivated C(sp3)-H bonds of 8-methyl quinolines with arylboronic acids to synthesize diarylmethane compounds is presented. The transformation shows a broad substrate scope with high chemoselectivity for the synthesis of 8-benzyl quinolines. In the preliminary mechanistic studies, control experiments, deuterium labeling experiments, and kinetic studies have been performed.

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.

Ru(II)/Rh(III)-Catalyzed C(sp3)-C(sp3) Bond Formation through C(sp3)-H Activation: Selective Linear Alkylation of 8-Methylquinolines and Ketoximes with Olefins.

Herein, [RuCl2(p-cymene)]2/[Cp*RhIIICl2]2-catalyzed direct alkylation of C(sp3)-H bond of 8-methylquinolines with olefins (acrylates, styrenes, and aliphatic) is reported. The alkylation also proceeds with other conjugated systems such as malemides and α,ß-unsaturated ketones. The reaction is highly regioselective, forms only a linear product, and tolerates a variety of functional groups on quinoline and olefin moieties. Control experiments, deuterium labeling, and kinetic studies have been carried out for preliminary understanding of the reaction pathway. The reaction possibly proceeds through five-membered metallacycle under redox-neutral condition. Diversification of alkylated product and late-stage functionalization of ketoxime derivatives of (-)-santonin have also been carried out to demonstrate the applicability of the developed method.