A DFT-based theoretical investigation of the mechanism of the PtCl2-mediated cycloisomerization of allenynes

The mechanism of Pt-II-catalyzed intramolecular cycloisomerization of allenyne systems has been extensively investigated by DFT calculations. Different mechanistic schemes have been proposed and discussed, including the Alder-ene reaction. The free energy results suggest that the kinetically preferred reaction pathway for precursors that are tri- and tetra-substituted on the allene moiety should proceed by a five-step mechanism. This would involve formation of a platina(IV)cyclopentene intermediate by selective engagement of the external bond of the allene, which would under-go regioselective beta-H elimination from the equatorially disposed methyl group. A metal-induced H migration leads to a second octahedral Pt-IV-chelate complex, which would yield the expected bicyclic system through an intramolecular migratory insertion step. Therefore, depending on the conformation of the initial eta(4)-reactant complex for tri-substituted patterns, two possible intermediates can be formed that would evolve through different paths. In these cases, the regio- and stereochemical outcomes predicted by the mechanistic scheme proposed agree with experimental data. Substituted precursors on the alkyne moiety follow a distinct, four-step, mechanism also involving an oxidative cyclometalation process to an octahedral Pt-IV intermediate complex. Theoretical results reveal the kinetic preference for beta-H elimination from the allylic group rather than from the gem-dimethyl group, which should account for the observed regioselectivity.