A DFT‐Based Theoretical Investigation of the Mechanism of the PtCl2‐Mediated Cycloisomerization of Allenynes

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Abstract
The mechanism of PtII‐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 tetrasubstituted 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 π bond of the allene, which would undergo regioselective β‐H elimination from the equatorially disposed methyl group. A metal‐induced H migration leads to a second octahedral PtIV–chelate complex, which would yield the expected bicyclic system through an intramolecular migratory insertion step. Therefore, depending on the conformation of the initial η4‐reactant complex for trisubstituted 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 PtIV intermediate complex. Theoretical results reveal the kinetic preference for β‐H elimination from the allylic group rather than from the gem‐dimethyl group, which should account for the observed regioselectivity.

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