(1a; R3 = Me3, R1 = Me, R2 = H)4b,9

[74542-82-8]  · C7H14Si  · 1-Methyl-1-(trimethylsilyl)allene  · (MW 126.30) (1b; R3 = t-BuMe2, R1 = Me, R2 = H)7

[99035-24-2]  · C10H20Si  · 1-Methyl-1-(t-butyldimethylsilyl)allene  · (MW 168.39) (1c; R3 = (i-Pr)3, R1 = Me, R2 = H)7

[120789-53-9]  · C13H26Si  · 1-Methyl-1-(triisopropylsilyl)allene  · (MW 210.48) (1d; R3 = Me3, R1 = H, R2 = Me)4b

[74542-82-8]  · C7H14Si  · 3-Methyl-1-(trimethylsilyl)allene  · (MW 126.30)

(propargylic anion equivalents;2,3 three-carbon synthons for [3 + 2] annulations leading to five-membered compounds including cyclopentenes,4 dihydrofurans,5 pyrrolines,5 isoxazoles,6 furans,7 and azulenes8)

Physical Data: (1a) bp 111 °C; bp 54-56 °C/90 mmHg; (1b) bp 62-65 °C/30 mmHg; (1d) bp 107-110 °C.

Solubility: sol CH2Cl2, benzene, THF, Et2O, and most organic solvents.

Form Supplied in: colorless liquid; not commercially available.

Analysis of Reagent Purity: (1a) IR (neat) 2955, 2910, 2900, 2860, 1935, 1440, 1400, 1250, 935, 880, 830, 805, 750, and 685 cm-1; 1H NMR (250 MHz, CDCl3) d 0.08 (s, 9H), 1.67 (t, 3H, J = 3.3), and 4.25 (q, 2H, J = 3.3); 13C NMR (67.9 MHz, CDCl3) d -2.1, 15.1, 67.3, 89.1, and 209.1.4b,9

Preparative Methods: 1-methyl-1-(trialkylsilyl)allenes can be conveniently prepared by the method of Vermeer.9,10 Silyl-substituted propargyl mesylates thus undergo SN2 displacement by the organocopper reagent generated from methylmagnesium chloride, Copper(I) Bromide, and Lithium Bromide. 1-Methyl-1-(trimethylsilyl)allene is produced in 52% yield from commercially available (trimethylsilyl)propargyl alcohol in this fashion (eq 1).4b,9 The t-butyldimethylsilyl and triisopropylsilyl analogs are synthesized by the same method in 90% and 58% yield, respectively.7 Propargyl alcohols bearing these and other trialkylsilyl groups can be prepared by treatment of Propargyl Alcohol with n-Butyllithium and the appropriate trialkylsilyl chloride.7 Allenylsilanes bearing other C-1 substituents can be prepared in an analogous manner by using the appropriate Grignard reagents.

1-Methyl-1-(trialkylsilyl)allenes can be alkylated at C-3 with a variety of alkyl halides by treatment of the allenylsilane with n-butyllithium and the desired alkyl halide (e.g. eq 2).7 In addition to ethyl bromide, alkylating agents such as n-heptyl bromide,6 1,2-dibromobutane,7 and 5-bromo-1-pentene8 have been employed in this reaction.

3-Methyl-1-(trimethylsilyl)allene (1d) is prepared by the direct silylation of the lithium derivative of 1,2-butadiene.4b Sequential treatment of a THF solution of 1,2-butadiene with 1.0 equiv of Lithium 2,2,6,6-Tetramethylpiperidide (-78 °C, 3 h) and 1.05 equiv of Chlorotrimethylsilane (-78 °C to 25 °C, 12 h) affords (1d) in 41% yield after distillation (eq 3).

Purification: allenes (1a), (1b), and (1c) are purified by distillation at reduced pressure or by column chromatography. Allene (1d) is distilled at atmospheric pressure. The allenylsilanes obtained by the Vermeer method typically contain up to 7-8% of the trialkylsilyl-1-butyne isomer produced by SN2 reaction. This mixture can be used directly in most subsequent reactions without further purification. If desired, however, the alkynyl contaminant can be selectively removed by treatment of the mixture with Silver(I) Nitrate in (10:1) methanol-water at room temperature for one hour.9 1-Methyl-1-(trimethylsilyl)allene is obtained in 79% yield after pentane extraction and distillation.

Handling, Storage, and Precautions: 1-methyl-1-(trimethylsilyl)allene is stable indefinitely when stored under nitrogen in the refrigerator.

Propargylic Anion Equivalents.

Due to the silicon b-effect, allenylsilanes react with electrophiles at the 3-position in a fashion analogous to the behavior of allyl- and propargylsilanes.11 Allenylsilanes can thus function as propargylic anion equivalents. Particularly important is the reaction of (trimethylsilyl)allenes with aldehydes and ketones to provide a regiocontrolled route to homopropargylic alcohols of a variety of substitution types. Allenylsilanes substituted at the 1-position undergo the addition to carbonyl compounds in the presence of Titanium(IV) Chloride to afford homopropargylic alcohols directly (eq 4).2

In contrast, allenylsilanes lacking a substituent at C-1 react with carbonyl compounds to produce a mixture of the desired homopropargylic alcohols and (trimethylsilyl)vinyl chlorides. This initial product can be converted to the desired alkyne using the method of Cunico and Dexheimer:12 exposure of the crude mixture of allenylsilane adducts to 2.5 equiv of Potassium Fluoride in DMSO furnishes the homopropargylic alcohols in good yield (eq 5).2

A number of methods have been reported for the preparation of homopropargylic alcohols.13 Alcohols having the substitution pattern represented in structure (3), for example, can be prepared using 3-alkyl-substituted allenyltitanium,14 -alanate,15 and -zinc compounds.16 Unfortunately, these methods are not applicable to the synthesis of type (4) products. Zweifel has shown that allenyldialkylboranes (generated via the reaction of lithium chloropropargylide with trialkylboranes) combine with aldehydes (but not ketones) to produce type (4) homopropargylic alcohols.17 The preparation of type (2) homopropargylic alcohols is discussed in the article on (Trimethylsilyl)allene.

Santelli has demonstrated that allenylsilanes without a C-1 substituent undergo conjugate addition to a,b-unsaturated acyl cyanides to give d,ε-alkynic acyl cyanides.3

Three-Carbon Synthon for [3 + 2] Annulations.

Danheiser and co-workers have exploited allenylsilanes as the three-carbon components in a [3 + 2] annulation strategy for the synthesis of a variety of five-membered carbocycles and heterocycles. The pathway by which a typical annulation proceeds is shown in eq 6. Reaction of the 2-carbon component (the allenophile) at C-3 of the allenylsilane is followed by rapid rearrangement of the silicon-stabilized vinyl cation. Ring closure then affords the five-membered product.

Synthesis of Five-Membered Carbocycles.

1-Substituted allenylsilanes react with a,b-unsaturated carbonyl compounds in the presence of titanium tetrachloride to produce cyclopentenes.4 For example, carvone and 1-methyl-1-(trimethylsilyl)allene react smoothly to give a cis-fused bicyclic system (eq 7).4d

As illustrated above, the reaction proceeds with a strong preference for suprafacial addition of the allene to the allenophile, thus permitting the stereocontrolled synthesis of a variety of mono- and polycyclic systems. Both cyclic and acyclic enones participate in the reaction. Spiro-fused products are obtained from a-alkylidene ketone substrates (eq 8).4b

When a,b-unsaturated acyl silanes are employed, the type of product formed varies depending on the trialkylsilyl substituent of the acyl silane: five-membered carbocycles are produced from reaction with t-butyldimethylsilyl derivatives, whereas six-membered carbocycles are obtained from trimethylsilyl compounds (eq 9).4c

Allenylsilanes lacking a C-1 alkyl substituent do not function efficiently as three-carbon synthons in the [3 + 2] annulation. This phenomenon is attributable to the relative instability of the terminal vinyl cation intermediate required according to the proposed mechanism for the annulations (eq 6). Fully substituted five-membered rings result from annulations employing allenylsilanes substituted at both C-1 and C-3.4

Synthesis of 1,3-Dihydrofurans.

(t-Butyldimethylsilyl)allenes combine with aldehydes to produce dihydrofurans (eq 10).5 In a typical reaction, the aldehyde and 1.1 equiv of titanium tetrachloride are premixed at -78 °C in methylene chloride for 10 min. The allenylsilane (1.2 equiv) is then added, and the reaction mixture is stirred in the cold for 15-45 min.

In reactions of a C-3 substituted allenylsilane with achiral aldehydes, cis-substituted dihydrofurans are the predominant products (eq 11).5

(Trimethylsilyl)allenes are unsuitable for this [3 + 2] annulation, as the intermediate carbocations undergo chloride-initiated desilylation to produce alkynic byproducts. This unwelcome reaction pathway is suppressed when the bulkier (t-butyldimethylsilyl)allenes are employed.

Synthesis of Pyrrolizinones.

Cyclic N-acyl imine derivatives combine with (t-butyldimethylsilyl)allenes to afford nitrogen heterocycles (eq 12).5 The N-acyliminium ions are generated from ethoxypyrrolidinones in the presence of titanium tetrachloride.

Synthesis of Furans and Isoxazoles.

Electrophilic species of the general form Y&tbond;X+ serve as heteroallenophiles, combining with allenylsilanes in a regiocontrolled [3 + 2] annulation method. As illustrated in the mechanism shown in eq 13, addition of the heteroallenophile at C-3 of the allenylsilane produces a vinyl cation stabilized by hyperconjugative interaction with the adjacent carbon-silicon s-bond. A 1,2-trialkylsilyl shift then occurs to generate an isomeric vinyl cation, which is intercepted by nucleophilic X. Elimination of H+ furnishes the aromatic heterocycle.

Isoxazoles are synthesized when the heteroallenophile is nitrosonium ion. Thus, reaction of commercially available Nitrosonium Tetrafluoroborate with allenylsilanes in acetonitrile at -30 °C affords silyl-substituted isoxazoles in good yield (eq 14).6

In a variation of the above procedure, (trimethylsilyl)allenes are employed in a one-pot procedure that produces 5-substituted and 3,5-disubstituted isoxazoles lacking the 4-silyl substituent (eq 15). Desilylation is encouraged by addition of water and warming the reaction mixture to 65-70 °C after the initial annulation.6 Alternatively, addition of electrophilic reagents to the reaction mixture leads to isoxazoles with C-4 substituents such as Br, COMe, etc.

The heteroaromatic strategy is extended to the synthesis of furans when acylium ions are employed as the heteroallenophile (eq 16).7 Acylium ions are generated in situ via the reaction of acyl chlorides and Aluminum Chloride. Typically, the allenylsilane is added to a solution of 1.0 equiv each of AlCl3 and the acyl chloride in methylene chloride at -20 °C. The reaction is complete in 1 h at -20 °C.

Intramolecular [3 + 2] annulation affords bicyclic furans (eq 17).7

(t-Butyldimethylsilyl)- and (triisopropylsilyl)allenes are superior to their trimethylsilyl counterparts for this annulation, presumably due to the ability of the larger trialkylsilyl groups to suppress undesirable desilylation reactions. In annulations involving allenylsilanes which lack C-3 substituents, the bulkier triisopropylsilyl derivatives are superior to t-butyldimethylsilyl analogs.

Synthesis of Azulenes.

Reaction of tropylium cations with allenylsilanes produces substituted azulenes.8 Typically, commercially available Tropylium Tetrafluoroborate (2 equiv) is employed. The second equivalent dehydrogenates the dihydroazulene intermediate to produce the aromatic product. Poly(4-vinylpyridine) (poly (4-VP)) or methyltrimethoxysilane is used to scavenge the HBF4 produced in the reaction.

The azulene synthesis proceeds best with 1,3-dialkyl (t-butyldimethylsilyl)allenes. (Trimethylsilyl)allenes desilylate to generate propargyl-substituted cycloheptatrienes as significant byproducts. As observed in the other [3 + 2] annulations discussed already, allenylsilanes lacking C-1 alkyl substituents do not participate in the reaction.

Synthesis of Silylalkynes via Ene Reactions.

(Trimethylsilyl)allenes undergo ene reactions with 4-Phenyl-1,2,4-triazoline-3,5-dione and other reactive enophiles to give silylalkenes.18

1. Review: Panek, J. S. COS 1991, 2, 579.
2. (a) Danheiser, R. L.; Carini, D. J. JOC 1980, 45, 3925. (b) Danheiser, R. L.; Carini, D. J.; Kwasigroch, C. A. JOC 1986, 51, 3870.
3. (a) Jellal, A.; Santelli, M. TL 1980, 21, 4487. (b) Santelli, M.; Abed, D. E.; Jellal, A. JOC 1986, 51, 1199.
4. (a) Danheiser, R. L.; Carini, D. J.; Basak, A. JACS 1981, 103, 1604. (b) Danheiser, R. L.; Carini, D. J.; Fink, D. M.; Basak, A. T 1983, 39, 935. (c) Danheiser, R. L.; Fink, D. M. TL 1985, 26, 2513. (d) Danheiser, R. L.; Fink, D. M.; Tsai, Y.-M. OS 1988, 66, 8.
5. Danheiser, R. L.; Kwasigroch, C. A.; Tsai, Y.-M. JACS 1985, 107, 7233.
6. Danheiser, R. L.; Becker, D. A. H 1987, 25, 277.
7. Danheiser, R. L.; Stoner, E. J.; Koyama, H.; Yamashita, D. S.; Klade, C. A. JACS 1989, 111, 4407.
8. Becker, D. A.; Danheiser, R. L. JACS 1989, 111, 329.
9. Danheiser, R. L.; Tsai, Y.-M.; Fink, D. M. OS 1988, 66, 1.
10. Westmijze, H.; Vermeer, P. S 1979, 390.
11. For reviews on allylsilanes and related systems, see Ref. 1 and: Fleming, I.; Dunogues, J.; Smithers, R. OR 1989, 37, 57; Fleming, I. COS 1991, 2, 563.
12. Cunico, R. F.; Dexheimer, E. M. JACS 1972, 94, 2868.
13. For reviews of the chemistry of propargylic anion equivalents, see: (a) Yamamoto, H. COS 1991, 2, 81. (b) Epsztein, R. In Comprehensive Carbanion Chemistry; Buncel, E.; Durst, T., Eds.; Elsevier: Amsterdam, 1984; Part B, pp 107-176. (c) Moreau, J.-L. In The Chemistry of Ketenes, Allenes, and Related Compounds; Patai, S., Ed.; Wiley: New York, 1978, pp 343-381.
14. (a) Furuta, K.; Ishiguro, M.; Haruta, R.; Ikeda, N.; Yamamoto, H. BCJ 1984, 57, 2768. (b) Ishiguro, M.; Ikeda, N.; Yamamoto, H. JOC 1982, 47, 2225.
15. Hahn, G.; Zweifel, G. S 1983, 883.
16. Zweifel, G.; Hahn, G. JOC 1984, 49, 4565.
17. Zweifel, G.; Backlund, S. J.; Leung, T. JACS 1978, 100, 5561.
18. Laporterie, A.; Dubac, J.; Manuel, G.; Deleris, G.; Kowalski, J.; Dunogues, J.; Calas, R. T 1978, 34, 2669.

Katherine L. Lee & Rick L. Danheiser

Massachusetts Institute of Technology, Cambridge, MA, USA

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