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of Lewis acids (such as Mg2+, Zn2+, Yb3+ and Sc3+ ions).14 Further, Su
et al. showed mesoporous carbon nitride (mpg-C3N4) polymer can
act as a photocatalyst to activate molecular oxygen for the selec-
tive oxidation of benzyl alcohols under visible light irradiation.15
Though several methods have been developed for the oxidation
of benzyl alcohols, they suffer certain drawbacks to some extent
such as the use of expensive and toxic metals, low selectivity,
and limited substrate scope. Hence there is a need to develop a
simple method for the oxidation of benzyl alcohols which can sur-
pass the above limitations.
This result encouraged us to optimize our reaction conditions.
First, we were interested to find out the optimum amount of the
photocatalyst, Rose Bengal, required for the photooxidation. For
this purpose, we have carried out the photooxidation of 1a with
varying amount of Rose Bengal. The result showed that as we
increase the concentration of Rose Bengal the yields improves sig-
nificantly. It is worthy to note that Rose Bengal (5 mol%) in acetoni-
trile gives the maximum product (76%) with high selectivity (100%)
(Table 1, Entry 2). The photooxidation of 1a was completely inhib-
ited in the absence of Rose Bengal (Table 1, Entry 13). On the other
hand, when we have used Eosin Y (Table 1, Entry 3) or Fluorescein
(Table 1, Entry 4) as the organic photocatalyst in place of Rose Ben-
gal, the photooxidations produced similar yield. The above experi-
ment confirms the need of organic photocatalyst. Second, we
investigated the optimum amount of NH4SCN required for the pho-
tooxidation of 4-Chlorobenzyl alcohol. It was found that 3 equiva-
lent of NH4SCN in acetonitrile yielded 4-Chlorobenzaldehyde (2a)
in maximum yield (76%) (Table 1, Entry 2).
In addition, to check the importance of pseudohalide precursor
NH4SCN, we performed the photooxidation of 1a using KSCN in
place of NH4SCN (Table 1, Entry 11). We found that photooxidation
was not effective like in the case of NH4SCN, and there was no pho-
toproduct formation in the absence of NH4SCN (Table 1, Entry 10).
Third, we noted that irradiation time has great influence on the
yield of photooxidised product. Increasing the irradiation time sig-
nificantly improve the yield of 2a. Maximum yield of 2a was
obtained after 20 h of irradiation. Finally, to understand the signif-
icance of solvent on the photooxidation, we have carried out the
photooxidation of 4-Chlorobenzyl alcohol in different solvents
such as acetonitrile (CH3CN), chloroform (CHCl3) and tetrahydrofu-
ran (THF). In CH3CN, we noted 1a produces 76% of 4-Chloroben-
zaldehyde (Table 1, Entry 2). However, when THF was used as a
solvent the oxidation of 1a produced 2a with 38% in yield (Table 1,
Entry 5). In CHCl3, this method oxidizes 1a only in trace quantities,
this might be due to the insolubility of thiocyanate salts in CHCl3
(Table 1, Entry 6). Further, we also showed that 1a can be oxidized
to 2a in CH3CN/H2O (1:1) solvent system, proving that our method
can be useful in the presence of water.
Recently, pseudohalide, thiocyanate ions (SCNÀ) have gained
considerable importance mainly in the areas like modulation of
catalytic activity,16a shape-controlled synthesis of metal
nanostructures16b and as a dopant in photovoltaic applications.16c
Pseudohalides, viz. cyanide, thiocyanate and azide anions, show
chemical properties analogous to single halogen atoms.17
It is well known that halide radicals are potential candidates for
hydrogen abstraction.18–20 This motivates us to reason that, like
halide radicals pseudohalide radicals can show similar property
of abstracting hydrogen.
In the literature we have found Pixu Li’s research group and
others showed the generation of thiocyanate radicals from thio-
cyanate anion on exposure to visible light in the presence of
organic dyes.21 Their studies prompted us to explore, for the first
time, thiocyanate radical for the oxidation of benzyl alcohols. We
hypothesize that thiocyanate radical can abstract hydrogen, like
the halide radicals, from the benzylic position of the alcohols,
which will then subsequently convert to their corresponding alde-
hydes in the presence of molecular oxygen. Based on the above
assumptions, we designed a metal free, non-toxic and cheap
method for the photooxidation of aromatic alcohols under visible
light. In this method, we have used a very cheap organic dye, Rose
Bengal (RB), as the photocatalyst, thiocyanate radical as the hydro-
gen abstractor and air as the oxidant (Scheme 1).
Results and discussion
Initially, we investigated the photooxidation of 4-Chlorobenzyl
alcohol. The reaction was performed using 4-Chlorobenzyl alcohol
(0.1 mmol), Rose Bengal (1 mol%), NH4SCN (1 eq.), acetonitrile as
the solvent in presence of air under the irradiation of a CFL light.
After 60 min of irradiation, we have detected 4-Chlorobenzalde-
hyde (2a). Further, increasing the irradiation time up to 20 h, we
noted that 4-Chlorobenzyl alcohol (1a) was oxidized to 4-
Chlorobenzaldehyde (2a) with a moderate yield (23%) and high
selectivity (100%) (Table S1, Supporting information). This method
is highly selective towards the conversion of the alcohols to their
corresponding aldehydes over other products (such as carboxylic
acid). We have calculated the selectivity according to the reported
equation (Supporting Information – page 3).
It is of significance that there is no photooxidation of alcohol in
absence of O2 (Table 1, Entry 9) and visible-light irradiation
(Table 1, Entry 14). After optimization of the reaction conditions,
we carried out the photooxidation of 1a in large scale, we noted
our method produced 2a in good yield with high selectivity
(Table 1, Entry 16). These results suggest that visible light, photo-
catalyst, oxygen and thiocyanate play their unique roles for the
successful oxidation of various alcohols.
The only limitation of our method is that it requires an excess
amount of thiocyanate anion for the oxidation of alcohol. This is
because of the thiocyanate radicals produced from thiocyanate
anion combines with each other to form an inorganic polymer
known to be polythiocyanogen (or parathiocyanogen) with the
empirical formula (SCN)x as a yellow solid after completion of
the reaction. The polymer was characterized by UV/Vis, FT-IR,
and MALDI-TOF mass spectroscopy. UV/Vis spectrum shows a kmax
at 450 nm (Supporting information, Fig. S1). For (SCN)x, the
MALDI-TOF mass spectroscopy showed a parent ion at 1030 and
a series of peaks with (SCN)2 repeat units (116 m/z); this result
implies that (SCN)2 may be the monomer unit of the polymer (Sup-
porting information, Fig. S2). Further, IR spectrum shows a peak
with the maximum at 1145 cmÀ1 (Supporting information,
Fig. S3). The observed data suggest that the polymer may be con-
sists of 1,2,4-dithiazole rings linked by nitrogen atoms.22
The parathiocyanogen itself is photoactive,23 so we hypothe-
sized that this polymer may contribute to the reaction to improve
the yield. When we replaced Rose Bengal with parathiocyanogen,
the oxidation of 1a produced 2a (Table 1, entry 15). Hence, this
Scheme 1. Photooxidation of benzyl alcohols to their corresponding benzaldehydes
under different conditions.