Showing papers on "Steric effects published in 1984"

Chelation or Non‐Chelation Control in Addition Reactions of Chiral α‐ and β‐ Alkoxy Carbonyl Compounds [New Synthetic Methods (44)]

Abstract: The addition of C-nucleophiles such as Grignard reagents or enolates to chiral α- or β-alkoxy aldehydes or ketones creates a new center of chirality and is therefore diastereogenic. In order to control stereoselectivity, two strategies have been developed: (1) Use of Lewisacidic reagents which form intermediate chelates, these being attacked stereoselectively from the less hindered side (chelation control); (2) use of reagents incapable of chelation, stereoselective attack being governed by electronic and/or steric factors (non-chelation control). Generally, the two methods lead to the opposite sense of diastereoselectivity. It is possible to predict the outcome by careful choice of organometallic reagents containing elements such as Li, Mg, B, Si, Sn, Cu, Zn, or Ti. For corrigendum see DOI:10.1002/anie.198407461

C-H and H-H Activation in Transition Metal Complexes and on Surfaces.

Abstract: : The breaking of the H-H bond in H2 and a C-H bond in CH4 on both discrete transition metal complexes and on Ni and Ti surfaces is studied, and the essential continuity and similarity of the physical and chemical processes in two cases is demonstrated. We begin with an orbital analysis of oxidative addition, delineating four basic interactions: H-H or C-H sigma M electron transfer the reverse M sigma* transfer (both weakening the sigma bond, forming the M-H bond), a repulsive interaction between sigma and metal filled orbitals, and a rearrangement of electron density at the metal. The molecular cases analyzed in detail are d6 ML5, d8 ML4 and CRhoM'L. Coordinative unsaturation is necessary, and consequently sigma M electron transfer dominates the early stages of the reaction. Steric effects are important for CH4 reaction. Activation in angular ML4 or CpM L is achieved through a destabilized yz MO, and d10 ML3, ML2 candidates for activation are described. For our study of the surface we develop tools such as projections of the density of states and crystal orbital overlap populations - the extended structure analogues of a population analysis. These allow a clear understanding of what happens when an H2 or a CH4 molecule approaches a surface. Because of the higher energy of the occupied metal orbitals on the surface the M sigma* interaction leads the reaction. There are great similarities and some differences between the activation acts in a discrete complex and on a surface. (Author)

Spectral properties and basicity of stilbazolium betaines containing bulky substituents on the quinoid ring

Abstract: Synthese, basicite et spectre visible de [palmityl-1-hydroxy-2' ethyl-1 et hydroxy-6' hexyl-1 dihydro-1,4 pyridylidene-4 methyl]-7 benzoquinonemethides-1,4

Ligation of phosphorus ligands to silver(I). 2. Electronic and steric effects in the coordination of one to four P(OR)3 and PR3 ligands

Abstract: Synthese et etude par RMN de 31 P des complexes {Ag[P(OPh) 3 ] 3 }BF 4 , {Ag[P(O-iPr) 3 ] 3 }BPh 4 , {Ag[P(O-i-Pr) 3 ] 4 }X (X=BF 4 , BPh 4 ), {Ag[P(O-t-Bu) 3 ] 2 }BF 4 {Ag[P(O-t-Bu) 3 X} (X=I, Cl, CN), {Ag[P(O-t-Bu) 3 ] 2 NO 3 } et {Ag[P(O-Me 2 C 6 H 3 -2,6) 3 ] BF 4 }

Molybdenum(VI)-dioxo complexes with sterically bulky ligands

Abstract: Synthese de nouveaux complexes MoO 2 LH 2 avec LH 4 =N,N'-bis(hydroxy-2 t-butyl-3 benzyl) ethylenediamine. Etude de la reduction coulometrique. Etude RX, RPE

Electron-transfer reactions of photoreduced flavin analogues with c-type cytochromes: quantitation of steric and electrostatic factors.

Abstract: We have found correlations between rate constants and the difference in redox potential of the reactants for electron-transfer reactions between oxidized cytochromes and either photoproduced riboflavin or flavin mononucleotide (FMN) semiquinones (the latter rate constants extrapolated to infinite ionic strength). The riboflavin-cytochrome rate constants are about 70% of those for reduction by lumiflavin, probably because of steric interference by the ribityl side chain. Reduction of cytochromes by FMN semiquinone was ionic strength dependent in all cases, due to electrostatic interactions. Extrapolation of rate constants to infinite ionic strength shows that the phosphate exerts a significant steric effect as well (rate constants average about 27% of those for lumiflavin, although part of this decrease is due to a difference in the semiquinone pK value). Differences in the magnitude of the FMN steric effect correlate well with surface topology differences for those cytochromes whose three-dimensional structures are known. Mitochondrial cytochromes c and the cytochromes c2 all showed attractive (plus-minus) interaction with FMN in spite of the fact that some of these proteins have large net negative charges. Four small c-type cytochromes (including Pseudomonas cytochrome c-551) show a weak repulsive interaction with FMN semiquinone. We conclude that flavosemiquinones interact at a site on the cytochromes that is near the exposed heme edge. There is a large positive electrostatic field at this site in mitochondrial cytochrome c and the cytochromes c2, but this region is primarily hydrophobic in Pseudomonas cytochrome c-551 and in the other small bacterial cytochromes.(ABSTRACT TRUNCATED AT 250 WORDS)

Organic counterions and micellar parameters: substituent effects in a series of benzoates

Abstract: Determination des concentrations critiques micellaires obtenues par la methode de tension de surface pour des micelles de decyltrimethylammonium dans des solutions 0,5000 M de 20 ions antagonistes aromatiques sous forme de sels de sodium

Positional selectivity of the methylation of 5-substituted tetrazolate anions

Abstract: The methylation of a series of 15 sodium 5-substituted tetrazolates using iodomethane in acetone/water (4 : 1) has been studied. The reaction yields both 1- and 2-methyl products, and the ratio of these products is discussed in terms of the nature of the 5-substituent. Electronic and steric effects dominate the reaction pathway; both increased substituent electro negativity and steric bulk lead to predominant methylation at N2. Sodium 5-ethoxycarbonyltetrazolate (3n) goes against this trend and an intermediate is proposed where the incoming electrophile is associated with the ester carbonyl group.

A substituent constant analysis of the interaction of substituted naphthalene monoimides with DNA.

Abstract: In a continuing analysis of substituent effects in intercalator-DNA interactions, an unsubstituted naphthalene monoimide, with a 3-(dimethylamino)propyl group on the imide nitrogen has been prepared along with 3- and 4-nitro- and 3- and 4-amino-substituted derivatives. These derivatives allow an evaluation of the importance of the Hammett substituent constant and of the substituent position on the binding of naphthalene monoimides to DNA. Viscosity and spectrophotometric analyses indicate that all five compounds bind to DNA by intercalation. The 4-nitro compound gives a smaller viscosity increase and binds only approximately one-third as strongly as the 3-nitro derivative. It is postulated that this difference is due to the significant angle that the 4-nitro group makes with the intercalated monoimide ring system. The 3-NO2 group can assume a coplanar configuration with the monoimide ring system, allowing more favorable interactions with DNA base pairs, larger viscosity increases, and stronger binding to DNA. The binding constants of the 3-substituted monoimides are in the order 2 greater than 4 greater than 1 and, thus, do not follow a substituent constant pattern. The Tm values from thermal melting of DNA, on the other hand, are in the order 2 greater than 1 greater than 4, suggesting that the enthalpy contributions are significantly different for the binding of the three compounds to DNA. van't Hoff plots support this finding and indicate that both enthalpy and entropy contribute significantly to the binding free energy of 1 and 2 while the binding of 4 is primarily an enthalpic process. Plots of Tm and 65 degrees C log K values as a function of substituent constant for 1, 2, and 4 are linear. CPK model building studies suggest that 4 can form a hydrogen bond with the 5' diester oxygen of the sugar-phosphate backbone of DNA in an intercalation complex. This would lead to more favorable energetics of binding but a loss of mobility and/or available binding configurations with a resulting enthalpy-entropy compensation in the binding free energy of 4. This series of compounds dramatically illustrates the steric and hydrogen bonding complexity that can arise in attempts to design drugs to favorably interact with a DNA intercalation site as a potential bioreceptor.

Steric effects on rates of dehalogenation of anion radicals derived from substituted nitrobenzyl halides

Abstract: One-electron reduction of nitrobenzyl halides produces the anion radicals, which subsequently undergo dehalogenation to form nitrobenzyl radicals. The rate constants for these processes have been studied by pulse radiolysis in aqueous alcoholic solutions. ..cap alpha..-Substitution with a methyl group increases the rate of dehalogenation by weakening the C-X bond. On the other hand, substitution with a tert-butyl group was found to decrease the rate of dehalogenation considerably. Since the tert-butyl is not expected to increase the C-X bond dissociation energy, it is concluded that its effect is a result of steric interaction, which rotates the C-X bond toward the ring plane and thus decreases the overlap between this bond the the ring ..pi.. system and slows down the tra

Regioselectivity in organo-transition-metal chemistry. A remarkable steric effect in π-allyl palladium chemistry

Abstract: The regioselectivity of the palladium catalysed allylic substitution by several representative nucleophiles was found to be highly dependent on very small steric differences that exist at the two ends of the allylic system: attack by stabilized nucleophiles occurred at the less hindered position while PhZnCl led to substitution at the more hindered position.

A novel preparative method for γ-butyrolactams via carbon–carbon bond formation: copper or ruthenium-catalysed cyclization of N-allyl trichloroacetamides

Abstract: Trichlorinated γ-butyrolactams are formed by copper or ruthenium-catalysed cyclization of Nallyl tichloroacetamides.

Large-amplitude bending motions in phenylalanine transfer RNA.

Abstract: Conformational energy calculations on yeast tRNAPhe reveal several likely modes of intramolecular bending, including both hingelike motions (rotations about a discrete point) and distributed flexibility (deformations that bend a double-helical segment along a smooth curve). By combining these modes of motion, the molecule can be bent from the L-shaped crystallographic structure to two extremes. It can be straightened into a nearly linear conformation at an energy cost of about 50 kcal/mol, and it can be doubled over to a conformation where the anticodon and the amino acid acceptor terminus are separated by about 40 A at an energy cost of less than 100 kcal/mol. A bending range of over 100° can be covered for 50 kcal/mol, and we estimate that this value could be cut in half with a minimization algorithm that produced optimum stereochemistry. These energies are comparable to those that would be associated with changes in solvation due to changes in surface area as the molecule bends, indicating that there are no major steric barriers to tRNA flexibility and that variations in solvent conditions and interactions with other molecules may produce large changes in the overall conformation of tRNA.

1,3-Adamantane−3,13-porphyrin−6,6-cyclophane: crystal structure of the free base and steric effects on ligation of the iron(II) complex

Abstract: La base non complexee cristallise dans le systeme triclinique, groupe P1 et sa structure est affinee jusqu'a R=0,065. Le macrocycle porphyrine est plan et la molecule dans son ensemble est sans contrainte. Etude du type de liaison de CO et O 2 en complexe (dicyclohexyimidazole) fer (II)