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Synlett 2003(4): 0516-0518
DOI: 10.1055/s-2003-37528
LETTER
© Georg Thieme Verlag Stuttgart · New York

Inversion of Planar Chirality vs Axial Isomerization of Axially Chiral Biaryl Chromium Complexes

Ken Kamikawaa,b, Tomohiro Sakamotoa,b, Motokazu Uemura*a,b
a Department of Chemistry, Faculty of Integrated Arts and Sciences, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
b Research Institute for Advanced Sciences and Technology, Osaka Prefecture University, Sakai, Osaka 599-8570, Japan
Fax: +81(72)2549931; e-Mail: uemura@ms.cias.osakafu-u.ac.jp;
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Publication History

Received 17 December 2002
Publication Date:
26 February 2003 (online)

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Abstract

syn-Biaryl chromium complexes having a coordinating ortho-substituent gave anti-biaryl chromium complexes 5 with an inversion of the planar chirality by heating in a non-aromatic solvent.

Key words

arene complexes - asymmetric synthesis - atropiso­merism - biaryls - isomerizations

    References

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5

General Procedure: A solution of syn-biaryl complex 3 (0.25 mmol) in mixture of n-Bu2O (2 mL) and (CH2Cl)2 (2 mL) was stirred at 120 °C for 2 h. The solvent was removed under reduced pressure, and the residue was purified by silica gel chromatography to give anti-biaryl complex 5.
Complex 5a: Mp 122 °C. [α]D 31 -142.0 (c 0.1, CHCl3). 1H NMR (300 MHz, CDCl3): δ = 1.72 (1 H, s, CH3), 2.09 (3 H, s, CH3), 3.66 (3 H, s, OCH3), 4.20 (2 H, d, J = 5.5 Hz, CH 2 OH), 5.06 (1 H, d, J = 6.5 Hz, Cr-Ar), 5.15 (1 H, d, J = 6.5 Hz, Cr-Ar), 5.79 (1 H, t, J = 6.5 Hz, Cr-Ar), 7.23-7.33 (3 H, m, Ar), 7.42-7.45 (1 H, m, Ar). IR (CHCl3): 1950, 1870, 1530, 1450, 1420, 1260, 1030 cm-1. Anal. Calcd for C18H16O5Cr: C, 59.34; H, 4.43; Found: C, 59.34; H, 4.50.

7

Enantiomeric excesses of anti-biphenyl chromium complexes, 4a and 5a were determined by chiral HPLC: Chiralcel OD, hexane/2-propanol (9/1), flow rate 0.5 mL/min, 40 °C, retention time; 13.8 min(4a) and 16.2 min(5a). For racemic 2-methyl-6-hydroxymethyl-2′-biphenyl; chiralcel OJ-H, hexane-2-propanol (50:1), flow rate 0.5 mL/min, 40 °C, retention time; 46.5 min and 53.7 min.