Selective synthesis of 1,1'-binaphthalene derivatives by oxidative coupling with TiCl4

Article abstract of DOI:10.1016/S0040-4039(00)00206-9

Oxidative coupling of naphthalene compounds with TiCl₄ in nitromethane under mild conditions gives symmetrical 1,1'-binaphthyls in good yields. The method is particularly useful when applied to substrates with electron donating groups. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00206-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 2545–2547
Selective synthesis of 1,10-binaphthalene derivatives by oxidative
coupling with TiCl4
Joël Doussot,∗ Alain Guy and Clotilde Ferroud
Laboratoire de Chimie Organique, associé au CNRS, Conservatoire National des Arts et Métiers, 292 rue Saint-Martin,
F-75141 Paris cedex 03, France
Received 20 December 1999; accepted 29 January 2000
Abstract
Oxidative coupling of naphthalene compounds with TiCl4 in nitromethane under mild conditions gives symme-
trical 1,10-binaphthyls in good yields. The method is particularly useful when applied to substrates with electron
donating groups. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: naphthalenes; coupling reaction; titanium; biaryls.
The biaryl substructure is a central building block in a very large number of natural products, such as
polyketides, terpenes, lignanes, coumarins, flavonoids, tannins, and many alkaloids.1 Homochiral 1,10-
binaphthalene derivatives have successfully been utilized as chiral inducers for highly stereoselective
reactions because of their axial dissymmetry and molecular flexibility.2 Therefore, it is essential to
Scheme 1.
∗
Corresponding author. Tel: 33 (0) 1 40 27 20 12; fax: 33 (0) 1 42 71 05 34; e-mail: doussotj@cnam.fr (J. Doussot)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00206-9
tetl 16486

2546
Table 1
Oxidative coupling of naphthalene compounds 1 into 1,10-binaphthyls 2
establish simple and convenient methods for the preparation of 1,10-binaphthalene derivatives. Several
procedures have been developed for this purpose, and the most straightforward method for the construc-
tion of binaphthyl skeletons is the oxidative coupling of corresponding naphthalene derivatives. A variety
of oxidants, e.g. FeCl3,3 Cu2+- and Fe3+-doped MCM-41 aluminosilicate,4 Cu(II)–amine complexes,5
6
or Mn(acac)3 have been utilized as coupling reagents. The reactions were carried out in organic
solvents,7 in aqueous Fe3+ solution,8 or in the solid state without solvent,9 sometimes accelerated by
microwave10 or ultrasound irradiation.11 Recently, Schreier et al. reported the oxidative coupling of
phenolic compounds catalyzed by horseradish peroxidase.12 Some Lewis acids such AlCl3, ZrCl4, SnCl4,
or SbCl3 are reported13 to be effective catalysts to produce biaryls from aromatic compounds via aromatic
radical cations. The latter reaction is known as the Scholl reaction,14 usually giving mixtures of isomeric
biaryls.
In this paper, we report the coupling reaction of naphthalene derivatives using the relatively weak
Lewis acid TiCl4 as oxidant. The reaction proceeds regioselectively in nitromethane under milder
conditions compared to those initially used in the Scholl reaction (Scheme 1).
Some factors which may influence the yield of the reaction were examined and are summarized in

2547
Table 1. A typical experimental procedure is as follows: at 5°C and under an argon atmosphere, the
dropwise addition of TiCl4 (number of equivalents is indicated in Table 1) to nitromethane (50 mL) gave
a bright yellow complex. To this complex, a solution of 1 (0.020 mol) in nitromethane (20 mL) was slowly
added at room temperature. The resulting dark brown mixture was stirred and heated if necessary for the
time indicated. The mixture was hydrolyzed with cold water and extracted with dichloromethane (3×50
mL). The combined organic layers were washed with water and dried over MgSO4. After removal of
the solvent under reduced pressure, the resulting product was first checked by 1H NMR (Bruker AC 300
spectrometer) and then purified by flash chromatography on silica gel (heptane:ethyl acetate). Analytical
data of binaphthyls 2a–i were identical to data reported in the literature.
As can be seen in Table 1 runs 3, 4, and 6, the yield of coupling product 2 is higher if the reaction
is conducted at 50°C instead of room temperature and the rate of the reaction increases significantly.
When the molar ratio of oxidant (Ti4+) to substrate 1 is changed from 2:1 to 1:2, the yield of reaction
decreases significantly (runs 3, 5, 10 vs 4, 6, 11). The naphthalene compounds substituted in 1-position
by an alkoxy or an hydroxy substituent (R2=OMe, OEt, OH) lead, beside to the coupling product, to a
significant quantity of naphthoquinone (runs 10, 12) or to an unidentifiable product mixture (run 14). In
the case of alkyl substituted substrates 1e or 1i, the rate of the reaction is slower (runs 9, 15).
By contrast, all attempts to couple compounds without electron donating substituents like naphthalene
or 1-chloronaphthalene were unsuccessful even at higher temperature. In other solvents such as trifluo-
roacetic acid, the reaction gave little conversion (about 20%) and required a very long reaction time (one
week).
In conclusion we have described for the first time the oxidative coupling of naphthalene derivatives
mediated by a Ti4+–nitromethane complex, which provides efficient and regioselective means for pre-
paration of 1,10-binaphthyls substituted by electron donating groups. A mechanistic study is now in
progress in our laboratory.
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