Trimethylaluminum Activates Zeolite-Confined Lanthanum Borohydrides to Enhance Catalytic C–H Borylation

AlMe₃-treatment of the borylation precatalyst La(BH₄)₂(THF)_2.5-Ph₃Si-HY₃₀ affords La(BH₄)₂(AlMe₃)-Ph₃Si-HY₃₀, which is the superior lanthanum-based precatalyst for benzene borylation with pinacolborane (HBpin), giving higher turnovers (>285) and improved yields (up to 42%) of phenylpinacolborane (PhBpin). Solid-state NMR spectroscopy, X-ray adsorption spectroscopy, and theoretical studies characterized the precatalytic sites in La(BH₄)₂(AlMe₃)-Ph₃Si-HY₃₀ as κ²-<i>O</i>,<i>O</i>-{≡SiO(=Al)≡SiO}La(BH₄)₂(AlMe₃), revealing that AlMe₃ had displaced the THF ligands. In contrast to the expected high reactivity of THF-free organolanthanum, the turnover frequency (TOF) for PhBpin formation catalyzed by La(BH₄)₂(AlMe₃)-Ph₃Si-HY₃₀ (2.7 h^-1) is slightly lower than that of untreated La(BH₄)₂(THF)_2.5-Ph₃Si-HY₃₀ (3.6 h^-1), implying that AlMe₃ is a stronger inhibitor than THF for lanthanum. On the other hand, AlMe₃-treatment of inactive La(BH₄)₂(THF)_2.2-SiO₂ generates an active benzene borylation catalyst. AlMe₃ also desorbs surface-O-B_<i>x</i>H_<i>y</i> species and quenches residual Brønsted acid sites (BAS) and silanols. Alumination of the BAS inhibits HBpin degradation, while alumination of silanols creates sites for that reaction. The 8-fold inhibition of the BAS-catalyzed HBpin decomposition rate by AlMe₃ treatment gives a kinetic advantage to the lanthanum-catalyzed C-H borylation, leading to increased yields and turnovers. Knowledge of the competing roles of sites in La(BH₄)₂(AlMe₃)-Ph₃Si-HY₃₀ and the catalytic rate law law enables identification of favorable conditions of low [HBpin] to maximize turnovers or PhBpin yield. Sterics affect the selectivity in borylation of substituted arenes and heteroarenes, which can proceed without the precoordination of a donor. These steric effects, as well as the AlMe₃ treatment having an opposite effect on the activity of lanthanum in HY vs SiO₂, point to confinement-activated sites.