Reactivities Of Substrates Research Articles

Transition States for the Carboalumination of Alkenes and Alkynes

Ab initio molecular orbital theory was used to characterize stationary points on the potential energy surface for the carboalumination of alkenes and alkynes. Activation energies and entropies of activation were calculated and found to be in good agreement with experimental values. A π-complex between the alkyl aluminum and the substrate is a stable precursor to the transition structure. The transition structures are four-centered. Substituent effects on the transition structure geometries are small and predictable, suggesting that standard values for geometrical parameters for this and related reactions are reasonable. The anomalous relative reactivities of substrates can be explained by considering the reaction to essentially be a nucleophilic attack, rather than an electrophilic one.

Comparative Reactivities of Phosphotriesters toward Iodosocarboxylates in Cationic Micelles

Comparative reactivities have been determined for the cleavages of p-nitrophenyl phosphotriester substrates 4−9, and 12, and 13 by iodosobenzoate (1) and iodosonaphthoate (11) in micellar cetyltrimethylammonium chloride at pH 8. The aggregate substrate reactivities can be manipulated by altering the phosphate substituents, with each PhO-to-MeO or -EtO change worth ∼1 order of magnitude decrease in the rate constant. With 2 PhO/EtO mutations, the reactivity of 4 is 470 times greater toward 1 than that of 5, and the differential increases to 1080 with nucleophile 11. Phosphodiester substrates such as 10 are inert to micellar 1. Reactivity differences can be amplified by covalent attachment of the substrate moiety to cationic surfactants (12 and 13), where the PhO/EtO changes induce 600−700-fold reactivity diminutions with nucleophiles 1 and 11. The differential reactivities of the various substrates reflect both innate reactivity differences and differences in binding to the micellar pseudophase.

Dissection of the role of the conserved G.U pair in group I RNA self-splicing.

Phylogenetic conservation among > 100 group I introns and previous in vitro studies have implicated a G.U pair as defining the 5'-splice site for exon ligation. The U residue defines the 3' end of the 5' exon, and the complementary G residue is part of the internal guide sequence (IGS) that base pairs to the 5' exon. We now quantitate the effect of this pair on individual reaction steps using the L-21ScaI ribozyme, which is derived from the group I intron of Tetrahymena thermophila pre-rRNA. The following results indicate that interactions with this G.U pair contribute to the binding of the 5'-exon, the positioning of the 5'-splice site with respect to the catalytic site, and the chemical step. The oligonucleotide, CCCUCU, binds to the ribozyme approximately 20-fold stronger than CCCUCC despite the fact that the U-containing oligonucleotide forms an approximately 5-fold less stable duplex with an oligonucleotide analog of the IGS, GGAGGG. This and two independent experimental observations indicate that the G.U pair contributes approximately 100-fold (3 kcal/mol, 50 degrees C) to tertiary interactions that allow the P1 duplex, which is formed between the 5'-exon and the IGS, to dock into the ribozyme's core. The approximately 50-80-fold increase in miscleavage of 5'-exon analogs upon replacement of the 3'-terminal U of CCCUCU with C or upon removal of the 3'-terminal U suggests that the tertiary interactions with the G.U pair not only contribute to docking but also ensure correct positioning of the 5'-splice site with respect to the catalytic site, thereby minimizing the selection of incorrect splice sites. Comparison of the rates of the chemical cleavage step with G.U vs G.C suggests that the G.U pair contributes approximately 10-fold to the chemical step. It was previously suggested that the 2'-hydroxyl of this U residue helps stabilize the 3'-oxyanion leaving group in the chemical transition state via an intramolecular hydrogen bond. Relative reactivities of oligonucleotide substrates with ribose and deoxyribose U and C are consistent with a model based on a recent X-ray crystallographic structure in which the exocyclic amino group of G helps orient the 2'-hydroxyl of U via a bridging water molecule, thereby strengthening the hydrogen bond donated from the 2'-hydroxyl group to the neighboring incipient 3'-oxyanion. Finally, kinetic and thermodynamic evidence for the formation of a G.C+ wobble pair is presented.(ABSTRACT TRUNCATED AT 400 WORDS)

Ion-dipole complex formation from deprotonated phenol fatty acid esters evidenced by using gas-phase labeling combined with tandem mass spectrometry

The behavior of para-hydroxy-benzyl and hydroxy-phenylethyl fatty acid esters and methoxy derivatives toward the NH 3/NH − 2 system was investigated. Under these negative ion chemical ionization (NICI) conditions, proton abstraction takes place mainly at the more acidic site (i.e., phenol); however, this reaction is not entirely regioselective. Using NICI-ND 3 conditions, both isomeric phenoxide and enolate molecular species are produced in competition from these phenol esters. Their respective low-energy collision-activated dissociation spectra are studied, and they strongly differ, showing that these molecular species are not convertible to a common structure. Analysis of specific fragmentations of the OD-enolate parent species labeled by ND 3 in the gas phase, indicates that by charge-promoted cleavage, isomerization into an ion—dipole intermediate takes place prior to dissociation. This complex, containing a ketene moiety, isomerizes into different isomeric forms via two consecutive proton transfers: the first, which is very exothermic, is irreversible in contrast to the second, less exothermic reaction, which occurs via a reversible process. It is evidenced by the loss of labeling at phenol or enolizable sites in the fragment ions. Such a stepwise process does not take place from the phenoxide parent ion, which preferentially yields a very stable carboxylate ion. A thermochemical approach, using estimated acidity values, yields a rationalization of the observed reactivities of the various substrates studied.

Quantitative expression of dynamic kinetic resolution of chirally labile enantiomers: stereoselective hydrogenation of 2-substituted 3-oxo carboxylic esters catalyzed by BINAP-ruthenium(II) complexes

Hydrogenation of chirally unstable 2-substituted 3-oxo carboxylic esters gives a mixture of four stereoisomeric hydroxy esters. Use of BINAP-Ru(II) complex catalysts allows selective production of one stereoisomer among four possible isomers. The stereoselectivity obtained by the dynamic kinetic resolution depends on facile in situ racemization of the substrates, efficient chirality recognition ability of the catalysts, and the structures of the ketonic substrate. The factors controlling the efficiency of the stereoselective hydrogenation are experimentally determined by reaction of racemic oxo esters using enantiomerically pure and racemic BINAP complexes. Quantitative expression of the dynamic kinetic resolution has been made by defining the product partition coefficients (w, x, y, and z), the relative reactivities of the enantiomeric substrates (k[sub fast]/k[sub slow]), and the relative ease with which stereoinversion and hydrogenation take place (k[sub inv]/k[sub fast]). The validity of the equations has been demonstrated by the graphical exhibition of the enantioselectivity and diastereoselectivity as a function of conversion of the substrates. 18 refs., 4 figs., 3 tabs.

Evidence for concerted acid hydrolysis of alkyl nitrites

The results of studying the acid hydrolysis of methyl, ethyl, isopropyl, butyl, tert-butyl, pentyl, 2-bromoethyl, 2-chloroethyl and 2-ethoxyethyl nitrites in water show that the reaction is not, as widely accepted in the literature, catalysed by nucleophiles (Cl–, Br–, etc.), but is however subject to general acid catalysis. These findings, and the observed values of the solvent isotope effect, suggest that the substrate is protonated in the rate-controlling step of the reaction. Further, the relative reactivities of the various substrates investigated suggest a concerted mechanism in which the proton transfer occurs concurrently with the breaking of the O–N bond via a slightly imbalanced transition state.

Nucleophilic Displacement Catalyzed by Transition Metal. IX. [Pd2(dba)3]·CHCl3–DPPF Catalyzed Cyanation of Aryl Halides and Aryl Triflates

Abstract A catalyst system comprising [Pd2(dba)3]·CHCl3 and DPPF was proved to be efficient for the cyanation of aryl iodides and aryl triflates with KCN. The reactivities of aryl substrates decreased in the order ArI>ArOTf>ArBr.

Transition metal ions supported on hydrogels as hydrogenation catalysts

Synthesis of new hydrogel polymers based on 2-hydroxyethyl methacrylate and other functional monomers is described. These copolymers are useful for anchoring palladium and rhodium species. The resulting supported catalysts are effective for the hydrogenation of alkenes, alkynes and dienes under mild conditions. The supported catalysts have been characterised by chemical analysis, particle size measurements, IR, TGA and X-ray photoelectron spectra. Kinetics of hydrogenation have been evaluated using the concepts applicable to slurry reactors. The hydrogenation proceed through the ‘unsaturate route’. Relative reactivities of a few substrates, solvent and particle size effects and selectivities by product analysis have been assessed. These catalysts possess excellent recycling efficiencies.

Acid-catalyzed hydrolysis of 2-methylene-1,3-dithiolane. 2. Remarkable effects of β substitution on reversibility of the carbon protonation

Hydrolyses of 2-ethylidene-(1b), 2-isopropylidene-(1c), and 2-benzylidene-1,3-dithiolane (1d) were kinetically investigated in aqueous solution. All the individual rate constants involved in this three-step reaction were evaluated. Initial carbon protonation is only partially reversible (k2/k−1 = 1.33, 0.68, and 1.02 for 1b, 1c, and 1d, respectively) at higher pH, while the protonation becomes completely reversible below pH 2 where the third step is rate determining. Complete H–D isotope exchange at the β-carbon of 1b and 1d was observed in deuterium media before appreciable hydrolysis took place. It was demonstrated that reversion from the tetrahedral intermediate 3 to 1 occurs extensively during the reaction in the latter acidity range. Relative stabilities and reactivities of the olefinic substrates 1 are discussed.

Kinetics studies of oxidations of catechol and l-dopa as mediated by IrCl i2−6 at polymer-coated glassy carbon electrodes

Rotating disk techniques were employed to measure the kinetics of the oxidations of catechol and l-dopa as mediated by IrCl 2− 6 at a glassy carbon rotating disk electrode coated with a mixture of two polycationic electrolytes. For both reactants the kinetic behavior matched that described by Andrieux et al. [1] for an “SR” case. Products of the cross-reaction rate constants and substrate partition coefficients derived from the data had values consistent with very similar reactivities of both substrates toward IrCl 2− 6 inside and outside of the coatings.

Mechanism of nucleophilic substitution of thiamine and its analogs: Methanol and water solvents

4-Amino-1,2-dimethyl-5-(substituted methyl)pyrimidinium ions undergo lyate ion-catalyzed nucleophilic substitution in methanol and in water. The substituents at position 5 of these thiamine analogs include phenols and pyridinium ions. The observation of separate rate- and product-determining steps indicates a multistep mechanism of substitution. Linear free energy relationships suggest that addition of lyate ion to the pyrimidinium ring is rate-limiting when phenoxide ions act as leaving groups and that a change occurs when pyridines depart. Elimination of the pyridine becomes the rate-limiting step following rapid addition of the lyate ion. Reactivities of phenol substrates in methanol and in water are similar but, when pyridine departs, changing from water to methanol gives rise to a large rate acceleration. An increase in ground state electrostatic destabilization of the dicationic substrate in the less polar alcohol provides the rate enhancement.

Reduction and dissolution of manganese(III) and manganese(IV) oxides by organics: 2. Survey of the reactivity of organics

Reduction and dissolution of manganese(III,IV) oxide suspensions by 27 aromatic and nonaromatic compounds resembling natural organics were examined in order to understand the solubilization reaction in nature. At pH 7.2 10/sup -3/ M formate, fumarate, glycerol, lactate, malonate, phthalate, propanol, propionaldehyde, propionate, and sorbitol did not dissolve appreciable amounts of oxide after 3 h of reaction. The following organics did dissolve manganese oxides under these conditions and are listed in order of decreasing reactivity: 3-methoxycatechol approx. catechol approx. 3,4-dihydroxybenzoic acidapprox. ascorbate > 4-nitro-catechol > thiosalicylate > hydroquinone > 2,5-dihydroxybenzoic acid > syringic acid > o-methoxyphenol > vanillic acidapprox. orcinol approx. 3,5-dihydroxybenzoic acid > resorcinol > oxalite approx. pyruvate approx. salicylate. Relative reactivities of organic substrates are discussed in terms of surface complex formation prior to electron transfer. Dissolution of manganese oxides by marine fulvic acid was enhanced by illumination, verifying that the reaction is photocatalyzed.

Rate measurements of certain Vilsmeier–Haack reactions. Part 3. The reactivities of various pyrrole substrates

The rates of reaction of six pyrrole substrates with the Vilsmeier–Haack reagent derived from NN-dimethylbenzamide (and in some cases from N-benzoylmorpholine or NN-diethylbenzamide) and phosphoryl chloride were measured. The deactivating effect on substitution at the 5-position of a groups when present at the 2-and 4-position. This net activation of the substrate was lost when the bulkier Vilsmeier–Haack complex derived from NN-diethylbenzamide was used. The presence of a methyl or benzyl group at the 1-position of the pyrrole resulted in an unexpected and very marked deactivation of the substrate. A highly specific orientation of reagent and substrate in the transition state is suggested to account for this effect.

Preparation and capillary gas chromatography of polymethyl- and ethylmethylbiphenyls and polymethyldiphenylmethanes

Trimethyl-, tetramethyl-, and ethylmethylbiphenyls with the alkyl groups at one aromatic ring have been prepared by the Gomberg reaction and identified by capillary gas chromatography. The products are either chemical individua or mixtures of up to three isomers depending on choice of the aromatic hydrocarbon. The ethylmethylbiphenyls have been prepared as mixed standards only. Most aromatic hydrocarbons used in the Gomberg reaction have been prepared by combination of rectification and sulphonation procedures. The HMO method has been used for calculation of the values of the radical superdelocalizabilities at individual centres of the 1,2,4- and 1,2,3-trimethylbenzene molecules which have been compared with product composition of the Gomberg reactions of these hydrocarbons. Reactivities of the aromatic substrates used in the Gomberg reaction have been compared by the competition technique. Capillary gas chromatography using three columns wetted with non-polar , medium, and polar stationary phases has been used to determine the Kovats indexes of the hydrocarbons prepared and parameters of the linear dependences IApiez.L = k.Istat.phase(2) + q (with the corresponding correlation coefficients). From the elution data of the isomeric methyl- and ethylbiphenyls the increments have been derived for methyl and ethyl groups, and possibility of prediction and assessment of the Kováts indexes of polyalkybiphenyls and dimethyldiphenylmethanes is discussed.

Synthesis of nucleoside 3'-(S-alkyl phosphorothioates) and their use as substrates for nucleases.

The synthesis of cytidine, uridine, guanosine, and adenosine 3'-(S-methyl phosphorothioates) by treatment of the 2',5'-di-O-(4-methoxytetrahydropyran-4-yl)ribonucleosides with 2-(methylthio 4H-1,3,2-benzodioxaphosphorin 2-oxide is described. These nucleotide analogues are stable compounds both in the solid state and the neutral aqueous solution. All four of these compounds are degraded by RNase T2 to the parent nucleotides and methanethiol. In addition, cytidine and uridine 3'-(S-methyl phosphorothioates) are substrates for bovine pancreatic ribonuclease and guanosine 3'-(S-methyl phosphorothioate) is a substrate for RNase T1 and RNase U1. When used in conjunction with a chromophore-producing reagent, nucleoside 3'-(S-methyl phosphorothioates) provide a means for direct kinetic measurement of ribonuclease activity over a wide pH range (pH 2-9). The reactivities of these substrates with ribonucleases are compared to the reactivities of other synthetic substrates as well as a number of natural substrates. The utility of ribonucleoside 3'-(S-methyl phosphorothioates) as substrates for the assay of ribonucleases is discussed.

Prenyl transferases from Micrococcus luteus: Characterization of undecaprenyl pyrophosphate synthetase

Three prenyl transferases in Micrococcus luteus were recovered in the soluble fraction following cell disruption. Undecaprenyl pyrophosphate (C 55-PP) synthetase chromatographed on DEAE-cellulose independently from geranylgeranyl-PP and octaprenyl-PP synthetases. Further purification of C 55-PP synthetase resulted in an approximate 250-fold purification over the crude lysate. The molecular weight of the synthetase was estimated to be between 47,000 and 49,000 by Sephadex G-100 chromatography. The enzyme had a broad specificity toward the allylic pyrophosphate substrate. The reactivities of the allylic substrates increased with chain length, C 10 < C 15 < C 20, except for trans-solanesyl-PP, which was unreactive. Moreover, the enzyme was active on allylic substrates having both cis- and trans-stereochemistry. Although C 55-PP and C 50-PP were the major products, some shorter chain products were also produced, when t,t-farnesyl pyrophosphate and Δ 3sopentenyl pyrophosphate (IPP) were used as substrates. The stereochemistries of the products formed with C 55-PP synthetase were established, using [ 14C]IPP and 2 R-[2- 3H] and 2 S-[2- 3H]IPP. Each new isoprene unit added had a cis-configuration. The enzyme was inactive in the absence of added effectors. It was stimulated by Triton X-100, egg lecithin, and a whole phospholipid extract from M. luteus. Cardiolipin and deoxycholate were poor activators of the enzyme. The product chain length distribution observed with the phospholipid-activated enzyme showed highly favored production of the C 55-PP product over the C 50-PP product.

有機反応理論の流れ

The organic electronic theory was constructed on the concepts of electron density and delocalization energy, and introduced some hypothetical latent factors to correlate the chemical reactivities of substrates with structures. The latent factors were used successfully to deduce reaction mechanism. However, with accumulation of facts being inexplicable with electronic theory, the theory had to give way to quantum chemistry based on perturbation method. Generally chemical studies take the processes being similar to the life of human being : birth, growth, senility and death. The causes turning the processes might be made obvious by means of case studies.

The Origin of the Polarity of a Radical in Hydrogen Abstraction Reactions

Abstract The electrophilicity of the H atom in hydrogen abstraction reactions was studied by the CNDO/2 method. The origin of the electrophilicity of the H atom was well understood using the stabilization energy due to the delocalization of electrons (SEDE) between the radical and substrate; this energy was calculated along the reaction path. The electrophilic energy (SEDE from the substrate to the radical) governs the relative reactivities of substrates, CH4&amp;lt;CH3CH3&amp;lt;CH3CH2CH3&amp;lt;(CH3)3CH which correspond to the order of the increase in the electron density. The electrophilic energy is also important in hydrogen abstraction by the H atom from aliphatic alcohols. In the case of the Cl2− radical, the nucleophilic energy (SEDE from the radical to the substrate) determines the relative reactivities of substrates, CH3OH&amp;lt;C2H5OH&amp;lt;i-C3H7OH. The polarity of a free radical is tentatively clarified by using its ionization potential.

Polarity of Free Radicals in Hydrogen Abstraction Reactions

Abstract The electrophilicity of the CF3 radical in hydrogen abstraction reactions was studied theoretically by the CNDO/2 method. The activation energy for hydrogen abstraction by the CF3 radical from aliphatic hydrocarbons decreases with an increase in the electron density on the hydrogen atom to be abstracted. The origin of the electrophilicity of the CF3 radical can be well understood by the stabilization energy due to delocalization of electrons (SEDE) between the radical and substrate. The electrophilic stabilization energy denned by the SEDE from substrate to the radical governs the relative reactivities of aliphatic hydrocarbons, CH4&amp;lt;CH3CH3&amp;lt;CH3CH2CH3&amp;lt;(CH3)3CH, which corresponds to the order of the increase in the electron density. The results indicate that the radical is an electrophile if the relative reactivities of substrates are governed by the electrophilic stabilization energy, and a nucleophile when the reactivities are governed by the nucleophilic stabilization energy (the SEDE from the radical to substrate). SEDE was calculated for the reactions of the OH and the CH3 radicals with CH4and CH3CN; the results indicated that the OH radical is electrophilic and the CH3 radical nucleophilic. These polarities of radical reagents are discussed in connection with their physicochemical properties such as ionization potentials and electron affinities.

Kinetics of the reaction of p-nitrophenyl acetate with amines in the presence of dodecylammonium propionate and aerosol-OT aggregates in benzene

Aminolysis of p-nitrophenyl acetate (NPA) by dodecylamine (DA) is 53 times faster in the presence of dodecyclammonium propionate (DAP) aggregates than in pure.benzene. Reaction with imidazole (Imz) and N-methylimidazole (Melmz) in the presence of DAP displays much more micellar catalysis. Thus addition of 0.2M-DAP to 0.08M-Imz in benzene reduced the reaction half-life from 25 h to 3 min. Addition of water decreases the reaction rates. This decrease is mainly due to DAP head-group hydration with subsequent loss of catalytic activity. Reaction between NPA and the amines, is faster in presence of DAP than Aerosol-OT(AOT). Addition of water to the amine–AOT system increases the rates. The results are discussed in terms of bifunctional catalysis by DAP and the effects of the surfactants on the reactivities of substrates solubilized in their micellar cores.