Insertion Reactions of Allenes Giving Vinyl Complexes

Metallaaromatic Chemistry: History and Development

Since the prediction of the existence of metallabenzenes in 1979, metallaaromatic chemistry has developed rapidly, due to its importance in both experimental and theoretical fields. Now six major types of metallaromatic compounds, metallabenzenes, metallabenzynes, heterometallaaromatics, dianion metalloles, metallapentalenes and metallapentalynes (also termed carbolongs), and spiro metalloles, have been reported and extensively studied. Their parent organic analogues may be aromatic, non-aromatic, or even anti-aromatic. These unique systems not only enrich the large family of aromatics, but they also broaden our understanding and extend the concept of aromaticity. This review provides a comprehensive overview of metallaaromatic chemistry. We have focused on not only the six major classes of metallaaromatics, including the main-group-metal-based metallaaromatics, but also other types, such as metallacyclobutadienes and metallacyclopropenes. The structures, synthetic methods, and reactivities are described, their applications are covered, and the challenges and future prospects of the area are discussed. The criteria commonly used to judge the aromaticity of metallaaromatics are presented.

Aromatic Osmacyclopropenefuran Bicycles and Their Relevance for the Metal-Mediated Hydration of Functionalized Allenes

The aromatic osmacyclopropenefuran bicycles [OsTp{κ³ -C¹ ,C² ,O-(C¹ H₂ C² CHC(OEt)O)}(Pⁱ Pr₃ )]BF₄ (Tp=hydridotris(1-pyrazolyl)borate) and [OsH{κ³ -C¹ ,C² ,O-(C¹ H₂ C² CHC(OEt)O)}(CO)(Pⁱ Pr₃ )₂ ]BF₄ , with the metal fragment in a common vertex between the fused three- and five-membered rings, have been prepared via the π-allene intermediates [OsTp(η² -CH₂ =CCHCO₂ Et)(OCMe₂ )(Pⁱ Pr₃ )]BF₄ and [OsH(η² -CH₂ =CCHCO₂ Et)(CO)(OH₂ )(Pⁱ Pr₃ )₂ ]BF₄ , and their aromaticity analyzed by DFT calculations. The bicycle containing the [OsH(CO)(Pⁱ Pr₃ )₂ ]⁺ metal fragment is a key intermediate in the [OsH(CO)(OH₂ )₂ (Pⁱ Pr₃ )₂ ]BF₄ -catalyzed regioselective anti-Markovnikov hydration of ethyl buta-2,3-dienoate to ethyl 4-hydroxycrotonate.

Development, Mechanism, and Scope of the Palladium-Catalyzed Enantioselective Allene Diboration

In the presence of a chiral phosphoramidite ligand, the palladium-catalyzed diboration of allenes can be executed with high enantioselectivity. This reaction provides high levels of selectivity with a range of aromatic and aliphatic allene substrates. Isotopic-labeling experiments, stereodifferentiating reactions, kinetic analysis, and computational experiments suggest that the catalytic cycle proceeds by a mechanism involving rate-determining oxidative addition of the diboron to Pd followed by transfer of both boron groups to the unsaturated substrate. This transfer reaction most likely occurs by coordination and insertion of the more accessible terminal alkene of the allene substrate, by a mechanism that directly provides the eta3 pi-allyl complex in a stereospecific, concerted fashion.