A novel NHC-catalyzed transformation of 2H-chromene-3-carboxaldehydes to 3-methyl-2H-chromen-2-ones

Article abstract of DOI:10.1039/c1ob05325f

An unexpected transformation of 2H-chromene-3-carboxaldehydes to coumarin derivatives, mediated by NHC, is reported.

Full text of DOI:10.1039/c1ob05325f

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PAPER
A novel NHC-catalyzed transformation of 2H-chromene-3-carboxaldehydes
to 3-methyl-2H-chromen-2-ones†
Vijay Nair,*a C. R. Sinu,a R. Rejithamol,a K. C. Seetha Lakshmia and Eringathodi Sureshb
Received 1st March 2011, Accepted 5th May 2011
DOI: 10.1039/c1ob05325f
An unexpected transformation of 2H-chromene-3-carboxaldehydes to coumarin derivatives, mediated
by NHC, is reported.
Introduction
The groundbreaking work on nucleophilic heterocyclic car-
bene (NHC) mediated benzoin condensation by Breslow in
1958 constitutes the first definitive example of such a catalytic
transformation.1 Inexplicably, with the exception of the Stetter
reaction,2 the unique catalytic properties of NHC remained
virtually unexplored for a long time. In recent years, however con-
sequent to the reemergence of interest in organocatalysis,3 NHC
catalyzed reactions have been attracting considerable attention.
Scheme 1
A number of novel protocols of synthetic value employing NHC
catalysis, such as asymmetric benzoin condensation,4 inter- and
intra-molecular Stetter reactions,5 redox reactions6 and transes-
terification reactions,7 have been published by different groups.
The first report on the unique NHC catalyzed generation of
homoenolate by Bode and Glorius was followed by a variety
of novel application of this species. Some of the key reactions
involving homoenolates are the formation of g-butyrolactones,8,9
g-lactams,10 spiro-g-lactones,11 d-lactones,12 cyclopentenes,13,14 b-
lactams,15 spirocyclopentanones,16 and g-amino butyric acid
(GABA) derivatives.17
formed from 1 would be trapped by the latter to form a g-
lactone consistent with known homoenolate chemistry. In the
event the reaction yielded none of the expected product, but
surprisingly 3-methyl-2H-chromen-2-one (3-methyl coumarin)
5 was formed in 25% yield. Coumarin derivatives are often
prepared by the application of the Knoevenagel reaction18 or
Perkin reaction19 with salicylaldehydes. A mechanistically related
process involving the latter and enals engaging NHC catalysis
leading to 3-alkyl coumarins in moderate yields was reported
recently.20 Although the expected reaction did not occur, intrigued
by the novelty of the reaction, we decided to pursue it in
some detail. Additional incentive for our studies was accrued
from the well documented and important biological properties
of coumarin derivatives.21 Inter alia they have been shown to
possess antithrombotic,22 vasodilatory23 and anti-inflammatory 24
properties.
Against the backdrop presented above, an experiment was
conducted in which the aldehyde 1 was exposed to the catalyst
3b, in DCM in the absence of 4-fluorobenzaldehyde. The reaction
mixture on column chromatography (95 : 5 hexane : ethyl acetate)
afforded the product 5 in 30% yield. Subsequent studies aimed at
catalyst screening revealed that the best result was obtained with
3b in THF (Tables 1 and 2).
Results and discussion
In the context of our continuing interest in the chemistry of
homoenolates and with a view to extending the scope of the latter,
we sought to generate endocyclic homoenolates and examine
their reactivity towards electrophiles. In a prototype experiment,
2H-chromene-3-carboxaldehyde 1 (Scheme 1) was exposed to
SIMes (1,3-dimesityl imidazolinium carbene), generated from
the chloride salt (3b) of SIMes by DBU, in the presence of
4-fluorobenzaldehyde. It was surmised that the homoenolate
aOrganic Chemistry Section, National Institute for Interdisciplinary Science
and Technology (CSIR), Trivandrum, 695 019, India. E-mail: vijaynair_
2001@yahoo.com
bCentral Salt and Marine Chemicals Research Institute, Bhavnagar, 364002,
India
In order to explore the generality of the reaction, a number of
substituted chromene aldehydes were treated with the catalyst, and
the results are presented in Table 3.
All the products were characterized by spectroscopic anal-
ysis. In addition, final proof for the structure of the product
† Electronic supplementary information (ESI) available: General exper-
imental procedure,1H and 13C spectra of compounds. CCDC reference
number 813059. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1ob05325f
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Table 1 Catalyst screening
Entry
Catalyst
Conditions
Yield (%)
Fig. 1 Single crystal X-ray structure of 5g (40% probability factor for the
thermal ellipsoids).
1
2
3
4
5
6
3a
3b
3c
3d
3e
—
THF, rt, 8 h
THF, rt, 8 h
THF, rt, 8 h
THF, rt, 8 h
THF, rt, 24 h
THF, rt, 8 h
27
94
—
50
—
—
Table 2 Optimization of conditions
Entry
Base
Solvent
Temp.
Time (h)
Yielda (%)
1
2
3
4
5
6
7
DBU
DBU
DBU
DBU
K2CO3
DMAP
K2CO3
DCM
THF
Toluene
CH3CN
CH3CN
THF
rt
rt
rt
rt
24
8
24
24
12
24
24
30
94
38
—
—
—
—
82 ◦
rt
C
THF
rt
Scheme 2 Postulated catalytic cycle.
a Isolated yield.
Conceivably the initially formed Breslow intermediate B trans-
forms to the homoenolate equivalent C which on proton transfer
delivers the species D. The latter undergoes a fragmentation,
reminiscent of the Grob fragmentaion, to generate E; it then
undergoes intramolecular acylation to afford F. Subsequent
elimination of the carbene followed by isomerization of the enone
delivers the 3-alkyl coumarin.
Table 3 Scope of the reaction
Notwithstanding the superficial resemblance of the key step in
the cascade process to Grob fragmentation,25 it is important to
note that there are major differences between the two transforma-
tions. For instance, almost always, the scission of a C–C s bond is a
fait accompli in the Grob fragmentation, whereas no such thing oc-
curs in the present process. Evidently a more detailed discussion of
the cascade reaction will have to await the results of further inves-
tigations. It may be pointed out that, as far as we know, this is the
first example of a cascade process of this type mediated by NHC.
As a prelude to examining the scope of the reaction and
to gain some support for the mechanistic postulate we in-
vestigated the reaction with 6-bromo-2-methyl-2H-chromene-3-
carbaldehyde (Scheme 3).
Entry
Product
R1
R2
R3
R4
Yielda (%)
1
2
3
4
5
6
7
8
5a
5b
5c
5d
5e
5f
H
H
H
H
H
H
H
H
H
Br
Cl
H
H
H
H
CH3
H
H
H
H
H
H
H
H
H
H
94
82
80
70
65
64
53
45
(CH3)2CH
H
OCH3
CH3
5g
5h
H
(CH3)2CH
a Isolated yield.
was obtained by single crystal X-ray determination on 5g
(Fig. 1).
While the mechanistic intricacies of the transformation de-
scribed here remain to be unravelled, a rationalization along the
following lines may be postulated (Scheme 2).
Scheme 3
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Gratifyingly, in this case also, analogous product was obtained.
6-Isopropyl-3-methyl-2H-chromen-2-one (5d). Colourless liq-
uid, IR (film) 1724, 1619, 1428 cm-1. 1H NMR (CDCl3, 500 MHz):
d 7.48 (s, 1H) 7.32–7.29 (m, 1H) 7.22–7.20 (m, 2H) 2.98–2.93 (m,
1H) 2.20 (s, 3H) 1.27 (d, 6H, J = 7 Hz). 13C NMR (CDCl3, 125
MHz): d 162.2, 151.5, 144.8, 139.2, 128.9, 125.5, 124.1, 119.3,
116.2, 33.5, 24.1, 17.2 ppm. LRMS-FAB calcd. for C13H14O2
(M+H)+: 203.11, found 203.21.
Conclusions
In conclusion, we have uncovered a novel NHC catalyzed
rearrangement of chromene-3-carboxaldehydes to 3-methyl
coumarins. It is conceivable that the process will find application to
the synthesis of biologically active coumarin derivatives. Further
studies are currently under way.
3,7-Dimethyl-2H-chromen-2-one (5e). white solid. Mp: 104–
◦
1
106 C, IR (film) 1710, 1623, 1437 cm-1. H NMR (CDCl3, 500
MHz): d 7.69(s, 1H) 7.33(d, 1H, J = 8 Hz) 7.15 (d, 1H, J = 8 Hz)
7.11 (s, 1H) 2.52 (s, 3H) 2.24 (s, 3H). 13C NMR (CDCl3, 125 MHz):
d 161.8, 153.8, 138.9, 135.9, 130.1, 126.5, 124.6, 117.1, 114.6, 21.7,
17.2 ppm. LRMS-FAB calcd. for C11H10O2 (M+H)+: 175.07, found
175.20.
Experimental
General
Melting points were recorded on a Bu¨chi melting point apparatus
and are uncorrected. NMR spectra were recorded at 500 (1H)
and 125 (13C) MHz respectively on a Bruker DPX-500 MHz
NMR spectrometer. Chemical shifts (d) are reported relative to
TMS (1H) and CDCl3 (13C) as the internal standards. Coupling
constants (J) are reported in hertz (Hz). Mass spectra were
recorded under EI/HRMS or FAB using a JEOL JMS 600H
mass spectrometer. IR spectra were recorded on a Bruker Alpha-
T FT-IR spectrophotometer. Gravity column chromatography
was performed using 100–200 mesh silica gel and mixtures of
petroleum ether–ethyl acetate were used for elution.
6-Methoxy-3-methyl-2H-chromen-2-one (5f). Mp 114–116 ◦C
[112 –115 ◦C],29 IR (film) 1704, 1630, 1538 cm-1. 1H NMR (CDCl3,
500 MHz): d 7.45 (s, 1H) 7.23 (d, 1H, J = 9 Hz) 7.03–7.00 (m, 1H)
6.82 (s, 1H) 3.83 (s, 3H) 2.21 (s, 3H). 13C NMR (CDCl3, 125 MHz):
d 162.0, 155.9, 147.7, 138.8, 126.3, 119.9, 117.8, 117.4, 109.2, 55.6,
17.3 ppm. LRMS-FAB calcd. for C11H10O3 (M+H)+: 191.07, found
191.27.
3,6-Dimethyl-2H-chromen-2-one (5g). Mp 114–116 ◦C, IR
1
(film) 1711, 1600 cm-1. H NMR (CDCl3, 500 MHz): d 7.43 (s,
1H) 7.24–7.23 (m, 1H) 7.19–7.17 (m, 2H) 2.39 (s, 3H) 2.19 (s, 3H).
13C NMR(CDCl3, 125 MHz): d 162.1, 151.4, 138.9, 133.7, 131.3,
126.7, 125.7, 119.3, 116.2, 20.8, 17.2 ppm. LRMS-FAB calcd. for
C11H10O2 (M+H)+: 175.07, found 175.23.
General procedure for the synthesis of 3-methyl coumarins
DBU (20 mol %) was added to a suspension of the carbene
precursor 1,3-dimesityl imidazolinium chloride (SIMesCl) (15 mol
%) and 2H-chromene-3-carboxaldehyde (0.50 mmol) in dry THF
(5 mL) and the resulting solution was stirred for 8 h–12 h. After
the removal of the solvent by distillation in vacuum using a rotary
evaporator, the residue was subjected to chromatography on a
silica gel (100–200 mesh) column using 95 : 5 petroleum ether–ethyl
acetate solvent mixtures to afford the 3-alkyl coumarin derivatives.
7-Isopropyl-3-methyl-2H-chromen-2-one (5h). Colourless liq-
uid IR (film) 1706, 1627, 1533 cm-1. 1H NMR (CDCl3, 500 MHz):
d 7.47 (s, 1H) 7.30 (d, 1H, J = 8 Hz) 7.17–7.15 (m, 1H) 7.11–7.09
(m, 1H) 3.02–2.94 (m, 1H) 2.20 (s, 3H) 1.28 (d, 6H, J = 7 Hz)
13C NMR (CDCl3, 125 MHz): d 162.3, 152.3, 138.9, 135.3, 130.4,
126.6, 125.1, 117.5, 114.1, 34.2, 23.7, 23.6, 17.2 ppm. LRMS-FAB
calcd. for C13H14O2 (M+H)+: 203.11, found 203.13.
3-Me◦thyl-2H-chromen-2-one (5a). White solid. Mp: 90–92 ◦C
[87–90 C],26 IR (film) 1709, 1638, 1447, 918 cm-1. 1H NMR
(CDCl3, 500 MHz): d 7.42 (s, 1H) 7.39–7.35 (m, 1H) 7.32 (d,
1H, J = 8 Hz) 7.2 (d, 1H, J = 8 Hz) 7.17–7.14 (m, 1H) 2.14 (s,
3H). 13C NMR (CDCl3, 125 MHz): d 161.9, 153.3, 139.0, 130.4,
126.8, 125.9, 124.1, 119.5, 116.4, 17.2 ppm. LRMS-FAB calcd. for
C10H8O2 (M+H)+: 161.06, found: 161.09.
6-Bromo-3-ethyl-2H-chromen-2-one (7). White solid. Mp:
◦
110–112 C, IR (film) 1719, 1628, 1599 cm-1. H NMR (CDCl3,
500 MHz): d 7.50 (d, 1H, J = 2 Hz) 7.48–7.46 (m, 1H) 7.31 (s, 1H)
7.13 (d, 1H, J = 8.5 Hz) 2.57–2.52 (m, 2H) 1.19 (t, 3H, J = 7 Hz)
13C NMR (CDCl3, 125 MHz): d 159.7, 150.9, 134.8, 132.1, 131.7,
128.4, 120.1, 117.1, 115.7, 22.9, 11.1 ppm. LRMS-FAB calcd. For
C11H9BrO2 (M+H)+: 252.99, found 253.28.
1
6-Bromo-3-methyl-2H-chromen-2-one (5b). Yellow solid. Mp:
152–153 ◦C [151–152 ◦C],27 IR (film) 1726, 1599, 1478, 1248, 922,
815 cm-1. 1H NMR (CDCl3, 500 MHz): d 7.55–7.53 (m, 2H)
7.41 (s, 1H) 7.19 (d, 1H, J = 9.5 Hz) 2.23 (s, 3H). 13C NMR
(CDCl3, 125 MHz): d 161.2, 152.1, 137.6, 133.2, 129.2, 127.3,
121.1, 118.2, 116.8, 17.3 ppm. LRMS-FAB calcd. for C10H7BrO2
(M+H)+: 238.97, found 239.11.
Acknowledgements
VN thanks the Department of Science and Technology (DST),
New Delhi for a Raja Ramanna Fellowship. The authors also
thank the Council of Scientific and Industrial Research (CSIR)
and the University Grants Commission (UGC) New Delhi, for
financial assistance.
6-Chlo◦ro-3-methyl-2H-chromen-2-one (5c). Yellow solid. Mp:
Notes and references
152–154 C [151–152 ◦C],28 IR (film) 1726, 1602, 1410, 1479, 925,
1
815 cm-1. H NMR (CDCl3, 500 MHz): d 7.43 (s, 1H) 7.41–7.38
1 R. Breslow, J. Am. Chem. Soc., 1958, 80, 3719–3726.
(m, 2H) 7.25 (s, 1H) 2.23 (s, 3H). 13C NMR (CDCl3, 125 MHz):
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5514 | Org. Biomol. Chem., 2011, 9, 5511–5514
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