Frustrated Lewis pairs (FLPs) are the combinations of both Lewis acids and bases motifs, where owing to steric hindrance, prevent the strong adduct formation. Accordingly, the ability of the FLPs in small molecules activation and capability of them in hydrogen cleavage, lead to their use in hydrogenation of a wide range of unsaturated substrates. Here, we have been investigated the ability of the three intramolecular phosphorus/boron FLPs as the bifunctional catalysts in metal-free hydrogenation of dimethyl acetylene to cis-alkene, theoretically. The mechanism of this hydrogenation reaction based on the boron acceptor including –OR substitutes (B(OR)2) and phosphorus donor, has been explored. Based on the obtained results, it was confirmed that 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 boron atom in FLPs on the hydrogenation reaction was investigated. The reduction steps of the studied mechanism showed an exothermic nature. This result revealed that the presence of the boron as 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 helpful.

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