Trichlorosilane1

Cl3SiH

[10025-78-2]  · Cl3HSi  · Trichlorosilane  · (MW 135.45)

(reducing agent; hydrosilating agent; precursor of trichlorosilane radical)

Physical Data: mp -126.5 °C; bp 31-32 °C; d 1.342 g cm-3.

Solubility: sol benzene, carbon disulfide, chloroform, carbon tetrachloride; dec by water.

Form Supplied in: high quality colorless liquid.

Purification: can be purified by treatment with quinoline to remove hydrogen chloride followed by distillation, first at atmospheric pressure and then under vacuum.

Handling, Storage, and Precautions: this reagent is highly flammable and corrosive; should be stored in the absence of moisture and handled in a fume hood.

Functional Group Reductions.

Trichlorosilane has often been used together with tertiary amines (usually Tripropylamine or Tri-n-butylamine) to reduce the carbonyl groups of aromatic aldehydes, ketones, acids, amides, acid chlorides, and anhydrides to give the corresponding benzylic trichlorosilanes (eq 1);1 this is referred to as reductive silylation. The mechanism of this reaction remains unclear; however, studies indicated the possible formation of the trichlorosilyl anion by the action of tertiary amines on trichlorosilane (eq 2).2 The adduct (1) was proposed based on the known propensity of tetracoordinated silicon to form five or six-coordinate complexes.3 This method provided a new way to form silicon-carbon bonds and, more importantly, the benzylic trichlorosilane products can be cleaved by base treatment to give toluenes. This transformation can be carried out without isolation of the intermediate organosilane (eq 3).4

Aromatic esters are unaffected during this reduction; consequently it is possible to effect selective reduction of half-esters (eq 4).5

In contrast to the above results, reduction of aldehydes and ketones with premixed trichlorosilane and lithium derivatives of phenols or 1,2-diols gives the corresponding alcohols; pentacoordinate hydridosilicates (such as 2) are believed to function as the reducing agent (eqs 5 and 6).6

It has also been found that the trichlorosilyl radical is generated in the presence of radical initiators (hn, 1,1-Di-t-butyl Peroxide). This radical readily reacts with aliphatic carboxylates (eq 7)7 and acetals8 to give dialkyl ethers, and with alkyl halides9 and acid chlorides10 to give alkanes. The reaction is believed to proceed by a radical chain mechanism.

Trichlorosilane also reduces enamines (eq 8)11 and imines12 to the corresponding amines; one-step hydroamidation of a Schiff base gives the acetamide (eq 9).13

Reduction of carbamates and isocyanates with trichlorosilane give isocyanides.13 This provides a relatively simple method for synthesis of vinyl isocyanides from alkenes (eq 10).14 Due to the mild conditions of the reductions, it has been applied in the resolution of alcohols. Along this line, diastereomeric carbamates obtained by treating racemic alcohols with optically pure isocyanates are separated and reduced to give the desired (R)- and/or (S)-alcohols (eq 11).15 This method has been extensively used, such as in the enantioselective syntheses of (+)-sterpurene,16 NRDC 182,17 ascofuranone, and ascofuranol.18

Hydrosilation.

The additions of trichlorosilane to alkenes and alkynes usually are highly stereoselective19 and can be effected under a wide variety of conditions. For example, the additions can be brought about thermally,20 or by using metal complexes (eq 12).21 In the presence of chiral catalysts, additions proceed enantioselectively.22 It has been demonstrated that the trichlorosilyl group is a synthetic equivalent for the OH, Cl, and Br groups.23

The addition can also be carried out under radical conditions.24 The trichlorosilyl radical is more electrophilic than trialkylsilyl radicals, and adds faster to both alkenes and carbonyl groups (eqs 13 and 14).25

Reaction with Halides.

Organic halides undergo a variety of transformations in the presence of trichlorosilane, depending upon the reaction conditions. One of the most important applications is the formation of allylsilanes (eqs 15 and 16).26

Reaction with Sulfur and Phosphine Compounds.

Trichlorosilane is also used for the reduction of phosphine oxides to phosphines (eq 17).27 Sulfenyl, sulfinyl, and sulfonyl chlorides are reduced to symmetrical disulfides.28


1. Benkeser, R. A. ACR 1971, 4, 94 and references therein.
2. (a) Benkeser, R. A.; Foley, K. M.; Grutzner, J. B; Smith, W. E. JACS 1970, 92, 697. (b) Bernstein, S. C. JACS 1970, 92, 699.
3. (a) Campbell-Ferguson, H. J.; Ebsworth, E. A. V. JCS(A) 1966, 1508. (b) Campbell-Ferguson, H. J.; Ebsworth, E. A. V. JACS 1967, 705. (c) Aylett, B. J. J. Inorg. Nucl. Chem. 1960, 15, 87.
4. (a) Benkeser, R. A., Gaul, J. M. JACS 1968, 90, 5307. (b) Li, G. S.; Ehler, D. F.; Benkeser, R. A. OSC 1988, 6, 747 and references therein.
5. Benkeser, R. A.; Ehler, D. F. JOC 1973, 38, 3660.
6. Kira, M.; Sato, K.; Sakurai, H. JOC 1987, 52, 948.
7. (a) Tsurugi, J.; Nakao, R.; Fukumoto, T. JACS 1969, 91, 4587. (b) Baldwin, S. W.; Haut, S. A. JOC 1975, 40, 3885. (c) Nagata, Y.; Dohmaru, T.; Tsurugi, J. JOC 1973, 38, 795.
8. Nakao, R.; Fukumoto, T.; Tsurugi, J. JOC 1972, 37, 4349.
9. Kerr, J. A.; Smith, B. J. A.; Trotman-Dickenson, A. F.; Young, J. C. JCS(A) 1968, 510.
10. Oka, K.; Nakao, R.; Abe, Y.; Dohmaru, T. JOM 1990, 381, 155. For more information about reduction with trichlorosilane under radical conditions, see references therein.
11. Snyder, D. C. JOM 1987, 320, 163.
12. Okamoto, H.; Kato, S. BCJ 1991, 64, 2128.
13. Baldwin, J. E.; Bottaro, J. C.; Riordan, P. D.; Derome, A. E. CC 1982, 942.
14. Baldwin, J. E.; Yamaguchi, Y. TL 1989, 30, 3335.
15. Pirkle, W. H.; Adams, P. E. JOC 1979, 44, 2169 and references cited therein.
16. Gibbs, R. A.; Okamura, W. H. JACS 1988, 110, 4062.
17. Hatch III, C. E.; Baum, J. S.; Takashima, T.; Kondo, K. JOC 1980, 45, 3281.
18. Mori, K.; Takechi, S. T 1985, 41, 3049.
19. Benkeser, R. A.; Burrous, M. L.; Nelson, L. E.; Swisher, J. V. JACS 1961, 83, 4385.
20. Barry, A. J.; DePree, L.; Gilkey, J. W.; Hook, D. E. JACS 1947, 69, 2916.
21. Kira, M.; Hino, T.; Sakurai, H. TL 1989, 30, 1099 and references there in.
22. Hayashi, T.; Matsumoto, Y.; Morikawa, I.; Ito, Y. TA 1990, 1, 151 and references therein.
23. Kumada, M.; Tamao, K.; Yoshida, J.-I. JOM 1982, 239, 115 and references therein.
24. (a) Sommer, L. H.; Pietrusza, E. W.; Whitmore, F. C. JACS 1947, 69, 188. (b) Burkhard, C. A.; Krieble, R. H. JACS 1947, 69, 2687.
25. Krause, G. A.; Liras, S. TL 1990, 5265.
26. (a) Carr. S. A.; Weber, W. P. JOC 1985, 50, 2782. (b) Trost, B. M.; Buch, M.; Miller, M. L. JOC 1988, 53, 4887.
27. Minami, T.; Okada, Y.; Nomura, R.; Hirota, S; Nagahara, Y; Fukuyama, K. CL 1986, 613.
28. Chan, T. H.; Montillier, J. P.; Van Horn, W. F.; Harpp, D. N. JACS 1970, 92, 7224.

Lianhong Xu

Abbott Laboratories, Abbott Park, IL, USA



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