Showing papers on "Intramolecular force published in 1990"
TL;DR: The crystal structure of thioredoxin from Escherichia coli has been refined by the stereochemically restrained least-squares procedure to a crystallographic R-factor of 0.165 at 1.68 A resolution.
571 citations
TL;DR: The role of hydrogen bonding in catalysis has been discussed in this article, although mainly in terms of the salicylate ion as a leaving group, and it seems likely that this will herald other systems where the role of a strong hydrogen bond may serve as the key step in a catalytic process.
Abstract: Publisher Summary This chapter discusses the hydrogen bonding, which has been recognized as the single most important intermolecular interaction, The chapter presents the hypothesis that there are three kinds of hydrogen bond: weak, strong, and very strong. These are determined by the shape of the potential energy well and the respective positions of hydrogen and deuterium within the well, which can be used to provide information about the well that applies in a particular example. The role of hydrogen bonding in catalysis has been discussed, although mainly in terms of the salicylate ion as a leaving group. With its strong intramolecular hydrogen bond playing an essential part in the reaction mechanism, it seems likely that this will herald other systems where the role of a strong hydrogen bond may serve as the key step in a catalytic process.
454 citations
TL;DR: Comparison of the structure of yeast iso-1-cytochrome c with those of tuna and rice cytochromes c shows that these three molecules have very high structural similarity, with the atomic packing in the heme crevice region being particularly highly conserved.
366 citations
341 citations
TL;DR: In particular, cyclometallated compounds, obtained by direct intramolecular C-H activation of a ligand by transition metals, are fairly reactive towards insertion of internal alkynes into their metal-carbon bond as mentioned in this paper.
Abstract: Cyclometallated compounds, obtained by direct intramolecular C–H activation of a ligand by transition metals, are fairly reactive [especially those derived from palladium(II)] towards insertion of internal alkynes into their metal-carbon bond. This reaction leads to either new organometallic compounds by inclusion of one to three alkynes into the M-C bond, or organic compounds which are either carbocyclic or heterocyclic, their nature depending upon the alkyne substituents as well as upon the cyclometallated ligand.
241 citations
TL;DR: In this article, the authors examined the mechanism of excited singlet state quenching by nitroxides in a series of nitroxide-fluorophore adducts.
Abstract: Absorbance and steady-state and time-resolved fluorescence measurements were employed to examine the mechanism(s) of excited singlet state quenching by nitroxides in a series of nitroxide-fluorophore adducts. This work establishes the following: (1) the absorption and emission energies of the fluorophores are unaffected by the presence of the nitroxide substituent(s), and the residual emission that is observed from the adducts arises from the locally excited singlet of the fluorophore, not from charge recombination; (2) rate constants for intramolecular quenching by the nitroxides (k{sub q}) are high (10{sup 8}-10{sup 10} s{sup {minus}1}) and decrease significantly with increasing nitroxide to fluorophore distance-however, relatively high rates of quenching (>10{sup 8} s{sup {minus}1}) are observed over distances as great as 12 {angstrom}; (3) Foerster energy transfer does not contribute significantly to the quenching due to the low values for the spectral overlap integrals; (4) the K{sub q}'s do not increase proportionally to the solvent-dependent increases in the Dexter overlap integral, indicating that energy transfer by the Dexter mechanism is not responsible for the quenching; (5) the values of k{sub q} show no obvious correlation with the calculated free energies for photoinduced electron transfer, suggesting that this quenching pathway is also unimportant; (6) for hematoporphyrin-nitroxide adducts,more » which contain a fluorophore whose singlet energy is below that of the first excited state energy of the nitroxide (thus precluding energy transfer), significant rates of quenching are still observed.« less
222 citations
TL;DR: Second-harmonic generation from powders of a series of three aromatic intramolecular charge transfer moieties of D 3h symmetry is reported in this paper, where one compound is shown to be active while its symmetric structure precludes any molecular non-linear contribution of a vectorial nature such as usually prevails in polar paranitroaniline-like systems.
160 citations
155 citations
TL;DR: In this article, the spectral characteristics, photophysical parameters and photochemical reactivities of 1,10-anthraquinone derivatives are reported and the effects of various parameters (substituents, intramolecular and intermolecular hydrogen bonds, solvents, concentration and chemical reaction) on the absorption and emission properties are described.
Abstract: The spectral characteristics, photophysical parameters and photochemical reactivities of 1,10-anthraquinone derivatives are reported. The effects of various parameters (substituents, intramolecular and intermolecular hydrogen bonds, solvents, concentration and chemical reaction) on the absorption and emission properties are described. The photochemical reactivities, photo-oxidations and photoreductions displayed by the various derivatives are given and compared.
149 citations
TL;DR: The photophysical properties of donor-acceptor molecules, such as push-pull polyenes and carotenoids, have been studied by absorption and fluorescence spectroscopy as discussed by the authors.
Abstract: The photophysical properties of donor-acceptor molecules, push-pull polyenes and carotenoids, have been studied by absorption and fluorescence spectroscopy. The compounds bear various acceptor and donor groups, linked together by chains of different length and structure. The position of the absorption and fluorescence maxima and their variation in solvents of increasing polarity are in agreement with long-distance intramolecular charge-transfer processes, the linker acting as a molecular wire. The effects of the linker length and structure and of the nature of acceptor and donor are presented.
TL;DR: In this article, it has been shown that nitrogen can demonstrate unusually high coordination when included as the central atom in such complexes, which suggests the possibility of forming polyaurated species with the metal atoms clustering about a central element.
Abstract: IT has been demonstrated recently that pentacoordinate and hexacoordinate carbon atoms can be prepared in complexes containing two-coordinate gold(I) species1–3. This unusual environment for carbon can be ascribed to the tendency of gold(I) to form clusters with short intramolecular contacts between the metal atoms4,5; the effect, termed 'aurophilicity', suggests the possibility of forming polyaurated species with the metal atoms clustering about a central element. Here we report that nitrogen, as well as carbon, can demonstrate unusually high coordination when included as the central atom in such complexes. We have prepared an aurated dication in which the nitrogen exhibits fivefold coordination. In keeping with theoretical predictions, we anticipate that it may prove possible to synthesize similar compounds containing other central elements, such as boron.
TL;DR: In this article, a series of palladium cyclizations of aryl iodides and vinyl bromides onto proximate alkenes or heteroaromatic rings (indole, pyrrole) lead to a wide variety of fused ring systems.
TL;DR: Partial asymmetric synthesis has been observed in intramolecular C-H insertion, aromatic cycloaddition, and cyclopropanation reactions of α-diazoketones catalysed by homochiral rhodium(II) mandelate carboxylates derived from L-proline as mentioned in this paper.
Abstract: Partial asymmetric synthesis has been observed in intramolecular C–H insertion, aromatic cycloaddition, and cyclopropanation reactions of α-diazoketones catalysed by homochiral rhodium(II) mandelate and rhodium(II) carboxylates derived from L-proline.
TL;DR: In this paper, the S1 state of 1,8-dihydroxy-9,10-anthraquinone showed the evidence of excited-state intramolecular proton transfer process.
Abstract: Photodynamics of the S1 state of the 1,4- and 1,8-disubstituted hydroxyquinones and aminoquinones have been studied in different organic solvents by absorption and fluorescence spectroscopy. The internal conversion process is the major deactivation path for the S1 state. The solvent and temperature dependence and deuterium isotope effect on the fluorescence dynamics suggest that hydrogen stretching vibrations in different intra- and inter-molecular hydrogen bonds are responsible for very fast non-radiative decay processes. Among the intra-molecular hydrogen-bonded molecules studied only the S1 state of 1,8-dihydroxy-9,10-anthraquinone showed the evidence of excited-state intramolecular proton-transfer process. Preliminary observations on the interaction of the S1 state of these quinones with benzene and other aromatic hydrocarbon solvents via the formation of an exciplex have also been reported.
TL;DR: The transition structures for the intramolecular hydrogen atom abstractions of the butoxy radical, the triplet and the radical cation states of butanal, as well as that of the thermal retro-ene reaction of butanol, were located with ab initio molecular orbital calculations as mentioned in this paper.
Abstract: The transition structures for the intramolecular hydrogen atom abstractions of the butoxy radical, the triplet and the radical cation states of butanal, and the radical cation of butanol, as well as that of the thermal retro-ene reaction of butanal, were located with ab initio molecular orbital calculations. These processes are the rate- and/or product-determining steps in common reactions like the Barton reaction, the Norrish type II photochemical reaction, and the McLafferty rearrangement of radical cations in mass spectrometry. The corresponding intermolecular hydrogen abstractions from methane by the methoxy radical, triplet formaldehyde, formaldehyde radical cation, and the methanol radical cation were located for comparison, using UHF theory and correlation theory corrections at the MP2 level. Differences in activation energy are related to the electronic differences between closed-shell, open-shell, and charged open-shell systems
TL;DR: The combination of palladium-catalyzed intramolecular biaryl coupling with enantioselective dihydroxylation of achiral stilbenes provides an efficient, two-step route to the chiral 9,10-dihydrophenanthrenediol unit of the pradimicins and benanomicins as mentioned in this paper.
TL;DR: In this paper, the insertion reaction of α-diazo β-keto esters has been achieved by exploiting homochiral rhodium(II) carboxylates, affording optically active cyclopentane derivatives in up to 46% ee.
TL;DR: In this article, the second-order Mo/ller-Plesset method was used to calculate the quadratic force constants for the full quartic fields. But the effects of using large basis sets were examined.
Abstract: Knowledge of a force field expanded through quartic displacements, together with a dipole field expanded through cubic displacements, yields all the harmonic and anharmonic molecular properties of interest to infrared spectroscopists. Such force fields may also explain much of the mechanism behind intramolecular vibrational energy redistribution. The ab initio quantum chemist can now calculate these fields, either at the self‐consistent field level or with the inclusion of electron correlation effects. For accurate predictions, it is important to include electron correlations effects for at least the quadratic part of the force fields. Here we report studies using the second‐order Mo/ller–Plesset method for the full quartic fields. We examine the effects of using large basis sets. The quadratic force constants are calculated analytically; cubic and quartic constants are calculated using central differences of second derivatives in reduced normal coordinates. Three molecules are studied. HCCF, for which a ...
TL;DR: In this article, a plan for the synthesis of tetronomycin A was proposed based on the intramolecular addition of a hydroxyl group to an alkene promoted by Pd(ll) leads to an alkyl-Pd intermediate which can undergo CO insertion and cleavage by MeOH.
Abstract: The intramolecular addition of a hydroxyl group to an alkene promoted by Pd(ll) leads to an alkyl-Pd intermediate which can undergo CO insertion and cleavage by MeOH. The result is 1,2-alkoxycarbonylation. While in the formation of six-membered rings the 2,6- substituents strongly prefer a cis arrangement, the selectivity in generating 2,5-disubstituted tetrahydrofurans is poor. However, nearby substituents can influence the selectivity, and by proper positioning of large (removeable) groups, both cis-2,5 and trans-2,5 disubstituted isomers can be prepared selectively. In the absence of CO, P-hydride elimination is fast, and a remarkable solvent effect has been observed. In DMSO, the kinetic product is formed very selectively, while in MeCN, for example, the thermodynamic olefin isomer is preferred. These selectivities are tested in a plan for the synthesis of tetronomycin A.
TL;DR: In this article, a stereoselective synthesis of artemisinin based on intramolecular Diels-Alder reaction of the triene 4 derived from (+)isolimonene 2 is described.
TL;DR: In this paper, a series of conformationally restricted por-phyrins bridged by aromatic spacers has been synthesized, and the geometry-dependence of the intramolecular singlet excita- tion energy transfer (EN) in the zinc-free base diporphyrin system and the intra-molecular electron transfer (ET) in zinc-ferric hybrid diporphrin system were studied by the pico-second time-resolved fluorescence spectroscopy.
Abstract: A series of conformationally restricted dimeric por- phyrins bridged by aromatic spacers has been synthesized. Soret bands of their bis-zinc complexes are split due to the exciton coupling, depending upon the geometrical relationship of the two porphyrin rings. With these dimeric porphyrins, the geometry-dependence of the intramolecular singlet excita- tion energy transfer (EN) in the zinc-free base diporphyrin system and the intramolecular electron transfer (ET) in the zinc-ferric hybrid diporphyrin system were studied by the pico-second time-resolved fluorescence spectroscopy and pico- second time-resolved transient absorption spectroscopy. It has been shown that the former process proceeds via Forster mechanism. In the latter system, both rates of the forward and backward ET were determined. The rates of the forward ET decreased upon the increase of the center-to-center distance of the two porphyrins, while the rates of the backward ET were nearly constant through the series in the range of 8-22 A of the center-to-center distance. An improved procedure has been developed for the synthesis of conformationally restricted trimeric and pentameric porphyrins as well as quinone-linked porphyrin aggregates, which will be very useful in studying the sequential and long-distance EN and ET processes.
TL;DR: In this paper, a tentative proposal for the steric course of the in situ reaction sequence involved in the carbocyclization of an enamine and an intermediate olefinic enamine is presented.
Abstract: The mechanism of the complex sequence of reactions, leading to the observed products, is discussed. The structures of the products and intermediates, which can be trapped under certain conditions, as well as previous investigations of single steps of the in situ reaction sequence involved in the carbocyclization are used to arrive at a tentative proposal for the steric course of these steps: (i) intermolecular coupling of the trigonal centers of enamine and nitroolefin with preferred relative topicity like, (ii) intramolecular coupling of the trigonal centers in an intermediate olefinic enamine, (iii) protonation of a nitronate anion moiety, and (iv) protonation of an enamine (in the case of the monocyclic products)
TL;DR: In this article, the reaction 2-pyridine ⇌ 2-hydroxypyridine (HP) was studied as a prototype of tautomerism of heteroaromatic compounds.
TL;DR: In this paper, two model cyclic pseudopentapeptides, cyclo(D-Phe-Proψ[CSNH]Gly-Pro-Gly) and cyclo (D-phe-pro-gly-proψ [CSNH]-GlyGly Proψ(GlyPro-Pro)-Gly), were synthesized and the results of the studies suggest that the thioamide is generally compatible with the β-and γ-turn features.
Abstract: Two model cyclic pseudopentapeptides, cyclo(D-Phe-Proψ[CSNH]Gly-Pro-Gly) and cyclo(D-Phe-Pro-Gly-Proψ[CSNH]Gly) are synthesized . The all-amide parent molecule has been shown by NMR to possess both a β- and a γ-turn intramolecular hydrogen bond. The results of our studies suggest that the thioamide is generally compatible with the β- and γ-turn features, but steric and solvation effects appear to have a more pronounced influence on conformation than in the case of the all-amide parent
TL;DR: Palaldium-Catalyzed Stereocontrolled Intramolecular 1,4-Additions to Cyclic 1,3-Dienes Involving Amides as Nucleophiles as discussed by the authors.
Abstract: Palaldium-Catalyzed Stereocontrolled Intramolecular 1,4-Additions to Cyclic 1,3-Dienes Involving Amides as Nucleophiles
TL;DR: The acid dissociation constants (Kapp) of p-t-butylcalix[n]arenes (1n: n=4, 6, and 8) and their noncyclic analogs (3n) were estimated in THF at 25°C.
Abstract: The acid dissociation constants (Kapp) of p-t-butylcalix[n]arenes (1n: n=4, 6, and 8), O-methylated p-t-butylcalix[4]arene (2Men), and their noncyclic analogs (3n) were estimated in THF at 25°C. As pH indicators, Et4N+ salts of p-nitrophenolate, 2,4-dinitrophenolate, and picrate were employed. The pKapp values for 1n were lowered by at least four pK units from that of p-t-butylphenol because of strong intramolecular hydrogen-bonding interactions. Compounds 2n involved both the strong and weak hydrogen bonds: the proton in the strong hydrogen bond showed the acidity comparable with that of 1n whereas that in the weak hydrogen bond showed the relatively high pKapp. These properties were discussed in relation to δOH (chemical shift in 1HNMR) and νOH(OH vibration band in IR). This is the first systematic discussion on “pKa” of p-t-butylcalix[n]arenes.