Organic Syntheses, CV 6, 1033
Submitted by Robert K. Boeckman, Jr.
12, David M. Blum
1, Bruce Ganem
3, and Neil Halvey
3.
Checked by William R. Baker and Robert M. Coates.
1. Procedure
A.
Vinyltrimethylsilane (
1). A
2-l., three-necked, round-bottomed flask fitted with a
mechanical stirrer, a
reflux condenser, and a
500-ml., pressure-equalizing dropping funnel (Note
1) is charged with
26.4 g. (1.09 g.-atom) of magnesium turnings and
800 ml. of dry tetrahydrofuran (Note
2). A solution of
107 g. (70.5 ml., 1.00 mole) of vinyl bromide (Note
3) in
200 ml. of tetrahydrofuran is placed in the
addition funnel and slowly added dropwise to the reaction vessel. After the reaction has begun (Note
4), the addition rate is regulated, maintaining a gentle reflux during the remainder of the addition period. The mixture is heated at reflux for an additional hour, and a solution of
108 g. (0.995 mole) of chlorotrimethylsilane (Note
5) in
100 ml. of tetrahydrofuran is added dropwise while the reaction is maintained at reflux with continued heating and stirring (Note
6). The suspension is stirred for another 2 hours under reflux, then cooled to room temperature and stirred overnight.
The condenser and
dropping funnel are removed, and the flask is equipped for distillation with a
30.5-cm. Vigreux column. The distillate (b.p.
60–65°) is collected, transferred to a
separatory funnel, and washed with 10–20 100-ml. portions of water (Note
7), yielding
67–78 g. (
67–78%) of
silane 1 as a colorless liquid containing small amounts of
tetrahydrofuran (Note
8).
B.
(1-Bromoethynl)trimethylsilane (2). A
1-l., three-necked, round-bottomed flask equipped with a mechanical stirrer and a
250-ml. dropping funnel is charged with
89.8 g. (approximately 0.90 mole, (Note 8)) of silane 1. The contents of the flask are stirred and cooled to −78°, and
168 g. (1.06 mole) of bromine is added dropwise over
ca. 1 hour. The cooling bath is removed, and the red viscous mixture is warmed to room temperature. The flask is fitted with an efficient,
water-cooled condenser, and
425 g. (600 ml., 5.82 moles) of diethylamine (Note
9) is cautiously (Note
10) added with continued stirring. After the addition is complete, the reaction mixture is heated at reflux for 12 hours, during which time a precipitate of
diethylamine hydrochloride forms. The salts are separated from the cooled suspension by filtration and washed with several
300-ml. portions of diethyl ether. The
ether filtrate is carefully washed, first with
100-ml. portions of 10% hydrochloric acid until the aqueous layer remains acidic (pH
ca. 2), then with 100 ml. of water and
200 ml. of saturated aqueous sodium chloride. The ether solution is dried with anhydrous
magnesium sulfate, concentrated with a
rotary evaporator, and distilled under reduced pressure through a
20.3-cm. Vigreux column, affording
104–110 g. (
65–68%) of
silane 2, b.p.
72–75° (120 mm.) (Note
11).
C.
3-Trimethylsilyl-3-buten-2-ol (3). A
500-ml., three-necked, round-bottomed flask equipped with two
30.5-cm. Liebig condensers connected in series, a
pressure-equalizing dropping funnel, and a
magnetic stirrer is charged with
9.2 g. (0.38 g.-atom) of magnesium turnings and
100 ml. of tetrahydrofuran (Note
1) and (Note
2). About
2 g. of 1,2-dibromoethane is added, initiating the formation of the Grignard reagent. When the supernatant solution becomes warm and begins to reflux from reduction of
1,2-dibromoethane, a solution of
50 g. (0.28 mole) of silane 2 in
75 ml. of tetrahydrofuran is added dropwise to the stirred mixture at a rate that maintains gentle reflux. After the addition is complete, the reaction mixture is kept at reflux for an additional hour before freshly distilled
acetaldehyde (25.0 g., 0.568 mole) is introduced. The temperature is maintained at reflux, and stirring is continued throughout the addition and for an additional hour. The flask is then fitted with a distillation head and heated until
ca. 100 ml. of distillate has been collected. The reaction mixture is cooled (
ice-water bath) and stirred, diluted with
100 ml. of ether, and hydrolyzed by addition of enough saturated
ammonium chloride (approximately 50 ml.) to dissolve the thick, sticky precipitate. The salts are filtered and washed with
ether, and the aqueous layer of the filtrate is extracted with three
150-ml. portions of ether. The combined
ether layers are washed with saturated aqueous
sodium chloride, dried over anhydrous
magnesium sulfate, and concentrated by distillation at atmospheric pressure (Note
12), giving
48–55 g. of crude
butenol 3 as a liquid that is used in the next step without further purification.
D.
3-Trimethylsilyl-3-buten-2-one (4). A solution of
55 g. of crude butenol 3 in
100 ml. of acetone is placed in a
500-ml., three-necked, round-bottomed flask equipped with a mechanical stirrer and a
250-ml. dropping funnel. The reaction vessel is immersed in an
ice-water bath, and
95 ml. of an aqueous solution containing chromium trioxide and
sulfuric acid (Note
13) is added to the stirred
acetone solution. After completion of the addition,
2-propanol is added to the reaction mixture until a green endpoint is reached, indicating consumption of excess oxidant. The contents are poured into
450 ml. of ether, 300 ml. of water are added, and the aqueous layer is saturated with
sodium chloride. The layers are separated, and the aqueous solution is extracted with five
150-ml. portions of ether. The combined
ether solutions are washed with two
150-ml. portions of saturated aqueous sodium chloride, dried with anhydrous
magnesium sulfate, and concentrated by distillation at atmospheric pressure through a 30.5-cm. Vigreux column. Continued distillation under reduced pressure gives, after separation of a low boiling forerun,
14.7–15 g. (
37–38%) of
butenone 4 as a pale yellow liquid, b.p.
98–103° (100 mm.) (Note
14) and (Note
15).
2. Notes
1. The apparatus is flamed dry under an
argon atmosphere and maintained under
argon during the reaction.
3. Gaseous
vinyl bromide is condensed in a
500-ml. flask cooled in an
acetone–dry ice bath and diluted with
200 ml. of tetrahydrofuran. The submitters used
vinyl bromide supplied by J. T. Baker Chemical Company; the checkers purchased this reagent from Linde Specialty Gases.
Vinyl bromide is also available from Aldrich Chemical Company, Inc.
4. Approximately 70 ml. is added over a 20-minute period before formation of the Grignard reagent begins. The total addition time is
ca. 1 hour.
5. The submitters used
practical grade chlorotrimethylsilane purchased from PCR, Inc., which was distilled before use.
Chlorotrimethylsilane from Aldrich Chemical Company, Inc., was employed by the checkers, both with and without prior distillation. Approximately the same yield was obtained in either case.
6. A white precipitate, presumed to be magnesium salts, is deposited as the solution of
chlorotrimethylsilane is added.
7. The layers must be allowed to separate completely to avoid sizable mechanical losses. The submitters used 10 water extractions, whereas the checkers continued the extractions until the organic layer reached an approximately constant weight.
8. The
1H NMR spectrum of the product obtained by the checkers revealed the presence of 6 ± 2% of
tetrahydrofuran.
9.
Diethylamine is available from Aldrich Chemical Company, Inc.
10. The reaction of the excess
bromine with
diethylamine is exothermic; consequently it may be necessary to moderate the reaction by cooling with an
ice-water bath during the early stages of the addition.
11.
1H NMR spectrum (CDCl
3), δ (multiplicity, coupling constant
J in Hz., number of protons, assignment): 0.16 [s, 9H, Si(C
H3)
3], 6.12 (d,
J = 2, 1H, C
H), 6.21 (d,
J = 2, 1H, C
H).
12. The checkers terminated the distillation when the head temperature reached 44°.
14. GC analysis of the product by the submitters on a
1.85-m. 3% silicone gum rubber (SE-30) column at 25° gave a single peak.
Butenone 4 has the following
1H NMR spectrum (CCl
4), δ (multiplicity, coupling constant
J in Hz., number of protons, assignment): 0.14 [s, 9H, Si(C
H3)
3], 2.23 (s, 3H, C
H3), 6.18 (d,
J = 2, 1H, C
H), 6.53 (d,
J = 2, 1H, C
H).
15. This material showed no tendency to deteriorate when stored under an
argon atmosphere at −20°.
3. Discussion
Butenone 4 has been obtained by Brook and Duff
4 from the reaction of
1-trimethylsilylvinylmagnesium bromide and
acetic anhydride at −120°. However, the product was a mixture of
butenone 4 and a dimeric substance which apparently resulted from subsequent conjugate addition of the Grignard reagent to the ketone. The procedures in the present reaction sequence for the preparation of silane
1, silane
2, and
1-trimethylsilylvinylmagnesium bromide are based on those reported by Ottolenghi, Fridkin, and Zilkha.
5 Similarly, other 1-trimethylsilylvinyl ketones may be prepared by reaction of the appropriate aldehyde with
1-trimethylsilylvinylmagnesium bromide and subsequent oxidation with
chromic acid. The 1-trimethylsilylvinyl ketones are remarkably stable and useful in a variety of conjugate addition reactions.
6
This preparation is referenced from:
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