Benzannulation of heterocyclic frameworks by 1,1-carboboration pathways.

A small series of S- and N-heterocyclic 1,2-bis(trimethylsilylethynyl)arenes (2, 9, and 12) react with the strongly electrophilic borane B(C6F5)3 in consecutive 1,1-carboboration sequences to benzannulated heterocyclic systems. With this approach, highly substituted carbazole (6), benzothiophene (10), and quinoline (14) derivatives can be synthesized. While benzannulation occurs in all three cases, the reactions are quite different in detail. Finally, one-pot deborylation reactions lead to hydroxy-hetarenes, as demonstrated for the hydroxy-carbazole 7 and the hydroxy-benzothiophene 11.