Cyclizations of Enynes Catalyzed by PtCl₂ or Other Transition Metal Chlorides:  Divergent Reaction Pathways

Abstract

1-en-6-ynes react with alcohols or water in the presence of PtCl2 as catalyst to give carbocycles with alkoxy or hydroxy functional groups at the side chain. The reaction proceeds by anti attack of the alkene onto the (eta2-alkyne)platinum complex. The formation of the C-C and C-O bonds takes place stereoselectively by trans addition of the electrophile derived from the alkyne and the nucleophile to the double bond of the enyne. Formation of five- or six-membered carbo- or heterocycles could be obtained from 1-en-6-ynes depending on the substituents on the alkene or at the tether. Although more limited in scope, Ru(II) and Au(III) chlorides also give rise to alkoxy- or hydroxycyclization of enynes. On the basis of density functional theory (DFT) calculations, a cyclopropyl platinacarbene complex was found as the key intermediate in the process. In the presence of polar, nonnucleophilic solvents, 1-en-6-ynes are cycloisomerized with PtCl2 as catalyst. Formation of a platinacyclopentene intermediate is supported by DFT calculations. The reaction takes place by selective hydrogen abstraction of the trans-allylic substituent. Cycloisomerization of enynes containing disubstituted alkenes could be carried out using RuCl3 or Ru(AsPh3)4Cl2 in MeOH.