Rearrangement sulfonium ylids

There are many variations on the Pummerer rearrangement but they all involve the same steps a leaving group is lost from the sulfur atom of a sulfonium ylid to create a cationic intermediate that captures a nucleophile at the a carbon atom. Often the starting material is a sulfoxide. 

Draw mechanisms for these reactions of a sulfonium ylid and the rearrangement of the first product. Why is BF4 chosen as the counterion ... 

Some sulfonium ylid chemistry, revision on rearrangements (Chapter 37), and some simple selenium reactions. 

One solution is a [2,3] sigmatropic rearrangement of a sulfonium ylid. A stable allylic thiol reacts cleanly with an a-haloketone 64 to give a sulfide and hence the sulfonium salt after ethylation. Only a weak base is need to make the ylid 65 as this is also an enolate. Notice the regioselectivity here the alternative ylid would also be stabilised but only by an alkene. The [2,3] shift joins the allylic fragment to the enolate of the ketone and at the same time turns it inside out. This automatically produces the more difficult product 63 after desulfurisation with Raney nickel. We shall see alternative solutions to this problem later in the chapter.9... 

Although each of these ions reacts differently toward butyllithium, treatment with phenyllithium always gives predominantly the Stevens rearrangement, and treatment with amide ion in liquid ammonia gives predominantly the orfAo-rearrangement. Sulfonium and benzyl sulfides 59) on treatment with potassium amide undergo a similar c Ao-rearrange-ment which proceeds via the sulfur ylid. 

A soln. of startg. diazoester and freshly prepared Rh(ll)-acetate in benzene stirred for 10 min at room temp., and heated for 20 min at 80° - product. Y 68%. [2.3]-Sigma-tropic rearrangement via an 8-membered sulfonium ylid appears to be unprecedented. F.e.s. F. Kido et al.. Tetrahedron Letters 30, 1575-8 (1989). 

Biosynthesis of Sterols - On the basis that enzymatic coupling of two farnesyl pyrophosphate units to yield squalene involves a thio derivative of the type 1, models have been prepared,converted to the corresponding sulfonium ylids, and rearranged easily to squalene-1 ike hydrocarbons. 

Without additional reagents Rearrangement of sulfonium ylids... 

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