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

Authors: MANSOOREH MOVAHEDI, NADER ZABARJAD HIRAZ, ALI EZABADI, MARJANEH SAMADIZADEH, MOHAMAD REZA TALEI BAVIL OLYAI

Abstract: The mechanism as well the stereochemistry of cascade cycloisomerization 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 (ΔG^#=35.38 kcal/mol), Diels-Alder cycloaddition (ΔG^# = 17.16 kcal/mol), and sigmatropic H-shift rearrangement (ΔG^# = 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 (ΔG^# = 63.90 kcal/mol). In the presence of palladium catalyst, Pd(0) formed an adduct by coordinating to C = C bonds and decreased the ΔG^# 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.

Keywords: DFT (density functional theory), triazatriacetyle, cycloisomerization, Ene?Reaction, Pd, [2+2+2] reaction

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