Hammett Substituent Parameters Research Articles

Substituent effects on the [1-closo-CB9H10]− anion geometry: experimental and DFT studies

Five new structures of the [closo-1-CB9H10]− anion substituted at the apical position(s) are reported. C(1) and B(10) apical substituents affect the {closo-1-CB9} cluster geometry in different ways, and the effect correlates with the Hammett substituent parameters.

Inherent Reactivity of Spiro‐Activated Electrophilic Cyclopropanes

The kinetics of the ring‐opening reactions of thiophenolates with geminal bis(acceptor)‐substituted cyclopropanes in DMSO at 20 °C was monitored by photometric methods. The determined second‐order rate constants of the SN2 reactions followed linear relationships with Mayr nucleophilicity parameters (N/s N) and Brønsted basicities (pK aH) of the thiophenolates as well as with Hammett substituent parameters (σ) for groups attached to the thiophenolates. Phenyl‐substituted cyclopropanes reacted by up to a factor of 15 faster than their unsubstituted analogues, in accord with the known activating effect of adjacent π‐systems in SN2 reactions. Variation of the electronic properties of substituents at the phenyl groups of the cyclopropanes gave rise to parabolic Hammett relationships. Thus, the inherent SN2 reactivity of electrophilic cyclopropanes is activated by electron‐rich π‐systems because of the more advanced C1−C2 bond polarization in the transition state. On the other hand, electron‐poor π‐systems also lower the energetic barriers for the attack of anionic nucleophiles owing to attractive electrostatic interactions.

Long Vibrational Lifetime R-Selenocyanate Probes for Ultrafast Infrared Spectroscopy: Properties and Synthesis.

Ultrafast infrared vibrational spectroscopy is widely used for the investigation of dynamics in systems from water to model membranes. Because the experimental observation window is limited to a few times the probe's vibrational lifetime, a frequent obstacle for the measurement of a broad time range is short molecular vibrational lifetimes (typically a few to tens of picoseconds). Five new long-lifetime aromatic selenocyanate vibrational probes have been synthesized and their vibrational properties characterized. These probes are compared to commercial phenyl selenocyanate. The vibrational lifetimes range between ∼400 and 500 ps in complex solvents, which are some of the longest room-temperature vibrational lifetimes reported to date. In contrast to vibrations that are long-lived in simple solvents such as CCl4, but become much shorter in complex solvents, the probes discussed here have ∼400 ps lifetimes in complex solvents and even longer in simple solvents. One of them has a remarkable lifetime of 1235 ps in CCl4. These probes have a range of molecular sizes and geometries that can make them useful for placement into different complex materials due to steric reasons, and some of them have functionalities that enable their synthetic incorporation into larger molecules, such as industrial polymers. We investigated the effect of a range of electron-donating and electron-withdrawing para-substituents on the vibrational properties of the CN stretch. The probes have a solvent-independent linear relationship to the Hammett substituent parameter when evaluated with respect to the CN vibrational frequency and the ipso 13C NMR chemical shift.

Correlations between Substituent Effects and Catalytic Activities: A Quantitative Approach for the Development of Halogen-Bonding-Driven Anion-Binding Catalysts.

A quantitative approach for the development of halogen-bonding-driven anion-binding catalysts was studied using 4-substituted perfluorinated iodobenzene. 19 F NMR titrations were used to determine the binding constants K for chloride, and their catalytic activities were evaluated in the allylation reaction of a N-activated pyridine. We discovered that the log K and product yields were linearly correlated, and that they were dependent on the Hammett substituent parameter, σmeta (r2 =0.99). This linear correlation provided a quantitative predictive model for both the binding constant and the reaction yield. Concomitantly, this efficiently permitted the development of a highly active anion-binding catalyst, namely 4-CNC6 F4 I (K=489±5 M-1 ). Additionally, the catalytic activity of 4-CNC6 F4 I was established in the allylation and crotylation of N-activated isoquinolines (7 examples). Overall, this approach highlights the value of quantitative analysis by exploring experimentally informed correlations in the development of halogen bond donor catalysts.

Trans Influence of Ligands on the Oxidation of Gold(I) Complexes.

Gold(I) complexes are considered active species toward oxidative addition; current understanding indicates a different mechanism in contrast to other late transition metals, but a rational understanding of the reactivity profile is lacking. Herein, we propose that the accessibility of the gold(I) center to tri- or tetra-coordination is critical in the oxidative process involving a tri- or tetra-coordinate gold(I) with the oxidizing reagent as one of the ligands as an intermediate. A computational study of the geometry of (Phen)R3PAu(I)NTf2 complexes shows that the accessibility of such tricoordinate species shows a good correlation with the "trans influence" of phosphine ligands: the weak σ-donating phosphine ligands promote tricoordination of gold(I) complexes. The oxidative addition to the asymmetric tricoordinate (Phen)R3PAu(I)NTf2 complexes with alkynyl hypervalent iodine reagents was built. The kinetic profile of the oxidative addition exhibits a good relationship to the Hammett substituent parameter (ρ = 3.75, R2 = 0.934), in which the gold(I) complexes bearing less σ-donating phosphine ligands increase the rate of oxidative addition. The positive ρ indicates a high sensitivity of the oxidative addition to the trans influence. The reactivity profile of oxidative addition to a linear bis(pyridine)gold(I) complex further supports that the oxidative addition to gold(I) complexes is promoted by ligands with small trans influence.

C(3) Functional Derivatives of the Blatter Radical.

A series of 3-substituted 1-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yls 1 was prepared by addition of PhLi to 3-substituted benzo[e][1,2,4]triazines 2 followed by aerial oxidation. The scope of the C(3) substituents in the reaction was investigated, and 10 structurally diverse radicals 1 were isolated, their stability was assessed and properties were investigated with spectroscopic and electrochemical methods. Two radicals were analyzed with single-crystal XRD methods. Experimental data are compared to DFT results and correlated with Hammett substituent parameters.

Kinetic Studies of Donor-Acceptor Cyclopropanes: The Influence of Structural and Electronic Properties on the Reactivity.

The kinetics of (3+2) cycloaddition reactions of 18 different donor-acceptor cyclopropanes with the same aldehyde were studied by in situ NMR spectroscopy. Increasing the electron density of the donor residue accelerates the reaction by a factor of up to 50 compared to the standard system (donor group=phenyl), whereas electron-withdrawing substituents slow down the reaction by a factor up to 660. This behavior is in agreement with the Hammett substituent parameter σ. The obtained rate constants from the (3+2) cycloadditions correlate well with data from additionally studied (3+n) cycloadditions with a nitrone (n=3) and an isobenzofuran (n=4). A comparison of the kinetic data with the bond lengths in the cyclopropane (obtained by X-ray diffraction and computation), or the 1 H and 13 C NMR shifts, revealed no correlation. However, the computed relaxed force constants of donor-acceptor cyclopropanes proved to be a good indicator for the reactivity of the three-membered ring.

NMR-based investigations of acyl-functionalized piperazines concerning their conformational behavior in solution.

Selected N-benzoylated piperazine compounds were synthesized to study their conformational behavior using temperature-dependent 1H NMR spectroscopy. All investigated piperazines occur as conformers at room temperature resulting from the restricted rotation of the partial amide double bond. In the case of selected mono-N-benzoylated and unsymmetrically N,N′-substituted derivatives, the appearance of the 1H NMR spectrum was further shaped by the limited interconversion of the piperazine chair conformations. Therefore, two different coalescence points TC were determined and their resulting activation energy barriers ΔG‡ were calculated to be between 56 and 80 kJ mol−1. In most of the cases, TC and ΔG‡ for the amide site appeared to be higher than the corresponding values for the ring inversion. The influences of substituents on rotational and inversion barriers were analyzed by correlation to Hammett constants. The obtained results are discussed and interpreted in the context of literature data. An additional aryl substituent connected at the amine site led to reduced rotational and inversion barriers compared to the free secondary amine. To support and evidence the findings from the NMR analyses, single crystals of selected piperazines were obtained and XRD analyses were performed. To underline the results, two potential TGase 2 inhibitors were investigated showing energy barriers with similar values.

Controlling the Conformational Energy of a Phenyl Group by Tuning the Strength of a Nonclassical CH···O Hydrogen Bond: The Case of 5-Phenyl-1,3-dioxane.

Anancomeric 5-phenyl-1,3-dioxanes provide a unique opportunity to study factors that control conformation. Whereas one might expect an axial phenyl group at C(5) of 1,3-dioxane to adopt a conformation similar to that in axial phenylcyclohexane, a series of studies including X-ray crystallography, NOE measurements, and DFT calculations demonstrate that the phenyl prefers to lie over the dioxane ring in order to position an ortho-hydrogen to participate in a stabilizing, nonclassical CH···O hydrogen bond with a ring oxygen of the dioxane. Acid-catalyzed equilibration of a series of anancomeric 2-tert-butyl-5-aryl-1,3-dioxane isomers demonstrates that remote substituents on the phenyl ring affect the conformational energy of a 5-aryl-1,3-dioxane: electron-withdrawing substituents decrease the conformational energy of the aryl group, while electron-donating substituents increase the conformational energy of the group. This effect is correlated in a very linear way to Hammett substituent parameters. In short, the strength of the CH···O hydrogen bond may be tuned in a predictable way in response to the electron-withdrawing or electron-donating ability of substituents positioned remotely on the aryl ring. This effect may be profound: a 3,5-bis-CF3 phenyl group at C(5) in 1,3-dioxane displays a pronounced preference for the axial orientation. The results are relevant to broader conformational issues involving heterocyclic systems bearing aryl substituents.

Monofunctionalized Cobaltocenium Compounds by Dediazoniation Reactions of Cobaltoceniumdiazonium Bis(hexafluorophosphate)

Monofunctionalized cobaltocenium salts are obtained for the first time from cobaltoceniumdiazonium bis(hexafluorophosphate) with various nucleophiles via Sandmeyer-type and related reactions. For successful conversions, reaction conditions are quite critical: either standard solution chemistry in nitromethane or solvent-free ball milling proved necessary, depending on the type of reactant. By this synthetic approach valuable synthons such as iodocobaltocenium and azidocobaltocenium salts are accessible that open up new vistas in cobaltocenium chemistry. Spectroscopic characterization by NMR, IR, HRMS, and single-crystal structure analysis as well as the results of electrochemical studies are reported. Derivatives with two reversible reductions show the expected relation of the half-wave potentials with the Hammett substituent parameter σp of the respective substituent with a slightly larger slope for the first reduction. The carboxylic acid (reductive deprotonation of the −COOH functionality), the iodo (p...

Receptor binding profiles and quantitative structure-affinity relationships of some 5-substituted-N,N-diallyltryptamines.

N,N-Diallyltryptamine (DALT) and 5-methoxy-N,N-diallyltryptamine (5-MeO-DALT) are two tryptamines synthesized and tested by Alexander Shulgin. In self-experiments, 5-MeO-DALT was reported to be psychoactive in the 12-20mg range, while the unsubstituted compound DALT had few discernible effects in the 42-80 mg range. Recently, 5-MeO-DALT has been used in nonmedical settings for its psychoactive effects, but these effects have been poorly characterized and little is known of its pharmacological properties. We extended the work of Shulgin by synthesizing additional 5-substituted-DALTs. We then compared them to DALT and 5-MeO-DALT for their binding affinities at 45 cloned receptors and transporter proteins. Based on in vitro binding affinity, we identified 27 potential receptor targets for the 5-substituted-DALT compounds. Five of the DALT compounds had affinity in the 10-80 nM range for serotonin 5-HT1A and 5-HT2B receptors, while the affinity of DALT itself at 5-HT1A receptors was slightly lower at 100 nM. Among the 5-HT2 subtypes, the weakest affinity was at 5-HT2A receptors, spanning 250-730 nM. Five of the DALT compounds had affinity in the 50-400 nM range for serotonin 5-HT1D, 5-HT6, and 5-HT7 receptors; again, it was the unsubstituted DALT that had the weakest affinity at all three subtypes. The test drugs had even weaker affinity for 5-HT1B, 5-HT1E, and 5-HT5A subtypes and little or no affinity for the 5-HT3 subtype. These compounds also had generally nanomolar affinities for adrenergic α2A, α2B, and α2C receptors, sigma receptors σ1 and σ2, histamine H1 receptors, and norepinephrine and serotonin uptake transporters. They also bound to other targets in the nanomolar-to-low micromolar range. Based on these binding results, it is likely that multiple serotonin receptors, as well as several nonserotonergic sites are important for the psychoactive effects of DALT drugs. To learn whether any quantitative structure-affinity relationships existed, we evaluated correlations among physicochemical properties of the congeneric 5-substituted-DALT compounds. The descriptors included electronic (σp), hydrophobic (π), and steric (CMR) parameters. The binding affinity at 5-HT1A, 5-HT1D, 5-HT7, and κ opioid receptors was positively correlated with the steric volume parameter CMR. At α2A, α2B, and α2C receptors, and at the histamine H1 receptor, binding affinity was correlated with the Hammett substituent parameter σp; higher affinity was associated with larger σp values. At the σ2 receptor, higher affinity was correlated with increasing π. These correlations should aid in the development of more potent and selective drugs within this family of compounds.

An experimental study on relationship between hammett substituent constant and electronic absorption wavelength of some azo dyes

In this study, absorption spectra of sixteen azo dyes have been recorded in various solvents. These azo dyes have substituents such as OH, SO3H, Cl, I, NO2, C2H5 and OCH3 in different positions of phenyl ring. There is a shift in ?max whose amount is dependent upon the type and position of substituent on the ring. The effects of substituent on the absorption spectra of azo dyes are interpreted by correlation of absorption maximum wavelengths (nm) with the hammett substituent parameters. Charge transfer transitions are observed in inverse direction for azo dyes with electron acceptor substituent compared to azo dyes with electron donor

Multinuclear magnetic resonance studies of 2-aryl-1,3,4-selenadiazoles

2-Aryl-1,3,4-selenadiazoles were studied by (1) H, (13) C, (15) N and (77) Se NMR spectroscopy. The results (chemical shifts and coupling constants) were correlated with Hammett substituent parameters as well as calculated chemical shifts and bond lengths.

Actual Structure, Thermodynamic Driving Force, and Mechanism of Benzofuranone-Typical Compounds as Antioxidants in Solution

5,7-Ditert-butyl-3-(3,4-dimethylphenyl)benzofuran-2(3H)-one (HP-136) (1H) and its 30 analogues (2H-5H) as benzofuranone-typical antioxidants were synthesized. The structures of the benzofuranones in solid and solution were examined by using experimental and theoretical methods. The results show that the dominant structure is the lactone form rather than the enol form both in solid and solution. The thermodynamic driving forces of the 31 benzofuranone-typical compounds to release protons [ΔG(PD)(XH)], hydrogen atoms [ΔG(HD)(XH)], and electrons [E(ox)(XH)] and the thermodynamic driving forces of the anions (X(-)) of the benzofuranones to release electrons [E(ox)(X(-))] were determined for the first time in DMSO. The ΔG(HD)(XH) scale of these compounds in DMSO ranges from 65.2 to 74.1 (kcal/mol) for 1H-4H and from 73.8 to 75.0 (kcal/mol) for 5H, respectively, which are all smaller than that of the most widely used commercial antioxidant BHT (2,6-ditert-butyl-4-methylphenol, 81.6 kcal/mol), suggesting that the 31 XH could be used as good hydrogen-atom-donating antioxidants. The ΔG(PD)(XH) were observed to range from 11.5 to 16.0 (kcal/mol) for 1H-4H and from 18.6 to 22.4 (kcal/mol) for 5H, indicating that benzofuranones (1H-4H) are good proton donors, and their analogues (5H) should belong to middle-strong proton donors. E(ox)(XH) of the 31 XH to release an electron vary from 1.346 to 1.962 (V versus Fc(+/0)), implying that the 31 XH are weak electron donors, whereas the quite negative E(ox)(X(-)) show that X(-) are good electron donors. The Gibbs free-energy changes of the radical cations (XH(+•)) to release protons [ΔG(PD)(XH(+•))] were evaluated according to the corresponding thermodynamic cycle, and the results reveal that XH(+•) are good proton donors. Further inspection of our experimental results showed the ΔG(HD)(XH), ΔG(PD)(XH), ΔG(PD)(XH(+•)), E(ox)(XH), and E(ox)(X(-)) of the five chemical and electrochemical processes are all linearly dependent on the sum of Hammett substituent parameters σ with very good correlation coefficients, indicating that for any one- or multisubstituted species at the para- and/or meta-position of benzofuranones and their various reaction intermediates, the five thermodynamic driving force parameters all can be easily and safely estimated from the corresponding Hammett substituent parameters. The rates of hydrogen atom transfer from XH to DPPH(•) were determined by using the UV-vis absorption spectroscopy technique. Combining these important thermodynamic parameters and dynamic determination results, the mechanism of hydrogen transfer from HP-136 and its analogues to DPPH(•) was studied. The results suggest that the hydrogen transfer from HP-136 and its analogues 2H to DPPH(•) actually includes two steps, proton transfer and the following electron transfer, but the proton transfer is rate-determined.

Comparison between the two different forms of iron(IV)-oxo porphyrin and the effects of neutral axial ligand on their catalysis: A theoretical study

Density functional theory calculations reveal that iron(IV)-oxo porphyrin cation radical (Cpd I) models are indeed much more reactive than iron(IV)-oxo porphyrin (Cpd II) models in the case of 1, 3-cyclohexdiene oxidation. The geometrical parameters and electronic configuration are quite different for Cpd I and Cpd II. This is the origin of the reactivity difference. The axial ligand indeed affects the geometries of Cpd I and Cpd II and the oxidation process, even though it cannot switch the reactivity of Cpd I and Cpd II. Interestingly, the axial ligand can switch the oxidation process from dehydrogenation to epoxidation by Cpd I, however, for Cpd II, it cannot switch the reaction process, but lessen the barrier difference of dehydrogenation and epoxidation, even though the decrease of barrier is not significant. The axial ligand effect is somewhat in relation to the Hammett substituent parameter ( σ P) of the substituent group in the axial ligand. The more negative the σ P is, the longer the Fe O bond of Cpd I is, and the higher the reaction barrier is.

Hydride Affinity Scale of Various Substituted Arylcarbeniums in Acetonitrile

Combined with the integral equation formalism polarized continuum model (IEFPCM), the hydride affinities of 96 various acylcarbenium ions in the gas phase and CH(3)CN were estimated by using the B3LYP/6-31+G(d)//B3LYP/6-31+G(d), B3LYP/6-311++G(2df,2p)//B3LYP/6-31+G(d), and BLYP/6-311++G(2df,2p)//B3LYP/6-31+G(d) methods for the first time. The results show that the combination of the BLYP/6-311++G(2df,2p)//B3LYP/6-31+G(d) method and IEFPCM could successfully predict the hydride affinities of arylcarbeniums in MeCN with a precision of about 3 kcal/mol. On the basis of the calculated results from the BLYP method, it can be found that the hydride affinity scale of the 96 arylcarbeniums in MeCN ranges from -130.76 kcal/mol for NO(2)-PhCH(+)-CN to -63.02 kcal/mol for p-(Me)(2)N-PhCH(+)-N(Me)(2), suggesting most of the arylcarbeniums are good hydride acceptors. Examination of the effect of the number of phenyl rings attached to the carbeniums on the hydride affinities shows that the increase of the hydride affinities takes place linearly with increasing number of benzene rings in the arylcarbeniums. Analyzing the effect of the substituents on the hydride affinities of arylcarbeniums indicates that electron-donating groups decrease the hydride affinities and electron-withdrawing groups show the opposite effect. The hydride affinities of arylcarbeniums are linearly dependent on the sum of the Hammett substituent parameters σ(p)(+). Inspection of the correlation of the solution-phase hydride affinities with gas-phase hydride affinities and aqueous-phase pK(R)(+) values reveals a remarkably good correspondence of ΔG(H(-)A)(R(+)) with both the gas-phase relative hydride affinities only if the α substituents X have no large electron-donating or -withdrawing properties and the pK(R)(+) values even though the media are dramatically different. The solution-phase hydride affinities also have a linear relationship with the electrophilicity parameter E, and this dependence can certainly serve as one of the most effective ways to estimate the new E values from ΔG(H(-)A)(R(+)) or vice versa. Combining the hydride affinities and the reduction potentials of the arylcarbeniums, we obtained the bond homolytic dissociation Gibbs free energy changes of the C-H bonds in the corresponding hydride adducts in acetonitrile, ΔG(HD)(RH), and found that the effects of the substituent on ΔG(HD)(RH) are very small. Simple thermodynamic analytic platforms for the three C-H cleavage modes were constructed. It is evident that the present work would be helpful in understanding the nature of the stabilities of the carbeniums and mechanisms of the hydride transfers between carbeniums and other hydride donors.

Accurate Estimation of the One-Electron Reduction Potentials of Various Substituted Quinones in DMSO and CH3CN

The one-electron reduction potentials of 116 important p- and o-quinones in DMSO and CH(3)CN were predicted for the first time by using the B3LYP/DZP++ method and the PCM cluster continuum model. The calculated gas-phase electron affinities and one-electron reduction potentials agree well with the available experimental observations, respectively. The study showed the one-electron reduction potentials of the 116 quinones range from -0.949 to 1.128 V in DMSO and from -0.904 to 0.971 V in CH(3)CN. The one-electron reduction potentials of p-quinones are generally smaller than those of the o-quinones by about 0.132 V. For quinones with aromatic properties, 2-substituted-1,4-naphthquinones have the largest one-electron reduction potentials, followed by substituted-1,4-anthraquinones and then by substituted-9,10-anthraquinones. The study also showed that the one-electron reduction potentials of quinones in DMSO are linearly dependent on the sum of the Hammett substituent parameters sigma(p): E(NHE)(p-Q/p-Q(*-)) = 0.45Sigma sigma(p) - 0.194 (V) and E(NHE)(o-Q/o-Q(*-)) = 0.45Sigma sigma(p) - 0.059 (V). Combined with the hydride affinities of quinones in the former paper [DeltaG(H)-(A)(p-Q) = -16.0Sigma sigma(p) - 70.5 (kcal/mol) and DeltaG(H)-(A)(o-Q) = -16.2Sigma sigma(p) - 81.5 (kcal/mol)] and the one-electron reduction potentials of quinones estimated in this work, we obtained the homolytic bond dissociation energies of the hydroquinone anions (QH(-)) and found that these thermodynamic parameters also have linear correlations against the sum of the Hammett substituent parameters sigma(p) if only the substituents have no larger electrostatic inductive force and no large steric hindrance: BDE(p-QH(-)) = 5.05Sigma sigma(p) + 63.18 (kcal/mol) and BDE(o-QH(-)) = 5.33Sigma sigma(p) + 71.30 (kcal/mol). Knowledge about the redox potentials of the quinones should be of great value for the understanding of the nature of chemical reactions of quinones, the designing of new electronic materials of quinones, and the examining of biological activities of quinones.

Cytosine in Context: A Theoretical Study of Substituent Effects on the Excitation Energies of 2-Pyrimidinone Derivatives

The ultrafast radiationless decay mechanism for cytosine has been shown to be in part dependent upon high vertical excitation, while slower fluorescence displayed in some cytosine analogs is generally linked to lower vertical excitation energies. To probe how excitation energies relate to pyrimidine structure, substituent effects on the vertical excitation energies for a number of derivatives of 2-pyrimidin-(1H)-one (2P) have been calculated using multireference configuration-interaction ab initio methods. Substitutions using groups with pi electron donating, withdrawing and conjugation-extending properties at the C(4) and C(5) positions on the 2P system give predictive trends for the first three singlet excited-state energies. The S(1) pipi* energies of 2P derivatives involving C4 substitution vary linearly with the Hammett substituent parameter sigma(P)+. Cytosine is shown to have the highest bright pipi* energy of the 2P derivatives presented, with that energy being strongly dependent on the position, orientation, and geometry of the C4-amino. A simple description of the predictive energetic trends for the bright pipi* energies using frontier molecular orbital theory is presented, based on the character of the HOMO and LUMO orbitals for each derivative. The results of this study expand the current understanding of the photophysical behavior of the DNA pyrimidine bases and could be useful in the design of analogs where particular spectral properties are desired.

Theoretical Prediction of the Hydride Affinities of Variousp- ando-Quinones in DMSO

The hydride affinities of 80 various p- and o-quinones in DMSO solution were predicted by using B3LYP/6-311++G (2df,p)//B3LYP/6-31+G* and MP2/6-311++G**//B3LYP/6-31+G* methods, combined with the PCM cluster continuum model for the first time. The results show that the hydride affinity scale of the 80 quinones in DMSO ranges from -47.4 kcal/mol for 9,10-anthraquinone to -124.5 kcal/mol for 3,4,5,6-tetracyano-1,2-quinone. Such a long scale of the hydride affinities (-47.4 to -124.5 kcal/mol) indicates that the 80 quinones can form a large and useful library of organic oxidants, which can provide various organic hydride acceptors that the hydride affinities are known for chemists to choose in organic syntheses. By examining the effect of substituent on the hydride affinities of quinones, it is found that the hydride affinities of quinones in DMSO are linearly dependent on the sum of the Hammett substituent parameters sigma: DeltaGH-(Q) approximately -16.0Sigmasigmai - 70.5 (kcal/mol) for p-quinones and DeltaGH-(Q) approximately -16.2Sigmasigmai - 81.5 (kcal/mol) for o-quinones only if the substituents have no large electrostatic inductive effect and large ortho-effect. Study of the effect of the aromatic properties of quinone on the hydride affinities showed that the larger the aromatic system of quinone is, the smaller the hydride affinity of the quinone is, and the decrease of the hydride affinities is linearly to take place with the increase of the number of benzene rings in the molecule of quinones, from which the hydride affinities of aromatic quinones with multiple benzene rings can be predicted. By comparing the hydride affinities of p-quinones and the corresponding o-quinones, it is found that the hydride affinities of o-quinones are generally larger than those of the corresponding p-quinones by ca. 11 kcal/mol. Analyzing the effect of solvent on the hydride affinities of quinones showed that the effects of solvent (DMSO) on the hydride affinities of quinones are mainly dependent on the electrostatic interaction of the charged hydroquinone anions (QH-) with solvent (DMSO). All the information disclosed in this work should provide some valuable clues to chemists to choose suitable quinones or hydroquinones as efficient hydride acceptors or donors in organic syntheses and to predict the thermodynamics of hydride exchange between quinones and hydroquinones in DMSO solution.

Positional Effect on the NMR Spectroscopy of Esters and Amides of 2- and 3-Furancarboxylic Acids

Eleven derivatives of esters and amides of 2- and 3-furancarboxylic acids and their NMR spectra were obtained in DMSO-d 6 . The spectra of esters were also obtained in chloroform-d. The chemical shift values show good correlation with the Hammett substituent parameters. The slopes, however, show contrasting phenomena depending on the position in the furan ring.