Thienothiophenes radical phenylation

The free radical phenylation experiments indicated that the order of reactivity is (3) > thiophene > (7). Calculations on all the three classical thienothiophenes, however, had predicted a different order of reactivity, viz. (8) > thiophene > (3) > (7). The most reactive positions were also expected to be the a-positions (76AHC(19)123). 

However, experimental studies of the effect upon thiophene or thienothiophenes 1 and 2 of phenyl radicals obtained by thermal decomposition of iV-nitrosoacetanilide, or from aniline and amyl nitrite, demonstrated a somewhat different experimental order of reactivity thieno[3,2-6]thiophene (2) > thiophene > thieno[2,3-6]thiophene (1). It was also found that the phenyl radical preferentially attacks position 2... 

To confirm the structures of the products formed in the reaction between thienothiophenes 1 and 2 and phenyl radicals, the corresponding 2-phenyl- and 3-phenyl-substituted thienothiophenes 1 and 2 were synthesized by independent methods (see Section II,B). In addition, 2-phenyl-substituted thienothiophenes 1 and 2 were prepared by photolysis of the corresponding 2-iodo compounds, under conditions previously used with the iodothiophenes [Eqs. (86), (87)]. 

Reaction of thienothiophene (3) with phenyl radicals generated by the thermal decomposition of N- nitrosoacetanilide led to both C-2 and C-3 phenylation, whereas C-2 phenylation occurred preferentially in the isomeric (7). This result was attributed to the stabilization of the initially formed a- complex by extensive delocalization as in (105), whereas the corresponding cr- complex (106) derived by initial attack at C-2 in thienothiophene (7) is less effectively stabilized (76AHC(19)123). 

---