Cp*CoIII-Catalyzed Efficient Dehydrogenation of Secondary Alcohols

Article abstract of DOI:10.1002/asia.201800697

A novel, well-defined molecular Cp*Co^III complex was isolated and structurally characterized for the first time. The efficiency of this cobalt catalyst was demonstrated in the alcohol dehydrogenation and dehydrative coupling of secondary alcohols under mild conditions into ketones and ethers, respectively.

Full text of DOI:10.1002/asia.201800697

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Cp*Co(III)-catalyzed Efficient Dehydrogenation of Secondary
Alcohols
Authors: Manoj Kumar Gangwar, Pardeep Dahiya, Balakumar
Emayavaramban, and Basker Sundararaju
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10.1002/asia.201800697
Chemistry - An Asian Journal
COMMUNICATION
Cp*Co(III)-catalyzed Efficient Dehydrogenation of Secondary
Alcohols
Manoj Kumar Gangwar, Pardeep Dahiya, Balakumar Emayavaramban and Basker Sundararaju*
Dedication ((optional))
Table 1. Reaction optimization and control experiments.
Abstract: A novel, well-defined molecular Cp*Co(III) complex was
isolated and structurally characterized for the first time. The
efficiency of this cobalt catalyst was demonstrated in alcohol
dehydrogenation and dehydrative coupling of secondary alcohols
under mild conditions into ketones and ethers, respectively.
Hydrogen is considered as a clean alternative source of
energy for the unavoidable depletion of fossil fuels.[1]
Acceptorless dehydrogenation of oxidized hydrocarbons such as
alcohols derived from biomass is of great interest in connection
with the production of hydrogen, and in this respect alcohols can
be regarded as hydrogen reservoirs.[2] In this regard, transition
metal catalyzed alcohol dehydrogenation into carbonyl
compounds has been intensively explored in the recent years
and has led to applications in petrochemicals, fine chemicals
and pharmaceutical industry.[3],[4] Noble-metal catalysts have
been used for such transformations and several efficient
catalytic systems have been reported earlier.[5] The scarcity of
existence of these metals in future is a cause of concern by
considering extensive use of noble metals for several
applications and its low-abundance. Hence, it may be
appropriate to look for substitute metal catalysts that are earth-
abundant, low-cost and environmental friendly. First row
transition metals such as Fe, Co and Mn could in principle be
utilized for this thermodynamically uphill process,[6-8] which is
largely explored using noble metals such as Ru, Rh and Ir.[9]
Among the 2nd and 3rd row late-transition metals, Cp*Ir(III)
Entry[a]
[Co] (3 mol %)
L (3 mol %)
Base
Yield (%)[b]
1
2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
-
-
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
KOH
48
50
38
24
31
57
42
35
n.d.
39
23
66
n.d
n.d
L1
L2
L3
L4
L5
L6
L5
L5
L5
L5
L5
BL
BL
proved to be
a
simple and efficient catalyst for
3
dehydrogenation/hydrogenation reactions compared to the
complexity involved in the preparation of privileged pincer
ligand/metal complexes.[10] A large variety of reactions were
explored for the syntheses of amines, amides and heterocycles
via dehydrogenation/rehydrogenation concept (commonly called
as hydrogen borrowing process) under Cp*Ir(III) catalysis.[10]
However, the corresponding less expensive, air-stable, first
group congener Cp*Co(III)[11] was not explored until now. In
continuation of our effort to develop sustainable chemistry under
cobalt catalysis,[12] we looked for catalytic activities of novel
Cp*Co(III) catalysts involving hydrogen transfer processes.
We began our investigation for alcohol dehydrogenation of
secondary alcohol into ketone 2a in the presence of 3 mol% of
Cp*Co(CO)I2 along with stoichiometric amount of base in
acetone (0.1 M) at 80 °C for 20 h.
4
5
6
7
8
9
K2CO3
tBuOK
tBuOK
tBuOK
10[c]
11[d]
12[e]
13
14
tBuOK
[a]
Dr. M. K. Gangwar, P, Dahiya, B. Emayavaramban and Prof. B.
Sundararaju
-
Cp*Co(CO)I2
Fine Chemicla Laboratory,
Department of Chemistry,
[a]
Standard reaction conditons:
Alcohol 1 (0.2 mmol), [Co] (3 mol%), L (3
Indian Institute of Technology Kanpur, Kanpur,
Uttar Pradesh, India – 208 016.
E-mail: basker@iitk.ac.in
mol%), tBuOK (0.3 mmol) in acetone (0.1 M) under Argon at 80 °C for 20 h. [b]
Isolated Yield. [c] 30 mol% of tBuOK was used. [d] 10 mol% of tBuOK was used.
[e]5 mol% each of [Co] and L5 was used and the reaction carried out for 24 h.
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The expected ketone was isolated in 48% yield. Encouraged by
this result, we further investigated various ligands as additive to
increase the rate of the reaction. Similar or even lower product
formation was observed when Ligand L1-L4 were screened
(Table1, entries 2-5). Further screening other ligands led to the
identification of 8-hydroxyquinoline as the best ligand affording
the expected ketone in 57% yield (entry 6-7). Next, we
t
examined the effect of base by changing BuOK by KOH and
K2CO3. Unfortunately, both of them were inefficient under our
standard conditions (entry 8-9). Further, decreasing the amount
of base was detrimental to the ketone formation (entry 10-11).
Next, we increased the catalyst loading from 3 to 5 mol% and
extended the reaction time up to 24 h (entry 12). To our delight,
the ketone 2 was isolated in 66% yield. Finally, we have
performed the control experiments to realize the need of both
cobalt catalyst and base, which results in no product formation
(entry 13-14).
With the best conditions in hand, we next aimed at isolating
the in situ formed complex by mixing stoichiometric amount of
8-hydroxyquinoline and Cp*Co(CO)I2 in the presence of base in
dichloromethane at room temperature for 24 h (scheme 1). The
expected cobalt(III) complex A was isolated after column
chromatography in 96% yield as
a green powder. This
diamagnetic complex was further confirmed by NMR, ESI-MS
and elemental analysis. To obtain solid-state crystal structure,
single crystals were grown by vapour diffusion of hexane into
concentrated dichloromethane solution.[13] The N1-Co-O1 angle
for five-membered metallacycle A is about 84.23°, which is
slightly larger as compared to reported N1-M-O1 angles of
78.40° (M= Rh) and 77.8 (M= Ir).[14] The Co-O bond and Co-N
bonds are 1.956 and 1.947 Å much shorter than those reported
for Rh-O/Ir-O bonds and Rh-N/Ir-N bonds.[14] The isolated
complex A shows the methyl proton of the Cp* further highfield δ
1.61 compared to Cp*Co(CO)I2 for which the Cp* resonance
appears at δ 2.23. The crystal structure further indicates that
the isolated cobalt complex
configuration.
A adopt 3-leg piano stool
Scheme 2. Scope of Alcohol
Then, we examined the reactivity of complex
A
for
dehydrogenation of secondary alcohols. To our delight, 1 mol%
of the well-defined cobalt complex A under standard conditions
afforded the acetophenone 2a in 94% yield after 24 h (Scheme
2, 2a). However, when the reaction was performed with
equimolar amount of base resulted in 71% of 2a suggesting that
the slight excess of base is required. Aromatic secondary
alcohols containing electron-donating groups such as Me-, OMe-
and free NH2- groups at the para-position were tolerated and
provided the corresponding para-substituted acetophenones in
excellent yields (2b-2d).
Aromatic alcohols containing an
electron-withdrawing group such as F-, Cl- and Br- were also
suitable for dehydrogenation and afforded ketone 2e-2g in good
yields. Further, various substituents at the meta-position were
equally tolerated, affording the dehydrogenative products in
Scheme 1. (a) Synthesis of cobalt complex A. (b) X-ray crystal structure of A
(50% probability of thermal ellipsoids; H atoms omitted for clarity.
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good yields (2h-j).
Even more sterically hindered alcohols
and sterically hindered ortho- position) were employed at room
temperature and the corresponding ethers were isolated in
moderate to good yield (3a-3j). To our delight, the scope can be
extended to other alcohols to obtain unsymmetrical ethers in
good yields (3k-3p). It is noteworthy that this reaction required
the presence of AgSbF6 (5 mol%) as iodide abstractor to
generate the active cationic cobalt species.[18]
In conclusion, we have developed an efficient, air-stable,
phosphine/privileged pincer ligand-free cobalt catalyst for
alcohol dehydrogenation under mild conditions. Novel, air-
stable Cp*Co(N,O)I (A) was isolated for the first time and the
efficiency of this novel catalyst was demonstrated for the
synthesis of ketones directly from secondary alcohols. We have
also established the use of Co(III) complex A for dehydrative
etherification at room temperature to access various symmetrical
ethers. Exploration of Cp*Co(III)-based complexes for other
applications using hydrogen borrowing strategy is currently
underway in our laboratory.
having substituents at the ortho-positions provided excellent
yields (2k-m). To our surprise, heteroaromatic secondary
alcohols, which often poison the catalyst by strong coordination
to the metal via heteroatom, were reactive providing the ketones
(2n-2p) in satisfactory yields. Finally, we have tested the
challenging aliphatic secondary alcohols for dehydrogenation. It
was found that cyclopentanone, cyclohexanone and
cycloheptanone were obtained in moderate yield (2q-s).
Acknowledgements
Financial support provided by CEFIPRA (5805-1) to support this
research work is gratefully acknowledged. MKG thanks to
SERB for National postdoctoral fellowship. PD and EB thanks to
CSIR and IITK for their fellowship. PK Kelkar young faculty
award to BS is gratefuly acknowledged.
Keywords: Dehydrogenation • Cobalt • alcohol • Ketones •
Ethers
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COMMUNICATION
M. K. Gangwar, P. Dahiya, B.
Emayavaramban, B. Sundararaju*
Page No. – Page No.
Cp*Co(III)-catalyzed Efficient
dehydrogenation of Secondary
Alcohols
A well-defined, air-stable Cp*Co(N,O)I complex was isolated for the first time and
demonstrated its activity for an efficient dehydrogenation of alcohols into ketones
and ethers under mild conditions.
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