Olefin Metatheses and Related Reactions Initiated by Carbene Derivatives of Metals in Low Oxidation States

Experiments carried out 24 years ago with tantalum carbenes have led to the much cited hypothesis that metals (other than ruthenium) must be in their highest oxidation states for their carbene derivatives to initiate olefin metatheses. The hypothesis legitimizes the uniqueness of high‐oxidation‐state molybdenum and tungsten carbenes as effective initiators, and it means that the Fischer tungsten carbenes that even earlier were found to initiate olefin metatheses and related transformations must be oxidized before they can be effective. The newer initiators have been termed “well‐defined”, the older “ill‐defined”. But what does the evidence show? Highs and lows: Tungsten carbenes do not have to be in their highest oxidation state to initiate olefin metatheses, acetylene polymerizations, and enyne metatheses. Although low‐oxidation‐state metal carbenes are less active than those currently used, the former exhibit greater stereoselectivity and a broader substrate range.