Definitions

naphthol

naphthol

[naf-thawl, -thol, nap-]
naphthol, C10H7OH, either of two crystalline monohydric alcohols. The naphthols are position isomers, differing in the location of the hydroxyl group, -OH, on the carbon skeleton of naphthalene; α-naphthol is 1-hydroxynaphthalene and β-naphthol is 2-hydroxynaphthalene: The naphthols have a number of similar properties. They melt at 95°C; and 122°C; and boil at 279°C; and 285°C;, respectively; both are soluble in alcohol and ether and slightly soluble in hot water. One way in which they differ is the form of their crystals—α-naphthol crystallizes in prisms and β-naphthol in plates. The naphthols are prepared by reacting naphthalene with sulfuric acid and hydrolyzing the resultant sulfate ester by heating it with sodium hydroxide solution. Both naphthols exhibit antiseptic properties. They are used in the synthesis of certain azo dyes and antioxidants for rubbers. Naphthol solutions are used in chemical analysis to detect the ferric ion; dissolved ferric ion turns an α-naphthol solution violet and turns a β-naphthol solution green.

1,1'-Bi-2-naphthol (BINOL) is an organic compound that is often used as a ligand for transition-metal catalysed asymmetric synthesis. BINOL has axial chirality and the two enantiomers can be readily separated and are stable toward racemisation. The specific rotation of the two enatiomers is +/- 35.5° (c=1 in THF). BINOL is a precursor for another chiral ligand called BINAP.

Preparation

The organic synthesis of BINOL is not a challenge as such but the preparation of the individual enantiomers is.

(S)-BINOL can be prepared directly from an asymmetric oxidative coupling of 2-naphthol with copper(II) chloride. The chiral ligand in this reaction is (+)-amphetamine.

Racemic BINOL can also be produced using iron(III) chloride as an oxidant. The mechanism involves complexation of iron(III) into the hydroxyl, followed by a radical coupling reaction of the naphthol rings initiated by iron(III) oxidizing into iron(II).

Optically active BINOL can also be obtained from racemic BINOL by optical resolution. In one method, the alkaloid N-benzylcinchonidinium chloride form a crystalline inclusion compound. The inclusion compound of the S-enantiomer is soluble in acetonitrile but that of the R-enantiomer is not.

In another method BINOL is reacted with the acid chloride pentanoyl chloride to obtain the di-ester compound. The enzyme cholesterol esterase is then added in the form of bovine pancreas acetone powder which is able to hydrolyse the (S)-di-ester but not the (R)-di-ester. The (R)-dipentanoate is hydrolysed in a second step with sodium methoxide.

Third method employs HPLC with chiral stationary phases. A very efficient phase is described here.

References

External links

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