The development of pyrimidine-based analogues of the well-known pyridinediimine (PDI) iron complexes enables access to a functional-group-tolerant methodology for the catalytic trimerization of terminal aliphatic alkynes. Remarkably, in contrast to established alkyne trimerization protocols, the 1,3,5-substituted arenes are the main reaction products. Preliminary mechanistic investigations suggest that the enhanced π-acidity of the pyrimidine ring, combined with the hemilability of the imine groups coordinated to the iron center, facilitates this transformation. The entry point in the catalytic cycle is an isolable iron dinitrogen complex. The catalytic reaction proceeds via a 1,3substituted metallacycle, which explains the observed 1,3,5-regioselectivity. Such a metallacycle could be isolated and represents a rare 1,3-substituted ferracycle obtained through alkyne cycloaddition.
We present an efficient iron-catalyzed method for synthesizing conformationally restricted cyclobutane-fused Nheterocycles from unactivated precursors. This method is orthogonal to the established photocatalytic methods, extends the range of substrates, and provides a single-step route to previously unattainable cyclobutane-fused piperidines and azepanes. Ring stereochemistry depends on size, with five-and sixmembered rings adopting a cis configuration and seven-membered rings preferring a trans configuration. A key aspect of this method is the use of a catalyst design based on an electron-deficient, redoxactive, pyrimidinediimine scaffold. Mechanistic investigations suggest that the π-acidic core significantly enhances catalyst stability against deleterious intramolecular C−H activation pathways, while the electron-rich flanking groups accelerate the reaction rate. Mechanistic insights were obtained by extracting kinetic profiles and establishing catalyst−activity relationships. Computational studies established that the oxidative cyclization step proceeds with the highest energy barrier, which is further confirmed by experimental Hammett analysis.
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