Silyl halides, reactions

Use of di-( -butyl)stannyl dichloride along with an acyl or silyl halide leads to addition of allylstannanes to the aldehydes.1728,172 Reaction is also promoted by butylstannyl trichloride.173 Both SnCl4 and SnCl2 also catalyze this kind of addition. 

These precursors are generally prepared by alkane elimination (Equation (7a)) or—especially useful with bulkier substituents—the coupling of metal chlorides with lithium pnictides or silyl arsines (Equation (7b)) or salt elimination or silyl halide elimination reactions (Equation (7c)) ... 

Preparation of [l-(diethoxyphosphinyl)ethoxy]dimethyl-silane — Reaction of an aldehyde with a trialkyl phosphite in the presence of a silyl halide... 

In the instances of phosphorous and phosphonous acid systems, the generation of a new C-P bond via the classical Michaelis-Arbuzov reactions as noted above leads to products that are esters themselves. Isolation of the free acid product requires cleavage of the ester linkage in a separate reaction step, generally after isolation and purification of the initial product. The advent of silyl phosphorus reagents for the Michaelis-Arbuzov reaction allowed free acid products to be isolated simply by water workup of the reaction system. Further, since the byproduct was a silyl-halide, the general concern that the by-product halide would participate in an extraneous Michaelis-Arbuzov reaction was obviated. 

Several approaches have been used to overcome the stereochemical difficulties for intramolecular "alkyl transfer." One of these is to use a "trapping agent" in the reaction mixture with which the oxyanion site of the intermediate can react. A silyl halide works nicely for this purpose the halide anion facilitates the required dealkylation.206-210... 

This work was initiated in 1988 when Villacorta et al.71a reported the asymmetric conjugate addition of a Grignard reagent to 2-cyclohexenone. This study showed that 1,4-adducts with 4-14% ee were obtained in the presence of aminotroponeimine copper complex.713 Enhanced results (74% ee) were obtained by adding HMPA or silyl halides.71b Several other copper complexes were also used for inducing asymmetric conjugate addition reactions. Moderate results were obtained in most cases when THF was used as the solvent and HMPA as the additive. 

The first demonstration of fluorous synthesis was in the preparation of small (8-12 members) isoxazo-line and isoxazole libraries by the three-step procedure outlined in Figure 8.1461 All reactions were purified by three-phase liquid-liquid extraction. The starting substrates were simple allylic alcohols which were tagged with the fluorous silyl halide 5 to make substrates 6 for an ensuing dipolar cycloaddition. This was conducted by the Mukaiyama method with a large excess of nitro compound and... 

The reaction is stereoselective giving the Z-isomer as the major product with alkyl halides and silyl halides. Interestingly, E selectivity is observed with enolphosphates. 

Simultaneous treatment of a carbonyl compound with a Lewis acid and a tertiary amine or another weak base ( soft enolization ) can sometimes be used to generate enolates of sensitive substrates which would have decomposed under strongly basic reaction conditions [434]. Boron enolates, which readily react with aldehydes at low temperatures, can also be prepared in situ from sensitive, base-labile ketones or carboxylic acid derivatives [293, 295, 299]. Unwanted decomposition of a carbanion may also be prevented by generating it in the presence of an electrophile which will not react with the base (e.g. silyl halides or silyl cyanides [435]). 

Reaction of silylene 85 with silyl halides is believed to proceed via a similar reaction mechanism. The reactions are however less clean giving other products as well, such as silyldihalides 85X2. Similarly, during the reaction with germanium or tin(iv) chlorides, the dichloride 85C12 and the element(ll) chloride are formed <2006JOM811>. 

The kinetics and equilibrium of the reaction 5 in carbon tetrachloride was investigated by 31P NMR spectroscopy. The process follows the kinetic law for a second order reversible reaction (first order in both the ester and the silyl halide). Values of rate and equilibrium constants 25°C and of activation parameters are =0.06 1 dm mol 1s-1, k. =0.0 8 dm3mo1 1s 1,... 

It has been demonstrated that quatemarization of nitrogen may be realized with alkyl halides or tosylates and iodide is found to be the best anion. Formation of N-unsubstituted pyrrolidines when using an alkyl chloride was tentatively explained by the formation of trimethylsilyl chloride in the reaction medium. This silyl halide participates in the quatemarization of nitrogen to give A-silyl pyrrolidine and finally 1V-H pyrrolidine under the hydrolytic conditions of the work-up. The fact that changing iodide for chloride allows formation of the N-unsubstituted pyrrolidine is a synthetically interesting feature.393... 

Sommer et al. (136) found that optically active silyl halides readily undergo racemization induced by halide ions. The reaction is related to halide ion exchange discovered by Allen and Modena (137). The first tandem kinetic studies of the racemization and halide exchange reaction led to conclusion that the reaction proceeds via a silylenium ion pair [Eq. (28)],... 

We should compare the S reaction at silicon with the S 2 reaction at carbon. There are some iportant differences. Alkyl halides are soft electrophiles but silyl halides are hard electrophiles. Alkyl halides react only very slowly with fluoride ion but silyl halides react more rapidly with fluoride [than with any other nucleophile. The best nucleophiles for saturated carbon are neutral and/or based on elements down the periodic table (S, Se, I). The best nucleophiles for silicon are charged and based on highly electronegative atoms (chiefly F, Cl, and O). A familiar example is the reaction of enolates at carbon with alkyl halides but at oxygen with silyl chlorides (Chapter 21). 

The first reaction of this type was reported by Lippard et al. in 1988 the reaction of 2-cyclo-hexenone (3) with Grignard reagents in the presence of the chiral aminotroponeimine copper complex 5 as catalyst gave the 1,4-adducts 4 with 4- 14 % ee [3a]. The selectivity was increased to 74 % ee by addition of hexamethyl-phosphoric triamide (HMPA) and silyl halides [3b]. 

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