Reactions of Metal–Carbon Bonds within Six‐Membered Metallaaromatic Rings

Abstract

Metallaaromatics represent a distinct and attractive class of heterocycles, which have greatly expanded the library of known aromatics. They have garnered extensive research interest on account of their unique properties, organometallic reactivity, and aromaticity. The metal-carbon bonds have been regarded as the most active reaction sites of metallaaromatics and a variety of related reactions have been demonstrated in the literature. This Minireview summarized recent findings on the reactivity of the metal-carbon bonds, mainly relevant to the representative metallaaromatics with six-membered ring scaffolds. A series of reactions occurred on the metal-carbon bonds, including the nucleophilic addition reactions, nucleophilic aromatic substitution reactions, electrophilic addition reactions, reactions with unsaturated compounds, and migratory insertion reactions, have been classified. These reactions demonstrate that the metal-carbon bonds of metallaaromatics can undergo typical reactions for both aromatic species and metal carbene/carbyne complexes. Meanwhile, the reactivity trends of the metal-carbon bonds are strongly affected by the metal centers and the effect of the corresponding substituents of metallacycles.

Metallaaromatic Chemistry: History and Development

“…Due to convenience and efficiency, NMR chemical shifts are the most widely used in metallaaromatics. − ,− …”

“…Although great progress has been achieved, the precise definition of “aromaticity” is somewhat nebulous and continues to evolve. − The continual discovery of new aromatic molecules expands the concept of aromaticity and provides new challenges for chemists. It is accepted that, when one of the CH groups in benzene is formally replaced by an isoelectronic main-group heteroatom, the resulting heterocycles, for instance, silabenzene, pyridine, phosphabenzene, arsabenzene, and pyrylium ion, still retain aromatic character through delocalization of p electrons. − On the other hand, when a CH group is formally replaced by a transition-metal fragment, the resulting metallabenzene should be different, because the metal p orbitals participate in σ bonding to the ligand and its d or f orbitals must participate in the π conjunction. − The resulting differences of structures and delocalization from traditional organic aromatic compounds result in various properties, attracting both synthetic and theoretical chemists.…”

Reactions of Cyclic Osmacarbyne with Coinage Metal Complexes

“…In the 1 H NMR, the signals of the two hydrogens from the metallacyclic skeleton in 2 were observed at 8.2 (C3 H ) and 8.8 (C5 H ) ppm, respectively, which were located in the typical region of metallaaromatics . In the 13 C NMR spectrum, the characteristic low field signal at 293.6 ppm was attributed to C1 (Table ), which was upshifted in comparison with that of complex 1 (325.8 ppm) but downshifted compared with those of previously reported osmapentalenes (210.0–250.5 ppm).…”

“…As a distinct class of aromatics, metallaaromatics can undergo characteristic reactions of aromatics . For example, metallabenzenes can react with other metal complexes as η 6 ligands .…”

Metalla-aromatics: Planar, Nonplanar, and Spiro

“…Then the first stable metalla-aromatic complex, osmabenzene, was isolated by Roper and co-workers in 1982, unfolded this fantastic area . Although numerous metalla-aromatics, such as metallabenzenes, − metallabenzynes, ,, metallapentalenes, metallapentalynes, heterometalla-aromatics, and their analogues, have been well-presented during the past decades, intriguing structures are still emerging in an endless stream.…”