SELECTIVE SYNTHESIS OF 4-MEMBERED AND 5-MEMBERED METALLAHETEROCYCLES VIA C-C BOND-FORMING REACTIONS OF ETA-2-IMINOACYL AND ETA-3-1-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 eta-2-iminoacyl and eta-3-1-azaallyl complexes Cp star (CO)2W[eta-2-C(NEt)Me] (1) and Cp star (CO)2W[eta-3-CH2...CH...NEt] (2) (Cp star = eta-5-C5Me5), respectively. A clean isocyanide insertion into the metal-iminoacyl-carbon bond occurs when 1 is heated with RNC (R = Me, Et), leading to the eta-2-1,4-diaza-3-methylbutadien-2-yl complexes Cp star (CO)2 Activated W[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 metal-azaallyl-carbon bond, to afford the metallacyclic aminocarbene complexes Cp star (CO)2 Activated W[C(NHR)C(H)C(H)NEt] (5: R = Me; 6: R = Et). A tautomeric equilibrium between the aminocarbene complexes and their imino counterparts Cp star (CO)2 Activated W[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 star (CO)2 Activated W[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 pi-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.