Carbene Addition to Furans and Thiophenes

The addition of dichlorocarbene to the C2-C3 double bonds of 2-methylfuran, 2-methyl thiophene, and 2-methylbenzofuran is similar to reaction of dichlorocarbene with indole [42]. Simple mono-adducts of dichlorocarbene are not isolated but rather products arising from these initial adducts by the following reaction sequence [43]. Ionization of this initial adduct followed by ring opening yields a pentadienylic car-bonium ion/chloride ion pair. Loss of a proton from the methyl group yields a ring expanded triene with an exocyclic double bond. A similar process has been observed in the addition of dichlorocarbene to 2-methylnorbornene. Finally, addition of a second equivalent of dichlorocarbene to the exocyclic double bond yields product (Eq. 2.26). 

The phase transfer catalyzed (BTEAC) addition of dichlorocarbene to 2-methylfuran (2.5%), 2-methyl thiophene (2.3%) and 2-methylbenzofuran (38%) yields products in accord with equation 2.26 [43]. 

---