Electrochemical Access to Aza-Polycyclic Aromatic Hydrocarbons: Rhoda-Electrocatalyzed Domino Alkyne Annulations

Article abstract of DOI:10.1002/anie.201914775

Nitrogen-doped polycyclic aromatic hydrocarbons (aza-PAHs) have found broad applications in material sciences. Herein, a modular electrochemical synthesis of aza-PAHs was developed via a rhodium-catalyzed cascade C?H activation and alkyne annulation. A mu

Full text of DOI:10.1002/anie.201914775

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Accepted Article
Title: Electrochemical Access to Aza-Polycyclic Aromatic
Hydrocarbons: Rhodaelectro-Catalyzed Domino Annulations
Authors: Wei-Jun Kong, Zhigao Shen, Lars H. Finger, and Lutz
Ackermann
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914775
Angew. Chem. 10.1002/ange.201914775
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10.1002/anie.201914775
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Electrochemical Access to Aza-Polycyclic Aromatic
Hydrocarbons: Rhodaelectro-Catalyzed Domino Annulations
Wei-Jun Kong,+ Zhigao Shen,+ Lars H. Finger, and Lutz Ackermann*
Abstract: Nitrogen-doped polycyclic aromatic hydrocarbons (aza-
PAHs) have found broad applications in material science. Herein,
a modular electrochemical synthesis of aza-PAHs was developed
via a rhodium-catalyzed cascade C−H activation and alkyne
annulations. A multifunctional N-methoxylimidamide ensured high
chemo- and regioselectivities. The isolation of two key rhodacyclic
intermediates allowed to delineate the exact order of three C−H
activation events. In addition, the metalaelectro-catalyzed
multiple C−H transformation was characterized by an outstanding
functional group tolerance, including highly reactive iodo and
azido groups.
transformations, which are inaccessible with common chemical
oxidants.
While most of the aza-PAHs synthesized by transition metal-
catalyzed C−H functionalization involve only one or two C−H
activation steps, a domino transformation in which three or more
C−H bonds are activated and larger π-extended systems
constructed continued to be rare.[11] To this end, we envisioned a
multifunctional N-methoxylimidamide motif to facilitate threefold
C−H activations by taking advantage of the imido group after the
first internal oxidative C−H functionalization. With our ongoing
interest in material syntheses by metalaelectrocatalysis, we have
now developed a one-step electrochemical assembly of aza-
PAHs via rhodaelectro-catalyzed cascade C−H annulations
(Figure 1).[12] Salient features of this electrocatalytic
transformation include: a) electricity as green oxidant, b) excellent
functional group tolerance, c) three C–H bonds activated and six
new bonds formed, and d) user-friendly scale-up.
Polycyclic aromatic hydrocarbons (PAHs) are broadly explored in
materials science[1] and synthetic chemistry.[2] The incorporation
of heteroatoms, such as nitrogen, into these conjugated π-
systems can tune or fundamentally alter their optoelectronic and
catalytic properties.[3] Therefore, the assembly of atomically
precise aza-PAHs in an efficient and economic manner has
received considerable attention. However, the synthesis of PAHs
or aza-PAHs generally relies on stepwise elaborations, largely
involving
Diels-Alder
cycloadditions,
dehydrogenative
cyclizations or transition metal-catalyzed cross-couplings that
require prefunctionalized substrates.[4] Transition metal-catalyzed
oxidative C−H activation/annulation has been proven to be a
powerful tool for PAHs assembly.[5] However, the inherent
sustainability of the C−H activation approach is compromised by
the frequent use of toxic and waste-generating stoichiometric
oxidants, such as copper(II) and silver(I) salts.
The merger of metal catalysis and electrosynthesis,[6]
metallaelectrocatalysis,[7] has been identified as a powerful
strategy towards sustainable synthesis, which was substantiated
by electrochemical metal-catalyzed C−H functionalizations in
recent years.[8] Very recently, we[9] and Xu[10] disclosed a distinct
catalytic regime, namely anodic oxidation-induced reductive
elimination, where the electricity not only functions as the terminal
oxidant, but is also responsible for anodic oxidation-induced
reductive elimination in rhodium-catalyzed C−N and C−P bonds
formations. These findings may lead to new catalytic activities and
Figure 1. Electrochemical synthesis of aza-Nanographene via rhodaelectro-
catalyzed domino C−H annulations. GF: graphite felt.
We initiated our studies by employing imidamide 1a and
diphenylacetylene 2a for the envisioned rhodaelectro-catalyzed
cascade C−H annulations (Table 1). When using KOAc as the
base and [Cp*RhCl2]2 as the catalyst precursor, the desired
product 3aa was isolated in 38% yield with MeOH as the solvent
and a constant current of 4.0 mA (entry 1). Other solvents, such
as trifluoroethanol (TFE), H2O, and acetonitrile failed to deliver
product 3aa (entries 2-5). Other bases showed inferior
performance compared with KOAc (entries 5-8). The addition of
catalytic amounts of carboxylic acids was found to be beneficial,
with AdOH giving the best result (entries 9-11). The reaction
performed better at a shorter reaction time, indicating that a high
potential and prolonged electrolysis may lead to undesired side
reactions or product decomposition (entry 12). Indeed, lowering
the applied current to 2.0 mA increased the yield to 75% with a
prolonged reaction time (entry 13). Cationic rhodium catalyst
[Cp*Rh(CH3CN)3](SbF6)2 further improved the efficacy, affording
product 3aa in 90% yield at 35 °C (entries 14 and 15). Control
experiments verified the necessity of electricity and the rhodium
[*]
Dr. Wei-Jun Kong, Zhigao Shen, Dr. Lars H. Finger, Prof. Dr. Lutz
Ackermann
Institut für Organische und Biomolekulare Chemie
Georg-August-Universität Göttingen
Tammannstraße 2, 37077 Göttingen (Germany)
E-mail: Lutz.Ackermann@chemie.uni-goettingen.de
Homepage: http://www.ackermann.chemie.uni-goettingen.de
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.201914775
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catalyst (entries 16 to 18). The chemical oxidant Cu(OAc)2 fell
short in affording product 3aa in a comparable yield (25%).[13] In
addition, when nickel foam was used as the cathode or platinum
as the anode, the reaction proceeded, albeit with inferior yields
(67% and 65%, respectively).[13]
probed its versatility with various imidamides 1 (Scheme 1). A
broad range of aryl imidamides 1 bearing electron-donating (1b,
1c and 1d) and electron-withdrawing groups (1e to 1h) proved
applicable to this electrocatalysis. For unsymmetrically
substituted substrates 1i and 1j, the corresponding products were
isolated with high levels of regioselectivity and good yields (3ia
and 3ja). Notably, the highly sensitive iodo substituent was well
tolerated under the electrochemical conditions, which facilitates
further post-synthetic modifications (3ka).
Table 1. Optimization of the rhodaelectro-catalyzed domino C–H activation.[a]
Entry Base
Additive Solvent I [mA] Time [h] 3aa [%]
1
KOAc
KOAc
KOAc
KOAc
KOAc
NaOAc
NaOPiv
CsOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
-
MeOH
TFE
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
2.0
2.0
2.0
_
10
10
10
10
10
10
10
10
10
10
10
6
38
0
2
-
3
-
H2O
0
4
-
MeCN
EtOH
0
5
-
4
6
-
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
25
29
9
7
-
8
-
9
PivOH
AdOH
AcOH
AdOH
AdOH
AdOH
AdOH
AdOH
AdOH
AdOH
43
46
43
56
75
90[b]
89[c]
trace[d]
0[e]
60[f]
10
11
12
13
14
15
16
17
18
12
12
12
12
12
12
2.0
2.0
[a] Undivided cell, graphite felt anode (GF) (10 mm x 15 mm x 6 mm), platinum
plate cathode (Pt) (10 mm x 15 mm x 0.25 mm), 1a (0.2 mmol), 2a (0.7 mmol),
[Cp*RhCl2]2 (2.5 mol %), base (2.0 equiv), additive (0.1 equiv), solvent (4.0 mL),
25 °C under air, isolated yield. [b] RhCp*(CH3CN)3(SbF6)2 (5.0 mol %), 35 °C.
[c] RhCp*(CH3CN)3(SbF6)2 (2.5 mol %), 35 °C. [d] RhCp*(CH3CN)3(SbF6)2 (5.0
mol %), without electricity, 35 °C. [e] Without rhodium catalyst, 35 °C. [f]
RhCp*(CH3CN)3(SbF6)2 (2.5 mol %), 35 °C, under N2.
After establishing the optimized reaction conditions for the
rhodaelectro-catalyzed cascade annulative C–H activation, we
Scheme 1. Rhodaelectro-catalyzed C–H activation with imidamides 1. Rh cat.
= [RhCp*(CH3CN)3](SbF6)2
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Scheme 2. Rhodaelectro-catalyzed C–H activation with alkynes 2.
The high efficacy of the rhodaelectro-catalyzed cascade C–
H activation for aza-PAHs synthesis motivated us to delineate its
mode of action. To this end, two C–H activated rhodacyclic
intermediates 4 and 5 were isolated, which correspond to the first
and second C–H activation step, respectively (Scheme 3a). Both
Subsequently, a variety of alkynes 2 was evaluated in the
rhodaelectro-catalyzed cascade C–H activations (Scheme 2).
Alkynes 2 with electron-donating group substituents on the arene
motif delivered the desired products (3ab, 3ac, and 3ad). The
trimethylsilyl group was well tolerated under the electrolysis
condition, serving as a handle for further transfomations.[14] The
annulative cascade reaction proceeded equally well with meta-
substituted alkyne 2f, affording the desired product 3af in high
yield and selectivity. Remarkably, challenging unsymmetrical
alkyne 2g delivered the corresponding product with only two
regioisomers and good selectivity. When alkyne 2h bearing
pendant hydroxyl groups was applied, the micelle-type PAH 3ah
with hydrophilic hydroxyl groups surrounding the PAH moiety was
obtained. Surprisingly, the highly functional azide group was well
tolerated in this metalaelectrocatalysis (3ai), setting the stage for
versatile post-modification with considerable potential for
functional materials and bioimaging.
rhodacycles
4 and 5 showed catalytic activity for the
electrocatalysis (Scheme 3b). When intermediate 5 was reacted
with one equivalent of alkyne 2b, the desired product 3ab was
selectively obtained in 32% yield. This evidenced that the three
C–H activation steps took place in the order of 1→2→3 (Scheme
3c). As N-methoxylamide was widely used in rhodium and
ruthenium catalyzed C–H annulation and functioned as an
internal oxidant,[15] a similar pathway might proceed for our new
N-methoxylamide directing group, as N–O bond cleavage was
observed on rhodacycle 5 and the products 3. In addition, when
cyclic voltammetry studies of complex 5 in the presence of alkyne
2b were conducted, a low oxidation peak at 0.23 V vs ferrocene
implied that an oxidatively-induced reductive elimination pathway
was involved for the second and third C–N bond formations
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Figure 2. Cyclic voltammetry of complex 5 (2 mM, black), the mixture of 5 (2 mM)
and 2b (10 mM, red), the mixture of 5 (2 mM) and 2b (10 mM) after heating to
40 °C for 20 minutes (green), and this mixture after addition of KOAc (10 mM,
blue), all in methanol with n-Bu4NPF6 (0.1 M) at 0.1 V/s.
(Figure 2). Based on these results, a detailed mechanism was
proposed.[13]
The obtained aza-PAHs 3 could be easily transformed to
valuable functional material analogs. Treating aza-PAH 3aa with
iodomethane afforded a cationic nitrogen-doped nanographene 6
in 93% yield, which showed a reversible redox behavior with its
radical form at a low potential of E1/2 = -1.72 V vs ferrocene
(Figure 3). This promises applicability as novel anolyte material in
organic redox-flow batteries.[16] Intrigued by the six pendant azide
groups on aza-PAH 3aj, an azide-alkyne Huisgen cycloaddition
reaction was performed with a terminal alkyne linked to protected
D-lactone 7 (Scheme 5). Thus, a dendrimer with hydrophobic core
and hydrophilic periphery 8 was assembled in high yield, which
has potential applications in imaging and drug delivery.[17]
Scheme 4. Synthesis of cationic aza-nanographene 6.
Scheme 3. Mechanistic studies.
Figure 3. Cyclic voltammetry of compound 6 (2 mM) at varied scan rates, in
dichloromethane with n-Bu4NPF6 (0.1 M).
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order of the cascade C–H activation events. The versatility of the
electrosynthesis was demonstrated by its broad substrate scope
and excellent functional group tolerance, including otherwise
reactive iodo and azido groups. The practicality of this reaction
was reflected by its mild conditions, user-friendly setup and ease
of scale up. The obtained aza-PAHs and their post-transformed
derivatives were characterized in terms of their photophysical and
electronic properties, which indicated future applications in
optoelectronics, biomaterials, and energy storage.
Acknowledgements
Generous support by the DFG (Gottfried-Wilhelm-Leibniz award
to LA) is gratefully acknowledged. We thank Dr. Christopher Golz
(Göttingen University) for assistance with the X-ray diffraction
analysis.
Keywords: Metallaelectrocatalysis • Rhodium • C–H activation •
aza-PAHs • Domino reaction
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Scheme 6. Gram scale synthesis of 3aa
In summary, we have herein reported on the modular
assembly of aza-PAHs enabled by rhodaelectro-catalyzed
Domino C–H activation. A transformable N-methoxylimidamide
was designed to guarantee the reactivity and selectivity. The
isolation of two C–H activated rhodacycles revealed the exact
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10.1002/anie.201914775
Angewandte Chemie International Edition
COMMUNICATION
Table of Contents
COMMUNICATION
Wei-Jun Kong,+ Zhigao Shen,+ Lars H.
Finger, and Lutz Ackermann*
Page No. – Page No.
Electrochemical Access to Aza-
Polycyclic Aromatic Hydrocarbons:
Rhodaelectro-Catalyzed Domino
Annulations
Text for Table of Contents
electro-aza-PAH: Electrochemical access to aza-PAHs was achieved via rhodaelectro-catalyzed Domino C–H functionalization with
outstanding functional group tolerance. The isolation of key intermediates unravelled the catalyst’s mode of action, leading among
others to novel nitrogen-doped nanographenes.
This article is protected by copyright. All rights reserved.