Cobalt-catalyzed C-H alkylation

Scheme 19.44 Cobalt-catalyzed C-H alkylation of benzo[h]quinoline with the Grignard reagents.

The scope of the cobalt-catalyzed C-H amination was extended to phosphoryl azide derivatives (Scheme 10.20) [49]. Amination of the benzylic C-H bond of an alkyl group ortho to the phosphoryl azide is efficiently promoted by a cobalt(II) complex [Co(Pl)j with a D2 -symmetric tetraarylporphyrin ligand, which features proximal amide functionalities, thus affording a six-membered cyclic product. The amide functionalities of the catalyst appear to play critical roles in the amination reaction presumably by facihtating catalyst-substrate... 

Wang, Shi, and coworkers [70] reported the cobalt-catalyzed directed C-H alkylation of benzo[h]quinoHne with alkyl Grignard s reagents (Scheme 19.44). The reaction occurred at room temperature but proved less efficient and general than the corresponding C-H arylation. Interestingly, when isopropylmagnesium bromide was employed, a mixture of Hnear and branched products was obtained. The... 

Scheme 19.45 Cobalt-catalyzed directed C-H alkylation of benzamides and phenylpyridines...

Scheme 19.92 Cobalt-catalyzed directed ortho-C-H alkylation.

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. 

The elaborate cobalt(II)-porphyrin catalyst [Co(Pl)] further enabled intramolecular amination of C(sp )-H bonds in simple alkyl chains, adjacent to electron-withdrawing groups, or at the allylic or propargylic positions through decomposition of sulfamoyl azide (Scheme 10.21) [50]. Thus, the catalyst promotes nitrene insertion into such C(sp )-H bonds located at the y-position of the sulfamoyl group to afford six-membered cyclic sulfamides in excellent yields, which serve as versatile 1,3-diamine scaffolds. On the basis of mechanistic experiments, a mechanism that is distinct from related Rh -catalyzed C-H amination was proposed. Thus, the 15-electron metalloradical catalyst [Co(Pl)j decomposes the sulfamoyl azide to form a Co -radical nitrene species, which... 

Scheme 10.22 Enantioselective intramolecular C-H alkylation with acceptor/acceptor-substituted diazo compound catalyzed by chiral cobalt(ll)-porphyrin complex.

More recently, Yoshikai [135] developed a cobalt-catalyzed chelation-assisted C-H bond alkylation with alkenes. The reaction was initially developed with 2-arylpyridines (Scheme 19.92) [136], The authors demonstrated the influence of the ligand on the reaction, leading selectively to the formation of the linear or the branched product. 

Cobalt hydrocarbonyl-catalyzed olefin hydroformylation with network 6.9 may serve as an example. Cobalt, with atomic number 27, contributes nine valence electrons to its complexes (the other eighteen occupy the inner 1-5, 2-s, 2-p, 3-5, and 3-p orbitals) H, the alkyl group, and the acyl group contribute one each, CO contributes two (of its fourteen electrons, four are shared by C and O in the double bond, an additional four each complete the inner octets of C and O), and an olefin ligand contributes the two Tr-electrons of its double bond. The contributions and totals for some key participants are ... 

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