Oxidation indole-based ligands

While many of the species discussed in this chapter are metal-centered radicals, it is clear that formation of ligand-centered radicals by oxidation of redox noninnocent radicals can also play an important role. This includes oxidation of the known redox noninnocent dithiole-thionate-, indole-, and catechol-based ligands, as well as the rather unexpected formation of aminyl radicals upon oxidation of Rh NR2) amido species. 

The intermolecular a-arylation of amides with aryl chlorides was unknown until very recently. Fortunately, the work developed by Walsh and coworkers [93] has demystified the theme. Aryl chlorides are generally less reactive than aryl bromides in oxidative additions nonetheless, aryl chlorides are less expensive and more readily accessible when compared with aryl bromides, making their use in cross-coupling reactions highly desirable. Walsh and coworkers reported the first deprotonative cross-coupling process for the intermolecular arylation of amides with aryl chlorides. Buchwald-type [94] precatalysts formed with Kwong s [95] indole-based phosphane ligand effectively catalyzes the... 

In 2005, Sames and co-workers reported the highly selective (>50 1) C2-arylation of NH-free indoles using a rhodium complex derived from an electron-deficient phosphine as the catalyst, CsOPiv as a base, and iodoar-enes as coupling partners (Scheme 37, 05JA4996). Mechanistic studies revealed that a highly electrophilic Ar-Rh(III) complex was formed in situ through the oxidative addition of aryl iodide to Rh(I) (Scheme 38). Then the displacement of the phosphine ligand by indoles takes place, followed by... 

Copper salts and complexes are used either as oxidant or as catalysts in the presence of a ligand/additive. Cu(acac)2 in THF has been used for oxidative coupling between an indole and a chiral nickel(II) complex (4) in the presence of lithium diisopropylamide (base). The resulting adduct, formed in 63% yield with >99% de, disproportionates readily to 3-indolylglycine derivatives in high yields. 

Palladium-catalyzed Direct Arylation of Indoles and Thiophenes. Five-membered ring heterocycles possessing only one heteroatom and no Af-oxide function can also be arylated using palladium(II) complexes, a phosphine ligand, and an inorganic base. In one example, a tandem palladium-catalyzed Heck coupling reaction and direct intramolecular C2 arylation reaction on a )V-(2-chlorobenzyl)-5-bromoindole was reported (eq 30). The procedure, which is catalyzed by palladium acetate, uses tn-tert-butylphosphonium tetrafluoroborate as a ligand and ferf-butyl acrylate as the alkene for the Heck reaction (eq 30). ... 

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