Nitriles tautomeric forms

When PAN-T samples are heated by a gradual increase of temperature, the mercapto groups of the tautomeric form of the thioamide groups, enter into an intramolecular reaction with the nitrile groups in accordance with the following scheme ... 

All dilithiated nitriles show one peak in the 1900-1800 cm range. They could also occur in several tautomeric forms (equation 5). 

When R is hydrogen, more tautomeric forms similar to those of monometalated acetonitriles can be written . These modes appear at too low wavenumbers to be acetylenic or nitrile absorptions and therefore they were attributed to ketenimine derivatives of form R(Li)C=C=NLi. For the ionic species, Gornowicz and West proposed a delocalized structure (Scheme 8) °. Klein and Brenner called it sesquiazacetylene in a previous study . 

It is proposed that 279 are formed in the presence of I11CI3 by trapping of the nitril-ium ion intermediate by the / -hydroxy group of the tautomeric form of the ketoximes (equation 87). 

The Af,0-enolato ligands 146 are usually prepared by deprotonation of the corresponding conjugated acids which, for ketimines, can be formulated as consisting of the tautomeric forms 147-149. However, some of them have been prepared (i) by reacting nitriles with the corresponding S-diketonato complexes, or with 2-oxoalkyl or enolato complexes (Section in.C, Chapter 6 ) or (ii) by oxidation of diketoamines with metal complexes in the presence of a base (Section n.B.3). Although the most abundant tautomeric form of a S-ketimine depends on the nature of the substituents, they wiU be represented as 148. 

There are many Q moieties but 1,3-cylohexanediones, pyrazolones and diketo-nitriles are the most important examples. It is essential to know that all compounds of the general structure 1 could exist in different tautomeric forms, as shown in Fig. 4.4.2 for Q = 1,3-cyclohexanedione. 

Intensive studies using NMR methods, kinetic experiments, and computational calculations were conducted to elucidate the catalytic mechanism and observed stereoinduction [22]. The data revealed that the hydrocyanation catalyzed by 33 presumably proceed over an initial amido-thiourea catalyzed proton transfer from hydrogen isocyanide to imine 32 to generate a catalyst-bound diastereomeric iminium/cyanide ion pair. Thereby, hydrogen isocyanide, as the tautomeric form of HCN, is stabilized by the thiourea moiety of 33. The stabilization degree of the formed iminium ion by the catalyst is seen as the basis for enantioselectivity. Subsequent collapse of the ion pair and bond formation between the electrophile and the cyanide ion forms the a-amino nitrile. It should be emphasized that the productive catalytic cycle with 33 does not involve a direct imine-urea binding, although this interaction was observed both kinetically and spectroscopically in the Strecker reaction catalyzed by 25 (see above) [19],... 

The cationic species 4 thus formed reacts with water to give the iminol 5, which tautomerizes to a more stable amide tautomer, the N-substituted carboxylic amide 2. Those steps correspond to the formation of amides by the Schmidt reaction. A side reaction can give rise to the formation of nitriles. 

Compounds (99) and (100) are thought to be formed by addition of cyanide ion to the ring ortho to the carbonyl, followed by protonation at oxygen, aromatization by tautomerization, hydrolysis of the nitrile, and lactonization upon acidification. The photolysis of 2-methoxyacetophenone, on the other hand, results in rearrangement to 3-methoxyacetophenone ... 

The 1,3-dipoles were generated by the addition of Et3N in 20% excess. Only imidazole was basic enough to generate a nitrile oxide in the absence of triethy-lamine. Due to prototropic tautomerism, reactions of triazoles and tetrazoles led to mixtures of two isomers. With unsubstituted pyrazole and imidazole only one hydroximoylazole was formed (117). 

Scheme 65) <60G1290>. Oxadiazolines (156) are produced by cycloaddition of cyanamides and nitrile oxides aryl (Scheme 66) <66JPR22l>. Thus, for un- and monosubstituted cyanamides, cycloaddition occurs to the C=N double bond of a tautomeric carbodiimide form rather than to a nitrile triple bond. On the other hand, dicyandiamide (157) and benzonitrile oxide furnish (158) (Scheme 67). 

The nitrosation of aliphatic carbon atoms, particularly of carbon atoms activated by adjacent carbonyl, carboxyl, nitrile, or nitro groups, has been reviewed in great detail [2]. Judging from this review, with few exceptions, nitrosation of active methylene compounds leads to the formation of oximes (unfortunately termed isonitroso compounds in the older literature). The few exceptional cases cited in which true nitroso compounds (or their dimers) were formed involved tertiary carbon atoms in which no hydrogen atoms were available to permit tautomerism to the oxime or involved a reaction which was carried out under neither acidic nor basic conditions. 

Huisgen has stated that the driving force behind the 1,3-dipolar addition is stronger the more the loss of T-bond energy in the reactants is overcompensated by the energy of the two new bond energy is O-N < N-N < C-N, azides do not add at all to aldehydes and ketones and add with more difficulty to nitriles than to olefins. Phenyl azide, for instance, adds preferentially to the C-C double bond of acrylonitrile.194 103 This is also the reason why the condensation products of aldehydes and primary amines, which essentially exist in the Schiff-base structure 46a, react in the tautomeric enamine form 46b.2 ... 

Benzonitrile oxide (C in Figure 15.44) is an isolable 1,3-dipole. It can be generated from benzaldoxime and anNaOH/Cl2 solution. Under these reaction conditions the oxime/nitroso anion (A B) is initially formed and chlorine disproportionates into Cl—O and chloride. An SN reaction of the negatively charged C atom of the anion A B at the Cl atom of Cl— O or of Cl—O—H affords the oc-chlorinated nitroso compound E, which tautomerizes to the hydroxamic acid chloride D. From that species, the nitrile oxide C is generated via a base mediated 1,3-elimination. Isoxazoles are formed in the reactions of C with alkynes (Figure 15.44), while isoxazolines would be formed in its reactions with alkenes. 

Many examples of the intramolecular [2 + 2] photocydoaddition of alkenes to benzene derivatives have been reported. The acetophenone derivative 41 undergoes an efficient [2 + 2] photocydoaddition, leading to the cyclobutane derivative 42 (Scheme 5.9, reaction 18) [46, 47]. It was shown that, in this case, a nn triplet state is involved. The presence of a nitrile group in compound 43 induces a [2 + 2] cycloaddition at position 1,2 of the aromatic moiety, leading to intermediate VIII (reaction 19) [48]. Following tautomerization, the final product 44 is formed. 

Diaza-1,4-dienes 106 (or the tautomeric enamine form 107) are obtained by the addition of imines to nitriles in the presence of aluminium chloride (equation 44)65. 

The mechanism of nitrile hydrolysis in both acid and base consists of three parts [1] nucleophilic addition of H2O or OH to form the imidic acid tautomer [2] tautomerization to form the amide, and [3] hydrolysis of the amide to form RCOOH or RCOO. The mechanism is shown for the basic hydrolysis of RCN to RCOO (Mechanism 22.11). 

Hydrolysis of the nitrile produces the imino form of the amide, which readily tautomerizes by the usual mechanism to form the amide. 

Alcohols can be added to nitriles in an entirely different manner from that of reaction 16-9. In this reaction, the alcohol is converted by a strong acid to a carbo-cation, which is attacked by the nucleophilic nitrogen atom to give 117. Subsequent addition of water to the electrophilic carbon atom leads to the enol form of the amide (see 118), which tautomerizes (p. 98) to the V-alkyl amide. 

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