ETA(2)-ALKYNYL AND VINYLIDENE TRANSITION-METAL COMPLEXES - 5 - REACTION OF THE METAL-ACETYLIDE [(ETA(5)-C5H5)(CO)(NO)WC-CR](-) WITH ETHYL IODOACETATE AS ELECTROPHILE - PREPARATION AND CRYSTAL-STRUCTURE ANALYSIS OF A METALLACYCLIC COMPLEX

Treatment of eta(1)-acetylide complexes [(eta(5)-C5H5)(CO)(NO)WC=CR](- ) (R = C(CH3)(3) (2a), C6H5 (2b), p-CH3C6H4 (2c)) With ethyl iodoaceta te in THF at -78 degrees C afforded, after protonation with diluted ac ids, the corresponding oxametallacyclopentadienyl complexes eta(5)-C5H 5)W(I)(NO)-[eta(2)-O=C(CH2COOC2H5)CH=CR] (3a-c). The formation of thes e metallafuran derivatives is rationalized by the electrophilic attack of ethyl iodoacetate on the metal center of 2 to create first [(eta(5 )-C5H5)(I)(CO)(NO)WC=CR]. Subsequent nucleophilic addition of the eate r enolate anion of [CH2COOC2H5]Li and a reductive-elimination step fol lowed by protonation lead to the product 3, One of the reaction interm ediates was trapped by the addition of CF3SO3CH3.