On the regioselective mechanism of novel rearrangements of 1,6-enynes catalyzed by PtCl2: a DFT study

In the present work, the density functional theory is employed to study qualitatively the mechanism of the rearrangement reaction of 1,6-enynes catalyzed by PtCl2 in gas phase. An efficient and reliable strategy has been adopted to search for the transition states of the reaction involved. As demonstrated, the rearrangement reaction process mainly involves two possible regioalternative mechanisms: cycloisomerization and the formation of cyclopropanes. Four reaction paths for the mechanism of cycloisomerization and one reaction path for the formation of cyclopropanes are investigated at the levels of B3LYP/LANL2DZ* and B3LYP/LANL2MB. The results of calculations show that the rate-determining step for the cycloisomerization is the hydrogen transfer from Pt to C7, and the rate-determining step for the formation of cyclopropanes is the formation of 3-member ring involving C6 and C7. This conclusion is different from the mechanism conjectured by Echavarren. According to our calculations, the result obtained is in agreement with the experiment. The coordination of Pt with the unsaturated bonds releases a large amount of heat, which provides the thermodynamical driving force for the reaction.