A study of Heck cyclization reactions to form phenanthridine ring systems

A survey of conditions for the palladium catalyzed intramolecular Heck cyclization of protected amines has shown that the Herrmann–Beller palladacycle can be exploited under ‘cationic’ conditions to provide a robust and rapid route (<2 h) to the synthesis of single double bond isomer phenanthridines in excellent yield (76–99%). In addition, the same cyclization can be performed under ‘neutral’ conditions to provide phenanthridines with a double bond isomer profile suitable for exploitation in diversity-based applications. We have also shown that the highly reactive (tBu3P)2Pd catalyst can induce cyclization at low temperatures (≤50 °C), giving similar results to the ‘neutral’ conditions, and offering an alternative pathway for sensitive substrates.