Coupling Phenols, Naphthols

Dyes. Sodium nitrite is a convenient source of nitrous acid in the nitrosation and diatozation of aromatic amines. When primary aromatic amines react with nitrous acid, the intermediate diamine salts are produced which, on coupling to amines, phenols, naphthols, and other compounds, form the important azo dyes (qv). The color center of the dye or pigment is the -N=N- group and attached groups modify the color. Many dyes and pigments (qv) have been manufactured with shades of the entire color spectmm. 

With a phenol, naphthol or keto-enol coupling component the mechanism is given by Scheme 4.4, in which blocking of the p-position forces coupling at the o-position. In certain cases involving the use of feebly reactive diazonium salts, loss of the proton from the transition state (4-1) in Scheme 4.4 may be slow but may often be facilitated by the addition of a tertiary base, such as pyridine, to the coupling mixture [7,8]. 

Phenols, Naphthols (e.g., 39-47). Phenols mainly couple at the 4-position, or at the 2-position if the 4-position is occupied, p-Hydroxybenzoic acid couples with elimination of C02 resorcinol couples twice initially at the 4-position, and with a second equivalent of diazonium compound at the 2-position under acid conditions or at the 6-position under alkaline conditions. a-Naphthols mainly couple at the 4-position, in addition to which varying quantities of 2- and 2,4-coupling products are obtained, depending on the diazo component. p-Naphthol couples at the 1-position. Substituents in the 1-position, such as S03H, COOH, Cl, CH2OH, or... 

Direct dyes (see Section 3.3) that contain aromatic amino groups can be diazo-tized on the fiber after dyeing and then coupled with a developer (a phenol, naphthol, or aromatic amine). Wetfastness, in particular, is improved by such an enlargement of the molecule, and the shade also changes (see Section 2.2). Conversely, water-soluble, substantive azo dyes which bear amino or hydroxyl groups capable of coupling can also be used, followed by aftertreatment with a diazo-nium compound. The resulting polyazo dye shows excellent wetfastness (see also Section 4.8). 

The investigations of acid-base pre-equilibria of active methylene compounds (C-acids) as coupling components began in 1968 i.e. about two to three decades later than those on phenols (naphthols) and aromatic amines. The most extensive and comprehensive paper on pre-equilibria in azo coupling of active methylene compounds was published by Hashida s group in 1971. They investigated no fewer... 

In dye chemistry, the term coupling" is used specifically to mean electrophilic attack by a diazonium salt on phenols, naphthols and anilines. 

An oxidative coupling of a naphthol by ferric chloride in a reaction that mimics nature s method of coupling phenolic substances into important pigments, such as hypericin (I) ... 

Organoboron reagents can also be used in the cross-coupling of naphthols in the presence of a stoichiometric amount of BEts [Eq. (7)] [52]. Although these naphthol activation methods are currently inapplicable to phenols, these results promise future development of the direct cross-couplings of C(aryl)-OH bonds. 

Coupling to phenols, naphthols, and related hydroxyl compounds is carried out in alkaline solution. Under alkaline conditions the hydroxy compound is soluble, and coupling usually is rapid. Ordinarily the coupling must be carried out in the cold to prevent decomposition of the diazonium salt. 

A fairly general procedure consists in coupling a phenol or naphthol with a diazotised amine, reducing the product to an aminophenol or aminonaphthol, and oxidising the hydroxy compound with acid ferric chloride solution. This method is illustrated by the preparation of (3 (or 1 2)-naphthoquinone ... 

The Colour Index (up to June 1991) Hsts 21 direct violets with disclosed chemical constitutions. Commercially important are Cl Direct Violet 9 [6227-14-1] (79) (Cl 27885) (sulfanihc acid coupled to cresidiue followed by alkaline coupling to V-phenyl J-acid) and Cl Direct Violet 66 [6798-03-4] (80) (Cl 29120) (a copper complex of 2-arniao-l-phenol-4-sulfonarnide (2 mol) coupled to 6,6 -imiQobis-l-naphthol-3-sulfonic acid). 

A diazonium salt is a weak electrophile, and thus reacts only with highly electron-rich species such as amino and hydroxy compounds. Even hydroxy compounds must be ionized for reaction to occur. Consequendy, hydroxy compounds such as phenols and naphthols are coupled in an alkaline medium (pH > of phenol or naphthol typically pH 7—11), whereas aromatic amines such as N,N diaLkylamines are coupled in a slightly acid medium, typically pH 1—5. This provides optimum stabiUty for the dia2onium salt (stable in acid) without deactivating the nucleophile (protonation of the amine). 

It can also be mentioned that polyphosphazenes substituted with aromatic groups, such as phenols or naphthols, can form inter- and intra- molecular excimers by coupling reaction of the planar aromatic rings of the substituents under illumination [467-471,473,725]. These species disappear as soon as the light is switched off. 

Hydroxyazo dyes fall into four main categories depending upon the nature of the coupling component used to make them, namely acyclics, phenols, heterocyclics, and naphthols. Those derived from naphthols comprise the largest and most important group commercially. 

The stoichiometric oxidative coupling of various phenols proceeds with moderate selectivity (76% ee) in the presence of (-)-sparteine CuCl2, Eq. 106 (127). As mentioned above, selectivity seems to be driven by solubility issues since isolation of the product from the precipitate or from solution results in different enantiose-lectivities. Indeed, this system performs far worse under catalytic conditions. The best result involves the use of silver chloride as reoxidant the heterooxidative coupling of two naphthols 182 and 183 affords the product 184 in 41% yield and 32% ee using 10 mol% catalyst, Eq. 107. 

A variety of phenol couplings have been described. Those reported before 1991 have been reviewed [66]. 2-Naphthol (27) was oxidized to l,l -binaphthol (28) in high current efficiency on a graphite felt electrode coated with a thin poly(acrylic acid) layer immobilizing 4-amino-2,2, 6,6-tetramethylpiperidinyl-l-oxy (4-amino-TEMPO) (Scheme 10) [67]. 

Oxidation of phenols and aromatic amines using HRP is generally of little synthetic value, as oligomers and polymers are the main products (5, 260). Under certain conditions oxidative coupling of phenols or naphthols to give biaryls can be achieved, but with low selectivity (262). In contrast, HRP can catalyze a number of useful oxidative N-and 0-deaIkyIation reactions that are relatively difficult to carry out synthetically. This area has been described in detail by Meunier (263). A method for the preparation of optically active hydroperoxides using HRP C has been developed (264). Optically pure (S)-hydroperoxides... 

The phenolic oxidative coupling process can also be demonstrated in laboratory experiments. Thus, treatment of 1-naphthol with alkaline potassium... 

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