Cyclizations of enynes catalyzed by PtCl2 or other transition metal chlorides: Divergent reaction pathways

1-En-6-ynes react with alcohols or water in the presence of PtCl2 as cataly st to give carbocycles with alkoxy or hydroxy functional groups at the side chain. The reaction proceeds by anti attack of the alkene onto the (eta (2 -)alkyne)platinum complex. The formation of the C-C and C-O bonds takes pla ce stereoselectively by trans addition of the electrophile derived from the alkyne and the nucleophile to the double bond of the enyne. Formation of f ive- or six-membered carbo- or heterocycles could be obtained from 1-en-6-y nes 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 alkox y- or hydroxycyclization of enynes. On the basis of density functional theo ry (DFT) calculations, a cyclopropyl platinacarbene complex was found as th e key intermediate in the process. In the presence of polar, nonnucleophili c solvents, 1-en-6-ynes are cycloisomerized with PtCl2 as catalyst. Formati on of a platinacyclopentene intermediate is supported by DFT calculations. The reaction takes place by selective hydrogen abstraction of the trans-all ylic substituent. Cycloisomerization of enynes containing disubstituted alk enes could be carried out using RuCl3 or Ru(AsPh3)(4)Cl-2 in MeOH.