Asymmetric Hydrogenation of Unprotected Indoles Catalyzed by η6‑Arene/N‑Me-sulfonyldiamine–Ru(II) Complexes

Protecting-group-free transformation is a challenging and important issue in atom-economical organic synthesis. The η6-arene/N-Me-sulfonyldiamine–Ru­(II)–BF4 complex-catalyzed asymmetric hydrogenation of 2-substituted unprotected indoles in weakly acidic hexafluoroisopropanol gives optically active indoline compounds with up to >99% ee. Under mild reaction media, halogen atoms and synthetically important protecting groups (e.g., silyl ether, acetal, benzyl ether, and ester) on indoles are maintained, which is advantageous for the synthesis of further complex indoline molecules.