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1,1'-Bi-2-naphthol

1,1'-Bi-2-naphthol


IUPAC name	1,1'-Binaphthalene-2,2'-diol
Other names	1,1'-Bi-2-naphthol 1,1-Binaphthol BINOL
Identifiers
CAS number	Racemic: [602-09-5] (R)-(+): [18531-94-7] (S)-(-): [18531-99-2]
SMILES	Oc1c(c2c(O)ccc3c2cccc3)c(cccc4)c4cc1
Properties
Molecular formula	C₂₀H₁₄O₂
Molar mass	286.32 g/mol
Melting point	205-211 °C
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

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). It is a precursor for another chiral ligand called BINAP.^[1]

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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.^[2]

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.^[2] The (R)-dipentanoate is hydrolysed in a second step with sodium methoxide.^[3]

References

^ "Binap: An industrial approach to manufacture"
^ ^a ^b "RESOLUTION OF 1,1'-BI-2-NAPHTHOL", Dongwei Cai, David L. Hughes, Thomas R. Verhoeven, and Paul J. Reider, in Organic Syntheses Coll. Vol. 10, p.93; Vol. 76, p.1
^ "(S)-(−)- AND (R)-(+)-1,1'-BI-2-NAPHTHOL", Romas J. Kazlauskas in Organic Syntheses, Coll. Vol. 9, p.77; Vol. 70, p.60