Origin of the Difference in Reactivity between Ir Catalysts for the Borylation of C–H Bonds

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.