Facile synthesis of 3-substituted imidazo[1,2-: A] pyridines through formimidamide chemistry

Article abstract of DOI:10.1039/c9ra05841a

A facile entry to 3-aryl/alkenyl/alkynyl substituted imidazo[1,2-a]pyridines (3a-p, 6a-d & 9a-9e) has been developed from readily available benzyl/allyl/propargyl halides and 2-amino pyridines as substrates via formimidamide chemistry that is devoid of caustic or expensive reagents, such as transition metal complexes. Quantum chemical calculations performed to understand the underlying mechanism of the transformation revealed a preference for intramolecular Mannich-type addition over pericyclic 1,5-electrocyclization for the systems reported herein that enable a Baldwin allowed 5-exo-trig cyclization instead of a formally anti-Baldwin 5-endo-trig process.

Full text of DOI:10.1039/c9ra05841a

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Facile synthesis of 3-substituted imidazo[1,2-a]
pyridines through formimidamide chemistry†‡
Cite this: RSC Adv., 2019, 9, 29659
a
Vamshikrishna Reddy Sammeta,a Yanchang Huang,a
*
Rasapalli Sivappa,
James A. Golena and Sergey N. Savinov
b
A facile entry to 3-aryl/alkenyl/alkynyl substituted imidazo[1,2-a]pyridines (3a–p, 6a–d & 9a–9e) has been
developed from readily available benzyl/allyl/propargyl halides and 2-amino pyridines as substrates via
formimidamide chemistry that is devoid of caustic or expensive reagents, such as transition metal
complexes. Quantum chemical calculations performed to understand the underlying mechanism of the
transformation revealed a preference for intramolecular Mannich-type addition over pericyclic 1,5-
electrocyclization for the systems reported herein that enable a Baldwin allowed 5-exo-trig cyclization
instead of a formally anti-Baldwin 5-endo-trig process.
Received 28th July 2019
Accepted 3rd September 2019
DOI: 10.1039/c9ra05841a
rsc.li/rsc-advances
mediated regioselective synthesis of 3-aryl imidazopyridines
using standard a-halo ketones as substrates.7 Enolizable alde-
Introduction
hydes have also been utilized as substrates for the 3-substituted
imidazopyridine synthesis under oxidative conditions.8–10 Uma
Maheshwari et al. have reported Et3N catalyzed oxidative ami-
nation and denitration reactions leading to 3-aryl imidazopyr-
idines using substituted nitrostyrenes as substrates.11
Imidazo[1,2-a]pyridines, nitrogen containing fused bicyclic
heterocycles, are considered to be privileged scaffolds in the
eld of medicinal chemistry, due to their ability to display
a wide range of pharmacological activities, such as antifungal,
antibacterial, antitumor, antipyretic, analgesic, antiulcer, anxi-
olytic, antineoplastic, cardiac stimulant and anti-osteoporotic
activities.1–5 Zolpidem (hypnotic), miroprofen (analgesic), DS-1
(GABAA positive allosteric modulator), zolimidine (peptic
ulcer), olprinone (cardiac stimulant) and minodronic acid (anti-
osteoporotic) are some of the currently marketed imidazopyr-
idine drugs (Fig. 1).
¨
Krohnke et al. reported synthesis of imidazopyridines using
benzyl-2-iminopyridines and carboxylic anhydrides.12 All these
methods have certain advantages, as well as disadvantages,
such as formation of regioisomeric mixtures, harsh and
demanding conditions, need for uncommon reagents or cata-
¨
lysts. Inspired by the lone example reported by Wurthwein et al.,
involving base-mediated 6-electron cyclization of imino pyr-
idinium salt to obtain 2,3-disubstituted imidazopyridine from
benzyl bromide and benzaldehyde,13 and buoyed by our
previous success in employing aminopyrimidine for-
mimidamide chemistry,14 we became interested in extending
Due to this promising potential to be a useful scaffold in
drug design, continuous synthetic efforts are directed to access
2/3-substituted imidazo[1,2-a]pyridines in novel and facile
ways. Several methods have been reported for the synthesis of
substituted imidazopyridines, as shown in Scheme 1. For
example, Jafarzadeh et al. have reported uconazole-
functionalized nanoparticle (Fe3O4@SiO2–Flu) catalyzed
synthesis of 3-aryl or 3-amino-imidazo-[1,2-a]pyridines using
standard a-halo ketones or aryl aldehydes and substituted iso-
nitriles, respectively.6 Meshram et al. have reported DABCO
aDepartment of Chemistry and Biochemistry, University of Massachusetts, 287 Old
Westport Rd, North Dartmouth, MA-02747, USA. E-mail: srasapalli@umassd.edu
bDepartment of Biochemistry and Molecular Biology, UMass Amherst, Amherst, MA-
01003, USA
† Dedicated to Prof. Alan P. Marchand in recognition of his contributions to
Physical Organic Chemistry on the occasion of 80th birthday.
‡ Electronic supplementary information (ESI) available: Complete experimental
procedures are provided, including copies of 1H and 13C NMR spectra of all new
compounds and HRMS analysis. X-ray data for 2a, 3d, 3i, 8d and 9d is also
provided. CCDC 1920094–1920096, 1922120, 1910925 and 1910928. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c9ra05841a
Fig. 1 Marketed imidazo[1,2-a]pyridine drugs.
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reaction between 2-aminopyridine and N,N-dime-
thylformamide dimethyl acetal (DMF-DMA) under reux in
toluene (see Table 1).14 We initially attempted to access
imidazopyridine 3a by exposing compound 1 and benzyl
ꢀ
bromide together to elevated temperatures (toluene/115 C
in the pressure tube), but we could only isolate the quater-
nary ammonium salt (2a). We have reasoned that the cycli-
zation of 2a may require the help of an acidic or basic
promoter, as the attempted cyclization of 2a even at highly
elevated temperature (i.e., 180 ꢀC/DMSO) also failed to
access compound 3a. Our efforts in acid catalysis to activate
the imine of formimidamide in compound 2a using catalytic
sulfuric acid or a super acid [i.e., triic acid (TfOH)] were
also unsuccessful, indicating the need for a base to activate
the benzylic carbon attached to the quaternary nitrogen of
compound 2a as a nucleophilic center. In the case of a-halo
ketones as N-alkylating electrophiles, simple heating is
enough to drive the cyclization as enolization and, thereby,
introduction of nucleophilicity at the carbon attached to the
quaternary nitrogen is facilitated by two anking electron-
withdrawing groups. In the case of benzyl halides as
substrates though, the methylene carbon attached to the
quaternary nitrogen is not acidic enough to drive the
thermal cyclization thus demanding a base promoter. We
have initially used sodium hydroxide as a base and water as
a reaction medium, which failed to provide compound 3a,
but yielded rather hydrolysed product 7 (Table 1) via
hydrolysis of the formimidamide group. The formation of
such compounds accounted for the loss of the desired
product in subsequent experiments, as any remaining H2O
in the reaction mixture resulted unavoidably in hydrolysis
products. So, we then resorted to non-nucleophilic bases
and water-free reaction conditions to suppress the same.
The use of DABCO or DBU resulted in failure to generate the
cyclised product 3a though traces of hydrolysis products
were still observed, along with unreacted starting material
and degradation products, as the above two bases are
hygroscopic and had collected H2O upon storage. The use of
potassium t-butoxide, however, was successful in providing
compound 3a in 16% yield which was encouraging to
intensify our efforts to nd a better condition. The later
attempts of using one molar equivalent of sodium hydride
(60% w/w in mineral oil) yielded 23% of the desired product
upon reux in THF for 0.5 h. Gratifyingly, doubling the
amount of sodium hydride increased the product yield to
44%, while further attempts of increasing molar excess of
NaH over the cyclization precursor had no effect on product
yield. This observation suggests that proton abstraction by
the base is the rate limiting and, therefore, yield-controlling
step in this process. With the above conditions in hand, we
have explored the scope of this reaction, using various
arylmethyl halides as substrates (Scheme 3) to access
compounds 3a–3p. A closer look at the observed yields
reveals that electron-withdrawing groups on the meta posi-
tion of the benzyl substituent tend to increase the product
recovery, especially for doubly substituted variants (c.f., 3m,
3n, 3e, 3k, 3f, 3h, and 3g).
Scheme
imidazopyridines.
1 Methods for the synthesis of 2/3-substituted
and furthering the scope of the later functionality to obtain 3-
substituted imidazopyridines. We reasoned that activating the
simple halides through formation of corresponding
formimidamide-pyridinium salts and thereby incorporating an
electrophilic carbon onto 2-aminopyridine would obviate the
need for a-halo ketones as the corresponding 1,2-bis-electro-
philes.15–17 In addition to the undesirable characteristics and
limitations of a-halo ketones, the regioselectivity of their cycli-
zation with 2-aminopyridines is a major concern (Scheme 2).
The results of our efforts towards the same are reported herein.
Results and discussions
Our efforts commenced with the synthesis of N,N-dimethyl-
N'-(pyridin-2-yl)formimidamide (1) accessed through
Scheme 2 Proposed chemistry for synthesis of 3-substituted imida-
zopyridines through formimidamide pyridinium salt to obviate the
need for a-halo ketones and 5-endo-trig cyclization.
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Table 1 Optimization of reaction conditions for the synthesis of 3a
Temperature/reaction
% yield
Entry
Solvent
Reagent/catalyst
Equivalents
time
% isolated yield of 3a
of hydrolysis product 7
1
2
3
4
5
6
7
8
DMSO
THF
THF
H2O
THF
THF
THF
THF
DCM
THF
—
—
1
1
1
1
1
1
1
1
1
2
2
2
2
2
180 ꢀC/6 h
65 ꢀC/12 h
65 ꢀC/12 h
RT/30 min
RT/1 h
Not observed
Not observed
Not observed
Not observed
Not observed
Not observed
Not observed
16%
14%
23%
44%
22%
Trace
—
—
41%
35%
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
H2SO4
Triic acid
NaOH
NaOH
DABCO
DBU
KOtBu
KOtBu
NaH
NaH
NaH
NaH
NaH
65 ꢀC/12 h
65 ꢀC/12 h
65 ꢀC/2 h
9
40 ꢀC/2 h
10
11
12
13
14
15
65 ꢀC/0.5 h
65 ꢀC/0.5 h
40 ꢀC/0.5 h
65 ꢀC/0.5 h
65 ꢀC/0.5 h
65 ꢀC/0.5 h
THF
DCM
DMF
Dioxane
DMA
20%
11%
21%
NaH
This yield trend follows closely the Hammett meta-effect That is, once the pyridinium ylide is formed, the cyclization and
constants,18 which correlate with both electron-withdrawing subsequent aromatization through b-elimination of dimethyl-
strength of substituents and acidity at the benzylic carbon, amine occurs with few (if any) interferences from unwanted side
conrming the central role that the base plays in this reaction. processes. In fact, the reaction can be followed visually since the
Scheme 3 Synthesis of substituted 3-aryl imidazo[1,2-a]pyridines – substrate scope.
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(unoptimized) due to, most likely, reduced acidity of the cycli-
zation precursors.
The X-ray crystallography data on some of the pyridinium
salts (2a & 2o, see ESI†) revealed that they are hygroscopic and
absorb moisture upon storage, which could negatively impact
the cyclization reaction as it requires anhydrous conditions to
supress the hydrolysis product. We then thought of condensing
the formation of imine, pyridinium salt, and the cyclization of
the corresponding ylide to a one-pot solvent-free procedure to
avoid the isolation of potentially hygroscopic pyridinium salts.
Thus, we have attempted this procedure to access compound 3a
(Scheme 6) and successfully synthesized it in 58% yield, which
is greater than the yield from the route shown in Scheme 3. We
have also witnessed the absence of the hydrolysed product 7 in
the one-pot synthesis. We are currently focusing on further
optimization of this procedure and extending this to diverse
substrates.
Scheme 4 Substrate scope of substituted 2-aminopyridines.
Reaction mechanism & computational studies
¨
Previous studies by Wurthwein et al. on a cyclization reaction of
similar imino pyridinium salts, which lack the dimethylamino
group, suggested that these precursors undergo an apparent
base-initiated anti-Baldwin24 (5-endo-trig) cyclization to provide
imidazo[1,2-a]pyridines in low yields, upon 4 h exposure to KOt-
Scheme 5 Substrate scope of alkenyl/alkynyl bromides.
orange colour of the ylide solution dissipates quite rapidly. It is
also quite notable that in the above reactions, no hydrolysis
product was detected.
We have also explored the scopes of substituted amino
pyridines as substrates (Scheme 4). While the listed yields are
unoptimized,
the
relatively
poor
performance
of
triuoromethyl-substituted 2-aminopyridine suggests that the
nucleophilicity of the pyridine nitrogen is important for the
overall reaction success. Notably, the alkylation step could be
optimized further via solvent optimization and/or catalyst use.
Finally, we have also explored the scope of allyl and propargyl
bromides to access the corresponding 3-vinyl and 3-ethynyl
imidazopyridines, as they provide attractive functional handles
for further manipulation in organic synthesis.19,20 Such
compounds are generally accessed through palladium-
mediated coupling reactions.21–23 Using our formimidamide
chemistry under basic conditions, we were able to access
compounds 9a–9e through transition metal-free formamidine-
pyridinium route (Scheme 5), albeit in relatively low yield
Scheme 7 Plausible mechanistic pathways for the formation of 3a
from compound 2a.
Scheme 6 One-pot synthesis of compound 3a.
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Bu (2 eq.) in heated THF.13 Quantum-chemical calculations and
subsequent analysis of reaction coordinates have suggested that
this sluggish anti-Baldwin reaction proceeds via a pericyclic 1,5-
electrocyclization process, rather than an intramolecular addi-
tion. The superior yields of imidazopyridines and broader scope
of our system prompted us to investigate the mechanism and
the role of the dimethylamino substituent in the key cyclization
step. The positioning of counter anions in the crystal structures
of pyridinium salts revealed that the dimethyl amino substit-
uent participates actively in the delocalization of positive charge
throughout the trinitrogen conjugated system (2a, 2o & 8d, see
ESI†), with a signicant resonance contribution from the
dimethylimino form. Thus, the formation of 3a could arise from
Fig. 2 FMO's of lowest energy transition state.
¨
either 1,5-electrocyclization, as in the Wurthwein et al. system,
Search for minima and transition states (TS) for the cyclization
reaction, their geometry optimizations and validation of stationary
points via vibrational frequencies indicated that the lowest energy
conformations of pyridinium ylide (Ra and Sa) have to undergo
signicant reorganization via bond rotations to bring into proximity
and align the reactive centers: nucleophilic carbon of the ylide and
distal electrophilic carbon of the formimidamide group (Scheme 8).
The activation barriers for achieving the TS geometries (6.0 and
7.6 kcal molÀ1 for those leading to trans and cis products, respec-
tively) were found to be signicantly lower than those reported by
or Mannich-type intramolecular nucleophilic addition, which
can now proceed via Baldwin-allowed 5-exo-trig ring closure, as
shown in Scheme 7.
Geometry optimization and assessment of pKa of benzylpyr-
idinium salt 2a (pKa z 27 in DMSO), using quantum-chemical
¨
approach implemented within program Jaguar (Schrodinger,
LLC), rationalized both the need for a relatively strong base, such as
KOtBu (pKa of conjugate acid z 29 in DMSO)25 and NaH (pKa of
conjugate acid $ 34),26 to initiate this reaction and the observed
dependence of the yield on acidity of the ylide.
¨
Wurthwein et al. for the dimethylamino-free imino pyridinium
system (10.5 kcal molÀ1) at the same level of Density Functional
Theory (B3LYP/6-31+G*). The less sterically congested TS leading to
the trans product is expected to favour its formation in preference to
cis. However, this could not be established experimentally due to
facile elimination of dimethylamine with concomitant aromatiza-
tion of the resulting bicycle under the reaction conditions.
The signicantly reduced activation barrier resulting in
improved imidazopyridine yield within the formimidamide
series over the imino one raises the question of mechanistic
differences in the two highly related cyclization processes.
Further analysis of the frontier molecular orbitals of the for-
mimidamide TS revealed that the highest molecular orbital
coefficients at the reactive centers (ylide carbon anion and distal
formimidamide carbon) reside on the highest occupied
molecular orbital (HOMO) and the second lowest occupied
molecular orbital (LUMO+1; LUMO is centered on pyridinium
ring), respectively (Fig. 2). Notably, none of the frontier molec-
ular orbitals extend through the 5-atom/6-electron system that
would be expected for a pericyclic process (path A in Scheme 7).
Both HOMO and LUMO+1, on the other hand, are fully
consistent with path B mechanism, whereby the carbanion,
reacts with dimethyliminium, both stabilized by extensive
conjugation with adjacent groups. Thus, the dimethylamine
substituent, which participates in delocalization of positive
charge, as seen in the crystal structures of formimidamides (see
above Scheme 5), appears to play a key role in the reaction
described herein. Notably, this group converts the formally anti-
Baldwin 5-endo-trig cyclization, forced to undergo a thermal
pericyclic process, into a formally allowed 5-exo-trig cyclization,
which yields a practical and versatile synthetic procedure for
a medicinally important class of aromatic heterobicycles.
Scheme 8 Minima and transition states for cyclization of ylide derived
from compound 2a to yield compound 3a.
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4 L. Almirante, L. Polo, A. Mugnaini, E. Provinciali, P. Rugarli,
A. Biancotti, A. Gamba and W. Murmann, J. Med. Chem.,
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Conclusions
We have successfully developed a facile entry to 3-aryl/alkenyl/
alkynyl substituted imidazo[1,2-a]pyridines using respective
benzyl/allyl/propargyl halides and 2-amino pyridines as
substrates (3a–p & 6a–d) via formimidamide chemistry that is
devoid of expensive or caustic reagents or transition metals.
Anhydrous conditions are necessary for suppressing major side
reaction and for efficient synthesis of 3-substituted imidazo-
pyridines. We have also developed a one-pot procedure, which
furnished the corresponding heterobicycle in a better yield, and
are currently focussing on further optimization and scope
extension. Quantum chemical analysis of the reaction mecha-
nism revealed that dimethyliminium group, acting as an exo
substituent in the cyclization reaction, is benecial in reducing
the activation energy by permitting a formally allowed Mannich-
type reaction over Baldwin-forbidden 1,5-electrocylization. We
have also been able to access both vinyl and ethynyl substituted
imidazopyridines (9a–e) that can, in turn, be used as starting
points for further synthetic elaboration of fused heterocycles
through the convenient functional handles. We have success-
fully expanded the repertoire of halides and 2-aminopyridines
as substrates in this chemical transformation to access the
imidazopyridine which otherwise was restricted to a-halo
ketones. Currently, we are focusing on optimizing reaction
conditions and on extending the scope of this formimidinium
way of transforming a formally anti-Baldwin 5-endo-trig cycli-
zation into a formally allowed 5-exo-trig cyclization to obtain
diversely substituted fused imidazosystems through, and the
results will be reported in due course.
´
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Conflicts of interest
There are no conicts to declare.
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This work has been supported by LSMF, and OTCV Phase-I
funds of UMass System and instrumentation grants from the
NSF (Grants CHE-1229339 and CHE-1429086 for funds to
purchase of the 400 MHz NMR spectrometer and X-ray diffrac-
tometer, respectively, used in this research).
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