Arylazo-2-naphthols

Gilli, P., Bertolasi, V., Pretto, L., Antonov, L., and Gilli, G., Variable-temperature X-ray crystallographic and DFT computational study of the N-H- -O/N- H-O tautomeric competition in l-(arylazo)-2-naphthols. Outline of a transition-state hydrogen bond theory, J. Am. Chem. Soc., Ill, 4943-4953 (2005). 

Aromatic amines can sometimes be chromatographed successfuly after their conversion to derivatives. For example, isomeric toluidines and aniline, which are poorly separated as salts, can be clearly separated after their conversion to bromo derivatives (105) a series of primary amines can be separated after conversion to arylazo-2-naphthole (p. 342), and for the separation of isomers the products of the reaction of amines with diazonium salts (p. 324) can also be employed. They can be chromatographed in a formamide/hexane system. Thin-layer chromatography of free bases can be carried out on nonadhering layers of alumina (106, 107), or on silica gel G layers using the mobile phase cyclohexane — carbon tetrachloride-ethyl ecetate (10 70 20) ... 

Enol imine-enaminone and phenol—quinone tautomerism in (arylazo) naphthols and in analogous Schiff bases were studied by Fabian et al. [92, 93]. In all these molecules there is a favorable N- -H- -O intramolecular hydrogen bond. Depending on the X-H sigma bond (X = N, O), there are two possible tautomers in solution. The solvent effect was calculated on the equilibrium [92], and a combined effect of the solvent and the benzene substituent was studied in [93]. While the FEP/MC simulations provided consistent organic solvent effects in accord with the experimental results [92], the wide spectrum of the solvent-effect calculation methods could predict rather diverse results for several groups of systems in [93]. 

The triphenylsilyl leaving group in 12.29 is interesting from a mechanistic viewpoint (Sunthankar and Gilman, 1950), as also is the replacement of the nitro group in l-nitro-2-naphthol by an arylazo group (Bunce, 1974). 

Burawoy, A., and A. R. Thompson The Effect of Solvents on the Tautomeric Equilibria of 4-Arylazo-l-Naphthols and the ortho-Effect. J. chem. 

In Table 15, 13C chemical shifts in arylazo derivatives of 2-naphthol (34 and 35) and 1-naphthol (36) are presented. 

Chromogens of different classes, viz., arylazo derivatives of resorcinol, naphthols, naphthylamines, chromotropic acid and its cyclic modification, and also some formazans, were investigated by Fedorov et al.95 using 13C CP/MAS NMR. Usually only negligible changes of 6(13C) were found on comparing solution and solid-state spectra. Table 14 shows solution and the solid-state data for azo resorcinols in alkaline media existing in paro-quinoidic structure 33, not in the ortho one typical of some other compounds. 

The formation of this product of a consecutive disubstitution is apparently not easy to understand. It is known that an arylazo residue diecreases the reactivity of a phenol or a naphthol for substitution by a second diazonium ion by 3 to 5 orders of ten. In phenols and naphthol the favorable effect of promoting the dissociation of a proton from the substrate does not work it was found only for C-acids (see discussion in the preceding Sect. 4.1). 

The o/p-ratio is of primordial industrial importance for the synthesis of technical dyes, particularly for coupling components like 1-naphthol and derivatives of l-naphthpl-3-sulfonic acid. The dyes with an arylazo group in the 2-position form an intramolecular hydrogen bond (149) which is, of course, impossible for the 4-isomer. 

Various 4-(arylazo)phenols and naphthols have been photooxidized using 02( A ) involving a type II mechanism . l,l -Binaphthol undergoes enantioselective oxidation (5.2% ee) when the chiral complex A-[Ru(4,4 -dimenthoxycarbonyl-2,2 -bipyridine)3] + is used as the photocatalyst . 

Some phenylhydrazones show hydrazone-azo tautomerism in the pure state as well as in non-polar solvents. For instance, 4-arylazo-l-naphthols (27) are capable of keto-enol as well as hydrazone-azo tautomerism - The position of equilibrium moves toward the... 

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