Para-substituted Styrenes Research Articles

Relative stabilities of trans-dichloro(olefin)(pyridine)platinum(II) complexes. Re-examination of hammett equation for para-substituted styrene complexes

Relative solution stabilities of several olefin (L) complexes, trans-[PtCl2(py)L](1)(py = pyridine) have been measured by means of 1H n.m.r. spectroscopy by observing two separate methyl proton singlets of free CH2CHC6H4Me-o and (1; L = CH2 CHC6H4Me-o). Data for (1; L = CH2CHC6H4Y-p, Y = NMe2, OMe, Me, H, Cl, or NO2) show the higher stability for the more electron-donating olefin complex, with the Hammett ρ+ value being –0.82. This is consistent with previous 13C n.m.r. and structural trends which suggest olefin-to-Pt σ donation to be much more important than π backbonding in determining the stability of (1).

Carbon-13 chemical shifts as a probe for conformational and electronic effects in alkyl vinyl ethers

The principle of additivity of substituent chemical shifts (SCS) is applied to the carbon-13 chemical shifts of β carbons (δβ) in a number of 1,1-disubstituted ethylenes and propenes, which includes some α-substituted alkyl vinyl ethers. An additivity relationship is observed for 34 compounds that indicates an independent polarization mechanism across the double bond for each geminal substituent. Positive deviations from additivity are observed for compounds bearing bulky substituents. The magnitude of deviations is taken as an indication of the severity of steric interactions among substituents.Reductions in efficiency of transmission of substituent effects in ring substituted α-phenyl alkyl vinyl ethers, relative to para-substituted styrenes, are interpreted as the result of a variable dihedral angle between the ring and vinyl group. An explanation of the relationship between conformation and transmission of substituent effects is presented as an alternative to existing views. An angular dependence of π-polarization and resonance effects is consistent with the chemical shift data.

Reaction paths in the cyclopalladated NN-dialkylbenzylamine–substituted styrene system in acetic acid as solvent. The structure of palladated 2-dialkylaminomethylstilbenes

The reaction between di-µ-chlorobis(NN-dialkylbenzylamine-2C,N)dipalladium(II)(1) and a para-substituted styrene at 50°C in 1 : 1 benzene–acetic acid affords 2-dialkylaminomethylstilbene (2) and its ring-palladated derivative di-µ-chlorobis(2-dialkylaminomethylstilbene-6C′,N)dipalladium(II)(3). The structure of (3) was established on the basis of 1H n.m.r. measurements. The relative amounts of (2) and (3) formed are strongly dependent both on the atmosphere and the presence of alkali-metal perchlorates. Free (2) dominates under argon, but (3) under oxygen. The salts MClO4(M = Na, Li) accelerate the reaction markedly. Formation of (2) is a Fujiwara–Moritani or Heck-type reaction, but at least two paths contribute to the appearance of (3) : (i) direct metallation of (2) by reoxidized palladium(II) and (ii) an exchange reaction between (1) and (2) affording (3).

Kinetics and mechanism of vinylation of ortho-palladated NN-dialkylbenzylamines by para-substituted styrenes

The kinetics of vinylation of ortho-palladated NN-dialkylbenzylamines (1a–g) by CH2CHC6H4R3-p(R3= H, Cl, Br, Me, and MeO) to from the respective 2-dialkylaminomethylstilbenes (2) has been studied mainly at 30°C in acetic acid solvent. The reaction follows second-order kinetics: d[(2)]/dt=k2[(1)][styrene]. The rate constants k2 increase markedly in the presence of both alkali-metal perchlorates (MClO4) and small concentrations of perchloric acid, k2 being directly proportional to [HClO4]. It is shown that the salt effect originates from the solvolytic reaction MClO4+ HOAc ⇌ MOAc + HClO4. Conversion of dimer (1a) into the respective monomer (3) stops the reaction, but the latter species readily forms π-complexes with styrenes. The stability constants of 1 : 1 π-complexes have been determined and their Hammett correlation with a slope of –1.87 is established. For a series of substituted styrenes lgk2 is the linear function of Hammett σp values with a slope ca.–1. For a series of ring-substituted complexes (1) lgk2 correlates with the pKa of the parent benzylamines with a slope of 0.98. The results are interpreted in terms of a mechanism involving intermediate protonation of (1), π-co-ordination of alkene, and subsequent rate-determining intramolecular insertion of styrene into a Pd–C bond through a four-centred transition state.

Synthese, eigenschaften und carbonyl-insertions-reaktionen von tetracarbonyleisen(0)-komplexen para-substituierter styrole

The Fe(CO) 4 η 2-complexes of para-substituted styrenes (substituent  H, CH 3, OCH 3, Cl, F) can be obtained in good yields. Complexation changes drastically the chemical shifts and coupling constants of the vinyl protons. Reaction of the complexes with one mole equivalent of bromine in methanol liberates styrenes, but using two equivalents esters and diesters are formed via intermediate complexes of iron(III). For these carbonyl insertion reactions no systematic influence of the substituent on yield and product distribution was observed.

Frontier-controlled cycloaddition reactions of cyclopentadienones having electron-donating or -attracting substituents: configuration of adducts and kinetic studies

The stereochemistry of the main adducts resulting from the cycloadditions of 2,5-dimethoxycarbonyl- and 2,5-diethyl-3,4-diphenylcyclopentadienones [CPC (1a) and EPC (1b)] with para-substituted styrenes (2a–h), norbornadiene (7), norbornene (8), 1,4-dihydro-1,4-epoxynaphthalene (9), maleic anhydride (10), and N-phenylmaleimide (11), was established as having the endo configuration. In order to define the substituent effects on the rate of the Diels–Alder reaction, a kinetic study of the reactivities of (1a, b) with a series of styrenes (2a–h) was carried out. The log k/kH values for the cycloadditions of (1a) with (2a–h) were correlated with Okamoto–Brown's σp+ constants and the ρ value was –0.941. This result suggests that the cycloaddition is classified as a Diels–Alder reaction with inverse electron demand. By contrast, in the correlation for the cycloaddition of (1b) with (2a–h), two lines with markedly different slopes were observed, which can be ascribed to a Diels–Alder reaction with neutral electron demand. The initial cycloadditions of (1a) and (1b) with tropone (29) gave the [4 + 6]π adducts (32a,b). Subsequently the adduct (32b) underwent oxy-Cope rearrangement to (35). The cycloadditions of (1a) and (1b) with 2-chlorotropone also gave oxy-Cope-type rearrangement products (34a,b).

A study of the steric effect in the radical copolymerization of para-substituted stilbenes with styrene

The specific optical exhaltation has been measured for a number of para-substituted stilbenes. The behaviour of two series of monomers ( para-substituted stilbenes and para-substituted styrenes) has been studied in copolymerization with styrene. The changes in the specific optical exhaltation, EΣ DX and in the dipole moment μ for these two series have been compared in the way in which they depend on the σ-constantts of the substituents. It has been shown that EΣ DX and are interconnected with the copolymerization constants and the steric effect in the copolymerization of para-substituted stilbenes with styrene has been quantitatively assessed by means of correlation equations.

13C NMR spectra of styrene derivatives: An undergraduate experiment involving the application of the Hammett equation

The basis for this experiment is the observation that for meta- and para-substituted styrene derivatives the magnitude of the 13C shielding value for the beta-carbon is linearly related to the alpha for the ring substituent.

Metabolism in vitro of para-substituted styrenes: Kinetic observations of substituent effects

The metabolism of six para-substituted styrene derivatives ( p-XC 6 H 4CH CH 2; X = CN, Br, Cl, Ph, CH 3, CH 3O) was studied in vitro using rat liver microsomes. With p-methyl-, p-methoxy- and p-phenylstyrene it was shown that metabolic attack was completely confined to the vinyl group (as is the case with styrene itself); aromatic hydroxylation, benzylic hydroxylation, and O-demethylation could not be detected. All six styrenes gave a type 1 difference binding spectrum with microsomes in vitro, and the role of cytochrome P450 in the metabolism of p-phenylstyrene was further confirmed by its inhibition by CO and stimulation by phenobarbital pretreatment. The spectral dissociation constants ( K s ) for the styrenes showed no dependence on the nature of the substituents and were confined to a narrow range of values, each being close to the K m value observed for the metabolism of that substrate. While the latter varied slightly more than K s , there was again no regular dependence on the properties of the substituent. Similarly the V m values obtained for these substrates showed no dependence on the nature of the substituents, which is in contrast to previously observed substituent effects on aromatic hydroxylation by P450 enzymes. These results, as well as related examples from other studies, are discussed in terms of chemical selectivity and mechanistic versatility on the part of a single multi-potential oxygen atom-transfer reagent generated by P450 enzymes.

Carbon-13 nuclear magnetic resonance study of the styrene–platinum(II) bond

13 C N.m.r. and i.r. spectroscopic studies of a series of para-substituted styrene–platinum(II) complexes indicate a significant ionic contribution to the styrene–platinum bond.

Relative Reactivities in the Addition of Free Trichloromethyl Radicals to Substituted Styrenes. An Attempt to Separate Polar and Resonance Effects

Abstract Relative reactivities in dibenzoyl peroxide-catalyzed addition of bromotrichloromethane to nine nuclearsubstituted styrenes have been investigated with unsubstituted styrene. The effect of meta substituents upon the relative rates was correlated with the simple Hammett equation using either σ or σ+ constants. The relative rates of para-substituted styrenes, however, did not obey the simple Hammett relation, with the results best explained on the basis of polar (ρσ+) and resonance (ED) terms. All para substituents examined increase the reactivity. log(k⁄kH)=ρσ++ED The ED term shows a striking correlation with logQ, of the corresponding para styrene in Alfrey-Price′s Q-e scheme of free-radical copolymerization. That the polar term originates in charge-transfer interactions between the olefin and the electrophilic trichloromethyl radical and can be expressed by ρσ+ is further supported by the evidence that charge-transfer energies of substituted styrenes showed good correlation with σ+. Factors influencing the reactivity have been discussed.

The polymerization of substituted styrenes by butyl lithium—I. The initiation reaction

The initiation of some substituted styrenes by butyl lithium has been studied. In hydrocarbon solvents the rates are given by R 1= k[M][BuLi] n where n lies between 1 4 and 1 6 for benzene solutions and is approximately 0·57 for n-hexane solution. In benzene solution the relative rates of initiation of the meta- and para-substituted styrenes give a linear Hammett plot of slope +1·0 with the exception of certain monomers which undergo side reactions. The addition of ethers to systems in hydrocarbon solvents increases the rate of initiation in a manner which may be accounted for by assuming the formation of mono and dietherate complexes of the catalyst.

Reactivity of para-substituted styrenes, containing elements of group IV B, in the addition of thiocyanogen

The reactivity of para-trialkylheteroelement-substituted (elements of Group IV B) styrenes in the addition of thiocyanogen is arranged in the following series depending on the nature of the elements: Sn>C>Ge>Si.

Spectrophotometric Investigations of Organic Complexes from Copolymerization Monomers: the Instantaneous Complex Formation of Parasubstituted Styrenes and Maleic Anhydride1

Spectrophotometric Investigations of Organic Complexes from Copolymerization Monomers: the Instantaneous Complex Formation of Parasubstituted Styrenes and Maleic Anhydride¹