TY - JOUR
T1 - Mechanism-Guided Design of Robust Palladium Catalysts for Selective Aerobic Oxidation of Polyols
AU - Ramsay-Burrough, Summer
AU - Marron, Daniel P.
AU - Armstrong, Keith C.
AU - Del Castillo, Trevor J.
AU - Zare, Richard N.
AU - Waymouth, Robert M.
N1 - Publisher Copyright:
© 2023 American Chemical Society.
PY - 2023/2/1
Y1 - 2023/2/1
N2 - The palladium complex [(L1)Pd(μ-OAc)]2[OTf]2 (L1 = neocuproine) is a selective catalyst for the aerobic oxidation of vicinal polyols to α-hydroxyketones, but competitive oxidation of the ligand methyl groups limits the turnover number and necessitates high Pd loadings. Replacement of the neocuproine ligand with 2,2′-biquinoline ligands was investigated as a strategy to improve catalyst performance and explore the relationship between ligand structure and reactivity. Evaluation of [(L2)Pd(μ-OAc)]2[OTf]2 (L2 = 2,2′-biquinoline) as a catalyst for aerobic alcohol oxidation revealed a threefold enhancement in turnover number relative to the neocuproine congener, but a much slower rate. Mechanistic studies indicated that the slow rates observed with L2 were a consequence of precipitation of an insoluble trinuclear palladium species─(L2Pd)3(μ-O)22+─formed during catalysis and characterized by high-resolution electrospray ionization mass spectrometry. Density functional theory was used to predict that a sterically modified biquinoline ligand, L3 = 7,7′-di-tert-butyl-2,2′-biquinoline, would disfavor the formation of the trinuclear (LPd)3(μ-O)22+ species. This design strategy was validated as catalytic aerobic oxidation with [(L3)Pd(μ-OAc)]2[OTf]2 is both robust and rapid, marrying the kinetics of the parent L1-supported system with the high aerobic turnover numbers of the L2-supported system. Changes in ligand structure were also found to modulate regioselectivity in the oxidation of complex glycoside substrates, providing new insights into structure-selectivity relationships with this class of catalysts.
AB - The palladium complex [(L1)Pd(μ-OAc)]2[OTf]2 (L1 = neocuproine) is a selective catalyst for the aerobic oxidation of vicinal polyols to α-hydroxyketones, but competitive oxidation of the ligand methyl groups limits the turnover number and necessitates high Pd loadings. Replacement of the neocuproine ligand with 2,2′-biquinoline ligands was investigated as a strategy to improve catalyst performance and explore the relationship between ligand structure and reactivity. Evaluation of [(L2)Pd(μ-OAc)]2[OTf]2 (L2 = 2,2′-biquinoline) as a catalyst for aerobic alcohol oxidation revealed a threefold enhancement in turnover number relative to the neocuproine congener, but a much slower rate. Mechanistic studies indicated that the slow rates observed with L2 were a consequence of precipitation of an insoluble trinuclear palladium species─(L2Pd)3(μ-O)22+─formed during catalysis and characterized by high-resolution electrospray ionization mass spectrometry. Density functional theory was used to predict that a sterically modified biquinoline ligand, L3 = 7,7′-di-tert-butyl-2,2′-biquinoline, would disfavor the formation of the trinuclear (LPd)3(μ-O)22+ species. This design strategy was validated as catalytic aerobic oxidation with [(L3)Pd(μ-OAc)]2[OTf]2 is both robust and rapid, marrying the kinetics of the parent L1-supported system with the high aerobic turnover numbers of the L2-supported system. Changes in ligand structure were also found to modulate regioselectivity in the oxidation of complex glycoside substrates, providing new insights into structure-selectivity relationships with this class of catalysts.
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U2 - 10.1021/jacs.2c10667
DO - 10.1021/jacs.2c10667
M3 - Article
C2 - 36657018
AN - SCOPUS:85146591350
SN - 0002-7863
VL - 145
SP - 2282
EP - 2293
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 4
ER -