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DOI:10.15227/orgsyn.069.0089

Organic Syntheses, Coll. Vol. 8, p.602 (1993); Vol. 69, p.89 (1990).

CONVERSION OF ESTERS TO ALLYLSILANES: TRIMETHYL(2METHYLENE-4-PHENYL-3-BUTENYL)SILANE[Silane, trimethyl (2-methylene-4-phenyl-3-butenyl)-]

Submitted by William H. Bunnelle and B. A. Narayanan1.
Checked by Jürgen Fischer and Ekkehard Winterfeldt.

1. Procedure
A 1-L, three-necked, round-bottomed flask with mechanical stirrer and vacuum outlet (Note 1) ischarged with powdered cerium (III) chloride (CeCl3 · 7H2O, 52.92 g, 0.142 mol) (Note 2). The flask isimmersed to the necks in an oil bath and evacuated to 0.2 mm. The solid is agitated by stirring as theflask is heated to 150°C for 2 hr. After the flask is cooled to room temperature, it is vented to theatmosphere and quickly purged with a stream of dry nitrogen for 2 min. At this stage, the flask is fittedwith a low-temperature thermometer and a graduated 250-mL addition funnel containing 300 mL of dry
tetrahydrofuran (THF) (Note 3), and capped with a rubber septum. Connection to a dry nitrogen linewith a pressure relief bubbler is made via a needle through the septum. Tetrahydrofuran is run into theflask with good stirring, so that an even suspension results (Note 4). The white suspension is stirred atroom temperature for 2 hr. Meanwhile, the addition funnel is charged withtrimethylsilylmethylmagnesium chloride (Me3SiCH2MgCl, 1 M solution in ethyl ether, 142 mL, 0.142mol) (Note 5) and (Note 6), transferred via stainless-steel cannula. The contents of the flask are cooled
to −65°C with a dry ice/2-propanol bath, and the Grignard solution is added dropwise over a period of
40–50 min, so that the temperature of the reaction mixture remains below −60°C. The addition funnel isremoved from the setup, with the septum/nitrogen inlet connected directly to the flask. After the cold
mixture is stirred for 15 min more, ethyl cinnamate (10.32 g, 0.0586 mol) (Note 7) is added via syringeover a 2–3-min period. Stirring is continued as the flask is allowed to warm slowly to room temperature(Note 8). The off-white to beige reaction mixture is cooled to <5°C (ice-water bath) and quenched by
the portionwise addition of chilled 1 M hydrochloric acid (200 mL), so that the internal temperatureremains below 20°C (Note 9). The layers are separated, and the aqueous phase is extracted twice withethyl ether (100 mL each). The combined organic layers are washed with saturated sodium bicarbonate
solution (Note 10) and dried over sodium sulfate, and the solvents are removed at a rotary evaporator(25°C at 60 mm). The residual oil is transferred to a 100-mL, round-bottomed flask and distilled bulbto-bulb (Note 11). The product (9.69–9.85 g, 76.5–78%) (Note 12) is collected at an air-bathtemperature of 110–112°C, 0.20 mm.

2. Notes
1. The vacuum outlet should be packed with glass wool to prevent loss of significant quantities of
cerium salt during the drying process. During some runs, cerium salts worked into the stirrer shaft
bearing, causing it to bind. This problem could be avoided by placing a straight adapter tube between
the bearing and the flask, to distance the bearing from the reaction mixture. The checkers found that
sublimation of the cerium salt occurred in all their runs, but did not appear to affect the yield.
2. Cerium(III) chloride heptahydrate was purchased from Aldrich Chemical Company, Inc., and groundwith a mortar and pestle just before use.
3. Tetrahydrofuran was freshly distilled under nitrogen from sodium and benzophenone. The (nominal)
250-mL funnel holds the entire volume of solvent.
4. On one occasion, when the solvent was added without stirring, the solid formed a hard cake on thebottom of the flask and resisted attempts to bring it into suspension. The full 2-hr "aging" period isnecessary for best results.

5. This clear, pale-yellow solution was purchased from the Aldrich Chemical Company, Inc. The
Grignard solution may be prepared from trimethylsilylmethyl chloride.2 Take care to prevent air
oxidation in opened bottles of the Grignard reagent—these solutions deteriorate rapidly, and older
samples are unsuitable for the present procedure.
6. The prescribed quantity of Grignard reagent (and cerium salt) is 25% more than stoichiometric. This
excess is required to ensure complete consumption of the ester.
7. Ethyl cinnamate was purchased from Eastman Kodak Company, and was distilled at 110–111°C/0.75
mm before use.
8. Warming takes approximately 3 hr. The reaction is complete by this time and can be worked up.
Alternatively, the mixture may be stirred at room temperature for at least 12 hr (overnight) without any
deleterious effect on yield.
9. If the reaction is allowed to become warm, substantial protodesilylation of the product takes place togive 3-methyl-1-phenyl-1,3-butadiene. The aqueous workup should be carried out rapidly to minimize
this side reaction. The use of hydrochloric acid has a particular advantage since all of the salts dissolve,
facilitating the extractions.
10. Some residual salts precipitate during this operation, but do not pose any handling difficulty.
11. Gas chromatographic analysis [0.53–mm i.d. × 10 m poly(dimethyl silicone) fused-silica column,
temperature programmed from 140° to 220°C] indicates that this product is 98% pure, with < 1% 3methyl-1-phenyl-1,3-butadiene and 1–2% of a less volatile, unidentified component. The checkers could
obtain a very pure product by normal vacuum distillation. They found the impurities to be higher boilingcompounds.
12. This material has the following spectral data: IR cm−1: 3068, 1593, 872, 853, 838; 1H NMR δ: 0.01
(s, 9 H), 1.80 (br, s, 2 H), 4.83 (br, s, 1 H), 5.00 (d, 1 H, J = 1), 6.45 (d, 1 H, J = 16), 6.77 (d, 1 H, J =16), 7.1–7.4 (m, 5 H); 13C NMR δ: −1.2, 22.2, 114.8, 126.4, 127.3, 128.6, 128.7, 132.0, 137.4, 143.8;
MS (m/e, %): 216 (40%). 73 (100%).

3. Discussion
Allylsilanes are exceptionally versatile compounds with a well-established function in organicsynthesis.3 4 General methods for their preparation, then, are valuable. The method described here iseffective for elaboration of esters to allylsilanes. The transformation is conceptually straightforward:
twofold addition of a trimethylsilylmethyl metal species to the ester, followed by Peterson-type
elimination from the resultant bis(trimethylsilylmethyl)carbinol, leads to the allylsilanes.
The Grignard reagent Me3SiCH2MgCl has been used to effect this conversion with simple,
unbranched esters, but the yields are only moderate ( 50%) and the process fails completely for moresterically congested esters.5 6 7 8 In these cases, the α-silylketone intermediate resists further addition,
instead suffering preferential enolization.9 The use of organocerium reagents to circumvent thisdifficulty has been developed by two groups.10,11 The reagent prepared from cerium(III) chloride andMe3SiCH2Li is quite effective for conversion of acid chlorides to allylsilanes, but does not react
efficiently with esters.10 Somewhat surprisingly in view of these results, a mixture of cerium(III)chloride with Me3SiCH2MgCl produces a powerful reagent for conversion of esters to allylsilanes in
excellent yields.11 Some examples, for reactions carried out on a 1-mmol scale, are collected in Table
I.11 Only the highly hindered methyl adamantanecarboxylate fails to react.

TABLE I
Ester

Allylsilane

Yield (%)
9590a9577920b

Starting material and product each a 1 : 1 mixture of diastereoisomers.
bStarting material recovered quantitatively.

aThe protocol described above is a scaled-up modification of that reported earlier.11 The title
compound was prepared previously by a nickel-catalyzed cross-coupling,12 and by theorganocerium/acid chloride route.10 The present procedure offers advantages in both convenience andyield compared to these other methods. Recently the method has been successfully applied toheteroatom-functionalized systems.13
This preparation is referenced from:zOrg. Syn. Coll. Vol. 10, 200

References and Notes
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.

Department of Chemistry, University of Missouri-Columbia, MO 65211.
Whitmore, F. C.; Sommer, L. H. J. Am. Chem. Soc. 1946, 68, 481.
Parnes, Z. N.; Bolestova, G. I. Synthesis 1984, 991;
Chan. T. H.; Fleming, I. Synthesis 1979, 761.
Petrov, A. D.; Ponomarenko, V. A.; Snegova, A. D. Dokl, Akad. Nauk S.S.S.R. 1957, 112, 79;
Chem. Abstr, 1957, 51, 11239a;
Fleming, I.; Pearce, A. J. Chem. Soc., Perkin Trans. 1 1981, 251;
Ochiai, M.; Fujita, E.; Arimoto, M.; Yamaguchi, H. J. Chem. Soc., Chem. Commun. 1982, 1108;
Yamazaki, T.; Ishikawa, N. Chem. Lett. 1984, 521.
Demuth, M. Helv. Chim. Acta 1978, 61, 3136.
Anderson, M. B.; Fuchs, P. L. Synth. Commun. 1987, 17, 621.
Narayanan, B. A.; Bunnelle, W. H. Tetrahedron Lett. 1987, 28, 6261.
Hayashi, T.; Fujiwa, T.; Okamoto, Y.; Katsuro, Y.; Kumada, M. Synthesis 1981, 1001.
Lee, T. V.; Channon, J. A.; Cregg, C.; Porter, J. R.; Roden, F. S.; Yeoh, H. T.-L. Tetrahedron
1989, 45, 5877.

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);(Registry Number)
Me3SiCH2Li
Me3SiCH2MgClhydrochloric acid (7647-01-0)
ethyl ether (60-29-7)
sodium bicarbonate (144-55-8)
sodium sulfate (7757-82-6)
nitrogen (7727-37-9)
Benzophenone (119-61-9)
sodium (13966-32-0)
Ethyl cinnamate (103-36-6)
Tetrahydrofuran (109-99-9)
cerium (III) chloride,cerium(III) chloride (7790-86-5)
methyl adamantanecarboxylate (711-01-3)
Trimethylsilylmethylmagnesium chloride (13170-43-9)
trimethylsilylmethyl chloride (2344-80-1)
Trimethyl(2-methylene-4-phenyl-3-butenyl)silane,
Silane, trimethyl (2-methylene-4-phenyl-3-butenyl)- (80814-92-2)
Cerium(III) chloride heptahydrate (18618-55-8)
3-methyl-1-phenyl-1,3-butadienebis(trimethylsilylmethyl)carbinol
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