Pyridinium triflate catalyzed intramolecular alkyne-carbonyl metathesis reaction of O-propargylated 2-hydroxyarylaldehydes

Article abstract of DOI:10.1016/j.tetlet.2020.152657

3,5-Dibromopridinium trifluoromethanesulfonate catalyzes the intramolecular alkyne-carbonyl metathesis reaction of a variety of O-propargylated 2-hydroxyarylaldehydes and ketones bearing alkyl, aryl and heteroaryl substituted internal alkynes to provide v

Full text of DOI:10.1016/j.tetlet.2020.152657

Journal Pre-proofs
Pyridinium Triflate Catalyzed Intramolecular Alkyne-Carbonyl Metathesis
Reaction of O-Propargylated 2-Hydroxyarylaldehydes
Manoj Kumar Saini, Harshita Singh Korawat, Shashi Kant Verma, Ashok K.
Basak
PII:
S0040-4039(20)31168-0
DOI:
Reference:
https://doi.org/10.1016/j.tetlet.2020.152657
TETL 152657
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
5 October 2020
4 November 2020
8 November 2020
Please cite this article as: Kumar Saini, M., Singh Korawat, H., Kant Verma, S., Basak, A.K., Pyridinium Triflate
Catalyzed Intramolecular Alkyne-Carbonyl Metathesis Reaction of O-Propargylated 2-Hydroxyarylaldehydes,
Tetrahedron Letters (2020), doi: https://doi.org/10.1016/j.tetlet.2020.152657
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Pyridinium Triflate Catalyzed Intramolecular Alkyne-Carbonyl Metathesis
Reaction of O-Propargylated 2-Hydroxyarylaldehydes
Manoj Kumar Saini, Harshita Singh Korawat, Shashi Kant Verma, Ashok K. Basak*
Department of Chemistry, Institute of Science Banaras Hindu University, Varanasi 221005, India
Corresponding author e-mail ID: akbasak.chem@bhu.ac.in
Abstract: 3,5-dibromopridinium trifluoromethanesulfonate catalyzes the intramolecular alkyne-
carbonyl metathesis reaction of a variety of O-propargylated 2-hydroxyarylaldehydes and
ketones bearing alkyl, aryl and heteroaryl substituted internal alkynes to provide various 3-
(hetero)aroyl 2H-chromenes in high yields.
Introduction: Over the last two decades, alkyne-carbonyl metathesis (ACM) reaction has
emerged as a powerful tool for the stereoselective synthesis of α,β-unsaturated ketones utilizing
an alkyne (mostly internal alkyne) and a carbonyl compound (aldehyde/ketone).1 The general
reaction is shown in Scheme 1. The ACM reaction is considered as a viable and atom-economic
alternative to the classical Wittig olefination and has been extensively utilized for the synthesis
of biologically active natural products.2 While the intermolecular version of the reaction provides
direct access to the synthetically useful acyclic α,β-unsaturated ketones, the intermolecular
version has widely been demonstrated as a unique strategy for annulation reactions which
delivers ring systems difficult-to-synthesize or nearly impossible to synthesize by other known
synthetic methods.3 This aspect of the intermolecular alkyne-carbonyl metathesis reaction has
been continuing attracting interest of synthetic organic chemists worldwide. The ACM reaction
(a) Intermolecular alkyne-carbonyl metathesis
R1
O
O
R4
LA/BA
+
R2
R4
R2
R1
R3
R3
(b) Intramolecular alkyne-carbonyl metathesis
R2
R1
O
R1
O
R2
LA/BA
Scheme 1: General scheme for alkyne-carbonyl metathesis reaction
is usually catalyzed by σ-electrophilic Lewis acids4 (LA) such as BF3.OEt2, FeCl3, Sc(OTf)3,
In(OTf)3, Mg(OTf)2, Sn(OTf)2, Zn(OTf)2, SbF5, π-electrophilic Lewis acids5 (AgSbF6, AgOTf,
1

AuCl3), combination of Lewis acid with alcohol6 and Bronsted acids7 (BA) such as TfOH,
CF3CO2H, HBF4, etc. In addition, Barluenga reagent in combination with HBF4 (Py2IBF4-HBF4)
has also been reported as an efficient catalyst for the intra- and intermolecular ACM reaction.8
On the other hand, pyridinium salts are commonly utilized as mild, neutral and
environmentally benign organocatalyst for various transformations such as acetal formation and
cleavage,9 H2O2 mediated oxidation of sulfide to sulfoxide10 and aziridination of olefins.11
Pyridinium p-toluenesulfonate (PPTS) is a suitable catalyst for deprotection of silyl ethers or
tetrahydropyranyl ethers when a substrate is not compatible to stronger acid catalysts. It is also a
commonly used catalyst for the preparation of acetals and ketals from aldehydes and ketones. In
addition to these reports, very recently Kancharla et al. showed that 2,4,6-tri-tert-
butylpyridinium chloride acts an efficient catalyst for highly stereoselective glycosylation
reactions of glycals.12 However, despite these reports, to the best of our knowledge, alkyne-
carbonyl metathesis reaction catalyzed by an organocatalyst, in particular, readily available
pyridinium salts has not been reported in the literatures.
Among the various intramolecular versions, alkyne-carbonyl metathesis reactions of
internal alkyne-tethered 2-hydroxyarylaldehydes have recently gained significant research
interest due to the important biological activities associated with the products derived thereof.
Although examples (mostly single example) of this intramolecular reaction were scattered in the
literature,5a,7a in 2011, Jana and co-workers carried out extensive work and demonstrated that
internal alkyne-tethered 2-hydroxyarylaldehydes can be converted into the corresponding 3-aroyl
2H-chromenes in high yields using 15 mol% FeCl3 in refluxing CH3CN (12 examples, 45-89%
yields).13 Later on, Saito et al. utilized Py2IBF4-HBF4 catalytic system for the intramolecular
ACM reaction (4 examples, 50-80% yields).8 Das and co-workers showed that stoichiometric
amount (1.2 eq) of BF3.OEt3 in trifluoroethanol promote the intramolecular metathesis reaction
to generate 3-aroyl 2H-chromenes in excellent yields at rt (13 examples, 90-97% yields).14
Adding to these development, in a very recent report, Ramesh et at. utilized acidic deep eutectic
solvent (choline chloride:p-toluene sulfonic acid) as a promoter for this useful transformation (13
examples, 29-92% yields).15 However, these reported methods have certain drawbacks such as
use of transition metal based catalyst, use of stoichiometric amount of Lewis acid and use of
special solvents which limits their wide applicability, practicality and environmental
compatibity. Moreover, all these reports are restricted to the substrates containing aryl and alkyl-
2

substituted internal alkynes. To the best of our knowledge, there is no report for this
intramolecular alkyne-carbonyl metathesis reaction of substrates bearing a heteroaryl substituted
internal alkynes. Herein, we disclose our results of the intramolecular alkyne-carbonyl
metathesis reaction of a fairly large number of O-propargylated 2-hydroxyarylaldehydes bearing
aryl, alkyl and heteroaryl substituted internal alkynes using 30 mol% 3,5-dibromopyridinium
triflate as the organocatalyst in MeOH at 80 oC.
(a) Shonogashira coupling
O
O
I
CF3
H
CuI, Pd(PPh3)4
Et3N, DMSO
H
+
O
CF3
3f
O
rt
2b
1a
88% yeild
(b) Etherification (O-alkylation)
O
O
O
H
Br
K2CO3, DMF
+
H
rt
3m
78% yield
2g
OH
1g
Scheme 2: Synthesis of O-propargylated benzaldehydes bearing internal alkyne
We began our investigation with 2-propargyloxybenzaldehyde 3f which was prepared in
two steps from commercially available starting materials (Scheme 2a). Initially, 2-
propargyloxybenzaldehyde 3f was treated with 10 mol% hydrochloride salt A in DCM at rt.
However, no detectable transformation was observed. Upon heating at 60 oC in a sealed tube for
24 h, formation of trace amount of product was observed. To our disappointment, pyridinium
o
salts B and C also provided similar results in DCM at 60 C (entries c-d, Table 1). However,
when we tested the intramolecular metathesis reaction using 10 mol% pyridinium triflate salt C
o
in MeOH at 60 C, we observed approximately 20% conversion after 24 h (entry e, Table 1).
o
Carrying out the reaction at 80 C for 24 h led to 30% conversion (entry f, Table 1). Using 30
mol% of catalyst C, a moderate conversion (45%) was observed (entry g, Table 1). While the
i
catalyst C exhibited lower activity in EtOH and PrOH, its activity in HFIP was comparable to
that in MeOH (entries h-j, Table 1). Next, screening of triflate salts of halogenated pyridines
were examined as the catalyst in MeOH at 80 oC. Triflate salts derived from 2-fluoropyridine and
2-chloropyridine provided inferior results when compared to that of 2-bromopyridine (entries k-
m, Table 1). Pyridinium salt G provided encouraging results (entry n, Table 1). When the triflate
salt H derived from 3,5-dibromopyridine
3

Table 1: Optimization study
O
H
O
O
CF3
catalyst
conditions
O
4f
CF3
3f
N
N
N
F
N
N
Cl
Cl
Ts
TfO
H
H
TfO H
H
TfO H
A
B
C
D
E
Br
Br
Cl
Cl
Br
N
Br
N
COOH
N
N
H
I
N
TfO H
TfO H
TfO
H
TfO
TfO H
F
J
G
H
entry
catalyst
conditionsa
DCM, rt, 24 h
yield (%)
NR
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
10 mol% A
10 mol% A
10 mol% B
10 mol% C
10 mol% C
10 mol% C
30 mol% C
30 mol% C
30 mol% C
30 mol% C
30 mol% D
30 mol% E
30 mol% F
30 mol% G
30 mol% H
30 mol% I
30 mol% J
30 mol% H
DCM, 60 oC, 24 h
DCM, 60 oC, 24 h
DCM, 60 oC, 24 h
MeOH, 60 oC, 24 h
MeOH, 80 oC, 24 h
MeOH, 80 oC, 24 h
EtOH, 80 oC, 24 h
iPrOH, 80 oC, 24 h
HFIP, 80 oC, 24 h
MeOH, 80 oC, 24 h
MeOH, 80 oC, 24 h
MeOH, 80 oC, 24 h
MeOH, 80 oC, 24 h
MeOH, 80 oC, 20 h
MeOH, 80 oC, 24 h
MeOH, 80 oC, 24 h
H2O, 80 oC, 24 h
trace
trace
trace
14 %
22%
36%
24%
18%
34%
18%
23%
45%
60%
85%
67%
62%
NR
aAll reactions were carried out in 0.20 mmol scale in appropriate solvent in
0.2M concentration; NR = no reaction.
was used as the catalyst, complete conversion took place within 20 h and product 4f was
obtained in 85% yield (entry o, Table 1). Catalyst I provided inferior results when compared
with catalyst H (entry p, Table 1). Interestingly, triflate salt of 2-picolinic acid also worked as
catalyst albeit in lower efficiency (entry q, Table 1). It is important to mention that no reaction
4

Table 2: Synthesis of 3-(hetero)aroyl 2H-chromenes
O
O
H
H
30 mol%
A
R
R'
A
B
R
MeOH, 80 oC
O
4
O
3
R'
B
entry
2-propargyloxyarylaldehyde
3-(hetero)aroyl 2H-chromene
3
4
O
O
H
a
O
3a
O
4a
80 oC, 24 h, 89%
O
O
Br
Cl
Br
H
H
b
O
O
4b
80 oC, 16 h, 90%
3b
O
O
Cl
c
d
e
f
O
O
4c
3c
Cl
Cl
80, 16 h, 92%
O
O
H
Et2N
O
Et2N
O
4d
3d
80 oC, 36 h, 84%
O
O
H
O
O
4e
3e
80 oC, 24 h, 91%
O
O
CF3
H
O
O
4f
CF3
80 oC, 20 h, 85%
3f
O
O
Br
Br
CF3
H
g
O
O
4g
CF3
80 oC, 24 h, 92%
3g
5

O
O
O2N
O2N
CF3
H
h
i
O
O
4h
CF3
3h
80 oC, 18 h, 86%
O
O
CF3
H
Et2N
O
Et2N
O
4i
CF3
80 oC, 36 h, 84%
3i
O
O
Cl
H
j
O
O
F
4j
Cl
F
100 oC, 24 h, 82%
3j
O
CH3
O
CH3
k
l
F
O
F
O
4k
3k
60 oC, 24 h, 62%
O
CF3
O
Br
Br
CF3
O
O
4l
60 oC, 12 h, 58%
3l
O
O
O
H
m
n
O
4m
90 oC, 12 h, 68%b
3m
O
O
Br
Br
H
O
O
4n
90 oC, 12 h, 72%b
3n
O
H
O
o
p
Et2N
O
Et2N
O
4o
90 oC, 24 h, 64%c
3o
O
O
O
H
O
O
4p
O
3p
80 oC, 8 h, 84%
6

O
O
S
H
H
q
r
O
O
4q
S
N
3q
80 oC, 10 h, 88%
O
O
N
O
O
4r
150 oC, 12 h, (35)c
3r
O
O
H
H
N
s
t
O
O
4s
150 oC, 16 h, (42)d
N
3s
O
O
OMe
O
O
3t
4t
80 oC, 24 h, 80%
MeO
aAll reactions were carried out in 0.25 mmol scale using 1.0 eq of 3, 30 mol% of
b
catalyst H, in MeOH (0.2 M); reaction was carried out in DCM-MeOH (1:1);
cNMR yield (40% conversion); dNMR yield (45% conversion).
o
was observed in water at 80 C possibly due to the insolubility of the starting material in water
(entry r, Table 1).
Identifying pyridinium salt H as an efficient catalyst for the high yielding conversion of
3f to 4f, we next sought to evaluate the scopes of this catalytic transformation. A series of 2-
propargyloxybenzaldehydes bearing (hetero)aryl and alky substituted internal alkynes were
synthesized following reported procedures. As shown in Scheme 2, a wide variety of O-
propargylated 2-hydroxyarylaldehydes bearing internal alkynes can be synthesized via
complementary methods, viz. Sonogashira coupling and O-alkylation. In addition, the
trifluoromethyl ketone containing O-propargylated compound 3l can be synthesized in two steps
(see the supplementary information, page S6) from compound 3b. As shown in Table 2, the
alkyne-carbonyl metathesis reaction of 2-propargyloxybenzaldehydes bearing phenyl and 3-
(trifluoromethyl)phenyl substituted internal alkynes underwent smooth metathesis reaction
furnishing 3-aroyl 2H-chromenes in very high yields (entries a-i, Table 2). Due to the presence
of two strong electronegative halogen atoms (F, Cl) in aryl ring B, substrate 3j required elevated
7

o
temperature (100 C) for the alkyne-carbonyl metathesis reaction to furnish 2H-chromene 4j in
82% yields (entry j, Table 2). Substrates bearing electron-withdrawing groups on the aryl ring A
showed better reactivity than those having strong electron-releasing group on the aryl ring A.
Substrates bearing methyl ketone as well as trifluoromethyl ketone exhibited lower reactivity
when compared to the substrates containing aldehydic group. The lower reactivity of these
substrates could be attributed to the steric and electronic effects exerted by the methyl and
trifluoromethyl groups in the transition state of the cycloaddition reaction. Nevertheless,
synthetically useful yields of the corresponding 3-aroyl 2H-chromenes were obtained by carrying
out the reactions at 60 oC (entries k-l, Table 2). Unlike phenyl substituted 2-
propargyloxybenzaldehydes, the cyclohexyl substituted substrates exhibited lower reactivity
towards the metathesis reaction. However, good yields of the products were obtained by carrying
o
out the reaction at 90 C (entries m-o, Table 2). Next, the scopes of heteroaryl substituted 2-
propargyloxybenzaldehydes were evaluated under the optimized reaction conditions. Electron-
rich heteroaryl (furan, thiophene) substituted 2-propargyloxybenzaldehydes underwent smooth
o
reaction at 80 C in short reaction time to provide the corresponding 3-(hetero)aroyl 2H-
chromenes in high yields (entries p-q, Table 2). On the contrary, 2- and 3-pyridyl substituted 2-
propargyloxybenzaldehydes failed to undergo metathesis reaction at 80 oC. However,
approximately 40% conversion took place when these substrates were heated with 1.0 eq. of
o
catalyst H in MeOH at 150 C (entries r-s, Table 2). Unfortunately, despite our best efforts, the
pyridyl products could not be separated from the starting materials to obtain a sample for
recording analytical data. It would be interesting to see the reactivity of the substrates containing
pyridine N-oxide which may exhibit better reactivity towards the metathesis reaction. It is worth
mentioning that substrate 3t bearing 2-methoxynaphthyl substituted alkyne also underwent
metathesis reaction to provide 2H-chromene 4t in high yield (80%). Due to the extended
conjugation, all the synthesized 3-(hetero)aryl 2H-chromenes could be visualized under long UV
(354 nm) at which 2-propargyloxybenzaldehydes usually do not show any absorption.
The mechanistic hypothesis of the pyridinium triflate catalyzed ACM reaction is outlined
in Scheme 3a. Pyridinium triflate H activates an alcohol molecule (solvent) via intermolecular
hydrogen bonding between the H-atom of pyridinium salt and O-atom of the alcohol molecule.
The H-atom of the alcohol molecule, in turn, activates the carbonyl group via intermolecular H-
bonding towards the intramolecular [2+2]-cycloaddition reaction leading to the oxetene
8

intermediate and release of the pyridinium catalyst. Formal (2+2)-cycloreversion of the unstable
oxetene intermediate leads to the 3-aroyl 2H-chromene with complete regioselectivity. Direct
activation of the carbonyl group by the pyridinium salt, as shown in Scheme 3b, is also
considerable in aprotic solvent (such as 1,2-dichloroethane). However, substrate activation by the
o
catalyst in aprotic solvent is less effective. For example, with catalyst H at 100 C for 24 h, 2-
propargyloxybenzaldehyde 3j underwent complete conversion in MeOH whereas only
approximately 60% conversions was observed in 1,2-dichloroethane. Recent literature reports by
Kancharla and co-workers12 further supports our catalytic hypothesis in MeOH (solvent).
Br
Br
N
H
O
O
R'
O
Br
Br
H
cat.
H
H
O
R'OH
(solvent)
N
H
H
O
R
O
O
(a)
(i) [2+2]-cycloadd.
(ii) cycloreversion
(b)
O
R
H
R
O
[2+2]-
H
cat. + R'OH
cycloaddition
R
H
+ cat
O
R
O
cyclo-
reversion
R
O
O
oxetene
Scheme 3: Plausible mechanism of the pyridinium salt catalyzed ACM reaction.
Conclusion: In summary, the intramolecular alkyne-carbonyl metathesis reaction of 2-
propargyloxyarylaldehydes containing (hetero)aryl and alkyl substituted internal alkynes
catalyzed by 3,5-dibromopyridinium triflate in MeOH has been demonstrated. The mild reaction
conditions are amenable to several functional groups (NO2, NEt2, CF3, enones) and provide a
wide variety of novel 3-(hetero)aroyl 2H-chromenes in good yields. 2-Pyridyl and 3-pyridyl
substituted 2-propargyloxybenzaldehydes exhibited poor reactivity under the standard reaction
conditions. It would be interesting to examine the reactivity of the substrates bearing pyridine N-
oxide substituted internal alkynes. The intramolecular alkyne-carbonyl metathesis reaction may
provide interesting results in the presence of a pyridine based bifunctional organocatalyst.16 We
will investigate on these in our future work.
Notes: Authors declare no competing financial interest.
9

Acknowledgement: Research grants from Council of Scientific and Industrial Research, New
Delhi (Grant No. 02(0346)/19/EMR-II) is gratefully acknowledged. MKS thanks CSIR, New
Delhi for a research fellowship. HSK and SKV thank UGC, New Delhi for research fellowships.
Associated content:
The Supplementary Information is available (Experimental procedures, analytical data and
copies of 1H & 13C NMR spectra).
*Corresponding Author:
Ashok K. Basak-Department of Chemistry, Institute of Science, Banaras Hindu University,
Varanasi 221005, India. E-mail ID: akbasak.chem@bhu.ac.in
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11

Graphical Abstract
Pyridinium Triflate Catalyzed Intramolecular
Alkyne-Carbonyl Metathesis Reaction of
O-Propargylated 2-Hydroxyarylaldehydes
Manoj Kumar Saini, Harshita Singh Korawat, Shashi Kant Verma, Ashok K. Basak*
Br
Br
30 mol%
R
O
O
O
N
H TfO
R'
R
MeOH, 80 o
C
O
R''
R''
R'
19 examples
upto 92% yield
R = H, CH3, CF3
R' = alkyl, aryl, heteroaryl
R'' = Cl, Br, NO2, NEt2
 Organocatalyzed alkyne-carbonyl metathesis reaction
 Activation via hydrogen bonding
 Broad substrate scope
 High yielding reactions
12