Trimethylsilyl enolates exchange reaction

The diastereoselectivity of the zinc iodide catalyzed reaction of the azetidinone I with the trimethylsilyl enolate derivatives of the chiral 3-(l-oxopropyI)oxazolidinones 6 was considerably lower (about 60 40), although independent generation of the zinc enolate, via exchange of the lithium enolate with zinc bromide, afforded the /9-Iactam carboximide derivatives in a ratio (RIS) 80 20177. 

An exchange reaction of trimethylsilyl enolates with dialkylboryl triflates will provide the corresponding boron enolates (Scheme 11) after removal of trimethylsilyl triflate. ... 

A metal exchange reaction between an alkenyloxysilane (silyl enolate) and a dialkylboryl triflate has already been described (Scheme 11). When a mixture of a silyl enolate and dibutylboryl triflate is allowed to react with an aldehyde, aldol product diastereoselectivity is negligible, presumably because trimethylsilyl triflate promotes an aldol reaction of the silyl enolate. When trimethylsilyl triflate is com-... 

Scheme 10.68. A representation of the reaction of enol exchange with 1-ethoxycyclohexene and 3-(trimethylsilyl)-l-propanol in the presence of p-toluenesulfonic acid catalyst followed by a Claisen-type rearrangement. Subsequent ring closure under the influence of titanium tetrachloride is also represented (see, e.g., Trost, B. M. Dong, G. Vance, J. A. J. Am. Chem. Soc., 2007,129,4540). HOTs = toluenesulfonic acid.

Reaction of triisopropylsilyl enol ether with a combination of iodosylbenzene 18 and trimethylsilyl azide at -15 °C gives directly the /J-azido triisopropylsilyl enol ether 38 in a high yield. A mechanism involving the reductive -elimination of a-iodanyl onium ion 37, probably produced by ligand exchange of in situ generated PhI(N3)OTMS with silyl enol ether, was proposed. Addition of azide to the resulting a,/l-unsaturated onium ion explains the formation of 38 [58,59]. 

Table XIX shows the results of reactions of silyl ketene acetals derived from propionates with crotonate, cinnamate, sorbate, and fumarate in the presence of aluminum ion-exchanged montmorillonite (Al-Mont) (62). The reactions proceeded at low temperatures. The Michael products could be isolated in the unstable form of a trimethylsilyl ketene acetal in good yield owing to an easy work-up procedure (removal of the solid catalyst). It is noteworthy that the montmorillonite-induced Michael addition to a polyenoate occurred regioselectively in a 1,4-fashion in the case of methyl sorbate (Table XIX, Entry 4), the preference for 1,4-addition (98%) over 1,6-addition (2%) is notable because the addition of a lithium enolate (a conventional...

As another example of novel catalysis employing montmorillonite, the clay was found to show excellent catalytic activity for the addition reaction of trimethylsilyl ketene acetal to a, -acetylenic esters (ynoates), which contrasted strikingly with the reactions induced by a homogeneous acid catalyst, trimethylsilyl triflate (TMSOTf), as well as the addition reactions of lithium enolates with ynoates [Eq. (17)] (89). Table XXIII summarizes the results of the reactions of the silicon and lithium enolates of methyl propionate (21) with ynoates (22a-c). Except for the reaction of 22c, ferric ion-exchanged montmorillonite (Fe-Mont), which is more acidic than Al-Mont, catalyzed exclusive 1,2-additions of trimethylsilyl ketene acetal to 22a and 22b to give 23 in... 

To keep the concentration of trimethylsilyl triflate as low as possible during the reaction a dichloromethane solution of silyl enolate and aldehyde vras added slovrly to the solution of the catalyst 80 (20 mol%), and the aldol product 92 vas obtained in good yields vith high enantioselectivity (Eq. (44)) [70]. Selectivity is improved by using propionitrile as solvent instead of dichloromethane. The rate of metal exchange betv een Sn-Si is faster in propionitrile than in dichloromethane [71]. 

Propagation. It must be stressed here that the propagation reaction which is represented hereafter occurs through monomer addition by the carbon form of the enolate whereas the exchange of trimethylsilyl groups between dormant and reactive chains resorts to the oxygen form (due to oxophilicity of silicon atom). 

---