Coupling of η3-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 tricarbonyl 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 structural analysis of the dithiophosphinate derivative 3b reveals that the alkenyl ligand is stabilized through intramolecular coordination of one oxygen of the ester group to the metal, forming a five-membered oxametallacycle. 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 organic moiety can be easily afforded by reaction with air or HCl gas to give the corresponding 2-allyl fumarate 4 in high yield. Regioselectivity studies employing 1-methylallyl complexes reveal that the reaction is strongly influenced by the dithio ligand bonded to molybdenum. In all cases reaction at the more substituted carbon of the allyl is favored.