Synthesis of Phenalenyl-Fused Pyrylium Cations: Divergent C?H Activation/Annulation Reaction Sequence of Naphthalene Aldehydes with Alkynes

Article abstract of DOI:10.1002/anie.201708127

Described herein is the synthesis of stable oxonium-doped polycyclic aromatic hydrocarbons (PAHs) by the rhodium-catalyzed C?H activation/annulations of naphthalene-type aldehydes with internal alkynes. This protocol provides four divergent reaction types, including two unexpected annulations with an oxygen transposition process, which lead to diverse types of phenalenyl-fused pyrylium cations comprising a four-, five-, or six-ring-fused π-conjugated core. The annulations exhibit an exquisite regioselectivity and a high tolerance of sensitive functional groups. These PAHs feature intriguing photophysical properties such as full-color tunable fluorescence emission, high quantum yield, and positively charged core, and can be reduced easily to the phenalenyl radicals.

Full text of DOI:10.1002/anie.201708127

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Angewandte
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Accepted Article
Title: Synthesis of Diverse Phenalenyl-Fused Pyrylium Cations via
Divergent Cascade C-H Activation/Annulations of Naphthalene
Aldehydes with Alkynes
Authors: Jingsong You, Jiangliang Yin, Meiling Tan, Di Wu, Ruyong
Jiang, and Chengming Li
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201708127
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COMMUNICATION
Synthesis of Diverse Phenalenyl-Fused Pyrylium Cations via
Divergent Cascade C–H Activation/Annulations of Naphthalene
Aldehydes with Alkynes
Jiangliang Yin, Meiling Tan, Di Wu, Ruyong Jiang, Chengming Li and Jingsong You*
[
8]
[9]
[10]
Abstract: Herein we establish, for the first time, a straightforward
pyrrole, pyridine, and pyridinium salt
has been developed
access to stable oxonium-doped polycyclic aromatic hydrocarbons
maturely, whereas only a few reports refer to the oxygen-doped
[
11]
[12]
(PAHs)
through
the
rhodium-catalyzed
cascade
C–H
heterocycles like furan and pyran. Oxonium cations are well
known to us for their widely practical values as fluorescent
activation/annulations of naphthalene-type aldehydes with internal
alkynes. This protocol provides four divergent reaction types
[
5,13]
[1b]
probe,
organic photoredox catalysts,
and radical
[
3]
(categorized as IA, IB, IIA and IIB), including two unexpected
precursors. Intuitive chemical knowledge would suggest that the
transition metal-catalyzed C–H bond activation/annulation of an
oxygen-containing functionality with an alkyne could be a concise
access to construct the pyrylium skeleton. However, so far no
literature is devoted to such a synthetic protocol, rendering this
type of annulation more challenging, which is probably attributed
annulations with an oxygen transposition process, which lead to
diverse types of phenalenyl-fused pyrylium cations comprising a four,
five or six-ring-fused π-conjugated core. The annulations exhibit an
exquisite regioselectivity and a high tolerance of sensitive functional
groups. These PAHs feature the intriguing photophysical properties
such as full-color tunable fluorescence emission, high quantum yield
and positively charged core, and can be reduced easily to the
phenalenyl radicals.
[
14]
to its instability (typically as the reaction intermediate)
easily occurring transformation.
and
[
15]
Stable polycyclic aromatic hydrocarbon (PAH) cations and
radicals have attracted considerable attention due to their widely
potential applications in the field of organic synthetic chemistry,
[
1-4]
photochemistry, materials science and biology.
The neutral
phenalenyl radical fragment is a promising building block for the
construction of molecular electronic devices and spin carriers.
Delocalized radicals such as phenalenyl radicals can be
stabilized electronically by merging heteroatom into the PAH core,
which was demonstrated recently by Bucher’s work via the
Scheme 1. Two bottom-up pathways to phenalenyl-fused pyrylium cations.
[
3]
incorporation of an oxygen atom. The synthetic protocol to
access phenalenyl-fused pyrylium counterparts requires multi-
step operation with harsh reaction conditions, involving the
Michael-type Grignard addition of methoxyarylmagnesium
bromide to the C9 position of phenalenone, demethylation and
3
cyclization using BBr , which leads to confined product types
[
3]
[5]
(
Scheme 1a). Another route to the oxonium-doped PAHs,
[
3b]
classified as a nonphenalenyl-type cation, involves an oxidation
process of the pyran skeleton, which encounters the similar
issues mentioned above (Scheme 1b). Thus, it is expected to
develop a more general, simple access to rapidly assemble a
structurally diverse family of phenalenyl-based oxonium cations.
Over the past few years, transition metal-catalyzed C–H
functionalization has emerged as one of the most efficient tools to
access complicated functional molecules, among which the Rh-
catalyzed C–H annulation with alkynes stands out for the
[
6,7]
construction of π-conjugated heterocycles (Scheme 2).
In this
hot field, the synthesis of nitrogen-containing heterocycles like
Scheme 2. Metal-catalyzed C–H activation/annulation with alkyne to construct
π-conjugated heterocycles.
[]
J. Yin, M. Tan, Prof. Dr. D. Wu, R. Jiang, C. Li, Prof. Dr. J. You
Key Laboratory of Green Chemistry and Technology of Ministry of
Education, College of Chemistry
Aldehyde is one of the most common and useful functional
groups broadly existing in organic compounds. Unlike other
directing groups, the aldehyde-assisted C–H activation examples
are significantly underrepresented due to its weak chelation to
Sichuan University
29 Wangjiang Road, Chengdu 610064, PR China
E-mail: jsyou@scu.edu.cn.
Supporting information for this article is given via a link at the end of
the document.
metal ions and almost all the successful cases are relevant to the
[16]
oxidative olefination.
Recently, we achieved a rhodium-
catalyzed regioselective [4+2] cyclization of indolyl aldehyde with
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alkyne to afford benzo-fused oxindoles through an aldehyde-
1-carbaldehyde (1g) with diaryl alkynes worked well, delivering
the six-ring-fused oxonium-doped PAHs (3ga and 3gd). More
importantly, the dialkyl alkyne, oct-4-yne, could react with pyrene-
1-carbaldehyde to give 3gl in 78% yield. In contrast, the reaction
of naphthalene aldehyde with dialkyl alkyne did not afford the
target 3al, implying that the aldehyde with a larger conjugation is
favorable to the annulation.
[
17]
directed C4−H activation. Inspired by this work, we envisaged
that the phenalenyl-fused pyrylium framework could be formed in
one step by two tandem [4+2] cyclizations of naphthalene
aldehyde with two equivalents of alkyne through sequential C8–H
[
12c]
and C2–H cleavages (Scheme 2).
We initiated our research by using naphthalene aldehyde and
diphenylacetylene as the substrates, [Cp*RhCl as the catalyst,
and Ag CO as the oxidant in the presence of 20 mol% of AgSbF
at 120 C (Table S1). To our delight, a trace amount of the
2 2
]
2
3
6
o
1
9
phenalenyl cation was formed in DCE (Table S1, entry 1). The
F
NMR spectrum and X-ray single-crystal diffraction indicated that
-
[18]
this oxonium-doped PAH contains SbF
6
as the counteranion.
With this consideration in mind, two equivalents of NaSbF
6
were
added, dramatically improving the yield of 3aa to 38% (Table S1,
entry 3). Further optimization of reaction parameters, such as
solvent, oxidant, reaction temperature and time, improved the
yield of 3aa to 77% (Table S1, entry 14).
Scheme 4. Discovery and substrate scope of reaction type IB.
Unexpectedly, 4-methoxyl-1-naphthaldehyde (1b) gave rise to
a mixture of 3ba and 4ba with a ratio of 1: 2.2 in 90% total yield
under the standard reaction conditions (Scheme 4a). The X-ray
single-crystal diffraction identified the structure of 4ba, in which
[
18]
the oxygen atom of the aldehyde group is expelled. The ratio of
ba and 4ba was increased from 1:2.2 to 1:4.6 in a similar total
3
yield when 10 equivalents of water were added into the reaction
system. We assumed that the addition of water may facilitate the
+
[19]
cleavage of the CH
3
–O bond, leading to the formation of 4ba.
4
-(Benzyloxy)-1-naphthaldehyde (1c) gave a similar result and
1
8
4ba was obtained in an isolated yield of 71 %. When O-labelled
water was added into the standard reaction system, the vast
1
8
majority of oxygen atom of 4ba was not replaced by O (Scheme
b). These observations indicate that the oxonium could come
from the oxygen atom of the benzyloxyl group rather than the
aldehyde group. This protocol could be extended to the reaction
of 1c with other diaryl alkynes (reaction type IB, Scheme 4),
giving the pyrylium cations (4ba-4cg). The ratios of the products
Scheme 3. Substrate scope of reaction type IA. Standard conditions: 1 (0.2
4
mmol), 2 (0.2 mmol), [Cp*RhCl
2
]
2
(5 mol %), AgSbF
6
(20 mol %), Ag
2
O (3.0
o
equiv), NaSbF (2.0 equiv) and DCE (1.0 mL) at 120 C under N
6
2
for 24 h.
With the optimized reaction conditions in hand, the substrate
scope was investigated (reaction type IA). As shown in Scheme 3, 4 and 3 are approximately 8:1 to 10:1. It is noteworthy that, in
the annulation of naphthalene aldehyde allows a wide scope of
alkynes, producing a family of four-ring-fused phenalenyl-based
pyrylium cations. The reactions of symmetrical diaryl alkynes with
electron-donating group such as p-methyl, m-methyl, p-methoxyl
and 2-naphthyl smoothly occurred in 55%, 69%, 67% and 90%
contrast to the 4-unsubstituted 1-naphthaldehyde (Scheme 3, 3al),
1c could react with dialkyl alkyne, affording 4cl in 59 % yield.
While the 4-alkyloxyl-substituted naphthaldehyde gave
a
mixture of 3ba and 4ba, the 4-bromo-substituted aldehyde 1d
only delivered 4ba in 46% yield (reaction type IIA, Scheme 5a).
Different from 1c as the substrate, the reaction of 1d with 2a
clearly involved an oxygen transposition process from the
carbonyl to the C4 position of the naphthalene skeleton given the
[
18]
yields, respectively (3ab-3ae).
electron-withdrawing group such as fluoro, chloro, CF
The diaryl alkynes bearing the
, CO Et,
3
2
and CHO could also be tolerated well (3af-3aj). 4-Phenyl-
substituted alpha-naphthaldehyde gave 3ea in 72% yield. For the
unsymmetric alkyl aryl alkyne 2k, the reaction successfully
1
8
absence of extra water. The O-labelling experiment led to the
1
8
18
O-labelled 4ba with an O incorporation of 64%, further
proceeded to afford 3ak in 68% yield with
regioselectivity.
a
complete
implying that the dehydration relative to the aldehyde group took
place (Scheme 5b).
[18]
To obtain larger π-conjugated oxonium-doped PAHs, the
aldehyde substrates with a larger conjugation degree were next
performed. To our delight, phenanthrene-9-carbaldehyde (1f)
could react with diaryl alkynes, affording the five-ring-fused
While phenanthrene-9-carbaldehyde (1f) reacted with diaryl
alkynes to afford the type IA-products 3fa and 3fd, the dialkyl
alkyne 2l unexpectedly delivered the oxygen-transferred pyrylium
cation 5fl (reaction type IIB, Scheme 5c), which was identified by
[
18]
[18]
18
oxonium-doped PAHs (3fa and 3fd). The annulation of pyrene-
the X-ray single crystal diffraction. When O-labelled water
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was added into the reaction system, the annulation of 1f with 2l
Next, we performed the intermolecular competition experiment
1
8
18
could afford the O-labelled 5fl with an O incorporation of 68%
Scheme 5d), disclosing that the oxonium in the product probably
between electronically differentiated 2d and 2g with 1a (Eq (1)).
1
(
The H NMR spectrum of the obtained products indicated that
originated from the oxygen atom of the carbonyl group.
3ad was a major product (approximately 75%) and 3ag was not
formed, suggesting that an electrophilic cyclization might be
favored in the catalytic cycle (Figure S1).
[
17,20]
Based on the above observations and the known reports
a
scenario of mechanism is proposed (Scheme 7; for detailed
description from A to D, see Scheme S1). For the reaction type IA
(
Path 1, R = H or Ph), the rhodium alkoxide intermediate D
probably undergoes a consecutive oxidation with Ag O/C2–H
2
cleavage to form a five-membered intermediate E directly. Next,
the second equivalent of alkyne inserts into the C2–rhodium bond
to generate a seven-membered intermediate F (detected by
MALDI-TOF-MS). In the previously reported literature, the
rhodium alkoxide intermediate like D typically undergoes a
[
20]
protonation process to form an alcohol or a ketone. Finally, the
oxonium product 3 is obtained through a reductive elimination of
F. For the reaction type IB (path 2, R = OMe or OBn), it is likely
that the intermediate D undergoes a protonation to provide an
Scheme 5. Reaction types IIA and IIB involving an oxygen transposition.
[
20a]
alcohol intermediate with the release of the Rh(III) species,
followed by an aromatization via dehydration/oxidation to yield the
+
[19]
intermediate I. After a cleavage of R–O bond, the type IB-
product is formed through the second rhodium-catalyzed
carbonyl-directed C–H annulation process. For the reaction type
IIA (Path 3, R = Br), the rhodium alkoxide intermediate D, starting
from the 4-bromo-substituted naphthaldehyde 1d, provides an
alcohol intermediate L via a protonolysis. Subsequently, the
sequential dehydration, aromatization and nucleophilic attack of
To probe the possible reaction mechanism, the control
experiments were performed by using the reaction of naphthalene
aldehyde (1a) with diphenylacetylene (2a). Unexpectedly, the
initially proposed intermediate 6aa was not observed in both the
[
20b]
water produce a possible alcohol M.
The resulting alcohol
presence and absence of NaSbF
trace amount of 7aa (isolatable). We proposed that the formation
of 7aa could involve the sequential C8–H
activation/annulation/aromatization via dehydration/nucleophilic
6
, except for the formation of a
undergoes an oxidation with Ag
2
O to form the intermediate I.
[
17]
addition of water/oxidation (For details, see Scheme S1). Thus,
it is reasonable to assume that the phenalenone derivative 6aa
could not be a direct intermediate and the double oxidative
insertions of alkyne could not yet be a step by step process.
These observations also imply that the C–H bond activation
initially occurred at the C8 position rather than the C2 position. In
addition, the reaction type IIB shown in Scheme 5c may undergo
a similar oxygen transposition process (Scheme 6). A reason why
type IIB reaction took place by using dialkyl alkyne 2l in place of
diaryl alkynes upon treatment with 1f could possibly be the
inferior reactivity of dialkyl alkyne in comparison to diaryl alkyne.
For dialkyl alkyne, the nucleophilic addition of water is faster than
the insertion of the second equivalent of alkyne.
Scheme 7. Proposed mechanism.
Scheme 6. Proposed pathway for reaction type IIB.
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Finally, the photophysical properties of the representative
phenalenyl-fused pyrylium cations were studied. These cations
exhibit high stability and full-color tunable emissions (from blue to
Keywords: C–H activation • rhodium-catalysis • annulation •
pyrylium cation • phenalenyl radical
2 2
NIR: λem 469~703 nm) in CH Cl . Most of these cations have
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ferrocene/ferrocenium (Figure S10). The cations investigated all
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Figure 1. (a) Emission spectra of the representative products in CH
photographic images of 4cl, 3ak, 3aa, 3ab and 3ga (from left to right) in CH
excited at 365 nm under a UV lamp). (b) EPR spectrum of reduced 3aa with
hyperfine couplings in the solid state at room temperature. (c) EPR spectra of
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2
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2
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In summary, we have established an unparalleled ease of
access to phenalenyl-fused pyrylium cations, including four
divergent Rh-catalyzed cascade C–H activation/annulations of
naphthalene-type aldehydes with alkynes. Considering that both
naphthalene-type aldehydes and alkynes are structurally diverse
and easily available, this protocol would be a more general and
rapid route to a diverse library of phenalenyl-based oxonium
cations. Future work in our lab will focus on the potential
applications of these compounds.
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(
1566042 (4ba) and 1566043 (5fl) contain the supplementary
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This work was supported by grants from the National NSF of
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COMMUNICATION
COMMUNICATION
Jiangliang Yin, Meiling Tan, Di Wu,
Ruyong Jiang, Chengming Li, and
Jingsong You*
Page No. – Page No.
Synthesis of Diverse Phenalenyl-
Fused Pyrylium Cations via Divergent
Cascade C–H Activation/Annulations
of Naphthalene Aldehydes with
Alkynes
The rhodium-catalyzed cascade C–H activation/annulations of naphthalene-type
aldehydes with internal alkynes have been developed to provide an unparalleled
ease of access to phenalenyl-fused pyrylium cations. Also disclosed is the open-
shell radical character of the reduced products.
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