Mechanistic Aspects of the Crabtree‐Pfaltz Hydrogenation of Olefins ‐ An Interplay of Experimentation and Quantum Chemical Computation

Abstract

Introduction of Crabtree's iridium‐based hydrogenation catalyst in 1977 marked a paradigm shift both with respect to the role of iridium in homogeneous catalysis as well as catalytic hy­dro­genation of olefins. In 1998, Pfaltz introduced an improved catalyst, by use of BARF‐ as anion, and established the first chiral variant of the Crabtree catalyst. This led to nume­rous practical highly enantioselective syntheses. Elucidation of mechanistic de­tails posed great problems because of instability of the crucial intermediates. A remarkable breakthrough was achieved by Brandt, Andersson et al. in 2003, based on dft calculations. These authors replaced a previously assumed IrI/IrIII catalytic cycle by a novel IrIII/IrV cycle. The proposal was experimentally verified by Pfaltz in 2014 and corroborated by advanced quantum chemical calculations. This essay is an attempt to de­scribe a fascinating interplay of experi­ments and quantum chemical calculations for an important syn­thet­ic method.