The role of linkers and frustrated lewis pairs catalysts in the formation of zwitterionic 1,2-anti-addition product with non-conjugated terminal diacetylenes: A computational study

Abstract

This study presents a computational investigation into the mechanistic pathway and the linker units involved in forming the zwitterionic 1,2-anti-addition product of non-conjugated diacetylenes, di(propargyl)ether (DPE), di(prop-2yn-1yl)sulfane (DPS) and 1,6-Heptadiyne (HD) catalyzed by the inter-molecular phosphine/borane frustrated Lewis pairs (FLPs), i.e., PPh₂[C₆H₃(CF₃)₂](P-CF)/[B(C₆F₅)₃]([B]) and P(o-tolyl)₃(P-tol)/[B(C₆F₅)₃]([B]). The potential energy surface (PES) calculations reveal that the anti-addition of P-CF to the internal C-atoms of acetylene units is energetically more favored than that of the addition of P-tol in DPE, DPS, and HD by ∼10.0, ∼9.2, and ∼6.0 kcal/mol, respectively. The calculations performed with DPE contain “—O—,” linker unit exhibits superior reactivity than DPS and HD, which suggests the electronegativity of linkers plays a significant role and facilitates the addition of Lewis bases. The higher electronegativity of linker units enables the 1,2-addition reaction by lowering the free energy activation barriers, as observed in the DFT calculations. The Molecular Electrostatic Potential (MESP) study shows that the electrostatic interactions favor the addition of P-CF to the active acetylene positions (C5/C4/C4) of [B]-DPE/DPS/HD-π complexes than the P-tol. The Distortion/Interaction (D/I) analysis reveals that transition states involving P-CF (TS1, TS3, and TS5) exhibit more interaction energy (ΔE_Int) and less distortion energies (ΔE^d) than that of the P-tol (TS2, TS4, and TS6). Further, the Energy Decomposition Analysis (EDA) also rationalizes the preferential approach of the electron-deficient Lewis base over the electron-rich one on the basis of the significant contribution of orbital interaction energies (ΔE_orbital) in the cases of P-CF; TS1, TS3, and TS5. This study suggests that the electronic effects of substrates and the FLPs are crucial to facilitate the desired products formed with non-conjugated terminal alkynes.