Bonding, and tautomerism

However, many reports in this field describe the intramolecular hydrogen bonding and tautomerism in Schiff bases bearing the hydroxy groups in both fragments. 

Shchavlev, A.E., Pankratov, A.N., and Shalabay, A.V., Theoretical studies on the intramolecular hydrogen bond and tautomerism of 8-mercaptoquinoline in the gaseous phase and in solution using modern DFT methods, J. Phys. Chem. A, 109, 4137 148 (2005). 

Some anthraquinone-sulphenic and -disulphenic acids and 1-metbyl-uracil-4-sulphenic add have been prepared by different routes . In these compounds intramolecular hydrogen-bonded and tautomeric structures are suggested to stabilize the sulphenic derivatives . Chemical and n.m.r. evidence for the existence of an aliphatic sulphenic acid in the... 

Scheme 13.14 Hydrogen bonding and tautomerism in sulfamethazinertheophyilline cocrystal.

Olivier L, Poupko R, Zimmermann FI and Luz Z 1996 Bond shift tautomerism of bibullvalenyl in solution and in the solid-state—a C-13 NMR-study J. Phys. Chem. 100 17 995-18 003... 

This 1,3-migration of hydrogen was also observed when 40 reacted with Lawesson s reagent to produce the dithiolactone 41. However, when y-hydroxy-a,P-unsaturated aldehyde 42 was reacted under similar conditions, thiophene 43 was prepared efficiently. These results are not surprising considering that the oxidation state of 42 is equivalent to the traditional saturated 1,4-dicarbonyl substrates of the Paal thiophene reaction via tautomerization of the double bond, and aromaticity is reestablished in the fully conjugated 43. 

The phenomenon of tautomerism comprises many different types of which the prototropic tautomerism that we consider here is only one. Prototropic tautomerism exists when the two tautomers differ only in the position of a proton (this is, of course, an approximation there are other differences between two tautomers, for example, in precise bond lengths). Other important types of tautomerism include the following (1) anioniotropy, where the two tautomers differ only in the position of an anion, which moves from one place to another in the molecule (2) cationiotropy, where the two tautomers differ in the position of a cation (other than a proton), which moves from one place to another in the molecule (3) ring-chain tautomerism and (4) bond-valence tautomerism. 

The mechanism of the first part of transamination is shown in Figure 29.14. The process begins with reaction between the a-amino acid and pyridoxal phosphate, which is covalently bonded to the aminotransferase by an iminc linkage between the side-chain -NTI2 group of a lysine residue and the PLP aldehyde group. Deprotonation/reprotonation of the PLP-amino acid imine in steps 2 and 3 effects tautomerization of the imine C=N bond, and hydrolysis of the tautomerized imine in step 4 gives an -keto acid plus pyridoxamine... 

Most methods of testing bond type involve the motion of nuclei. The chemical method, such as substitution at positions adjacent to a hydroxyl group in testing for double-bond character, as used in the Mills-Nixon studies, is one of these. This method gives only the resultant bond type over the period required for the reaction to take place. Since this period is much longer than that of ordinary electronic resonance, the chemical method cannot be used in general to test for the constituent structures of a resonating molecule. Only in case that the resonance frequency is very small (less than the frequencies of nuclear vibration) can the usual methods be applied to test for the constituent structures and in this case the boundary between resonance and tautomerism is approached or passed. 

There are two main factors to consider in order to understand the bonding and structures of metal complex azo colorants, namely (i) azo/hydrazone tautomerism and (ii) the nature of the azo-to-metal bonding. 

Thus, ion-complex tautomerism occurs when the cyclic form is transformed into the acyclic form. In this case the rupture of two bonds and the formation of two new ones occur. All the atoms retain their coordination only the type of bonds changes. Redistribution of electronic density... 

Both ionic forms have the same set of characteristic bands in their IR spectra and thus cannot be identified in solution. The 3IP NMR spectrum contains only one averaged signal. The IR study revealed tautomeric transformations in solution. The characteristic absorption band of the P—H bond and carbonyl group appeared in solution spectra that is possible only on dissociation of the a-hydroxyalkyl fragment, present only in the second tautomeric form. An X-ray single-crystal study showed the compound (128) to be in a cyclic form. 

No data are available for compounds with C=Se and C=Te bonds and their equilibria with the seleno- and telluro-enol tautomer. For simple selenoacetone, its tautomer, and telluro-analogues, quantum mechanical calculations have been carried out using different methods.5 These calculations have shown that on passing from O to Te, the energy difference between the tautomeric forms is reduced, a trend that parallels the calculated C=E bond energies, which decrease from O to Te. 

However, the introduction of sterically hindered substituents at the p-car bon atom of nitroalkene (42) completely changes the ring-chain tautomerism of conjugated nitroalkenes. Apparently, steric hindrance caused by two bulky Bu groups in product (42a) (Scheme 3.47) prevents effective conjugation of the jt systems of the C,C double bond and the nitro group, thus causing its deviation from the plane of the C=C bond as a result of which isomer (47a) becomes thermodynamically more favorable. 

This transformation proceeds through coordination of the isocyanide group to the ruthenium complex (structure 172), followed by insertion of the C-bound ruthenium into the benzylic C-H bond (intermediate 173). After ruthenium-mediated addition of the benzylic carbon to the isonitrile carbon and tautomerization, the desired product was obtained via elimination of the ruthenium complex. 

Generally, the imine substrates are prepared from the corresponding ketone and amine and are hydrogenated as isolated (and purified) compounds. However, reductive animation where the C = N function is prepared in situ is attractive from an industrial point of view, and indeed there are some successful examples reported below [18, 19]. It is reasonably certain that most catalysts described in this chapter catalyze the addition of H2 directly to the C=N bond and not to the tautomeric enamine C = C bond, even though enamines can also be hydrogenated enantioselectively. 

Nitro dyes exhibit benzenoid-quinonoid tautomerism (1.25) and their colour is attributed mainly to the o-quinonoid form, since this can be stabilised by hydrogen bonding. The tautomeric o-nitrosonaphthols (1.26) readily form chelate complexes with metals. A few yellow nitro disperse dyes, including Cl Disperse Yellow 1 (1.25), and brown acid dyes remain of significance. The remaining nitro and nitroso colorants, such as (1.26) and its 1 3 iron (II) complex (1.27), are no longer of commercial interest. 

The aggregation behaviour and tautomerism of three 0,0 -dihydroxy and one o-hydroxy-o -methoxy monoazo dyes have been studied by UV-visible spectroscopy [17]. Evidence of monomer-dimer equilibria was obtained for all four of these mordant dye structures. Intermolecular hydrogen bonding between the hydroxy groups and hydrophobic interaction between aryl nuclei contribute to the dimerisation effect. 

Nor can there be any question of real tautomerism in the case of phenol. In its chemical properties phenol resembles the aliphatic enols in all respects. We need only recall the agreement in the acid character, the production of colour with ferric chloride, and the reactions with halogens, nitrous acid, and aromatic diazo-compounds (coupling), caused by the activity of the double bond and proceeding in the same way in phenols and aliphatic enols. The enol nature of phenol provides valuable support for the conception of the constitution of benzene as expressed in the Kekule-Thiele formula, since it is an expression of the tendency of the ring to maintain the aromatic state of lowest energy. In this connexion the hypothetical keto-form of phenol (A)—not yet obtained—would be of interest in comparison with... 

Dibenzoylmethane (8b) has been the subject of much interest as regards the possibility that its polymorphism is associated with keto-enol tautomerism. Chemical and spectroscopic studies showed that this is not so (33a). This compound had previously been reported to be trimorphic (33b), but one form appears, in fact, to be a eutectic mixture of the other two. The molecules in these two polymorphs are both in the same state of tautomerism they differ in the torsional angle about the (CH)-(CO) bond and in the type of hydrogen bonding in which they participate. It is noteworthy that solutions prepared from these forms at low temperature have differences in chemical and spectroscopic properties that are maintained for some time. For example, such solutions prepared and held at —35° react at different rates with FeCl3. 

The most unusual bond in this system is the N-Cl bond. The nucleophilic substitution step must involve cleavage of this bond. No base is present, but S is an excellent nucleophile, even in its neutral form, so the first step probably entails formation of an S9-N2 bond. Now we have to make the C4-C10 bond and make the S9-N2 bond. Deprotonation of C4 gives an ylide, which as discussed in problem 4.15 is likely to undergo a [2,3] sigmatropic rearrangement. Tautomerization to rearomatize then gives the product. 

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