Integrating Organic Lewis Acid and Redox Catalysis: The Phenalenyl Cation in Dual Role

Article abstract of DOI:10.1021/jacs.8b04786

In recent years, merging different types of catalysis in a single pot has drawn considerable attention and these catalytic processes have mainly relied upon metals. However, development of a completely metal free approach integrating organic redox and organic Lewis acidic property into a single system has been missing in the current literature. This study establishes that a redox active phenalenyl cation can activate one of the substrates by single electron transfer process while the same can activate the other substrate by a donor-acceptor type interaction using its Lewis acidity. This approach has successfully achieved light and metal-free catalytic C-H functionalization of unactivated arenes at ambient temperature (39 entries, including core moiety of a top-selling molecule boscalid), an economically attractive alternative to the rare metal-based multicatalysts process. A tandem approach involving trapping of reaction intermediates, spectroscopy along with density functional theory calculations unravels the dual role of phenalenyl cation.

Full text of DOI:10.1021/jacs.8b04786

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Article
Integrating Organic Lewis Acid and Redox
Catalysis: The Phenalenyl Cation in Dual Role
Jasimuddin Ahmed, Soumi Chakraborty, Anex Jose, Sreejyothi P, and Swadhin K. Mandal
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04786 • Publication Date (Web): 08 Jun 2018
Downloaded from http://pubs.acs.org on June 8, 2018
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Integrating Organic Lewis Acid and Redox Catalysis: The Phenalenyl Cation in Dual Role
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Jasimuddin Ahmed, Soumi Chakraborty, Anex Jose, Sreejyothi P and Swadhin K. Mandal*
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*Eꢀmail: swadhin.mandal@iiserkol.ac.in
Department of Chemical Sciences, Indian Institute of Science Education and ResearchꢀKolkata,
Mohanpurꢀ741246, India.
______________________________________________________________________________
ABSTRACT: In recent years, merging different types of catalysis in a single pot has drawn
considerable attention and these catalytic processes have mainly relied upon metals. However,
development of a completely metal free approach integrating organic redox and organic Lewis
acidic property into a single system has been missing in the current literature. This study
establishes that a redox active phenalenyl (PLY) cation can activate one of the substrates by
single electron transfer (SET) process while the same can activate the other substrate by a donor–
acceptor type interaction using its Lewis acidity. This approach has successfully achieved light
and metalꢀfree catalytic CꢀH functionalization of unactivated arenes at ambient temperature (39
entries; including core moiety of a topꢀselling molecule boscalid), an economically attractive
alternative to the rare metalꢀbased multiꢀcatalysts process. A tandem approach involving trapping
of reaction intermediates, spectroscopy along with DFT calculations unravels the dual role of
phenalenyl cation.
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INTRODUCTION
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In recent years, there have been emerging trends in integrating multiple catalysts in one pot. In
this way, concepts of “tandem”, “merged”, “cascade”, or “relay” catalysis were introduced.1ꢀ7
MacMillan and coꢀworkers have reported merging photoredox catalysis with organocatalysis for
direct asymmetric alkylation of aldehydes where two different catalysts in a same reaction pot
act on two different substrates.4 These catalytic combinations frequently exploit Lewis acidity
(inorganic Lewis acids, mostly metal based) and/or redox activity (by metal based catalysts)
and/or nucleophilicity (predominantly metalꢀfree catalyst). In this regard, development of a
completely metal free catalyst integrating such properties into a single metal free system might
overcome these shortcomings, however such approach has been missing in the current literature.
For example, role of a Lewis acid in a variety of catalytic transformations is wellꢀestablished and
traditionally it takes advantage of various readily available inorganic Lewis acids. The classic
Lewis acid AlCl3 activates the aromatic ring in FriedelꢀCraft reaction by formation of a charge
transfer complex with aromatic πꢀelectron cloud.8 On the other hand, redox activity of a typical
transition metal based catalyst is the basis of most of the transition metal mediated catalytic
processes. Integrating these two important features namely Lewis acidity with redox activity
might lead to an unprecedented catalytic outcome. For example, Glorius and coꢀworkers have
introduced a pioneering catalytic protocol combining redox active ruthenium catalyst stimulated
by light and Lewis acidic gold catalyst where 1ꢀpenteneꢀ5ꢀol reacted with aryldiazonium salt to
afford oxyarylated product.9 Using the combination of Ru or Ir based photoredox catalyst and
Lewisꢀacidic gold catalyst, a series of organic transformations has been accomplished and the
area was reviewed recently.10, 11
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a) Previous strategy:
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b) This strategy:
c)
Figure 1. (a, b) Summary of past and present work. (c) Molecular drawings of phenalenyl
cations (I and II) showcasing that the redox active property originates from their lowꢀlying
empty NBMO.
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At the same time Yoon has reviewed the topic of photochemical stereocontrolled organic
transformations by using tandem photoredox–chiral Lewis acid catalyst.12 Very recently, Lee and
coꢀworkers have utilized this dual Au/Ru catalytic protocol in the arylation of arenes under light
stimulation (Figure 1a).13 Despite this success, these multicatalytic combinations suffer from the
following major drawbacks: a) use of expensive and rare metals, b) chance of catalyst poisoning
of one catalyst by another; a common issue with merging two different catalysts in one pot.
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To achieve the goal of merging redox catalysis with Lewis acidic catalysis without using
expensive and rare metals, we resorted to the phenalenyl cation which is known from entirely
different perspective since almost six decades.14, 15 The early Hückel calculations14, 15 showed
that the phenalenyl cation consisting of odd number (13) of carbon atoms has an empty
nonbonding molecular orbital (NBMO), which can readily accept electron(s); an observation also
revealed experimentally by the cyclic voltammograms (CVs) of the phenalenyl cations I and II
(Figure 1c) recently.16 This property of phenalenyl cation in fact inspired Haddon to pioneer the
idea of creating phenalenyl radical based conductors.17ꢀ20 The singly populated phenalenyl
radicals (generated from phenalenyl cation) heralded a new era in designing organic magnets,
conductors, molecular switch, molecules with elusive Resonating Valence Bond (RVB) ground
state as postulated by Pauling and Anderson, molecular battery, quantum spin simulators, spin
electronic device etc.19ꢀ28 During the last two decades, a handful of phenalenyl based radicals
were isolated and characterized in solidꢀstate by Haddon18ꢀ20, 23, Nakasuzi24ꢀ25, Takui21, Kubo26,
Morita29 and coꢀworkers. The relevance of phenalenyl based radicals in catalysis is emerging
only in recent years and the topic has been very recently reviewed.30 As the second advantage, by
virtue of the same empty NBMO of phenalenyl cation, it can act as an organic Lewis acid. We
envisage that the major advantage associated with phenalenyl cation as an organic Lewis acid as
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well as redox active catalyst might originate from easy accessibility of its empty NBMO (Figure
1c), which does not compromise the stability of the associated transition state owing to its
minimal reorganization energy required in populating the empty NBMO as also proposed by
Haddon in different perspective long back.17
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In this work, we have integrated the redox activity of phenalenyl cation along with its
organic Lewis acidity. The phenalenyl cation on reduction by a single electron can generate the
phenalenyl based radical, which in turn can participate in a single electron transfer process (SET)
to activate the aryl diazonium coupling partner into a highly reactive aryl radical. On the other
hand, upon the SET process, it produces back the phenalenyl cation which subsequently can act
as a Lewis acid to activate the unbiased arene coupling partner just like the classic Lewis acid
AlCl3 in FriedelꢀCraft reaction by forming a charge transfer complex.8 This dual mode of
activation through phenalenyl cations (I and II) opens up possibility of an alternative yet very
effective strategy for replacing the existing expensive and rare metals based catalytic
combination. Herein, we report for the first time, light and metal free CꢀH arylation of a diverse
range of arenes at room temperature resulting in excellent yield (upto 91%, 39 examples) using
the phenalenyl based cation (Figure 1b).
RESULTS AND DISCUSSION
The phenalenyl based cations I and II (Figure 1c) were synthesized according to literature
method.31, 32 The current investigation was started with direct CꢀH arylation of arenes which has
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drawn considerable attention in recent years.13,
At first, metallic potassium (K) was
considered as the reducing agent and benzene was taken as an electronically unbiased arene to
proceed with our reaction optimization study in the presence of 4ꢀchlorophenyl diazonium salt
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coupling partner (see ESI, Table S1). Subsequently, an organic electron donor
tetrakis(dimethylamino)ethylene (TDAE) was used instead of metal (K) to carry out the same
reaction which resulted in the formation of desired product with almost similar efficiency. After
optimizing with different combinations (see ESI, Table S1), we found that 2.5 mol % of catalyst
I in the presence of 5 mol % of TDAE in DMSO for 12h is the most productive conditions for
arylation of benzene (1a) with diazonium coupling partner 2a to afford 73% yield of the
biarylated product 3a at room temperature (ESI, Table S1, entry 4). Two sets of blank reactions
were carried out: one is without catalyst, which does not afford biarylated product and another is
without TDAE, which affords <5% arylated product (ESI, Table S1, entries 10 and 11). To check
the effect of light (if there any), the reaction was also performed under dark condition. This
reaction led to 75% yield of 3a, which clearly discarded the possibility of any light stimulation
(ESI, Table S1, entry 12). Before proceeding towards general scope of this protocol using
various arene and diazonium coupling partner combinations, our previously reported protocol38
was tested using a neutral phenalenyl ligand PLY(N,O) and KOtBu. Three different reactions
were performed with three arenes (benzene, anisole and nitrobenzene) using the neutral
phenalenyl ligand PLY(N,O) and KOtBu under the same conditions reported earlier37 and these
reactions afforded the desired biarylated product only in trace quantity (<5% conversion, see SI,
Scheme S1). These unsuccessful reactions clearly demonstrate the advantage of the present
catalytic protocol using PLY cation as catalyst over previously reported method38 and validate
our current catalytic results.
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Next, arylation of benzene (1a) was carried out with five different aryldiazonium salt
coupling partners (2aꢀf) under the optimized metal free catalytic conditions (ESI, Table S1, entry
4), which resulted in 49ꢀ82% yield of corresponding biarylated products (3aꢀf, Scheme 1).
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Moderate to excellent yields (58ꢀ82%) of para, ortho, metaꢀfunctionalized biarylated products
(4a-f) were observed when toluene (1b) was used as the arene partner. Further, this protocol was
utilized in orthoꢀselective (other regioisomers were formed in less than 5% yield) arylation of
anisole (1c) with different aryldiazonium salt coupling partners (2aꢀd, 2g), resulting in good to
excellent yield (64ꢀ91%) of the C2 arylated products of anisole (5a-d, 5g). Successful arylation
of these two arenes (toluene and anisole) at ambient conditions shows efficiency of our catalytic
protocol for electron rich aromatic systems. Additionally, we started investigating the
applicability of this protocol towards electron deficient aromatic systems such as nitrobenzene or
benzonitrile. Arylation of nitrobenzene (1d) resulted in moderate to excellent yield (43ꢀ91%) of
biarylated products (6b, 6eꢀg) of nitrobenzene at preferably orthoꢀposition (Scheme 1).
Subsequently, chlorobenzene (1e) was taken as the arene partner for arylation under metal free
conditions at room temperature. Biarylated products of chlorobenzene (7b, 7f) were obtained as
the mixture of ortho and para arylated products with 82% and 63% overall yield, respectively.
Whereas benzonitrile (1f) shows an ortho selectivity in this arylation process with less than 10%
other regioꢀisomers, affording biarylated products 8a, 8f-g with 50ꢀ64% yields. Additionally, the
preparation of a biologically active biarylated product was carried out by arylation of
methylbenzoate (1g) under metal free catalytic protocol at room temperature.
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F
OMe
NO2
Cl
Cl
4a; 78%, 74%b
3b; 81%
63% (Catalyst II)
3e; 82%
3f; 49%
52% (Catalyst II)
3d; 78%
50% (Catalyst II)
3a; 73%
68% (Catalyst II)
o:p:m = 1:0.36:0.24
Cl
NO2
F
OMe
NO2
4b; 82%, 80%b
4d; 58%, 60%b
OMe
OMe
4e; 73%
4f; 62%, 65%b
o:p:m = 1:0.42:0.30 o:p:m = 10.45:0.31
5a; 91%
5b; 71%
o:p:m = 1:0.38:0.26
o:p:m = 1:0.62:0.41
NO2
NO2
NO2
CN
Me
NO2
F
OMe
OMe
5d; 70%
OMe
5c; 87%
OMe
5g; 64%
6b; 91%, 87%b
o:p = 1:0.45, m: trace
6f; 68%, 63%b
6e; 87%
o:p = 1:0.26; m: trace
o:p = 1:0.35; m: trace
NO2
Cl
Cl
Cl
Me
OMe
NO2
Cl
OMe
NO2
6a; 84%, 80%b
7f; 63%
; 82%
7b
6g; 43%, 48%
CN
8a; 64%
CN
8f; 73%
o:p = 0.27:1; m: trace
o:p = 1:0.5; m: trace
o:p = 1:0.54; m: trace
Cl
Me
OMe
CN
Me
COOMe
9g; 53% d
COOMe
9d; 68%, 59%c
COOMe
9f; 72%, 68%c
CN
8g; 50%
COOMe
9c; 76%, 74%c
10a; 84%, 80%b
52% (Catalyst II)
OMe
70% (Catalyst II)
OMe
NO2
OMe
MeO
Cl
NO2
MeO
OMe
OMe
11b; 89%, 82%b
10d; 65%, 60%b
10f; 67%, 70%b
10b; 80%
11a; 84%, 77%b
87% (Catalyst II)
76% (Catalyst II)
OMe
OMe
OMe
OMe
Me
OMe
MeO
MeO
MeO
MeO
OMe
11d; 43%, 40%b
OMe
11g; 41%, 45%b
OMe
11f; 50%, 47%b
OMe
11h; 28%, 21%b
Scheme 1. Aromatic CꢀH arylation of different arenes (1) at room temperature with various
diazo coupling partners. [a] [a] Reaction conditions: 1a-b (2.4 mmol)/1c-i (1.2 mmol), 2a (0.24
mmol), catalyst (0.006 mmol), solvent (1 mL), rt = room temperature. [b] With lower equivalent
of arenes and potassium as electron donor. [c] With 3 equiv. (0.72 mmol) arene. [d] NMR
conversion.
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Orthoꢀposition selective arylation of methylbenzoate was accomplished (Scheme 1) with four
different aryl diazonium salts (2c, d, f, g), which resulted in 53ꢀ76% yield of biarylated products
(9c, d, f, g). Almost similar yield was obtained when 3 equivalent of methyl benzoate was taken
(Scheme 1). Next, these reactions were carried out with 1, 3, 5 substituted benzene with
different aryldiazonium salt coupling partners. When mesitylene (1h) was the coupling partner,
84ꢀ65% yield of the arylated products (10aꢀf) was obtained and earlier similar yield with
meitylene was achieved using Ru/Au dual photoredox catalysts (Scheme 1).13 Arylation of 1, 3,
5ꢀtrimethoxy benzene shows a good reactivity towards the arylation with different arydiazonium
salt coupling partners (2a, b) to afford 84 and 89% yield of arylated products 11a and 11b,
respectively. However, this shows poor reactivity towards arylation with coupling partners 2d,
2fꢀh affording 28ꢀ50% yield of arylated products 11d, 11fꢀh (Scheme 1). An alternative strategy
of this arylation protocol using an inorganic reducing agent (metallic potassium) for toluene (1a,
5 equiv.), nitrobenezene (1d, 3 equiv.), mesitylene (1h, 3 equiv.) and 1, 3, 5 trimethoxybenzne
(1i, 2 equiv.) successfully resulted in similar yield of the arylated products. This study further
revalidate underlying concept of this protocol (Scheme 1). It may be noted that the product 6a is
the core part of a very important fungicide boscalid (marketed by BASF with over 150 million
euro turnover per year38), has been prepared by the arylation of nitrobenzene with 4ꢀchloro
aryldiazonium salt coupling partner in 84% yield, and there has been no report of such metal free
catalytic reaction at room temperature, which promises costꢀeffective preparation of a high value
product such as boscalid.
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Keeping this elegant catalytic protocol in hand, the mechanistic investigation of this
reaction was initiated. When this reaction was carried out in the presence of higher equivalence
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of a radical scavenger 2,2,6,6ꢀtetramethylpiperidinoxyl (TEMPO) the reaction was almost
completely arrested (Scheme 2a). This experiment indicates a radical mediated mechanistic
path.39 Additionally, we have been able to trap the aryl radical formed from the aryl diazo
coupling partner by TEMPO to unequivocally establish the involvement of the aryl radical as an
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intermediate (Scheme 2b). The trapped product was characterised by spectroscopy (1H and C
NMR) and matched the data with the earlier report.39 Furthermore, an EPR spectrum of catalyst I
has been recorded before and after the addition of TDAE. This EPR experimental result clearly
indicates that catalyst I (PLY cation) is EPR silent before addition of TDAE and upon TDAE
addition, it undergoes a sharp colour change from orange to deep green resulting in an EPR
active compound (g = 2.002), which suggests formation of a phenalenyl based radical37 by taking
an electron from the organic reducing agent TDAE (Scheme 2c). Such electron transfer
capability of TDAE to cationic phenalenyl generating phenalenyl based radical compound is
wellꢀestablished in literature.40 All these experiments (Scheme 2) confirm the formation of a
phenalenyl based radical. Next we became curious to explore the insight of the activation process
of electronically unbiased arenes by the present phenalenyl cations. It was envisaged that the
phenalenyl cation as an organic Lewis acid,16 can form a donorꢀacceptor type complex on
interaction with the aromatic πꢀelectron cloud of the arenes. This type of interaction between
aromatic πꢀelectron donor and acceptor molecule bearing NDI (naphthalene diimide) containing
moieties has been reported previously41ꢀ43 and effect of molecules having varied electron
donating capacity has been investigated by estimating quenching of absorbance as well as
fluorescence.43 This area of catalyst designing by nonꢀcovalent πꢀinteraction has been reviewed
recently by Sigman, Toste and coꢀworkers.44 Herein, to understand the πꢀelectronic displacement
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from unperturbed arene to the NBMO of phenalenyl cation, its effect on absorbance and
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luminescence of phenalenyl cation II in presence of different arenes was investigated.
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(a) Reaction inhibition in the presence of TEMPO.
(b) TEMPO trapped intermediate formation.
Catalyst I (0.5 equiv)
O
TDAE (0.5 equiv)
N
O2N
N2BF4
+
N
O
DMSO, RT, 12 h
O2N
41% (12)
TEMPO (2 equiv.)
2b (1 equiv.)
(c) Investigation of phenalenyl based radical in solution phase.
Scheme 2. Mechanistic investigation: (a) Reaction inhibition on benzene arylation by TEMPO.
(b) Trapping of reaction intermediate by TEMPO, (c) A schematic representation of phenalenyl
radical generation by electron transfer from TDAE in DMSO and the EPR spectrum of radical
species A
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This study was carried out with strongly luminescent phenalenyl cation II. At first, 2.5 mL of 18
µM solution of II in acetonitrile was subjected for absorbance and fluorescence measurements.
Next, maintaining the same concentration of II in 2.45 mL of acetonitrile, 50 µL of different
arenes were added. UVꢀvis spectroscopy revealed a clear quenching in absorbance of phenalenyl
cation II (Figure 2a). Further, it unambiguously establishes that the relative quenching efficiency
is higher for stronger electron donating arenes (benzene < toluene < mesitylene). Similar trend in
relative quenching was also noted in emission spectra of II, where luminescence intensity of II
diminished most effectively by mesitylene as compared to toluene and benzene (Figure 2b). This
type of quenching in both the UV and emission spectra can be attributed to the πꢀelectronic
charge transfer from arene to the NBMO of phenalenyl cation II. CD spectra of the same
samples eliminated the possibility of such quenching originating from other effect such as Cotton
effect or monochromophoric effect (see ESI, Figure S3). Furthermore, appearance of a new
absorption peak near 350 nm in the UVꢀvis spectra (Figure 2c) can be considered as the signature
for such charge transfer from aromatic π to NBMO of phenalenyl cation. From the normalized
UVꢀvis spectra (Figure 2c), it can be clearly observed that this peak (near 350 nm) becomes
gradually prominent on moving from benzene to mesitylene. Subsequently, the TimeꢀCorrelated
Single Photon Counting (TCSPC) measurement displays a steady decrease in fluorescence lifeꢀ
time of II in the presence of arenes. The gradual decrease in lifeꢀtime of II from 2.41 ns (in neat
acetonitrile) to 2.31 ns in benzene to 0.53 ns in toluene was noted (Figure 2d). In case of
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mesitylene, this lifeꢀtime decreased as low as 0.43 ns (Figure 2d). Furthermore, the H NMR
spectrum of II in presence of benzeneꢀd6 in CD3CN (relative arene concentration is ~15%) shows
a clear upꢀfield shift of the proton peak positions when compared with that of free phenalenyl
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2.5x106
2.0x106
1.5x106
1.0x106
ACN (2.5 mL)
a)
b)
ACN (2.5 mL)
0.5
0.4
0.3
0.2
0.1
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ACN (2.45 mL) + Benzene (50
ꢀ
L)
L)
ACN (2.45 mL) + Benzene (50
ꢀ
L)
L)
ACN (2.45 mL) + Toluenee (50
ꢀ
ACN (2.45 mL) + Toluenee (50
ꢀ
ACN (2.45 mL) + Mesitylene (50 ꢀL)
ACN (2.45 mL) + Mesitylene (50 ꢀL)
λ
exc = 450 nm
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5.0x105
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Wavelength (nm)
Wavelength (nm)
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ACN (2.5 mL)
IRF
PLYACN
PLYBEN
PLYTOL
PLYMES
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Fit
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ACN (2.45 mL) + Mesitylene (50 ꢀL)
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π
n
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Wavelength (nm)
Delay Time (ns)
e)
5 mg catalyst II
in 600µL CD3CN +
100 µL C6D6
5 mg catalyst II
in 700µL CD3CN
Figure 2: Spectroscopic measurements. (a) Absorption spectra of phenalenyl cation II in
CH3CN, (b) Emission spectra of phenalenyl cation II with different arenes in CH3CN, (c)
Normalized UV spectra, (d) TCSPC spectra of phenalenyl cation II with different arenes in
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CH3CN, (e) Stack plot of H NMR spectra of phenalenyl cation II in absence and presence of
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cation II in acetonitrileꢀd3 (Figure 2e). Such H NMR spectra at various concentrations of arene
(benzene or toluene or mesitylene) in CD3CN were also measured. For example, at ~2 %
concentration of arene as well as at ~7% concentration of arene in CD3CN, we have noticed a
1
similar upꢀfield H NMR shift of the PLY(O,O) cation (see SI, Figures S7ꢀS8). Recently, a
similar 1H NMR spectroscopic investigation was carried out to understand πꢀcation interaction in
silylation reaction of heteroarenes.45 Further to investigate the intermolecular interaction between
the PLY(O,O) cation (catalyst II) and arene molecule, Nuclear Overhauser Effect (NOE)
experiments were performed. These experiments were carried out with different arenes (50 µL of
benzene or toluene or mesitylene) in 600 µL acetontrileꢀd3 (see SI, Figures S9ꢀ12) and clearly
revealed a significant NOE interaction between PLY cation and arene. This result clearly
supports a strong intermolecular interaction between PLY cation and arene molecule. Combining
all these spectroscopic results, it may be concluded that the aromatic πꢀelectron is transferred
from arenes to phenalenyl cation II establishing Lewis acidic property of phenalenyl cation.
To further understand the nature of interaction between the phenalenyl cation and arenes,
we resorted to DFT calculations. Figure 3a shows the binding energy of such areneꢀphenalenyl
cation complex, intermolecular distance between aromatic planes and Mulliken charge
distributions for three complexes studied. In all the cases, the intermolecular distances were
calculated (3.26 to 3.17 Å) to be wellꢀbelow the sum of Van der Waal’s radii of carbon atoms
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[PLY(O,O)…Mesitylene]+
[PLY(O,O)…Benzene]+
[PLY(O,O)…Toluene]+
Binding energy:
ꢀ11.54 kcal molꢀ1
Binding energy:
ꢀ7.95 kcal molꢀ1
Binding energy:
ꢀ9.34 kcal molꢀ1
Distance: 3.17 Å
Mulliken charge:
Distance: 3.26 Å
Mulliken charge:
Distance: 3.22 Å
Mulliken charge:
+0.788 (PLY cat)
+0.212 (Mesitylene)
+0.870 (PLY cat)
+0.130 (Benzene)
+0.824 (PLY cat)
+0.176 (Toluene)
b)
HOMO-LUMO
HOMO-LUMO
HOMO-LUMO
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Figure 3: a) Optimized geometry of charge transfer complexes between phenalenyl cation II and
different arenes revealing strong interaction. b) Frontier molecular orbitals of the optimized
species.
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(3.42 Å). Such distance below the sum of Van der Waal’s radii is hallmark of nonꢀclassical
bonding interaction that has been demonstrated in several phenalenyl based compounds earlier
based on experimental findings as well as from theoretical calculations.19, 20, 25, 46 It may be
interesting to note that, in the present phenalenyl cationꢀarene complexes, the intermolecular
bond distance gradually decreases from 3.26 Å in case of complex with benzene to 3.22 Å (with
toluene) to 3.17 Å (with mesitylene). This trend clearly correlates with the relative electron
donation ability of benzene to mesitylene forming gradually stronger charge transfer complex
with phenalenyl cation. Accordingly, the complex of cation II with mesitylene has the highest
binding energy (ꢀ11.54 kcal molꢀ1) followed by tolueneꢀ (ꢀ9.34 kcal molꢀ1) and benzeneꢀcomplex
(ꢀ7.95 kcal molꢀ1). Furthermore, Mulliken charge analysis clearly indicated the charge transfer
from arene to phenalenyl cation in all these complexes. The extent of charge transfer is
maximum for mesityleneꢀcomplex and lowest for benzeneꢀcomplex. All these parameters
obtained from calculations corroborate well with the observation noted from our photophysical
studies as discussed above (Figure 2). Thus it is evident that the arene acts as donor while
phenalenyl cation serves as an acceptor due to its πꢀacidity. Figure 3b depicts the frontier
molecular orbital analysis. In all complexes, HOMO is delocalized on arene and LUMO is
delocalized over phenalenyl cation, which is indicative of a charge transfer from arene to
phenalenyl cation. Thus, the computational studies clearly indicate that mesitylene forms the
strongest charge transfer interaction with phenalenyl cation and binding interaction between the
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donor and acceptor within the charge transfer complexes increases gradually in the following
order: benzene<toluene<mesitylene. This conclusion correlates very well with the trend observed
for quenching of absorbance and luminescence of phenalenyl cation in presence of different
arenes. Most electron rich arene among the series namely mesitylene forms complex with
maximum charge transfer and thus it results the quenching of the absorbance and luminescence
of phenalenyl cation to the highest among the three arenes studied (Figure 2). In addition to this,
owing to this donor acceptor type interaction between the phenalenyl cation and arene, the arene
develops a net +(ve) charge because of some of its πꢀelectron cloud is drained out. Theoretical
studies calculate a net charge of +0.130 on benzene, +0.176 on toulene and +0.212 on mesitylene
moieties in these complexes, respectively. This +(ve) charge generated on the arene coupling
partners (such as benzene or toluene or mesitylene) on forming a donorꢀacceptor type complex
activates them towards the attack by the in situ generated aryl radical having predominant
nucleophilic feature47 (see below). Combining all these observations and evidences, we propose
a mechanistic cycle (Figure 4a) where the phenalenyl based radical (A) is formed in situ by
accepting an electron from TDAE. Subsequently, this phenalenyl radical transfers an electron via
a SET process to the aryl diazonium coupling partner. To further establish the redox role of PLY
radical, the relative rate of electron transfer for the SET process was measured using UVꢀvis spectrometry
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o
at ꢀ20 C. From this study, it was revealed that the electron transfer from TDAE to PLY cation is five
times faster (0.0592 Sceꢀ1) than that of TDAE to diazonium salt (0.0113 Sceꢀ1). This result indicates that
the electron transfer from PLY radical to aryl diazonium partner is more feasible than that of direct
electron transfer from TDAE. This SET (SET 1) from the phenalenyl based radical (A) to the aryl
diazonium coupling partner produces an aryl radical (III)37 and regenerates the phenalenyl cation
(I or II). The generation of aryl radical has been proved by trapping it using TEMPO and
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b)
Figure 4. a) Proposed reaction mechanism, b) Schematic presentation of the different single
electron transfer process with relative frontier molecular orbital energy calculated by DFT at
B3LYP/6ꢀ31g+(d) level (EI: energy of catalyst I and EII: energy of catalyst II).
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characterized by NMR spectroscopic measurements. Further, the PLY radical was isolated and
reacted it with aryl diazonium partner to generate the aryl radical. This aryl radical was also
trapped with TEMPO to establish that the PLY radical is involved in the SET process to generate
the aryl radical (see SI, Scheme S2). However, the possibility of direct electron transfer from TDAE to
the diazonium salt cannot be completely excluded. Subsequently, this regenerated phenalenyl cation
can interact with the other substrate (arene) of this reaction via formation of species IV.
Formation of IV is evident from spectroscopic and theoretical studies (vide supra, Figures 2 and
3). The formation of species IV facilitates a πꢀelectron transfer from arene (πꢀbonding orbital) to
the nonꢀbonding orbital of phenalenyl cation, which creates a deficiency of electron on the arene
carbon centers making it susceptible towards nucleophilic attack by aryl radical formed in situ
generating a radical species V. Following this, the species V undergoes another SET process
(SET 2) to regenerate the phenalenyl based radical species A and a cationic biaryl species VI.
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ꢀ
Highly acidic proton of VI is next taken up by BF4 to form HBF4 affording the desired
biarylated product. In such radical mechanism a chain propagation process will always be there,
which has been shown by SET 3 process.39 In such process, the electron transfers from V to aryl
diazonium salt coupling partner 2 to produce aryl radical which subsequently can attack to the
activated carbon center of arenes to afford the biaryl coupled product. Further, the electronic
structure calculations on species V (with benzene as arene partner and in situ generated phenyl
radical from phenyl diazonium salt partner) unraveled the optimized geometry (Figure 4a).
Binding energy of species V was found to be ꢀ8.72 kcal/mol. The registry of the diarene
intermediate by forming a complex with phenalenyl cation in complex V facilitates the SET 2
process where an electron is transferred from diarene intermediate to the LUMO of phenalenyl
cation. Thus the phenalenyl radical species A can be regenerated, and the catalytic cycle
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continues. The frontier molecular orbital (FMO) energy calculations using DFT at B3LYP/6ꢀ
31g+(d) level authenticates the feasibility of the energy landscape involving various proposed
single electron transfer processes involved during the catalysis. The relative FMO energies of the
active species involved in these SET processes are presented schematically in Figure 4b.
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CONCLUSIONS
In conclusion, for the first time without employing any metals, we have been able to integrate
two important features widely used in catalyst design namely redox activity and Lewis acidity
within a molecule. The empty NBMO of phenalenyl cation can induce a SET process after it is
filled with a single electron by a chemical reductant thus it uses its redox activity to activate one
of the coupling partners. On the other hand, the same empty NBMO of the phenalenyl cation
plays a key role as a purely organic Lewis acceptor in activating the unbiased arene, which is
another coupling partner of this reaction. We have developed a metal free catalytic protocol yet
efficient and versatile method as a replacement of expensive metal based catalysis in CꢀH
arylation of arenes at room temperature without any light stimulations.
Methods
All solvents and arenes were distilled from Na/benzophenone or calcium hydride under inert
condition prior to use. All chemicals were purchased and used as received. The H and 13C {1H}
1
NMR spectra were recorded on 400 and 500 MHz spectrometers in CDCl3 with residual
undeuterated solvent (CDCl3, 7.26/77.0) as an internal standard. Chemical shifts (δ) are given in
ppm, and J values are given in Hz. All chemical shifts were reported in ppm using
tetramethylsilane as a reference. Chemical shifts (δ) downfield from the reference standard were
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assigned positive values. Openꢀcolumn chromatography and thinꢀlayer chromatography (TLC)
were performed on silica gel (Merck silica gel 100ꢀ200 mesh). Evaporation of solvents was
performed under reduced pressure using a rotary evaporator.
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Reaction procedure for optimization study on C-H arylation of benzene.
Catalyst of required amount was taken with 1 mL DMSO solvent in 25 mL pressure tube,
reducing agent of required amount was added to the catalyst solution. Benzene (2.4 mmol) and
diazo coupling partner (2a, 0.24 mmol) were added to the resulting solution of catalyst inside a
nitrogen filled glovebox. The final reaction mixture was allowed to stir for required time at room
temperature. After completion of the reaction, product was extracted in 25 mL dichloromethane
(DCM) and dried over anhydrous sodium sulphate. The solvent was removed under reduced
pressure and crude product was purified by column chromatography on silica gel (100ꢀ200
mesh) using hexane/EtOAc mixture to yield the pure desired products.
General procedure for C-H arylation of arenes
Catalyst I/catalyst II (0.006 mmol) was taken with 1 mL DMSO solvent in a 25 mL pressure
tube, TDAE (0.012 mmol) was added to the catalyst solution. Arene (1a-b 2.4 mmol/1c-I 1.2
mmol) and diazo coupling partner (2, 0.24 mmol) were added to the resulting solution of catalyst
inside a nitrogen filled glovebox. The final reaction mixture was allowed to stir for required time
at room temperature. After completion of the reaction, product was extracted in 25 mL
dichloromethane (DCM) and dried over anhydrous sodium sulphate. The solvent was removed
under reduced pressure and crude product was purified by column chromatography over silica
gel (100ꢀ200 mesh) using hexane/EtOAc mixture to yield the pure desired products.
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ACKNOWLEDGEMENTS
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We thank SERB, India (Grant No. SR/S1/ICꢀ25/2012) for financial support. J. A, S. C and A. J
thank IISERꢀKolkata for fellowship. IISERꢀKolkata computation facility. JA thanks Mr Shantanu
Pattanayak for his help in measuring rate of electron transfer. Authors thank anonymous
reviewers for their constructive suggestions.
ASSOCIATED CONTENT
Supporting Information. Tables, figures and details of the experiments. “This material is
available free of charge via the Internet at http://pubs.acs.org.”
Competing financial interests
The authors declare no competing financial interests.
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TOC GRAPHIC
Integrating Organic Lewis Acid and Redox Catalysis: The Phenalenyl Cation in Dual Role
Jasimuddin Ahmed, Soumi Chakraborty, Anex Jose, Sreejyothi P and Swadhin K. Mandal*
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