A RU CATALYZED ADDITION OF ALKENES TO ALKYNES

The potential of the Alder ene reaction which possesses high atom economy is not realized because of limitations of scope and selectivity. Thus, the thermal bimolecular addition of unactivated alkenes to unactivated alkynes has not been reported. This addition now becomes possible through the advent of ruthenium catalysis. Several ruthenium complexes are effective including (PhOCH(3))(Ph(3)P)RuCl2, (p-cymene)(Ph(3)P)RuCl2, (p-cymene)[(C4H9)(3)P]RuCl2, and CpRu(COD)Cl, but the latter gives the highest conversions and regioselectivities. The reaction best proceeds in aqueous DMF at 100 degrees C or methanol at reflux. Both internal and terminal alkynes react. Monosubstituted alkenes are required. The reaction exhibits extraordinary chemoselectivity and control of product double bond geometry. A mechanism envisioning formation of a ruthenacyclopentene accounts for the experimental observations.