Analyse conformationnelle a l'aide de la notation des angles de torsion—III: Controle conformationnel de certaines reactions de type sn2' ou sni' de derives cyclohexeniques faisant intervenir un deplacement allylique de l'olefine

To take place stereoselectively S_N2' reactions of cyclohexene derivatives require an axial leaving group either on a half-chair (suprafacial reaction) or on a 1,3-diplanar form or a boat (antarafacial reaction). These conclusions appear generally relevant to concerted reactions of cyclohexenic derivatives involving an allylic migration of the olefinic bond.     

Groupes de départ inhabituels lors de cyclisations dans la série des quinoxalines,II [1]

Quinoxaline-3-ketones substituted by different groups in position 2 (I) are easily cyclized by hydroxylamine and phenylhydrazine to form isoxazolo[4, 5-b] quinoxalines (II) and pyrazolo [3, 4-b] quinoxalines (III) respectively. The reactions proceed via the oximes resp. phenylhydrazones. Groups displaced are not only the customary leaving groups of aromatic S_N2 reactions (halogens, OH), but likewise H, COOH, CONH₂, CO-Ar, and, less easily, benzyl groups; methyl and phenyl groups were not displaced. The displacement of hydride ion in the presence of excess of hydroxylamine resp. phenylhydrazine is explained in terms of an extension of the theory of osazone formation.     

Convergent synthesis of panclicin-D via intramolecular SN2 displacement approach

A convergent enantioselective synthesis of panclicin-D has been reported from simple octanal using syn aldol reaction via intramolecular S_N2 displacement reaction for the first time towards the construction of anti-β-lactones in panclicin-D. The key steps involved are C-allylation, asymmetric aldolization under Crimmins condition, intramolecular S_N2 displacement, and Mitsunobu esterification reaction.     

Nucleophilic and organometallic displacement reactions of allylic compounds: stereo-and regiochemistry

Allylic alcohols, esters, halides and related compounds have been of mechanistic and synthetic interest for years. This Report focuses on the stereo- and regiochemical aspects of three reaction types: the S_N2' reaction (bimolecular nucleophilic substitution with allylic rearrangement); displacement reactions effected by organometallic reagents; the conversion of allylic alcohols into halides.     

Stereochimie de l'action des organomagnesiens et des organolithiens sur les dimethyl-1,2 alcoxy-1 silacyclobutanes

The reaction of organomagnesium organolithium compounds (alkyl, aryl, allyl and benzyl derivatives) with 1,2-dimethyl-1-alkoxy-l-silacyclobutanes proceeds with retention of configuration at the silicon atom. Stereochemical results are discussed in terms of the S_N2Si mechanism. The proposed configurations are supported by ¹H and ¹³C NMR data.     

Étude conformationnelle et spectres de vibration de la β alanyl ciliatine

β-Alanyl-ciliatine (β-Ala-Cilia) studied in the solid state by X-ray diffraction and IR spectrometry, is shown to exist as the monobydrated zwitterion H₃N⁺CH₂CH₂CONHCH₂CH₂PO₃H^?, H₂O. Its conformation in the crystal (P2₁/c space group, Z = 4) is defined by: τ₁ = +66.1°, ψ = ?157.4°. φ = +140.3°, τ₂ = +175.4°, τ₃ = +50.0°, τ₄ = +80.1° and is compared with those of similar molecules. A partial interpretation of the IR spectra is given. A conformational energy calculation of the isolated zwitterion shows different conformations stabilized by various types of chelation.     

Beiträge zur Chemie der Thiazylhalogenide. 9. Zur Chemie des Cyclopentathiazeniums S5N5+

Contributions to the Chemistry of Thiazylhalides. 9. On the Chemistry of the Cyclopentathiazenium, S₅N₅⁺ Preparation, properties, and reactions of a new thiazylchloride, S₅N₅⁺Cl^?, are reported. The chloride is a product of the direct reaction of S₄N₄ and (NSCl)₃. Using S₅N₅Cl, further new cyclopentathiazenium compounds (S₅N₅Br, S₅N₅Br₃, S₅N₅I₂Cl, and S₅N₅SbCl₄) could be prepared. Their properties show a strong dependence of the stability of the compound on the size of the anion present.     

Mecanismes de la protonolyse des organo-chromiques issus de bromures propargyliques.

The reaction of CrCl₂ with propargylic bromides leads to an allenic organo—chromic derivative protonated without rearrangement by carboxylic acids. Alcohols like methol transform this metallic to its propargylic isomer and then protonate it with rearrngement (S_Ei′ type mechanism).     

Theoretical investigation of ion pair SN2 reactions of alkali isothiocyanates with alkyl halides. Part 1. Reaction of lithium isothiocyanate and methyl fluoride with inversion mechanism

The gas‐phase ionic S_N2 reactions NCS^‐ + CH₃F and ion pair S_N2 reaction LiNCS + CH₃F with inversion mechanism were investigated at the level of MP2(full)/6‐311+G**//HF/6‐311+G**. Both of them involve the reactants complex, inversion transition state, and products complex. There are two possible reaction pathways in the ionic S_N2 reaction but four reaction pathways in the ion pair S_N2 reaction. Our results indicate that the introduction of lithium significantly lower the reaction barrier and make the ion pair displacement reaction more facile. For both ionic and ion pair reaction, methyl thiocyanate is predicted to be the major product, but the latter is more selective. More‐stable methyl isothiocyanate can be prepared by thermal rearrangement of methyl thiocyanate. The theoretical predictions are consistent with the known experimental results. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Nucleophilic displacement reactions on fluorine. II. The role of σ- and π-bond nucleophiles

S_N2 displacement reactions on fluorine by σ- and π-bond nucleophiles are discussed and shown to constitute a general class of reaction. Mechanisms are suggested for reactions mentioned in the literature which are consistent with this generalization.     

Unconventional Cellulose Products Through Nucleophilic Displacement Reactions

Summary: The paper highlights the synthesis of unconventionally functionalized cellulose derivatives obtained by nucleophilic displacement (S_N) reactions. The key intermediates, cellulose sulphonates, are discussed due to the fact that sulphonate groups play the major role as leaving group in S_N reactions with the biopolymer. A short overview about “classical” S_N reactions will be given. The main part of the review is focused on novel cellulose products in particular aminodeoxy cellulose derivatives showing monolayer formation, click chemistry with cellulose starting from deoxyazido (copper-catalyzed Huisgen reaction) yielding products with methylcarboxylate-, 2-aniline-, 3-thiophene moieties, e.g., and new selectively dendronized cellulose based materials. Structure characterization and typical applications are briefly reviewed as well.     

SILICON-PHOSPHORUS ANALOGIES THE MECHANISM OF NUCLEOPHILIC SUBSTITUTION AT SILICON AND PHOSPHORUS ACTIVATED BY EXTERNAL NUCLEOPHILES EXTENSION OF COORDINATION OF THE REACTIVE SPECIES

Abstract

A comparison is made concerning the mechanism of S_N2 reactions of silicon and phosphorus compounds, catalyzed by nucleophiles. The rate-law of the catalyzed solvolysis of halogenophosphorus esters corresponds to a mechanism with extension of coordination of the reactive species. The overall inversion observed with chlorophosphonates affords a distinction between a nucleophilic activated substitution S_NA (P) and a nucleophilic double displacement at phosphorus.     

Novel Nucleophilic Substitution Reactions of 5‐Halo‐4‐Sulfur‐Substituted 5,6‐Dihydro‐2‐Pyridones

The title compounds ( 3 , 8 , 9 and 10 ) were efficiently synthesized, and their substitution reactions with various nucleophiles were carried out. The effects of leaving group, sulfur‐substituent, solvent, reaction temperature, and the nature of the nucleophiles on the reactivity and S_N2/S_N2′ regioselectivity were studied and rationalized with semi‐empirical calculations.     

Nucleophilic displacement reactions on fluorine. I. General phenomenon and lone-pair nucleophiles

S_N2 displacement reactions involving halogen atoms are progressively more facile in going from F to I. Diverse and hitherto largely mechanistically uncorrelated examples of S_N2(F) reactions are presented and its is suggested that they form part of a general phenomenon in fluorine chemistry.     

Etude de la stabilisation du polychlorure de vinyle avec des molecules modeles—IV. Composes epoxydes

Chloro-4-hexene-2 (C4H2), a model compound for the study of polyvinylchloride stabilization, reacts with epoxy compounds in dichloroethane but not in tetrahydrofuran. In the latter case, the reaction is catalysed by zinc chloride but this compound is also a catalyst for the polymerization of the epoxy function. The zinc chloride is then modified so that the kinetic laws for etherification and dehydrochlorination of C4H2 are more complex than when the catalyst is CdCl₂, as previously studied by Anderson and McKenzie. With zinc and calcium stearate, one also observes esterification of C4H2, the three reactions being competitive. The synergistic effect of epoxy in the presence of stearate is induced chiefly by modification of ZnCl₂ through the initiation reaction for the polymerization. Therification, as well as esterification, may be reversed by HCl which regenerates C4H2; this fact explains the catastrophic character of degradation after the stabilizer has been consumed. The results of the study on model compounds can be applied to the degradation of PVC upon mastication in a Brabender plastograph.     

Etude de la nature des especes ioniques intervenant au cours de la polymerisation anionique de la vinyl-4 pyridine

Initiation of anionic polymerization of 4-vinyl-pyridine (4-VP) in tetrahydrofuran solution, using organo-metallic compounds of Na and K, involves the same type of side reactions as the polymerization of 2-vinyl pyridine (2-VP). The living polymers of 2- and 4-VP have similar u.v. spectra. The values of the dissociation constants of ionic species of 2- and 4-VP are similar when the chain [poly(styrene-b vinyl-4 pyridine)] contains one 4-VP unit (K_n ≈ 10^?9). But when the size of 4-VP block increases, the conductivity goes up to the value observed for living polystyrene (K_D ≈ 10^?7). This phenomenon could be due to a solvation of the counter-ion by the poly 4-vinyl pyridine chain, acting as a polydentate complexing agent.     

Nucleophilic substitution at phosphorus: stereochemistry and mechanisms

This review is devoted to the stereochemistry of nucleophilic substitution reactions at phosphorus. The study of the reactions of phosphoryl group transfer is important for biological and molecular chemistry. The stereochemistry and mechanisms of S_N1(P) monomolecular and S_N2(P) bimolecular nucleophilic substitution reactions of organophosphorus compounds are discussed. It has been shown that hydrolysis of many natural phosphates proceeds according to the monomolecular S_N1(P) mechanism via the formation of metaphosphate intermediate (PO₃^?). S_N2(P) nucleophilic substitution at chiral trivalent or pentavalent phosphorus compounds proceeds via the formation of penta-coordinated transition state or pentacoordinate intermediate.     

Etude stereochimique de l'alkylation d'anions par des derives p-nitrobenzyliques optiquement actifs : Competition SN2-SNR1

Competition between S_RN1,S_N2 and S_RN1 mechanisms is discussed according to the stereochemical results in the alkylation of anions by optically active secondary p-nitrobenzyl reagents. Results from alkylation of the anions of benzylcyanide C and α-aminonitrile A by p-nitrobenzyl chloride 2 rule out S_N2 and S_RN2 mechanisms. On the other hand, the S_N2 process becomes exclusive in O-and C-alkylation of the acetoacetic ester anion B by the p-nitrobenzyl phosphonium salt, and this result shows that it is possible to obtain p-nitrobenzyl alkylation products without racemisation. C-Alkylation of the anion B by halide 2 involves an S_N2-electron transfer competition. The whole result illustrates that the stereochemical method provides precise information on the mechanism of these reactions.     

Etude de la degradation thermique des oligomeres du polyacrylonitrile

To obtain more information on the reactions during the pyrolysis of polyacrylonitrile at 290° in air, we studied the thermal degradation of its trimers, tetramers and pentamers having the general formula: R  CH(CN)  CH₂  CH(CN)_n?1  R where R = H or CH₃ and n = 3,4,5. Study by gas chromatography of the volatile compounds produced during pyrolysis at 290° and 770° showed that the first step of the degradation is homolytic scission of the chain. There are then intra and intermolecular polymerisations of the nitrile groups and formation of a ladder polymer. This reaction is clearly exothermic only for pentamers. This increase of the reactivity with DP is borne out by a kinetic study by EPR of the pyrolysis in air at 290°, oxidation of the ladder polymer giving a paramagnetic solid. Its EPR characteristics (band width and number of spins per g) are very dependent of the DP and nature of the end-groups, H or CH₃, of the pyrolysed oligomer.     

Triflate Induced C-N Coupling of Sugars with Heterocyclic Bases: Synthesis of a New Class of Compounds

A new class of pharmacologically interesting compounds has been synthesized through a novel S_N2 displacement of trifluoromethanesulfonyloxy group in protected sugar (1) with a variety of secondary heterocyclic cases (2–6). The reaction pathway also provides a possible route to alklodial N-glycosides.