Mechanistic insights into intramolecular carbon–carbon bond formation and carbon–boron cyclization of non-conjugated terminal diacetylenes by frustrated Lewis Pairs: A DFT study

Frustrated Lewis pairs (FLPs) exhibit highly variable reactivity with non-conjugated terminal diacetylenes, leading to 1,2-anti-addition products and also to intramolecular C-C coupling followed by C-B cyclization with the variation in the Lewis bases. However, the detailed mechanisms of such reactions have not been examined. This study reports a detailed mechanistic investigation of C-C coupling followed by C-B cyclization of 1,6-heptadiyne by FLPs. Density functional theory calculations unveil three different mechanistic pathways, including 1,1-carboboration, and two alternative variations of 1,2-carboboration pathways. The 1,1-carboboration involves C-C coupling on geminal carbon [pathway (a)], while 1,2-carboboration involves vicinal C-C coupling [pathways (b) and (c)]. The computed results reveal that the 1,1-carboboration is energetically favoured and the formation of regioselective 8-membered product is governed by the kinetic effect in the reaction. The role of different Lewis bases (PPh₂[C₆H₃(CF₃)₂) and linkers [di(propargyl)ether] suggests the ease to form the 8-membered zwitterionic heterocyclic products via 1,1-carboboration pathway.