Molybdenum-Promoted Synthesis of Isoquinuclidines with Bridgehead CF3 Groups

Three-Component Synthesis of Multiply Functionalized 5,6-Dehydroisoquinuclidines through Dearomatization of Pyridine

A three-component synthesis of multiply functionalized 5,6-dehydroisoquinuclidines is described. After the formation of an N-alkylpyridinium salt, Grignard addition led to the formation of the corresponding 1,2-dihydropyridine bearing an alkyl, alkene, aryl, or alkynyl group. Subsequent Diels-Alder reaction with a dienophile provided functionalized dehydroisoquinuclidines in high yields (up to 93%) with endo selectivities (73:27 to >99:1). This reaction was applicable to the synthesis of an N-(4-methoxybenzyl)pyridinium salt, where the 4-methoxybenzyl group was switched to a benzyloxycarbonyl group after the formation of the dehydroisoquinuclidine.

Amination of Aminopyridines via η6-Coordination Catalysis

Pyridine, a widespread aromatic heterocycle, features a sp2-hybridized nitrogen atom that can readily coordinate to metals, leading to distinctive achievements in catalysis. In stark contrast, π-coordination of pyridine and derivatives with transition metals is notably scarce, and the involvement of such activation mode in catalysis remains to be developed. Herein, we present amination reactions of aminopyridines that leverages the reversible π coordination with a ruthenium catalyst as the arenophilic π acid, rather than relying on the conventional κ-N coordination. Specifically, a transient η6-pyridine complex functions as the electrophile in the nucleophilic aromatic substitution with amines, providing a diverse array of products via the cleavage of the pyridyl C-N bond. In addition, this method can be employed to incorporate chiral amines and 15N-labeled amines.

Recent Strategies in the Nucleophilic Dearomatization of Pyridines, Quinolines, and Isoquinolines

Dearomatization reactions have become fundamental chemical transformations in organic synthesis since they allow for the generation of three-dimensional complexity from two-dimensional precursors, bridging arene feedstocks with alicyclic structures. When those processes are applied to pyridines, quinolines, and isoquinolines, partially or fully saturated nitrogen heterocycles are formed, which are among the most significant structural components of pharmaceuticals and natural products. The inherent challenge of those transformations lies in the low reactivity of heteroaromatic substrates, which makes the dearomatization process thermodynamically unfavorable. Usually, connecting the dearomatization event to the irreversible formation of a strong C-C, C-H, or C-heteroatom bond compensates the energy required to disrupt the aromaticity. This aromaticity breakup normally results in a 1,2- or 1,4-functionalization of the heterocycle. Moreover, the combination of these dearomatization processes with subsequent transformations in tandem or stepwise protocols allows for multiple heterocycle functionalizations, giving access to complex molecular skeletons. The aim of this review, which covers the period from 2016 to 2022, is to update the state of the art of nucleophilic dearomatizations of pyridines, quinolines, and isoquinolines, showing the extraordinary ability of the dearomative methodology in organic synthesis and indicating their limitations and future trends.

Molybdenum-Catalyzed Asymmetric Hydrogenation of Fused Arenes and Heteroarenes

The synthesis of enantioenriched molybdenum precatalysts for the asymmetric hydrogenation of substituted quinolines and naphthalenes is described. Three classes of pincer ligands with chiral substituents were evaluated as supporting ligands in the molybdenum-catalyzed hydrogenation reactions, where oxazoline imino(pyridine) chelates were identified as optimal. A series of 2,6-disubstituted quinolines was hydrogenated to enantioenriched decahydroquinolines with high diastereo- and enantioselectivities. For quinoline derivatives, selective hydrogenation of both the carbocycle and heterocycle was observed depending on the ring substitution. Spectroscopic and mechanistic studies established molybdenum η⁶-arene complexes as the catalyst resting state and that partial hydrogenation arises from dissociation of the substrate from the coordination sphere of molybdenum prior to complete reduction. A stereochemical model is proposed based on the relative energies of the respective coordination of the prochiral faces of the arene determined by steric interactions between the substrate and the chiral ligand, rather than through precoordination by a heteroatom.

Synthesis and structure of a pyridine-stabilized silanone molybdenum complex and its reactions with PMe3 and acetone

The synthesis, structure and reactivity of a pyridine-stabilized silanone molybdenum complex are described.