Polar Substituent Effects Research Articles

Oxidation of alcohols by dimethyldioxirane

The kinetics of oxidation of a series of monoatomic alcohols (methanol, 2-propanol, 2-butanol, 2-methyl-1-propanol, 2-chloroethanol, 1,3-dichloro-2-propanol, benzyl alcohol), hydroxyacetic acid, and 1,3-butandiol (ROH) by dimethyldioxirane (1) was studied. The reaction kinetics obeys the second order equationw=k[ROH][1]. The rate constants were measured in the range of 7–50†C, and the activation parameters were found. To describe the reaction rate constants as a function of the ROH structure, the two-parametric Taft equation was used, which takes into account both the polar and resonance substituent effects. Alcohol oxidation produces the corresponding carbonyl compounds,viz., ketones from secondary alcohols and aldehydes from primary alcohols, in yields of at least 80%. The results were explained by the competition of the molecular (oxenoid) and radical mechanisms. The introduction of electron-withdrawing substituents into the alcohol molecule increases the contribution of the radical channel of the reaction.

Influence of the olefin structure on the rate of hydroformylation and C=C bond hydrogenation in the presence of Co2(CO)8 as the catalyst precursor

The rate of C=C bond hydrogenation and the yield of paraffins increase in the order 1-hexene < propylene dimers < propylene trimers < propylene tetramers. The kinetic equation of the reaction was derived on the basis of the Breslow-Heck mechanism. The equation is consistent with the experimental data. The ratio between the rates of hydroformylation and hydrogenation correlates to the polar and steric effects of substituents in olefin molecules.

Transmission of polar substituent effects across the bicyclo[1.1.1]pentane ring system as monitored by19F NMR shifts

A series of 3-substituted(X)bicyclo[1.1.1]pent-1-yl fluorides (1) together with a more limited series of 3-substituted(X)bicyclo[1.1.1]pent-1-yl trifluorides (4) were synthesized and their 19F and 13C NMR spectra were recorded. Correlation of the 19F substituent chemical shifts (SCS) of 1 against the 19F SCS of 4-substituted(X)bicyclo[2.2.2]oct-1-yl fluorides (5), together with a linear multiple regression analysis of the SCS against polar substituent parameters, provide evidence for the importance of strong electronegativity effects (σχ) in 1. The shift pattern for 3 is also more consistent with a dominant substituent electronegativity influence than an electrostatic field effect (σF). Most importantly, the signs of the 19F SCS of 1 and 4 are diametrically opposed. The origin of the σχ effect in 1 is ascribed to hyperconjugative and homohyperconjugative interactions governing the degree of electron population of the σ* orbital of the C– F bond. By contrast, the perturbation of the fluorine π-electron density appears to be the pertinent molecular parameter in 4. The 1J(C,F) values of 1 were not subject to any sensible empirical analysis. Copyright © 2000 John Wiley & Sons, Ltd.

Use of competition kinetics with fast reactions of grignard reagents

Competition kinetics are useful for estimation of the reactivities of Grignard reagents if the reaction rates do not differ widely and if exact rates are not needed. If the rate of mixing is slower than the rate of reaction, the ratios between the rates of fast and slow reagents are found to be too small. This is concluded from experiments in which results obtained by competition kinetics are compared with results obtained directly by flow stream procedures. A clearer picture of the reactivity ratios is obtained when the highly reactive reagent is highly diluted with its competitor. A fast reagent may account for almost all the product even when present as only 1 part in 100 parts of the competing agent. In this way allylmagnesium bromide is estimated to react with acetone, benzophenone, benzaldehyde, and diethylacetaldehyde ca. 1.5 x 10(5) times faster than does butylmagnesium bromide. The rates found for the four substrates do not differ significantly, and it seems possible that there is a ceiling over the rate of reaction of this reagent, for example, caused by diffusion control. This may explain that competition kinetics using allylmagnesium bromide have failed to show kinetic isotope effects or effects of polar substituents with isotopically or otherwise substituted benzophenones. A recently reported alpha-deuterium secondary kinetic isotope effect for the reaction of benzaldehyde with allylmagnesium bromide was observed at -78 degrees C, but was absent at room temperature. It is suggested that the reaction of benzophenone and benzaldehyde with allylmagnesium bromide has a radical-concerted mechanism since no radical-type products are produced and since no color from an intermediate ketyl is observed even at -78 degrees C.

Polar and Steric Substituent Effects in 5-exo-trig Cyclizations of 1-Aryl-4-penten-1-oxyl Radicals

A series of 1-aryl-substituted 4-penten-1-oxyl radicals 2 were generated from parent N-alkoxypyridinethiones 6. The intermediates 2 undergo 5-exo-trig cyclizations which are sensitive to polar and steric substituent effects. Thus, para groups in the aromatic ring alter the reactivity of radicals 2c–e by one order of magnitude without changing the stereochemical course of the 5-exo-trig reaction (cis:trans = 50:50). Substitution of the ortho methyl groups for hydrogen atoms of the phenyl group affects the cis-ring closures more than the trans reactions which lead to trans-selective alkoxyl radical reactions. Methyl groups at the terminal olefinic double bond cause a significant acceleration of K5-exo in general and of the trans reactions in particular. Thus, trans-2-isopropyl-5-phenyltetrahydrofuran (8i) was the major product in the photoreaction of pyridinethione 6i in the presence of tri-n-butyltin hydride (98% yield, cis:trans, 30:70).

Polar and Steric Substituent Effects in 5-exo-trig Cyclizations of 1-Aryl-4-penten-1-oxyl Radicals

By mistake wrong concentrations were given in the footnote of Table 1 and in the Discussion. Instead of c0 = 1.2 M read c0 = 0.12 M if 2-naphthalenethiol was used as hydrogen donor and instead of c0 = 1.8 M read c0 = 0.18 M if Bu3SnH was used to trap photoproducts. In Table 2 relative rate constants of radical 2d refer to radical 2e and vice versa as correctly stated in the Discussion. In the Experimental Section (Iodocyclization) read 2.25 g (8.86 mmol) of iodine instead of 2.25 g (1.10 mmol) of iodine.

Correlation analysis of UV spectral data of some phenylhydrazones and semicarbazones by the dual-parameter equation. Observation of three types of behaviors of the ?max values induced by the polar effects of substituents

The K-band λmax values of the UV spectra of five types of p-Y-benzaldehyde derivatives, namely phenylhydrazones (1-Ys), 2,4-dinitrophenylhydrazones (2-Ys), 4-nitrophenylhydrazones (3-Ys), semicarbazones (4-Ys) and N-phenylsemicarbazones (5-Ys), were measured and the corresponding wavenumbers (νmax) calculated. Correlation analyses of the νmax values by the dual-parameter equation show that the νmax values of these compounds are affected, albeit to different degrees, by both spin-delocalization effects (σ˙) and polar effects (σx) of the substituents. The spin-delocalization effects of substituents always facilitate bathochromic shifts of the λmax values (ρ˙ negative). However, polar effects of the substituents on these λmax data depend on the mode of polarization of these hydrazone or semicarbazone molecules, i.e. electron-pair acceptor substituents (Ys) facilitate bathochromic shifts for 1-Ys (ρx negative), but hypsochromic shifts for 2-Ys and 3-Ys (ρx positive), and induce very small shifts for 4-Ys and 5-Ys. As expected, donor Ys facilitate hypsochromic shifts for 1-Ys and bathochromic shifts for 2-Ys and 3-Ys and induce very small shifts for 4-Ys and 5-Ys. Possibly, a higher degree of polarization of the substrate molecule would demand a higher degree of polar assistance from the substituents at the transition states. © 1998 John Wiley & Sons, Ltd.

Photochemical C−C Bond Cleavage of 1,2-Diarylcyclopropanes Bearing an Acetylphenyl Group. Generation and Observation of Triplet 1,3-Biradicals

The photochemical properties of 1,2-diarylcyclopropanes bearing an acetylphenyl group were studied by product analysis and laser flash photolysis. All cyclopropanes showed efficient cis−trans photoisomerization followed by inefficient isomerization to 1,3-diarylpropenes. All the products were unquenched by the addition of triplet quencher, 2-methyl-1,3-butadiene (ET ≃ 60 kcal mol-1), whereas molecular dioxygen gave oxygenated products. Triplet 1,3-biradicals generated from a short-lived acetophenone-like triplet (≲1 ns) were observed as intermediates in these reactions through nanosecond laser flash photolysis. Polar substituent effects on the lifetime of the 1,3-biradicals were small, except for a heavy atom effect in the case of Br. Spin−orbit coupling calculations on model 1,3-biradicals show a negligible effect on polar substituent and thus predict a negligible effect on the intersystem crossing rate. Conjugated biradicals in general now seem unlikely to show the “ionic character effect” on intersyste...

Transmission of polar substituent effects in the adamantane ring system as monitored by19F NMR

An extensive series of (E)/(Z)-5-substituted(X)adamant-2-yl fluorides (2 and 3, respectively) and (E)/(Z)-4-substituted(X)adamant-1-yl fluorides (4 and 5, respectively) were synthesized and characterized and their 19F chemical shifts measured in several solvents. Correlation of the 19F substituent chemical shifts (SCS) against polar field parameters (σF) together with comparisons against the 19F SCS of 4-substituted(X)bicyclo[2.2.2]oct-1-yl fluorides (1) provide unequivocal evidence for the importance of electrostatic field and ‘through-three-bond’ electron delocalization (double hyperconjugation) effects as long-range modes of transmission of polar effects in these saturated systems. The former effect is shown to be a function not only of spatial factors (angles and distance) but also the ‘stiffness’ of the C—F σ bond. The latter electronic mechanism is clearly the dominant factor regulating the 19F SCS of 2 and 4 but is switched ‘off’ in 3 and 5 as a result of specific stereoelectronic requirements. ©1998 John Wiley & Sons, Ltd.

Proton affinity of some substituted 2,6-dimethylbenzamides and their N,N-dimethyl derivatives: the susceptibility of the amide group to polar substituent effects

The proton affinity (PA) of 2,6-dimethylbenzamide and its derivatives substituted by an amino or a nitro group at the para- or meta-position, as well as of the corresponding N,N-dimethylbenzamides, have been determined by the kinetic method using bracketing by a pair of reference bases B-i which give a slightly more or less intense signal of the protonated base [BHi](+) compared with that of the protonated amide [AH](+), The amide group of the 2,6-dimethylbenzamides is twisted out of the plane of the aromatic ring by the methyl substituents at both ortho-positions which interrupts the pi-electron conjugation between amide group and benzene ring. The amino group and the nitro group have been chosen as prototypical polar substituents with electron donating and with electron withdrawing properties to reveal the effect of steric hindrance of pi-conjugation on the PA of benzamides, A comparison with the PA of the corresponding sterically unhindered benzamides shows that this effect is generally small and manifests itself clearly only in the case of para-amino substituents. This result is discussed considering different conformations of the benzamides using semi-empirical PM3 calculation. The absolute PA values calculated by PM3 agree only modestly with the experimental values, but the calculated conformation and the relative PA values corroborate a weak interaction between amide group and benzene ring as suggested by experiments. Finally, the modest susceptibility of the amide group and protonated amide group to polar substituent effects in the gas phase agrees with comparable small substituent effects observed on the pK(a) in solution and shows that the amide group is singular with respect to a resonance saturation.

Through-space transmission of polar substituent effects in the bicyclo[1.1.1]pentane ring system as monitored by119Sn NMR chemical shifts: a13C and119Sn NMR study of 3-substituted-(X)bicyclo[1.1.1]pent-1-yltrimethylstannanes

A series of 3-substituted(X)bicyclo[1.1.1]pent-1-yltrimethylstannanes (3) were synthesized and their 119Sn and 13C NMR spectra were recorded. The 119Sn substituent chemical shifts (SCS) and the one-bond carbon–tin coupling constants [1J(13C,119Sn)] were analyzed in terms of possible substituent–probe interactions. The study clearly reveals that the 119Sn NMR parameters of 3 are remarkably sensitive to modulation of the through-space back- or rear-lobe interactions between the bridgehead bond molecular orbitals by the electronic character of the substituent. An extraordinarily large four-bond tin–tin coupling constant in 3 (X=SnMe3) highlights the significance of through-space bridgehead–bridgehead interactions in this system. © 1997 John Wiley & Sons, Ltd.

Localized triplet diradicals as a probe for electronic substituent effects in benzyl-type radicals: The ΔD scale

Abstract

ChemInform Abstract: Polar and Steric Substituent Effects in 5‐exo‐trig Cyclizations of 1‐ Aryl‐4‐penten‐1‐oxyl Radicals.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Polar Substituent and Solvent Effects on the Kinetics of Radical Reactions with Thiols.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Polar and Steric Substituent Effects in 5‐exo‐trig Cyclizations of 1‐Aryl‐4‐penten‐1‐oxyl Radicals

AbstractA series of 1‐aryl‐substituted 4‐penten‐1‐oxyl radicals 2 were generated from parent N‐alkoxypyridinethiones 6. The intermediates 2 undergo 5‐exo‐trig cyclizations which are sensitive to polar and steric substituent effects. Thus, para groups in the aromatic ring alter the reactivity of radicals 2c–e by one order of magnitude without changing the stereochemical course of the 5‐exo‐trig reaction (cis:trans = 50:50). Substitution of the ortho methyl groups for hydrogen atoms of the phenyl group affects the cis‐ring closures more than the trans reactions which lead to trans‐selective alkoxyl radical reactions. Methyl groups at the terminal olefinic double bond cause a significant acceleration of K5‐exo in general and of the trans reactions in particular. Thus, trans‐2‐isopropyl‐5‐phenyltetrahydrofuran (8i) was the major product in the photoreaction of pyridinethione 6i in the presence of tri‐n‐butyltin hydride (98% yield, cis:trans, 30:70).

Polar substituent and solvent effects on the kinetics of radical reactions with thiols

The rates of thiol trapping of radicals depend upon polar substituents (Table 1) and solvent effects (Table 2). The rate accelerating effect of water on the reactions of biologically relevant radicals is of special importance.

Reactivity of nitroso oxides: effect of polar substituents and reaction mechanism

The structure and reactivity of nitroso oxide intermediates has been studied by trapping and tracer experiments in the photooxidation of substituted phenyl azides. O-Transfers with p-methoxyphenyl nitroso oxide were efficient and showed the reactivity order of PhNO > Ph2S > Ph2SO > PhH. Sulfides and sulfoxides were oxidized electrophilically and the partial rate factor for the hydroxylation of benzenes afforded a negative ρ-value of –1.40 (σ+). In the case of the p-mtrophenyl isomer, the radical reactivity was much more significant as exemplified in the hydrogen abstraction from toluene. Both isomers behave as electrophilic radicals as shown by the relative rates of hydrogen abstraction from aliphatic C–H bonds, i.e., 1°:2°:3°= 1:7:50–60. An 18O-tracer study revealed that the intramolecular cyclization of aryl nitroso oxides is competitive with intermolecular O-transfers.

The aminolysis and hydrolysis of N-(4,6-diphenoxy-1,3,5-triazin-2-yl) substituted pyridinium salts: concerted displacement mechanism

Reaction of the title pyridinium salts in aqueous buffers containing amines involves nucleophilic displacement of the pyridine by the amine nitrogen in addition to background hydrolysis. The second-order rate constants for attack of a series of unhindered primary amines on the (4,6-diphenoxy-1,3,5-triazin-2-yl)pyridinium cation obey a Bronsted-like equation with slope βnuc= 0.68. Six-membered-ring cyclic secondary amines have a similar Bronsted selectivity but are approximately 10-fold more reactive than primary amines of similar pKa.The reactivity of ethanolamine against the title pyridinium salts obeys a Bronsted-like equation with βlg=–0.47. In aminolysis of the triazinyl pyridinium salts the changes in bond fission and bond formation signalled by the polar substituent effects give Leffler αlg and αnuc values of 0.38 and 0.54, respectively, which indicate strong coupling between the bonding changes consistent with a concerted process for the displacement reaction. The adduct in the putative stepwise mechanism is considered too reactive to possess a significant barrier for it to exist as a discrete molecule.

Cyclic Voltammetric and E.P.R. Spectroscopic Studies on Apical Substituent Effects in Radical Anions of 9-Substituted and 9,10-Disubstituted Nitroethanoanthracenes

Cyclic voltammetry and electron paramagnetic resonance spectroscopy were used to examine apical substituent effects on the properties of Me2SO solutions of the radical anions from 9-substituted and 9,10-disubstituted 2- and 3-nitro-9,10-dihydro-9,10-ethanoanthracenes (1)-(24). The reductions of the nitro group are, in general, reversible at 100 mV s-1 and at 20°, except where there are coupled intra-or inter-molecular electron or proton transfer reactions with aliphatic bridgehead substituents, such as a carboxylic acid or iodine. The substituent effects for the meta- and para-nitroethanoanthracene systems are both similar and additive. This similarity in the meta and para polar substituent effects is attributed to the orthogonality of the π*(ArNO2/ArNO2-) orbital with the σ* orbital of the carbon substituent (C-X) groups at the bridgehead positions. Overall, the meta substituent effect was slightly lower than that for corresponding simple meta-nitrobenzyl systems, but the para-nitro systems showed substituent effects that are a factor of 2 smaller than that for corresponding simple para-nitrobenzyl systems. These linear correlations between the substituent effects and redox potentials have been used to estimate the redox potentials of irreversible systems, which are required for digital simulations of reactions involving nitroaryl radical anions. Only small substituent effects are present in hyperfine coupling with the aliphatic and aromatic protons of the nitroaromatic radical anions of ethanoanthracenes (1)-(13), but a clear trend to lower nitrogen hyperfine values was observed with increasing electron-withdrawing ability of the apical substituent. In addition, no spin density was transferred to the benzylic bridgehead substituent in any of the nitro radical anions studied, clearly demonstrating that the bond between the bridgehead substituent and the carbon at a benzylic position is orthogonal to the π-system of the nitroaromatic ring bearing the odd electron.

Transfer Hydrogenation of Nitro‐, Nitroso‐ and Azoarenes by Homolytic Retrodisproportionation

AbstractNitro‐, nitroso‐ and azobenzene are reduced almost quantitatively to aniline when heated to 230–300°C with 9,10‐dihydroanthracene (DHA), xanthene or tetralin. From the effect of polar substituents and polar solvents on the reactivity and from the isotope effect kH/kD ≈︁ 2.4 (280°C), a hydrogen‐atom transfer from the H donor to the acceptor (retrodisproportionation) is proposed as the rate‐determining step. The lower reactivity of xanthene compared with 9,10‐dihydroanthracene eliminates the possibility of a rate‐determining hydride transfer. The observation of an intense ESR signal of 9‐xanthyl radicals during the reaction in diphenyl ether and the typical products support the proposed homolytic mechanism.