Indole cobalt catalysts

The resulting heterocycles in the complex may be further reduced or desilylated (either in the complex or after demetallation). Further synthetic potential exists in the use of the primary products, obtained by cobalt-mediated cycloadditions, as synthons in organic chemistry. For example, indole derivatives have been co-cyclized at the j/ -Cp-cobalt catalyst to give 4a,9a-dihydro-9//-carbazoles or, after oxidation, precursors for strychnine [50]. Remarkably, the cycloaddition of acrolein in the presence of a small amount of methyl acetate occurs at the carbonyl, rather than at the C=C double bond, to give vinylpyran selectively (eq. (19)) [48]. 

Cobaltn-Schiff base complexes, e.g. Co(salen),567 Co(acacen)568 and cobalt(II) porphyrins,569 e.g. Co(TPP), are effective catalysts for the selective oxygenation of 3-substituted indoles to keto amides (equation 249), a reaction which can be considered as a model for the heme-containing enzyme tryptophan-2,3-dioxygenase (equation 21).66 This reaction has been shown to proceed via a ternary complex, Co-02-indole, with probable structure (175), which is converted into indolenyl hydroperoxide (176). Decomposition of (176) to the keto amide (174) readily occurs in the presence of Co(TPP), presumably via formation of a dioxetane intermediate (177).569,56 Catalytic oxygenolysis of flavonols readily occurs in the presence of Co(salen) and involves a loss of one mole of CO (equation 251).570... 

The Beckmann rearrangement of ketoximes to the corresponding amides (31), the Fischer indole cyclization, isomerization of epoxides to the corresponding aldehydes, ketones, or alcohols, hydration and ammo-nolysis of epoxides, oxygen-sulfur interchange, formation of diaryl-ureas and -thioureas from condensation of aniline and carbonyl sulfide, and olefin carbonylation occur over zeolite catalysts (62). The oxo reaction over rhodium and cobalt containing zeolites recently has been claimed (22). 

VolLhardt and coworkers further investigated the scope of these [2+2+2] reactions [99]. They explored the intermolecular reactions between simple A-substituted indoles and diynes. With these substrates 263, the CpCo(CO)2 catalyst used above led only to diyne trimers and ohgomers, but no desired cycloaddition products. However, the use of CpCo(C2H4)2 led to the formation of the desired cycloaddition cobalt complexes in moderate yields. It is interesting to note the difference in reactivity observed for 263 when treated with unsubstituted and trimethylsUyl-substituted diynes (Scheme 70). While the use of unsubstituted 1,6-heptadiyne (264) failed to produce the desired cycloaddition product, the cycloaddition with 1-trimethylsilyl-1,6-heptadiyne (265) resulted in the formation of a single regioisomeric product 266 in 36% yield. In contrast, the reaction of 1,7-octadiyne (267) with 263 produced... 

C—H borylation of indoles has been reported by a number of groups. Chirik and coworkers utilized pincer-ligated cobalt complexes with N-methyhndole and recorded the C-2 borylated indole (133) as the major product (2014JAC4133). The catalysts employed demonstrated high catalysis turnover and low catalytic loading and also demonstrated efficacy with other electron-rich heteroarenes (furan, thiophene, benzofuran) as well as electron-deficient pyridines. More recendy, platinum-NHC complexes have been used in the selective C—H borylation of indoles (2015JAC12211). The authors reported higher isolated yields with this... 

Additionally, the intramolecular cyclization of 2-halo-AT-allylanilines provides optional proeedures for indole derivative synthesis. Many transition metal eatalysts have been developed in this field, such as cobalt, nickel and palladium, but stoichiometric amounts of catalysts are needed. In 1987, a eatalytie version was developed by Laroek and Babu. In the pres-enee of a eatal54ie amount of Pd(OAe)2, indoles, indolines, oxindoles, qui-nolones, isoquinolines and isoquinolones were formed in high yields in short reaetion times under mild temperatures (Seheme 2.106). 

Cobalt NHC catalysts have been reported to promote intramolecular olefin hydroary-lation of Ai-homoallyl indoles (158), having a C(3) aldimine directing group, to afford the 5-mco-trig (159) and 6-enr/o-trig (160) products. The course can be controlled by the choice of the NHC ligand. 

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