Base-catalysed hydrogen-exchange between organogermanium hydrides and methanol

First order rate constants have been determined for the loss of tritium from tritiated germanium hydrides in MeOH/MeONa at 20–40°C. The rate constant for Ph₃GeT (T = ³H) is greater than that for exchange at the 9-position of fluorene under similar conditions. Values of k_rel, the rate relative to that for Ph₃-GeH, for (XC₆H₄)₃GeT compounds at 30°C are: (X =) m-Cl, 380; p-Cl, 71; m-Me, 0.48; p-Me, 0.078; o-Me, 0.081; p-OMe, 0.025; o-Me, 0.018, and there is a good linear correlation between log k_rel and σ. For the compounds (XC₆H₄)-Ph₂GeT the values of k_rel are (X =) p-NO₂, 1280; p-CN, 680; m-Cl, 10; and p-F, 1.1, and a plot of log k_rel against σ^? constants is much better than that against σ constants. The results imply that there is substantial delocalization of charge from the anionic germanium centre into the aromatic rings, and thus na important degree of (pp)_π bonding between the germanium atom and these rings in the anion.The rates of exchange fall on going from Ph₃GeT to Ph₂GeHT, to PhGeH₂T (the values of k_rel being 1, 0.11, and ca. 0.015 respectively), implying an order of kinetic acidities opposite to the reported order of equilibrium acidities in liquid ammonia. Replacement of Ph by Et groups also leads to a fall in kinetic acidity, values of k_rel for EtPh₂GeT and Et₃GeT being 6 × 10^?2 and ca. 3 × 10^?3, respectively. Solvolysis, involving generation of hydrogen, is significant for PhGeH₃ under the reaction conditions, and important for Et₃GeH.The value of the inverse solvent isotope effect, k_MeOD/k_MeOH is ca. 1.7 for both Ph₃GeT and (m-ClC₆H₄)₃ GeT at 20°C. This value, and the activation parameters determined for some of the (XC₆H₄)₃GeT compounds are consistent with a transition state for the rate-determining step in which the hydrogen is roughly half transferred from germanium to methoxide ion, with partial liberation of the methanol molecules initially solvating the latter.     

Efficient microwave-assisted synthesis of 1-aryl-4-dimethylamino methyleno-pyrrolidine-2,3,5-triones

A microwave-assisted synthesis of a series of 1-aryl-4-dimethylaminomethylene pyrrolidine-2,3,5-triones from the cyclocondensation reaction of ethyl 5,5,5-trichloro-3-dimethylamino methylene-2,4-dioxopentanoate with aniline, 3- or 4-substituted anilines [where the substituents = 3-Me, 3-OMe, 3-OH, 4-Me, 4-F, 4-Cl, 4-Br, 4-NO₂, and 4-COMe] is reported. This process is an efficient alternative to the conventional thermal heating and furnishes the products in a short reaction time (ca. 13 min), under mild conditions and in moderate to good yields (50–76%).     

A mechanistic study of the acid- and base-catalysed cleavages of β-trimethylstannylstyrenes

The cleavages of cis- and trans-PhCHCHSnMe₃ in 1/10 AcOD-MeOD have been shown to give cis- and trans-PhCHCHD, respectively. The rates of cleavage of some XC₆H₄CHCHSnMe₃ compounds in 1/10 AcOH-MeOH at 50°C have been measured; there is no significant difference between the rates for cis- and trans-PhCHCHSnMe₃, and the relative rates of cleavage as X is varied are (X  H), 1.0; p-OMe, 7.0; p-Me, 2.3; m-Cl, 0.34; m-Br, 0.36. There is an excellent correlation with σ⁺ constants, with a ρ value of 1.1. The results are interpreted in terms of rate-determining proton transfer to the β-carbon atoms, and it is suggested that acid cleavages of vinylHgX bonds involve analogous mechanisms. PhSnMe₃ is cleaved 20 times as slowly as PhCHCHSnMe₃ in the 1/10 AcOH/MeOH.The rates of cleavage of XC₆H₄CHCHSnMe₃ compounds by a mixture of MeOH (3 vol.) and 2 M aqueous NaOH (2 vol.) have been measured; trans-PhCHCHSnMe₃ is cleaved about 1.3 times as rapidly as its cis-isomer, and about 12 times as rapidly as a mixture of cis- and trans-PhCHCHSnEt₃. The relative rates for the various XC₆H₄CHCHSnMe₃ compounds (mainly trans-isomers) are (X ) H, 1.0; p-OMe, 0.99; p-Me, 0.92; m-Cl, 1.67; m-Br, 1.65. Cleavage of trans-PhCHCHSnMe₃ by NaOD/D₂O/MeOD gives exclusively trans-PhCHCHD. For cleavages in methanolic NaOMe the values of the rate isotope effects, (the ratio k_meOH/k_MeOD) are 2.3–2.6, and those of the product isotope effects, PIE (the product ratio RH/RD on cleavage of RSnMe₃ by NaOMe in 1 : 1 MeOH/MeOD) are 4.5–5.0.The results are interpreted in terms of proton transfer from the solvent to the leaving carbon atom in the rate determining step as the SnC bond breaks as a result of the attack of the base anion at tin in a prior or synchronous process. PhCHCHSnMe₃ is cleaved by the aqueous alcoholic base about 5 times as rapidly as PhSnMe₃.Cleavage of trans-PhCHCHSnMe₃ by PHCOCl in presence of AlCl₃ in CH₂Cl₂ gives trans-PhCHCHCOPh, and cleavage of a 90/10 mixture of trans- and cis-PhCHCHSnMe₃ by bromine in CCl₄ gives a corresponding mixture of trans- and cis-PhCHCHBr.     

Gas phase 1,3-deoxystannylation reactions of γ-substituted organotin alcohols utilizing chemical ionization mass spectrometry

The gas phase 1,3-deoxystannylation reactions of γ-substituted organotin alcohols have been studied by methane and isobutane chemical ionization mass spectrometry. It was found that γ-hydroxybutyltributyltin and γ-hydroxybutyldibutyltin chloride undergo the 1,3-deoxystannylation reaction to a greater extent than the corresponding 1,4-deoxystannylation using the δ-substituted analogues of the above named compounds. This result substantiates the unusual reactivity of γ-substituted organotin alcohols under gas phase protonolysis conditions.The electronic factors affecting the stabilization of the transition state were ascertained with γ-phenyl-γ-hydroxypropyltrimethyltin derivatives, in which the γ-phenyl group was substituted with groups such as H, p-OMe, p-Me, p-Cl, p-F, m-OMe, m-Me, m-Cl and m-CF₃. We observed a reasonably linear Hammett relationship when plotting the log [P ? 17]_x⁺/[P ? 17]_H⁺ vs. σ⁺ with rho (ρ) equal to ?1.0.Thus electron-donating groups stabilize the [P ? 17]⁺ ion and carbon—tin sigma (σ) electrons can either, by a neighbouring group effect, attack the nucleofugic center, or the carbonium ion can attack the carbon—tin σ electrons to form the trimethyltin cation and a cyclopropane derivative. Consequently, we propose that a two-step mechanism for the 1,3-deoxystannylation reaction is operating in the gas-phase with this type of compound. The factors contributing to this gas phase reaction will be discussed.     

Reactions of aromatic compounds with nucleophilic reagents in liquid ammonia: XVII. Effect of ionic association on the orientation in aryloxydefluorination of 2,4-difluoronitrobenzene

The analysis of dependence of fluorine replacement orientation in 2,4-difluoronitrobenzene effected by nucleophiles Y-C₆H₄OM (Y =p-OMe,p-Me,m-Me, H,p-F,p-Cl,m-F,m-Cl; M = Li, Na, K, Et₄N, Bu₄N) in liquid ammonia medium at -35‡C on the nature of M, Y, on nucleophile concentration, on addition of crown ethers, diaminoalkanes, and dimethoxyethane indicates the occurrence of ionic association at M = K, Na. For communication XVI, see [1].     

Base cleavage of substituted [phenyl(2-thienyl)methyl]- and [phenyl(2-furyl)methyl]-trimethylsilane. Stabilization of carbanionic centres by 2-thienyl and 2-furyl groups

Rates of cleavage by NaOMEMeOH at 25°C have been determined for (2-thienyl)₂CHSiMe₃ and for the compounds Ph(2-thienyl)CHSiMe₃ and Ph(2-furyl)CHSiMe₃ and some of their derivatives with a substituent in the m- or p-position of the phenyl group or the 5-position of the heterocyclic group. The results indicate that the 2-thienyl and 2-furyl groups stabilize a carbanionic centre more effectively than a phenyl group, and the following approximate pK_a values can be derived: Ph₂CH₂, 33.4; Ph(2-thienyl)CH₂, 30.0; Ph(2-furyl)CH₂, 29.6; (2-thienyl)₂CH₂, 27.1. The effect of the 2-Cl substituent in the thiophen ring is close to that of the p-Cl substituent in the benzene ring, and the effects of the p-Me substituents on the benzene ring are very close to those of the 2-Me substituents on the thiophen or furan rings. The product and rate isotope effects (determined by use of MeOD) are consistent with separation of the carbanion in the rate-determining step.     

Cleavage of benzyltrimethylstannanes by aqueous methanolic perchloric acid

The rates of cleavage of some XC₆H₄CH₂SnMe₃ bonds by aqueous-methanolic perchloric acid have been measured spectrophotometrically, and the rate of cleavage of the MeSn bonds of PhCH₂SnMe₃ and Me₄Sn by monitoring the methane evolution. The results indicate that for X = H, p-Me, o-Me, p-Bu^t, o-, m- and p-F and -Cl, and o-Br, the cleavage of the CH₂SnMe₃ bond involves attack of the acid at the benzylic atom, and is not much faster than that of the SnMe bonds, but that a mechanism involving ring-protonation is important for X = m-Me, and greatly predominant for X = m-OMe.     

Synthesis of Diaryl Ethers: A Long-Standing Problem Has Been Solved

A breakthrough in the synthesis of diaryl ethers has been achieved as shown in Equation (1). The coupling of phenols with aryl boronic acids in the presence of copper(II ) acetate and a base proceeds under very mild conditions (room temperature) as described simultaneously by Evans et al. and Chan et al. Examples: R¹=2-Cl, 2-I, 2-OMe, 4-tBu, 4-CH₂CH(NHCOOtBu)CO₂Me, 3,5-tBu₂; R²=2-Me, 3-OMe, 3-NO₂, 4-H, 4-F, 4-OMe.     

ESR study of the free radical intermediates formed during the electrolytic process: III. Electrooxidation of substituted anilines

Active free radicals formed by the electrooxidation of substituted anilines RC₆H₄NH₂ (R = H, p-Br, p-Cl, p-I, p-Me, p-COOH, p-MeCO, p-NO₂, m-CO₂H, and m-Cl) are trapped by spin trap 2-methyl-2-nitroso propane (MNP). A multiple ESR signal of the solution containing electrolytic aniline and MNP is identified with the spin adduct of MNP and radical cation 1 by theoretical simulation of observed spectrum. Furthermore, ESR spectra of para- or meta-substituted anilines give a reasonable explanation about spin adducts of MNP and the cation 2 or 3 by the same method.     

Kinetics and mechanism of the aminolysis of O-ethyl S-aryl dithiocarbonates in acetonitrile

The aminolysis reactions of O-ethyl S-(Z-phenyl) dithiocarbonates (Z=p-CH₃, H, p-Cl, and p-NO₂) with anilines (AN) and N,N-dimethylanilines (DMA) in acetonitrile at 30.0°C are investigated. Relatively small values of β_X (β_nuc,0.4 ca. 0.7) and β_Z (β_lg −0.1 ca. −0.4) for both ANs and DMAs, significantly large k_H/k_D values (1.1 ca. 1.9) involving deuterated anilines, and large negative ρ_XZ values for ANs (−0.56) are interpreted to indicate a concerted mechanism for both ANs and DMAs but with a hydrogen bonded four-center type transition state (TS) for ANs. The relative leaving ability, k(Z=p-NO₂)/k(Z=p-CH₃), is smaller for ANs than for DMAs, especially for a weaker nucleophile (1.9 and 4.7 for AN and DMA, respectively, with X=p-Cl). This suggests that the rate enhancement by the hydrogen-bond formation in the four-center type TS for AN is greater for a weaker nucleofuge (Z=p-CH₃), especially when the nucleophile (X=p-Cl) is weaker. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 419–423,1998     

Correlation analysis of reactivity in the reduction of trans-chloro(RC6H4NH2)bis(1,2-diaminoethane)cobalt(III) complexes by hexacyanoferrate(II) in aqueous methanol

The structural and solvent effects on the reduction of trans-chloro(RC₆H₄NH₂)bis(1,2-diaminoethane)cobalt(III) ion (R=H, p-Me, p-OMe, p-OEt, p-F, m-Me and m-OMe) by the hexacyanoferrate(II) complex was investigated in aqueous MeOH. The second order rate constants for these reactions were determined spectrophotometrically at three different temperatures and activation parameters have been computed. Reduction proceeding through ion-pair formation is proposed on the basis of the Laidler–Eyring and Grunwald–Winstein equations. The influence of added co-solvent on the reactivity has been analysed using multiple regression equations viz. Kamlet–Taft and Swain. The reduction rates were correlated with Hammett's substituent constants yielding a negative reaction constant.     

Hammet correlations for the solvolysis of silylpyrroles

The siliconnitrogen bond in (XC₆H₄)Me₂SiNC₄H₄ (X = p-MeO, p-Me, m-Me, H, p-Cl) is cleaved in absolute ethanol containing sulphuric acid or amine buffers. The logarithms of the rate constants correlate with σ^o-constants, the p-values being -0.97 for the acid-catalysed reaction, and +1.8 and +2.6 for the reactions catalysed by amine and ethoxide anion, respectively. Mechanisms involving pentacoordinate silicon are suggested.     

Cleavage of α-halo-substituted alkyl groups from silicon. The mechanism of catalysis by ammonia buffer

Kinetic studies have been made of the base-catalysed cleavage of α-dihalo-substituted alkyl groups from silicon in n-propanol—water in the presence of ammonia buffer. The separate rate constants for the concurrent base-catalysed and base-plus-nucleophile-catalysed processes have been determined for the compounds XC₆H₄(Me₂)SiCHCl₂ with X = H, p-MeO, p-Me, p-Cl, m-Cl, m-CF₃ and a good correlation with σ-constants found for both types of catalysis. Solvent isotope effects and steric effects of substituents have also been studied. Possible mechanisms are discussed.     

Paradoxes in Thermal Stability of the Liquid-Crystal Phase of Phenylpropargyl Phenyl Ethers: I. Synthesis and Mesogenic Properties of Phenylpropargyl Phenyl Ethers

The ethers PhCCCH₂OC₆H₃RR' were prepared in 51-83% yields by reactions of phenylpropargyl tosylate in alkaline solution with appropriate p- and o-substituted phenols. Below are given R, R', and the ranges of existence of the nematic mesophase (°C): p-I, H, 104-137; p-Cl, H, 65-117; p-F, H, 33-53; p-OMe, H, 79-120; H, H, 44-115; p-(Me)₃C, H, 74-82; p-COOH, H; p-NO₂, H, 77-96; o-NO₂, H, 83-139; o-CHO, H, 75-115; p-Br, o-NO₂, 95-132; p-Cl, o-NO₂, 71-123; p-F, o-NO₂, 79-120; o-Cl, H, none; p-COOPr, H, none; and p-COOPh, H, 116-134. In contrast to the traditional views, the presence of the o-nitro group enhances, rather than distorts, the thermal stability of the mesophase. The stability increases in parallel with the -R effect of the o-substituent.     

N-(Substituted phenyl) itaconimides as organic stabilizers for plasticized poly(vinyl chloride) against photo-degradation

Several N-(substituted phenyl)itaconimide derivatives, N-(RPh)II (R: -NO₂, -COOH, -H, -OH, -OMe, -Me, -Cl, or -Br), have been investigated as organic photo-stabilizers for poly(vinyl chloride) (PVC) plasticized with dioctyl phthalate (DOP). Their stabilizing efficiencies are evaluated by measuring the length of the induction period (Ts), the period during which no detectable amounts of hydrogen chloride gas could be observed, and also from the rate of dehydrochlorination as measured by continuous potentiometric determination, and the extent of discolouration of the degraded polymer. The efficiencies are also evaluated by determining the amount of gel formation as well as the intrinsic viscosity of the insoluble and the soluble fractions of the degraded polymer, respectively. The results have proved the greater stabilizing efficiency of the N-(RPh)II derivatives relative to that of the phenyl salicylate UV absorber which is a commonly used industrial stabilizer. A radical mechanism is proposed to account for the stabilizing action of the investigated products.     

Thermodynamic studies of sitting-atop complexation between free base meso-tetraarylporphyrins and antimony(III) chloride in chloroform

Interaction of para, meta and ortho-substituted meso-tetraarylporphyrins, (H₂t(X)pp, X: OMe, Me, H and Cl) with SbCl₃ in chloroform solution afforded 1 : 1 sitting-atop (SAT) complexes ([(SbCl₃)(H₂t(X)pp)]). The formation constants were calculated by KINFIT and found to decrease with increasing temperature. The thermodynamic parameters, ΔH°, ΔS° and ΔG°, were obtained. Formation constants of these complexes change with changing substituent (X) on the aryl rings of H₂t(X)pp in the following order: (SbCl₃)H₂t(4-OMe)pp >?(SbCl₃)H₂t(4-Me)pp >?(SbCl₃)H₂tpp >?(SbCl₃)H₂t(4-Cl)pp >?(SbCl₃)H₂t(3-OMe)pp >?(SbCl₃)H₂t(3-Me)pp> (SbCl₃)H₂t(2-OMe)pp >?(SbCl₃)H₂t(2-Me)pp.     

Study of substituted aryldiazonium salts and their crown ethers complexes by XPS

The complexes of fourteen substituted aryldiazonium salts RC₆H₄N₂+BF₄^? (R?H, p-CH₃, p-NO₂, p-I, p-Cl, p-F, m-Br, m-Cl. m-CH₃, o-CH₃, o-OCH₃, o-NO₂, o-Br, o-Cl) with crown ethers 18-C-6 (1) and dibenzo-24-c-8 (2) have been studied by XPS. The results show that the chemical shifts of α-N1s and β-N1s of substituted aryldiazonium salts are closely related to the induction and conjugation effects of R groups. It is interesting to note that charge transfer(β-N→O) take place upon complexation of substituted aryldiazonium salts with crown ethers. Therefore the decrease of binding energy of crown ether oxygen may be used as a measurement of the stabilities of these complexes.     

Binary Mixtures of Liquid Crystalline Ester Bismaleimides

Abstract

A binary system consisting of a chlorohydroquinone-based ester bismaleimide (3-Cl), T _m = 238°C, and a methylhydroquinone-based ester bismaleimide (3-Me), T _m = 251°C, was investigated for the purpose of improving processability by widening the nematic phase range before polymerization. Calculations based on the Schroeder-van Laar equation predicted a system eutectic composition of 41% 3-Me monomer and a eutectic temperature of 202°C. Experiments found the eutectic composition at 35% 3-Me and the eutectic temperature at 218.5°C. Discrepancies between experimental results and theoretical predictions are likely due to error in measured heats of fusion either due to impurities in the samples or due to the reactive nature of the components being considered. Thermal cycling was also found to have a significant melting point depression effect. While significant depression of the system melting point was achieved, polymerization still occurred immediately after melting in all systems evaluated. All mixtures could be polymerized from the nematic phase to yield a solid which retained the nematic orientation of the starting polymer melt.     

The palladium-catalysed conjugate addition type reaction of arylmercury compounds with α,β-unsaturated ketones in a two-phase system

Pd-catalysed reaction of arylmercury compounds with α,β-enones in an acidic two-phase system provides a mild and selective way to β-aryl ketones. The present conjugate addition type reaction may accommodate a wide variety of functional groups. Thus, aryl units containing electron-donating and electron-withdrawing substituents such as -Me, -Cl, -CHO, -COMe, -COOMe, -COOH, -OH, -OMe, -NHCOMe and -NO₂ were successfully transferred to the β C atom of benzalacetone. A number of α,β-enones were also treated with 3-formylphenyl mercury chloride to give the corresponding β-(3-formylphenyl) derivatives. The main limitation seems to arise from steric hindrance in the starting α,β-enonic system. Substituents in the aryl moiety of the organomercury compounds were found to affect the transmetalation step in the direction expected for a rate determining σ-complex formation.     

Amide group catalysis of ester hydrolysis

Hydrolysis of phenyl-N-acetylanthranilate 1 (Ar = Ph) to N-acetyl anthranilie acid 3 is base catalysed and occurs through the intermediacy of 2-methyl-3,1-benzoxazin-4-one 4. Above pH 6 the formation of 4 from 1 is faster than the base (or acid) catalysed ring opening of 4 to 3. Electron-withdrawing substituents in the ester moiety (e.g. 1, Ar = p-NO₂C₆H₄) aid cyclization to 4 relative to direct hydroxide catalyzed hydrolysis (to 3). Concomitantly, neutral amide participation is observed so that cyclization of 1 (Ar = p-NO₂, m-NO₂, m-Cl, p-ClC₆H₄) occurs even in acidic solution; the Hammett ? values for neutral and base catalysed cyclizations are compared. Hydrolysis of methyl-N-acetylanthranilate 6 to 3 occurs slowly in base, possibly without the formation of the intermediate 4.