[RhIL]-catalyzed cyclotetramerization of 1,3-butadiene: a theoretical investigation of alternative mechanistic paths for the generation of the [RhIII(octadienediyl)(PR3)]+ complex

Abstract

A detailed theoretical investigation of alternative mechanistic paths for the formation of the [Rh^III(octadienediyl)(PiPr₃)]⁺ complex is presented, employing a gradient-corrected density functional theory (DFT) method (BP86). This process represents most likely the first step in the recently reported [Rh^IL]-catalyzed cyclotetramerization of butadiene (M. Bosch, M. S. Brookhart, K. Ilg and H. Werner, Angew. Chem., Int. Ed., 2000, 39, 2304). The favorable route for oxidative addition under C–C-bond formation starts from the prevalent [Rh^I(butadiene)₂(PiPr₃)]⁺ form of the active catalyst through oxidative coupling between two cis-η⁴-butadienes. This affords the [Rh^III(bis-η³-anti-octadienediyl)(PiPr₃)]⁺ compound as the kinetic coupling product that consecutively undergoes transformation into the thermodynamically favorable bis-η³-syn-octadienediyl–Rh^III isomer via facile allylic conversions occurring in the octadienediyl framework. The computationally predicted energy profile is almost in quantitative agreement with the experimentally determined kinetics and allows a consistent rationalization of the experimental observations.