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

Unusual alpha-regioselective C(sp(3))-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 alpha 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 alpha-borylated product. In this unprecedented mechanism, the AlB ligand plays important roles; its bulkiness suppresses the oxidative addition of the alpha C(sp(3))-H bond and its planar geometry corresponding to the aromatic resonance form stabilizes the anionic iridium(III) complex [(AlB)Ir(H)(Bpin)(2)](-). The beta C(sp(3))-H borylation occurs via oxidative addition of the beta C(sp(3))-H bond to the Ir(III) atom, followed by reductive elimination of the beta-borylated product because the primary beta C(sp(3))-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 alpha C(sp(3))-H borylation and 35.7 kcal mol(-1) for the beta one; the calculated alpha/beta 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 alpha- and beta-regioselective C(sp(3))-H borylations of alkylamines are discussed.

Automated summary

The unusual alpha-regioselective and beta-regioselective C(sp(3))-H borylation reactions of NEt3 were investigated using density functional theory calculations. The study revealed that the AlB ligand plays a crucial role in the reaction mechanism. This research is significant for understanding the factors controlling alpha and beta regioselectivity in C(sp(3))-H borylation reactions.