Tautomerism Schiff bases

The effect of a covalently linked carboxylic group on the Schiff base tautomerism has been studied using the labeled model system 50 dissolved in CD2CI2... 

The nucleophilicity of amine nitrogens is also differentiated by their environments. In 2,4,5,6-tetraaminopyrimidine the most basic 3-amino group can be selectively converted to a Schiff base. It is meta to both pyrimidine nitrogens and does not form a tautomeric imine as do the ortho- and /xira-amino groups. This factor is the basis of the commercial synthesis of triamterene. 

The NMR spectroscopy has been widely used in the studies of different types of equilibria like ring-chain tautomerism, racemisation or stereomutation and proton transfer equilibrium in Schiff bases. 

Talancon et al.40 investigated the effects of substituents on the ring-chain tautomerism of Schiff bases being derivatives of norephedrine on the basis of 1H and 13C NMR measurements [19]. 

The ring-chain tautomerism has been also studied for di-Schiff base derivatives of C-substituted serinols [20].41... 

The 8N chemical shift can be used in studies of tautomeric equilibrium for optically active Schiff bases, however, it depends not only on the position of equilibrium but also on the types of substituents. Usually values higher than —200 ppm (relative to nitromethane) indicate the presence of pure NH-form or equilibrium strongly shifted towards NH tautomer. Lower values of 815N imply the presence of tautomeric equilibrium. ... 

The anomeric and tautomeric equilibria of the Schiff bases being derivatives of D-2-glucosamine have been studied by Kotodziej et al. [28]. 58... 

Diphenylcyclopropene thione also reacts with Schiff bases 379 via the tautomeric vinylamine form to give betaines 380, which tautomerize to the bicyclic thio amides 3 /247 ... 

Likewise, Schiff bases of type 379 react with triafulvenes again (see p. 75) via the tautomeric vinylamine form to give products of C—C-insertion 253. In the presence of a COOR group at triafulvene C4, the cross-conjugated system 536 undergoes facile cyclization to 2-pyridones 537 by loss of alcohol. 

The various tautomers and rotamers of alloxan have been examined in detail by the MNDO method and it is predicted that the keto form is most important in the gas phase, although in solution the monohydroxy forms are also thought to contribute. A mass spectral study has been used to investigate the enol-keto tautomeric equilibria of a series of substituted salicylaldehyde and 2-hydroxynaphthaldehyde Schiff bases. In neutral, ethanolic solutions, the cis- and trans-tm forms of 4,5-dimethyl-2-(2 -hydroxyphenyl)imidazoles (393) and (394) have been found to exist in equilibrium in the ground state. However, in neutral aqueous solutions, the trans-eao and keto forms (394) and (395) were the only species detected. Deuterium isotope effects on... 

Schiff bases are obtained from 2-aminoimidazothiadiazoles with aldehydes (77M665). Hydrazones and oxime ether formation of imidazothia-diazole-5-carbaldehydes and -5-ketoesters was reported (84FES585 95EUP662477). Reduction of 122 with Al/FIg yields the tautomeric imine 128 of the corresponding amine (83JHC1003). 

The iV-unsubstituted 2-aryltetrahydro-l,3-oxazines exhibit ring-chain tautomerism in solution. Depending on the steric and electronic characters of the substituents, the ring form and the open-chain Schiff base form occur in different ratios. For a more detailed discussion, see Section IV,A,4. 

Both single and double cyclizations of the Schiff bases 65A and 66A occur in the multicomponent tautomeric mixtures derived from as- and /ra t-cyclohexane aminodiols 69 and 70, to give monocyclized ring forms of C-2-epimeric perhydro-l,3-benzoxazines (B and G) and oxazolidines (D and E), besides 7,ll-dioxa-9-azatricyclo[7.2.1.0 °]-dodecane diastereomers 67 and 68 (Scheme 10). The tautomeric ratios are found to satisfy by Equation (1) (Table 1) and are influenced by the cis- or rra t-geometry of the cyclohexane substituents in the Schiff base (A). For the equilibria... 

In a number of nonenzymatic reactions catalyzed by pyridoxal, a metal ion complex is formed—a combination of a multivalent metal ion such as cupric oi aluminum ion with the Schiff base formed from the combination of an amino acid and pyridoxal (I). The electrostatic effect of the metal ion, as well as the electron sink of the pyridinium ion, facilitates the removal of an a -hydrogen atom to form the tautomeric Schiff base, II. Schiff base II is capable of a number of reactions characteristic of pyridoxal systems. Since the former asymmetric center of the amino acid has lost its asymmetry, donation of a proton to that center followed by hydrolytic cleavage of the system will result in racemic amino acid. On the other hand, donation of a proton to the benzylic carbon atom followed by hydrolytic cleavage of the system will result in a transamination reaction—that is, the amino acid will be converted to a keto acid and pyridoxal will be converted to pyridoxamine. Decarboxylation of the original amino acid can occur instead of the initial loss of a proton. In either case, a pair of electrons must be absorbed by the pyridoxal system, and in each case, the electrostatic effect of the metal ion facilitates this electron movement, as well as the subsequent hydrolytic cleavage (40, 43). 

Ir and nmr analysis of the condensation products of primary amines with aldehydes and ketones are in favor of the Schiff-base structure (46a). Nevertheless, these substances react with aryl azides in chloroform solution in the tautomeric enamine form (46b) yielding aminotriaiolines.206 225- 227 It proves that the dipolarophilic activity of an enamine olefin bond is much greater than that of an azomethine bond. 

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 ... 

Beta replacement is catalyzed by such enzymes of amino acid biosynthesis as tryptophan synthase (Chapter 25),184 O-acetylserine sulfhydrylase (cysteine synthase),185 186a and cystathionine (3-synthase (Chapter 24).187 188c In both elimination and (3 replacement an unsaturated Schiff base, usually of aminoacrylate or aminocrotonate, is a probable intermediate (Eq. 14-29). Conversion to the final products is usually assumed to be via hydrolysis to free aminoacrylate, tautomerization to an imino acid, and hydrolysis of the latter, e.g., to pyruvate and ammonium ion (Eq. 14-29). However, the observed stereospecific addition of a... 

The EXAFS technique has been especially useful for metalloproteins. It has often provided the first clues as to the identity of atoms (O, N, S) surrounding a metal atom and either covalently bonded to it or coordinated with it (Chapter 16). Interpretations are often difficult, and a common approach is to try to simulate the observed spectrum by calculation from a proposed structure.118 Tautomerism in crystalline Schiff bases (see Eq. 23-24) has been studied by nearedge X-ray absorption fine structure (NEXAFS) employing soft X-rays.119... 

Thus, with methyl and also ethyl cyanoacetate, the anilinomethylene chromonedione (220) (this form has been found94 to predominate in dimethylsulfoxide solution rather than the tautomeric structure of the formal SchifFs base from 3-formyl-4-hydroxycoumarin and aniline) yields, initially, with potassium hydroxide in DMF, following hydrolysis, the pyronocoumarin 221. 

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