Guidelines for Epoxidation for Epoxidation.pdf · 5 with 1 M aqueous Na 2S2O3 and brine, dried (Na...
Transcript of Guidelines for Epoxidation for Epoxidation.pdf · 5 with 1 M aqueous Na 2S2O3 and brine, dried (Na...
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Guidelines for Asymmetric Epoxidation
Prepared by O. Andrea Wong 2009
For reviews on chiral ketone-catalyzed asymmetric epoxidation, see: (a) Wong, O.A.;
Shi, Y. Chem. Rev. 2008, 108, 3958. (b) Shi, Y. Acc. Chem. Res. 2004, 37, 488.
I. Catalyst Selection
1a. Substrate Scope of Ketone 1
O
OO
OO
O
1
PhPh
O
85% yield98% ee
C6H13
C6H13
89% yield95% ee
O
Ph
O
94% yield96% ee
trans-olefins
PhPh
O
54% yield97% ee
PhO
Ph
94% yield98% ee
C10H21
O
94% yield89% ee
trisubstituted olefins
Ph
O
OH
85% yield94% ee
Ph
O
90% yield91% ee
OH
hydroxyalkenes
O
82% yield90% ee
OH
PhTMS
O
74% yield94% ee
TMSO
51% yield90% ee
2,2-disubstituted vinylsilanes
HO TMSO
71% yield93% ee
Ph
O
77% yield97% ee
Ph
conjugated dienes and enynes
O
78% yield93% ee
O
82% yield95% ee
OEt
OBzOO
Ph
OAcO
82% yield93% ee
66% yield91% ee
enol esters
BzOO
87% yield91% ee
Trans- and trisubstituted olefins: Wang, Z-X.; Tu, Y.; Frohn, M.; Zhang, J-R.; Shi, Y. J. Am. Chem. Soc. 1997, 119, 11224. Enol esters: (a) Zhu, Y.; Tu, Y.; Yu, H.; Shi, Y. Tetrahedron Lett. 1998, 39, 7819. (b) Zhu, Y.; Manske, K.J.; Shi, Y. J. Am. Chem. Soc. 1999, 121, 4080. (c) Feng, X.; Shu, L.; Shi, Y. J. Am. Chem. Soc. 1999, 121, 11002. (d) Zhu, Y.; Shu, L.; Tu, Y.; Shi, Y. J. Org. Chem. 2001, 66, 1818. Hydroxyalkenes: Wang, Z-X.; Shi, Y. J. Org. Chem. 1998, 63, 3099.
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2,2-Disubstituted vinyl silanes: Warren, J.D.; Shi, Y. J. Org. Chem. 1999, 64, 7675.
Conjugated Dienes: Frohn, M.; Dalkiewicz, M.; Tu, Y.; Wang, Z-X.; Shi, Y. J. Org. Chem. 1998, 63, 2948. Conjugated Enynes: (a) Cao, G-A.; Wang, Z-X.; Tu, Y.; Shi, Y. Tetrahedron Lett. 1998, 39, 4425. (b) Wang, Z-X.; Cao, G-A; Shi, Y. J. Org. Chem. 1999, 64, 7646.
1b. Synthesis of Ketone 1 and ent-1
O
OO
OO
O
O
HOOH
OHOH
OH
O
OO
OO
OH
O
D-Fructose1
H+
[O]
Tu, Y.; Frohn, M.; Wang, Z-X.; Shi, Y. Org. Synth. 2003, 80, 1.
OOH
HO OH
OH
OH OMe
OMe
DME, SnCl2
O OO
O
O OH
MsCl, pyO O
O
O
O OMs
1) H2SO4, rt2) NaOH
3) NaOH, 90-100 oC
4) H2SO4, 70-80 oC
O
HOOH
OHOH
OH
L-Fructose
O
OO
OO
O
O
OO
OO
OH
O
ent-1
H+
[O]
L-Sorbose
Zhao, M-X.; Shi, Y. J. Org. Chem. 2006, 71, 5377.
1c. Commercial Sources of Ketone 1 (CAS #18422-53-2)
a. Alfar Aesar (1 g, 5 g) b. Aldrich (5g)
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1d. Representative Epoxidation Procedure (small-scale, Oxone/K2CO3, see Figures 1
and 2)
trans-β-Methylstyrene (0.118 g, 1 mmol) was dissolved in acetonitrile-DMM (15 mL, 1:2
v/v). Buffer (10 mL, 0.05 M solution of Na2B4O7·10H2O in 4 x 10-4 M aqueous
Na2EDTA, tetrabutylammonium hydrogen sulfate (15 mg, 0.04 mmol), and ketone 1
(0.0774 g, 0.3 mmol) were added with stirring. The mixture was cooled to about –10 oC
(inside) (bath temperature was about –12 to –15 oC) using an NaCl-ice bath. A solution
of Oxone (0.85 g, 1.38 mmol) in aqueous Na2EDTA (4 x 10-4, 6.5 mL) and a solution of
K2CO3 (0.8 g, 5.8 mmol) in water (6.5 mL) were added dropwise separately over a period
of 2 h (via syringe pump). At this point, the reaction was immediately quenched by the
addition of pentane and water. The mixture was extracted with pentane (3 x 30 mL),
washed with brine, dried over Na2SO4, purified by flash column chromatography [the
silica gel was buffered with 1% Et3N in pentane; pentane-ether (1:0 to 50:1 v/v) was used
as eluent] to afford trans-β-methylstyrene oxide as a colorless liquid (0.126 g, 94% yield,
95.5% ee). Wang, Z.-X.; Tu, Y.; Frohn, M.; Zhang, J.-R.; Shi, Y. J. Am. Chem. Soc.
1997, 119, 11224. See Section VI for general tips and troubleshooting information.
Note:
(a) 0.05 M Na2B4O7·10H2O solution can be replaced by K2CO3-AcOH (pH = 9.3)
buffer, see Section III for buffer preparation procedures. References: (a) Cao, G-
A.; Wang, Z-X.; Tu, Y.; Shi, Y. Tetrahedron Lett. 1998, 39, 4425. (b) Wang, Z-
X.; Cao, G-A; Shi, Y. J. Org. Chem. 1999, 64, 7646.
Additional references: (a) Wang, Z.-X.; Tu, Y.; Frohn, M.; Zhang, J.-R.; Shi, Y. J. Am. Chem. Soc. 1997, 119, 11224. (b) Warren, J.D.; Shi, Y. J. Org. Chem. 1999, 64, 7675. (c) Wang, Z-X.; Shi, Y. J. Org. Chem. 1998, 63, 3099. (d) Frohn, M.; Dalkiewicz, M.; Tu, Y.; Wang, Z-X.; Shi, Y. J. Org. Chem. 1998, 63, 2948. (e) Wang, Z-X.; Cao, G-A; Shi, Y. J. Org. Chem. 1999, 64, 7646. (f) Zhu, Y.; Shu, L.; Tu, Y.; Shi, Y. J. Org. Chem. 2001, 66, 1818.
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1e. Representative Epoxidation Procedure (large-scale, Oxone/KOH or Oxone/K2CO3,
see Figures 3, 4, and 5)
A 2L, three-necked, round-bottomed flask equipped with a 5-cm, egg-shaped, Teflon-
coated magnetic stir bar and two addition funnels is cooled in an ice bath. The flask is
charged with trans-β-methylstyrene (5.91 g, 50.0 mmol), 500 mL of a 2:1 mixture of
DMM:MeCN, 300 mL of K2CO3-AcOH buffer solution (the buffer solution is prepared
by adding 4.5 mL of glacial acetic acid to 1 L of a 0.1 M solution of K2CO3),
tetrabutylammonium hydrogen sulfate (0.375 g, 1.1 mmol), and ketone 1 (4.52 g, 17.5
mmol, 35 mol%). One addition funnel is charged with a solution of Oxone (46.1 g, 75.0
mmol) in 170 mL of aqueous 4 × 10 −4 M Na2EDTA solution and the other addition
funnel is charged with 170 mL of 1.47 M aqueous KOH solution. The two solutions in
the addition funnels are added dropwise at the same rate over 2.5 h to the cooled reaction
mixture which is stirred vigorously at 0 °C. The resulting suspension is stirred at 0 °C for
an additional hour and then 250 mL of pentane is added. The aqueous phase is separated
and extracted with two 250 mL portions of pentane, and the combined organic phases are
dried over Na2SO4, filtered, and concentrated at 0 °C. The resulting oil is loaded onto 50
g of Whatman 60 Å (230-400 mesh) silica gel packed in a 5-cm diameter column. The
silica gel is first washed with 200 mL of hexanes to remove trace amounts of unreacted
olefin, then the product is eluted with 200 mL of 10:1 hexane:ether to afford 6.02-6.31 g
(90-94%) of trans-β-methylstyrene oxide. Wang, Z-X.; Shu, L.; Frohn, M.; Tu, Y.; Shi,
Y. Org. Synth. 2003, 80, 9. See Section VI for general tips and troubleshooting
information.
Note:
(a) In principle, the Oxone/KOH solutions can be replaced by solutions of Oxone
(42.5 g, 69 mmol, 325 mL) and K2CO3 (40.0 g, 290 mmol, 325 mL).
1f. Representative Epoxidation Procedure (MeCN/H2O2, see Figure 6)
To a solution of trans-stilbene (0.180 g, 1.0 mmol) and ketone 1 (0.0774 g, 0.30 mmol) in
MeCN-DMM (1:2 v/v) (6.0 mL) was added a solution of 2.0 M K2CO3 in 4 x 10-4 M
Na2EDTA (1.5 mL) followed by H2O2 (30%, 0.4 mL, 4 mmol) at 0 oC. Upon stirring at 0 oC for 10 h and at rt for 14 h, the reaction mixture was extracted with hexanes, washed
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with 1 M aqueous Na2S2O3 and brine, dried (Na2SO4), filtered, concentrated, and purified
by chromatography (silica gel was buffered with 1% Et3N in hexanes, using
hexanes/ether 50/1 as eluent) to afford trans-stilbene oxide as a white solid (0.151 g, 77
% yield, 99% ee). Shu, L.; Shi, Y. Tetrahedron 2001, 57, 5213. See Section VI for
general tips and troubleshooting information.
Note:
(a) Vigorous stirring is crucial for epoxidation efficiency in the H2O2 epoxidation,
particularly for less reactive (i.e. nonpolar) substrates.
(b) Mixed solvent systems such as MeCN-EtOH-DCM, MeCN-nPrOH-DCM,
MeCN-nPrOH-PhCH3, and MeCN-nPrOH-PhH are beneficial for olefins with
poor solubility.
(c) The quality of H2O2 is important. Old or contaminated H2O2 cannot provide
maximum conversion.
Additional references: (a) Shu, L.; Shi, Y. Tetrahedron Lett. 1999, 40, 8721. (b) Shu, L.; Shi, Y. Tetrahedron 2001, 57, 5213. (c) Wang, Z-X.; Shu, L.; Frohn, M.; Tu, Y.; Shi, Y. Org. Synth. 2003, 80, 9.
1g. Examples of Synthetic Applications of Ketone 1
OMe
OMe
O O
Ketone 1
OMe
OMe
O O
OH
OMe
OMe
N O
(-)-Mesembrine
Oxone
OMe
OMe
O O
OMgCl
73% two steps
96% ee
H
Taber, D.F.; He, Y. J. Org. Chem. 2005, 70, 7711.
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OH O
O
Ph
HOH
1) Ketone 1Oxone
2) Nicotinic acidDCC, DMAP
O O
O
Ph
HO
O
N
O
ON
(+)-Nigellamine A2
51% two steps
Bian, J.; Van Wingerden, M.; Ready, J.M. J. Am. Chem. Soc. 2006, 128, 7428.
OMe
MeH
MeMe
TMSO
H
H
ent -Abudinol
O
HHO
MeMe H
Me
OMeH
ent-Nakorone
Me Me
Me
Me
SO2Tol
TMS Me Me
Me
Me
SO2Tol
TMS
O OKetone 1
Oxone76%
OMe
MeH
MeMe
HO
H
H OH
MeH
OH
MeMe
>20:1 dr
Tong, R.; Valentine, J.C.; McDonald, F.E.; Cao, R.; Fang, X.; Hardcastle, K.I. J. Am. Chem. Soc. 2007, 129, 1050.
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OH
HOOH
OH OH
HOOH
OH
O O
O OKetone 1
Oxone
O
O O
OO
OHOH
HMe
HMe
H
HMe
HMe
H
3 steps
Glabrescol
66%
(a) Corey, E.J.; Xiong, Z. J. Am. Chem. Soc. 2000, 122, 4831. (b) Corey, E.J.; Xiong, Z.
J. Am. Chem. Soc. 2000, 122, 9328.
H H
HH
H Me
Me H
Me
O
H H
HH
H Me
Me
O
H
Me
O O O O1) BF3
.OEt2
2) Ac2Opyridine O
O
O
O
O
O
H H MeMe
OAcH
Me
Me H H
Ketone 1
O
O
Me2N
O
Me2N
OxoneH H
HH
H Me
Me H
MeHO
1) SharplessEpoxidation
2) n-BuLiMe2NC(O)Cl
(a) McDonald, F.E.; Wang, X.; Do, B.; Hardcastle, K.I. Org. Lett. 2000, 2, 2917. (b) McDonald, F.E.; Bravo, F.; Wang, X.; Wei, X.; Toganoh, M.; Rodríguez, J.R.; Do, B.; Neiwert, W.A.; Hardcastle, K.I. J. Org. Chem. 2002, 67, 2515. (c) Bravo, F.; McDonald, F.E.; Neiwert, W.A.; Do, B.; Hardcastle, K.I. Org. Lett. 2003, 5, 2123. (d) Valentine, J.C.; McDonald, F.E.; Neiwert, W.A.; Hardcastle, K.I. J. Am. Chem. Soc. 2005, 127, 4586.
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O
TBSOH
H
O
O
O
H HH
H HH
MeKetone 1
Oxone O
TBSOH
H
MeO
O
O
OMe
HOH
H
TBAF
THF
H2O
70 oC, 72 h
71%O
HOH
H
MeO
O
O
Vilotijevic, I.; Jamison, T.F. Science 2007, 317, 1189. 2a. Substrate Scope of Ketone 2
Ph
O
92% yield81% ee
OCl
74% yield83% ee
O
90% yield85% ee
ClPh
O
78% yield95% ee
styrenes trisubstituted olefins
PhO
55% yield80% ee
Ph
O
87% yield91% ee
O
76% yield92% ee
O
74% yield92% ee
F
O
88% yield83% ee 88% yield
84% ee
O
O
NCO
61% yield91% ee
conjugated cis-olefins
O
Ph
O
nC6H13
82% yield91% ee
77% yield87% ee
O
O
OO
O
O
O
O
O
61% yield97% ee
47% yield96% ee
88% yield94% ee
unconjugated olefins
F
OO
73% yield86% ee
O
61% yield81% ee
Cl
n-C5H11
O
O
O
76% yield92% ee
O
93% yield71% ee
n-C6H13
MeO
O
79% yield61% ee
71% yield85% ee
Me
OO
O
OO
NBocO
O
O
2
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Styrenes: (a) Tian, H.; She, X.; Xu, J.; Shi, Y. Org. Lett. 2001, 3, 1929. (b) Tian, H.; She, X.; Yu, H.; Shu, L.; Shi, Y. J. Org. Chem. 2002, 67, 2435.
Trisubstituted, conjugated cis-olefins, unconjugated olefins: (a) Tian, H.; She, X.; Shu, L.; Yu, H.; Shi, Y. J. Am. Chem. Soc. 2000, 122, 11551. (b) Tian, H.; She, X.; Yu, H.; Shu, L.; Shi, Y. J. Org. Chem. 2002, 67, 2435. (c) Burke, C.P.; Shi, Y. Org. Lett. 2009, 11, 5150.
2b. Synthesis of Ketone 2
O
OH
OH
HO OH
OH
D-glucose
O
OH
HO OH
OHNBn2
(MeO)3CH
CH3COCH3
HCl
O
OH
OHNBn2
OO
H2, Pd/C
HOAc
O
OH
OHNH3OAc
OO
COCl2
NaHCO3
O
OO
NBocO
O
O
O
OO
NHO
OH
O
PDC
Bn2NH
HOAc
(Boc)2O
DMAP
O
OO
NBocO
OH
O
Shu, L.; Shen, Y.M.; Burke, C.; Goeddel, D.; Shi, Y. J. Org. Chem. 2003, 68, 4963.
2c. Representative Epoxidation Procedure (small-scale, Oxone/K2CO3, see Figures 1
and 2)
To a solution of cis-β-methylstyrene (0.059 g, 0.5 mmol) and ketone 2 (0.026 g, 0.075
mmol) in DME-DMM (3:1, v/v) (7.5 mL) were added buffer (0.2 M K2CO3-AcOH in 4 x
10-4 M aqueous Na2EDTA, buffer pH = 8.0) (5 mL) and Bu4NHSO4 (0.0075 g, 0.02
mmol) with stirring. After the mixture was cooled to about –10 oC (bath temperature)
using an NaCl-ice bath, a solution of Oxone (0.21 M in 4 x 10 –4 M aqueous Na2EDTA,
4.2 mL) (0.548 g 0.89 mmol) and K2CO3 (0.48 M in 4 x 10-4 M aqueous Na2EDTA, 4.2
mL) (0.278 g, 2.01 mmol) were added dropwise separately over a period of 3.5 h via
syringe pump. The reaction was then quenched with the addition of pentane and
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extracted with pentane. The combined organic layers were washed with brine, dried
(Na2SO4), filtered, concentrated, and purified by flash chromatography [the silica gel was
buffered with 1% Et3N in pentane; pentane-ether (1/0 to 50/1) was used as eluent] to give
cis-β-methylstyrene oxide as a colorless liquid (0.058 g, 87% yield, 91% ee). Tian, H.;
She, X.; Shu, L.; Yu, H.; Shi, Y. J. Am. Chem. Soc. 2000, 122, 11551. See Section VI for
general tips and troubleshooting information.
Additional references: (a) Tian, H.; She, X.; Shu, L.; Yu, H.; Shi, Y. J. Am. Chem. Soc. 2000, 122, 11551. (b) Tian, H.; She, X.; Xu, J.; Shi, Y. Org. Lett. 2001, 3, 1929. (c) Tian, H., She, X.; Yu, H.; Shu, L.; Shi, Y. J. Org. Chem. 2002, 67, 2435.
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3a. Substrate Scope of Ketones 3
Ph
O
99% conv.84% ee
cis-ββββ -methylstyrenes
O
100% conv.88% ee
Me
O
94% conv.96% ee
NC
2,2-dimethylchromenes
O
O
O
ONC
100% conv.84% ee
83% conv.93% ee
O
O
95% conv.88% ee
CN
O
72%yield86% ee
styrenes
O
86%yield90% ee
NC
benzylidenecyclobutanes and 2-aryl-cyclopentanones
48% yield91% ee
H
O
O
H
Arvia
O
benzylidenecyclopropanes
54% yield87% ee
Me
O
O
Me
ArviaO
CO2Et
O
O
CO2Et
74% yield94% ee
64% yield94% ee
cis-dienes and trienes
Ph
O
66% yield85% ee
O
78% yield93% ee
Ph
O
Ph76% yield93% ee
cis-enynesO
O
O
O
54% yield87% ee
unconjugated olefins
O
nHex
OH
89% yield82% ee
O
nHex
HO
71% yield79% ee
O
OH
n-C5H11
O
75% yield91% ee
O
OO
NO
O
O
R
3a = Me3b = Et3c = nBu
Ph
OR O
R
Ph
Et2AlCl
R=H, 93% (90% ee)R=Me, 94% (84% ee)
R=H, 90% (90% ee)R=Me, 93% (84% ee)
O
82% conv.93% ee
Br
O
nHex
52% yield64% ee
O
OCl
100% conv.93% ee
cis-β-Methylstyrenes: Shu, L.; Shi, Y. Tetrahedron Lett. 2004, 45, 8115.
Unconjugated Olefins: Burke, C.P.; Shi, Y. Org. Lett. 2009, 11, 5150. 2,2-Dimethylchromenes: Wong, O. A.; Shi, Y. J. Org. Chem. 2006, 71, 3973. Styrenes: Goeddel, D.; Shu, L.; Yuan, Y.; Wong, O. A.; Wang, B.; Shi, Y. J. Org. Chem. 2006, 71, 1715.
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Benzylidenecyclobutanes and 2-Aryl-Cyclopentanones: (a) Shen, Y.-M.; Wang, B.; Shi, Y. Angew. Chem., Int. Ed. 2006, 45, 1429. (b) Shen, Y.-M.; Wang, B.; Shi, Y. Tetrahedron Lett. 2006, 47, 5455.
Benzylidenecyclopropanes: Wang, B.; Shen, Y.-M.; Shi, Y. J. Org. Chem. 2006, 71, 9519.
cis-Enynes: Burke, C. P.; Shi, Y. J. Org. Chem. 2007, 72, 4093.
cis-Dienes and Trienes: Burke, C. P.; Shi, Y. Angew. Chem., Int. Ed. 2006, 45, 4475.
3b. Synthesis of Ketones 3
O OH
OH
OH
HO OH
1) ArNH2
HOAc
2) CH(OMe)3acetoneH2SO4
D-Glucose
OOH
OHOO
NHAr 1) COCl2
2) PDC
O NO
O
O
O
O
3
R
(a) Goeddel, D.; Shu, L.; Yuan, Y.; Wong, O.A.; Wang, B.; Shi, Y. J. Org. Chem. 2006, 71, 1715. (b) Zhao, M-X.; Goeddel, D.; Li, K.; Shi, Y. Tetrahedron 2006, 62, 8064.
3c. Commercial Sources of Ketone 3a (CAS #403501-30-4)
Aldrich (100 mg, 500 mg)
3d. Representative Epoxidation Procedure (small-scale, Oxone/K2CO3, see Figures 1
and 2)
To a mixture of 2,2-dimethylchromene (0.4 mmol), Bu4NHSO4 (0.001 g, 0.003 mmol),
and ketone 3b (0.028 g, 0.08 mmol) was added DME/DMM (v/v 3:1) (6.0 mL). After the
mixture was stirred at rt for 20 min, buffer (0.1 M K2CO3-AcOH in 4 x 10-4 M aqueous
Na2EDTA, pH 9.3) (4.0 mL) was added. After being stirred at rt for 10 additional min,
the mixture was cooled using an ice bath (0 °C). Oxone (5.04 mL, 0.212 M in 4 x 10-4 M
aqueous Na2EDTA) and K2CO3 (5.04 mL, 0.84 M in 4 x 10-4 M aqueous Na2EDTA)
were added simultaneously and separately via syringe pump over 6 hours. The reaction
was quenched by the addition of diethyl ether and extracted with diethyl ether. The
13
combined organic layers were washed with brine, dried over Na2SO4, filtered,
concentrated, and purified by flash chromatography (silica gel was buffered with 1%
NEt3) to afford the corresponding epoxide. Wong, O.A.; Shi, Y. J. Org. Chem. 2006, 71,
3973. See Section VI for general tips and troubleshooting information.
3e. Representative Epoxidation Procedure (MeCN/H2O2, see Figure 6)
A mixture of 2-methyl-1-phenyl-1-propene (0.135 g, 1.00 mmol) and ketone 3b (0.051
g, 0.15 mmol) in n-BuOH (3.0 mL) was cooled to 0 °C with an ice bath. CH3CN (0.20
mL, 3.8 mmol) and 0.30 M K2CO3 in 4 x 10-4 M aqueous Na2EDTA (3.0 mL) were then
added, followed by H2O2 (30%, 0.30 mL, 3.0 mmol) with vigorous stirring at 0 °C. The
mixture was stirred at 0 °C for 24 h and was then poured into petroleum ether and
extracted with petroleum ether. The combined organic layers were washed with water
and saturated aqueous Na2S2O3, dried (Na2SO4), filtered, concentrated, and purified by
flash chromatography (silica gel was buffered with 1% Et3N in petroleum ether;
petroleum ether was used as eluent) to give 2,2-dimethyl-3-phenyloxirane as a colorless
oil (0.121 g, 82% yield, 92% ee). Burke, C.P.; Shu, L.; Shi, Y. J. Org. Chem. 2007, 72,
6320. See Section VI for general tips and troubleshooting information.
Notes:
a) The quality of the ketone catalyst is very important when H2O2 is used as the
oxidant. For example, it was observed that lower conversions were obtained with
the ketones 3a-c prepared from the TEMPO-bleach oxidation of the alcohol
precursor. It appears that some residual impurities from TEMPO-bleach
oxidation somehow affect the epoxidation.
b) Vigorous stirring is crucial for epoxidation efficiency in the H2O2 epoxidation,
particularly for less reactive (i.e. nonpolar) substrates.
c) The quality of H2O2 is important. Old or contaminated H2O2 cannot provide
maximum conversion.
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4a. Substrate Scope of Ketone 4
O OO
AcO O
AcO 4
OOBz
97% yield95% ee
O
PhPh
63% yield93% ee
78% yield86% ee
NOnBu
O
O
trans and trisubstituted olefins
Ph
O
TBS
60% yield90% ee
Ph
O
OTBS
62% yield70% ee
O
O
O
ONC
75% yield88% ee
73% conv.90% ee
cis-olefins
95% yield75% ee
NOnBu
O
O
PhCO2Et
O
73% yield96% ee
CO2EtO
MeO57% yield90% ee
PhCO2Et
O
93% yield96% ee
O
CO2Et
CO2EtO
Ph
CO2EtO
77% yield93% ee
96% yield94% ee
64% yield82% ee
αααα,ββββ-unsaturated esters fluoroolefins
Ph
O
86% yield91% ee
60% yield90% ee
Ph
O
68% yield92% ee
Ph
F
F
Ph
OF
n-Bu
O
77% yield91% ee
n-Bu
F
trans, cis, and Trisubstituted Olefins: Wang, B.; Wu, X.-Y.; Wong, O.A.; Nettles, B.; Zhao, M.-X.; Chen, D.; Shi, Y. J. Org. Chem. 2009, 74, 3986.
α,β-Unsaturated Esters: Wu, X-Y.; She, X.; Shi, Y. J. Am. Chem. Soc. 2002, 124, 8792. Fluoroolefins: Wong, O.A.; Shi, Y. J. Org. Chem., 2009, 74, 8377-8380
15
4b. Synthesis of Ketone 4 (hydrate of ketone 4, 4••••H2O, can be used in place of ketone 4)
O OO
AcO O
AcO4
O
OO
OO
O
1
DDQ, MeCN/H2O
H2O, Ac2O
Ac2O, DMAPO O
O
HO O
OH
O OO
AcOOH
AcO4.H2O
O
OO
OO
O
1
AcOH, ZnCl2
H2O, Ac2O
OH
OR
Wang, B.; Wu, X.-Y.; Wong, O.A.; Nettles, B.; Zhao, M.-X.; Chen, D.; Shi, Y. J. Org. Chem. 2009, 74, 3986.
4c. Representative Epoxidation Procedure (small-scale, Oxone/K2CO3, see Figures 1
and 2)
To a solution of trans-β-methylstyrene (0.059 g, 0.5 mmol), ketone 4·H2O (0.015 g,
0.046 mmol), and tetrabutylammonium hydrogen sulfate (0.01 g, 0.03 mmol) in MeCN-
DMM (v/v, 1/2) (9 mL) was added buffer (0.05 M aq Na2HPO4-0.05 M aq KH2PO4, pH
7.0) (3 mL) with stirring. Upon cooling to 0 oC, a solution of Oxone (0.212 M in 4 x 10-4
M aq Na2EDTA, 4.8 mL) and a solution of K2CO3 (0.42 M in 4 x 10-4 M aq Na2EDTA,
4.8 mL) were added dropwise simultaneously and separately over 8 h via syringe pump.
The reaction was quenched by addition of pentane and extracted with pentane. The
combined organic layers were dried over Na2SO4, filtered, concentrated, and purified by
flash chromatography (silica gel was buffered with 1% Et3N in organic solvent, first
pentane, then pentane/Et2O = 20/1) to give the epoxide as a colorless oil (0.054 g, 81%
yield, 86% ee). Wang, B.; Wu, X.-Y.; Wong, O.A.; Nettles, B.; Zhao, M.-X.; Chen, D.;
Shi, Y. J. Org. Chem. 2009, 74, 3986. See Section VI for general tips and
troubleshooting information.
16
4d. Representative Epoxidation Procedure (NaHCO3/Oxone, see Figure 7)
Aqueous Na2(EDTA) (1×10-4 M, 2.5 mL) and a catalytic amount of tetrabutylammonium
hydrogen sulfate (0.010 g, 0.03 mmol) were added to a solution of ethyl trans-4-
methylcinnamate (0.095 g, 0.5 mmol) in CH3CN (2.5 mL) with vigorous stirring at 0 ˚C.
A mixture of Oxone (1.537 g, 2.5 mmol) and NaHCO3 (0.651 g, 7.75 mmol) was
pulverized, and a small portion of this mixture was added to the reaction mixture to bring
pH to >7.0. Then a solution of ketone 4 (0.038 g, 0.125 mmol) in CH3CN (1.25 mL) was
added. The remainder of the Oxone and NaHCO3 was added to the reaction mixture
portionwise over a period of 4.5 h. Upon stirring for an additional 7.5 h at 0 oC and 12 h
at rt, the resulting mixture was diluted with water, and extracted with ethyl acetate. The
combined extracts were washed with brine, dried over Na2SO4, filtered, concentrated, and
purified by flash chromatography (silica gel, hexane/EtOAc = 1/0 to 95/5) to give the
epoxide as a colorless oil (0.094 g, 91% yield, 97% ee). Wu, X-Y.; She, X.; Shi, Y. J.
Am. Chem. Soc. 2002, 124, 8792. See Section VI for general tips and troubleshooting
information.
17
5a. Substrate Scope of Ketone 5
5
O
Ph
60% yield62% ee
O
Ph
62% yield77% ee
O
Ph
43% yield86% ee
OF
72% yield88% ee
OOH
Ph
93% yield77% ee
OOH
Ph
76% yield87% ee
Me Me
OOH
nC6H13
78% yield60% ee
Me Me
1,1-disubstituted olefins
Ph
O
60% yield85% ee
O
ONC
87% yield84% ee
O
Ph
89% yield80% ee
PhO
56% yield90% ee
cis-olefins
OOH
Ph
86% yield88% ee
O
OO
O
N
O
O
1,1-Disubstituted Olefins and cis-Olefins: Wang, B.; Wong, O.A.; Zhao, M.-X.; Shi, Y. J. Org. Chem. 2008, 73, 9539.
18
5b. Synthesis of Ketone 5
O OH
OH
OH
HO
OH
D-Glucose
p-Toluidine
H+
OOH
NHTol-p
OH
OH
HO
Me2C(OMe)2
acetoneH2SO4
O
BrBr
Et3N, NaH
OOH
O
O
NHTol-p
OH
OO
O
O
NTol-p
OH
O
OO
O
O
NTol-p
O
O
PDC
CH2Cl25
Wang, B.; Wong, O.A.; Zhao, M.-X.; Shi, Y. J. Org. Chem. 2008, 73, 9539.
5c. Representative Epoxidation Procedure (small-scale, Oxone/K2CO3, see Figures 1
and 2)
To a solution of α-methylstyrene (0.024 g, 0.20 mmol), tetrabutylammonium hydrogen
sulfate (0.0038 g, 0.010 mmol), and ketone 5 (0.0208 g, 0.06 mmol) in dioxane (3 mL)
was added buffer (0.1 M K2CO3-AcOH in 4 x 10-4 M aqueous Na2EDTA, pH = 9.3; 2
mL) with stirring. After the mixture was cooled to -10 oC (bath temperature), a solution
of Oxone (0.20 M in 4 x 10-4 M aqueous Na2EDTA, 1.6 mL) (0.197 g, 0.32 mmol) and a
solution of K2CO3 (0.84 M in 4 x 10-4 M aqueous Na2EDTA, 1.6 mL) (0.185 g, 1.344
mmol) were added separately and simultaneously via syringe pump over a period of 2 h.
The reaction mixture was quenched with hexane, extracted with EtOAc, dried over
Na2SO4, filtered, concentrated, and purified by flash chromatography (silica gel was
buffered with 1% Et3N in organic solvent) to give α-methylstyrene oxide (60% yield,
62% ee). Wang, B.; Wong, O.A.; Zhao, M.-X.; Shi, Y. J. Org. Chem. 2008, 73, 9539.
See Section VI for general tips and troubleshooting information.
19
II. General Guidelines for Epoxidation Setup Small-scale epoxidation set up (Figures 1 and 2)
Figure 1. Epoxidation using Oxone/K2CO3 (syringe)
a) Fill with Oxone dissolved in 4x10-4 M aq. Na2EDTA b) Fill with K2CO3 dissolved in 4x10
-4 M aq. Na2EDTA c) Syringes are taped together for stability. d) Rubber tubing and plastic tip, do not use metal. e) Holes are spaced to prevent mixing at the tube tips and to
prevent the solutions from dripping down the side of the flasks where they freeze.
Figure 2. Epoxidation using Oxone/K2CO3
a) Syringe pump. Oxone and K2CO3 should be added at a constant rate over the desired reaction time. Air at the tip of
20
the syringe should be pushed out to ensure the simultaneous addition of Oxone and K2CO3 as pH control is important for the epoxidation reactions. It has been found that the oxidation activity of the purchased Oxone occasionally varies with different batches.
b) Reaction flask is suspended into Dewar with a clamp. All glassware should be washed with soap to be free of any trace metals, which catalyze the decomposition of Oxone. Cap should not be airtight.
c) Dewar flask. For –10 oC epoxidations, bath temperature should not be below –15 oC which will freeze the reaction. Low conversions will result from frozen reactions.
d) Stir plate. Stirring should be vigorous but without splashing.
Large-scale epoxidation set up (Figures 3-5)
Figure 3. Epoxidation using Oxone/ K2CO3
a) Fill with Oxone dissolved in 4x10-4 M aq. Na2EDTA b) Fill with K2CO3 dissolved in 4x10
-4 M aq. Na2EDTA c) Ice bath. For –10 oC epoxidations, bath temperature should
not be below –15 oC which will freeze the reaction. Low conversions will result from frozen reactions.
d) Stir plate. Stirring should be vigorous but without splashing.
21
Figure 4. Epoxidation using Oxone/ K2CO3 or Oxone/ KOH
a) Fill with Oxone dissolved in 4x10-4 M aq. Na2EDTA b) Fill with KOH or K2CO3 dissolved in 4x10
-4 M aq. Na2EDTA
c) Ice bath. For –10 oC epoxidations, bath temperature should not be below –15 oC which will freeze the reaction. Low conversions will result from frozen reactions.
d) Stir plate. Stirring should be vigorous but without splashing.
22
Figure 5. Epoxidation using Oxone/ K2CO3 or Oxone/ KOH (addition funnel)
a) Addition funnel equipped with needle stopcock is preferred for precise addition-rate control.
Figure 6. Epoxidation using MeCN/H2O2
a) The ketones 3a-c used in the H2O2 epoxidation were synthesized by the PDC oxidation. It was observed that lower conversions were obtained with the ketones prepared from the TEMPO-bleach oxidation of the alcohol precursor. It appears that some residual impurities from TEMPO-bleach oxidation somehow affect the epoxidation.
23
b) Vigorous stirring is crucial for epoxidation efficiency in the H2O2 epoxidation, particularly for less reactive (i.e. nonpolar) substrates.
c) For the H2O2 epoxidation with ketone 1, a mixed solvent such as MeCN-EtOH-DCM is beneficial for olefins with poor solubility.
Figure 7. Epoxidation using Oxone/NaHCO3 (solid mixture)
a) Oxone needs to be pulverized before use. b) The pH of the reaction mixture needs to be >7 before
adding ketone. c) For substrates with poor solubility, the amount of organic
solvent used can be doubled.
24
III. Buffer Preparations
Buffer solution Ratio of components
pH 9.3 buffer (0.05 M Na2B2O7
.10H2O) 1.907 g Na2B2O7·10H2O,
100 mL 4 x 10 –4 M aq Na2(EDTA) solution
pH 9.3 buffer (0.1 M K2CO3-AcOH)
13.82 g K2CO3 and 4.80 mL AcOH in 1L H2O
pH 8.0 buffer (0.1 M K2CO3-AcOH)
13.82 g K2CO3 and 5.96 mL AcOH in 1L H2O
pH 7.0 buffer 7.1 g Na2HPO4, 6.8 g KH2PO4 in 1L H2O
IV. K2CO3 Concentration Chart (Oxone concentration = 0.21 M)
Catalyst K2CO3 Concentration (M)
1 0.89
2 0.48
3a-c 0.84
4·H2O 0.42
5 0.84
25
V. Preparation of Buffers for pH Study
Buffer solution Ratio of components
pH 8.0 buffer 13.82 g K2CO3 and 5.96 mL AcOH in 1 L H2O
pH 8.5 buffer 13.82 g K2CO3 and 5.70 mL AcOH in 1 L H2O
pH 9.0 buffer 13.82 g K2CO3 and 5.30 mL AcOH in 1 L H2O
pH 9.3 buffer 13.82 g K2CO3 and 4.80 mL AcOH in 1 L H2O
pH 9.5 buffer 13.82 g K2CO3 and 4.40 mL AcOH in 1 L H2O
pH 10.0 buffer 13.82 g K2CO3 and 2.80 mL AcOH in 1 L H2O
26
VI. General Tips and Troubleshooting Information
a) Oxone and K2CO3 should be added at a constant rate over the desired reaction
time. Air at the tip of the syringe should be pushed out to ensure the simultaneous
addition of Oxone and K2CO3 as pH control is important for the epoxidation
reactions. It has been found that the oxidation activity of the purchased Oxone
occasionally varies with different batches.
b) Reaction flask is suspended into Dewar with a clamp. All glassware should be
washed with soap to be free of any trace metals, which catalyze the
decomposition of Oxone. Avoid using metal needles for the addition of aqueous
Oxone solution.
c) For –10 oC epoxidations, bath temperature should not be below –15 oC which will
freeze the reaction. Even momentary freezing will result in low conversions.
d) Stirring should be vigorous but without splashing.
e) Relatively insoluble substrates should be stirred in only organic solvent at room
temperature for about 20-30 min before adding buffer and cooling to ensure
maximum conversion.
f) For less reactive substrates, conversion can also usually be improved with a
slower addition of Oxone and/or higher reaction temperature (0 °C or rt).
However, a decrease of ee may result from higher reaction temperature.
g) Purification columns are usually performed with solvents buffered with 1% Et3N
as the epoxides are acid-sensitive. However, it was found that certain epoxides
decompose in the presence of Et3N (e.g. Fluoroepoxides: Wong, O. A.; Shi, Y. J.
Org. Chem., 2009, 74, 8377-8380). Moreover, Et3N maybe difficult to
completely remove for extremely volatile epoxides.
h) The quality of H2O2 is important in the case where H2O2 is used as the oxidant .
Old or contaminated H2O2 cannot provide maximum conversion.
i) The quality of the ketone catalyst is very important when H2O2 is used as the
oxidant. For example, it was observed that lower conversions were obtained with
the ketones 3a-c prepared from the TEMPO-bleach oxidation of the alcohol
27
precursor. It appears that some residual impurities from TEMPO-bleach
oxidation somehow affect the epoxidation.
j) Certain reactive olefins may contain racemic epoxide resulting from oxidation
during storage. To ensure maximum enantioselectivities, those olefins should be
purified before asymmetric epoxidation.