Pyridine, reactions

These compounds are soluble in ether, are comparatively stable, and exhibit many of the reactions of Grignard reagents but are more reactive. Because of their greater reactivity, organohthium compounds can often be used where Grignard reagents fail thus they add to the azomethine linkage in pyridines or  [c.928]

In the absence of specific interactions, ligands tend to reduce the efficiency of copper(II) and nickel(II) catalysis of the Diels-Alder reaction between 3.8c and 3.9. This trend is a consequence of diminished equilibrium constants for binding of the chenophile to the catalyst as well as decreased rate constants for the reaction of the resulting complex with the diene in the presence of the ligand. In special cases, specific interactions can cause a deviation from this behaviour. Most interestin y, the binding of the dienophile to copper complexes of aromatic -amino acids benefits considerably (up to 5.6 kJ/mole) from an enthalpy-driven arene - arene interaction between the aromatic ring of the -amino acid and the pyridine ring of the dienophile, leading to ligand - accelerated catalysis. There are indications that the arene - arene interaction is governed mainly by London-dispersion and electrostatic forces. These interactions dictate the conformation of the ternary dienophile - copper(II) - ligand complex leading to shielding of one face of the dienophile for approach by the diene. Consequently, enantioselectivity (up to 74% ee) is induced in the Diels-Alder reaction of the ternary dienophile - copper(II) - aromatic -amino acid complexes with diene 3.9. Interestin y, enantioselectivity benefits markedly from the use of water as the solvent. Use of this solvent also leads to cleaner reactions and reduced reaction times.  [c.101]

The interest attaching to the nitration of pyridine i-oxide and its derivatives has already been mentioned ( 8.2.5). Some data for these reactions are given in tables 8.1, 8.2 and 8.4. The 4-nitration of pyridine I-oxide is shown to occur through the free base by comparison with the case of i-methoxypyridinium cation ( 8.2.2), by the nature of the rate profile ( 8.2.1), and by consideration of the encounter rate ( 8.2.3). - Some of these criteria have been used to show that the same is true for  [c.192]

For many heterocycies straightforward syntheses can be proposed employing only simple nitrogen containing reagents. Regioselectivity is mostly no problem in these synthetic reactions, since the nitrogen reagents react symmetrically. The oxidation states of the ring atoms in the heterocycle to be synthesized are determined by the educts. Another important fact is, that formation of heteroaromatic systems (e.g. imidazoles, pyrroles, pyridines) is thermodynamically favored and thus often achieved under acidic or oxidative reaction conditions. Oxygen as a substituent on nitrogen heterocycies, however, tends to be double-bonded, and the N-protonated tautomers generally prevail in equilibrium.  [c.148]

The pyridine-like nitrogen of the 2H-pyrrol-2-yiidene unit tends to withdraw electrons from the conjugated system and deactivates it in reactions with electrophiles. The add-catalyzed condensations described above for pyrroles and dipyrromethanes therefore do not occur with dipyrromethenes. Vilsmeier formylation, for example, is only successful with pyrroles and dipyrromethanes but not with dipyrromethenes.  [c.255]

Like pyridines (334), thiazoles undergo addition reactions with dimethyl acetylenedicarboxylate leading to 2 1 molar adducts, the structure of which has been a matter of controversy (335-339).  [c.95]

These results show that in the phenylation of thiazole with benzoyl peroxide two secondary reactions enter in competition the attack of thiazole by benzoyloxy radicals, leading to a mixture of thiazolyl benzoates, and the formation of dithiazolyle through attack of thiazole by the thiazolyl radicals resulting from hydrogen abstraction on the substrate and from the dimerization of these radicals. This last reaction is less important than in the case of thiophene but more important than in the case of pyridine (398).  [c.109]


Two modified sigma constants have been formulated for situations in which the substituent enters into resonance with the reaction center in an electron-demanding transition state (cr+) or for an electron-rich transition state (cr ). cr constants give better correlations in reactions involving phenols, anilines, and pyridines and in nucleophilic substitutions. Values of some modified sigma constants are given in Table 9.4.  [c.1004]

The physical properties of pyridines are the consequence of a stable, cycHc, 6- TT-electron, TT-deficient, aromatic stmcture containing a ring nitrogen atom. The ring nitrogen is more electronegative than the ring carbons, making the two-, four-, and six-ring carbons more electropositive than otherwise would be expected from a knowledge of benzenoid chemistries. The aromatic TT-electron system does not require the participation of the lone pair of electrons on the nitrogen atom hence the terms weakly basic and TT-deficient used to describe pyridine compounds. The ring nitrogen of most pyridines undergoes reactions typical of weak, tertiary organic amines such as protonation, alkylation (qv), and acylation.  [c.322]

Chemical reactivity of pyridines is a function of ring aromaticity, presence of a basic ring nitrogen atom, TT-deficient character of the ring, large permanent dipole moment, easy polarizabiUty of the TT-electrons, activation of functional groups attached to the ring, and presence of electron-deficient carbon atom centers at the a- and y-positions. Depending on the conditions of the chemical transformation, one or more of these factors can give rise to the observed chemistry. The chemistry of pyridines can be divided iato two categories reactions at the ring-a tomic centers, and reactions at substituents attached to the ring-atomic centers.  [c.324]

Benzenesulphonyl chloride reacts with primary and secondary, but not with tertiary, amines to yield substituted sulphonamides (for full discussion, see Section IV,100,3). The substituted sulphonamide formed from a primary amine dissolves in the alkaline medium, whilst that produced from a secondary amine is insoluble in alkali tertiary amines do not react. Upon acidifying the solution produced with a primary amine, the substituted sulphonamide is precipitated. The reactions form the basis of the Hinsberg procedure for the separation of amines see Section IV,100,(viii) for details. Feebly basic amines, such as o-nitroaniline, react slowly in the presence of allcali in such cases it is best to carry out the reaction in pyridine solution see Section IV,100,3.  [c.1073]

At the outeet of the work described in this thesis, a number of questions were formulated. Given the substantia] benefits of water with respect to the uncatalysed Diels-Alder reaction, the most important question addressed the possibilities of transferring these benefits to the Lewis-acid catalysed reaction. It soon became obvious that this could not easily be achieved, since the majority of Diels-Alder reactants have a negligible tendency to interact with a Lewis-acid catalyst in water. Fortunately, the affinity of a Diels-Alder reactant for Lewis-acids can increase dramatically if the possibility of forming a chelate exists. In Chapter 2 it was demonstrated that, by following this approach, Lewis-acid catalysis of a Diels-Alder reaction in water is feasible. Moreover, it turned out that part of the beneficial effect of water on the uncatalysed Diels-Alder reaction is retained in the Lewis-acid catalysed counterpart. The studies in Chapter 2, and also in Chapter 3, employed a dienophile that has been specifically designed for bidentate binding to the Lewis-acid catalyst. To accomplish this a pyridine ring was fused to a a,p-unsaturated ketone fragment allowing coordination through formation of a 5-membered ring chelate. The encouraging results obtained for the Diels-Alder reaction of this molecule prompted investigation of the possibilities of extending these results to other Diels-Alder reactions. Attempts in this direction are described in this chapter, but first the literature claims of Lewis-acid catalysis of Diels-Alder reactions in water are critically examined.  [c.107]

Extension of the scope of Lewis-acid catalysis of Diels-Alder reactions in water through the introduction of a temporary chelating auxiliary is possible. With the aid of strongly chelatirg 2-(N-methylaminomethyl)pyridine (4.50), Lewis-acid catalysis of the Diels-Alder reaction of benzylidene ace tone (4.39) with cyclopentadiene (4.6) in aqueous solution is feasible, producing Diels-Alder adduct 4.54. Unfortunately, removal of the chelating auxiliary from this compound by a retro Mannich reaction was not successful. Instead, an intramolecular base-assisted elimination produced oc, (3-unsaturated ketone 4.56 in 24 % overall yield. We envisage that other dienophiles that are capable of undergoing a Mannich reaction with 4.50 can be treated with reactive dienes in the presence of a Lewis-acid catalysts in water.  [c.119]

Having observed the beneficial effects of water on the rate and enantioselectivity of one particular Diels-Alder reaction, an answer to the third question, addressing the scope of Lewis-acid catalysis of Diels-Alder reactions in water, became all the more desirable. Chapter 4 describes an investigation of the limitations of ftiis process. It is concluded that in water efficient catalysis is feasible only for Diels-Alder reactants capable ofbi- or multidentate binding to the catalyst. Unfortunately, hardly any common diene or dienophile fulfils this requirement. In an attempt to extend the scope of Lewis-acid catalysis in water we made use of a strongly chelating diamine as a coordinating auxiliary, introduced via a Mannich reaction. This approach may well be expected to subject those dienes or dienophiles capable of undergoing a Mannich reaction with 2-(methylaminomethyl)pyridine to Lewis-acid catalysis in water. However, enantioselective Lewis-acid catalysis employing the copper - aromatic a-amino acid complexes introduced in Chapter 3 is unlikely to be successful for these compounds. The Mannich adducts coordinate in a tridentate fashion. In this arrangement a geometry resembling that shown in Scheme 3.10 is unlikely. In conclusion, at this moment the scope of Lewis-acid catalysis in aqueous solution is still rather limited.  [c.162]

Chapter 3 describes an investigation into the effects of ligands on a Lewis-acid catalysed Diels-Alder in water. In the literature there are only a limited number of examples of systematic studies of ligand effects on Lewis-acid catalysed reactions in water. These studies mainly focns on the metal-ion catalysed decarboxylation of oxaloacetate. This reaction was observed to benefit from the presence of aromatic diamine ligands. This inspired us to investigate the influence of this class of ligands on the rate and endo-exo selectivity of the Lewis-acid catalysed Diels-Alder off with 2. We have selected 2,2 -bipyridine and 1,10-phenanthroline as target ligands and have included ethylenediamine, dimethylethylenediamine and 2-(aminomethyl)pyridine for comparison purposes. Unfortunately, none of these ligands was capable of inducing a significant increase in the ecpiilibrium constant (K ) for  [c.175]

We found a simple way to carry out anhydrous hydrogen fluoride reactions at atmospheric pressure in ordinary laboratory equipment (polyolefin or even glass) by using the remarkably stable complex formed between pyridine and excess hydrogen fluoride. HF (70%) and pyridine (30%) form a liquid complex, C5H5NFI (HF)xFT showing low vapor pressure at temperatures up to 60°C. This reagent (pyri-dinium polyhydrogen fluoride, sometimes called Olah s reagent) thus enables one to carry out a wide variety of synthetically very useful fluorination reactions safely and under very simple experimental conditions.  [c.103]

The most practical route to acyl azides starts with the hydrazinolysis of esters. The usually rather poorly soluble and poorly stable hydrazides are dissolved in mixtures of organic solvents (e.g. THF, DMF, AcOH) and strong acids (e.g. HQ, TFA) and then mixed with an equimolar amount of sodium nitrite or amyl nitrite at -10 °C to yield the azide almost instantaneously. The coupling step with amines at room temperature may require several days. A great advantage of the acyl azide method is the lack of a-racemization (see p. 230f.). The acyl chloride method is quicker and may also be applied for the preparation of esters and amides. Here the free acid is used as starting material, and aptotic solvents (CHCI3, DMF, pyridine) must be applied in the chlorination. Thionyl chloride and oxalyl chloride are the most common agents, and low temperatures are again advantageous. Nitiosation or chlorination of activated CH groups by nitrites or SOClj, resp., are sometimes troublesome side reactions.  [c.143]

Furthermore, the strongly metallic character of selenium weakens the C-Se bond and thus favors reactions involving opening of the ring. The basicity of the three heterocycles is approximately in the same order, the nitrogen atom of selenazoie and thiazole possessing much the same properties as the heteroatom of pyridine. Of the two carbon atoms ortho to nitrogen, that is, the 2-carbon and the 4-carbon, only the one in the 2-position is fairly active as a result of its interaction with selenium or sulfur. The 4- and 5-positions of thiazole and selenazoie are more susceptible to electrophilic substitution than the 3- and 5-positions of pyridine. This is particularly true of the 5-position of selenazoie. Thus it can be said that the 2- and 5-positions of the selenazoles and thiazoles  [c.239]

In all its reactions the lone pair of thiazole is less reactive than that of pyridine. Table 1-61 shows three sets of physicochemical data that illustrate this difference. These are (1) the thermodynamic basicity, which is three orders of magnitude lower for thiazole than for pyridine (2) the enthalpy of reaction with BF3 in nitrobenzene solution, which is 10% lower for thiazole than for pyridine and (3) the specific rate of quaterni-zation by methyl iodide in acetone at 40°C, which is about 50% lower for  [c.125]

Because of its dissymmetrical geometry, the thiazole ring affords an unique substrate for the study of the steric implications of 5 2 reactions. In the case of the Menschutkin quaternization reaction, both 2- and 4-positions give the possibility of introducing at the very proximity of the electrophilic carbon center various substituents whose electronic and especially steric effects could be finely adjusted. Thus it was possible to give experimental evidence for the geometrical variations in the transition state of Sn2 reactions induced by changing the leaving group from a comparative study of quaternization rates of or/Ito-substituted thiazoles and pyridines with various methylating agents (Mel, MeSOjF, MeOTs) it was experimentally established that an increase in the basicity of the leaving group moves the transition state of the Srg2 reaction closer to the products (457).  [c.126]

Although acetonitrile is one of the more stable nitriles, it undergoes typical nitrile reactions and is used to produce many types of nitrogen-containing compounds, eg, amides (15), amines (16,17) higher molecular weight mono- and dinitriles (18,19) halogenated nitriles (20) ketones (21) isocyanates (22) heterocycles, eg, pyridines (23), and imidazolines (24). It can be trimerized to. f-trimethyltriazine (25) and has been telomerized with ethylene (26) and copolymerized with a-epoxides (27).  [c.219]

Reactions with Ammonia and Amines. Acetaldehyde readily adds ammonia to form acetaldehyde—ammonia. Diethyl amine [109-87-7] is obtained when acetaldehyde is added to a saturated aqueous or alcohoHc solution of ammonia and the mixture is heated to 50—75°C in the presence of a nickel catalyst and hydrogen at 1.2 MPa (12 atm). Pyridine [110-86-1] and pyridine derivatives are made from paraldehyde and aqueous ammonia in the presence of a catalyst at elevated temperatures (62) acetaldehyde may also be used but the yields of pyridine are generally lower than when paraldehyde is the starting material. The vapor-phase reaction of formaldehyde, acetaldehyde, and ammonia at 360°C over oxide catalyst was studied a 49% yield of pyridine and picolines was obtained using an activated siHca—alumina catalyst (63). Brown polymers result when acetaldehyde reacts with ammonia or amines at a pH of 6—7 and temperature of 3—25°C (64). Primary amines and acetaldehyde condense to give Schiff bases CH2CH=NR. The Schiff base reverts to the starting materials in the presence of acids.  [c.50]

Displacement reactions with oxygen nucleophiles are of potential commercial interest. Alkaline hydrolysis provides 2-fluoro-6-hydroxypyridine [55758-32-2], a precursor to 6-fluoropyridyl phosphoms ester insecticides (410—412). Other oxygen nucleophiles such as bisphenol A and hydroquinone have been used to form aryl—pyridine copolymers (413).  [c.336]

The elimination of two ring hydrogens accompanied by the formation of an aryl—aryl bond under Friedel-Crafts conditions is known as the SchoU reaction (1). The dehydrogenating condensations can take place by either iater- or iatra-molecular pathways. Intermolecular SchoU reactions are numerous and iaclude such reactions as formation of biphenyl from ben2ene, of perylene from naphthalene (through biaaphthyl), of 2,2 -dipyridyl from pyridine, and the formation of high molecular weight polycondensed aromatics.  [c.556]

Miscellaneous Reactions. Some hydantoin derivatives can serve as precursors of carbonium—immonium electrophiles (57). 5-Alkoxyhydantoins are useful precursors of dienophiles (17), which undergo Diels-Alder cycloadditions under thermal conditions or in the presence of acid catalysis (58). The pyridine ring of Streptonigrine has been constmcted on the basis of this reaction (59).  [c.253]

Iron(II) phthalocyanine [132-16-1] (3), a green compound, was first prepared by accident during the manufacture of phthalimide. Phthalocyanines are an important group of blue/green pigments that have excellent color intensity, photochemical and thermal stabiUty, and chemical inertness (see Phthalocyanine compounds). They find use in dyes, inks, paints, toners, and optical recording media. Iron(II) phthalocyanine is prepared by reductive cyclization of phthalonittile with finely divided iron in a high boiling solvent such as 1-chloronaphthalene and is purified by sublimation at 450°C under partial vacuum. The iron in the complex has a square planar coordination geometry and an intermediate spin, 5 = 1, ground state. The complex is insoluble in most noncoordinating organic solvents, but dissolves in very strong acids such as sulfuric and chlorosulfonic acids owing to protonation of the basic bridging a2a groups. The compound does not dissolve in hot hydrochloric acid, but instead reacts with it to form a material called chloroferric phthalocyanine [14285-56-4] the nature of which is not fully resolved. Iron(II) phthalocyanine forms adducts in coordinating solvents or in the presence of bases, for example phthalocyariinatobis(pyridine)iron [20219-84-5] which can be low spin. Water-soluble iron phthalocyanine complexes are obtained by sulfonating the phenyl residues to obtain tetrasodiumphthalocyaninetetrasulfonatoferrate [41867-66-7]. Purer materials may be obtained, however, by cyclization of sulfonated phthaUc acid or nitrile monomers. Iron(II) phthalocyanine may be reduced by up to four electrons. The complex finds use as a catalyst for a variety of chemical and electrochemical redox reactions.  [c.439]

The reactions of primary amines and maleic anhydride yield amic acids that can be dehydrated to imides, polyimides (qv), or isoimides depending on the reaction conditions (35—37). However, these products require multistep processes. Pathways with favorable economics are difficult to achieve. Amines and pyridines decompose maleic anhydride, often ia a violent reaction. Carbon dioxide [124-38-9] is a typical end product for this exothermic reaction (38).  [c.450]

Detoxifica.tlon. Detoxification systems in the human body often involve reactions that utilize sulfur-containing compounds. For example, reactions in which sulfate esters of potentially toxic compounds are formed, rendering these less toxic or nontoxic, are common as are acetylation reactions involving acetyl—SCoA (45). Another important compound is. Vadenosylmethionine [29908-03-0] (SAM), the active form of methionine. SAM acts as a methylating agent, eg, in detoxification reactions such as the methylation of pyridine derivatives, and in the formation of choline (qv), creatine [60-27-5] carnitine [461-06-3] and epinephrine [329-65-7] (50).  [c.379]

Synthesis. Peroxyesters are prepared by the reaction of alkyl hydroperoxides R OOH, with acylating agents, eg, acid chlorides, anhydrides, ketenes, organosulfonyl chlorides, phosgene (qv), alkyl chloroformates, oxalyl chloride, alkyl chlorooxalates, isocyanates, carbamoyl chlorides, carboxyhc acids, and esters, under appropriate reaction conditions, according to Figure 2. Reactions with acylating agents that generate hydrogen chloride are carried out in the presence of a base, eg, pyridine or sodium hydroxide, or by using the sodium or potassium salt of the hydroperoxide.  [c.126]

Peroxyesters may also be prepared by condensation of hydroperoxides with carboxyUc acids using condensing agents, eg, dicyclohexylcarbodiimide (214), imida2ohdes (213), and -toluenesulfonyl chloride with pyridine (213). Suitable esters, eg, monoesters of ethylene glycol, have been used to prepare peroxyesters by ester interchange with alkyl hydroperoxides (215). Generally, reactions of isocyanates to form peroxycarbamates are cataly2ed by dibutyltin dilaurate (213,216).  [c.127]

Halophenols without 2,6-disubstitution do not polymerize under oxidative displacement conditions. Oxidative side reactions at the ortho position may consume the initiator or intermpt the propagation step of the chain process. To prepare poly(phenylene oxide)s from unsubstituted 4-halophenols, it is necessary to employ the more drastic conditions of the Ullmaim ether synthesis. A cuprous chloride—pyridine complex in 1,4-dimethoxybenzene at 200°C converts the sodium salt of 4-bromophenol to poly(phenylene oxide) (1)  [c.330]

Polymerization of Dianhydrides and Diisocyanates. Phthahc anhydride does not react readily with an aromatic isocyanate (70). /V-Pheny1phtha1imide is obtained from phthaUc anhydride and phenyUsocyanate only in refluxing pyridine, suggesting the reaction may be catalyzed by base. The reaction proceeds in dipolar solvents at moderate temperatures and protic compounds such as water, alcohols, and amines serve as catalysts (71). High molecular weight polyimides have been obtained ia dipolar solvents by several iavestigators (72—76). High molecular weight polyetherimides are synthesized from bis(ether anhydride)s and diisocyanates by melt polymerization or high temperature solution polymerization ia nonpolar aromatic solvents (77). In the absence of polar solvent, the complex side reactions are minimal but the reaction rate is slow. However, ia the presence of a stable base such as alkaU carbonates, the polymerization proceeds efftciendy at temperatures above 200°C. PMDA and an aromatic diisocyanate with bulky substituents have been polymerized (78). High molecular weight polyimides have formed also ia high boiling nonpolar aromatic solvents such as nitrobenzene, henzonitrile, and anisole, whereas polymers with only moderate molecular weights were obtained ia dipolar solvents such as NMP.  [c.403]

Pyridine was first synthesized in 1876 (1) from acetjiene and hydrogen cyanide. However, a-picoline (2) was the first pyridine compound reported to be isolated in pure form (2). Interestingly, it was the market need for (2) that motivated the development of synthetic processes for pyridines during the 1940s, in preference to their isolation from coal-tar sources. The basis for most commercial pyridine syntheses in use can be found in the early work of Chichibabin (3). There are few selective commercial processes for pyridine (1) and its derivatives, and almost all manufacturing processes produce (1) along with a series of alkylated pyridines in admixture. The chemistry of pyridines is significantly different from that of benzenoids. Pyridines undergo some types of reaction that only highly electron-deficient benzenoids undergo, and do not undergo some facile reactions of benzenoids, such as Friedel-Crafts alkylation and C-acylation, for example.  [c.322]

See pages that mention the term Pyridine, reactions : [c.42]    [c.160]    [c.274]    [c.406]    [c.2788]    [c.915]    [c.109]    [c.73]    [c.219]    [c.337]    [c.508]    [c.47]    [c.478]    [c.521]    [c.325]   
Practical organic chemistry (1960) -- [ c.377 ]

Practical organic chemistry (1978) -- [ c.377 ]