Phenalenyl Based Aluminum Compound for Catalytic C-H Arylation of Arene and Heteroarenes at Room Temperature

Article abstract of DOI:10.1021/acs.joc.8b02699

Main group metal based catalysis has been considered to be a cost-effective alternative way to the transition metal based catalysis, due to the high abundance of main group metals in the Earth's crust. Among the main group metals, aluminum is the most abundant (7-8%) in the Earth's crust, making the development of aluminum based catalysts very attractive. So far, aluminum based compounds have been popularly used as Lewis acids in a variety of organic reactions, but chemical transformation demanding a redox based process has never utilized an Al(III) complex as a catalyst. Herein, we tuned the redox noninnocence behavior of a phenalenyl ligand by coupling with Al(III) ion, which subsequently can store the electron upon reduction with K to carry out direct C-H arylation of heteroarenes/mesitylene at ambient temperature. A mechanistic investigation revealed that a three-electron reduced phenalenyl based triradical aluminum(III) complex plays the key role in such catalysis. The electronic structure of the catalytically active triradical species has been probed using EPR spectroscopy, magnetic susceptibility measurements, and electronic structure calculations using a DFT method.

Full text of DOI:10.1021/acs.joc.8b02699

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Article
Phenalenyl based Aluminum Compound for Catalytic C-
H Arylation of Arene and Heteroarenes at Room Temperature
Pavan K Vardhanapu, Jasimuddin Ahmed, Anex Jose, Bikash Kumar
Shaw, Tamal K. Sen, Amita A. Mathews, and Swadhin K. Mandal
J. Org. Chem., Just Accepted Manuscript • Publication Date (Web): 03 Dec 2018
Downloaded from http://pubs.acs.org on December 3, 2018
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Phenalenyl based Aluminum Compound for Catalytic C-H
Arylation of Arene and Heteroarenes at Room Temperature
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Pavan K. Vardhanapu†, Jasimuddin Ahmed†, Anex Jose†, Bikash Kumar Shaw†, Tamal K. Sen†,
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Amita A. Mathews‡ and Swadhin K. Mandal*,†
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†Department of Chemical Sciences, Indian Institute of Science Education and Research-Kolkata,
Mohanpur-741246, India.
‡Department of Chemical Sciences, Loyola College, University of Madras, Nungambakkam,
Chennai-600034, India.
*E-mail: swadhin.mandal@iiserkol.ac.in
TOC/Abstract Graphic:
ABSTRACT: Main group metal based catalysis has been considered to be a cost effective
alternative way to the transition metal based catalysis, due to the high abundance of main group
metals in the Earth crust. Among the main group metals, aluminum is the most abundant (7-8%)
in the Earth crust making the development of aluminum based catalyst very attractive. So far
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aluminum based compounds have been popularly used as Lewis acids in a variety of organic
reactions but chemical transformation demanding a redox based process has never utilized
Al(III) complex as a catalyst. Herein, we tuned the redox non-innocence behavior of phenalenyl
ligand by coupling with Al(III) ion, subsequently can store the electron upon reduction with K to
carry out direct C-H arylation of heteroarenes/mesitylene at ambient temperature. A mechanistic
investigation revealed that a three-electron reduced phenalenyl based triradical aluminum(III)
complex plays the key role in such catalysis. The electronic structure of the catalytically active
triradical species has been probed using EPR spectroscopy, magnetic susceptibility
measurements, and electronic structure calculations using a DFT method.
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INTRODUCTION
The characteristics of most of the transition metal mediated catalysis are facile substrate
activation and their rearrangement triggered by switching of different accessible redox states.1
On the other hand, substrate activation by Lewis acidic non-transition metals such as alkali,
alkaline earth metals or by other main group metals is very well-known which works through
entirely different mechanism than that of a typical transition metal mediated process involving
redox switching.2 It has long been discussed and considered as an open challenge to mimic the
transition metal like behavior by a main group element in catalysis.3 The major impetus of
mimicking transition metals by a main group element stems from its low-toxicity, less expenses
and high abundance. However, designing redox chemistry to devise a catalytic organic
transformation with the compounds of main group elements is extremely challenging. One
school of thought introduced by Power, Roesky, Bertrand and others considered that the use of
low-valent main group complexes may introduce feasibility on the oxidative addition process,
however, the reverse reductive elimination was not favorable which is essential to complete the
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catalytic cycle as it regenerates the active catalytic species.3,4,5 As a result, the chemistry of low-
valent main group elements can mimic the transition metals only half-way in which the small
molecules activation by oxidative addition could be accomplished.6 However, the reductive
elimination reaction which is the hallmark of transition metal mediated catalytic process; often
the last step of a typical catalytic cycle remained elusive in such design using low-valent main
group chemistry. It thus warrants devising conceptually new strategy to achieve such goal.
Keeping this aim in mind, herein, we have taken advantage of well-known redox non-innocent
ligand phenalenyl (PLY) and synthesized an Al(III) based complex (Scheme 1) for designing
direct C-H arylation catalysis at ambient temperature.
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Early HMO calculations predicted that phenalenyl can exist in three redox active states
such as cation, neutral radical and anion using its unique non-bonding molecular orbital
(NBMO).7-12 In recent years, we have explored the cationic state of phenalenyl by coordinating
Scheme 1. Redox catalysis of Al(III) complex, 1
Ar2-N2BF4
Reducing
agent
3-
Al(III)L3
Al(III)L3
Ar1-Ar2
L
=
Electronic structure
O
O
determined by CV, EPR,
Magnetic measurement
Ar1-H
* Transition metal f ree
* No Light Irradiation
* Functional group tolerance
* Room teemperature
* 21- Substrates
with appropriate metal ions and it is capable of accepting electron(s) to form phenalenyl based
radical.13 The in situ generated phenalenyl radical can show spectacular range of properties
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spanning from spin based device, fuel cell application and transition metal mimicking
catalysis.14-19
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In the present work, the phenalenyl based Al(III) complex was used to store electrons by
reducing it with a chemical reductant. Subsequently the stored electron was transferred through a
single electron transfer (SET) process to generate the aryl radical from aryl diazonium coupling
partner which can couple with arene/heteroarene partner delivering a facile route to C-H
functionalization under mild conditions. There has been seminal effort by König and co-workers
to avoid transition metal as catalyst to couple aryl diazonium salt with heteroarene partner under
light stimulation, however this method failed to activate arene partner.20 It may be noted that the
coupling between aryl diazonium salt and arene partner was successfully accomplished in 2017
by Lee and co-workers using a dual gold and ruthenium based photoredox catalyst, which
induces electron transfer process to activate the aryl diazonium coupling partner while the Au
based catalyst acts as an Lewis-acid for electrophilic activation of arene partner.21 More
recently, this coupling reaction between aryl diazonium partner and arene/heteroarene partner
was further modified by Ackerman and co-workers using an efficient Mn(I) based catalyst under
blue LED irradiation.22 Recently, we have reported the catalytic protocol for this direct C-H
arylation of arenes and heteroarenes by redox active phenalenyl assisted Fe(III) complex under
ambient conditions.23 However, all these recent reports failed to provide a completely transition
metal free direct C-H arylation catalysis at ambient conditions without using external light
stimulation activating both arene and heteroarene coupling partner. The present work addresses
this concern.
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RESULTS AND DISCUSSION
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First we prepared the aluminum(III) complex (1) with 9-hydroxyphenalenone (PLY)
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ligand using anhydrous AlCl3 precursor following earlier method after modification.24 Complex
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o
1 was obtained as yellow precipitate after 16 h reflux in dichloroethane at 100 C. The product
was crystallized from CHCl3/hexane and yielded 77% of crystallized product. Complex 1 was
further characterized by 1H and 13C NMR spectroscopy and matched with the earlier report.24
To understand the redox behavior of complex 1, cyclic voltammetric (CV) experiment was
carried out in chloroform which shows three, quasi-reversible, single-electron reductions at -1.28
V (E11/2), -1.52 V (E21/2) and -1.78 V (E31/2), (Figure 1b).25 The CV result clearly indicates that
there are three consecutive one electron reductions which may be attributed to gradual
acceptance of three electrons into three phenalenyl units. This observation is in consonance with
previous study carried out on a similar PLY based complexes.26-28 It clearly reveals that three
consecutive one electron reduction processes are generating a radical, bi-radical and tri-radical
species, respectively (vide infra, Figure 1a). It may be noted that the differences between the
consecutive reduction potentials (∆E2-1 = -0.24 V and ∆E3-2 = -0.26 V, respectively) matches
very well with earlier electrochemical results.25 Its worth to note that the ∆E is considered as the
disproportionation potential and is directly correlated to the on-site Coulomb correlation energy
(U).27 The observed ∆E values are in close agreement to bis-phenalenyl-boron complexes (∆E2-1
values are -0.29 V and -0.28 V),27 tris-phenalenyl-silicon (∆E2-1 = -0.24 V and ∆E3-2 = -0.32 V,
respectively) and tris-phenalenyl-germanium complexes (∆E2-1 = -0.22 V and ∆E3-2 = -0.28 V,
respectively)28 bearing the same O,O-phenalenyl ligand. These identical ∆E values observed for
1 strongly suggests that successive electron reduction processes occur into phenalenyl moieties.
After having this information on redox behavior of complex 1, three electrons were chemically
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injected to 1 by reduction with 3 equiv. K yielding a tri-radical paramagnetic compound IV
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(Figure 2a). Further, the DFT calculation at the level B3LYP/6-31+g* reveals that three electrons
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(a)
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E21/2 = -1.52 V
E11/2 = -1.28 V
e
e
O
O
O
O
O
O
e
e
(III)
(III)
Al
Al
II
(III)
O
O
O
O
Al
O
O
O
O
O
O
O
O
III
Complex 1
E31/2 = -1.78 V
e
(b)
e
3
O
O
(III)
O
Al
O
O
O
IV
Figure 1. (a) Redox process for 1. (b) Cyclic voltammetry of complex 1 in chloroform at a scan
rate of 0.1 V s−1 acquired using Pt working electrode, Pt-wire as the counter electrode and
reference electrode, internally referenced to Ag/AgCl.
after reduction populates on PLY moieties as shown in spin density plot (Figure 2b). The natural
bond orbital (NBO) analysis of three electrons reduced species IV revealed empty 3s and 3p
orbitals on aluminum, which clearly supports the presence of Al(III) center and is consistent with
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other experimental results (see below). To further explore the electronic spin structure of the
catalytically active species IV, we have performed variable temperature dc magnetic
susceptibility measurements in an applied field of 0.01 Tesla in temperature range 2 ‒ 300K. The
μRT value obtained from the variation of effective magnetic moment (μeff) as a function of
temperature (Figure 3a) is 3.92 μB. This is close to the theoretical value (3.87 μB) of tri-radical
species IV generated after successive reduction of three PLY moieties present in complex 1. The
existence of a significant antiferromagnetic coupling between these radical spins is revealed from
the continuous decrease in the μeff values upon lowering down the temperature to 2K.
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3
(a)
a) K (3 equiv.), CH3CN
RT, 12 h
O
O
O
O
Al III
b) Solvent evaporation
Al
O
O
O
O
O
O
O
O
Complex 1
IV
(b)
Figure 2. (a) Preparation of three electron reduced species IV from complex 1. (b) Spin density
plot of IV, Isovalue = 0.02, density = 0.0004 e-/au3.
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To understand the strength of this super-exchange coupling, the experimental data were fitted
using the following spin Hamiltonian (1) considering three PLY units as equilateral triangular
sites.29 The molar susceptibility is plotted in the form of χMT vs. T. The sharp fall in the χMT
values indicates a significant contribution of temperature independent paramagnetism (TIP) in
the species IV. The best fit is obtained considering the super-exchange coupling J between the
three S = ½ PLY•¯centers and the Lande g factor (from EPR studies, Figure 3b) as ~ 1.99, the
extracted parameters are found as J = ‒ 25.38 (2) cm-1, TIP = 6 x 10-2cm3 mol-1. Good agreement
factor (R2) in the order of 10–5 is obtained in the least squares fitting procedure (Figure 3a).
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H  (JS S  JS S  JS S )...............(1)
i j
j
i
k
k
where, the specific terms carry their usual meaning [Si =Sj=Sk = ½;(PLY•¯)]
Furthermore, the X-band EPR spectroscopy of species IV reveals a sharp singlet line with
resonance at g = ~ 1.9998 (Figure 3b). This EPR signal at g = ~2 is indicative of a phenalenyl
based radical.30 The magnetic susceptibility along with EPR measurements, DFT studies,
catalytic results with IV and CV result of complex 1 provide a consistent picture of the tri-radical
species IV. This observation encouraged us to check if such reduced species IV can initiate any
single electron transfer (SET) process. In this context, we studied direct C-H arylation of arenes
and heteroarenes with aryl diazonium salt coupling partners under ambient conditions using 1 in
the presence of a chemical reductant. In order to design such reaction, furan (1a) was taken as
the arene partner and 4-chloroaryl diazonium salt 2a was taken as the aryl diazonium-coupling
partner to obtain the arylated product 3a (Table 1) under different conditions using phenalenyl
supported aluminium complex 1.
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(a)
(b)
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Figure 3. (a) Variation of μeff values as a function of temperature for reduced tri-radical species
IV (Blue). Variation of χMT values with temperature (Black). Solid line (Red) represents the
theoretically fitted curve and the points are the experimental data (Black). (b) EPR spectrum of
complex 1 and gradual change in the EPR spectrum of complex 1 on addition of 1/2/3
equivalents of K. The signal intensities in the EPR spectra are not normalized.
The first reaction was carried out by using 10 mol % of 1 and 40 mol % of K in DMSO for 24 h
which afforded 74% of the arylated product 3a (Table 1, entry 1). However, the reaction did not
proceed at all when it was carried out without addition of any reducing agent (K) (Table 1, entry
2). Use of 40 mol% of an organic amine tetrakis diamino ethylene (TDAE) as the reducing agent
with 10 mol % of complex 1, afforded less than 10% yield of the arylated product 3a (Table 1,
entry 3), indicating the necessity of a strong reducing agent (such as K) to reduce 1. The reaction
was optimized under various catalyst and potassium loadings (Table 1, entries 4-8), we found
that 5 mol% of 1 and 4 equiv. K (20 mol %) are the effective combinations to afford 72% yield
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of 3a (Table 1, entry 4). Taking these combinations of catalyst and K, reactions were performed
in DMF, THF and DMA and we obtained lesser yield in other solvents than in DMSO (Table 1,
entries 9-11). A blank reaction without 1 was carried out keeping other conditions fixed which
afforded only 16% yield of product 3a (Table 1, entry 12), confirming the necessity of complex 1
to succeed this reaction. When this reaction was carried out for 12 h instead of 24 h, only 41%
yield of 3a (Table 1, entry 13), was obtained. As a control, the reaction using catalytic amount of
Al(acac)3 having identical coordination environment around Al(III) ion in complex 1, was
performed keeping other parameters same, when it resulted in only 26% yield of product 3a
(Table 1, entry 14). This result (Table 1, entry 14) clearly establishes the role of phenalenyl
ligand to initiate redox process for the direct C-H arylation of heteroarene. Overall, we found 5
mol % of complex 1 with 20 mol % of K in DMSO solvent can afford the best isolated yield72%
of C-H arylated product of furan within 24 h.
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With this optimized reaction conditions, the substrate scope was screened over the range
of heteroarene (furan and thiophene) partners as well as in different aryl diazonium salt coupling
partners. The C-H arylation of heteroarenes has been accomplished by König and co-workers
with aryl coupling partners using organic dye in the presence of light.24 At first, C-H arylation of
furan (1a) was carried out with differently substituted aryl diazonium salt coupling partners.
Electron withdrawing group containing aryl diazonium salt coupling partners (2a, 2b, 2c and 2d)
yielded 72-82% of arylated products 3a, 3b, 3c and 3d, respectively whereas electron donating
group containing aryl diazonium salt coupling partners (2f and 2g) afforded slightly less yield
(69% and 65%) of corresponding products 3f and 3g, respectively. Phenyl group containing aryl
diazonium salt coupling partner (2e) yielded 68% of product 3e (Table 2). Next C-H arylation of
thiophene (1b) was carried out with differently substituted aryl diazonium salt coupling partners.
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Table 1. Optimization study for C-H arylation of furana
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Catalyst (X mol %)
K (Y mol %)
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Cl
N2BF4
H
O
O
Solvent, RT, t
Cl
2 equiv.
1a
1 equiv.
2a
3a
Entry Catalyst (mol %) Reductant (mol %) Solvent Time (h) Yieldb (%)
1
2
1 (10)
1 (10)
1 (10)
1 (5)
K (40)
None
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
24
24
24
24
24
24
24
24
24
24
24
24
12
24
74
NR
<10
72
3
TDAE (40)
K (20)
K (8)
4
5
1 (2)
21
6
1 (5)
K (5)
15
7
1 (5)
K (10)
K (15)
K (20)
K (20)
K (20)
K (20)
K (20)
K (20)
19
8
1 (5)
52
9
1 (5)
15
10
11
12
13
14
1 (5)
THF
NR
<10
16
1 (5)
DMA
None
1 (5)
DMSO
DMSO
DMSO
41
Al(acac)3
26
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aReaction conditions: Furan (0.64 mmol), 2a (0.32 mmol), K (x mol % loading). b Isolated yield,
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NR stands for “No Reaction.
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Table 2. Substrate scopes for C-H arylation of heteroarenes/mesitylene using aluminum
complex 1 under ambient conditions.a
Catalyst 1 (5 mol %)
K (20 mol%)
X
or
N2BF4
or
X
R
DMSO, RT, 24h
R
R
X = O (3)
S (4)
X = O (1a)
S (1b)
5
2
1c
O
O
O
O
O
R
NO2
CN
OMe
R = Cl, 3a (72%)b
R = Br, 3b (67%)b
3c (82%)b
3d (80%)b
3f (69%)b
3e (68%)b
O
S
S
S
S
CH3
Cl
Br
CN
NO2
3g (65%)b
4d (79%)b
4c (74%)b
4a (70%)b
4b (63%)b
S
S
S
OMe
I
Br
Cl
4g (69%)b
4f (62%)b
4e (63%)b
5a(55%)b
5b (56 %)b
F
I
OMe
CH3
NO2
5c (60%)b
5d (64%)b
5e (61%)b
5f (52%)b
5g (42%)b
aReaction conditions: Heteroarene/mesitylene (0.64 mmol), 2 (0.32 mmol), catalyst 1 (5 mol %,
0.016 mmol), K (2.49 mg, 20 mol % loading, 0.064 mmol). bIsolated yield.
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(a) Competition reaction between furan and mixture of two electronically biased aryl
diazonium salts 2a and 2f.
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MeO
Cl
Complex 1 (10 mol %)
K (40 mol%)
O
O
O
DMSO, RT, 24h
N2BF4
2a (1 equiv.)
N2BF4
Cl
3a (58%)
OMe
3f (50%)
1a (4 equiv.)
2f (1 equiv.)
(b) Reaction inhibition in presence of TEMPO.
Complex 1 (5 mol %)
K (20 mol%)
Cl
N2BF4
O
DMSO, RT, 24h
O
Cl
1a (1 equiv.)
2a (0.5 equiv)
3a
TEMPO (1 equiv.):
TEMPO (2 equiv.):
15%
<5%
(c) TEMPO trapped intermediate formation.
O
Catalyst 1 (5 mol%)
N
O2N
N2BF4
N
O
K (20 mol%)
DMSO, RT, 12 h
O2N
6 (56%)
TEMPO (1 equiv.)
2c (1 equiv.)
Figure 4. Mechanistic investigation: (a) Competition reaction between furan and mixture of two
aryl diazonium salts 2a and 2f. (b) Reaction inhibition of furan arylation in presence of TEMPO.
(c) Trapping of aryl radical by TEMPO.
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Electron withdrawing group containing aryl diazonium salt coupling partners (2a, 2b, 2c and 2d)
yielded 70-79% of arylated products 4a, 4b, 4c and 4d, respectively whereas electron donating
group containing aryl diazonium salt coupling partner (2f) afforded slightly less yield of 62% of
corresponding product 4f, respectively. Phenyl group containing aryl diazonium salt coupling
partner (2e) yielded 63% of product 4e (Table 2). Aryl diazonium salt coupling partner (2g)
containing ortho-substitution afforded 69% of corresponding product 4g.
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After successful C-H arylation of heteroarenes, direct C-H arylation of arene (mesitylene) was
attempted. Electron withdrawing groups such as Cl, Br, F, I, NO2, containing aryl diazonium salt
coupling partners yielded 55-64% of arylated products 5a, 5b, 5c, 5d and 5e, respectively
whereas aryl diazonium salt coupling partners containing electron donating groups such as OMe,
or Me afforded less yield of 52% or 42% of corresponding product 5f or 5g (Table 2). Almost
similar yields of arylated products of mesitylene have been reported by Lee and co-workers
under Au and Ru based dual catalysis in the presence of light with aryl diazonium salt as the
coupling partners21 whereas the present method avoids use of any expensive and rare metals and
avoids light stimulation. With this versatile and efficient C-H arylation of arenes and
heteroarenes in hand, the mechanistic pathway for the reaction was investigated. At first, an
intermolecular competitive reaction was carried out for the arylation of furan with substituted
aryl diazonium salt coupling partners of which one bears an electron donating substituent (2f)
while the other bears an electron withdrawing substituent (2a), to afford respective arylated
products 3f (50%) and 3a (58%, Figure 4a). This indicates the present catalytic system is biased
towards the aryl diazonium coupling partners having electron withdrawing groups than the aryl
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diazonium coupling partners containing electron donating groups. This result indicates that a
radical intermediate might be involved during the catalytic cycle.30
7
8
9
Next, the reaction between furan 1a and aryl diazonium salt coupling partner 2a was
carried out in the presence of a well-known radical scavenger 2,2,6,6-tetramethylpiperidinoxyl
(TEMPO) under different TEMPO loadings (Figure 4b). In presence of 1 equiv. TEMPO, only
15% of arylated product 3a was obtained while in presence of 2 equiv. TEMPO, the reaction was
arrested almost completely (<5%). This reaction inhibition in presence of a radical scavenger,
confirms a radical mediated pathway for this arylation process. Furthermore, the reaction
between TEMPO and aryl diazonium salt coupling partner 2c was performed with our optimized
conditions in absence of any arene or heteroarene partner, which resulted in the formation of a
TEMPO-trapped product 6 with 56% yield (Figure 4c). The arrest of TEMPO-trapped aryl
radical 6 clearly establishes a radical mediated SET process is operative which can further be
supported by previous reports for direct C-H arylation.31-34 As described earlier, we were able to
isolate the active catalytic species IV by performing stoichiometric reaction between Al(PLY)3
complex 1 and K. Using this triradical species IV in 5 mol % without further addition of the
reducing agent (K) (see above for details on electronic structure of IV), a catalytic reaction was
carried out under the optimized conditions and the direct C-H arylation of furan afforded 74%
yield of the arylated product 3a. This result establishes that species IV (Figure 2a) is the
catalytically active species for this arylation reaction.
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Taking these results in account along with the previous literature supports,20,33 we sketch
a plausible mechanistic cycle for the catalytic reaction (Scheme 2). Complex 1 forms the
catalytically active triradical species IV upon reduction with K metal. The electronic structure of
this species is well understood by magnetic, spectroscopic and theoretical studies as described
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above. In the first step of mechanistic cycle, species IV transfers an electron(SET1) to the aryl
5
6
diazonium salt and forms the aryl radical. This aryl radical was trapped and isolated as
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Scheme 2. Plausible mechanistic cycle for C-H arylation using complex 1.
K
O
O
+3e
Al(III)
O
O
O
O
3
Al-(PLY)3
Complex 1
or
X
O
O
R
R
Al(III)
O
O
O
HBF4
O
N2BF4
BF4
N2BF4
[Al-(PLY)3]3-
IV
R
H
R
2
2
or
X
2
H
R
VIIa
SET 1
R
SET 2
VIIb
SET 3
R
R
N2
BF4
N2
BF4
O
O
or
Al(III)
X
O
O
V
V
R
O
O
VIa
R
VIb
[Al-(PLY)3]2-
III
or
X
1
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compound 6 using TEMPO and characterized by H and C NMR spectroscopy. During this
process, species IV transforms into species III as also indicated by electrochemistry data and
theoretical calculations. We have studied the electron transfer process in this key step of catalytic
cycle. The process takes place by transfer of an electron from HOMO of IV (Energy = 3.99 eV)
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to the low-lying LUMO of substrate 2 (Energy = -8.18 eV). Figure 5 displays the HOMO of IV,
which clearly shows that it is delocalized over PLY moieties. Hence the first electron transfer
from IV to the diazonium salt takes place from PLY moiety of IV.
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Figure 5. Energy levels and plot of HOMO of IV and LUMO of 2 (Isovalue = 0.02 e-/au3)
It is found from DFT calculations that the orbital occupancy on aluminum atom remains the
same in IV and III supporting the fact that the electron transfer takes place from a PLY unit.
Further, the biarylated radical intermediate VI is formed by reaction of aryl radical V with the
arene/heteroarene partner. However, the exact pathway of the arene activation is not clear, which
traditionally requires a Lewis acid as reported earlier.19,21 The biarylated radical VI forms a
cationic biaryl intermediate VII by transferring an electron to species III (SET2) thus
regenerating the catalytically active species IV for the catalytic cycle to continue. Further, the
-
BF4 counter anion abstracts a proton from the cationic biaryl intermediate VII to yield the
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desired product.20 There can be two probable fate of the second electron released from species
VII. It can either be consumed by species III to regenerate the active catalyst species IV or it can
be captured by aryl diazonium salt coupling partner (2) and used in radical propagation process
forming species V, which will further continue the reaction forming species VI. But the
inefficiency of this chain propagation process (through SET 3) has been confirmed by the
reaction optimization study in absence of catalyst but in presence of 20 mol % of K yielding only
16% of product 3a (Table 1, entry 12).
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CONCLUSION
In conclusion, we have shown an aluminum supported phenalenyl complex is highly accessible
for redox transformation reaction and thus mimics the transition metal mediated catalysis. A
radical based SET process plays key role for such direct arylation of arenes and heteroarenes.
Magnetic susceptibility, EPR measurement, CV, catalytic experiment and DFT studies indicate
involvement of a tri-radical species during this catalysis. This study establishes that by taking the
advantage of the non-innocence behavior of phenalenyl ligand, it is possible to design catalysts
of main group elements, which may take part in redox catalysis, such a phenomena is usually
considered as the hallmark of transition metals mediated catalysis.
EXPERIMENTAL SECTION
General methods and Instrumentation. Unless stated otherwise, reactions were performed in
flame–dried glassware under inert atmosphere (N2), using standard Schlenk techniques or inside
a MBraun Glovebox maintained under nitrogen. All solvents were distilled from
Na/benzophenone or calcium hydride prior to use. All other chemicals were purchased from
commercial sources and used as received. Deuterated solvents were purchased from Cambridge
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Isotope Laboratories. NMR spectra were recorded on a JEOL ECS 400 MHz spectrometer and
on a Bruker Avance III 500 MHz spectrometer. All chemical shifts were reported in ppm using
tetramethylsilane as a reference. Chemical shifts (δ) downfield from the reference standard were
assigned positive values. Thin-layer chromatography (TLC) and open-column chromatography
were performed on silica gel (Merck silica gel 100-200 mesh). Evaporation of solvents was
performed under reduced pressure using a rotary evaporator.
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General Procedure for the Synthesis of Aryldiazonium tetrafluoroborate.35 Substituted
aniline (10 mmol) was dissolved in 5 mL of distilled water at room temperature then 4 mL of
46% hydrofluoroboric acid was added to the mixture. The resulting reaction mixture was cooled
o
down to 0-5 C using an ice bath. Sodium nitrite (0.69 g) was dissolved in 1.5 mL of water and
o
cooled down to 0-5 C separately, this cold sodium nitrite solution was added drop-wise to the
main reaction mixture. The resulting mixture was stirred for 30 min maintaining the temperature
o
0-5 C and the precipitate was collected by filtration after washing the residue with ice cold
water. The final residue was dissolved in minimum amount of acetone and then diethyl ether was
added. Pure product was collected as crystals from this mixture of acetone and diethyl ether
solvent. The final aryldiazonium tetrafluoroborate crystals were washed with diethyl ether and
dried under vacuum.
Synthetic procedure of Complex 1.24 In an oven dried Schlenk flask, 9-hydroxyphenalenone (1
g, 5.09 mmol) was dissolved in 50 mL of dichloroethane and anhydrous aluminum chloride
(0.225 g, 1.69 mmol) was added under N2 atmosphere. Then the reaction mixture was refluxed
o
for 16h at 100 C. The yellow precipitate was filtered and washed thoroughly with fresh
dichloroethane. The crude product was purified by column chromatography and by
recrystallization from CHCl3/hexane yielding to pure yellow powder. Single crystals of the
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complex 1 were obtained in chloroform, the molecular structure was resolved by X-ray single-
crystal diffraction and matched with literature report (yellow crystals, 77 % yield, 0.796 g, 1.301
mmol). 1H NMR (400 MHz, CDCl3, 298 K): δ 8.01(d, J = 9.3 Hz, 2H), 7.94 (d, J = 7.6 Hz, 2H),
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7.51 (d, J = 7.6 Hz, 1H), 7.13 (d, J = 9.2 Hz, 2H) ppm; C NMR (100 MHz, CDCl3, 298 K): δ
179.0, 141.2, 132.7, 127.7, 127.6, 125.1, 122.7, 113.2 ppm.
General Procedure for C-H Arylation of Heteroarenes. In a glove box, catalyst 1 (9.80 mg, 5
mol %, 0.016 mmol) and K (2.49 mg, 20 mol %, 0.064 mmol) were taken in a 25 mL pressure
tube and subsequently DMSO (1 mL) was added in to the reaction mixture, which resulted in a
dark green colored solution. Subsequently, furan/thiophene (1a/1b, 0.64 mmol) and diazo
coupling partner 2 (0.32 mmol) were added to the reaction mixture and allowed to stir for 24 h at
room temperature. After completion of the reaction, product was extracted in dichloromethane
(25 mL) and dried over anhydrous sodium sulphate. The solvent was removed under reduced
pressure and crude product was purified by column chromatography on silica gel (100-200
mesh) using hexane/EtOAc mixture to yield the pure desired product.
General Procedure for C-H Arylation of Mesitylene. In a glove box, catalyst 1 (9.80 mg, 5
mol %, 0.016 mmol) and K (2.49 mg, 20 mol %, 0.064 mmol) were taken in a 25 mL pressure
tube, subsequently DMSO (1 mL) was in to the reaction mixture, which resulted in a dark green
colored solution. Subsequently, mesitylene 1c (0.64 mmol) and diazo coupling partner 2 (0.32
mmol) were added to the reaction mixture and allowed to stir for 24 h at room temperature. After
completion of the reaction, product was extracted in dichloromethane (25 mL) and dried over
anhydrous sodium sulphate. The solvent was removed under reduced pressure and crude product
was purified by column chromatography on silica gel (100-200 mesh) using hexane/EtOAc
mixture to yield the pure desired product.
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General Procedure for Inhibition of C-H Arylation Reaction in Presence of TEMPO
(Radical Scavenger). Furan (1a, 0.64 mmol), diazo coupling partner (2a, 0.32 mmol), catalyst 1
(9.80 mg, 5 mol%, 0.016 mmol), K (2.49 mg, 20 mol%, 0.064 mmol) and required amount of
TEMPO (with respect to diazo coupling partner 2a) were taken in a 25 mL pressure tube and
subsequently added DMSO (1 mL) in to the reaction mixture inside a nitrogen filled glovebox.
Then, the reaction mixture was allowed to stir for 24 h at room temperature. After completion of
the reaction, product was extracted in dichloromethane (25 mL) and dried over anhydrous
sodium sulphate. The solvent was removed under reduced pressure and product 3a was purified
by column chromatography on silica gel (100-200 mesh) using hexane/EtOAc.
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General Procedure for Synthesis of TEMPO-Trapped Intermediate (2,2,6,6-tetramethyl-1-
(4-nitrophenoxy) piperidine), (6).36 TEMPO (0.32 mmol), diazo coupling partners 2c (0.32
mmol), catalyst 1 (9.80 mg, 5 mol%, 0.016 mmol) and K (2.49 mg, 20 mol%, 0.064 mmol) were
taken in a 25 mL pressure tube, subsequently DMSO (1 mL) was added in to the reaction
mixture inside the glovebox. Subsequently, the reaction mixture was allowed to stir for 12 h at
room temperature. After completion of the reaction, product was extracted in dichloromethane
(25 mL) and dried over anhydrous sodium sulphate. The solvent was removed under reduced
pressure and product 6 was purified by column chromatography on silica gel (100-200 mesh)
1
using hexane/EtOAc. Colorless solid, Yield: 0.0498 g (56%, 0.179 mmol). H NMR (400 MHz,
CDCl3, 298 K) δ 8.14 (d, J = 8.5 Hz, 2H), 7.26 (s, 2H), 1.65 – 1.60 (m, 5H), 1.45 – 1.42 (m, 1H),
1.24 (s, 6H), 1.0 (s, 6H) ppm; 13C NMR (100 MHz, CDCl3, 298 K) δ 168.7, 141.2, 125.5, 114.2,
60.8, 39.6, 32.2, 20.4, 16.8 ppm.
General Procedure of Sample Preparation for Magnetic Study. Catalyst 1(100 mg, 0.163
mmol) and K (19.10 mg, 0.490 mmol) were added in a 25 mL Schlenk flask, subsequently
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acetonitrile (3 mL) was added in to the reaction mixture inside the nitrogen filled glovebox.
Then, the reaction mixture was allowed to stir for 12 h at room temperature. After solvent
evaporation and after multiple washing with acetonitrile, a dark brown colored solid was
obtained for magnetic study.
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General Procedure of Sample Preparation for EPR Study. A series of reactions was
performed with catalyst 1 (100 mg, 0.163 mmol) by varying the equivalents of K (1, 2, 3
equivalent) with respect to catalyst 1 in a 25 mL Schlenk flask, subsequently acetonitrile (3mL)
was added into the reaction mixture inside a glovebox under N2 atmosphere. The reaction
mixture drastically changes from yellow to brown color. After solvent evaporation, a brown
colored solid was obtained which was taken for EPR measurement.
1
2-(4-chlorophenyl)furan (3a).37 Colorless solid. Yield: 0.0415 g (72%, 0.234 mmol); H NMR
(400 MHz, CDCl3, 298 K): δ 7.60 – 7.58 (m, 2H), 7.46 (m, 1H), 7.36 – 7.33 (m, 2H), 6.64 – 6.63
(m, 1H), 6.47 – 6.46 (m, 1H) ppm; 13C NMR (100 MHz, CDCl3, 298 K): δ 152.9, 142.3, 132.9,
129.3, 128.8, 125.0, 111.7, 105.4 ppm.
1
2-(4-Bromophenyl)furan (3b).38 Colorless solid. Yield: 0.0478 g (67%, 0.214 mmol); H NMR
(400 MHz, CDCl3, 298 K): δ 7.55 – 7.47 (m, 5H), 6.65 (d, J = 3.3 Hz, 1H), 6.48 – 6.47 (m, 1H)
13
ppm; C NMR (100 MHz, CDCl3, 298 K): δ 152.9, 142.3, 131.7, 129.7, 125.2, 121.0, 111.7,
105.5 ppm.
1
2-(4-nitrophenyl)furan (3c).38 Yellow solid. Yield: 0.0496 g (82%, 0.262 mmol); H NMR (500
MHz, CDCl3, 298 K): δ 8.24 – 8.22 (m, 2H), 7.80 – 7.76 (m, 2H), 7.56 (d, J = 1.72 Hz, 1H), 6.87
– 6.86 (m, 1H), 6.55 – 6.54 (m, 1H) ppm; 13C NMR (125 MHz, CDCl3, 298 K): δ 151.7, 146.4,
144.1, 136.4, 124.2, 123.9, 112.4, 108.9 ppm.
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2-(4-Cyanophenyl)furan (3d).20 Brown solid. Yield: 0.0433 g (80%, 0.256 mm0l); 1H NMR (400
MHz, CDCl3, 298 K) δ 7.74 (d, J = 8.3 Hz, 2H), 7.65 (d, J = 8.6 Hz, 2H), 7.53 (d, J = 1.5 Hz,
1H), 6.81 (d, J = 3.4 Hz, 1H), 6.52 (dd, J1 = 3.7 Hz, J2 = 2.1 Hz, 1H) ppm; 13C NMR (100 MHz,
CDCl3, 298 K) δ (ppm) 152.0, 143.6, 134.6, 132.5, 124.0, 119.0, 112.2, 110.2, 108.1.
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2-Phenylfuran (3e).20 Pale yellow solid. Yield: 0.0314 g (68%, 0.217 mmol); H NMR (400
MHz, CDCl3, 298 K) δ 7.70 (d, J = 7.9 Hz, 2H), 7.48 (m, 1H), 7.41 (t, J = 7.8 Hz, 2H), 7.30-7.25
(m, 1H), 6.69 (d, J = 3.2 Hz, 1H), 6.50-6.48 (m, 1H) ppm; 13C NMR (100 MHz, CDCl3, 298 K) δ
154.0, 142.0, 131.0, 128.6, 127.3, 123.7, 111.6, 105.0 ppm.
2-(4-Methoxyphenyl)furan (3f).39 Colorless solid. Yield: 0.0384 g (69%, 0.221 mmol); H NMR
1
(400 MHz, CDCl3, 298 K): δ 7.60 (d, J = 8.7 Hz, 2H), 7.42 (s, 1H), 6.92 (d, J = 8.8 Hz, 2H), 6.51
(d, J = 3.2 Hz, 1H), 6.45 – 6.44 (m, 1H), 3.83 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3, 298 K):
δ 159.0, 154.2, 141.3, 125.2, 124.0, 114.1, 111.5, 103.3, 55.31 ppm.
1
2-(p-tolyl)-furan (3g).39 Colorless solid. Yield: 0.0329 g (65%, 0.208 mmol); H NMR (400
MHz, CDCl3, 298 K) δ 7.57 (d, J = 8.1 Hz, 2H), 7.45 (d, J = 1.6 Hz, 1H), 7.19 (d, J = 8.0 Hz,
13
2H), 6.60 (d, J = 3.3 Hz, 1H), 6.47 – 6.46 (m, 1H), 2.37 (s, 3H) ppm; C NMR (100 MHz,
CDCl3, 298 K): δ 154.2, 141.6, 137.1, 129.3, 128.2, 123.7, 111.5, 104.1, 21.2 ppm.
2-(4-Chlorophenyl)thiophene (4a).37 Colorless solid. Yield: 0.0436 g (70%, 0.224 mmol); H
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NMR (400 MHz, CDCl3, 298 K) δ 7.55-7.53 (m, 2H), 7.36-7.34 (m, 2H), 7.30-7.29 (m, 2H),
7.10-7.07 (m, 1H) ppm; 13C NMR (100 MHz, CDCl3, 298 K) δ (ppm) 143.0, 133.1, 132.9, 128.9,
128.1, 127.0, 125.1, 123.4 ppm.
2-(4-Bromophenyl)thiophene (4b).39 Colorless solid. Yield: 0.0482 g (63%, 0.201 mmol); 1H NMR
(400 MHz, CDCl3, 298 K) δ 7.51-7.47 (m, 4H), 7.29 (d, J = 4.1 Hz, 2H), 7.09-7.06 (m, 1H) ppm;
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13C NMR (100 MHz, CDCl3, 298 K) δ (ppm) 143.1, 133.1, 131.9, 128.1, 127.4, 125.2, 123.4,
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121.2 ppm.
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2-(4-nitrophenyl)thiophene (4c).39 Yellow solid. Yield: 0.0486 g (74%, 0.237 mmol); H NMR
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(400 MHz, CDCl3, 298 K): δ 8.28 – 8.22 (m, 2H), 7.76 – 7.73 (m, 2H), 7.49 – 7.43 (m, 2H), 7.16
– 7.14 (m, 1H) ppm; 13C NMR (100 MHz, CDCl3, 298 K): δ 146.63, 141.6, 140.6, 128.6, 127.6,
126.0, 125.7, 124.4 ppm.
2-(4-Cyanophenyl)thiophene (4d).40 Yellow solid. Yield: 0.0468 g (79%, 0.253 mmol); 1H NMR
(400 MHz, CDCl3, 298 K) δ 7.73-7.64 (m, 4H), 7.43-7.40 (m, 2H), 7.14-7.12 (m, 1H) ppm; 13C
NMR (125 MHz, CDCl3, 298 K) δ (ppm) 142.0, 138.6, 132.7, 128.5, 127.0, 126.1, 125.1, 119.0,
110.5 ppm.
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2-Phenylthiophene (4e).40 Colorless solid. Yield: 0.0323 g (63%, 0.201 mmol); H NMR (400
MHz, CDCl3, 298 K) δ 7.63 – 7.59 (m, 2H), 7.40-7.36 (m, 2H), 7.32-7.28 (m, 2H), 7.26 – 7.25
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(m, 1H), 7.10-7.07 (m, 1H) ppm; C NMR (100 MHz, CDCl3, 298 K): δ 144.4, 134.4, 128.8,
128.0, 127.4, 125.9, 124.7, 123.1 ppm.
2-(4-Methoxyphenyl)thiophene (4f).36 Pale yellow solid, Yield: 0.0377 g (62%, 0.198 mmol); 1H
NMR (400 MHz, CDCl3, 298 K) δ 7.60 (d, J = 8.8 Hz, 2H), 7.28-7.26 (m, 2H), 7.12-7.10 (m,
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1H), 6.99 (d, J = 8.7 Hz, 2H), 3.90 (s, 3H) ppm; C NMR (125 MHz, CDCl3, 298 K) δ 159.1,
144.3, 127.9, 127.3, 127.2, 123.8, 122.0, 114.2, 55.3 ppm.
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2-(2-Iodo-phenyl)thiophene (4g).41 Colorless oil, Yield: 0.063 g (69%, 0.220 mmol); H NMR
(400 MHz, CDCl3, 298 K) δ 7.97–7.53 (m, 1H), 7.44–7.36 (m, 3H), 7.18–7.17 (m, 1H), 7.11–
7.10 (m, 1H), 7.05–7.02 (m, 1H) ppm; 13C NMR (100 MHz, CDCl3, 298 K) 144.9, 139.9, 139.3,
131.2, 129.3, 128.0, 127.6, 126.7, 125.8, 99.5 ppm.
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4'-Chloro-2,4,6-trimethylbiphenyl (5a).42 Yellow oil, Yield: 0.0406 g (55%, 0.176 mmol); H
NMR (400 MHz, CDCl3, 298 K): δ 7.39 (d, J = 8.3 Hz, 2H), 7.07 (d, J = 8.3 Hz, 2H), 6.94 (s,
2H), 2.33 (s, 3H), 2.00 (s, 6H) ppm; 13C NMR (100 MHz, CDCl3, 298 K): δ 139.4, 137.7, 136.9,
135.8, 132.4, 130.7, 128.6, 128.1, 21.00, 20.6 ppm.
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2′-Bromo-2,4,6-trimethylbiphenyl (5b).43 Colorless liquid. Yield: 0.0493 g (56%, 0.179 mmol);
1H NMR (400 MHz, CDCl3, 298 K): δ 7.67 (d, J = 8.1 Hz, 1H), 7.38 (t, J = 7.5 Hz, 1H), 7.22 (t, J
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= 6.7 Hz, 1H), 7.14 (d, J = 7.2 Hz, 1H), 6.95 (s, 2H), 2.35 (s, 3H), 1.96 (s, 6H) ppm; C NMR
(100 MHz, CDCl3, 298 K): δ 141.8, 137.9, 137.2, 135.7, 132.6, 130.8, 128.5, 127.9, 127.6,
124.1, 21.1, 20.1 ppm.
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2′-Fluoro-2,4,6-trimethylbiphenyl (5c).44 Colorless oil, Yield: 0.0411 g (60%, 0.192 mmol); H
NMR (400 MHz, CDCl3, 298 K): δ 7.38 – 7.33 (m, 1H), 7.23 – 7.21 (m, 1H), 7.20 – 7.13 (m,
2H), 6.98 (s, 2H), 2.36 (s, 3H), 2.05 (s, 6H) ppm; 13C NMR (100 MHz, CDCl3, 298 K): δ 159.6
(d, JC–F = 242 Hz), 137.3, 136.5, 131.6, 128.8 (d, JC–F = 7.8 Hz), 128.1, 124.1, 115.6 (d, JC–F
22.4 Hz), 21.1, 20.3 ppm; 19F NMR (CDCl3, 376 MHz, 298 K): δ 115.0 – 115.03 (m) ppm.
=
1
2′-Iodo-2,4,6-trimethylbiphenyl (5d).45 Colorless solid, Yield: 0.0659 g (64%, 0.204 mmol); H
NMR (400 MHz, CDCl3, 298 K): δ 7.95 –7.93 (m, 1H), 7.42 –7.39 (m, 1H), 7.14 –7.12 (m, 1H),
7.04 –7.01(m, 1H), 6.95 (s, 2H), 2.35(s, 3H), 1.93(s, 6H); 13C NMR (100 MHz, CDCl3, 298 K): δ
146.0, 141.1, 139.0, 137.3, 135.5, 129.7, 128.5, 128.4, 128.1, 100.8, 21.2, 20.3 ppm.
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4′-Nitro-2,4,6-trimethylbiphenyl (5e).21 Yellow solid, Yield: 0.0471 g (61%, 0.195 mmol); H
NMR (400 MHz, CDCl3, 298 K): δ 8.29 (d, J = 8.8 Hz, 2H), 7.33 (d, J = 8.8 Hz, 2H), 6.97 (s,
2H), 2.34 (s, 3H), 1.99 (s, 6H) ppm; 13C NMR (100 MHz, CDCl3, 298 K): δ 148.5, 146.8, 137.6,
136.7, 135.2, 130.4, 128.3, 123.7, 21.0, 20.5 ppm.
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4′-Methoxy-2,4,6-trimethylbiphenyl (5f).46 Yellow oil, Yield: 0.0376 g (52%, 0.166 mmol); H
NMR (400 MHz, CDCl3, 298 K): δ 7.05 (d, J = 8.9 Hz, 2H), 6.95 (d, J = 8.6 Hz, 2H), 6.93 (s,
2H), 3.85 (s, 3H), 2.32 (s, 3H), 2.01 (s, 6H) ppm; 13C NMR (100 MHz, CDCl3, 298 K): δ 158.1,
138.8, 136.4, 136.0, 133.2, 130.30, 128.0, 113.7, 55.2, 21.0, 20.8 ppm.
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4′-Methyl-2,4,6-trimethylbiphenyl (5g).21 Colorless oil, Yield: 0.0283 g (42%, 0.134 mmol); H
NMR (400 MHz, CDCl3, 298 K): δ 7.22 (d, J = 7.7 Hz, 2H), 7.02 (d, J = 8.0 Hz, 2H), 6.93 (s,
2H), 2.40 (s, 3H), 2.32 (s, 3H), 2.00 (s, 6H) ppm; 13C NMR (100 MHz, CDCl3, 298 K): δ 139.0,
138.0, 136.3, 136.1, 136.0, 129.1, 129.0, 128.0, 21.2, 21.0, 20.7 ppm.
Computational Methods
All the theoretical calculations performed to interpret the experimental observation were
performed using Gaussian09 quantum chemistry package.47 All the Density Functional Theory
(DFT) calculations were performed at B3LYP/6-31+g* level of theory.48 Unrestricted
calculations were performed for open shell molecules. Frequency calculations were performed to
confirm the optimization to a minimum for all structures.
ASSOCIATED CONTENT
The Supporting Information is available free of charge on the ACS Publications website, at
DOI:…………
General experimental procedures, computational details and spectroscopic data of all compounds
(PDF).
AUTHOR INFORMATION
Corresponding Authors
*E-mail: swadhin.mandal@iiserkol.ac.in
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Author Contributions
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The manuscript was written through contributions of all authors. All authors have given approval
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to the final version of the manuscript.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank SERB, India (Grant No. (EMR/2017/000772). PKV thanks UGC, India for a research
fellowship. JA, AJ, and AAM thank IISER-Kolkata for fellowship. BKS thanks SERB (DST) for
providing NPDF Fellowship (PDF/2016/000213) for financial support.
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