D. A. Evans Diastereoaselective Hydrogenation: Cationic Catalysts Chem 206 mechanism of Hydrogenation Cationic Rhodium-(()Catalysts D.A. Evans&M. M. Morrissey JACS 106, 3866(1984) 0 Catalyst Mol% Catalyst H2 Pressure trans: cis(Yield) 18e 15 psi H2 200:1(89%) Rh(DIPHOS-4) 375 psi H2 300:1(95%) Irpyr) PCy3 15 psi H2 15 psi H2 Which hydrogen P-Rh- 19:1 Rh(DIPHOS-4) H2 1000 psi CH2Ch Excessive steric Hindrance A potential stereoelectronic effect H Retigeranic Acid That H atom lying parallel to the pi-system(HA) should migrate preferentially if the dihydride is an intermediate THF is important to success of nxn to buffer the Lewis acidity of the catalyst which causes elimination of roh under normal conditionsRh Ph2P PPh2 S S + – BF4 CH2OH CH3 H2 CH3 OH CH3 CH2OH H2 CH2Cl2 CH3 OH OH CH3 H2 OH CH3 H Rh H S S Ph2P PPh2 R2 H H2 C H P Rh P OH H C OH H H Rh P P HB HA CH3 C H H Rh P P H2 2H2 HA Rh HB P P OH C H H R2 R2 O Rh P P HB HA R2 H H H Me Me Me H Me H CO2H H Me H Me OH Me Me Me Me OH Me Me OH Me Me Me Me OH Me H HA H2 C H H OH CH2 R2 R2 CH3 OH Rh Ph2P PPh2 THF is important to success of rxn to buffer the Lewis acidity of the catalyst which causes elimination of ROH under normal conditions D. A. Evans Diastereoaselective Hydrogenation: Cationic Catalysts Chem 206 – BF4 16-e- 18-e￾Mechanism of Hydrogenation Cationic Rhodium-(I) Catalysts. + + + Which hydrogen migrates ?? A potential stereoelectronic effect + That H atom lying parallel to the pi-system (HA) should migrate preferentially if the dihydride is an intermediate. Rh(DIPHOS-4)+ 200 : 1 (89%) 300 : 1 (95%) 50 : 1 (82%) 150 : 1 (85%) Catalyst H2 Pressure trans:cis (Yield) 15 psi H2 375 psi H2 15 psi H2 15 psi H2 Mol% Catalyst 17.5 3.5 20.0 2.5 Ir(pyr)PCy3 19 : 1 Rh + 65 : 1 } Rh(DIPHOS-4)+ H2 1000 psi CH2Cl2 D. A. Evans & M. M. Morrissey JACS 106, 3866 (1984) Retigeranic Acid Excessive Steric Hindrance Rh + 75 : 1 (95%) Rh(DIPHOS-4)+ H2 800 psi THF Rh + +2 S