Improving the Dakin Reaction by Using an Ionic Liquid Solvent
24 July 2003 (online)
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.
oxidations - phenols - aldehydes - ionic liquid - Dakin reaction
DakinHD. Org. Synth., Coll. Vol. 1 Wiley; New York: 1941. p.149
MatsumotoM. KobayashiH. HottaY. J. Org. Chem. 1984, 49: 4740
SyperL. Synthesis 1989, 167
McKillopA. SandersonWR. Tetrahedron 1995, 51: 6145
RoyA. ReddyKR. MohantaPK. IlaH. JunjappaH. Synth. Commun. 1999, 29: 3781
- 6 For a recent review, see:
Olivier-BourbigouH. MagnaL. J. Mol. Cat. A: Chem. 2002, 182-183: 419
VarmaRS. NamboodiviVV. Chem. Commun. 2001, 643
HowartJ. HanlonK. FayneD. McCormacP. Tetrahedron Lett. 1997, 38: 3097
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].10
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.