An experimental and theoretical study of the mechanism of stannylcupration of alpha,beta-acetylenic ketones and esters

The title reaction has been investigated by experimental and computational (DFT) techniques, and subsequently compared to the corresponding carbocupration reaction, with particular emphasis on the stereoselectivity. For stannylcupration of an ynone substrate, only the anti-addition product is observed, whereas for the corresponding ynoate substrate, the stereoselectivity can be affected by the reaction conditions: in the presence of methanol as proton donor, the initial syn-addition product can be trapped, whereas a syn/anti mixture is obtained in a non-protic solvent. This is in sharp contrast to the carbocupration of the same ynone substrate with a cyanocuprate (RCu(CN)Li), which is highly selective for syn-addition. The product selectivities can be understood from a detailed computational characterization of the reaction paths, and in particular from the relative stabilities of the vinyl cuprate and allenolate intermediates. It is suggested that the stereodetermining step is protonation of vinyl cuprate intermediates.     

Factor groups,point groups,space groups: A review of some central concepts with application to vibrational spectroscopy on crystalline solids

Although crystallographers and solid-state spectroscopists have much in common, differences in background and tradition raise obstacles for an easy communication between the two groups of scientists. The present article features the concept of theFactor Group, which has its roots in group theory and crystallography, and is essential in the analysis of vibrational spectra of solids. A unified exposition of many concepts related to the factor group is given. The calcite structure of sodium nitrate is chosen as an illustrative example. Finally, some complications in the interpretation of vibrational spectra of simple solids like the alkali metal nitrates are discussed.     

Molecular modeling of bifunctional chelate peptide conjugates. 1. Copper and indium parameters for the AMBER force field

In this work we describe the development of parameters for In(III) and Cu(II) for the AMBER force field as found in the modeling package MacroModel. These parameters were developed using automated procedures from a combination of crystallographic structures and ab initio calculations. The new parameters were added in the form of AMBER substructures containing specific metal-ligand parameters to the existing force field. These new parameters have produced results in good agreement with experiment without requiring additional changes to the existing AMBER parameters. These parameters were then utilized to examine the conformational effects caused by the conjugation of InDTPA (DTPA = diethylenetriaminepentaacetic acid) and CuDOTA (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) to the cyclic octapeptide octreotide.     

A DFT exploration of the enantioselective rearrangement of cyclohexene oxide to cyclohexenol

In this paper, we present computational results for the (1S,3R,4R)-3-(pyrrolidinyl)-methyl-2-azabicyclo[2.2.1]heptane mediated rearrangement of cyclohexene oxide. The results nicely explain the differences in enantioselectivities between catalytic and stoichiometric mode between different ligands, and provides a rational for the identification of non-stereospecific background reactions as the major cause of decreased enantioselectivity in catalytic reactions for sterically hindered diamines.     

Me2Zn-mediated addition of acetylenes to aldehydes and ketones

[reaction: see text] Contrary to expectations, commercially available 2 M Me2Zn in toluene is able to promote the addition of phenylacetylene to aldehydes and ketones. This reactivity is determined by a new, unprecedented mechanism, which involves activation of the zinc reagent via coordination with carbonyl substrates that behave "ligand like". Broad scope, high tolerance to functional groups, and a simple procedure make this new method highly interesting for the synthetic chemist.     

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.     

Design and synthesis of a new type of ferrocene-based planar chiral DMAP analogues. A new catalyst system for asymmetric nucleophilic catalysis

A new first-generation catalyst system for nucleophilic catalysis has been developed. It is based on a planar chiral ferrocene skeleton with either the potent nucleophile 4-(dimethylamino)pyridine (DMAP) or the related 4-nitropyridine N-oxide attached in either the 2- or the 3-position. The syntheses are short, efficient, and enantioselective and X-ray crystal structures of both DMAP-derived catalysts are presented. The DMAP-based catalysts were tested in asymmetric reactions and the 3-derivative 14 showed good activity and a moderate level of enantioselectivity. The sense of induction (selectivity) was studied using molecular modeling and the results pointed at new directions for future generations of catalysts based on this design.     

Quantum chemical investigation of mechanisms of silane oxidation.

Several mechanisms for the peroxide oxidation of organosilanes to alcohols are compared by quantum chemical calculations, including solvation with the PCM method. Without doubt, the reaction proceeds via anionic, pentacoordinate silicate species, but a profound difference is found between in vacuo and solvated reaction profiles, as expected. In the solvents investigated (CH(2)Cl(2) and MeOH), the most favorable mechanism is addition of peroxide anion to a fluorosilane (starting material or formed in situ), followed by a concerted migration and dissociation of hydroxide anion. In the gas phase, and possibly in very nonpolar solvents, concerted addition-migration of H(2)O(2) to a pentacoordinate fluorosilicate is also plausible.     

The mechanism of the phosphine-free palladium-catalyzed hydroarylation of alkynes

The mechanism of the Pd-catalyzed hydroarylation and hydrovinylation reaction of alkynes has been studied by a combination of experimental and theoretical methods (B3LYP), with an emphasis on the phosphine-free version. The regioselectivity of the hydroarylation and hydrovinylation shows unexpected differences, which could be attributed mainly to the higher steric demand of the cyclohexenyl group as compared to the phenyl group. Hydroarylation of alpha,beta-acetylenic carbonyl substrates yields a very unusual anti-Michael selectivity, which is shown to result from reaction of the nonconjugated double bond, leaving the conjugation intact. In all cases were the regioselectivities reproduced by the calculations.     

First order transitions to a lyotropic biaxial nematic

The phase diagram of the sodium dodecylsulphate/decanol/water system is studied by²H NMR spectroscopy in the range between the calamitic nematic (N⁺_C) and discotic nematic (N^-_D) phases. In this narrow range a nematic biaxial phase (N_BX) is observed. The phase transitions between the nematic phases are all of first order. The shape of the surfactant aggregates in the nematic phases varies with composition and temperature.