A mechanistic study on the origin of the difference in reactivity between Ir catalysts for C-H borylation reactions is reported. Catalytic reactions of B₂pin₂ with a series of substrates that require high temperatures and long reaction times were conducted. These reactions catalyzed by the combination of [Ir(COD)(OMe)]₂ and 3,4,7,8-tetramethylphenanthroline (tmphen) occur in yields that are substantially higher than those of reactions catalyzed by [Ir(COD)(OMe)]₂ and 4,4'-di-tert-butylbipyridine (dtbpy). The electronic properties of Ir catalysts ligated by dtbpy or tmphen and their stoichiometric reactivity were investigated. It was found that a longer lifetime rather than higher reactivity of the catalyst leads to higher yields of reactions catalyzed by Ir-tmphen. The catalyst ligated by dtbpy decomposes principally by dissociation of the ligand and rapid borylation at the positions alpha to nitrogen. Thus, the greater stability of the catalyst containing tmphen results from its greater binding constant.

An in-depth study of iridium catalysts for the borylation of alkyl C-H bonds is reported. Although the borylation of aryl C-H bonds can be catalyzed by iridium complexes containing phen or bpy ligands at mild temperatures and with limiting arene, the borylation of alkyl C-H bonds remains underdeveloped. We prepared a library of phenanthrolines that contain varying substitution patterns. The corresponding phen-Ir trisboryl carbon monoxide complexes were synthesized to determine the electron-donating ability of these ligands, and the initial rates for the borylation of the C-H bonds in THF and diethoxyethane β to oxygen catalyzed by Ir complexes containing these ligands were measured. For some subsets of these ligands, the donor ability correlated positively with the rate of C-H borylation catalyzed by the complexes containing ligands within a given subset. However, across subsets, ligands possessing similar donor properties to one another form catalysts for the borylation of alkyl C-H bonds with widely varying activity. This phenomenon was investigated computationally, and it was discovered that the stabilizing interactions between the phenanthroline ligand and the boryl ligands attached to Ir in the transition state for C-H oxidative addition could account for the differences in the activity of the catalysts that possess similar electron densities at Ir. The effect of these interactions on the borylation of secondary alkyl C-H bonds is larger than it is on the borylation of primary alkyl C-H bonds.

The selective functionalization of strong, typically inert carbon-hydrogen (C-H) bonds in organic molecules is changing synthetic chemistry. However, the undirected functionalization of primary C-H bonds without competing functionalization of secondary C-H bonds is rare. The borylation of alkyl C-H bonds has occurred previously with this selectivity, but slow rates required the substrate to be the solvent or in large excess. We report an iridium catalyst ligated by 2-methylphenanthroline with activity that enables, with the substrate as limiting reagent, undirected borylation of primary C-H bonds and, when primary C-H bonds are absent or blocked, borylation of strong secondary C-H bonds. Reactions at the resulting carbon-boron bond show how these borylations can lead to the installation of a wide range of carbon-carbon and carbon-heteroatom bonds at previously inaccessible positions of organic molecules.