The Chemistry of Aromatic Osmacycles

Abstract

A romatic compounds, such as benzene and its derivatives, porphyrins, fullerenes, carbon nanotubes, and graphene, have numerous applications in biomedicine, materials science, energy science, and environmental science. Metalla-aromatics are analogues of conventional organic aromatic molecules in which one of the (hydro)carbon segments is formally replaced by an isolobal transition-metal fragment. Researchers have studied these transition-metalcontaining aromatic molecules for the past three decades, particularly the synthesis and reactivity of metallabenzenes. Another focus has been the preparation and characterization of other metalla-aromatics such as metallafurans, metallapyridines, metallabenzynes, and more. Despite significant advances, remaining challenges in this field include the limited number of convenient and versatile synthetic methods to construct stable and fully characterized metalla-aromatics, and the relative shortage of new topologies.To address these challenges, we have developed new methods for preparing metalla-aromatics, especially those possessing new topologies. Our synthetic efforts have led to a large family of closely related metalla-aromatics known as aromatic osmacycles. This Account summarizes the synthesis and reactivity of these compounds, with a focus on features that are different from those of compounds developed by other groups. These osmacycles can be synthesized from simple precursors under mild conditions. Using these efficient methods, we have synthesized aromatic osmacycles such as osmabenzene, osmabenzyne, isoosmabenzene, osmafuran, and osmanaphthalene. Furthermore, these methods have also created a series of new topologies, such as osmabenzothiazole and osmapyridyne. Our studies of the reactivity of these osma-aromatics revealed unprecedented reaction patterns, and we demonstrated the interconversion of several osmacycles.Like other metalla-aromatics, osma-aromatics have spectroscopic features of aromaticity, such as ring planarity and the characteristic bond lengths between a single and double bond, but the osma-aromatics we have prepared also exhibit good stability towards air, water, and heat. Indeed, some seemingly unstable species proved stable, and their stability made it possible to study their optical, electrochemical, and magnetic properties. The stability of these compouds results from their aromaticity and the phosphonium substituents on the aromatic plane: most of our osma-aromatics carry at least one phosphonium group. The phosphonium group offers stability via both electronic and steric mechanisms. The phosphonium acts as an electron reservoir, allowing the circulation of electron pairs along metallacycles and lowering the electron density of the aromatic rings. Meanwhile, the bulky phosphonium groups surrounding the aromatic metallacycle prevent most reactions that could decompose the skeleton.

Catalytic Cyclization of o‐Alkynyl Phenethylamines via Osmacyclopropene Intermediates: Direct Access to Dopaminergic 3‐Benzazepines

“…[13] Tr aditionally,ithas been used to stabilize models of reactive intermediates proposed for reactions catalyzed by ruthenium and other metals. [13] Tr aditionally,ithas been used to stabilize models of reactive intermediates proposed for reactions catalyzed by ruthenium and other metals.…”

Understanding the Structure, Substituent Effect, Natural Bond Analysis and Aromaticity of Osmabenzyne: A DFT Study

“…Metallabenzynes, [1][2][3][4][5][6] which are organometallic compounds derived from formal replacement of a carbon atom or a C-H group in benzyne with an isolobal transition metal fragment, are closely related to metallabenzenes. At initial view, one might expect that metallabenzynes may be too unstable to be isolated, because organic compounds with a C≡C bond in the six-membered ring, for example, benzyne and cyclohexyne, are thermally highly unstable because of ring strain.…”

Synthesis and Characterization of Dirhenadehydro[12]annulenes

“…Formal replacement of carbon atoms or CH groups in annulenes and dehydroannulenes by an isolobal transitionmetal fragment would give metallaannulenes and metalladehydroannulenes.W hile annulenes with the number of ring carbon atoms up to 30 and dehydroannulenes with the number of ring carbon atoms up to 38 are known, reported examples of metallaannulenes and metalladehydroannulenes are mainly those with six or less ring atoms,f or example, metallacyclobutadienes (3), [2,3] metallabenzenes (4), [4,5] dimetallabenzenes (5), [6] and metallabenzynes (6). [7,8] Larger metallaannulenes and metalladehydroannulenes are very rare.V ery recently,X ia nd his co-workers reported the synthesis and characterization of the first dimetalla- [10]annulenes (7).…”