Org. Synth. 1962, 42, 75
DOI: 10.15227/orgsyn.042.0075
[2,4-Pentanedione, 3-methyl-]
Submitted by A. W. Johnson, E. Markham, and R. Price1.
Checked by Virgil Boekelheide and M. Kunstmann.
1. Procedure
A mixture of 65.2 g. (0.65 mole) of pentane-2,4-dione, 113 g. (0.80 mole) of methyl iodide, 84 g. of anhydrous potassium carbonate (Note 1), and 125 ml. of acetone is placed in a 500-ml. round-bottomed flask fitted with a reflux condenser and a calcium chloride guard tube. The mixture is heated under reflux for 20 hours and is then allowed to cool. The insoluble material is removed by filtration and washed with acetone (Note 2). The combined filtrate and acetone washings are concentrated on the steam bath (Note 3), and the residual oil is distilled. There is collected 56-57 g. (75-77%) of a colorless oil, b.p. 170–172°/760 mm.., nD24 1.4378 (Note 4).
2. Notes
1. The potassium carbonate is dried at 100° for 2 hours before use.
2. Thorough washing of the inorganic residues is essential and requires about 200 ml. of acetone.
3. During removal of the acetone, potassium iodide is deposited and it is advisable to decant the crude 3-methylpentane-2,4-dione from this material before distillation.
4. It has been reported by A. M. Roe and J. B. Harbride (private communication) that this procedure yields a product containing 20–25% of 3,3-dimethylpentane-2,4-dione as shown by gas chromatography. The amount of the dialkylation product is said to be reduced to 5–10% when the reflux period is shortened from 20 to 4.5 hours. The impurity is not readily removed, but it does not interfere with the preparation of 2,3,4,5-tetramethylpyrrole [this volume, p. 1022].
The same authors report that the work-up of the reaction may be improved by adding 250 ml. of petroleum ether (b.p. 40–60°) to the cold reaction mixture before filtering, and washing the solids with a 1:1 mixture of acetone and petroleum ether. With this change it is not necessary to decant the product from the precipitated potassium iodide as recommended in (Note 3).
3. Discussion
3-Methylpentane-2,4-dione has been prepared by the reaction of the sodium derivative of pentane-2,4-dione with methyl iodide in a sealed tube at 140°,2 and from the sodium3 and potassium4 derivatives of pentane-2,4-dione and methyl iodide in alcoholic solution. It has also been prepared by the reaction of methyl iodide and pentane-2,4-dione in the presence of potassium carbonate in alcoholic or ethereal solution5 and in acetone solution,6,7 and by heating 2-aminopenten-4-one with methyl iodide at 100°.8 The present modification affords improved yields.
4. Merits of Preparation
The method presented here has also been used for the preparation of 3-ethyl- and 3-isopropylpentane-2,4-diones and is probably of general applicability in the preparation of 3-alkylpentane-2,4-diones.
This preparation is referenced from:

References and Notes
  1. Department of Chemistry, University of Nottingham, Nottingham, England.
  2. W. R. Dunstan and T. S. Dymond, J. Chem. Soc., 59, 428 (1891).
  3. J. Salkind, Chem. Zentr., 1905, II, 753.
  4. W. H. Perkin, J. Chem. Soc., 61, 848 (1892).
  5. L. Claisen, Ber., 27, 3184 (1894).
  6. K. von Auwers and H. Jacobsen, Ann., 426, 229 (1922).
  7. A. M. Roe and J. B. Harbridge, Chem. & Ind. (London), 182 (1965).
  8. A. Combes and C. Combes, Bull. Soc. Chim. France, [3] 7, 785 (1892).

Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

petroleum ether

sodium derivative of pentane-2,4-dione

3-ethyl- and 3-isopropylpentane-2,4-diones

potassium carbonate (584-08-7)

potassium iodide (7681-11-0)

acetone (67-64-1)

sodium (13966-32-0)

potassium (7440-09-7)

Methyl iodide (74-88-4)

pentane-2,4-dione (123-54-6)

2,3,4,5-Tetramethylpyrrole (1003-90-3)

2,4-Pentanedione, 3-methyl- (815-57-6)

3,3-dimethylpentane-2,4-dione (3142-58-3)