Silyl Pummerer rearrangement

Vinyl sulfoxides (221), which are aldehyde a-cation equivalents, and vinylthiolium ions (230), which are a.jj-unsaturated carbonyl 3-cation equivalents, are also suitable acceptors for silyl ketene acetals and enol silyl ethers (Scheme 36). Kita reports that the bulky r-butyldimethylsilyl ketene acetals and tri-methylsilyl ketene acetals form 1 1 adducts (224) and 1 2 adducts (225) with (221), respectively 91 mechanistically, these additions proceed via an initial Pummerer rearrangement The vinylthiolium ion additions are notable for their synthetic flexibility for example, additions to the ketene dithioacetal (229) proceed with higher diastereoselectivity than the corresponding enolate additions to a,3-unsaturated esters.9 lc,91d... 

The first example of the sila-Pummerer rearrangement, which consists in the thermal conversion of sulfoxide 169 into O-silylated cyclic 0,S-acetal 170, has been described <1999TL185>. As shown in Scheme 29, a 1,3-migration of silicon atom to a sulfoxide oxygen resulted in ring expansion. 

Use of trimethylsilyl triflate to bring about Pummerer rearrangement requires the presence of a base such as a tertiary amine (vide supra equations IS and 26). In some instances, involving attempts to alkylate Pummerer intermediates with silyl enol ethers under such conditions, the base has been found to compete as a nucleophile. In the absence of the silyl enol ether, amine iuldition can be very efficient. For example, treatment of methallyl phenyl sulfoxide with diisopropylethylamine and trimethylsilyl triflate in dichloromethane (equation 29) at 0 C yields the anunonium triflate indicated in 91% yield. Other tertiary amines which undergo this reaction include triethylamine and A(//-diethyltrimethylsil-amine. In the latter case with allyl phenyl sulfoxide as the substrate and a mildly acidic work-up, the Mannich derivative shown in equation (30) can be obtained in 90% yield. ... 

Sharpless oxidation, 8 Sila-Pummerer rearrangement, 314 Silica, 18, 185, 342, 346-347 Silicon(IV) chloride, 347 Silver carbonate, 350 Silver chloride, 347-348 Silvcr(II) dipicolinate, 348 Silver fluoride-Pyridine, 348 Silver imidazolate, 349-350 Silver(I) nitrate, 350 Silver(l) oxide, 350-351 Silver(II) oxide, 80, 352-354 Silver perchlorate, 354-355 Silver(I) trifluoracetate, 365 Silybin, 351,352 2-Silyl-l,3-dithianes, 380 Silyl nitronates, 381 SMEAH, 357... 

The normal protic Pummerer rearrangement has its metallic equivalent in the stannyl (and silyl) Pummerer reaction,46 9 for example 47... 

Indolizidinones are created from 6-(3-benzenesuUinylpropyl)-2-piperidone when exposed to Znl2 and a ketene silyl ether. This method entails a Pummerer rearrangement to generate a sulfur-stahilized carbocation for inducing the ring closure (N—C bond formation). 

Pummerer rearrangement. The O-silyl ketene acetal-induced rearrangement of sulfoxides also requires a catalyst such as Znl2. 

In another example, addition of phenyl vinyl selenoxide into a mixture of an indanedione, hexamethyldisilazane, and TMSCl in dichloromethane affords the a-trimethylsiloxyselenide in 70% yield (eq 67). The reaction proceeds through a sequence of in situ formation of TMS silyl enol ether, Michael addition onto the phenyl vinyl selenoxide, and seleno Pummerer rearrangement of the resulting selenoxide. Trifluoroacetic anhydride and various other trialkylchlorosilanes give the same product for this reaction, but in much lower yields. 

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