Manganese-salen catalyzed oxidative benzylic chlorination

Article abstract of DOI:10.1007/s12039-018-1511-7

Abstract: Metalloporphyrins are well-known to serve as the model for mimicking reactivities exhibited by cytochrome P450 hydroxylase. Recent developments on selective C–H halogenation using Mn-porphyrins provided the way for understanding the reactivity as well as mechanism of different halogenase enzymes. In this report, we demonstrated a method for benzylic C–H chlorination using easily prepared Mn(salen) complex as the catalyst, which shows a complementary reactivity of Mn-porphyrins. Here, NaOCl has been used as a chlorinating source as well as the oxidant. Efforts towards understanding the mechanism suggested the formation of the high-valent Mn(V)=O species which is believed to be the key intermediate to conduct this transformation. Graphical abstract: SYNOPSIS Mn(salen)-catalyzed selective benzylic chlorination protocol has been developed using aqueous NaOCl solution. Reactions proceeded efficiently at room temperature and displayed good functional group tolerance. The mechanistic investigation demonstrated that Mn (V) = O species is likely to be the key intermediate which is responsible to generate benzylic radical. EPR and ESI-MS studies confirmed the in situ formation of Mn(IV)-species.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s12039-018-1511-7

J. Chem. Sci. (2018) 130:88
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1511-7
REGULAR ARTICLE
Special Issue on Modern Trends in Inorganic Chemistry
Manganese-salen catalyzed oxidative benzylic chlorination
SHEULI SASMAL, SUJOY RANA, GOUTAM KUMAR LAHIRI∗ and DEBABRATA MAITI∗
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra 400 076, India
E-mail: lahiri@chem.iitb.ac.in; dmaiti@chem.iitb.ac.in
MS received 5 April 2018; revised 28 May 2018; accepted 8 June 2018
Abstract. Metalloporphyrins are well-known to serve as the model for mimicking reactivities exhibited by
cytochrome P450 hydroxylase. Recent developments on selective C–H halogenation using Mn-porphyrins
provided the way for understanding the reactivity as well as mechanism of different halogenase enzymes. In
this report, we demonstrated a method for benzylic C–H chlorination using easily prepared Mn(salen) complex
as the catalyst, which shows a complementary reactivity of Mn-porphyrins. Here, NaOCl has been used as a
chlorinating source as well as the oxidant. Efforts towards understanding the mechanism suggested the formation
of the high-valent Mn(V)=O species which is believed to be the key intermediate to conduct this transformation.
Keywords. High-valent manganese; salen; hypochlorite; benzylic chlorination.
1. Introduction
the various known metalloporphyrins, Mn-porphyrins
have been known for its incredible reactivity towards
the oxidation of both saturated and unsaturated hydro-
carbons including oxidation of complex molecules.12
Recently, Mn-porphyrin complexes had also shown
its ability to catalyze carbon-halogen bond formation
(Scheme 1a).13 Pertaining to chlorination, in 2010, J.
T. Groves and co-workers demonstrated Mn-porphyrin
mediated C–H bond chlorination of simple hydrocar-
bons which depicted only single instance of benzylic
chlorination.13a Herein, we are interested to investigate
the less explored domain of benzylic sp3 C–H chlo-
rination using a manganese salen complex as catalyst
(Scheme 1b). This catalyst was mainly developed by
Jacobson/Katsuki for enantioselective epoxidations.14 It
has also been utilized effectively towards other carbon-
heteroatom bond formations.15 The attractive features
of these catalysts lie in their easy synthesis and facile
structural modifications.
Chlorinated benzylic compounds are very impor-
tant class of substrates since their chloromethyl group
can be easily converted to other group like hydrox-
ymethyl, cyanomethyl, etc.16 Further, various bioactive
molecules, such as ibuprofen, vitamin E analogues like
steroids, terpenoids including various amino acids con-
tain benzylic position that are amenable for late-stage
diversification (Figure 1).
Naturehasdevelopedseveralfascinatingenzymaticpath
to incorporate hydroxyl or halogen group oxidizing
specific C–H bond of bioactive molecules.1 In this per-
spective, heme-dependent enzymes such as cytochrome
P450, haloperoxidase are known to accomplish hydrox-
ylation and halogenation in natural molecules2 where
a high-valent iron-oxo species is believed to be the
active intermediate.1,3 In the realm of practical chem-
istry, the feasibility of halogenation viz. chlorination
of benzylic substrates is a bit challenging because of
the competitive ring halogenation or over-halogenation.
The reported procedures for benzylic sp3 C–H chlorina-
tion uses reagents such as free Cl2,4 PCl5,5 aquaregia,6
Et4NCl,7 benzyltrimethylammonium tetrachloroiodate
BTMAICl4,8 nano-Ag/AgCl,9 t BuOCl,10 etc., which
are explosive, toxic as well as corrosive. Further, most of
the cases require irradiation or heating resulting in poor
chemoselectivity and regioselectivity. Additionally, use
of nano-Ag/AgCl,9 t BuOCl10 for chlorination purpose
produced oxygenated side product in major quantity. To
address these issues, researchers designed various met-
alloporphyrin as biocatalyst which successfully served
as a model compound to mimic the reactivities of
cytochrome P450 over the past few decades.11 Among
*
For correspondence
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-018-1511-7) contains
supplementary material, which is available to authorized users.

88 Page 2 of 9
J. Chem. Sci. (2018) 130:88
per million (ppm) and measured relative to the signals for
residual chloroform (7.26 ppm) in a deuterated solvent, unless
otherwise stated. All 13C NMR spectra were reported in ppm
relative to CDCl3 (77.23 ppm), unless otherwise stated, and
all were obtained with 1H decoupling. All the products were
analyzed by GC-MS. GC-MS was performed on a Thermo
Scientific ISQ QD Mass Spectrometer attached with Thermo
Scientific TRACE 1300 gas chromatograph using an HP-5ms
capillary column (30 m × 0.25 mm × 0.25 µm, J&W Scien-
tific) with helium as the carrier gas All GC analyses were
performed on an Agilent 7890A GC system with a flame ion-
ization detector using a J&W DB-1 column (10 m × 0.1 mm,
i.d.). UV-Vis studies were performed in Agilent 8453 diode
array based UV-Vis Spectrophotometer. ESI-MS spectra were
performed in Bruker QTOF ESI-MS instrument. First order
rate constants (k1) were calculated based on non-linear expo-
nential fit in OriginPro8 software. Single crystal of complex
A2 was diffracted in Rigaku X-ray single crystal diffractome-
ter. The EPR measurements were carried out with an X-band
(9.5 GHz) Bruker EMX Plus at 100 K.
a
b
2.2 Synthesis of salen ligand (H2Salen)
Scheme 1. (a) Mn-porphyrine catalyzed C–H halogenation.
(b) Mn-salen catalyzed benzylic C–H chlorination.
Derivative of Salisaldehyde (2 equiv.) as solid was added in
portions to absolute ethanol solution of diamine derivative.
The reaction mixture was heated to reflux for an hour under
nitrogen atmosphere and then cooled to room temperature.17
The yellow colored product was then collected by filtration
washed with cold ethanol and dried under vacuum.
2.2a N,N-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclo-
hexanedi-amine: 1H NMR (500 MHz, CDCl3) δ 13.70 (s,
2H), 8.29 (s, 2H), 7.30 (d, J = 2.5 Hz, 2H), 6.98 (d, J =
2.5 Hz, 2H), 5.04–4.91 (m, 2H), 3.35–3.29 (m, 2H), 1.94 (d,
J = 14.5 Hz, 2H), 1.90–1.84 (m, 2H), 1.73 (d, J = 11.0 Hz,
2H), 1.50–1.45(m, 2H), 1.41(s, 18H), 1.23(s, 18H).13CNMR
(101 MHz, CDCl3) δ 166.0, 158.2, 140.0, 136.5, 126.9, 126.2,
118.0, 72.6, 35.1, 34.2, 33.4, 31.6, 29.6, 24.5.
Figure 1. Examples of bioactive molecules containing ben-
zylic C–H bond.
2. Experimental
2.3 Synthesis of Mn(salen) complex (MnI I I
2.1 Materials and instrumentation
(salen)X)17
Unless otherwise stated, all of the reactions were carried H2salen ligand (1.6 mmol) in 10 ml of toluene was added
out at room temperature in a 10 mL screw-capped reac- to a suspension of Mn(OAc)2.4H2O (3 equiv., 5 mmol) in
tion tube under N2 atmosphere. NaOCl was purchased from 20 mL of boiling ethanol. Then the reaction mixture was
Sigma Aldrich. Other necessary Chemicals and solvents were refluxed overnight under nitrogen. After passing a stream
purchased from Sigma Aldrich, Merck, Alfa Aesar and Spec- of air through the reaction mixture. 20 mL saturated aque-
trochem. A gradient elution using petroleum ether and ethyl ous NaCl or KBr solution was added under vigorous stirring
acetate was performed, based on Merck Aluminum TLC and the reaction mixture was allowed to cool at room tem-
sheets (silica gel 60F254). Products were characterized by 1H perature. After addition of 100 mL of toluene and 20 mL
and 13C NMR spectroscopy, gas chromatography (GC) and CH2Cl2, the organic layer was extracted with water and sat-
gas chromatography-mass spectrometry (GC-MS). n-decane urated NaCl solution. Then, it was dried with Na2SO4. Then
was used as standard. NMR spectra were recorded either on the solvent was evaporated under reduced pressure. The prod-
a Bruker 500/400 MHz or on a Varian 400 MHz instrument. uct was dissolved in CH2Cl2 and then collected by filtration
Copiesofthe1H, 13CspectraareincludedinSupportingInfor- as a brown powder and dried under vacuum. The complex
mation. All 1H NMR spectra were reported in units of parts was characterized by X-ray crystallography (A2), ESI-MS

J. Chem. Sci. (2018) 130:88
Page 3 of 9 88
study (m/z = 599.3 for C36H52MnN2O2) (A1/A2) and UV- MHz, CDCl3) δ 7.44 (d, J = 7.5 Hz, 4H), 7.41–7.35 (m,
Vis spectroscopy (λmax = 440 nm and 510 nm) (A1/A2).
2.4 General procedure for benzylic chlorination
4H), 7.32 (q, J = 7.1 Hz, 2H), 6.16 (s, 1H).
2.4g Methyl 2-(4-(1-chloro-2-methylpropyl)phenyl)
propane (4): Compound 4 was obtained from ibuprofen
methyl ester 3, isolated by column chromatography (hex-
NaOCl (0.66 M, 4 mL) was added to a solution of [Mn(Salen)
X] (2 mol%) catalyst, tetrabutylammonium chloride (TBACl,
5 mol%), substrate (2 mmol) in dichloromethane in a 10 mL
sealed vial. Here, TBACl acts as phase transfer catalyst and
NaOCl is oxidant and chlorinating source. This mixture was
stirred smoothly under a nitrogen atmosphere at room tem-
perature and carried out for 12 h. Then the reaction mixture
was extracted with CH2Cl2 and water to remove the catalyst
and unreacted NaOCl. The solution was analyzed by GC/MS.
The yield was calculated with respect to reactant added.
1
ane/ethyl acetate). H NMR (400 MHz, CDCl3) δ 1H NMR
(400 MHz, CDCl3) δ 7.31–7.28 (m, 2H), 7.28–7.25 (m, 2H),
4.62 (d, J = 7.6 Hz, 1H), 3.72 (q, J = 7.2 Hz, 1H), 3.66 (s,
3H), 2.22 (dq, J = 13.5, 6.7 Hz, 1H), 1.49 (d, J = 7.2 Hz,
3H), 1.10 (d, J = 6.6 Hz, 3H), 0.87 (d, J = 6.7 Hz, 3H). 13C
NMR (101 MHz, CDCl3) δ 175.1, 140.3, 140.1, 127.9, 127.6,
70.7, 52.3, 45.3, 36.8, 20.4, 19.7, 18.7. HRMS [ESI, (+) ve]:
calcd. for (C14H19ClNaO2) 277.0962, found 277.0966.
2.4a Benzyl chloride (2b)18: Compound 2b was obtai-
ned from toluene 1b, isolated by column chromatography
2.5 Preparation of oxidized [Mn(salen)]+ complex
for ESI-MS and UV-Vis study
1
(hexane/ether). H NMR (500 MHz, CDCl3) δ 7.40 (t, J =
5.4 Hz, 3H), 7.38–7.31 (m, 2H), 4.61 (s, 2H).
In the CH2Cl2 solution of complex A1/A2 (0.3 × 10−3 M),
50 fold of excess NaOCl was added and shaken vigorously
at room temperature, after 15 s the color of the solution
changed from orange-brown to green. Then, it was imme-
diately subjected to UV-Vis spectroscopy. For ESI-MS study,
to the CH2Cl2 solution of complex A1 (10−3 M), 15 fold of
excess NaOCl was added and shaken vigorously for a minute
at room temperature, then directly used for ESI-MS study at
a different time.
2.4b 1-bromo-4-(chloroethyl)-benzene (2c): Compo-
und 2c was obtained from 1-bromo-4-ethylbenzene 1c, iso-
lated by column chromatography (hexane/ethyl acetate).
1HNMR (400 MHz, CDCl3) δ 7.48 (m, 2H), 7.3 (m, 2H),
5.04 (q, 1H), 1.83 (d, 3H). 13C NMR (101 MHz, CDCl3) δ
142.0, 131.1, 128.4, 122.3, 58.0, 26.6.
2.4c 4-(chloromethyl)-1,1ꢀ-biphenyl (2d): Compound
2d was obtained from 4-methyl-1,1ꢀ-biphenyl 1d, isolated
by column chromatography (hexane/ethyl acetate). 1H NMR
(400 MHz, CDCl3) δ 7.65–7.58 (m, 4H), 7.51–7.45 (m, 4H),
7.38 (ddd, J = 7.3, 3.8, 1.1 Hz, 1H), 4.66 (s, 2H). 13C NMR
(101 MHz, CDCl3) δ 141.6, 140.7, 136.7, 129.3, 129.0, 127.7,
127.7, 127.3, 46.3.
2.6 Kinetic study
A stock solution of complex A2 (1.5×10−4 M) was prepared
in CH2Cl2. To this solution 50 equivalent of excess NaOCl
was added and shaken vigorously at 0 ◦C to get the green color
solution. Then 4 mL of this solution was taken and 1 mmol of
ethylbenzene was added and immediately subjected to time-
dependent UV-Vis study. The spectra were recorded in every
5 s. The rate constant for decay of Mn(V)=O complex was
determined from time trace at λmax, 640 nm which shows an
exponential decay pattern. Kinetic data analysis showed that
this decay followed a pseudo-first order reaction in presence
of excess substrate with decay rate constant k1 = 4.94 ×
2.4d 4ꢀ-methyl-[1,1ꢀ-biphenyl]-2-carbonitrile (2e):
Compound 2e was obtained from 4ꢀ-methyl-[1, 1ꢀ- biphenyl]-
2-carbonitrile 1e, isolated by column chromatography (hex-
1
ane/ethyl acetate) H NMR (400 MHz, CDCl3) δ 7.77 (d,
J = 7.7 Hz, 1H), 7.72–7.61 (m, 2H), 7.59–7.54 (m, 2H), 7.52
(dd, J = 5.3, 3.0 Hz, 2H), 7.46 (td, J = 7.7, 1.1 Hz, 1H),
4.65 (s, 1H).13C NMR (101 MHz, CDCl3) δ 144.9, 138.4,
138.1, 134.0, 133.1, 130.2, 129.4, 129.3, 129.1, 128.0, 126.8,
118.8, 111.4, 45.9.
10−2 s−1
.
2.7 EPR study to detect the formation of MnIV (salen)
complex
2.4e (1-chloromethyl)naphthalene (2f): 19 Compound
2f was obtained from 1-methyl naphthalene 1f, isolated by
column chromatography (hexane/ethyl acetate). 1H NMR
(500 MHz, CDCl3) δ 8.16 (d, J = 8.4 Hz, 1H), 7.88 (dd,
J = 17.0, 8.2 Hz, 2H), 7.61 (t, J = 7.6 Hz, 1H), 7.57–7.52
(m, 2H), 7.47–7.41 (m, 1H), 5.07 (s, 2H).
A solution of MnIII(salen) [complex A1 (10 mM)] in 2.5 mL
of CH2Cl2 was prepared. To this solution, 5 equiv. of PhIO
was added and the solution was stirred for 5 min. When the
colour of the solution changed from brown to dark green, 10
equiv. of ethylbenzene (substrate) was added. The solution
2.4f (Chloromethylene)-dibenzene (2g): Compound was shaken for a few seconds and an aliquot of 250 μL of this
2g was obtained from diphenyl methane 1g, isolated by col- solution was transferred to an EPR tube, frozen at 100 K and
1
umn chromatography (hexane/ethyl acetate) H NMR (500 the EPR spectrum of this solution was recorded.

88 Page 4 of 9
J. Chem. Sci. (2018) 130:88
Table 1. Optimization of Mn-salen catalyzed benzylic C–H monitored by GC-MS and yields were determined by
chlorination.
GC with respect to the substrate. Control reaction in
absence of the Mn(salen) catalyst did not produce any
desired product.
With optimized reaction conditions, we examined the
scope of this methodology (Scheme 2). For toluene (1b)
we got benzyl chloride (2b) with TON 10. Interestingly,
NaOCl
amount
(mL)
DCM
amount
(mL)
wedidnotobserveanychlorinationonthearomaticring.
In the presence of a halide substituent (1c), the reaction
proceeded smoothly to give benzylic chlorinated prod-
uct (2c). The biphenyl system (1d) under the present
reaction conditions afforded the respective product (2d)
with a relatively high TON 16. Even the biphenyl sys-
tem containing electron withdrawing group such as
cyano group (1e) was tolerated to give corresponding
benzylic chlorinated product (2e). Naphthalene system
(1f) also worked well to give the expected chlorinated
product (2f). Diphenylmethane and isobutyl benzene
were chlorinated at benzylic site (2g and 2h) along
with formation of some dimerized products. In case
of α-tetralone, along with benzylic chlorinated product
(2i) some amount of α,β-unsaturated ketone (2i’) was
formed in the ratio 2.5:1.
To demonstrate the synthetic potential of our method,
we applied Mn(salen) chlorination method to a bioac-
tive molecule (protected analogue). Ibuprofen methyl
ester 3, a nonsteroidal anti-inflammatory drug, when
subjected to this method afforded a regioselective ben-
zylic chlorinated product 4 with 40% isolated yield
(Scheme 3).
Entry
Yield (%)[a]
TON
1
2
0.5
1
1
1
1
1
1
1
1
1
1
2
3
4
2
1
4
8
4
1.5
2
6
3
5
10
12
12
14
28
22
12
16
17
5
6
2.5
3
6
7
6
8
3.5
4
7
9
14
11
6
10
11
12
13
5
4
4
8
4
8.5
aTheyieldoftheproductisdeterminedbyGC. TON(turnover
number) for the catalyst is calculated as the mol of product
formed per mol of catalyst used.
Mn-salen catalyst (A2) was characterized by X-ray
crystallography (Figure 2) and ESI-MS (m/z = 599.3).
Mn(salen)X complex (A1/A2) gave an orange-brown
3. Results and Discussion
To check the efficacy of Mn(salen) complex towards coloredsolutioninCH2Cl2 whichshowedanabsorbance
benzylicsp3 C–Hchlorination, ethylbenzenewastreated band in UV-Vis spectroscopy at λmax, 440 nm, (εmax
∼
with N, N-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclo- 6.4 × 103M−1 cm−1) along with another weak band at
hexanediamino-manganese chloride A1 as the catalyst λmax, 510 nm (Supporting Information).20
and aqueous NaOCl solution (both as oxidant and chlo-
The orange-brown color of the solution of com-
rinating agent) in CH2Cl2. For this catalytic reaction plex A1/A2 turned to green color upon addition of
tetrabutylammonium chloride (TBACl) was used as an excess of oxidant aqueous NaOCl solution. UV-
phase transfer catalyst. This biphasic system afforded Vis spectrum of this solution showed a new broad
1-chloroethyl benzene with turn over number (TON) 14 absorption band around λmax, 640 nm, which indi-
under a nitrogen atmosphere with time duration of 12 h cates the formation of high-valent Mn(V)-oxo species
at room temperature. Replacing the anion counterpart, (Figure 3).20,21
Mn(salen)Br (A2) also produced the same result. Other
Further, time-dependent UV-Vis spectra showed a
Mn(salen) complexes were found to give inferior results gradual decrease of this band at 640 nm upon reac-
in comparison to either A1 or A2 (see Supporting Infor- tion with ethylbenzene at room temperature (Figure 4).
mation). Variation of the amount of sodium hypochlorite Finally, the color of the solution changes from green to
(NaOCl) showed that 4 mL of NaOCl (0.6 M) is opti- brown indicating the regeneration of the starting com-
mal for this reaction, and lower concentration of NaOCl plex [Mn(salen)]+. A kinetic analysis of disappearance
retarded the reactivity of this reaction. We also found of Mn-oxo species during oxidation of ethylbenzene
that dilution with solvent CH2Cl2 had an adverse effect revealed a pseudo-first order rate dependency with rate
on this reaction (Table 1). The reaction mixtures were constant k1 = 4.9 × 10−2 s−1.

J. Chem. Sci. (2018) 130:88
Page 5 of 9 88
Scheme 2. Substrate scope for benzylic chlorination. TON for
catalyst was calculated as the mol of product formed per mol of
catalyst used. [a] yield of the product was determined by GC. [b]
yield of the product was determined from isolated product. [c]
5–6% of dimerized product was formed.
Further, we conducted ESI-MS study to identify the (m/z = 615.31) and [MnIV(salen)(OH)]+(m/z =
intermediates.22 We have recorded the mass spectrum of 616.33) (formed due to the hydrogen atom abstrac-
the [MnV(salen)(O)]+ prepared from a [MnIII(salen)]+ tion of MnV = O from CH2Cl2). Interestingly both the
solution in CH2Cl2 upon addition of excess aque- experimentally obtained spectra matched well with the
ous NaOCl. We have detected both [MnV(salen)(O)]+ simulated spectra (Figure 5).

88 Page 6 of 9
J. Chem. Sci. (2018) 130:88
Scheme 3. Selective benzylic chlorination of ibuprofen
methyl ester. Chlorination to oxidation product ratio is 4:1.
Figure 4. UV-Vis spectral changes during oxidation of
ethylbenzene. Inset: Decay of absorption at 640 nm.
Figure 2. ORTEP diagram of [Mn(salen)Br]. Mn has
square pyramidal geometry.
Figure 5. (a)
ESI-MS
spectrum
of
[MnV(salen)(O)]+; (b) ESI-MS spectrum of
[MnIV(salen)(OH)]+(experimental (blue) and
simulated (red)).
complexes are EPR-silent at the usual X-band fre-
quencies.22 For the EPR study, a 10 mM solution of
MnIII(salen) in CH2Cl2 was prepared, subsequently, 5
equiv. of PhIO was added which generated green color
of MnV=O species. Then 10 equiv. of ethylbenzene
Figure 3. UV-Vis spectral changes upon addition of excess
NaOCl into CH2Cl2 solution of Mn(salen)X (green line).
UV-Vis spectrum of Mn(salen)X in CH2Cl2 (red line).
We choose EPR spectroscopy for the detection was added to this solution (noteworthy, the resulting
of MnIV(salen) species as Mn(IV) is a paramagnetic solution provided C–H oxidation product of ethylben-
system (d3 electronic configuration) and shows well- zene, 1-phenylethanol and acetophenone via HAA step).
characterized EPR signals, whereas MnV and MnIII This solution displays an EPR spectrum with g values

J. Chem. Sci. (2018) 130:88
Page 7 of 9 88
basic NaOCl to form MnV=O (B) complex (supported
by ESI-MS and UV/Vis), which then abstracts hydrogen
from substrate generating free benzyl radical and MnIV–
OH (C) complex (supported by ESI-MS and EPR). For
a few substrates (isobutylbenzene, diphenylmethane),
the formation of dimerized products indicates the for-
mation of a stable benzyl radical. Then MnIV–OCl (D)
is being generated from MnIV–OH by ligand exchange
with NaOCl. In the next step, benzyl radical abstracts
chlorine from MnIV–OCl to form chlorinated product
and regenerate MnV=O complex.
4. Conclusions
In summary, we report a method for benzylic C–H chlo-
rination catalyzed by easily synthesizable and tunable
Mn-(salen) complex where cheap NaOCl has been used
Figure 6. X-band EPR spectra of the MnIV(salen) com-
plexes in CH2Cl2 at 100 K (frequency 9.5 GHz) generated
from the MnIII(salen) Cl complex A1 + PhIO (5 equiv.) + as the chlorinating agent. Here, we are proposing that
ethylbenzene (10 equiv.).
the reaction is passing through an Mn(V)=O species
which can abstract a hydrogen atom from substrate gen-
erating benzylic radical. The formation of high-valent
manganese species is supported by ESI-MS and UV-Vis
studies. A further development of this methodology can
accomplish late-stage derivatization of various bioactive
molecules containing benzylic site.
Supplementary Information (SI)
1
All additional information, H NMR, 13C NMR, Mass and
GC-MS spectral data for the characterization of compounds
are given in the Supporting Information. Supplementary
Information is available at www.ias.ac.in/chemsci.
Acknowledgements
This activity is supported by SERB, India (EMR/2015/
000164). Financial support has been received from CSIR-
India (Fellowship to S.R.).
Figure 7. Proposed mechanistic cycle.
of 5.17, 2.19 and 1.58 implicating the formation of a
MnIV intermediate during manganese salen-catalyzed
benzylic chlorination. These g values are characteris-
tic of a d3 MnIV-system with S = 3/2 which arise from
the ms = 1/2 with rhombic symmetry.21,23 The signal
at g = 5.17 also displays six-line hyperfine splitting as
expected for I = 5/2 of 55Mn nucleus (Figure 6).
In the absence of ethylbenzene, we got similar
EPR signals under the same experimental condition.
This can be addressed by the proton abstraction of
MnV=O species (generated upon addition of PhIO to
MnIII(salen)Cl in CH2Cl2) from solvent CH2Cl2.
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