K. Selvam et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 7 (2010)
835
Table 5. Photocatalytic Cyclization of 2-Aminothiophenol (2-ASH) with Propylene Glycol, Ethylene Glycol, and
Primary Alcoholsa)
Conversion/%
Run
1
Substrate
Irradiation time/h
Products/% (Yield of UV, Solar)
UV
79
Solar
2-ASH + PG
8
61
2-Methyl benzothiazole (38, 33),
benzothiazole (15, 10),
2,2¤-diaminodiphenyl disulfide (25, 18),
2-aminothiophenol (11, 38)
2
2-ASH + EG
8
61
46
Benzothiazole (15, 10),
2,2¤-diaminodiphenyl disulfide (35, 28),
2-aminothiophenol (11, 38),
2-benzothiazolemethanol (11, 8)
2,2¤-Diaminodiphenyl disulfide (60, 58)
2,2¤-Diaminodiphenyl disulfide (73, 76)
2,2¤-Diaminodiphenyl disulfide (77, 81)
3
4
5
2-ASH + EtOH
2-ASH + PrOH
2-ASH + BuOH
8
8
8
60
73
77
58
76
81
a) All reactions were performed with a mole ratio of 1:2 in acetonitrile (20 mL) solvent; TiO2: 200 mg, airflow rate:
¹3
¹1
8.1 mL s¹1, I0 = 1.381 © 10 einstein L¹1 s (4 lamps).
compared to propanol is due to the steric effect of the bulky
propyl group.
sulfate,26 hydrogen peroxide in trifluoroethanol,27 and dimethyl
sulfoxide.28 Enzymatic29 and electrochemical30 methods are
also known to perform this oxidative transformation. In this
regard, this non-toxic clay composite catalyst is considered as
an ideal “green” oxidant for the formation of disulfides by the
controlled oxidation of 2-aminothiophenol without the forma-
tion of by-products.
The photocatalytic cyclization of OPD with 2-phenyl-1,2-
propanediol gave 2-hydroxy-2-phenylpropionic acid and benz-
imidazole (Run 33). When OPD was irradiated with toluene,
2-phenylbenzimidazole and 2,2¤-diaminoazobenzene were
formed (Run 32).
In almost all OPDs, the benzimidazoles and 2-alkylbenz-
imidazoles were selectively obtained in moderate to good
yields. However, when 1,2-diaminoanthraquinone (Run 34)
and 3,4-diaminobenzophenone (Run 35) were employed
instead of o-phenylenediamine with PG, only a trace amount
of the respective condensation product was detected by GC-
MS.
But in the case of 2-aminophenol under the same reaction
conditions, the desired benzoxazoles was obtained in very low
yield along with several side products as identified from the
GC-MS analysis. Hence no further attempts were made to
optimize the reaction with this substrate.
When we compare the efficiency of this catalyst in UV and
solar processes, Ag-TiO2 is slightly more efficient in solar light
than in UV light. The higher activity of Ag-TiO2 catalyst in
solar light has already been proved in the degradation of azo
dyes.14 The Ag particles have increased absorption in the
visible region and also act as electron traps, enhancing the
electron-hole separation by the transfer of the trapped electron
to the adsorbed O2.31
Plausible Reaction Mechanism. A mechanism for the
formation of 2-methylbenzimidazole and benzimidazole is
proposed based on the experimental results (Scheme 1). When
titanium dioxide is exposed to UV light, electrons are promoted
to the conduction band and positive holes are formed in the
valence band. It is already established that alcohols can be
easily photooxidized on semiconductor surface to correspond-
ing aldehyde or ketone.32 They may be further oxidized to acids
by the hydroxyl radicals produced by irradiated semicon-
ductors. It is also known that the oxidation depends on the
acidity of semiconductor/clay composite catalyst. In this
case o-phenylenediamines are adsorbed preferentially than
PG (or alcohols) in the internal surface area of the clay pores.
The preferential adsorption of OPDs causes PG to undergo
oxidation to ketoaldehyde and then to acetic acid and formic
acid (step 1). Acetic acid and formic acid formed undergo
cyclization with OPDs to give 2-methylbenzimidazole and
benzimidazole respectively (step 2). HCOOH is reported to be
a hole scavenger and so it can undergo easy oxidation. This
reduces the availability of free HCOOH for condensation.
To extend the scope of this method, we also examined the
coupling of 2,3-diaminopyridine (Run 36) and 1,2-diamino-
cyclohexane with propylene glycol under these conditions.
Irradiation of these diamines with PG using Ag-TiO2 failed to
produce desired products. Although acids might have been
formed in this reaction mixture, they could not precede further
to form benzimidazole compounds as in the Philips synthesis.18
Similar to OPD, 2-aminothiophenol may also undergo
cyclization with the oxidizing products of glycols and alcohols.
We had carried out the reaction of 2-aminothiophenol with PG,
EG, EtOH, PrOH, and BuOH and results are given in Table 5.
2-Aminothiophenol also underwent condensation with the
oxidative products of PG and EG in presence of TiO2/clay
composite catalysts, affording the corresponding 2-methyl-
benzothiazoles and benzothiazole along with disulfide (Runs 1
and 2). 2-Aminothiophenol with other three primary alcohols
(Runs 3-5) gave only sulfide and no benzothiazole was formed.
This catalyst selectively oxidizes aminothiols to amino sulfides
and the yield increases from ethanol (60%) to butanol (81%)
both in UV and solar light.
Selective oxidation of thiols to the corresponding disulfides
is of interest from the point of view of biological and chemical
process.19-22 Thiols are easily over-oxidized and therefore
extensive study has been carried out for the controlled
oxidation of thiols using iodine/hydrogen iodide,23 bromine,24
potassium dichromate,25 potassium permanganate/copper(II)