Ketene Bis(trimethylsilyl) Acetal1,2

(R = Me)

[24697-35-6]  · C8H20O2Si2  · Ketene Bis(trimethylsilyl) Acetal  · (MW 204.46) (R = Et)

[24697-36-7]  · C14H32O2Si2  · Ketene Bis(triethylsilyl) Acetal  · (MW 288.64)

(stable equivalent of acetic acid dianion; enolate surrogate useful for aldol reactions6 and similar Mukaiyama reactions; inverse-electron-demand dienophile for Diels-Alder reactions8)

Alternate Name: 1,1-bis(trimethylsiloxy)ethylene.

Physical Data: R = Me, bp 50 °C/10 mmHg; R = Et, bp 77 °C/1 mmHg.

Solubility: freely sol organic solvents.

Form Supplied in: clear colorless liquid.

Analysis of Reagent Purity: NMR in dry (molecular sieves) solvent (data in Ainsworth and Kuo3). Likely impurities are trimethylsilyl (trimethylsilyl)acetate (bp 61 °C/10 mmHg) and a-trimethylsilylacetic acid.

Preparative Methods: disilylation of acetic acid dianion and silylation of the enolate of trimethylsilyl acetate are problematic because a mixture of O- and C-silyl compounds is generated in roughly equal proportions (eq 1)3 and these isomers are nearly inseparable.

The reaction conditions can be modified to produce either of these compounds in good yields (eqs 2 and 3).2 Rapid generation, and the subsequent quenching of the highly dissociated sodium enolate, at low temperature, is important for the preponderant formation of the O-silyl product.4 In a related reaction, trimethylsilyl acetate upon reaction with excess of Triethylamine and Trimethylsilyl Trifluoromethanesulfonate gives a mixture of two bis-silylated isomers (eq 4).5

The first synthesis of the ketene bis(triethylsilyl) acetal involved the treatment of triethylsilyl a-(triethylstannyl)acetate (3 in eq 6) with triethylsilyl iodide.1 The intermediate (3) in turn was prepared from ketene and triethylsilyloxytriethylstannane (eq 5).

Handling, Storage, and Precautions: flammable; extremely moisture-sensitive. Use in a fume hood.

O to C Migration.

In contrast to the alkyl trialkylsilyl ketene acetals, the thermal O to C migration is sluggish in the case of ketene bis(trialkylsilyl) acetals. Yet the migration can be effected by heating the compound to temperatures above 170 °C. The reaction can be facilitated by addition of catalytic amounts of HgI2, Chlorotrimethylsilane, or Iodotrimethylsilane (eq 7).1

Electrophilic Reactions.

Bis(trimethylsilyl) acetal (1) adds to cinnamaldehyde to give exclusive 1,2-addition upon treatment with Titanium(IV) Chloride (eq 8). The same product can be prepared by reaction of the C-silyl compound (2) with the aldehyde in the presence of Tetra-n-butylammonium Fluoride (eq 8).6

Ketene acetal (1) reacts with dry Sulfur Dioxide to give trimethylsilyl [(trimethylsilyloxy)carbonyl]methanesulfinate (eq 9).7 Reaction with SiCl4 produces 1,1-bis(trimethylsilyloxy)-2-trichlorosilylethylene (eq 10).1

Cycloaddition Reaction.

Bis(trialkylsilyl) acetal (1) is an excellent inverse-electron-demand dienophile, as illustrated by the [4 + 2] cycloaddition reaction (eq 11).8 Note that the initial adduct readily loses N2 and subsequently, the elements of TMS-OH. With 3,6-bis(methoxycarbonyl)-1,2,4,5-tetrazine, compound (1) reacts 6 times faster than ethylene and about 30 times faster than styrene or 1-trimethylsilyloxyethylene at 20 °C in dioxane.

Related Reagents.

Dilithioacetate; Ethyl Lithioacetate; Ethyl Trimethylsilylacetate; Ketene t-Butyldimethylsilyl Methyl Acetal; 1-Methoxy-2-methyl-1-(trimethylsilyloxy)propene; 1-Methoxy-1-(trimethylsilyloxy)propene; 1-Methoxy-2-trimethylsilyl-1-(trimethylsilyloxy)ethylene; Trimethylsilylacetic Acid; Tris(trimethylsilyloxy)ethylene.

1. Rudakova, T. A.; Dudukina, O. V.; Baukov, Y. I.; Lutsenko, I. F. JGU 1969, 39, 1982.
2. Bellassoued, M.; Gaudemar, M. TL 1990, 31, 209.
3. Ainsworth, C.; Kuo, Y. JOM 1972, 46, 73.
4. Under almost identical conditions to those shown in eq 2, except for the time between the formation of the enolate and subsequent quenching with TMS-Cl, previous workers reported significant contamination with the C-silyl compound.7
5. Emde, H.; Domsch, D.; Feger, H.; Frick, U.; Gotz, A.; Hergott, H. H.; Hofmann, K.; Kober, W.; Krageloh, K.; Oesterle, T.; Steppan, W.; West, W.; Simchen, G. S 1982, 1. See also: Emde, H.; Simchen, G. S 1977, 867.
6. Bellassoued, M.; Mladenova, M. PS 1991, 60, 295.
7. Sergeev, V. N.; Baukov, Y. I. JGU 1980, 50, 851.
8. Hierstetter, T.; Tischler, B.; Sauer, J. TL 1992, 33, 8019.

T. V. (Babu) RajanBabu

The Ohio State University, Columbus, OH, USA

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