General C-H Arylation Strategy for the Synthesis of Tunable Visible Light-Emitting Benzo[a]imidazo[2,1,5-c,d]indolizine Fluorophores

Article abstract of DOI:10.1021/acs.joc.6b02928

Herein we report the discovery of the benzo[a]imidazo[2,1,5-c,d]indolizine motif displaying tunable emission covering most of the visible spectrum. The polycyclic core is obtained from readily available amides via a chemoselective process involving Tf₂O-mediated amide cyclodehydration, followed by intramolecular C-H arylation. Additionally, these fluorescent heterocycles are easily functionalized using electrophilic reagents, enabling divergent access to varied substitution. The effects of said substitution on the compounds' photophysical properties were rationalized by density functional theory calculations. For some compounds, emission wavelengths are directly correlated to the substituent's Hammett constants. Easily introduced nonconjugated reactive functional groups allow the labeling of biomolecules without modification of emissive properties. This work provides a straightforward platform for the synthesis of new moderately bright fluorescent dyes remarkable for their chemical stability, predictability, and unusually high excitation-emission differential.

Full text of DOI:10.1021/acs.joc.6b02928

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Article
General C–H Arylation Strategy for the Synthesis of Tunable Visible Light
Emitting High Stokes Shift Benzo[a]imidazo[2,1,5-c,d]indolizine Fluorophores
Éric Lévesque, William Schulz Bechara, Léa Constantineau-Forget, Guillaume
Pelletier, Natalie M. Rachel, Joelle N. Pelletier, and André B. Charette
J. Org. Chem., Just Accepted Manuscript • Publication Date (Web): 25 Apr 2017
Downloaded from http://pubs.acs.org on April 25, 2017
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General C–H Arylation Strategy for the Synthesis of Tunable
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Visible Light Emitting Benzo[a]imidazo[2,1,5-c,d]indolizine
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Fluorophores
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Éric Lévesque, William S. Bechara†, Léa Constantineau-Forget†, Guillaume Pelletier, Natalie
M. Rachel, Joelle N. Pelletier and André B. Charette*
Centre in Green Chemistry and Catalysis, Faculty of Arts and Sciences, Department of Chemistry, Université de
Montréal, P.O. Box 6128, Station Downtown, Montréal, Québec, Canada, H3C 3J7
ABSTRACT: Herein we report the discovery of the benzo[a]imidazo[2,1,5-c,d]indolizine motif displaying tunable emis-
sion covering most of the visible spectrum. The polycyclic core is obtained from readily available amides via a
chemoselective process involving Tf2O-mediated amide cyclodehydration followed by intramolecular C–H arylation.
Additionally, these fluorescent heterocycles are easily functionalized using electrophilic reagents, enabling divergent
access to varied substitution. The effects of said substitution on the compounds’ photophysical properties were rational-
ized by DFT calculations. For some compounds, emission wavelengths are directly correlated to the substituent’s
Hammett constants. Easily introduced non-conjugated reactive functional groups allow the labelling of biomolecules
without modification of emissive properties. This work provides a straightforward platform for the synthesis of new mod-
erately bright fluorescent dyes remarkable for their chemical stability, predictability and unusually high excitation-
emission differential.
INTRODUCTION
Organic fluorophores are privileged structures that have found pivotal applications in biological imaging, clinical diagnosis, and
drug discovery.1 Such small molecules are employed in stimulated emission depletion (STED) microscopy2 and single-
molecule microscopy,3 tools extensively used in biochemistry for the precise tracking of specific molecules in complex biologi-
cal systems.4 In addition, the handling of small-molecule fluorophores5 (i.e. fluorescein derivatives, benzopyrylium salts,
BODIPY, etc.) is operationally attractive in contrast to fluorescent metal complexes6 or proteins, 7 which broadens their appli-
cation in both medical and material sciences. Specifically, fluorescent molecules have gained considerable popularity recently
in material science due to their successful incorporation into organic light-emitting diodes (OLEDs).8 Despite the wide spec-
trum of applications, few examples of organic, metal-free fluorescent cores with tunable emission are available in the litera-
ture. Most of the commonly employed dyes suffer from a narrow variability of the excitation and emission wavelengths, and
substituent effects are often difficult to predict.9 Furthermore, the methodical design of bright10 fluorescent scaffolds with a
specific predictable emission wavelength is still a difficult task.11
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The constant efforts to characterize and to understand each fluorophore’s distinct properties are essential endeavors for both
fundamental and commercial applications. A good example of such innovation is characterized by the development of a series
of tunable fluorescent 1,2-dihydropyrrolo[3,4-b]indolizin-3-ones, also called Seoul-Fluors, initiated by the Park group.12 Librar-
ies of fluorophores sharing a single molecular framework were synthesized using combinatorial techniques. Significant varia-
bility of photophysical properties was achieved by installing substituents on the fluorescent core. Density Functional Theory
(DFT) calculations helped pinpoint the specific positions most sensitive to electronic effects. Unfortunately, a challenging 1,3-
dipolar cycloaddition reaction was needed to access the key tricyclic framework.
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In order to address the challenges associated with small molecule fluorophore synthesis and applications, we report the dis-
covery and characterization of a previously unknown fluorescent tetracyclic core.13 Further chemical modifications of this tet-
racyclic scaffold, through a rational approach based on calculations, enable straightforward tuning of their emissive properties.
These benzo[a]imidazo[2,1,5-c,d]indolizines are readily synthesized and functionalized in a practical, high-yielding and diver-
gent synthetic pathway. This led to the elaboration of a library of chemically stable fluorescent molecules with emission wave-
lengths covering most of the visible spectrum. The excitation spectra feature two distinct bands with a similar light output, one
of which displaying a high excitation-emission differential14 (up to 1,62 µm-1). The synthesis of these fluorophores is described,
followed by mechanistic, computational and photophysical studies. Non-conjugated reactive functional groups can be append-
ed to the aromatic core without affecting its innate photophysical properties. Such a group enabled the tagging of two target
proteins through an enzyme-catalyzed glutamine transamidation, demonstrating the utility of these fluorophores in probing
biological macromolecules.
RESULTS AND DISCUSSION
Synthetic methodology. Our group initially reported the synthesis of a single example of the benzo[a]imidazo[2,1,5-
c,d]indolizine scaffold, obtained in 3 simple steps from 6-bromo-2-pyridinemethanamine.15 However, this commercially availa-
ble primary amine is prohibitively expensive. In order to broaden the accessibility of this methodology, an alternative and easi-
ly scalable 6-step procedure from commercially available and inexpensive chemicals was designed (Scheme 1).
Scheme 1. General Synthetic Pathway Toward Benzo[a]imidazo[2,1,5-c,d]indolizines Fluorophores
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Commercially available 6-bromopicolinaldehyde (2) can either be purchased or generated from 2,6-dibromopyridine (1).16 The
previously reported synthetic pathway is intercepted by condensation of 2 with hydroxylammonium sulfate to yield the corre-
sponding oxime (3), followed by treatment with powdered zinc metal in acetic acid which reduces the oxime to the ammonium
acetate salt (4). These last two steps quantitatively yield the primary ammonium on a multi-gram scale. The fluorophore’s
fused benzene ring is then introduced along with any desired substitution by acylation of the primary ammonium (4) with a
benzoyl chloride derivative. The resulting secondary amides (5) are treated under the optimized triflic anhydride-mediated cy-
clodehydration/aromatization conditions previously developed by our research group15 to afford the corresponding imid-
azo[1,5-a]pyridines (6) (Table 1). This step is the first in the sequence where some products require chromatographic purifica-
tion.
The second cyclization, giving access to the fluorescent core (7), takes place via a palladium catalyzed intramolecular C–H
arylation. This high-yielding transformation is triggered by a Pd(0) precatalyst, a bulky trialkylphosphine ligand,17 and an ex-
cess of carbonate base (Table 1). We decided to investigate this transformation by performing kinetic isotopic assays. A reac-
tion rate comparison between fully deuterated (6a-d5) and non-deuterated (6a) substrates results in a kH/kD ratio of 2.02. By
analogy, an intramolecular competition reaction (6a-ortho-d1 as substrate) yields the C–H and C–D insertion products in a ratio
of 2:1, which represents a 2.0 KIE value. These results support a mechanism where the turnover limiting step is either the C–
H insertion step or the reductive elimination (in which case the former would be in pre-equilibrium) (Scheme 2).18 A concerted
metalation-deprotonation (CMD) mechanism is probable.19 The need for a bulky phosphine to reach completion supports the
hypothesis of a turnover-limiting, irreversible reductive elimination.17
Scheme 2. Proposed mechanism for the intramolecular C-H arylation
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After efficient reaction conditions for both intramolecular cyclization steps were established, a variety of benzo[a]imidazo[2,1,5-
c,d]indolizines fluorophores were prepared (Table 1). Gratifyingly, the overall process was efficient in the presence of various
functional groups (R2 in Scheme 1), with both reactions giving high yields regardless of the aromatic group’s electronic prop-
erties (Table 1, entries 2-16). Oxygen and sulfur-containing heterocycles 7q and 7r could even be formed, albeit in lower
yields for the C–H arylation step (entries 17-18). Alternatively, the methodology can be extended to α,β-unsaturated amides
(entries 19-20). In this case, the second cyclization likely occurs via a carbometallation-elimination sequence. Accordingly, the
reaction is inefficient with an alkene whose geometry forces a trans β-hydride elimination (entry 20).
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Table 1. Synthesis of Unsubstituted Benzo[a]imidazo[2,1,5-c,d]indolizines: Cyclodehydration and C–H Aryla-
tion
yield
(6)
(%)a
96
yield
(7)
(%)b
100
100
89
Benzoimidazo
entry
amide (5)
R2 = H
imidazopyridine (6)
indolizines (7)
1
2
5a
5b
5c
5d
5e
5f
6a
6b
6c
6d
6e
6f
7a
7b
7c
7d
7ec
7f
Ph
t-Bu
87
3
91
4
OBn
91
90
89c
5
O(CH2)2OBz
NMe2
F
52
6
84
98
7
5g
5h
5i
6g
6h
6i
7g
7h
7i
90
98
8
Cl
89
94
9
CO2Me
CN
81
95
99d
10
5j
6j
7jd
88
11
5k
6k
7k
88
93
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5l
6l
7l
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92
66
5m
6m
7m
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14
15
5n
5o
6n
6o
7n
7o
86
86
98
97
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17
5p
5q
6p
6q
7p
7q
65
84
61
40
18
19
20
5r
5s
5t
6r
6s
6t
7r
7s
7t
90
74
94
65
87
28
a Isolated yield for the corresponding imidazo[1,5-a]pyridine (6) (%). b Isolated yield for the corresponding ben-
zo[a]imidazo[2,1,5-c,d]indolizines (7) (%). c Yield of saponified product (primary alcohol) over 2 steps. d Made with no chroma-
tographic purification over 6 steps. Overall yield from 2,6-dibromopyridine : 29%
Scheme 3. Elaboration of the C-2-Aryl Series: C–H arylation or Electrophilic Bromination/Suzuki-Miyaura Cou-
pling
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a 2.0 equiv of iodide used. b Corrected for recovered starting material.
The benzo[a]imidazo[2,1,5-c,d]indolizines (7) generated from the previous sequence are referred to as the unsubstituted se-
ries. Similarly to the parent imidazo[1,5-a]pyridines,20 they have an inherently nucleophilic character. For example, they suc-
cessfully react at position C-2 under the cationic palladium-catalyzed arylation conditions reported by the Murai group21
(Scheme 3, method A). This allows for the synthesis of a second category of fluorophores, dubbed the C-2-aryl series (8).
This arylation can also be achieved in two steps via a regioselective electrophilic bromination by the Br2-dioxane complex fol-
lowed by a Suzuki-Miyaura coupling20 (Scheme 3, method B). Both approaches provide good to excellent yields for most cou-
pling partners. Interestingly, when tricycle 7s was subjected to the direct arylation conditions, a significant amount of the dia-
rylated derivative 8q was isolated along with a modest yield of 8p, highlighting the tricyclic and tetracyclic scaffolds’ distinct
electronic properties.
Scheme 4. Alkylation of Exocylic Nitrogen
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The N-alkylation or protonation of the exocyclic nitrogen produces fluorescent imidazoindolizinium salts (Scheme 4). Under
electrophilic benzylation conditions, the desired salt precipitates quantitatively and can be filtered out, requiring no further puri-
fication.
Figure 1. (a) Crystal structure of 7k (b) Approximate bond orders (ref 22) and atom numbering. Representative bond
lengths obtained from the crystal structure of 10c.
Structural properties. The high crystallinity of benzo[a]imidazo[2,1,5-c,d]indolizines allows for the easy growth of single crys-
tals (in the case of compound 7k, a single 200 mg piece was obtained). X-ray diffraction spectroscopy revealed the main core
is completely flat, with most outer bonds of order between 1.0 and 1.5 (elongated) (Figure 1). 22 The perfect planarity of the
internal nitrogen indicates sp2 hybridization, meaning the imidazopyridine’s 10-electron aromaticity is conserved. However, the
fused benzene ring is distorted, the bridging bond being slightly longer than the other five. This suggests at least some 14-
electron conjugation around all four fused rings.
Photophysical properties. The members of the unsubstituted series generally exhibit blue fluorescence, with a λmax of emis-
sion between 461 nm and 489 nm. The excitation curves feature two to three well-defined peaks that all produce the same
emission peak, ruling out fluorescence from higher excited states (Table 2, 7a).23 The lowest wavelength peak exhibits an
unusually high excitation-emission differential of 1,44 µm-1.24 A direct excitation to a higher electronic level, followed by nonra-
diative relaxation to the lowest electronic excited state (S1) and radiative relaxation to ground state (S0) is a likely explana-
tion.23
Functional group modifications on the fused benzene ring can affect the emissive properties, especially if resonance is possi-
ble. The most noticeable effect is a bathochromic shift of emission caused by electron-donating groups at the C-7 or C-9 posi-
tions (para and ortho positions of the initial benzoyl chloride). Interestingly, adding two more methoxy groups on the meta po-
sitions C-6 and C-8 (compound 7k) had the opposite effect, bringing back the emission band close to that of the unsubstituted
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7a (Table 3) via inductive effects. The presence of an electron-withdrawing group at C-7 (Table 2, 7i) does not shift the emis-
sion band significantly, but alters the excitation curve to feature three well-defined peaks.
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8
Arylation of the core with a phenyl group at the C-2 position (2-aryl series) causes a bathochromic shift of both spectra and
some changes to the excitation spectrum (Table 2, 8a). The high-energy excitation band (315 nm) widens and exhibits higher
light output (brightness)10 than its lower energy counterpart (469 nm).
9
Imidazole ring N-alkylation (imidazoindolizinium series) significantly alters the photophysical properties (Table 2, 9a). The
emission and lowest-energy excitation peaks undergo a hypsochromic shift while the latter becomes more intense. An almost
identical spectrum can be obtained by exposing unsubstituted 7a to aqueous HCl. When acquiring spectra in aqueous buffer
solutions of varying pH, this spectrum change can be observed taking place between pH = 5 and pH = 3 (Figure 2). This ob-
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servation reveals benzo[a]imidazo[2,1,5-c,d]indolizine is slightly less basic than the parent imidazo[1,5-a]pyridine (pKa
=
5.5425). The neutral species dominates the mixture at pH = 5 or higher. Similar cationic compounds with interesting fluorescent
properties were previously reported.26
pH:
Figure 2. Excitation curves (λem = 464nm) of 7a in aqueous buffers. a Relative normalized fluorescence intensity.
These empirical observations prompted us to turn to computational methods in order to locate the frontier molecular orbitals
for all three systems. This should facilitate substituent effect prediction, enabling the rational modification of the fluorescent
core to obtain specific emissive properties.27
Table 2. Photophysical and Computational Data of Representative Benzo[a]imidazo[2,1,5-c,d]indolizines.
Structure
and colora
Fluorescence Data
Calculated HOMOb
Calculated LUMOb
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tuted)
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7i
(Unsubsti-
tuted)
8a
(C-2-Aryl)
9ad
(Imidazoin-
dolizinium)
a
b
Observed under an UVA-emitting low pressure mercury-vapor gas-discharge lamp (“blacklight blue”). B3LYP 6-31G(d,p)
hybrid basis set was used.27 c Normalized fluorescence intensity. d The N-methyl derivative was used in calculations to shorten
computing time.
The resulting orbital representation of unsubstituted 7a (Table 2, 7a) shows that both the HOMO and LUMO are mostly cen-
tered on the imidazole ring. This explains why the two meta methoxy groups in compound 7k counteract the bathochromic
shift observed for the para or ortho alkoxy groups (compare with 7d and 7n). Moreover, the LUMO representation features a
node at the C-2 position.
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Frontier orbitals on the C-2-aryl core exhibit very little change, suggesting that the extra aryl group and its substituents can be
a good handle to control the C-2 position’s electron density. Substitution at this position should influence the HOMO’s energy
level with minimal effect on the LUMO. This hints at a straightforward way to modulate the emission wavelength, as the S1-S0
1
2
3
4
5
6
transition energy is often linked to the energy difference between frontier molecular orbitals.9a,13c,
28
7
8
9
Time-dependent DFT (TD-DFT) calculations yield theoretical excitation wavelengths, which are in general agreement with the
experimental results (Table 2, black arrows on graphs), adding validity to the employed model.29
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Similar calculations performed on the imidazoindolizinium core show, in agreement with the measured fluorescence spectra, a
completely different system. While the LUMO is still located on the imidazole ring, the neutral system’s HOMO is stabilized by
the positive charge on the ring and gets relegated to the HOMO-7 position. A π-orbital on the fused benzene ring takes the
place of the HOMO, yielding a system where the frontier orbitals are separated, indicating the possibility of intramolecular
charge transfer (ICT) fluorescence.30
These frontier orbital representations are further validated by substituent effects on the fused benzene ring. Since the HOMO
is on that ring in the imidazoindolizinium series, all positions can contribute to its electron enrichment, not just the ortho (C-9)
and para positions (C-7). Indeed, the addition of three methoxy groups at the positions C-6, C-7 and C-8 (9c) induces a batho-
chromic emission shift of 45 nm (Table 3, 9a and 9c). This is in stark contrast with the unsubstituted series, where that same
modification had almost no effect on the photophysical properties (Table 3, 7a and 7k).
Table 3. Different effect of fused benzene ring substitution on unsubstituted and imidazoindolizinium series.
Structure
Colora
λem (nm)
464
7a
7k
9a
9c
471
458
503
a Observed under an UVA-emitting low pressure mercury-vapor gas-discharge lamp (“blacklight blue”).
As predicted for the C-2-aryl series, para-substitution on the exocyclic benzene ring has a significant effect on the emission
peak, with donating groups effectively lowering the emission energy. In this series, substitution at both C-7 and the para posi-
tion of the exocyclic C-2 phenyl with electron-donating groups results in a bathochromic shift of the emission wavelength. The-
se substituents effects appear cumulative (Table 4, compare 8a, 8d and 8g). In fact, the emitted photons’ energy was found to
have a linear relationship with the sum of the Hammett constants31 of the C-2 and C-7 substituents, allowing the mathematical
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prediction of compound 8r’s emission maximum (Figure 3) (Predicted λem: 525 nm; measured λem: 524 nm). A similar but non-
cumulative correlation was previously observed in the structurally related but less bright alkynyl-3-arylimidazo[1,5-
a]pyridines.32 This is particularly useful as it enables precise knowledge of a fluorophore’s emissive properties before synthe-
sis, avoiding lengthy trial-and-error when attempting to obtain probes tailor-made for specific applications.
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Figure 3. Linear relationship between the substituents’ Hammett constants and emitted photon energy.
Furthermore, the excitation band producing the highest brightness10 (light output) consistently resides between 315 nm and
350 nm for most examples. It is thus associated with a consistently high excitation-emission differential. High excitation-
emission differential fluorophores are desirable24 as their use greatly reduces interference from excitation light diffusion. They
can also be combined in a single sample with low Stokes shift probes having similar excitation maxima. This enables the acti-
vation of multiple, differently colored probes with a single wavelength of incident light.
As shown in Figures 4 and 5, simple one- or two-step structural modifications on the benzo[a]imidazo[2,1,5-c,d]indolizine core
enable the synthesis of a fluorophore library whose emission maxima cover most of the visible spectrum. In most similar librar-
ies, conjugated fused cycles must be added to access the higher wavelengths,33 significantly lengthening the synthetic path-
way.
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Figure 4. Emission spectra of selected compounds. aNormalized fluorescence intensity.
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Figure 5. Luminescence of selected compounds under an UVA-emitting low pressure mercury-vapor gas-discharge lamp
(“blacklight blue”).
Table 4. Photophysical Data of Selected Fluorophoresa
Excitation-
Exctinction coef-
ficients
ε (103M-1cm-1)
Excitation maxi- Emission maxi-
Fluorescence
quantum yields
ΦF
emission differ-
entials
Brightness10
(103M-1cm-1)
Compound
ma
ex (nΦm)b
ma
λ
λ
em (nm)
(µm-1)
278
421
295
438
286
340
420
289
420
299
411
315
469
352
469
317
469
325
469
339
1.44
0.22
1.34
0.24
1.33
0.77
0.21
1.34
0.26
1.12
0.21
1.22
0.18
0.83
0.12
1.29
0.26
1.28
0.33
1.34
24.8
5.9
0.10c
0.34c
0.15c
0.31c
0.09c
0.22c
0.27c
0.11c
0.31c
0.04c
0.09c
0.20d
0.37d
0.28c
0.25c
0.17d
0.27d
0.14d
0.21d
0.02d
2.4
2.3
2.2
1.5
1.7
1.4
1.6
3.1
2.3
0.3
0.4
4.7
4.2
1.4
0.7
3.8
3.0
3.1
1.7
0.4
7a
7e
464
489
14.8
4.7
18.0
0.22
0.27
28.0
7.3
7i
464
7k
7s
8a
8c
8d
471
450
512
498
535
7.2
4.9
22.4
11.0
5.1
2.8
22.0
11.1
23.0
8.1
8g
8h
555
620
20.4
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343
469
285
468
285
484
398
399
399
287
333
420
280
424
1.19
0.41
1.62
0.25
1.60
0.16
0.33
0.22
0.52
1.33
0.85
0.23
1.43
0.22
29.7
0.07d
0.10d
0.11d
0.34d
0.22d
0.37d
0.45c
0.47c
0.18c
0.18c
0.44c
0.49c
0.14c
0.43c
2.1
0.7
3.0
5.1
4.9
4.0
2.7
6.0
1.6
4.7
2.9
4.0
1.5
1.3
8j
8q
8r
579
529
524
1
2
3
4
5
6
7
8
7.1
27.1
14.7
22.2
10.3
6.1
9a
9b
9c
458
437
503
12.8
8.9
9
25.9
6.7
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10b
464
8.4
10.6
3.0
10e
467
aExcitation-emission fluorescence spectra were acquired in MeOH at a concentration of 5 µM. b Maxima of major well-defined
bands. cA 5 µM solution of quinine in 0,5 M H2SO4 was used as quantum yield standard (ΦF = 0.60).34b d A 0.1 µM solution of
fluorescein in 0,1 M NaOH was used as quantum yield standard (ΦF = 0.92).34c
Tether groups and biolabelling. Most applications of small molecule fluorophores involve their grafting onto biomolecules
via a covalent tether. The fused benzene ring of benzo[a]imidazo[2,1,5-c,d]indolizines is a very convenient attaching point for
tether group precursors introduced through the acyl chloride building block (Scheme 5). Unconjugated substitution has very
little effect on photophysical properties (Table 4, compare 7a with 10e). The fluorescent core tolerates heat, acids or bases,
allowing traditional functional group manipulations without degradation. The nitrile 7j is readily reduced to the corresponding
primary benzylic ammonium chloride, making the molecule highly water soluble.35 The methyl ester 7i can be hydrolyzed to
the acid 10a, thermally amidated (10b) or reduced to the benzylic alcohol 10c. The sodium salt of 10a is also highly water
soluble.35 A primary glycol monoether (7e) can be obtained from hydrolysis of a benzoyl protecting group, allowing the tether-
ing of electron-richer fluorophores similar to compounds 7d and 8f-8i.
Scheme 5. Post-cyclization derivatizations to include reactive tether groups.
In light of these results, we sought to demonstrate the applicability of our fluorophores as probes for biological targets. Site-
specific protein labelling remains a field of high interest within biotechnology, with biocatalysis representing a promising alter-
native to traditional metal-catalyzed conjugation.36 Advantages include high specificity and compatibility with biological media.
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Microbial transglutaminase (MTG) was selected to serve this purpose.37 The acyl-transfer reaction catalyzed by MTG forms an
amide linkage between a primary amine and a glutamine residue on the substrate protein. Here, we were able to take ad-
vantage of MTG’s broad specificity for primary amines38 in conjunction with the highly customizable nature of our fluorophores.
The primary ammonium salt 10e was both reactive with MTG and soluble in aqueous media. We selected α-lactalbumin and
the immunoglobulin-binding domain B1 of streptococcal protein G (GB1) as model glutamine-containing protein substrates.
The reactivity of MTG with α-lactalbumin has been extensively characterized, including for chemoenzymatic protein labelling.39
Its reactivity with protein GB1 has not yet been reported. To prevent non-specific labelling with other glutamine-containing
substrates, the reactions were carried out in vitro with purified protein substrates. (Table 5) Both proteins were successfully
labelled, illustrating the biological compatibility of benzo[a]imidazo[2,1,5-c,d]indolizines and the potential that these fluoro-
phores have in the field of biocatalyzed bioconjugation.
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Table 5. Fluorophore-conjugated α-LA and GB1 proteins.
Target
GB1
α-LA
Protein
MTG
-
+
-
+
Coomassie
staining
UVA
irradiation
Target protein and 10e were incubated overnight in presence or absence of MTG in pH 7.4 buffer and resolved by gel electro-
phoresis.
In summary, we have developed an efficient divergent synthesis yielding a series of strained heteroaromatic fluorophores.
Choice of building blocks for the amidation and electrophilic arylation steps enables the introduction of diverse functional
groups. A linear relationship with Hammett constants allow the prediction of emission maxima before synthesis. Coverage of
most of the visible spectrum was achieved by varying the substituents on three series of molecules based on the same core.
Reactive functional groups can be linked to the tetracyclic core without affecting photophysical properties, enabling covalent
tethering to biomolecules and materials. Some of the insights gathered in this article are likely applicable to other similar fluo-
rescent polyheterocycles,13 broadening their impact. Thorough determination of properties as well as the predictive model will
help researchers chose the optimal fluorescent target to suit their specific needs. We believe benzo[a]imidazo[2,1,5-
c,d]indolizines’ high chemical stability, excitation-emission differential and tuneability make them promising tools for the fields
of fluorescent microscopy and photoactive materials and devices.
EXPERIMENTAL SECTION
General remarks. Unless otherwise stated, all glassware was stored in the oven and/or was flame-dried prior to use. All reac-
tions were set up under an argon atmosphere40 while adding reagents and were run with the exclusion of moisture. All reac-
tion flasks were kept closed with a septum during the reaction time. Anhydrous solvents were obtained either by filtration
through alumina or molecular sieves (THF, CH2Cl2, DMF, CH3CN, toluene) or by distillation over CaH2 (MeOH), BaO (DMA) or
Na (1,4-Dioxane). Anhydrous EtOH and AcOH were used as is from commercial bottles. Analytical thin-layer chromatography
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(TLC) was performed on precoated, glass-backed silica gel. Visualization of the developed chromatogram was performed by
UV absorbance (254 nm), UV fluorescence (368 nm), aqueous potassium permanganate (KMnO4) or cerium ammonium mo-
lybdate (CAM). Flash column chromatography was performed on an automatic purification system using prepacked normal
phase silica gel columns (12 g, 24 g, 40 g, 80 g and 120 g). Melting points are uncorrected. Chemical shifts for 1H NMR spec-
tra are recorded in parts per million from tetramethylsilane using the central peak of chloroform (CHCl3) (δ = 7.26 ppm), MeOH
(δ = 3.31 ppm) or HDO (δ = 4.79 ppm) as the internal standard. Chemical shifts for 13C NMR spectra are recorded in parts per
million from tetramethylsilane using the central peak of CDCl3 (δ = 77.16 ppm) or MeOH (δ = 49.15 ppm), as the internal
standard. All 13C NMR spectra were obtained with complete proton decoupling. Chemical shifts for 19F NMR spectra are rec-
orded in parts per million from trichlorofluoromethane using the central peak of trifluorotoluene (δ = -63.72 ppm) as the internal
standard. Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, qn = quintet,
sx = sextet, h = heptet, m = multiplet and br = broad), coupling constant (J) in Hz and integration. Infrared spectra are reported
in reciprocal centimeters (cm-1). High resolution mass spectra were performed on a TOF analyser. Excitation-emission fluo-
rescence spectra were acquired on a PTI fluorimeter (excitation bandwidth = 2.6 nm, emission bandwidth = 1.3 nm) with 1 mm
slits and temperature set at 20 °C. Fluorophores were dissolved in MeOH or H2O (no significant spectrum change observed,
both solvents tested for for 7a, 9a and 10e) at a concentration of 5 µM. Absorbance spectra were acquired at a 25 µM concen-
tration in MeOH.
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Reagents. Unless otherwise stated, commercial reagents were used without purification. Triethylamine (Et3N) was distilled
over calcium hydride and kept under argon before use. Trifluoromethanesulfonic anhydride (Tf2O) was made by heating TfOH
on P4O10, distilled and kept under argon in a Schlenk flask before use. 6-bromopyridine-2-carbaldehyde41, 4-(2-hydroxy-
ethoxy)-benzoic acid42, benzoic-2-d acid43 and [Pd(phen)2][PF6]2 were synthesized according to previously reported proce-
21d
dures.
Synthesis of starting material
4-[2-(Benzoyloxy)ethoxy]benzoic acid: To a flame-dried 50 mL round bottom flask equipped with a magnetic stirbar and a
rubber septum were dissolved 4-(2-hydroxyethoxy)benzoic acid (0.75 g, 4.1 mmol, 1.0 equiv), pyridine (0.83 mL, 10 mmol, 2.5
equiv) and 4-dimethylaminopyridine (50 mg, 0.41 mmol, 0.10 equiv) in CH2Cl2 (12 mL, 0.34 M). Benzoyl chloride (1.0 mL, 8.6
mmol, 2.1 equiv) was added dropwise and mixture was stirred at room temperature for 20 h. 1.0 M HCl (20 mL) was added
and mixture was stirred for 1 h. Phases were separated and the aqueous layer was extracted with CH2Cl2 (3x). The combined
organic layers were washed with 2.0 M HCl and dried over anhydrous MgSO4, filtered over a sintered funnel and evaporated
to dryness. The residue was purified by flash chromatography using a gradient of 0% to 15% MeOH in 0.5 % AcOH/CH2Cl2.
The fractions containing the pure product were combined and evaporated to dryness. 4-[2-(benzoyloxy)ethoxy]benzoic acid
was isolated as a white solid (0.85 g, 72% yield). mp: 170-173 °C; 1H NMR (CDCl3, 500 MHz): δ 8.04-8.01 (m, 2H), 8.00-7.96
(m, 2H), 7.63-7.58 (m, 1H), 7.49-7.45 (m, 2H), 7.07-7.03 (m, 2H), 4.68 (t, J = 4.5 Hz, 2H), 4.43 (t, J = 4.5 Hz, 2H); 13C NMR
(CDCl3, 125 MHz): δ 169.9, 167.9, 164.0, 134.4, 132.9, 131.2, 130.6, 129.6, 124.8, 115.4, 67.4, 64.6; FTIR (cm-1) (neat):
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2931, 1716, 1685, 1603, 1425, 1278, 1248, 1170, 708; HRMS (ESI, Pos): calcd for C16H15O5 [M+H]+: 287.0914 m/z, found
287.0914 m/z.
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Revised synthesis of (6-bromopyridin-2-yl)methanamine
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6-Bromopyridine-2-carbaldehyde (2): Following a literature procedure41 the product was obtained as a beige solid (4.7 g,
80% yield). The crude mixture was purified by recrystallization from boiling hexanes (reflux to -20°C) instead of chromatog-
raphy. 1H NMR (CDCl3, 500 MHz): δ 10.01 (d, J = 0.5 Hz, 2H), 4.68 (dd, J = 7.0 Hz, J = 1.5 Hz, 2H), 7.78-7.71 (m, 2H).
6-Bromopyridine-2-carbaldehyde oxime (3a): To a 10 mL round-bottom flask equipped with a magnetic stirbar and a rubber
septum were added 6-bromopyridine-2-carbaldehyde (0.30 g, 1.6 mmol, 1.0 equiv), hydroxylammonium sulfate
((NH3OH)2SO4) (0.19 g, 1.1 mmol, 0.7 equiv) and sodium acetate (0.20 g, 2.4 mmol, 1.5 equiv). EtOH (4.0 mL, 0.4 M) was
added and heterogenous mixture was vigorously stirred at room temperature for 16h. MgSO4 was added and mixture was
filtered on cotton and concentrated under vacuum to yield a white solid. This was suspended in 2% MeOH/CH2Cl2 and filtered
through a short silica plug. The filtrate was concentrated under vacuum to yield 6-bromopyridine-2-carbaldehyde oxime as a
white solid (325mg, quantitative yield). mp: 164-166 °C; 1H NMR (DMSO, 500 MHz): δ 8.03 (s, 1H), 7.83 (dd, J = 7.5 Hz, J =
1.0 Hz, 1H), 7.69 (dt, J = 8.0 Hz, J = 0.5 Hz, 1H), 7.55 (dd, J = 8.0 Hz, J = 1.0 Hz, 1H); 13C NMR (CDCl3, 125 MHz): δ 155.1,
148.8, 142.5, 140.6, 129.3, 120.3; FTIR (cm-1) (neat): 3200, 3003, 1545, 1157, 1119, 976, 943, 786, 704; HRMS (ESI, Pos):
calcd for C6H6BrN2O [M+H]+: 200.9658 m/z, found 200.9665 m/z.
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6-Bromopyridin-2-yl)methanammonium acetate (4a): To a 5 mL round-bottom flask equipped with a magnetic stirbar and a
rubber septum were added 6-bromopyridine-2-carbaldehyde oxime (50 mg, 0.25 mmol, 1.0 equiv) and AcOH (2.0 mL, 0.13
M). Zinc (49 mg, 0.75 mmol, 3.0 equiv) was poured in the flask in one portion. Mixture was stirred at room temperature for 10
minutes and filtered over Celite. The filtrate was concentrated under vacuum to yield and off-white solid (140 mg) containing 6-
bromopyridin-2-yl)methanammonium acetate (58 mg, 0.24 mmol, 95% yield) and zinc acetate (82 mg, 0.45 mmol). mp: 200-
210 °C; 1H NMR (CDCl3, 500 MHz): δ 7.81 (t, J = 8.0 Hz, 1H), 7.66 (dd, J = 8.0 Hz, J = 0.5 Hz, 1H), 7.46 (dd, J = 7.5 Hz, J =
1.0 Hz, 1H), 4.23 (s, 2H), 1.98 (s, 3H); 13C NMR (CDCl3, 125 MHz): δ 180.2, 159.3, 142.1, 141.7, 129.3, 122.7, 44.2, 22.3;
FTIR (cm-1) (neat): 3264, 2997, 1544, 1388, 1333, 1169, 1019, 675, 613; HRMS (ESI, Pos): calcd for C6H8BrN2 [M+H]+:
186.9870 m/z, found 186.9871 m/z.
Literature synthesis of (6-bromopyridin-2-yl)methanamine15, 44
2-[(6-Bromopyridin-2-yl)methyl]-2,3-dihydro-1H-isoindole-1,3-dione (3b): To a flame-dried 1000 mL round bottom flask
equipped with a magnetic stirbar and a rubber septum was added 2-bromo-6-hydroxymethylpyridine (8.7 g, 46 mmol, 1.0
equiv). The alcohol was diluted with anhydrous THF (0.46 L, 0.10 M) and then 1,1'-(azodicarbonyl)dipiperidine (15 g, 60 mmol,
1.3 equiv) was added. The orange solution was then stirred at room temperature for 2 minutes. Then, triphenylphosphine (16
g, 60 mmol, 1.3 equiv) and phthalimide (8.9 g, 60 mmol, 1.3 equiv) were added successively. The reaction was then stirred for
24 h at room temperature (the solution was clear and orange). The reaction was then cooled to 0 °C and a solid precipitate
was formed. The cooled suspension was then filtered on a Buchner filter and the cake was washed thoroughly with cold THF.
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The filtrate was then evaporated to dryness. The residue was then quenched by addition of a saturated aqueous solution of
NaHCO3 (30 mL) and diluted with EtOAc (100 mL). The biphasic layers were transferred to a 250 mL extraction funnel and the
aqueous layer was then extracted with EtOAc (3x). The organic layers were combined and dried over anhydrous Na2SO4,
filtered over a sintered funnel and evaporated to dryness. The residue was then purified by flash chromatography using a gra-
dient of 30% to 100% EtOAc in hexanes while using a drypack to inject the crude mixture. The fractions containing the pure
product were combined and evaporated to dryness. 2-[(6-Bromopyridin-2-yl)methyl]-2,3-dihydro-1H-isoindole-1,3-dione was
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obtained as a crystalline white solid (12.3 g, 84% yield). mp: 118-119 °C; H NMR (CDCl3, 500 MHz): δ 7.92-7.91 (m, 2H),
7.78-7.76 (m, 2H), 7.50 (t, J = 7.5 Hz, 1H), 7.38 (d, J = 8.0 Hz, 1H), 7.18 (d, J = 7.5 Hz, 1H), 5.01 (s, 2H). Product corresponds
to previously reported characterization data.15
(6-Bromopyridin-2-yl)methanamine (4b): A suspension of 2-[(6-bromopyridin-2-yl)methyl]-2,3-dihydro-1H-isoindole-1,3-
dione (12.3 g, 38.6 mmol, 1.0 equiv) in absolute ethanol (0.39 L, 0.10 M) was charged in a 1000 mL round bottom flask
equipped with a magnetic stirbar and a condenser and heated to reflux until complete dissolution (around 15 min). To the ho-
mogeneous solution was added hydrazine monohydrate 65% (11.5 mL, 154 mmol, 4.0 equiv). Within a minute, the reaction
mixture became yellow. The reaction mixture was heated at reflux for 7 h during which time the mixture solidified into a white
thick suspension. An additional 50 mL of ethanol was added, and the mixture was kept stirring for an extra 2 h. The reaction
was cooled down to 0 °C to precipitate the hydrazine/phthalimide adduct. The precipitate was filtered on a Buchner and
washed with absolute ethanol. The filtrate was partially concentrated. Remaining solids in the suspension were again filtered
1
off and washed with cold EtOH. The filtrate was concentrated to dryness to yield an orange oil. (6.3 g, 87% yield). H NMR
(CDCl3, 500 MHz): δ 7.57-7.50 (m, 1H), 7.40-7.35 (m, 1H), 7.31-7.27 (m, 1H), 3.98 (br s, 2H), 1.73 (br s, 2H). Product corre-
sponds to previously reported characterization data.15
General procedures for the synthesis of amides
Procedure A:
To a round bottom flask equipped with a magnetic stirrer and a rubber septum was added the (6-bromopyridin-2-
yl)methanamine (1.0 equiv) 4a as previously synthesized. The amine was diluted with CH2CL2 (0.2 M) and cooled to 0 °C
using an ice/water cooling bath. Triethylamine (1.2 equiv) was added via syringe followed by the acyl chloride (1.1 equiv). The
reaction mixture was warmed to room temperature and stirred for 16 h. The reaction was quenched by addition of a saturated
aqueous solution of Na2CO3, phases were separated and the aqueous layer was extracted with CH2Cl2 (3x). The combined
organic layers were dried over anhydrous Na2SO4, filtered and evaporated to dryness.
Procedure B:
To a round bottom flask equipped with a magnetic stirrer and a rubber septum was added the (6-bromopyridin-2-
yl)methanamine (1.0 equiv) 4b as previously synthesized. The amine was diluted with CH2Cl2 (0.2 M). Triethylamine (1.5
equiv) was added via syringe followed by the carboxylic acid (1.1 equiv) and N,N,N′,N′-tetramethyl-O-(1H-benzotriazol-1-
yl)uronium hexafluorophosphate (HBTU) (1.5 equiv). The reaction was quenched by addition of a saturated aqueous solution
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of Na2CO3, phases were separated and the aqueous layer was extracted with CH2Cl2 (3x). The combined organic layers were
dried over anhydrous Na2SO4, filtered and evaporated to dryness.
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4
Procedure C:
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8
To a round bottom flask equipped with a magnetic stirrer and rubber septum was added the (6-bromopyridin-2-
yl)methanammonium acetate salt (1.0 equiv) 4a as previously synthesized. The amine was diluted with CH2Cl2 (0.2 M) and
cooled to 0 °C using an ice/water cooling bath. Triethylamine (5.0 equiv) was added via syringe followed by the acyl chloride
(1.3 equiv). The reaction mixture was warmed to room temperature and stirred for 16 h. The reaction was quenched by addi-
tion of a saturated aqueous solution of Na2CO3, phases were separated and the aqueous layer was extracted with CH2Cl2
(3x). The combined organic layers were dried over anhydrous MgSO4, filtered and evaporated to dryness.
General procedure for acyl chloride synthesis:
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To a round-bottomed flask equipped with a magnetic stirrer and rubber septum was added the carboxylic acid (1.0 equiv),
DMF (0.05 equiv) and CH2Cl2 (0.26 M). Oxalyl chloride (2.0 equiv) was added dropwise (gas evolution). Mixture was stirred for
2h, evaporated to dryness and used as is.
Characterization data of amides
N-((6-Bromopyridin-2-yl)methyl)benzamide (5a): Following procedure A. The crude amide was dissolved in a minimal
amount of hot EtOAc and excess hexanes were added. The resulting precipitate was recovered by filtration and washed with
hexanes, resulting in a white solid (0.81 g, 93% yield). mp: 130-131 °C, lit:15 130-131 °C; 1H NMR (CDCl3, 300 MHz): δ 7.87-
7.81 (m, 2H), 7.55-7.29 (m, 7H), 4.71 (d, J = 5.5 Hz, 2H). Product corresponds to previously reported characterization data.15
N-((6-Bromopyridin-2-yl)methyl)-[1,1'-biphenyl]-4-carboxamide (5b): Following procedure A. The crude amide was puri-
fied by flash chromatography over silica gel using a gradient of 30% to 100% EtOAc in hexanes. Fractions containing 5b were
concentrated to dryness, resulting in a white solid (0.78 g, 71% yield). mp: 168-169 °C; 1H NMR (CDCl3, 500 MHz): δ 7.93-
7.91 (m, 2H), 7.69-7.67 (m, 2H), 7.63-7.61 (m, 2H), 7.55 (t, J = 8.0 Hz, 1H), 7.48-7.45 (m, 2H), 7.43.7.34 (m, 2H), 7.35 (d, J =
7.5 Hz, 1H), 7.29 (br s, 1H), 4.76 (d, J = 5.0 Hz, 2H); 13C NMR (CDCl3, 125 MHz): δ 136.2, 158.3, 144.5, 141.7, 140.0, 139.2,
132.7, 128.9, 128.0, 127.6, 127.3, 127.2, 126.9, 121.2, 44.5; FTIR (cm-1) (neat): 3259, 1637, 1541, 781, 736, 690, 675; HRMS
(ESI, Pos): calcd for C19H15BrN2O [M+H]+: 367.0440 m/z, found: 367.0444 m/z.
N-((6-Bromopyridin-2-yl)methyl)-4-(tert-butyl)benzamide (5c): Following procedure A. The crude amide was purified by
flash chromatography over silica gel using a gradient of 30% to 70% EtOAc in hexanes. Fractions containing 5c were concen-
trated to dryness, resulting in a white solid (1.0 g, 96% yield). mp: 162-164 °C; 1H NMR (CDCl3, 500 MHz): δ 7.79-7.77 (m,
2H), 7.54 (t, J = 8.0 Hz, 1H), 7.48-7.47 (m, 2H), 7.41 (d, J = 8.0 Hz, 1H), 7.33 (d, J = 7.5 Hz, 1H), 7.15 (br s, 1H), 4.73 (d, J =
5.5 Hz, 2H), 1.34 (s, 9H); 13C NMR (CDCl3, 125 MHz): δ 167.4, 158.5, 155.2, 141.6, 139.2, 131.2, 126.9, 126.8, 125.6, 121.2,
44.5, 35.0, 31.2; FTIR (cm-1) (neat): 3357, 3313, 2959, 2928, 2902, 1634, 1555, 1543, 1309, 982, 780, 658; HRMS (ESI, Pos):
calcd for C17H19BrN2O [M+H]+: 347.0754 m/z, found: 347.0755 m/z.
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4-(Benzyloxy)-N-((6-bromopyridin-2-yl)methyl)benzamide (5d): Following procedure B. The crude amide was purified by
flash chromatography over silica gel using a gradient of 0% to 80% EtOAc in hexanes. Fractions containing 5d were concen-
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trated to dryness, resulting in a white solid (0.90 g, 76% yield). mp: 133-135 °C; H NMR (CDCl3, 500 MHz): δ 7.82-7.79 (m,
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2H), 7.53 (t, J = 7.5 Hz, 1H), 7.45-7.37 (m, 5H), 7.36-7.31 (m, 2H), 7.11 (br t, J = 4.5 Hz, 1H), 7.03-7.00 (m, 2H), 5.12 (s, 2H),
4.71 (d, J = 5.0 Hz, 1H); 13C NMR (CDCl3, 125 MHz): δ 166.9 161.5, 158.5, 141.6, 139.2, 136.4, 129.0, 128.7, 128.2, 127.5,
126.8, 126.6, 121.2, 114.7, 70.1, 44.5; FTIR (cm-1) (neat): 3362, 1637, 1604, 1537, 1501, 1248, 1113, 767, 734; HRMS (ESI,
Pos): calcd for C20H17BrN2O2 [M+H]+: 397.0546 m/z, found: 397.0541 m/z.
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2-(4-(((6-Bromopyridin-2-yl)methyl)carbamoyl)phenoxy)ethyl benzoate (5e): Following procedure C. The crude amide
was dissolved in a minimal amount of hot EtOAc and excess hexanes were added. The resulting precipitate was recovered by
filtration and washed with hexanes. The residue was dissolved in 25% EtOAc/CH2Cl2, washed with saturated NaHCO3 (2X),
dried with MgSO4 and filtrated on a short silica gel column (washing with 25% EtOAc/CH2Cl2). Filtrate was concentrated to
dryness, resulting in a beige solid (0.66 g, 90% yield). mp: 121-123 °C; 1H NMR (CDCl3, 500 MHz): δ 8.04 (dd, J = 1.0 Hz, J =
8.5 Hz, 1H), 7.81 (d, J = 8.5 Hz, 1H), 7.58-7.50 (m, 2H), 7.45-7.37 (m, 3H), 7.32 (d, J = 7.5 Hz, 1H), 7.24 (br t, J = 5.0 Hz, 1H),
6.97 (d, J = 8.5 Hz, 2H), 4.71-4.66 (m, 4H), 4.35 (t, J = 5.0 Hz, 2H); 13C NMR (CDCl3, 125 MHz): δ 167.0, 166.6, 161.4, 158.7,
141.6, 139.3, 133.3, 129.8 (2), 129.1, 128.5, 126.9 (2), 121.3, 114.6, 66.2, 63.2, 44.6; FTIR (cm-1) (neat): 3313, 2968, 1708,
1278, 1248, 1119, 1069, 713, 665; HRMS (ESI, Pos): calcd for C22H20BrN2O4 [M+H]+: 455.0608 m/z, found: 455.0601m/z.
N-((6-bromopyridin-2-yl)methyl)-4-(dimethylamino)benzamide (5f): Following procedure A. The crude amide was purified
by flash chromatography over silica gel using a gradient of 30% to 70% EtOAc in hexanes. Fractions containing 5f were con-
centrated to dryness, resulting in a white solid (0.56 g, 56% yield). mp: 155-157 °C; 1H NMR (CDCl3, 400 MHz): δ 7.74 (d, J =
8.8 Hz, 2H), 7.52 (t, J = 7.5 Hz, 1H), 7.38 (d, J = 7.5 Hz, 1H), 7.32 (d, J = 7.5 Hz, 1H), 7.00 (br t, J = 5.5 Hz, 1H), 6.68 (d, J =
8.8 Hz, 2H), 4.70 (d, J = 6.0 Hz, 2H), 3.02 (s, 6H); 13C NMR (CDCl3, 125 MHz): δ 167.4, 159.1, 152.6, 141.5, 139.1, 128.6,
126.6, 121.2, 120.8, 111.1, 44.5, 40.1; FTIR (cm-1) (neat): 3276, 2921, 1626, 1604, 1514, 1302, 1115, 769, 665; HRMS (ESI,
Pos): calcd for C15H17BrN3O [M+H]+: 334.0550 m/z, found: 334.0544 m/z.
N-((6-Bromopyridin-2-yl)methyl)-4-fluorobenzamide (5g): Following procedure A. The crude amide was purified by flash
chromatography over silica gel using a gradient of 0% to 50% EtOAc in hexanes. Fractions containing 5g were concentrated
1
to dryness, resulting in a white solid (0.87 g, 94% yield). mp: 111-113 °C; H NMR (CDCl3, 500 MHz): δ 7.87-7.83 (m, 2H),
7.54 (t, J = 7.5 Hz, 1H), 7.41 (d, J = 8.0 Hz, 1H), 7.32 (d, J = 7.5 Hz, 1H), 7.25 (br s, 1H), 7.15-7.10 (m, 2H), 4.71 (d, J = 5.5
Hz, 2H); 13C NMR (CDCl3, 125 MHz): δ 166.4, 164.9 (d, JC-F = 250 Hz), 158.0, 141.7, 139.2, 130.3. 130.3, 129.4 (d, JC-F = 8.9
Hz), 126.9, 121.2, 115.7 (d, JC-F = 21.8 Hz); 19F NMR (CDCl3, 282 MHz): δ -108.0 (s, F); FTIR (cm-1) (neat): 3355, 1638, 1504,
847, 764, 599; HRMS (ESI, Pos): calcd for C13H10BrFN2O [M+H]+: 309.0033 m/z, found: 309.0033 m/z.
N-((6-Bromopyridin-2-yl)methyl)-4-chlorobenzamide (5h): Following procedure C. The crude amide was dissolved in a
minimal amount of hot EtOAc and excess hexanes were added. The resulting precipitate was recovered by filtration and
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washed with hexanes, resulting in a yellowish solid (30 mg, 92% yield). mp: 105-107 °C; H NMR (CDCl3, 300 MHz): δ 7.79
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(d, J = 8.7 Hz, 2H), 7.57 (t, J = 7.8 Hz, 1H), 7.43 (d, J = 8.7 Hz, 3H), 7.34 (d, J = 7.5 Hz, 1H), 4.72 (d, J = 5.4 Hz, 2H); 13C
NMR (CDCl3, 125 MHz): δ 166.5, 158.2, 141.7, 139.4, 138.0, 132.5, 129.0, 128.7, 127.0 121.3, 44.6; FTIR (cm-1) (neat): 3328,
2923, 1636, 1542, 1310, 1093, 847, 774, 658; HRMS (ESI, Pos): calcd for C13H10BrClN2NaO [M+Na]+: 346.95572 m/z, found:
346.95592 m/z.
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Methyl 4-(((6-bromopyridin-2-yl)methyl)carbamoyl)benzoate (5i): Following procedure A. The crude amide was purified
by flash chromatography over silica gel using a gradient of 30% to 100% EtOAc in hexanes. Fractions containing 5i were con-
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centrated to dryness, resulting in a white solid (0.88 g, 84% yield). mp: 149-151 °C; H NMR (CDCl3, 500 MHz): δ 8.04-8.02
(m, 2H), 7.84-7.82 (m, 2H), 7.48 (t, J = 7.5 Hz, 1H), 7.35 (dd, J = 0.5, 8.0 Hz, 1H), 7.25 (dd, J = 0.5, J = 7.5 Hz, 1H), 4.66 (d, J
= 5.5 Hz, 2H), 3.87 (s, 3H); 13C NMR (CDCl3, 125 MHz): δ 166.7, 166.4, 158.0, 141.8, 139.4, 138.1, 133.0, 130.0,127.3,
127.1, 121.3, 52.5, 44.7; FTIR (cm-1) (neat): 3275, 3040, 2959, 2864, 2844, 1721, 1637, 1279, 716; HRMS (ESI, Pos): calcd
for C15H13BrN2O3 [M+H]+: 349.0198 m/z, found: 349.0198 m/z. Compound 5i was also synthesized via procedure C (0.21 g,
59% yield).
N-((6-Bromopyridin-2-yl)methyl)-4-cyanobenzamide (5j): Following procedure C. The crude amide was dissolved in a
minimal amount of hot EtOAc and excess hexanes were added. The resulting precipitate was recovered by filtration and
1
washed with hexanes, resulting in a beige solid (0.90 mg, 71% yield). mp: 127-128 °C; H NMR (CDCl3, 500 MHz): δ 7.89-
7.87 (m, 2H), 7.70-7.68 (m, 2H), 7.50 (t, J = 7.5 Hz, 1H), 7.37-7.36 (m, 2H), 7.25 (dd, J = 0.5, J = 7.5 Hz, 1H), 4.66 (d, J = 5.0
Hz, 2H); 13C NMR (CDCl3, 125 MHz): δ 165.7, 157.5, 141.8, 139.4, 138.1, 132.6, 127.9, 127.2, 121.3, 118.1, 115.4, 44.6;
FTIR (cm-1) (neat): 3304, 3066, 2973, 2939, 2933, 2864, 2844, 2825, 2230, 1643, 1554, 1306, 672; HRMS (ESI, Pos): calcd
for C14H10BrN3O [M+H]+: 316.0080 m/z, found: 316.0087 m/z.
N-((6-Bromopyridin-2-yl)methyl)-3,4,5-trimethoxybenzamide (5k): Following procedure A. The crude amide was dis-
solved in a minimal amount of hot EtOAc and excess hexanes were added. The resulting precipitate was recovered by filtra-
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tion and washed with hexanes, resulting in a beige solid (1.0 g, 89% yield). mp: 115-117 °C; H NMR (CDCl3, 400 MHz): δ
7.55 (t, J = 7.6 Hz, 1H), 7.41 (d, J = 8.0 Hz, 1H), 7.30-7.35 (m, 2H), 7.08 (s, 2H), 4.70 (d, J = 5.6 Hz, 2H), 3.90 (s, 6H), 3.88 (s,
3H); 13C NMR (CDCl3, 125 MHz): δ 167.2, 158.4, 153.3, 141.6, 141.1, 139.5, 129.5, 127.0, 121.5, 104.5, 61.0, 56.4, 44.6;
FTIR (cm-1) (neat): 3303, 2937, 1580, 1496, 1410, 1325, 1230, 1119, 1000; HRMS (ESI, Pos): calcd for C16H18BrN2O4 [M+H]+:
381.0445 m/z, found: 381.0451 m/z.
N-((6-Bromopyridin-2-yl)methyl)-2-ethoxy-4-nitrobenzamide (5l): Following procedure B. The crude amide was purified
by flash chromatography over silica gel using a gradient of 0% to 20% EtOAc in CH2Cl2. Fractions containing 5l were concen-
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trated to dryness, resulting in a white solid (0.75 g, 66% yield). mp: 136-138 °C; H NMR (CDCl3, 400 MHz): δ 8.93 (s, 2H),
8.40 (d, J = 8.4 Hz, 1H), 7.91 (dd, J = 2.0, J = 8.4 Hz, 1H), 7.85 (s, 1H), 7.56 (t, J = 7.6 Hz, 1H), 7.43 (d, J = 8.0 Hz, 1H), 7.31
(d, J = 7.6 Hz, 1H), 4.80 (d, J = 4.8 Hz, 1H), 4.39 (q, J = 6.8 Hz, 1H), 1.65 (t, J = 6.8 Hz, 1H); 13C NMR (CDCl3, 125 MHz): δ
163.5, 158.0, 157.3, 150.4, 141.9, 139.3, 133.5, 127.0, 126.8, 121.1, 115.8, 107.6, 66.1, 45.0, 14.9; FTIR (cm-1) (neat): 3347,
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1653, 1522, 1433, 1229, 1024, 866, 789, 735; HRMS (ESI, Pos): calcd for C15H15BrN3O4 [M+H]+: 380.0241 m/z, found:
380.02518m/z.
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N-((6-Bromopyridin-2-yl)methyl)-2,3,4,5-tetrafluorobenzamide (5m): Following procedure A. The crude amide was puri-
fied by flash chromatography over silica gel using a gradient of 50% to 80% EtOAc in hexanes. Fractions containing 5m were
concentrated to dryness, resulting in a white solid (0.91 mg, 84% yield). mp: 126-128 °C; 1H NMR (CDCl3, 700 MHz): δ 7.82-
7.76 (m, 2H), 7.59 (t, J = 10.5 Hz, 1H), 7.46 (dd, J = 0.7, 11.2 Hz, 1H), 7.33 (dd, J = 0.7, 10.5 Hz, 1H), 4.75 (d, J = 4.9 Hz,
2H); 13C NMR (CDCl3, 175 MHz): δ 160.5, 157.1, 147.9 (d, JC-F = 7.0 Hz), 147.0 (d, JC-F = 9.3 Hz), 146.48 (d, JC-F = 9.8 Hz),
145.6 (d, JC-F = 9.5 Hz), 143.4 (ddd, JC-F = 3.5, 12.4, 16.5 Hz), 141.9 (ddd, JC-F = 3.5, 12.4, 16.5 Hz), 141.4 (ddd, JC-F = 3.5,
12.8, 18.9 Hz), 140.0 (ddd, JC-F = 3.2, 12.4, 18.4 Hz), 139.3, 127.1, 120.9, 117.2, 112.8 (d, JC-F = 2.8 Hz), 44.7; 19F NMR
(CDCl3, 282 MHz): δ -153.7 to -153.6 (m, F), -148.9 to -148.8 (m, F), -138.8 to -138.7 (m, F), -136.8 to -136.7 (m, F).; FTIR
(cm-1) (neat): 3392, 3078, 1667, 1503, 1474, 792, 772; HRMS (ESI, Pos): calcd for C13H7BrF4N2O [M+H]+: 362.9751 m/z,
found: 362.9752 m/z.
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Benzyl 4-(((6-bromopyridin-2-yl)methyl)carbamoyl)benzenesulfonate (5n): Following procedure B. The crude amide was
purified by flash chromatography over silica gel using a gradient of 0% to 100% EtOAc in hexanes. Fractions containing 5n
were concentrated to dryness, resulting in a white solid (0.40 g, 42% yield). mp: 90-91 °C; 1H NMR (CDCl3, 500 MHz): δ 9.11
(br t, J = 6.0 Hz, 1H), 8.24 (dd, J = 2.0, 8.0 Hz, 1H), 7.53 (t, J = 7.5 Hz, 1H), 7.46 (ddd, J = 1.5, 7.0, 7.5 Hz, 1H), 7.39 (dd, J =
0.5, 8.0 Hz, 1H), 7.32 (dd, J = 0.5, 7.5 Hz, 1H), 7.08 (dd, J = 1.5, 8.0 Hz, 1H), 7.00 (dd, J = 1.0, 8.0 Hz, 1H), 4.78 (d, J = 6.0
Hz, 2H), 4.07 (s, 3H); 13C NMR (CDCl3, 125 MHz): δ 165.5, 159.0, 158.0, 141.6, 139.2, 133.1, 132.4, 126.6, 121.3, 121.2,
121.1, 111.5, 56.1, 44.8; FTIR (cm-1) (neat):3348, 2922, 1638, 1511, 1434, 1233, 1158, 1023, 780, 753, 618; HRMS (ESI,
Pos): calcd for C14H14BrN2O2 [M+H]+: 321.0233 m/z, found: 321.0225 m/z.
N-((6-Bromopyridin-2-yl)methyl)-1-naphthamide (5o): Following procedure A. The crude amide was purified by flash
chromatography over silica gel using a gradient of 10% to 50% EtOAc in hexanes. Fractions containing 5o were concentrated
to dryness, resulting in a white solid (0.90 g, 86% yield). mp: 154-155 °C; 1H NMR (CDCl3, 500 MHz): δ 8.41-8.37 (m, 1H),
7.94 (d, J = 8.5 Hz, 1H), 7.89-7.86 (m, 1H), 7.70 (dd, J = 1.5, 7.0 Hz, 1H), 7.59-7.51 (m, 3H), 7.50-7.46 (m, 1H), 7.43 (dd, J =
0.5, 7.5 Hz, 1H), 7.38 (dd, J = 0.5, 7.5 Hz, 1H) 7.04 (br s, 1H), 4.83 (d, J = 5.5 Hz, 2H); 13C NMR (CDCl3, 125 MHz): δ 169.6,
158.2, 141.7, 139.3, 134.0, 133.7, 130.9, 130.2, 128.3, 127.2, 126.9, 126.5, 125.5, 125.3, 124.8, 121.1, 44.5; FTIR (cm-1)
(neat): 3244, 1638, 1536, 1427, 1404, 1308, 778, 767, 714; HRMS (ESI, Pos): calcd for C17H13BrN2O [M+H]+: 341.0284 m/z,
found: 341.0292 m/z.
N-((6-Bromopyridin-2-yl)methyl)-9-oxo-9H-fluorene-2-carboxamide (5p): Following procedure A. The crude amide was
purified by flash chromatography over silica gel using a gradient of 20% to 80% EtOAc in hexanes. Fractions containing 5p
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were concentrated to dryness, resulting in a yellow solid (0.75 g, 74% yield). mp: 192-194 °C; H NMR (CDCl3, 500 MHz): δ
8.10-8.06 (m, 2H), 7.71 (d, J = 5.0 Hz, 2H), 7.65-7.52 (m, 4H), 7.43 (d, 7.5 Hz, 1H), 7.39-7.33 (m, 2H), 7.22 (s, 1H), 4.75 (d, J
= 5.5 Hz, 2H); 13C NMR (CDCl3, 125 MHz): δ 192.8, 166.2, 158.0, 147.3, 143.5, 141.8, 139.3, 135.0, 134.9, 134.6, 134.3,
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130.0, 127.0, 124.6, 122.4, 121.1 (2), 120.6, 44,6; FTIR (cm-1) (neat): 3405, 1703, 1656, 1614, 1578, 1265, 738, 583; HRMS
(ESI, Pos): calcd for C20H13BrN2O2 [M+H]+: 393.0233 m/z, found: 393.0242 m/z.
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N-((6-Bromopyridin-2-yl)methyl)furan-2-carboxamide (5q): Following procedure A. The crude amide was purified by flash
chromatography over silica gel using a gradient of 40% to 70% EtOAc in hexanes. Fractions containing 5q were concentrated
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to dryness, resulting in a white solid (0.73 g, 87% yield). mp: 117-118 °C; H NMR (CDCl3, 500 MHz): δ 7.50-7.42 (m, 3H),
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7.35 (d, J = 7.5 Hz, 1H), 7.28 (dd, J = 0.5, 7.5 Hz, 1H), 7.09 (dd, J = 1.0, 3.5 Hz, 1H), 6.46 (dd, J = 1.5, 3.0 Hz, 1H), 4.65 (d, J
= 5.5 Hz, 2H); 13C NMR (CDCl3, 125 MHz): δ 158.5, 158,5, 147.6, 144.3, 141.6, 139.2, 126.8, 120.9, 114.5, 112.1, 43.9; FTIR
(cm-1) (neat): 3235, 1636, 1526, 1014, 771, 762, 629, 601; HRMS (ESI, Pos): calcd for C11H9BrN2O2 [M+H]+: 280.9920 m/z,
found: 280.9921 m/z.
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N-((6-Bromopyridin-2-yl)methyl)thiophene-2-carboxamide (5r): Following procedure A. The crude amide was purified by
flash chromatography over silica gel using a gradient of 40% to 100% EtOAc in hexanes. Fractions containing 5r were con-
centrated to dryness, resulting in a white solid (0.75 g, 84% yield). mp: 123-127 °C; 1H NMR (CDCl3, 500 MHz): δ 7.57 (dd, J
= 1.5, 4.0 Hz, 1H), 7.54 (t, J = 8.0 Hz, 1H), 7.49 (dd, J = 1.0, 5.0 Hz, 1H), 7.41 (d, J = 7.5 Hz, 1H), 7.33 (d, J = 7.5 Hz, 1H),
7.10-7.05 (m, 2H), 4.69 (d, J = 5.5 Hz, 2H); 13C NMR (CDCl3, 125 MHz): δ 161.9, 158.1, 141.6, 139.3, 138.5, 130.3, 128.4,
127.7, 126.9, 121.2, 44.4; FTIR (cm-1) (neat): 3319, 3083, 2981, 2968, 2934, 1552, 1305, 721, 679; HRMS (ESI, Pos): calcd
for C11H9BrN2OS [M+H]+: 296.9692 m/z, found: 296.9690 m/z.
N-((6-Bromopyridin-2-yl)methyl)-1-naphthamide (5s): Following procedure A. The crude amide was purified by flash
chromatography over silica gel using a gradient of 40% to 100% EtOAc in hexanes. Fractions containing 5s were concentrat-
ed to dryness, resulting in a white solid (0.74 g, 72% yield). mp: 53-54 °C; 1H NMR (CDCl3, 500 MHz): δ 7.66 (d, J = 16.0 Hz,
1H), 7.54-7.50 (m, 3H), 7.40-7.34 (m, 4H), 7.30 (dd, J = 1.0, 7.5 Hz, 1H), 6.77 (br s, 1H), 6.52 (d, J = 15.5 Hz, 1H), 4.66 (d, J =
5.5 Hz, 2H); 13C NMR (CDCl3, 125 MHz): δ168.4, 158.4, 141.6, 139.6, 139.2, 126.8, 121.1, 120.2, 44.2, 18.6; FTIR (cm-1)
(neat): 3327, 3083, 2924, 1657, 1616, 1583, 1556, 1521, 1435, 1405, 1121, 776; HRMS (ESI, Pos): calcd for C10H11BrN2O
[M+H]+: 255.0128 m/z, found: 255.0140 m/z.
N-((6-Bromopyridin-2-yl)methyl)cinnamamide (5t): Following procedure A. The crude amide was purified by flash chroma-
tography over silica gel using a gradient of 40% to 100% EtOAc in hexanes. Fractions containing 5t were concentrated to dry-
1
ness, resulting in a white solid (0.77 g, 80% yield). mp: 120-122 °C; H NMR (CDCl3, 500 MHz): δ 7.66 (d, J = 16.0 Hz, 1H),
7.54-7.50 (m, 3H), 7.40-7.34 (m, 4H), 7.30 (dd, J = 1.0, 7.5 Hz, 1H), 6.77 (br s, 1H), 6.52 (d, J = 15.5 Hz, 1H), 4.66 (d, J = 5.5
Hz, 2H); 13C NMR (CDCl3, 125 MHz): δ 166.0, 158.3, 141.6, 141.6, 139.2, 134.7, 129.8, 128.8, 127.9, 126.8, 121.2, 120.28,
44.4; FTIR (cm-1) (neat): 3232, 3064, 3032, 2982, 2970, 2946, 1652, 1613, 1578, 1551, 731, 684, 671; HRMS (ESI, Pos):
calcd for C15H13BrN2O [M+H]+: 317.0284 m/z, found: 317.0283 m/z.
N-((6-Bromopyridin-2-yl)methyl)benzamide-2-d (5a(ortho-d1)): Following procedure C. The crude amide was dissolved in
a minimal amount of hot EtOAc and excess hexanes were added. The resulting precipitate was recovered by filtration and
washed with hexanes. resulting in a beige solid (0.11 g, 75% yield). mp: 127-128 °C; 1H NMR (CDCl3, 500 MHz): δ 7.84 (dd, J
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= 1.0 Hz, J = 8.0 Hz, 1H), 7.54 (t, J = 10.0 Hz, 1H), 7.53-7.50 (m, 1H), 7.48-7.43 (m, 2H), 7.41 (d, J = 10.0 Hz, 1H), 7.33 (dd, J
= 1.0 Hz, J = 7.5 Hz, 1H), 7.23 (br s, 1H), 4.73 (d, J = 5.5 Hz, 2H); 13C NMR (CDCl3, 125 MHz): δ 167.6, 158.4, 141.8, 139.3,
134.1, 131.8, 128.8, 128.7, 127.2, 127.0 126.9 (t, JC-D = 23.4 Hz), 121.3, 44.6; FTIR (cm-1) (neat): 3287, 3061, 2926, 1638,
1537, 1436, 1403, 1121, 770; HRMS (ESI, Pos): calcd for C13H11DBrN2O [M+H]+: 292.01903m/z, found: 292.0191 m/z. Prod-
uct is 90% ortho-deuterated, as was the starting acyl chloride.
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N-((6-Bromopyridin-2-yl)methyl)benzamide-2,3,4,5,6-d5 (5a-d5): Following procedure A. The crude amide was was dis-
solved in a minimal amount of hot EtOAc and excess hexanes were added. The resulting precipitate was recovered by filtra-
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tion and washed with hexanes, resulting in a beige solid (0.38 g, 85% yield). mp: 128-129 °C; H NMR (CDCl3, 400 MHz): δ
7.55 (t, J = 7.6 Hz, 1H), δ 7.42 (d, J = 8.0 Hz, 1H), δ 7.34 (d, J = 7.2 Hz, 1H), δ 7.19 (s, 1H), δ 7.40 (d, J = 5.2 Hz, 2H); 13C
NMR (CDCl3, 125 MHz): δ 167.6, 158.4, 141.7, 139.3, 134.0, 131.3 (t, JC-D = 24.3 Hz), 128.2 (t, JC-D = 24.5 Hz), 127.0, 126.8
(t, JC-D = 24.3 Hz), 121.3, 44.6; FTIR (cm-1) (neat): 3287, 3061, 2926, 1638, 1538, 1403, 1121, 770, 629; HRMS (ESI, Pos):
calcd for C13H7D5BrN2O [M+H]+: 296.0441 m/z, found: 296.0453m/z.
General procedure for the synthesis of imidazo[1,5-a]pyridines
Procedure D
To a flame-dried and argon-flushed glass microwave (sealable) vial (VWR®) equipped with a magnetic stirrer and a rubber
septum was added the amide (1.0 equiv) and anhydrous CH2Cl2 (0.50 M). 2-methoxypyridine (2-MeOPyr) (1.1 equiv) was
added via syringe and Tf2O (1.2 equiv) was added over 2 min via syringe. The reaction usually changed color (dark purple or
dark blue) and was exothermic upon addition of the anhydride. The reaction mixture was slowly heated to 50 °C using an oil
bath and stirred for 16 h. The mixture was cooled to room temperature and quenched by addition of saturated aqueous
Na2CO3 and then stirred for 5 min. Phases were separated and the aqueous layer was extracted with CH2Cl2 (3x). The com-
bined organic layers were dried over anhydrous MgSO4, filtered and evaporated to dryness.
Characterization data of imidazo[1,5-a]pyridines
5-Bromo-3-phenylimidazo[1,5-a]pyridine (6a): Following procedure D. The crude imidazo[1,5-a]pyridine was purified by
flash chromatography over silica gel using a gradient of 10% to 80% EtOAc in hexanes. Fractions containing 6a were concen-
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trated to dryness, resulting in a green solid (0.50 g, 96% yield). mp: 80-82°C, lit:15 80-81 °C; H NMR (CDCl3, 400 MHz): δ
7.66 (s, 1H), 7.57-7.52 (m, 3H), 7.50-7.41 (m, 3H), 6.85 (app dt, J = 1.0, 7.0 Hz, 1H), 6.61 (dd, J = 7.0, 9.0 Hz, 1H); Product
corresponds to previously reported characterization data.15
3-([1,1'-Biphenyl]-4-yl)-5-bromoimidazo[1,5-a]pyridine (6b): Following procedure D. The crude imidazo[1,5-a]pyridine was
purified by flash chromatography over silica gel using a gradient of 10% to 70% EtOAc in hexanes. Fractions containing 6b
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were concentrated to dryness, resulting in a brown solid (0.61 g, 87% yield). mp: 164-166 °C; H NMR (CDCl3, 400 MHz): δ
7.70-7.66 (m, 5H), 7.63-7.60 (m, 2H), 7.53 (dd, J = 0.8, 9.2 Hz, 1H), 7.49-7.45 (m, 2H), 7.40-7.36 (m, 1H), 6.85 (dd, J = 1.2,
6.8 Hz, 1H), 6.60 (dd, J = 6.8, 8.8 Hz, 1H); 13C NMR (CDCl3, 100 MHz): δ 141.6, 140.6, 140.2, 133.8, 131.8, 131.4, 128.9,
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127.6, 127.2, 125.8, 121.3, 119.6, 119.0, 118.0, 111.8; FTIR (cm-1) (neat): 3057, 2964, 842, 807, 758, 691; HRMS (ESI, Pos):
calcd for C19H13BrN2 [M+H]+: 349.0335 m/z, found: 349.0344 m/z.
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5-Bromo-3-(4-(tert-butyl)phenyl)imidazo[1,5-a]pyridine (6c): Following procedure D. The crude imidazo[1,5-a]pyridine
was purified by flash chromatography over silica gel using a gradient of 10% to 70% EtOAc in hexanes. Fractions containing
6c were concentrated to dryness, resulting in a yellow solid (0.60 g, 91% yield). mp: 128-129 °C; Rf: 0.45 (70% EtOAc in hex-
anes); 1H NMR (CDCl3, 400 MHz): δ 7.61 (s, 1H), 7.49 (dd, J = 0.8, 8.8 Hz, 1H), 7.46-7.40 (m, 4H), 6.80 (dd, J = 1.2, 6.8 Hz,
1H), 6.55 (dd, J = 6.4, 8.8 Hz, 1H), 1.37 (s, 9H); 13C NMR (CDCl3, 100 MHz): δ 152.1, 140.7, 133.6, 131.1, 129.5, 124.1,
121.0, 119.3, 118.7, 118.0, 111.8, 34.8, 31.4; FTIR (cm-1) (neat): 2966, 2949, 2865, 2845, 822, 757, 686; HRMS (ESI, Pos):
calcd for C17H17BrN2 [M+H]+: 329.0648 m/z, found: 329.0663 m/z.
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3-(4-(Benzyloxy)phenyl)-5-bromoimidazo[1,5-a]pyridine (6d): Following procedure D. The crude imidazo[1,5-a]pyridine
was purified by flash chromatography over silica gel using a gradient of 15% to 75% EtOAc in hexanes. Fractions containing
6d were concentrated to dryness, resulting in a green solid (0.73 g, 91% yield). mp: 117-119 °C; 1H NMR (CDCl3, 400 MHz): δ
7.61 (s, 1H), 7.50-7.44 (m, 5H), 7.40 (t, J = 5.6 Hz, 2H), 7.34 (d, J = 5.6 Hz, 1H), 7.02 (d, J = 7.2 Hz, 2H), 6.80 (dd, J = 0.8, J =
5.2 Hz, 1H), 6.55 (dd, J = 5.2, J = 7.2 Hz, 1H), 5.13 (s, 2H); 13C NMR (CDCl3, 125 MHz): δ 159.5, 140.4, 136.8, 133.6, 132.8,
128.7, 128.1, 127.7, 125.2, 120.9, 119.4, 118.8, 118.1, 113.6, 111.9, 70.2; FTIR (cm-1) (neat): 3057, 1450, 1238, 1172, 1027,
827, 814, 772, 691, ; HRMS (ESI, Pos): calcd for C20H16BrN2O [M+H]+: 379.0441 m/z, found: 379.0458 m/z.
2-(4-(5-Bromoimidazo[1,5-a]pyridin-3-yl)phenoxy)ethyl benzoate (6e): Following procedure D. The crude imidazo[1,5-
a]pyridine was purified by flash chromatography over silica gel using a gradient of 10% to 75% EtOAc in hexanes. Fractions
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containing 6e were concentrated to dryness, resulting in a green solid (0.20 g, 52% yield). mp: 109-111 °C; H NMR (CDCl3,
500 MHz): δ 8.11-8.06 (m, 2H), 7.62 (s, 1H), 7.61-7.42 (m, 6H), 7.01-6.96 (m, 2H), 6.82 (dd, J = 1.5, 11.5 Hz, 1H), 6.61-6.54
(m, 1H), 4.71 (dd, J = 8.0 Hz, 2H), 4.38 (dd, J = 8.0 Hz, 2H); 13C NMR (CDCl3, 125 MHz): δ 166.7, 159.4, 140.2 133.6, 133.3,
132.9, 130.0, 129.9, 128.5, 125.3, 120.9, 119.5, 118.9, 118.1, 113.4, 111.9, 66.2, 63.4; FTIR (cm-1) (neat): 1712, 1455, 1273,
1241, 1112, 1062, 834, 804, 706; HRMS (ESI, Pos): calcd for C22H18BrN2O3 [M+H]+: 437.0501 m/z, found: 437.0495 m/z.
4-(5-Bromoimidazo[1,5-a]pyridin-3-yl)-N,N-dimethylaniline (6f): Following procedure D. The crude imidazo[1,5-a]pyridine
was purified by flash chromatography over silica gel using a gradient of 15% to 75% EtOAc in hexanes. Fractions containing
6f were concentrated to dryness, resulting in a yellow solid (0.33 g, 84% yield). mp: 167-168 °C; 1H NMR (CDCl3, 400 MHz): δ
7.59 (s, 1H), 7.46 (d, J = 8.4 Hz, 1H), 7.37 (d, J = 8.8 Hz, 2H), 6.78 (dd, J = 0.8 Hz, J = 6.8 Hz, 1H), 6.73 (dd, J = 2.0 Hz, J =
6.8 Hz, 2H), 6.52 (dd, J = 6.8, J = 8.8, Hz, 1H), 3.02 (s, 6H); 13C NMR (CDCl3, 75 MHz): δ 150.9, 141.5, 133.5, 132.4, 120.8,
120.0, 119.3, 118.5, 118.1, 112.2, 110.7, 40.5; FTIR (cm-1) (neat): 3077, 2889, 1609, 1439, 1359, 1231, 822, 805, 692; HRMS
(ESI, Pos): calcd for C15H15BrN3 [M+H]+: 316.0444 m/z, found: 316.0459 m/z.
5-Bromo-3-(4-fluorophenyl)imidazo[1,5-a]pyridine (6g): Following procedure D. The crude imidazo[1,5-a]pyridine was
purified by flash chromatography over silica gel using a gradient of 10% to 70% EtOAc in hexanes. Fractions containing 6g
were concentrated to dryness, resulting in a grey solid (0.52 g, 90% yield). mp: 78-80 °C; 1H NMR (CDCl3, 500 MHz): δ 7.61
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(s, 1H), 7.51-7.48 (m, 3H), 7.12-7.09 (m, 2H), 6.82 (dd, J = 1.0, 7.0 Hz, 1H), 6.58 (dd, J = 7.0, 9.0 Hz, 1H); 13C NMR (CDCl3,
125 MHz): δ 163.3 (d, JC-F = 296.9 Hz), 133.7, 133.3 (d, JC-F = 10.0 Hz), 128.6 (d, JC-F = 3.9 Hz), 121.0, 119.6, 119.0, 118.0,
114.2 (d, JC-F = 26.1 Hz), 111.5; 19F NMR (CDCl3, 471 MHz): δ -112.2 (t, JH-F = 8.5 Hz, 1F); FTIR (cm-1) (neat): 3057, 2964,
1214, 798; HRMS (ESI, Pos): calcd for C13H8BrFN2 [M+H]+: 290.9928 m/z, found: 290.9933 m/z.
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5-Bromo-3-(4-chlorophenyl)imidazo[1,5-a]pyridine (6h): Following procedure D. The crude imidazo[1,5-a]pyridine was
purified by flash chromatography over silica gel using a gradient of 10% to 75% EtOAc in hexanes. Fractions containing 6h
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were concentrated to dryness, resulting in a green solid (45 mg, 95% yield). mp: 99-101 °C; H NMR (CDCl3, 400 MHz): δ
7.63 (s, 1H), 7.52 (d, J = 8.8 Hz, 1H), 7.47 (d, J = 8.4 Hz, 2H), 7.40 (d, J = 8.4 Hz, 2H), 6.85 (d, J = 8.8 Hz, 1H), 6.61 (dd, J =
8.8 Hz, 6.8 Hz, 1H); 13C NMR (CDCl3, 75 MHz): δ 139.3, 135.2, 134.0, 132.8, 131.1, 127.5, 121.4, 119.8, 119.3, 118.2, 111.7;
FTIR (cm-1) (neat): 3084, 1450, 1161, 1117, 1087, 967, 832, 791, 689; HRMS (ESI, Pos): calcd for C13H9BrClN2 [M+H]+:
306.9632 m/z, found: 306.9641 m/z.
Methyl 4-(5-bromoimidazo[1,5-a]pyridin-3-yl)benzoate (6i): Following procedure D. The crude imidazo[1,5-a]pyridine was
purified by flash chromatography over silica gel using a gradient of 10% to 70% EtOAc in hexanes. Fractions containing 6i
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were concentrated to dryness, resulting in a brown solid (0.54 g, 81% yield). mp: 134-135 °C; H NMR (CDCl3, 400 MHz): δ
8.14-8.11 (m, 2H), 7.70 (s, 1H), 7.68-7.65 (m, 2H), 7.48 (dd, J = 0.8, 9.2 Hz, 1H), 6.90 (dd, J = 1.2, 6.8 Hz, 1H), 6.67 (dd, J =
6.8, 8.8 Hz, 1H), 4.0 (s, 3H); 13C NMR (CDCl3, 100 MHz): δ 166.8, 139.2, 136.8, 134.1, 131.3, 130.3, 128.3, 121.7, 119.9,
119.4, 118.0, 111.6, 52.3; FTIR (cm-1) (neat): 3086, 3071, 2987, 2942, 2923, 2858, 2845, 1753, 1276, 692; HRMS (ESI, Pos):
calcd for C15H11BrN2O2 [M+H]+: 331.0077 m/z, found: 331.0076 m/z.
4-(5-Bromoimidazo[1,5-a]pyridin-3-yl)benzonitrile (6j): Following procedure D. The crude imidazo[1,5-a]pyridine was dis-
solved in a minimal amount of hot EtOAc and excess hexanes were added. The resulting precipitate was recovered by filtra-
tion and washed with hexanes. resulting in a green solid (0.58 g, 88% Yield). mp: 148-149 °C; 1H NMR (CDCl3, 500 MHz): δ
7.72-7.65 (m, 5H), 7.57 (dd, J = 1.0, 9.0 Hz, 1H), 6.91 (dd, J = 1.0, 6.5 Hz, 1H), 6.68 (dd, J = 7.0, 9.0 Hz, 1H); 13C NMR
(CDCl3, 125 MHz): δ 138.2, 136.8, 134.4, 131.8, 130.9, 122.1, 120.2, 119.8, 118.7, 118.1, 112.4, 111.3; FTIR (cm-1) (neat):
2981, 2972, 2947, 2923, 2863, 2844, 2827, 842, 799, 781; HRMS (ESI, Pos): calcd for C14H8BrN3 [M+H]+: 297.9974 m/z,
found: 297.9973 m/z.
5-Bromo-3-(3,4,5-trimethoxyphenyl)imidazo[1,5-a]pyridine (6k): Following procedure D. The crude imidazo[1,5-a]pyridine
was purified by flash chromatography over silica gel using a gradient of 15% to 75% EtOAc in hexanes. Fractions containing
6k were concentrated to dryness, resulting in a green solid (0.80 g, 88% Yield). mp: 101-102 °C; 1H NMR (CDCl3, 500 MHz):
δ 7.61 (s, 1H), 7.51 (dd, J = 1.0 Hz, 9.0 Hz, 1H), 6.84 (dd, J = 1.0 Hz, 6.5 Hz, 1H), 6.75 (s, 2H), 6.59 (dd, J = 6.5, 9.0 Hz, 1H),
3.91 (s, 3H), 3.87 (s, 6H); 13C NMR (CDCl3, 125 MHz): δ 152.2, 140.3, 139.1, 133.7, 127.6, 120.9, 119.7, 119.1, 118.1, 111.9,
109.4, 61.2, 56.3; FTIR (cm-1) (neat): 2935, 1582, 1463, 1409, 1233, 1118, 999, 760, 691; HRMS (ESI, Pos): calcd for
C16H16BrN2O3 [M+H]+: 363.0339 m/z, found: 363.0349 m/z.
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5-Bromo-3-(2-ethoxy-4-nitrophenyl)imidazo[1,5-a]pyridine (6l): Following procedure D. The crude imidazo[1,5-a]pyridine
was purified by flash chromatography over silica gel using a gradient of 15% to 75% EtOAc in hexanes. Fractions containing
6l were concentrated to dryness, resulting in a green solid (0.42 mg, 94% Yield). mp: 158-159 °C; 1H NMR (CDCl3, 400 MHz):
δ 7.93 (dd, J = 2.0, 8.4 Hz, 1H), 7.74 (d, J = 10.0 Hz, 2H), 7.73 (d, J = 14.4 Hz, 1H) 7.55 (d, J = 9.2 Hz, 1H), 6.89 (dd, J = 1.0,
6.8 Hz, 1H), 6.67 (dd, J = 7.2, 9.2 Hz, 1H), 4.08 (q, J = 6.8 Hz, 2H), 1.20 (t, J = 6.8 Hz, 3H); 13C NMR (CDCl3, 75 MHz): δ
158.7, 149.6, 135.6, 134.3, 132.0, 129.2, 122.0, 119.8, 119.4, 118.1, 115.2, 112.3, 105.7, 64.6, 14.5; FTIR (cm-1) (neat): 3053,
1512, 1340, 1319, 1083, 1213, 868, 802, 734; HRMS (ESI, Pos): calcd for C15H13BrN3O3 [M+H]+: 362.0145 m/z, found:
362.0135 m/z.
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5-Bromo-3-(2,3,4,5-tetrafluorophenyl)imidazo[1,5-a]pyridine (6m): Following procedure D. The crude imidazo[1,5-
a]pyridine was purified by flash chromatography over silica gel using a gradient of 10% to 70% EtOAc in hexanes. Fractions
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containing 6m were concentrated to dryness, resulting in a beige solid (0.64 g, 93% Yield). mp: 88-89 °C; H NMR (CDCl3,
500 MHz): δ 7.69 (s, 1H), 7.56 (d, J = 9.0 Hz, 1H), 7.26-7.21 (m, 1H), 6.92 (d, J = 6.5 Hz, 1H), 6.70 (dd, J = 7.0, 9.0 Hz, 1H);
13C NMR (CDCl3, 175 MHz): δ 147.4-147.1 (m), 146.0-145.7 (m), 142.3 (ddd, JC-F = 3.3, 12.6, 15.9 Hz), 141.1-140.8 (m),
139.5 (ddd, JC-F = 4.2, 12.6, 16.6 Hz), 134.5, 131.1, 122.1, 120.1, 119.9, 118.1, 117.7-117.5 (m), 113.9 (dd, JC-F = 2.3, 19.6
Hz); 19F NMR (CDCl3, 470 MHz): δ -156.9 to -155.8 (m, 1F), -153.2 to -153.0 (m, 1F), -139.7 to -139.6 (m, 1F), -134.7 to -
134.6 (m, 1F); FTIR (cm-1) (neat): 3107, 3039, 3012, 2956, 2847, 960, 799, 749, 700, 679; HRMS (ESI, Pos): calcd for
C13H5BrF4N2 [M+H]+: 344.9645 m/z, found: 344.9645 m/z.
5-Bromo-3-(2-methoxyphenyl)imidazo[1,5-a]pyridine (6n): Following procedure D. The crude imidazo[1,5-a]pyridine was
purified by flash chromatography over silica gel using a gradient of 15% to 75% EtOAc in hexanes. Fractions containing 6n
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were concentrated to dryness, resulting in a green solid (0.30 g, 86% Yield). mp: 129-131 °C; H NMR (CDCl3, 500 MHz): δ
7.64 (s, 1H), 7.51-7.43 (m, 3H) 7.03 (dt J = 1.0, 9.0 Hz, 1H), 6.90 (d, J = 8.4 Hz, 1H), 6.79 (d, J = 6.4 Hz, 1H), 6.79 (dt, J = 1.0,
9.0 Hz, 1H), 3.72 (s, 3H); 13C NMR (CDCl3, 125 MHz): δ 159.3, 137.6, 133.7, 132.3, 131.0, 122.4, 121.0, 120.0, 118.9 (2),
118.0, 112.3, 110.0, 55.3; FTIR (cm-1) (neat): 2925, 1443, 1283, 1261, 1239, 1100, 1020, 796, 746; HRMS (ESI, Pos): calcd
for C14H12BrN2O [M+H]+: 303.0128 m/z, found: 303.0137 m/z.
5-Bromo-3-(naphthalen-1-yl)imidazo[1,5-a]pyridine (6o): Following procedure D. The crude imidazo[1,5-a]pyridine was
purified by flash chromatography over silica gel using a gradient of 10% to 70% EtOAc in hexanes. Fractions containing 6o
1
were concentrated to dryness, resulting in a light greenish solid (0.76 g, 82% yield). mp: 152-153 °C; H NMR (CDCl3, 500
MHz): δ 7.99 (d, J = 8.5 Hz, 1H), 7.91 (d, J = 8.5 Hz, 1H), 7.75 (s, 1H), 7.62 (dd, J = 1.0, 7.0 Hz, 1H), 7.58 (dd, J = 1.0, 9.0 Hz,
1H), 7.56-7.52 (m, 1H), 7.50-7.46 (m, 1H), 7.42-7.38 (m, 1H), 7.23-7.20 (m, 1H), 6.77 (dd, J = 1.0, 6.5 Hz, 1H), 6.64-6.60 (m,
1H); 13C NMR (CDCl3, 125 MHz): δ 138.5, 134.8, 133.5, 132.9, 130.2 (2), 129.8, 128.2, 126.7, 126.0, 125.9, 124.5, 121.0,
119.2, 118.0, 111.7; FTIR (cm-1) (neat): 2966, 2865, 1055, 1033, 1013, 775, 758, 693; HRMS (ESI, Pos): calcd for C17H11BrN2
[M+H]+: 323.0178 m/z, found: 323.0194 m/z.
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2-(5-Bromoimidazo[1,5-a]pyridin-3-yl)-9H-fluoren-9-one (6p): Following procedure D. The crude imidazo[1,5-a]pyridine
was purified by flash chromatography over silica gel using a gradient of 10% to 70% EtOAc in hexanes. Fractions containing
6p were concentrated to dryness, resulting in a dark orange solid (0.45 g, 58% yield). mp: 198-199 °C; 1H NMR (CDCl3, 500
MHz): δ 7.83-7.82 (m, 1H), 7.74-7.70 (m, 2H), 7.68 (s, 1H), 7.64-7.60 (m, 2H), 7.58-7.53 (m, 2H), 7.36 (dt, J = 1.0, 7.5 Hz,
1H), 6.90 (dd, J = 1.0, 6.5 Hz, 1H), 6.68-6.64 (m, 1H); 13C NMR (CDCl3, 125 MHz): δ 144.6, 144.1, 139.1, 137.1, 134.8, 134.6,
134.0, 133.3, 132.9, 129.4, 127.1, 124.5, 121.5, 120.7, 119.9, 119.4, 119.2, 118.1, 111.6; FTIR (cm-1) (neat): 1707, 1612,
1597, 1184, 769, 742, 685, 671; HRMS (ESI, Pos): calcd for C20H11BrN2O [M+H]+: 375.0128 m/z, found: 375.0134 m/z.
5-Bromo-3-(furan-2-yl)imidazo[1,5-a]pyridine (6q): Following procedure D. The crude imidazo[1,5-a]pyridine was purified
by flash chromatography over silica gel using a gradient of 10% to 70% EtOAc in hexanes. Fractions containing 6q were con-
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centrated to dryness, resulting in a beige solid (0.44 mg, 84% yield). mp: 123-124 °C; H NMR (CDCl3, 400 MHz): δ 7.64 (s,
1H), 7.60 (dd, J = 0.8, 1.6 Hz, 1H), 7.51 (dd, J = 0.8, 8.8 Hz, 1H), 6.89 (dd, J = 1.2, 6.8 Hz, 1H), 6.73 (dd, J = 0.8, 3.6 Hz, 1H),
6.64 (dd, J = 6.8, 8.8 Hz, 1H), 6.55 (dd, J = 2.0, 3.6 Hz, 1H); 13C NMR (CDCl3, 100 MHz): δ 143.0, 142.9, 134.2, 130.4, 121.4,
120.0, 119.4, 117.7, 114.8, 112.0, 111.5; FTIR (cm-1) (neat): 3126, 3109, 3091, 3076, 2968, 2939, 2922, 2867, 2844, 805,
760; HRMS (ESI, Pos): calcd for C11H7BrN2O [M+H]+: 262.9815 m/z, found: 262.9819 m/z.
5-Bromo-3-(thiophen-2-yl)imidazo[1,5-a]pyridine (6r): Following procedure D. The crude imidazo[1,5-a]pyridine was puri-
fied by flash chromatography over silica gel using a gradient of 10% to 70% EtOAc in hexanes. Fractions containing 6r were
concentrated to dryness, resulting in a brown solid (0.50 g, 90% yield). mp: 95-96 °C; 1H NMR (CDCl3, 500 MHz): δ 7.64 (s,
1H), 7.53-7.50 (m, 2H), 7.25 (dd, J = 1.5, 4.0 Hz, 1H), 7.10 (dd, J = 3.5, 5.0 Hz, 1H), 6.87 (dd, J = 1.5, 7.0 Hz, 1H), 6.62 (dd, J
= 7.0, 9.0 Hz, 1H); 13C NMR (CDCl3, 125 MHz): δ 134.2, 132.9, 132.7, 132.0, 128.1, 126.1, 121.4, 119.9, 119.5, 118.0, 112.0;
FTIR (cm-1) (neat): 3080, 3065, 3008, 2981, 2967, 2949, 2938, 2922, 2866, 2844, 2829, 802, 712, 689; HRMS (ESI, Pos):
calcd for C11H7BrN2S [M+H]+: 278.9586 m/z, found: 278.9589 m/z.
5-Bromo-3-(prop-1-en-2-yl)imidazo[1,5-a]pyridine (6s): Following procedure D. The crude imidazo[1,5-a]pyridine was puri-
fied by flash chromatography over silica gel using a gradient of 5% to 60% EtOAc in hexanes. Fractions containing 6s were
1
concentrated to dryness, resulting in a green solid (0.76 g, 74% yield). mp: 76-77 °C; H NMR (CDCl3, 500 MHz): δ 7.51 (s,
1H), 7.43 (dd, J = 1.0, 9.0 Hz, 1H), 6.81 (dd, J = 1.0, 7.0 Hz, 1H), 6.56-6.52 (m, 1H), 5.61-5.58 (m, 1H), 5.22-5.20 (m, 1H),
2.26 (d, J = 10.0 Hz, 3H); 13C NMR (CDCl3, 75 MHz): δ 141.4, 136.6, 133.2, 122.1, 120.6, 119.2, 118.8, 117.8, 111.6, 25.2;
FTIR (cm-1) (neat): 3076, 3064, 1623, 1358, 1312, 1278, 1181, 814, 800, 762, 689; HRMS (ESI, Pos): calcd for C10H9BrN2
[M+H]+: 237.0025 m/z, found: 237.0029 m/z.
(E)-5-Bromo-3-styrylimidazo[1,5-a]pyridine (6t): Following procedure D. The crude imidazo[1,5-a]pyridine was purified by
flash chromatography over silica gel using a gradient of 10% to 70% EtOAc in hexanes. Fractions containing 6t were concen-
trated to dryness, resulting in a green solid (0.57 g, 94% yield). mp: 91-93 °C; 1H NMR (CDCl3, 300 MHz): δ 8. 31 (d, J = 15.9
Hz, 1H), 7.61-7.55 (m, 4H), 7.45 (dd, J = 0.9, 8.7 Hz, 1H), 7.40-7.35 (m, 2H), 7.31-7.27 (m, 1H), 6.88 (dd, J = 1.2, 6.9 Hz, 1H),
6.52 (dd, J = 6.6, 8.7 Hz, 1H); 13C NMR (CDCl3, 75 MHz): δ 139.7, 137.3, 134.4, 131.5, 128.7, 127.9, 126.7, 122.5, 120.2,
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The Journal of Organic Chemistry
118.8, 118.4, 117.0, 111.5; FTIR (cm-1) (neat): 3107, 3055, 3039, 3012, 2954, 2847, 960, 799, 749, 700, 679; HRMS (ESI,
Pos): calcd for C15H11BrN2 [M+H]+: 299.0178 m/z, found: 299.0183 m/z.
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5-Bromo-3-(phenyl-2-d)imidazo[1,5-a]pyridine (6a-ortho-d1): Following procedure D. The crude imidazo[1,5-a]pyridine
was purified by flash chromatography over silica gel using a gradient of 10% to 75% EtOAc in hexanes. Fractions containing
1
6a-ortho-d1 were concentrated to dryness, resulting in a green solid (60 mg, 74% yield). mp: 59-61°C; H NMR (CDCl3, 500
MHz): δ 7.64 (s, 1H), 7.54-7.51 (m, 2H), 7.48-7.40 (m, 3H), 6.83 (dd, J = 1.0, 6.5 Hz, 1H), 6.59 (dd, J = 6.5, 9.1 Hz, 1H); 13
C
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NMR (CDCl3, 100 MHz): δ 140.5, 133.7, 132.3, 131.6, 131.3 (t, JC-D = 25.0 Hz), 129.1, 127.3, 127.2 121.0, 119.7, 119.1 118.1,
111.8; FTIR (cm-1) (neat): 1623, 1445, 1360, 1320, 1185, 1105, 965, 804, 760, 688, 633; HRMS (ESI, Pos): calcd for
C13H9DBrN2 [M+H]+: 274.0085 m/z, found 274.0095 m/z.
5-Bromo-3-(phenyl-d5)imidazo[1,5-a]pyridine (6a-d5): Following procedure D. The crude imidazo[1,5-a]pyridine was puri-
fied by flash chromatography over silica gel using a gradient of 15% to 75% EtOAc in hexanes. Fractions containing 6a-d5
were concentrated to dryness, resulting in a green solid (0.26 g, 92% yield). mp: 72-74°C; 1H NMR (CDCl3, 500 MHz): δ 7.64
(s, 1H), 7.52 (dd, J = 1.0, 9.0 Hz, 1H), 6.83 (dd, J = 1.0, 7.0 Hz, 1H), 6.59 (dd, J = 6.5, 9.0 Hz, 1H); 13C NMR (CDCl3, 125
MHz): δ 140.5, 133.7, 132.4, 131.1 (t, JC-D = 24.4 Hz), 128.5 (t, JC-D = 24.3 Hz), 126.7 (t, JC-D = 24.3 Hz), 121.1, 119.6, 119.0,
118.1, 111.8; FTIR (cm-1) (neat): 3351, 3083, 1750, 1478, 1094, 809, 689, 562; HRMS (ESI, Pos): calcd for C13H5D5BrN2
[M+H]+: 278.0336 m/z, found 278.0342 m/z.
General procedure for the synthesis of imidazo[2,1,5-c,d]indolizines
Procedure E:
To a flame-dried microwave (sealable) vial (VWR®) equipped with a magnetic stirrer was added the corresponding 5-
bromoimidazo[1,5-a]pyridine (1.0 equiv), as previously synthesized. Then, Pd2(dba)3 (0.025 equiv), HP(t-Bu)3BF4 (0.10 equiv),
and K2CO3 (2.0 equiv) were added to the vial. The vial was capped with a rubber septum and purged with argon. Anhydrous
DMF (0.20M) was added and the reaction was quickly heated to 120 °C using an oil bath and stirred for 2.5 h. The reaction
was cooled to room temperature and the crude mixture was diluted in EtOAc and a saturated aqueous solution of NaCl was
added. The layers were separated and the aqueous layer was extracted with EtOAc (3x). The combined organic layers were
washed with brine (2x) dried over anhydrous MgSO4, filtered over a pad of silica gel, and evaporated to dryness. *Note: Pre-
paring the reaction mixture in a glovebox produced similar results for several runs and is likely unnecessary for most sub-
strates.
Procedure F:
To a flame-dried microwave (sealable) vial (VWR®) equipped with a magnetic stirrer was added the corresponding 5-
bromoimidazo[1,5-a]pyridine (1.0 equiv), as previously synthesized. The vial was then purged (2 x 10 min) with argon and
transferred in an Ar-filled glovebox. Then, Pd2(dba)3 (0.025 equiv), HP(t-Bu)3BF4 (0.10 equiv), and K2CO3 (2.0 equiv) were
added to the vial. The vial was capped with an aluminum microwave cap (VWR® with Teflon seal) and removed from of the
glovebox. Anhydrous DMF (0.20 M) was added. The reaction was slowly heated to 110 °C using an oil bath and stirred for 16
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