Novel ruthenium-pyridinedicarboxylate complexes of terpyridine and chiral bis(oxazolinyl)pyridine: A new catalytic system for alkene epoxidation with [bis(acetoxy)iodo]benzene as an oxygen donor

Article abstract of DOI:10.1039/a705109c

Ruthenium-pyridine-2,6-dicarboxylate (pydic) complexes 1-3 of terpyridine and chiral bis(oxazolinyl)pyridines (pybox-ip and pybox-ph), which can be synthesized from [Ru(p-cymene)Cl₂]₂ and the corresponding ligands, exhibit catalytic activity for epoxidation of trans-stilbene in combination with [bis(acetoxy)iodo]benzene: asymmetric induction with 2 was observed in 74% ee for trans-stilbene.

Full text of DOI:10.1039/a705109c

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Novel ruthenium–pyridinedicarboxylate complexes of terpyridine and chiral
bis(oxazolinyl)pyridine: a new catalytic system for alkene epoxidation with
[bis(acetoxy)iodo]benzene as an oxygen donor
Hisao Nishiyama,* Tomoo Shimada, Hirofumi Itoh, Hirotaka Sugiyama and Yukihiro Motoyama
School of Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi 441, Japan
Ruthenium–pyridine-2,6-dicarboxylate (pydic) complexes
–3 of terpyridine and chiral bis(oxazolinyl)pyridines
pybox-ip and pybox-ph), which can be synthesized from
Ru(p-cymene)Cl and the corresponding ligands, exhibit
catalytic activity for epoxidation of trans-stilbene in combi-
nation with [bis(acetoxy)iodo]benzene: asymmetric induc-
tion with 2 was observed in 74% ee for trans-stilbene.
3
CHCl (ca. < 10 mg in 200 ml). Chiral complexes 2 and 3 were
also prepared by the same procedure as shown in Scheme 1.‡
1
(
[
terpyridine or
1
(76%)
2
]
2
pybox-R
0.5 [Ru(p-cymene)Cl2]2 + C5H3N-2,6-(CO2Na)2
2 (78%)
3 (71%)
MeOH–H2O
reflux
4
5
Scheme 1
Ruthenium complexes with nitrogen-based ligands have been
intensively investigated in order to develop catalysts for organic
oxidation processes and to simulate the mechanism of bio-
organic oxidation. This is because ruthenium complexes act as
oxidation catalysts, often via ruthenium-oxo species, oxidizing
We first examined epoxidation of trans-stilbene 6 (0.5 mmol)
with iodosobenzene (PhIO) (1.5 mmol) in CH Cl (5 ml) in the
2
2
presence of 1 (5 mol%) (Scheme 2) (Table 1, run 1) trans-
Stilbene oxide 8 was obtained in good to excellent yield (67%)
with a small amount of benzaldehyde (8%), and some 6 (5%)
was recovered. After screening of several oxidants, we found
1,2
alcohols or alkanes and epoxidizing alkenes.
especially interested in Balavoine’s alkene epoxidation using
RuCl –bipyridine–NaIO , in which addition of bipyridine can
efficiently minimize cleavage of the alkene, even in the
We are
that [bis(acetoxy)iodo]benzene 7 [PhI(OAc)
efficient oxygen donor assisted by complex 1; PhI(OAc)
better solubility in CH Cl . When 7 (1.0 mmol) was added to a
suspension of complex 1 (14 mg, 5 mol%) in a CH Cl (5 ml)
2
]§ served as an
3
4
2
has
2
2
3
4
presence of NaIO . In connection of our studies of transition-
2
2
metal catalysis with terdentate bis(oxazolinyl)pyridine (Pybox)
ligands,4 we have been studying an applicaton of Ru–
bisoxazoline catalysts for catalytic epoxidation of alkenes
solution of 6 (0.5 mmol), complex 1 spontaneously reacted,
dissolved and gave a dark brown solution. The catalytic
epoxidation proceeded slowly at room temperature for 72 h to
give trans-stilbene oxide in excellent yield (92%) (run 3).
5
accompanied by asymmetric induction. In this respect, Wae-
gell reported an asymmetric alkene epoxidation with Ru–
However, use of NaIO
4
resulted in exclusive cleavage of the
6
oxazoline catalysts with low asymmetric induction, and Che
alkene (run 5). Epoxidation under aerobic epoxidation with
described an asymmetric and stoichiometric oxygen transfer
t
t
2
Bu CHO/O (1 atm). (run 6) and use of Bu OOH (run 7) resulted
from a chiral ruthenium complex to styrene; 56% ee for
in yields of 86 and 81%, respectively.
7
4
-chlorostyrene. As a new catalyst design, we adopted the
introduction of dual closed meridional stereotopes around an
active metal and consequently chose pyridine-2,6-dicarboxylate
H
H
O
1 (5 mol%)
Ph
Ph
+
Phl(OAc)2
Ph
Ph
(
pydic) as the counterpart. Here we disclose a synthesis of
Ru(pydic)(terpy) (terpy = 2,2A:6A,2B-terpyridine) 1 as a non-
CH2Cl2 (5 ml),
H
H
room temp., 72 h
6
7
8
(0.5 mmol)
(1.0 mmol)
(92% yield)
Scheme 2
O
O
N
N
We then examined asymmetric epoxidation of trans-stilbene
with Ru(pydic)(pybox-ip) 2 (5 mol%), which also exhibited
catalytic activity with PhIO (3 equiv.) and PhI(OAc) (3 equiv.)
2
N
O
N
N
O
N
6
Ru
Ru
O
O
R
R
O
O
O
O
N
N
Table 1 Catalytic epoxidation of trans-stilbene with Ru(pydic)(terpy) 1a
Oxidant:trans-
stilbene/mmol t/h Yield (%) alkene (%)
Recovered
1
2 R = Pri
R = Ph
Run Oxidant
3
1
2
3
4
5
6
7
PhIO
PhI(OAc)
PhI(OAc)
PhI(OAc)
NaIO
1.5:0.5
0.5:0.5
1.0:0.5
1.0:0.5
2.5:0.5
2.5:0.5
2.0:0.5
72 67
96 49
72 92
96 90
5
39
0
5
0
2
2
2
i
chiral catalyst and Ru(pydic)(pybox-R) 2 (R = Pr ) and 3
(
b
R = Ph) as chiral catalysts, and their catalytic activity for
alkene epoxidation is also demonstrated.
A mixture of [Ru(p-cymene)Cl 4, disodium pyridine-
,6-dicarboxylate 5, and 2,2A:6A,2B-terpyridine was treated in
MeOH–H O at reflux for 1 h under argon atmosphere. After
cooling the reaction mixture, Ru(pydic)(terpy) 1 was obtained
in 78% yield as a dark violet precipitate by filtration (Scheme
).† Complex 1, which is stable in air, is slightly soluble in
c
d
4
96
0
t
e
O
2
/Bu CHO
48 86
24 81
1
1
2 2
]
t
Bu OOH
2
2
a
2 2
Catalyst 1 (0.025 mmol, 5 mol% of trans-stilbene), CH Cl (5 ml), at room
temperature. The yields are based on 0.5 mmol of trans-stilbene.
b
Benzaldehyde was obtained in the range 2–10%. Acetone (5 ml) was used
as a solvent. NaIO (3.5 mmol), H O (2.5 ml), room temp. Benzaldehyde
(27%) was obtained. Bu CHO (2.5 mmol), O
1
c
d
4
2
e
t
MeOH (ca. 10 mg in 25 ml), but almost insoluble in CH
2
2
Cl and
2
(1 atm).
Chem. Commun., 1997
1863

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Table 2 Asymmetric catalytic epoxidation of trans-stilbene with Ru(pydic)(pybox-ip) 2a
Oxidant:trans-
trans-Stilbene oxide
Recovered
alkene (%)
Run
Oxidant
stilbene/mmol
Solvent
T °C
t/h
Yield (%)
Ee (%)
1
2
3
4
5
6
7
8
PhIO
1.5:0.5
1.5:0.5
1.5:0.5
1.5:0.5
1.5:0.5
2.5:0.5
2.5:0.5
2.0:0.5
PhMe
PhMe
PhMe
25
25
0
25
25
5
96
96
96
96
96
96
96
72
67
80
63
43
40
trace
67
24
63
74
52
36
—
5
9
3
PhI(OAc)
PhI(OAc)
PhI(OAc)
PhI(OAc)
NaIO
2
2
2
2
18
22
27
trace
3
C
H
6 6
CH
2
Cl
2
2
b
4
PhMe
PhMe
t
c
O
2
/Bu CHO
25
25
t
Bu OOH
CH
2
Cl
38
16
38
a
Catalyst 2 (0.025 mmol, 5 mol% of trans-stilbene), solvent (10 ml). The yields are based on 0.5 mmol of trans-stilbene. Benzaldehyde was obtained in the
b
c
4 2
range 2–14%. The ees were determined by chiral LC (Daicel Chiralcel OD). All epoxides had (1S,2S) configuration. NaIO (3.5 mmol), H O (2.5 ml).
Bu CHO (2.5 mmol), O (1 atm).
2
t
MeOH (8 ml) was added a solution of disodium pyridine-2,6-dicarboxylate
(0.48 mmol) in MeOH–H O (2:1, 7.2 ml) under argon atomosphere. The
5
2
O
O
N
mixture was stirred at 60 °C for 1 h. The dark violet precipitate was
collected by filtration to give 1 (183 mg, 0.37 mmol) in 76% yield. Calc. for
C22H14N O Ru(H O): C, 49.35; H, 3.39; N, 10.46. Found: C, 49.11; H,
H
OAc
N
N
S
Ph
Ru
4
4
2
Ph
Pri
Pri
3.52; N, 10.65%; nmax(KBr)/cm 1650, 1620.
21
S
H
O
AcO
‡ Synthesis of Ru(pydic)(pybox-ip) 2. To a solution of [Ru(p-cymene)Cl ]
2 2
N
4
(306 mg, 0.5 mmol) and pybox-ip (301 mg, 1.0 mmol) in MeOH (7 ml)
was added a solution of disodium pyridine-2,6-dicarboxylate 5 (1.0 mmol)
in MeOH–H O (2:1, 15 ml) under argon atmosphere. The mixture was
stirred at 60 °C for 1 h. The mixture was extracted with CH Cl (40 ml). The
combined organic layers were concentrated and the residue was purified by
silica gel column chromatography with CH Cl –MeOH (50:1) to give 2 as
a dark violet solid (444 mg, 0.78 mmol) in 78% yield; mp > 240 °C
(270 MHz, CDCl , Me Si) 0.48 (d, J 6.8, 6 H), 0.62 (d, J 6.8,
H), 1.09 (m, 2 H), 3.71 (m, 2 H), 4.61 (dd, 2 H), 4.70 (dd, 2 H), 7.64 (t,
H), 7.88 (d, 2 H), 8.11 (t, 1 H), 8.34 (d, 2 H). Calc. for C24 Ru:
(
S,S)-8
2
9
2
2
2
2
to give 67 and 80% yields of (1S,2S)-trans stilbene oxide (S,S)-8
24 and 63% ee), respectively (Table 2, runs 1 and 2). The best
result for this catalytic system was obtained at 0 °C (74% ee, run
). When performed in toluene rather than CH
epoxidation using complex 2 (5 mol%) and PhI(OAc)
equiv.) gave higher yields (runs 2, 4 and 5). Aerobic oxidation
and oxidation with Bu OOH proceeded but gave relatively low
yields and enantiomeric excesses (runs 7 and 8).
The Ru(pydic)(pybox-ph) complex 3 gave slightly lower
enantioselectivities than 2; oxidation of trans-stilbene under the
same conditions as in run 2 of Table 2 gave the epoxide in 84%
yield (58% ee).
(
(
6
1
decomp.) d
H
3
4
3
2
Cl
2
or C
6
H
(3
6
,
26 4 6
H N O
2
C, 50.79; H, 4.62. Found: C, 50.59; H, 4.72%.
§ PhI(OAc) was purchased from ACROS. See Encyclopedia of Reagents
2
t
for Organic Synthesis, ed. L. A. Paquette, Wiley, New York, 1995, vol. 2,
p. 1479.
1 For review, see G. A. Barf and R. A. Sheldon, J. Mol. Catal. A: Chem.,
1995, 102, 23 and references cited therein; C.-M. Che, Pure Appl. Chem.,
1995, 67, 225; T. Mukaiyama and T. Yamada, Bull. Chem. Soc. Jpn.,
1995, 68, 17; K. A. J o¨ rgensen, Chem. Rev., 1989, 89, 431; S. I.
In comparison, trans-(AcO)
pared by reaction of [Ru(p-cymene)(OAc)
pyridine, was examined as an oxidation catalyst with
PhI(OAc) . The complex 9 (5 mol%) similarly catalysed the
epoxidation of 6 under the same conditions as in run 2 of Table
to give racemic trans-stilbene oxide in 49% yield; 15% of the
2
Ru(pybox-ip)(pyridine) 9, pre-
Murahashi, Angew. Chem., Int. Ed. Engl., 1995, 34, 2443.
8
2 2
] , pybox-ip and
2
For example: G. A. Barf, D. van den Hoek and D. Sheldon, Tetrahedron,
1
996, 52, 12 971; A. M. J. Jorna, A. E. M. Boerrijk and H. J. Hoorn,
2
React. Funct. Polym., 1996, 29, 10; H. Aneetha, J. Padmaja and
P. S. Zacharias, Polyhedron, 1996, 15, 2445; M. H. Robbins and
R. S. Drago, J. Chem. Soc., Dalton Trans., 1996, 105; W.-Y. Ru,
W.-C. Cheng and C.-M. Che, Polyhedron, 1994, 13, 2963.
2
alkene was recovered. This finding implies that the meridional
tridentate connected structure of pydic on 2 and 3 must be rigid
enough during the catalysis to help pybox maintain the chiral
environment, inducing the enantioselection.
Thus, we have found a new alkene epoxidation methodology
utilizing ruthenium(ii) complexes having the dual meridional
system of pydic with terpyridine and pybox, in combination
3
G. Balavoine, C. Eskenazi, F. Meunier and H. Rivi e` re, Tetrahedron Lett.,
1
984, 25, 3187; C. Esk e´ nazi, G. Balavione, F. Meuneir and H. Rivi e` re,
J. Chem. Soc., Chem. Commun., 1985, 1111.
4
5
H. Nishiyama, Y. Itoh, Y. Sugawara, H. Matsumoto, K. Aoki and K. Itoh,
Bull. Chem. Soc. Jpn., 1995, 68, 1247 and references cited therein.
H. Nishiyama, S.-B. Park, M. Haga, K. Aoki and K. Itoh, Chem. Lett.,
1
994, 1111.
with PhI(OAc)
2
as the oxygen donor. Our studies are now
6 C. Augier, L. Malara, V. Lazzeri and B. Waegell, Tetrahedron Lett.,
1995, 36, 8775 and references for ruthenium-catalysed oxidation cited
therein. For a review, see G. A. Barf and R. A. Sheldon, J. Mol. Catal. A:
Chem., 1995, 102, 23.
focused on the scope and limitations of this alkene epoxidation
and its mechanism.
7
W.-H. Fung, W.-C. Cheng, W.-Y. Yu, C.-M. Che and T. C. W. Mak,
J. Chem. Soc., Chem. Commun., 1995, 2007.
Footnotes and References
8
D. A. Tocher, R. O. Gould and T. A. Stephenson, J. Chem. Soc., Chem.
Commun., 1983, 1571.
*
†
E-mail: hnishi@tutms.tut.ac.jp
Synthesis of Ru(pydic)(terpy) 1. To a solution of [Ru(p-cymene)Cl ] 4
2 2
(150 mg, 0.24 mmol) and 2,2A:6A,2B-terpyridine (144 mg, 0.48 mmol) in
Received in Cambridge, UK, 17th July 1997; 7/05109C
1864
Chem. Commun., 1997