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-O,O-{≡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-BxHy 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.