sp3 C–H Borylation Catalyzed by Iridium(III) Triboryl Complex: Comprehensive Theoretical Study of Reactivity, Regioselectivity, and Prediction of Excellent Ligand

Iridium-catalyzed C–H borylation of THF was theoretically investigated as example of sp3 C–H functionalization. DFT computations show that β-regioselective borylation occurs more easily than does α-regioselective, as reported experimentally, through oxidative addition of C–H bond to iridium­(III) species and reductive elimination of B–C bond. The reductive elimination is both a rate-determining step and a regioselectivity-determining step. The lower energy transition state (TS) of the reductive elimination of β-boryloxolane arises from the Ir···(β-oxolanyl) interaction at TS being stronger than the Ir···(α-oxolanyl) one. The Ir···(β-oxolanyl) interaction being stronger than the Ir···(α-oxolanyl) one is a result of the valence orbital energy of the α-oxolanyl group being higher than that of the β-oxolanyl group due to antibonding overlap of the valence orbital with O 2p orbital, where SOMO of oxolanyl radical is taken as valence orbital hereinafter. Reactivity of substrate decreases following the order primary (β) C–H of ethyl ether > primary C–H of n-pentane ∼ secondary (β) C–H of THF > secondary C–H of cyclopentane > secondary (α) C–H of THF ∼ secondary C–H of n-pentane > secondary (α) C–H of ethyl ether. The primary C–H bond is more reactive than the secondary one because of its smaller steric repulsion and lower energy valence orbital of the primary alkyl group. The β-C–H bond of THF is more reactive than the secondary C–H bond of cyclopentane because of valence orbital energy of the β-oxolanyl group being lower than that of the cyclopentyl group. Both steric and electronic factors are important for determining reactivity of substrate. Bidentate ligand consisting of pyridine and N-heterocyclic carbene is predicted to be better than 3,4,7,8-tetramethyl-1,10-phenanthroline used experimentally.