Baeckvall J.-E. (ed.) Modern Oxidation Methods
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84
j
2 Transition Metal-Catalyzed Epoxidation of Alkenes
3
Organocatalytic Oxidation. Ketone-Catalyzed Asymmetric
Epoxidation of Alkenes and Synthetic Applications
Yian Shi
3.1
Introduction
The asymmetric epoxidation of alkenes constitutes a powerful approach to
enantiomerically enriched epoxides, a class of highly versatile intermediates in
organic synthesis [1]. Various effective epoxidation systems have been developed,
including epoxidation of allylic [2, 3] and homoallylic [4] alcohols, metal-catalyzed
epoxidation of unfunctionalized alkenes [5–7], and the nucleophilic epoxidation of
electron-deficient alkenes [8]. During the past 10–15 years, much effort has been
devoted to chiral ketone-catalyzed asymmetric epoxidation (Scheme 3.1). The
subject has been described in great detail in the first edition [9] and other
reviews [10]. This chapter provides an update on progress in this area since the
first edition [9].
R
1
R
3
O
R
1
R
3
2
-
-
-
*R'*R
OO
*R'*R
O
*R'*R
HO O
O
SO
3
*R'*R
O O
O
SO
3
-
-
-
R
2
R
2
HSO
5
OH
SO
4
*
*
Scheme 3.1 Chiral ketone-catalyzed asymmetric epoxidation of alkenes.
j
85
3.2
Catalyst Development
Dioxiranes are generated from an oxidant such as Oxone (2KHSO
5
KHSO
4
K
2
SO
4
)
and ketones, and can be used in situ (Scheme 3.1) [11, 12]. While one may expect that
asymmetric epoxidation could be possible using a chiral ketone as catalyst, it has been
quite challenging to develop an effective catalyst with a broad substrate scope. As
summarized in Schemes 3.2–3.7 , many types of chiral ketones of varying structures
have been investigated and reported by various laboratories [13–44].
Trans and trisubstituted alkenes conjugated with aromatic groups appear to be
more effective substrates, likely because of the large size of aromatic substituents,
thus providing more efficient stereodifferentiation. A number of chiral ketones can
provide good to high ees for this class of alkenes (Table 3.1). However, trans and
O
O
O
O
O
R
O
O
O
O
O
R
3
R
3
R
1
R
1
R
2
R
2
O
O
O
O
O
O
F
Ph
Me
Me
O
H
Me
F
Me
O
3
7
12
Y
CO
2
R
X
Me
Me
O
Me
7 (ref. 13a)
O
F
*
R
( )
n
8 (ref. 14)
O
Cl
Me
X
O
F
Me
X
Y
9 (ref. 30)
10 (ref. 29b, 33)
11 (ref. 35)
12 (ref. 18) 13 (ref. 37a-h)
14 (ref. 37c-f) 15 (ref. 37e,i)
16 (ref. 39)
Scheme 3.3 Selected examples of carbocyclic ketones.
N
N
O
O
O
O
Ph
Ph
H
3
C
H
H
3
C
H
Ph
Me
O
Me
H
H
3
C
O
O
CF
3
H
3
C
CH
3
O
OCH
3
CF
3
*
CH
3
NO
CF
3
O
O
Ph
CH
3
O
O
O
1 (ref. 13a) 2 (ref. 13b)
3 (ref. 13b)
4 (ref. 35b)
5 (ref. 36)
6 (ref. 43a)
Scheme 3.2 Selected examples of ketones with an attached chiral moiety.
86
j
3 Organocatalytic Oxidation. Ketone-Catalyzed Asymmetric Epoxidation of Alkenes
trisubstituted alkenes without conjugated aromatic substituents as well as cis and
terminal alkenes have generally proven to be extremely challenging (Scheme 3.8),
and very few chiral ketones are effective for these alkenes.
Fructose-derived ketone 42 has been shown to be a highly general ketone catalyst
(typically 20–30 mol% used) for epoxidation of a wide variety of trans and trisubsti-
O
O
O
O
O
X
X
O
O
O
O
Ph
Ph
Ph
Ph
O
26 (ref. 32)
Ph
Ph
O
O
O
25 (ref. 31)
O
O
O
18 (ref. 17c, 31)
Me
Me
F
F
O
Me
Me
O
F
F
O
O
F
F
F
F
F
N
N
SO
2
Ph
SO
2
Ph
O
O
O
O
19 (ref. 38b)
20 (ref. 38b)
N
N
O
Ph
Ph
SO
2
R
SO
2
R
O
N
O
O
H
N
N
O
O
O
O
O
H
H
17 (ref. 17)
21 (ref. 10a, 16)
22 (ref. 10a, 16)
23 (ref. 40)
24 (ref. 40)
27 (ref. 38a)
28 (ref. 38c)
29 (ref. 38c)
R
R
Scheme 3.4 Selected examples of C
2
-symmetric binaphthyl-based and related ketones.
O
N
Me
n
-C
12
H
25
Me
30 (ref. 10a)
N
O
Me Me
+
+
32 (ref. 10a, 15c)
N
N
Me Me
Me
Me
O
33 (ref. 10a)
+
+
N
N
O H
+
+
34 (ref. 10a)
2 TfO
-
2 TfO
-
NN
O
++
35 (ref. 10a)
NN
O
+
+
36 (ref. 10a)
Ph
Ph
Ph
Ph
2 TfO
-
2 TfO
-
N
Me Me
F
O
31 (ref. 15b, 16)
+
TfO
-
TfO
-
TfO
-
Scheme 3.5 Selected examples of ammonium and bis(ammonium) ketones.
3.2 Catalyst Development
j
87
tuted alkenes with or without aromatic substituents [20c], including vinyl silanes
[20i], hydroxyalkenes [20d], enol silyl ethers and esters [20g,j,l,45], conjugated dienes
[20e], and enynes (Scheme 3.9) [20f,h]. A reliable transition state model has been
established for epoxidation with ketone 42. The epoxidation of trans and trisubsti-
tuted alkenes proceeds mainly through spiro A, with planar B competing (Figure 3.1)
[20]. The competition between spiro A and planar B is dependent on the substituents
on the alkenes [10d,h,20,25]. In general, transition state spiro A is favored by
decreasing the size of R
1
and/or increasing the size of R
3
, thus giving high
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OMe
O
O
O
O
O
O
O
R
1
R
2
R
4
O
O
O
O
O
51 (ref. 29a)
47 (ref. 29a) 48 (ref. 29b)
R
3
O
O
O
AcO
O
AcO
O
O
O
RN
O
O
O
O
O
O
NR
O
O
O
O
O
O
NR'
O
O
O
RR
O
O
O
O
R
O
R
49 (ref. 29c)
O
O
O
O
O
O
R
R
50 (ref. 29c)
O
O
O
PivO
O
O
O
O
O
OBn
OR
RO
O
O
O
O
O
OR
CHO
42 (ref. 20, 21) 46 (ref. 28)43 (ref. 22)
44 (ref. 24) 45 (ref. 27)
52 (ref. 29a)
53 (ref. 42a)
54 (ref. 42b)
55 (ref. 42c-f)
56 (ref. 44)
O
O
O
O
O
TBSO
H
(Me)
Scheme 3.7 Selected examples of carbohydrate-based and related ketones.
N
CO
2
Et
F (OAc)
O
O
F (OAc)
O
O
O
F (OAc, SO
2
Me)
O
O
OAc
F
O
O
F
R
37 (ref. 34a,b,d,e)
38 (ref. 34b,c,g)
39 (ref. 34g)
40 (ref. 34g)
41 (ref. 41)
Scheme 3.6 Selected examples of bicyclo[3.2.1]octan-3-ones and related ketones.
88
j
3 Organocatalytic Oxidation. Ketone-Catalyzed Asymmetric Epoxidation of Alkenes
enantioselectivity for the epoxidation [20c]. Spiro A is also favored by conjugating
groups such as phenyl, alkenyl, alkynyl, and so on. These groups enhance the
stabilizing secondary orbital interaction in the spiro transition state by lowering the
energy of the p
orbital of the reacting alkene [46, 47].
Ketones 43 and 46 (Scheme 3.7), obtained by replacing the fused ketal of 42 with
more electron-withdrawing diacetates and an oxazolidinone respectively, show
significantly enhanced reactivity. Only 1–5 mol% of ketone 46 is required to achieve
good conversions for the epoxidation of a variety of trans and trisubstituted alkenes
with high ees [28]. A variety of a,b-unsaturated esters can be epoxidized with ketone 43
Table 3.1 Representative Asymmetric Epoxidation of Aryl Alkenes with Chiral Ketones.
Entry Ketone
Ph
Ph
Ph
Ph
Me (Ph)
Ph
Ph
Ref.
11–3 9.5–18% ee 13a,b
2 4 25% ee 31–33% ee 34% ee 35b
3 6 36% ee 35% ee 26% ee 43a
4 7 12.5% ee 13a
5 9 (R ¼ CMe
2
OH) 96% ee 92% ee 85% ee 80% ee 30
610 32–85% ee 25–85% ee 38–46% ee 29b,33
712 42–87% ee 18
8 13, 14 60–90% ee 37a–h
9 15 86% ee 70% ee 37e,i
10 16 43–98% ee 20–85% ee 15–78% ee 39
11
a)
17 32–84% ee 67–81% ee 64–71% ee 17c
12 18 12–26% ee 29% ee 17c,31a
13 21, 22 94% ee 59% ee 85–88% ee 10a,16
14 23, 24 83–86% ee 40
15 25 59% ee 20% ee 31a
16 26 65% ee 81% ee 32
17 27 20–30% ee 38a
18 28, 29 6–64% ee 70–73% ee 78–82% ee 57–62% ee 38c
19 31 58% ee 7% ee 35% ee 16
20 32 58% ee 34% ee 10a
21 33–36 8–40% ee 10a
22 37 (F) 76% ee 73–83% ee 69% ee 56% ee 34d
23
b)
37 (OAc), 38 83–93% ee
max
98% ee
max
82% ee
max
70% ee
max
34b–d,g
24 41 (F) 68% ee 66% ee 34% ee 41
25 42 98% ee 96–97% ee 98% ee 95% ee 20c
26 53 11–71% ee 47% ee 29% ee 42a
27 54 67% ee 43–68% ee 40% ee 42b
28 55 67–83% ee 80–90% ee 60–85% ee 69–72% ee 42d
29 56 64–94% ee 53–92% ee 48–67% ee 44
a) Up to 95% ee obtained for 4,4
0
-substituted stilbenes.
b) ee
max
(100 product ee/ketone ee).
3.2 Catalyst Development
j
89
in good yields and high ees (Scheme 3.10). Interestingly, high ees have also been
obtained for some cis-alkenes with 43 (Scheme 3.10) [22,23c].
In efforts to extend the asymmetric epoxidation to other types of alkenes, glucose-
derived oxazolidinone ketone 44 was developed. In initial studies [24a–d], Boc-
ketone 44a was found to be highly enantioselective for various cis-alkenes, with no
isomerization being observed in the epoxidation of acyclic systems (Scheme 3.11)
[24a,c]. Certain terminal alkenes were also epoxidized in encouragingly high ees
(Scheme 3.11) [24b,c]. It appears that the stereodifferentiation for cis and terminal
alkenes with 44a probably results from an apparent attractive interaction between the
R
p
group of the alkene and the oxazolidinone moiety of the ketone catalyst in spiro
transition state C (Figure 3.2) [24a–c]. Further studies show that a carbocylic analog of
ketone 44a provides higher ees for styrenes (89–93% ee) than 44a [25]. The replace-
ment of the pyranose oxygen with a less electronegative carbon raises the energy of
the nonbonding orbital of the dioxirane and consequently enhances the stabilizing
interaction of the nonbonding orbital of the dioxirane with the p
orbital of the
alkene, thus further favoring the spiro transition state over the planar one [25].
Encouraged by the enantioselectivity obtained with ketone 44a, a series of N-aryl-
substituted ketones (examples shown in Scheme 3.12) were investigated to further
understand the N-substituent effect on the epoxidation and to search for more
practical ketone catalysts [24e,i]. Ketones such as 44b,c can be readily prepared in four
steps in large quantities from inexpensive glucose and anilines [24l].
Like ketone 44a, these N-aryl-substituted ketones have also been found to be highly
effective for a variety of alkenes, including cis-b-methylstyrenes [24f], chromenes
O
1, 12% ee (ref. 13a)
n-C
5
H
11
O
C
6
H
13
-
n
O
S
-3, 20% ee (ref. 13b)
S
-3, 16% ee (ref. 13b)
O
17 (X = H), <5% ee (ref. 17a)
O
Y
17 (X = H), Y = Cl, 18% ee (ref. 17a)
Me
O
17 (X = H), 18% ee (ref. 17a)
C
6
H
13
C
6
H
13
O
9 (R = CH
2
OAc), 42% ee
(ref. 30b)
28, 27% ee (ref. 38c)
21, Y = Cl, 43% ee (ref. 16)
31, Y = Cl, 29% ee (ref. 16)
Me
O
OBn
21, 68% ee (ref. 16)
31, 6% ee (ref. 16)
n
-C
4
H
9
n
-C
4
H
9
O
37 (F), 44% ee (ref. 34d)
R
O
37 (F), R =
n
-Pr, 41% ee
(ref. 34d)
53 (H), R = Et, 22% ee
(ref. 42a)
OH
37 (F), Y = H, 29% ee (ref. 34d)
37 (F), 22% ee (ref. 34d)
37 (F), 18% ee (ref. 34d)
55 (R = Me), 32% ee (ref. 42e)
9 (R = CH
2
OAc), 52% ee
(ref. 30b)
9 (R = CMe
2
OH), Y = H, 69% ee
(ref. 30b)
OBn
55 (R = neopentyl), 67% ee
(ref. 42d)
O
9 (R = CH
2
OAc), 21% ee (ref. 30b)
6, Y = Cl, 40% ee (ref. 43a)
6, 17% ee (ref. 43a)
6, 37% ee (ref. 43a)
56 (R = Bn), 27% ee (ref. 44)
16, 76% ee (ref. 39e)
16, Y = H, 70% ee (ref. 39e)
Scheme 3.8 Examples of epoxidation of alkenes with chiral ketones.
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3 Organocatalytic Oxidation. Ketone-Catalyzed Asymmetric Epoxidation of Alkenes
Ph
Ph
85% (98% ee)
Ph
94% (95% ee)
Ph
87% (94% ee)
OTBS
O
OO
OTBS
O
83% (94% ee)
C
6
H
13
C
6
H
13
O
OO
O
Ph
OMe
O
O
89% (95% ee)
92% (92% ee) 68% (92% ee)
Ph
Ph
89% (96% ee)
Ph
Ph
54% (97% ee)
O
O
Ph
O
Ph
94% (98% ee)
Ph
C
10
H
21
92% (97% ee)
O
Ph
C
10
H
21
O
94% (89% ee)
CO
2
Me
O
89% (94% ee)
O
O
O
41% (97% ee)
O
77% (81% ee)
Ph
TMS
O
74% (94% ee)
Ph TMS
O
66% (93% ee)
TMS
O
51% (90% ee)
TMS
O
71% (93% ee)
HO
Ph OH
O
85% (94% ee)
OH
O
68% (91% ee)
OH
O
93% (94% ee)
OH
O
85% (92% ee)
Ph OH
O
75% (74% ee)
OH
O
82% (90% ee)
BzO
O
82% (93% ee)
BzO
O
79% (80% ee)
BzO
O
87% (91% ee)
BzO
O
82% (95% ee)
BzO
O
92% (88% ee)
Ph
O
AcO
66% (91% ee)
Ph
Ph
O
77% (97% ee)
O
54% (95% ee)
OH
O
68% (90% ee)
OEt
O
O
82% (95% ee)
OEt
O
O
61% (94% ee)
OEt
O
O
89% (94% ee)
TMS
O
60% (92% ee)
O
78% (93% ee)
O
CH
2
OTBS
98% (96% ee)
Ph
O
TMS
59% (96% ee)
O
TMS
71% (89% ee)
O
TMS
84% (95% ee)
O
60% (93% ee)
OTBS
O
68% (96% ee)
(ent-42)
Scheme 3.9 Selected examples of epoxidation of trans and trisubstituted alkenes with ketone 42.
3.2 Catalyst Development
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91
O
O
O
O
O
O
R
3
R
1
R
2
O
O
O
O
O
O
R
1
R
2
Planar (B)
O
O
Spiro (A)
R
3
R
1
R
3
R
2
O
R
3
R
1
R
2
O
Favored
Figure 3.1 The spiro and planar transition states for epoxidation with ketone 42.
Ph
O
NC
O
O
O
O
61% (91% ee)77% (91% ee)
87% (91% ee)
88% (94% ee)
O
74% (83% ee)
O
93% (71% ee)
O
O
O
NBoc
O
44a
O
O
82% (91% ee)
Cl
O
F
74% (92% ee)
O
88% (84% ee)
O
O
O
Ph
Ph
O
65% (94% ee)
Scheme 3.11 Epoxidation of alkenes with ketone 44a (0.15–0.30 equiv.).
Ph
CO
2
Et
73% (96% ee)
Ph
CO
2
Et
93% (96% ee)
CO
2
Et
77% (93% ee)
CO
2
Et
n
-Bu
74% (98% ee)
O
O
O
O
O
O
O
AcO
O
43
CO
2
Et
MeO
O
57% (90% ee)
AcO
O
O
NC
Ph
TBS
O
73% (90% ee)
60% (90% ee)
N
O
O
n
-Bu
O
78% (86% ee)
Ph
Ph
O
63% (93% ee)
O
OBz
97% (95% ee)
n
-C
6
H
13
n
-C
6
H
13
O
53% (88% ee)
O
Ph
82% (92% ee)
Scheme 3.10 Epoxidation of alkenes with ketone 43 (0.1–0.3 equiv.).
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3 Organocatalytic Oxidation. Ketone-Catalyzed Asymmetric Epoxidation of Alkenes