Chelated hydrogen bond

Table 8.8. Three-center chelated hydrogen-bond configurations observed in the crystal structure of the purines, pyrimidines, nucleosides and nucleotides (normalized from X-ray data)...

Monothio -diketones can be prepared by the action of hydrogen sulfide and hydrogen chloride on the appropriate -diketone in alcohol solution. Nevertheless the conditions are rather critical. At room temperature -dike-tones are in tautomeric equilibrium between the diketo form (I) and the chelated hydrogen-bonded form (II), and in polar solvents the concentration of the diketo form (I) is increased. Reaction with hydrogen sulfide occurs only with the diketo tautomer (I). Consequently, higher concentrations of hydrogen chloride are required for those -diketones which exist predominantly in the... 

Hydroxybiphenyls 277—see also Octachlorodihydroxybiphenyls, 6-Hydroxycarbonyl compounds, chelated hydrogen bonding in 377 Hydroxycarboxyhc acids, decarboxylation of 1079 reactions in presence of titanium oxide 1080... 

Fig. 14.28. Schematic representation of the hydrogen bonding between molecules and of the chelate hydrogen bonding within the molecules in the structure of salicylic acid, o-C6H4.OH.COOH.

The structure of 2-(o-hydroxyphenyl)benzothiazole, a compound forming insoluble chelates with bivalent metal ions, has also been determined by Af-ray analysis. The observed bond distances and angles deviate little from the expected values. The figures reveal a strong intramolecular chelating hydrogen bond O—H" N, with an O—N distance of 2.605 A. ... 

Hydrogen bonding plays a major role in pyrazolone tautomerism, and the formation of a chelate structure can shift the equilibrium towards the chelated form. Structures (135) and (136) are two representative examples of such stabilized tautomers. Structure (137) is a hypothetical example of stabilization of the NH tautomer. 

Acyl-, 4-alkoxycarbonyl- and 4-phenylazo-pyrazolin-5-ones present the possibility of a fourth tautomer with an exocyclic double bond and a chelated structure. The molecular structure of (138) has been determined by X-ray crystallography (Table 5). It was shown that the hydroxy group participates in an intramolecular hydrogen bond with the carbonyl oxygen atom of the ethoxycarbonyl group at position 4 (8OCSCII21). On the other hand, the fourth isomer is the most stable in 4-phenylazopyrazolones (139), a chelated phenyl-hydrazone structure. 

After 19 hours, no reaction between the zinc chelate 2 and benzaldehyde can be detected at 20 °C. However, 10 mol % of the zinc chelate effectively catalyzes theenantioselective addition of diethylzinc to aromatic aldehydes. The predominant formation of the S-configurated products, effected by this conformationally unambiguous catalyst, can be explained by a six-mem-bered cyclic transition state assembly17. The fact that the zinc chelate formed from ligand M is an equally effective catalyst clearly demonstrates that activation of the aldehyde moiety does not occur as a consequence of hydrogen bond formation between the ammonium proton of the pyrrolidine unit and the aldehydic oxygen. 

Spiroketals based upon such structures as l,7-dioxaspiro[5.5]undecane (18), occur frequently in natural products. Accordingly, an extensive amount of literature relates to the isolation and total synthesis of this type of compound. This literature was reviewed104 in 1989. The authors of Ref. 104 listed three factors that influence conformational preferences in these systems. They are (7) steric influences, (2) anomeric and related effects, and (3) intramolecular hydrogen bonding and other chelation effects. 

Figure 9J2a Hydrolysis of the zinc eugenolate bis chelate to the hydrogen bonded eugenol dimer and zinc hydroxide. After Wilson Mesley (1974).

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