Selective synthesis of four- and five-membered metallaheterocycles via CC bond-forming reactions of η²-iminoacyl and η³-1-azaallyl complexes of tungsten with alkyl isocyanides

Efficient procedures for the synthesis of four- and five-membered tungstenaheterocycles are reported, starting from the readily accessible η²-iminoacyl and η³-1-azaallyl complexes Cp*(CO)₂W[η²-C(NEt)Me] (1) and {A figure is presented} (2) (Cp* = η⁵-C₅Me₅), respectively. A clean isocyanide insertion into the metaliminoacyl-carbon bond occurs when 1 is heated with RNC (R = Me, Et), leading to the η²-1,4-diaza-3-methylbutadien-2-yl complexes Cp*(CO)₂W[C(NR)C(NEt)Me] (3: R = Me; 4: R = Et). Similarly complex 2 reacts with RNC (R = Me, Et), by insertion of the isocyanide into the metalazaallyl-carbon bond, to afford the metallacyclic aminocarbene complexes Cp*(CO)₂W[C(NHR)C(H)C(H)NEt] (5: R = Me; 6: R = Et). Atautomeric equilibrium between the aminocarbene complexes and their imino counterparts Cp*(CO)₂W[C(NR)CH₂C(H)NEt] (R = Me, Et) is suggested by the H/D exchange of the amino and one vinyl hydrogen observed in the acid-catalyzed reaction of 5 with D₂O to yield Cp*(CO)₂W[C(NDR)C(D)C(H)NEt] 5′). Determination of the solid-state structure of 6 by a single-crystal X-ray diffraction study has revealed a "four-legged piano stool" complex with a planar five-membered metallaheterocycle. The intraannular bond lengths indicate π-electron delocalization within the cycle. A close analogy between 5 and 6 and 3-aminopyrroles is discussed in terms of reactivity and spectroscopic and structural data.