Synthesis of Carbazoles, Dibenzofurans and -thiophenes via Benzynes

Significance

The synthesis of 1-substituted carbazoles, dibenzofurans and dibenzothiophenes by benzyne-mediated anionic ring closure is reported. Thus, in a three-step sequence, Ar-Y-Ar systems derived by classical nucleophilic aromatic substitution, Ullmann, or modern C-N-coupling tactics, were converted into iodo derivatives by a standard Sandmeyer reaction. Interestingly, upon treatment with t-BuLi, these undergo both low-temperature-directed ortho metallation driven by the F-directed metallation group and iodo-lithium exchange, to give intermediates which, following warming and electrophilic quench, afford the condensed heterocycles in moderate to good yields. A reasonable scope of electrophile introduction was established; unfortunately, use of higher-substituted starting materials was apparently not tested.