Highly Enantioselective Catalytic Ketone Allylation with Sn(CH₂CH=CH₂)₄/RSn(CH₂CH=CH₂)₃ Mixtures (R = Et, Bu)

Abstract

In the presence of the monothiobinaphthol (MTB) ligand aryl ketones are allylated by mixtures of Sn(CH₂CH=CH₂)₄/RSn(CH₂CH=CH₂)₃ (R = Et, Bu) in high e.e. The presence of water suppresses racemic background allylation.

Key words

tin - asymmetric catalysis - ketones - allylation - thiols

References

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10

Other Et_nSn(CH₂CH=CH₂)_4-n, n = 2-4 species are also present as very minor contaminants in these preparations. Control reactions using d₅-EtBr confirm that the ethyl group arises from the EtBr used for magnesium activation in ref. 9. Fresh (silver-coloured) as opposed to aged (>2 years, black-coloured) magnesium leads only to pure Sn(CH₂CH=CH₂)₄. Electron microscopy in conjunction with EDS studies reveal that the surface of aged magnesium has an appreciable oxygen content (presumably MgO) and it appears to be this that favours production of EtMgBr over CH₂=CHCH₂MgCl.

14

Experimental Procedure: All operations were performed under argon; toluene was distilled from sodium. A toluene solution of MTB 2 (24 mg, 0.08 mmol in 1.0 mL) was treated with a mixture of Sn(CH₂CH=CH₂)₄/SnR(CH₂CH=CH₂)₃ (0.7:0.3 mol fraction mix, 0.46 mmol total Sn content, R = Et, Bu). For the ‘dry’ catalyst this mixture (R = Et) was heated directly for 2 h at 52 °C. For the ‘wet’ catalyst water (3 µL, 0.16 mmol) was added to the mixture (R = Bu) prior to the heating period. The mixtures were cooled to ambient temperature, the ketone 1 (or 4) (0.4 mmol) added and the mixture stirred whilst its composition was monitored by GC, HPLC, or ¹H/¹¹⁹Sn NMR spectroscopy. Flash chromatography afforded the known alcohols 3 (or 5) as essentially single products. Enantioselectivities were determined by GC (CYCLODEX-B for 3c, oktakis(6-O-methyl-2,3-di-O-pentyl)-γ-cyclodextrin for 3a, 3b, 3e, 3f, 5) or HPLC (Chiralcel AD for 3d, Chiralcel OD for 3g).