Checked by Choon Sup Ra and Leo A. Paquette.
1. Procedure
A.
Ethyl 2-(diphenylmethylsilyl)decanoate. A
500-mL, three-necked, round-bottomed flask equipped with a
magnetic stirrer, a
nitrogen inlet, a
100-mL pressure-equalizing dropping funnel, and a
no-air stopper is flame-dried under a vigorous flow of
nitrogen, cooled under an atmosphere of
nitrogen to −78°C with a
dry ice–acetone bath, and charged with
39.2 mL (52.5 mmol) of a 1.34 M solution of butyllithium in hexane (Note
1). To this solution is added
7.4 mL (5.31 g; 52.5 mmol) of diisopropylamine (Note 2) in 7 mL of tetrahydrofuran (Note
3). The resulting solution is warmed to ambient temperature and held for 30 min. The solution is diluted with
50 mL of dry tetrahydrofuran and cooled again to −78°C. To this solution is added
11.6 mL (10.0 g; 50 mmol) of ethyl decanoate (Note 4) in 45 mL of tetrahydrofuran dropwise over a 30-min period. The mixture is kept at −78°C for 30 min to allow the enolate to form, and then
10.3 mL (11.6 g; 50 mmol) of diphenylmethylchlorosilane (Note 5) in 40 mL of tetrahydrofuran is added over a 5-min period. The reaction mixture is allowed to reach ambient temperature and stir at that temperature for 8 hr. It then is cooled to 0°C, diluted with
hexane (150 mL), washed with cold water (2 × 100 mL), dried over
magnesium sulfate, filtered, and concentrated at reduced pressure (Note
6). The crude product, which is ca. 95% pure (Note
7), is purified by rapid filtration through 50 g of silica gel (Note
8) with
1% ethyl acetate–hexane (Note
9) as eluant. There is obtained
18.4–18.7 g (
93–94%) of
ethyl 2-(diphenylmethylsilyl)decanoate (Note
10). Similar results are obtained on a larger scale (Note
11).
B.
2-Methyl-2-undecene. A
1-L, three-necked, round-bottomed flask equipped with a magnetic stirrer, a nitrogen inlet, a
500-mL pressure-equalizing dropping funnel, and a no-air stopper is flame-dried under vacuum, cooled to room temperature under an atmosphere of
nitrogen, and charged with
87 mL (260 mmol) of 3 M methylmagnesium bromide in ether (Note
12). This solution is cooled to 0°C (
ice bath) and
52 g (130 mmol) of ethyl 2-(diphenylmethylsilyl)decanoate in 260 mL of tetrahydrofuran (Note
3) is added over an 8-min period. After the addition is complete, the reaction mixture is warmed to room temperature and heated to reflux for 24 hr. The reaction mixture is again cooled to 0°C and
244 mL (390 mmol) of 1.6 M methyllithium in tetrahydrofuran (Note
13) is added over a 30-min period. After the addition is complete, the reaction mixture is heated to reflux for 24 hr, cooled to 0°C (ice bath), and
29.2 g (260 mmol) of solid potassium tert-butoxide (Note
14) is added in three portions (Note
15). The reaction mixture is heated to reflux for 1 hr, cooled to 0°C, diluted with
hexane (100 mL), and hydrolyzed by the dropwise addition of
1 M hydrochloric acid (240 mL), followed by about
150 mL of 3 M hydrochloric acid until a pH of 4 is reached (Note
16). The organic layer is separated and the aqueous layer is extracted with
hexane (3 × 100 mL). The combined organic layers are dried over anhydrous
magnesium sulfate, filtered, and concentrated under reduced pressure (Note
6) to give 55 g of crude material (Note
17). This material is diluted with
150 mL of dry hexane (Note
18) and applied to a silica gel column (Note
8). The product is obtained by eluting with
hexane and collecting 200-mL fractions. This material, which contains small amounts of
dimethyldiphenylsilane and
diphenylmethylsilanol, is chromatographed under 3–5 psi on a silica gel column (50 × 2.8 cm) eluting with
hexane (Note
19) to give
11.3 g (
51.8%) of the olefin (Note
20).
2. Notes
1.
Butyllithium was purchased from Foote Mineral Company and titrated by the method of Watson and Eastham.
2
3.
Tetrahydrofuran was a gift from Pfizer Pharmaceuticals of Puerto Rico purchased by them from Dupont Company. It was distilled from
sodium/benzophenone prior to use.
4.
Ethyl decanoate was purchased from Aldrich Chemical Company, Inc. and used without further purification.
6. A high-volume house vacuum system was used for this step.
8. Chromatographic silica gel, 70–230 mesh, from Matheson-Coleman-Bell was used.
9. Alternatively, the product can be distilled in an Aldrich Kugelrohr apparatus (pot temperature 130–135°C at 0.2 mm) to give slightly lower (
80–90%) yields.
10. The physical properties are as follows:
nD20 1.5190; IR (neat) cm
−1: 3068, 3045, 2950–2850, 1714, 1589, 1254, 790;
1H NMR (CDCl
3, 80 MHz) δ: 0.66 (s, 3 H), 0.95 (t, 3 H,
J = 1), 1.21 (brs, 14 H), 2.56 (m, 1 H), 3.86 (m, 2 H), 7.29–7.62 (m, 10 H);
13C NMR (CDCl
3) δ: 3.39, −5.57, 14.01, 22.63, 25.02, 27.56, 29.20, 29.35, 30.51, 31.87, 36.39, 59.75, 127.71, 129.50, 129.56, 134.32, 134.64, 134.78, 134.83, 175.02; MS 70 eV m/e (relative abundance) 398 (10), 397 (19), 396 (33), 353 (21), 351 (20), 319 (27), 298 (39), 297 (75), 284 (23), 227 (33), 199 (30), 198 (43), 197 (100), 195 (30), 183 (26), 181 (27), 121 (35), 105 (39), 93 (20), 73 (24), 69 (21), 55 (36), 53 (16). Anal. calcd. for C
25H
36O
2Si: C, 75.76, H, 9.09. Found: C, 75.59, H, 9.19.
11. The submitters report that a 187.5-mmol-scale reaction gave an
89% yield of product.
13.
Methyllithium was purchased from Aldrich Chemical Company, Inc., and titrated prior to use.
2
14.
Potassium tert-butoxide was purchased from Aldrich Chemical Company, Inc., and used without further purification.
15.
Caution! Some foaming occurs because of an exothermic reaction.
16. Litmus paper was used to determine the pH.
17. Gas-chromatographic analysis of this material (6 ft × 1/8 in. 10% SP-2401 on 100–120-mesh Supelcoport; 100–200°C program at 10°C/min; flow rate of 20 psi) showed the presence of
ethyl decanoate,
2-undecanone,
dimethyldiphenylsilane, and
2-methyl-2-undecanol in addition to the desired olefin. Small amounts of unidentified products were also present.
18. A mixture of hexanes (Mallinkrodt anhydrous) was used. If the crude product is placed directly on the silica gel column, the column plugs and the compound does not elute.
19. Attempts to purify the product by spinning-band distillation from the crude material gave only about
20% yield.
20. The product is greater than 97% pure by GLC (Note 17). It showed
nD20 1.4360;
1H NMR (CDCl
3, 80 MHz) δ: 0.88 (br t, 3 H), 1.28 (br s, 12 H), 1.61 (br s, 3 H), 1.69 (br s, 3 H), 1.93–2.00 (m, 2 H), 5.14 (m, 1 H);
13C NMR (CDCl
3) δ: 14.05, 17.62, 22.74, 25.64, 28.14, 29.41, 29.65, 29.99, 31.98, 125.08, 131.00; MS (70 eV) m/e (relative abundance) 169 (2), 168 (14), 112 (6), 84 (11), 83 (13), 82 (6), 70 (23), 69 (100), 68 (10), 67 (13), 57 (34), 56 (68), 55 (34), 53 (9).
3. Discussion
Compound
1 represents one example of several α-(diphenylmethylsilyl) esters prepared by the method presented herein.
3 Other examples include the α-diphenylmethylsilylated derivatives of
ethyl acetate,
ethyl propionate,
ethyl 10-undecenoate,
ethyl palmitate, and
ethyl stearate, all obtained in greater than 70% yield. Other alcohols, principally methyl, isopropyl,
tert-butyl and 1-menthyl, also have been employed in this reaction without marked differences. The reasons as to why the lithium enolates of esters are silylated at the
carbon terminus with
diphenylmethylchlorosilane as opposed to the usual silylation on the oxygen terminus is not clear. The direct
C-silylation of the lithium enolates of α,β-disubstituted esters is not possible, except with
ethyl cyclopropanecarboxylate and
ethyl cyclobutanecarboxylate.
4
The α-(diphenylmethylsilyl) esters have been shown to be vinyl dication equivalents 3, and as such are precursors to terminal olefins and deuterated olefins,
5 1,1-disubstituted olefins,
6 and tri- and tetrasubstituted olefins.
7 They are precursors to β-ketosilanes and ketones,
8 9 wherein the overall transformation results in an ester to ketone conversion. They can also be deprotonated and the enolate anion condensed with aldehydes and ketones to give α,β-unsaturated esters,
10 in particular α-alkylated-α,β-unsaturated esters.
11 Their γ-lactone counterparts,
α-(diphenylmethylsilyl)-γ-butyrolactone 4a and
α-(diphenylmethylsilyl)-γ-valerolactone 4b, are precursors to 4-oxo acids,
12 1,4-diketones
13 and α-ylidene-γ-lactones.
14
Copyright © 1921-2002, Organic Syntheses, Inc. All Rights Reserved