Trapping of Unsaturated Small Molecules through Cyclization with Iron−Iminophosphorane Complexes

The iron−iminophosphorane−carbonyl complex Cp*(CO)2Fe{P(NPh)(OMe)2} (1; Cp* stands for η5-C5Me5) reacted with the activated alkyne dimethyl acetylenedicarboxylate (DMAD) to give in which the alkyne was trapped between the imino
nitrogen and one carbonyl carbon to form a six-membered metallacycle. In contrast, 1 reacted with CO2
to afford aza-Wittig-type metathesis products, PhNCNPh and oxophosphorane (phosphonate) complex
Cp*(CO)2Fe{P(O)(OMe)2}, probably via a four-membered aza-phosphacycle as an intermediate. Free
acetonitrile did not react with 1, while the coordinated acetonitrile in [Cp*(CO)(NCMe)Fe{P(NHPh)(OMe)2}]PF6 was incorporated into a five-membered metallacycle through a base-catalyzed rearrangement
to give in which the nitrile carbon was bonded to the imino
nitrogen. It was proposed that this intramolecular cyclization reaction is initiated by the formation of the
neutral iminophosphorane complex Cp*(CO)(NCMe)Fe{P(NPh)(OMe)2} as an intermediate, followed
by the prompt nucleophilic attack of the resulting imino nitrogen to the carbon atom of the coordinated
acetonitrile.