Effect of 6,6′-Substituents on Bipyridine-Ligated Ni Catalysts for Cross-Electrophile Coupling

Abstract

A family of 4,4′- t Bu 2 -2,2′-bipyridine ( t Bu bpy) ligands with substituents in either the 6-position, 4,4′- t Bu 2 -6-Me-bpy ( t Bu bpy Me ), or 6 and 6′-positions, 4,4′- t Bu 2 -6,6′-R 2 -bpy ( t Bu bpy R2; R = Me, i Pr, s Bu, Ph, or Mes), was synthesized. These ligands were used to prepare Ni complexes in the 0, I, and II oxidation states. We observed that the substituents in the 6 and 6′-positions of the t Bu bpy ligand impact the properties of the Ni complexes. For example, bulkier substituents in the 6,6′-positions of t Bu bpy better stabilized ( t Bu bpy R2 )Ni I Cl species and resulted in a cleaner reduction from ( t Bu bpy R2 )Ni II Cl 2 . However, bulkier substituents hindered or prevented the coordination of t Bu bpy R2 ligands to Ni 0 (cod) 2 . In addition, by using complexes of the type ( t Bu bpy Me )NiCl 2 and ( t Bu bpy R2 )NiCl 2 as precatalysts for different XEC reactions, we demonstrated that the 6 or 6,6′-substituents lead to major differences in the catalytic performance. Specifically, while ( t Bu bpy Me )Ni II Cl 2 is one of the most active catalysts reported to date for XEC and can facilitate XEC reactions at room temperature, lower turnover frequencies were observed for catalysts containing t Bu bpy R2 ligands. A detailed study on the catalytic intermediates ( t Bu bpy)Ni(Ar)I and ( t Bu bpy Me2 )Ni(Ar)I revealed several factors that likely contributed to the differences in the catalytic activity. For example, whereas complexes of the type ( t Bu bpy)Ni(Ar)I are low spin and relatively stable, complexes of the type ( t Bu bpy Me2 )Ni(Ar)I are high-spin and less stable. Furthermore, ( t Bu bpy Me2 )Ni(Ar)I captures primary and benzylic alkyl radicals more slowly than ( t Bu bpy)Ni(Ar)I, consistent with the lower activity of the former in catalysis. Our findings will assist in the design of tailor-made ligands for Ni-catalyzed transformations.

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