An unprecedented oxidative intermolecular homo coupling reaction between two sp3C–sp3C centers under metal-free condition

Article abstract of DOI:10.1016/j.tetlet.2016.06.092

An unprecedented formation of benzylic sp³C–sp³C coupled dibenzylic products has been illustrated. The reactions have been carried out in the presence of three oxidizing reagents, i.e., diacetoxy-iodobenzene (IBDA), N-fluorobenzenesulfonimide (NFSI), and pyridine (Py) using toluene derivatives.

Full text of DOI:10.1016/j.tetlet.2016.06.092

Tetrahedron Letters 57 (2016) 3476–3480
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
An unprecedented oxidative intermolecular homo coupling reaction
between two sp3C–sp3C centers under metal-free condition
,1
⇑
Santosh K. Sahoo
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
a r t i c l e i n f o
a b s t r a c t
An unprecedented formation of benzylic sp3C–sp3C coupled dibenzylic products has been illustrated. The
reactions have been carried out in the presence of three oxidizing reagents, i.e., diacetoxy-iodobenzene
(IBDA), N-fluorobenzenesulfonimide (NFSI), and pyridine (Py) using toluene derivatives.
Ó 2016 Elsevier Ltd. All rights reserved.
Article history:
Received 28 May 2016
Revised 19 June 2016
Accepted 21 June 2016
Available online 27 June 2016
Keywords:
Metal free
CAC coupling
Dibenzylic
Alkanes
CDC reaction
Applications of radical chemistry in the development of modern
organic synthesis are well documented in the literature.1 At pre-
sent times, most of the radical reactions are carried out utilizing
robust toxic metal complex such as tin hydride reagents and toxic
radical initiators like peroxides.1a Major setback of such reactions
is the formation of large amounts of unexpected residual metal
catalysts and the inherent difficulty in purification of the desired
products. These disadvantages greatly restrict the application of
radical chemistry in the areas of pharmaceutical industry.1b–d
Thus; it poses a challenge to update these traditional methods to
greener and sustainable organic synthesis. As a result of its easy
availability, environmental benignity, and high reactivity,
hypervalent iodine(III) reagent initiated radical processes under
metal-free conditions are undoubtedly the ideal and promising
route to address the aforementioned challenges. Under metal-
free conditions, the utilization of hypervalent iodine(III) reagents
for CAN, CAO bond formations have been thoroughly
investigated.1b Of late, hypervalent iodine(III) reagents have been
found to show remarkable relativities for CAC bond formation1e,f
following which, bi-aryl compounds and heterocycles could be
obtained successfully.
used as key intermediates both for the synthesis of dyes, paints,
resins, polymers, agrochemicals, and pharmaceuticals, and for the
preparation of a number of natural products as stilbenzyl or di-
benzyl derivatives.2,3 However, the wide use of toxic transition
metal catalysts/reagents in low oxidation state for such homo cou-
pling reactions imposes considerable environmental prob-
lem.4,5a,4m Though metal free sp3C–sp3C homo coupling reactions
using toluene as substrate is reported in literature,5b there are no
reports of a generalized reaction for the same. Thus, a new, metal
free, sp3C–sp3C homo coupling methodology for the synthesis of
di-benzyl compounds using toluene derivatives has been
developed herein.
The regioselective functionalization of pyridines, such as the
C2-selective addition of highly reactive nucleophiles and C2-selec-
tive deprotonation with strong base followed by reactions with
electrophiles, has long been investigated by many researchers.6
In literature, lewis basic sp2 nitrogen atom in the pyridine ring
was utilized as the directing group, and CAH bond activation by
transition-metal catalysts occurred selectively at the C2 position.6
However, reports of C3- or C4-selective catalytic CAH bond func-
tionalization of pyridines without an additional directing group
are limited.6,7 In 2011 Yu et al. reported the first Pd-catalyzed C-
3 selective CAH olefination enabled by a bidentate ligand that
weakens the coordination of the Pd catalyst with the pyridyl N
atom through the trans-effect. Two major challenging factors
addressed in this report were that, low reactivity of pyridyl CAH
due to the poor electron density of the pyridyl ring and the strong
coordination of the pyridine N atom with the Pd(II) center which
Compounds containing the di-benzyl moiety as the core struc-
ture constitute an interesting group of molecules which have been
⇑
Tel.: +91 22 2572 4163.
E-mail address: santoshsahoo.iit@gmail.com
Present address: Post-doctoral fellow, Department of Applied Chemistry, National
1
Chiao Tung University, 1001 University Road, Hsin-Chu 30010, Taiwan.
http://dx.doi.org/10.1016/j.tetlet.2016.06.092
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

S. K. Sahoo / Tetrahedron Letters 57 (2016) 3476–3480
3477
Table 1
prevents the catalyst from interacting with pyridine C(3)AH and
Control experiment for the exact role of reagentsa,b,c
C(4)AH bonds. In 2013, Kanai and co-workers reported a challeng-
ing C4- selective direct alkylation protocol of pyridine substrates
through hydrometalation/nucleophilic addition/rearomatization
sequence catalyzed by Co(II) and hydride sources.6a In addition,
hypervalent iodine mediated oxidative amination to arene,8
alkene,9 and alkyne10 based substrates have been reported. In line
with these observations, a hypothesis to apply pyridine based
molecules for C-4 based amination reaction by using metal free
hypervalent oxidizing reagent has been envisaged (Scheme 1a).
To test the hypothesis on the regioselective C-4 amination of
pyridine scaffolds, N-fluorobenzenesulfonimide (NFSI) and iodo
benzene diacetate (IBDA) were initially employed to react with
Pyridine (Py) at 100 °C using various solvents like DCE, Dioxane,
DMSO, DMF, THF, EtOH, and toluene. Unfortunately, under these
conditions, the desired product was not observed. However grati-
fyingly, in the presence of toluene solvent, an unexpected diben-
zylic product 1a was observed instead of the expected C-4
aminated product 10a (Scheme 1). Following this observation, a
thorough search for the importance of dibenzylic product resulted
in many more of its applications and synthetic utility under robust
and toxic metal catalyzed conditions11 including a side dibenzylic
product reported in metal catalyzed and unfriendly peroxide con-
dition.12 Delightfully, this is an interesting methodology to observe
homo coupling sp3C–sp3C bond formation using different oxidizing
reagents (Scheme 1b).
CH3
CH3
IBDA, NFSI
Py, 100 0C, 15 h
H3C
H3C
(2)
(2a)
Entry
NFSI (mmol)
IBDA (mmol)
Py (mmol)
NMR yield (%)
1
2
3
4
5
6
7
1
1
1
X
1
X
X
2
2
X
1
X
1
X
1
X
1
1
X
X
1
72
02
01
01
01
01
01
X = Not used.
Bold refer to better yield compare to other method.
a
p-Xylene (2) 30 mmol, 100 °C, 15 h.
Yield was calculated comparing the amount of NFSI used.
Tetra chloro ethane (TCE) internal standard for NMR yield.
b
c
CH3
2% Grubbs 1
DCM, 40 oC
Overnight H3C
H3C
(2')
(2'a) 30% yield
The reaction conditions were then implemented to p-xylene (2)
substrate to generalize whether the reaction promotes the benzylic
homo coupling dimerized product. Gratifyingly, almost major
amount of bibenzylic product 2a was observed along with unchar-
acterized trace amount of side product (Table 1, entry 1). For cross
verification, the formation of bibenzylic product was further con-
firmed by the reduction of (E)-1,2 dip-tolylethene 20a substrate
H2 Baloon
Pd on activated
carbon 5 wt%
EtOAc (0.5 mL), RT
Over night
CH3
(Scheme 2).13
.
H3C
A controlled experiment was performed to ascertain the exact
role of all NFSI, IBDA, and Py reagents on this reaction condition
which confirmed that all the three abovementioned reagents are
playing vital role in this reaction (Table 1). The reaction was fur-
ther standardized by varying the stoichiometry of the three
reagents. It is found that combination of 1 mmol NFSI, 2 mmol
IBDA, and 1 mmol Py treated with p-xylene (2) gave better yield
(NMR yield 72%, isolated yield 70%) compared to other conditions
(see Supporting information (SI, Table S2.1)). It is worth mention-
ing that the yield of benzylic dimerized product (2a) is calculated
compared to the amount of NFSI as oxidizing reagent used in the
corresponding reaction condition. In this reaction condition, excess
amount of p-xylene (2) was used (for the role of both solvent and
substrate); hence NFSI was used as a limiting reagent. The reaction
condition was tested with various solvents (see SI Table S2.2) to
decrease the amount of p-xylene (2). Unfortunately, there was no
improvement in the yield of the product. To find out the role of
IBDA, the reaction was tested with different hypervalent reagents
and oxidants (see SI Table S2.3), and it was concluded that IBDA
(2a) 92% yield
Scheme 2. Indirect synthesis of dibenzylic product 2a.
is a better oxidant under the present reaction condition. Further,
the reaction was treated with various fluorinating, halogenated,
and aminating reagents (see SI Table S2.4) and it was observed that
NFSI is a more efficient reagent for the formation of the corre-
sponding dimerized product 2a. Further, Py proved to be a better
supporting reagent for the formation of dimerized product 2a com-
pared to other pyridine derivative reagents (see SI Table S2.5).
Subsequently, it was found that 100 °C is the optimum temper-
ature for this reaction (see SI Table S2.6). Likewise, the reaction
was optimized with different time interval and found that 15 h is
required (see SI Table S2.7) to furnish better yield of the product
2a. Another noteworthy observation was that only 30 equiv
(15 equiv of dimer product compare to NFSI) of the substrate 2a
was required to produce the expected dimerized product 2a in
quantitative yield (see SI Table S2.8).
Having acquired this decent optimization condition, the sub-
strate scope was tested with a variety of toluene derivatives
(Table 2). The substrates m-xylene (3) and o-xylene (4) also gave
their corresponding homo coupled sp3C–sp3C dimerized products
3a and 4a in good yields. An interesting observation in case of sub-
strate mesitylene (5) was the coupling occurred at only one methyl
position to furnish the dimerized product 5a while the other two
methyl substituents remained intact. The substrate 4-tert-butyl
toluene (6) showed well tolerance under this condition giving
moderate yield of the corresponding homo coupled dimerized
product 6a. Most importantly, the compound 4-methylanisole (7)
having two possible positions for radical formation, one at
IBDA
OAc
OAc
a)
b)
+
NFSI
NR2
1'a
IBDA = Ph
I
Toluene
(solvent)
N
(Py)
N
(
)
C-4 amination
SO2Ph
SO2Ph
CH3
NFSI =
F
N
IBDA, NFSI
Py
1a
(
)
1
(
)
Scheme 1. (a) Working hypothesis (sp2CAN bond formation). (b) Reaction obser-
vation (metal free sp3C–sp3C bond formation).

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S. K. Sahoo / Tetrahedron Letters 57 (2016) 3476–3480
OMe-substituted side and another at Me-substituted side, gave
dimerized product 7a corresponding to dimerization at only Me-
position. The chlorine containing toluene derivative substrates (8,
9, 10, and 11) provided their respective homo coupled dimerized
products (8a, 9a, 10a, and 11a) in moderate to good yields. In addi-
tion, the fluorine containing toluene substituents (12, 13 and 14)
furnished their sp3C–sp3C homo coupling products in decent
yields. It is worth noting that; methyl group containing naph-
thalene substituent (15) provided the respective dimerized pro-
duct (15a) in moderate yield. The diarene group possessing
methane substrates (16 and 17) gave their desired products (16a
and 17a) in moderate yields. Unfortunately, substrates like ethyl
benzene, cumene, cyclohexane, linear, and branched alkanes did
not yield their expected sp3C–sp3C homo coupling products under
the similar conditions. Notably, the solid substrates like fluorene
(18), xanthene (19), and dibenzosuberane (20) yielded the
Figure 1. ORTEP diagram of 9-(9H-fluorene-9-yl)-9H-fluorene (18a) (50% ellipsoid).
Table 2
Substrate scope for the formation dimerized producta,b,c
H
H
H
R
IBDA, NFSI
Py, 100 oC
R
R
R = Functional group
(1−20)
(1a−20a)
CH3
H3C
CH3
H3C
1a
2a
3a
, 59%
, 63%
, 70%
CH3
CH3
CH3
CH3
H3C
H3C
CH3
H3C
H3C
CH3
CH3
CH3
4a, 75%
5a, 64%
8a, 53%
6a, 41%
9a, 56%
Cl
OCH3
Cl
H3CO
Cl
Cl
7a, 46%
F
Cl
Cl
H3C
CH3
Cl
Cl
F
10a
13a
11a
14a
12a
, 70%
, 51%
, 58%
, 61%
, 65%
, 26%
F
F
F
F
15a
F
F
F
F
16a
17a
, 23%
, 27%
O
b
O
18a, 25%
19a, 28%
20a
, 32%
a
b
c
Reaction condition: substrate 30 mmol, IBDA 2 mmol, Py 1 mmol, NFSI 1 mmol, 100 °C, 15 h.
Substrate 1 mmol, IBDA 2 mmol, Py 10 mmol, NFSI 1 mmol, 100 °C, 15 h.
Isolated yield calculated based on the amount of NFSI used.

S. K. Sahoo / Tetrahedron Letters 57 (2016) 3476–3480
3479
H
H
H
H
O
O
OAc
OAc
OAc
H3C
S
Ph
Ph
-HF
I
H
2
+
+
AcO
I
+
F
N
N(SO2Ph)2
SET
S
H3C
r.d.s
O
O
(A)
(IBDA)
(B)
(NFSI)
Formation of pyridinium
salt, helping for oxidation
+
HF
N
(Py)
N
.
HF
(C)
OAc
H
H
I
CH3
N(SO2Ph)2
Oxidative
dimerization
CH2
Thermodynamic Control
H3C
H3C
Kinetic Control
AcO
SET
H3C
AcO
(B)
C-N bond
formation
Observed product
(2a)
+
AcOH
(PhSO2)2NH +
+
PhI
N(SO2Ph)2
Side products, observed in GC MS
H3C
One of the expected product
(2b)
Scheme 3. Proposed mechanism for bibenzylic product.
dimerized products 18a, 19a, and 20a respectively. Slight variation
Supplementary data
in the reaction condition was applied to solid substrates 18–20
wherein an excess amount of pyridine used as supportive reagent
as well solvent to promote the reactions efficiently. The formation
of dimerized product 18a was further confirmed by single X-ray
crystallography which is depicted in the Figure 1 (See Table 2).
Based on these observations, it is predicted that benzylic sp3
hydrogen abstraction might be involved in the rate-limiting step
(Scheme 3).14 A radical scavenger study (see SI Table S2.9) and con-
trol experiment study (Table 1) revealed that the reagents IBDA
and NFSI react together forming very active oxidative intermediate
A in the presence of substrate 2. The benzyl radical B is formed by a
SET process.14 The intermediate B is over oxidized to form the
kinetically favored sp3C–sp3C homo coupling dimerize product
2a instead of the thermodynamic controlled sp3C–N coupling pro-
duct 2b. Perhaps, the role of pyridine in forming pyridium salt C,
probably helps promote the oxidative homo coupling product for-
mation and restrict acidification of the reaction medium. However,
study of the mechanism of this transformation is currently
underway.
In summary, a well-defined protocol has been demonstrated for
the effective synthesis of the homo coupling reactions of toluene
derivatives, in the presence of IBDA, NFSI, and Py reagents. The oxi-
dation of toluene derivatives resulted in moderate to good yields of
the corresponding dibenzylic products. Functional groups on the
aromatic ring such as t-butyl, methoxy, and diphenyl methane
groups tolerated well under the reaction conditions. Attributes of
this new methodology are in overcoming the use of toxic metal
catalyst and lachrymatory benzylic halide substrates which usually
used to furnish sp3C–sp3C homo coupled dibenzylic products. The
present protocol is metal free, atom economic, and environmen-
tally friendly compared to metal catalyzed reactions. Potential
applications of these highly efficient homo coupling reactions in
material synthesis are being explored in industries.
Supplementary data (optimization details, reaction procedure,
characterization data and 1H, 13C NMR spectra) associated with
this article can be found, in the online version, at http://dx.doi.
org/10.1016/j.tetlet.2016.06.092.
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