Showing papers on "Steric effects published in 2003"

Hydrogen-hydrogen bonding: a stabilizing interaction in molecules and crystals.

Abstract: Bond paths linking two bonded hydrogen atoms that bear identical or similar charges are found between the ortho-hydrogen atoms in planar biphenyl, between the hydrogen atoms bonded to the C1–C4 carbon atoms in phenanthrene and other angular polybenzenoids, and between the methyl hydrogen atoms in the cyclobutadiene, tetrahedrane and indacene molecules corseted with tertiary-tetra-butyl groups. It is shown that each such H–H interaction, rather than denoting the presence of “nonbonded steric repulsions”, makes a stabilizing contribution of up to 10 kcal mol−1 to the energy of the molecule in which it occurs. The quantum theory of atoms in molecules—the physics of an open system—demonstrates that while the approach of two bonded hydrogen atoms to a separation less than the sum of their van der Waals radii does result in an increase in the repulsive contributions to their energies, these changes are dominated by an increase in the magnitude of the attractive interaction of the protons with the electron density distribution, and the net result is a stabilizing change in the energy. The surface virial that determines the contribution to the total energy decrease resulting from the formation of the H–H interatomic surface is shown to account for the resulting stability. It is pointed out that H–H interactions must be ubiquitous, their stabilization energies contributing to the sublimation energies of hydrocarbon molecular crystals, as well as solid hydrogen. H–H bonding is shown to be distinct from “dihydrogen bonding”, a form of hydrogen bonding with a hydridic hydrogen in the role of the base atom.

Bite angle effects in diphosphine metal catalysts: steric or electronic?

Abstract: The effects of wide bite angles of bidentate phosphine ligands on three catalytic reactions are reviewed: rhodium catalysed hydroformylation, nickel catalysed hydrocyanation, and palladium catalysed reactions of ethene, carbon monoxide and methanol leading to polyketone or methyl propanoate. The P–M–P bite angle plays a crucial role in determining the selectivity and rate in all three reactions. In this review an attempt is made to separate the mode of action into a steric and an electronic one. The regioselectivity of hydroformylation seems to be governed by steric factors, while the rate of reaction is determined by the electronic influence of the bite angle. The rates in hydrocyanation and polyketone formation were previously thought to be determined by orbital effects, but that should be questioned. Selectivity in the palladium carbonylation reaction is mainly due to steric factors.

A homo [3+2] dipolar cycloaddition: the reaction of nitrones with cyclopropanes.

Abstract: Cycloadditions are among the most trusted of chemical transformations available to the synthetic chemist. The predictability of the transition state based on steric and electronic considerations allows for the strategic construction of complex molecular assemblies. While the Diels–Alder reaction has received the most attention, 1,3-dipolar cycloadditions have also maintained a place of prominence in the toolbox of the synthetic chemist. During recent studies of the synthetic reactions of cyclopropanediesters, we were struck by the fact that simple 1,1cyclopropane diesters behaved very much like a,b-unsaturated carbonyl compounds in their ability to react with nucleophiles in what could be considered a homo-Michael addition (an observation not unnoticed by others). The strained bonds in the cyclopropane ring have long been considered to have a significant p character and in 1,1cyclopropane diesters this bond can be polarized and further weakened by coordination of a Lewis acid to one or both of the ester moieties. This reactivity suggests that such compounds are one-carbon homologues of electron-deficient olefins, and led us to investigate their reactions with nitrones in what is naively a dipolar homo [3+ 2] cycloaddition (Scheme 1) resulting in the formation of tetrahydro-1,2oxazines. In this communication we disclose the first reaction of a nitrone with a cyclopropane to produce a

Enantioselective oxidative biaryl coupling reactions catalyzed by 1,5-diazadecalin metal complexes: efficient formation of chiral functionalized BINOL derivatives.

Abstract: Chiral 1,5-diaza-cis-decalins have been examined as ligands in the enantioselective oxidative biaryl coupling of substituted 2-naphthol derivatives Under the optimal conditions employing 25-10 mol % of a 1,5-diaza-cis-decalin copper(II) catalyst with oxygen as the oxidant, enantioselective couplings (44-96% ee) could be achieved for a range of 3-substituted 2-naphthols including the ester, ketone, phosphonyl, and sulfonyl derivatives The relationship between the substitution of the naphthalene starting materials and reactivity/selectivity is determined by several factors which act in concert: (1) the effect of substituents on the oxidation potential of the substrate, (2) the ability of the substrate to participate in a chelated copper complex which depends on (a) the inherent coordinating ability of the 3-substituent and (b) substituent steric interactions that affect chelation between the 2-hydroxyl and 3-substituent, (3) the effect of substituents on dissociation of the product from the copper catalyst

Stereochemistry of Nucleophilic Substitution Reactions Depending upon Substituent: Evidence for Electrostatic Stabilization of Pseudoaxial Conformers of Oxocarbenium Ions by Heteroatom Substituents

Abstract: Lewis acid-mediated nucleophilic substitution reactions of substituted tetrahydropyran acetates reveal that the conformational preferences of six-membered-ring cations depend significantly upon the electronic nature of the substituent. Nucleophilic substitutions of C-3 and C-4 alkyl-substituted tetrahydropyran acetates proceeded via pseudoequatorially substituted oxocarbenium ions, as would be expected by consideration of steric effects. Substitutions of C-3 and C-4 alkoxy-substituted tetrahydropyran acetates, however, proceeded via pseudoaxially oriented oxocarbenium ions. The unusual selectivities controlled by the alkoxy groups were demonstrated for a range of other heteroatom substituents, including nitrogen, fluorine, chlorine, and bromine. It is believed that the pseudoaxial conformation is preferred in the ground state of the cation because of an electrostatic attraction between the cationic carbon center of the oxocarbenium ion and the heteroatom substituent. This analysis is supported by the observation that selectivity diminishes down the halogen series, which is inconsistent with electron donation as might be expected during anchimeric assistance. The C-2 heteroatom-substituted systems gave moderately high 1,2-cis selectivity, while small alkyl substituents showed no selectivity. Only in the case of the tert-butyl group at C-2 was high 1,2-trans selectivity observed. These studies reinforce the idea that ground-state conformational effects need to be considered along with steric approach considerations.

An electronic effect on protein structure.

Abstract: The well-known preference of the peptide bond for the trans conformation has been attributed to steric effects. Here, we show that a proline residue with an N-formyl group (Hi−1−C′i−1=Oi−1), in which Hi−1 presents less steric hindrance than does Oi−1, likewise prefers a trans conformation. Thus, the preference of the peptide bond for the trans conformation cannot be explained by steric effects alone. Rather, an n → π* interaction between the oxygen of the peptide bond (Oi−1), and the subsequent carbonyl carbon in the polypeptide chain (C′i) also contributes to this preference. The Oi−1 and C′i distance and Oi−1···C′i=Oi angle are especially favorable for such an n → π* interaction in a polyproline II helix. We propose that this electronic effect provides substantial stabilization to this and other elements of protein structure.

Chiral Ru-Based Complexes for Asymmetric Olefin Metathesis: Enhancement of Catalyst Activity through Steric and Electronic Modifications

Abstract: Design, synthesis, characterization, and catalytic activity of six enantiomerically pure Ru-based metathesis catalysts are disclosed (3a-3f). The new chiral catalysts were prepared through steric and electronic alterations of the parent catalyst system (3). The present studies indicate that the effect of structural modifications of chiral complex 3 does not always correspond to those of the related achiral complexes. The present findings illustrate that modified Ru complexes (3e and 3f) deliver reactivity levels that are more than 2 orders of magnitude higher than 3. Reactivity and physical data are provided that shed light on the origin of activity differences. Some members of the new generation of chiral Ru catalysts promote asymmetric ring-opening (AROM) and ring-closing (ARCM) metatheses that cannot be effected by the first generation chiral catalyst (3).

The case for steric repulsion causing the staggered conformation of ethane.

Abstract: The barrier to rotation in ethane (see picture) is considered a prototypical case of steric hindrance. This view has been recently contradicted, and hyperconjugation has been put forward as the right explanation. The authors argue that there is no reason to abandon the steric repulsion explanation, and discuss why different electronic structure analyses yield different views.

A model for predicting likely sites of CYP3A4-mediated metabolism on drug-like molecules.

Abstract: We have developed a rapid semiquantitative model for evaluating the relative susceptibilities of different sites on drug molecules to metabolism by cytochrome P450 3A4. The model is based on the energy necessary to remove a hydrogen radical from each site, plus the surface area exposure of the hydrogen atom. The energy of hydrogen radical abstraction is conventionally measured by AM1 semiempirical molecular orbital calculations. AM1 calculations show the following order of radical stabilities for the hydrogen atom abstractions: sp2 centers > heteroatom sp3 centers > carbon sp3 centers. Since AM1 calculations are too time intensive for routine work, we developed a statistical trend vector model, which is used to estimate the AM1 abstraction energy of a hydrogen atom from its local atomic environment. We carried out AM1 and trend vector calculations on 50 CYP3A4 substrates whose major sites of metabolism are known in the literature. A plot of the lowest hydrogen radical formation energy versus its sterically accessible surface area exposure for these 50 substrates shows that only those hydrogen atoms with solvent accessible surface area exposure g 8.0 A 2 are susceptible to CYP3A4-mediated metabolism. This approach forms the basis for our general model, which predicts sites on drugs that are susceptible to cytochrome P450 3A4-mediated hydrogen radical

Halogen-bond geometry: a crystallographic database investigation of dihalogen complexes

Abstract: X-ray crystal structures of 141 halogen-bonded complexes Y—X⋯B formed between homo- and heteronuclear dihalogens Cl2, Br2, I2, IBr and ICl with O, S, Se, N, P and As Lewis bases show remarkable and constant geometrical features. The metrics of the halogen bond found in the gas phase for simple complexes [Legon (1999a). Angew Chem. Int. Ed. Eng. 38, 2686–2714] is supported (i) in the solid state, (ii) for new Lewis acids (I2 and IBr), (iii) for new basic centers (Se, As and =N—) and (iv) for more complicated bases. The Y—X⋯B arrangement is more linear than the corresponding Y—H⋯B hydrogen bond and the axis of the Y—X molecule lies in the plane of the B lone pair(s), with a preference for the putative lone-pair direction within that plane. However, exceptions to this lone-pair rule are found for sterically hindered thiocarbonyl and selenocarbonyl bases. A bond-order model of the halogen bond correctly predicts the observed correlation between the shortening of the X⋯B distance and the lengthening, Δd(Y—X), of the Y—X bond. The expectation that the solid-state geometric parameters d(X⋯B) and Δd(Y—X) reflect the strength of the interaction is supported by their significant relationships with the solution thermodynamic parameters of Lewis acidity and basicity strength, such as the Gibbs energy of 1:1 complexation of Lewis bases with diiodine. This analysis of halogen-bonded complexes in the solid state reinforces the similarities already known to exist between hydrogen and halogen bonding.

Novel crystal structure, cis-trans isomerization, and host property of meta-substituted macrocyclic azobenzenes with the shortest linkers.

Abstract: Azobenzenophanes, in which two (2), three (3), and four (4) azobenzene units are connected cyclically by methylene linkers at the meta positions, were synthesized. The effect of ring strain on the structure and the photochemical and the thermal isomerization of azobenzene were investigated. Complex formation with guest species such as alkali metal cations and solvent molecules was observed. X-ray crystal analyses revealed the crystal structure of all three isomers of 2. Compound 2(t,c) was distorted due to the ring strain and 2(c,c) was not deformed. Both 3 and 4 formed supramolecular channel structures in which an azobenzenophane molecule holds a solvent molecule in the molecular cavity. Upon exposure to light, 2, 3, and 4 exhibited stepwise trans-cis photoisomerization in solutions. The quantum yields for all isomerization steps of 2 were determined, and these values suggested that interactions between chromophores such as energy transfer and/or steric distortion affect the isomerization process. The lifetime of 2(c,c) (19.7 days) was longer than that of 2(t,c) (6.1 days) at room temperature. The relative stability of these isomers was explained by the isokinetic (or compensation) relationship between deltaH++ and deltaS++ values and by the effect of the ring strain. The relative energies of three isomers of 2 were estimated by HF/6-31G** calculations, and these values indicated that 2(t,c) has the largest ring strain. Complexes (1:1 and 2:1) between macrocycles and alkali metal cations (Li+, Na+, K+, Rb+, Cs+) were observed by ESIMS. The cation selectivity was shifted upon photoirradiation especially in 4.

Dynamic Covalent Approach to [2]- and [3]Rotaxanes by Utilizing a Reversible Thiol–Disulfide Interchange Reaction

Abstract: A dynamic covalent approach to disulfide-containing [2]- and [3]rotaxanes is described. Symmetrical dumbbell-shaped compounds with two secondary ammonium centers and a central located disulfide bond were synthesized as components of rotaxanes. The rotaxanes were synthesized from the dumbbell-shaped compounds and dibenzo-[24]crown-8 (DB24C8) with catalysis by benzenethiol. The yields of isolated rotaxanes reached about 90 % under optimized conditions. A kinetic study on the reaction forming [2]rotaxane 2 a and [3]rotaxane 3 a suggested a plausible reaction mechanism comprising several steps, including 1) initiation, 2) [2]rotaxane formation, and 3) [3]rotaxane formation. The whole reaction was found to be reversible in the presence of thiols, and thermodynamic control over product distribution was thus possible by varying the temperature, solvent, initial ratio of substrates, and concentration. The steric bulk of the end-capping groups had almost no influence on rotaxane yields, but the structure of the thiol was crucial for reaction rates. Amines and phosphines were also effective as catalysts. The structural characterization of the rotaxanes included an Xray crystallographic study on [3]rotaxane 3 a.

Catalytic Asymmetric Michael Reaction of β-Keto Esters: Effects of the Linker Heteroatom in Linked-BINOL

Abstract: We describe the development of a general catalytic asymmetric Michael reaction of acyclic β-keto esters to cyclic enones, in which asymmetric induction occurs at the β-position of the acceptors Among the various asymmetric catalyst systems examined, the newly developed La−NR-linked-BINOL complexes (R = H or Me) afforded the best results in terms of reactivity and selectivity In general, the NMe ligand 2 was suitable for the combination of small enones and small β-keto esters, and the NH ligand 1 was suitable for bulkier substrates (steric tuning of the catalyst) Using the La−NMe-linked-BINOL complex, the Michael reaction of methyl acetoacetate (8a) to 2-cyclohexen-1-one (7b) gave the corresponding Michael adduct 9ba in 82% yield and 92% ee The linker heteroatom in linked-BINOL is crucial for achieving high reactivity and selectivity in the Michael reaction of β-keto esters The amine moiety in the NR-linked-BINOL can also tune the Lewis acidity of the central metal (electronic tuning of the catalyst),

Complexity with simplicity: a steric continuum of chelating diamines with copper(I) and dioxygen

Abstract: Copper(I) complexed by simple peralkylated diamines reacts with O2 at low temperature in aprotic solvents to create a diverse set of thermally sensitive, polynuclear copper products. Structural and spectroscopic characterization of several complexes provides keen insights into the complexity of this ostensibly simple, yet important, biological and industrial reaction. A correlation of a series of closely related copper(I) complexes with their oxygenated products highlights the preeminent, though not absolute, role of the ligand in these multi-step formation reactions. A steric continuum of diamine ligands provides a sensitive probe into this copper(I)/O2 reactivity. The diverse product distribution is interpreted through a single unifying mechanism.

Elucidation of the Solution Structures of Transition Metal Complex Ion Pairs by NOE NMR Experiments

Abstract: Our results from structural investigations of transition metal complex ion pairs in solution by homonuclear and heteronuclear 1D- and 2D-NOE NMR spectroscopy are reviewed. Both model (A) and catalytically active (B) complexes have been taken into account; their general formulas are: trans-[ML2(N,X)(Y)(Z)]+A− (A) {M = FeII, RuII and OsII, L = PMe3, Y = CO, Z = COMe, A− = BPh3R−, CF3SO3−, BF4−, PF6− and B[3,5-(CF3)2C6H3]4−; M = IrIII, L = PR3, Y = Z = H and A− = BPh4−, CF3SO3−, BF4−, PF6−} and [M′(N,X)(R,olefin)]+A− (B) {M′ = Pt, R = Me, olefin = ethene, propene and CH2=CHCOOMe, A− = BF4−; M′ = Pd, (R,olefin) = η1,η2-5-methoxycyclooctenyl, A− = BPh4−, CF3SO3−, BF4−, PF6−, SbF6− and B[3,5-(CF3)2C6H3]4−}, where N,X = bis(pyrazolyl)methane, bipyridine, α-diimines, α-diamines or α-oxo imines. The detection of dipolar interionic interactions in 1H-NOESY and 19F,1H-HOESY NMR spectra allowed relative anion-cation orientations to be determined, these being well defined in most cases, with one orientation predominating in solution. We often found that the anion approaches the complexes from the side of the N,X-ligands, due to an accumulation of positive charge at the junction of the moieties containing the N- and X-donor atoms. In square-planar complexes B, the apparently favoured apical positions were often protected either by electronic factors [N,X = bis(2-pyridyl)amine or bis(pyrazolyl)methane] or by a combination of electronic and steric factors (N,X = α-diimines). For M′ complexes, bearing Ar−N=C(Me)−C(Me)=N−Ar ligands, the position of the counter-anion is finely modulated by the steric hindrance of the ortho-aryl substituents. The accessibility of the metal centre, which is directly associated with the catalytic performances of the complexes, was determined by using the counteranion as a probe. Finally, average interionic distances were estimated for model complexes A, bearing unsymmetrical BPh3R− counter-anions, based on the quantification of interionic NOEs. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2003)

Influence of the nitroxide structure on the homolysis rate constant of alkoxyamines: a Taft-Ingold analysis.

Abstract: Alkoxyamines and persistent nitroxyl radicals are important regulators of living radical polymerizations. Because polymerization times decrease with the increasing rate of the homolytic C−O bond cleavage between the polymer chain and the nitroxide moiety, the factors influencing the homolysis rate are of considerable interest. Here, we present an analysis of the cleavage rate constants for 28 alkoxyamines carrying the styryl (PhEt) group as leaving alkyl radical in terms of polar inductive/field (σL) and steric (Es) effects of the nitroxide substituents, using the Taft−Ingold equation, i.e., log(k/k0) = ρLσL + δEs. The rate constants are shown to increase with the increasing electron-donating capacities, the steric demand, and the intramolecular (hydrogen) bonding capabilities of the substituents. A good correlation, (R2 = 0.95, 23 data) log kd = −3.07σL − 0.88Es − 5.88, is obtained, which should facilitate the design of new nitroxyl radicals and alkoxyamine regulators.

Electronic and Steric Effects on the Oxygenation of Organic Sulfides and Sulfoxides with Oxo(salen)chromium(V) Complexes

Abstract: The kinetics of oxygenation of several para-substituted phenyl methyl sulfides and sulfoxides with a series of 5-substituted and sterically hindered oxo(salen)chromium(V) complexes have been studied by a spectrophotometric technique. Though the reaction of sulfides follows simple second-order kinetics, sulfoxides bind strongly with the metal center of the oxidant and the oxygen atom is transferred from the oxidant−sulfoxide adduct to the substrate. The reduction potentials, Ered, of eight Cr(V) complexes correlate well with the Hammett σ constants, and the reactivity of the metal complexes is in accordance with the Ered values. The metal complexes carrying bulky tert-butyl groups entail steric effects. Organic sulfides follow a simple electrophilic oxidation mechanism, and the nonligated sulfoxides undergo electrophilic oxidation to sulfones using the oxidant−sulfoxide adduct as the oxidant. Sulfoxides catalyze the Cr(V)−salen complexes' oxygenation of organic sulfides, and the catalytic activity of sulfo...

New Phenyl-Substituted PPV Derivatives for Polymer Light-emitting Diodes−Synthesis, Characterization and Structure−Property Relationship Study

Abstract: Three new PPV derivatives with dialkoxyphenyl substituents, BEH2P−PPV, BEH3P−PPV, and BEH4P−PPV have been synthesized. The polymers were characterized by FT-IR, 1H NMR, and elemental analysis. The polymers possess excellent solubility, high molecular weights, high photoluminescence efficiencies and good thermal stability. The influence of substitution pattern on the formation of structural defects has been investigated by measuring the signal due to tolane−bisbenzyl moieties (TBB) in the proton NMR spectra. BEH2P−PPV with a steric phenyl group at the ortho-position on the side phenyl ring shows the lowest amount of TBB, which indicates suitable steric hindrance can be applied to suppress the formation of irregular head-to-head and tail-to-tail linkage in the polymer chains. In addition, the polarity of solvents used for the Gilch polymerization will also affect the amount of irregular structure in the polymers. Polar solvents such as THF will result in polymers with low TBB content. Energy level measureme...

Electronegativity, Resonance, and Steric Effects and the Structure of Monosubstituted Benzene Rings: An ab Initio MO Study

Abstract: The deformation of the carbon skeleton of the benzene ring under substituent impact has been analyzed from the structures of 74 monosubstituted derivatives, as determined by ab initio MO calculations. The geometry of the substituted ring is shown to contain valuable information on the electronegativity, resonance, and steric effects of the substituent, and also on other, more subtle effects, affecting primarily the length of the Cipso−Cortho bonds. The results obtained substantially augment previous knowledge from the analysis of experimental geometries (Domenicano, A.; Murray-Rust, P.; Vaciago, A. Acta Crystallogr., Sect. B 1983, 39, 457). Varying the electronegativity of the substituent causes a concerted change of the ring angles at the ipso, ortho, and para positions, coupled with a change in the Cipso−Cortho bond length. The values of the ipso angle span a remarkably wide range, 113−126°. Enhancing the resonance interaction between a substituent and the ring causes a complex pattern of angular distor...

1H chemical shifts in NMR: Part 19. Carbonyl anisotropies and steric effects in aromatic aldehydes and ketones†

Abstract: The 1H chemical shifts of benzaldehyde, 2-chloro-, 2-hydroxy- and 2-methoxybenzaldehyde, acetophenone, 2-methoxy- and 2-hydroxyacetophenone, indanone, anthraquinone, fluorenone, anthrone, -tetralone, 2,4,6-trimethylacetophenone, 9-acetylanthracene, 9-anthranaldehyde and benzosuberone were obtained and completely assigned in CDCl3 and DMSO solution. In anthrone a keto-enol tautomerism (anthrone-9-hydroxyanthracene) was observed by NMR in hydrogen bonding solvents but not chloroform. The percentage of enol is linearly dependent on the Kamlett hydrogen bonding parameter of the solvent, and not the solvent relative permittivity. The chemical shift data allowed the determination of the carbonyl substituent chemical shifts (SCS) in these molecules. These were analysed in terms of the carbonyl electric field, magnetic anisotropy and steric effects for long-range protons together with a model (CHARGE7) for the calculation of the two- and three-bond effects. The SCS of the carbonyl bond was reproduced with an asymmetric magnetic anisotropy acting at the midpoint of the carbonyl bond with values of par and perp of 6.36 and -11.88 (10-30 cm3 molecule-1) plus a steric term from the oxygen atom and the CO electric field effect. The short-range effects of the carbonyl group on the aldehyde proton were modelled using the appropriate functions in the CHARGE routine. For the 9-substituted anthracenes the Huckel calculation was modified to account for the 1H chemical shifts of the H-10 protons. This model gave a comprehensive calculation of the 1H chemical shifts of these aromatic aldehydes and ketones. For the data set of 129 chemical shifts ranging from 2.5 to 11.5 the r.m.s. error of the observed vs calculated shifts was 0.094 ppm. The CO anisotropy and oxygen shielding differ appreciably from the corresponding values for the aliphatic aldehydes and ketones but are similar to the values for the CO group of amides, illustrating the effect of conjugation on these parameters. The model was used in the conformational analysis of some related compounds. In 2-chlorobenzaldehyde the chemical shift calculations support a non-planar molecule with the aldehyde-ring dihedral angle in the trans conformer of ca 25°. In the strained seven-membered ring of benzosuberone, the model was used to test calculated geometries. The ab initio geometry at the B3LYP(6-31++G(d,p)) level gave the best agreement with the observed shifts. Copyright © 2002 John Wiley & Sons, Ltd.

Design of radical-resistant amino acid residues: a combined theoretical and experimental investigation.

Abstract: Ab initio calculations have been used to design radical-resistant amino acid residues. Optimized structures of free and protected amino acids and their corresponding α-carbon-centered radicals were determined with B3-LYP/6-31G(d). Single-point RMP2/6-31G(d) calculations on these structures were then used to obtain radical stabilization energies, to examine the effect of steric repulsion between the side chains and amide carbonyl groups on the stability of α-carbon-centered peptide radicals. Relative to glycine, the destabilization for alanine and valine residues was found to be approximately 9 and 18 kJ mol-1, respectively, which correlates with the reactivity of analogous amino acid residues in peptides toward hydrogen atom abstraction in conventional free radical reactions. To design amino acid residues that would resist radical reactions, strategies by which the steric effects could be magnified were considered. This resulted in the identification of tert-leucine and 3,3,3-trifluoroalanine as suitable ...

Diastereoselective photochromism of a bisbenzothienylethene governed by steric as well as electronic interactions.

Abstract: Introduction of an asymmetric center to C-2 of one of the benzothiophene rings of bisbenzothienylperfluorocyclopentene results in a highly diastereoselective photochromic system. The stereogenic center bears a hydrogen atom, a methyl group, and a methoxymethoxy group. The steric as well as the electronic repulsions gave an 87−88% diastereomer excess in various solvents at room temperature with 80−85% conversion to the colored form. The enantioselective synthesis was also carried out. Upon photoirradiation in hexane, a change in optical rotation at 820 nm, where neither the open form nor the colored form absorbs light, was observed repeatedly.

Steric Effects on the Adsorption of Alkylthiolate Self-Assembled Monolayers on Au (111) †

Abstract: Steric effects on the adsorption of self-assembled monolayers (SAMs) formed by alkylthiolates on the Au(111) surface were investigated using density functional theory. Based on the (√3 × √3) R30° structure, the current results on methylthiolate (CH3S) show that the adsorption prefers the face-centered cubic-bridge and hexagonal close-packed-bridge sites. Furthermore, the adsorption energy decreases slightly compared to the CH3S adsorption on the p(2 × 2) structure due to lateral interactions. Comparison between the results on CH3S and 1-propylthiolate (C3H7S) illustrates that the adsorption energy increases with chain length. Strong steric effects were found due to the chain length of the alkylthiolates and the hydrogen atoms in the CH2 unit adjacent to the S atom (α hydrogens). The energetically favored tilt angle is 20° for C3H7S. The preferred geometry for both CH3S and C3H7S adsorptions has the two α hydrogens pointing toward the bridge Au atoms of the surface. The results suggest a flat potential ene...