Abstract
Due to the linear property around an acetylenic carbon, the introduction of such an atom to a small cycle would result in high ring strain. Currently, the smallest isolated rings are five-membered, including metallacycloalkynes and metallapentalynes. Both types contain at least one unusual small bond angle around the acetylenic carbon, thus exhibiting abnormal reactivities. This feature article gives a comprehensive overview on these two kind complexes. The synthesis and reactivities are extensively described, the source of stability is presented, and the future prospect is discussed. The article aims to provide a better development for the chemical diversity of five-membered metallacycloalkynes and metallapentalynes.
Highlights
Due to the sp hybridization of acetylenic carbon, the X≡C-Y moiety is normally linear
In normal zirconocene-alkyne complexes, the lengths of coordinated triple bonds are in the range for double bonds [28], which is contradictory to the C2≡C3 distance in trans-5a
Organic alkynes can react with electrophiles to give alkenes, in contrast, the reactions between five-membered metallacycloalkynes and electrophiles normally result in ring-opening, due to their high ring strain
Summary
Due to the sp hybridization of acetylenic carbon, the X≡C-Y moiety is normally linear. Cyclopentyne is too unstable to be trapped [10]; in contrast, two types of five-membered metallacycles containing at least one acetylenic carbon atom have been developed (metallacycloalkyne A and metallapentalyne B, Figure 2) [11,12,13,14,15,16] These five-membered metallacycles are interesting due to their abnormal structures and unusual reactivities, and because five-membered metallacycles are intermediates in many transition-metal-catalyzed organic reactions [11,12,13,14,15,16,17,18,19]. Through the summarization and comparison on the synthesis and reactivities of A and B, it is favorable for the development of their chemical diversity, which is the aim of this account
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