The Role of Water in Platinum-Catalyzed Cycloisomerization of 1,6-Enynes: A Combined Experimental and Theoretical Gas Phase Study

Pt-II-catalyzed cycloisomerization of a 1,6-enyne proceeds quickly in CHCl3 at room temperature, and also in the gas phase. However, calculations predict a slow reaction, because of the formation of a very stable chelate that undergoes oxidative cyclization at a high energy cost. The electrophilic activation of the alkyne followed by nucleophilic attack of the double bond to generate a cyclopropyl carbene would lead to a faster reaction, but the decomplexation of the double bond to generate an unsaturated platinum species is also disfavored. However, decomplexation, can be envisaged if an adventitious molecule of water enters the coordination sphere of the metal. This crucial role of water in Pt-II-catalyzed cycloisomerization of enynes, initially sensed some years ago (Echavaren et al., J. Am. Chem. Soc. 2001, 123, 10511) is now supported by mass spectrometry studies.