Coupling Titanium and Chromium Catalysis in a Reaction Network for the Reprogramming of [BH4 ]- as Electron Transfer and Hydrogen Atom Transfer Reagent for Radical Chemistry

Abstract

We describe a unique catalytic system with an efficient coupling of Ti- and Cr-catalysis in a reaction network that allows the use of [BH₄ ]^- as stoichiometric hydrogen atom and electron donor in catalytic radical chemistry. The key feature is a relay hydrogen atom transfer from [BH₄ ]^- to Cr generating the active catalysts under mild conditions. This enables epoxide reductions, regiodivergent epoxide opening and radical cyclizations that are not possible with cooperative catalysis with radicals or by epoxide reductions via Meinwald rearrangement and ensuing carbonyl reduction. No typical S_N 2-type reactivity of [BH₄ ]^- salts is observed.

Keywords: Borohydrides; Chromium; Cooperative Catalysis; Epoxide; Titanium.