Ligand Effect in Platinum-Catalyzed Cycloisomerization of 1,6-Enynes: Water or Carbon Monoxide, a Similar Role despite Distinct Electronic Properties?

The PtCl2-catalyzed cycloisomerization of 1,6-enynes is considerably accelerated in the presence of carbon monoxide. The effect of CO has been explained by the reinforcement of the electrophilicity of the alkyne moiety once ligated to the pi-acidic [PtCl2(CO)] fragment. However, platinum is also described as wealdy capable of back-donation toward CO. A theoretical study has been carried out to shed light on this apparent contradiction. An alternative explanation based on the approach of CO onto the [PtCl2(eta(4)-(1,6-enyne))] complex is proposed. When CO is ligated to PtCl2, the triple-bond coordination is favored, yet there is no positive influence on the formation of the cyclopropylcarbene key intermediate. The role of CO is actually comparable to that of a water molecule.