Improving the Dakin Reaction by Using an Ionic Liquid Solvent

Abstract

The oxidation of aromatic aldehydes to phenols (Dakin reaction) has been demonstrated to proceed readily on both activated and non-activated aldehydes in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMI][PF₆], 1) with high selectivity, easy product separation and excellent chemical yields.

Key words

oxidations - phenols - aldehydes - ionic liquid - Dakin reaction

References

• 1 Dakin HD. Org. Synth., Coll. Vol. 1   Wiley; New York: 1941.  p.149 
• 2 Matsumoto M. Kobayashi H. Hotta Y. J. Org. Chem.  1984,  49:  4740 
• 3 Syper L. Synthesis  1989,  167 
• 4 McKillop A. Sanderson WR. Tetrahedron  1995,  51:  6145 
• 5 Roy A. Reddy KR. Mohanta PK. Ila H. Junjappa H. Synth. Commun.  1999,  29:  3781 
• 6 For a recent review, see: Olivier-Bourbigou H. Magna L. J. Mol. Cat. A: Chem.  2002,  182-183:  419 
• 7 Varma RS. Namboodivi VV. Chem. Commun.  2001,  643 
• 8 Howart J. Hanlon K. Fayne D. McCormac P. Tetrahedron Lett.  1997,  38:  3097 

9

CAS #: 2b [108-95-2], 3b [106-44-5], 4b [150-76-5], 5b [574-00-5], 6b [533-31-3], 7b [106-48-9].

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Typical Procedure: To a solution of the aromatic aldehyde (1 mmol) in [BMI][PF₆] (1, 1 mL) was added boric acid
(3-5 mmol) with stirring, followed by sulfuric acid (0.1 mmol) and a solution of hydrogen peroxide (35%, 2 mmol). The resulting reaction mixture was stirred for 1-2 h at room temperature and then toluene (2 mL) was added. The two-phase liquid mixture was stirred and then allowed to stand. The toluene phase (upper phase) was decanted off and the ionic phase was extracted with a second portion of toluene (2 mL). The combined toluene layers were dried (MgSO₄) and then evaporated to give a yellowish solid. This solid was redissolved in CH₂Cl₂, filtered through a pad of silica gel (eluted with hexane/ethyl acetate 10:1), and dried in vacuo to afford a white-yellowish solid in 65-89% yield. The product was analyzed by ¹H NMR or GC-MS.