[86934-32-9] · C12H24O4Si · Bis(1-methoxy-2-methyl-1-propenyloxy)dimethylsilane · (MW 260.40)
(very useful bifunctional protecting agent for various types of H-acidic materials under mild conditions)
Physical Data: 89-90 °C/6.5 mmHg.
Form Supplied in: colorless liquid; not commercially available.
Preparative Method: prepared by treating 2 equiv of methyl lithioisobutyrate with 1 equiv of dichlorodimethylsilane at low temperature in dry THF, followed by distillation of the colorless liquid.
Handling, Storage, and Precautions: moisture sensitive; should be stored under nitrogen in a refrigerator.
Bifunctional compounds containing alcohol, thiol, acid, and amine units may be protected as cyclic dimethylsilylene derivatives, which are analogous to isopropylidene derivatives, by treatment with the title reagent (1).2 Cleavage of the silylene protective group is easily achieved by hydrolysis or solvolysis. It should be noted that the dimethylsilylene group is highly sensitive to hydrolysis and is unstable to column chromatography.
Silylene derivatives have become very useful in synthetic organic chemistry. For example, the cyclic dimethylsilylene derivative (2) of o-hydroxyacetophenone is useful for [4 + 2] cycloadditions (eq 1).3 A glycosyl donor and acceptor have been linked as the dimethylsilylene derivative (3) and used for stereoselective glycosylation (eq 2).4 Also, an efficient and stereocontrolled aldol reaction of silylene enolate (4) with aldehydes has been reported (eq 3).5
Dichlorosilanes,6 bis(trifluoromethanesulfonyl)silanes,7 and dimethoxysilanes8 have been used for the synthesis of dialkylsilylene derivatives. These methods, however, have some disadvantages: (i) a strong base or acid catalyst is required; (ii) forcing reaction conditions (high temperature for a long period) are sometimes required; (iii) large amounts of side-products such as inorganic salts, or amine or acid salts, are produced that must be separated from the moisture-sensitive silylene derivatives; and (iv) the yields of the products are not always high. Reagent (1), which is much more reactive than other difunctional silyl reagents, allows the ready silylenation of H-acidic materials in the absence of base, and allows isolation of pure products in almost quantitative yields without aqueous workup. The only reaction side-product is methyl isobutyrate, which is volatile.
The reaction of (1) with H-acidic materials is generally carried out by employing a slight excess of the reagent in an inert solvent such as CH2Cl2 or MeCN and usually gives a high yield of the corresponding silylene derivatives. Furthermore, addition of catalytic amounts of dichlorodimethylsilane induces a slightly exothermal reaction, leading to the rapid formation of the silylenes. A wide range of 1,2-, 1,3-, and 1,4-difunctional compounds give the corresponding silylenes in high yields (eqs 4-8). Monofunctional alcohols and carboxylic acids can also be linked via the dimethylsilylene group forming dialkoxy- and diacyloxydimethylsilane, respectively (eqs 9 and 10).
Osaka University, Japan