Showing papers on "Steric effects published in 2000"
TL;DR: In this article, the effect of Co or Ni on the HDS activity of Mo/alumina was studied by using dibenzothiophene (DBT) and 4,6-dimethyldibenzethiophene(46DMDBT), and the reaction was carried out at 340°C under a 4 MPa total pressure.
396 citations
TL;DR: It is found that, while base-pairing selectivity does depend on Watson - Crick hydrogen bonding in the natural pairs, it is possible to design new bases that pair selectively and stably in the absence of hydrogen bonds.
Abstract: The physical and chemical factors that allow DNA to perform its functions in the cell have been studied for several decades. Recent advances in the synthesis and manipulation of DNA have allowed this field to move ahead especially rapidly during the past fifteen years. One of the most common chemical approaches to the study of interactions involving DNA has been the use of DNA base analogues in which functional groups are added, deleted, blocked, or rearranged. Here we describe a different strategy, in which the polar natural DNA bases are replaced by nonpolar aromatic molecules of the same size and shape. This allows the evaluation of polar interactions (such as hydrogen bonding) with little or no interference from steric effects. We have used these nonpolar nucleoside isosteres as probes of noncovalent interactions such as DNA base pairing and protein - DNA recognition. We have found that, while base-pairing selectivity does depend on Watson - Crick hydrogen bonding in the natural pairs, it is possible to design new bases that pair selectively and stably in the absence of hydrogen bonds. In addition, studies have been carried out with DNA polymerase enzymes to investigate the importance of Watson - Crick hydrogen bonding in enzymatic DNA replication. Surprisingly, this hydrogen bonding is not necessary for efficient enzymatic synthesis of a base pair, and significant levels of selectivity can arise from steric effects alone. These results may have significant impact both on the study of basic biomolecular interactions and in the design of new, functionally active biomolecules.
288 citations
TL;DR: In this article, the enthalpies of reaction for nucleophilic carbenes depend on the stereoelectronic properties of the ligands affecting the availability of the carbene lone pair.
Abstract: Publisher Summary Specific applications benefit from or require the use of sterically demanding phosphine ligation to stabilize reactive intermediates. Therefore, there is a need for strongly nucleophilic, bulky ligands that are resistant to oxidizing agents and form stable bonds with metals. The enthalpies of reaction can be converted to relative enthalpies of reaction on a mole of product basis by dividing the enthalpies by 4 which represents the number of bonds made in the course of reaction. The enthalpies of reaction for nucleophilic carbenes depend on the stereoelectronic properties of the ligands affecting the availability of the carbene lone pair. Increase in the steric congestion around the carbene carbon atom hinders a closer approach of the ligand, therefore affording smaller metal-lone pair overlap. The nucleophilic carbenes are phosphine-mimics but they are much more. They reside at the upper end of the Tolman electronic and steric parameter scales. Much remains to be explored with these ligands.
209 citations
TL;DR: In this article, the Gibbs energy change ΔGHB for the formation of 1:1 hydrogen bonding complexes between hydrogen bond acceptors (bases) and a reference hydrogen bond donor (4-fluorophenol) in tetrachloromethane at 298 K was measured.
Abstract: Both proton transfer and hydrogen bonding play important roles in biological systems. In order to measure hydrogen bond basicity, we are building a new scale that differs significantly from the pKa scale of proton transfer basicity. The strength of hydrogen bond acceptors (HBAs) is measured from the Gibbs energy change ΔGHB for the formation of 1:1 hydrogen bonding complexes between hydrogen bond acceptors (bases) and a reference hydrogen bond donor (4-fluorophenol) in tetrachloromethane at 298 K. The pKHB database (1.364 pKHB =–ΔGHB (kcal mol-1)) comprises ca. 1000 hydrogen bond acceptors. The HBA strength depends on (i) the position of the acceptor atom in the periodic table, (ii) polarizability, field/inductive and resonance effects of substituents around the acceptor atom, and (iii) proximity effects including steric hindrance of the acceptor site, intramolecular hydrogen bonding and lone-pair–lone-pair repulsions. The ranking of oxygen and sp nitrogen bases does not depend very much on the solvent and the reference hydrogen bond donor, but sp2 and sp3 nitrogen bases gain strength in solvents of higher reaction field than CCl4 and lose strength toward CH and weak NH donors. The complete scatter pattern exhibited by the pKa versus pKHB plot demonstrates the non-equivalence of the two scales. The HBA strength scale is applied to the prediction of the hydrogen bonding site in polybasic drugs (e.g strychnine and carbimazole), and to the calculation of octanol–water partition coefficients. A possible relationship between HBA strength and antihistaminic activity is studied for the `push–pull' drugs cimetidine, ranitidine and famotidine.
179 citations
TL;DR: These compounds show unique photophysical properties due to the p(pi)-pi conjugation through the p-orbital on the boron atom, including a significant solvatochromism of fluorescence from blue to orange colors.
149 citations
TL;DR: A revision of the most typical examples used to illustrate the existence of secondary orbital interactions (SOI) has been achieved, but analysis indicates that no conclusive evidence can be obtained from these cases.
Abstract: A revision of the most typical examples used to illustrate the existence of secondary orbital interactions (SOI) has been achieved. However, our analysis indicates that no conclusive evidence can be obtained from these cases. All five examples proposed by Woodward and Hoffmann in The Conservation of Orbital Symmetry have been revisited. A combination of well-known mechanisms (such as solvent effects, steric interactions, hydrogen bonds, electrostatic forces, and others) can be invoked instead to justify the endo/exo selectivity of Diels−Alder reactions.
133 citations
TL;DR: A series of phosphinoalkyl-substituted cyclopentadienyl chromium complexes, e.g., (η1:η5-R2PC2H4C5H4)CrCl2, have been synthesized.
126 citations
TL;DR: In this article, the steric constraints imposed by the solid matrix on the Si−O−Si linkages and their nearest surroundings were considered in the cluster approach at the DFT (B3LYP) and MP2 levels of theory.
Abstract: Hydrolysis of Si−O−Si linkages of β-cristobalite by a single H2O molecule is studied within the cluster approach at the DFT (B3LYP) and MP2 levels of theory. The 6-31G(d) and 6-311G(d) basis sets are used. Cluster models, including from 6 up to 14 Si atoms, of the (001) and (111) surface planes are considered. These models are specially designed to take into account the steric constraints imposed by the solid matrix on the Si−O−Si linkages and their nearest surroundings. For comparison, the hydrolysis of the Si−O−Si bridge of the free (HO)3Si−O−Si(OH)3 molecule is also calculated. The computed activation energy of the reaction (ΔEa) for the (001) and (111) planes of β-cristobalite is larger by 5 and 16 kcal/mol, respectively, than for (HO)3Si−O−Si(OH)3 (17 kcal/mol). The higher energy barrier for the surface is due to the resistance of the lattice to the relaxation of the activated complex of the reaction. The difference in ΔEa between the (001) and (111) planes suggests that the larger the number of Si−O...
122 citations
TL;DR: The rotational barriers for biphenyl, 2,2′-bipyridine and 2-phenylpyridine were calculated with HF, B3LYP, BPW91, and MP2 and double-zeta polarized basis sets as mentioned in this paper.
120 citations
116 citations
TL;DR: In this paper, various dialkyldibenzothiophenes containing methyl, ethyl, diisobutyl, and diisopropyl substituents in the fourth and sixth positions were synthesized and their reactivities compared over an NiMo/Al 2 O 3 industrial catalyst.
TL;DR: In the chemistry of main-group elements the pentamethylcyclopentadienyl (Cp*) group represents a very important substituent as discussed by the authors, and the steric requirements of the σ- or π-bound Cp* group enable the kinetic stabilization of otherwise highly reactive species.
Abstract: In the chemistry of main-group elements the pentamethylcyclopentadienyl (Cp*) group represents a very important substituent. Owing to its quite variable bonding modes, ranging from mainly ionic to mainly covalent and from η5via η2/3 to η1, an effective adjustment to the electronic situation at the respective element centre is feasible. Furthermore, the steric requirements of the σ- or π-bound Cp* group enable the kinetic stabilization of otherwise highly reactive species. Covalent Cp*–element bonds are comparatively weak, consequently allowing fast sigmatropic and haptotropic rearrangement processes. A further consequence is that the Cp* radical as well as the Cp*− anion are potential leaving groups. This phenomenon forms the basis of a rich faceted substitution and elimination chemistry. As proof of this statement, several examples from phosphorus, silicon, and gallium chemistry are presented.
TL;DR: In this article, the factors that influence the overall cycloaddition reactions between imidozirconocene complexes Cp2(THF)ZrNR (R = 2,6-i-Pr2C6H3, t-Bu, i-Pr, SiMe3, cyclohexyl, p-tolyl) and small unsaturated organic molecules were examined.
TL;DR: In this paper, the electronic and steric effects of substituents on silicon triple bonds are investigated to make disilynes synthetically accessible as stable compounds for RSiSiR.
TL;DR: The proposed semi-quantitative model, which explains most hydrogen-bonded motifs as a combination of the effects of substituents at C3 and C5, has been examined as a function of the steric and polarizability effects of these substituent effects represented by molar refractivity.
Abstract: The secondary structure of 1H-unsubstituted pyrazole derivatives bearing only one hydrogen donor group and one or more acceptor groups has been analyzed in terms of some descriptors representing the substituents at C3 and C5. The substituent at C4 appears to affect mainly the tertiary or quaternary structure of these compounds. The proposed semi-quantitative model, which explains most hydrogen-bonded motifs as a combination of the effects of substituents at C3 and C5, has also been examined as a function of the steric and polarizability effects of these substituents represented by molar refractivity. The model also applies to other five-membered rings (1,2,4-triazoles, 1,2,4-diazaphospholes and 1,2,4-diazaarsoles). Furthermore, ab initio calculations at RHF/6-31G* have been performed to discover the relative stability of three of the four hydrogen-bond patterns displayed by several symmetrical pyrazoles (dimers, trimers, tetramers). The fourth motif, catemers, has only been discussed geometrically.
TL;DR: In this paper, an experimental study of the stability and coordination of oxygen-and nitrogen-containing ligands in association with Mg2+ in the gas phase has been undertaken.
Abstract: An experimental study of the stability and coordination of oxygen- and nitrogen-containing ligands in association with Mg2+ in the gas phase has been undertaken. The ligands chosen exhibit a wide range of physical properties in terms of their ionization energies, dipole moments, and polarizabilities, and a simple electrostatic model reveals a semiquantitative trend between these properties and the ability of each ligand to stabilize Mg2+. The model clearly demonstrates why water is extremely effective at stabilizing Mg2+, and in this respect, CO2 also proves to be a good ligand. Evidence of a discrete first solvation shell is apparent only for those ligands which do not display hydrogen bonding. For water, methanol, and ethanol, hydrogen bonding leads to extended solvation units for which the boundaries are less obvious. However, for more complex alcohols, steric interactions appear to negate the influence of hydrogen bonding. Discrete solvation shells are observed for most aprotic ligands, and the optimu...
TL;DR: Correlation analyses show the presence of an inverse relationship between reactivity and selectivity in the reaction of various sulfides with a given oxo(salen)chromium(V) complex and vice versa, which shows that this redox system falls under strong reactivity-selectivity principle (RSP).
Abstract: The selective oxidation of organic sulfides to sulfoxides by oxo(salen)chromium(V) complexes in acetonitrile is overall second-order, first-order each in the oxidant and the substrate. The rate constant, k2, values of several para-substituted phenyl methyl sulfides correlate linearly with Hammett σ constants and the ρ values are in the range of −1.3 to −2.7 with different substituted oxo(salen)chromium(V) complexes. The reactivity of different alkyl sulfides is in accordance with Taft's steric substituent constant, ES. A mechanism involving direct oxygen atom transfer from the oxidant to the substrate rather than electron transfer is envisaged. Correlation analyses show the presence of an inverse relationship between reactivity and selectivity in the reaction of various sulfides with a given oxo(salen)chromium(V) complex and vice versa. Mathematical treatment of the results shows that this redox system falls under strong reactivity−selectivity principle (RSP).
TL;DR: In this article, the influence of substituents on intramolecular charge transfer (ICT), absorption and emission spectra were measured for several derivatives of 2-styrylquinoline (2-StQ), 2-stylylthiophene (2StT) systems.
Abstract: To understand the influence of substituents on intramolecular charge transfer (ICT), absorption and emission spectra were measured for several derivatives of 2-styrylquinoline (2-StQ), 2-styrylpyridine (2-StP) and 2-styrylthiophene (2-StT) systems. A strong donor, such as an N,N-dimethylamino group can produce an ICT compound in these systems, and the excited state dipole moments were measured using a solvatochromic method. Quinoline and pyridine can only act as acceptors. Depending on substituents, the thiophene ring could be a donor or an acceptor. For same donor in our systems, the strength order of different acceptors is quinoline>naphthalene>pyridine>thiophene. Besides electronic effect, steric conformation is also an important factor in ICT molecules. Protonation and hydrogen bonding at acceptor site show red-shift in absorption maxima. The degree of shift depends on the substituents. Interaction at a donor site gives the opposite result. There is a linear relation between excited state dipole moments and absorption energy.
TL;DR: In this article, the reactivity of the mononuclear 16-electron iridadithiolene ring complex Cp*Ir(S2C2B10H10) (1) toward alkynes, a diazoalkane, and quadricyclane has been investigated.
TL;DR: This analysis and the observed apparent dissimilarity in the reactions of model four-coordinate aluminum alkyls with O2 clearly show that the stereoelectronic prerequisites are responsible for the fundamentally different reactivity.
Abstract: The interaction of dioxygen with various tetrahedral aluminum alkyls, (tBu)3Al.OEt2 (1), tBu2Al(mu-OtBu)2AltBu2 (6), (tBu)2Al(mesal) (2) [mesal=methyl salicylate anion], R2Al(mu-pz)2AlR2 [pz=deprotonated pyrazole, R= Me (3a), Et (3b), and tBu (3c)], R2Al(mu-3,5-Me2pz)2AIR2[3,5-Me2pz = deprotonated 3,5-dimethylpyrazole, R= Me (4a), and Et (4b)], and Et2B(mu-pz)2AlEt2 (5), has been investigated. We were particularly interested in the effect of steric hindrances both caused by the metal-bonded substituents and those that result from the nature of the bifunctional ligand used in the oxygenation reaction. In the reaction of 1 with O2, only the formation of the monoalkoxide compound6 was observed. The latter di-tert-butyl compound as well as all planar aluminapyrazoles, that is, the tert-butyl derivative 3c and lower alkylaluminum derivatives with the more demanding 3,5-dimethylpyrazoyl ligands 4a and 4b, are stable under an atmosphere of dry oxygen and ambient conditions. Inspection of the space-filling representation of these compounds has undoubtedly shown that the bulky tert-butyl groups or pyrazolyles ligands, respectively, provide steric protection for the metal center from the dioxygen attack. In contrast, the dialkylaluminum derivatives of pyrazole, 3a and 3b, and the diethylaluminum bis(1-pyrazolyl)borate complex 5, all with the metal center eclipsed with respect to the plane defined by the four nitrogen atoms, react smoothly with O2 to form the alkyl(alkoxy)aluminum complexes. In the reaction of 5 with O2 for example, the Et-B bonds remained intact, and the dimeric five-coordinate compound [Et2B(mu-pz)2 Al(mu-OEt)Et]2 (9) was isolated in good yield. The interaction of mononuclear di-tert-butyl chelate complex 2 with O2 at -15 degrees C gives (tBuOO)(tBuO)Al(mu-OtBu)2Al(mesal)2 (7) in high yield, and the presence of the alkylperoxo moiety is a particularly significant point in the resulting product. All the compounds have been characterized spectroscopically, and the structures of 3c, 4a, 6, 7, and 9 have been confirmed by X-ray crystallography. Structural features of 1-6 are discussed and are considered in relation to the possible approach pathways of the O2 molecule to the four-coordinate metal center. This analysis and the observed apparent dissimilarity in the reactions of model four-coordinate aluminum alkyls with O2 clearly show that the stereoelectronic prerequisites are responsible for the fundamentally different reactivity.
TL;DR: Palladium catalysed cascade mono- and bis-cyclisation–anion capture involving a wide variety of starter species, terminating species and organotin(IV) anion capture reagents are reported leading to a range of bridged, fused and spirocyclic products.
TL;DR: Four novel calixarene-based cuprous complexes are described, which present a biomimetic tris(imidazole) coordination core associated with a hydrophobic cavity that wraps the apical binding site and were promoted by the strong conformational coupling between the metal center and the host structure.
Abstract: Four novel calix[6]arene-based cuprous complexes are described. They present a biomimetic tris(imidazole) coordination core associated with a hydrophobic cavity that wraps the apical binding site. Each differs from the other by the methyl or ethyl substituents present on the phenoxyl groups (OR1) and on the imidazole arms (NR2) of the calix[6]arene structure. In solution, stable CO complexes were obtained. We have investigated their geometrical and dynamic properties with respect to the steric demand. IR and NMR studies revealed that, in solution, these complexes adopted two distinct conformations. The preferred conformation was dictated only by the size of the OR1 group. When R1 was an ethyl group, the complex preferentially adopted a flattened C3-symmetrical structure. The corresponding helical enantiomers were in conformational equilibrium, which, however, was slow on the 1H NMR time scale at -80 degrees C. When R1 was a methyl group, the low-temperature NMR spectra revealed the partial inclusion of one tBu group. The complex wobbled between three dissymmetric but equivalent conformations. Hence, small differences in the steric demand of the calixarene's skeleton changed the geometry and dynamics of the system. Indeed, this supramolecular control was promoted by the strong conformational coupling between the metal center and the host structure. Interestingly, this was not only the result of a covalent preorganization, but also stemmed from weak interactions within the hydrophobic pocket. The vibrational spectra of the bound CO were revealed to be a sensitive gauge of this supramolecular behavior, similar to copper proteins in which allosteric effects are common.
TL;DR: In this article, the relative rate of hydrolysis of these compounds is rationalized by considering the influence of steric, inductive and stereoelectronic effects on the reaction mechanism.
TL;DR: In this article, the application of silica-supported BF3 as a mild acid catalyst to the C- and O-alkylation of various aryl alcohols is described.
Abstract: The application of silica-supported BF3 as a mild solid acid catalyst to the C- and O-alkylation of various aryl alcohols is described. In the case of O-alkylation, the reactivity of the ether product in both heterogeneous and homogeneous systems is compared, and it is shown that the former can be used to affect selective C-alkylation of ethers, while in the latter system, ether rearrangement to ring poly-alkylated phenols also occurs. Ether rearrangement is thought to require coordination of the ether to an available Lewis acid site, which, in the case of the heterogeneous system, is apparently impossible. The difference between the homogeneous and the supported BF3 may be attributed to steric restrictions and/or relatively weak Lewis acidity of the supported system. Effects of steric hindrance at the reaction centre are examined. Possible routes to ethers and ring-alkylated products are also discussed.
TL;DR: A crystal structure analysis of the title compound NiTBuOEP reveals a highly nonplanar conformation with an average deviation from planarity for the 24 macrocycle atoms of 0.462 A and displacements of the meso carbon atoms from the 4N-plane of 1.044 A.
Abstract: Highly substituted porphyrins exhibit significantly distorted, nonplanar conformations in the solid state. The crystallographically determined degree of nonplanarity correlates with a bathochromic shift of the absorption maxima in solution. In addition, nonplanar porphyrins with meso aryl groups show increasing in-plane rotation of the meso aryl groups, which potentially could account for the observed changes in spectroscopic and physicochemical properties of nonplanar porphyrins. A crystal structure analysis of the title compound NiTBuOEP reveals a highly nonplanar conformation with an average deviation from planarity for the 24 macrocycle atoms of 0.462 A and displacements of the meso carbon atoms from the 4N-plane of 1.044 A. The average Ni–N bond distance in the crystal (1.873(3) A) is in good agreement with the Ni–N bond distance in solution (1.87 A) that was determined by EXAFS. Compared to more planar reference compounds, NiTBuOEP exhibits significantly red-shifted absorption spectra in solution, correctly predicted by INDO/s calculations. As the shortness of the Ni–N bonds has been shown to be an excellent indicator for the degree of conformational distortion in porphyrins, this proves that the highly nonplanar conformation of sterically strained porphyrins is maintained in solution. Thus, the physical and chemical properties measured in solution do indeed reflect the stereochemistry of the single crystals. In addition, the use of only alkyl substituents in NiTBuOEP to cause nonplanarity obviously circumvents potential electronic effects due to aryl ring interactions.
TL;DR: A number of 2-aryl substituted 6-pyrrolidino-4(3H)-quinazolinones were synthesized in four steps starting from commercially available 5-chloro-2-nitrobenzoic acid.
Abstract: A number of 2-aryl substituted 6-pyrrolidino-4(3H)-quinazolinones are reported. They were synthesised in four steps starting from commercially available 5-chloro-2-nitrobenzoic acid. The key cyclis...
TL;DR: The influence of substituents on which cyclopropyl bond cleaves in the cycloisomerization of cycloprostylenynes catalyzed by CpRu(N&tbd1;CCH(3))(3)(+)PF(6)(-) is compared to the corresponding Rh-catalyzed reaction.
TL;DR: In this article, density functional theory is applied to the calculation of the steric and electronic factors which might affect the Co−CR bond activation in coenzyme B12, and it is shown that stereoelectronic properties of cobalamins can only be explained on the basis of density functional theories.
Abstract: Density functional theory (DFT) is applied to the calculation of the steric and electronic factors which might affect the Co−CR bond activation in coenzyme B12. The six-coordinate cobalamins (B−[CoIII(corrin)]−R, models of coenzyme B12) include the actual corrin macroring as the equatorial ligand and imidazole (Im), dimethylbenzimidazole (DBI) or water (H2O), as the α-trans ligand (B). The β axial ligand (R) represents a series of alkyl groups with different steric bulkiness ranging from −C⋮N, −C⋮CH through methyl, ethyl, isopropyl, tert-butyl to 5‘-deoxy-5‘-adenosyl. Each trans ligand (Im, DBI or H2O) produces a positive correlation of the Co−CR and Co−NB bond lengths. The increasing complexity of the R group leads to two major structural correlations: a positive correlation between the Co−CR and Co−NB bond lengths and an inverse correlation between the Co−CR bond length and the flatness of the corrin ring. It is shown that stereoelectronic properties of cobalamins can only be explained on the basis of ...
TL;DR: The results of a systematic study of structural isomerism in more than 30 noncovalent hydrogen-bonded assemblies are described in this article, where it is shown that the covalent capture of the dynamic assemblies via a ring-closing metathesis (RCM) reaction provides a novel analytical tool to distinguish between the D3 and C3h isomeric forms of the assembly.
Abstract: The results of a systematic study of the structural isomerism in more than 30 noncovalent hydrogen-bonded assemblies are described. These dynamic assemblies, composed of three calix[4]arene dimelamines and six barbiturates/cyanurates, can be present in three isomeric forms with either D3, C3h, or Cs symmetry. The isomeric distribution can be readily determined via a combination of 1H NMR and 13C NMR spectroscopy. In one case it is shown that the covalent capture of the dynamic assemblies via a ring-closing metathesis (RCM) reaction provides a novel analytical tool to distinguish between the D3 and C3h isomeric forms of the assembly. For the D3 isomer the RCM results in the formation of a cyclic trimer, comprising three dimelamines, whereas for the C3h isomer a cyclic monomer is formed. Molecular dynamics simulations in chloroform are qualitatively in agreement with the experimental data and reveal that the isomeric distribution is determined by a combination of steric, electronic, and solvation effects. A wide range of isomeric distributions covering all extremes has been found for the studied assemblies. Those with 5,5-disubstituted barbituric acid derivatives exclusively form the D3 isomer, because steric hindrance between the barbiturate substituents prevents formation of the C3h and Cs isomers. In contrast, assemblies with isocyanuric acid derivatives exhibit increased stability of the C3h and Cs isomers upon increasing the size of the isocyanurate substituent. The outcome of the assembly process can be controlled to a large extent via chiral substituents in the calix[4]arene dimelamines, due to the preferred orientation of the chiral centers.