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

AlMe-treatment of the borylation precatalyst La(BH)(THF)-PhSi-HY affords La(BH)(AlMe)-PhSi-HY, 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(BH)(AlMe)-PhSi-HY as κ-,-{≡SiO(=Al)≡SiO}La(BH)(AlMe), revealing that AlMe 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(BH)(AlMe)-PhSi-HY (2.7 h) is slightly lower than that of untreated La(BH)(THF)-PhSi-HY (3.6 h), implying that AlMe is a stronger inhibitor than THF for lanthanum. On the other hand, AlMe-treatment of inactive La(BH)(THF)-SiO generates an active benzene borylation catalyst. AlMe also desorbs surface-O-BH 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 AlMe 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(BH)(AlMe)-PhSi-HY 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 AlMe treatment having an opposite effect on the activity of lanthanum in HY vs SiO, point to confinement-activated sites.