Employing the simple monosilylcopper reagent,Li[PhMe2SiCuI], in 1,4-addition reactions

Conjugate addition reactions using the simple Li[PhMe2SiCuI] reagent to a variety of alpha,beta-unsaturated carbonyl compounds is described; dimethyl sulfide from the purification of CuI plays a key role for very high yields as well as high stereoselectivities in the formation of beta-silyl carbonyl compounds.     

Sequential functionalization of pyrazole 1-oxides via regioselective metalation: synthesis of 3,4,5-trisubstituted 1-hydroxypyrazoles

A range of 3,5-diarylated and 3,4,5-triarylated 2-(4-methoxybenzyl)pyrazole 1-oxides have been prepared by regioselective deprotonation at C-5 or bromine-magnesium exchange at C-3 or C-4 followed by transmetalation with ZnCl(2) and palladium(0)-catalyzed cross-coupling. Furthermore, the metalated pyrazole 1-oxides could be trapped with electrophiles. The sequential metalation/functionalization of the pyrazole 1-oxides may follow the order C-5, C-3, C-4, or alternatively the order C-3, C-5, C-4. The 4-methoxybenzyl group of the functionalized 2-(4-methoxybenzyl)pyrazole 1-oxides could be removed by treatment with TFA and i-Pr(3)SiH in CH(2)Cl(2), providing the corresponding functionalized 1-hydroxypyrazoles.     

Long-range electron transfer in porphyrin-containing [2]-rotaxanes: tuning the rate by metal cation coordination

A series of [2]-rotaxanes has been synthesized in which two Zn(II)-porphyrins (ZnP) electron donors were attached as stoppers on the rod. A macrocycle attached to a Au(III)-porphyrin (AuP+) acceptor was threaded on the rod. By selective excitation of either porphyrin, we could induce an electron transfer from the ZnP to the AuP+ unit that generated the same ZnP*+-AuP* charge-transfer state irrespective of which porphyrin was excited. Although the reactants were linked only by mechanical or coordination bonds, electron-transfer rate constants up to 1.2x10(10) x s(-1) were obtained over a 15-17 A edge-to-edge distance between the porphyrins. The resulting charge-transfer state had a relatively long lifetime of 10-40 ns and was formed in high yield (>80%) in most cases. By a simple variation of the link between the reactants, viz. a coordination of the phenanthroline units on the rotaxane rod and ring by either Ag+ or Cu+, we could enhance the electron-transfer rate from the ZnP to the excited 3AuP+. We interpret our data in terms of an enhanced superexchange mechanism with Ag+ and a change to a stepwise hopping mechanism with Cu+, involving the oxidized Cu(phen)22+ unit as a real intermediate. When the ZnP unit was excited instead, electron transfer from the excited 1ZnP to AuP+ was not affected, or even slowed, by Ag+ or Cu+. We discuss this asymmetry in terms of the different orbitals involved in mediating the reaction in an electron- and a hole-transfer mechanism. Our results show the possibility to tune the rates of electron transfer between noncovalently linked reactants by a convenient modification of the link. The different effect of Ag+ and Cu+ on the rate with ZnP and AuP+ excitation shows an additional possibility to control the electron-transfer reactions by selective excitation. We also found that coordination of the Cu+ introduced an energy-transfer reaction from 1ZnP to Cu(phen)2+ (k = 5.1x10(9) x s(-1)) that proceeded in competition with electron transfer to AuP+ and was followed by a quantitative energy transfer to give the 3ZnP state (k = 1.5x10(9) x s(-1)).     

Excited states and two-photon absorption of some novel thiophenyl Pt(II)-ethynyl derivatives

The photophysical characterization of two new compounds related to bis((4-(phenylethynyl)phenyl)ethynyl)bis(tributylphosphine)platinum(II), here abbreviated Pt1, is reported. For the first new compound ATP1, the inner phenyl rings (closer to the Pt atom) in Pt1 are replaced by thiophene rings bridging at the 2,5-positions. In compound ATP2, the outer phenyl groups are replaced by thiophene rings bonded at the 2-position. Specifically, we report on the fluorescence quantum yield, two-photon absorption, triplet decay times and two-photon absorption induced emission spectra of the molecules in THF solutions. The results were compared with those of Pt1 and Pt1 capped with an acetonide-protected 2,2-bis(methylol)propionic acid (bis-MPA) ester group (Pt1-G1). The photophysical properties of the organic dye 7-(diethylamino)coumarin (Coumarin 110) were determined and used as a reference material. The two-photon absorption cross section around 720-740 nm of ATP1 and ATP2 was found to be of the same order of magnitude as for Pt1-G1, i.e., between 5 and 10 GM, but slightly larger for ATP1 than for ATP2 (1 GM = 1 G?ppert-Mayer = 10(-50) (cm(4) s)/photon). The fluorescence decay time of all compounds was found to be very short (subnanosecond) with quantum yields 0.0045, 0.0007, 0.0011 and 0.0020 for ATP1, ATP2, Pt1-G1 and Pt1, respectively, measured at excitation wavelength 373 nm. Just as Pt1 and Pt1-G1, both thiophenyl derivatives showed large intersystem crossing capabilities and phosphorescence, characteristic for a triplet state that can enhance the nonlinear absorption and optical power limiting by triplet state absorption. The phosphorescence emission of the thiophenyl derivatives was red-shifted in comparison with Pt1 and Pt1-G1, and the phosphorescence decay times were on the order of 200-500 ns, as for the Pt1 compound.     

A novel solid-phase synthesis of di- and trisubstituted N-acyl ureas

A novel, mild method for the synthesis of di- and trisubstituted N-acyl ureas on solid support is described. Addition of carboxylic acids to a resin-bound carbimidoyl chloride gave, initially, an O-acyl isourea which subsequently rearranged to the corresponding N-acyl urea. Trisubstituted N-acyl ureas were assembled on a Wang resin from a wide range of Fmoc amino acids, secondary amines and carboxylic acids. Acid mediated cleavage yielded the products in good yields and excellent purities. In addition, the regioselective synthesis of disubstituted N-acyl ureas is demonstrated with four examples.     

A new strategy for the improvement of photophysical properties in ruthenium(II) polypyridyl complexes. Synthesis and photophysical and electrochemical characterization of six mononuclear ruthenium(II) bisterpyridine-type complexes

The synthesis and characterization of six ruthenium(II) bistridentate polypyridyl complexes is described. These were designed on the basis of a new approach to increase the excited-state lifetime of ruthenium(II) bisterpyridine-type complexes. By the use of a bipyridylpyridyl methane ligand in place of terpyridine, the coordination environment of the metal ion becomes nearly octahedral and the rate of deactivation via ligand-field (i.e., metal-centered) states was reduced as shown by temperature-dependent emission lifetime studies. Still, the possibility to make quasi-linear donor-ruthenium-acceptor triads is maintained in the complexes. The most promising complex shows an excited-state lifetime of tau = 15 ns in alcohol solutions at room temperature, which should be compared to a lifetime of tau = 0.25 ns for [Ru(tpy)2]2+. The X-ray structure of the new complex indeed shows a more octahedral geometry than that of [Ru(tpy)2]2+. Most importantly, the high excited-state energy was retained, and thus, so was the potential high reactivity of the excited complex, which has not been the case with previously published strategies based on bistridentate complexes.     

The direct catalytic asymmetric alpha-aminooxylation reaction: development of stereoselective routes to 1,2-diols and 1,2-amino alcohols and density functional calculations

Proline-catalyzed direct asymmetric alpha-aminooxylation of ketones and aldehydes is described. The proline-catalyzed reactions between unmodified ketones or aldehydes and nitrosobenzene proceeded with excellent diastereo- and enantioselectivities. In all cases tested, the corresponding products were isolated with >95 % ees. Methyl alkyl ketones were regiospecifically oxidized at the methylene carbon atom to afford enantiomerically pure alpha-aminooxylated ketones. In addition, cyclic ketones could be alpha,alpha'-dioxidized with remarkably high selectivity, furnishing the corresponding diaminooxylated ketones with >99 % ees. The reaction mechanism of the proline-catalyzed direct asymmetric alpha-aminooxylation was investigated, and we performed density functional theory (DFT) calculations in order to investigate the nature of the plausible transition states further. We also screened other organocatalysts for the asymmetric alpha-oxidation reaction and found that several proline derivatives were also able to catalyze the transformation with excellent enantioselectivities. Moreover, stereoselective routes for the synthesis of monoprotected vicinal diols and hydroxyketones were found. In addition, short routes for the direct preparation of enantiomerically pure epoxides and 1,2-amino alcohols are presented. The direct catalytic alpha-oxidation is also a novel route for the stereoselective preparation of beta-adrenoreceptor antagonists.     

Accurate Backbone 13C and 15N Chemical Shift Tensors in Galectin-3 Determined by MAS NMR and QM/MM: Details of Structure and Environment Matter

Chemical shift tensors obtained from solid-state NMR spectroscopy are very sensitive reporters of structure and dynamics in proteins. While accurate ¹³C and ¹⁵N chemical shift tensors are accessible by magic angle spinning (MAS) NMR, their quantum mechanical calculations remain challenging, particularly for ¹⁵N atoms. Here we compare experimentally determined backbone ¹³C^α and ¹⁵N^H chemical shift tensors by MAS NMR with hybrid quantum mechanics/molecular mechanics/molecular dynamics (MD-QM/MM) calculations for the carbohydrate-binding domain of galectin-3. Excellent agreement between experimental and computed ¹⁵N^H chemical shift anisotropy values was obtained using the Amber ff15ipq force field when solvent dynamics was taken into account in the calculation. Our results establish important benchmark conditions for improving the accuracy of chemical shift calculations in proteins and may aid in the validation of protein structure models derived by MAS NMR.     

Different types of microfibrillated cellulose as filler materials in polysodium acrylate superabsorbents

Three types of microfibrillated cellulose(MFC)with differences in structure and surface charge were used at low concentration as filler materials in polysodium acrylate superabsorbents(SAPs).The swelling of the composite hydrogels was determined in 0.9%NaCl solution as well as in deionized water.The shear modulus of the samples was determined through uniaxial compression analysis after synthesis and after swelling in 0.9%NaCl solution.Furthermore,the ability to retain filler effects after washing was investigated.The results showed that all of the investigated MFCs had a strong reinforcing effect on the shear modulus after synthesis.The filler effect on swelling and on the associated shear modulus of swollen samples showed a more complicated dependence on structure and surface charge.Finally,it was found that the filler effects were reasonably retained after washing and subsequent drying.The results confirm that MFC holds great potential as a filler material in superabsorbent applications.Furthermore,the results provide some insight on how the structural properties and surface charge of MFC will affect gel properties depending on swelling conditions.This information should be useful in evaluating the use of different types of MFC in future applications.