Highly Regioselective Domino Benzannulation Reaction of Buta-1,3-diynes To Construct Irregular Nanographenes

Article abstract of DOI:10.1002/anie.201808043

The properties of nanographenes can be tuned by changing their shapes, therefore the development of new methods suitable for the synthesis of various nanographenes is highly desirable. Described herein is an intramolecular InCl₃/AgNTf₂-catalyzed regioselective domino benzannulation reaction of buta-1,3-diynes to build irregular nanographenes. Different nanographene compounds were easily obtained in moderate to high yields through careful design of the precursor compounds. This new domino reaction was successfully applied to a fourfold alkyne benzannulation of dimethoxy-1,1′-binaphthalene derivatives to arrive at novel chiral butterfly ligand precursors. The regioselectivity of the benzannulation reaction was unambiguously confirmed by X-ray crystallography. Moreover, this new method enables us to synthesize different nanographene isomers and study their optical properties as a function of shape.

Full text of DOI:10.1002/anie.201808043

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Accepted Article
Title: Highly Regioselective Domino Benzannulation Reaction of
Buta-1,3-diynes to Construct Irregular Nanographenes
Authors: Wenlong Yang, Radha Bam, Vincent J. Catalano, and Wesley
A. Chalifoux
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201808043
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10.1002/anie.201808043
Angewandte Chemie International Edition
COMMUNICATION
Highly Regioselective Domino Benzannulation Reaction of Buta-
1,3-diynes to Construct Irregular Nanographenes
Wenlong Yang, Radha Bam, Vincent J. Catalano, and Wesley A. Chalifoux*
Abstract: The properties of nanographenes can be tuned by
changing their shapes, therefore the development of new methods
that are suitable for the synthesis of various nanographenes is highly
desirable. Herein, we describe an intramolecular InCl3-AgNTf2-
catalyzed regioselective domino benzannulation reaction of buta-1,3-
diynes to build irregular nanographenes. Different nanographene
compounds were easily obtained in moderate to high yields through
careful design of the precursor compounds. This new domino reaction
was successfully applied to a four-fold alkyne benzannulation of
dimethoxy-1,1'-binaphthalene derivatives to arrive at novel chiral
butterfly ligand precursors. The regioselectivity of the benzannulation
reaction was unambiguously confirmed by the X-ray crystallography.
Moreover, this new method enables us to synthesize different
nanographene isomers and study their optical properties as a function
of shape.
(Figure 1a). Mꢀllen, Feng, and coworkers reported the
benzannulation of conjugated buta-1,3-diynes, in which two
cyclization events occurred individually (Figure 1b).[16] To the best
of our knowledge, the domino benzannulation of conjugated 1,3-
diynes has never been reported (Figure 1c). Herein, we report the
first domino benzannulation reaction of buta-1,3-diynes catalyzed
by InCl3-AgNTf2, to build different irregular nanographenes. This
protocol is applicable to efficiently make different nanographene
structures through variation of either the building block structures
or the substituent positions within the precursors.
Extended polycyclic aromatic hydrocarbons (PAHs), often
referred to as nanographenes, have recently received
a
considerable amount of attention due to their interesting optical
and electronic properties.[1] The properties of PAHs can be tuned
by changing their degree of π-extension, shape, width, and edge
topologies.[2] Bottom-up approaches for the production of well-
defined nanographenes have attracted significant interest of
chemists.[3] The Scholl reaction,[4] transition-metal-[5] or photo-
catalyzed[6] directed intramolecular arylation reactions, and
alkyne benzannulation reactions (inter- and intramolecular)[7] are
widely used in the key C-C bond-forming reaction of
nanographene syntheses. Moreover, the preparation of carbon-
rich materials from precursors containing many C-C triple bonds
is gaining more attention.[8] For example, Tanaka and coworkers
reported the synthesis of helically chiral 1,1′-bitriphenylenes via
rhodium-catalyzed double [2+2+2] cycloaddition of biaryl-linked
tetraynes with 1,4-diynes.[9] Rubin and coworkers synthesized
graphene nanoribbons (GNRs) via a topochemical polymerization
and subsequent aromatization of a diacetylene precursors.[10] In
2016, our group reported the bottom-up synthesis of soluble and
narrow GNRs using alkyne benzannulations.[11] The multi-fold
intramolecular benzannulation reaction of alkynes have been
shown to proceed using a variety of π-Lewis acids,[12] Brønsted
acids,[13] radical reagents,[14] and other electrophiles.[15] However,
we found that almost all of previously reported alkyne
benzannulation reactions have been limited to monoyne groups
Figure 1. Intramolecular benzannulation reaction of alkynes. (a) 6-endo-dig
cyclizations of monoyne moieties. (b) Random sequential 6-endo-dig
cyclizations of buta-1,3-diyne. (c) Domino benzannulation reaction of buta-1,3-
diynes.
Previous studies have shown that alkyne benzannulation
reactions are more facile with electron-rich ethynylaryl groups.[7b,
17] Therefore, we initiated this study by screening suitable catalytic
conditions for the domino benzannulation reaction of electron-rich
buta-1,3-diyne 1a as a model substrate with the results
summarized in Table 1. Transition metals, such as In(III),[12b, 18]
Au(I),[19] Au(III),[12b] and Pt(II),[12b, 20] have been successfully used
in the Lewis acid-catalyzed benzannulation reaction of alkynes
and these metals have provided a starting point for our study.
Recently, we have shown that a wide scope of peropyrenes and
teropyrenes could be synthesized through two- and four-fold
alkyne benzannulations using InCl3 as a catalyst in toluene at high
[*]
Dr. W. Yang, R. Bam, Prof. V. J. Catalano, Prof. W. A. Chalifoux
Department of Chemistry, University of Nevada, Reno
1664 N. Virginia St., Reno, NV 89557 (USA)
E-mail: wchalifoux@unr.edu
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate)
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10.1002/anie.201808043
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temperature.[17] However, in this case, catalytic amounts of InCl3
only resulted in mono-cyclized intermediate 2a with no formation
of 3a being detected (entry 1, Table 1). We found that PtCl2 and
AuCl3 were less effective with AuCl(PPh3) giving no conversion at
all (entries 2-4). Among all the addition of Ag(I) salts (entries 5-9),
addition of AgNTf2 (Tf = trifluoromethanesulfonyl) exhibited
highest catalytic ability for this transformation with desired
annulated product 3a being formed exclusively in 83%yields,
(entry 9). Compared to using InCl3/AgOTf catalyst system, similar
yield was obtained while In(OTf)3 was used as a catalyst (entry
10). The catalytic activity of InCl3/AgNTf2 was high and thus the
reaction could even be carried out at a reduced catalyst loading
of 10 mol% InCl3 and 10 mol% AgNTf2 to provide improved yield
of 94% (entry 12). The reaction resulted in many side products
and low yield when conducted in 1,2-dichloroethane (DCE) (entry
13). We conducted a control experiment and no reaction occurred
with only AgNTf2 present in the reaction mixture (entry 14). We
also explored a Brønsted acid [i.e., triflic acid (TfOH)], and full
conversion was observed after stirring 1a with 2 equivalents of
TfOH in dichloromethane (DCM) at –40 ºC for 30 mins giving the
target compound 3a in 51% yield (entry 15). It should be noted
that the first alkyne benzannulation is highly regioselective with
the structure of 3a being unambiguously confirmed by X-ray
crystallographic analysis (vide infra).
12
13
14
15
InCl3 (10)
InCl3 (10)
-
AgNTf2 (10)
AgNTf2 (10)
AgNTf2 (10)
-
toluene
DCE
001
001
1:0:0
001
94
25[d]
0
toluene
DCM
TfOH (200)
51[e]
[a] Determined by 1H NMR. [b] Isolated yield. [c] Not determined. [d] 80°C. [e]
Reaction was quenched after 30 minutes.
With optimized reaction conditions in hand (entry 12, Table 1),
we turned our attention to exploring the substrate scope of parent
PAHs 5 (Table 2). A hexyl chain was incorporated instead of
isobutyl group not only to enhance the solubility of both precursors
1 and fully cyclized products 3 but also provide good crystallinity
of the products for later possible X-ray analysis. Gratifyingly,
naphthalene, anthracene, phenanthrene, and pyrene were all
applicable building blocks in this reaction to afford the
corresponding larger PAHs in moderate to high yields (3b-3g).
Heteroaromatic systems were also investigated to further expand
the scope. The cyclization of quinoline-based precursors (1h and
1h') did not produce the desired fully annulated products 3h and
3h'. Treating the benzothiophene-containing precursor 1i to the
optimized reaction conditions resulted in no desired doubly
cyclized product 3i. Using InCl3 as a catalyst in DCE gave 87%
yield of mono-cyclized intermediate 6. Subjecting compound 6 to
the optimized reaction conditions gave only intractable material.
We propose that this is because the distance between the alkyne
and aryl carbons are too far apart in 6 to participate in a second
6-endo-dig cyclization reaction (~3.5 Å, insert figure in Table 2).[21]
Table 1. Optimization of the domino benzannulation reaction conditions.
Table 2. Substrate scope with respect to parent PAHs 5.[a]
catalyst
(mol%)
activator
(mol%)
ratio[a]
(1a:2a:3a)
3a
(%)[b]
entry
solvent
1
2
3
InCl3 (20)
PtCl2 (20)
AuCl3 (20)
-
-
-
toluene
toluene
toluene
130
810
2210
0
0
0
AuCl(PPh3)
(20)
4
5
6
-
toluene
toluene
toluene
100
0
InCl3 (20)
InCl3 (20)
AgBF4 (60)
29010
361
0
AgCO2CF3
(60)
ND[c]
7
InCl3 (20)
InCl3 (20)
InCl3 (20)
In(OTf)3 (20)
InCl3 (10)
AgSbF6 (60)
AgNTf2 (60)
AgOTf (60)
-
toluene
toluene
toluene
toluene
toluene
001
001
001
001
001
50
83
71
72
85
8
[a] Reaction conditions: InCl3 (10 mol%), AgNTf2 (10 mol%), toluene, 100 ºC,
12 h. [b] K2CO3, Pd(PPh3)4, THF/H2O, 80 ºC, 24 h. [c] Isolated yield. [d] InCl3
(10 mol%), DCE, 80 ºC, 12 h.
9
10
11
Recently, we have demonstrated that ethynylthiophene
moieties can undergo a facile benzannulation reaction to produce
thiophene-functionalized pyrene, peropyrene, and teropyrene
AgNTf2 (30)
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[22]
compounds.
After treating precursor 7 under optimized
reaction conditions the thiophene-functionalized benzo[a]pyrene
8 was isolated in 82% yield (Scheme 1a). Summarizing all the
above results, we show that electron-rich buta-1,3-diynes work
well for the domino benzannulation reaction. We switched to test
alkyne substituents terminated with electron-deficient and
electron-neutral aryl groups: compounds 9 and 11, respectively.
Unfortunately, no reaction occurred after treating 9 to our
optimized reaction conditions, even in mesitylene at 150 °C
(Scheme 1b). The electron-neutral precursor 11 gave a trace
amount of the desired product 12 when the reaction was carried
out at 100 °C (Scheme 1c). Interestingly, the major product was
the addition product of molecular toluene to the monocyclized
intermediate to provide compound 13. In an attempt to avoid this
side reaction, more sterically hindered mesitylene was used as a
solvent. While no reaction occurred while treating the precursor
11 in mesitylene at 100 ºC for 12 hours, full conversion of starting
material was observed after keeping the reaction at 150 ºC for 24
hours. The NMR results indicated that mesitylene had added
across the remaining triple bond of the monocyclized intermediate
to form compound Z-14 with the stereochemistry being
unambiguously confirmed by X-ray crystallographic analysis
(Scheme 1c). A possible explanation for this solvent trapping
reaction is that this is a formal Friedel-Crafts reaction that
proceeds through a cationic intermediate, analogous to cationic
Au(I)-catalyzed cyclizations.[23] After mono-cyclization of
compound 11, the more stable cation intermediate generated
after In(III) complexation would be that which places the positive
charge on the carbon next to the newly generated chrysene
moiety (see Scheme S7). The two possible outcomes would be
either a 5-exo-dig cyclization (not observed) or trapping with a
nucleophile (i.e., the solvent). The benzannulation in a non-
aromatic solvent (DCE) also failed. It appears that the barrier of
the second cyclization step is too high for electron-neutral alkynes,
with other pathways – such as solvent trapping being more
favorable.
Scheme 1. Scope of buta-1,3-diynes terminated with (a) ethynylthiophene
moiety, (b) electron-deficient ethynylphenyl, (c) electron-neutral ethynylphenyl
group.
During the investigation of substrate scope, we found that this
domino benzannulation reaction also worked well in stilbene
derivatives. After treating stilbene derivative 1j to the optimized
reaction conditions, the benz[a]anthracene derivative 3j was
obtained in 84% yield (Scheme 2a). Since naphthyl derivatives 1a
and 1b can cyclize in relative high yield (vide supra), we decided
to try applying this method in dimethoxy-1,1'-binaphthalene
derivative 1k. The annulated product 3k was isolated in 92% yield
(Scheme 2b). To our delight, the annulated compound 3m was
obtained in 63% yield after four-fold alkyne benzannulation
reaction of bisbuta-1,3-diynes 1m (Scheme 2c). This
demonstrates a new method to afford novel chiral butterfly ligands
for use in enantioselective catalysis.
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Figure 2. X-ray structures (ellipsoids at 50% probability, hydrogens removed for
clarity) and packing structures (hydrogens and side chains removed for clarity)
of (a-c) 3a, (d-e) 3d, (f-h) 3e, and (i-k) 3f.
The UV-Vis absorption and emission spectra of selected
compounds 3c, 3d, 3e, 3f, and 3g are shown in Figure 3. All of
these compounds display vibrational fine structure in their
respective UV-vis spectra. The observed emission spectra are
mirror images of the absorption spectra, which indicates that the
same states are involved in absorption and emission. Interestingly,
compounds 3c, 3d, and 3e are dibenzochrysene structural
isomers, with the same number of fused rings, but show very
different absorption and emission profiles (Figure 3a). Compared
to compounds 3d and 3e (λmax = 396 and 401 nm, respectively),
a dramatic bathochromic shift of ~60 nm for compound 3c was
observed (λmax = 459 nm) with a smaller Stokes shift. Similarly,
compounds 3f and 3g can also be considered as anthrapyrene
isomers, both having seven fused benzene rings. However,
compared to compound 3f with planar backbone, twisted
compound 3g showed broad absorption and emission spectra
with a relatively small bathochromic shift being observed (Figure
3b).
Scheme 2. Domino benzannulation reaction of alkynes in (a) stilbene derivative,
(b-c) dimethoxy-1,1'-binaphthalene derivatives.
Single crystals of 3a, 3d, 3e, and 3f suitable for X-ray
crystallographic analysis were obtained by solvent diffusion
methods, which unambiguously confirmed the regioselectivity of
the domino benzannulation reaction of buta-1,3-diynes.[24] Their
structures and corresponding crystal packing are shown in Figure
2. Compounds 3a and 3e showed slip-stacking (Figure 2b, 2g)
while 3f showed face-to-face packing (Figure 2j) due to its relative
larger planar backbone, with a close interplanar spacings of ~3.4-
3.5 Å for these compounds (Figure 2c, 2h, and 2k). Compound 3d
showed distorted structure because of its helical backbone
(Figure 2d-2e) and exhibited a face-to-face packing with a
distance of 3.8 Å (Figure 2e).
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In summary, we have developed the first domino
benzannulation reaction of buta-1,3-diynes catalyzed by InCl3-
AgNTf2. A series of irregular PAHs were successfully synthesized
through this new strategy. Moreover, we can use this method to
produce a broad scope of nanographene isomers efficiently,
which allows one to study their interesting optical properties. An
electron-donating group attached to the buta-1,3-diyne
precursors was crucial for the second benzannulation reaction to
occur. The double 6-endo-dig cyclization reaction was favored
and the regioselectivity of the first alkyne benzannulation reaction
was unambiguously confirmed by the X-ray crystallographic
analysis of the resulting PAH products. The benzannulation
reaction of buta-1,3-diynes was also applicable in stilbene and
dimethoxy-1,1'-binaphthalene derivatives, allowing for the
synthesis of novel chiral butterfly ligand precursors. The domino
benzannulation reaction of buta-1,3-diynes described here has
immense potential for application towards the precise synthesis
of large PAHs with irregular shapes.
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Acknowledgements
WAC acknowledges the National Science Foundation (NSF) for
supporting this work through a CAREER Award (CHE-1555218).
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Keywords: domino • benzannulation • alkyne • nanographenes •
[24] CCDC 1843508, 1843509, 1843513, 1843510, 1843511, and 1843512
(3a, 3d, 3e, 3f, 6, and 14) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre.
regioselectivity
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Entry for the Table of Contents
Layout 1:
COMMUNICATION
A
series
of
irregular
were
InCl3-
highly
domino
of
Wenlong Yang, Radha Bam, Vincent J. Catalano,
and Wesley A. Chalifoux*
nanographenes
synthesized
AgNTf2-catalyzed
regioselective
by
Page No. – Page No.
Highly Regioselective Domino Benzannulation
Reaction of Buta-1,3-diynes to Construct
Irregular Nanographenes
benzannulation
conjugated diynes. A novel
chiral butterfly ligand
precursor was successfully
obtained through this
strategy after a four-fold
alkyne benzannulation.
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