Origins of Regio- and Chemoselectivity in Iron-PDAI-Catalyzed [2+2+2] Cycloaddition Syntheses of 4,6-Disubstituted 2-Aminopyridines

DFT Struggles to Predict the Energy Landscape for Iron Pyridine Diimine-Catalyzed [2 + 2] Cycloaddition of Alkenes: Insights into the Problem and Alternative Solutions

In recent years, noninnocent pyridine diimine (PDI) complexes featuring first-row transition metals have emerged as prominent catalysts, demonstrating efficacy in a diverse range of vital organometallic transformations. However, the inherent complexity of the fundamental reactivity paradigm in these systems arises from the presence of a noninnocent ligand and the multispin feasibility of 3d metals. While density functional theory (DFT) has been widely used to unravel mechanistic insights, its limitations as a single-reference method can potentially misrepresent spin-state energetics, compromising our understanding of these intricate systems. In this study, we employ extensive high-level ab initio state averaged-complete active space self-consistent field/N-electron valence state perturbation theory (SA-CASSCF/NEVPT2) calculations in combination with DFT to investigate an iron-PDI-catalyzed [2 + 2] cycloaddition reaction of alkenes. The transformation proceeds through two major steps: oxidative cyclization and reductive elimination. Contrary to the predictions of DFT calculations, which suggest two-state reactivity in the reaction and identify reductive elimination as the turnover-limiting step, SA-CASSCF/NEVPT2-corrected results unequivocally establish a single-state reactivity scenario with oxidative cyclization as the turnover-limiting step. SA-CASSCF/NEVPT2-based insights into electronic ground states and electron distribution elucidate the intriguing interactions between the PDI ligand and the iron center, revealing the highly multiconfigurational nature of these species and providing a precise depiction of metal-ligand cooperativity throughout the transformation. A comparative assessment of several widely recognized DFT functionals against SA-CASSCF/NEVPT2-corrected data indicates that single-point energy calculations using the modern density functional MN15 on TPSSh geometries offer the most reliable density functional methodology, in scenarios where SA-CASSCF/NEVPT2 computational cost is a consideration.

Mechanistic Investigation on the Regioselectivity of Electrochemical Co(II)-Catalyzed [2 + 2 + 2] Cycloaddition of Terminal Acetylenes

In this paper, the regioselectivity of electrochemical Co(II)-catalyzed [2 + 2 + 2] cycloaddition of terminal alkynes was investigated using density functional theory. We explored in detail the energy profiles for both 1,2,4- and 1,3,5-regioselectivity pathways and revealed the origin of the regioselectivity. Two kinds of conformational isomers derived from the different coordination modes of alkynes with cobaltacyclopentadiene have been found, which were formed through electrochemically mediated redox processes. The regioselectivity of the reaction depends on the two coordination modes. When the Co(II) center attacks α-C of the third alkyne, while β2-C in cyclopentadiene bonds to β-C of the alkyne, the reaction favors the formation of 1,2,4-products. In contrast, when the Co(II) center connects to β-C of the alkyne, it forms only the 1,3,5-products via [4 + 2] cycloaddition because of the steric repulsion between the bulky ligand on Co(II) and the phenyl group in the alkyne.

Rapid assembly of structurally diverse cyanamides and disulfanes via base-mediated aminoalkylation of aryl thiourea

A general method for the preparation of cyanamides and disulfanes from aryl thiourea and halide through a base-mediated strategy is described. Mercaptan and N-aryl cyanamide are the key intermediates in the reaction. The current method is convenient, eco-friendly, and has high yields for the synthesis of substituted cyanamide and functional disulfanes in a one-pot procedure from readily available starting materials.

Iron/Photosensitizer Hybrid System Enables the Synthesis of Polyaryl-Substituted Azafluoranthenes

Photosensitization of organometallics is a privileged strategy that enables challenging transformations in transition-metal catalysis. However, the usefulness of such photocatalyst-induced energy transfer has remained opaque in iron-catalyzed reactions despite the intriguing prospects of iron catalysis in synthetic chemistry. Herein, we demonstrate the use of iron/photosensitizer-cocatalyzed cycloaddition to synthesize polyarylpyridines and azafluoranthenes, which have been scarcely accessible using the established iron-catalyzed protocols. Mechanistic studies indicate that triplet energy transfer from the photocatalyst to a ferracyclic intermediate facilitates the thermally demanding nitrile insertion and accounts for the distinct reactivity of the hybrid system. This study thus provides the first demonstration of the role of photosensitization in overcoming the limitations of iron catalysis.

Regio- and Chemoselective Copper-Catalyzed Formal [2+2+2] Cycloaddition of Primary Amines with Arylacetylenes to 2,4,5-Trisubstituted Pyridines

Disclosed herein is an efficient strategy for the synthesis of 2,4,5-trisubstituted pyridines via CuI/NBS-catalyzed formal intermolecular [2+2+2] cycloaddition of easily available primary amines and nonactivated terminal alkynes. Moreover, this given reaction features a new mode of cycloaddition with high regio- and chemoselectivity, good atom- and step-economy, broad substrate scope, and wide functional group compatibility. Further mechanism studies indicate that this transformation starts with oxidative alkynylation of the amine to form a propargylamine intermediate, followed by radical addition to the alkyne and intramolecular cycloaddition, delivering the pharmacologically interesting multisubstituted pyridines.

Synthesis of 3-Trifluoromethyl-1,2,4-triazolines and 1,2,4-Triazoles via Tandem Addition/Cyclization of Trifluoromethyl N-Acylhydrazones with Cyanamide

A tandem addition/cyclization reaction between trifluoromethyl N-acylhydrazones and cyanamide is described, which provides a novel and efficient process for the synthesis of polysubstituted 3-trifluoromethyl-1,2,4-triazolines and their derivatives. The method has the advantages of mild reaction conditions, a broad substrate scope, good product yields, and atom economy.