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

Abstract

A family of 4,4'-^tBu₂-2,2'-bipyridine (^tBubpy) ligands with substituents in either the 6-position, 4,4'-^tBu₂-6-Me-bpy (^tBubpy^Me), or 6 and 6'-positions, 4,4'-^tBu₂-6,6'-R₂-bpy (^tBubpy^R2; R = Me, ⁱPr, ^sBu, 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 ^tBubpy ligand impact the properties of the Ni complexes. For example, bulkier substituents in the 6,6'-positions of ^tBubpy better stabilized (^tBubpy^R2)Ni^ICl species and resulted in cleaner reduction from (^tBubpy^R2)Ni^IICl₂. However, bulkier substituents hindered or prevented coordination of ^tBubpy^R2 ligands to Ni⁰(cod)₂. In addition, by using complexes of the type (^tBubpy^Me)NiCl₂ and (^tBubpy^R2)NiCl₂ as precatalysts for different XEC reactions, we demonstrated that the 6 or 6,6' substituents lead to major differences in catalytic performance. Specifically, while (^tBubpy^Me)Ni^IICl₂ 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 ^tBubpy^R2 ligands. A detailed study on the catalytic intermediates (^tBubpy)Ni(Ar)I and (^tBubpy^Me2)Ni(Ar)I revealed several factors that likely contributed to the differences in catalytic activity. For example, whereas complexes of the type (^tBubpy)Ni(Ar)I are low spin and relatively stable, complexes of the type (^tBubpy^Me2)Ni(Ar)I are high-spin and less stable. Further, (^tBubpy^Me2)Ni(Ar)I captures primary and benzylic alkyl radicals more slowly than (^tBubpy)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.

Keywords: Cross-electrophile coupling; Ni complexes; bipyridine ligands; catalysis; mechanistic studies.