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Showing papers on "Steric effects published in 2011"


Journal ArticleDOI
15 Sep 2011-Nature
TL;DR: The preparation of hydrocarbons with extremely long C–C bonds (up to 1.704 Å) is reported, the longest such bonds observed so far in alkanes, and the prepared compounds are unexpectedly stable—noticeable decomposition occurs only above 200 °C.
Abstract: The synthesis of alkane hydrocarbons containing extremely long carbon–carbon (C–C) bonds, the longest observed in alkanes to date, is reported. Long C–C bonds are usually weaker than short ones, but these new alkanes are surprisingly stable, with decomposition occurring only above 200 °C. Quantum mechanical calculations show that, remarkably, the compounds are stabilized by attractive interactions between bulky groups at either end of the long C–C bonds. The stabilizing interactions observed in these compounds could prove useful in the development of new materials that utilize attractive dispersion interactions. Steric effects in chemistry are a consequence of the space required to accommodate the atoms and groups within a molecule, and are often thought to be dominated by repulsive forces arising from overlapping electron densities (Pauli repulsion). An appreciation of attractive interactions such as van der Waals forces (which include London dispersion forces) is necessary to understand chemical bonding and reactivity fully. This is evident from, for example, the strongly debated origin of the higher stability of branched alkanes relative to linear alkanes1,2 and the possibility of constructing hydrocarbons with extraordinarily long C–C single bonds through steric crowding3. Although empirical bond distance/bond strength relationships have been established for C–C bonds4 (longer C–C bonds have smaller bond dissociation energies), these have no present theoretical basis5. Nevertheless, these empirical considerations are fundamental to structural and energetic evaluations in chemistry6,7, as summarized by Pauling8 as early as 1960 and confirmed more recently4. Here we report the preparation of hydrocarbons with extremely long C–C bonds (up to 1.704 A), the longest such bonds observed so far in alkanes. The prepared compounds are unexpectedly stable—noticeable decomposition occurs only above 200 °C. We prepared the alkanes by coupling nanometre-sized, diamond-like, highly rigid structures known as diamondoids9. The extraordinary stability of the coupling products is due to overall attractive dispersion interactions between the intramolecular H•••H contact surfaces, as is evident from density functional theory computations with10 and without inclusion of dispersion corrections.

331 citations


Journal ArticleDOI
TL;DR: A detailed electronic structure analysis of alkane hydroxylation by an oxo-iron(IV) species on different spin-state potential energy surfaces is performed and shows that the lengthening of the Fe–oxo bond in ferryl reactants, which is the part of the reaction coordinate for H-atom abstraction, leads to the formation of oxyl- iron(III) species that then perform actual C–H bond activation.
Abstract: Oxo-iron(IV) species are implicated as key intermediates in the catalytic cycles of heme and nonheme oxygen activating iron enzymes that selectively functionalize aliphatic C–H bonds. Ferryl complexes can exist in either quintet or triplet ground states. Density functional theory calculations predict that the quintet oxo-iron(IV) species is more reactive toward C–H bond activation than its corresponding triplet partner, however; the available experimental data on model complexes suggests that both spin multiplicities display comparable reactivities. To clarify this ambiguity, a detailed electronic structure analysis of alkane hydroxylation by an oxo-iron(IV) species on different spin-state potential energy surfaces is performed. According to our results, the lengthening of the Fe–oxo bond in ferryl reactants, which is the part of the reaction coordinate for H-atom abstraction, leads to the formation of oxyl-iron(III) species that then perform actual C–H bond activation. The differential reactivity stems from the fact that the two spin states have different requirements for the optimal angle at which the substrate should approach the (FeO)2+ core because distinct electron acceptor orbitals are employed on the two surfaces. The H-atom abstraction on the quintet surface favors the “σ-pathway” that requires an essentially linear attack; by contrast a “π-channel” is operative on the triplet surface that leads to an ideal attack angle near 90°. However, the latter is not possible due to steric crowding; thus, the attenuated orbital interaction and the unavoidably increased Pauli repulsion result in the lower reactivity of the triplet oxo-iron(IV) complexes.

223 citations


Journal ArticleDOI
TL;DR: Adding 12 bulky tert-butyl groups, one to each of the 12 meta positions, gives a stabile ethane derivative and this unexpected stabilization is shown to result from attractive dispersion interactions between the substituents.
Abstract: 12 not so angry men: Hexaphenylethane is unstable, a phenomenon traditionally attributed to steric repulsion between the six phenyl rings. However, adding 12 bulky tert-butyl groups, one to each of the 12 meta positions, gives a stabile ethane derivative (see space-filling model and potential energy curve for the dissociation of the central C-C bond). This unexpected stabilization is shown to result from attractive dispersion interactions between the substituents.

205 citations


Journal ArticleDOI
TL;DR: Merging cooperative Si-H bond activation and electrophilic aromatic substitution paves the way for C-3-selective indole C-H functionalization under electronic and not conventional steric control.
Abstract: Merging cooperative Si−H bond activation and electrophilic aromatic substitution paves the way for C-3-selective indole C−H functionalization under electronic and not conventional steric control. The Si−H bond is heterolytically split by the Ru−S bond of a coordinatively unsaturated cationic ruthenium(II) complex, forming a sulfur-stabilized silicon electrophile. The Wheland intermediate of the subsequent Friedel−Crafts-type process is assumed to be deprotonated by the sulfur atom, no added base required. The overall catalysis proceeds without solvent at low temperature, only liberating dihydrogen.

205 citations


Journal ArticleDOI
TL;DR: The extension of the frustrated Lewis pair (FLP) concept to the transition series with cationic zirconocene-phosphinoaryloxide complexes is demonstrated, and these transition metal FLPs are markedly more reactive than main group systems in many cases.
Abstract: The extension of the frustrated Lewis pair (FLP) concept to the transition series with cationic zirconocene–phosphinoaryloxide complexes is demonstrated. Such complexes mimic the reactivity of main group FLPs in reactions such as heterolytic hydrogen cleavage, CO2 activation, olefin and alkyne addition, and ring-opening of tetrahydrofuran. The interplay between sterics and electronics is shown to have an important role in determining the reactivity of these compounds with hydrogen in particular. The Zr–H species generated from the heterolytic activation of hydrogen is shown to undergo insertion reactions with both CO2 and CO. Crucially, these transition metal FLPs are markedly more reactive than main group systems in many cases, and in addition to the usual array of reactions they demonstrate unprecedented reactivity in the activation of small molecules. This includes SN2 and E2 reactions with alkyl chlorides and fluorides, enolate formation from acetone, and the cleavage of C–O bonds to facilitate SN2 ty...

204 citations


Journal ArticleDOI
TL;DR: A combination of studies involving ultraviolet photoemission spectroscopy and density functional theory shows that the narrowing of the band gap upon borane coordination to the pyridal nitrogen on PT is a result of lowering the energies of both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbitality (LUMO) of the optically relevant fragments; however, the LUMO is decreased to a greater extent, thereby giving rise to the narrowed of the gap.
Abstract: We report on the interaction of Lewis acids with narrow band gap conjugated copolymers containing donor and acceptor units. Examination of the widely used poly[(4,4-bis(2-ethylhexyl)cyclopenta-[2,1-b:3,4-b′]dithiophene)-2,6-(diyl-alt-benzo[2,1,3]thiadiazole)-4,7-diyl] (1) shows weaker binding with B(C6F5)3 when compared with a small molecule that contains a cyclopenta-[2,1-b:3,4-b′]dithiophene (CDT) unit flanked by two benzo[2,1,3]thiadiazole (BT) fragments. Studies on model compounds representative of 1, together with a comparison between B(C6F5)3 and BBr3, indicate that the propensity for Lewis acid coordination is decreased because of steric encumbrance surrounding the BT nitrogen sites. These observations led to the design of chromophores that incorporate an acceptor unit with a more basic nitrogen site, namely pyridal[2,1,3]thiadiazole (PT). That this strategy leads to a stronger B−N interaction was demonstrated through the examination of the reaction of B(C6F5)3 with two small molecules bis(4,4-bis(...

195 citations


Journal ArticleDOI
TL;DR: The work has shown that the M-C(cage) bonds in transition metal-carboranyl complexes are generally inert toward electrophiles, and hence significantly different from traditional M-cage bonds, which can be ascribed to steric effects resulting from the carboranyl moiety.
Abstract: The construction and transformation of metal−carbon (M−C) bonds constitute the central themes of organometallic chemistry. Most of the work in this field has focused on traditional M−C bonds involving tetravalent carbon: relatively little attention has been paid to the chemistry of nontraditional metal−carbon (M−Ccage) bonds, such as carborane cages, in which the carbon is hypervalent. We therefore initiated a research program to study the chemistry of these nontraditional M−Ccage bonds, with a view toward developing synthetic methodologies for functional carborane derivatives. In this Account, we describe our results in constructing and elucidating the chemistry of transition metal−carboryne complexes.Our work has shown that the M−Ccage bonds in transition metal−carboranyl complexes are generally inert toward electrophiles, and hence significantly different from traditional M−C bonds. This lack of reactivity can be ascribed to steric effects resulting from the carboranyl moiety. To overcome this steric p...

178 citations


Journal ArticleDOI
30 Sep 2011-Science
TL;DR: Three-dimensional free energy relationships correlating steric and electronic effects were applied to design and optimize a ligand class for the enantioselective Nozaki-Hiyama-Kishi propargylation of ketones.
Abstract: Chemical reaction outcomes are often rationalized on the basis of independent analyses of steric and electronic effects. We applied three-dimensional free energy relationships correlating steric and electronic effects to design and optimize a ligand class for the enantioselective Nozaki-Hiyama-Kishi propargylation of ketones. The resultant mathematical model describing the steric and electronic parameter relationship is highly reliant on the synergistic interactions of these two effects.

161 citations


Journal ArticleDOI
TL;DR: It is demonstrated that substitution of the carboxylate for a carbonate X-type ligand leads to a complete reversal in site selectivity for many arene substrates.
Abstract: This paper presents a detailed investigation of the factors controlling site selectivity in the Pd-mediated oxidative coupling of 1,3-disubstituted and 1,2,3-trisubstituted arenes (aryl-H) with cyclometalating substrates (L~C-H). The influence of both the concentration and the steric/electronic properties of the quinone promoter are studied in detail. In addition, the effect of steric/electronic modulation of the carboxylate ligand is discussed. Finally, we demonstrate that substitution of the carboxylate for a carbonate X-type ligand leads to a complete reversal in site selectivity for many arene substrates. The origins of these trends in site selectivity are discussed in the context of the mechanism of Pd-catalyzed oxidative cross-coupling.

149 citations


Journal ArticleDOI
TL;DR: A soluble isolated wheat protein fraction (sIWP) prepared from commercially deamidated wheat protein was incubated with dextrans D10 or D65 at 60 °C and 75% relative humidity to form Maillard type complexes, which provided enhanced steric stabilization of the emulsions at acidic pH (∼pH 4) and the location of conjugation was dependent on the size of the dextran.

135 citations


Journal ArticleDOI
TL;DR: Compounds 3, 5, and 6 have been structurally characterized using single crystal X-ray diffraction and represent the first structure determinations for compounds featuring B-C(6)Cl(5) bonds; each exhibits a trigonal planar geometry about B, despite having different ligand sets.
Abstract: A new family of electron-deficient tris(aryl)boranes, B(C6F5)3–n(C6Cl5)n (n = 1–3), has been synthesized, permitting an investigation into the steric and electronic effects resulting from the gradual replacement of C6F5 with C6Cl5 ligands. B(C6F5)2(C6Cl5) (3) is accessed via C6Cl5BBr2, itself prepared from donor-free Zn(C6Cl5)2 and BBr3. Reaction of C6Cl5Li with BCl3 in a Et2O/hexane slurry selectively produced B(C6Cl5)2Cl, which undergoes B–Cl exchange with CuC6F5 to afford B(C6F5)(C6Cl5)2 (5). While 3 forms a complex with H2O, which can be rapidly removed under vacuum or in the presence of molecular sieves, B(C6Cl5)3 (6) is completely stable to refluxing toluene/H2O for several days. Compounds 3, 5, and 6 have been structurally characterized using single crystal X-ray diffraction and represent the first structure determinations for compounds featuring B–C6Cl5 bonds; each exhibits a trigonal planar geometry about B, despite having different ligand sets. The spectroscopic characterization using 11B, 19F, ...

Journal ArticleDOI
TL;DR: The Pd(0)-catalyzed intermolecular C-H amination of unactivated C(sp 3 )-H bonds using aryl amines as the nitrogen source is disclosed in this article.
Abstract: The Pd(0)-catalyzed intermolecular C–H amination of unactivated C(sp 3 )–H bonds using aryl amines as the nitrogen source is disclosed. Either the C–N cross-coupling product or the C–H amination product could be accessed selectively by adjusting the steric environment of the substrate.

Journal ArticleDOI
TL;DR: The new protocol expands the scope of catalytic azine functionalization as the excellent regioselectivity at the 3- and 4-positions well complements the existing methods for C-H arylation and Ir-catalyzed borylation, as well as classical functionalization of pyridines.
Abstract: We report a new catalytic protocol for highly selective C–H arylation of pyridines containing common and synthetically versatile electron-withdrawing substituents (NO2, CN, F and Cl). The new protocol expands the scope of catalytic azine functionalization as the excellent regioselectivity at the 3- and 4-positions well complements the existing methods for C–H arylation and Ir-catalyzed borylation, as well as classical functionalization of pyridines. Another important feature of the new method is its flexibility to adapt to challenging substrates by a simple modification of the carboxylic acid ligand or the use of silver salts. The regioselectivity can be rationalized on the basis of the key electronic effects (repulsion between the nitrogen lone pair and polarized C–Pd bond at C2-/C6-positions and acidity of the C–H bond) in combination with steric effects (sensitivity to bulky substituents).

Journal ArticleDOI
18 Feb 2011-Science
TL;DR: An in-depth experimental study of reagent alignment effects in reactions of unpolarized, excited CHD3 with fluorine (F) and O(3P) and the decisive role of long-range anisotropic interactions in steric control of this chemical reaction is elucidated.
Abstract: Exciting the CH-stretching mode of CHD 3 (where D is deuterium) is known to promote the C-H bond’s reactivity toward chlorine (Cl) atom. Conventional wisdom ascribes the vibrational-rate enhancement to a widening of the cone of acceptance (i.e., the collective Cl approach trajectories that lead to reaction). A previous study of this reaction indicated an intriguing alignment effect by infrared laser–excited reagents, which on intuitive grounds is not fully compatible with the above interpretation. We report here an in-depth experimental study of reagent alignment effects in this reaction. Pronounced impacts are evident not only in total reactivity but also in product state and angular distributions. By contrasting the data with previously reported stereodynamics in reactions of unpolarized, excited CHD 3 with fluorine (F) and O( 3 P), we elucidate the decisive role of long-range anisotropic interactions in steric control of this chemical reaction.

Journal ArticleDOI
TL;DR: A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized and it was indicated that the unpaired electron is mostly localized on the imide nitrogen for the Ni(III) complexes.
Abstract: A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. Oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III)-imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl. The two Ni(III)-imide compounds showed different variable-temperature magnetic properties but analogous EPR spectra at low temperatures. To account for this discrepancy, a low-spin/high-spin equilibrium was proposed to take place for the alkyl-substituted Ni(III)-imide complex. This proposal was supported by DFT calculations. DFT calculations also indicated that the unpaired electron is mostly localized on the imide nitrogen for the Ni(III) complexes. The results of reactions carried out in the presence of hydrogen donors supported the findings from DFT calculations that the adamantyl substituent was a significantly more reactive hydrogen-atom abstractor. Interestingly, the steric properties of the 2,6-dimesitylphenyl substituent are important not only in protecting the Ni═N core but also in favoring one rotamer of the resulting Ni(III)-imide, by locking the phenyl ring in a perpendicular orientation with respect to the NiPP plane.

Journal ArticleDOI
TL;DR: It is somewhat surprised to find that an adduct is readily formed simply upon mixing 1 with the 9-BBN bistriflimide reagent 5a despite the transannular steric demands of the 9 -BBN core and the need to form adjacent quaternary bonds to boron as well as nitrogen.
Abstract: During our studies on aromatic borylation,[1] we considered the combination of a highly electrophilic R2BNTf2 reagent with a base that would neutralize the HNTf2 byproduct of borylation without deactivating the electrophile. In principle, these requirements might be satisfied by 1,8-bis(dimethylamino)naphthalene (1), a hindered and exceptionally basic aniline that finds numerous applications as a basic catalyst or reagent due to its legendary lack of nucleophilicity.[2, 3] Strong electrophiles interact weakly, if at all, with the amine nitrogens, and very few examples are known where stable bonds to nitrogen can be formed between 1 and electrophilic groups larger than hydrogen.[2, 4-7] Among these exceptional cases, cyclic boronium structures 2 and 3 are relatively stable because the subunits BH2 and BF2 have minimal steric requirements.[5] However, the more hindered BMe2 derivative 4 has not been detected and no analogous BR2 structures are known.[5a, 8] In view of this long history, we were somewhat surprised to find that an adduct is readily formed simply upon mixing 1 with the 9-BBN bistriflimide reagent 5a despite the transannular steric demands of the 9-BBN core and the need to form adjacent quaternary bonds to boron as well as nitrogen.[9, 10] The remarkable structural features and unusual reactivity of this adduct are the subject of the following communication.

Journal ArticleDOI
TL;DR: Investigation of regioselectivities of N-heterocyclic carbene ligands in Ni-catalyzed alkyne-aldehyde reductive coupling reactions with silane reducing agents indicates that the regiOSElectivities are directly affected by the shape and orientation of the N-substituents on the ligand.
Abstract: The regioselectivities of N-heterocyclic carbene (NHC) ligands in Ni-catalyzed alkyne-aldehyde reductive coupling reactions with silane reducing agents are investigated using density functional theory. Reversal of regioselectivity can be achieved by varying the steric bulkiness of the ligand. The steric influences of NHC ligands are highly anisotropic. Regioselectivity is primarily controlled by the steric hindrance at the region of the ligand close to the alkyne. Analysis of 2D contour maps of the NHC ligands indicates that the regioselectivities are directly affected by the shape and orientation of the N-substituents on the ligand.

Journal ArticleDOI
TL;DR: To rationalize the reactivity of the carbene-borane Lewis pairs, the thermodynamics of adduct formation with B(C(6)F(5))(3) were calculated for 10 different carbenes; the stability (or instability) of these adducts can be used as a good measure of the degree of "frustration".
Abstract: A variety of Lewis acid–base pairs consisting of tris(pentafluorophenyl)borane, B(C6F5)3, in combination with sterically demanding five- and six-membered N-heterocyclic carbenes (NHCs) of the imidazolin-2-ylidene, imidazolidin-2-ylidene, and tetrahydropyrimidin-2-ylidene types were investigated with respect to their potential to act as frustrated Lewis pairs (FLP) by reaction with dihydrogen (H2) and tetrahydrofuran (THF). A sufficient degree of “frustration” was usually established by introduction of a 1,3-di-tert-butyl or 1,3-diadamantyl carbene substitution pattern, which allows an unquenched acid–base reactivity and thus leads to heterolytic dihydrogen activation and ring-opening of THF. In contrast, 1,3-bis(2,6-diisopropylphenyl)-substituted carbenes showed ambiguous behavior, and the corresponding five-membered imidazolin-2-ylidene formed a stable carbene-B(C6F5)3 adduct, whereas fast C–F activation and formation of a zwitterionic pyrimidinium-fluoroborate was observed for the six-membered tetrahydr...

Journal ArticleDOI
TL;DR: The low reactivity of peptide-prolyl-thioesters in native chemical ligation is not due to steric effects at the β-carbon, but rather to the presence of a carbonyl moiety on the nitrogen atom of the proline.

Journal ArticleDOI
TL;DR: In this paper, molecular dynamics simulations with four different models based on interatomic potentials have been carried out to determine the atomic-level structure of three hematite-water interfaces.

Journal ArticleDOI
TL;DR: In this paper, the authors discuss the low-friction mechanism of diamond-like carbon (DLC) by using their tight-binding quantum chemical molecular dynamics method, which employs a DLC film sliding simulation in order to explore the effect of hydrogen atoms on the carbon-based transfer film.
Abstract: Diamond-like carbon (DLC) has recently attracted much attention as a solid-state lubricant, because of its resistance to wear, low friction, and low abrasion. Several factors, such as the hydrogen atoms in DLC and transfer film formation are important for improving the tribological characteristics of DLC. In this paper, we discuss the low-friction mechanism of DLC by using our tight-binding quantum chemical molecular dynamics method. The method employs a DLC film sliding simulation in order to explore the effect of hydrogen atoms on the carbon-based transfer film. The formation of C–C bonds between DLC films increases friction, while surface hydrogen atoms suppress C–C bond formation, which results in the low-friction state. Moreover, the steric effect of hydrogen molecule generation was found to remove the load from the substrate, inhibiting C–C bond formation. In addition, we determined that surface hydrogen atoms play a key role in the cleavage of C–C bonds formed during sliding of DLC films.

Journal ArticleDOI
TL;DR: In this article, the enthalpies of reaction have been computed for the conjugate additions of MeSH to six α,β-unsaturated ketones, and substituent effects on the activation energies for the rate-determining step of the thiol addition (reaction of the enone with MeS−) were also computed.
Abstract: CBS-QB3 enthalpies of reaction have been computed for the conjugate additions of MeSH to six α,β-unsaturated ketones. Compared with addition to methyl vinyl ketone, the reaction becomes 1–3 kcal mol–1 less exothermic when an α-Me, β-Me, or β-Ph substituent is present on the C═C bond. The lower exothermicity for the substituted enones occurs because the substituted reactant is stabilized more by hyperconjugation or conjugation than the product is stabilized by branching. Substituent effects on the activation energies for the rate-determining step of the thiol addition (reaction of the enone with MeS–) were also computed. Loss of reactant stabilization, and not steric hindrance, is the main factor responsible for controlling the relative activation energies in the gas phase. The substituent effects are further magnified in solution; in water (simulated by CPCM calculations), the addition of MeS– to an enone is disfavored by 2–6 kcal mol–1 when one or two methyl groups are present on the C═C bond (ΔΔG⧧). The...

Journal ArticleDOI
TL;DR: The turn-on of emission in fluorescent protein chromophores sequestered in an "octaacid" capsule is controlled by stereoelectronic effects described by a linear free energy relationship.
Abstract: The turn-on of emission in fluorescent protein chromophores sequestered in an "octaacid" capsule is controlled by stereoelectronic effects described by a linear free energy relationship. The stereochemical effects are governed by both the positions and volumes of the aryl substituents, while the electronic effects, including ortho effects, can be treated with Hammett σ parameters. The use of substituent volumes rather than A values reflects packing of the molecule within the confines of the capsule.

Journal ArticleDOI
TL;DR: It is shown that the axial-equatorial energy difference in general, with the anomeric effect as a special case, is dictated by two factors of the stereoelectronic origin, steric hindrance and classical electrostatic interactions, synchronously working together.
Abstract: The anomeric effect (the tendency of heteroatomic substituents adjacent to a heteroatom within the cyclohexane ring to prefer the axial orientation instead of the sterically less hindered equatorial position) is traditionally explained through either the dipole moment repulsion or the hyperconjugation effect. In this work, by employing our recent work in density functional steric analysis, we provide a novel two-component explanation, which is consistent with the common belief in chemistry that the effect has a stereoelectronic origin. With α-D-glucopyranose as the prototype, we systematically explore its conformational space and generate 32 isomers, leading to a total of 80 axial-equatorial conformation pairs. The energy difference analysis of these pairs shows that while statistically speaking the tendency is valid, the anomeric effect is not always true and can be violated. Three energy components, exchange-correlation, classical electrostatic, and density functional steric, are found to be directly proportional to the total energy difference between axial and equatorial isomers. We also found that the total dipole moment change, not the hyperconjugation effect, is a reasonable indicator of the total energy difference. However, all these correlations alone are not strong enough to provide a compellingly convincing explanation for the general validity of the effect. With the help of strong correlations between energy components, an explanation with two energy components, steric and electrostatic, was proposed in this work. We show that the axial-equatorial energy difference in general, with the anomeric effect as a special case, is dictated by two factors of the stereoelectronic origin, steric hindrance and classical electrostatic interactions, synchronously working together. Another explanation in terms of exchange-correlation and electrostatic interactions has also been obtained in this work.

Journal ArticleDOI
TL;DR: DFT calculations unveiled that the relative orientation of the tethers in the 1-ene/yne-VCPs plays a key role in controlling the stereochemistry of the [3+2] cycloadducts.
Abstract: The mechanisms, structures of all stationary points involved, and kinetic and thermodynamic parameters of the Rh(I)-catalyzed intramolecular [3+2] cycloaddition reactions of 1-ene- and 1-yne-vinylcyclopropanes (1-ene-VCPs and 1-yne-VCPs) have been investigated using density functional theory (DFT) calculations. The computational results showed that the [3+2] reactions of 1-ene/yne-VCPs studied here occur through a catalytic cycle of substrate−catalyst complex formation, cyclopropane cleavage, alkene/alkyne insertion, and reductive elimination. Alkene/alkyne insertion is the rate- and stereoselectivity-determining step of these multistep [3+2] cycloadditions. The experimentally observed high reactivity of 1-yne-VCPs compared to 1-ene-VCPs is well rationalized by the differences of steric effects in the alkyne/alkene insertion transition states. DFT calculations unveiled that the relative orientation of the tethers in the 1-ene/yne-VCPs plays a key role in controlling the stereochemistry of the [3+2] cycloa...

Journal ArticleDOI
TL;DR: Comparison of the effects of fluorinated alcohols as promoting solvents in three reactions is reported, and the positive correlation between rate constants and H-bonding donation ability for sulfide oxidation and imino Diels-Alder reaction brings to light the role of this property.
Abstract: The influence of substituents on the properties of tri- and hexafluorinated alcohols derived from 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) was examined. Measurements of specific solvent−solute interactions revealed that H-bond donation (HBD) of fluorinated alcohols is sensitive to the steric hindrance of the OH group, whereas their Bronsted acidity is dependent only on the number of fluorine atoms. For hexafluorinated alcohols (HFAs), their association with amines characterized by X-ray diffraction showed that the balance between HBD and acidity is influenced by their structure. Moreover, the ability of HFAs to donate H-bonds is exerted in synclinal (sc), synperiplanar (sp), and also antiperiplanar (ap) conformations along the C−O bond. Comparison of the effects of fluorinated alcohols as promoting solvents in three reactions is reported. The positive correlation between rate constants and H-bonding donation ability for sulfide oxidation and imino Diels−Alder reaction brin...

Journal ArticleDOI
TL;DR: It is possible to perform virtual screening of nitroxides to optimize their stability, which can help to rationally design novel nitroxide for their potential use in vivo.
Abstract: We have synthesized several nitroxides with different substituents which vary the steric and electronic environment around the N−O moiety and have systematically investigated the role of substituents on the stability of the radicals. Our results demonstrated the reactivity toward ascorbate correlates with the redox potential of the derivatives. Furthermore, ab initio calculations also indicated a correlation between the reduction rate and the computed singly occupied molecular orbital−lowest unoccupied molecular orbital energy gap, but not with solvent accessible surface area of the N−O moiety, supporting the experimental results and suggesting that the electronic factors largely determine the radicals’ stability. Hence, it is possible to perform virtual screening of nitroxides to optimize their stability, which can help to rationally design novel nitroxides for their potential use in vivo.

Journal ArticleDOI
TL;DR: Two changes in relative equilibrium and rate constants cause the rates of reaction of ethylene with the two THF-ligated species having different steric properties to be similar to each other.
Abstract: We report a detailed examination of the effect of the steric and electronic properties of the ancillary ligand and the alkene reactant on the rate of migratory insertion of unactivated alkenes into the palladium-nitrogen bond of isolated palladium amido complexes. A series of THF-bound and THF-free amidopalladium complexes ligated by cyclometalated benzylphosphine ligands possessing varied steric and electronic properties were synthesized. The THF-free complexes react with ethylene at -50 °C to form olefin-amido complexes that were observed directly and that undergo migratory insertion, followed by β-hydride elimination to generate enamine products. The effect of the steric properties of the ancillary ligand on the binding of the alkene and the rate of migratory insertion were evaluated individually. The relative binding affinity of ethylene vs THF is larger for the less sterically hindered complex than for the more hindered complex, but the less hindered complex undergoes the insertion of ethylene more slowly than does the more hindered complex. These two changes in relative equilibrium and rate constants cause the rates of reaction of ethylene with the two THF-ligated species having different steric properties to be similar to each other. Reactions of the complexes containing electronically varied ancillary ligands showed that the more electron-poor complexes underwent the migratory insertion step faster than the more electron-rich complexes. Reactions of a THF-ligated palladium-amide with substituted vinylarenes showed that electron-poor vinylarenes reacted with the amido complex slightly faster than electron-rich vinylarenes. Separation of the energetics of binding and insertion indicate that the complex of an electron-rich vinylarene is more stable in this system than the complex of a more electron-poor vinylarene but that the insertion step of the bound, electron-rich vinylarene is slower than the insertion step with the bound, electron-poor vinylarene.

Journal ArticleDOI
TL;DR: A butyl-substituted radical gave single crystals, in which a π-dimeric structure, not a σ-bonded dimer was observed, even though steric protection was absent.
Abstract: We have prepared and isolated neutral polycyclic hydrocarbon radicals. A butyl-substituted radical gave single crystals, in which a π-dimeric structure, not a σ-bonded dimer, was observed, even though steric protection was absent. Thermodynamic stabilization due to the highly spin-delocalized structure contributes effectively to the suppression of σ-bond formation.

Journal ArticleDOI
TL;DR: The role of ligand-based steric effects in the polymerization of 4bromo-2,5-bis(hexyloxy)phenylmagnesium chloride was investigated in this article.
Abstract: The role of ligand-based steric effects was investigated in the polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride Three different Ni(L-L)Cl2 catalysts were synthesized using comm