The Catalyst Review June 2024 - 14

Hydrogenation of Esters Catalyzed by Bis(N-Heterocyclic Carbene) Molybdenum Complexes
Catalytic hydrogenation of carbonyl compounds typically involves using catalysts based on noble metals (mainly Ru). Investigation of less costly
alternatives such as Mn, Fe, and Co has led to the successful development of hydrogenation catalysts based on these metals. From the author's
perspective, Mo may also possess catalytic potential since it plays a vital role in the active centers of various redox-active enzymes, is inexpensive,
environmentally benign, and offers a rich coordination chemistry. Herein, these workers explored the development of Mo-based catalysts for the
reduction of carboxylic acid esters.
It has recently been shown that Mn complexes bearing strongly electron-donating, bidentate bis(NHC) ligands can catalyze the hydrogenation of esters.
These authors, therefore, believed that comparable Mo complexes could also function as hydrogenation catalysts. Bis(NHC) Mo complexes 2 and 3
were therefore prepared by the reaction of Mo(CO)6
or Mo(CO)3(CH3CN)3 with bis(NHC)-BEt3 adducts 1 (Scheme 1), which are obtained from their
corresponding bis(imidazolium) salts.
These complexes were then characterized
by NMR and IR spectroscopy and single
crystal XRD analysis before being evaluated
as (pre)catalysts for the hydrogenation of
esters. Complexes 3 were found to be far
more efficient than Complexes 2 for the
catalytic hydrogenation of aliphatic and
aromatic esters (>35 examples) operating
at low catalyst loadings (0.5−2 mol %) and
temperatures (80−120 °C). Precatalyst 3b
(R1
=H) possessed a broad scope
of substrate activity, including electronrich
and -poor aliphatic and aromatic
esters. Excellent chemoselectivity toward
other reducible functional groups like
alkenes, nitriles, and N-heteroarenes was
also demonstrated. Various additional
functional groups like alcohols, amines,
and halides were well tolerated, and
substrates containing bulky substituents
in the proximity of the ester, lactones
with larger ring sizes, and diesters were
readily hydrogenated. A non-bifunctional
outer-sphere hydrogenation mechanism
is proposed based on NMR spectroscopic
investigations and DFT calculations
(Scheme 2).
= iPr, R2
In conclusion, the authors present the
first example of a homogeneous Mo-based
catalyst for the hydrogenation of esters,
which operates at low catalyst loadings
compared to known molecularly defined
Mo-based catalysts. The presented catalyst
activities and substrate scope highlight
the potential of Mo-based catalysts for
replacing noble metals in (homogeneous)
hydrogenation catalysis. Both NF, Thaens J,
Spannenberg A, et al. (2024.) ACS Catal. 20,
14: 4082−4092.
Scheme 1. General Synthesis of Bis(NHC) Mo Complexes 2 and 3a
aReaction Conditions: For 2: toluene (56 mM), 90 °C, 16 h; for 3: CH3
CN (75 mM), 65 °C, 16 h.
Scheme 2. DTF-Supported Mechanistic Rationale for the Mo-Catalyzed Hydrogenation of Esters
14
The Catalyst Review
June 2024
The Catalyst Review June 2024

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