Making Fe(BPBP)-catalyzed C-H and CC oxidations more affordable

Abstract

The limited availability of catalytic reaction components may represent a major hurdle for the practical application of many catalytic procedures in organic synthesis. In this work, we demonstrate that the mixture of isomeric iron complexes [Fe(OTf)2(mix-BPBP)] (mix-1), composed of Λ-α-[Fe(OTf)2(S,S-BPBP)] (S,S-1), Δ-α- [Fe(OTf)2(R,R-BPBP)] (R,R-1) and Δ/Λ-β-[Fe(OTf) 2(R,S-BPBP)] (R,S-1), is a practical catalyst for the preparative oxidation of various aliphatic compounds including model hydrocarbons and optically pure natural products using hydrogen peroxide as an oxidant. Among the species present in mix-1, S,S-1 and R,R-1 are catalytically active, act independently and represent ca. 75% of mix-1. The remaining 25% of mix-1 is represented by mesomeric R,S-1 which nominally plays a spectator role in both C-H and C=C bond oxidation reactions. Overall, this mixture of iron complexes displays the same catalytic profile as its enantiopure components that have been previously used separately in sp3 C-H oxidations. In contrast to them, mix-1 is readily available on a multi-gram scale via two high yielding steps from crude dl/meso-2,2′-bipyrrolidine. Next to its use in C-H oxidation, mix-1 is active in chemospecific epoxidation reactions, which has allowed us to develop a practical catalytic protocol for the synthesis of epoxides.