Silver-mediated fluorodestannylation of an electron-rich thieno[3,2-b]thiophene diverts to oxygenation: an α-fluorothiophenone instead of an aryl fluoride.

Abstract

Fluorination of the terminal acceptor unit is a reliable way to deepen the LUMO level and enhance the electron mobility of quinoidal n-type organic semiconductors, yet no fluorinated analogue of the thieno[3,2-b]thiophene-fused acceptor TTC has been reported. Two routes to such an acceptor were examined. In a late-stage approach, the tricyclic 1,3-dione core was assembled first; under Pd(0)-catalyzed stannylation conditions, however, its doubly activated methylene group rather than the C-Br bond was engaged, as confirmed by a control experiment with a monoketone analogue. In an early-stage approach, a bicyclic thienothiophene stannane was subjected to silver-mediated fluorodestannylation with SelectfluorTM. Instead of the expected aryl fluoride, an α-fluorothiophenone was isolated and its structure established by HRMS and multinuclear NMR spectroscopy. Its formation is consistent with the interception of the aryl-silver intermediate by oxygen, followed by tautomerization and α-fluorination. In the absence of silver, the stannane underwent oxidative homocoupling without fluorine incorporation. These results reveal a limitation of silver-mediated fluorodestannylation on π-excessive fused thiophenes.

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