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.