Diels-alder reactions with 2-(Arylsulfinyl)-1,4-benzoquinones: effect of aryl substitution on reactivity, chemoselectivity, and pi-facial diastereoselectivity

Diels-Alder reactions of (SS)-2-(2'-methoxynaphthylsulfinyl)-1, 4-benzoquinone (1b), 2-(p-methoxyphenylsulfinyl)-1,4-benzoquinone (1c), and 2-(p-nitrophenylsulfinyl)-1,4-benzoquinone (1d) with cyclopentadiene are reported. These cycloadditions allowed the highly chemo- and stereoselective formation of both diastereoisomeric endo-adducts resulting from reaction on the unsubstituted double bond C(5)-C(6) of quinones working under thermal and Eu(fod)(3)- or BF(3).OEt(2)-catalyzed conditions. The synthesis of endo-adduct [4aS,5S,8R,8aR,SS]-9d resulting from cycloaddition on the substituted C(2)-C(3) double bond was achieved in a chemo- and diastereoselective way from quinone 1d in the presence of ZnBr(2). The reactivity and selectivity of the process proved to be dependent on the electron density of the arylsulfinyl group.     

Deliberate design of ligand architecture yields dramatic enhancement of metal ion affinity

Evaluation of the malonamide substructure with respect to binding site preorganization and complementarity for lanthanide metal ions suggests a new ligand architecture specifically designed to enhance lanthanide ion affinity. Consideration of conformational reorganization, restricted bond rotation, and donor group orientation suggests that typical malonamide structures, for example, N,N,N'N'-tetrahexylpropane-1,3-diamide (1), N,N'-dibutyl-N,N'-dimethyl-2-tetradecylpropane-1,3-diamide (2), or N,N,N'N'-tetramethylpropane-1,3-diamide (6), are poorly organized for metal ion complexation. Molecular mechanics analyses show that the unfavorable enthalpic and entropic terms are eliminated by the use of the novel bicyclic architecture found in 3,9-diaza-3,9-dimethylbicyclo[4.4.0]decane-2,10-dione (7). Diamide 7 was prepared, and the X-ray crystal structure of the complex [Eu(7)(2)(NO(3))(3)] exhibits the same chelate conformation predicted by the molecular mechanics model. A hydrophobic derivative, 3,9-diaza-3,9-dioctylbicyclo[4.4.0]decane-2,10-dione (8), was prepared, and solvent extraction studies reveal that the preorganized architecture of 8 gives a dramatic enhancement in binding affinity, exhibiting Eu(3+) distribution coefficients that are 7 orders of magnitude larger than a typical malonamide ligand, 1.     

Electron and mass transport in hybrid redox polyether melts contacted with carbon dioxide

Films of neat metal salts with covalently attached oligoether side chains ([Co(bpy(CO(2)MePEG-350)(2))(3)](ClO(4))(2); bpy is 2,2'-bipyridine, and MePEG-350 is methyl-terminated oligomeric ethylene oxide with an average molecular weight of 350 Da) undergo marked changes in physical and electrochemical properties upon contact with CO(2). Electrochemical measurements indicate that the physical diffusion coefficient (D(PHYS)) of the Co(II) species, the observed rate constant for Co(II/I) self-exchange (k(EX)), and the physical diffusion coefficient of the perchlorate counterion (D(ClO4)) increase from 2.4 x 10(-11) to 7.0 x 10(-10) cm(2)/s, 6.8 x 10(5) to 4.5 x 10(6) M(-1) s(-1), and 3.4 x 10(-10) to 4.3 x 10(-9) cm(2)/s, respectively, as CO(2) pressure is increased from 0 to 2000 psi at 23 degrees C. A reduction in activation energy accompanies the enhancement of each of these properties over this pressure range. Increasing CO(2) pressure from ambient to 1000 psi causes the films to swell 13%, and free-volume theory explains the enhanced mass transport properties of the films. The origin of increases in electron-transfer kinetics is considered. Plots of log(k(EX)) versus log(D(PHYS)) and log(k(EX)) versus log(D(ClO4)) are both linear. This suggests that electron self-exchange is controlled by factors that also affect log(D(PHYS)) or log(D(ClO4)). One explanation is based on plasticization of the oligoether side-chain motions by CO(2) that affect ether dipole repolarization and Co complex diffusion rates. A second explanation for the changes in k(EX) is based on control of electron transfer by relaxation of counterions neighbor to the Co complexes, which is measured by D(ClO4). Both explanations represent a kind of solvent dynamics control of k(EX).     

High stereocontrol in aldol reaction with ketones: enantioselective synthesis of beta-hydroxy gamma-ketoesters by ester enolate aldol reactions with 2-acyl-2-alkyl-1,3-dithiane 1-oxides

The asymmetric aldol reaction of 1,2-diketones, masked as nonracemic 2-acyl dithiane oxides, with lithium enolates derived from several esters and lactones, proceeds with a high degree of stereocontrol at both carbonyl and enolate prochiral centers, the stereocontrol mainly determined by the configuration of the sulfoxide sulfur atom. The sense of induced stereochemistry observed for ester enolates is different from that seen for lactone enolates. Hydrolysis of the dithiane oxide units of the major diastereoisomerically pure aldol products affords enantiomerically pure tertiary alpha-substituted beta-hydroxy-gamma-ketoesters.     

Cross-linked poly(4-vinylpyridine-N-oxide) as a polymer-supported oxygen atom transfer reagent

Oxygen atom transfer (OAT) reagents are common in biological and industrial oxidation reactions. While many heterogeneous catalysts have been utilized in OAT reactions, heterogeneous OAT reagents have not been explored. Here, cross-linked poly(4-vinylpyridine-N-oxide), called x-PVP-N-oxide, was tested as a heterogeneous OAT reagent and its oxidation chemistry compared to its molecular counterpart, pyridine-N-oxide. The insoluble oxidant x-PVP-N-oxide demonstrated comparable reactivity to pyridine-N-oxide in direct oxidation reactions of phosphines and phosphites in acetonitrile, but x-PVP-N-oxide did not react in other solvents. The polymer backbone of x-PVP-N-oxide, however, allowed for easy filtering and recycling in sequential oxidation reactions. In addition, x-PVP-N-oxide was tested as the stoichiometric oxidant in a copper-catalyzed OAT reaction to α-diazo-benzeneacetic acid methyl ester. The heterogeneous oxidant was much less reactive than pyridine-N-oxide, indicating that interaction with the metal catalyst was challenging. These results demonstrated a proof-of-concept that recyclable, polymer-supported OAT reagents could be a viable OAT reagents in direct oxidation reactions without metal catalysts.     

Concomitant magnetic field gradients and their effects on imaging at low magnetic field strengths

Low-field NMR imaging systems which use large amplitude field gradient pulses (e.g., in flow velocity encoding) may be subject to the undesirable effects of concomitant gradients. We demonstrate the effects of these extra gradients, which arise from Maxwell's equations, and show that the resultant image phase shifts and amplitude changes are consistent with theory.     

Luminescent nonacoordinate cationic lanthanide complexes as potential cellular imaging and reactive probes

Nonacoordinate delta- and lambda-Eu and Tb complexes have been tested as imaging and reactive probes in mouse fibroblast (NIH 3T3) cells. The uptake of these complexes by the cells was assessed by fluorescence microscopy. Complex-induced DNA damage was studied by gel electrophoresis and shown to be a function of complex chirality.