Ylids special

There can now be little doubt that the reaction of singlet carbenes and car-benoids with thiophene proceeds by attack of the carbene at the ring sulfur atom to generate the S,C-ylid. However, only in the special cases indicated above do these ylids enjoy any real stability. In the majority of cases, other products usually result from rearrangement of the intermediate ylids and the nature of the products formed is remarkably sensitive to both steric and electronic effects. Broadly speaking, six major reaction pathways have been observed (1) 2-substituted thiophene formation, (2) 2H-thiopyran formation, (3) formation of derivatives of 2-thiabicyclo[3.1.0]hex-3-ene, (4) 3-substituted thiophene formation, (5) oxathiocin formation, and (6) carbenic fragmentation. 

Oxathiocin formation occurs only in very special circumstances when the thiophene ring is substituted with chlorine at the 2- and 5-positions of the ring (88CC138). Reaction of ethyl diazoacetoacetate with 2,5-dichloro-thiophene under rhodium(II) acetate catalysis results in the formation of 2-methyl-3-ethoxycarbonyl-5,8-dichloro-l,4-oxathiocin (26, R = H, R = COjEt, R = CHj) (Scheme 3). In this reaction the intermediate ylid is not observed and the only isolated product is the oxathiocin. However, when diazodimedone (27) is used, the intermediate ylid has been isolated and... 

Ylids (or ylides) are zwitterions in which the charges are on adjacent atoms— we mentioned phosphorus ylids in Chapters 14 and 31. A whole chapter, Chapter 46, will be devoted to sulfur ylids and ylid-like species, because they have a special type of chemistry. 

Betaine or oxaphosphetane intermediates are not usually isolable, although an oxaphosphetane derived from a highly specialized ylid was isolated as an stable solid and its structure was determined by X-ray crystallography. 35 -p e existence of betaine derivatives as discrete intermediates has been called into question and this issue will be addressed in connection with mechanistic discussions of the Wittig reaction in Section 8.8.A.ii. 

H-D exchange via an electrophilic addition process, such as operates for benzene, does not take place with pyridine. A special mechanism allows selective exchange at the two a-positions in DCI-D2O or even in water at 200 C, the key species being an ylid formed by 2/6-deprotonation of the l//-pyridinium cation (see also section 5.12). ... 

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