Disubstituted Stilbenes Research Articles

Aerobic Copper-Catalyzed Intramolecular Cascade Oxidative Isomerization/[4+4] Cyclization of 2,2'-Disubstituted Stilbenes.

An aerobic copper-catalyzed cascade oxidative isomerization/[4+4] cyclization of 2,2'-disubstituted stilbenes is described. Under the mild CuCl/DBED/air catalytic system, various 5,10-heteroatom-containing tetrahydroindeno[2,1-a]indenes were efficiently prepared through the difunctionalizations of alkenes in a highly atom economic manner. Mechanistic investigations suggested the bicyclic product was likely formed through a sequence of rapid single-electron oxidation/[4+4] cyclization from 2,2'-disubstituted stilbene. The antarafacial manner of the thermally allowed [4+4] cyclization was further proven by series of control experiments and density functional theory calculations. Our findings provide an important addition to the aerobic copper-catalyzed oxidative cyclization.

Determining the excited‐state substituent constants of ortho‐substituents from 2,4’‐disubstituted stilbenes

AbstractThere are mainly three kinds of substituent electronic effects derived from aromatic rings: ground‐state, radical‐state, and excited‐state effect. However, excited‐state substituent constants of ortho‐substituents have not been reported up to now. In this paper, a method for extracting the excited‐state substituent constant of ortho‐substituents was provided. Forty‐nine 2,4’‐disubstituted stilbenes were synthesized, and the ultraviolet absorption spectra of the synthesized stilbenes were determined. The values of ortho‐substituents involving MeO, Me, F, Cl, Br, CF3, and CN were quantified by curve fitting method based on the measured ultraviolet absorption spectra. The rationality of the values were further experimentally confirmed by measuring the ultraviolet absorption spectra of other 10 samples of synthesized 2,3’‐disubstituted stilbenes and by applying these values in regression analysis of the ultraviolet absorption spectra for a complicated data set of 284 organic compounds.

The functional group on (E)-4,4′–disubstituted stilbenes influences toxicity and antioxidative activity in differentiated PC-12 cells

This work probes the relationship between stilbene functional group and biological activity. The biological activity of synthesized stilbenes (E)-4,4′-dicyanostilbene, (E)-4,4′-diacetylstilbene, (E)-4,4′-diaminostilbene, a novel stilbene, 1,1′-(vinylenedi-p-phenylene)diethanol, and (E)-stilbene was assessed at biologically relevant nanomolar concentrations using the MTS cell viability assay in differentiated PC-12 cells under optimal culture conditions and conditions of oxidative stress. Under optimal culture conditions the synthesized stilbene derivatives were found to be non-toxic to cells at concentrations up to 10μg/ml. To mimic oxidative stress, the activity of these stilbene derivatives in the presence of 0.03% H2O2 was investigated. Stilbene derivatives with electron-withdrawing functional groups were 2–3 times more toxic than the H2O2 control, indicating that they may form toxic metabolites in the presence of H2O2. Fluorescence data supported that stilbene derivatives with electron-withdrawing functional groups, (E)-4,4′-dicyanostilbene and (E)-4,4′-diacetylstilbene, may react with H2O2. In contrast, the stilbene derivative with a strong electron-donating functional group, (E)-4,4′-diaminostilbene, rescued neurons from H2O2-induced toxicity. The DPPH assay confirmed that (E)-4,4′-diaminostilbene is able to scavenge free radicals. These data indicate that the Hammett value of the functional group correlates with the biological activity of (E)-4,4′-disubstituted stilbenes in differentiated PC-12 cells.

Determining the excited‐state substituent constants of meta‐substituent from 3,4'‐disubstituted stilbenes

In this paper, 61 samples of 3,4'‐disubstituted stilbenes and 18 samples of 3,3'‐disubstituted stilbenes were synthesized, and their UV data were measured in anhydrous ethanol. Based on the UV absorption energy (wavenumber) of 3,4'‐disubstituted stilbenes, the excited‐state substituent constants of meta‐substituent were determined by means of curve‐fitting. The availability of was confirmed by the good correlation with the UV absorption energy of 3,4'‐disubstituted stilbenes and 4,4'‐disubstituted stilbenes. Further, using the obtained constants and the correlation equation, we calculated the UV wavenumbers of 3,3'‐disubstituted stilbenes, and the calculated wavenumbers are in good agreement with the experimental values. These results verified that the excited‐state substituent constants of meta‐substituent are reliable parameter to scale the effect of meta‐substituent on the UV absorption energy. Copyright © 2012 John Wiley & Sons, Ltd.

Effects of substituent and solvent on the UV absorption energy of 4,4′-disubstituted stilbenes

Twenty five samples of 4,4′-disubstituted stilbene derivatives were synthesized, and their UV absorption max wavelengths were determined in over 10 kinds of solvents including cyclohexane, ether, chloroform, acetonitrile and ethanol, in which 242 experimental data were recorded. The effects of substituents and solvents on the energy of their UV absorption max wavelengths were discussed. The research results showed: the energy of UV absorption max wavelengths of 4,4′-disubstituted stilbenes was mainly affected by their intramolecular structure (substituent effect) in a given solvent, that is, the energy is dominated by both of excited-state substituent parameter σ CC ex and polar substituent constant σ p. While their energy was dominated by the substituent effect and solvent effect in different kinds of solvents. An equation quantifying the energy of UV absorption max wavelengths of 4,4′-disubstituted stilbenes was developed. In addition, it is found that the n-octanol/water partition coefficient (logP) is more effective than the solvatochromic dye (E T(30)) in scaling the solvent effect. The equation employed the parameter logP has a better correlation and more specific physical meaning. Further, the energies of UV absorption max wavelengths of some reported compounds were predicted by the obtained equation, which are in agreement with their experimental values.

Excited‐state substituent constants σ from substituted benzenes

AbstractIn this work, 13 compounds of 4,4′‐disubstituted stilbenes and 5 compounds of 3‐methyl‐4′‐substituted stilbenes were prepared and their UV spectra were measured. A new substituent effect constant, namely excited‐state substituent constant, was proposed, which was calculated directly from the UV absorption energy data of substituted benzenes. The investigation result shows that the proposed constant is different from the existing polar substituent constants and radical substituent constants in nature. The availability of the new constant was confirmed by the good correlations with the UV absorption energy of four kinds of compounds, 1,4‐disubstituted benzenes, 4,4′‐disubstituted stilbenes, substituted ethenes, and m‐Y‐substituted aromatic compounds. It is expected that the excited‐state substituent constant can be applied in QSPR study on organic compounds at the excited state. Copyright © 2008 John Wiley & Sons, Ltd.

Anodic oxidation of stilbenes bearing electron-withdrawing ring substituents

A number of disubstituted stilbenes bearing either two strong electron-withdrawing groups or one electron-withdrawing and one electron-donating group were synthesized and anodically oxidized in a divided cell in methanol at a carbon anode. A variety of types of products were obtained, most of which have never been observed upon oxidation of alkenes not bearing electron-withdrawing groups. A mechanistic scheme involving 2-methoxy-1,2-diarylethyl cations as key intermediates can account for all of the observed products. The nature of the products from each alkene is strongly correlated with the sum of the Hammett σ + values of the ring substituents.

Photophysical characterization of trans-4,4′-disubstituted stilbenes

Absorption and emission spectra of twenty trans-4,4′-disubstituted stilbenes have been measured in four solvents: cyclohexane (CH), chlorobenzene (CB), 2-butanone (methylethyl ketone, MEK) and dimethylsulfoxide (DMSO) at room temperature. Fluorescence quantum yields ( Φ f) and fluorescence lifetimes ( τ f) have been measured for these stilbenes. The lifetimes and quantum yields of fluorescence were found to be dependent on the donor-acceptor properties of the substituents and correlate with the Hammett ν-constants. In addition, we experimentally observed the appearance of a second emitting state which is close energetically to the lowest exited singlet state 1t ∗ in cases of strong donor-acceptor substituents in polar solvents.

Characterization of cation radical reactions. Aminium salt-catalysed Diels–Alder reactions

Kinetic studies of the Diels–Alder cycloadditions of a series of trans-stilbenes (2) to 2,3-dimethylbuta-1,3-diene (3), catalysed by tris(4-bromophenyl)aminium hexachloroantimonate (1˙+), reveal a curved Hammett–Brown plot, consistent with a mechanistic transition from reversible to rate determining ionization of 2 to 2˙+ as the substituents become progressively more electron donating. Further, substituent effects in mono- and di-substituted stilbenes are essentially multiplicative, describing a distribution of positive charge which is symmetrical with respect to both aryl rings. The symmetry of the charge development is inconsistent with an electrophilic addition to the substrates, proceeding via a carbocation. The combination of the magnitude and symmetry of the positive charge distribution provides strong evidence for a cation radical mechanism.

Second-Order Optical Nonlinearities of Substituted Stilbenes and the Related Compounds Containing a Trifluoromethyl as the Electron-Withdrawing Group

Abstract The second-order optical nonlinearities and transparencies of seventeen 4,4′-disubstituted stilbenes and ten related compounds containing a trifluoromethyl group at the para position, as the electron-withdrawing group, were examined. The introduction of a trifluoromethyl group produces a shorter-wavelength shift of the UV-vis absorption spectra without any decrease in the magnitude of the second-order molecular hyperpolarizability (β), as compared with the corresponding nitro-derivatives. The absorption cutoff (λcutoff) of 4-methoxy-4′-trifluoromethylstilbene (1) was observed at 375 nm, which is by ca. 90 nm shorter than that (463 nm) of p-nitroaniline (p-NA); the β value (12.2 × 10−30 esu) of 1, however, is almost equal to that (12.6 × 10−30 esu) of p-NA. The bromo- or methylthio-substituted stilbenes (12 and 16) with a cyano group on the central double bond show SHG activity. The relative intensities of SHG are 3.0 and 1.5, respectively.

13C NMR spectra of some symmetrically 2,2′-disubstituted stilbenes: conjugational and conformational effects

In the 13C NMR spectra of symmetrically 2,2′-disubstituted cis -stilbenes the decrease of α-effects and increase of γ-effects at ortho -carbons with respect to tarns -isomers were observed. However the most significant differences in substituent effects between trans- and cis -isomers were found for γ-effects, at unsubstituted ortho -C-6 and C-4 atoms. The one bond C-H splitting of olefinic carbons is greater in cis -than trans -stilbenes. The differences in substituent effects and apparent coupling constants are assumed to originate mainly from the different conjugational interactions in cis -and trans -stilbenes. the conformational and configurational differences of these molecules were substantiated by molecular mechanics calculations.

Mutual correlation between nonaqueous oxidation-reduction potentials and electronic spectra, and its extension to describe the substituent effect on electronic spectra by means of substituent constants.

In order to confirm and to extend the scope of the equation (Eoxd1/2-Ered1/2)=k11, 3 EUVho-lu+k2, presented previously by us, the polarographic half-wave reduction (Ered1/2) and oxidation (Eoxd1/2) potentials in nonaqueous media and the electronic spectra of 4, 4'-disubstituted stilbenes have been measured and analyzed by means of the equation with reasonable results. The substituent effect on Eoxd1/2 and Ered1/2 values has been well described by using the Hammett-type substituent constants. This result has been combined with the above equation, and thus a general equation describing the substituent effect on electronic spectra in terms of substituent constants has been derived as EUVho-lu=aF+bR+c, F and R being Swain and Lupton's substituent constants. This equation has been experimentally examined using the strong longest-wavelength π-π* bands of 4, 4'-disubstituted stibenes and azoxybenzenes. The results were quite reasonable. Finally, the description of the substituent effect on electronic spectra in terms of substituent constants is discussed on the basis of molecular orbital theory.

Wittig‐Reaktionen mit Cyanhydrinen sowie Redoxreaktion des 4‐Nitromandelsäurenitrils

Wittig Reactions of Cyanohydrines and Redox Reaction of 4-Nitromandelonitrile Substitued mandelonitriles 5 (R1  H, OCH3, NO2) react in the presence of sodium tert- butoxide as a base like the corresponding aldehydes with substituted benzylidenetriphenylphosphoranes via the Wittig reaction to yield 4,4′-disubstituted stilbenes 8, for which the (E)/(Z) stereoisomer ratios are given. Our results refer to a reaction with a preceding cyanohydrine-aldehyde equilibrium. 4-Nitromandelonitrile (5a) forms in a basic methanolic solution via a redox reaction a mixture of dimethyl 4,4′-azoxybenzenedicarboxylate (11) and methyl 4-nitrobenzoate (12).

Effect of ortho substituents in the electron impact induced fragmentation of 2,2′‐disubstituted stilbenes

AbstractThe effect of ortho substituents NH2, OCH3, CH3, Cl and NO2 on the fragmentation of five symmetrically and five unsymmetrically 2,2′‐disubstituted stilbenes under electron impact was investigated. The fragmentation patterns deduced were supported by metastable transitions in the first and second field free regions and by exact mass measurements of prominent ions. In general, the fragmentation was found to be in accord with that of stilbene and the corresponding monosubstituted stilbenes. There are, however, some deviations from the general fragmentation scheme caused by direct through‐space interactions of the ortho, ortho′ substituents with concomitant loss of neutral fragments. It is supposed that the formation of 7‐membered cyclic or heterocyclic ions is the result of such through‐space reactions.

ChemInform Abstract: The Preparation of Some Unsymmetrically 2,2′‐Disubstituted Stilbenes.

AbstractDie 2,2′‐ asymm. disubstituierten Stilbene (III) werden nach Wittig aus den Phosphoniumbromiden (I) und den Aldehyden (II) dargestellt, wobei (IIIa)‐(IIIc) als cis/trans‐Gemische vorliegen.

The Preparation of Some Unsymmetrically 2,2′ -Disubstituted Stilbenes

The new unsymmetrically 2,2′-disubstituted stilbenes with NO2, F, Cl, Br, OCH3, CH3 and NH2 groups as substituents have been prepared by WITTIG or PERKIN reactions. The compounds 1-8 were prepared due to the studies of fragmentation on electron impact, photochemistry and photoelectron spectroscopy. Their geometrical configuration has been established by 1H NMR and IR spectra.

Some Observations on the Reactions of α,α′-Dimethylstilbene and Related Compounds

Abstract Cis- and trans-α,α′-Dimethylstilbene, heated with a little sulfuric acid at 210°, were isomerized to an equilibrium mixture consisting of 55% of the trans and 45% of the cis isomers. Heated with iodine at 210°, they yielded meso-2,3-diphenylbutane. The action of bromine produced 1,4-dibromo-2,3-diphenyl-2-butene; under certain experimental conditions the concurrent cis-trans isomerization was observed. This fact, together with the isolation of a small smount of α–bromoacetophenone as a by-product, supports the view that the reaction proceeds by a mechanism involving bromine atoms. The action of bromine on some α,α′-disubstituted stilbenes was examined. The reactions observed were discussed in terms of the steric effect exerted by the bulky groups at the ethylenic linkage.