A New Highly Selective Method for the Catalytic Epoxidation of Chiral Allylic Alcohols by Sandwich-Type Polyoxometalates with Hydrogen Peroxide

Abstract

Oxidatively and solvolytically persistent sandwich-type polyoxometalates, namely [WZnM₂(ZnW₉O₃₄)₂]^q- [M = Mn(II), Ru(III), Fe(III), Zn(II)], catalyze chemoselectively, diastereoselectively and regioselectively the epoxidation of chiral allylic alcohols with 30% hydrogen peroxide through a tungsten peroxo species, in which the allylic alcohol is coordinated as alcoholate (template effect).

Key words

polyoxometalate - catalytic epoxidation - chemoselectivity - diastereoselectivity - regioselectivity

References

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7

General Procedure for Catalytic Epoxidations of Allylic Alcohols 1 by Sandwich-Type Polyoxometalates: Substrate 1 (0.50 mmol) and dimethyl isophthalate (0.20 mmol), as internal standard, were dissolved in a stock solution of the particular methyltricaprylammonium M-POM catalyst (0.50 mL, 1.0 mM). The reaction was initiated by addition of 30% hydrogen peroxide (110 mL, 1.0 mmol) to the above solution at ca. 20 °C, and the resulting biphasic mixture was stirred at a constant rate (ca. 1000 rpm) for all runs. The reaction progress was monitored by ¹H NMR spectroscopy or TLC, and after the required reaction time, the organic phase of the reaction medium was separated, dried over Na₂SO₄, and the solvent removed at reduced pressure (20 °C/50 mbar). The conversions, product ratios and diastereomeric ratios of the epoxides 2 were determined by ¹H NMR analysis directly on the crude mixture and the data are given in Table [1] and Table [2] . The epoxides 2a-e, obtained by M-POM-catalyzed epoxidation, are literature known and have been identified on the basis of their NMR data.