Trimethylaluminum Activates Zeolite-Confined Lanthanum Borohydrides to Enhance Catalytic C–H Borylation
Year: 2025
Type: article
Abstract: High Resolution Image Download MS PowerPoint Slide AlMe 3 -treatment of the borylation precatalyst La(BH 4 ) 2 (THF) 2.5 -Ph 3 Si-HY 30 affords La(BH 4 ) 2 (AlMe 3 )-Ph 3 Si-HY 30, 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 4 ) 2 (AlMe 3 )-Ph 3 Si-HY 30 as κ 2 - O, O -{≡SiO(=Al)≡SiO}La(BH 4 ) 2 (AlMe 3 ), revealing that AlMe 3 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 4 ) 2 (AlMe 3 )-Ph 3 Si-HY 30 (2.7 h –1 ) is slightly lower than that of untreated La(BH 4 ) 2 (THF) 2.5 -Ph 3 Si-HY 30 (3.6 h –1 ), implying that AlMe 3 is a stronger inhibitor than THF for lanthanum. On the other hand, AlMe 3 -treatment of inactive La(BH 4 ) 2 (THF) 2.2 -SiO 2 generates an active benzene borylation catalyst. AlMe 3 also desorbs surface–O-B x H y 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 3 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 4 ) 2 (AlMe 3 )-Ph 3 Si-HY 30 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 3 treatment having an opposite effect on the activity of lanthanum in HY vs SiO 2, point to confinement-activated sites.
DOI: 10.1021/jacs.4c18569
Language: English
Subfield: Organic Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $4,500
OpenAlex ID: W4409122433