HfCl4–phosphoric acid catalyzed reduction of in situ generated carbocations with hydrosilane: facile approaches to 2,4-dihydrocyclopenta[b]indoles

Abstract

A binary-acid complex HfCl₄/diphenyl phosphate PA could promote in situ generation of stabilized carbocations from γ-indolyl-β,γ-unsaturated α-keto esters, and they were further reduced by hydrosilane (e.g., PhMe₂SiH). Under mild conditions, the reaction afforded 2,4-dihydrocyclopenta[b]indoles in up to 90% yield.