Iron-Catalyzed/Mediated C–N Bond Formation: Competition between Substrate Amination and Ligand Amination

Abstract

Iron catalyzed carbon−nitrogen bond formation reactions of a wide variety of nucleophiles and aryl halides using well-defined iron-complexes featuring redox noninnocent 2-(arylazo)-1,10-phenanthroline (L 1 ) ligands are reported. Besides substrate centered C−N coupling, C−N bond formation reactions were also observed at the ortho-and para-positions of the phenyl ring of the coordinated azoaromatic scaffolds affording new tetradentate ligands, 2-Naryl-(2-arylazo)-1,10-phenanthroline (L 2 ), and tridentate ligands, 4-N-aryl-(2-arylazo)-1,10-phenanthroline (L 3 ), respectively. Control experiments and mechanistic studies reveal that the complex [FeL 1 Cl 2 ] (1) undergoes in situ reduction during the catalytic reaction to produce the monoanionic complex [1] − , which then acts as the active catalyst. The metal (iron) and the coordinated ligand were found to work in a cooperative manner during the transfer processes involved in the fundamental steps of the catalytic cycle. Detailed experimental and theoretical (DFT) studies were performed to get insight into the competitive substrate versus ligand centered amination reactions.

Achieving Nickel Catalyzed C–S Cross-Coupling under Mild Conditions Using Metal–Ligand Cooperativity

“…The use of redox noninnocent ligands in combination with the base metals has emerged as an attractive alternative. , The ability of redox noninnocent ligands to store and release electrons when required allows multielectron transformations (mostly preferred for noble metals like palladium) with inexpensive 3d-base metals. In recent times, using the redox noninnocent properties of imine, diamine, and aminophenol based scaffolds, various new stoichiometric and catalytic reactions were developed where these organic scaffolds were found to participate actively during electron transfer processes involved during catalytic turnover . However, no systematic studies were carried out to explore catalytic C–S coupling reactions using transition metal complexes of redox noninnocent ligands.…”

Iron-Catalyzed Alkyne-Based Multicomponent Synthesis of Pyrimidines under Air

“…Two Fe­(II) complexes, [Fe­(L 1 )­Cl 2 ] ( 1 ) and [Fe­(L 2 )­Cl 2 ] ( 2 ), bearing tridentate arylazo pincers, 2-((4-chlorophenyl)­diazenyl)-1,10-phenanthroline ( L 1 ) and 2-(phenyldiazenyl)-1,10-phenanthroline ( L 2 ), respectively, were used in this work (Scheme ). , To begin with, we studied the reaction between benzyl alcohol ( 3a ), 1-phenylacetylene ( 4a ), and guanidine ( 5a ) as the standard reaction to obtain the optimal reaction conditions using 1 as the catalyst (Table ). The reaction proceeds satisfactorily in toluene and xylene.…”

“…Finally, a set of control reactions were carried out to investigate the intermediates and the plausible reaction sequence. Previously, we noticed that catalyst 1 , in the presence of t BuOK, undergoes single-electron reduction to form monoanionic species [1] − containing an azo anion radical ligand . Further, [1] − was found to act as the active catalyst during 1 -catalyzed alcohol dehydrogenation reactions. , During dehydrogenation of alcohols, the one-electron-reduced azo chromophore in [1] − abstracts the β-hydrogen atom from the iron–alkoxy intermediate to form a ketyl-type radical intermediate, which subsequently produces the corresponding carbonyls upon intramolecular single-electron transfer.…”

“…A plausible mechanism for the present three-component synthesis of trisubstituted pyrimidines is outlined in Scheme . The reaction starts with the 1 -catalyzed dehydrogenation of primary alcohols to the corresponding aldehydes. , The reaction of the in situ formed aldehyde with the alkyne then produces ( E )-chalcone ( 9 ) via the intermediacy of 8 . , Finally, base-promoted condensation of 9 with amidines followed by intramolecular cyclization via selective C–N bond formation yields the desired 2,4,6-trisubstituted pyrimidines.…”

“…Recently, using a bench-stable Fe­(II) catalyst, [FeL 1 Cl 2 ] ( 1 ) featuring a redox-noninnocent tridentate azo-aromatic ligand, 2-((4-chlorophenyl)­diazenyl)-1,10-phenanthroline ( L 1 ), we reported the synthesis of a variety of polysubstituted pyrimidine derivatives via dehydrogenative functionalization of alcohols with amidines under comparatively mild aerobic conditions . Mechanistic investigation revealed that iron and the arylazo scaffold participate cooperatively during this reaction, allowing us to execute the multicomponent coupling at 100 °C under air. , To further explore the potential of our metal–ligand cooperative approach herein, we report the synthesis of various multisubstituted pyrimidines via multicomponent dehydrogenative functionalization of primary alcohols with alkynes and amidines.…”