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| - Cyclizations of Enynes Catalyzed by PtCl2 or Other Transition MetalChlorides: Divergent Reaction Pathways
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| - 1-En-6-ynes react with alcohols or water in the presence of PtCl2 as catalyst to give carbocycleswith alkoxy or hydroxy functional groups at the side chain. The reaction proceeds by anti attack of the alkeneonto the (η2-alkyne)platinum complex. The formation of the C−C and C−O bonds takes place stereoselectivelyby trans addition of the electrophile derived from the alkyne and the nucleophile to the double bond of theenyne. Formation of five- or six-membered carbo- or heterocycles could be obtained from 1-en-6-ynes dependingon the substituents on the alkene or at the tether. Although more limited in scope, Ru(II) and Au(III) chloridesalso 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 thepresence of polar, nonnucleophilic solvents, 1-en-6-ynes are cycloisomerized with PtCl2 as catalyst. Formationof a platinacyclopentene intermediate is supported by DFT calculations. The reaction takes place by selectivehydrogen abstraction of the trans-allylic substituent. Cycloisomerization of enynes containing disubstitutedalkenes could be carried out using RuCl3 or Ru(AsPh3)4Cl2 in MeOH.
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