Thiophene 5-ylids, formation rearrangement

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. 

Formally, the insertion of a carbene(oid) into the 2,3-doubIe bond of the thiophene ring should result in the formation of the 2-thiabicyclo [3.1.0] hex-3-ene ring system. Copper(II)-catalyzed reaction of thiophene with diazomethane results in the formation of 24 (R = H) in modest yield (63TL1047). Analogously, the reaction of thiophene with ethyl diazoacetate yields 24 (R = COjEt) (22LA154). Although these reactions appear to be simple carbene insertion reactions, it is probable that this simple mechanism is not in operation. Rather, the cyclopropane derivatives 24 probably result from the initial formation of the ylid (e.g., 18), which subsequently rearranges. 

In light of the known reactivity of the thiophene S,N-ylids, a detailed examination of the reaction of thiophene with ethyl azidoformate has been undertaken. A careful product analysis revealed the presence of previously unreported products, such as 68, 70, and 75, which can be explained satisfactorily only in terms of the intermediacy of the thiophene S,N-ylid 66 (Scheme 12) (86TL1105). Thus, if the initial reaction of thiophene with the ethoxycarbonylnitrene generates 66, the products 68-70 may be rationalized in terms of a Diels-Alder dimerization to 67. Cheleotropic elimination of 69 from 67, followed by aromatization, would result in the formation of 68. Alternatively, 66 could undergo rearrangement by way of a bicyclic transition state (71) (cf. 33, Scheme 7) to a dipolar intermediate 72 (analogous to 35, Scheme 7). Proton transfer in 72 would then furnish 73 analogous to the 2-... 

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