Theoretical Investigation\nof the Steric Effects on\nAlkyne Semihydrogenation Catalyzed by Frustrated Lewis Pairs

Frustrated Lewis\npairs (FLPs) have received increasing attention\nfor offering a distinctive pathway for the activation and conversion\nof small molecules. Here, we employ density functional calculations\nto investigate the electronic and steric effects of FLPs and alkynes\non the activity toward alkyne semihydrogenation, a crucial reaction\nin the production of pharmaceuticals and polymers. We investigated\nthe steric effect of FLPs by replacing the LA-LB linker (<b>d</b>) of the model catalyst FLP, 1-NMe<sub>2</sub>-2-B(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>-C<sub>6</sub>H<sub>4</sub> (<b>d1</b>),\nwith various linkers. Additionally, we studied the steric effect of\nalkynes by varying the substrate from acetylene to but-2-yne, hex-3-yne,\n2,5-dimethylhex-3-yne, 2,2,5,5-tetramethylhex-3-yne, and 1,2-diphenylethyne.\nOur computational results suggest that the overall activity of alkyne\nand alkene hydrogenation is influenced by both electronic and steric\neffects when using FLPs with varied LA-LB linkers. To achieve better\nactivity, one could increase the steric hindrance of the LA neighboring\nenvironment and reduce the electron density of the LB site of FLPs.\nStructure–activity relationships for each elementary step involving\nalkyne semihydrogenation were identified in this work too. In contrast,\nwhen the reaction is catalyzed by the same FLP, the activity is solely\ngoverned by the steric hindrance of the alkynes. The overall activity\nexhibits a volcano-shaped trend as a function of the buried volume\nof the alkynes. FLP (<b>d1</b>) shows the highest activity toward\nhex-3-yne, as predicted by the volcano correlation, while it exhibits\nlower activity toward alkynes with either less or greater steric hindrance\ncompared to hex-3-yne.