Aromatization modulates the activity of small organic molecules as promoters for carbon-halogen bond activation

Article abstract of DOI:10.1039/c7sc04450j

The combination of small organic molecules and a base serves as a unique system for the activation carbon-halogen bonds in haloarenes by single electron transfer (SET). However, most of the molecules employed as promoters only allow for the activation of aryl iodides, and efficient activation of aryl bromides and chlorides under this mode is still rather challenging. Herein, we report the discovery of a structurally simple yet powerful promoter molecule, indoline, which exhibits unusually high activity in promoting the activation of haloarenes by SET. In the presence of t-BuOK and a trace amount of oxygen, indoline promotes the formation of aryl radicals not only from aryl iodides and bromides, but also from unactivated aryl chlorides (e.g., chlorobenzene) under relatively mild conditions. Mechanistic studies reveal the molecular basis for its high activity, for which the aromatization process plays a key role in modulating the electron transfer process.

Full text of DOI:10.1039/c7sc04450j

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Aromatization modulates the activity of small
organic molecules as promoters for carbon–
halogen bond activation†
Cite this: DOI: 10.1039/c7sc04450j
Huan Yang, De-Zhao Chu and Lei Jiao
*
The combination of small organic molecules and a base serves as a unique system for the activation
carbon–halogen bonds in haloarenes by single electron transfer (SET). However, most of the molecules
employed as promoters only allow for the activation of aryl iodides, and efficient activation of aryl
bromides and chlorides under this mode is still rather challenging. Herein, we report the discovery of
a structurally simple yet powerful promoter molecule, indoline, which exhibits unusually high activity in
promoting the activation of haloarenes by SET. In the presence of t-BuOK and a trace amount of
oxygen, indoline promotes the formation of aryl radicals not only from aryl iodides and bromides, but
also from unactivated aryl chlorides (e.g., chlorobenzene) under relatively mild conditions. Mechanistic
studies reveal the molecular basis for its high activity, for which the aromatization process plays a key
role in modulating the electron transfer process.
Received 15th October 2017
Accepted 13th December 2017
DOI: 10.1039/c7sc04450j
rsc.li/chemical-science
activation of the C–Cl bond in unactivated chloroarenes is still
Introduction
12
rather challenging. This is due to the stepwise C–Br/C–Cl bond
13
Aryl halides constitute an important category of intermediates activation process with a slow C–X cleavage step, as well as the
in organic chemistry, and the activation of carbon–halogen (C– increasing difficulty for bromo-/chloroarene activation with
14
15
X) bonds in aryl halides plays a signicant role in organic respect to both reduction potential and bond strength
1
2
synthesis. In addition to stoichiometric metalation or (Scheme 1b). Herein, we report the discovery of a highly active
transition-metal catalysis for C–X bond activation, in recent promoter, indoline, based on the structural modication of
3
years the development of small-organic-molecule-promoted existing promoters for carbon–halogen bond activation. It
C–X bond activation has attracted great attention. A series of ranked among the most active small molecule promoters to
4
small organic molecules, such as 1,10-phenanthrolines, 1,2- date, which enables the activation of bromo- and chloroarenes,
5
6
7
diamines, 1,2-diols and N-methylanilines, were found to
promote the activation of haloarene in the presence of a base,
producing a reactive aryl radical intermediate which undergoes
substitution processes (referred to as base-promoted homolytic
8
aromatic substitution, BHAS). Careful mechanistic studies
revealed that the promoter is transformed into a “super electron
donor” under the reaction conditions, which then initiates the
cleavage of the C–X bond by single electron transfer (SET) to the
9
haloarene substrate (Scheme 1a). This process represents
a new mode of carbon–halogen bond activation, which enables
a series of synthetic methods utilizing haloarenes as starting
10
materials in a transition-metal-free manner.
Despite these achievements, efficient activation of relatively
inert C–X bonds in the BHAS reaction remains a major chal-
lenge. To date, most promoters could merely activate iodoar-
5
11
enes, and only a few could activate bromoarenes. The
Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua
University, Beijing 100084, China. E-mail: Leijiao@mail.tsinghua.edu.cn
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7sc04450j
Scheme
1 Reaction mode of carbon–halogen bond activation
1
promoted by small organic molecules and challenges therein.
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and more attractively, a series of single electron reduction Furthermore, compared with several of the most active organic
reactions traditionally carried out using alkali metal reductants. promoters reported to date, indoline exhibited the highest
ꢀ
activity under 80 C (Fig. 1). The unprecedented high activity of
indoline in the BHAS reaction attracted our interest and
prompted us to elucidate its molecular basis.
Results and discussion
First, we found that indole was generated in a high yield as
Our previous study showed that, N-methylaniline serves as an the end-product in this indoline-promoted BHAS-type reaction.
efficient promoter for the activation of iodoarenes in the pres- This transformation involves both oxidation and aromatization,
ence of t-BuOK, where N-methylanilide anion acts as an electron which might be responsible for the observed SET-initiated C–X
7
donor. However, attempts to enable the activation of bro- activation. Second, aniline-type promoters that were not able to
moarene 1a by structural tuning of the N-methylaniline form indole through aromatization were found to be much less
promoter proved unsuccessful (Scheme 2a). Although electron- active (Table 1). 3,3-Dimethylindoline (2j), tetrahydroquinoline
rich anilines exhibited slightly better activities (2a–e), the (2k), and N-isobutylaniline (2l) were found to exhibit activity for
results were far from satisfactory, and the attachment of an iodoarene, but not for bromo- and chloroarenes. These results
additional aromatic ring to aniline (either as a substituent or as suggest that the activation of iodoarenes and bromo-/
a fused cycle, 2f–h) with the aim to assist SET by stabilizing the chloroarenes is rather different, and the aromatization
formed N-centered radical, was also useless. With the hypoth- process, rather than the ring-strain, plays a key role in the
esis that ring strain might help to increase the activity, we activation of more challenging C–X bonds.
attempted to use a cyclic aniline derivative, indoline (2i), as the
promoter. To our surprise, the indoline/t-BuOK system exhibi-
ted great activity for activating iodo-, bromo- and even chlor-
oarenes (Scheme 2b). A screen of substrates showed that the
indoline/t-BuOK system is able to promote the BHAS reaction of
a series of bromo- and chloroarenes, although chloroarenes are
generally less reactive than bromoarenes (Scheme 2c).
Fig. 1 Yields of the coupling products from the reaction of ArBr 1a
(red) and ArCl 1c (blue) employing various promoters. Reaction
conditions: 1a or 1c (0.5 mmol), promoter (20 mol%), t-BuOK (1.5
ꢀ
mmol), 4 mL benzene, argon atmosphere, 80 C for 24 h.
a
Table 1 Effect of promoter structure on activity
ArI (1b)
ArBr (1a)
ArCl (1c)
Entry Promoter
Conv. Yield Conv. Yield Conv. Yield
1
95%
99%
75%
73%
19%
16%
6%
7%
10%
7%
<1%
<1%
2
3
76%
59%
16%
1%
10%
<1%
a
Reaction conditions: haloarene 1 (0.5 mmol), promoter 2 (20 mol%), t-
ꢀ
Scheme 2 Effect of the promoter structure on carbon–halogen bond BuOK (1.5 mmol), 4 mL benzene, Ar atmosphere, 80 C for 12 h.
activation.
Conversions and yields were determined by gas chromatography.
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Another observation also provided important clues for the anion A, which serves as a weak electron donor. For iodoarenes,
initiation mechanism. The kinetic prole of the reaction anion A could directly activate the C–I bond by SET, and radical
revealed that the indoline-promoted BHAS reaction of 4-chlor- intermediate B is formed simultaneously. Subsequent proton
oanisole (1d) had a signicant induction period under argon, abstraction by t-BuOK affords intermediate C (equivalent to
whereas the reaction exhibited no induction period in the a radical anion), which acts as a better electron donor and is
presence of O
2
(Fig. 2A). The generation of indole from indoline able to activate another iodoarene molecule. This process is
similar to the initiation mechanism of the BHAS reaction
in these reactions followed the same trend. Injection of pure O
2
into the reaction system directly initiated the reaction, but led promoted by N-methylaniline. On the other hand, for bromo-/
to a low nal conversion (Fig. 2B). The control experiment also chloroarenes, anion A is not able to act as a direct electron
showed that under rigorous oxygen-free conditions, the reac- donor. Alternatively, in the presence of a trace amount of O2,
tions of chloro- and bromoarene were almost completely sup- single electron oxidation of A takes place to generate B, which
pressed, but the reaction of iodoarene could still proceed
(
Scheme 3). The transformation of indoline to indole proceeded
without a haloarene substrate with trace O , but could not
proceed under O -free conditions (Scheme 3).
2
2
The above experimental results indicate that the aromatiza-
tion of indoline plays a critical role in the activation of chloro-
and bromoarene, and trace oxygen existing in the reaction
system serves as a key factor to initiate the activation process.
Based on this, the mechanism of carbon–halogen bond activa-
tion in the indoline-promoted BHAS reaction is proposed in
Scheme 4. Deprotonation of indoline by t-BuOK led to anilide
Scheme 4 Proposed initiation mechanism in the indoline-promoted
BHAS reaction.
Fig. 2 Kinetic profiles of the indoline-promoted BHAS reaction of 4-
chloroanisole (1d) to produce biaryl 3a under Ar and in the presence of
O
2
(A) and under Ar with injection of O
2
(B). Reaction conditions: 1d (1
ꢀ
mmol), indoline (40 mol%), t-BuOK (3 mmol), 8 mL benzene, 80 C.
Scheme 3 Effect of oxygen on the C–X activation activity of indoline.
a] “Trace O ” condition refers to setting up the reaction in a Schlenk
tube sealed with a rubber septum under Ar. [b] “O -free” condition
[
2
2
refers to carefully degassing the solvent by three freeze–pump–thaw
cycles and then setting up the reaction in a reaction tube sealed with
a Teflon screw valve under Ar.
Scheme 5 Cyclopropyl substituted indoline as the mechanistic probe.
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then affords C as a more efficient electron donor. Due to the anion-type intermediate E in the reaction system (Scheme 5b).
slow C–X bond cleavage step, the electron transfer between C Once formed, radical anion E could undergo either SET to ArX
ꢁ
and the ArBr/ArCl substrate to form ArXc and isoindole D is to afford indole 5m, or cyclopropane ring-opening to produce
0
reversible. Under this circumstance, the isomerization of D to the ring-opened radical F, which is then transformed to 5m by
indole is rather important because the irreversible aromatiza- hydrogen atom transfer or reduction. In this kinetic competi-
tion favors the electron transfer equilibrium and thus enables tion scenario, the ratio of the two end-products is expected to be
the activation of more challenging bromo-/chloroarenes. Thus, dependent on [ArX]. This was proven to be true by experiments
the proposed mechanism well rationalizes the superior activity performed with different [1a] (Fig. 3), which clearly indicate that
of indoline over many aniline-type promoters, and claries the intermediate E acts as the key electron donor in the SET-
key role of trace O in the initiation process.
induced carbon–halogen activation.
2
This mechanism was further supported by the reaction
DFT calculation was conducted to provide a quantitative
16
employing 2-cyclopropylindoline (2m) as a probe (Scheme 5a). understanding of the indoline-promoted C–X bond activation
In the reactions of bromo- and chloroarenes 1a and 1c (Fig. 4). Starting from the deprotonated indoline Int-1 (equiva-
promoted by 2m, both 2-cyclopropylindole (5m) and 2-propy- lent to intermediate A), direct electron transfer to PhI is
0
ꢁ1
lindole (5m ) were observed as the end-product of indoline 2m. possible, though endergonic by 25.0 kcal mol in terms of
Under O -free conditions no reaction occurred and none of the Gibbs free energy, to produce phenyl radical and radical
2
end-products were formed. The formation of the ring-opening complex Int-2 (equivalent to intermediate B). Deprotonation of
0
product 5m serves as key evidence for the existence of radical Int-2 at the C–H bond neighboring the N-centered radical via
TS-1 is rather facile, affording the radical anion complex Int-3
(
2
equivalent to intermediate C). Though endergonic by
4.8 kcal mol , the direct activation of PhI by anilide Int-1 is
ꢁ
1
feasible. In contrast, for PhBr and PhCl direct electron transfer
is not possible since the SET step is rather endergonic
ꢁ
1
(
>60 kcal mol , see the ESI†). In such cases, trace oxygen reacts
with Int-1 to afford radical complex Int-2 with a much more
17
favorable free energy barrier, and Int-3 is subsequently
generated by deprotonation.
As expected, complex Int-3 is a more efficient electron donor,
enabling exergonic SET to PhI to form the phenyl radical and
isoindole complex Int-4 (equivalent to intermediate D). The
following aromatization process from Int-4 to Int-5 is also
Fig. 3 Ratio between the indole products 5m and 5m in the BHAS exergonic (by 18.6 kcal mol ). For the reaction of bromo-/
0
ꢁ1
reaction of 1a employing indoline 2m as a mechanistic probe.
chloroarenes, this aromatization process is of great
Fig. 4 Partial potential energy surface of different initiation pathways calculated at the M06-2X/6-311+G(d,p)//B3LYP/6-31G(d) level of theory.
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other reactions to modulate the electron transfer processes
therein.
Conflicts of interest
The authors declare no competing nancial interest.
Acknowledgements
The National Natural Science Foundation of China (Grant No.
21390403) and the Thousand Talents Plan for Young Profes-
sionals are acknowledged for nancial support. The technology
platform of CBMS and the Tsinghua Xuetang Talents Program
are acknowledged for providing instrumentation and compu-
tational resources.
Scheme 6 Reduction and reductive cleavage reactions using the
2
indoline/t-BuOK/O system.
Notes and references
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ꢁ1
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