Aerobic oxidation of alkenes to esters of vicinal diols with a syn-configuration catalyzed by I2 and the H5PV 2Mo10O40 polyoxometalate

Article abstract of DOI:10.1055/s-2005-917069

A new method for the synthesis of vicinal diols from alkenes has been developed. Reaction of molecular iodine in the presence of a polyoxometalate as oxidation catalyst under aerobic conditions in acetic acid solvent leads to the oxidative iodoacetoxylation of an alkene, i.e. formation of a 1,2-iodoacetate. Further in situ substitution of the iodide by water yields the 1,2-diol monoacetate with a predominantly (ca. 4.5:1) cis-configuration. Further esterification under the reaction's acidic conditions leads also to the cis-diacetate. The method may be valuable for the synthesis of cis-vicinal diols without use of toxic osmium catalysts. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-917069

LETTER
2525
Aerobic Oxidation of Alkenes to Esters of Vicinal Diols with a
syn-Configuration Catalyzed by I2 and the H5PV2Mo10O40 Polyoxometalate
Aerobic
O
xidation
l
of Alk
e
enes to
E
st
n
er
s
of Vicina
a
l
D
iols Branytska, Ronny Neumann*
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, 76100, Israel
Fax +972(8)9344142; E-mail: Ronny.Neumann@weizman.ac.il
Received 15 August 2005
tion. It should be noted that the source of water for the
Abstract: A new method for the synthesis of vicinal diols from
alkenes has been developed. Reaction of molecular iodine in the
presence of a polyoxometalate as oxidation catalyst under aerobic
conditions in acetic acid solvent leads to the oxidative iodoacetoxyl-
hydrolysis of the 1,3-dioxolenium cation is the reoxida-
tion reaction of the reduced polyoxometalate by molecu-
lar oxygen, the catalyst’s water of crystallization and
ation of an alkene, i.e. formation of a 1,2-iodoacetate. Further in situ residual water in the glacial acetic acid solvent. The
substitution of the iodide by water yields the 1,2-diol monoacetate
with a predominantly (ca. 4.5:1) cis-configuration. Further esterifi-
cation under the reaction’s acidic conditions leads also to the cis-di-
since H5PV2Mo10O40 is a highly acidic polyoxometalate.
acetate. The method may be valuable for the synthesis of cis-vicinal
diacetate ester of the cis-diol may be formed by a further
acid-catalyzed esterification reaction with acetic acid
diols without use of toxic osmium catalysts.
I2 + 2 H5PVV2Mo10O40
2 I+H5PVIVVVMo10O40
Key words: aldehydes, arenes, electrophilic aromatic substitutions,
oxidations, quinones
2 I+H5PVIVVVMo10O40 + 2 CH3COOH
2 H6PVIVVVMo10O40 + 1/2 O2
2 I+ + 2 CH3COO–
+
2 H6PVIVVVMo10O40
2 H5PVV2Mo10O40 + H2O
Commonly, cis-1,2-diols are generally prepared using
highly toxic osmium tetraoxide as a stoichiometric
oxidant or catalytically by use of amines as ligands and an
oxygen donor such as N-methylmorpholine oxide.1 cis-
Dihydroxylation of alkenes has also been observed with
some Fe, Ru, Mn and Re complexes.2 The metal based
catalysts seem to have in common the formation of
reactive metal dioxo intermediates and probably react via
[3+2] addition of the dioxo species to the alkene. For
trans-1,2-diols there are more diverse synthetic approach-
es, however, the most commonly used synthesis include
the well known hydrolysis under acidic conditions of
epoxides obtained via epoxidation of alkenes, and reduc-
tive coupling of aldehydes or ketones using a large variety
of reagents and reaction conditions.3
R1
I
R2
O
R1
R2
R1
O
+ I+
O
O
I
R2
R2
R1
R1
R2
R2
R1
O
R1
I
R2
O
H2O
–HI
O
O
O
O
HO
O
I–
O
HO
R1
H5PVV2Mo10O40
R2
+ CH3COOH + 1/2 O2 + I2 + H2O
2
R2
R1
O
+ 2 HI
2
HO
O
Scheme 1 Proposed pathway for the catalytic aerobic iodine-
mediated syn-acetoxylation of alkenes
In order to develop a new method for cis-diol synthesis,
we now extend the concept of aerobic oxidative iodination
of arenes catalyzed by the H5PV2Mo10O40 polyoxo-
metalate4 to the oxygen-mediated oxidative iodination of
alkenes, in what can be described as a catalytic alternative
to the Woodward method of diastereoselective cis-diol
formation.5,6 As outlined in the proposed reaction path-
way, Scheme 1, a catalytically generated I+-type species
may add to a double bond of an alkene yielding a cyclic
iodonium cation, which then is attacked by an acetate
anion giving corresponding O-acyliodohydrin. The O-
acyliodohydrin undergoes an intramolecular nucleophilic
attack that yields an intermediate 1,3-dioxolenium cation,
and finally, the hydrolysis of the 1,3-dioxolenium cation
yields the monoacetate of the diol with a syn-configura-
In order first to test for the reactivity of the proposed
aerobic I2/H5PV2Mo10O40 catalytic system for diacetoxyl-
ation of alkenes, reactions were carried out on terminal
alkenes (Equation 1). A typical procedure involved the re-
action of one equivalent of the alkene substrate with one-
half equivalent of iodine and a catalytic amount, 5 mol%,
of H5PV2Mo10O40 in acetic acid under molecular oxygen
(1–2 bar). As is observable from the results presented in
Table 1 for the diacetoxylation of terminal alkenes, these
alkenes were oxidized to the corresponding diacetates in
high yields and >90% selectivity except for styrene as
substrate where there was significant polymerization.
Other substituted styrene substrates (not shown in
Table 1) with electron-withdrawing moieties (Cl, Br,
NO2) gave similar results whereas form 4-methoxystyrene
only polymer was formed. Polymerization was deduced
SYNLETT 2005, No. 16, pp 2525–2527
0
4
.1
0
.2
0
0
5
Advanced online publication: 21.09.2005
DOI: 10.1055/s-2005-917069; Art ID: G25505ST
© Georg Thieme Verlag Stuttgart · New York

2526
O. Branytska, R. Neumann
LETTER
OH
OAc
OAc
OAc
OAc
I
from an incomplete mass balance upon addition of a
standard for the GC analysis. This polymerization is most
likely due to acid-catalyzed cationic polymerization of
styrene.
OAc
OAc
~100%
Step 1
Step 2
Step 3
10
49
57
<2
16
12
73
9
11
0
OAc
I
OH
0
0
R
R
R
R
OAc
OAc
OAc
1
4
2
3
Scheme 2 The oxidation of cyclohexene and the product distri-
Equation 1
bution after each stage as determined by GC
Table 1 Aerobic Diacetoxylation of Terminal Alkenes Mediated by
I2/H5PV2Mo10O40
OH(Ac)
OAc(H)
OAc
OAc
Substrate, conversion (mol%)
~100%
Product R = n-Bu R = n-Pent R = n-Hex R = n-Oct R = Ph
Step 1
Step 2
Step 3
19
36
63
47
52
0
90
99
99
99
99b
2a
3a
4a
96 (54)c
93 (64)c
91 (72)c
97 (72)c
70 (75)c
4
–
5
2
9
–
3
–
2
–
OAc
I(OAc)
OAc(I)
OAc
Reaction conditions: 1 mmol alkene, 0.5 mmol I2, 0.01 mmol
H5PV2Mo10O40, 1 mL AcOH, O2 (2 atm), 80 °C, 5 h.
a Selectivity to given product relative to all products.
b The remaining product was not eluted by GC and is assumed to be
polymeric material.
Step 1
Step 2
Step 3
7
8
31
8
c Isolated yields are given in parenthesis for reactions carried out with
5 mmol alkene, 2.5 mmol I2, 0.05 mmol H5PV2Mo10O40, 5 mL AcOH,
O2 (2 atm), 80 °C, 5 h.
15
7
Scheme 3 The oxidation of 4-methylcyclohexene and the product
distribution after each stage as determined by GC
After showing that the I2/H5PV2Mo10O40-mediated
aerobic oxidations of terminal alkenes were efficient for
formation of 1,2-mono- and diesters, we wished to vali-
date the method and pathway for formation of syn-substi-
tuted products as formulated in Scheme 1. Towards this
purpose we reacted cyclic alkenes using cyclohexene and
4-methylcyclohexene as model substrates. Under typical
reaction conditions as described for the oxidation of
terminal alkenes significant amount of iodine containing
products were obtained. Therefore the synthesis was
modified to a three stage procedure (i) reaction of 1 mmol
alkene, 0.5 mmol I2, 0.01 mmol H5PV2Mo10O40 in 1 mL
acetic acid under 2 atm O2 at 80 °C for four hours fol-
lowed by (ii) addition of 1.1 mmol pyridine (to generate
the AcO– anion) and 1 mmol H2O and further heating at
80 °C for 16 hours and then (iii) completion of the esteri-
fication reaction (to simplify the analytical analysis) by
addition of 5 mmol acetic anhydride and heating for an
additional two hours. The results are summarized in
Scheme 2 and Scheme 3.
4-methylcyclohexene is possibly due to the steric con-
straints that facilitate elimination of iodine atom resulting
in the intermediate 1,3-dioxolenium cation. The forma-
tion of the trans-isomers may be explained by reaction of
other possible carbocation structures of the 1,3-dioxo-
lenium cation (Equation 2).
R1
O
R2
O
R2
R1
R2
R1
O
O
O
O
O
O
O
O
Equation 2
In view of the possible synthetic utility of iodohydrins, we
have also further investigated the catalytic aerobic
iodohydroxylation of terminal alkenes. The typical proce-
dure involves the reaction of one equivalent of alkene
with one-half equivalent of iodine and a catalytic amount,
1 mol%, of H5PV2Mo10O40 in water–acetonitrile mixture
under O2. The results obtained are summarized in
Equation 3. We observed that iodohydroxylation of the
terminal alkenes proceeds in high yields (92–95%) and
to selectively yield 1-hydroxy-2-iodo- and 1-iodo-2-hy-
droxy products in a ca. 85:15 ratio.
The main and most important result is that the two sub-
strates showed a similar behavior and gave a final ratio ca.
4.5:1 syn- vs. anti-addition product. After the first step,
cyclohexene yielded to the O-acyliodohydrin, while 4-
methyl-1-cyclohexene gave a mixture of O-acyliodo-
hydrin, hydroxy acetates and diacetates. This more sig-
nificant formation of hydroxy acetates and diacetates for
Synlett 2005, No. 16, 2525–2527 © Thieme Stuttgart · New York

LETTER
Aerobic Oxidation of Alkenes to Esters of Vicinal Diols
2527
I
OH
References
R = n-Bu
R = n-Pent
R = n-Hex
+
R
R
R
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Equation 3
In conclusion a new catalytic aerobic method has been
established for the preparation of acetate esters of 1,2-
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Acknowledgment
This research was supported by the Israel Science Foundation,
the European Commission (G1RD-CT-2000-00347) SUSTOX
program and the Helen and Martin Kimmel Center for Molecular
Design. R. N. is the Rebecca and Israel Sieff Professor of Organic
Chemistry.
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Synlett 2005, No. 16, 2525–2527 © Thieme Stuttgart · New York