(X = H)

[77129-88-5]  · C7H14OSi  · 1-Trimethylsilyl-1-methoxyallene  · (MW 142.30) (X = Li)

[82200-98-4]  · C7H13LiOSi  · 3-Lithio-1-trimethylsilyl-1-methoxyallene  · (MW 148.23)

(precursor to acylsilanes, 2-trimethylsilylfurans, and dihydrofurans)

Physical Data: bp 40-43 °C/28 mmHg.2

Solubility: sol THF, ether.

Preparative Method: from methoxyallene by deprotonation with n-Butyllithium and quenching the allenyl anion with Chlorotrimethylsilane.3

Handling, Storage, and Precautions: best stored at -20 °C or below, over anhydrous K2CO3. Hydrolysis, desilylation, or polymerization take place in the presence of acid.

a,b-Unsaturated Acylsilanes.

Deprotonation at C-3 of 1-trimethylsilyl-1-methoxyallene (1) takes place with n-BuLi in THF. Trapping the lithioallene with an alkyl halide, followed by hydrolysis of the methyl enol ether with Trifluoroacetic Acid, leads to the (E)-acylsilane (eq 1).3 Fluorodesilylation with Tetra-n-butylammonium Fluoride prior to the hydrolysis step produces the corresponding a,b-unsaturated aldehyde.

Furan and Dihydrofuran Synthesis.

Trapping the 3-lithio derivative of (1) with decanal leads to the allenic alcohol (eq 2). Aqueous acid under carefully controlled conditions produces 2-trimethylsilyl-5-nonylfuran.2 The addition product of the 3-lithio derivative of (1) with cyclopentanone undergoes acid-catalyzed conversion to the butenolide (2) (eq 3). This unusual transformation apparently takes place through formal loss of Me3Si- from a cyclic intermediate.2

Deprotonation of (1) with Lithium Diethylamide takes a different course (eq 4). The initially formed allenyl anion isomerizes to the acetylide, which is trapped with ketones or aldehydes to produce propargyl alcohols (3).4 The isomerization is postulated to take place through proton transfer steps mediated by diisopropylamine. This is consistent with the observation that no such isomerization takes place with alkyllithium reagents. The propargyl alcohols (3) are converted to methoxydihydrofurans (4) with catalytic Potassium Hydride in DMSO.4

Related Silanes.

1-Trimethylsilyl-1-(ethoxyethoxy)allene (5) undergoes efficient reaction with electrophiles to produce substituted unsaturated acylsilanes (eq 5).5 The reaction products can be converted to diverse 2-silyloxy-1,3-butadienes. Exposure of (5) to Boron Trifluoride Etherate leads to rearrangement in which an oxonium ion is intercepted at C-2 of the allene (eq 5).6 This rearrangement proceeds via an intermolecular pathway. Acylstannanes can also be prepared through this method.

Epoxidation of t-butyldimethylsilyl-1-(ethoxyethoxy)allene (6) with m-Chloroperbenzoic Acid leads to an a-keto acylsilane,6 presumably through an allene oxide intermediate (eq 6). Deprotonation of (6) at C-3 and trapping of the anion with Selenium, followed by Iodomethane, produces an allenyl selenide. The reaction of this material with peracid follows a different course, leading to an acetylenic acylsilane, presumably via [2,3]-sigmatropic rearrangement of an allenyl selenide (eq 6).7

1. (a) Huche, M. BSF(2) 1978, 313. (b) Zimmer, R. S 1993, 165. (c) Schuster, H. F.; Coppola, G. M. Allenes in Organic Synthesis; Wiley: New York, 1984; pp 215-224. (c) Ricci, A.; degl'Innocenti, A. S 1989, 647.
2. Pappalardo, P.; Ehlinger, E.; Magnus, P. TL 1982, 23, 309.
3. Clinet, J.-C.; Linstrumelle, G. TL 1980, 21, 3987.
4. Kuwajima, I.; Sugahara, S.; Enda, J. TL 1983, 24, 1061.
5. Reich, H. J.; Kelly, M. J.; Olson, R. E.; Holtan, R. C. T 1983, 39, 949.
6. Ricci, A.; degl'Innocenti, A.; Capperucci, A.; Faggi, C.; Seconi, G.; Favaretto, L. SL 1990, 471.
7. Reich, H. J.; Kelly, M. J. JACS 1982, 104, 1119.

Marcus A. Tius

University of Hawaii, Honolulu, HI, USA

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