A cyclometalated Ir(iii)-NHC complex as a recyclable catalyst for acceptorless dehydrogenation of alcohols to carboxylic acids

Article abstract of DOI:10.1039/d0dt02341h

In this work, we have synthesized two new [C, C] cyclometalated Ir(iii)-NHC complexes, [IrCp?(C∧C:NHC)Br](1a,b), [Cp? = pentamethylcyclopentadienyl; NHC = (2-flurobenzyl)-1-(4-methoxyphenyl)-1H-imidazoline-2-ylidene (a); (2-flurobenzyl)-1-(4-formylphenyl)-1H-imidazoline-2-ylidene (b)] via intramolecular C-H bond activation. The molecular structure of complex 1a was determined by X-ray single crystal analysis. The catalytic potentials of the complexes were explored for acceptorless dehydrogenation of alcohols to carboxylic acids with concomitant hydrogen gas evolution. Under similar experimental conditions, complex 1a was found to be slightly more efficient than complex 1b. Using 0.1 mol% of complex 1a, good-to-excellent yields of carboxylic acids/carboxylates have been obtained for a wide range of alcohols, both aliphatic and aromatic, including those involving heterocycles, in a short reaction time with a low loading of catalyst. Remarkably, our method can produce benzoic acid from benzyl alcohol on a gram scale with a catalyst-to-substrate ratio as low as 1?:?5000 and exhibit a TON of 4550. Furthermore, the catalyst could be recycled at least three times without losing its activity. A mechanism has been proposed based on controlled experiments and in situ NMR study.

Full text of DOI:10.1039/d0dt02341h

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14 March 2017
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DOI: 10.1039/D0DT02341H
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Cyclometalated Ir(III)-NHC complex as recyclable catalyst for acceptorless
dehydrogenation of alcohols to carboxylic acids
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Dhrubajit Borah,a,b Biswajit Saha,c,d Bipul Sarma,e Pankaj Das*a
a Department of Chemistry, Dibrugarh University, Dibrugarh, Assam, India, 786004.
b Department of Chemistry, N. N. Saikia College, Titabar, Assam, India, 785630.
c Advanced Materials Group, Materials Sciences and Technology Division, CSIR-North East
Institute of Science and Technology, Jorhat 785006 Assam, India
d Academy of Scientific and Innovative Research (AcSIR), CSIR-NEIST Campus, Jorhat
785006, India
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e Department of Chemical Sciences, Tezpur University, Napaam, Tezpur, Assam, India,
784028.
*Corresponding author E-mail address: pankajdas@dibru.ac.in.
Abstract
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In this work, we have synthesized two new [C,C] cyclometalated Ir(III)-NHC complexes,
[IrCp*(CC:NHC)Br](1a,b), [Cp* = pentamethylcyclopentadienyl; NHC = (2flurobenzyl)
1(4methoxyphenyl)1Himidazoline2ylidene (a); (2flurobenzyl)1(4formylphe-
nyl)1Himidazoline2ylidene) (b)] via intramolecular C-H bond activation. The
molecular structure of the complex 1a was determined by Xray single crystal analysis. The
catalytic potentials of the complexes were explored for acceptorless dehydrogenation of
alcohols to carboxylic acids with concomitant hydrogen gas evolution. Under similar
experimental conditions, the complex 1a was found to be slightly more efficient than the
complex 1b. Using 0.1 mol% of complex 1a, good-to-excellent yields of carboxylic
acids/carboxylates have been obtained for a wide range of alcohols, both aliphatic and
aromatic including those involved heterocycles, in a short reaction time with a low loading of
catalyst. Remarkably, our method can produce benzoic acid from benzyl alcohol in gram
scale with a catalysttosubstrate ratio as low as 1:5000 and exhibiting a TON of 4550.
Furthermore, the catalyst could be recycled at least three times without losing its activity. A
mechanism has been proposed based on controlled experiments and in situ NMR study.
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Keywords: Cyclometalated complex; intramolecular C-H bond activation; N-heterocyclic
carbene; iridium(III); acceptorless dehydrogenation; recyclable catalyst
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Introduction
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Oxidantfree catalytic oxidation of alcohols to carbonyl compounds with concomitant
hydrogen gas evolution, socalled acceptorless dehydrogenation reaction, has recently been
emerged as a highly attractive process in organic synthesis.1 This approach of hydrogen
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generation from an organic substrate is not only an atom economical and enviDroOIn: 1m0.1e0n39ta/Dl0lyDT02341H
benign method to access carbonyl compounds but also has the potential to be exploited in
“Hydrogen economy” as liquid hydrogen storage material.1b,2 Though alcohols can be easily
dehydrogenated to corresponding aldehydes or ketones with the help of various noble metal
catalyst,3direct conversion of alcohols to carboxylic acids/carboxylates via dehydrogenative
transformation without using any oxidant has got relatively less attention.The reaction was first
reported by Milstein's group in 2013 using a pincerbased rutheniumPNN catalyst.4 Since
then, several Ru-based systems have been reported that contained tridentate PNP5, CNP6 or
NNN3g,m,7 type pincerligands or N, N8- / NHC9- based nonpincer ligands. Although
remarkable advancements have been seen in terms of product yields or substratescope
improvements, in most cases, the catalytic systems suffer from crucial disadvantages like
excessive metal loadings (up to 5 mol%),8,9c low turnover numbers (TON),8,9c prolonged
reaction times (40h),9d nonrecyclable nature, etc. Nevertheless, recent works by the groups
of Madsen (1 mol%, 6h),10 Szymczak (0.2 mol%, 18h),11 Li12(10.002 mol%; 24h) and
others13 showed some improvements over catalyst loading or TON, however, in almost all
the cases, catalytic systems used noninnocent phosphines, either as a principal ligand
system6 or as a supporting ligand along with other phosphinefree systems like NHC,10,12
bipyridine,13,14 etc. It is noteworthy to mention that from economical and environmental
perspectives, phosphines are no longer considered as the most desired ligand system in
organometallic chemistry because of their inherent toxicity, air, and moisture sensitive
property, handling inconvenience, and synthetic inaccessibility,15 etc. Thus, it is extremely
important to design novel recyclable phosphinefree catalytic systems that could exhibit
superior activity for acceptorless dehydrogenation of alcohols to acids with a low loading of
catalyst.
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In the past few years, Nheterocyclic carbenes (NHCs) has emerged as a highly sustainable
alternative to traditional phosphine ligands in organometallic catalysis.16,17c Like phosphines,
the stereoelectronic environment of NHC ligands can be easily tuned by changing the
pendant groups attached to the imidazole moiety.17 One unique advantage of using NHCs as
ligands is that due to their strong σdonors and poor πacceptors properties, they often
facilitate intramolecular CH bond activation leading to highly stable cyclometalated
complex.18 Recently, cyclometalated NHCbased IrIII systems have received enormous
attentions because of their applications in diverse fields that include organometallic
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catalysis,19 catalysts for water oxidation,20 as chemosensors materials,17c as photophyVsieiwcaArlticle Online
DOI: 10.1039/D0DT02341H
devices in organic light-emitting diodes,21 as anticancer agents,22 etc.
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Na+
-
SO3
TfO
O
(OTf)2
Me2N
N
Ir
P
OH2
OH2
N
Ir
Br
N
R
OH
Ir
F
Ir
O
N
N
N
N
N
N
N
Ir(CO)2
NMe2
Fujita 2018
R= OCH3 , CHO
Fujita 2017
> 80% yield
~ 40 TON,20h
Williams 2018
> 98% yield
~800 TON,20h
Toluene, KOH
Kostal 2018
92% Yield
This work
> 94% yield
> 95% yield
~ 980 TON, 4h.
Toluene, KOH
~ 376 TON ,18 h
Water, NaOH
293249 TON(mw, 8h)
Gly/water, KOH
Water, without base
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Fig. 1 Example of iridium comlexes employed in acceptorless dehydrogenation of alcohols
to acids.
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In 2012, for the first time, Fukuzumi’s group has reported catalytic evolution of hydrogen
from aliphatic alcohols to afford aldehyde or ketone using a [C, C] cyclometalated Ir (III)
catalyst.23 Since then, several cyclometalated Ir(III) systems were reported that served as
effective catalysts for various types of dehydrogenative reactions to afford aldehydes and
ketones14, amines or imines, and dehydrogenation of formic acid2a,19e,24. However, to the best
of our knowledge, [C, C] cyclometalated Ir(III)NHC systems has not been explored so far
for direct conversion of alcohols to carboxylic acids. Thus, intending to develop robust
phosphinefree catalytic system for acceptorless dehydrogenations of alcohols to carboxylic
acids, herein, we have synthesized two new cyclometalated IrIIINHC complexes and
explored their potentials as catalysts. It may be worth to mention that until now only a
handful of iridium−based systems are reported for direct dehydrogenations of alcohols to
carboxylic acids including conversion of glycerol to lactic acid.9c,9d,25 Among them, a
dicationic Ir(III) complex9ccontaining a functional NHC ligand reported by Fujita [Fig. 1] is
the most significant as it can produce carboxylic acids from alcohols using water as a solvent
under a base-free environment, however the protocols showed efficiency only with benzyl
alcohols with a relatively high catalyst loading (2-5 mol%). The catalyst loading as well as
the substrate scope were significantly improved using a monocationic Ir(I) system9d by
Williams [Fig. 1] in the presence of stoichiometric quantity of KOH. Although, this method
provides aliphatic and benzylic carboxylates in high yield with a catalyst loading as low as
0.2 mol%, however the use of phosphine-based ligand was its principal limitation.Hence,
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from the practical viewpoint, our system is advantageous as we could produce carboxylic
DOI: 10.1039/D0DT02341H
acids from alcohols using a recyclable phosphine-free system with a low loading of catalyst
(0.1 mol%) for a wide range of substrates. Interestingly, this is the first example of a [C, C]
cyclometalated IrIIINHC system explored for this transformation.
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Results and Discussion
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Synthesis of cyclometalated iridium complexes
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The ligand precursor L1 was recently reported by us.26 The same procedure was adopted for
synthesizing L2 by reacting 1(4formylphenyl)1Himidazole with 10 equivalent of
2fluoro benzyl bromide in CH3CN for 48 h. The formation of L2 was confirmed by ESIMS
spectrum which showed a base peak at m/z =281.106 corresponds to [MBr]+ ion. The
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1
characteristics imidazolium proton (NCHN) resonance in the H NMR spectrum was
observed at 10.25 ppm. In the 13C NMR spectrum, a signal appeared at 139.04 ppm attributed
to NCN carbon.27 The complexes (1a and 1b) were synthesized by in situ deprotonation of
imidazolium salts (L1 and L2) in CH3CN by treating a mixture of Cs2CO3, NaOAc and
[Cp*IrCl2]2 in presence of excess potassium bromide (Scheme 1).19g The formation of the
complexes 1a and 1b was confirmed by ESIHRMS spectra which rendered base peaks at
m/z 609.186 and 607.258 respectively corresponds to [MBr]+ ions. Successful coordination
of the NHC ligands to the iridium centers was confirmed by the disappearances of the NCHN
protons in the 1H NMR spectroscopy. The signal corresponds to the methyl groups of the Cp*
ligands appeared at around 1.84 ppm. The 13C resonances of the IrCNHC carbons of the
complexes 1a and 1b appeared at 164.01 and 167.57 ppm respectively and the values are in
good agreement with reported IrNHC complexes.28 The resonances due to orthometalated
carbon (IrCPh) appeared at 142.79 and 141.83 ppm for the complexes 1a and 1b
respectively.
Crystal Structure analysis
Suitable single crystals of the complex 1a were grown for X-ray studies by layering a
saturated dichloromethane solution with hexane. Unfortunately, after repeated attempts, good
quality single crystals could not be generated for the complex 1b.The ORTEP of the complex
1a is displayed in Fig. 2 which confirms the expected connectivity pattern. Crystallographic
parameters are documented in Table S1 in the Supporting Information. The iridium center of
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the complex 1a adopts typical piano stool geometry where three “legs” of the piano stool are
DOI: 10.1039/D0DT02341H
comprised of Nheterocyclic carbene carbon, carbon from adjacent phenyl ring and a
bromide. Residual Cp ligand is connected to the metal center in 5 fashion. The bond
distance of IrCNHC is 2.04 (1) Å falls within the expected range.28 The IrCPh (2.08(1) Å)
bond length is found to be slightly longer compared to the IrCNHC bond length. The
cyclometalated fivemember ring Ir(1)C(28)N(1)C(4)C(29) of the complex 1a is almost
planar and the dihedral angle between the two planes of C(29)Ir(1)C(28) and
C(28)N(1)C(4)C(29) is found to be 1.61.The planarity of the cyclometalated ring is
further confirmed by observing the fact that the sum of the inner angles of the fivemember
ring is approximately 540.
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Br-
Ir
F
R
F
R
Br
N
Cs2CO3, NaOAc, Excess KBr
N
N
N
[IrCp*Cl2]2, CH3CN
Reflux, 10h
1a
1b
, R= -OCH3
, R= -CHO
L1,
L2,
R= -OCH3
R= -CHO
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Scheme 1 Synthesis of Cyclometalated NHC IrIII Complexes 1a and 1b
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Fig. 2 ORTEP plot of the complex 1a (50% probability thermal ellipsoid). Hydrogen atoms
have been omitted for clarity. Selected bond distances (Å) and angles (deg): Ir1 Br1
2.527(2), Ir1C28 2.04(1), Ir1C29 2.08(1), C(28)N1 1.38(1), C(4)N(1) 1.44(2),
C(4)C(29) 1.37(2), Br1Ir1C28 88.6(3), Br1Ir1C29 91.0(3), C28Ir1C29 76.9(4),
N1C28N2 104.3(9).
Catalytic studies
Optimization of reaction variables
To investigate the efficacies of the Ir(III) complexes 1a and 1b as catalysts for acceptorless
dehydrogenation of alcohols to acids, benzyl alcohol was chosen as a test substrate. A model
reaction was carried out with benzyl alcohol (1mmol) using complex 1a (1mol %) as a
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catalyst, in presence of KOH (1.1mmol) as a base in refluxing toluene with a reaction time of
DOI: 10.1039/D0DT02341H
24 h. To our delight, benzyl alcohol was effectively converted to potassium benzoate in
100% yield, accompanied by the evolution of hydrogen gas as a byproduct (Table 1, entry 1).
Almost quantitative formation of benzoic acid was achieved by treating the benzoate salt with
hydrochloric acid. However, the yield drops to 82% when the same reaction was carried out
using complex 1b as catalyst (entry 16).The higher catalytic performance of the complex 1a
compared to 1b could be attributed to more electron donating ability of the ligand a over b.9c
Moreover, the presence of a formyl group at the para position in one of the phenyl groups
attached to N- atom in the complex 1b is expected to make the complex relatively unstable
under our catalytic condition. To check the stability of the complex 1b, a reaction was carried
out with complex 1b under our standard conditions in absence of benzyl alcohol for 24h and
the reaction mixture was analyzed by HR-MS spectrometry which showed an intense peak at
607.17 corresponding to the fragment [M-Br]+ with the isotopic pattern in a perfect match
with the original compound 1b (ESI, Fig S 25). This observation clearly suggests that ligand
site of the complex remains intact during the course of reaction. Although, initially, the
model reaction with the complex 1a was continued for 24 h, a timedependent conversion
study reveals that the reaction gets completed within 4 h (entry 3, time profile conversion
graph, ESI Fig S24). Moreover, the decrease in catalyst loadings from 1 mol% to 0.1 mol%
does not have any impact on the yield (95 %, entries 3 and 5). It is noteworthy to highlight
that under comparable conditions, William’s catalyst (Fig.1) required a double amount of
catalyst (0.2 mol%) along with 10 fold increase in reaction time (40 h) to achieve similar
conversion, suggesting the superiority of our system over the reported one. A temperature
optimization study with the complex 1a reveals that a refluxing condition is indispensable for
the smooth performance of the catalyst. An attempt to decrease the reaction temperature from
1100C to 800C significantly slow down the process and only 65% yield was obtained (entry
6). Further decrease in temperature from 800C to 500C resulted in a complete cessation of the
reaction and no desired carboxylic acid was obtained (entry 7). Lower conversion of benzyl
alcohol was also observed when the base was replaced from KOH to NaOH (entry 8) and the
reaction completely terminated in presence of weak bases such as K2CO3 and Cs2CO3 (entries
9 & 10). Not only that, the amount of base has also had some impact in our catalytic system
as decreasing the quantity of KOH from 1.1 mmol to 0.75 mmol resulted in a sharp drop in
activity (entry 15). It may be pertinent to mention here that there are literature precedents
available where bases can also promote a transfer dehydrogenation reaction with alcohol.29
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However, in our system, such a possibility would be extremely remote as we have not VuiesweAdrticle Online
DOI: 10.1039/D0DT02341H
any acceptor and it is very unlikely that toluene would serve as a hydrogen acceptor.30
After the initial optimizations of bases, temperatures, and catalyst loadings, the influence of
various solvents on acceptorless dehydrogenation of alcohols has been studied. The reaction
also proceeded well in a nonpolar solvent like xylene and afforded 80% yield ( entry 11)
while polar solvents like water, THF, isopropanol are not effective and delivered much lower
yields (entries 1214). Therefore, the optimization condition of the dehydrogenation reaction
is found to be 0.1% of 1a, 1.1 equivalent of KOH, toluene and 1100C (bath temperature,
120C). Noteworthy to mention that under similar experimental conditions, the parent
complex [Ir(Cp*)Cl2]2 gave only 35% benzoic acid (entry 19) indicating the predominant
role played by the cyclometalated NHC ligand in the complex 1a or 1b.
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Reaction scopes
With this optimized condition, we have attempted to extend the scope of our catalytic system
for a broad range of primary alcohols. Both benzyl (Table 2, Entries 113) and aliphatic
alcohols (without branched chains) (entries 16 and 17) can be easily converted to their
corresponding carboxylate salts in high yields. Under optimal conditions, the sterically
hindered 2- methylbutanol was found to be inactive for the reaction (entry 18). In general,
aliphatic alcohols required a longer reaction time than benzyl alcohols (entries 16 and 17).
Benzyl alcohols bearing electron−donating or withdrawing groups at the meta or para
positions furnished corresponding benzoic acids (entries 28) in high yields, however a nitro
group at para position provided only moderate yield (46%, entry 9) accompanied with 4-
nitrobenzaldehyde as a side product (ESI, Fig. S44-45). In fact, the incompetency of
nitrosubstituted compounds in alcohol dehydrogenation reactions has also been noticed
24 earlier.9d Orthosubstituted alcohols such as 2-methylbenzyl alcohol and 2-chlorobenzyl
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alcohol were also amenable for acceptorless dehydrogenation reactions to give corresponding
acids in high yields (Entries 10 & 11). In contrast, alcohol bearing coordinating functional
groups at ortho positions such as 2-amino and 2-methoxybenzyl alcohols gave the expected
products in moderate yields i.e.54 and 36 % respectively. (entries 12 & 13). In addition; our
catalyst can convert highly challenging alcohols that contain aromatic heterocycles like furan
and pyridine to their corresponding acids in high yields (entries 14 and 15). It needs to
mention that all the carboxylic acids presented in Table 2 were precipitated as potassium salts
in refluxing toluene during the reaction and hence were easily isolated by filtration.
Potassium carboxylates have been transformed into corresponding acids by treatment of
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aqueous HCl and the products were isolated by extraction with ethyl acetate and subsequent
DOI: 10.1039/D0DT02341H
removal of solvents under vacuum. This gave pure products for analysis and did not require
further purification. The filtrate contains the catalyst which is further used for fresh catalytic
cycles.
Table 1. Optimization of reaction variables for acceptorless dehydrogenation of benzyl
alcohola
OH
COOH
Catalyst, base, solvent
2 H2
+
Temperature, then HCl
Temperature
Catalyst
(mol%)
Isolated
Entry
Solvent
Base
Time (h)
yield (%)
(C)
110
110
110
110
110
80
1
2
3
4
5
6
7
1a (1)
1a (1)
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
KOH
KOH
KOH
KOH
KOH
KOH
KOH
24
2
95(100)
80 ( 83)
94(100)
95
1a (1)
4
1a (0.5)
1a (0.1)
1a (0.1)
1a (0.1)
4
4
95
4
65
50
4
00
8
9
1a (0.1)
1a (0.1)
1a (0.1)
1a (0.1)
Toluene
Toluene
Toluene
Xylene
NaOH
K2CO3
Cs2CO3
KOH
110
110
110
110
4
4
4
4
75
Traced
Traced
80
10
11
12
13
14
15
16
17
18
1a (0.1)
1a (0.1)
1a (0.1)
1a(0.1)
1b (1)
Isopropranol
THF
KOH
KOH
KOH
KOH
KOH
KOH
KOH
82
4
4
20
10
32
67b
82
81
70
66
Water
110
110
110
110
110
4
Toluene
Toluene
Toluene
Toluene
4
24
24
4
1b (0.1)
1b ( 0.1)
[IrCpCl2]2
19
20
Toluene
Toluene
KOH
KOH
8
110
110
4
4
35(42)
(0.1)
--
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aReaction Conditions: alcohol (1mmol), catalyst (10.1 mol %), base (1.1mmol), DsOoIl:v10e.1n0t3s9/(D100DT02341H
mL) in the N2 atmosphere. Values within parenthesis indicate conversion of benzyl alcohol
to potassium benzoate determined by 1H NMR spectroscopy.b KOH (0.75 mmol)
1
2
Table 2. Acceptorless Dehydrogenation of Alcohols to Acidsa using complex 1a as a catalyst
1a
(0.1 mol%), KOH (1.1 eqv)
1.
O
O
Toluene, reflux,
+ 2H2
Or
R
OK
R
OH
R
OH
2. HCl, H2O
Entry
1
Substrate
Product
Isolated yield (%)
O
OH
OH
94
O
2
3
OH
OH
80
F
F
O
OH
OH
76
87
F3C
F3C
O
4
5
6
OH
OH
O
OH
OH
Cl
85
81
Cl
O
OH
OH
H3CO
H3CO
H2N
COOH
O
OH
7
8
H2N
67
72
H3CO
H3CO
OH
H3CO
H3CO
OH
OCH3
OCH3
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O
OH
9
OH
O2N
46
84
O2N
O
10
11
12
OH
OH
OH
OH
O
OH
Cl
Cl
83
54
O
OH
NH2
NH2
O
13
OH
OCH3
OH
OCH3
36
80
14
15
OH
OH
O
N
O
N
O
OK
OH
OH
71
82
O
COOK
16
17
18
COOK
75
00
OH
COOK
OH
aReaction Conditions: alcohol (2 mmol), KOH (2.2 mmol),1a (0.1 mol%), toluene (10 mL),
reflux, 415 h, (entries 1-6, 8, 10, 11, 13 ; 4h, entries 7, 9, 12, 14, 18; 10h, entries 16-17,
15h.) then aq HCl.
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The catalyst can be reused at least three times without a discernible decrease in the product
yield (94%, 92% and 92%, ESI Fig S65). It is interesting to note that the acceptorless
dehydrogenation reaction with our catalyst can be scaled up easily. Benzyl alcohol could be
smoothly converted to benzoic acid in 91% yield in a gram scale experiment with a substrate
to catalyst ratio of 5000:1 and a TON of 4550 has been achieved.
Reaction mechanism
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A series of controlled experiments were carried out to gain mechanistic insightsDoOfI: 1r0e.1a0c3Vt9iie/oDw0nADr.tiTc0le2O3n4l1inHe
At first, a mercury poisoning test was conducted by reacting benzyl alcohol (2 mmol) with
KOH (2.2 mmol) in presence of catalyst 1a (0.1 mol%) and Hg( 10 mmol) at 110C. After 4h
of reaction, 80% yield of benzoic acid was obtained which suggests metallic Hg has virtually
no impact on our catalytic system demonstrating that the reaction proceeds with molecular
intermediate.
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Then, to garner evidence on the evolution of hydrogen gas during dehydrogenation of
alcohols to acids, we have designed a hydrogenation experiment using styrene as a substrate
in presence of Pd/C catalyst without using any external hydrogen source. The reaction was
carried out in a closed vessel using benzyl alcohol (1 mmol), KOH (1.1 mmol), complex 1a
(0.1 mol %), styrene (1mmol), toluene and Pd/C (5 mol %). After 4h of refluxing, it has been
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observed by H NMR that 89% of styrene was converted to ethylbenzene. Based on this
outcome, we conclude that hydrogen gas was generated during the transformation of benzyl
alcohol to potassium benzoate. To measure the amount of hydrogen evolved, the reaction
vessel was connected with a gas burette filled with water. It has been observed that when 0.5
mmol of benzyl alcohol was allowed to react, a total gas volume of 23.5 mL (approx. 0.96
mmol) was obtained which is consistent with the release of 2 equivalents of hydrogen.10,12
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As we know that the catalytic dehydrogenation of alcohols to corresponding acids can
proceed via an aldehyde or ester intermediate and therefore, to distinguish between these two
pathways, we have conducted two experiments. Firstly, benzyl alcohol was reacted with
KOH in presence of catalyst 1a at 110C (bath temperature 120C) without using any solvent
and after 4h of reactions, benzaldehyde was observed as the only product by GC and no
corresponding ester was formed.9a Similarly, when the same reaction was carried out using
toluene as solvent at 50C, only benzaldehyde without any ester formation was observed
indicating that ester is the less likely intermediate and the reaction might proceed through a
Cannizzaro pathway.9d To examine this, two parallel experiments were conducted with or
without the catalyst 1a taking benzaldehyde as a substrate under similar experimental
condition for a reaction time of 1h (Scheme 2). The reaction in presence of the catalyst 1a
produces 88% benzoic acid, while in absence of catalyst produces only 56%.These results
suggest that both the Cannizzaro and catalytic dehydrogenation processes are taking place in
the case of benzylic alcohol derivatives
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1
KOH ( 1.1 equiv ), Toluene ( 10 mL)
1100 C,(bath temperature,120C), 1h
O
OH
OH
+
2
3
O
42%
56%
H
1
(0.1 mol % )
KOH ( 1.1 equiv ), Toluene ( 10 mL)
1100 C,(bath temperature,120C), 1h
4
O
OH
OH
+
5
88%
9%
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7
Scheme 2 Cannizzaro and dehydrogenation reactions of benzaldehyde.
8
9
Further, to assess whether the F atom present in the wingtip of the imidazole moiety of the complex
1a has any role in the catalytic pathway, we synthesized an analogous complex without F atom by the
reaction of 3-benzyl-1-(4-methoxyphenyl)-1H-imidazolium bromide(See ESI Fig S19-S23) with
[Ir(Cp*)Cl2] following a similar experimental procedure as reported for 1a or 1b and performed the
dehydrogenation reaction of benzyl alcohols by using 1c (0.1 mol%) as a catalyst. Under optimal
conditions, the performance of the catalyst 1c is more or less comparable with the catalyst 1a (85% vs
94%) which suggests that F atom attached with wingtip of imidazole moiety in the complex 1a may
not play a significant role in catalytic activity of the complex 1a in the dehydrogenation reaction.
Moreover, it may be worthy to note that, there are instances where Cp* ligand dissociated
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from a metal center during catalytic conditions.31 However, to investigate this, we have
recorded LC-MS of the complex 1a isolated from a post catalytic reaction mixture, and we
observed a strong peak at m/z 609.42 which is very close to the [ M-Br]+ peak of the original
complex 1a. These results, along with almost no loss in activity on recycling the catalyst
suggest, although not conclusively, that the complexes remains intact during our catalytic
conditions and Cp* may not get dissociated.
To get more rationality about the mechanism, we have examined the catalytic
1
dehydrogenation process of benzyl alcohol through an in situ H NMR. A mixture of benzyl
alcohol (1mmol), KOH (1.1 mol), and the complex 1a (0.01mmol) was dissolved in 0.8 mL
of deuterated toluene in an NMR tube and the headspace of the tube was tightly closed with
Teflon cap. The NMR tube was placed in a preheated oil bath at 50C for 10 minutes and
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then the H NMR spectrum was recorded. Indeed, a highly shielded peak was observed at
17.5 ppm which indicated the presence of an iridium hydride species ( ESI, Fig
S22).Moreover, we have performed a similar experiment taking complex 1a in deuterated
toluene (0.8 mL) in presence of KOH base at 120C for 20 minutes(under standard catalytic
conditions). The 1H NMR spectrum of the reaction displayed the characteristic Ir-H peak at -
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17.5 along with the signals for Cp* and NHC ligands(ESI, Fig S23). These experiments
suggest, although not conclusively, Ir-H is likely intermediate of the reaction.
Thus, based on control studies and literature precedents of metal-catalyzed acceptors
dehydrogenation of alcohols,9c,10,12,13 a plausible mechanism of the reaction is proposed (ESI,
Scheme 1)
Conclusions
7
8
In summary, we have synthesized and characterized two new cyclometalated Cp*Ir(III)-NHC
complexes and explored their potentials as catalysts for acceptorless dehydrogenation of
alcohols to carboxylic acids. By using one of the complexes (1a), a wide range of alcohols
could be efficiently converted to corresponding acids in high yields accompanied by the
evolution of hydrogen gas. It is interesting to note that the catalyst 1a could be easily
recycled at least three times without compromising with its activity. Besides, complex 1a is
compatible to carry out large scale synthesis of benzoic acid from benzyl alcohol.
Interestingly, this is the first example of a [C, C] cyclometalated IrIIINHC system exploited
for acceptorless dehydrogenation of alcohols to carboxylic acids.
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Acknowledgements
18 We gratefully acknowledge the financial support from the Department of Science and
19 Technology, New Delhi (Grant no: CRG/2018/001669) and UGC, New Delhi for
20 SAP-DRS grant to the Department of Chemistry, Dibrugarh University (DU). DB is
21 thankful to UGC, NERO for providing teacher’s fellowship under UGC FDP
22 programme. Authors are grateful to Mr. Bibekananda Gogoi, JRF, DST project for
23 experimental assistance respectively.
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Supporting information
1
The supporting information is available free of charge on the RSC publication website. H,
13C NMR, 19F, mass spectra of the ligands and complexes, crystallographic data of the
complex 1a; 1H and 13-NMR spectra of all the catalytic reaction product.
Experimental Section
General Information: All reactions were carried out under a nitrogen atmosphere using the
schlenk technique. All solvents were purified through the standard purification method. All
the reagents and deuterated solvents, CDCl3, (CD3)2SO, CD3OD, and D2O were purchased
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from commercial suppliers and used as received. H, 13C, and 19F NMR sDpOeIc: t10ra.103w9/eDr0eDT02341H
recorded on the Bruker AVANCE III HD500 MHz spectrometer. HRMS was measured on
Shimadzu LCMSITTOF mass spectrometer. The ligand 3-(2-flurobenzyl)-1-(4-
methoxyphenyl)-1H-imidazolium bromide (L1) was prepared as per the previously reported
procedure.26
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Synthesis of 3-(2-flurobenzyl)-1-(4-formylphenyl)-1H-imidazolium bromide (L2)
Ligand L2 was prepared by following the same procedure of L1 where
(4formylphenyl)−1H−imidazole (172 mg, 1mmol) and 2−fluorobenzyl bromide (1.89g, 10
mmol) were employed. The ligand L2 was isolated as white powder. Yield: 250 mg (69%).
1H NMR (500 MHz, DMSO-d6) δ 10.25 (s, 1H, NCHN), 10.12 (s, 1H, CHO), 8.51 (s, 1H,
Ar), 8.20 (d, J = 8.7 Hz, 2H, Ar), 8.11 – 8.07 (m, 3H, Ar), 7.64 (t, J = 7.7 Hz, 1H, CH = CH,
imidazole), 7.51 (q, J = 7.4 Hz, 1H, Ar), 7.37 – 7.28 (m, 2H,Ar), 5.65 (s, 2H, benzyl CH2 ).
13C NMR (126 MHz, DMSO) δ 192.55 (s), 161.63 (d,1JC,F = 248.22 Hz), 139.04 (s), 136.79
(d, 3JC,F = 13.1 Hz), 131.89 (d, 3JC,F = 8.2 Hz), 131.51 (s), 125.41 (d, 4J C,F= 3.4 Hz), 124.04
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2
(s), 122.86 (s), 121.88 (s), 121.59 (d, JC,F = 14.4 Hz), 116.23 (s), 116.07 (s), 47.13
(s).HRMS: [MBr]+ = 281.106, calculated: 281.11.
Synthesis of 3-benzyl-1-(4-methoxyphenyl)-1H-imidazolium bromide (L3)
Ligand L3 was prepared by following the same procedure of L1 where
(4methoxyphenyl)−1H−imidazole (174 mg, 1mmol) and benzyl bromide (1.71g, 10 mmol)
were employed. The ligand L3 was isolated as white powder. Yield : 254 mg (73.5%).
1H NMR (500 MHz, CD3OD) δ 9.63 (s, 1H), 8.02 (s, 1H), 7.81 (s, 1H), 7.66 (dd, J = 9.1, 2.7
Hz, 2H), 7.50 (dt, J = 23.2, 8.3 Hz, 5H), 7.17 (d, J = 8.5 Hz, 2H), 5.55 (s, 2H), 3.90 (s,
3H).13C NMR (126 MHz, CD3OD) δ 160.96, 134.89, 133.56, 128.99, 128.29, 127.77, 124.77
, 123.53 , 122.71, 122.14 , 114.91 , 52.99 . LC-MS : [ M-Br]+ = 265.24, calculated : 265.13.
Synthesis of complex 1a
A mixture of [IrCp*Cl2]2 (84mg, 0.105 mmol), ligand precursor L1(76 mg, 0.21mmol),
Cs2CO3 (202mg, 0.62mmol), NaOAc (172 mg, 2.092mmol) and excess KBr were taken in
100 mL schlenk flask and the mixture was refluxed for 10 h in dry CH3CN under nitrogen
atmosphere. After it cooled to room temperature, the mixture was filtered through celite and
washed using CH3CN three times. The solvent was removed under reduced pressure. The
crude product was purified through column chromatography (SiO2, CH2Cl2/ methanol (9: 1).
1
The complex was isolated as brown solid. Yield: 110 mg (76%). H NMR (500 MHz,
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CDCl3) δ 7.55 (t, J = 7.9 Hz, 1H, CH=CH,imidazole), 7.41 – 7.27 (m, 3H,Ar), 7.15-7.10 (m,
DOI: 10.1039/D0DT02341H
2H,Ar), 7.06 (d, J = 10.1 Hz, 1H, CH = CH, imidazole), 6.84 (d, J = 13.3 Hz, 1H, Ar), 6.51
(dd, J = 13.3, 7.5 Hz, 1H, Ar), 5.52 (s, 2H, benzyl CH2), 3.86 (s, 3H, OCH3), 1.84 (s, 15H,
3
1
4
Cp*).13C NMR (126 MHz, CDCl3) δ 164.04 (s), 161.39 (d, JC,F=246.96 Hz), 156.83 (s),
5
142.68 (s), 140.22 (s), 131.64 (d, 3JC,F = 3.2 Hz), 130.09 (d, 3JC,F = 8.2 Hz), 124.97 (d, 4JC,F
=
2
6
3.4 Hz), 123.29 – 122.85 (m), 122.40 (s), 121.90 (s), 119.14 (s), 115.16 (d, JC,F = 20.9Hz),
2
7
110.56 (d, JC,F = 16.2 Hz), 106.82 (s), 106.61 (s), 91.08 (s), 55.28 (s), 9.76 (s).MS (ESI;
8
CH3CN), m/z, [MBr]+ =609.186 (Found), 609.19 (Calculated).
9
Synthesis of complex 1b
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Complex 1b was synthesized by following same procedure of 1a where [IrCp*Cl2]2 (84 mg,
0.105 mmol), ligand precursor L2 (76 mg, 0.21 mmol), Cs2CO3 (202 mg, 0.62 mmol),
NaOAc (172 mg, 2.092 mmol) and excess KBr were employed. Complex 1b was isolated as
brown solid. Yield: 104 mg (72%). 1H NMR (500 MHz, CDCl3) δ 9.92 (s, 1H, CHO), 8.22 (s,
1H, Ar), 7.53 – 7.49 (m, 1H, Ar), 7.45 (dd, J = 8.0, 1.7 Hz, 1H,Ar), 7.32 (d, J = 2.3 Hz, 1H,
Ar), 7.11 – 7.04 (m, 3H, Ar), 6.84 (d, J = 2.2 Hz, 1H, CH = CH, imidazole), 6.72 (s, 1H, CH
= CH, imidazole), 5.49 (s, 2H,CH2, benzyl), 1.80 (s, 15H, Cp*).13C NMR (126 MHz, CDCl3)
1
δ 192.48 (s), 167.57 (s), 161.45 (d, JC,F= 246.96 Hz), 151.53 (s), 141.83 (s), 139.08 (s),
136.43 (s), 133.62 (s), 131.79 (d, 3JC,F= 2.9 Hz), 130.40 (d, 3JC,F = 8.2 Hz), 120.41 (s), 115.99
– 115.65 (m), 115.47 (d,2JC,F = 41.2 Hz), 115.14 (s), 110.57 (s), 91.78 (s), 47.03 (s), 9.73 (s).
m/z, [MBr]+ =607.258 (Found), 607.17 (Calculated)
Synthesis of complex 1c
Complex 1c was synthesized by following same procedure of 1a where [IrCp*Cl2]2 (84 mg,
0.105 mmol), ligand precursor L3 (72 mg, 0.21 mmol), Cs2CO3 (202 mg, 0.62 mmol),
NaOAc (172 mg, 2.092 mmol) and excess KBr were employed. Complex 1c was isolated as
brown solid. Yield: 97 mg (69%) (1H NMR (500 MHz, CDCl3) δ 7.49 (d, J = 6.8 Hz, 2H,
Ar), 7.41 – 7.35 (m, 4H, Ar), 7.27 (d, J = 2.1 Hz, 1H, CH=CH,imidazole), 7.08 (d, J = 8.4
Hz, 1H), 6.74 (d, J = 2.1 Hz, 1H,CH=CH, imidazole), 6.53 (dd, J = 8.4, 2.7 Hz, 1H,Ar), 5.65
(d, J = 14.4 Hz, 1H, benzyl CH), 5.27 (d, J = 14.4 Hz, 1H, benzyl CH), 3.88 (s, 3H,OCH3),
1.85 (s, 15H, Cp*).13C NMR (126 MHz, CDCl3) δ 163.88, 156.84, 142.69, 140.26, 135.98,
128.85, 128.62, 128.18, 122.46, 119.26, 114.89, 110.44, 106.66, 91.04, 55.30, 9.79 . m/z,[ M-
Br]+ = 591.20 ( Found), 591.20 (calculated)
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Single crystal X-ray diffraction
Diffraction quality single crystals of the complex 1a (CCDC No :1973746) were grown by
slow diffusion of hexane into a saturated dichloromethane solution. Single crystal X-ray
diffractions were collected on a Bruker SMART APEX-II CCD diffractometer using Mo Kα
(λ =0.71073 Å) radiation.32 Bruker SAINT software has been employed for reducing the data
and SADABS for correcting the intensities of absorption.33 All cocrystal structures were
solved and refined using SHELXL with anisotropic displacement parameters for non-H
atoms. In all crystal structures, H-atoms are located experimentally, whereas C–H atoms were
fixed geometrically using the HFIX command in SHELXT.34 Not, any missed symmetry
observed in the final check of the CIF file using PLATON.35,36
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General Procedure for Alcohol Dehydrogenation A mixture of alcohol (2 mmol), KOH (2.2
mmol), and iridium complex 1a was taken in a 100 mL two neck roundbottomed flask and
allowed to dissolve in dry toluene (10 mL). The mixture was refluxed (bath temperature
120C) for the required time under nitrogen. The reaction was monitored by TLC. After
completion of the reaction, the solvent was removed under reduced pressure, affording the
crude potassium carboxylate.
Isolation Method A. Dichloromethane (10 mL) was added to potassium carboxylate and
allowed to stir for 10 min to dissolve the catalyst and the solvent was decanted. Thereafter,
the solid residue was washed with ethyl acetate several times and filtered. The resulting solid
mass was dissolved in deionized water (40 mL) and the solution was acidified with 1M HCl
and extracted with ethyl acetate. The organic phase was separated and dried over (Na2SO4).
Finally, the ethyl acetate was removed under vacuum, affording pure carboxylic acid.
Isolation Method B. Dichloromethane (10 mL) was added to potassium carboxylate and
allowed to stir for 10 min to dissolve the catalyst and the solvent was decanted, followed by
washed with ethyl acetate thrice (3X15 mL) and filtered. The solid mass was dissolved in
methanol (40 mL) and filtered. The solution was removed to dryness, under vacuum,
affording pure potassium carboxylate.
Benzoic acid (1). White solid, Yield: 0.23g (94 %).1H NMR (500 MHz, DMSO−d6) δ 12.94
(s, 1H), 7.95 (d, J = 7.8 Hz, 2H), 7.63 (t, J = 7.4 Hz, 1H), 7.51 (t, J = 7.7 Hz, 2H).13C NMR
(126 MHz, DMSO−d6) δ 167.7, 133.2, 131.1, 129.6, 128.9
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3Fluorobenzoic acid (2). White solid, Yield: 0.224 g (80 %).1H NMR (500 MHz,
DMSO−d6) δ 7.79 (d, J = 7.7 Hz, 1H), 7.65 (d, J = 12.1 Hz, 1H), 7.54−7.59 (m,1H),
7.46−7.50 (m, 1H). 13C NMR (126 MHz, DMSO) δ 166.66 , 163.39 (s), 161.45 (s), 133.67 (d,
J = 7.1 Hz), 131.29 (d, J = 8.0 Hz), 125.89 (d, J = 2.7 Hz), 120.45 (s), 120.34 (d, J = 21.2
Hz), 116.15 (d, J = 16.6 Hz).
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4(Trifluoromethyl) benzoic Acid (3). White solid, Yield: 0.289g (76%). H NMR (500
7
MHz, DMSO−d6) δ 8.14 (d, J = 8.0 Hz, 1H), 7.87 (d, J = 8.2 Hz, 1H). 13C NMR (126 MHz,
DMSO) δ 166.68, 135.04, 133.08 (s), 132.83, 130.57, 126.06 (q, J = 3.6 Hz), 125.35.
4− Methylbenzoic acid (4). White solid, Yield: 0.237 g, (87%). 1H NMR (500 MHz, DMSO)
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δ12.76 ( br, 1H), 7.85(d, J= 10Hz, 2H), 7.31( d, J= 5Hz, 2H), 2.37(s, 3H), C NMR (126
MHz, DMSO) δ 167.62, 143.39, 129.67, 129.47, 128.34, 21.46.
4− Chlorobenzoic acid (5). White solid, Yield: 0.267 g (85%).1H NMR (500 MHz,
13
DMSO−d6) δ 13.24 (s, 1H), 7.95( d, J = 10 Hz, 2H), 7.57(d, J = 5 Hz, 2H). C NMR (126
MHz, DMSO−d6) δ 166.7, 138.09, 131.48, 129.96, 129.08 .
1
4− Methoxybenzoic acid (6). White solid, Yield: 0.247 g (81%). H NMR (500 MHz,
DMSO−d6) δ12.66 (br, 1H), 7.91 (d, J = 10Hz, 2H), 7.03(d, J = 10Hz , 2H), 3.83 (s, 3H). 13C
NMR (126 MHz, DMSO−d6), δ 167.36, 163.17, 131.69, 123.28, 114.15, 55.77
1
4 Amino benzoic acid (7). White solid, Yield: 0.184 g (67%). H NMR (500 MHz,
DMSO−d6) δ 11.98 (s, 1H), 7.62 (d, J = 8.6 Hz, 2H), 6.55 (d,J = 8.3 Hz, 2H), 5.86 (s, 2H).
13C NMR (126 MHz, DMSO−d6), 167.86, 153.48,131.56,117.27, 112.91
1
3,4,5trimethoxybenzoic Acid (8). White solid, Yield: 0.153g (72%) H NMR (500 MHz,
DMSO) δ 8.14 (d, J = 8.0 Hz, 1H), 7.87 (d, J = 8.2 Hz, 1H). 13C NMR (126 MHz,
DMSO−d6),δ 171.76, 152.98, 142.98, 124.11, 107.41, 60.98, 56.26.
4 Nitrobenzoic acid (9). White solid, Yield: 0.154 g (46%). 1H NMR (500 MHz, DMSO) δ
8.28 (d, J = 6.7 Hz, 2H), 8.13 (d, J = 7.0 Hz, 2H). 13CNMR (126 MHz, DMSO−d6) δ 166.36,
150.30, 136.52, 131.19, 123.77.
1
4 Nitrobenzaldehyde (9a).Pale yellow solid, Yield: 0.030 g (10%) H NMR (500 MHz,
CDCl3) δ 10.19 (s, 1H), 8.42 (d, J = 8.7 Hz, 2H), 8.10 (d, J = 8.8 Hz, 2H). 13CNMR (126
MHz, CDCl3−d1) δ 190.15, 151, 140.13, 130.3, 124.21.
1
2−Methylbenzoic Acid (10). White solid, Yield: 0.229 g (84%). H NMR (500 MHz,
DMSO−d6) δ 12.84 (s, 1H), 7.82 (d, J = 7.6 Hz, 1H), 7.44 (t, J = 7.5 Hz, 1H), 7.29 (d, J =
13
10.5 Hz, 2H), 2.54 (s, 3H). C NMR (126 MHz, DMSO−d6) δ 169.02, 139.33, 132.06,
131.84, 130.76, 130.51, 126.17, 21.58.
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2−Chlorobenzoic acid (11). White solid, Yield: 0.26 g (83%). H NMR (500 MHz,
DOI: 10.1039/D0DT02341H
DMSO−d6) δ 13.33 (s, 1H), 7.76 (d, J = 7.3 Hz, 1H), 7.55 – 7.49 (m, 2H), 7.44 – 7.39 (m,
1H). 13C NMR (126 MHz, DMSO-d6) δ 167.3, 131.9, 131.8, 131.1, 130.9, 127.6
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2 Aminobenzoic acid (12). White solid, Yield: 0.148 g (54%). H NMR (500 MHz,
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DMSO−d6) δ 8.51 (br, 1H), 7.69 (d, J = 8.0 Hz, 1H), 7.22 (t, J = 7.7 Hz, 1H), 6.74 (d, J = 8.3
Hz, 1H), 6.50 (t, J = 8.0 Hz, 1H).13C NMR (126 MHz, DMSO−d6) δ 169.93, 151.84, 134.07,
131.50, 116.66, 114.91, 109.93.
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2- Methoxybenzoic acid (13). White solid, Yield : 0.110 g, (36%), H NMR (500 MHz,
9
DMSO) δ 12.60 (s, 1H), 7.65 (dd, J = 7.6, 1.8 Hz, 1H), 7.52 – 7.46 (m, 1H), 7.11 (d, J = 8.2
13
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13
14
Hz, 1H), 6.99 (t, J = 7.5 Hz, 1H), 3.81 (s, 3H). C (126 MHz, DMSO-d6) δ 167.60, 158.40,
133.36, 130.98, 121.60, 120.34, 112.71, 55.84.
1
2-Furancarboxylic acid (14). White solid, Yield: 0.179 g (80%). H NMR (500 MHz,
DMSO−d6) δ 7.92 – 7.91 (m, 1H), 7.22 (dd, J = 3.5, 0.8 Hz, 1H), 6.65 (dd, J = 3.5, 1.7 Hz,
1H). 13C NMR (126 MHz, DMSO−d6) δ 159.66, 147.39, 145.21, 118.07, 112.44.
15 Potassium Pyridine2carboxylate (15). This compound was isolated by method B as a
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17
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white powder. Yield: 0. 229 g (71%). H NMR (500 MHz, CD3OD) δ 8.58 (d, J = 4.5 Hz,
13
1H), 8.01 (d, J = 7.8 Hz, 1H), 7.86 (t, J = 8.5 Hz, 1H), 7.45 – 7.40 (m, 1H). C NMR (126
MHz, CD3OD) δ 171.61, 154.98, 147.97, 136.66, 124.40, 123.43.
Potassium Butyrate (16). This compound was isolated by method B as a white powder Yield:
0.207g (82%). 1H NMR (500 MHz, D2O) δ 2.08 (t, J = 8.7 Hz, 2H), 1.49 (m, 2H), 0.82 (t, J =
8.7 Hz, 3H). 13C NMR (126 MHz, D2O) δ 184.12, 39.57, 19.30, 13.24
Potassium Octanoate (17). This compound was isolated by method B as a white powder.
1
Yield: 0.137 g (75%). H NMR (500 MHz, D2O) δ 2.06 (t, J = 7.5 Hz, 2H), 1.49 – 1.37 (m,
13
2H), 1.17 (m, 8H), 0.75 (t, J = 7.0 Hz, 3H). C NMR (126 MHz, D2O) δ 184.22, 37.51,
30.85, 28.55, 28.10, 25.31, 21.84, 13.24.
Procedure for the large scale synthesis of benzoic acid
A mixture of benzyl alcohol (1.08 g, 10 mmol), KOH (617.1 mg, 11mmol), and iridium
complex 1a (1.38 mg, 0.002 mmol) was taken in a 100 mL two neck roundbottomed flasks
and allowed to dissolve in dry toluene (20 mL). The mixture was refluxed (bath temperature
120C) for 72h under nitrogen. The solvent was removed under reduced pressure, affording
the crude potassium carboxylate. Dichloromethane (10 x 3 mL) was added to potassium
carboxylate and allowed to stir for 10 min to dissolve the catalyst and the solvent was
decanted. Thereafter, the solid residue was washed with ethyl acetate several times and
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filtered. The resulting solid mass was dissolved in deionized water (60 mL) andDtOhIe: 10s.o10l3u9t/iDo0nDT02341H
was acidified with 1M HCl and extracted with ethyl acetate. The organic phase was separated
and dried over (Na2SO4). Finally, the ethyl acetate was removed under vacuum, affording
pure carboxylic acid as a white solid (1.12 g, 91%)
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4
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Volumetric Estimation of Evolved Hydrogen. A 100 mL schlenk flask was flame dried and
purged with nitrogen. Thereafter, the flask was charged with alcohol (0.5 mmol), 1a (0.1
mol%), and potassium hydroxide (0.11 mmol) in 10 mL of toluene. The reaction mixture was
placed in a preheated oil bath (bath temperature 120C). The sidearm of the schlenk flask was
connected to a gas burette and headspace was closed tightly with glass stopcock. The reaction
was allowed to continue until the evolution of gas ceased. To get consistent readings, the
experiment was repeated three times and the number of moles of hydrogen was calculated by
applying ideal gas law. Vapour pressure of water at 293 K = 17.5424 Torr, Atmospheric
pressure = 761.3126 Torr, R = 62.3635 L Torr K−1 mol−1, Volume of water displaced 23.5
mL. n(H2) = [(Patm − Pwater)V]/RT = 0.00096 mol, expected value 0.001 mol.
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Hg Poisoning Experiment:
A mixture of benzyl alcohol (2 mmol), KOH (2.2 mmol), and iridium complex 1a, (0.1
mol%) Hg (10mmol) was taken in a 100 mL two neck roundbottomed flask and allowed to
dissolve in dry toluene (10 mL). The mixture was refluxed (bath temperature 120C) for 4h
under nitrogen. After completion of the required time, the solvent was removed under
reduced pressure, affording the crude potassium carboxylate. Corresponding benzoic acid
was found in 80 % (0.195 g) in yield by following the isolation procedure A.
Control experiments and reusability
Verification of ester formation in the dehydrogenation of alcohols
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a) Benzyl alcohol (3 mL) and KOH (1.1 mmol) were taken in a 100 mL two neck
roundbottomed flask and placed in a preheated oil bath ( bath temperature
120C).The mixture was stirred for 4 h under an N2 atmosphere. The reaction mixture
was allowed to cool to room temperature and the reaction mixture was analyzed by
GC. However, the formation of ester was not observed, only a small amount of
benzaldehyde was detected.
b) Benzyl alcohol (1 mmol) and KOH (1.1 mmol) were dissolved in toluene (10 mL) in
a 100 mL two neck roundbottomed flask and placed in a preheated oil bath ( bath
temperature 120C).The mixture was stirred for 4h under an N2 atmosphere. The
reaction mixture was allowed to cool to room temperature and the reaction mixture
was analyzed by GC. However, the formation of ester was not observed, only a small
amount of benzaldehyde was detected.
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The reaction of benzaldehyde with KOH.
Benzaldehyde (1 mmol) and KOH (1.1 mmol) were dissolved in toluene (10 mL) in a 100
mL two neck roundbottomed flask and placed in a preheated oil bath ( bath temperature
120C).The mixture was refluxed for 1 h under an N2 atmosphere. The reaction mixture was
allowed to cool to room temperature and the solvent was removed under vacuum. Thereafter,
the solid residue was washed with ethyl acetate several times and filtered. The resulting solid
mass was dissolved in deionized water (30 mL) and the solution was acidified with 1M HCl
and extracted with ethyl acetate. The organic phase was separated and dried over (Na2SO4).
Finally, the ethyl acetate was removed under vacuum, affording pure carboxylic acid as a
white solid (68 mg, 56%). Residual ethyl acetate solution (washing part) was collected in a
round bottom flask and the solvent was removed under vacuum to recover benzyl alcohol.
The reaction of benzaldehyde with KOH in the presence of catalyst 1a
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Benzaldehyde (1mmol), 1a (0.1 mol%), and KOH (1.1 mmol) were dissolved in toluene (10
mL) in a 100 mL two neck roundbottomed flask and placed in a preheated oil bath (bath
temperature 120C).The mixture was refluxed for 1 h under an N2 atmosphere. The reaction
mixture was allowed to cool to room temperature and the solvent was removed under
vacuum. Dichloromethane (10 x 3 mL) was added to a mixture of potassium carboxylate and
benzyl alcohol and allowed to stir for 10 min to dissolve the catalyst and the solvent was
decanted. Thereafter, the solid residue was washed with ethyl acetate several times and
filtered. The resulting solid mass was dissolved in deionized water (30mL) and the solution
was acidified with 1M HCl and extracted with ethyl acetate. The organic phase was separated
and dried over (Na2SO4). Finally, the ethyl acetate was removed under vacuum, affording
pure carboxylic acid as a white solid (107mg, 88%). Residual ethyl acetate and
dichloromethane solution were collected in a round bottom flask and the solvent was
removed under vacuum. To the collected mixture small amount of dichloromethane was
added and to this solution, diethyl ether was added in stirring conditions to get precipitate of
the catalyst. The solution is filtered and the filtrate part is removed under vacuum to recover
benzyl alcohol (9%).
Reusability Experiment:
Benzyl alcohol (2 mmol), 1a (0.1 mol%), and KOH ( 1.1 mmol) were dissolved in toluene (
10 mL) in a 100 mL two neck round-bottomed flask and placed in a preheated oil bath ( bath
temperature 120). The mixture was refluxed for 4h under an N2 atmosphere. The reaction
mixture was allowed to cool to room temperature and the mixture was filtered. To the filtrate,
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benzyl alcohols (2 mmol), KOH ( 1.1mmol) are added and again the reaction was carried out
DOI: 10.1039/D0DT02341H
for 4h. The process was repeated three times.
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Hydrogenation
Complex 1a (0.005mmol), KOH(1.1mmol), styrene (1 mmol), Pd/C (5 mol %) and benzyl
alcohol (1 mmol) were dissolved in toluene (10 mL) in a 100 ml Schlenk tube. The solution
was allowed to reflux for 4h under an N2 atmosphere at 120°C. The mixture was cooled to
room temperature and the solution was filtered to remove potassium carboxylate. The solvent
was removed under vacuum and the mixture was dissolved in ethyl acetate and filtered. Ethyl
acetate was removed in vacuum and the sample was submitted for 1H NMR analysis.
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254x190mm (96 x 96 DPI)