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-cyclohexane-diamine (H2tBu-salch) and bis(3,5-dibromosalicylidene)-1,2-cyclohexane-diamine (H2Br-salch) are described. Reactions of [Os(VI)tBu-salch)O2] (1a) and [Os(VI)(Br-salch)O2] (1b) with PPh(3), p-X-arylamines (X = NO2, CN), N2H4 x H2O, Ph2NNH2, SOCl2, CF3CO2H, Br2, and I2 under reducing conditions gave [Os(II)(Br-salch)(OPPh3)2] (2), [Os(IV)(Br-salch)(p-X-C6H4NH)2] (3), [mu-O-{Os(IV)(tBu-salch)(p-NO2C6H4NH)}2] (4), [Os(II)(Br-salch)(N2)(H2O)] (5), [Os(IV)(tBu-salch)(OH)(Cl)] (6), [Os(IV)(tBu-salch)(OH)2] (7), [Os(IV)(tBu-salch)Cl2] (8), [Os(IV)(tBu-salch)(CF3CO2)2] (9), [Os(IV)(tBu-salch)Br2] (10), and [Os(IV)(tBu-salch)I2] (11), respectively. X-ray crystal structure determinations of [Os(IV)(Br-salch)(p-NO2C6H4NH)2] (3a), [Os(IV)(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) A for the bis(amido) complexes, Os-Cl distances of 2.333(8)-2.3495(1) A for 6 and 8, Os-O(CF3CO2) distances of 2.025(6)-2.041(6) A for 9, and Os-I distances of 2.6884(6)-2.6970(6) A 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.

Sterically encumbered 2,6-dibenzhydryl-4-methylphenyl derived ligand systems: synthesis and structures

“…Complexes employing bulky Schiff base derivatives have been used as efficient and selective catalysts for several organic transformations, stabilisation of low valent complexes as well as for biological applications. [16][17][18][19][20][21][22][23][24] Of particular mention here are the nickel and palladium complexes of Schiff base derivatives containing 2,6-Dibenzhydryl-4-methylphenyl moiety which have been employed as excellent catalysts for ethylene polymerisation. [25][26][27] Further, such Schiff base derivatives containing bulky 2,6-Dibenzhydryl-4-methylphenyl moiety have been employed in the stabilization of highly low valent transition as well as main group metal complexes such as those containing Zn, Bi, Sb, As, Mg and Mn.…”

Crystal structure of bis(4-nitroaniline-κN1)(5,10,15,20-tetraphenylporphyrinato-κ4N)cobalt(III) chloride dichloromethane monosolvate

“…For the synthesis of the title compound, see: Madure & Scheidt (1976). For related structures, see: Dhifet et al (2010); Konarev et al (2003); Jentzen et al (1995); Mansour et al (2013); Zhang et al (2005); Feng (2012). For a description of the Cambridge Structural Database, see: Allen (2002 Hydrogen-bond geometry (Å , ).…”

Cyclopropanation

“…Chiral Ir(III) salen complexes, which contain σ-coordinated aryl ligands, were found to catalyze the cyclopropanation of styrene derivatives with TDA. Higher yields (99%), higher diastereoselectivity (t/c = 1:99), and enantioselectivity (99% ee) were observed when the reactions were conducted at lower temperature (−78 • C) (98 = bis(3,5-di-tert-butylsalicylidene)-1R,2R-cyclohexane-diamine) catalyzed the cyclopropanation of a series of substituted styrenes with EDA, with moderate to good enantioselectivities (up to 79%) and slightly lower trans-selectivities (99). For the rhenium catalysts, asymmetric cyclopropanation was first performed with a number of chiral bipyridine and terpyridine catalysts.…”