Suzuki-Miyaura cross-coupling of 2-nitroarenediazonium tetrafluoroborates: Synthesis of unsymmetrical 2-nitrobiphenyls and highly functionalized Carbazoles

The Suzuki-Miyaura cross-coupling of 2nitrodiazonium tetrafluoroborate salts with substituted boronic acids is an effective and efficient means of preparing highly functionalized 2-nitrobiphenyls in modest to excellent yield under extremely mild reaction conditions. Cross-coupling of 2-nitrodiazonium tetrafluoroborate salts with ortho-methoxy- and benzyloxyphenylboronic acids was also demonstrated leading to the ortho-ortho-2-nitrobiphenyls. Reductive cyclization of the 2-nitrobiphenyl products allows for the overall three-step synthesis of uniquely substituted carbazoles from readily available 2-nitroanilines. The methodology was further highlighted by the short total synthesis of the carbazole alkaloids clausine V, N, C, and glycoborine.