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).

polyoxometalate - catalytic epoxidation - chemoselectivity - diastereoselectivity - regioselectivity

References

For reviews, see

1a Mizuno N. Misono M. Chem. Rev.  1998,  98:  199 

1b Hill CL. Prosser-McCartha CM. Coord. Chem. Rev.  1995,  143:  407 

1c Kozhevinikov IV. Chem. Rev.  1998,  98:  171 

1d Neumann R. Prog. Inorg. Chem.  1998,  47:  317 

For leading examples, see

2a Hill CL. Brown RB. J. Am. Chem. Soc.  1986,  108:  536 

2b Mansuy D. Bartoli JF. Finke RG. J. Am. Chem. Soc.  1991,  113:  7222 

2c Mizuno N. Nozaki C. Kiyoto I. Misono M. J. Am. Chem. Soc.  1998,  120:  9267 

2d Bösing M. Nöh A. Loose I. Krebs B. J. Am. Chem. Soc.  1998,  120:  7252 

3 Neumann R. Gara M. J. Am. Chem. Soc.  1995,  117:  5066 

4 Neumann R. Khenkin AM. Inorg. Chem.  1995,  34:  5753 

5 Adam W. Wirth T. Acc. Chem. Res.  1999,  32:  703 

6 Tourné CM. Tourné GF. Zonnevijlle F. J. Chem. Soc., Dalton Trans.  1991,  143 

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.

It has been reported that the epoxidation activity of the peroxo species of tungsten or molybdenum is inhibited by strongly coordinating solvents:

8a Mimoun H. De Roch IS. Sajus L. Tetrahedron  1970,  26:  37 

8b Mimoun H. Angew. Chem., Int. Ed. Engl.  1982,  21:  734 

9 A peroxo species of tungsten has also been proposed inref. 3; a hydroperoxy species may not be excluded as oxidant, see: Neumann R. Khenkin AM. J. Mol. Catal.  1996,  114:  169 

10 Prat D. Delpech B. Lett P. Tetrahedron Lett.  1986,  27:  711 

11 Ishii Y. Yamawaki K. Ura T. Yamada Y. Yoshida T. Ogawa M. J. Org. Chem.  1988,  53:  3587 

12 Aubry C. Chottard G. Platzer N. Brègault J.-M. Thouvenot R. Chauveau F. Huet A. Ledon H. Inorg. Chem.  1991,  30:  4409 

13 Adam W. Mitchell CM. Paredes R. Smerz AK. Veloza LA. Liebigs Ann. Recl.  1997,  1365