Photocatalyzed oxidation of triphenyl derivatives of P, As, Sb & Bi and reduction of their oxides

Full text of DOI:10.1021/jo951990n

J . Org. Chem. 1996, 61, 2895-2896
2895
attempts have been made to reduce the oxides, Ar3MO.9
Handa et al.10 reduced Ph3PO to Ph3P using the SmI2-
THF-HMPA system. Our report here includes deoxy-
genation of Ar3MO on the surface of illuminated TiO2 in
an organic solvent in the presence of triethylamine (TEA).
P h otoca ta lyzed Oxid a tion of Tr ip h en yl
Der iva tives of P , As, Sb & Bi a n d
Red u ction of Th eir Oxid es
Narayanan Somasundaram and
Chockalingam Srinivasan*
Resu lts a n d Discu ssion
Department of Materials Science, Madurai Kamaraj
University, Madurai 625 021, India
Oxid a tion . Triphenylphosphine and triphenylstibine
have been oxidized to their corresponding oxides in yields
>75% in 30 min in methanol (Table 1). The oxidation
product of triphenylstibine was found to be a polymer
having high mp (>280 °C) in accordance with the
previous report.12 On the other hand, triphenylbismuth-
ine and tri-p-tolylarsine were oxygenated to the extent
of 50-65%. However, the yield of the oxygenated prod-
uct increased with longer irradiation time.
By considering the yield of the oxide at 30 min, the
relative order of reactivity is found to be Ph3P > Ph3Sb
> (p-CH3C6H4)3As > Ph3Bi. During our extensive studies
on the mechanism of oxidation of Ar3M by isostructural
and isoelctronic oxidants peroxodisulfate13 and peroxo-
diphosphate,12 we observed the same order of reactivity.
We propose the following mechanism for the photo-
catalyzed oxidation of Ar3MO, analogous to the oxidation
of organic sulfides.14
Received November 8, 1995
In tr od u ction
Heterogeneous photocatalysis provides the means not
only for utilizing solar energy but also for new synthetic
routes1,2 and pathways of degradation of industrial
wastes.3 Particulate semiconductors suspended in solu-
tion or metal-doped semiconductors2 such as TiO2, CdS,
ZnO, Fe2O3, WO3, Pt/TiO2, Ag/TiO2, colloidal TiO2, etc.,
have attracted the attention of many chemists as several
reactions which are inaccessible by other means may be
accomplished through heterogeneous photocatalysis. For
instance the formidable reduction of sulfonyl group in
aryl methyl sulfones has been achieved by us smoothly
on the surface of irradiated TiO2.4
TiO2 is widely used as the semiconductor phtotocata-
lyst in many organic reactions due to its nontoxic nature,
chemical stability, availability, and capability of repeated
use without substantial loss of catalytic activity.2 Ir-
radiation of TiO2 using low energy UV radiation (λ >350
nm) induces the formation of an electron-hole pair (e--
h+). The electron is promoted from the valence band to
the conduction band readily available for transference
while the positive equivalent, the hole (h+) in the valence
band, is ready to accept the electron from the substrate.5
The substrate that receives the electron from the semi-
conductor would be reduced whereas that which donates
the electron to the semiconductor is oxidized.
Though semiconductor-photoinduced oxidation of sev-
eral organic compounds has been studied,2 there are only
a few reports of photosensitized reductions such as
reduction of aldehydes and ketones,6 nitroaromatics,7 and
aryl methyl sulfones.4 The limited success of organic
reductions on semiconductor surfaces may be attributed
to the modestly negative potential of electrons at the
conduction band of the semiconductor and the ease of
oxygen reduction by electrons.5
TiO2
O2 + e- f O2
9
hν8 TiO2(h+) + e-
•-
Ar3M + TiO2(h+) f Ar3M•+ + TiO2
Ar3M•+ + O2•- f [Ar3M+OO-] Ar M8 2Ar3MO
3
While the photocatalyzed oxidation of Ar3As and Ar3-
Bi yields 14% and 16% of other products in 30 min, these
products decrease with a longer period of irradiation,
implying that they have been converted to Ar3MO. One
possible explanation is that in these cases, the miscel-
laneous products may contain a major amount of Ar2MO2-
Ar which may decompose on prolonged irradiation to give
Ar3MO. Though we are investigating this aspect in
detail, in the photolysis of Ph3Sb in the presence of
oxygen in benzene, the formation of Ph2SbO2Ph has been
postulated.15
To the best of our knowledge there appears to be no
report on the photocatalyzed oxidation of organo-group
VA compounds, Ar3M, and herein we present our results
on the TiO2-sensitized photooxidation of triphenylphos-
phine, tri-p-tolylarsine, triphenylstibine, and triphenyl-
bismuthine.
Though triphenylphosphine has been extensively em-
ployed as a deoxygenating agent,8 not much attention has
been paid to the reduction of the oxide. Only a few
Red u ction . Photolysis of a solution of Ar3MO in an
organic solvent purged with nitrogen in the absence of
either TiO2 or TEA or both has not resulted in any
reduced product. Only in the presence of both photo-
catalyst and TEA, the deoxygenated products have been
obtained (Table 2). TEA functions as a sacrificial elec-
tron donor which prevents electron-hole recombination
process by trapping the hole (h+) effectively.2 While 4 h
(9) Freeman, L. D.; Doak, G. D. J . Organomet. Chem. 1980, 203,
359 and references cited therein.
(10) Handa, Y.; Inanaga, J .; Yamaguchi, M. J . Chem. Soc., Chem.
Commun. 1989, 298.
(1) Bard, A. J . Science 1980, 207, 4427.
(2) (a) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (b) Rao,
N. N.; Natarajan, P. Curr. Sci. 1994, 66, 742.
(3) Muneer, M.; Das, S.; Manilal, V. B.; Haridas, A. J . Photochem.
Photobiol. A: Chem. 1992, 63, 107.
(4) Somasundaram, N.; Pitchumani, K.; Srinivasan, C. J . Chem.
Soc., Chem. Commun. 1994, 1473.
(5) Al-Ekabi, H. Photochemistry in Condensed Phases; Ramamurthy,
V., Eds.; VCH Press: New York, 1991; p 495.
(6) Pruden, C. J .; Pross, J . K.; Li, Y. J . Org. Chem. 1992, 57, 5087.
(7) Mahdavi, F.; Bruton, T. C.; Li, Y. J . Org. Chem. 1993, 58, 744.
(8) Amose, R. A. J . Org. Chem. 1985, 50, 1311.
(11) Srinivasan, C.; Chellamani, A. Indian. J . Chem. 1984, 23A, 684.
(12) Srinivasan, C.; Pitchumani, K. Can. J . Chem. 1985, 63, 2285
and references cited therein.
(13) Srinivasan, C.; Pitchumani, K. Int. J . Chem. Kinet. 1982, 14,
1315.
(14) Davidson, R. S.; Pratt, J . E. Tetrahedran Lett. 1983, 52, 5903.
(15) Glushakova, V. N.; Kol’yakova, G. M.; Skorodumova, N. A.;
Alferov, V. A.; Aleksandrov, Yu.A.; Razuvaev, G. A. Zh. Obshch. Khim.
1980, 50, 1811.
0022-3263/96/1961-2895$12.00/0 © 1996 American Chemical Society

2896 J . Org. Chem., Vol. 61, No. 8, 1996
Notes
Ta ble 1. P h otooxid a tion a of Tr ia r yl Gr ou p VA
Com p ou n d s (Ar 3M) on TiO2 in Meth a n ol
Sch em e 1. P la u sible Mech a n ism of th e
P h otoin d u ced Red u ction of Ar 3MO (M ) P , As, Sb,
product mixture (%)b
or Bi) in th e P r esen ce of TEA on TiO2
substrate
Ar
irr time
(min)
M
Ar3M
Ar3MO
Xc
phenyl
P
P
15
30
30
30
30
16
9
75
82
77d
64
52
74
74
9
9
8
14
16
11
5
phenyl
phenyl
p-tolyl
phenyl
p-tolyl
phenyl
Sb
As
Bi
As
Bi
15
22
32
15
21
90
120
a
Irradiated using 350 nm/400 W annular reactor. Catalyst
b
(TiO2): 1%. Solvent: 15 mL, oxygen-saturated. GC analysis
results. Error limit: (5%. c Other products including diaryl
derivatives. The oxidation product was a polymer.12
d
photoinduced reduction of Ar3MO in the presence of TEA
on TiO2. The formation of oxygen has been confirmed
by GC analysis of the evolved gas.
Ta ble 2. TiO2-P h otosen sitized a Deoxygen a tion of
Tr ia r yl Gr ou p VA Oxid es (Ar 3MO) in th e P r esen ce of
Tr ieth yla m in e
substrate
product mixture (%)b
Exp er im en ta l Section
irr
time
Ar
M
solvent
Ar3M
Ar3MO
Xc
Triphenylphosphine (Riedel, pyrosynth), triphenylstibine (Al-
drich), and triphenylbismuthine (Aldrich) were recrystallized
from ethanol. Tri-p-tolylarsine was prepared by reported meth-
ods.11 Hydrogen peroxide oxidation of the above subtrates
yielded the oxides.12 TiO2 (anatase, Aldrich, 99.9%) was used
as the photocatalyst. All the solvents employed were AR/HPLC
grade. The commercial BDH sample of triethylamine was
purified by distillation.
phenyl
phenyl
p-tolyl
phenyl
phenyl
p-tolyl
phenyl
phenyl
P
4 h
4 h
4 h
4 h
30 min
30 min
30 min
30 min
MeOH
MeOH
MeOH
MeOH
DMF
DMF
DMF
DMF
25
19
25
36
35
26
20
24
57
69
56
46
46
55
68
57
18
12
19
18
19
19
12
19
Sb
As
Bi
Bi
As
Sb
P
a
Oxid a tion . One millimolar solution of Ar3M in methanol (15
mL) containig suspended TiO2 (0.15 g) was saturated with
oxygen and irradiated with a 400 W medium pressure mercury
vapor lamp (λ >350 nm) in an annular reactor with water filter.
Irradiated using 350 nm/400 W immersion reactor. Catalyst
(TiO2): 1%. TEA: 0.5%. Solvent: 200 mL. Nitrogen purging for
45 min. GC analysis results. Error limit: (5%. c Other products
including diaryl derivatives.
b
Red u ction . Reduction of Ar3MO (1 mM) was performed in
presence of TEA (0.5 M) using 2 g of TiO2 suspended in methanol
or acetonitrile (200 mL). An immersion type reactor with a
medium pressure mercury vapor lamp (400 W, 350 nm) with a
water filter was employed here, and the reaction mixture was
continuously stirred using a magnetic stirrer. Before irradiation,
the solution was purged with nitrogen for 45 min.
of irradition of Ar3MO in methanol yields about 20-30%
of deoxygenated product, it is interesting to note that the
same result is accomplished in DMF in 30 min (Table
2). It may be due to the electron-donating capacity of
DMF and the availability of more electrons for reduc-
tion.16 The sacrificial electron donor TEA has been
converted to diethylamine and acetaldehyde as reported
in the reduction of organic substrates by illuminated CdS
in the presence of TEA.16
P r od u ct An a lysis. The reaction mixture, after removing the
catalyst by centrifugation followed by filtration, was analyzed
by gas chromatograph (NETEL, India) fitted with Dexil 300 GC
column and FID.
On the basis of the experimental results, we propose
a plausible mechanism as shown in Scheme 1 for the
Ack n ow led gm en t. The authors thank CSIR, New
Delhi, for financial support.
(16) Shiramugi, T.; Fukami, S.; Wada, Y.; Yanagida, S. J . Phys.
Chem. 1993, 97, 12882 and references cited therein.
J O951990N