Sulfur Ylides and Related Nucleophiles

1. more reactive than less stable than Sulfur Ylides and Related Nucleophiles Sulphur ylides are next to phosphorus ylides in importance as synthetic reagents. Dimethylsulfonium methylide and dimethylsulfoxonium methylide are especially useful. These sulfur ylides are prepared by deprotonation of the corresponding sulfonium salts, both of which are commercially available. Dimethylsulfonium methylide is both more reactive and less stable than dimethylsulfoxonium methylide, so it is generated and used at a lower temperature.

2. FOUR-CENTER ELIMINATION PHOSPHORUS ALKENES INTRAMOLECULAR DISPLACEMENT SULPHUR EPOXIDES • There is an important difference between the reactions of these sulfur ylides and those of phosphorus ylides. • Whereas phosphorus ylides normally react with carbonyl compounds to give alkenes, dimethylsulfonium methylide and dimethylsulfoxonium methylide yield epoxides. • Instead of a four-center elimination, the adducts formed from the sulphur ylides undergo intramolecular displacement of the sulfur substituent by oxygen. YLIDES

3. EPOXIDES CYCLOPROPANES A sharp distinction between the two ylides emerges in their reactions with a,b-unsaturated carbonyl compounds. Dimethylsulfonium methylide yields epoxides, whereas dimethylsulfoxonium methylide reacts by conjugate addition to give cyclopropanes It appears that the reason for the difference in their behavior lies in the relative rates of the two reactions available to the betaine intermediate: (a) reversal to starting materials or (b) intramolecular nucleophilic displacement.

4. Presumably, both reagents react most rapidly at the carbonyl group. In the case of dimethylsulfonium methylide, the intramolecular displacement step is faster than the reverse of the addition, and epoxide formation takes place.

5. With the more stable dimethylsulfoxonium methylide, the reversal is relatively more rapid, and product formation takes place only after conjugate addition.

6. Another difference between dimethylsulfonium methylide and dimethylsulfoxonium methylide concerns the stereoselectivity in formation of epoxides from cyclohexanones. Dimethylsulfonium methylide usually adds from the axial direction whereas dimethylsulfoxonium methylide favors the equatorial direction. This result may also be due to reversibility of addition in the case of the sulfoxonium methylide. The product from the sulfonium ylide would be the result of the kinetic preference for axial addition by small nucleophiles. In the case of reversible addition of the sulfoxonium ylide, product structure would be determined by the rate of displacement, and this may be faster for the more stable epoxide.

7. Dimethylsulfonium methylide reacts with reactive alkylating reagents such as allylic and benzylic bromides to give terminal alkenes. A similar reaction occurs with primary alkyl bromides in the presence of LiI. The reaction probably involves alkylation of the ylide, followed by elimination.