Checked by Ronald C. Newbold and Andrew S. Kende.
1. Procedure
A.
Preparation of 4-(trimethylsilyl)-3-butyn-1-ol. A
flame-dried, three-necked, 2-L, round-bottomed flask is fitted with a
1-L pressure-equalizing addition funnel, a
mechanical stirrer, and a
nitrogen inlet. The flask is flushed with dry
nitrogen and charged with
3-butyn-1-ol (Note 1) (freshly distilled, 31.4 g, 0.448 mol) and
900 mL of anhydrous tetrahydrofuran (Note
2). The stirred solution is cooled to 0°C under
nitrogen and to it is added over 1 hr a solution of
ethylmagnesium bromide in tetrahydrofuran (493 mL of 2.0 M solution, 0.986 mol) (Note
1). The resulting heterogeneous mixture is rapidly stirred at 0°C for 1 hr, allowed to warm to room temperature for 1 hr, and then recooled to 0°C. To this mixture is slowly added over 30 min with rapid stirring freshly distilled (Note
3)
chlorotrimethylsilane (125 mL, 0.986 mol). The mixture is stirred for 1 hr at 0°C and allowed to warm to room temperature over 1–2 hr. The entire reaction mixture is poured slowly with rapid stirring into a
4-L Erlenmeyer flask that contains
1 L of ice-cold 3 M hydrochloric acid, and is stirred at 25°C for an additional 2 hr. The organic phase is separated and the aqueous phase is extracted with three
200-mL portions of ether.
The combined organic phases are washed with two 200-mL portions of water, four
200-mL portions of saturated sodium bicarbonate solution, and two
200-mL portions of saturated sodium chloride. The organic phase is dried over anhydrous
magnesium sulfate, filtered, and concentrated under reduced pressure at room temperature using a
rotary evaporator. The crude product is distilled through a short-path distillation apparatus under reduced pressure to give
45.2 g (0.318 mol,
71% yield) of
4-(trimethylsilyl)-3-butyn-1-ol, bp
78–79°C (10 mm), as a colorless liquid (Note
4) and (Note
5).
B.
Preparation of (Z)-4-(trimethylsilyl)-3-buten-1-ol. A
dry, 250-mL, round-bottomed flask with a
stirring bar is charged with
8.84 g (0.062 mol) of 4-(trimethylsilyl)-3-butyn-1-ol,
0.4 g of 5% palladium on barium sulfate (Note
6),
0.45 g of synthetic quinoline (Note
7), and
78 mL of methanol. The flask is placed in a hydrogenation apparatus equipped with a
gas burette, and the stirred mixture is thoroughly purged with
nitrogen. The
nitrogen is then replaced by
hydrogen and the reaction mixture is stirred at atmospheric pressure and room temperature until
1.46 L (0.065 mol) of hydrogen is consumed. The flask is flushed with
nitrogen and the solution is filtered through a thick pad of Celite. The filtrate is concentrated on a rotary evaporator at room temperature to afford 10–15 mL of an oil, which is diluted with
150 mL of ether. The
ether solution is thoroughly washed once with
200 mL of ice-cold 0.2 M sulfuric acid, then once with
20 mL of 5% sodium bicarbonate solution. The
ether layer is dried over anhydrous
magnesium sulfate, filtered, and concentrated to yield
8.4 g (0.058 mol) of the crude buten-1-ol (Note
8). Short-path distillation under reduced pressure gives
7.60 g (0.0527 mol,
85% yield) of
(Z)-4-(trimethylsilyl)-3-buten-1-ol, bp
95–100°C (25 mm) as a colorless liquid (Note
9) and (Note
10).
2. Notes
4. The product,
4-(trimethylsilyl)-3-butyn-2-ol, shows the following proton NMR spectrum at 300 MHz in CDCl
3 δ: 0.03 (s, 9 H, SiCH
3), 1.8 (broad s, 1 H, OH), 2.47 (t, 2 H, CH
2), 3.67 (m, 2 H, CH
2OH); and infrared spectrum (neat) cm
−1: 3350 (very broad), 2178, 1250, 1031, 894, 842, 760.
7. Synthetic
quinoline was purchased from Aldrich Chemical Company, Inc. and was distilled prior to use.
8. The proton NMR spectrum of the crude buten-1-ol was essentially identical to that of the distilled product, except for traces of solvent. This crude silylbuten-1-ol was of sufficient purity for the
tetrahydropyridine synthesis described in the next procedure.
9. Distilled product showed a proton NMR at 250 MHz in CDCl
3 as follows δ: 0.14 (s, 9 H, SiCH
3), 1.61 (broad s, 1 H, OH), 2.37–2.46 (m, 2 H, CH
2CH
2OH), 3.68 (broadened t, 2 H,
J = 6.5, CH
2OH), 5.66–5.71 (dt, 1 H,
J = 14.1,
J = 1.2, Me
3SiCH=CHR), 6.29 (overlapping dt, 1 H,
J = 14.1,
J = 7.1, R
3SiCH=CHR). Gas chromatographic analysis using a
25-m 5% methylphenylsilicone column showed that this sample was a 92:8 mixture of
Z and
E isomers and contained <2% of other impurities.
10. The submitters report that
Z-4-(trimethylsilyl)-3-buten-1-ol of >98% isomeric purity can be obtained in ca.
60% overall yield by a more lengthy sequence involving hydroalumination–protonolysis
3 of the
tetrahydropyranyl (THP) ether of 4-(trimethylsilyl)-3-butyn-1-ol4 followed by cleavage
5 of the
THP ether with pyridinium p-toluenesulfonate in methanol. This sequence is less convenient for the
tetrahydropyridine synthesis described in the next procedure, since the isomeric purity of the
vinylsilane is not important for the cyclization reaction.
6
3. Discussion
The direct silylation of
3-butyn-1-ol follows the Danheiser modification
7 of the Westmuze–Vermeer
8 method. The subsequent semihydrogenation is a modification
9 of the Lindlar procedure and yields the
Z-alkene isomer in >90% isomeric purity.
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