(PORPHYRINATO)OSMIUM(II) YLIDE COMPLEXES FROM THE ADDITION OF PYRIDINE-DERIVATIVES TO (PORPHYRINATO)OSMIUM(II) ALKYLIDENE COMPLEXES - SPECTROSCOPIC PROPERTIES AND REACTIVITY TOWARD CYCLOPROPANATION

Addition of 4-substituted pyridine derivatives (4-R-C5H4N; R = Me, i-P r, NMe2) to the tetratolylporphyrinato (TTP) osmium carbene complex (T TP)Os=CH(CO2Et) affords stable osmium ylide complexes, trans-(4-R-C5H4 N)(TTP)Os-CH(CO2Et)(4-R-C5H4N). Dynamic NMR studies of the 4-picoline and 4-(dimethylamino)pyridine adducts show that the free energies for the barrier to rotation around the C(alpha)-N axis are respectively 14 +/- 1 kcal/mol at 298 +/- 5 K and 13.0 +/- 0.5 kcal/mol at 268 +/- 2 K. The barrier to rotation around the C-NMe2 axis of the (dimethylamin o)pyridine ylide fragment is 13.0 +/- 0.5 kcal/mol at 240 +/- 2 K. Add ition of 4-picoline (4-pic) to (TTP)Os=CH(SiMe3) leads to the equilibr ium formation of trans-(4-pic)(TTP)Os=CH(SiMe3) and the osmium ylide c omplex trans-(4-pic)-(TTP)Os-CH(SiMe3)(4-pic). H-1 NMR spin saturation transfer experiments confirm the existence of this equilibrium. Disub stituted carbene complexes (TTP)Os=CR2 do not yield osmium ylide compl exes upon treatment with 4-picoline. Prolonged treatment of (TTP)-Os=C (CO2Et)2 with 4-picoline affords mainly (TTP)Os(4-pic)2. The bis(picol ine) complex (TTP)Os(4-pic)2 crystallizes in the monoclinic space grou p P2(1/n) with 1.74 molecules of benzene (C60H50N6Os.1.74C6H6, a = 10. 800(2) angstrom, b = 21.332(2) angstrom, c = 12.868(1) angstrom, beta = 108.89(1)-degrees, V = 2805 angstrom3, Z = 2, R = 3.08%, R(w) = 3.80 %). Key metrical parameters include an Os-N(pic) distance of 2.078(3) angstrom and a torsional angle between the plane of the axial ligand a nd the closest Os-N(pyrrole) bond of 40.1(3)-degrees. Treatment of the osmium ylide complexes with styrene yields cyclopropane derivatives w ith high stereoselectivities and conversions ranging from 65 to 95%. T he trans/cis ratios for these reactions range from 23 to 28.