[2+2] Cycloadditions with an Iron Carbene: A Critical Step in Enyne Metathesis

Ruthenium
carbenes, famously used in olefin metathesis, have impacted
numerous research areas, ranging from synthesis to materials and biology.
Although in the same group as ruthenium, iron carbenes showing similar
reaction patterns have not been reported. Such targets are of high
interest because the use of a sustainable metal would lead to a decreased
cost, toxicity, and environmental impact of the corresponding compounds.
Herein, we report the synthesis of an iron carbene complex, [{PC­(sp2)­P}­Fe­(N2)­(PMe3)] ([PC­(sp2)­P] = (bis­[2-(di-isopropyl­phosphino)­phenyl]­methylene),
which is capable of performing [2+2] cycloaddition reactions in the
presence of alkynes. Specifically, η3-vinyl carbenes
are formed stoichiometrically through a [2+2] cycloaddition between
the alkyne and the metal carbene. Additional reactivity of the η3-vinyl carbenes with alkynes yields a second insertion product
containing a new iron carbene moiety.