Rhenium hydride/boron lewis acid cocatalysis of alkene hydrogenations: Activities comparable to those of precious metal systems

Article abstract of DOI:10.1021/ja107187r

Dibromonitrosyl(dihydrogen)rhenium(I) complexes [ReBr₂(NO) (PR₃)₂(η²-H₂)] (1; R = iPr, a; Cy, b) and Me₂NH?BH₃ (DMAB) catalyze at either 90 °C or ambient temperature under 10 bar of H₂ the hydrogenation of various terminal and cyclic alkenes (1-hexene, 1-octene, cyclooctene, styrene, 1,5-cyclooctadiene, 1,7-octadiene, α-methylstyrene). Maximum turnover frequency (TOF) values of 3.6 × 10⁴ h^-1 at 90 °C and 1.7 × 10⁴ h^-1 at 23 °C were achieved in the hydrogenation of 1-hexene. The extraordinary catalytic performance of the 1/DMAB system is attributed to the formation of five-coordinate rhenium(I) hydride complexes [Re(Br)(H)(NO)(PR₃)₂] (2; R = iPr, a; Cy, b) and the action of the Lewis acid BH₃ originating from DMAB. The related 2/BH₃?THF catalytic system also exhibits under the same conditions high activity in the hydrogenation of various alkenes with a maximum turnover number (TON) of 1.2 × 10⁴ and a maximum TOF of 4.0 × 10⁴ h^-1. For the hydrogenations of 1-hexene with 2a and 2b, the effect of the strength of the boron Lewis acid was studied, the acidity being in the following order: BCl₃ > BH₃ > BEt₃ ≈ BF₃ > B(C₆F₅) ₃ > BPh₃ ? B(OMe)₃. The order in catalytic activity was found to be B(C₆F₅)₃ > BEt₃ ≈ BH₃?THF > BPh₃ ? BF₃?OEt₂ > B(OMe)₃ ? BCl ₃. The stability of the catalytic systems was checked via TON vs time plots, which revealed the boron Lewis acids to cause an approximate inverse order with the Lewis acid strength: BPh₃ > BEt₃ ≈ BH₃?THF > B(C₆F₅)₃. For the 2a/BPh₃ system a maximum TON of 3.1 × 10⁴ and for the 2a/B(C₆F₅)₃ system a maximum TOF of 5.6 × 10⁴ h^-1 were obtained in the hydrogenation of 1-hexene. On the basis of kinetic isotope effect determinations, H ₂/D₂ scrambling, halide exchange experiments, Lewis acid variations, and isomerization of terminal alkenes, an Osborn-type catalytic cycle is proposed with olefin before H₂ addition. The active rhenium(I) monohydride species is assumed to be formed via reversible bromide abstraction with the "cocatalytic" Lewis acid. Homogeneity of the hydrogenations was tested with filtration and mercury poisoning experiments. These "rhenium(I) hydride/boron Lewis acid" systems demonstrate catalytic activities comparable to those of Wilkinson- or Schrock-Osborn-type hydrogenations accomplished with precious metal catalysts.