Ir(NHC)
(NHC, N-heterocyclic carbene)-catalyzed
dehydrogenative coupling of sustainable ethylene glycol and various
bioalcohols can produce industrially valuable α-hydroxy acids
(AHAs). This study is the first to report the sustainable synthesis
of higher C
n
AHAs, in addition to glycolic
acid (C2 AHA) and lactic acid (C3 AHA). This
catalytic system can be recycled to the seventh cycle while maintaining
good yields. A reaction mechanism, including facile dehydrogenation
of each alcohol and fast cross-coupling of dehydrogenated aldehydes
forming products, was proposed based on 18O- and 2H-labeling experiments and electron spray ionization-mass spectrometry
(ESI-MS) and NMR spectral analyses.
Multi N‐heterocyclic carbene(NHC)‐modified iridium catalysts were employed in the β‐alkylation of alcohols; dimerization of primary alcohols (Guerbet reaction), cross‐coupling of secondary and primary alcohols, and intramolecular cyclization of alcohols. Mechanistic studies of Guerbet reaction, including kinetic experiments, mass analysis, and density functional theory (DFT) calculation, were employed to explain the fast reaction promoted by bimetallic catalysts, and the dramatic reactivity increase of monometallic catalysts at the late stage of the reaction.
Sustainable iridium‐catalyzed transfer hydrogenation using glycerol as the hydride source was employed to convert levulinic acid to γ‐valerolactone (GVL) with exceptionally high turnover numbers (TONs) (500,000) and turnover frequencies (TOFs) (170,000 h−1). The highly efficient triscarbene‐modified iridium catalysts demonstrated good catalytic activities with low catalyst loadings (0.7 ppm) and good recyclability with an accumulated TON of over two million in the fourth reaction. In addition to glycerol, propylene glycol (PG), ethylene glycol (EG), isopropanol (IPA), and ethanol (EtOH) successfully transferred hydrides to levulinic acid, producing GVL with TONs of 339,000 (PG), 242,000 (EG), 334,000 (IPA), and 208,000 (EtOH), respectively. Deuterium‐labeling experiments were conducted to gain insight into the reaction mechanism.
Novel tri-N-heterocyclic carbene (triNHC)-coordinated
iridium catalysts (Ir(triNHC)) were employed for generating clean
hydrogen and useful C
n
acids from biomass-derived
alcohols. The high efficiency and fast rate of hydrogen production
(maximum turnover frequency = 13,080 h–1 and 485
L of H2/g-cat·h) from sustainable ethylene glycol
were elucidated by the increased electron-richness of Ir(triNHC) complexes
where three NHC ligands were coordinated to an iridium(I) ion. Elaborated
mechanistic studies support the proposed reaction mechanisms forming
H2 and C
n
acids. This Ir(triNHC)-catalyzed
process converting sustainable alcohols to useful fuels and chemicals
is a promising carbon-neutral process.
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