Sulfonium ylides with «,/3-unsaturated carbonyl

Dimethylsulfonium methylide and dimethylsulfoxonium methylide also differ in their reachons with a,p-unsaturated carbonyl compounds. The sulfonium ylide reacts at the carbonyl group to form an epoxide, but with the sulfoxonium ylide a cyclopropane derivative is obtained by Michael addihon to the carbon-carbon double bond. The difference is again due to the fact that the kinehcally favoured reachon of the sulfonium yhde with the carbonyl group is irreversible, whereas the corresponding reaction with the sulfoxonium yhde is reversible, allowing preferenhal formahon of the thermodynamically more stable product from the Michael addihon. For example, the cyclopropane 112 is obtained from the reaction of dimethylsulfoxonium methylide with the enone 111 (1.105). Other methods for the formahon of cyclopropanes include carbene and Simmons-Smith-type... 

Due to the high reactivity of sulfonium ylide 2 for a,P-unsaturated ketone substrates, it normally undergoes methylene transfer to the carbonyl to give the corresponding epoxides. However, cyclopropanation did take place when 1,1-diphenylethylene and ethyl cinnamate were treated with 2 to furnish cyclopropanes 53 and 54, respectively. 

Both S ylides from Figure 9.1 react with a,/3-unsaturated esters to give cyclopropanes (Section 9.2). Sulfoxonium ylides also react with a,j8-unsaturated carbonyl compounds to give cyclopropanes (Section 9.2). Sulfonium ylides cannot do this because they react to form epoxides. 

Fig. 9.5. Comparison of sulfonium and sulfoxonium ylides II—chemoselectivity in the reaction with an a,/3-unsaturated carbonyl compound. The sulfoxonium ylide reacts through the enolate intermediate A, whereas the sulfonium ylide reacts through the alkoxide intermediate B.

The greater stability of sulfoxonium compared to sulfonium ylides and the associated consequence that product-development control influences, which of several conceivable zwitterionic intermediates is formed preferentially, is also noticeable in reactions of S ylides with a,/3-unsaturated carbonyl compounds. This can be seen in Figure 9.5 using the example of two reactions with cyclohexenone. Because of product-... 

Unstabilized sulfonium ylides and stabilized sulfoxonium ylides show different reactions with a,P-unsaturated carbonyl compounds the former give epoxides and the latter give cyclopropanes. The epoxide formation (i.e. 1,2-addition) is kinetically favourable while cyclopropane formation (i.e. 1,4-addition, Michael addition) is energetically favourable. 

The two types of sulfur ylides also differ in their reactions with a,p-unsaturated carbonyl compounds. The highly reactive sulfonium ylides react rapidly by 1,2-addition across the carbon-oxygen double bond to yield the epoxides. On the other hand, the less reactive sulfoxonium ylides react by slower conjugate addition (1,4-addition) to give substituted ketocyclopropanes. Thus, dimethylsulfonium methylide (21) reacts rapidly with benzylideneacetophenone (chalcone) (37)... 

This process follows the mechanistic pattern of a 1,3-elimination and formally corresponds to a (1 + 2)-cycloaddition of the ylidic (carbene-like) CH2 group to the carbonyl group. It should be mentioned that a, fi-unsaturated ketones like benzalacetophenone react with sulfonium yhdes to give oxiranes, but with sulfoxonium ylides to give cyclopropanes [13]. 

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