Synthesis, resolution and rates of racemisation of 1-(2'-methyl-3'- indenyl)-2-naphthylamine and -2-naphthol

Article abstract of DOI:10.1016/S0040-4039(00)00618-3

Axially chiral ligands 1-(2'-methyl-3'-indenyl)-2-naphthylamine 7 and - 2-naphthol 8 have been prepared in three and four steps, respectively, from 2-nitro-1-naphthol. Following resolution by chiral HPLC, the absolute configurations were assigned by circu

Full text of DOI:10.1016/S0040-4039(00)00618-3

Tetrahedron Letters 41 (2000) 4471±4473
Synthesis, resolution and rates of racemisation of
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-(2 -methyl-3 -indenyl)-2-naphthylamine and -2-naphthol
Robert W. Baker* and James A. Taylor
School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia
Received 29 March 2000; accepted 10 April 2000
Abstract
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Axially chiral ligands 1-(2 -methyl-3 -indenyl)-2-naphthylamine 7 and -2-naphthol 8 have been prepared
in three and four steps, respectively, from 2-nitro-1-naphthol. Following resolution by chiral HPLC, the
absolute con®gurations were assigned by circular dichroism as (aR)-(^)-7 and (aR)-(+)-8. The enantiomers
ꢀ
of both ligands have signi®cant thermal stability, with half-lives for racemisation of 73 h at 144 C and 220
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h at 110 C, for 7 and 8, respectively. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: planar chirality; axial chirality; indenes; circular dichroism.
Planar chiral cyclopentadienylmetal complexes o€er enormous potential as stereoselective catalysts,
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but obtaining them in stereochemically pure form is often problematic. We have recently
demonstrated that a unique ligand designÐaxially chiral chelating cyclopentadienyl ligandsÐhas
the ability to allow the enantiospeci®c synthesis of planar chiral cyclopentadienylmetal complexes
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containing pendant donor atoms or a second cyclopentadienyl moiety. In the former case, the
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reaction of axially chiral (aR)-1-(2 -methyl-3 -indenyl)naphthalene-2-methanol, (aR)-1, with
2
Zr(NEt ) was shown to enantiospeci®cally provide planar chiral complex (pR)-2 (Eq. (1)). In
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extending this chemistry, particularly attractive targets were ligands in which the second metal
coordination site is directly attached to the aromatic bridge, since metal complexes of such
ligands would be expected to show a higher level of stereorigidity. In this Letter we report the
facile synthesis of two such ligands, containing pendant nitrogen and oxygen donor atoms, and
the determination of the absolute con®gurations and rates of racemisation of their enantiomers.
ꢁ1†
*
Corresponding author. Tel: +61 2 9351 4049; fax: +61 2 9351 6650; e-mail: r.baker@chem.usyd.edu.au
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00618-3

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The preparation of the ligand rac-1-(2 -methyl-3 -indenyl)-2-naphthylamine, rac-7, was achieved
in just three steps from commercially available 2-nitro-1-naphthol 3, as outlined in Scheme 1. The
naphthol 3 was ®rst converted to the tosylate 4 (1 equiv. TsCl, 1.1 equiv. triethylamine, CH Cl ,
2
2
rt, 12 h) in 85% yield. Slow addition over 2 h of a THF solution of the tosylate 4 to a solution of
-methylindenyllithium (1.5 equiv., generated by treatment of 2-methylindene with BuLi) in THF
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at ^20 C, followed by a further 1 h at ^20 C, provided rac-1-(2 -methyl-1 -indenyl)-2-nitro-
naphthalene as a single stable rotamer, ac*(R*)-5, in 40% yield. Allowing the reaction to warm to
rt, then stirring for an additional 12 h, led to in situ base-catalysed isomerisation of ac*(R*)-5 to
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rac-1-(2 -methyl-3 -indenyl)-2-nitronaphthalene, rac-6, also isolated in 40% yield. Finally, reduction
Scheme 1.
Figure 1. CD spectra (acetonitrile) of (aR)-(^)-7, (aR)-(+)-8 and (aR)-(^)-9

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of rac-6 in EtOAc solution with hydrogen at atmospheric pressure over a platinum catalyst
Adam's catalyst or Pt on C) during 12 h, provided rac-7 in 90% yield. Conversion of rac-7 to
(
rac-1-(2 -methyl-3 -indenyl)-2-naphthol, rac-8, was achieved by diazotization with sodium nitrite
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in 50% aqueous sulfuric acid at 0 C, followed by heating at re¯ux overnight, in 45% yield.
Resolution of the ligands rac-7 and rac-8 was achieved by semi-preparative chiral HPLC using
a Pirkle Type-1A column [ionically-bound (R)-N-(3,5-dinitrobenzoyl)phenylglycine]. In the case
of 7 only the enantiomer eluting ®rst was obtained in enantiomerically pure form, while both
enantiomers of 8 could be obtained in enantiomerically pure form. The enantiomer of 7 eluting
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rst had [ꢀ] ^53.5 (c 1.45, toluene), while the enantiomer of 8 eluting ®rst had [ꢀ] +165 (c 0.64,
D
D
toluene). Comparison of the CD spectra of the two enantiomers with that of methyl (aR)-1-
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(
2 -methyl-3 -indenyl)naphthalene-2-carboxylate, (aR)-(^)-9 (the absolute con®guration of which
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has been previously determined), indicates that both have (aR) absolute con®gurations (Fig. 1).
The rates of racemisation of the resolved ligands were determined by monitoring the ee of the
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ligands by chiral HPLC; in the case of 7 the half-life for racemisation for a solution in xylenes at
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re¯ux (144 C) was 73 h; in the case of 8 the half-life for racemisation for a solution in toluene at
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re¯ux (110 CÐthe ligand decomposed in re¯uxing xylenes) was 220 h. Thus, both ligands have
useful stability towards thermal racemisation. The preparation of transition metal complexes
using these new ligands will be reported in due course.
References
1
2
3
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. Halterman, R. L. Chem. Rev. 1992, 92, 965, and references cited therein.
. Baker, R. W.; Wallace, B. J. Chem. Commun. 1999, 1405.
. Baker, R. W.; Wallace, B. J.; Turner, P. J. Chem. Soc., Dalton Trans. 2000, 431.
. A 10Â250 mm column (Pirkle Type-1A, Regis) was used with 1% 2-propanol/hexanes as eluent at a ¯ow rate of
^
1
3.0 ml min , detection 280 nm; t
for (aS)-8.
R
: 19.2 min for (aR)-7 and 21.6 min for (aS)-7, 18.6 min for (aR)-8 and 25.3 min
5
6
. Baker, R. W.; Reek, J. N. H.; Wallace, B. J. Tetrahedron Lett. 1998, 39, 6573.
. A 4.6Â250 mm column (Pirkle Type-1A, Regis) was used with 1% 2-propanol/hexane as eluent at a ¯ow rate of
^
1
1
.0 ml min , detection 254 nm, t : 6.0 min for (aR)-7 and 6.6 min for (aS)-7, 11.4 min for (aR)-8 and 12.9 min for
R
(
aS)-8.