sp 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 sp 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.