Platinum-Catalyzed Cycloisomerization Reactions of Enynes

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Abstract
PtCl2 constitutes an efficient and practical catalyst for a set of different atom economical rearrangement reactions of enynes. This includes (i) a formal enyne metathesis reaction delivering 1,3-dienes, (ii) the formation of polycyclic vinylcyclopropane derivatives, and (iii) an unprecedented O→C allyl shift reaction if unsaturated ethers are employed. Although these transformations produce significantly different structural motifs, they share a common mechanism comprising a cationic manifold triggered by the π-complexation of Pt(II) onto the alkyne unit of the substrates. Strong experimental support for the proposed mechanism comes from deuterium-labeling studies, a careful analysis of the product distribution pattern, and the fact that in some cases PtCl2 can be replaced by simple Lewis or Brønsted acids as the catalysts.