Ylids, sulfonium

With acyl and alkoxycarbonyl azides, 2 -triazoles are isolated, although the l.ff-isomers are detected as intermediates. The reaction is not very general it fails with the corresponding sulfonium ylids, and fairly minor changes in substituents on the phosphorus ylid result in the formation of diazoketones rather than triazoles. ... 

So what has this got to do with cyclopropanes If sulfur ylids react with enones either the epoxide 74 or the cyclopropane 76 may be formed.19 The general rule is that sulfonium ylids from 69 give epoxides but sulfoxonium ylids give cyclopropanes 76. 

Sulfonium salts can be deprotonatcd to give sulfonium ylids... 

The positive charge carried by the sulfur atom means that the protons next to the sulfur atom in a sulfonium salt are significantly more acidic than those in a sulfide, and sulfonium salts can be depro-tonated to give sulfonium ylids. 

In Chapter 31 we discussed the Wittig reaction of phosphonium ylids with carbonyl compounds. Sulfonium ylids react with carbonyl compounds too, but in quite a different way—compare these two reactions. 

We could compare sulfonium ylids with the carbenoids we discussed in Chapter 40—both are nucleophilic carbon atoms carrying a leaving group, and both form three-membered rings by insertion into n bonds. Sulfonium ylids are therefore useful for making epoxides from aldehydes or ketones other ways you have met of making epoxides (Chapters 20 and 45) started with alkenes that might be made with phosphorus ylids. 

The simplest route to certain potential (3-blocker drugs is from an epoxide, and the chemists working on their synthesis decided that, since 4-cyclopropylbenzaldehyde was more readily available than 4-cyclopropyl styrene, they would use the aldehyde as the starting material and make the epoxide in one step using a sulfonium ylid. 

You will recall from Chapter 31 that we divided phosphorus ylids into two categories, stabilized and un stabilized , in order to explain the stereochemistry of their alkene-forming reactions. Again, there is a similarity with sulfonium ylids the same sort of division is needed—this time to explain the different regioselectivities displayed by different sulfonium ylids. Firstly, an example. 

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 ... 

The key part of the molecule for reactivity is the thiazolium salt in the middle. The proton between the N and S atoms can be removed by quite weak bases to form an ylid. You saw sulfonium ylids in Chapter 46, and there is some resemblance here, but this ylid is an ammonium ylid with extra stabilization from the sulfur atom. The anion is in an sp2 orbital, and it adds to the reactive carhonyl group ofpyruvate. 

The ylid forms in the usual way but can t reach across the ring to attack the carbonyl group so it has to do conjugate addition instead. It also has to attack from the top face as it is teth -- there. Completion of the cyclopropane-forming reaction leaves the sulfur still attached to fc angular methyl group, from which it is freed by Raney nickel. This reaction shows that si— sulfonium ylids can do conjugate addition - they just prefer not to. 

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

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