[75232-81-4]  · C7H12OSi  · (Trimethylsilyl)vinylketene  · (MW 140.28)

(reactive diene for [4 + 2] cycloadditions1)

Alternate Name: 2-(trimethylsilyl)-1,3-butadien-1-one.

Physical Data: bp 49.0-50.5 °C/1.6 mmHg.

Solubility: sol CH2Cl2, CHCl3, CCl4, toluene, benzene, hexane, most organic solvents.

Form Supplied in: yellow-green liquid; not commercially available.

Analysis of Reagent Purity: IR (CDCl3) 2085, 1610 cm-1; 1H NMR (CDCl3) d 0.25 (s, 9 H), 4.82 (dd, J = 1, 10 Hz, 1 H), 4.88 (dd, J = 1, 17 Hz, 1 H), 5.92 (dd, J = 10, 17 Hz, 1 H) ppm; 13C NMR (CDCl3) d -1.0 (q), 22.3 (s), 111.6 (t), 125.1 (d), 183.7 (s) ppm.

Preparative Methods: conveniently prepared as outlined in eq 1. Treatment of 1-(trimethylsilyl)propyne (2)2 with 1.1 equiv of Diisobutylaluminum Hydride (25 °C, 21 h) and 1.1 equiv of Methyllithium (0 °C, 0.5 h) in ether-hexane,2 followed by reaction of the resulting vinylalanate with anhydrous Carbon Dioxide,3 yields (Z)-2-(trimethylsilyl)-2-butenoic acid (3) in 68% yield. Exposure of the potassium salt of this acid (4) to 1.1 equiv of Oxalyl Chloride in pentane containing a catalytic amount of N,N-Dimethylformamide (0-25 °C, 1.5 h) then produced a mixture of the acid chloride (5) and its geometric isomer, which was dehydrohalogenated without further purification. A solution of (5) in pentane was added dropwise over 1-2 h to a solution of 0.9 equiv of Triethylamine in pentane at 25 °C, and the resulting mixture was heated at reflux for 15-24 h and then filtered with the aid of pentane. Solvent was evaporated at -50 °C (0.5 mmHg), and the residue was distilled at 25 °C (1 mmHg) and then again at 5 mmHg into a receiver cooled at -78 °C. In this manner a yellow-green liquid of (trimethylsilyl)vinylketene (1) was obtained in 39-50% overall yield (from 3).

Handling, Storage, and Precautions: purified vinylketenes can be stored under nitrogen in solution at 0 °C without appreciable decomposition for 1-2 weeks.

Reactive Enophile in [4 + 2] Cycloadditions.

Vinylketenes are not effective as dienes in Diels-Alder reactions because they undergo only [2 + 2] cycloaddition with alkenes, as predicted by frontier molecular orbital theory. However, silylketenes exhibit dramatically different properties from those found for most ketenes.4 (Trimethylsilyl)vinylketene (1) is a relatively stable isolable compound which does not enter into typical [2 + 2] cycloadditions with electron-rich alkenes. Instead, (1) participates in Diels-Alder reactions with a variety of alkenic and alkynic dienophiles. The directing effect of the carbonyl group dominates in controlling the regiochemical course of cycloadditions using this diene. For example, reaction of (1) with Methyl Propiolate produced methyl 3-(trimethylsilyl)salicylate with the expected regiochemical orientation. Protodesilylation of this adduct with Trifluoroacetic Acid in chloroform (25 °C, 24 h) afforded methyl salicylate in 78% yield (eq 2).

Diels-Alder addition of (1) to alkenic dienophiles furnishes cyclohexenone derivatives (eqs 3 and 4). Addition of (1) to naphthoquinone afforded a mixture of several cycloadducts which could be oxidized to a single anthraquinone (eq 5).

The reactivity of (trimethylsilyl)vinylketene compares favorably to previously reported vinylketene equivalents. Both vinylketene acetals5 and thioacetals6 have been reported to undergo [4 + 2] cycloadditions in good yield. However, due to the limited reactivity of hindered (Z)-dienes, only highly electrophilic dienophiles react. Specially activated vinylketene thioacetal substrates have been made with an electron-donating group at the 3-position of the diene to promote reaction with weaker dienophiles. Unfortunately, due to their strong nucleophilic character, these dienes were found to participate in a stepwise Michael-type process. Concerted [4 + 2] cycloadditions were only seen with a few dienophiles.6c Vinylketenimines are shown to be isolable by distillation, are stable at room temperature, and undergo [4 + 2] cycloadditions in moderate to good yield. Again, reaction only occurs with electron-deficient dienophiles.7

A general route to substituted (silyl)vinylketenes has been developed and several substituted ketenes have been prepared. The synthesis of 1-(1-cyclohexenyl)-2-triisopropylsilylketene is representative of this general approach (eq 6).10

Thus detrifluoroacetylative diazo transfer8 furnished the diazo ketone in 80% yield, which was converted to the silyl diazo ketone in 75% yield by the method of Maas.9 The key photolysis step was effected by irradiating a degassed 0.1 M solution of the silyl diazo ketone in hexane in a Vycor tube using a low-pressure mercury lamp (300 nm) for 2 h. Concentration and chromatographic purification furnished the desired (silyl)vinylketene in 73% yield.10

1. Danheiser, R. L.; Sard, H. JOC 1980, 45, 4810.
2. (a) Eisch, J. J.; Damasevitz, G. A. JOC 1976, 41, 2214. (b) Uchida, K.; Utimoto, K.; Nozaki, H. JOC 1976, 41, 2215.
3. Zweifel, G.; Steele, R. B. JACS 1967, 89, 2754.
4. Allen, A. D.; Tidwell, T. T. TL 1991, 32, 847.
5. (a) McElvain, S. M.; Morris, L. R. JACS 1952, 74, 2657. (b) Banville, J.; Grandmaison, J. L.; Lang, G.; Brassard, P. CJC 1974, 52, 80. (c) Banville, J.; Brassard, P. JCS(P1) 1976, 1852. (d) Banville, J.; Brassard, P. JOC 1976, 41, 3018. (e) Grandmaison, J. L.; Brassard, P. T 1977, 33, 2047. (f) Grandmaison, J. L.; Brassard, P. JOC 1978, 43, 1435. (g) Roberge, G.; Brassard, P. JCS(P1) 1978, 1041. (h) Gompper, R.; Sobotta, R. TL 1979, 11, 921.
6. (a) Carey, F. A.; Court, A. S. JOC 1972, 37, 1926, 4474. (b) Kelly, T. R.; Goerner, R. N.; Gillard, J. W.; Prazak, B. K. TL 1976, 43, 3869. (c) Danishefsky, S.; McKee, R.; Singh, R. K. JOC 1976, 41, 2934.
7. (a) Sonveaux, E.; Ghosez, L. JACS 1973, 95, 5417. (b) Differding, E.; Vandevelde, O.; Roekens, B.; Van, T. T.; Ghosez, L. TL 1987, 28, 397.
8. Danheiser, R. L.; Miller, R. F.; Brisbois, R. G.; Park, S. Z. JOC 1990, 55, 1959.
9. Maas, G.; Bruckman, R. JOC 1985, 50, 2801.
10. Danheiser, R. L.; Loebach, J. L. unpublished results.

Jennifer L. Loebach & Rick L. Danheiser

Massachusetts Institute of Technology, Cambridge, MA, USA

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