Catalysis of Organic..

Riermeier et al. 459Reaction of the diphosphines with [Rh(COD)2]BF4 afforded precatalysts of thetype [Rh(COD)(ligand)]BF4. DFT calculations and X-ray structural analysesrevealed that the P-Rh-P bite angles of complexes based on 1a-c are significantlylarger than that of the corresponding DuPHOS complex. Compared to the benzenebackbone in DuPHOS, the five-membered and four-membered backbone,respectively of catASium fi M ligands cause a larger exo PC=C angle. This in turnwidens the bite angle in the rhodium complexes.Corresponding rhodium precatalysts were tested in the enantioselectivehydrogenation. In the first trials, as a benchmark test standard substrates like methyl(Z)-N-acetamido cinnamate or itaconates were employed. In general, catalysts basedon new ligands afforded good or excellent enantioselectivities. In some cases adecline in the enantioselectivity was noted by application of the ligand with thesquaric acid backbone 1c. The corresponding Rh-complex has the largest bite angleamong the catalyst investigated.Obviously, the enantioselective hydrogenation has an optimum for each ligandand solvent used. Figure 5 exemplifies a typical fine tuning approach employingfour related bisphospholane ligands. After three levels of testing, 99 % ee could beachieved in the hydrogenation.NHAc[Rh(COD)(Ligand)]BF 4 , H 2 (1 bar) solventNHAcPhCOOMeLigand:RYRPhCOOMePPRR1.LevelR: Mesolvent: MeOH2.LevelR: Mesolvent: THF3.LevelR: Etsolvent: THFY%ee%ee%ee97.697.4-OMeNO96.997.3-OOO94.498.699.0OO83.982.3-Figure 5. Fine tuning of enantioselective hydrogenation (only the best ligand afterlevel 2 was taken to level 3 of the optimization)