Triethylsilyl Perchlorate


[18244-91-2]  · C6H15ClO4Si  · Triethylsilyl Perchlorate  · (MW 214.75)

(potent silylating agent; Lewis acid catalyst)

Physical Data: distillable colorless liquid, bp 45-46 °C/1 mmHg1 or 43-45 °C/0.5 mmHg.2

Solubility: sol most organic solvents; CH2Cl2 is most commonly used

Preparative Methods: prepared from Chlorotriethylsilane using Silver(I) Perchlorate1 (eq 1) or from Triethylsilane by treatment with silver perchlorate3 or Triphenylmethyl Perchlorate2 (eq 2). Of these methods, the trityl perchlorate procedure may be safer and more cost-effective.

Handling, Storage, and Precautions: reacts rapidly with water and other protic solvents. As for any perchlorate derivative, appropriate precautions should be taken when handling or storing.

General Discussion.

Triethylsilyl perchlorate, a covalent silyl ester of perchloric acid, is a more reactive silylating agent than is chlorotriethylsilane. This is evidenced from the fact that the silyl perchlorate ester is cleaved rapidly and quantitatively by Sodium Borohydride, while the chloride derivative requires the use of Lithium Aluminum Hydride (eq 3).4

Wilcox and Babston reported that alkyl esters of aliphatic acids react with Et3SiClO4 in the presence of Hünig's base (Diisopropylethylamine) or 2,2,6,6-Tetramethylpiperidine to provide (Z)-silyl ketene acetals selectively (eq 4).5 The stereoselectivity of these reactions is dependent on a number of factors, including the nature of the base, the solvent polarity, and workup conditions. This procedure offers higher (Z) selectivity than does the usual deprotonation-silylation protocol using Lithium Diisopropylamide/HMPA.

While alkyl esters generally undergo O-silylation in reactions with triethylsilyl perchlorate, ethyl diazoacetate was found to give exclusively the C-alkylation product (eq 5).6 This procedure, which presumably involves the intermediacy of a diazonium perchlorate salt, affords higher yields of the a-silyl diazoester relative to other methods.

Triethylsilyl perchlorate has been shown to catalyze the Mukaiyama-Michael reaction of silyl ketene acetals with hindered a-enones, enabling 1,5-keto esters having contiguous quaternary centers to be prepared in high yield (eq 6).7 Upon treatment of an equimolar mixture of unsubstituted and b,b-dimethyl silyl ketene acetals with the enone and a catalytic quantity of Et3SiClO4, only the more highly substituted adduct was obtained. To account for this, a mechanism was postulated in which the Lewis acid mediates the transfer of an electron from the silyl ketene acetal to the enone.

1. Wannagat, U.; Liehr, W. AG 1957, 69, 783.
2. Barton, T. J.; Tully, C. R. JOC 1978, 43, 3649.
3. Eaborn, C. JCS 1955, 2517.
4. Barton, T. J.; Hovland, A. K.; Tully, C. R. JACS 1976, 98, 5695.
5. Wilcox, C. S.; Babston, R. E. TL 1984, 699.
6. Wilcox, C. S.; Babston, R. E. S 1985, 941.
7. Sato, T.; Wakahara, Y.; Otera, J.; Nozaki, H.; Fukuzumi, S. JACS 1991, 113, 4028.

Edward Turos

State University of New York at Buffalo, NY, USA

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