Aromaticity

σ-Aromaticity in an Unsaturated Ring: Osmapentalene Derivatives Containing a Metallacyclopropene Unit

In general, aromaticity can be clarified as π- and σ-aromaticity according to the type of electrons with major contributions. The traditional π-aromaticity generally describes the π-conjugation in fully unsaturated rings whereas σ-aromaticity may stabilize fully saturated rings with delocalization caused by σ-electron conjugation. Reported herein is an example of σ-aromaticity in an unsaturated three-membered ring (3 MR), which is supported by experimental observations and theoretical calculations.

Unexpected 1,2-Migration in Metallasilabenzenes: Theoretical Evidence for Reluctance of Silicon to Participate in π Bonding

Density functional theory (DFT) calculations were carried out to investigate the 1,2-migration in metallasilabenzenes. The results suggested that the chloride migration of metallabenzenes is unfavorable due to the loss of aromaticity in the nonaromatic analogues. In sharp contrast, such a migration in metallasilabenzenes is favorable due to the reluctance of silicon to participate in π bonding. The migration of hydride and methyl group from the metal center to the silicon atom in metallasilabenzenes is computed to be also feasible.

Theoretical study on the interconversion of silabenzenes and their monocyclic non-aromatic isomers via the [1,3]-substituent shift: Interplay of aromaticity and Bent's rule

Density functional theory (DFT) calculations were performed to examine the substituent effects on the interconversion of silabenzenes and their monocyclic non-aromatic isomers. A previous study suggested that aromaticity is the driving force for this process. Interestingly, our systematic calculations reveal that the contribution from aromaticity can be evaluated quantitatively (ca. 30 kcal mol-1). Thus it is the interplay of aromaticity and Bent's rule that determine their relative stabilities.

A Metal-Bridged Tricyclic Aromatic System: Synthesis of Osmium Polycyclic Aromatic Complexes

Aromaticity is one of the most important concepts in organic chemistry. A variety of metalla-aromatic compounds have been recently prepared and in most of those examples, the metal participates only in a monocyclic ring. In contrast, metal-bridged bicyclic aromatic molecules, in which a metal is shared between two aromatic rings, have been less developed. Herein, we report the first metal-bridged tricyclic aromatic system, in which the metal center is shared by three aromatic five-membered rings. These metalla-aromatics are formed by reaction between osmapentalyne and arene nucleophiles.

Theoretical Study on the Stability and Aromaticity of Metallasilapentalynes

Antiaromatic compounds and small cyclic alkynes or carbynes are both challenging for synthetic chemists because of the destabilization caused by their antiaromaticity and highly distorted triple bonds, respectively. These dual destabilizations could be the reason why pentalyne (I), a highly antiaromatic and extremely strained cyclic alkyne, has never been synthesized.

Planar Möbius aromatic pentalenes incorporating 16 and 18 valence electron osmiums

Aromaticity, a highly stabilizing feature of molecules with delocalized electrons in closed circuits, is generally restricted to ‘Hückel’ systems with 4n+2 mobile electrons. Although the Möbius concept extends the principle of aromaticity to 4n mobile electron species, the rare known examples have complex, twisted topologies whose extension is unlikely. Here we report the realization of osmapentalenes, the first planar Möbius aromatic complexes with 16 and 18 valence electron transition metals.

Computations Offer an Unconventional Route to Metallaphosphabenzene from a Half-Phosphametallocene

Metallaaromatics have attracted continuing interest of both theoretical and experimental chemists since the first metallabenzene was predicted by Hoffmann and isolated by Roper. In sharp contrast to metallabenzenes, metallaphosphabenzene (MPB) is much less developed and has not been synthesized so far. Thus, developing synthetic approaches is urgent. Here we present thorough density functional theory (DFT) calculations on the thermodynamics and kinetics of the rearrangement between MPBs and the corresponding η5-phosphacyclopentadiene (η5-PCp) complexes.

Aromaticity Effects on the Profiles of the Lowest Triplet-State Potential-Energy Surfaces for Rotation about the CC Bonds of Olefins with Five-Membered Ring Substituents: An Example of the Impact of Baird's Rule

A density functional theory study on olefins with five-membered monocyclic 4n and 4n+2 π-electron substituents (C4H3X; X=CH+, SiH+, BH, AlH, CH2, SiH2, O, S, NH, and CH−) was performed to assess the connection between the degree of substituent (anti)aromaticity and the profile of the lowest triplet-state (T1) potential-energy surface (PES) for twisting about olefinic CC bonds. It exploited both Hückel’s rule on aromaticity in the closed-shell singlet ground state (S0) and Baird’s rule on aromaticity in the lowest ππ* excited triplet state.

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