Triple aryne-tetrazine reaction enabling rapid access to a new class of polyaromatic heterocycles

Article abstract of DOI:10.1039/c5sc01726b

One of the most challenging goals of modern synthetic chemistry is to develop multi-step reactions for rapid and efficient access to complex molecules. We report a triple aryne-tetrazine reaction that enables rapid access to a new class of polyaromatic heterocycles. This new reaction, which couples diverse reactivity modes between simple aryne and tetrazine starting materials, proceeds in a single operation and takes less than 5 minutes in air with no metal catalyst.

Full text of DOI:10.1039/c5sc01726b

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Triple aryne–tetrazine reaction enabling rapid
access to a new class of polyaromatic
heterocycles†
Cite this: DOI: 10.1039/c5sc01726b
Sung-Eun Suh, Stephanie A. Barros and David M. Chenoweth*
One of the most challenging goals of modern synthetic chemistry is to develop multi-step reactions for
rapid and efficient access to complex molecules. We report a triple aryne–tetrazine reaction that enables
rapid access to a new class of polyaromatic heterocycles. This new reaction, which couples diverse
reactivity modes between simple aryne and tetrazine starting materials, proceeds in a single operation
and takes less than 5 minutes in air with no metal catalyst.
Received 12th May 2015
Accepted 23rd June 2015
DOI: 10.1039/c5sc01726b
www.rsc.org/chemicalscience
that a simple change in the functionality on tetrazine (3a, Fig. 1)
can unveil latent reactivity in the presence of arynes, providing
Introduction
New polyaromatic heterocycles are important for the discovery rapid and direct access to a new class of dibenzo[de,g]cinnoline
and development of lead bioactive molecules and chemical polyaromatic heterocycles 5a. This new class of dibenzo[de,g]
probes. Our interest in developing new chemical probes and cinnoline exhibits pH responsiveness resulting in dramatic
selective nucleic acid modulators motivated us to explore direct shis in photophysical properties upon protonation. This new
1,2
methods for rapidly accessing novel polyaromatic systems.
class of dibenzocinnolines may have several potential applica-
Recent advances in benzyne/aryne chemistry have paved the tions as cellular imaging agents or bioactive molecules.
way for use of these reactive intermediates in new synthetic
3
–12
13–20
methods,
complex natural product synthesis,
and aryne
Results and discussion
21
polymerizations (Fig. 1). In principal, multiple-consecutive
aryne additions to a central core molecule could provide a novel The reaction transforming tetrazines into dibenzocinnolines
and efficient approach for rapid access to new polyaromatic is operationally simple and requires less than 5 minutes
systems. Metal catalyzed methods have been reported for from start to nish. Recently, important new methods devel-
22
21
cyclotrimerization and polymerization of arynes. However, oped by Devaraj and co-workers have paved the way for the
harnessing the reactivity of arynes for controlled multiple- efficient synthesis of tetrazines, providing easy access to many
consecutive intermolecular reaction processes still remains a
signicant challenge. An alternative strategy for directly
accessing higher order polyaromatic systems could be envi-
sioned using arynes, such as benzyne, in combination with a
reactive core molecule, such as a tetrazine, acting as a scaffold
for iterative additions.
The reaction between benzyne and tetrazine 1 was reported
approximately 50 years ago and is known to proceed through a
Diels–Alder/retro-Diels–Alder process, resulting in phthalazine
23
2
as the terminal product (Fig. 1). However, if the phthalazine
product is thought of as an intermediate, one could envision
further aryne additions to produce polyaromatic systems
through sequential aryne addition reactions. Here, we show
Department of Chemistry, University of Pennsylvania, 231 South 34th Street,
Philadelphia, PA 19104, USA. E-mail: dcheno@sas.upenn.edu
†
Electronic supplementary information (ESI) available: Experimental protocols,
characterization data, X-ray crystallographic data (CIF) and NMR spectra of all
new compounds. CCDC 1400529. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c5sc01726b
Fig. 1 Triple aryne–tetrazine reaction. Red bold lines represent new
bonds formed during the course of the reaction.
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24
substitution patterns. A source of uoride anion serves as a initial [4 + 2] bicyclo intermediate I from 3a. Cycloreversion with
mild reagent to rapidly initiate the reaction via desilylation of a concomitant loss of nitrogen gas produces intermediate
25,26
23
masked benzyne precursor,
obviating the necessity for a phthalazine heterocycle 6. Next, phthalazine 6 undergoes
metal catalyst, external heat source, or inert gas. The reaction nucleophilic addition to benzyne, affording key intermediate II
was performed in a vial, under air, charged with tetrazine and followed by proton transfer to produce s-cis diene intermediate
the benzyne precursor. Aer dropwise addition of a 1.0 M III. A third aryne engages the newly formed diene through one
solution of tetrabutylammonium uoride (TBAF) in THF at 24 of two possible mechanisms. This rst possibility is a second
ꢀ
C, dibenzo[de,g]cinnoline 5a was produced (Fig. 1). The struc- [4 + 2] cycloaddition reaction, resulting in dihydrophenan-
ture of dibenzocinnoline 5a was conrmed using X-ray crystal- threne-like intermediate V. The alternate possibility is a non-
lography (Table 2 and ESI†). Dibenzo[de,g]cinnolines, which concerted pathway where a third equivalent of benzyne is
have not been previously reported, share signicant structural attacked by the nucleophilic enamine like intermediate III to
2
7,28
homology to several natural products.
produce iminium IV, followed by intramolecular conjugated
29
A plausible mechanism for the reaction proceeding through addition to afford V. Two mechanistic possibilities are envi-
six elementary steps is shown in Fig. 2A. Aryne formation sioned for the nal oxidation, where the rst mechanism
commences aer addition of TBAF followed by formation of the involves a direct 1,4-oxidation to yield 5a. Although this mech-
anism is plausible, we favor a second possible pathway in which
11
a base, such as uoride, hydroxide, or water, facilitates an
initial 1,4- to 1,2-dihydro rearrangement of cross-conjugated
intermediate V to restore aromaticity in the [de] ring leading to
intermediate VI. This step is then followed by either direct
oxidation or aryne promoted desaturation leading to the fully
aromatized dibenzocinnoline 5a. The feasibility of the aryne
promoted desaturation process was recently demonstrated in
12
seminal studies by Hoye and coworkers. To test the aryne
desaturation pathway, we conducted the tandem reaction in the
absence of air. The heterocyclic product was formed, providing
support for the desaturation pathway.
To trap intermediate phthalazine 6, 2 equivalents of 3a were
used with 1 equivalent of benzyne precursor 4a (Fig. 2B), since 6
was not observed in the optimized conditions shown in Table 1,
entry 7. Using these conditions, 6 was obtained in 7% yield and
was resubjected to the optimized reaction conditions (Fig. 2B
Table 1 Selected optimization experiments of the reaction
a
Entry
Solvent
THF
Conc. (M)
Yield (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
0.1
0.5
1.5
2.0
—
22
25
14
14
1
25
26
6
Et
2
O
Hexanes
Xylenes
DMF
CH
CH
CH
CH
CH
CH
—
3
2
2
2
2
2
CN
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
7
1
1
1
0
1
2
10
13
19
Fig. 2 (A) Proposed mechanism of the reaction. DA, Diels-Alder; DAinv
,
inverse electron demand Diels-Alder. (B) Reaction conditions trapping
intermediate 6. (C) Resubjection of isolated intermediate 6 to the
original reaction conditions. (D) Reaction of benzyne with diphe-
nyltetrazine 1 to produce 9,10-diphenylanthracene 7.
a
HPLC yield. 9,10-Diphenylanthracene was used as an internal
standard.
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and C). The desired product 5a was produced in 27% yield. Table 2 Substrate scope for the synthesis of dibenzo[de,g]cinnolines
a,b
and X-ray crystal structure of 5a
Although further inverse Diels–Alder reaction between inter-
mediate 6 and a benzyne is plausible, 9,10-dimethylanthracene
was not observed. However, when diphenyltetrazine 1 reacted
ꢀ
with benzyne precursor 4a and TBAF in THF at 50 C for 2
hours, 9,10-diphenylanthracene 7 and 1,4-diphenylphthalazine
2
were both obtained. This result was consistent with our
hypothesis that a benzylic proton unveils masked reactivity at
phthalazines diverting the reaction down alternate competing
mechanistic pathways leading to the formation of dibenzo[de,g]
cinnolines.
Reaction optimization studies were conducted using
nonvolatile tetrazine 3d (Table 1). Among the several desilylat-
ing reagents used (see Table S1 in ESI†), TBAF afforded 5d in
2
6% yield (Table S1,† entry 1), while other uoride anion
sources, such as tetrabutylammonium diuorotriphenylsilicate
TBAT), CsF, HF, and KF, resulted in no reaction (Table S1,†
(
entries 2–12). In the presence of 18-crown-6, KF resulted in a low
yield of product 5d (Table S1,† entry 13). As expected, the
reaction did not occur when other additives including tetrabu-
tylammonium chloride or tetrabutylammonium bromide were
added in the absence of a uoride source (Table S2 in ESI†).
The highest yields were obtained using CH
solvent. However, several alternative solvents such as THF,
Et O, xylene, hexane, and CH CN resulted in a reasonable yield
2 2
Cl as the
2
3
of product 5d (Table 1, entries 1–7). The reaction was sensitive
to the concentration with the highest conversion to product
achieved at a nal concentration of 1.0 M (Table 1, entries 7–12).
In addition, excess benzyne precursor and TBAF reagents were
required to achieve optimal yields (see Table S3 in ESI†). A
decrease in the temperature and an increase in the reaction
a
Reactions were performed under the optimized condition shown in
b
time resulted in decreased product yields (see Table S4 in ESI†). Table 1, entry 7. Isolated yield.
The addition of TBAF to the reaction mixture over the course of
60 seconds resulted in the best yields. During the TBAF addi-
tion, a steady increase in the internal temperature of the reac- methyl group. In addition, isolated 5a or 5b did not react further
ꢀ
tion mixture to a maximum of 51 C aer 70 seconds followed with excess benzyne in CH Cl or THF under reuxing condi-
2
2
ꢀ
by a decrease to 24 C was observed. Aer a total time of 300 tions. This result is consistent with the nitrogen–benzyne
seconds, the reaction was complete (see Fig. S1 in ESI†). adduct creating a sterically congested environment that
Interestingly, 5d was isolated in 27% yield when 4a was used precludes addition to the adjacent nitrogen atom, as seen from
as a limiting reagent, TBAF was added to 3d over a period of 4 the crystal structure of 5a in Table 2. To further test the inu-
ꢀ
hours at 0 C, and the solution was stirred for an additional 20 ence of steric hindrance, the bulky adamantyl tetrazine 3c was
hours (see Table S4,† entry 7). Reduction in the amount of used. The bulky adamantyl group was expected to shut down
benzyne precursor used resulted in longer reaction times with the Diels–Alder reaction, preventing appropriate alignment of
unreacted tetrazine and required lower temperatures. Further the aryne with the central tetrazine carbon atoms. To our
optimization is currently underway in our group.
surprise, product 5c was produced in 10% yield.
Using the optimized conditions described here, several
To broaden the scope of the reaction, difunctionalized tet-
dibenzocinnolines 5a–5j were synthesized. Given that the razines were explored. We found that the reaction is not limited
reaction forms four new C–C bonds and one new C–N bond to substitution at the R1 position. Difunctionalized tetrazines
during the course of 6–8 steps, yields from 10–52% are quite with substitution at R2 resulted in reasonable yields of diben-
reasonable. Alkyl, aryl, and heteroaryl substituents are tolerated zocinnoline products 5k–5m (Table 3). Encouraged by these
in position R1. Differentially dialkylated tetrazines show selec- results, alternate benzyne precursors were tested with a set of
tivity for reaction at the least sterically hindered position. For tetrazines (Table 4). We found that addition of 4,5-dimethyl-o-
example, 3-methyl-6-nonyl-1,2,4,5-tetrazine 3b has two benzylic benzyne to tetrazines afforded 5n–5p in reasonable yields (24–
positions; however, only product 5b (Table 2) was observed, 31%; Table 4).
consistent with nonyl group sterics dictating addition of the
second benzyne to the least sterically hindered nitrogen atom only initiates the reaction but also decomposes the tetrazine
see intermediate II in Fig. 2), which is much bulkier than the starting materials in a competing but slower background
Depending on the tetrazine used, we found that TBAF not
(
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Table Scope of tetrazines for the synthesis of dibenzo[de,g]
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3
a,b
cinnolines
2 2 2 2
Fig. 3 Structure of 5q. Solutions of 5q in CH Cl (left) and in CH Cl /
TFA (right) irradiated at 365 nm with a UV lamp (left).
a
Reactions were performed under the optimized conditions shown in
b
Table 1, entry 7. Isolated yield.
a,b
Table 4 Addition of 4,5-dimethyl-o-benzyne
Fig. 4 Emission spectra for the neutral and protonated forms of
compounds 5b, 5d, 5i, and 5q.
dinaphthocinnoline 5q exhibited a bathochromic emission
shi in the presence of acid unlike 5b, 5d, and 5i (Fig. 3). Taken
together, these results show that the photophysical properties
are tunable across a large window of the visible spectrum
(Fig. 4) and we expect even more interesting photophysical
phenomena as we explore new derivatives for applications in
a
Reactions were performed under the optimized condition shown in
b
Table 1, entry 7. Isolated yield reported.
reaction. For mono-substituted tetrazines, such as 3-methyl-
1,2,4,5-tetrazine, decomposition was faster than the initial
Diels–Alder reaction, resulting in neither the desired product
nor the phthalazine intermediate. Additionally, 3-benzyl-6-
methyl-1,2,4,5-tetrazine decomposed faster due to the increased
acidity of the methylene accelerating the tetrazine decomposi-
tion. Also, the addition of dinaphthyne precursor to 3-methyl-6-
phenyl-1,2,4,5-tetrazine 3d led to dinaphthocinnoline 5q.
Dibenzo[de,g]cinnolines exhibit interesting photophysical
properties (see ESI†). Dibenzo[de,g]cinnolines 5b, 5d, and 5i
are emissive in the solid state and solution state under
Fig. 5 Live-cell imaging of HeLa cells in the presence of 5e. (A)
Cultured HeLa cells were incubated with 5e for 2 hours and imaged
using confocal microscopy. (B) Cells were incubated with 5e for 2
hours, then counterstained with Hoechst 33342 and MitoTracker Red
FM. Extranuclear cyan corresponds to compound 5e. Using an exci-
tation of 405 nm and emission from 450 to 500 nm allows for
simultaneous visualization of 5e, nuclear localized Hoechst 33342, and
365 nm UV radiation. Additionally they exhibited hypsochromic
emission response in the presence of acid. In contrast, mitochondrial localized MitoTracker Red FM.
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materials, sensing, and imaging. Preliminary cellular imaging
with 5e shows that upon excitation of the acidic form at 405 nm,
intracellular vesicles can be selectively stained (Fig. 5a). When
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5
1
Conclusions
4
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Acknowledgements
This work was supported by funding from the University of
Pennsylvania. We thank Pat Carroll for X-ray crystallographic
assistance. Instruments supported by the National Science
Foundation and the National Institutes of Health include
HRMS (Grant NIH RR-023444) and X-ray diffractometer (Grant
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