Organic Syntheses, CV 6, 445
Submitted by Michael E. Jung and Charles A. McCombs
1.
Checked by Ping Sun Chu and George H. Büchi.
1. Procedure
Caution! Part A should be carried out in a hood, since the reagents are noxious.
A.
2-Trimethylsilyloxy-1,3-butadiene (1). An
oven-dried, 500-ml., three-necked, round-bottomed flask is fitted with two
oven-dried addition funnels, a
glass stopper, and
magnetic stirrer, and placed in a 80–90°
oil bath. Under an inert atmosphere,
methyl vinyl ketone (25.0 g., 0.357 mole) in
25 ml. of N,N-dimethylformamide and
chlorotrimethylsilane (43.4 g., 0.400 mole) in
25 ml. of N,N-dimethylformamide are added over 30 minutes to a magnetically stirred solution of
triethylamine (40.5 g., 0.400 mole) in
200 ml. of N,N-dimethylformamide (Note
1). The reaction gradually darkens from colorless to yellow or dark brown, and supports a white precipitate of
triethylamine hydrochloride. The reaction is set up to run overnight, or
ca. 14 hours.
The reaction is cooled to room temperature, filtered (Note
2), and transferred to a
2-l. separatory funnel containing
300 ml. of pentane. To this solution is added
1 l. of cold 5% sodium hydrogen carbonate solution to facilitate the separation of phases and remove the
N,N-dimethylformamide. The
pentane layer is separated, and the aqueous layer extracted twice with
300-ml. portions of pentane. The
pentane extracts are combined, washed with 200 ml. of
cold distilled water (Note
3), dried over powdered anhydrous
sodium sulfate, and filtered into a
2-l., round-bottomed flask.
The
pentane and other volatiles are removed by fractional distillation, using a
5-cm., steel wool-packed column and heating the pot in a 70° oil bath (Note
4). A water aspirator vacuum is applied, and
18.2–22.9 g. (
36–45%) of the diene
1 is distilled as a colorless oil, b.p.
50–55° (50 mm.) (Note
5). On a smaller scale, yields of up to
50% have been obtained.
B.
Diethyl trans-4-trimethylsilyloxy-4-cyclohexene-1,2-dicarboxylate (2). A
25-ml., round-bottomed flask equipped with a
reflux condenser is charged with
diene 1 (7.1 g., 0.050 mole) and
diethyl fumarate (5.7 g., 0.033 mole) (Note
6). The mixture is stirred under a
nitrogen atmosphere in an oil bath kept at 130–150° for 24 hours (Note
7). Direct vacuum distillation using a short-path distillation apparatus affords a small amount of lower-boiling material, then
8.02 g. (
77%) of
cyclohexene 2, b.p.
127–128° (0.5 mm.) (Note
8) and (Note
9).
2. Notes
2. Filtration was conveniently performed through a plug of glass wool directly into the separatory funnel.
3. The hydrogen carbonate extractions must be performed quickly, since the product slowly hydrolyzes in the presence of water. The best yields were obtained when the phases were shaken briskly for 10 seconds and separated as soon as foaming ceased. Foaming also occurred when the
pentane extracts were washed with water, but did not prevent the separation of phases.
4. The checkers used a
18-cm. Vigreux column to remove low-boiling material, and then transferred the residue to a
250-ml., round-bottomed flask for fractional distillation under reduced pressure.
5. The distilled diene
1 has been obtained with 99% purity by the submitters. The product has the following spectral properties:
1H NMR: (CCl
4 with benzene as internal standard). δ (multiplicity, number of protons): 0.4 (s, 9H), 4.4 (broad s, 2H), 5.1 (d of m, 1H), 5.5 (d of d, 1H), 6.3 (d of d, 1H). The impurities were
triethylamine and
hexamethyldisiloxane. When stored in a serum-capped flask, in a
desiccator, and removed
via syringe, the butadiene is stable for 2 months.
7. The submitters allowed the reaction to proceed for 24 hours after monitoring by GC (1.83 m. × 0.64 cm., 10% SE-30/60–80 mesh Chromosorb W, 150°) indicated that the disappearance of
diethyl fumarate was not complete after 5.5 hours. The submitters made no attempt to optimize the reaction time. The checkers found the reaction to be complete after 7 hours at 135–145°.
8. The checkers distilled cyclohexene
2 after 7 hours reaction time using a
Büchi Kügelrohr distillation oven to obtain
9.9–10.0 g. (
95–96%) of cyclohexene
2.
9. Cyclohexene
2 has the following spectral properties:
IR (liquid film) cm
−1: 1730, 1670;
1H NMR: (CCl
4), δ (multiplicity, coupling constant
J in Hz., number of protons, assignment): 0.16 [s, 9H, Si(C
H3)
3], 1.17 (t,
J = 7, 6H, 2OCH
2C
H3), 2.00–3.16 (m, 6H, ring C
H), 4.07 (q,
J = 7, 4H, 2OC
H2CH
3), 4.70 (broad s, 1H, olefinic C
H).
3. Discussion
Butadienes substituted with alkoxy groups in the 2-position,
e.g.,
2-ethoxy-1,3-butadiene,
6 7 have been prepared from
methyl vinyl ketone, but required several conversions and a tedious spinning-band distillation to purify the product. This slight modification of the House procedure has been used to conveniently prepare
2-trimethylsilyloxy-1,3-butadiene from the readily available
methyl vinyl ketone. This one-step procedure has provided large amounts of a new and reactive diene for Diels–Alder reactions, as illustrated in Table I.
TABLE I
PREPARATION OF TRIMETHYLSILYLOXYCYCLOHEXENES
|
|
X |
Y |
Z |
Yield(%) |
|
COCH3 |
H |
H |
60 |
CO2CH3 |
H |
H |
46 |
CO2CH3 |
H |
CO2CH3 |
71 |
CO2C2H5 |
H |
CO2C2H5 |
77 |
CO2C2H5 |
CO2C2H5 |
H |
39 |
|
Copyright © 1921-2002, Organic Syntheses, Inc. All Rights Reserved