DFT (density functional theory) studies on cycloisomerization of 15-membered triazatriacetylenic macrocycle.

The mechanism as well the stereochemistry of cascadecycloisomerization of 15-membered triazatriacetylenic macrocycle was investigated theoretically by using M062X/6-31+G(d,p) and M062X/LANL2DZ calculations. The results showed that the mechanism and outcome of the reaction depended on the absence and presence of a transition metal catalyst. So that, in thermal-induced condition, the reaction had to experience several suprafacial concerted reactions including Ene-reaction (DG=35.38 kcal/mol), Diels-Alder cycloaddition (DG = 17.16 kcal/mol), and sigmatropic H-shift rearrangement (DG = 56.21 kcal/mol) to produce diastereoselective fused cis-tetracyclic aromatic bearing a pyrrole moiety by following kinetic considerations. Also, the [2+2+2] cycloaddition mechanism was neglected in thermal-induced conditions because of high activation free Gibbs energy (DG = 63.90 kcal/mol). In the presence of palladium catalyst, Pd(0) formed an adduct by coordinating to C = C bonds and decreased the DG of the process to 29.58 kcal/mol, and consequently provided a facilitated media for the reaction to follow the [2+2+2] to produce more stable fused tetracyclic benzenoid aromatic by passing through the lower energy barrier.