Five-membered all-C- and hetero-metallacycloallenoids of group 4 metallocenes

The formation of highly unsaturated small carbocycles is challenging and due to their high ring strain such five-membered compounds are often not isolable. However, the incorporation of metal atoms stabilizes the ring system. Therefore, unsaturated small metallacycles of transition metals are an interesting class of compounds. After the synthesis of the five-membered metallacyclocumulenes (1-metallacyclopenta-2,3,4-trienes) and metallacycloalkynes (1-metallacyclopent-3-ynes) of group 4 metallocenes was accomplished the formation of analogous metallacycloallenes (1-metallacyclopenta-2,3-dienes) was predicted. This review covers the successful formation and isolation of these compounds. The synthesis of the all-C and hetero-substituted (N, S) variants is examined as well as their structure. Furthermore, this review surveys the reactivity of the all-C complexes towards diverse substrates such as CO2, ketones, nitriles, alkynes or 1,3-enynes. Especially interesting in this regard is the reaction with nBuMgCl, which afforded stable monomeric magnesium hydrides.