Activation of nitrogen for organic synthesis

Abstract

Nitrogen fixation using transition metals is a fascinating process. We have already reported the incorporation of molecular nitrogen into organic compounds using a titanium–nitrogen complex reported by Yamamoto. A novel titanium-catalyzed nitrogenation procedure was developed using TiCl 4 in the presence of an excess amount of Li and TMSCl. One atm pressure of nitrogen gas can be used in this reaction, and the reaction proceeds at room temperature. The procedure is very simple: a THF solution of TiCl 4 or Ti(OiPr) 4, Li, and TMSCl is 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 been determined yet, the complexes are thought to consist of N(TMS) 3, TiX 2N(TMS) 2 and XTi NTMS. To this solution was added a compound having a keto-carbonyl group and the solution was refluxed overnight to give heterocycles. Indole, quinoline, pyrrole, pyrrolizine, indolizine derivatives and lactams could be synthesized from molecular nitrogen in good to moderate yields by a one-pot reaction. Furthermore, nitrogen in an atmosphere could be fixed using this nitrogen fixation method. pumiliotoxin C and lycopodine could be synthesized from nitrogen in air as a nitrogen source. Transmetalation of nitrogen on a titanium–nitrogen complex to a palladium complex could be realized, and aniline, benzamide, allylamide and allylamine derivatives could be synthesized from titanium–nitrogen complexes, a palladium catalyst and the corresponding aryl or allyl halide in the absence or in the presence of carbon monoxide.