Palladium-Catalyzed Direct Cyclopropylation of Heterocycles
In 2014, Professor Jianrong (Steve) Zhou’s group at the Nanyang Technical University in Singapore reported a palladium-catalyzed process for the alkylation of unsaturated heterocycles. The chemistry capitalized on Osborn’s initial discovery in the 1970s that phosphine complexes of Pd(0) react with alkyl halides to produce alkyl radicals via single-electron-transfer processes. Under the reaction conditions developed in Zhou’s lab, the alkyl radicals that were involved in the catalytic bond formation can be trapped by TEMPO. The key step of direct radical addition to heteroarenes without intervention by metal catalysts was further supported by DFT calculations, and many types of substrates were suitable for this transformation. For the current study, the researchers decided to investigate whether a similar radical process was possible by using cyclopropyl iodides together with heterocycles.
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