Chemical and Electrochemical Asymmetric Dihydroxylation of Olefins in I2-K2CO3-K2OsO2(OH)4 and I2-K3PO4/K2HPO4-K2OsO2(OH)4 Systems with Sharpless' Ligand

Article abstract of DOI:10.1021/jo952137r

Iodine-assisted chemical and electrochemical asymmetric dihydroxylation of various olefins in I2-K2CO3-K2OsO2(OH)4 and I2-K3PO4/K2HPO4-K2OsO2(OH)4 systems with Sharpless' ligand provided the optically active glycols in excellent isolated yields and high enantiomeric excessses.Iodine (I2) was used stoichiometrically for the chemical dihydroxylation, and good results were obtained with nonconjugated olefins in contrast to the case of potassium ferricyanide as a co-oxidant.The potentialily of I2 as a co-oxidant under stoichiometric conditions has been proven to be effective as an oxidizing mediator in electrolysis systems.Iodine-assited asymmetric electro-dihydroxylation of olefins in either a t-BuOH/H2O(1/1)-K2CO3/(DHQD)2PHAL-(Pt) or t-BuOH/H2O(1/1)-K3PO4/K2HPO4/(DHQD)2PHAL-(Pt) system in the presence of potassium osmate in an undivided cell was investigated in detail.Irrespective of the substitution pattern, all the olefins afforded the diols in high yields and excellent enantiomeric excesses.A plausible mechanism is discussed on the basis of cyclic voltammograms as well as experimental observations.

Full text of DOI:10.1021/jo952137r

J . Org. Chem. 1996, 61, 3055-3060
3055
Ch em ica l a n d Electr och em ica l Asym m etr ic Dih yd r oxyla tion of
Olefin s in I2-K2CO3-K2OsO2(OH)4 a n d I2-K3P O4/
K2HP O4-K2OsO2(OH)4 System s w ith Sh a r p less’ Liga n d
Sigeru Torii,*,† Ping Liu,† Narayanaswamy Bhuvaneswari,† Christian Amatore,‡ and
Anny J utand‡
Department of Applied Chemistry, Faculty of Engineering, Okayama University, Okayama 700, J apan,
and Ecole Normale Superieure, Departement de Chimie, URA CNRS 1679, 24 rue Lhomond,
75231 Paris Cedex 05, France
Received December 1, 1995X
Iodine-assisted chemical and electrochemical asymmetric dihydroxylation of various olefins in I2-
K2CO3-K2OsO2(OH)4 and I2-K3PO4/K2HPO4-K2OsO2(OH)4 systems with Sharpless’ ligand provided
the optically active glycols in excellent isolated yields and high enantiomeric excesses. Iodine (I2)
was used stoichiometrically for the chemical dihydroxylation, and good results were obtained with
nonconjugated olefins in contrast to the case of potassium ferricyanide as a co-oxidant. The
potentiality of I2 as a co-oxidant under stoichiometric conditions has been proven to be effective as
an oxidizing mediator in electrolysis systems. Iodine-assisted asymmetric electro-dihydroxylation
of olefins in either a t-BuOH/H2O(1/1)-K2CO3/(DHQD)2PHAL-(Pt) or t-BuOH/H2O(1/1)-K3PO4/
K2HPO4/(DHQD)2PHAL-(Pt) system in the presence of potassium osmate in an undivided cell was
investigated in detail. Irrespective of the substitution pattern, all the olefins afforded the diols in
high yields and excellent enantiomeric excesses. A plausible mechanism is discussed on the basis
of cyclic voltammograms as well as experimental observations.
In tr od u ction
potassium chlorate5a and hydrogen peroxide5b were the
first to be introduced, but in some cases these reagents
led to diminished yields of diols due to overoxidation.
Subsequently, some organic co-oxidants such as alkaline
tert-butyl hydroperoxide6 and N-ethylmorpholine N-ox-
ide7 were introduced, which afforded comparatively better
results. Recently, potassium ferricyanide in combination
with potassium carbonate has been demonstrated to be
a powerful system for osmium-catalyzed asymmetric
dihydroxylation of olefins.8-10 The effective chiral ligands
for enantioselective dioxyosmylation of olefins have been
investigated in detail.11 Among the reported asymmetric
dihydroxylation systems, an H2O/t-BuOH-K3Fe(CN)6/
Asymmetric induction has been frequently used in
syntheses of optically active natural products and biologi-
cally active compounds since the asymmetric centers
present in these compounds may be provided by chiral
transfer to readily available prochiral moieties.1 The
synthetic strategy is mostly left to the imagination of
chemists and not restricted by the availability of certain
starting materials. Among the most prominent examples
of asymmetric reactions, the “Sharpless process” for the
asymmetric dihydroxylation of olefinic compounds is
widely recognized in recent years.2 Our efforts in this
field have led to the establishment of chemical and
electrocatalytic asymmetric dihydroxylation of olefins
featured by (1) novel co-oxidant systems for conjugated
olefins as well as nonconjugated terminal olefins, (2) the
use of less amount of the co-oxidant, and (3) performing
the electrolysis in an undivided cell.
Criegee’s pioneering work3 showed that osmium tet-
raoxide in a stoichiometric amount could be used for
effective cis-dihydroxylation of olefins and that this
method is more reliable than other diol syntheses despite
its high cost and toxicity. Catalytic hydroxylation4
emerged in the field due to the growing economical
requirement. Inorganic co-oxidants such as sodium or
(5) (a) Hofmann, K. A. Chem. Ber. 1912, 45, 3329. (b) Milas, N. A.;
Trepagnier, J . H.; Nolan, J . T., J r.; Iliopulos, M. I. J . Am. Chem. Soc.
1959, 81, 4730.
(6) Sharpless, K. B.; Akashi, K. J . Am. Chem. Soc. 1976, 98, 1986.
(7) (a) Schneider, W. P.; McIntosh, A. V. U.S. Patent 2,769,824, Nov
6, 1956; Chem. Abstr. 1956, 51, 8822e. (b) Van Rheenen, V.; Kelly, R.
C.; Cha, D. Y. Tetrahedron Lett. 1976, 1973.
(8) Minato, M.; Yamamoto, K.; Tsuji, J . J . Org. Chem. 1990, 55, 766.
(9) (a) Gao, Y.; Charles, M. Z. U.S. Patent 5,302,257, Apr 12, 1994;
Chem. Abstr. 1994, 120, 119437j. (b) Amundsen, A. R.; Balko, E. N. J .
Appl. Electrochem. 1992, 22, 810.
(10) Torii, S.; Liu, P.; Tanaka, H. Chem. Lett. 1995, 319.
(11) (a) Hentges, S. G.; Sharpless, K. B. J . Am. Chem. Soc. 1980,
102, 4263. (b) Sharpless, K. B.; Amberg, W.; Beller, M.; Chen, H.;
Harturg, J .; Kawanami, Y.; Lu¨bben, D.; Manoung, E.; Ogino, Y.;
Shibata, T.; Ukita, T. J . Org. Chem. 1991, 56, 4585. (c) Sharpless, K.
B.; Amberg, W.; Bennani, Y. L.; Crispino, G. A.; Hartung, J .; J eong,
K.-S.; Kwong, H.-L.; Morikawa, K.; Wang, Z.-M.; Xu, D.; Zhang, X.-L.
J . Org. Chem. 1992, 57, 2768. (d) Andersson, P. G.; Sharpless, K. B.
J . Am. Chem. Soc. 1993, 115, 7047. (e) Wang, S.-M.; Kakiuchi, K.;
Sharpless, K. B. J . Org. Chem. 1994, 59, 6895. (f) Becker, H.; King, S.
B.; Taniguchi, M.; Vanhessche, K. P. M.; Sharpless, K. B. J . Org. Chem.
1995, 60, 3940. (g) Tomioka, K.; Nakajima, M.; Koga, K. J . Am. Chem.
Soc. 1987, 109, 6213. (h) Tomioka, K.; Nakajima, M.; Iitaka, Y.; Koga,
K. Tetrahedron Lett. 1988, 29, 573. (i) Yamada, T.; Narasaka, K. Chem.
Lett. 1986, 131. (j) Tokles, M.; Snyder, J . K. Tetrahedron Lett. 1986,
27, 3951. (k) Hirama, M.; Oishi, T.; Ito, S. J . Chem. Soc., Chem.
Commun. 1989, 665. (l) Annunziata, R.; Cinquini, M.; Cozzi, F.;
Raimondi, L.; Stefanelli, S. Tetrahedron Lett. 1987, 28, 3139. (m) Corey,
E. J .; J ardine, P. D.; Virgil, S.; Yuen, P.-W.; Connell, R. D. J . Am. Chem.
Soc. 1989, 111, 9243. (n) King, S. B.; Sharpless, K. B. Tetrahedron
Lett. 1994, 35, 5611.
* Tel:
81-86-251-8072.
Fax:
81-86-255-3424.
E-mail:
toriiken@cc.okayama-u.ac.jp.
† Okayama University.
‡ URA CNRS 1679.
X Abstract published in Advance ACS Abstracts, April 1, 1996.
(1) For a review, see: (a) Asymmetric Catalysis in Organic Synthesis;
Noyori, R.; Wiley-Interscience Publications: New York, 1993. (b)
Catalytic Asymmetric Synthesis; Ojima, I., Ed.; VCH Publications: New
York, 1993. (c) Berrisford, D. J .; Bolm, C.; Sharpless, K. B. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1059.
(2) Kolb, H. C.; VanNieuwenhze, M. S.; Sharpless, K. B. Chem. Rev.
1994, 94, 2483.
(3) (a) Criegee, R. J ustus Liebigs Ann. Chem. 1936, 522, 75. (b)
Criegee, R. Angew. Chem. 1937, 50, 153. (c) Criegee, R. Angew. Chem.
1938, 51, 519. (d) Criegee, R.; Marchand, B.; Wannowis, H. J ustus
Liebigs Ann. Chem. 1942, 550, 99.
(12) Mukaiyama, T.; Imagawa, K.; Yamada, T.; Takai, T. Chem. Lett.
1992, 231.
(4) Schroder, M. Chem. Rev. 1980, 80, 187.
S0022-3263(95)02137-2 CCC: $12.00 © 1996 American Chemical Society

3056 J . Org. Chem., Vol. 61, No. 9, 1996
Torii et al.
Ta ble 1. Electr o-d ih yd r oxyla tion a of Styr en e w ith
Va r iou s Co-Oxid a n ts
amount/
mmol
conversion
yield/%
enantiomeric
excess/% ee
entry
co-oxidant
1
2
3
4
5
6
7
K3Fe(CN)6
KCl
KBr
0.1
1.0
1.0
4.0
2/1
95.0
48.0
65.3
41.4
87.6
91.6
93.7
97.3
92.0
92.8
86.8
96.5
95.0
97.9
KI
KI/I2
KI/I2
I2
2/0.5
0.5
a
Carried out in a mixed solution of t-BuOH (5 mL) and H2O (5
mL) in the presence of (DHQD)2PHAL (0.01 mmol), K2OsO2(OH)4
(0.002 mmol), and K2CO3 (3.0 mmol) under passage of 2.5-5.0
F/mol of electricity for 25-30 h.
K2CO3 system with chiral ligands such as dihydroquini-
dine (DHQD) and/or dihydroquinine (DHQ) derivatives
has been found to be superior.11c
In the preceding paper,10 we reported the electrochemi-
cal asymmetric dihydroxylation of olefins using Sharp-
less’ ligand in the presence of potassium osmate by
recycling a catalytic amount of potassium ferricyanide
by electrochemical oxidation. We also mentioned a
possibility of the choice of other co-oxidants as well as
bases in the electrolytic conversion.
In continuation of our research in chemical and elec-
trocatalytic asymmetric dihydroxylation reactions, now,
we found that I2 and iodine-iodide combinations are
efficient co-oxidants for the osmate recycling. Namely,
the novel co-oxidant-base systems involving an I2-
K2CO3-K2Os2(OH)4 and I2-K3PO4/K2HPO4-K2Os2(OH)4
combination, which can realize efficient enantioselective
dioxyosmylation of olefins, were proved to be more
effective both under chemical and electrochemical condi-
tions.
F igu r e 1. Electrochemical asymmetric dihydroxylation of
styrene in an I2-K2CO3-K2OsO2(OH)4 system in the presence
of (DHQD)2PHAL. Isolated yield, % (b), obtained by changing
the amount of iodine under the following conditions: styrene
(1.0 mmol), (DHQD)2PHAL (0.01 mmol), K2OsO2(OH)4 (0.002
mmol), K2CO3 (3.0 mmol) in t-BuOH (5 mL)-H2O (5mL) at 0
°C in an undivided cell with 5.11 F/mol electricity (1.5-3.0 V,
5 mA/cm2). Enantiomeric excess, % ee (4), determined by
HPLC analysis of the diol (CHIRALCEL OB-H column, 10%
i-PrOH in hexane, 0.5 mL/min).
with 96.2% ee when the molar ratio of styrene and I2 was
1.5. However, the isolated yield diminished in proportion
with the amount of I2. Interestingly, under these cir-
cumstances, the value of enantiomeric excess was always
around 96%. Next, we investigared electrochemical
osmium-catalyzed asymmetric dihydroxylation in the
same system by recycling use of a catalytic amount of I2
(electrochemical procedure I). As shown in Figure 1, it
is clear that the I2-K2CO3-K2OsO2(OH)4-(DHQD)2-
PHAL system operated well under electrolysis conditions
and that the I2 played the expected role as a recyclable
co-oxidant for potassium osmate. Actually, more than
90% of isolated yields (96% ee) could be attained in the
presence of 0.3 equiv of I2. Under a similar electrolysis
condition, a large amount of styrene (5 equiv) can be
oxidized by changing the electrolysis time and current
density to afford 3 in 89% yield (91.9% ee).
Resu lts a n d Discu ssion
Initially, we studied the potentiality of I2 as an
alternative co-oxidant in the K2CO3-K2OsO2(OH)4-
(DHQD)2PHAL system for asymmetric dihydroxylation
of styrene. Halides have been found to be an efficient
catalyst in mediatory systems, which spured us to gather
precise data in the isolated and enantiomeric excess
yields taking advantage of the halide-assisted electro-
dihydroxylation of styrene (Table 1, entries 2-7). Ap-
parently, most co-oxidant systems provided the diols in
high yields of % ee. However, different efficiencies were
noticed in conversion yields. In comparison with the case
of potassium ferricyanide (Table 1, entry 1), the potas-
sium halides (entries 2, 3, and 4) were less effective even
when they were used in excesses more than 10 times
larger than that of potassium ferricyanide. This situa-
tion, however, seems to be improved by addition of I2
(entries 5 and 6). The most exciting results were
obtained when I2 alone was used in an I2-K2CO3-K2-
OsO2(OH)4 system as a co-oxidant (entry 7) under
electrochemical conditions. The correlation of the iso-
lated and enantiomeric excess yields in respect to the
amount of I2 is demonstrated in Figure 1. Apparently,
the use of 0.3 equiv of I2 still afforded sufficiently high
yield of the chiral diol from styrene. Without passing
electricity, the isolated yield sharply decreased in propor-
tion to the amount of I2, being less than 1.5 equiv. These
trends were not changed with different systems.
Encouraged by our initial results, we sought an alter-
native co-oxidant-base combination for the K2OsO2(OH)4-
(DHQD)2PHAL system active at pH values around 11.0-
11.5 which is required for the hydrolysis of the chiral
monoglycolate intermediate 2. The I2-K3PO4/K2HPO4
combination was proven to be ideal for the present
purpose, and the results are indicated in Figure 2.
Electrolysis of 1 in a I2(0.5 equiv)-K3PO4/K2HPO4(1.88/
The chemical asymmetric dihydroxylation with I2
smoothly proceeded in 96.6% isolated yield together

Asymmetric Dihydroxylation of Olefins
J . Org. Chem., Vol. 61, No. 9, 1996 3057
products.14 A successful application of the present pro-
cedures for the synthesis of chiral l-shikonin has been
carried out which will be published soon elsewhere.15
Electr och em ica l Mea su r em en t of th e Red ox Sys-
tem K2Os(VI)O2(OH)4/I2 in Wa ter . The cyclic voltam-
mogram of a solution of K2OsO2(OH)4 (2 mM) in water
containing K2CO3 (0.3 M) as supporting electrolyte
exhibits two oxidation peaks (Figure 3). The first elec-
tron transfer is controlled by the diffusion and takes place
at Epa(O1) ) -0.115 V (eq 1, at a platinum electrode at
the scan rate of 0.2 V s-1). The first oxidation produces
a species that is adsorbed at the electrode surface, and
therefore the second electron transfer at Epa(O2) ) +0.225
V (eq 3) is not controlled by the diffusion. The species
resulting from the first electron transfer is also reduced
at Epc(R1) ) -0.225 V (eq 2), and the shape of the
reduction peak is also characteristic of a species adsorbed
on the electrode surface.
Os(VI)O2(OH)4 -e8
2-
Os(VII)O3(OH)32- + H+
O1 (1)
R1 (2)
+
H
2-
2-
F igu r e 2. Electrochemical asymmetric dihydroxylation of
styrene in an I2-K3PO4/K2HPO4-K2OsO2(OH)4 system in the
presence of (DHQD)2PHAL. Isolated yield, % (b), obtained by
changing the amount of iodine under the following condi-
tions: styrene (1.0 mmol), (DHQD)2PHAL (0.01 mmol),
K2OsO2(OH)4 (0.002 mmol), K3PO4/K2HPO4 (1.88/1.12 mmol)
in t-BuOH (5 mL)-H2O (5mL) at 0 °C in an undivided cell
with 5.11 F/mol electricity (1.5-3.0 V, 5 mA/cm2). Enantio-
meric excess, % ee (4), determined by HPLC analysis of the
diol (CHIRALCEL OB-H column, 10% i-PrOH in hexane, 0.5
mL/min).
Os(VII)O3(OH)3
,e8 Os(VI)O2(OH)4
Os(VII)O3(OH)3 -e8
Os(VIII)O4(OH)22- + H+
2-
O2 (3)
Addition of 1 equiv of I2 results in a decay of the
oxidation peak O1 while two new oxidation peaks appear
at more positive potential at Epa(O3) ) +0.295 V and
E
pa(O4) ) +0.506 V (eqs 4 and 5; Figure 3). These
oxidation peaks are characteristic of the oxidation of
iodide anions (I-) as evidenced by the increase in their
respective magnitude when potassium iodide is added to
the solution. Therefore we assume that I2 oxidizes the
osmium(VI) salt in water and itself is reduced to I-.
1.12 equiv)-K2OsO2(OH)4-(DHQD)2PHAL system af-
forded the diol 3 in 96.2% yield (96.5% ee) (see electro-
chemical procedure II). As shown in Figure 2, the
presence of 0.2 equiv of I2 could afford the diol in more
than 81.8% yields in isolated yield with 95.2% enantio-
meric excess.
I2 + K2Os(VI)O2(OH)4 f
Table 2 further illustrates the results obtained from
the asymmetric dihydroxylation of a variety of olefins
using iodine-base combination systems under both
chemical and electrochemical conditions in comparison
with the previous results.11 It is obvious that both the
I2-K2CO3 and I2-K3PO4/K2HPO4 systems are well suited
for various olefins under the above conditions. Espe-
cially, terminal as well as di- and trisubstituted olefins
bearing alkyl and aryl groups were good candidates for
the asymmetric dihydroxylation. Some exceptions were
encountered when the present I2-K2CO3 system was
extended to the asymmetric dihydroxylation of R,â-
unsaturated esters (entries 4, 11, and 12), which exhib-
ited relatively lower conversion yields probably due to
hydrolysis of the ester groups under the employed condi-
tions. However, the use of K2CO3/KHCO3 (2.5/0.5 mmol)
in place of K2CO3 (3 mmol) was proven to be effective for
the improvement of the conversion yields. The scale-up
electrolysis performed in a microflow cell (ElectroCell AB,
Sweden) provided the same results obtained in a glass
apparatus (EOS-22, J apan). The diols obtained from
olefins (entries 5, 13, and 14) are well-known as chiral
synthons for the synthesis of chiral â-blockers,13 chiral
auxiliaries,11n and a variety of optically active natural
2I- + K2Os(VIII)O4(OH)2 + 2H+ (4)
or 2KI + Os(VIII)O4 + 2H2O
(5)
Electr och em ica l Mea su r em en t of th e Red ox Sys-
tem K2Os2(OH)4/I2 in a Mixtu r e (1/1) of Wa ter a n d
ter t-Bu tyl Alcoh ol. A mixture of water and tert-butyl
alcohol (1/1) containing K2CO3 (0.3 M) as a supporting
electrolyte results in the formation of two phases.
K2Os(VI)O2(OH)4 (2 mM) is only soluble in the aqueous
phase as attested by its pale violet color. No oxidation
peak is detected when voltametry is performed in the
organic phase. In the aqueous phase, the two oxidation
peaks of K2Os(VI)O2(OH)4 are detected at Epa(O1) )
+0.040 V and Epa(O2) ) +0.205 V, i.e., at potentials
similar to that mentioned above in pure water. When I2
is added to this solution, the pale violet color character-
istic of K2Os(VI)O2(OH)4 disappears and a yellow color
characteristic of Os(VIII)O4 appears in the organic phase.
However, no reduction peak that could characterize
Os(VIII)O4 is observed (carried out with an authentic
sample of Os(VIII)O4 alone). In the presence of 5 equiv
of the ligand, the oxidation peaks of K2Os(VI)O2(OH)4 are
(13) Rama Rao, A. V.; Gurjar, M. K.; J oshi, S. V. Tetrahedron:
Asymmetry 1990, 1, 697.
(14) Dumount, V. R.; Phander, H. Helv. Chim. Acta 1983, 66, 814.
(15) Torii, S.; Akiyama, K., Chem. Lett. 1996, in press.

3058 J . Org. Chem., Vol. 61, No. 9, 1996
Ta ble 2. Asym m etr ic Dih yd r oxyla tion of Olefin s Usin g a Va r iety of Iod in e-Ba se System s
Torii et al.
a
Carried out in a mixture of t-BuOH (5 mL) and H2O (5 mL) in the presence of K2CO3 (3.0 mmol) or K3PO4/K2HPO4 (1.88/1.12 mmol),
b
(DHQD)2PHAL (0.01 mmol), I2 (1.5 mmol) and K2OsO2(OH)4 (0.002 mmol) based on 1 mmol of olefin at 0 °C or rt for 30 h. Performed
by use of substrate (1 mmol), t-BuOH (5 mL), H2O (5 mL), K2CO3 (3.0 mmol) or K3PO4/K2HPO4 (1.88/1.12 mmol), (DHQD)2PHAL (0.01
mmol), I2 (0.5 mmol), K2OsO2(OH)4 (0.002 mmol) in an undivided cell at 0 °C or rt for 5.11 F/mol of electricity (1.5-3.0 V). c Reported by
Sharpless et al., with (DHQD)2PHAL.11c Enantiomeric excess, ee %, determined by 19F-NMR and 1H-NMR analyses of mono- and bis-
d
MTPA esters of the corresponding diols. e The yields reported here are isolated yields. f Determined by comparison of optical rotation
values with reported ones. g Ee % determined by HPLC analysis of the diols (CHIRALCEL OB-H column, 10% i-PrOH in hexane). Instead
h
of K2CO3, a mixed base of K2CO3/KHCO3 (2.5/0.5 mmol) was used.
detected in the aqueous phase. Addition of I2 results in
the oxidation of K2Os(VI)O2(OH)4 as attested by the decay
of the oxidation peaks of K2Os(VI)O2(OH)4 and the
appearance of oxidation peaks of I- (Epa(O3) ) +0.325 V
and Eoa(O4) ) +0.535 V). But no reduction peaks
characteristic of the new complex Os(VIII)O4L* can be
detected in the organic phase. The reduction of the
ligand alone can be observed in THF. Its cyclic voltam-
mogram exhibits two irreversible reduction peaks at
-2.00 and -2.03 V and a third reversible one at -2.275
V. The reduction of the ligand cannot be observed in the
mixture of water/tert-butyl alcohol since in this system
the reduction of the solvent takes place at around -0.9
V (carried out with an authentic sample of Os(VIII)O4 +
ligand (5 equiv) in water/tert-butyl alcohol). Therefore,
the oxidation of the osmium(VI) salt by I2 and the
recycling of I- to I2 by oxidation at the anode proceed in
the aqueous phase whereas the osmium(VIII) complex
produced by oxidation of the osmium(VI) salts is trans-
ferred to the organic phase.
oxidation leads to adsorption on the electrode that may
stop the process. It is why a mediator is necessary to
perform the oxidation of K2Os(VI)O2(OH)4 in the solution.
Iodine plays this role, and the resulting iodide anions (I-)
are oxidized at the anode back to I2. Since K2Os(VI)-
O2(OH)4 is more easily oxidized than I-, it is not possible
to oxidize I- to I2 in the presence of K2Os(VI)(OH)4. This
can explain why performing the reaction using a mixture
of K2Os(VI)O2(OH)4 and potassium iodide leads to lower
yield than starting from the mixture of K2Os(VI)O2(OH)4
and I2. Indeed, it is necessary to start the electrolysis
with a mixture of K2Os(VI)O2(OH)4 and an excess of I2.
Thus, I2 is able to convert quantitatively the K2Os(VI)-
O2(OH)4 to Os(VIII). This depletes the Os(VI) concentra-
tion in the diffusion layer and replenishes it with a
concentration of iodide ions which can then be freely
oxidized to regenerate I2. The process could only work
if I- is alone in the diffusion layer (or with Os(VIII) but
without the Os(VI)). This implies that I2 should oxidize
the Os(VI) by a very fast reaction compared to the
electrolysis.
Op tim a l Exp er im en ta l Con d ition s for th e Elec-
tr och em ica l P r ocess. Although K2Os(VI)O2(OH)4 can
be oxidized at the anode, its direct electrochemical
2-
In the aqueous electrolysis, the osmate Os(VI)O2(OH)4
unambiguously undergoes a two-electron oxidation with

Asymmetric Dihydroxylation of Olefins
J . Org. Chem., Vol. 61, No. 9, 1996 3059
NMR analysis via Mosher derivatives (mono-, or bis-MTPA
esters) prepared by standard procedures.16 1H NMR, 13C NMR,
and 19F NMR spectra were recorded in deuteriochloroform
(CDCl3) at 200 and 500 MHz. Optical rotations were taken
at 25 °C at the sodium line.
Electr olysis Ap p a r a tu s. Unless otherwise noted, an
undivided cell (EOS-V22, TECHNOSIGMA, J apan, 2.2 cm in
diameter and 6 cm in height (ca. 20 mL)) fitted with a stirring
bar and a gas outlet pipe or a flow cell (MICRO FLOW CELL,
ElectroCell AB, Sweden) was used. In the EOS-V22, two
platinum foil electrodes were placed parallel to each other 10
mm apart. The vessel was immersed in a ice-water bath or
kept at room temperature.
Electr och em ica l Setu p a n d Electr och em ica l P r oce-
d u r e for Cyclic Volta m m etr y. Cyclic voltammetry was
performed with a homemade potentiostat and a waveform
generator, PAR Model 175. Cyclic voltammograms were
recorded with a Nicolet 3091 digital oscilloscope. Experiments
were carried out in a three-electrode cell connected to a
Schlenk line. The counterelectrode was a platinum wire of
ca. 1 cm2 apparent surface area; the reference was a saturated
calomel electrode (Tacussel) separated from the solution by a
bridge (3 mL) filled with a 0.3 M K2CO3 solution in a mixture
of water and tert-butyl alcohol (1/1). A 12 mL volume of a
mixture of water and tert-butyl alcohol (1/1) containing 0.3 M
K2CO3 was poured into the cell. A 8.84 mg amount (2 × 10-3
M) of K2Os(VI)O2(OH)4 (commercial from Aldrich) was then
added. Cyclic voltammetry was performed in the aqueous
phase at a stationary disc electrode (a platinum disc made from
a cross section of wire, 0.5 mm in diameter, sealed into glass)
F igu r e 3. (s) Cyclic voltammetry of K2Os(VI)O2(OH)4 (2 mM)
in water containing K2CO3 (0.3M) at a stationary platinum
disk electrode (d ) 0.5 mm) with a scan rate of 0.2 V s-1 at 20
°C. (- - -) Cyclic voltammetry of K2Os(VI)O2(OH)4 (2 mM) in
the presence of I2 (2 mM) performed under the same conditions
as above.
at a scan rate of 0.2 V s-1
. Suitable amounts of required
reagents (e.g., 5.6 mg amount (2 × 10-3 M) of I2 and/or 93.4
mg amount (10-2 M) of the ligand (DHQD)2PHAL) were then
added to the cell, and the cyclic voltammetry was performed
again.
Electr och em ica l P r oced u r e I w ith th e I2-K2CO3-
K2OsO2(OH)4 System . To a stirred solution of tert-butyl
alcohol (5 mL), I2 (127 mg, 0.5 mmol), K2CO3(410 mg, 3.0
mmol), (DHQD)2PHAL (7.8 mg, 0.01 mmol), and K2OsO2(OH)4
(0.74 mg, 0.002 mmol) at 0 °C in an undivided cell (EOS-V22)
was added styrene (1 mmol) at once. The mixture was
electrolyzed under a constant current of 5 mA/cm2 under
vigorous stirring (applied voltage 1.0-3.0 V). After passage
of 5.11 F/mol of electricity for ca. 27 h, the reaction (monitored
by TLC) was quenched with a saturated sodium thiosulfate
solution. Usual workup followed by column chromatography
gave (-)-(R)-1-phenyl-1,2-ethanediol (3, 129.3 mg; 93.7%);
electrochemically produced active iodine-oxidizing species
to form Os(VIII)O6(OH)22- in a basic buffering conditions
(eq 6). The octavalent Os species would change into OsO4
by eliminating hydroxy anions (eq 7). This process seems
to be reversible; osmium tetraoxide, however, is more
soluble in an organic phase. The combination of OsO4
with nitrogen-containing chiral ligands L* proceeds
promptly to provide a rather tight complex which may
provide a binding pocket for olefin (eq 8). Subsequently,
olefin would be trapped by the binding pocket to afford
the olefin-Os-L* complexes as a precursor of the chiral
diol (eq 9). The hydrolysis of the complexes may give
chiral diols, chiral ligands, and osmate(VI) (eq 10).
[R]25 ) -62.03° (c ) 1.98 (CDCl3)). The enantiomeric excess
D
of the R-diol was determined by HPLC analysis (CHIRALCEL
OB-H; Daicel) to be 97.9%.
Electr och em ica l P r oced u r e II w ith th e I2-K3P O4/
K2HP O4-K2OsO2(OH)4 System . To a stirred solution of tert-
butyl alcohol (5 mL), water (5 mL), I2 (127 mg, 0.5 mmol),
K3PO4 (398 mg, 1.88 mmol), K2HPO4 (196 mg, 1.12 mmol),
(DHQD)2PHAL (7.8 mg, 0.01 mmol), and K2OsO2(OH)4 (0.74
mg, 0.002 mmol) at 0 °C in an undivided cell (EOS-V22) was
added styrene (1 mmol) at once. The mixture was electrolyzed
(5.0 mA/cm2) under vigorous stirring (applied voltage 1.0-3.0
V). After passage of 5.11 F/mol of electricity for ca. 27 h, the
mixture (monitored by TLC) was worked up in a usual manner
and chromatographed to give (-)-(R)-1-phenyl-1,2-ethanediol
Os(VI)O2(OH)42- f Os(VIII)O4(OH)22- + 2e- + 2H+
(6)
Os(VIII)O4(OH)22- f Os(VIII)O4 + 2OH- (7)
Os(VIII)O4 + L* f Os(VIII)O4L*
(8)
Os(VIII)O4L* + olefin f olefin-Os(VIII)O4L* (9)
olefin-Os(VIII)O4L* + OH-
(3, 130 mg; 96%); [R]25 ) -61.8° (c ) 2.46 (CDCl3)). The
D
enantiomeric excess of the R-diol was determined by HPLC
f
analysis (CHIRALCEL OB-H, Daicel) to be 97.1%.
2-
Electr och em ica l P r oced u r e III in a F low Cell System .
A mixture of tert-butyl alcohol-water (25/25 mL), methane-
sulfonamide (5 mmol), K2CO3 (15 mmol), I2 (2.0 mmol),
K2OsO2(OH)4 (0.01 mmol), (DHQD)2PHAL (0.05 mmol), and
trans-stilbene (5 mmol) was electrolyzed in an undivided flow
cell (MICRO FLOW CELL) equipped with two platinum
electrodes (3 × 4 cm2). After passage of 2.33 F/mol of electricity
at room temperature (progress was monitored by TLC), 5 g of
diol* + L* + Os(VI)O2(OH)4 (10)
Exp er im en ta l Section
Gen er a l. Starting olefinic substrates, unless specially
stated, were purchased from commercial sources and not
further purified prior to use. All reactions were monitored by
thin layer chromatography (TLC) and judged complete when
starting material was no longer detected in TLC. The enan-
tiomeric excesses of the chiral diols were determined by HPLC
analysis of diols on a CHIRALCEL OB-H column (Daicel) or
(16) Dale, J . A.; Dull, D. L.; Mosher, H. S. J . Org. Chem. 1969, 34,
2543.

3060 J . Org. Chem., Vol. 61, No. 9, 1996
Torii et al.
solid sodium sulfite was added and the solution was stirred
for 40 min. The two phases were separated, and the aqueous
phase was then extracted (3 × 25 mL) with EtOAc. The
combined organic phases were washed with 2 M NaOH and
dried over MgSO4. Concentration and flash chromatography
afforded the (+)-(1R,2R)-1,2-phenyl-1,2-ethanediol in 82.4%
hexane, 0.5 mL/min] to be 99.5%. The alkaloid ligand was
recovered in 88.0% yield.
Ch em ica l P r oced u r e II w ith th e I2-K3P O4/K2HP O4-
K2OsO2(OH)4 System . To a stirred solution of tert-butyl
alcohol (5 mL), I2 (382 mg, 1.5 mmol), K3PO4 (1.88 mmol),
K2HPO4(mg, 1.12 mmol), (DHQD)2PHAL (7.8 mg, 0.01 mmol),
and K2OsO2(OH)4 (0.74 mg, 0.002 mmol) at 0 °C was added
styrene (1 mmol) at once. The mixture was stirred vigorously
for ca. 30 h (progress was monitored by TLC and GLC), and
the reaction was quenched with a saturated sodium thiosulfate
solution (5 mL). Usual workup followed by column chroma-
tography gave (-)-(R)-1-phenyl-1,2-ethanediol (3) in 96.5%
yield, [R]25 ) +91° (CDCl3). The enantiomric excess of the
D
RR-diol was determined by HPLC analysis [CHIRALCEL
OB-H column (Daicel), 10% i-PrOH in hexane, 0.5 mL/min]
to be 99.5%. The alkaliod ligand was recovered in 84.0% yield.
Ch em ica l P r oced u r e I w ith th e I2-K2CO3-K2OsO2-
(OH)4 System . To a stirred solution of tert-butyl alcohol/water
(50/50 mL), K2CO3 (30 mmol), I2 (15 mmol), methanesulfona-
mide (10 mmol), K2OsO2(OH)4 (0.02 mmol), and (DHQD)2-
PHAL (0.1 mmol) was added trans-stilbene (10 mmol) at once
at 0 °C, the mixture was stirred vigorously for more than 30
h (monitored by TLC), and the reaction was quenched with
10 g of solid sodium sulfite. The two phases were separated,
and the aqueous phase was then extracted (3 × 50 mL) with
EtOAc. The combined organic phases were washed with 2 M
NaOH and dried over MgSO4. Concentration and flash
chromatography afforded the (+)-(1R,2R)-1,2-phenyl-1,2-
yield, [R]25 ) -61.5° (c ) 1.98 (CDCl3)). The enantiomeric
D
excess of the R-diol was determined by HPLC analysis
(CHIRALCEL OB-H; Daicel) to be 96.2%.
Ack n ow led gm en t. It is a pleasure to acknowledge
support of this investigation by the Grants-in-Aid for
Scientific Research 05235107 and 05403025 for the
Ministry of Education, Science and Culture, J apan. We
are grateful to the 500 MHz SC-NMR laboratory of
Okayama University for the NMR experiments.
ethanediol in 98.4% yield, [R]25 ) +91° (CDCl3). The enan-
D
tiomeric excess of the RR-diol was determined by HPLC
analysis [CHIRALCEL OB-H column (Daicel), 10% i-PrOH in
J O952137R