Ketosilanes, rearrangement

Recently it has been shown (4) that cr/3-dihydroxysilanes. particularly t-butyldimethyl species, undergo an acid-catalysed silapinacol rearrangement to /3-aldehydo- and / -ketosilanes, in most respectable yields. The implications of this rearrangement on the acid-catalysed rearrangement of u/i-epoxysilanes to carbonyl compounds are discussed in Chapter 4. 

Direct treatment of TIPS enol ethers of a variety of cyclic and acyclic ketones with the strong-base combination of n-BuLi/KO-t-Bu leads to /3-ketosilanes (2) after aqueous work-up. In contrast with the earlier method, this rearrangement appears to proceed through allylic, rather than vinylic, metallation, since enol ethers lacking an allylic a-proton are unreactive. 

We now propose that the proton transfer process could be occurring via an intramolecular 4-centered transition state between the newly formed silyl ketal (Figure 10). A similar 4-centered transition state has been previously proposed by Sommer and Fujimoto to explain retention of the stereochemistry at an asymmetric silicon center during the exchange of alkoxide groups (Scheme 37).34 Brook and co-workers also proposed a 4-centered transition state for the thermal rearrangement of (3-ketosilanes (Scheme 38).35... 

Scheme 38. 4-Centered transition state showing the rearrangement of p-ketosilanes.

The above mechanism involves a-opening of the epoxysilane, followed by a 1,2-elimination of a /3-hydroxysilane (Peterson olefination. Chapter 10). However, it has recently been shown that a/3-dihydroxysilanes, particularly t-butyldimethylsilyl species, undergo an acid-catalysed sila-pinacol rearrangement to produce /3-aldehydo- and /3-ketosilanes (5) ... 

TMS enol ethers may be prepared by rearrangement of a-ketosilanes in the presence of catalytic TMSOTf (eq 6). ... 

The rearrangement of a/8-epoxysilanes to jS-ketosilanes is catalysed by magnesium iodide. 

Recently Baukov has also reported that j3-ketosilanes are rearranged, using mercuric iodide as catalyst, to the isomeric siloxyalkene (93,102). The reaction was suggested to occur in two stages, the first yielding trialkylsilyl... 

Recently we have observed that jS-ketosilanes, but not /3-ketogermanes, can be isomerized thermally to the analogous oxyalkenes (57). This kind of rearrangement had previously been proposed by Brook and Pierce to... 

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