[18292-28-9]  · C7H16Si  · Crotyltrimethylsilane  · (MW 128.29) (E)

[17486-12-3] (Z)


(carbon nucleophile for the introduction of 1-methylallyl groups by Lewis acid-catalyzed reaction with aldehydes, iminium ions, enones, and similar carbon electrophiles, with some heteroatom electrophiles, and as a starting material for synthesis of a-branched allylsilanes)

Physical Data: bp 110-115 °C; (E) isomer: n25D 1.4159; (Z) isomer: n25D 1.4208.2

Solubility: freely sol all organic solvents.

Analysis of Reagent Purity: dH(CCl4) (E) isomer: 5.4-5.1 (2H, m), 1.55 (3H, d, J = 4), 1.26 (2H, d, J = 5), and 0.11 (9H, s); (Z) isomer: 5.36 (2H, m), 1.9-1.2 (5H, br m), and 0.05 (9H, s).2

Preparative Methods: both (E)- and (Z)-crotyltrimethylsilane can be prepared separately from the exo- and endo-crotylpotassium compounds3 and Chlorotrimethylsilane,4 by nickel(II)-catalyzed coupling of (E)- or (Z)-1-bromopropene with (Chloromethyl)trimethylsilane,5 and by reaction of methylmagnesium iodide on 1,3-bis(trimethylsilyl)propene oxide followed by acid- or base-catalyzed (Peterson) desilylative elimination.6 A mixture of the (E) and (Z) isomers in a 9:1 ratio can be prepared by the reaction of Trichlorosilane and Triethylamine with Crotyl Chloride, followed by treatment with methylmagnesium iodide.7 Coupling of the crotyl Grignard reagent with chlorotrimethylsilane similarly gives mixtures of both stereoisomers, but also some of the regioisomer.8 The three components may be separated by GC. The pure (Z) isomer can be prepared by hydrosilylation of 1,3-Butadiene with trimethylsilane using Hexacarbonylchromium.9 The regio- and stereoisomers can be equilibrated by Tetra-n-butylammonium Fluoride (TBAF) in refluxing THF.10 Methods for the synthesis of allylsilanes in general have been reviewed.11

Handling, Storage, and Precautions: crotyltrimethylsilane is inflammable.

As a Carbon Nucleophile in Uncatalyzed and in Lewis Acid-Catalyzed Reactions with Carbon Electrophiles.

Crotyltrimethylsilane is an alkene some 105 times more nucleophilic than propene and some 20 times more nucleophilic than Allyltrimethylsilane, as judged by its reactions with diarylmethyl cations.7,12 It reacts with cationic carbon electrophiles at the alkenic carbon remote from the silyl group to give an intermediate cation, and the silyl group is lost to create a double bond at the terminus, i.e. with allylic inversion, as illustrated for the tropylium cation (eq 1).13

More usually the electrophile is created by coordination of a Lewis acid to a functional group. Among the stereochemically straightforward electrophiles are Trichloroacetonitrile (eq 2),14 ketones (eq 3),15 linear methoxyethoxymethyl mixed acetals and monothioacetals,16 and alkoxyalkyl halides.18

Somewhat more complicated are those reactions where two adjacent stereogenic centers are set up simultaneously, as in the reactions with aldehydes (eq 4),5 acetals (eq 5),17 the corresponding alkoxymethyl halides,18 and a,b-unsaturated ketones and esters (eq 6).19

There is a trend for the (E)-crotylsilane to produce the syn and the (Z)-crotylsilane to produce the anti arrangement of substituents along the backbone, but in several reactions (for example, eq 5, R = i-Bu) the stereochemistry of the starting material appears to make no difference. The (E)-crotylsilane is more diastereoselective than the (Z)-crotylsilane. In the reaction with acetals (eq 5, R = Ph), substituents on the phenyl ring affect the diastereoselectivity.17

Suitable chiral electrophiles allow enantiocontrol as well as diastereocontrol. There are examples with aldehydes (eq 7),20 glycal 3-acetates (eq 8),21 acyliminium ions (eq 9),22 and a,b-unsaturated sulfoxides.23 Some tailoring of the substituents on the silicon atom can be helpful in achieving higher levels of diastereo- and enantiocontrol.19,21

Fluoride ion-catalyzed reactions of crotylsilanes are not regiospecific (eq 10), the ratio of the two products (3) and (4) being much the same whether crotyltrimethylsilane (1) or its regioisomer (2) is used.24

As a Carbon Nucleophile in Reactions with Heteroatom Electrophiles.

Crotyltrimethylsilane reacts with nitrogen25 and sulfur electrophiles with the usual allylic shift (eq 11). In the corresponding reactions with selenium,26 tellurium,27 and thallium28 electrophiles the products are themselves allylically unstable, giving overall the products that appear simply to have replaced the silyl group with the electrophile.

As a Source of the 1-Methyl-3-trimethylsilylallyl Anion.

The (E)- or (Z)-crotylsilanes react with n-Butyllithium and Potassium t-Butoxide (Schlosser's base) to give the corresponding allyl anions (5) and (6). Left for a few hours, these anions equilibrate to give largely the isomer (6). Alkylation of either anion takes place a to the silyl group, to give new (E) or (Z) a-branched allylsilanes (eq 12).4

Related Reagents.

Allyltrimethylsilane; Crotyltributylstannane.

1. Fleming, I.; Dunogués, J.; Smithers, R. OR 1989, 37, 57.
2. Seyferth, D.; Jula, T. F.; Dertouzos, H.; Pereyre, M. JOM 1968, 11, 63.
3. Hartmann, J.; Muthukrishnan, R.; Schlosser, M. HCA 1974, 57, 2261.
4. Mordini, A.; Palio, G.; Ricci, A.; Taddei, M. TL 1988, 29, 4991.
5. Hayashi, T.; Kabeta, K.; Hamachi, I.; Kumada, M. TL 1983, 24, 2865.
6. Shimizu, N.; Imazu, S.; Shibata, F.; Tsuno, Y. BCJ 1991, 64, 1122.
7. Hagen, G.; Mayr, H. JACS 1991, 113, 4954.
8. (a) Slutsky, J.; Kwart, H. JACS 1973, 95, 8678. (b) Matarosso-Tchiroukhine, E.; Cadiot, P. JOM 1976, 121, 169. (c) Sakurai, H.; Kudo, Y.; Miyoshi, H. BCJ 1976, 49, 1433. (d) Imai, T.; Nishida, S. JOC 1990, 55, 4849.
9. Wrighton, M. S.; Schroeder, M. A. JACS 1974, 96, 6235.
10. Hosomi, A.; Shirahata, A.; Sakurai, H. CL 1978, 901.
11. Sarkar, T. K. S 1990, 969 and 1101.
12. Mayr, H. AG(E) 1990, 29, 1371.
13. Picotin, G.; Miginiac, P. TL 1988, 29, 5897.
14. Hamana, H.; Sugasawa, T. CL 1985, 921.
15. Hosomi, A.; Sakurai, H. TL 1976, 1295.
16. (a) Nishiyama, H.; Itoh, K. JOC 1982, 47, 2496. (b) Nishiyama, H.; Narimatsu, S.; Sakuta, K.; Itoh, K. CC 1982, 459.
17. Hosomi, A.; Ando, M.; Sakurai, H. CL 1986, 365.
18. Sakurai, H.; Sakata, Y.; Hosomi, A. CL 1983, 409.
19. (a) Tokoroyama, T.; Pan, L.-R. TL 1989, 30, 197. (b) Pan, L.-R.; Tokoroyama, T. CL 1990, 1999.
20. Grossen, P.; Herold, P.; Mohr, P.; Tamm, C HCA 1984, 67, 1625.
21. (a) Danishefsky, S. J.; DeNinno, S.; Lartey, P. JACS 1987, 109, 2082. (b) Danishefsky, S. J.; Selnick, H. G.; Zelle, R. E.; DeNinno, M. P. JACS 1988, 110, 4368. (c) Danishefsky, S. J.; Armistead, D. M.; Wincott, F. E.; Selnick, H. G.; Hungate, R. JACS 1989, 111, 2967.
22. Polniaszek, R. P.; Belmont, S. E. JOC 1991, 56, 4868.
23. Pan, L.-R.; Tokoroyama, T. TL 1992, 33, 1469 and 1473.
24. Hosomi, A.; Shirahata, A.; Sakurai, H. TL 1978, 3043.
25. Olah, G. A.; Rochin, C. JOC 1987, 52, 701.
26. Nishiyama, H.; Hagaki, K.; Sakuta, K.; Itoh, K. TL 1981, 22, 5285.
27. Hu, N. X.; Aso, Y.; Otsubo, T.; Ogura, F. TL 1988, 29, 4949.
28. Ochiai, M.; Fujita, E.; Arimoto, M.; Yamaguchi, H. CPB 1983, 31, 86.

Ian Fleming

Cambridge University, UK

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