Cobalt-catalyzed heterocyclic synthesi

Cobalt-Catalyzed Heterocyclic Synthesis via Carbonylative C-H Activation 15.2.1... 

Cobalt-catalyzed C—C and C-heteroatom bond formation in synthesis of heterocycles 97SL876. 

Review. Vollhardt6 has reviewed the use of cobalt-catalyzed [2 + 2 + 2] cycloadditions for synthesis of annelated benzenes, pyridines, and other heterocycles, as well as of complex natural products. 

Paquin, J.F., Defieber, C., Stephenson, C.R.J., and Carreira, E.M., Asymmetric synthesis of 3,3-diarylpropanals with chiral diene-rhodinm catalysts, J. Am. Chem. Soc. 127 (31), 10850, 2005. Ritter, T., Kvaemo, L., Werder, M., Hanser, H., and Carreira, E.M., Heterocyclic ring scaffolds as small-molecule cholesterol absorption inhibitors, Org. and Biomol. Chem. 3 (19), 3514, 2005. Waser, J., Gonzalez-Gomez, J.C., Nambn, H., Hnber, P., and Carreira, E.M., Cobalt-catalyzed hydro-hydrazination of dienes and enynes access to allyhc and propargylic hydrazides, Org. Lett. 7 (19), 4249, 2005. 

SCHEME 3.11 Cobalt-catalyzed synthesis of nitrogen heterocycles under air [10],... 

Heterocycle Synthesis via Low-Valent Cobalt-Catalyzed C-H Activation 319... 

While major advances in the area of C-H functionalization have been made with catalysts based on rare and expensive transition metals such as rhodium, palladium, ruthenium, and iridium [7], increasing interest in the sustainability aspect of catalysis has stimulated researchers toward the development of alternative catalysts based on naturally abundant first-row transition metals including cobalt [8]. As such, a growing number of cobalt-catalyzed C-H functionalization reactions, including those for heterocycle synthesis, have been reported over the last several years to date (early 2015) [9]. The purpose of this chapter is to provide an overview of such recent advancements with classification according to the nature of the catalytically active cobalt species involved in the C-H activation event. Besides inner-sphere C-H activation reactions catalyzed by low-valent and high-valent cobalt complexes, nitrene and carbene C-H insertion reactions promoted by cobalt(II)-porphyrin metalloradical catalysts are also discussed. 

C-H alkylation and amination reactions involving metal-carbenoid and metal-nitrenoid species have been developed for many years, most extensively with (chiral) dirhodium(ll) carboxylate and carboxamidate complexes as catalysts [45]. When performed in intramolecular settings, such reactions offer versatile methods for the (enantioselective) synthesis of hetero- and carbocy-cles. In the past decade, Zhang and coworkers had explored the catalysis of cobalt(II)-porphyrin complexes for carbene- and nitrene-transfer reactions [46] and revealed a radical nature of such processes as a distinct mechanistic feature compared with typical metal (e.g., rhodium)-catalyzed carbenoid and nitrenoid reactions [47]. Described below are examples of heterocycle synthesis via cobalt(II)-porphyrin-catalyzed intramolecular C-H amination or C-H alkylation. 

In this chapter, the transition metal-catalyzed carbonylative C-H functionalization toward heterocycle synthesis has been summarized. Four metal catalysts such as cobalt, ruthenium, rhodium, and palladium were explored. Among them, palladium obtained more attention than the other three metals. The main challenge for the Pd-catalyzed carbonylation is the excellent reducing ability of CO introducing... 

Scheme 2.38 Synthesis of 5,6,7,8-tetrahydro-l,6-naphthyridines and related heterocycles by cobalt-catalyzed [2-I-2-I-2] cyclization.

Although in principle the thermal [2-I-2-I-2] cycloaddition process is allowed by orbital symmetry rules, there are problems with the entropy component, which may be overcome by using transition metal catalysis. This approach (Scheme 2.35) is one of the most convenient for the synthesis of pyridines 2.100. Metal-induced cycloaddition of two alkyne and one nitrile molecules has been described in general reviews of cycloaddition reactions [3,4]. However in some reviews on heterocycles the nitriles are considered as equivalent to alkyne in the [2+2+2] cyclotrimerization reaction [76], in particular, for the synthesis of pyridines and pyridinones in the reactions catalyzed by cobalt, ruthenium, titanium, and zirconium. 

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