Showing papers on "Steric effects published in 2001"
TL;DR: A series of ethane structure optimizations is reported, where successive removal of different interactions indicates that ethane's staggered conformation is the result of preferential stabilization through hyperconjugation.
Abstract: Many molecules can rotate internally around one or more of their bonds so that during a full 360° rotation, they will change between unstable and relatively stable conformations Ethane is the textbook example of a molecule exhibiting such behaviour: as one of its two methyl (CH3) groups rotates once around the central carbon–carbon bond, the molecule will alternate three times between an unstable eclipsed conformation and the preferred staggered conformation This structural preference is usually attributed to steric effects1,2,3,4,5,6,7; that is, while ethane rotates towards an eclipsed structure, the electrons in C–H bonds on the different C atoms are drawing closer to each other and therefore experience increased repulsion, introducing a rotation barrier that destabilizes the eclipsed structure8,9 Stabilization of the staggered structure through rotation-induced weakening of the central C–C bond10 and hyperconjugation11,12 has been considered to be involved, but evaluation of the contributions of these effects to ethane's internal rotation barrier and conformational preference remains difficult13,14 Here we report a series of ethane structure optimizations, where successive removal of different interactions indicates that ethane's staggered conformation is the result of preferential stabilization through hyperconjugation Removal of hyperconjugation interactions yields the eclipsed structure as the preferred conformation, whereas repulsive forces, either present or absent, have no influence on the preference for a staggered conformation
453 citations
TL;DR: X-ray analysis has revealed that 4,4-bipyridine, 1,2-bis(4-pyridyl)ethylene, and hexamethylenetetramine form donor-acceptor complexes with 1, 4-diiodobenzene and tetraiodoethylene in which the N···I distance is longer than that for the corresponding I2 complexes as discussed by the authors.
Abstract: X-ray analysis has revealed that 4,4‘-bipyridine, 1,2-bis(4-pyridyl)ethylene, and hexamethylenetetramine form donor−acceptor complexes with 1,4-diiodobenzene, 1,4-diiodotetrafluorobenzene, and tetraiodoethylene in which the N···I distance is longer than that for the corresponding I2 complexes. As opposed to the corresponding I2 complexes that are molecular adducts, these complexes have extended structures and longer N···I distances. Steric and electronic effects influence the strength of the N···I interaction and the crystal packing. An additional complex of 1,2-bis(4-pyridyl)ethane and 1,4-dibromotetrafluorobenzene has also been investigated for comparison of N···I and N···Br interactions.
311 citations
TL;DR: Several diorganoscandium complexes stabilized by the β-diketiminato ligands (Ar)NC(R)CHC(R),N(Ar) (Ar = 2,6-iPr-C6H3; R = CH3 (ligand a), R = tBu (ligands b)) have been synthesized as discussed by the authors.
198 citations
TL;DR: Chain length-related factors that stabilize organosoluble peptoid helices of this class are discussed, which are important for the design of helical, biomimetic peptoids sharing this structural motif.
Abstract: Oligomeric N-substituted glycines or “peptoids” with α-chiral, aromatic side chains can adopt stable helices in organic or aqueous solution, despite their lack of backbone chirality and their inability to form intrachain hydrogen bonds. Helical ordering appears to be stabilized by avoidance of steric clash as well as by electrostatic repulsion between backbone carbonyls and π clouds of aromatic rings in the side chains. Interestingly, these peptoid helices exhibit intense circular dichroism (CD) spectra that closely resemble those of peptide α-helices. Here, we have utilized CD to systematically study the effects of oligomer length, concentration, and temperature on the chiral secondary structure of organosoluble peptoid homooligomers ranging from 3 to 20 (R)-N-(1-phenylethyl)glycine (Nrpe) monomers in length. We find that a striking evolution in CD spectral features occurs for Nrpe oligomers between 4 and 12 residues in length, which we attribute to a chain length-dependent population of alternate struct...
184 citations
TL;DR: A theoretical study of the alkylation reaction of toluene with methanol catalyzed by the acidic Mordenite is reported, and the activation energy barrier of p-xylene formation is found to be approximately 20 kJ/mol lower than the corresponding values for the formation of its isomers.
Abstract: A theoretical study of the alkylation reaction of toluene with methanol catalyzed by the acidic Mordenite (Si/Al = 23) is reported. Cluster DFT as well as periodical structure DFT calculations have been performed. Full reaction energy diagrams of the elementary reaction steps that lead to the formation of the three xylene isomers are given. The use of periodical structure calculations allows one to account for zeolite framework electrostatic contributions and steric constraints that take place in zeolitic catalysts. Especially the steric constraint energy contribution has a significant effect on the energies and bond formation paths. The activation energy barrier of p-xylene formation is found to be ∼20 kJ/mol lower than the corresponding values for the formation of its isomers. Computed host−guest binding energies according to the DFT method need a correction due to the absence of the dispersive interaction with the zeolite wall. Apparent activation energies obtained with this correction are in good agre...
164 citations
TL;DR: A new program, Mercury, has been used to survey 144 monoalcohol (C(m)H(n)OH) and 101 dialcohol and results show that their hydrogen-bonding patterns are strongly correlated with steric effects.
Abstract: A new program, Mercury, has been used to survey 144 monoalcohol (CmHnOH) and 101 dialcohol [CmHn(OH)2] crystal structures. Results show that their hydrogen-bonding patterns are strongly correlated with steric effects. Primary monoalcohols have a strong preference to form infinite ⋯OH⋯OH⋯ chains. Secondary monoalcohols form chains and rings of hydrogen bonds with about equal facility. Tertiary monoalcohols very often form isolated OH⋯O hydrogen bonds or structures containing no OH⋯O hydrogen bonds at all. In the latter case, however, the structures almost invariably contain CH⋯O and/or OH⋯π interactions. Substitution on the β-carbon(s) of monoalcohols has a profound effect on packing patterns, with increased substitution disfavouring chains and rings. Dialcohols show a much stronger preference for chains of hydrogen bonds, compared with monoalcohols. This is particularly so when at least one of the hydroxyl groups is primary, in which case chains are overwhelmingly preferred. Once again, substitution on the β-C atoms is influential, heavy substitution tending to lead to packing arrangements that involve isolated or intramolecular OH⋯O hydrogen bonds. Dialcohols almost never crystallize without at least some OH⋯O hydrogen-bond formation. In both monoalcohols and dialcohols, chains show a stronger preference to be helical (usually threefold helices) as steric hindrance increases. Hydrogen-bonded rings usually contain four OH⋯O hydrogen bonds. It is possible that empirical observations such as these may aid crystal-structure prediction.
162 citations
TL;DR: This system possesses a high degree of chiroptical activity, which allows the differentiation of one of the smallest homologous elements of organic chemistry, that is, the methyl and ethyl groups attached to the asymmetric carbon, and additionally, which senses a remote chiral center at a position beta to the amine binding group.
Abstract: The achiral syn folded (face-to-face conformation) host molecule of the ethane-bridged bis(zinc porphyrin) transforms into the corresponding chiral extended anti bis-ligated species in the presence of enantiopure amine guests. The mechanism of the supramolecular chirogenesis is based upon the screw formation in bis(zinc porphyrin), arising from steric interactions between the largest substituent at the ligand's asymmetric carbon and peripheral alkyl groups of the neighboring porphyrin ring pointing toward the covalent bridge. The screw direction is determined by the guest's (amines) absolute configuration resulting in a positive chirality induced by (S)-enantiomers due to formation of the right-handed screw, and a negative chirality produced by the left-handed screw of (R)-enantiomers. The screw magnitude is strongly dependent upon the structure of the chiral guests. The amines with bulkier substituents result in stronger CD signals and larger 1H NMR resonance splittings of enantiotopic protons. This syst...
153 citations
TL;DR: Molecular dynamics simulations of the triangle and square structures revealed critical steric effects and restricted conformational flexibilities of the phosphane and diazadibenzoperylene ligands that help explain the distinct dynamic behavior observed in variable-temperature NMR studies.
Abstract: Tetraaryloxy-substituted diazadibenzoperylene bridging ligands 1a,b were employed in transition metal-directed self-assembly with Pd(II) and Pt(II) phosphane triflates 2a,b which resulted in complex dynamic equilibria between molecular triangles 3a−d and molecular squares 4a−d in solution. Characterization of the equilibria and assignment of the metallacycles was accomplished by 1H and 31P{1H} NMR spectroscopy in combination with electrospray ionization Fourier transform ion cyclotron mass spectrometry (ESI-FTICR-MS). It was found that the equilibria depend on several factors, such as the metal ion (Pd2+ or Pt2+), the solvent, and the steric demand of the phenoxy substituents of the diazadibenzoperylene ligands 1a,b. Introduction of bulky tert-butyl groups in 1b shifts the equilibrium significantly in the direction of the molecular squares. Molecular dynamics simulations of the triangle and square structures revealed critical steric effects and restricted conformational flexibilities of the phosphane and ...
151 citations
TL;DR: These assemblies show increased catalytic activity in the palladium-catalyzed Heck reaction and rhodium-Catalyzed hydroformylation and M=transition metal catalyst.
Abstract: Enforced ligand dissociation as a result of steric interactions between ZnII porphyrin units and the N atoms of pyridylphosphane ligands determines the catalytic properties of the encapsulated transition metal complexes. These assemblies show increased catalytic activity in the palladium-catalyzed Heck reaction and rhodium-catalyzed hydroformylation. M=transition metal catalyst.
151 citations
TL;DR: Experimental data on site-directed mutants show that the main determinant of ligand affinity changes is the polarity of the binding pocket and that H-bonding by the distal histidine accounts for about 85% of the O(2)/CO discrimination while steric hindrance accounts for the remaining 15%.
Abstract: Early reports of a severely bent CO adduct in myoglobin inspired the idea that heme proteins discriminate against CO, relative to O2, via steric hindrance imposed by a distal histidine residue. Recent results showing that the bound CO is only slightly distorted do not by themselves overthrow the steric hypothesis, because the steric energy could be stored in displacements of the protein. However, experimental data on site-directed mutants show that the main determinant of ligand affinity changes is the polarity of the binding pocket and that H-bonding by the distal histidine accounts for about 85% of the O2/CO discrimination while steric hindrance accounts for the remaining 15%.
133 citations
TL;DR: Variable-temperature NMR experiments proved that the observed increase in CD amplitude at lower temperatures derives from conformational changes in the preferred offset geometry between two porphyrin rings.
Abstract: A structural study of complexes formed between a dimeric zinc porphyrin tweezer (host) and chiral monoalcohols and monoamines derivatized by a bidentate carrier molecule (guest) confirmed that their CD couplets arise from the preferred porphyrin helicity of 1:1 host -guest complexes. NMR experiments and molecular modeling of selected tweezer complexes revealed that the preferred conformation is the one in which the L (larger) group protrudes from the porphyrin sandwich; this preferred helicity of the complex determines the CD of the complexes. It was found that the porphyrin ring-current induced 1 H chemical shifts and molecular modeling studies of the complex lead to the assignments of relative steric size of the L (large)/M (medium) substituents attached to the stereogenic center. The assignments, in turn, are correlated with the sign of the CD exciton couplet that establishes the absolute configuration at the stereogenic center. Variable- temperature NMR experiments proved that the observed increase in CD amplitude at lower temperatures derives from conformational changes in the preferred offset geometry between two porphyrin rings.
TL;DR: Structural investigations demonstrate that large distortions of the ligands attached to Pn occur when the pnictinyl radicals unite to form the corresponding dipnictine dimers, and allows a deeper understanding of the functioning of sterically demanding substituents.
Abstract: The molecular structures of the stable phosphinyl and arsinyl radicals, •PnR2 [Pn = P (2); As (4); R = CH(SiMe3)2], have been determined by gas-phase electron diffraction (GED) in conjunction with ab initio molecular orbital calculations. The X-ray crystal structures of the corresponding dipnictines, the “dimers”, R2PnPnR2 [Pn = P (1), As (3)], and the chloro derivatives R2PnCl [Pn = P (5), As (6)] have also been determined. Collectively, these structural investigations demonstrate that large distortions of the ligands attached to Pn occur when the pnictinyl radicals unite to form the corresponding dipnictine dimers. Principally, it is the shape and flexibility of the CH(SiMe3)2 ligands that permit the formation of the P−P and As−As bonds in 1 and 3, respectively. However, theoretical studies indicate that in the process of pnictinyl radical dimerization to form 1 and 3, both molecules accumulate substantial amounts of potential energy and are thus primed to spring apart upon release from the solid state ...
TL;DR: It was concluded that alpha-L-Fuc-(1-->3)-beta-D-GlcNAc and Lewis(x) bound to both lectins, whereas sialyl Lewis (x) only bound to AAA, and that only the L-fucose residues are in intimate contact with the protein.
Abstract: The interaction of sialyl Lewis(x), Lewis(x), and alpha-L-Fuc-(1-->3)-beta-D-GlcNAc with isolectin A from Lotus tetragonolobus (LTL-A), and with Aleuria aurantia agglutinin (AAA) was studied using NMR experiments and surface plasmon resonance. Both lectins are specific for fucose residues. From NMR experiments it was concluded that alpha-L-Fuc-(1-->3)-beta-D-GlcNAc and Lewis(x) bound to both lectins, whereas sialyl Lewis(x) only bound to AAA. Increased line broadening of 1H NMR signals of the carbohydrate ligands upon binding to AAA and LTL-A suggested that AAA bound to the ligands more tightly. Further comparison of line widths showed that for both lectins binding strengths decreased from alpha-L-Fuc-(1-->3)-beta-D-GlcNAc to Lewis(x) and were lowest for sialyl Lewis(x). Surface plasmon resonance measurements were then employed to yield accurate dissociation constants. TrNOESY, QUIET-trNOESY, and trROESY experiments delivered bioactive conformations of the carbohydrate ligands, and STD NMR experiments allowed a precise epitope mapping of the carbohydrates bound to the lectins. The bioactive conformation of Lewis(x) bound to LTL-A, or AAA revealed an unusual orientation of the fucose residue, with negative values for both dihedral angles, phi and psi, at the alpha(1-->3)-glycosidic linkage. A similar distortion of the fucose orientation was also observed for sialyl Lewis(x) bound to AAA. From STD NMR experiments it followed that only the L-fucose residues are in intimate contact with the protein. Presumably steric interactions are responsible for locking the sialic acid residue of sialyl Lewis(x) in one out of many orientations that are present in aqueous solution. The sialic acid residue of sialyl Lewis(x) bound to AAA adopts an orientation similar to that in the corresponding sialyl Lewis(x)/E-selectin complex.
TL;DR: The coordination chemistry of titanium(IV) complexes of amine bis(phenolate) ligands was investigated by synthesizing various types of complexes and analyzing them specroscopically and structurally.
Abstract: The coordination chemistry of titanium(IV) complexes of amine bis(phenolate) ligands was investigated by synthesizing various types of complexes and analyzing them specroscopically and structurally. Steric effects of tridentate [ONO]- and tetradentate [ONNO]-type ligands were studied by reacting the ligand precursors with titanium tetra(isopropoxide). [ONNO]-type ligands featuring an amine donor located on a pendant arm led to octahedral bis(isopropoxide) complexes, regardless of the steric bulk around the metal. Several such complexes having varying steric crowding were thus synthesized. On the other hand, steric effects were found to play a major role in determining the complex constitution when [ONO]-type ligands, featuring no side donor, were involved. Relatively sterically undemanding ligands led to octahedral bis(homoleptic) complexes, whereas increased steric bulk resulted in the formation of pentacoordinate bis(isopropoxide) complexes. These pentacoordinate complexes readily lead to bis(heteroleptic) complexes by reaction with nonsterically demanding [ONO]- and [ONNO]-type ligand precursors. In the latter case the sidearm nitrogen remains uncoordinated to the metal. The bis(isopropoxide) complexes of the [ONNO]-type ligands may also lead to bis(heteroleptic) complexes, however, these reactions are much slower.
TL;DR: In this article, the authors show that two axial ligands are required in order for the dz2 orbital (with the trichelating ligand defining the equatorial xy plane) to be destabilized and for the singlet to be the ground state.
TL;DR: In this article, the authors discuss the remarkable progress in the chemistry of heavy ketones leading to the systematic comparison of their structures and properties that are capable of existence as stable species if their highly reactive M=X bond is adequately protected toward dimerization.
Abstract: Publisher Summary This chapter discusses the remarkable progress in the chemistry of heavy ketones leading to the systematic comparison of their structures and properties that are capable of existence as stable species if their highly reactive M=X bond is adequately protected toward dimerization. Kinetic stabilization is superior to thermodynamic stabilization to elucidate the intrinsic nature of heavy ketones. A more general account of the whole chemistry of stable double bonds between heavier Group 14 and Group 16 elements is reviewed. The structural features of their M=X bonds (M = Si, Ge, Sn; X = chalcogen atom), such as trigonal planar geometry and bond shortening compared to the corresponding M–X single bonds are similar to those of ketones, heavy ketones have much higher reactivity because of their weak π bonds. The systematic study on heavy ketones containing silicon through lead using steric protection with the Tbt group reveals that a heavy ketone containing lead, R 2 Pb═X, is unique in that it is less stable than its isomeric lumbylene RPb–XR.
TL;DR: The coordination number of the metal in iron(II) beta-diketiminate complexes can be tuned through the size of the alkyl substituents on the ligand backbone.
TL;DR: On the basis of computational studies of ligands for the nicotinic acetylcholine receptors the structure-activity relationships are discussed and a possible binding mode suggested.
Abstract: Agonists of the alpha4beta2 nicotinic acetylcholine receptors have been synthesised as potential drugs for treatment of a variety of diseases. In this review, the published nicotinic agonists are presented and, on the basis of the molecular structure, the compounds are divided into three compound classes, nicotinoids (structurally close to nicotine), bicyclic compounds (structurally close to epibatidine and anatoxin-a), and analogues of imidacloprid (structurally close to the insecticide imidacloprid). The structure-activity relationships are discussed within and in between the classes. On the basis of computational studies of ligands for the nicotinic acetylcholine receptors the structure-activity relationships are discussed and a possible binding mode suggested. The binding mode encompasses: (A) an interaction between an anionic site in the receptor and the protonated nitrogen atom in the ligand, (B) a hydrogen bond between a hydrogen bond donor in the receptor and a hydrogen bond acceptor in the ligand, (C) an interaction between a pi-system (heteroaromatic ring, carbonyl bond) in the ligand and another pi-system or a positively charged amino acid residue in the binding site, (D) a pi-cation interaction between aromatic residues in the receptor binding site and the protonated nitrogen atom in the ligand, and (E) steric interactions of positive and negative character around the aliphatic and the heteroaromatic part of the ligand.
TL;DR: In this paper, the copper-II arenedisulfonate complexes with the general formula Cu(N4) have been synthesized in aqueous solution and structurally characterized by X-ray single crystal diffraction, IR and UV spectroscopy, TGA and DSC.
Abstract: Six copper(II) arenedisulfonate complexes with the general formula Cu(N4)(nds/bpds)·nH2O (nds = naphthalenedisulfonate, bpds = biphenyldisulfonate) have been synthesized in aqueous solution and structurally characterized by X-ray single crystal diffraction, IR and UV spectroscopies, TGA and DSC. In Cu(en)2(1,5nds)·2H2O 1, Cu(N-meen)2(2,6nds)·2H2O 2 (N-meen = N-methylethylenediamine), Cu(dpn)2(bpds) 3 (dpn = 2,3-diaminopropane), and Cu(cylam)(1,5nds) 4 the SO3− group coordinates monodentately to the axial position of Cu2+ which is equatorially coordinated by four amino nitrogens. In 1 and 4 intramolecular hydrogen bonds are formed between the SO3− oxygens and amino hydrogens which reinforce interaction between the SO3− group and Cu2+ complex cation. In Cu(dpn)2(1,5nds)·2H2O 5 and Cu(N,N′-meen)2(1,5nds)·3H2O 6 (N,N′-meen = N,N′-dimethylethylenediamine), due to the steric hindrance caused by the methyl substitution of the amino ligands and the rigidity of the naphthalene ring, water molecules, instead of the bulky SO3− group, coordinate to the axial position of Cu2+. Structures 1–4 represent the first synthesis and structural characterization of polymeric 1-dimensional structures constructed by arenedisulfonates and divalent transition metals. Also, 1 crystallizes in the enantiomorphic space group P41212 and displays interesting chiral grids constructed by ionic hydrogen bonds. The coordination geometries of Cu2+, as well as the coordination modes of arenedisulfonates, are discussed in detail.
TL;DR: The 1,3-dipolar cycloaddition of methyl azide to C60 and the subsequent nitrogen elimination from the formed triazoline intermediate to yield the aziridine adduct have been studied using semiempirical and density functional methods.
Abstract: The 1,3-dipolar cycloaddition of methyl azide to C60 and the subsequent nitrogen elimination from the formed triazoline intermediate to yield the aziridine adduct have been studied using semiempirical and density functional methods. The results obtained show that the addition of methyl azide to C60 takes place in the ring junction between two six-membered rings leading to a closed [6,6]-triazoline intermediate with an energy barrier of about 20 kcal mol-1 and an exothermicity of ca. 2 kcal mol-1 at the B3LYP/6-31G**//AM1 level of theory. The subsequent thermal loss of N2 takes place through a stepwise mechanism in which the cleavage of the N−N single bond precedes the breaking of the N−C bond, with a total activation energy of approximately 45 kcal mol-1. The N2 loss occurs simultaneously with the formation of the new N−C bond. During the process, the steric effects of the leaving N2 molecule prevent the addition of the nitrene substituent to the [6,6]-ring junction attacked initially and force the additi...
TL;DR: The reactions of group 6 (alkoxy)(aryl)- and group 6 carbene complexes with organolithium compounds, metal enolates, and enamines are summarized in this article.
TL;DR: PAr2 (Ar = ortho-substituted phenyl group) is a catalysts for copolymerisation of CO and C2H4 as discussed by the authors, and is a very efficient catalysts.
TL;DR: In this paper, a spectroscopic scale of hydrogen-bond basicity is constructed from the IR frequency shift Δν(OH) of methanol hydrogenbonded to secondary amines.
Abstract: Using 4-fluorophenol as a reference hydrogen-bond donor, equilibrium constants K for the formation of 1 ∶ 1 hydrogen-bonded complexes have been obtained by FTIR spectrometry for 33 secondary amines in CCl4 and/or C2Cl4 at 298 K. A spectroscopic scale of hydrogen-bond basicity is constructed from the IR frequency shift Δν(OH) of methanol hydrogen-bonded to secondary amines. The comparison of the pKHB (log K), Δν(OH), and pKa scales points to the sensitivity of pKHB to steric effects, and of Δν(OH) to the p character of the nitrogen lone pair. The pKHB scale of secondary amines extends from 2.59 for pyrrolidine to −0.45 for (Me3Si)2NH. The main effects explaining the pKHB variations are (i) the
opposite polarizability and steric effects in alkylamines, (ii) field-inductive effects (e.g.NCCH2NHMe), (iii) intramolecular hydrogen bonding, e.g. in (MeOCH2CH2)2NH, and (iv) the ring size giving the order: pyrrolidine = azetidine > piperidine > 2-methylaziridine > azepane. IR spectra show the attachment of 4-fluorophenol to the nitrile nitrogen of NCCH2NHMe and NCCH2CH2NHMe, to the oxygen of morpholine and (MeOCH2CH2)2NH, and to the sulfur of thiomorpholine and thiazolidine, in addition to attachment to the amino nitrogen. The correlation of pKHB with the minimum electrostatic potential on the nitrogen lone pair is used for unravelling the basicity of each nitrogen of 1-methyl-1,4-diazepane.
15 Dec 2001
TL;DR: In this article, the steric effect of hydrogen bonding between the two groups is attributed to the presence of Nacyl-N-methylamide and carboxylate groups in the surfactant molecule.
Abstract: The simultaneous presence of N-acyl-N-methylamide and carboxylate groups in the surfactant molecule produces some unique interfacial properties in aqueous media. These are ascribed to the steric effect of hydrogen bonding between the two groups. Properties of sodium N-dodecanoyl-N-methyl-β-alanate (SDMA) and N-dodecanoylsarcosinate (SDSa) were compared with those of sodium N-dodecanoyl-N-methyltaurinate (SDMT) and sodium decyloxyacetate (SDOA) in water in 0.1 M NaCl at 30°C. Evidence for the steric effect on micellization includes the order of increasing Amin (the minimum area occupied by the surfactant molecule at the air/water interface) and values of the critical micellar concentration (cmc)/C20 ratio (a measure of the factors inhibiting micellization relative to adsorption at the air/water interface): SDOA
TL;DR: Three series of P-chiral diphosphines based on ferrocene and biferrocenyl skeletons, including novel ligands 1f and 3c, were employed in palladium-catalyzed allylic substitution reactions, defining the regioselectivity of nucleophile addition.
Abstract: Three series of P-chiral diphosphines based on ferrocene (1a−f, 2a−c) and biferrocenyl skeletons (3a−c), including novel ligands 1f and 3c, were employed in palladium-catalyzed allylic substitution reactions. Steric effects imposed by the phosphine residues were studied using C2-symmetrical donors 1 (1 = 1,1‘-bis(arylphenylphosphino)ferrocene with aryl groups a = 1-naphthyl, b = 2-naphthyl, c = 2-anisyl, d = 2-biphenylyl, e = 9-phenanthryl, and f = ferrocenyl), whereas para-methoxy- and/or para-trifluoromethyl substitution of the phenyl moieties in 1a enabled investigation of ligand electronic effects applying ferrocenyl diphosphines 2a−c. Ligands 3 (3 = 2,2‘-bis(arylphenylphosphino)-1,1‘-biferrocenyls with aryl substituents a,c = 1-naphthyl (diastereomers) and b = 2-biphenylyl) allowed for comparison of backbone structure effects (bite angle variation) in catalysis. Linear and cyclic allylic acetates served as substrates in typical test reactions; upon attack of soft carbon and nitrogen nucleophiles on (...
TL;DR: In this paper, the unique electronic and steric features of the ligand allow the formation of a very stable multicyclic aggregate by reaction of the new halogeno β-diketiminato complex LScCl2 with NaN(SiMe3)2.
TL;DR: Higher level molecular theory is needed to provide a more quantitative description of these kinetics, as the 1,6- and 1,9-DCDD formed from 2, 6-dichlorophenol via Smiles rearrangement is favored at low temperatures more than thermodynamically predicted.
TL;DR: It is proposed that five parameters influence diminished reactivity toward oxygen, i.e., benzylic resonance stabilization, unpaired spin delocalization on oxygen, favorable stereoelectronic effects, electron-withdrawing effects, and steric effects.
TL;DR: A QM and QM/MM study of steric and electronic effects in the main steps of Rh-catalyzed carbonylation reactions indicates that electronic effects are not particularly relevant in determining the energetic of oxidative addition.
Abstract: We present a QM and QM/MM study of steric and electronic effects in the main steps of Rh-catalyzed carbonylation reactions All the considered systems adopt a square-planar geometry prior to CH3I oxidative addition As regards the octahedral complexes after CH3I oxidative addition, a comparison between the various models indicates that the energy gain due to the CH3I oxidative addition is reduced by the steric pressure of the substituents on the ligand The substantially similar results obtained with the QM/MM and QM models indicate that electronic effects are not particularly relevant in determining the energetic of oxidative addition As regards the P,P-Ph octahedral complex, the geometries in which the CO group is trans to the added CH3 group, or trans to one of the P atoms, are of similar energy A comparison between the various models indicates that the energy barrier of the CO insertion reaction is lowered by the presence of substituents on the chelating ligands This effect is related to a relief o
TL;DR: This study has demonstrated that the readily available and inexpensive 3-(trans-3'-alkyl/arylpropenoyl)oxazolidin-2-ones, featuring high electrophilicity and conformational homogeneity, are synthetically superior Michael acceptors over the conventionally used alkyl enoylates, allowing for a remarkable improvement in reactivity and diastereoselectivity.
Abstract: This study has demonstrated that the readily available and inexpensive 3-(trans-3‘-alkyl/arylpropenoyl)oxazolidin-2-ones, featuring high electrophilicity and conformational homogeneity, are synthetically superior Michael acceptors over the conventionally used alkyl enoylates, allowing for a remarkable improvement in reactivity and, in most cases, diastereoselectivity of the addition reactions with a Ni(II) complex of the chiral Schiff base of glycine with (S)-o-[N-(N-benzylprolyl)amino]benzophenone. Kinetically controlled diastereoselectivity in the corresponding Michael addition reactions between the Ni(II) complex of glycine and the oxazolidin-2-ones was systematically studied as a function of steric, electronic, and position effects of the substituents on the starting Michael acceptor. In both aliphatic and aromatic series the simple diastereoselectivity was found to be virtually complete, affording the products via the corresponding TSs with the approach geometry like. The face diastereoselectivity of...