Nitrogen Fixation: Synthesis of Heterocycles Using Molecular Nitrogen as a Nitrogen Source

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Abstract Nitrogen fixation using transition metals is a fascinating process. We have already reported on the incorporation of molecular nitrogen into organic compounds using a titanium–nitrogen complex reported by Yamamoto. We developed a novel titanium-catalyzed nitrogenation procedure using TiCl4 in the presence of an excess amount of Li and TMSCl. In this reaction, a 1 atm pressure of nitrogen gas can be used and the reaction proceeds at room temperature. The procedure is very simple. A THF solution of TiCl4 or Ti(OiPr)4 (1 equiv.), Li (10 equiv.), and TMSCl (10 equiv.) was stirred under an atmosphere of nitrogen at room temperature overnight to give titanium–nitrogen complexes. Although the structures of the titanium–nitrogen complexes have not yet been determined, they would consist of N(TMS)3, X2TiN(TMS)2, and XTi=NTMS. Using this procedure, various heterocycles, such as indole, quinoline, pyrrole, pyrrolizine, and indolizine derivatives, could be synthesized from molecular nitrogen in good-to-moderate yields as a stoichiometric reaction based on a titanium complex by a one-pot reaction. Furthermore, monomorine I and pumiliotoxin C were synthesized from molecular nitrogen as a nitrogen source. This procedure was further extended for the syntheses of heterocycles using a catalytic amount of titanium complex; also, indole and pyrrole derivatives were obtained in high yields.