Torsten Büttner

Ethanol as Hydrogen Donor: Highly Efficient Transfer Hydrogenations with Rhodium(I) Amides

Homogeneously catalyzed transfer hydrogenation has become a powerful tool in synthetic chemistry, and a wide range of unsaturated substrates can be employed in this reaction. Impressive activities (turnover frequencies TOF> 1 # 10 h ) and selectivites have been reached. Ruthenium(II) arene complexes and rhodium(III) cyclopentadienyl complexes in combination with 2-propanol or formic acid/ triethylamine mixtures as hydrogen donors are among the most popular catalytic systems. Ethanol is a renewable resource and has spurred considerable interest as an alternative to fossil fuels and as a potential feedstock for the chemical industry. Although reduced organometallic complexes are often prepared by reacting a complex with the metal in a higher oxidation state with ethanol (for example, Rh!Rh or Ru!Ru), ethanol has not been investigated systematically as a hydrogen source in transfer hydrogenation. This may be due to the fact that ethanol frequently poisons the catalyst by forming stable and inactive carbonyl complexes and that under basic conditions, aldol condensation products are easily formed with acetaldehyde. We reported that the d Rh diolefin amide [Rh(trop2N)(PPh3)] (2a) is an active catalyst for ketone and imine hydrogenation with H2 (trop2N= bis(5-H-dibenzo[a,d]cyclohepten-5-yl)amide). We report herein that such Rh amide complexes are very efficient catalysts for the reaction in Equation (1).

Amine olefin rhodium(I) complexes: pKa and NH bond strength

A combination of the rigid bis(5H-dibenzo[a,d]cyclohepten-5-yl)amine (trop(2)NH) and 5-amino-5H-dibenzo[a,d]cycloheptene (tropNH(2)) ligand allowed the synthesis of the stable pentacoordinated 18 electron amine olefin rhodium(i) complex [Rh(trop(2)NH)(tropNH(2))]O(3)SCF(3)(); this complex can be cleanly deprotonated [pK(a)(DMSO) = 20.6(1)] to the corresponding amide [Rh(trop(2)N)(tropNH(2))](6) which is reversibly oxidised at -0.466 V (vs. Fc/Fc(+)). The coordinated NH bond strength in is estimated to be 379 +/- 10 kJ mol(-1).