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

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.

Dinitrogen Fixation: Rationalizing Strategies Utilizing Molecular Complexes

“…Hydrolysis or removal of the silyl groups using CsF allowed for the isolation of a wide variety of N ‐heterocycles, such as indole, quinoline, pyrrole, pyrrolizine and indolizine, [247] even using dry air instead of pure dinitrogen (Scheme 63). [248, 249] …”

“…The molecular nature of the active catalysts has not been unambiguously determined, but the isolation of the titanium‐nitride complex [TiNMg 2 Cl 2 (thf)] upon reaction of TiCl 4 or TiCl 3 with magnesium under a dinitrogen atmosphere [250] suggested a dissociative mechanism. Nevertheless, the nature of the Ti complex involved in the transfer of the nitrogen synthons to the heterocycles has not been identified so far [248] …”

Dinitrogen functionalization with bis(cyclopentadienyl) complexes of zirconium and hafnium

“…Addition of strong electrophiles, typically in the form of mineral acids, has historically been the most common method for functionalizing coordinated N 2 . 6 In group 4 transition metal dinitrogen chemistry, electrophilic functionalization has also been used to prepare ammonia, 34 hydrazine 3 and even heterocycles 35 directly from atmospheric nitrogen. For dinitrogen complexes such as 1-N 2 that contain weakly activated, end-on N 2 ligands, strong mineral acids such as HCl are required to form hydrazine.…”

High-Efficiency “Working-in-Tandem” Nitrogen Photofixation Achieved by Assembling Plasmonic Gold Nanocrystals on Ultrathin Titania Nanosheets

“…3,11 Trivalent titanium (Ti 3+ ), another possible type of transition metal cations for activation, has been reported to be able to activate N 2 through electron donation and to form a Ti 4+ -azo complex with the NN bond, 10,12 which can be cleaved by strong reducing agents such as Li and Mg to produce a Ti 4+ -hydrazo complex. 13 Further reduction of the Ti 4+ -hydrazo complex produces NH 3 with the regeneration of Ti 3+ . Ti 3+ is extensively present in TiO 2 that contains oxygen vacancies (OVs).…”

“…The rate-determining step of the N 2 fixation reaction lies in the cleavage of the NN bond . Intriguingly, transition metal (Mo, Fe, Ru, W) complexes can weaken the NN bond and hence activate N 2 toward further reactions by increasing its lowest unoccupied molecular orbital (LUMO) electron density due to the back-donation of the metal dp electrons and by depleting its highest occupied molecular orbital (HOMO) electron density upon strong coordination. , Trivalent titanium (Ti 3+ ), another possible type of transition metal cations for activation, has been reported to be able to activate N 2 through electron donation and to form a Ti 4+ -azo complex with the NN bond, , which can be cleaved by strong reducing agents such as Li and Mg to produce a Ti 4+ -hydrazo complex . Further reduction of the Ti 4+ -hydrazo complex produces NH 3 with the regeneration of Ti 3+ .…”