Insight into the preferential N-binding versus O-binding of nitrosoarenes to ferrous and ferric heme centers

Recent advances in the synthetic applications of nitrosoarene chemistry

Nitroso groups are widely present in biologically active compounds in medicinal chemistry, and nitroso compounds serve as important building blocks in organic chemistry and materials science. Nitrosoarenes, in particular, showcase remarkable versatility, functioning as both electrophilic and nucleophilic reagents in a broad spectrum of organic reactions, thereby holding significant relevance in organic chemistry. This review aims to provide a comprehensive overview of the latest advancements in nitrosoarene reactions spanning a decade. Special attention is given to the synthesis of products derived from nitrosoarenes and the conditions that promote these reactions, as well as the type of catalysts. The exploration covers various facets of nitrosoarene chemistry, including cyclization, reactions involving attacks at the oxygen or nitrogen terminus, dimerization, rearrangement, coordination, and other significant reactions. By delving into these diverse reaction pathways and mechanisms, this review aspires to serve as a valuable resource for researchers seeking to deepen their understanding of nitrosoarene chemistry and its applications in both fundamental and applied scientific research.

Insights into Nitrosoalkane Binding to Myoglobin Provided by Crystallography of Wild-Type and Distal Pocket Mutant Derivatives

Nitrosoalkanes (R-N═O; R = alkyl) are biological intermediates that form from the oxidative metabolism of various amine (RNH2) drugs or from the reduction of nitroorganics (RNO2). RNO compounds bind to and inhibit various heme proteins. However, structural information on the resulting Fe-RNO moieties remains limited. We report the preparation of ferrous wild-type and H64A sw MbII-RNO derivatives (λmax 424 nm; R = Me, Et, Pr, iPr) from the reactions of MbIII-H2O with dithionite and nitroalkanes. The apparent extent of formation of the wt Mb derivatives followed the order MeNO > EtNO > PrNO > iPrNO, whereas the order was the opposite for the H64A derivatives. Ferricyanide oxidation of the MbII-RNO derivatives resulted in the formation of the ferric MbIII-H2O precursors with loss of the RNO ligands. X-ray crystal structures of the wt MbII-RNO derivatives at 1.76-2.0 Å resoln. revealed N-binding of RNO to Fe and the presence of H-bonding interactions between the nitroso O-atoms and distal pocket His64. The nitroso O-atoms pointed in the general direction of the protein exterior, and the hydrophobic R groups pointed toward the protein interior. X-ray crystal structures for the H64A mutant derivatives were determined at 1.74-1.80 Å resoln. An analysis of the distal pocket amino acid surface landscape provided an explanation for the differences in ligand orientations adopted by the EtNO and PrNO ligands in their wt and H64A structures. Our results provide a good baseline for the structural analysis of RNO binding to heme proteins possessing small distal pockets.

Generation of a Sulfinamide Species from Facile N-O Bond Cleavage of Nitrosobenzene by a Thiolate-Bridged Diiron Complex

The activation of nitrosobenzene promoted by transition-metal complexes has gained considerable interest due to its significance for understanding biological processes and catalytic C-N bond formation processes. Despite intensive studies in the past decades, there are only limited cases where electron-rich metal centers were commonly employed to achieve the N-O or C-N bond cleavage of the coordinated nitrosobenzene. In this regard, it is significant and challenging to construct a suitable functional system for examining its unique reactivity toward reductive activation of nitrosoarene. Herein, we present a {Fe₂S₂} functional platform that can activate nitrosobenzene via an unprecedented iron-directed thiolate insertion into the N-O bond to selectively generate a well-defined diiron benzenesulfinamide complex. Furthermore, computational studies support a proposal that in this concerted four-electron reduction process of nitrosobenzene the iron center serves as an important electron shuttle. Notably, compared to the intact bridging nitrosoarene ligand, the benzenesulfinamide moiety has priority to convert into aniline in the presence of separate or combined protons and reductants, which may imply the formation of the sulfinamide species accelerates reduction process of nitrosoarene. The reaction pattern presented here represents a novel activation mode of nitrosobenzene realized by a thiolate-bridged diiron complex.