N-heterocyclic carbene catalyzed intramolecular hydroacylation of activated alkynes: Synthesis of chromones

Article abstract of DOI:10.1002/adsc.201000828

An organocatalytic intramolecular Stetter-type hydroacylation reaction between an aldehyde and an activated alkyne has been developed. This study induces salicylaldehyde-derived alkyne derivatives to assemble into a series of chromone derivatives using a

Full text of DOI:10.1002/adsc.201000828

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DOI: 10.1002/adsc.201000828
N-Heterocyclic Carbene Catalyzed Intramolecular Hydroacylation
of Activated Alkynes: Synthesis of Chromones
Seenuvasan Vedachalam,a Qian-Ling Wong,a Biswajit Maji,a Jing Zeng,a Jimei Ma,a
and Xue-Wei Liua,*
a
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological
University, Singapore 637371
Fax : (+65)-67911961; e-mail: xuewei@ntu.edu.sg
Received: November 4, 2010; Published online: February 8, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000828.
Abstract: An organocatalytic intramolecular Stet-
ter-type hydroacylation reaction between an alde-
hyde and an activated alkyne has been developed.
This study induces salicylaldehyde-derived alkyne
derivatives to assemble into a series of chromone
derivatives using a catalytic amount of thiazolium-
based carbene catalyst.
unactivated olefin to construct the chromone scaf-
folds.
Chromones are useful heterocyclic motifs found
commonly in pharmaceutical compounds and they
often exhibit fascinating therapeutic effects.[9] Our in-
terest in drug discovery[10] motivated us to devise new
methodologies for the chromone synthesis. Recently,
we reported an NHC-catalyzed intramolecular cross-
coupling between the aldehyde and the nitrile func-
tion to afford 3-aminochromones in high yields[11]
[Scheme 1, Eq. (1)]. A comparison of this chemistry
to that of the Stetter reaction [Scheme 1, Eq. (2)]
prompted us to investigate the possibility of a newly
designed carbon-carbon bond forming reaction
Keywords: chromones; hydroacylation; N-heterocy-
clic carbenes; organocatalysis; Stetter reaction
Over the past few decades, the development of N-het-
erocyclic carbenes (NHC) has made important head-
ways due to their extensive applicability in several re-
actions.[1] Besides the role as excellent ligands in the
metal-catalyzed reactions,[2a–c] their ability to efficient-
ly catalyze a number of organic reactions, most nota-
bly benzoin condensation,[2d–j] Stetter reaction,[3] trans-
esterification[3h] and homo-enolate addition,[4] has con-
tributed significantly to organic transformations. In
1973, Stetter established the first conjugate addition
of aldehydes to a,b-unsaturated ester adopting the cy-
anide ion as the initial catalyst,[5a] and subsequently
using a thiazolium-based carbene catalyst.[5b,c] The
recent intermolecular variants of the Stetter reaction
set in motion an expanding usage of NHCs as organo-
catalysts for carbon-carbon bond formation.[6] By ex-
ploiting an intramolecular Stetter reaction, Trost[7a]
constructed a tricyclic system in order to synthesize
hirsutic acid. Pioneering studies carried out by Ciga-
nek,[7b] Enders[7c] and Rovis[7d,e] resulted in the devel-
opment of intramolecular cyclizations as an access to
various chromones. In addition, Glorius[8a,b] and She[8c]
embarked on intramolecular acyl anion additions to
À
Scheme 1. NHC-catalyzed C C bond formation strategy.
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219

Seenuvasan Vedachalam et al.
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Figure 1. Conformational analysis of the exocyclic-Stetter type products (I and II) and the stable aromatized product (III).
[Scheme 1, Eq. (3)]. Our proposed strategy involves omatized chromones are formed from the less favored
the intramolecular reaction of an aldehyde and an ac- exo-cyclic double bond. The calculated energy pro-
tivated alkyne (e.g., acetylenic ester) based on our files[12] (Figure 1, DFT, B3LYP/6-31G* level) of the
previous work[10] and a DFT calculation, in which ar- exocyclic Stetter product and the aromatized product
Table 1. Optimization of reaction conditions.
Entry[a]
Catalyst (equiv.)
Base[b]
Sovent (0.1M)
Yield [%][c]
1
2
3
4
5
6
7
8
A (0.2)
B (0.2)
C (0.2)
D (0.2)
E (0.2)
F (0.2)
G (0.2)
H (0.2)
D (0.2)
D (0.2)
D (0.2)
D (0.2)
D (0.2)
D (0.2)
D (0.2)
D (0.2)
D (0.2)
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DIPEA
Et3N
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
THF
t-BuOH
toluene
CH3CN
DMF
DMF
DMF
DMF
DMF
53
30
-
64
-
-
-
20
58
53
35
52
77
78
83
74
62
9
10
11
12
13
14
15
16
17
Cs2CO3
KHMDS
[a]
Unless otherwise specified, all of the reactions were carried out with freshly distilled dry solvents at room temperature
for 24 h.
[b]
[c]
Equal mol% with respect to catalyst.
Yield of isolated product.
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N-Heterocyclic Carbene Catalyzed Intramolecular Hydroacylation of Activated Alkynes
show that the exocyclic double bond I is higher in a promising yield of 77% (entry 13). Using catalyst D
energy (DE, 12.8 kcalmolÀ1) than the aromatized III, and DMF as the solvent, we varied the bases (en-
thus providing a driving force for the isomerization to tries 14–17) and found that mild bases afforded higher
chromone III. Indeed, the exocyclic double bond yields than strong bases. Et3N was observed to be the
could be as in conformation I, because the conforma- most suitable base (entry 15). Thus, the optimized
tion II is associated with a higher energy (DE, conditions were established to be as follows: thiazoli-
3.5 kcalmolÀ1) than conformation I. To test the feasi- um salt D as the pre-catalyst, Et3N as the base, DMF
bility of the above reaction [Scheme 1, Eq. (3)], a as the solvent and stirring at room temperature for
number of NHC catalysts were screened and the re- 24 h.
sults are summarized in Table 1.
We found that the aforementioned optimized con-
We began our investigation by using simple phenyl ditions are applicable for the vast majority of sub-
substrate 1b as a control variable in order to optimize strates tested in the chromone-forming Stetter-type
the reaction conditions. The initial screening was con- hydroacylation reaction. The access to the starting
ducted at room temperature for 24 h with 20 mol% of materials,[12] that is, activated alkyne derivatives was
the imidazolium, triazolium or thiazolium catalysts developed from corresponding salicylaldehyde deriva-
(A–H) utilizing 20 mol% of DBU as the base and di- tives which dramatically expanded the substrate scope
chloromethane (DCM, 0.1M) as the solvent (Table 1, (Table 2 and Table 3). In preliminary efforts, a range
entries 1–8). Thiazolium salt D (entry 4) afforded the of substrates with varied alkyl substituents on the
chromones in highest yield (64%) among the catalyst phenyl group of the salicylaldehyde-derived acetylen-
precursors. Subsequently, the effects of various sol- ic esters afford good yields (Table 2, entries 1–5).
vents on the reaction were examined using catalyst D While investigating the substrate scope, we extended
as a control (entries 9–13). The use of DMF exhibited the study to electron-donating substituents and these
Table 2. Reaction scope for chromone derivatives.
Entry
1
Substrate[a]
Product
Yield [%][b]
83
1b
2b
1c
2c
2
86
3
4
3b
4b
3c
4c
84
90
5
5b
5c
76
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Seenuvasan Vedachalam et al.
Yield [%][b]
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Table 2. (Continued)
Entry
Substrate[a]
Product
6
6b
7b
6c
7c
92
88
7
8
8b
8c
85
80
78
9
9b
9c
10
10b
10c
11
12
11b
12b
13b
11c
12c
13c
76
80
78
13
[a]
See the Supporting Information for a detailed experimental procedure.
Yield of isolated product.
[b]
delivered chromones in good to excellent yields (80– sessing a ketone moiety as the electron-withdrawing
92%, entries 6–9). Similar extension of the present group such as acetyl, trimethylacetyl, benzoyl and
methodology to electron-withdrawing substituents, phenylacetyl functionalities, which were found to fur-
such as the halides, also gave the desired chromones nish products to further broaden the synthetic utility
in good yields (75–82%, entries 10–12). However, we and tolerance of the NHC-catalyzed reaction
were not able to synthesize nitro-substituted reactants (Table 3, entries 1–4). All in all, this new method
to investigate their reactivities, although current data allows the preparation of a range of chromone deriva-
suggest that variations made to the phenyl motif have tives in a straightforward manner.
minimal impact on this carbon-carbon bond forming
With our current synthetic methodology, the suc-
reaction. Substituting the two a-hydrogens of the cess of the reaction mainly relies on the electron-with-
alkyne moiety with two methyl groups also has little drawing group of the propargyl moiety. We had at-
impact on the product yield (Table 2, entry 13). As tempted to use the simple propargyl surrogate 1a[8a]
highlighted in Table 3, we were pleased to find that without electron-withdrawing groups but it was ob-
the reaction was also suitable for the substrates pos- served that the intermolecular benzoin product in-
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N-Heterocyclic Carbene Catalyzed Intramolecular Hydroacylation of Activated Alkynes
Table 3. Reaction scope of carbonyl Michael acceptors.
and an aldehyde using a quantitative amount of imi-
dazolium or thiazolium NHC catalyst to obtain highly
functionalized furanone derivatives which highlights
the absence of Stetter-type products in the reaction.
Based on our system, we infer that both reactive spe-
cies are in close proximity to each other, allowing the
formation of the Stetter-type product instead of other
products which arises from the hydroxy enamine
having a harder oxygen or the alkyne as the first re-
acting species. Significantly, this type of intramolecu-
lar hydroacylation reaction allowed us to develop a
new class of heterocycles at room temperature. A rea-
sonable mechanism may be formulated as shown in
Scheme 2. We propose that the carbene first performs
a nucleophilic attack on the electrophilic carbonyl
carbon of the aldehyde. Subsequent proton transfer
results in an acyl anion equivalent[14] (Breslow inter-
mediate) which acts as a nucleophilic carbon to attack
the Michael acceptor (activated alkyne) thus forming
Entry Substrate[a] EWG
Product Yield [%][b]
À
1
2
3
4
14b
15b
16b
17b
CH3CO
t-BuCO
14c
15c
16c
17c
74
72
66
68
À
À
PhCO
PhCH2CO
À
[a]
See the Supporting Information for a detailed experi-
mental procedure.
[b]
Yield of isolated product.
stead of the desired hydroacylation product was a carbon-carbon bond. Proton exchange followed by
formed. However, Glorius et al. recently reported a elimination of the catalyst results in the exocyclic ki-
hydroacylation reaction using a hindered carbene netic product. This exocyclic hydroacylation product
from the same surrogate (1a) and salicylaldehyde-de- undergoes isomerization to give the aromatized prod-
rived unactivated olefin which might involve a Conia- uct, driven by the energetics of the system as previ-
ene-type reaction at higher temperature.[8a,b] In con- ously discussed.
trast to our reaction system, the reaction progress
In conclusion, the present methodology demon-
does not terminate in the exocyclic Stetter-type prod- strates the unique reactivity of these catalytically gen-
uct and subsequent isomerization forms the aromat- erated nucleophilic precursors and their subsequent
ized chromone at room temperature. Indeed, several Stetter character with activated alkynes. This versatile
control experiments were carried out with the inter- mechanistic platform can be applied not only to chro-
molecular reaction between benzaldehyde and methyl mone products, but can also be further extended for
phenylpropiolate or dimethyl acetylenedicarboxylate use in other novel synthetic methodologies. The cata-
(DMAD) under the optimized conditions and which lytic activity of the catalyst D is remarkable especially
could not support the viability of this pathway. Appa- in view of its low cost and commercial availability.
rently, Nair and Ma[13] demonstrated the intermolecu- This method also adds to the expanding use of orga-
lar reaction between an activated alkyne (DMAD) nocatalyst-based heterocycle synthesis which has been
Scheme 2. Plausible reaction mechanism.
Adv. Synth. Catal. 2011, 353, 219 – 225
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Seenuvasan Vedachalam et al.
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Experimental Section
Typical Procedure for the Intramolecular Chromone-
Forming Stetter-Type Hydroacylation Reaction, as
Exemplified for the Formation of Ethyl 2-(4-Oxo-4H-
chromen-3-yl)acetate (1c)
Precatalyst D (12 mg, 0.043 mmol, 0.2 equiv.) and 1b (50 mg,
0.216 mmol, 1 equiv.) were suspended with anhydrous DMF
(1 mL) in an oven-dried, round-bottom flask under a nitro-
gen atmosphere at room temperature. Et3N (6 mL,
0.043 mmol, 0.2 equiv.) was added via micro syringe to the
reaction mixture which was then allowed to stir for 24 h at
room temperature. The progress of the reaction was moni-
tored using TLC. Upon completion, the reaction mixture
was diluted with ethyl acetate (20 mL), washed with water
(3ꢁ20 mL), followed by brine (2ꢁ10 mL), and then dried
over Na2SO4. The solvent was removed and the crude prod-
uct was purified by column chromatography on silica gel
(eluent: hexanes/ethyl acetate 5:1) to afford 1c as a pale
1
white solid; yield: 41.5 mg (83%); m.p. 79–818C; H NMR
(300 MHz, CDCl3): d=8.21 (dd, J1 =8.0 Hz, J2 =1.6 Hz,
1H), 7.94 (s, 1H), 7.68–7.63 (m, 1H), 7.45–7.36 (m, 2H),
4.18 (q, J=7.1 Hz, 2H), 3.47 (s, 2H), 1.27 (t, J=7.1 Hz,
3H); 13C NMR (75 MHz, CDCl3): d=176.9, 170.6, 156.5,
153.7, 133.6, 125.9, 125.1, 123.6, 118.4, 118.0, 61.0, 30.9, 14.1;
FT-IR (KBr): vmax =3078, 2978, 1739, 1610, 1477, 1344, 1205,
1028, 759 cmÀ1; HR-MS (ESI): m/z=233.0815, calcd. for
C13H13O4 [M+H]+: 233.0814.
Acknowledgements
Financial supports from Nanyang Technological University
(RG50/08) and the Ministry of Health, Singapore (NMRC/
H1N1R/001/2009) are gratefully acknowledged. We thank Dr.
Yong-Xin Li for X-ray analyses and also thank Drs. Guan
Leong Chua and Guofu Zhong for proofreading this manu-
script.
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