Org. Synth. 1968, 48, 72
DOI: 10.15227/orgsyn.048.0072
DIMETHYLKETENE β-LACTONE DIMER
[3-Pentenoic acid, 3-hydroxy-2,2,4-trimethyl-, β-lactone]
Submitted by Robert H. Hasek
1, R. Donald Clark
1, and Gerald L. Mayberry
2.
Checked by V. Boekelheide, J. Witte, and G. Singer.
1. Procedure
Caution! Dimethylketene β-lactone dimer is a mild but deceptively persistent lachrymator.
In a 500-ml. three-necked flask equipped with a thermometer, a mechanical stirrer, and a reflux condenser are placed 200 g. (1.43 moles) of tetramethyl-1,3-cyclobutanedione (Note 1) and 50 g. of chlorobenzene (Note 2). The mixture is heated to 135° with stirring while a total of 1.8 g. of reagent grade anhydrous aluminum chloride is added in 0.3-g. portions over a 3-hour period (Note 3). After the addition is complete, heating is continued for an additional 5 hours (Note 4).
The reaction mixture is then cooled to 35–40° and poured into a stirred solution of 230 g. of sodium chloride and 6.0 g. of sodium acetate in 600 ml. of water at 40°. The mixture is stirred for 15 minutes and then is transferred to a separatory funnel; the layers are separated, and the lower, aqueous layer is discarded. The crude product is distilled at reduced pressure through a stainless steel spinning-band column (Note 5). The yield of the β-lactone dimer of dimethylketene is 122–132 g. (61–67%); b.p. 69–71.5° (14 mm.) (Note 6). The product may be redistilled and the fraction boiling at 119.5–120° (150 mm.), n20D 1.4380, collected.
2. Notes
1.
Tetramethyl-1,3-cyclobutanedione is available from Eastman Organic Chemicals. It melts at
115–116° and is typically 99% pure by vapor-phase chromatography.
2.
More highly chlorinated aromatic solvents such as
1,2,4-trichlorobenzene can be used with similar results.
3.
No special drying precautions are required; however, traces of water can be conveniently removed before the catalyst is added by distilling a small portion of the solvent until cloudiness disappears. The checkers found that the distillation of part of the solvent as indicated was necessary in order to obtain satisfactory yields. When the catalyst is added incrementally, no appreciable exotherm is observed.
4.
The isomerization is usually complete in 5 hours and can easily be followed by vapor-phase chromatography. (Heating periods up to 20 hours are not detrimental. The only failure among numerous preparations occurred when
tetramethyl-1,3-cyclobutanedione contaminated with
4% of isobutyric acid was used. In case of partial conversion after 5 hours, additional increments
(0.5 g.) of aluminum chloride should be added to complete the reaction.
5.
The product may be distilled directly from the crude reaction mixture after addition of
sodium acetate. The results are similar.
6.
The distilled product is 99% pure by vapor-phase chromatography.
3. Discussion
The
β-lactone dimer of dimethylketene can be prepared by pyrolysis of its polyester, which is formed by the base-catalyzed polymerization of
dimethylketene.
3,4,5 In addition to the rearrangement of the normal dimer described above,
6 the direct dimerization of
dimethylketene in the presence of
aluminum chloride3 or trialkyl phosphites
7 leads to the β-lactone dimer.
4. Merits of the Preparation
The
β-lactone dimer of dimethylketene reacts with alcohols, phenols, mercaptans, and amines to form derivatives of 2,2,4-trimethyl-3-oxovaleric acid.
3 In this respect it is a more powerful acylating reagent than the normal dimer,
tetramethyl-1,3-cyclobutanedione. The preparation of
2,2,4-trimethyl-3-oxovaleranilide, for example, is accomplished easily with the lactone dimer, but is extremely difficult with the normal dimer.
8In the presence of catalytic amounts of
sodium methoxide,
dimethylketene β-lactone dimer is polymerized at moderate temperature to a polyester.
3 At higher temperatures (above 100°), disproportionation to the cyclic trimer,
hexamethyl-1.3,5-cyclohexanetrione, takes place.
9 Addition of a stoichiometric amount of
sodium methoxide to the lactone dimer generates the
sodium enolate of methyl 2,2,4-trimethyl-3-oxovalerate. This reaction provides a convenient entry into certain ester anion chemistry that formerly required the use of a strong base like
tritylsodium.
10Although these reactions can be duplicated in most cases with the normal dimer of
dimethylketene,
11 the more reactive lactone dimer is the preferred reagent. The liquid form of this dimer is convenient to handle. A distinct difference in behavior of the
dimethylketene dimers is noted when they are pyrolyzed. The normal dimer is dissociated at 600° to
dimethylketene,
12 but the lactone dimer is decarboxylated almost quantitatively at 450° to
tetramethylallene.
13This preparation is referenced from:
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
Dimethylketene β-lactone dimer
β-lactone dimer of dimethylketene
sodium enolate of methyl 2,2,4-trimethyl-3-oxovalerate
sodium acetate (127-09-3)
sodium chloride (7647-14-5)
sodium methoxide (124-41-4)
chlorobenzene (108-90-7)
aluminum chloride (3495-54-3)
tritylsodium (4303-71-3)
isobutyric acid (79-31-2)
1,2,4-trichlorobenzene (120-82-1)
tetramethyl-1,3-cyclobutanedione (933-52-8)
Dimethylketene (598-26-5)
3-Pentenoic acid, 3-hydroxy-2,2,4-trimethyl-, β-lactone (3173-79-3)
2,2,4-trimethyl-3-oxovaleranilide
hexamethyl-1.3,5-cyclohexanetrione (778-18-7)
tetramethylallene (1000-87-9)
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