Supra-supra, supra-antara, and stepwise-diradical pathways for an observed 16-electron double-[4 + 4] cycloaddition...

Quantum chemistry calculations are used to provide insight into the cycloaddition of two dialkyne chains in initially monocyclic organoplatinum dimers of the type (PtX(2))(2)(μ-R(2)PC(4)PR(2))(2), where X = Cl or Me and R = Ph or Me. Previous experimental studies showed that the cycloaddition occurs with {X, R} = {Cl, Ph} but not {Me, Ph}. Two concerted pericyclic paths, a D(2h)-symmetry double-[π4s+π4s] "Hückel path" and a D(2)-symmetry double-[π4s+π4a] "Möbius path", were explored via orbital energy correlation diagrams (OECDs) computed using a singly occupied molecular orbital technique developed earlier. In accord with pericyclic reaction theory, the 16e(-) rearrangement is forbidden along the D(2h) Hückel path; four electrons would need to change their orbital symmetries. The D(2) Möbius path, afforded by the natural twist in the reactant structure which allows the desired Möbius orbital connectivity for a 4n rearrangement, is concluded to be a borderline forbidden pathway. This Möbius path creates avoided crossings in the OECD, which allows consistent orbital populations throughout the reaction, but it does not cause a change in intended orbital correlation, and the predicted activation barrier is rather high (∼50 kcal mol(-1)). The avoided crossings show strong coupling, but a clear HOMO-LUMO gap for the reaction is not produced. A stepwise path is also presented, with evidence of its diradical character.