Chiral Osmium Complexes with Sterically Bulky Schiff-Base Ligands. Crystal Structures of Os(IV) Derivatives and the Reactivity and Catalytic Cyclopropanation of Alkenes with EDA

The syntheses and reactivities of sterically encumbered trans-dioxoosmium(VI) complexes containing Schiff-base
ligands bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-diamine (H2tBu-salch) and bis(3,5-dibromosalicylidene)-1,2-cyclohexane-diamine (H2Br-salch) are described. Reactions of [OsVI(tBu-salch)O2] (1a) and [OsVI(Br-salch)O2] (1b)
with PPh3, p-X−arylamines (X = NO2, CN), N2H4·H2O, Ph2NNH2, SOCl2, CF3CO2H, Br2, and I2 under reducing
conditions gave [OsII(Br-salch)(OPPh3)2] (2), [OsIV(Br-salch)(p-X-C6H4NH)2] (3), [μ-O-{OsIV(tBu-salch)(p-NO2C6H4NH)}2] (4), [OsII(Br-salch)(N2)(H2O)] (5), [OsIV(tBu-salch)(OH)(Cl)] (6), [OsIV(tBu-salch)(OH)2] (7), [OsIV(tBu-salch)Cl2] (8), [OsIV(tBu-salch)(CF3CO2)2] (9), [OsIV(tBu-salch)Br2] (10), and [OsIV(tBu-salch)I2] (11), respectively. X-ray
crystal structure determinations of [OsIV(Br-salch)(p-NO2C6H4NH)2] (3a), [OsIV(Br-salch)(p-CNC6H4NH)2] (3b), 6, 8,
9, and 11 reveal the Os−N(amido) distances to be 1.965(4)−1.995(1) Å for the bis(amido) complexes, Os−Cl
distances of 2.333(8)−2.3495(1) Å for 6 and 8, Os−O(CF3CO2) distances of 2.025(6)−2.041(6) Å for 9, and Os−I
distances of 2.6884(6)−2.6970(6) Å for 11. Upon UV irradiation, (1S,2S)-(1a) reacted with aryl-substituted alkenes
to give the corresponding epoxides in moderate yields, albeit with no enantioselectivity. The (1R,2R)-6 catalyzed
cyclopropanation of a series of substituted styrenes exhibited moderate to good enantioselectivity (up to 79% ee)
and moderate trans selectivity.