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31.40.3.2.2.5.3 C—H Activation of Indoles

DOI: 10.1055/sos-SD-131-00300

Virieux, D.; Ayad, T.; Pirat, J.-L.; Volle, J.-N.Science of Synthesis Knowledge Updates, (20181297.

C—H activation processes drastically increase the organic chemistry toolbox for the introduction of functional groups at dedicated positions. Yang and co-workers have reported the copper-catalyzed cross-coupling reaction of indoles with various H-phosphinates to give indole derivatives 68 (Scheme 46).[‌73‌] Copper(I) chloride appears to be the best catalyst, and monodentate ligands such as triphenylphosphine are more efficient than bidentate phosphines [e.g., BINAP, 1,2-bis(diphenylphosphino)ethane, and 1,1′-bis(diphenylphosphino)ferrocene]. In terms of the mechanism, radical processes can clearly be excluded. It is suggested that the reaction occurs through the release of hydrogen and the formation of a copper–hydride complex. This is confirmed by the reaction of diphenylphosphine oxide in the presence of Stryker’s reagent (i.e., hexameric triphenylphosphine–copper hydride cluster). The reaction also displays a wide tolerance of various functional groups, including electrophilic carbonyl groups (e.g., ketones and aldehydes), as well as the chlorine atom. Stereoselectivity remains high and occurs with retention of configuration at the phosphorus center if chiral menthyl phenyl-H-phosphinate is used.

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