Cationic Divalent Metal Sites (M = Mn, Fe, Co) Operating as Both Nitrene-Transfer Agents and Lewis Acids toward Mediating the Synthesis of Three- and Five-Membered N-Heterocycles

The synthesis of a series of high-spincationic M(II) sites(M = Mn, Fe, Co) supported by tripodal ([N3N]) and bipodal([N2N]) ligands featuring superbasic tetramethylguanidinylresidues provides catalysts for the aziridination of olefins in acetonitrilesolutions. In chlorinated solvents, the same cationic metal sites(especially the Fe(II) congeners) are capable of mediating aziridinering opening in situ by virtue of their Lewis acidity followed bycycloaddition of dipolarophiles (nitriles, olefins, ketones) to affordfive-membered N-heterocycles. The tripodal compounds [(TMG(3)trphen)M-II-solv](PF6)(2) (M = Mn, Fe, Co;solv = MeCN, DMF) and bipodalanalogues [(TMG(2)biphen)M-II(NCMe)( x )](PF6)(2) (x = 3 for Mn, Fe; x = 2 for Co) and [(TMG(2)biphen)(MCl2)-Cl-II] have been synthesized with ligandsthat feature a triaryl- or diarylmethyl-amine framework and superbasictetramethylguanidinyl residues (TMG). The dicationic M(II) sites mediatecatalytic nitrene-transfer reactions between the imidoiodinane PhI NTs(Ts = tosyl) and a panel of styrenes in MeCN to afford aziridinesand low yields of imidazolines (upon MeCN insertion) with an orderof productivity that favors the bipodal over the tripodal reagentsand a metal preference of Fe > Co & GE; Mn. In CH2Cl2, the more acidic Fe(II) sites favor formation of 2,4-diaryl-N-tosylpyrrolidines by means of an in situ (3 + 2) cycloadditionof the initially generated 2-aryl-N-tosylaziridinewith residual styrene. In the presence of ketone, 1,3-oxazolidinescan be formed in practicable yields, involving a single-pot cycloadditionreaction of alkene, nitrene, and ketone (2 + 1 + 2). Mechanistic studiesindicate that the most productive bipodal Fe(II) site mediates stepwiseaddition of nitrene to olefins to generate aziridines with good retentionof stereochemistry and further enables aziridine ring opening to unmaska 1,3-zwitterion that can undergo cycloaddition with dipolarophiles(MeCN, alkene, ketone) to afford five-membered N-heterocycles.

Automated summary

A series of high-spin cationic M(II) sites (M = Mn, Fe, Co) supported by tripodal ( [N3N]) and bipodal ([N2N]) ligands featuring superbasic tetramethylguanidinyl residues have been synthesized. These catalysts can be used for the aziridination of olefins in acetonitrile solutions. In chlorinated solvents, the Fe(II) congeners are capable of mediating aziridine ring opening in situ followed by cycloaddition to afford five-membered N-heterocycles. The dicationic M(II) sites mediate nitrene-transfer reactions between imidoiodinane and styrenes to afford aziridines. The Fe(II) sites favor the formation of 2,4-diaryl-N-tosylpyrrolidines in CH2Cl2. The bipodal Fe(II) site mediates stepwise addition of nitrene to olefins to generate aziridines with good stereochemistry retention.