Showing papers on "Nucleophile published in 1981"
TL;DR: In this paper, a theory of charge transfer stabilization of the transition state for nucleophilic addition to a carbonyl group by electron donors is proposed, using the Bell-Evans-Polanyi principle.
Abstract: A theory of charge-transfer stabilization of the transition state for nucleophilic addition to a carbonyl group by electron donors is proposed. It is postulated, using the Bell-Evans-Polanyi principle, that the feature of this transition state critical for stereoselectivity of the reaction is the existence of a low-lying vacant orbital u** associated with the u bond being formed in the reaction and that electron delocalization into that orbital will stabilize the transition state and may thereby enhance the reaction rate: the kinetic anomeric effect and the kinetic a effect are considered as the examples of such a stabilizing interaction. Stereochemistry of nucleophilic addition to cyclohexanone is determined by two factors according to this model: steric hindrance which favors the equatorial approach and electron donation from the cyclohexanone ucc and ucH bonds into the u** orbital, which favors the axial approach since the carbon-hydrogen bonds are better electron donors. Consequently, nucleophile structure, metal cations complexing the carbonyl oxygen, solvent, and counterions or other solutes may influence stereoselectivity of the reaction by changing the e(.**). Furthermore, it is shown that this model offers a simple and consistent way to rationalize kinetic and stereochemical effects of the so-called "remote polar substituents" which cannot be explained in terms of steric or electrostatic interactions but appear to be controlled by the overlap and energy gap between the remote electron-donor orbitals and the u** orbital.
456 citations
TL;DR: The complement protein C3, when activated by limited proteolysis, forms a short-lived reactive intermediate fragment, 'nascent' C3b, which is known to bind covalently to certain surfaces, and the characteristics of the covalent binding reaction have been studied by using Sepharose-trypsin as a combined proteolytic activator and binding surface for C3.
Abstract: The complement protein C3, when activated by limited proteolysis, forms a short-lived reactive intermediate fragment, 'nascent' C3b, which is known to bind covalently to certain surfaces The characteristics of the covalent binding reaction have been studied by using Sepharose-trypsin as a combined proteolytic activator and binding surface for C3 Binding of C3 to Sepharose-trypsin is saturable, with a maximum of 25-26 molecules of C3b bound per molecule of trypsin A minimum life-time of about 60 microseconds for the reactive intermediate has been calculated from binding of C3 at saturation Initial binding efficiencies of over 30% can be obtained at physiological pH and ionic strength The efficiency of C3 binding to Sepharose-trypsin decreases as pH increases and also shows a slight decline at high ionic strength The covalent binding of C3 to Sepharose-trypsin can be inhibited by a range of oxygen and nitrogen nucleophiles Activation of C3 in the presence of radioactive forms of four such nucleophiles, phenylhydrazine, methylamine, glycerol and glucosamine results in apparent covalent incorporation of the nucleophile into the C3d fragment of C3 The quantity of radioactive nucleophile bound can be predicted from the observed potency of the nucleophile as an inhibitor of the binding of C3 to Sepharose-trypsin The radioactive nucleophiles may be considered as 'active-site' labels for C3
290 citations
248 citations
TL;DR: It is proposed that the site for incorporation of methylamine in each subunit is a thiol ester, which in S-alpha 2M (the electrophoretically "slow" form) is sterically shielded from reaction with large nucleophiles, but is revealed as a highly reactive group, free from steric hindrance, after the proteolytic cleavage.
Abstract: It is shown that non-proteolytic proteins can become covalently linked to alpha 2M (alpha 2-macroglobulin) during its reaction with proteinases, and that this probably occurs by the mechanism that leads to the covalent linking of proteinases described previously [Salvesen & Barrett (1980) Biochem. J. 187, 695-701]. The covalent linking of trypsin was at least partly dependent on the presence of unblocked lysine side chains on the protein. The covalent linking of proteinases was inhibited by nucleophiles of low Mr, and these compounds were themselves linked to alpha 2M in a molar ratio approaching one per quarter subunit. Peptide "mapping" indicated that the site of proteinase-mediated incorporation of the amines was the same as that at which methylamine is incorporated in the absence of a proteinase. The nucleophile-reactive site revealed in alpha 2M after reaction with a proteinase was shown to decay with a t1/2 of 112 s, at pH 7.5. After the reaction with a proteinase or with methylamine, a free thiol group was detectable on each subunit of alpha 2M. We propose that the site for incorporation of methylamine in each subunit is a thiol ester, which in S-alpha 2M (the electrophoretically "slow" form) is sterically shielded from reaction with large nucleophiles, but is revealed as a highly reactive group, free from steric hindrance, after the proteolytic cleavage. We have designated the activated species of the molecule "alpha 2M".
178 citations
TL;DR: The electrophilic reactivity of arenes coordinated to the chromium tricarbonyl unit has been developed into several distinct methods for coupling carbon nucleophiles with aromatic rings as mentioned in this paper.
160 citations
TL;DR: In this paper, 1,3-Diene monoepoxides react with nucleophiles in the presence of palladium complex as a catalyst under neutral conditions to give 1,4-adducts selectively.
149 citations
118 citations
TL;DR: In this article, a mechanism for the thermal decomposition of zinc dialkyldithiophosphate additives has been proposed that accounts for the major degradation products and their dependence on the nature of the alkyl group.
Abstract: A mechanism for the thermal decomposition of zinc dialkyldithiophosphate additives has been proposed that accounts for the major degradation products and their dependence on the nature of the alkyl group. The proposed mechanism generally follows well-known rules of phosphorus chemistry. The major points are as follows. (1) The decomposition is initiated by a general migration of alkyl groups from oxygen atoms to sulfur atoms. The rate of this reaction is governed by the ease with which nucleophilic substitution can take place at the α-carbon atom of the alkyl group. The well-established correlation between decreasing number of β-hydrogen atoms and increasing thermal stability is, therefore, explicable in terms of increasing steric hindrance, by the β-subtituents, of nucleophilic attack on the α-carbon atom. (2) Olefin elimination successfully competes with substitution where the alkyl group structure is favorable (i.e. secondary alkyl groups), leading to the formation of phosphorus acids. (3) Nucleophilic...
117 citations
TL;DR: In this paper, an efficient method for the regioselective enhancement of the nucleophilicity of polyhydroxy compounds has been developed, based on partial stannylation of carbohydrate with (Bu3Su)2O and subsequent electrophilic attack with benzoyl chloride.
103 citations
103 citations
TL;DR: Findings lead to a view that the proton-catalyzed nucleophilic displacement of O2- from MbO2 by an entering water molecule, or SN2 mechanism with proton assistance, is the basis for most of the autoxidation reaction under normal conditions.
TL;DR: The generation of flavanyl-4-carbo-cations from flavan-3,4-diols and their condensation with nucleophilic flavanol-3-ols to form [4,6]- and [ 4,8]-biflavanoids at ambient temperatures and under mildly acidic aqueous conditions apparently simulates the initial step in condensed tannin formation in a number of natural sources.
Abstract: The generation of flavanyl-4-carbo-cations from flavan-3,4-diols and their condensation with nucleophilic flavan-3-ols to form [4,6]- and [4,8]-biflavanoids at ambient temperatures and under mildly acidic aqueous conditions apparently simulates the initial step in condensed tannin formation in a number of natural sources. The stereospecificity (or stereoselectivity) of the reaction is conditioned mainly by the 2,3-cis or 2,3-trans stereochemistry of the parent flavan-3,4-diol, but also by the nucleophilicity of the flavan-3-ol, and its regiospecific (or regioselective) course by steric factors arising from variation in substitution of the receptive A-ring of the flavan-3-ol.
TL;DR: The reactions of CH3COOH and CH3CHO with oxygen adsorbed on a Ag(110) surface were examined by temperature-programmed reaction spectroscopy as mentioned in this paper.
TL;DR: Silicon halides in the presence of nucleophilic catalysts react with epoxides to form O-protected vicinal halohydrins with enhanced regioselectivity as discussed by the authors.
TL;DR: In this paper, LiDisopropylamide reacts with 2-fluoropyridine at low temperature: regioselectivity is excellent and metallation occurs without side reactions such as nucleophilic attack.
TL;DR: Results indicate a competitive partitioning of an acyl-enzyme intermediate between water and nucleophile and a strong inhibition of the rates of hydrolysis of methyl butyrate and triacetin by nucleophiles is in agreement with a rate-limiting acylation step.
TL;DR: The use of these reagents for Lewis acid catalyzed ene, Diels-Alder and cycloaddition reactions and Claisen rearrangements is described in this paper.
TL;DR: In contrast to their titanium analogues, vinylzirconium reagents are stable enough to be employed. as discussed by the authors showed that aldehyde and ketone carbonylophiles of exceedingly low basicity can be solved from aldehydes and aldeheeds using trialkoxyzircimiconium chloride.
Abstract: Solutions of the title compounds are accessible from organolithium reagents and trialkoxyzirconium chloride (equation 2). In contrast to their titanium analogues, vinylzirconium reagents are stable enough to be employed. Generally, organozirconium reagents are highly selective aldehyde and ketone carbonylophiles of exceedingly low basicity (Tables 1, 2, 3 and typical procedure).
TL;DR: In this paper, a correlation of the coordination shifts with the reactivity of the coordinated naphthalene is proposed, and the coordinated ring 13C-resonance is found to undergo a smooth transition to higher field with increasing donor character of the coligands L.
Abstract: The 13C-NMR. spectra of the series of complexes ?6-naphthalene · CrL3 (L--CO (1), PF3 (2), PF2OMe (6), P(OMe)3 (3), C10H8 (= 3 L) (4) and PMe3 (5)) are reported. Definite assignments of the 13C-NMR. resonances were made through the synthesis of [2, 3, 6, 7-2H4]-naphthalene complexes. The coordinated ring 13C-resonance are found to undergo a smooth transition to higher field with increasing donor character of the coligands L. A correlation of the coordination shifts with the reactivity of the coordinated naphthalene is proposed. In complexes containing strong acceptor ligands the naphthalene is activated to attack by nucleophiles. Sequential treatment of complexes 1-4, 6 and [C10H8FeC5H5]+[PF6]- (7) with stabilized carbanions and I2 or Ce(IV)-salt yields a-substituted naphthalenes in the case of 1, 2, 6 and 7 but not in the case of 3 and 4. Treatment of 3 with an excess of HBF4 results not in the expected metal protonation but in a novel ligand transformation to yield 6.
TL;DR: In this article, allyl vinyl ether derivatives with organoaluminiumamphoteric reagent, R 3 Al or R 2 AlSPn, results in the title reaction at room temperature under uptake of R, H or SPh as a nucleophile on the aldehydic carbon.
TL;DR: Nonactivated aryl iodides readily react with areneselenolates, in the presence of copper(I) iodide, in hexamethylphosphoric triamide to produce the corresponding diaryl selenides in good yields.
Abstract: Nonactivated aryl iodides readily react with areneselenolates, in the presence of copper(I) iodide, in hexamethylphosphoric triamide to produce the corresponding diaryl selenides in good yields.
TL;DR: Aliphatic primary alcohols, when intercalated in certain ion-exchanged montmorillonites, react preferentially via an intermolecular nucleophilic displacement of water to give high yields of di-(alk-1-yl) ethers, rather than the competitive intramolecular dehydration to alkenes.
Abstract: Aliphatic primary alcohols, when intercalated in certain ion-exchanged montmorillonites, react preferentially via an intermolecular nucleophilic displacement of water to give high yields of di-(alk-1-yl) ethers, rather than the competitive intramolecular dehydration to alkenes; an essentially similar process yields polymeric material, poly(phenylenemethylene), from benzyl alcohol, but aliphatic secondary and tertiary alcohols, in contrast, yield alkenes almost exclusively.
TL;DR: The results indicate that a deprotonated amine is necessary for hydroxylation and that the amine may be present in the original drug rather than an external nucleophile, and the redox properties of several different phenothiazines are presented.
Abstract: The cation radical of each of the phenothiazine tranquilizers is a likely intermediate in the metabolism of the drugs to at least two of the three major metabolic classes, the sulfoxides and the hydroxylated derivatives. Previous work has shown that the reactions of the radical are highly dependent on the environment, particularly the presence of nucleophiles. The present report discusses the effect of cation radical structure on the formation of sulfoxide and hydroxylated metabolites in vitro. Cyclic voltammetry, spectrophotometry, and liquid chromatography were used to examine reactions of various phenothiazine radicals in aqueous buffers. A radical with a three-carbon aliphatic side chain (e.g., chlorpromazine) forms solely sulfoxide and parent unless amine nucleophiles are present, in which case hydroxylation occurs. A shorter side chain (e.g., promethazine) causes radical dimerization and pronounced hydroxylation, regardless of external nucleophiles. A piperazine side chain (e.g., fluphenazine) promotes hydroxylation, with some sulfoxide observed. The results indicate that a deprotonated amine is necessary for hydroxylation and that the amine may be present in the original drug rather than an external nucleophile. In addition to information about cation radical reactions, the redox properties of several different phenothiazines are presented.