Acceptorless and Base-Free Dehydrogenation of Alcohols Mediated by a Dipyridylamine-Iridium(III) Catalyst

Article abstract of DOI:10.1002/ejoc.202000584

Several dipyridylamine-Ir^III and dipyridylamine-Ru^II complexes have been evaluated in the acceptorless dehydrogenation of alcohols in the absence of base additives. Iridium catalysts were found superior to ruthenium complexes, and the nature of the bridging nitrogen in dipyridylamine ligands was also evidenced as a key parameter. Catalytic reactions were conducted in toluene, but more sustainable solvents such as anisole and p-cymene were found suitable for this transformation.

Full text of DOI:10.1002/ejoc.202000584

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Dehydrogenation of Alcohols
Acceptorless and Base-Free Dehydrogenation of Alcohols
Mediated by a Dipyridylamine-Iridium(III) Catalyst
Harikrishnan Jayaprakash,[a] Liwei Guo,[a] Shengdong Wang,[a] Christian Bruneau,[a] and
Cédric Fischmeister*[a]
Abstract: Several dipyridylamine-IrIII and dipyridylamine-RuII nature of the bridging nitrogen in dipyridylamine ligands was
complexes have been evaluated in the acceptorless dehydro- also evidenced as a key parameter. Catalytic reactions were con-
genation of alcohols in the absence of base additives. Iridium ducted in toluene, but more sustainable solvents such as anis-
catalysts were found superior to ruthenium complexes, and the ole and p-cymene were found suitable for this transformation.
Introduction
chelating ligands are employed. Alternatively, Kanai reported
an original process for the room temperature acceptorless de-
hydrogenation of alcohols using a hybrid ternary catalytic
system.[16] Recently, we have been interested in the synthesis
of catalysts bearing chelating dipyridylamine ligands[17,18] in a
number of catalytic transformations. 2,2-Dipyridylamine is a
non-expensive commercially available compound and 2,2′-di-
pyridylamine derived ligands are easily accessible using simple
synthetic procedures. We have developed a number of dpa-
based catalysts that showed very good performances in
hydrogenation reactions[19] and base-free dehydrogenation of
formic acid[20] and nitrogen heterocycles.[21] These previous
studies clearly demonstrated the high efficiency of this type of
catalysts in hydrogenation and dehydrogenation processes and
prompted us to investigate their potential in the acceptorless
and base-free dehydrogenation of secondary and primary alco-
hols.
The catalytic dehydrogenation of alcohols,[1] often quoted as
oxidant free oxidation, is for many reasons a very appealing
oxidation process. Following the pioneering studies made by
Robinson,[2] this reaction was quickly identified as a valuable
route for the production of hydrogen.[3–5] The dehydrogenation
of alcohols for the production of carbonyl compounds is more
recent and has found numerous interesting applications in tan-
dem and hydrogen-borrowing reactions for the synthesis of
organic compounds[6] but also as a catalytic tool for the produc-
tion of (bio)-alcohols via the Guerbet reaction.[7] The oxidation
of alcohols through acceptorless dehydrogenation is a clean
process as dihydrogen is the unique coproduct of the reaction.
This reaction proceeds therefore with high atom economy un-
like many stoichiometric oxidants (Cr and Mn compounds,
Dess–Martin, Swern reagents ...,) used in organic synthesis and
it does not require any oxidant (O2, H2O2, RCO3H, ...) when com-
pared to other catalytic oxidation processes. A number of transi-
tion metal catalysts are known to promote the acceptorless de-
hydrogenation of alcohols[8–15] leading to ketone from second-
ary alcohols whereas primary alcohols do not always deliver the
expected aldehydes and rather furnish ester derivatives. Among
the catalyst reported so far, those operating under base- and
additive-free conditions[9,11,13,15] are the most attractive as they
generate less wastes and they tolerate reagents bearing base-
sensitive functional groups. A survey of the catalysts operating
under base-free conditions shows that bifunctional catalysts
bearing basic ligands such as pincer, carboxylate and other
Results and Discussion
The iridium-chloro catalysts Ir1–3 and the zwitterionic iridium
complexes Ir4–5 were first evaluated as catalysts for the de-
hydrogenation of the model substrate 1-phenylethanol (Fig-
ure 1).
These catalysts were first screened in the base-assisted de-
hydrogenation of 1-phenylethanol at 130 °C in toluene
(Scheme 1). This first set of experiments revealed the modest
activity of these catalysts in particular for Ir1, Ir4 and Ir5 and
demonstrated that Ir–Cl complexes Ir2 and Ir3 were more effi-
cient than their Ir–OSO3 counterparts. This result contrasts with
our previous studies on the dehydrogenation reactions of
formic acid and N-heterocycles for which Ir–OSO3 catalysts sur-
passed their Ir–Cl counterparts.[20,21] The nature of the reagent
may of course have a strong influence on the catalytic pathway,
but other parameters should also be considered as key parame-
ters. Indeed, previous studies on formic acid and N-heterocycle
dehydrogenations were conducted in water where ligand ex-
change (OSO3/H2O) was likely proceeding whereas decoordina-
[a] H. Jayaprakash, L. Guo, S. Wang, Dr. C. Bruneau, Dr. C. Fischmeister
Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) -
UMR 6226,
35000 Rennes, France
E-mail: cedric.fischmeister@univ-rennes1.fr
https://iscr.univ-rennes1.fr/cedric-fischmeister
https://iscr.univ-rennes1.fr/christian-bruneau
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.202000584.
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Table 1. Dehydrogenation of 1-phenylethanol with Ir2. Screening of bases.[a]
Entry
Base
Conversion [%][b]
1
2
3
4
5
6
Cs2CO3
Na2CO3
K2CO3
KOAC
NaOAc
K3PO4
45
29
20
20
11
No conversion
[a] Reaction conditions: 1-phenylethanol (40 mg, 0.33 mmol), Ir2 (4 mg,
0.006 mmol, 2 mol-%), base (15 mol-%), toluene (3 mL), 130 °C (oil bath),
17 h, argon flow. [b] Determined by gas chromatography.
tee the integrity of base-sensitive functional groups while
minimizing wastes production. The dehydrogenation of 1-phen-
ylethanol was attempted under base-free conditions and con-
sidering the issues raised here above, i.e. deprotonation of
bridging N-H in dipyridylamine ligand, all the catalysts were
again evaluated under these pH neutral conditions. Our investi-
gations demonstrated that the base-free dehydrogenation of
1-phenylethanol could be achieved with high conversion pro-
vided a higher reaction temperature and a longer reaction time
were implemented. All reactions were conducted under a flow
of argon as a simple open system failed to furnish good results
(see Supporting Information). This experimental observation is
consistent with a gas (H2) release from the reaction medium. Of
note, Ir2 led to full conversion of 1-phenylethanol in refluxing
toluene for 24 h (Table 2). It should be noted that despite the
neutral condition, Ir1, and Ir4 bearing the NH- dipyridylamine
ligand were yet inefficient as the zwitterionic Bn-dipyridylamine
Ir5. These results confirmed that zwitterionic complexes were
not suitable for this transformation and that the NH-dipyridyl-
amine ligand was also unsuitable. In the latter case this result
can no longer be explained by the deprotonation of the NH-
dipyridylamine ligand but may be related to the lower electron
density at the metal in Ir1 vs. Ir2 and Ir3, which was identified
as an important parameter for the dehydrogenation of formic
acid.[20]This result provides some hints for a possible reaction
mechanism in which the basicity of the dipyridylamine ligand
would be a key parameter.
Figure 1. Ir-dpa complexes evaluated in the dehydrogenation of 1-phenyleth-
anol.
tion of the –OSO3 ligand might be more difficult in toluene.
These results also emphasized the deleterious influence of a
bridging N–H vs. N–R in the dipyridylamine ligand (Ir1 and Ir2)
hence also contrasting with the dehydrogenation of formic acid
where the N–H moiety was necessary to ensure the best cata-
lytic performances.[20]
Scheme 1. Dehydrogenation of 1-phenylethanol (conversion determined by
gas chromatography).
Another parameter to consider in rationalizing these results
is the reaction pH. Whereas the dehydrogenation of formic acid
was performed under acidic conditions,[20] the dehydrogen-
ation of 1-phenylethanol is conducted under alkaline media
likely leading to the deprotonation of the bridging N–H hence
transforming the neutral dipyridylamine ligand into an anionic
dipyridylaminate[22] and consequently to a new catalytic spe-
cies. However, as reported hereafter in the base-free dehydro-
genation study, this hypothesis may not be the unique answer
to the poor activity of Ir1 and Ir4 since these two complexes
presented again poor performances under base-free conditions
(vide infra).
Having identified Ir2 and Ir3 as the most potent catalysts for
the dehydrogenation of 1-phenylethanol, we turned our atten-
tion to the nature of the base. As depicted in Table 1, none of
the inorganic bases tested led to better results than Cs2CO3.
The same conclusion was obtained when Ir3 was evaluated
with the same inorganic bases (see Supporting Information).
Table 2. Base-free dehydrogenation of 1-phenylethanol.[a]
Entry
Catalyst
Conversion [%][b]
1
2
3
4
5
Ir1
Ir2
Ir3
Ir4
Ir5
7
100 (94)[c](97)[d]
80
1
7
[a] Reaction conditions: 1-phenylethanol (40 mg, 0.33 mmol), Catalyst
(0.006 mmol, 2 mol-%), refluxing toluene (140 °C, oil bath temperature), 24 h,
argon flow. [b] Determined by gas chromatography. [c] 17 h at 140 °C.
[d] 17 h at 150 °C in xylenes.
Further tests were performed with Ir2 in order to identify
the optimized reaction conditions (Table 3). This series of exper-
iments demonstrated that 2 mol-% of catalyst was the lowest
catalyst loading acceptable to ensure high conversion in a rea-
As emphasized in the introduction, base-free dehydrogen- sonable amount of time and that a 1
M concentration of sub-
ation of alcohols is strongly desired as these conditions guaran- strate was also an important requirement (Table 3, entries 4–6).
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Finally, greener and more sustainable solvents[23] such as anis- (Figure 3, 2g–2i). To the best of our knowledge, the synthesis of
ole and p-cymene were evaluated and found to be suitable for compound 2i by dehydrogenation of alcohol is unprecedented.
this transformation whereas the reaction conducted in cyclo- Despite full conversion of the hydroxyl-precursor, the synthesis
pentylmethyl ether (CPME) led to poor conversion of 1-phenyl- of 2h was hampered by side reactions thus leading to a moder-
ethanol (Table 3, entries 7–9).
ate yield of 55 %. Notably, aliphatic secondary alcohols were
efficiently converted into the corresponding ketones (Figure 3,
2k–2l). The challenging dehydrogenation of primary alcohols
was also investigated. In this case, 2m was obtained in a mod-
est 50 % yield whereas cinnamyl alcohol was fully converted
but with poor selectivity as two products resulting from partial
(2nb) of full reduction (2nc) of the expected product (2na) were
detected by GC/MS leading to the isolation of 2na in a modest
45 % yield.
Table 3. Experimental condition optimization.[a]
Entry
Catalyst [mol-%]
[1] (mol L–1
)
t [h]
Conversion [%][b]
1
2
3
4
5
6
2
1
0.5
2
2
2
2
2
2
0.1
0.1
0.1
0.1
0.01
1
0.1
0.1
0.1
24
24
24
17
17
17
17
17
17
100
85
54
69
33
99
12
92
75
7[c]
8[d]
9[e]
[a] Reaction conditions: 1-phenylethanol (40 mg, 0.33 mmol), Ir2, solvent,
140 °C (oil bath temperature), 24 h, argon flow. [b] Determined by gas chro-
matography. [c] CPME (110 °C). [d] Anisole. [e] p-Cymene.
With respect to other transition metals, iridium-based com-
plexes are in general the most performing catalysts in dehydro-
genation of alcohols. However due to the high cost of iridium
metal, less expensive transition metals should also be amenable
to perform this reaction. A few examples of ruthenium catalysts
promoting the dehydrogenation of alcohols under base-free
conditions have been reported.[11] They require in general high
temperature (up to 165 °C) and long reaction times (up to 5
days) to reach high conversions. The ruthenium catalysts
Ru1–3 were evaluated in the base-free and base-assisted de-
hydrogenation of 1-phenylethanol (Figure 2). Under both type
of experimental conditions, these catalysts displayed poor per-
formances (see Supporting Information). These results are in
agreement with our previous studies on hydrogenation and de-
hydrogenation reactions where iridium catalysts always sur-
passed ruthenium catalysts.
Figure 3. Reaction scope with isolated yields. Reaction conditions: substrate
(0.33 mmol), catalyst (2 mol-%), argon flow. [a] refluxing toluene (140 °C, oil
bath temperature), 24 h; [b] refluxing toluene, 17 h; [c] anisole, 150 °C, 24 h.
Based on the experimental results obtained during the
screening of the various catalysts implemented in this study, a
possible reaction mechanism is presented in Figure 4. As re-
ported earlier in this manuscript, the activity of the catalyst was
drastically improved when the dipyridylamine ligand was made
of a bridging tertiary amine vs. secondary amine. This prompted
us to propose a mechanism in which the basicity of the bridg-
Figure 2. Ruthenium complexes.
The scope and limitations of the base-free dehydrogenation ing nitrogen was a key parameter. Just like other bifunctional
of alcohols with Ir2 was then investigated using a range of catalysts based on pincer ligands, the bridging nitrogen would
secondary alcohols. As depicted in Figure 3, 1-phenylethanol play the role of a base abstracting the acidic proton of the
derivatives bearing an electron-donating or an electron-with- alcohol thus leading to the possible intermediate b. Subsequent
drawing group at the para-position were efficiently transformed ꢀ-hydride elimination would release the ketone product leading
into the corresponding alcohols (Figure 3, 2a–2d) whereas to c and hydrogen extrusion would then regenerate Ir2. An-
steric hindrance at the ortho-position was problematic only other mechanism involving the decoordination of one of the
with an electron-withdrawing substituent. Steric hindrance on pyridine rings, which would act as a basic center, is also con-
the aliphatic side chain also showed some limitations upon ceivable. Further experimental and theoretical studies will be
moving from an isopropyl to a very bulky tert-butyl substituent necessary to shed light on the mechanism.
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Figure 4. Proposed mechanism.
Conclusions
We have implemented an iridium catalyst for the acceptorless
and base-free dehydrogenation of alcohols. This catalyst dis-
plays high efficiency in a number of alcohol dehydrogenation
reactions performed under experimental conditions similar to
those reported until now. However, by contrast to many of the
recently reported catalysts based on complex ligand architec-
tures, this catalyst is easily prepared from accessible com-
pounds. It is noteworthy that the nature of dipyridylamine li-
gand, particularly the substitution pattern at the bridging nitro-
gen, turned to be a key parameter, as previously observed in
the dehydrogenation of formic acid. We will take advantage of
the tunability of the dipyridylamine ligand to further improve
the catalyst performances in organic media and in water.
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Acknowledgments
The authors acknowledge the China Scholarship Council for
grants to S. W. and L. G.
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Keywords: Dehydrogenation · Alcohols · Homogeneous
catalysis · Iridium · Reaction mechanisms
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Received: April 28, 2020
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Full Paper
doi.org/10.1002/ejoc.202000584
EurJOC
European Journal of Organic Chemistry
Dehydrogenation of Alcohols
The acceptorless and base-free de-
hydrogenation of secondary alcohols
is achieved with an iridium(III) catalyst
bearing a dipyridylamine ligand.
H. Jayaprakash, L. Guo, S. Wang,
C. Bruneau, C. Fischmeister* ........... 1–6
Acceptorless and Base-Free De-
hydrogenation of Alcohols Medi-
ated by
a
Dipyridylamine-Irid-
ium(III) Catalyst
doi.org/10.1002/ejoc.202000584
Eur. J. Org. Chem. 0000, 0–0
www.eurjoc.org
6
© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim