Associative substitution mechanisms of clusters: the relationship between sites of nucleophilic attack and leaving group dissociation

1995 ◽  
pp. 1905 ◽  
Author(s):  
Richard A. Henderson
Keyword(s):  
Nucleophilic Attack ◽  
Leaving Group ◽  
The Relationship

Empirical σ Molecular Orbital Parameters and their Correlation with Rates of Substitution Reactions in Quadrate Cr(III) and Co(III) Complexes

1971 ◽  
Vol 49 (9) ◽  
pp. 1497-1501 ◽  
Author(s):  
C. H. Langford
Keyword(s):  
Field Theories ◽  
Complete Analysis ◽  
Substitution Reactions ◽  
Spectral Parameters ◽  
Empirical Measures ◽  
Orbital Parameters ◽  
Spectral Bands ◽  
Leaving Group ◽  
The Relationship

Empirical measures of σ bonding involving metal 3d orbitals are derived from Perumareddi's (4) complete analysis of the quartet spectral bands of quadrate complexes in the families Cr(NH3)5Xn+ and Cr(OH2)5Xn+. These are shown to correlate with lability of X in the Cr(III) complexes and in Co(NH3)5Xn+ complexes in a sense indicating that relative reactivity is controlled by variation of ligand metal 3d σ interaction. The relationship between the two Cr(III) series implies that the non-labile ligands can labilize the leaving group in proportion to their σ donor capacities. This observation bears on some well-known difficulties in crystal field theories of reactivity. In evaluating the correlation of spectral parameters with reactivity, the role of solvation in reactivity of Cr(III) and Co(III) complexes is discussed with emphasis on the surprisingly small solvent effects that have been observed.

The reactions of mercaptide anions with chlorinated sulfones

1977 ◽  
Vol 55 (12) ◽  
pp. 2316-2322 ◽  
Author(s):  
Richard F. Langler ◽  
James A. Pincock
Keyword(s):  
Substituent Effects ◽  
Nucleophilic Attack ◽  
Leaving Group

Mercaptide anions react with chlorinated sulfones in two modes, i.e. nucleophilic attack on carbon with chlorine as the leaving group and/or nucleophilic attack on chlorine with concomitant carbanion formation. Mercaptide anion pKb, degree of chlorination of the sulfone substrate, and substituent effects are qualitatively assessed in terms of the propensity for nucleophilic attack at carbon or chlorine.

Reactivity Optimization of Prevulcanization Inhibitors

1983 ◽  
Vol 56 (5) ◽  
pp. 1061-1079 ◽  
Author(s):  
A. B. Sullivan ◽  
L. H. Davis ◽  
O. W. Maender
Keyword(s):  
Side Reactions ◽  
Inhibitor Activity ◽  
Proper Selection ◽  
Leaving Group ◽  
Leaving Groups ◽  
The Relationship ◽  
Selection Of

Abstract A series of phthalimide-based prevulcanization inhibitors has been used to probe the relationship between MBT reactivity and inhibitor performance efficiency in NR accelerated with TBBS. Based on these results, there appears to be a reactivity window for best performance. For the phthalimide series, it occurs with secondary alkyl mercaptans. Inhibitors with MBT reactivity below the optimum are less effective because they do not compete with the accelerator for the autocatalyst. The activity also falls off at higher reactivities. There are two explanations for this behavior. First, as proposed by others, the very reactive inhibitors may not completely survive mixing and precuring, so that some of the inhibitor is lost to unproductive side reactions. Over and above this effect, the performance declines because the MBT is trapped less securely when the BtSSR is derived from a more reactive mercaptan. Although other leaving groups were not evaluated in this work, it is clear that reactivity optimization can be achieved with any leaving group, L, by proper selection of -SR. For example, in the work of Morita and Sullivan with thioketal type prevulcanization inhibitors, it was demonstrated that primary alkyl and aromatic mercaptans were required to achieve optimum inhibitor activity.

Studies of azo and azoxy dyestuffs. Part 18. Kinetics and mechanism of the hydrolysis of 3- and 4-(p′-methoxyphenylazo)pyridines in aqueous sulfuric acid media

1986 ◽  
Vol 64 (11) ◽  
pp. 2115-2126 ◽  
Author(s):  
Erwin Buncel ◽  
Ikenna Onyido
Keyword(s):  
Sulfuric Acid ◽  
Transition States ◽  
Activation Parameters ◽  
Nucleophilic Attack ◽  
Acid Solutions ◽  
Acid Media ◽  
Charge Delocalization ◽  
Leaving Group ◽  
The Difference ◽  
Hydrolysis Of

The kinetics of hydrolysis of 4-(p′-methoxyphenylazo)pyridine, 1, and its 3-isomer, 2, have been studied in moderately concentrated sulfuric acid media at 25 °C. In all the acid solutions investigated, 1 reacted faster than 2; rate differences between the two compounds varied from ca. 1000-fold in the dilute region of acidity to ca. 250-fold in the more concentrated acid solutions. The observed first-order rate constants, kψ, for both substrates exhibit a maximum, at ca. 42% H2SO4 and 47% H2SO4 for 1 and 2 respectively. Activation parameters have also been determined. The pKa values for the second protonation equilibria of 1 and 2 have been evaluated and structures of the diprotonated species are discussed. Hydrolysis is shown to occur from the diprotonated substrates and two main mechanisms are operative. The first is an A-2 type mechanism, which involves rate-limiting attack of H2O on the aryl carbon center giving delocalized transition states and intermediates in which the pyridinium and azonium functions are involved in charge delocalization. Subsequent transfer of a proton and detachment of the leaving group are fast processes. In the second A-SE2 type mechanism, nucleophilic attack and transfer of the proton are fast steps preceding the slow general acid catalyzed separation of the leaving group. The difference in reactivity of the two compounds is attributed to differences in extent of charge delocalization in the transition states of the reactions: for 1 both the pyridinium and protonated azonium functions are involved whereas for 2 only the azonium function participates in charge delocalization.

Glycosyl Group Transferases

2007 ◽  
Author(s):  
Perry A. Frey ◽  
Adrian D. Hegeman
Keyword(s):  
Great Majority ◽  
Nucleophilic Attack ◽  
Axial Position ◽  
Equatorial Position ◽  
Leaving Group ◽  
Dilute Aqueous Solutions ◽  
Anomeric Carbon ◽  
Basic Solutions ◽  
Hydrolysis Of ◽  
Acceptor Molecules

Glycosyl group transfer underlies the biosynthesis and breakdown of all nucleotides, polysaccharides, glycoproteins, glycolipids, and glycosylated nucleic acids, as well as certain DNA repair processes. Glycosyl transfer consists of the transfer of the anomeric carbon of a sugar derivative from one acceptor to another, as in, which describes the transfer of a generic pyranosyl ring between nucleophilic atoms :X and :Y of acceptor molecules. The stereochemistry at the anomeric carbon is not specified in eq. 12-1, but the leaving group occupies the axial position in an α-anomer or the equatorial position in a β-anomer. The overall transfer can proceed with either retention or inversion of configuration. In biochemistry, the acceptor atoms can be oxygen, nitrogen, sulfur, or in the biosynthesis of C-nucleosides even carbon. The great majority of biological glycosyl transfer reactions involve transfer between oxygen atoms of different acceptor molecules. Enzymes catalyzing glycosyl transfer are broadly grouped according to whether the acceptor :Y–R2 in is water or another molecule. In the actions of glycosidases, the acceptor is water, and glycosyl transfer results in hydrolysis of a glycoside, a practically irreversible process in dilute aqueous solutions. In the action of glycosyltransferases, the acceptors are molecules with hydroxyl, amide, amine, sulfhydryl, or phosphate groups. The simplest nonenzymatic glycosyl transfer reaction is the hydrolysis of a glycoside, and early studies revealed the fundamental fact that glycosides are much less reactive toward hydrolysis in basic solutions than in acidic solutions. This fact underlies much that is known about the mechanism of glycosyl transfer; that is, the anomeric carbon of a glycoside is remarkably unreactive toward direct nucleophilic attack, but it becomes reactive when one of the oxygens is protonated by an acid, as illustrated in fig. 12-1 for the acid-catalyzed hydrolysis of a generic glycoside. The reaction by both mechanisms in fig. 12-1 proceeds by pre-equilibrium protonation of the glycoside to form oxonium ion intermediates, which are subject to hydrolysis by water. The two mechanisms in fig. 12-1 are of interest. The mechanism proceeding through exocyclic cleavage of the glycoside has historically been regarded as the more likely, and for this reason, the route through endocyclic cleavage has received little consideration.

scholarly journals Expedient synthesis of 1,6-anhydro-α-D-galactofuranose, a useful intermediate for glycobiological tools

2014 ◽  
Vol 10 ◽  
pp. 1651-1656 ◽  
Author(s):  
Luciana Baldoni ◽  
Carla Marino
Keyword(s):  
Hydroxy Group ◽  
Building Blocks ◽  
Nucleophilic Attack ◽  
Step Procedure ◽  
Leaving Group ◽  
Anomeric Carbon ◽  
Good Precursor

A new and efficient three-step procedure for the synthesis of 1,6-anhydro-α-D-galactofuranose is described. The key step involves the formation of the galactofuranosyl iodide by treatment of per-O-TBS-D-Galf with TMSI, the selective 6-O-desilylation by an excess of TMSI, and the simultaneous nucleophilic attack of the 6-hydroxy group on the anomeric carbon, with the iodide as a good leaving group. This compound is a good precursor for building blocks for the construction of 1→6 linkages.

A mechanistic study of the [La2(OCH3)2]4+- and [(1,5,9-triazacyclodo-decane):Zn:(OCH3)]+-catalyzed methanolysis of carbonates: possible application for the recycling of bisphenol A polycarbonates

2013 ◽  
Vol 91 (11) ◽  
pp. 1139-1146 ◽  
Author(s):  
Alexei A. Neverov ◽  
Leanne D. Chen ◽  
Sean George ◽  
David Simon ◽  
Christopher I. Maxwell ◽  
...  
Keyword(s):  
Bisphenol A ◽  
Metal Ion ◽  
Catalytic Decomposition ◽  
Nucleophilic Attack ◽  
Alkyl Derivative ◽  
Mechanistic Study ◽  
Tetrahedral Intermediate ◽  
Rate Limiting Step ◽  
Leaving Group ◽  
Rate Limiting

The kinetics of the methanolysis of seven methyl aryl carbonates (3) and two methyl alkyl carbonates (4) promoted by [12[ane]N3:Zn:(OCH3)]+ and [La2(OCH3)2]4+ catalysts (1 and 2, respectively) have been studied at 25.0 °C. Brønsted plots of the [Formula: see text] values for methanolysis versus aryloxy and alkoxy leaving group (LG) [Formula: see text] or [Formula: see text] values (the pKa values of the parent ArOH or ROH in methanol) for substrates 3 and 4 show an apparent downward break at [Formula: see text] ∼16.6 and 15.2 with [12[ane]N3:Zn:(OCH3)]+ and [La2(OCH3)2]4+, respectively. The breakpoint is not due to a change in rate-limiting step in a two-step process involving metal ion delivery of a coordinated methoxide to a transiently associated substrate and the subsequent breakdown of a tetrahedral intermediate to form product. The more satisfactory explanation is that the break arises when one correlates the rate constants for two dissimilar sets of substrates, namely aryloxy- and alkoxy-substituted 3 and 4. DFT calculations for the 1-promoted reactions of methyl 4-nitrophenyl carbonate (3b), which has a good aryloxy leaving group, and methyl isopropyl carbonate (4b), which has a relatively poor alkyl one, indicate that the catalyzed processes involve two steps. Accordingly, the methanolysis of all 3 having [Formula: see text] values for the parent phenols ≤15.3 involves rate-limiting nucleophilic attack and fast breakdown. For the isopropyl alkyl derivative (4b) having a [Formula: see text] > 18.13, the rate-liming step is the metal ion promoted breakdown of a tetrahedral intermediate. The catalytic system employing 2 has utility for the catalytic decomposition of poly(bisphenol A carbonate). In a semi-optimized system where 1000 mg of poly(bisphenol A carbonate), treated at 100 °C for 30 min in 2 mL of 60:40 chloroform−methanol containing La(OTf)3:NaOMe (5:7.5 mmol L−1), the reaction gave an 84% yield of bisphenol A, corresponding to >300 turnovers per catalyst.

The relationship between the rate–equilibrium coefficient α and transition state properties: a second-order Møller–Plesset perturbation study of SN2 reactions

1992 ◽  
Vol 70 (2) ◽  
pp. 450-455 ◽  
Author(s):  
Zheng Shi ◽  
Russell J. Boyd
Keyword(s):  
Charge Transfer ◽  
Transition State ◽  
Sn2 Reactions ◽  
Leaving Group ◽  
The Difference ◽  
Equilibrium Relationships ◽  
Moller Plesset ◽  
Definition Of ◽  
The Relationship ◽  
Abinitio Calculations

Abinitio calculations including electron correlation are used to study rate–equilibrium relationships in gas-phase SN2 reactions. The difference between the "intrinsic" α and "group" α is emphasized. In general, the "group" α cannot be used as a measure of the transition state structure. The relationships between the "intrinsic" α and other properties, such as reaction endothermicity, geometry change, and the charge transfer, are discussed. A geometry change parameter Rα, defined by analogy with the definition of the "intrinsic" α, is shown to be linearly related to the "intrinsic" α. The charge transfer at the transition state is related not only to energy changes but also to the electronegativities of the entering nucleophile and leaving group in the product and reactant, respectively, and to the electronic structures at the transition state. Thus, the charge transfer parameter Qα, unlike the "intrinsic" α and Rα, is affected by the electronegativities of the groups involved in the reaction. The systems studied are SN2 reactions of the type N− + CH3X → CH3N + X−, where X = H, NH2, OH, OOH, F, CCH, CN, NC, PH2, SH, and Cl when N = H, and where X = H, NH2, OH, F, CN, NC, PH2, SH, and Cl when N = F. Keywords: SN2 reactions, rate–equilibrium relationships, transition state properties.

scholarly journals A computational and experimental study of O-glycosylation. Catalysis by human UDP-GalNAc polypeptide:GalNAc transferase-T2

2014 ◽  
Vol 12 (17) ◽  
pp. 2645-2655 ◽  
Author(s):  
Hansel Gómez ◽  
Raúl Rojas ◽  
Divya Patel ◽  
Lawrence A. Tabak ◽  
José M. Lluch ◽  
...  
Keyword(s):  
Experimental Study ◽  
Nucleophilic Attack ◽  
Front Side ◽  
Leaving Group

GalNAc-T2 catalyses GalNAc O-glycosylation via a front-side nucleophilic attack in which stabilization of the UDP leaving group is crucial.

An enantioconvergent halogenophilic nucleophilic substitution (SN2X) reaction

Science ◽  
2019 ◽  
Vol 363 (6425) ◽  
pp. 400-404 ◽  
Author(s):  
Xin Zhang ◽  
Jingyun Ren ◽  
Siu Min Tan ◽  
Davin Tan ◽  
Richmond Lee ◽  
...  
Keyword(s):  
Organic Chemistry ◽  
Nucleophilic Substitution ◽  
Steric Hindrance ◽  
Substitution Reaction ◽  
Common Variant ◽  
Nucleophilic Attack ◽  
Mechanistic Studies ◽  
Leaving Group

Bimolecular nucleophilic substitution (SN2) plays a central role in organic chemistry. In the conventionally accepted mechanism, the nucleophile displaces a carbon-bound leaving group X, often a halogen, by attacking the carbon face opposite the C–X bond. A less common variant, the halogenophilic SN2X reaction, involves initial nucleophilic attack of the X group from the front and as such is less sensitive to backside steric hindrance. Herein, we report an enantioconvergent substitution reaction of activated tertiary bromides by thiocarboxylates or azides that, on the basis of experimental and computational mechanistic studies, appears to proceed via the unusual SN2X pathway. The proposed electrophilic intermediates, benzoylsulfenyl bromide and bromine azide, were independently synthesized and shown to be effective.

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