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

Abstract

The syntheses and reactivities of sterically encumbered trans-dioxoosmium(VI) complexes containing Schiff-base ligands bis(3,5-di-tert-butylsalicylidene)-1,2-cydohexane-diamine (H 2Bu- salch) and bis(3,5-dibromosalicylidene)-1,2-cyclohexane-diamine (H 2Br-salch) are described. Reactions of [Os VI(Bu-salch) O 2] (1a) and [Os VI(Br-salch)O 2] (1b) with PPh 3, p-X-arylamines (X = NO 2, CN), N 2H 4·H 2O, Ph 2NNH 2, SOCl 2, CF 3CO 2H, Br 2, and I 2 under reducing conditions gave [Os II(Br-salch)(OPPh 3) 2] (2), [Os VI(Br-salch)(p-X-C 6H 4NH) 2] (3), [μ-O{Os IV(Bu-salch)(p-NO 2C 6H 4-NH)} 2] (4), [Os II(Br-salch)(N 2)(H 2O)] (5), [Os IV(Bu-salch)(OH)(Cl)] (6), [Os IV(Bu-salch)(OH) 2] (7), [Os IV(Bu-salch)-Cl 2] (8), [Os IV(Bu-salch)(CF 3CO 2) 2] (9), [Os IV(Bu-salch)Br 2] (10), and [Os IV(Bu-salch) I 2] (11), respectively. X-ray crystal structure determinations of [Os IV(Br-salch)(p-NO 2C 6H 4NH) 2] (3a), [Os IV(Br-salch)(p-CNC 6H 4NH) 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(CF 3CO 2) 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. © 2005 American Chemical Society.link_to_subscribed_fulltex