Metal vinylidenes in catalysis

Modern chemistry aims at the discovery of selective combinations of simple, environmentally friendly substrates under mild conditions and with atom economy in order to create new efficient synthetic methods. The epic of vinylidenemetal complexes is an illustration of this type of research. An unforeseen reaction produced the first metal vinylidene moiety, and this novelty motivated chemists to discover more efficient stoichiometric ways to generate metal vinylidene intermediates or complexes and understand their reactivity. During the past decade, the direct simple formation of metal vinylidene intermediates from terminal alkynes has made possible the transfer from stoichiometric to catalytic reactions, and now organometallic vinylidenes are at the heart of selective transformations of terminal alkynes with atom economy. In this Account, after a brief presentation of some examples of the different routes to produce metal vinylidene species and their reactivity, we will focus on their implications in catalytic reactions according to the type of fundamental mechanism involved: (i) [2+2] cycloaddition, (ii) dimerization of alkynes, (iii) nucleophilic addition to alkynes, and (iv) radical cycloaromatization.