Asymmetric Hydrogenation of Quinoxalines, Benzoxazines, and a Benzothiazine Catalyzed by Chiral Ruthenabicyclic Complexes

The ruthenabicyclic complex RuCl[(R)-daipena][(R)-dm-segphos] with potassium tert-butoxide catalyzes the hydrogenation of 2-alkylquinoxalines and a 3-methyl-2H-1,4-benzoxazine in toluene under 20-100atm of hydrogen at 40 degrees C to afford S-configured cyclic amino products in greater than 97% enantiomeric excess {DAIPENA=anion of DAIPEN at the 2-position of an anisyl group, DAIPEN=1,1-di(4-anisyl)-2-isopropyl-1,2-ethylenediamine, DM-SEGPHOS=(4,4-bi-1,3-benzodioxole)-5,5-diylbis[di(3,5-xylyl)phosphine]}. The high catalytic activity results in a turnover number as high as 9400. Hydrogenation of the benzoimine heterocycles with the RuCl[(R)-daipena][(R)-segphos]/potassium tert-butoxide system yields the R-configured products in high enantiomeric excess [SEGPHOS=(4,4-bi-1,3-benzodioxole)-5,5-diylbis(diphenylphosphine)]. The mode of enantioselection is discussed based on transition state models involving six-membered pericyclic structures.