Beyond Fischer and Schrock carbenes: non-heteroatom-stabilized group 6 metal carbene complexes – a general overview

Non-heteroatom-stabilized group 6 metal carbene complexes are a particular type of Fischer carbene complex that lacks stabilizing heteroatoms. The aim of this review is to provide a comprehensive overview of the usefulness of these organometallic reagents in organic synthesis. Thus, the diverse methodologies that have been developed for their synthesis are initially disclosed, followed by their reactivities. In this regard, the decisive role of these complexes in early studies of alkene metathesis and cyclopropanation and enyne metathesis is highlighted, as well as their participation in insertion reactions. However, the major part of the review highlights recent achievements in both stoichiometric and catalytic processes, which have led to the synthesis of structurally diverse carbo- and heterocycles, acyclic compounds, and even metal carbene complexes of other groups by transmetallation reactions.