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

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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• 4 Schmalz et al. reported that the diastereoeselectivity of direct complexation of 1-tetralol derivatives with Cr(CO)₆ under thermal conditions decreased in longer reaction time. The decreased diastereoselectivity was explained by the chromium migration to the inverted arene face under thermal conditions: Schmalz H.-G. Millies B. Bats JW. Dürner G. Angew. Chem., Int. Ed. Engl.  1992,  31:  631 
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• 8a

  The chromium-arene bond is weakened by assistance with the coordinating benzylic oxygen. Tricarbonyl(1-exo-vinyl-1-endo-indanol)chromium complexes were heated in the presence of functionalized arenes, 2-methyl-1,3-cyclopentadione and a catalytic amount of Triton-B to give the corresponding tricarbonylchromium migration products to the existing arenes.

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• 9a

  A coordinating donor solvent accelerates the ligand exchange.

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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.