Substituent effects in acid-catalyzed hydration of alkenes, measured under consistent reaction conditions

A set of convenient conditions for the title reaction is used to determine directly relative reactivities of alkenes with widely-varying steric requirements. Previous determinations did not use one set of conditions for all alkenes, but developed parameters in order to compare alkene reaction rates run in different solvents and in different concentrations; such an indirect reactivity comparison can introduce errors. Relative rates established under this common set of reaction conditions are compared to those previously calculated by using correction parameters. Comparison reveals that the approximate method overestimated effects of some groups attached to CC, such as methyl and chloro.     

A theoretical study on the alkene insertion step in Rh-Yanphos catalyzed hydroformylation

This paper studied the mechanism of the alkene insertion elementary step in the asymmetric hydroformylation （AHF） catalyzed by RhH（CO）2[（R,S）-Yanphos] using four alkene substrates （CH2=CH- Ph, CH2=CH-Ph-（p）-Me, CH2=CH-C（==O）OCH3 and CH2=CH-OC（=O）-Ph, abbreviated as A1-A4）. Interestingly, the equatorial vertical coordination mode （A mode） with respect to the Rh center was found for AI and A2 but not for A3 and A4, although the equatorial in-plane coordination mode （E mode） was found for A1 -A4. The relative energy of the E mode of the -q2-intermediates is lower than that of the A mode. In the alkene insertion step, Path 1 is more favorable than Path 2 for this system. As for AI and A2, there could be a transformation between 2eq and 2ax.     

Substituent effects on the13C and17O NMR,IR, and UV spectral parameters for propynals

Conclusions Trialkylsilyl(germanyl)propynals show a lower triple bond polarization compared with their carbon analogs because of the electron acceptor effects of the R substituents and the aldehyde group. This increases the inertness of the CC bond towards nucleophiles (RSH, R₂NH) and lowers the regioselectivity of dipolar attack. Lowering the relative basicity of the substituted propynals decreases the integrated intensity of the C=0 absorption band and shifts the ¹⁷O signal to low field.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 321–325, February, 1988.     

DFT calculation and AIM-analysis of the substituent influence on the structure of (1-azabuta-1,3-diene)tetracarbonyliron(0) complexes

1-Azabuta-1,3-dienes can coordinate to the tetracarbonyliron(0) moiety in four ways, to form (1-azabuta-1,3-diene)tetracarbonyliron(0) complexes with the ligand bonded in an η² fashion through the alkene, η² coordinated through its CN bond, σ-bonded to the lone pair of the nitrogen atom, or η³ coordinated through the CC-C moiety under concomitant coupling of the imine nitrogen with one of the carbonyl ligands to a carbamoyl species. In the experiment, the equilibrium between these species strongly depends on factors such as the nature of the substituents at the ligand, the solvent and the temperature. In this work, DFT calculations (B3LYP/LANL2DZ/6-31G* and 6-311++G**) and an AIM-analysis of the topology of the charge density were used to investigate the influence of the substituents at the 1-azabuta-1,3-diene ligand on the structural, electronic and energetic properties of these constitutional isomers. In most cases, the calculations correctly predict the observed structure, even in situations where the energy differences between related species are rather small. Substituents larger than CH₃ at N and H at C2 disfavour the structures with an η² coordination to the CN bond to such an extent that they cease to exist as minimum energy structures. Also the σ-N forms distort significantly with the introduction of substituents at N or C2 and become energetically less favourable. The geometries of the η²-alkene form do not change much upon substitution, whereas the η³ form tolerates steric strain best and becomes most favourable when the substituent at C2 is large. The activation barrier between the η²-alkene and the η³-allyl form is low (7.5-1.4 kcal/mol) and allows for an equilibration between these species. The conversion of the η²-alkene into the σ-N form requires almost complete dissociation of the ligand from the Fe(CO)₄ moiety. Accordingly, its activation barrier is higher (approx. 14 kcal/mol) and fairly independent of the nature of the substituents at the azabutadiene ligand.     

Importance of selenium in antioxidant behavior of ebselen: A theoretical study

The electronic structure, geometrical parameters, proton affinities, lone pair occupancies, and orbital interactions have been explored to understand the antioxidant behavior of ebselen at B3LYP/6-31+G1 level. The role of substituents in the nucleophilicity of selenium in eight model molecules containing SeN bond has been analyzed in terms of relative proton affinities. The free energy changes associated with probable catalytic reaction steps have also been evaluated.     

Effects of the simultaneous presence of nitro and amine substituents in cubane and some azacubanes

We have carried out an ab initio self-consistent-field molecular orbital study of the structures and bond orders (as measures of relative bond strengths) of nitro and aminonitro derivatives of cubane and some azacubanes. The NO₂ group is found to have, in general, a conformation-dependent effect on relative bond strengths. When the plane of the nitro group is perpendicular to that of an endocyclic bond, that bond is weakened. An aza nitrogen adjacent to C-NO₂, however, eliminates the bond-weakening due to a perpendicular nitro group, except when a C-NH₂ is also adjacent to the C-NO₂. The NH₂ group has a direction-specific bond-weakening effect on one [and only one] endocyclic bond; the particular bond undergoing this weakening is determined by the orientation of the amine nitrogen lone pair. The influences of the NO₂ and NH₂ are essentially independent; when these substituents are on adjacent carbons the effects are reinforcing. Thus, the combination of a perpendicular NO₂ and an NH₂ on adjacent carbons in cubane and azacubanes produces a marked bond weakening. Our findings suggest that strained cage systems that have alternating aza nitrogens and C-NO₂ groups should be further investigated as potential high-energy systems.     

Effects of Trifluoromethyl and Alkoxycarbonyl Groups on the Structure and Reactivity of Acrylates

Ab initio calculations with full geometry optimization have been performed for -CF₃-substituted cis- and trans-methyl crotonates, methyl-,-bis(trifluoromethyl) acrylate, and dimethylhexafluoroisopropylidene malonate using the restricted Hartree-Fock (RHF) method (6-31G* basis set). The effects of substituents at the multiple bond on the geometrical and electronic structure, dipole moments, polarizability and first molecular hyperpolarizability tensors, normal vibration frequencies, one-electron levels, electrostatic potentials, and local electron density at the C=C bond have been studied. Two substituents in the acrylic system prevent the conjugation of C=C and C=O bonds. The major effect on the structure and properties of the title compounds is produced by - interactions of the CF₃ group with the multiple bond and by incoherent exchange interactions of substituents at the C=C bond.     

Acetylene-allene rearrangement of propargyl systems X—CH2—C≡CH (X = H,Me, NMe2, OMe,F, SMe): an ab initio study

Acetylene—allene rearrangement in propargyl systems XCH₂CHCH (X = H, Me, NMe₂, OMe, F, and SMe) was studied using the ab initio approach. The relative stabilities of the starting and final propyne structures and the corresponding allenes as well as the structure of intermediate carbanions were considered. n--Conjugation was shown to dominate in allene stabilization while the inductive effect of heteroatomic substituents makes at least comparable contribution to stabilization (or destabilization) of the propynyl structure. In particular, relative instability of 1-methoxypropyne can be rationalized by high electronegativity of O atom, which leads to dramatic decrease in the total electron density in the region of the neighboring CC triple bond. The influence of substituents on the mobility of the migrating proton was considered for the gas phase and with solvation effects included. Calculations involving electron correlation at the MP2 level of theory were shown to be insufficient for correct reproduction of the energy differences between the corresponding propynes and allene structures. The results of MP4 calculations with inclusion of ZPE correction are in good agreement with the available experimental data.     

Isomerisierung von Methoxy- und Carbomethoxysubstituierten Cycloalkyl- und Alken-radikalkationen

The very complex isomerization patterns of methoxy and carbomethoxy substituted cycloalkanes (3- to 7-membered rings) have been investigated using collisional activation, metastable ion characteristics and field ionization kinetics. The extent of isomerization depends on both the ring size and the substituent. Irrespective of the electronic properties of the substituent, ring opening involves exclusively the C-1? C-2 bond whereby linear alkene radical cations are formed. In the case of OCH₃- and COOCH₃ substituents the position of the resulting double bond (terminal or α,β-unsaturated) is determined more by the ring size of the precursor molecules and less by the electronic properties of the substituents. Contrary to these findings alklyl substituted cycloalkanes (3- to 5-membered rings) rearrange exclusively to terminal alkene radical cations. The barrier for double bond isomerization seems to be substantially influenced by substituents.     

Using correlations to compare additions to alkenes: homogeneous hydrogenation by using Wilkinson's catalyst

Plots of the logarithms of relative rates of homogeneous catalytic hydrogenation of alkenes (log k(rel) values) by using Wilkinson's catalyst versus their ionization potentials (IPs) and versus their lowest unoccupied molecular orbital energy levels (LUMOs) display good-to-excellent correlations. The correlations indicate that the rate-determining step of this reaction is a nucleophilic addition to the alkene double bond, which is dependent upon both electronic effects and steric effects. This conclusion is in agreement with only two of three previously proposed mechanisms for the reaction, effectively ruling out one in which the rate-determining step involves electrophilic addition to the alkene. Characteristics of the analysis using these correlations are compared and contrasted with other additions to alkenes, such as the Wacker oxidation, to probe patterns in transition state characteristics.     

F and C spectra of fluorinated and partially fluorinated vinyl alkyl ethers

Fluorine, hydrogen, and ¹³C NMR spectral data have been obtained for vinyl alkyl ethers containing fluorines. Some of the molecules are perfluorinated and others include hydrogen, bromine, and chlorine substituents. New generalizations regarding FF spin-spin coupling are developed and used, along with previously recognized correlations, in the confirmation of structures and the assignments of resonances. ¹³C spectroscopy, especially the analysis of ¹³C¹⁹F coupling, is critical in several of the structure determinations. Chlorine isotope effects on fluorine chemical shifts are observed when the chlorine and fluorine are attached to the same carbon, and are also used in the structure analyses. Long-range couplings between fluorines in the vinyl group and fluorines in the alkyl group are interpreted in terms of molecular geometry which allows certain of the alkyl fluorines to “touch” the fluorines cis and gem to the ether oxygen but not the fluorine trans to the oxygen. Two bond ¹³C¹⁹F coupling across the vinyl double bond is found to vary dramatically with the electronegativity of the vinyl substituents in the ethers, in accordance with previous observations for olefins.     

Synthesis, characterization and photovoltaic properties of thiophene copolymers containing conjugated side-chain

Five side-chain conjugated polythiophene derivatives, P1-P5, were synthesized by Stille coupling reaction. The effects of side-chain structures, bearing CC double bond, CC triple bond as well as different number of methoxy substituents on the benzene ring of the side-chains, on the optical, electrochemical, and photovoltaic properties of the polymers were investigated. From P1 to P3, the effect of CC triple bond and CC double bond was compared. The results indicate that the content of the thiophene units with the CC triple bond in their conjugated side-chains not only influences the absorption shape and intensity, but also influences the energy bandgap and the photovoltaic properties of the polymers. From P3 to P5, the effect of methoxy substituents on the benzene ring of the conjugated side-chains was compared. On increasing the number of the methoxy groups on the benzene ring of the conjugated side chains, the visible π-π^∗ absorption of the conjugated polymer backbone become stronger both in solution and in film. Electron-donating ability of the methoxy groups decreased the bandgap of the polymers. The best polymer solar cell based on P5 with a structure of ITO/PEDOT:PSS/Polymer:PCBM (1:1 wt/wt)/Mg/Al showed a power conversion efficiency of 1.45% under the illumination of AM1.5, 80 mW/cm².     

Substituted 5-Dinitromethyl-3-phenyl-1,2,4-oxadiazoles in Reactions with Arylethenes

Reactions of substituted 5-dinitromethyl-3-phenyl-1'2'4-oxadiazoles with arylethenes of variousnucleophilicity, number, position of substituents attached to the double bond give rise to nitroalkenes andsecondary products resulting from -nitroketones or nitroalcohols O-alkylation. The direction of transforma-tions in the arising ion pair is governed predominantly by steric effects.     

Direct13C-13C spin-spin coupling constants for the triple bond in activated acetylenes

Conclusions ¹³C-¹³C SSCC values across the triple bond in activated acetylenes are determined primarily by the -electronic properties of substituents. The enhanced sensitivity of¹J_cc in acetylene derivatives to substituent effects also asserts their possible utility in estimating the electronegative properties of substituents.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1287–1292, June, 1988.     

Addition of Dialkyl Hydrogen Phosphites to Alkenes in the Presence of Carbonyl Complexes of Chromium Subgroup Metals and Iron

Depending on the reactant ratio and order of their mixing, reactions of dialkyl hydrogen phosphite with alkenes in the presence of catalytic amounts of homoligand carbonyl complexes of iron or chromium subgroup metals yield phosphonates by two pathways: reaction of dialkyl hydrogen phosphite with -coordinated alkene and addition to alkene of the product of reaction of dialkyl hydrogen phosphite with the transition metal carbonyl. The products of reactions of Fe(CO)₅ and W(CO)₆ with dialkyl hydrogen phosphites contain the phosphorus-metal bond.     

Spectroscopic study of compounds possessing a bond of an element of group IVB with hydrogen

Conclusions The frequencies of the valence vibrations of the Si-H bond in trisubstituted silanes depends not only on the inductive effect of substituents, but also on the effects of d-p interaction, ,- and p,-conjugation. An analysis was made of the relative contributions of these effects, depending on the nature of the substituent.For the preceding communications, see [1, 2].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 544–550, March, 1971.     

Quantification of the push-pull effect in tolanes and a revaluation of the factors affecting the 13C chemical shifts of the carbon atoms of the CC triple bond

Variously substituted tolanes were employed to show that the push-pull effect is also active in CC triple bonds by the successful correlation of the occupation quotient pi/pi of the pi orbital in resonance with the substituted phenyl moieties of tolanes versus the bond length of the CC triple bond. In addition, the influences of the ortho phenyl ring substituents on the 13C chemical shifts of the triple bond carbon atoms, which were estimated by Rubin et al.(4) to be "inapplicable for describing triple bond polarization", were re-evaluated, leading to the conclusion that, while anisotropic effects of ortho substituents are negligible, the steric ortho-substituent effects do in fact dominate the deviations obtained. A detailed theoretical NBO/NCS study has been employed to illuminate the facts of this case.     

Kinetic Study of the Heck Reaction by the Method of Competing Reactions

The method of competing reactions is used to measure the isotope effect in styrene phenylation by iodobenzene. The kinetics of styrene, methyl acrylate, and methyl methacrylate phenylation is studied. Experimental data suggest that the step of alkene insertion is relatively fast, while the subsequent step that involves C–H bond dissociation is rate-limiting. The kinetic parameters of the process point to the dependence of the rate-limiting step of the catalytic cycle on the concentrations of base and alkene. These results are consistent with the existing hypothesis that the step of -elimination of palladium hydride is present in the mechanism.     

Ethene polymerization with mixed-ring metallocene catalysts,Cp′Cp″MCl2, M = Ti and Zr

Mixed-ring metallocenes CpCpMCl₂ (Cp and Cp = cyclopentadienyl and its substituted derivatives; M = Ti and Zr) have been prepared by reacting CpMCl₃ with the Cp anion salt, in which the Cp–M bond is stronger than the Cp–M bond. Ethene has been polymerized with these complexes in the presence of methyl aluminoxane (MAO) as co-catalyst. The catalytic properties of titanocenes were influenced mainly by the steric effects of cyclopentadienyl ring substituents. Both steric and electronic effects of substituents influenced the catalytic properties of zirconocenes.