New catalysts for the undirected borylation of alkyl C-H bonds can lead to new strategies for the synthesis or derivatization of organic molecules. Catalysts composed of phenanthroline ligands, in combination with iridium precursors, have led to the borylation of aryl and some alkyl C-H bonds. However, catalysts that are more active or that react with distinct selectivity toward alkyl C-H bonds, or both, are needed. A new class of catalyst ligated by N-heterocyclic carbenes was predicted by computation to be more active for the borylation of alkyl C-H bonds than those with phenanthroline ligands. We show that these ligands generate catalysts that react in ways that are different from the pathways predicted. Specifically, we show that they rearrange to LX-type ligands by cyclometalation and then catalyze the borylation of THF at the position α-to oxygen in the presence of alkoxide, and that they do not react with alkyl C-H bonds in the absence of alkoxide. From computational and experimental studies, we provide strong evidence that the reactions in the presence of alkoxide occur by a catalytic cycle involving an anionic [iridium(III)trisboryl]^-Na⁺ complex that gives rise to the unique selectivity for the borylation of this saturated oxygen heterocycle and substantiate this conclusion by showing that the borylation of arylpyridines occurs by distinct pathways in the presence and absence of alkoxide.