N-Heterocyclic carbene as a Br?nsted base catalyst for the amination of naphthol derivatives and alcoholysis of glutaric anhydrides

Article abstract of DOI:10.1016/j.tetlet.2019.151131

A non-covalent Br?nsted-basic N-heterocyclic carbene catalyzed (NHC) Friedel-Crafts type amination of naphthol derivatives using dialkyl azodicarboxylates as the aminating source and alcoholysis of various glutaric anhydrides using alcohol as pronucleophile is presented. Both of these reactions are performed in the presence of either commercially available free-carbene catalyst or in situ-generated carbene catalyst. Friedel-Crafts type amination reaction is an example of a hydroxy group facilitated amination reaction. Both reactions proceed via in situ activations of –OH group by the carbene catalyst through hydrogen bonding interaction and furnish the relevant products in moderate to excellent yields.

Full text of DOI:10.1016/j.tetlet.2019.151131

Tetrahedron Letters xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
N-Heterocyclic carbene as a Brønsted base catalyst for the amination of
naphthol derivatives and alcoholysis of glutaric anhydrides
1
1
⇑
K.R. Jayakrishnan , M. Tamilarasu , K.V. Jincy, Alagiri Kaliyamoorthy
School of Chemistry, Indian Institute of Science Education and Research, Thiruvananthapuram, Vithura, Kerala 695551, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A non-covalent Brønsted-basic N-heterocyclic carbene catalyzed (NHC) Friedel-Crafts type amination of
naphthol derivatives using dialkyl azodicarboxylates as the aminating source and alcoholysis of various
glutaric anhydrides using alcohol as pronucleophile is presented. Both of these reactions are performed in
the presence of either commercially available free-carbene catalyst or in situ-generated carbene catalyst.
Friedel-Crafts type amination reaction is an example of a hydroxy group facilitated amination reaction.
Both reactions proceed via in situ activations of –OH group by the carbene catalyst through hydrogen
bonding interaction and furnish the relevant products in moderate to excellent yields.
Ó 2019 Elsevier Ltd. All rights reserved.
Received 5 July 2019
Revised 7 September 2019
Accepted 9 September 2019
Available online xxxx
Keywords:
N-Heterocyclic carbene
Amination
2-Naphthol
Alcoholysis
Organocatalysts usually the small organic molecules have been
generally used to bring out various synthetic transformations that
are exceptional. Besides, they are broadly employed to develop
complementary reactions that are catalyzed by metal-catalysts
[1]. In recent years, N-heterocyclic carbenes (NHCs) have been
widely used as organocatalysts to construct various C–C and C–
heteroatom bonds in organic synthesis [2]. Owing to the Lewis
basic propensity of NHC, they are predominantly exploited as the
nucleophilic catalyst. In general, the activation of starting precur-
sors by NHC takes place through various modes of activation, in
which the carbene catalyst and the functional group present on
the substrate play a crucial role. Most of the Lewis base catalysis
by NHC involves a reversible covalent-interaction between the car-
bene catalyst and the substrate [2,3]. In contrast, the substrate acti-
vation via non-covalent interaction by Brønsted basic NHC is less
documented, although, NHCs are known for their characteristic
Brønsted basic property [4].
alcohols to conjugate acceptors [4e]. Apart from this, various
intra- and inter-molecular Michael addition reactions were
realized
through
non-covalent
activation
of
different
pronucleophiles using a catalytic amount of NHC [4f–m].
Conventionally, aromatic amines are produced by reduction of
nitroaromatics and cross-coupling reaction between aromatic
halides and amines [5]. Direct amination of electron-rich arenes
was first reported by Deils and Back [6a], and then, catalytic
synthesis of the same was narrated in the presence of several
metal-catalysts [6,7]. Jørgenson’s group depicted the direct amina-
tion of naphthol derivatives using cinchona-alkaloid derived cata-
lyst [7a,b]. Despite these developments, evolving new prospects for
the synthesis of various aromatic amines via direct amination
method is highly desirable. In this context, we envisioned the
direct amination of 2-naphthol derivatives employing NHC as the
catalyst by capitalizing on characteristic Brønsted basic nature of
free carbenes.
In 2002, the application of NHC as a Brønsted base catalyst for
transesterification reaction was showcased independently by
Nolan [4a] as well as Waymouth and Hedrick [4b]. Later,
Movassaghi and co-workers [4c] described amidation of esters
mediated by NHC catalyst, and their NMR studies supported the
activation of alcohol pronucleophile by NHC via hydrogen
bonding interaction between an alcohol and carbene catalyst. Of
late, Scheidt et al., reported NHC-catalyzed conjugate addition of
We began the optimization study by choosing 2-naphthol 1a as
a precursor and di-tert-butyl azodicarboxylate (DtBuAD, 2 equiv)
2a as an aminating source. In the beginning, the catalytic use of
commercially available free carbene catalysts such as IMes (1,3-
Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, A) and IPr (1,3-Bis
(2,6-diisopropylphenyl)imidazol-2-ylidene, B) in CH2Cl2 rendered
the desired product 3aa in 87% and 88% yields respectively (Sche-
me 1a). Also, we probed the role of carbene catalyst in the activa-
tion of 1a toward Friedel-Crafts type amination reaction through
1H NMR experiment. Mixing of 1:1 mixture of 1a and IMes (A) in
C6D6 (0.05 M at rt) resulted in the considerable downfield shift
for the hydroxyl proton of 1a (Scheme 1b), which reveals the pres-
⇑
Corresponding author.
E-mail address: alagiri@iisertvm.ac.in (A. Kaliyamoorthy).
1
These authors contributed equally.
https://doi.org/10.1016/j.tetlet.2019.151131
0040-4039/Ó 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: K. R. Jayakrishnan, M. Tamilarasu, K. V. Jincy et al., N-Heterocyclic carbene as a Brønsted base catalyst for the amination of naph-
thol derivatives and alcoholysis of glutaric anhydrides, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151131

2
K.R. Jayakrishnan et al. / Tetrahedron Letters xxx (xxxx) xxx
H
N
tBuO2C
catalyst A or B
t
OH
N
CO2 Bu
OH
20 mol%
t
N
CO2 Bu
tBuO2C
N
a)
CH2Cl2, rt, 4 h
1a
2a
1 equiv
2 equiv
3aa
N
N
Ar
Ar
A: 87%
A: Ar = mesityl (IMes)
B: Ar = 2,6-iPr-Ph (IPr)
B: 88%
Mes
N
Mes
N
OH
C6D6, rt
+
H O
b)
N
Mes
0.05 M
rt
N
Mes
ꢀ
ꢀ
H = 7.35 ppm
H = 4.07 ppm
A
1a
A–1a
Scheme 1. a) Friedel-Crafts type amination of 2-naphthol mediated by free carbene catalyst. b) Activation of 2-naphthol via carbene-2-naphthol complex (A-1a).
ence of hydrogen bonding interaction between 1a and A, and this
scrutiny is in good accordance with the analogous study described
by Movassaghi’s group [4c].
Based on these promising findings, we envisaged optimizing the
reaction conditions using in situ-generated carbene catalyst from
NHC precatalyst and base as it has a more practical advantage
Table 1
Optimization of Reaction conditions.
H
tBuO2C
precatalyst (10 mol%)
N
t
OH
KOtBu (20 mol%)
solvent, rt, 12 h
N
CO2 Bu
OH
t
N
CO2 Bu
tBuO2C
N
+
1a
2a
1 equiv
2 equiv
3aa
entry
precatalyst
solvent
yield (%)a
1
2
3
4
4
5
6
7
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
35
57
52
40
X
I
N
N
N
N
R
R
4 R=mesityl, X=Cl
5 R=2,6-iPr-Ph, X=Cl
6 R=cyclohexyl, X=BF4
7
5
6
7
8
9
10
8
9
10
11
12
13
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
63
57
71
44
64
55
Cl
Mes
N
N
Mes
8
N
S
Ar
Cl
9 Ar=mesityl
11
12
13
14
15
14
15
16
16
16
CH2Cl2
CH2Cl2
CH2Cl2
THF
72
67
80
71
88
BF4
N
OH
I
N
S
N
N
Ar
Et2O
10
11 Ar=C6F5
12 Ar=mesityl
13 Ar=phenyl
16
17
18
19
20
21
16
16
16
16
CH3CN
DMF
toluene
CHCl3
CHCl3
CHCl3
85
62
94
Ph
I
N
N
N
N
X
N
N
96b
75b,c
14d
Ph
Ph
16
14
15 X=Cl
16 X=PF6
_
a
b
c
Isolated yield.
4 h reaction time.
8 mol% of KOtBu.
d
In the absence of precatalyst and base.
Please cite this article as: K. R. Jayakrishnan, M. Tamilarasu, K. V. Jincy et al., N-Heterocyclic carbene as a Brønsted base catalyst for the amination of naph-
thol derivatives and alcoholysis of glutaric anhydrides, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151131

K.R. Jayakrishnan et al. / Tetrahedron Letters xxx (xxxx) xxx
3
and also lucrative. On the basis of this hypothesis, the use of vari-
ous imidazolium pre-catalysts (4 to 7) together with KOtBu
(20 mol%) afforded the anticipated amination product 3aa in mod-
erate yields (entries 1–4, Table 1). Furthermore, NHC pre-catalysts
such as imidazolinium (8), thiazolium (9 and 10) and triazolium
(11 to 15) were less efficient and delivered 3aa in considerable
yields (entries 5–12). Satisfyingly, the aminated 2-naphthol 3aa
was obtained in 80% yield when the triazolium pre-catalyst 16
was engaged as the catalyst (entry 13), which is presumably due
to the increased acidity of triazolium precatalysts than other pre-
catalysts, wherein the in situ produced conjugate acid of the cat-
alytic species could act as proton source (see Scheme 2) [3l].
Several bases were screened for the amination of 2-naphthol
derivatives in the presence of 16, among them KOtBu furnished
the desired product in excellent yield (see ESI for the base screen-
ings). Remarkably, the amination reaction proceeded well when
CHCl3 was used as a solvent in the presence of pre-catalyst 16
and furnished the requisite product in excellent yield (96% yield,
entry 19). To ensure the complete utilization of base during
in situ-generation of active carbene catalyst, 10 mol% of pre-cata-
lyst 16 and 8 mol% of KOtBu was employed, and the expected pro-
duct 3aa was obtained in 75% yield (entry 20) [9].
With the optimized reaction conditions in hand, we went on to
scrutinize the scope of various naphthol derivatives and dialkyl
azodicarboxylates. A series of di-alkyl azodicarboxylates such as
di-tert-butyl azodicarboxylate (DtBuAD, 2a) diethyl azodicarboxy-
late (DEAD, 2b), diisopropyl azodicarboxylate (DIAD, 2c) and
dibenzyl azodicarboxylate (2d) reacted with 1a to afford the corre-
sponding aminated products (entries 3ab-ad, Table 2) in good
yields. Similarly, excellent yields were obtained in the case of 2-
naphthol bearing an aryl substituent (1b) (Table 2, entries 3ba-
bd). Whereas, an electron- donating group (such as a methoxy
group) on the 2-naphthol (1c) ring resulted in the formation of
analogous aminated product in appreciable yields (entries 3ca-
cd). Besides, the direct amination reaction proceeded well for the
substrate bearing bromo-substituent at 3- and 6-position (1d and
1e) and offered the concomitant products in better yields (entries
3da and 3ea-ed, Table 2). Electron-withdrawing groups such as
cyano, ester, amide, and ketone at the various position of the naph-
thol ring (1f-1j) reacted smoothly and delivered the requisite
products in moderate to excellent yields (entries 3fa-3ja), how-
ever, 2-naphthol bearing nitro substituent at the peri-position
failed to undergo amination reaction presumably due to strong
electron-withdrawing nature of nitro group which makes 8-
nitro-2-naphthol less nucleophilic towards amination reaction.
Additionally, 2-naphthol holding an allylic ether functionality at
7-position handed the desired product in moderate yield (56%,
entry 3ka). Interestingly, substituents at the peri-position of 2-
naphthol (1l and 1m) with DtBuAD (2a) furnished the desired
products 3la and 3ma in 74% and 68% yields, respectively. Next,
we explored 2-naphthol derivative encompassing a hetero-atom
on its ring (1n) for the amination reaction, and the corresponding
target products were obtained in moderate to good yields (entries
3na-3nd). Ultimately, the efficacy of the reaction was tested with
hydroxycoumarin (1o) with 2a as an aminating source, and the
corresponding aminated product was attained in high yield (93%,
entry 3oa).
Herein, we propose a plausible mechanism for an NHC-pro-
moted Friedel-Crafts type amination of 2-naphthol derivatives
based on the literature precedence (Scheme 2) [4]. At the outset,
the hydrogen bonding interaction between free-carbene catalyst
(I) and 2-naphthol (1a) activates the 1a for nucleophilic attack
(as shown in step II), which then, undergoes Friedel-Crafts type
amination reaction with 2a to form the intermediate III. Next,
intramolecular-proton exchange, followed by protonation of resul-
tant oxy-anion by triazolium precatalyst produces the target pro-
duct 3aa and regenerates the active free-carbene (I) in the
catalytic cycle (step IV).
NHC-promoted alcoholysis of glutaric anhydrides
Then, we envisaged an alcoholysis of glutaric anhydrides using
alcohols as pronucleophile by embracing Brønsted basic activation
of alcohol by a carbene catalyst demonstrated by Scheidt’s group
[4e]. The ensuing hemiesters (having chemically two distinctive
carbonyl groups) are important synthetic intermediate for several
bioactive molecules. Various metal catalysts and cinchona-
alkaloid based catalysts were previously known to mediate
asymmetric desymmetrization of meso-anhydrides [8].
To validate our speculation, a model reaction was performed in
the presence of a commercially available free-carbene catalyst
such as A (IMes, 10 mol%) using 3-phenylglutaric anhydride (4a)
as a precursor, MeOH (5a, 10 equiv) as the pronucleophile in
toluene. Gratifyingly, the desired hemiester (6aa) was obtained
in good yield (Scheme 3a). Next, we endeavoured the same reac-
tion by exercising carbene catalyst which is in situ generated from
NHC precatalyst (4) and DBU, and as expected the desired hemie-
ster was obtained in excellent yields (Scheme 3b) [10].
With these findings in hand, further alcoholysis experiments
were performed using various glutaric anhydrides. As summarized
in Table 3, methanolytic desymmetrization of phenyl glutaric
anhydride (4a) furnished the desired hemiester in excellent yield
(entry 6aa, Table 3). Increasing the carbon chain of the alcohol
scaffold such EtOH (5b), 2-propanol (5c), allylic alcohol (5d), and
propargyl alcohol (5e) resulted in slight deterioration of the pro-
duct yields, surmising that steric factor and acidity of the alcohols
may be the possible reasons (entries 6ab-6ae, Table 3) [4b]. Then,
we investigated the alcoholysis of glutaric anhydride bearing
substituent at the para position of the phenyl ring, for instance,
3-(4-fluorophenyl)glutaric anhydride (4b) on methanolysis
afforded the corresponding hemiester in good yield (Table 3,
entry 6ba). Next, we explored a methanolysis of unsymmetrical
cyclic anhydride like phenylsuccinic anhydride (4c), and in this
case, the hemiester was isolated as the sole product, presumably
due to steric hindrance exerted by the phenyl group (entry 6ca,
Table 3). In addition, preliminary investigations on asymmetric
version of this reaction were performed in the presence of
X
N
N
R
R
X=N or CH
base
X
N
HO
HO
tBuO2C
N
R
R
N
t
N
H
CO2 Bu
3aa
I
1a
R
N
R
N
H
H
O
O
II
X
X
N
R
N
R
t
CO2 Bu
tBuO2C
IV
N
N
t
N
H
CO2 Bu
N
2a
tBuO2C
O
H
t
N
N
CO2 Bu
tBuO2C
III
Scheme 2. Plausible mechanistic pathway.
Please cite this article as: K. R. Jayakrishnan, M. Tamilarasu, K. V. Jincy et al., N-Heterocyclic carbene as a Brønsted base catalyst for the amination of naph-
thol derivatives and alcoholysis of glutaric anhydrides, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151131

4
K.R. Jayakrishnan et al. / Tetrahedron Letters xxx (xxxx) xxx
Table 2
NHC-catalyzed amination of naphthol derivatives.a
aIsolated yield. bCHCl3:DMF (9:1).
10 mol% of chiral NHC precatalysts such as 17 or 18 and 8 mol% of a
base such as DBU at À20 °C delivered 6aa in 65% and 94% yield
respectively with 0% ee (Scheme 4A and 4B). Besides,
engagement of proton-shuttling strategy [4m,n] which proceeds
through an assembled transition state for the methanolysis of
phenylglutaric anhydrides rendered 6aa as a racemic mixture in
69% yield (Scheme 4C). Based on the literature precedence and
also by our observations, we contemplate that either lack of rigid
transition state or reversibility of the reaction may be the
possible reason for the absence of asymmetric induction [4e,f].
Additionally, if the deprotonation step for in situ generations of
carbene catalyst from precatalyst and a base is reversible, then
the reaction could possibly be promoted by the base rather than
NHC, which eventually will result in a racemic product [10].
Scheme 3. a) Free carbene-catalyzed alcoholysis of 3-phenylglutaric anhydride, b)
alcoholysis of phenylglutaric anhydride mediated by in situ-generated carbene
catalyst.
Please cite this article as: K. R. Jayakrishnan, M. Tamilarasu, K. V. Jincy et al., N-Heterocyclic carbene as a Brønsted base catalyst for the amination of naph-
thol derivatives and alcoholysis of glutaric anhydrides, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151131

K.R. Jayakrishnan et al. / Tetrahedron Letters xxx (xxxx) xxx
5
Table 3
opening reaction on the glutaric anhydride to furnish the requisite
hemiester (Scheme 5).
N-Heterocyclic carbene-catalyzed alcojolysis of glutaric anhydrides.a
In conclusion, we have documented a non-covalent Brønsted
basic N-heterocyclic carbene enabled Friedel-Crafts type amination
of 2-naphthol derivatives using various dialkyl azodicarboxylates
as an aminating source. Subsequently, we probed the alcoholysis
of glutaric anhydrides to hemiesters using N-heterocyclic carbene
as a non-covalent Brønsted basic catalyst. We showcased that both
the methodologies work well in the presence of commercially
available free-carbene catalyst, which clearly backs the Brønsted
basic role of NHC in these reactions, and also, the same reactions
were demonstrated by deploying in situ-generated carbene cata-
lyst. Future scope in this direction includes development of an
asymmetric version of the direct amination reaction of naphthols
using chiral NHC catalyst.
aIsolated yield.
Acknowledgments
The authors thank Science and Engineering Research Board
(SERB), Government of India (File No. ECR/2016/000202) and IISER
Thiruvananthapuram for financial support. One of the authors (J.K.
R) thanks CSIR, New-Delhi for senior research fellowship.
NHC precatalyst 17 or 18
(10 mol%)
Ph
O
Ph
O
base (8 mol%)
MeOH (10 equiv)
solvent, 48 h
Appendix A. Supplementary data
HO
O
OMe
*
O
O
O
4a
6aa
O
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2019.151131.
F
N
N
N
N
F
N
N
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Cl
F
F
18
17
F
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anhydrides.
Please cite this article as: K. R. Jayakrishnan, M. Tamilarasu, K. V. Jincy et al., N-Heterocyclic carbene as a Brønsted base catalyst for the amination of naph-
thol derivatives and alcoholysis of glutaric anhydrides, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151131

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Please cite this article as: K. R. Jayakrishnan, M. Tamilarasu, K. V. Jincy et al., N-Heterocyclic carbene as a Brønsted base catalyst for the amination of naph-
thol derivatives and alcoholysis of glutaric anhydrides, Tetrahedron Letters, https://doi.org/10.1016/j.tetlet.2019.151131