Theoretical Insight into Catalysis of the Aluminabenzene–Iridium Complex for C(sp3)–H Borylation of NEt3: How to Control α- and β‑Regioselectivities?

Unusual α-regioselective C­(sp3)–H borylation of NEt3 by a unique aluminabenzene iridium­(I) complex (AlB)­Ir­(cod) (AlB = aluminabenzene anion; cod = cyclooctadiene) was investigated using density functional theory calculations. (AlB)­Ir­(Bpin)2 (HBpin = pinacolborane) is an active species. The first step is hydride transfer from the α CH2 group of NEt3 to (AlB)­Ir­(Bpin)2 to afford an unprecedented ion-pair intermediate [(AlB)­Ir­(H)­(Bpin)2]−[Et2NCHCH3]+. The next step is the CN double bond insertion of [Et2NCHCH3]+ into the Ir–Bpin bond to yield an α-borylated product. In this unprecedented mechanism, the AlB ligand plays important roles; its bulkiness suppresses the oxidative addition of the α C­(sp3)–H bond and its planar geometry corresponding to the aromatic resonance form stabilizes the anionic iridium­(III) complex [(AlB)­Ir­(H)­(Bpin)2]−. The β C­(sp3)–H borylation occurs via oxidative addition of the β C­(sp3)–H bond to the Ir­(III) atom, followed by reductive elimination of the β-borylated product because the primary β C­(sp3)–H bond easily approaches the Ir atom due to small steric hindrance. In oxidative addition, AlB has a non-planar geometry corresponding to the ambiphilic resonance form to stabilize the iridium­(V) intermediate. The activation energy is 35.0 kcal mol–1 for the α C­(sp3)–H borylation and 35.7 kcal mol–1 for the β one; the calculated α/β ratio is 70:30, which is close to the experimentally reported ratio (83:21). Ligand and metal effects on catalytic activity and regioselectivity are explored by computational analysis of (AlB)­Rh­(Bpin)2, (Phen)­Ir­(Bpin)3, (Cp*)­Ir­(Bpin)2, and its Rh analogue. Key factors in controlling α- and β-regioselective C­(sp3)–H borylations of alkylamines are discussed.