Coupling of η3-Allyl and Alkyne in Molybdenum Carbonyl Complexes

The complexes [Mo(η 3 -allyl)(CO) 2 (S 2 PX 2 )(NCMe)] (X = OEt ( 1a ), Ph ( 1b )) react with DMAD (dimethyl acetylenedicarboxylate) to give the tricarbonyl complexes [Mo(CO) 3 (S 2 PX 2 ){OC(OMe)C(allyl) CCO 2 Me}] ( 2a,b ) in a reaction involving the coupling of allyl and alkyne. Subsequent addition of PEt 3 affords crystalline, air-stable dicarbonyl complexes [Mo(CO) 2 (PEt 3 )(S 2 PX 2 ){OC(OMe)C(allyl) CCO 2 Me}] ( 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 η 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.