Visible light photocatalysis with CBr4: A highly selective aerobic photooxidation of methylarenes to aldehydes

Article abstract of DOI:10.1039/c5ra26623h

A metal-free, operationally simple and efficient means of aerobic photooxidation of methylarenes to aromatic aldehydes at room temperature employing visible light photocatalysis with CBr₄ is reported. The reaction is highly selective as it proceeds without any over oxidation to carboxylic acids. The utilisation of visible light and molecular oxygen is inexpensive, readily available, non-toxic and the sustainable reagents make the protocol compatible with green chemistry demands. A plausible mechanism for the formation of aldehydes from methylarenes is also provided.

Full text of DOI:10.1039/c5ra26623h

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Visible light photocatalysis with CBr4: a highly
selective aerobic photooxidation of methylarenes
to aldehydes†
Cite this: RSC Adv., 2016, 6, 14547
*
Shubhangi Tripathi, Sachchida N. Singh and Lal Dhar S. Yadav
A metal-free, operationally simple and efficient means of aerobic photooxidation of methylarenes to
aromatic aldehydes at room temperature employing visible light photocatalysis with CBr4 is reported.
The reaction is highly selective as it proceeds without any over oxidation to carboxylic acids. The
utilisation of visible light and molecular oxygen is inexpensive, readily available, non-toxic and the
Received 13th December 2015
Accepted 21st January 2016
DOI: 10.1039/c5ra26623h
sustainable reagents make the protocol compatible with green chemistry demands.
mechanism for the formation of aldehydes from methylarenes is also provided.
A plausible
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compounds is quite demanding concerning both reactivity and
selectivity issues.
Introduction
Traditionally, inorganic oxidizing agents like chromates,
permanganates and cerrates, employed in stoichiometric
amounts, suffer from noxious heavy-metal waste generation
and their outlay.8 Therefore, development of a cleaner and cost-
effective methodology would be a valuable contribution to this
industrially important eld.
The fundamental objective of chemists is to develop new
sustainable routes for chemical transformations having signif-
icant conceptual and practical interest under mild reaction
conditions.1 In this context, visible light-mediated photoredox
catalysis has emerged as a state-of-the-art alternative to advance
this goal.2 In recent years, visible light-induced synthetic
methodologies have received central attention from various
research groups, especially because of its eco-compatibility,
easy availability, safe handling and everlasting abundance as
an energy source.3 Consequently, a number of synthetic proto-
cols have been developed that utilise the powerful photo-
catalytic properties of highly expensive and potentially toxic
ruthenium and iridium complexes. Therefore, the development
of synthetically useful chemical transformations employing
inexpensive visible light photocatalysts would be warmly
welcomed, especially in terms of economical and environ-
mental issues.
Among various methods, oxidation reactions having unique
advantage to activate inert bonds, especially C–H bonds, play
a pivotal role in the chemical industry for designing and real-
ising the chemical intermediates and ne chemical special-
ties.4,5 However, selectivity of oxidation is still an open challenge
for chemists. Similarly, carbonyl compounds, the vital
substrates in organic synthesis,6,7 are still synthesized by non-
eco-compatible methods with very low atom economy
involving copious amount of waste generation. Hence, selective
oxidation of methylarenes to synthetically important carbonyl
In contrast to traditional methods several transition-metal
based catalytic processes using selenium,9 silver10 and
copper11 metal catalysts have also been reported for selective
oxidation of methylarenes. Though these metal catalysts are
very effective for selective oxidation, most of them are very
expensive and hazardous to the environment. To overcome the
hazards associated with metal catalysts, several non-metallic
catalytic pathways for selective oxidation of methylarenes were
reported viz. 9,10-dicyanoanthracene/methyl viologen and
oxygen mediated photocatalysis12 and enzyme laccase/ABTS,8
DDQ/NBS,13 H2O2/HBr,14 Br2/DMSO15 or IBX/DMSO16 catalysis
but these processes also suffer from one or more drawbacks of
substrate limitations, handling problems, poor selectivity and
yield issues. Hence, development of a metal-free, environmen-
tally benign, cost effective and sustainable method for selective
oxidation of methylarenes is necessarily a welcome move.
Recently, a stoichiometric amount of CBr4 has been used to
generate bromine free radicals under visible light irradiation in
several synthetically useful reactions.18d,19b We reasoned that
a photoreaction employing CBr4 as a source of bromine radicals
and producing HBr as a by-product could be rendered catalytic
with respect to CBr4 in the presence of an oxidant which in situ
regenerates the bromine free radical from HBr.
Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad
211 002, India. E-mail: ldsyadav@hotmail.com; Fax: +91-532-246-0533; Tel: +91-
532-250-0652
Inspired by the aforementioned facts and our continued
efforts to devise metal-free efficient synthetic routes,18 we
hypothesised visible light-mediated one-pot selective oxidation
of methylarenes up to aromatic aldehydes only employing CBr4
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra26623h
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Results and discussion
To realise our hypothesis and determine the optimal reaction
conditions, we materialised the study with a model reaction of
toluene (1a, 1 mmol) with catalytic amount of CBr4 (2, 10 mol%)
in acetonitrile under an oxygen atmosphere (O2 balloon) and
visible light irradiation using 18 W CFL (380–740 nm) for 2 h at
rt. To our delight, the desired product benzaldehyde (3a) was
obtained as the sole product in an excellent yield of 89%. With
this encouraging result, we performed a series of control and
screening experiments and rstly observed that the reaction did
not proceed in the dark (Table 1, entry 2). Similarly, the absence
of CBr4 under the reaction conditions leads to the same
conclusion (Table 1, entry 3). These results suggest the necessity
of both light and CBr4 for the reaction. Next, to determine the
role of oxygen, the screening was performed under degassed
condition and an inert argon atmosphere (Table 1, entries 4 and
5) and the results revealed that the molecular oxygen is equally
essential for the present oxidation reaction. The system also
worked under an air atmosphere instead of an oxygen balloon
Scheme
aldehydes.
1 Visible light-assisted oxidation of methylarenes to
Table 2 Optimization of solventa
as a photocatalyst and molecular oxygen as an oxidant
(Scheme 1c). Implication of visible light and molecular oxygen
as reagents makes the protocol greener, more viable and
sustainable than existing methodologies. Moreover, visible light
is a traceless reagent and oxygen is believed to generate water as
the sole by-product. The present protocol on selective oxidation
of methylarenes being inexpensive, metal-free, employing
economical acquisition of reagents would be a valuable addi-
tion to this eld.7 A plausible mechanistic pathway for the
selective oxidation of methylarenes to aldehydes is given in
Results and Discussion Section.
Entry
Solvent
Time (h)
Yield (%)
1
2
3
4
5
6
CH3CN
CH3OH
CH3CH2OH
tBuOH
Hexane
THF
2
8
8
8
8
8
89
40
38
27
35
18
a
Reaction conditions: toluene (1a, 1.0 mmol) and CBr4 (2, 10 mol%) in
a solvent (3 mL) were irradiated under a molecular oxygen atmosphere
at rt using 18 W CFL for 2–8 h.
Table 1 Optimization of reaction conditionsa
Entry
Reaction conditions
Irradiation time (h)
Yieldb (%)
1
CBr4 (10 mol%), O2 balloon, 18 W CFL
CBr4 (10 mol%), O2 balloon, in the dark
Without CBr4, O2 balloon, 18 W CFL
CBr4 (10 mol%), degassed, 18 W CFL
CBr4 (10 mol%), argon, 18 W CFL
2
8
8
8
8
4
4
2
18
18
89
nd
nd
nd
nd
62
45
89
26
6
2c
3
4d
5e
6
7
8
CBr4 (10 mol%), air, 18 W CFL
CBr4 (5 mol%), O2 balloon, 18 W CFL
CBr4 (15 mol%), O2 balloon, 18 W CFL
NBS (10 mol%), O2 balloon, 18 W CFL
BDMS (10 mol%), O2 balloon, 18 W CFL
9
10
a
Reaction conditions: toluene (1a, 1.0 mmol) and CBr4 (2, 10 mol%) in CH3CN (3 mL) were irradiated under a molecular oxygen atmosphere (O2
balloon) at rt using 18 W CFL for 2–18 h. b Isolated yield of the product 3a; nd ¼ not detected. c Reaction was carried out in the dark. d Reaction was
carried out under degassed condition. e Reaction was carried out under an argon atmosphere.
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Table 3 Substrate scope for the selective oxidation of methylarenes to aldehydesa
a
For general procedure, see ESI. b All compounds are known and were characterized by comparison of their spectral data with those reported in the
c
literature (see ESI).7c,20 Isolated yield of puried products 3.
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but delivered a considerably lower yield of 3a (Table 1, entry 6). due to the far less stability of an alkyl radical than a benzyl
Cumulatively, on decreasing the amount of CBr4 from 10 mol% radical.
to 5 mol%, the yield of 3a was signicantly reduced (Table 1,
Congruent with the above observations and the literature
entry 1 vs. 7). However, use of 15 mol% CBr4 did not affect the precedents,17,19 a plausible mechanism for the visible light and
yield of the product (Table 1, entry 8). Other bromine sources, CBr4-promoted selective oxidation of methylarenes to aromatic
viz. NBS, and BDMS (bromodimethylsulfonium bromide) did aldehydes is depicted in Scheme 3. The mechanism might
not work satisfactorily under the present reaction conditions follow a radical pathway, as on addition of TEMPO (2,2,6,6-
(Table 1, entries 1 vs. 9 and 10).
tetramethyl-1-piperidinyloxyl), a traditional radical scavenger,
Given the importance of solvent activity in chemical trans- the reaction was quenched (Scheme 2) and that the formation of
formations, we performed a series of experiments for optimi- benzyl-TEMPO adduct was conrmed by its MS (HRMS (EI)
zation. The results reveal the superiority of acetonitrile over the calcd for C16H25NO: 247.1936, found 247.1934). On irradiation
t
other tested solvents, viz. CH3OH, CH3CH2OH, BuOH, hexane with visible light using 18 W CFL under a molecular oxygen
and THF (Table 2, entry 1 vs. 2–6).
environment, CBr4 generates the bromine radical, which reacts
Having established the optimal reaction conditions for with methylarene 1 to form resonance stabilised benzyl radical
visible light-mediated oxidation of methylarenes, we surveyed 4 along with HBr. The benzyl radical 4 combines with O2 to
the generality and scope of the present protocol over a wide generate peroxy radical 5. The peroxy radical 5 abstracts
range of methylarenes (Table 3).
hydrogen atom from HBr and regenerates the bromine radical
In general, methylarenes bearing either an electron-donating along with the hydroperoxide 6, which eliminates H2O to afford
(Me or OMe) or an electron-withdrawing substituent (NO2, F, Cl, the desired product 3.
Br, I, CN, MeSO2, PhSO2, CONH2, COOEt or COPh) are well
tolerated in the present methodology and afford carbonyl
compounds in good to excellent yields of 75–96%. However,
Conclusion
methylarenes bearing an electron-donating substituent on the In conclusion, we have developed a novel, highly efficient,
aromatic ring appear to react faster and afford marginally metal-free, one-pot synthesis of aromatic aldehydes by aerobic
higher yield than those bearing an electron-withdrawing group. oxidation of methylarenes employing visible light photo-
Also, evaluation of aliphatic hydrocarbons in the present catalysis with CBr4. Notably, water is the only by-product in the
methodology does not lead to the satisfactory results probably reaction. The protocol overrides the other methods for highly
selective oxidation of methylarenes only up to the aldehyde
stage. Advantageously, it utilises visible light and molecular
oxygen as clean, inexpensive and sustainable reagents and does
not require any additives, heating or inert conditions.
Acknowledgements
We sincerely thank SAIF, Punjab University, Chandigarh, for
providing microanalyses and spectra.
Scheme 2 Quenching of the reaction with TEMPO.
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