preparation of thermodynamic silyl enol ethers

Tetrahedron Letters,Vol.24,No.l3,pp 1345-1348,1983

There now exist a variety of mild and very selective procedures for the kinetic
deprotonation of unsymmetrical ketones employing alkali metal dialkylamides. The "kinetic"
enolates produced in this way may be efficiently trapped by trimethylsilyl chloride to
regiospecifically provide the less substituted trimethylsilyl enol ethers.1 Despite the
recent introduction of several new methods, there are still no procedures available which
allow direct3 regiospecific preparation of the more substituted "thermodynamic" enolates or
trimethylsilyl enol ethers.

 

this paper described a way using MeMgBr/diisopropylamine to generate the thermodynamic enolate at rt, which worked out as described.(HMPA is necessary, without it, no rxn at all).

preparation of 2-Bromo-3-(trimethylsillyl)-1-propene

(2-Bromoallyl)trimethylsilane
Expensive.
two methods to prepare it.

1. coupling of 2,3-dibromopropene with a trimethylsilyl organocopper reagent generated by addition
of 1.5 equiv of cuprous cyanide to (trimethylsily1)lithium in a 3:l THF-HMPA mixture at 0 C.
2. Alternatively, the bromosilane was conveniently prepared according to eq 2 in 71% yield. This method has the advantage of avoiding the use of HMPA.

The resulting bromosilane can be distilled and stored in the dark for prolonged periods.

Reference:
J. Am. Chem. SOC. 1991, 113, 7350-7362.

A novel synthesis of silyl enol ethers from a-silylbenzylthiols and carboxylic acid derivatives via C---C bond formation

Tetrahedron Letters
Volume 42, Issue 52, 24 December 2001, Pages 9221-9223

A new procedure for the synthesis of silyl enol ethers from S--silylbenzyl thioesters without need for either bases or catalysts via C---C bond formation is described. Solutions of S--silylbenzyl thioesters were simply heated at 180°C for 24 h in a sealed tube to give silyl enol ethers in good yields with high stereoselectivity. Cyclization of the dipoles generated by thermal rearrangement of the silyl group and elimination of sulfur afforded silyl enol ethers.

The method represents a new preparative method of silyl enol ethers under completely neutral conditions with no need for any catalyst or additives.

enantioselective reduciton of ketones by thichlorosilane

Org. Lett., 8 (17), 3789 -3792, 2006

Enantioselective reduction of ketones has been one of central topics in asymmetric synthesis over the past two decades.¹ Although a variety of reducing reagents have been used for the purpose,² there have been few studies on the use of trichlorosilane (Cl₃SiH),³ which is an economical and easy to handle reagent.⁴⁵ Because Cl₃SiH does not have the ability to reduce ketones by itself, appropriate activators are necessary for the reduction of ketones by Cl₃SiH with high efficiency. Organic chiral activators are in particular of interest in this respect because they provide a route for asymmetric reduction of ketones. We have reported a first enantioselective reduction of ketones by Cl₃SiH using chiral N-formylpyrrolidines (1q,r) as organic activators (up to 43% ee),⁴ and recently isoquinolinyloxazoline (2) was reported to work well as a chiral activator (up to 94% ee).⁶⁷

C-C bond forming by Si-C cleavage

J. Org. Chem., 61 (20), 6901 -6905

Carbon-carbon bond formation is often considered the most difficult challenge in synthetic organic chemistry, and new or improved solutions to carbon-carbon bond-forming reactions are continuously being sought. One of the most useful categories of reagents for this purpose is the organometallic carbon nucleophiles, including organolithium, organomagnesium, and organocopper compounds.¹ Unfortunately, these reagents do have limitations. Since they are extremely strong bases as well as potent nucleophiles, their use with base sensitive substrates is precluded. Organolithium and -magnesium reagents are incompatible with halo, nitro, and cyano functionalities. Finally, benzylic and allylic organometallics are notoriously difficult to generate and prone to homocoupling.¹

An alternative methodology that circumvents these limitations is the generation of stabilized carbanions or carbanoids by cleavage of silicon-carbon bonds using fluoride anion.² The most commonly used fluoride source for this purpose is tetrabutylammonium fluoride (TBAF). The superiority of tetrabutylammonium triphenyldifluorosilicate (TBAT) to TBAF as a fluoride source for nucleophilic fluorination³ is described.