Gold(I)-Catalyzed Cycloisomerization of (Arene)chromium Complexes with Enyne Bonds Directed Towards Axially Chiral Biaryls

Abstract

Planar chiral arene-chromium complexes with enyne bonds gave stereoselectively axially chiral biaryl-chromium complexes by gold(I)-catalyzed cycloisomerization in good yields.

Key words

arene complexes - catalysis - chirality - chromium - cycloisomerization - gold

References and Notes

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• 6 Reaction conditions: (i) trimethylsilylacetylene, (Ph₃P)₂PdCl₂, CuI, Et₃N, THF; (ii) NaOH, H₂O, MeOH; (iii) (2-iodophenyl)-1-methylpropene-1, Pd(PPh₃)₄, CuI, Et₃N. See: Müller TJJ. Lindner HJ. Chem. Ber.  1996,  129:  607 
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• The peri proton (at C-8′ position) of the anti-biaryl-chromium complex 7b appeared at δ = 7.67 ppm. The corresponding proton of stereoisomeric syn-biaryl-chromium complex is shifted to lower field due to an anisotropic effect of the tricarbonylchromium fragment. See:
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9

The stereochemistry at spiro quaternary carbon was assigned by reaction path; the double bond attack from exo side to the tricarbonylchromium fragment.

10

Typical Experimental Procedure
A mixture of chromium complex 1g (50 mg, 0.12 mmol), (PPh₃)AuNTf₂ (4.5 mg) in CH₂Cl₂ (5 mL) was heated at 40 °C for 5 h under argon. After filtration through Celite and evaporation under vacuum, the residue was purified by SiO₂ chromatography (Et₂O-hexane, 1:5) to give 48 mg (98%) of 2g; mp 209 °C (dec). ¹H NMR (300 MHz, CDCl₃): δ = 1.92 (3 H, s), 2.18 (3 H, s), 2.66 (3 H, s), 4.83 (1 H, d, J = 6.3Hz), 5.00 (1 H, d, J = 6.3Hz), 5.72 (1 H, t, J = 6.3Hz), 7.44-7.57 (2 H, m), 7.88 (1 H, s), 7.94 (1 H, d, J = 7.8 Hz), 8.04 (1 H, d, J = 7.8Hz). ¹³C NMR (75 MHz, CDCl₃): δ = 15.27, 17.05, 19.64, 56.01, 70.67, 85.08, 95.06, 107.42, 113.45, 123.82, 125.18, 126.46, 129.22, 130.87, 131.84, 132.04, 132.07, 132.10, 132.80, 143.40, 233.78. IR (CHCl₃): 1961, 1880
cm^-1. Anal. Calcd for C₂₃H₂₀O₄Cr: C, 66.98; H, 4.89. Found: C, 66.89; H, 4.97. The obtained chromium complex was exposed to sunlight in Et₂O at 0 °C to give chromium-free biaryl compound in a quantitative yield.