COUPLING OF ETA(3)-ALLYL AND ALKYNE IN MOLYBDENUM CARBONYL-COMPLEXES

The complexes [Mo(eta(3)-allyl)(CO)(2)(S2PX2)(NCMe)] (X = OEt (1a), Ph (1b)) react with DMAD (dimethyl acetylenedicarboxylate) to give the t ricarbonyl complexes [Mo(CO)(3)(S2PX2){OC(OMe)C(allyl)=CCO2Me}] (2a,b) in a reaction involving the coupling of allyl and alkyne. Subsequent addition of PEt3 affords crystalline, air-stable dicarbonyl complexes [Mo(CO)(2)(PEt3)(S2PX2){OC(OMe)C(allyl)=CCO2Me}] (3a,b). An X-ray stru ctural analysis of the dithiophosphinate derivative 3b reveals that th e alkenyl ligand is stabilized through intramolecular coordination of one oxygen of the ester group to the metal, forming a five-membered ox ametallacycle. The alkenyl ligand shows unusual trans stereochemistry in contrast to the cis disposition usually found in previous examples of metal-mediated eta(3)-allyl-alkyne coupling. Demetalation of the or ganic moiety can be easily afforded by reaction with air or HCl gas to give the corresponding 2-allyl fumarate 4 in high yield. Regioselecti vity studies employing 1-methylallyl complexes reveal that the reactio n is strongly influenced by the dithio ligand bonded to molybdenum. In all cases reaction at the more substituted carbon of the allyl is fav ored.