Transition-Metal Catalysis of Triene 6π Electrocyclization: The π-Complexation Strategy Realized

Triene 6π electrocyclization, wherein a conjugated triene undergoes a concerted stereospecific cycloisomerization to a cyclohexadiene, is a reaction of great historical and practical significance. In order to circumvent limitations imposed by the normally harsh reaction conditions, chemists have long sought to develop catalytic variants based upon the activating power of metal–alkene coordination. Herein, we demonstrate the first successful implementation of such a strategy by utilizing [(C₅H₅)Ru(NCMe)₃]PF₆ as a precatalyst for the disrotatory 6π electrocyclization of highly substituted trienes that are resistant to thermal cyclization. Mechanistic and computational studies implicate hexahapto transition-metal coordination as responsible for lowering the energetic barrier to ring closure. This work establishes a foundation for the development of new catalysts for stereoselective electrocyclizations.     

Novel silver(I)-organic coordination polymers: conversion of extended structures in the solid state as driven by argentophilic interactions

Structurally distinct coordination polymers [Ag(bpp)]ClO4 1 and [Ag(bpp)]PF6 2 [bpp = 1,3-bis(4-pyridyl)propane] have been assembled; the conversion of 1 into 2 on treatment with NaPF6 is driven by argentophilic interactions and is the first such transformation reported for silver(I)-organic coordination polymers.     

Formation of a tetranuclear copper (II) cluster assembled by metal-assisted hydrolysis and desulfurization of bis(3,5-dimethylpyrazolyl)methylphosphine sulfide, MeP(S)(3,5-Me2Pz)2

The reaction of methylphosphine sulfide, MeP(S)(3,5-Me2Pz)2, 1, with anhydrous CuCl2 affords a tetranuclear copper cluster[Cu2Cl2(3,5-Me2Pz)3(MePO3)]2, 2, and a dinuclear compound Cu2Cl4(3,5-Me2Pz)4, 3. This reaction involves a metal-assisted desulfurization along with concomitant hydrolysis of P-N bonds. The X-ray structures of 1-3 have been determined, and the crystal parameters for these are the following. 1: space group = C2/c, a = 15.2552(1) A, b = 8.7364(2) A, c = 21.4490(3) A, beta = 93.349(1) degrees, V = 2853.74(8) A3, and Z = 8. 2: space group = P2(1)/n, a = 12.5964(4) A, b = 15.7773(4) A, c = 13.9781(4) A, beta = 116.6280(10) degrees, V = 2483.32(12) A3, and Z = 2. 3: space group = P2(1)/c, a = 8.7137(8) A, b = 13.5493(14) A, c = 11.8847(12) A, beta = 106.179(2) degrees, V = 1347.6(2) A3, and Z = 2. The structure of 2 shows that it comprises two dinuclear copper cores bridged together by two tripodal methylphosphinate, MePO3, ligands. 2 is weakly antiferromagnetically coupled, as revealed by variable temperature magnetic susceptibility measurements.     

New synthetic routes to cationic rhenium tricarbonyl bipyridine complexes with labile ligands

Triflate abstraction from the complex [Re(OTf)(CO)(3)(bipy)] (1) using the salt NaBAr'(4) (Ar' = 3,5-bis(trifluoromethyl)phenyl) in dichloromethane solution in the presence of L = PPh(3), NCMe, NCPh, imines, ketones, Et(2)O, THF, MeOH, and MeI affords cationic complexes [Re(L)(CO)(3)(bipy)](+) as their BAr'(4)(-) salts. The new complexes have been characterized spectroscopically and, for [Re(eta(1)-O=C(Me)R)(CO)(3)(bipy)]BAr'(4) (R = CH(3), 6a; R = Ph, 6b), and [Re(THF)(CO)(3)(bipy)]BAr'(4) (9), also by single-crystal X-ray diffraction. Compared with conventional methodologies, the route reported here allows the coordination of a broader range of weakly coordinating ligands and requires considerably milder conditions. On the other hand, the reactions of lithium acetylides with [Re(THF)(CO)(3)(bipy)]BAr'(4) (9) can be used for the high-yield syntheses of rhenium alkynyls [Re(Ctbd1;CR)(CO)(3)(bipy)] (R = Ph, 12; R = SiMe(3), 13). Complex 9 was found to catalyze the aziridination of benzylideneaniline with ethyl diazoacetate.     

Synthesis and structural characterisation of a novel tris-methylene bridged compound (NO) 4Fe2 Se(μ-CH2)3

The tris-methylene bridged compound (NO)₄Fe₂Se(μ-CH₂)₃ has been isolated. It has been characterised by IR and ¹H, ¹³C, and ⁷⁷Se NMR spectroscopy. Its structure has been determined by single-crystal X-ray diffraction methods. The structure consists of a heavy atom triangle consisting of one Se and two Fe atoms. The Fe-Fe and the two Fe-Se edges are bridged by methylene groups.     

Stereoselective formation of glycosyl sulfoxides and their subsequent equilibration: ring inversion of an alpha-xylopyranosyl sulfoxide dependent on the configuration at sulfur

A series of four S-allyl D-thiopyranosides, alpha- and beta-manno and xylo, were oxidized with MCPBA at low temperature to give seven of the eight possible sulfoxides, whose configuration at sulfur was determined either directly by X-ray crystallography or by correlation with closely related structures. For the axial thioglycosides oxidation leads very predominantly to the (R)(S)-diastereomer in the xylo series and exclusively so in the manno series; the configuration at C2 is of little importance in determining the stereoselectivity of oxidation of axial thioglycopyranosides. In the equatorial series the configuration at C2 has a significant effect on the outcome of the reaction as, although both series favored the (S)(S)-sulfoxide, selectivity was significantly higher in the case of the beta-mannoside than of the beta-xyloside. The two alpha-xylo sulfoxides have different conformations of the pyranoside ring with the (R)(S)-isomer adopting the (1)C(4) chair and the (S)(S)-diastereomer the (4)C(1). Each pair of diastereomeric sulfoxides was thermally equilibrated in C(6)D(6) and in CD(3)OD. In the mannose series the kinetic isomers are also thermodynamically preferred. In the xylose series, on the other hand, the nature of the thermodynamic isomer in both the alpha- and beta-anomers is a function of solvent with a switch observed on going from C(6)D(6) to CD(3)OD. The results are rationalized in terms of the exo-anomeric effect, steric shielding provided by H3 and H5 in the axial series, the interaction of the C2-O2 and sulfoxide dipoles, and increased steric interactions on hydrogen bonding of the sulfoxides to CD(3)OD.     

Bimetallic reactivity. On the use of oxadiazoles as binucleating ligands

Two (1,3,4)-oxadiazole ligands have been prepared. In one case the oxadiazole ring is flanked by two o-aniline groups, and in the other case it is an extension of the first where the amines are condensed with 2-picolyl groups. A monometallic copper(II) complex of the former has been prepared, and its crystal structure was determined. A number of bimetallic copper(II), cobalt(II), and nickel(II) complexes of the di-deprotonated latter ligand were prepared and isolated. The crystal structure of the cobalt(II) complex bearing two acetate bridges is reported. The work demonstrates that the seldom-employed oxadiazole ring can be used effectively for generating bimetallic complexes.