Semihydrogenation of Alkynes Catalyzed by a Pyridone Borane Complex: Frustrated Lewis Pair Reactivity and Boron–Ligand Cooperation in Concert

• Published in: Chemistry : a European journal 2020-10, Vol.26 (59), p.13445-13450
• Main Authors: Wech, Felix, Hasenbeck, Max, Gellrich, Urs
• Format: Article
• Language: English
• Subjects: Alkynes; Boranes; Boron; boron–ligand cooperation; Catalysts; Chemistry; Cooperation; Coordination; density functional calculations; frustrated Lewis pair; Hydroboration; hydrogenation; Ligands; Mode of action; Stereoselectivity
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202001276

The metal‐free cis selective hydrogenation of alkynes catalyzed by a boroxypyridine is reported. A variety of internal alkynes are hydrogenated at 80 °C under 5 bar H2 with good yields and stereoselectivity. Furthermore, the catalyst described herein enables the first metal‐free semihydrogenation of terminal alkynes. Mechanistic investigations, substantiated by DFT computations, reveal that the mode of action by which the boroxypyridine activates H2 is reminiscent of the reactivity of an intramolecular frustrated Lewis pair. However, it is the change in the coordination mode of the boroxypyridine upon H2 activation that allows the dissociation of the formed pyridone borane complex and subsequent hydroboration of an alkyne. This change in the coordination mode upon bond activation is described by the term boron‐ligand cooperation. A case of important cooperation: The semihydrogenation of alkynes catalyzed by a pyridonate borane complex is reported. While the catalyst can be described as an intramolecular frustrated Lewis pair, it is the change in the coordination mode of the pyridonate substituent upon H2 bond activation that enables dissociation of the formed pyridone borane complex and hydroboration as the first C−H bond‐forming step. This mode of action is referred to as boron‐ligand cooperation.