Enantioselective Epoxidation of ß,ß-Disubstituted Enamides with a Manganese Catalyst and Aqueous Hydrogen Peroxide.

Enantioselective epoxidation of ß,ß-disubstituted enamides with aqueous hydrogen peroxide and a novel manganese catalyst is described. Epoxidation is stereospecific and proceeds fast under mild conditions. Amides are disclosed as key functional groups to enable high enantioselectivity.

Structural Preferences in Phosphanylthiolato Platinum(II) Complexes.

The transition-metal complexes of heterotopic phosphanylthiolato ligands are useful in various reactions which depend on the stereochemistry of the complexes. Bis-chelate complex [Pt(SCH2CH2PPh2-κ(2) P,S)2] (1) was obtained in good yields by direct base-free substitution reaction of the corresponding phosphanylthiol (HSCH2CH2PPh2) with K2PtCl4 or by oxidative addition of the same phosphanylthiol to Pt(PPh3)4. In agreement with the antisymbiosis rule, complex 1 shows a cis-P,P arrangement in solid state crystallizing in the monoclinic system (C2/c). Density functional theory (DFT) calculations on 1 reveal the right characteristics for the preferred cis-P,P arrangement, rationalizing its formation. Direct base-free reaction of [PtCl2(1,5-cyclooctadiene)] with one equivalent of the same phosphanylthiol produce the trinuclear complex [PtCl(µ-SCH2CH2PPh2-κ(2) P,S)]3 (2) instead of the binuclear structure common in palladium and nickel derivatives. Crystals of 2 are triclinic (P 1‾ ) showing a sulfur-bridging edge-sharing cyclic trinuclear complex with square-planar coordination geometry around the platinum atoms and a Pt3S3 cycle in skew-boat conformation. This preference for the trinuclear structure was rationalized mechanistically and through conceptual DFT.

Structural Preferences in Phosphanylthiolato Platinum(II) Complexes.

Invited for this month's cover picture are the groups of Prof. Alfonso Polo and Dr. Albert Poater at the Universitat de Girona, as well as their collaborators from the Universitat Autònoma de Barcelona and the Institute of Chemical Research of Catalonia. The cover picture shows phosphanylthiolate ligand coordination on a platinum(II) center to give only the bischelate cis -P,P isomer when the ligand/Pt ratio is 2, whereas a trinuclear unexpected complex is achieved with a ligand/Pt ratio of 1. Here, the synthesis and structural determination is combined with density functional theory (DFT) calculations to rationalize the reaction mechanistically and through conceptual DFT. The exciting point of this study is that it opens the door to test new experimental pathways to monitor the preferred cis or trans arrangement of bidentate ligands to platinum. (Legend: H-white, C-black, P-purple, S-yellow, Cl-green, Pt-blue.) For more details, see the Full Paper on p. 51 ff.

Platinum phosphinothiolato hydride complexes: synthesis, structure and evaluation as tin-free hydroformylation catalysts.

Ligand 2-diphenylphosphinothiophenol (Hsarp) reacted with Pt(PPh3)4 to yield trans-[PtH(sarp)(PPh3)], which undergoes fast exchange with free PPh3 on the NMR time scale and very slowly and reversibly formed some cis-[PtH(sarp)(PPh3)] over time in solution (11%, 24 h). Reaction of trans-[PtH(sarp)(PPh3)] with Hsarp in boiling toluene gave cis- and trans-[Pt(sarp)2]; the cis isomer being more stable. These complexes were characterized by (1)H and (31)P NMR and also analyzed by XRD in the case of trans-[PtH(sarp)(PPh3)], trans-[Pt(sarp)2], and cis-[Pt(sarp)2]. trans-[PtH(sarp)(PPh3)] was evaluated as a preformed, tin-free hydroformylation catalyst on styrene and found active at 100 °C, at pressures over 75 bar, yielding phenylpropanal (regioselectivities up to 83% in 2-phenylpropanal), with total conversions to aldehydes up to 100% at styrene/platinum ratios from 400/1 to 1000/1 and minimal hydrogenation products.

Palladium Phosphinothiolato Complexes. Syntheses and Crystal Structures of Mononuclear [PdCl(SC(2)H(4)PPh(2))PPh(3)] and Binuclear [Pd(2)Cl(2)(&mgr;-SC(3)H(6)PPh(2))(2)] and Their Performance in Catalytic Carbonylation.

The phosphinothiolato complexes [PdCl(dppet)PPh(3)] (3), [Pd(dppet)(2)] (4), and [Pd(2)Cl(2)(dpppt-P,&mgr;-S)(2)] (6) (Hdppet = HSC(2)H(4)PPh(2); Hdpppt = HSC(3)H(6)PPh(2)) have been synthesized in good yield by various base-free ligand exchange reactions on Pd(II), but [Pd(dpppt)(2)] (5) could only be obtained as a pure product employing an oxidative route from Pd(0). Both complexes 3 ((31)P NMR, delta(P) 66.8 (dppet), 24.1 (PPh(3)); (2)J(PP) = 459 Hz) and 5 ((13)C NMR) are trans-P,P. Crystals of 3.CH(2)Cl(2) are orthorhombic (P2(1)2(1)2(1)), with a = 9.247(3) Å, b = 17.956(9) Å, c = 19.869(9) Å. In 3 the Pd-S distance of 2.270(2) Å is short compared to the very different Pd-P distances of 2.280(2) (Pd-PPh(2)R) and 2.343(2) Å (Pd-PPh(3)). Crystals of 6.CH(2)Cl(2) are monoclinic (P2(1)/n) with a = 12.701(3) Å, b = 12.040(4) Å, c = 22.495(2) Å, and beta = 97.36(1) degrees. The structure of 6 consists of two Pd(dpppt)Cl moieties meeting at an angle of 105.81(3) degrees, linked by asymmetric thiolato bridges in a syn-endo configuration. The difference in the chelate angles of dppet (85.98(9) degrees ) and dpppt (96.60(3) degrees and 97.42(3) degrees ) seems to be crucial for palladium to form an unusual mononuclear complex (3) or a binuclear complex (6). Bischelate complexes 4 and 5 are inactive, but 3 and 6 catalyze the hydroesterification of styrene with CO (30 bar) and MeOH at moderate temperatures (60 and 80 degrees C) with no additives. The velocities are slow, but with 6 and with 3 at the lower temperature no decomposition to Pd metal is observed. Only esters are produced and regioselectivities of ca. 84% toward 2-phenylpropanoic acid methyl ester are achieved.