Kinetics and mechanism of benzoin oxidation using iron(III) in the presence of ferrozine or 2,2′-bipyridine in acidic medium

Article abstract of DOI:10.1002/kin.20092

Kinetics and mechanism of oxidation of benzoin (H₂B) by ferrozine (Fz) or 2,2′-bipyridine (bipy) have been carried out in aqueous HNO₃ medium. The rate shows first-order dependence on [H ₂B] and [Fe³⁺| and inverse second-order dependence on (H⁺). The rate of reaction increased with increase in the ligand concentration. The increase in dielectric constant will increase the rate, while increase in [HNO₃] decreased it. Substituent and temperature effects on the rates have been investigated. The rate laws derived are in excellent agreement with the experimental results. Plausible mechanisms are suggested.

Full text of DOI:10.1002/kin.20092

Kinetics and Mechanism of
Benzoin Oxidation Using
Iron(III) in the Presence of
ꢀ
Ferrozine or 2,2 -Bipyridine
in Acidic Medium
KHALDOUN A. AL-SOU’OD, BASEM F. ALI, RAJAB ABU-EL-HALAWA,
ABD-AL-HAKEEM H. ABU-NAWAS
Chemistry Department, Al al-Bayt University, Mafraq, Jordan
Received 6 December 2004; accepted 25 February 2005
DOI 10.1002/kin.20092
Published online in Wiley InterScience (www.interscience.wiley.com).
ꢀ
ABSTRACT: Kinetics and mechanism of oxidation of benzoin (H B) by ferrozine (Fz) or 2,2 -
2
bipyridine (bipy) have been carried out in aqueous HNO medium. The rate shows first-order
3
3+
+
dependence on [H B] and [Fe ] and inverse second-order dependence on [H ]. The rate of
2
reaction increased with increase in the ligand concentration. The increase in dielectric constant
will increase the rate, while increase in [HNO ] decreased it. Substituent and temperature ef-
3
fects on the rates have been investigated. The rate laws derived are in excellent agreement with
the experimental results. Plausible mechanisms are suggested. ꢁC 2005 Wiley Periodicals, Inc.
Int J Chem Kinet 37: 444–449, 2005
INTRODUCTION
transfer through inner sphere [18,19] or outer sphere
20] mechanisms. In a continuation of our work on
[
The oxidation of benzoin (H2B) to benzil by a vari-
ety of oxidizing agents such as 1,10-phenanthrolein
benzoin, we report herein the oxidation kinetics of ben-
zoin with iron(III) in the presence of ferrozine (Fz) or
2,2 -bipyridine (bipy) in acidic medium. The structure
ꢀ
[1], sulfur [2], bismuth triacetate [3], hexacyanofer-
rate(III) [4], and nitric acid [5] has been a subject of
a considerable number of studies [1–10]. The mecha-
nisms and kinetics of the oxidation of benzoins have
been previously studied [11–14]. Oxidation of organic
compounds by complex ion may occur via free radical
and further oxidation to yield the products [15–17].
Also, the oxidation processes may involve electron
of ferrozine is shown in Fig. 1.
EXPERIMENTAL
Materials
Benzoin (H2B) derivatives were prepared as reported
in the literature [21–23]. All other chemicals used
were of analytical grade. Solutions were prepared in
doubly distilled deionized water (second distillation
being from permanganate). All glass vessels were
Pyrex.
Correspondence to: K. A. Al-Sou’od; e-mail: khaldoun@aabu.
edu.jo.
Contract grant sponsor: Al al-Bayt University.
ꢁ
c 2005 Wiley Periodicals, Inc.

KINETICS AND MECHANISM OF BENZOIN OXIDATION USING IRON(III)
445
The resulting benzil (R = H) from oxidation was
separated by extraction using CHCl3 and then by
evaporation of solvent. The product recrystallized
from EtOH/H2O. For (C6H5) C(O)C(O) (C6H5),
◦
◦
m.p. = 92–94 C (lit. m.p. = 94.8 C); UV/VIS (EtOH):
λmax (log ε) 256 nm (4.24). Qualitative identification
of benzil was also made from TLC by comparing the
Rf values for the product from the oxidation reaction
and a standard pure sample of benzil.
Figure 1 Structure of ferrozine.
Kinetic Measurements
The kinetic measurements were carried out at 30 C
◦
+
3
in HNO3 acidic medium under the conditions [Fe ],
Fz], [bipy], [H2B]. All runs were carried out under
Iron(III) Dependence
[
3
+
pseudo-first-order conditions of [H2B] ꢂ [Fe ] and
at constant ionic strength (I) maintained by the addi-
tion of sodium nitrate. The progress of the reaction was
monitored by measuring the absorbance of the formed
complexes at 562 and 255 nm against time for iron(III)-
The kinetic measurements were carried out under
pseudo-first-order conditions. Initial concentration of
−
4
iron(III) was 2.0 × 10 M. A plot of ln(A∞ − At )
against time was linear indicating a first-order depen-
dence on iron(III) (see Fig. 2).
ꢀ
ferrozine and iron(III)-2,2 -bipyridine, using a Unicam
UV/VIS spectrophotometer with 1.0 cm cell. The iso-
lation method was used in this study where the con-
centration of the substance to be monitored is changed
with the others kept constant. Aqueous solutions are
prepared by making a constant total volume for all
runs. These solutions were made by mixing appropriate
amount of reagents in a sequence (Fe(III) first, followed
byHNO3 andthenNaNO3, ligand, andfinallybenzoin).
The blank solution is prepared in a similar manner with
no benzoin added. The solutions and blank were equi-
H B Dependence
2
Isolation method was used to determine the order with
respect to H2B. Different initial concentrations of H2B
were prepared while keeping the other parameters con-
stant. It was found that kobs increased as [H2B] in-
creased (Table I). Plot log kobs versus log[H2B] was
linear with a slope = 1, indicating a first order with re-
spect to H2B. Plot of 1/kobs versus 1/[H2B] was linear
with a positive intercept (Fig. 3).
◦
librated at 30 C for 24 h (completion of reaction) and
then absorbance is measured (A∞).
Ligands Dependence
The pseudo-first-order rate constant (kobs) was ob-
tained from the slope of the ln(A∞ − At ) against time
plot (where A∞ and At are absorbances at infinite time
and at t respectively). The temperature was kept con-
stant using a liquid circulating bath C85A.
Effect of ligand on the pseudo-first-order rate con-
stant (kobs) was studied at 30 C by varying the lig-
◦
−
3
−3
and concentrations from 3.0 × 10 to 7.0 × 10
M
and keeping the other parameters constant. Rate con-
stant increases with increase in ligand concentrations
(Table I). These kinetic results reflect the higher
RESULTS AND DISCUSSION
Stoichiometry and Product Analysis
Known amount of H2B reacted completely with ex-
◦
cess of iron(III) at 30 C in 0.01 M HNO3 (50% aque-
ous ethanol (v/v)) in the presence of excess Fz or bipy
for 24 h. Estimation of unreacted iron(III) showed that
2
moles of iron(III) was consumed for each mol of H2B
in accordance with the following equation:
3
+
Figure 2 Plot of ln(A∞ − At) vs. time at [Fe ] = 2.0
−
4
−3
−3
×
10 M, [H2B] = 2.0 × 10 M, [L] = 7.0 × 10 M,
+
−2
[
H ] =1.0 × 10 M, I = 0.06 M, 50% ethanol–water (v/v)
◦
and 30 C.

446
AL-SOU’OD ET AL.
+
◦
at 30 C.
Table I Effect of [H B], [L], and [H ] on k
2
obs
[
(
Fe3+] = 2.0 × 10−4 M, I = 0.06 M, 50% ethanol-water
v/v)
+
[
1
H2B] × [bipy] × [H ] ×
3
3
2
−1
kobs (min )
0 (M) 10 (M) 10 (M)
2.02 × 10− ± 5.59 × 10
3
−5
−5
−5
−5
−5
−5
−5
−5
−5
−5
−5
−5
−5
−5
−5
2
3
4
5
6
4
4
4
4
4
4
4
4
4
4
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
7.0
7.0
7.0
7.0
7.0
3.0
4.0
5.0
6.0
7.0
4.0
4.0
4.0
4.0
4.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.4
2.0
2.6
3.2
−
3
3.01 × 10 ± 5.77 × 10
−3
3.93 × 10 ± 6.35 × 10
−
3
4.80 × 10 ± 8.67 × 10
−3
5.74 × 10 ± 8.93 × 10
−
3
1.71 × 10 ± 4.88 × 10
−3
2.44 × 10 ± 6.63 × 10
2
3+
−4
,
−
3
Figure 4 Plot of 1/k vs. 1/[L] at [Fe ] = 2.0 × 10
3.10 × 10 ± 9.01 × 10
obs
+
−2
−3
[H ] = 1.0 × 10 M, I = 0.06 M, 50% ethanol–water (v/v)
3.63 × 10 ± 4.81 × 10
◦
and 30 C.
−
3
4.01 × 10 ± 7.08 × 10
−3
3.73 × 10 ± 3.36 × 10
−
3
3.13 × 10 ± 5.71 × 10
−3
+ 2
2.71 × 10 ± 6.34 × 10
The plot of 1/kobs versus [H ] was linear with a posi-
−3
2.32 × 10 ± 6.98 × 10
2.01 × 10− ± 6.01 × 10
tive intercept (Fig. 5).
3
Substituent Effects
Substituent effects on the oxidation rates of H2B have
been studied at 30 C (Table II). It was found that
oxidizing ability of the 1:2 formed complex [FeL2]2
+
◦
compared to uncomplexed Fe(III). This may be due
to the ability of ligand to increase the stability of the
the electron-withdrawing groups enhance the oxida-
tion rate, whereas the electron-donating groups inhibit
it. A plot of log(k/kH) versus substituent constant (ꢀ)
was linear (Fig. 6) in accord with Hammett’s equation.
The reaction constant (ꢁ) was equal to ≈1.2. This indi-
cates that the reaction site has a higher electron density
in the transition state compared to the starting material.
2
+
complex in the lower oxidation state of iron [FeL2]
over the higher one [FeL2]3 , through back donation
15]. The plots of 1/kobs versus 1/[L] were linear with
+
2
[
positive slopes and intercepts (Fig. 4).
Hydrogen Ion Dependence
Ionic Strength Dependence
The values of kobs were determined at different hydro-
gen ion concentrations (Table I) with ionic strength and
other reactants concentrations kept constant. kobs was
found to decrease with increase in acid concentration.
The influence of ionic strength on kobs was studied at
◦
3
0 C. Different initial concentrations of KNO3 were
prepared while keeping the other parameters constant.
+
+ 2
−3
Figure 3 Plot of 1/kobs vs. 1/[H2B] at [H ] = 1.0 ×
Figure 5 Plot of 1/k vs. [H ] at [H B] = 4.0 × 10 M,
obs
2
−2
3+
−4
3+
−4
1
0
M, [Fe ] = 2.0 × 10 , I = 0.06 M, 50% ethanol–
I = 0.06 M, [Fe ] = 2.0 × 10 , 50% ethanol–water (v/v)
◦
◦
water (v/v) and 30 C.
and 30 C.

KINETICS AND MECHANISM OF BENZOIN OXIDATION USING IRON(III)
447
Table II Substituent Effect on kobs at [Fe3 ] = 2.0 × 10 M, [H B] = 4.0 × 10 M, [L] = 0.004 M (Fz), and 0.007 M
+
−4
−3
2
+
◦
(
bipy), [H ] = 0.01 M, I = 0.06 M, 50% ethanol-water (v/v) at 30 C
3
−1
−1
Substrate
Substituent Constant (σ)
kobs× 10 (min ) (Fz)
kobs (min ) (bipy)
−
3
−5
−5
−5
−5
−5
−5
−3
−5
−5
−5
−5
−5
−5
H
0.0
0.11
0.37
3.41 × 10 ± 6.64 × 10
4.02 × 10 ± 7.32 × 10
ꢀ
ꢀ
−3
−3
m,m -CH3O
m,m -Cl
pp -Cl
p,p -CH3
5.13 × 10 ± 6.99 × 10
5.31 × 10 ± 6.59 × 10
−3
−3
1.14 × 10 ± 4.72 × 10
1.20 × 10 ± 3.05 × 10
ꢀ
−3
−3
0.22
7.20 × 10 ± 8.43 × 10
7.64 × 10 ± 9.11 × 10
ꢀ
ꢀ
−3
−3
−0.17
2.41 × 10 ± 5.51 × 10
2.61 × 10 ± 4.44 × 10
−3
−3
p,p -CH3O
−0.28
1.93 × 10 ± 5.66 × 10
2.13 × 10 ± 3.81 × 10
From data in Table III it can be observed that higher
kobs are attained at higher ionic strengths. Using kobs
data at different ionic strengths, a graph was obtained
employing the Brønsted–Bjerrum equation [24] based
on the extended law of Debye–H u¨ ckel (Fig. 7). The
10–4 M while the hydrogen ion concentration was in
the range 0.01–0.018 M. Therefore, it is quite reason-
able to assume that there is no appreciable dimerization
undertheseconditions[27]andtheconcentrationofhy-
droxylated Fe(III) is neglected due to low-hydrolysis
constant. The hydrolysis is also suppressed, since the
ligand (Fz or bipy) was present in large excess. The sec-
ond step in the mechanism may proceed by the outer
sphere route (step 3a in Scheme 1) or by inner sphere
reaction (step 3b in Scheme 1). Steps 4 and 5 describe
the formation of final products. In step 4, the formed
−
3
rate constant at infinite dilution (k0) is 1.32 × 10
−3
−1
and 1.24 × 10 min for Fz and bipy, respectively.
The product of the charges ZA ZB ≈ 3. This is the same
expected value considering the proposed mechanism.
Thermodynamic Parameters
·
3+
radicals HB react with 1 mol of [FeL2] to form ben-
The reaction was studied at different temperatures
2+
zil (B). Then in step 5, 1 mol of [FeL2] reacts with
(298–313 K). Plots of ln kobs and ln(kobs.h/kBT ) versus
+
another mol of HL to form a more stable complex
1
/T yield the activation energy Ea, enthalpy of acti-
2+
[FeL3]
.
ꢃ=
ꢃ=
vation ꢀH , entropy of activation ꢀS , and Gibbs
energy of activation ꢀG (Table IV).
The derived rate laws from the proposed mechanism
are given below.
ꢃ
Since the pKa values for bipy [25] and Fz [26] are
For Inner Sphere Mechanism
4
.33 and 3.13, respectively, they exist mainly as proto-
+
nated species (HL ) in the examined acid range (0.01–
3+
−
d[Fe
]
0
.018 M). It is reasonable to consider that the first step
Rate =
dt
in the mechanism involves the fast formation of the lig-
andprotonatedspecies(Scheme1). Inthepresentstudy,
the Fe(III) concentration employed was on the order of
2
3+
kK1 K2[L] [Fe ][H2B]
=
(1)
2
2
+ 2
K [L] + K K [H B][L] + [H ]
1
1
2
2
Fast
ꢁ
+
+
1
2
3
. H + L ꢀ HL
K1
ꢁ
. Fe3 + 2HL+ ꢀ [FeL2] + 2H+
+
3+
k
a. [FeL2]3 + H2B −→ [FeL2] + H + HB
+
2+
+
·
slow
or
K2
ꢁ
3
b. [FeL2]3 + H2B ꢀ [FeL2BH2]
+
3+
k
FeL2BH2]3 −→ [FeL2] + H + HB
+
+
·
[
slow
3
+
Fast
2+
+
4
. HB + [FeL2] −→ [FeL2] + H + B
Figure 6 Plot of ꢀ vs. log(kobs/kH) at [Fe3 ] = 2.0 ×
+
2+
+
Fast
2+
5
. [FeL2] + HL −→ [FeL3] + H+
−4
−3
1
0
0
M, [H2B] = 4.0 × 10 M, [L] = 0.004 M (Fz), and
+
.007 M (bipy), [H ] = 0.01 M, I = 0.06 M, 50% ethanol–
Scheme 1 Proposed mechanism of benzoin oxidation by
◦
3+
water (v/v) at 30 C.
Fe in the presence of Fz or bipy.

448
AL-SOU’OD ET AL.
at [Fe3 ] = 2.0 × 10 M, [H B] = 4.0 × 10 M, [L] = 0.004 M (Fz), and
+
−4
−3
Table III Effect of Ionic Strength on k
0
obs
2
◦
+
.007 M (bipy), [H ] = 0.01 M, I = 0.06 M, 50% ethanol–water (v/v) at 30 C
Ionic
Strength
(M)
0.020
0.030
0.040
0.050
kobs(bipy) 2.60 × 10 ± 4.98 × 10−5 3.14 × 10−3± 5.79 × 10−5 3.44 × 10−3 ± 7.35 × 10−5 3.74 × 10−3 ± 6.01 × 10−5
−
3
−
1
min
kobs(Fz) 2.84 × 10− ± 3.18 × 10−5 3.39 × 10−3 ± 4.71 × 10−5 3.81 × 10−3 ± 5.99 × 10−5 4.1 × 10−3 ± 7.64 × 10−5
3
−
1
min
according to Eq. (1), kobs can be obtained:
This rate law demands that the plots of 1/kobs ver-
2
sus 1/[H2B], 1/kobs versus 1/[L] , and 1/kobs versus
2
+ 2
kK1 K2[L] [H2B]
[H ] should be all linear with positive intercepts on
kobs =
(2)
(3)
2
2
+ 2
the 1/kobs axis. This is actually found to be the case
as shown in Figs. 3–5 suggesting the compatibility
of inner sphere mechanism with experimental results
K1[L] + K1 K2[H2B][L] + [H ]
the inverse of kobs is given by Eq. (3):
(Eq. (3)). Values of k, K1, and K2 calculated from the
+
1
1
1
[H ]2
interceptsandslopesoflinesinFigs. 3and5arelistedin
Table IV.
= k
+
+
kK2[H2B]
+
kK1 K2[L] [H2B]
2
kobs
ꢀ
ꢁ
The estimated activation parameters (Table IV) in-
dicate that the oxidation rate of benzoin readily occurs
in the presence of bipy compared to Fz. The large nega-
+
1
1
1
[H ]2
= k
+
[H2B] kK2
kK1 K2[L]2
ꢃ
tive ꢀS values indicate the formation of the compact
For outer sphere mechanism, the rate law, kobs and
its inverse are given by Eqs. (4)–(6).
activated complex in accord with inner sphere mecha-
nism.
2
3+
kK1[L] [Fe ][H2B]
Rate =
(4)
(5)
2
+ 2
K1[L] + [H ]
kK1[L] [H2B]
CONCLUSION
2
kobs =
2
+ 2
K1[L] + [H ]
Oxidation of benzoin in acidic medium using Fe(III)
in the presence of Fz and bipy was studied along with
kinetics and mechanisms. Kinetics was carried out un-
der pseudo-first-order conditions and at constant ionic
strength. The order of oxidation reaction with respect
to reactants was studied and found to be of a first
order with respect to both Fe(III) and H2B. More-
over, the effect of ligand as well as the hydrogen ion
concentrations on the pseudo-first-order rate constant
was examined. These studies show that the rate con-
stant increases with increase in ligand concentration,
+
1
=
1
[H ]2
+
2
kobs
k[H2B]
kK1[L] [H2B]
ꢀ
ꢁ
1
1
[H ]2
+
= [
+
(6)
kK1[L]2
H2B] k
+
while it decreases with increase in [H ]. The H2B sub-
stituent effects on the oxidation were also investigated
in accord with Hammett’s equation. Results revealed
that electron-withdrawing groups enhance oxidation,
while the electron-donating groups inhibit it. Further-
more, the ionic strength dependence was studied using
the Brønsted–Bjerrum equation. The higher the ionic
strength the larger is the rate constant. According to
the results deduced from plots of 1/k versus 1/[H B],
3
+
Figure 7 Plot of ionic strength vs log kobs at [Fe ] = 2.0 ×
obs
2
2
+ 2
−4
−3
+
1
0
M, [H2B] = 4.0 × 10 M, [H ] = 0.01 M, I = 0.06 M,
1/kobs versus 1/[L] , and 1/kobs versus [H ] (Figs. 3–5)
and the estimated activation parameters (Table IV), the
◦
5
0% ethanol–water (v/v) at 30 C.

KINETICS AND MECHANISM OF BENZOIN OXIDATION USING IRON(III)
449
ꢃ
ꢃ
ꢃ
3+
3+
Table IV Values of k, K , K , Ea , ꢀH , ꢀS and ꢀG for [Fe(Fz)2] and [Fe(bipy)2]
1
2
ꢃ
ꢃ
ꢃ=
ꢀG
(kJ/mol)
Ea
(J/mol K)
ꢀH
ꢀS
k
K1
−1
K2
−1
Complex
(kJ/mol)
(J/mol K)
(min 1)
−
(M
)
(M
)
Fe(Fz)2]3
+
55.31
49.70
52.77
47.16
−148.09
−169.3
98.04
98.92
0.0448
0.0923
8.514
9.869
40.966
16.827
[
[
3
+
Fe(bipy)2]
oxidation process is most likely to proceed via inner
sphere mechanism.
14. Sun, W.; Norikazu, U.; Nakamura, A. Tetrahedron 1993,
49, 1357–1370.
1
5. Rao, P. V. S.;Suryanarayana, M.;Subbaiah, K. V.;Murty,
P. S. N. J Inorg Nucl Chem 1980, 42, 1181–1184.
6. Subbaiah, K. V.; Rao, P. V. S.; Murty, B. A. N.; Murty,
P. S. N. J. Indian Chem Soc 1979, 56, 1213–1216.
BIBLIOGRAPHY
1
1
2
. El-Barghouthi, M. I.; Al-Sou’od, K. A.; Abu-Nawas,
A. H.; Abu-El-Halawah, R. Int J Chem 2004, 14, 77–
17. Pelizzetti, E.; Mentasi, E.; Pramauro, E. Inorg Chem
1976, 15, 2898–2900.
18. Sharaydeh, B. F.; Abu-Eid, M. A.; Zatar, N. A.;
Abu-Obied, A.; Khamis, M.; Kanan, K. Sci Technol
1994, 22, 355–358.
8
4.
. Zmachinskii, E.; Malishevskaya, L. I. J Gen Chem 1937,
, 2693–2695.
7
3
4
. Rigby, W. J Chem Soc, Perkin Trans II 1951, 793–795.
. Jarrar, A.; El-Zaru, R. A. J Chem Educ 1977, 54, 326–
19. Frank, M. S.; Ramaiah, A. K.; Rao, P. V. K. Ind J Chem,
Sect A 1979, 18, 369–371.
3
27.
. Weiss, M.; Appel, M. J Am Chem Soc 1932, 70, 3666–
670.
20. Pelizzetti, E.; Mentasi, E.; Pramauro, E. Inorg Chem
1978, 17, 1688–1690.
21. Lutz, R. E.; Murphey, R. S. J. J Am Chem Soc 1949, 71,
478–481.
22. Mubarak, M. S. Kinetics of Oxidation of Benzoin by
Hexacyanoferrate(III) in Alkaline Medium; Mubarak,
M. S, (Ed.); University of Jordan: Amman, Jordan,
1977.
23. Yousef, A. A. Solvent Effects on the Oxidation of Ben-
zoins by Hexacyanoferrate(III); Yousef, A. A, (Ed.);
University of Jordan: Amman, Jordan, 1989.
24. N o´ berga, J. A.; Rocha, F. R. P. J Chem Educ 1997, 74,
560–562.
5
6
7
8
9
3
. Yamarnoto, J.; Murakami, M.; Kameoka, N. Bull Chem
Soc Jpn 1982, 55, 345–346.
. Hatanaka, Y.; Imamoto, T.; Yokoyama, M. Tetrahedron
Lett 1983, 24, 2399–2400.
. Firouzabadi, H.; Iranpoor, N.; Kiaeezadeh, F.; Toofan, J.
Tetrahedron 1986, 42, 719–725.
. Binoy, J.; Vishnu, M. V.; Sreedharan, P.; Vadakkan, J.
Synth Commun 2002, 32, 2495–2497.
1
1
1
1
0. Dey, I.; Nongkynrih, I.; Mahanti, M. K. Oxidation Com-
munications 1993, 16, 124–127.
1. Chaplin, R. P.; Walpole, A. S.; Zadro, S.; Vorlow, S.;
Wainwright, M. S. J Mol Cat 1984, 22, 269–271.
2. Belkov, I. A.; Kuzbas, F. G. Kinet I Katal 1988, 29,
25. Baxendale, J. H.; George, P. Trans Faraday Soc 1950,
46, 55–63.
26. Thompson, J. C.; Mottola, H. A. Anal Chem 1984, 56,
755–757.
27. Milburn, R. M.; Vosburgh, W. C. J Am Chem Soc 1955,
77, 1352–1358.
1
038–1040.
3. Hodali, H. A.; El-Zaru, R. A. Tetrahedron 1990, 9, 2299–
304.
2