Studies on the air oxidation of some arsenic(III) compounds

Article abstract of DOI:10.1080/10426500500271402

The air oxidation of As(III) oxides [(PhAsO)_x and Ph ₂As-O-AsPh₂] and thioesters [Ph-As(SPh)₂, Ph₂As-SPh Me-As(SPh)₂, Me₂As-SPh], in chloroform and in methanol was studied. The air oxidation in chloroform was faster probably because the solubility of dioxygen is greater than in methanol, and it is favored by the electron-withdrawing phenyl groups bound to As(III). The products obtained were the arsonic or arsinic acids and diphenyl disulfide. In one case, diphenyl disulfide and thiophenol were produced. The results can be rationalized by assuming first hydrolysis of the As(III) compounds to arsonous or arsinous acids followed by their oxidation to arsonic and arsinic acids, which should involve the binding of dioxygen to As(III). The other hypothesis assumes first the binding of dioxygen to As(III) of these oxides and thioesters followed by the decomposition of the adducts. The binding of the ground state dioxygen to As(III) may have biochemical implications for toxicity or chemotherapy of arsenic(III) compounds. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500500271402

Phosphorus, Sulfur, and Silicon, 181:751–762, 2006
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500500271402
Studies on the Air Oxidation of Some Arsenic(III)
Compounds
Theodore D. Sideris
Panayiotis V. Ioannou
Department of Chemistry, University of Patras, Patras, Greece
The air oxidation of As(III) oxides [(PhAsO)x and Ph As-O-AsPh ] and thioesters
2
2
[
Ph-As(SPh) , Ph As-SPh Me-As(SPh) , Me As-SPh], in chloroform and in
2 2 2 2
methanol was studied. The air oxidation in chloroform was faster probably be-
cause the solubility of dioxygen is greater than in methanol, and it is favored by the
electron-withdrawing phenyl groups bound to As(III). The products obtained were
the arsonic or arsinic acids and diphenyl disulfide. In one case, diphenyl disulfide
and thiophenol were produced. The results can be rationalized by assuming first
hydrolysis of the As(III) compounds to arsonous or arsinous acids followed by their
oxidation to arsonic and arsinic acids, which should involve the binding of dioxy-
gen to As(III). The other hypothesis assumes first the binding of dioxygen to As(III)
of these oxides and thioesters followed by the decomposition of the adducts. The
binding of the ground state dioxygen to As(III) may have biochemical implications
for toxicity or chemotherapy of arsenic(III) compounds.
Keywords Arsine oxides; binding; dioxygen; dithioarsonites; thioarsinites
INTRODUCTION
The air oxidation of As(III) compounds has practical, e.g. storage, and
probably biochemical significance. The literature data indicate that the
ligands bound to As(III) affect its autoxidation. Thus, alkaline arsenite,
3
3
−
1
“
AsO ”, was autoxidized at 0.12% per 24 h, while in the presence of
sodium sulfite and under proper conditions one molecule of arsenate
2
was produced for every molecule of sulfate formed. Arsenosoalkanes
(
(
alkylarsine oxides), (RAsO)x, are oxidized by air, but arsenosoarenes
arylarsine oxides), (ArAsO)x, require stronger oxidizing agents to give
3
arsonic acids. Cacodyl oxide, Me2As-O-AsMe2, in aqueous sulfuric acid
is oxidized by air to cacodylic acid, while attempts at the crystalliza-
4
tion of Ph2As-O-AsPh2 without the exclusion of air gave diphenylarsinic
Received May 24, 2005; accepted May 24, 2005.
Address correspondence to Panayiotis V. Ioannou, University of Patras, Department
of Chemistry, Patras, Greece. E-mail: ioannou@chemistry.upatras.gr
751

7
52
T. D. Sideris and P. V. Ioannou
5
acid (monomeric and dimeric). Basic hydrolysis of Me2As-SPh gave the
cacodylate from the very rapid atmospheric oxidation of the dimethy-
larsinous acid, Me2As-OH.6 Indirect evidence suggests that Ph2As-
OH in methanol is air oxidized back to diphenylarsinic acid during
the reduction of this acid with excess ascorbic acid under iodine
7
catalysis.
A study on the air oxidation of aromatic trithioarsenites, (ArS)3As,
showed that para-substituted phenyl groups with electron-donating
groups do not favor autoxidation, while electron-withdrawing groups
8
favor the autoxidation according to Eq. (1):
4(ArS)3As + 3O2 → 2As2O3 + 6ArSSAr
(1)
Likewise, the air oxidation of aliphatic trithioarsenites, (RS)3As, de-
pends on the nature of R. Thus, the esters (EtS)3As, (n-PrS)3As, and (n-
BuS)3As are easily oxidized9 [65% oxidation is observed when air is
,10
10
passed through (n-BuS)3As for 2 h ]. The more polar As(SCH2CH2OH)3
is relatively stable to autoxidation.1 We have found that esters (RS)3As
bearing terminal-COOH groups are autoxidized according to Eq. (1)
1
−
in a chain-dependent manner; those bearing COO or zwitterionic
+
−
12
CH(NH3 )COO groups are not autoxidized. From the class of
alkyldithioarsonites, R-As(SL)2 (L = R or Ar), the oily Et-As(SEt)2, de-
1
3
posited crystalline Et-AsO3H2 on contact with air, and the diphenyl
allyl- and benzyldithioarsonites, R-As(SPh)2, formed during the reduc-
tion of allyl- and benzylarsonic acid by thiophenol, was decomposed by
air to, e.g., As2O3, (PhS)3As, PhCH2OH and PhCH O, implying a direct
binding of dioxygen to As(III).14 The more polar Me-As(SCH2CH2OH)2
and Me-As(Scysteine)2 in water are easily autoxidized to Me-AsO3H2
and disulfide, RSSR.15 Thioarsinites are also susceptible to air ox-
idation. Thus, Ph2As-S-AsPh2 deposited (Ph2AsO2H)2·2Ph2As(S)OH
on attempted recrystallization in air,16 while Me2As-(Scysteine) and
Me2As-(Sglutathione) in water were air oxidized to cacodylic acid and
1
5
disulfides.
In this article, we report exploratory experiments on the air oxida-
tion of the oxygen-bearing 1 and 2 and of sulfur-bearing esters 3–6 in
chloroform and in methanol in order to see the effect of the solvent and
the nature of the bound to As(III) ligands and to probe on the probable
mechanism of the air oxidation of these As(III) compounds.
(
PhAsO)x 1,
PhAs(SPh)2 3,
Ph2AsSPh 4,
MeAs(SPh)2 5
Me2AsSPh 6
Ph2As-O-AsPh2 2,

Air Oxidation of Some Arsenic(III) Compounds
753
RESULTS AND DISCUSSION
Air Oxidation of the Oxygen-Bearing As(III) Compounds
and 2
1
After stirring a chloroform or a methanol solution of 1 in air for 12 days,
phenylarsonic acid was not detected by IR, implying that less than 20%
oxidation had taken place, according to Eq. (2):
2
(PhAsO)x + xO2 + 2xH2O → 2xPhAsO3H2
(2)
Cohen and colleagues17 measured the rate of oxidation of substituted
with electron-donating and with electron-withdrawing phenylarsine ox-
ides based on the reaction, Eq. (3):
−
−
−
−
Ar-AsO2H + RSSR + 2HO → Ar-AsO3H + 2RS + H2O
(3)
Their data are difficult to explain because they do not fall on a straight
line when the rates are plotted against Hammett’s substituent constant
18
σ. It should be mentioned that the arsonous acid in this system does
not add (di)oxygen on the As(III), but acts as a nucleophile, along with
−
19
the HO , on the disulfide.
The oxide 2 was air oxidized, according to Eq. (4), to diphenylarsinic
acid faster in chloroform than in methanol (Figure 1). This may be due,
3
in part, to the greater solubility of dioxygen in chloroform (∼20 cm /
3
20
1
00 mL) than in methanol (∼5 cm /100 mL).
Ph2As-O-AsPh2 + O2 + H2O → 2Ph2AsO2H
(4)
The greater reactivity of 2 compared to 1 for air oxidation in chloroform
5
(
this work) and in benzene/pentane is obviously due to the two (weak)
electron-withdrawing phenyl groups in 2 compared to one phenyl group
in 1.
Conceivably, there are two routes for the air oxidation of 2 (Scheme 1).
The first, “hydrolysis-oxidation” gives diphenylarsinous acid, which
3
then binds to O2 to give the peracid Ph2AsO3H, which can oxidize
the diphenylarsinous acid, thus producing two molecules of dipheny-
larsinic acid. The second route is “binding-decomposition.” In this, a
direct binding of 3O2 to As(III) is assumed, giving a diradical, which
rearranges to a zwitterion. The latter, by adding a water molecule, gives
a hydroperoxy compound, which collapses into two diphylarsinic acid
molecules. This route also rationalizes the air oxidation of Ph2As-S-
AsPh2 in MeCN.16 The low reactivity of 2 dissolved in methanol is dif-
ficult to explain. Probably, in methanol, the intramolecularly hydrogen
bonded 6-membered intermediate, Scheme 1, is not formed. Our data
do not permit a clear selection of a favorable route for the air oxidation
of 2.

7
54
T. D. Sideris and P. V. Ioannou
FIGURE 1 Air oxidation of (PhAsO)
x 2 2
, 1 and Ph As-O-AsPh , 2 in chloroform
and in methanol, and of 2 + H
2
O (1:1 mol/mol) in methanol. Phenylarsonic acid,
produced from the air oxidation of 1, was not detected by IR.
While the oxides 1 and 2 in methanol were found to be air oxidized to
a very small extent, their hydrated forms (arsonous and arsinous acids)
seem to be more easily oxidized by air. Thus, a substituted phenylar-
sonous acid in water was oxidized to its arsonic acid, and inhibition of
2
1
the oxidation was observed after acidification or addition of glycine.
The protective effect of these reagents is most likely due to a weak hy-
drogen bond formation with the lone pair of As(III), which can prevent
3
the binding of O2 to As(III). The case of air oxidation of diphenylarsi-
7
nous acid in methanol, mentioned in the introduction, may be due to
the fact that upon dioxygen binding, the adduct is converted very easily
into a hyperoxy acid, Ph2AsO3H, which can oxidize the arsinous acid
(
Scheme 1), the ascorbic acid, or the hydriodic acid.
Air Oxidation of the Sulfur-Containing As(III) Esters 3–6
Figures 2 and 3 show the air oxidation of the esters 3–6 in chloroform
and in methanol. Again, the air oxidations were faster in chloroform
which, in part, may be attributed to the greater solubility of dioxygen
2
0
in chloroform. From Figure 2, it can be seen that 4, having two (weak)
electron-withdrawing phenyl groups, is air oxidized faster than 3, which

Air Oxidation of Some Arsenic(III) Compounds
755
SCHEME 1
has one phenyl group bound to As(III). Conversely, 6, which has two
weak) electron-donating methyl groups, is air oxidized slower than 5,
(
which has one methyl group bound to As(III) (Figure 3).
Comparing the pairs 3/5, and 4/6 (Figures 2 and 3), we see that 3
is air oxidized slightly faster than 5, while 6 is quite resistant to air
oxidation compared not only to 5 but also to 3 and 4. Thus, it is clear
that the nature of the groups bound to As(III) governs its ability to be
oxidized by air.
Thiophenol is air oxidized after 12 days to an extent of ∼3% in chlo-
roform and <10% in methanol. Therefore, if thiophenol were produced
during the oxidation of 3–6, it could have been detected due to its very
slow rate of oxidation to disulfide. The air oxidation of 3 in both chlo-
roform and methanol gave phenylarsonic acid. Phenylarsine oxide, 1,

7
56
T. D. Sideris and P. V. Ioannou
2 2
FIGURE 2 Air oxidation of PhAs(SPh) , 3 and Ph AsSPh, 4 in chloroform and
in methanol.
was not detected. Thiophenol was not detected, either by smell or by
loss of weight, while diphenyl disuslfide was detected by TLC. The air
oxidation of 4 in chloroform gave diphenylarsinic acid and diphenyl
disulfide, while in methanol, diphenylarsinic acid, disulfide, and small
amounts of thiophenol were produced. The presence of diphenyl disul-
fide did not affect the air oxidation of 5 (Figure 3). Therefore, we studied
the air oxidation of equimolar mixtures of 5 and PhSSPh as well as 6
and PhSSPh (simply obtained by the reduction of equivalent amounts
of methylarsonic or dimethylarsinic acids and thiophenol in methanol
and drying in vacuum). The air oxidation of 5 gave methylarsonic acid
while that of 6 gave dimethylarsinic acid.
As in the case of the arsine oxides, there are two routes for the expla-
nation of the data shown in Figures 2 and 3. The first is the “hydrolysis-
1
5
oxidation,” suggested by Cullen and colleagues, which invokes the hy-
drolysis of the arsenic(III) thioesters to arsonous or arsinous acids and
thiol, followed by the oxidation to arsonic or arsinic acids and disulfide.
This route probably is not followed during the air oxidation of the esters
8
12
(
ArS)3As and certain (RS)3As, but it must be considered for the air
oxidation of 1–6. It should be noted, however, that this route eventually

Air Oxidation of Some Arsenic(III) Compounds
757
FIGURE 3 Air oxidation of MeAs(SPh)
2
, 5, and of equimolar mixtures of
5
/PhSSPh and Me AsSPh, 6/PhSSPh in chloroform and in methanol.
2
requires the binding of dioxygen to arsonous or arsinous acids in order
to produce arsonic and arsinic acids. Such a binding will produce di-
radicals, which will rearrange to the peracids RAsO4H2 and R2AsO3H
(
see Scheme 1), which can oxidize the thiol to disulfide, which otherwise
22
requires a radical initiator and is fast in nonprotic solvents and takes
place faster in alkaline solution.23 The other route, which can be named
“
binding-decomposition,” assumes the direct binding of dioxygen to
+
As(III) of 3–6 to give a diradical (see Scheme 1) or a zwitterion, As(V) -
O-O , which can be interconverted via an arsenadioxirane, As(V)(O2).
−
8
The zwitterion, by addition of one water molecule, can be converted
into a hydroperoxy compound, As(V)(OH)(OOH) (see Scheme 1). The
8
diradical from 3 (and 5) can decompose to the unstable esters of As(V),
R-As(O)(SPh)2, which can either be hydrolyzed to R-AsO3H2 and PhSH
or by expulsion of PhSSPh to give the oxides (RAsO)x. Since thiophenol,
(
PhAsO)x or (MeAsO)x were not detected, this route may not be followed
during the air oxidation of 3 and 5 in chloroform or in methanol. The
air oxidation of 4 and 6 in methanol may follow this mechanism be-
cause the hydrolysis of the formed Ph2As(O)SPh gives diphenylarsinic
acid and thiophenol, both of which were detected along with diphenyl

7
58
T. D. Sideris and P. V. Ioannou
disulfide. Overall, our data do not permit a clear indication on which
substrate the dioxygen binds: the arsonous acid, the arsinous acid, or
the thioesters 3–6.
To summarize the air oxidation of 1–6 should involve the binding
3
of O2 either to intact 1–6 or their hydrolysis products (arsonous and
arsinous acids) producing a diradical. For a diradical to be formed, a
∗
3
24
transfer of a π electron from O2 into a contracted, empty, 4d or-
bital of As(III) is envisioned.25 It follows, then, that suitable electron-
withdrawing groups on As(III) will contract the 4d orbitals to a point
that the electron transfer is favorable, while electron-donating groups
will have the opposite effect. It seems that a decision on which As(III)
compound, i.e., on the intact oxides 1 and 2, their hydrated forms and
esters 3–6, or their hydrolysis products, dioxygen binds more easily can
be resolved only by theoretical calculations.
In spite of the uncertainty on the timing of 3O2 binding on As(III)
compounds, the ability of As(III) to bind and activate dioxygen may have
implications in chemotherapy14 or the induction of carcinogenesis
26
2
7,28
because both may involve reactive oxygen species.
As far as
work from this lab-
oratory indicates that the preparation of As(III) compounds [(ArS)3As/
8
,12,14
chemotherapy is concerned, this and previous
ꢀ
ꢀ
ꢀ
ꢀ
(
RS)3As, Ar-As(SR )2/R-As(SR )2, and Ar2As-SR /R2As-SR ] suitable for
binding (and activating) O2 can be planned.
3
EXPERIMENTAL
Phenylarsine oxide, 1, was prepared by the reduction of phenylar-
2
9
sonic acid (Aldrich) with ascorbic acid/iodine. Bis(diphenylarsenic)
3
0
oxide, 2, was prepared by the reduction of diphenylarsinic acid
7
◦
31
◦
with ascorbic acid/iodine, m.p. 91–93 C (lit. 92.5–93.5 C). Diphenyl
phenyldithioarsonite, 3, and diphenyl methyldithioarsonite, 5, were
prepared according to the literature.32 Methylarsonic acid was pre-
pared by the neutralization of its disodium salt by an equivalent
amount of hydrochloric acid and extracting with methanol, m.p. 158–
3
3
◦
34
◦
1
60 C (lit. 160–161 C). Cacodylic acid (dimethylarsinic acid) was from
Serva. Equimolar quantities of 5/PhSSPh and of 6/PhSSPh were pre-
3
2
pared by reducing equivalent quantities of the acids with thiophenol.
Silica gel 60 H for thin layer chromatography (TLC) was from
Merck.
TLC was run on microslides always using appropriate standards. The
spots were made visible by iodine vapors. Melting points were obtained
on an Electrothermal model 9100 apparatus. IR spectra were obtained
on a Perkin-Elmer model 16PC FT-IR spectrometer.

Air Oxidation of Some Arsenic(III) Compounds
759
Preparation of Phenyl Diphenylthioarsinite, 4
To a de-aerated solution of the oxide 2 (142 mg, 0.3 mmol) in methanol
(
1 mL), thiophenol (65 µl, 0.6 mmol) was added. The product 4 pre-
◦
cipitated at once as an oil, which was solidified by cooling at –20 C
◦
for 15 min. The solid was broken at r.t., cooled at +4 C for 2 h, cen-
◦
trifuged while cold, washed with methanol (1 mL), cooled at +4 C and
centrifuged while cold, to give the product 4 (180 mg, 89%) as a white
◦
35
◦
powder, m.p. 40–42 C (lit. 40 C). IR (KBr): 3062 w, 1576 m, 1476 vs,
430 s, 1300 w, 1156 w, 1076 s, 1020 s, 996 s, 734 vs, 690 vs.
1
Air Oxidation of the Oxides 1 and 2
Air Oxidation of 1
Samples of 1 (20.0 mg) dissolved in chloroform or in methanol (3 mL)
40 mM) were stirred in air at r.t., replenishing the solvent from time to
(
time. After a given time, a sample was evaporated giving an oil which
was triturated with ether (0.5 ml) in which both 1 and phenylarsonic
acid are insoluble. The solid obtained was analysed by IR. Phenylar-
sonic acid was not detected after 12 days stirring at r.t. Control experi-
ments showed that 20% phenylarsonic acid in 1 (dissolved in methanol,
evaporation and drying) can be clearly detected by IR (As O stretching
−
1
at 878 cm ).
Air Oxidation of 2
Samples of 2 (20.0 mg) were dissolved in chloroform or in methanol
3 ml) (14 mM) and stirred at r.t. as in 1. To the glass obtained, ether
0.5 ml) was added to crystallize the diphenylarsinic acid and to ex-
(
(
tract the oxide 2. The ether was syringed off, the solid washed with
ether (0.5 ml), dried, weighed and, in the case of chloroform, charac-
◦
30
◦
terized by m.p. (165–170 C; lit. 174 C) and IR. Results are shown in
Figure 1.
Air Oxidation of the Thioesters 3–6
Air Oxidation of 3
Samples of 3 (20–40 mg) were dissolved in chloroform or in methanol
3–6 ml) (18 mM) and stirred in air at r.t. replenishing the solvent
(
from time to time. The smell of thiophenol was not perceptible during
the runs. Evaporation and drying in vacuum gave a glass, weighing
more than the weighed 3, implying that thiophenol was not present in
the solution. Calculations (based on the phenylarsonic acid isolated in
the next step) showed that the weight matched the expected weight

7
60
T. D. Sideris and P. V. Ioannou
from the phenylarsonic acid plus diphenyl disulfide produced. TLC
petroleum ether) showed the presence of PhSSPh. Therefore the gain
(
in weight was due to the formation of phenylarsonic acid. This was ver-
ified by extracting the disulfide with boiling ether, leaving a solid which
◦
34
◦
was phenylarsonic acid by m.p. (150–153 C; lit. 158–162 C) and IR.
The ether insoluble (PhAsO)x was not detected by IR. Results are shown
in Figure 2.
Air Oxidation of 4
Samples of 4 in chloroform or in methanol (∼20 mM) were air oxi-
dized as in the case of 3. The smell of thiophenol was not perceptible
during the runs. The crude solids obtained after evaporation and drying
in vacuum weighed more than the weighed 4, indicating that dipheny-
larsinic acid and diphenyl disulfide were the components of the crude
solids. However, calculations based on the amounts of the dipheny-
larsinic acid, isolated in the next step, showed that in methanol, but
not in chloroform, thiophenol also was produced. The extraction of the
disulfide with boiling ether left diphenylarsinic acid by IR and m.p.
◦
30
◦
(
165–169 C; lit. 174 C), from which the percent air oxidation of 4,
shown in Figure 2, was calculated.
Air Oxidation of 5
The experiments were done using either pure 5 or an equimolar mix-
ture of 5 and PhSSPh. The solid obtained was methylarsonic acid by IR
◦
34
◦
and m.p. (155–158 C; lit. 160–161 C). The (MeAsO)x has an m.p. of
◦
36
95 C.
Air Oxidation of 6
This air oxidation was done using an equimolar mixture of 6 and
PhSSPh. The solid obtained was dimethylarsinic acid (by IR only be-
cause of the small amounts of the isolated solid).
Air Oxidation of Thiophenol
A solution of thiophenol (51 µl, 0.5 mmol) in chloroform (3 mL) (0.17 M)
was stirred in air for 12 days, replenishing the solvent from time to time.
Then a solution of mercuric acetate (80 mg, 0.25 mmol) in methanol
(
4 mL) was added, and the precipitated mercaptide (86 mg) was cen-
trifuged. The supernatant (16 mg) contained PhSSPh (by TLC), and by
extraction with ether gave PhSSPh (2.5 mg) corresponding to ∼3% oxi-
dation of thiophenol. The oxidation in methanol was done similarly, but
before the addition of mercuric acetate, the methanol was evaporated

Air Oxidation of Some Arsenic(III) Compounds
761
and the solid dissolved in chloroform (3 mL). Otherwise most of the
mercaptide remains in methanol. A <10% air oxidation of thiophenol
in methanol was estimated.
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