PALLADIUM-CATALYZED C–H BOND FUNCTIONALIZATION OF HETEROCYCLES AND AMINES
Nadres, Enrico 1979-
MetadataShow full item record
Palladium-catalyzed functionalization of C–H bonds is becoming an important synthetic tool that allows the preparation of desired substances in fewer steps and higher yields compared to traditional synthetic routes. The C–H bonds can be directly converted to C–C or C–heteroatom bonds. However, the ubiquity of C–H bonds in organic compounds can lead to problems in chemo- and regioselectivity. In heterocycles, the control of regioselectivity of the reaction is governed by the difference in acidity of the heterocyclic ring C–H bonds. An economical method for the arylation of C–H bonds of pyrroles and furans by aryl chlorides was developed. The method employs a palladium acetate catalyst, 2-(dicyclohexylphosphino)biphenyl ligand, and an inorganic base. Electron-rich, electron-poor, and heterocyclic aryl chloride coupling partners can be used and arylated heterocycles are obtained in moderate to good yields. The functionalization of sp2 and sp3 C–H bonds can be promoted by the use of directing groups that coordinate the Pd catalyst and activate the desired C–H functionalities. Use of Pd(OAc)2 in conjunction with cesium acetate or potassium carbonate bases allows functionalization of sp2 and sp3 C–H bonds in amides possessing picolinic acid directing group. Stoichiometric silver additive is not required in contrast with previously published procedure. Arylations are effective for sp2 as well as primary and secondary sp3 C–H bonds. Alkylations of sp2 C–H bonds are successful in most cases. Both primary and secondary alkyl iodides are reactive but secondary alkyl iodides afford low yields. Alkylation of sp2 C–H bonds is low yielding and the reaction requires further optimization. Alkyl and aryl iodides as well as benzyl bromides are reactive. Aryl and alkyl bromides afford no product. Direct conversion of C–H bonds to C–N bonds was also developed. Pd-catalyzed method for pyrrolidine, indoline, and isoindoline formation by cyclization via C–H/N–H coupling is presented. The method employs a picolinamide directing group, PhI(OAc)2 oxidant, and toluene solvent at 80–120 °C. Cyclization is effective for sp2 as well as aliphatic and benzylic sp3 C–H bonds.