Theoretical Evaluation of the Efficiency of Novel Frustrated Lewis Pairs in the cis-Hydrogenation Reaction of Dimethylacetylene

Frustrated Lewis pairs (FLPs) are the combination of Lewis acid and base motifs where steric hindrance prevents strong adduct formation. Accordingly, the ability of FLPs in small molecule activation and their capability in hydrogen cleavage led to their use in the hydrogenation of a wide range of unsaturated substrates. Here, we investigated theoretically the ability of three intramolecular phosphorus/boron FLPs as bifunctional catalysts in the metal-free hydrogenation of dimethylacetylene to cis-alkene. The mechanism of this hydrogenation reaction, based on the boron acceptor [including –OR substituents (B(OR)2), where R is an aliphatic or aromatic branch] and phosphorus donor, has been explored. Based on the results obtained, it was confirmed that the H2 splitting reaction and the formation of the phosphonium–borohydride motifs for these FLPs are endothermic. It has been shown that these FLPs have a moderate ability in H–H bond splitting. Also, the capability of the boron atom in FLPs on the hydrogenation reaction was investigated. The reduction steps of the mechanism showed an exothermic nature. This result revealed that the presence of the boron as a Lewis acid, with a very limited Lewis acidity, improves the catalytic hydrogenation reaction significantly. Finally, it was confirmed that the proposed FLPs in the cis-hydrogenation of alkynes will be effective.