AuCl3/AgSbF6-catalyzed rapid epoxide to carbonyl rearrangement

Article abstract of DOI:10.1016/j.tetlet.2012.07.056

An efficient epoxide to carbonyl rearrangement using catalytic AuCl ₃/AgSbF₆ has been presented. The reactions are fast and high yielding. β-Hydrogen migration takes place exclusively when hydrogen and methyl or substituted methyl groups are present at β-carbon of epoxide. When phenyl/acetyl/benzoyl and hydrogen are available at same carbon atom, migration of the former is preferred over the latter.

Full text of DOI:10.1016/j.tetlet.2012.07.056

Tetrahedron Letters 53 (2012) 5243–5247
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Tetrahedron Letters
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AuCl3/AgSbF6-catalyzed rapid epoxide to carbonyl rearrangement
⇑
Vanajakshi Gudla, Rengarajan Balamurugan
School of Chemistry, University of Hyderabad, Gachibowli, Hyderabad 500046, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient epoxide to carbonyl rearrangement using catalytic AuCl3/AgSbF6 has been presented. The
reactions are fast and high yielding. b-Hydrogen migration takes place exclusively when hydrogen and
methyl or substituted methyl groups are present at b-carbon of epoxide. When phenyl/acetyl/benzoyl
and hydrogen are available at same carbon atom, migration of the former is preferred over the latter.
Ó 2012 Published by Elsevier Ltd.
Received 14 June 2012
Revised 11 July 2012
Accepted 13 July 2012
Available online 20 July 2012
Keywords:
Epoxides
Rearrangement
Meinwald rearrangement
Gold catalysis
Epoxides are versatile and valuable synthetic building blocks for
the synthesis of complex organic structures due to their ready
availability in both racemic and enantiomerically pure forms and
their high reactivity to undergo different organic transformations.
Epoxide to carbonyl rearrangement which is also called as Mein-
wald rearrangement is a synthetically useful protocol to obtain
carbonyl compounds in an atom-economical way. Different cata-
lytic systems have been developed which include Lewis acids
and Brønsted acids for the epoxide rearrangement.1,2 For the syn-
thesis of various bioactive natural products, this epoxide rear-
rangement has been applied widely.3 Even several tandem
reactions have been reported which exploit the synthetic efficiency
of Meinwald rearrangement to generate in situ aldehyde or ketone
intermediates from epoxides for further transformations.4 These
tandem processes are particularly valuable when the carbonyl
compounds are sensitive or unstable or difficult to handle. Mainly
Lewis acid mediated epoxide rearrangement became an attractive
approach for the carbon-skeletal reorganization of epoxide. The
yields and regioselectivity of the rearrangement are sensitive to
reaction conditions (Lewis acid employed and solvent) and migrat-
ing aptitude of substituents attached to the ring. For example,
Pd,1b–d In,1e V,1f Bi,1g,h Ir,1i Fe,1j–m Er,1n and Cu1o,p favor the hydro-
gen migration, while Al,1a,2a B,2b Cr,2c,d Ga,2e and Si2f promote the
alkyl migration. Among these systems only a few are both regiose-
lective and truly catalytic in nature. However they pose poor sub-
strate scope. Hence, there is significant scope for the development
of better catalyst which would address the factors like efficiency,
regioselectivity, good substrate scope, and less catalyst amount.
In recent years, several gold-catalyzed reactions have been re-
ported, where gold in +1 or +3 oxidation states catalyzes a variety
of organic transformations with high efficiency in chemo, regio,
and stereoselective ways.5 In some of the gold catalyzed reactions
epoxides have been used as nucleophiles to attack gold-activated
alkyne or alkene bond.6,7 Gold can also activate oxirane ring
towards nucleophiles as it possesses oxophilicity in addition to
carbophilicity.8–11 Liang et al. investigated the rearrangement of
a-hydroxy epoxides into 1,5 or 1,6-diketones and monoketones
where gold(III) activates oxirane ring.8 Stratakis et al. recently re-
ported the isomerization of epoxides into allyl alcohols mediated
by TiO2-supported gold nano particles in which cationic Au(I) is
stabilized by the surface of TiO2.9 Hashmi et al. in the conversion
of 2-alkynylaryl epoxides into 3-acylindenes, observed the gold(I)
catalyzed isomerization of oxirane ring to a carbonyl compound
as a minor side product.7 In our study of opening of gold activated
epoxides with acetone for subsequent transformation into cyclic
acetals, we observed the epoxide to carbonyl rearrangement prod-
uct in minor amount from stilbene oxide. Here the oxidation state
of gold was +3.10 Based on these reports, we became interested to
study the gold-catalyzed epoxide rearrangement to carbonyl com-
pounds. Moreover, gold being capable of promoting a variety of
reactions, the intermediate or product of the reaction could be
exploited for further synthetic manipulations. We wished to use
gold(III) salts for this transformation because it is more oxophilic
compared to gold(I).11 The reaction is expected to proceed faster
as gold does not have strong affection to oxyanion (generated dur-
ing the reaction) like the other trivial Lewis acid catalysts have.12
As believed gold(III) catalyzed oxirane rearrangement occurred
with high efficiency as the reactions completed faster even with
0.1 mol % catalyst at room temperature.
⇑
Corresponding author.
E-mail address: rbsc@uohyd.ernet.in (R. Balamurugan).
0040-4039/$ - see front matter Ó 2012 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tetlet.2012.07.056

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V. Gudla, R. Balamurugan / Tetrahedron Letters 53 (2012) 5243–5247
Optimization of the reaction was carried out using the substrate
comparison, selected best literature data available for the conver-
sion are presented along the corresponding entries in Table 2. 1-
Aryl substituted epoxides rearranged exclusively by b-hydrogen
migration to give the corresponding aryl-substituted acetaldehy-
des or arylmethyl ketones. In the case of trans-stilbene oxide 1d
which is a 1,2-diaryl substituted epoxide, phenyl migration domi-
nated exclusively over hydrogen migration which lead to the for-
mation of diphenylacetaldehyde 2d as the sole product (entry 4).
However, cis-stilbene oxide 1d resulted in a mixture of rearranged
products due to both phenyl and hydrogen migrations. Still, diphe-
nylacetaldehyde 2d was the major product and the hydrogen mi-
grated product, phenylacetophenone was being 11% only from 1H
NMR of the mixture (entry 5). The reactions of epoxides bearing
electron withdrawing groups were also studied. b-Acetyl styrene
oxide 1e and b-benzoyl styrene oxide 1f lead to the formation of
acetyl and benzoyl migrated products, respectively, 2e and 2f (en-
tries 6 and 7). Same migration pattern has been reported in the
1a which has both H and alkyl groups available for migration. Sev-
eral reactions were carried out using AuCl3 by changing the solvent
and adding co-catalyst AgSbF6. Our first attempt with 2 mol % of
AuCl3 in CH2Cl2 resulted in 56% of ketone 2a, formed by hydrogen
migration and 13% of aldehyde 3 resulting from alkyl migration
along with 31% of the unreacted starting material in the crude mix-
ture. In fact there was no progress in the reaction after 15 min due
to the catalyst precipitation (Table 1, entry 1). Since the aldehyde 3
decomposed during the silica gel column purification of the crude
mixture, the isolated yield of 3 could not be presented in Table 1.
When 2 mol % of AuCl3 in combination with 6 mol % of AgSbF6
was used in CH2Cl2, 82% of 2a and 18% of 3 were obtained with
complete consumption of the starting material 1a (entry 2). In
dioxane, while there was no reaction with AuCl3 alone (entry 5),
the reaction proceeded nicely when AgSbF6 was used along with
AuCl3 (entry 6). The reaction was fast and completed in less than
15 min resulting the product 2a in 92% along with 8% of 3. Then
the reaction was carried out with 1 mol % of AuCl3/3AgSbF6 which
was found to be equally efficient as that of the reaction with
2 mol % of AuCl3/3AgSbF6 (entry 7). These results led us to perform
the reaction with a mixture of 0.1 mol % of AuCl3 and 0.3 mol % of
AgSbF6. The reaction worked well and completed in 20 min to yield
2a in 93% (entry 8). Interestingly, there was no reaction when
AuCl3 or AgSbF6 alone were used as catalysts in dioxane. Perhaps
the cationic gold species generated in AuCl3/AgSbF6 mixture might
be the active catalyst responsible for the reaction in dioxane. Also
NaAuCl4 in dioxane did not catalyze the reaction (entry 11). Other
solvents like CH3CN, toluene, and THF were not found to be suit-
able solvents.
reactions involving InCl3 or Bi(OTf)3ÁxH2O.1h Even, the substitu-
1e
ent –CO2Me on epoxide ring did not affect the reaction as it yielded
the hydrogen migrated product, as observed with BF3 in the rear-
rangement of ethyl b-phenylglycidate.13 As we were unable to iso-
late the pure product due to its decomposing nature in silica gel,14
the crude reaction mixture was treated with NaBH4 to reduce the
keto group to get the hydroxyl ester derivative 4 (entry 8). It is
interesting to note that InCl3 did not promote the rearrangement
in this case but cleaved the epoxide ring to result in the formation
of chlorohydrin by nucleophilic attack of ClÀ from InCl3.1e However
epoxides bearing cyano or nitro group at b-carbon are resistant to
the reaction conditions as the starting materials were recovered. It
is worth mentioning that these substrates do not undergo rear-
rangement with other Lewis acid catalysts also. When there is
competition between hydrogen and methyl or substituted methyl,
selectively hydrogen migration is preferred to produce methyl ke-
tones (entries 1, 9–14, and 16). In the case of silyl (TBS) protected
epoxy alcohol 1h, reaction worked well and resulted in the product
2h corresponding to hydrogen migration without damaging the si-
lyl protection (entry 9). Aryl epoxides attached to propyl, isopropyl,
and tertiary butyl group were also treated with gold catalyst (en-
tries 11–14). In all the cases reaction worked through hydrogen
migration selectively. Only when tertiary butyl group is attached
to oxirane ring, very small amount (3%) of alkyl group migrated
product was observed (entry 14). If there are two methyl groups
attached to the same carbon of the epoxide ring as seen in 1n,
methyl migration occurred because of unavailability of hydrogen
for migration (entry 15). It is pleasing to note that epoxide derived
from cinnamyl alcohol 1o resulted in the hydrogen migrated prod-
uct 2o in 76% yield in just 10 min (entry 16). The same conversion
took 60 h with Pd(PPh3)4 and heating in toluene at 140 °C.1b
The rearrangement of non-aromatic epoxides was also checked
with the present catalytic system. Compared to aromatic epoxides,
these required longer reaction times and comparatively more cat-
alyst (2 mol % of AuCl3/3AgSbF6). Cyclohexene oxide 1p rearranged
by hydrogen migration and resulted in cyclohexanone 2p in good
yield although it has to proceed via the formation of secondary car-
bocation which does not form as readily as benzylic/tertiary carbo-
cation (entry 17). This observation is special as the reaction of
cyclohexene oxide with IrCl3ÁxH2O yielded only 4% of cyclopene-
tene carboxaldehyde,1i with InCl3 resulted in chlorohydrin by the
nucleophilic attack of ClÀ,1e with BiOClO4ÁxH2O,1g or Bi(OTf)3Áx-
H2O1h there were no reaction and with mixed-valent iron catalyst
polymerization happened.1m These results show the superiority of
gold catalyst over other catalysts known for epoxide rearrange-
ment. Epoxide 1q derived from allyl benzene on treatment with
AuCl3/3AgSbF6 resulted in the formation of hydrocinnamaldehyde
2q (entry 18). In this case also the reaction proceeded through a
secondary carbocation intermediate.
After finding that the rearrangement is highly efficient at room
temperature using 0.1 mol % of AuCl3/3AgSbF6 in dry dioxane, the
generality of AuCl3/3AgSbF6 catalytic system was examined in
the reactions of various substituted and structurally diverse epox-
ides. Table 2 represents the outcome of these experiments. As ex-
plicit from Table 2, the reactions were faster and clean. For
Table 1
Catalyst and solvent screening for the epoxide rearrangement
CHO
catalyst
O
Ph
OMe
OMe
Ph
OMe
solvent, rt
Ph
O
1a
2a
3
Entry
Catalyst
Solvent
Time
Yield (%)h
2ai
3
1ai
1
2
3
4
5
6
7
8
AuCl3
CH2Cl2
CH2Cl2
CH3CN
CH3CN
Dioxane
Dioxane
Dioxane
Dioxane
THF
24 hg
15 min
24 h
24 h
24 h
15 min
15 min
20 min
30 min
24 h
56(48)
82(74)
No reaction
No reaction
No reaction
92(84)
92(83)
93(83)
42(41)
13
18
31(27)
0
a
AuCl3a/AgSbF6
b
b
b
a
AuCl3
AuCl3a/AgSbF6
a
AuCl3
AuCl3a/AgSbF6
8
8
7
9
0
0
0
AuCl3c/AgSbF6
d
AuCl3e/AgSbF6
f
a
9
AuCl3
49(47)
b
10
11
12
AuCl3a/AgSbF6
NaAuCl4
AgSbF6
Toluene
Dioxane
Dioxane
No reaction
No reaction
No reaction
a
24 h
24 h
b
a
b
c
d
e
f
2 mol %.
6 mol %.
1 mol %.
3 mol %.
0.1 mol %.
0.3 mol %.
g
h
i
After 15 min there was no progress in reaction.
% derived from 1H NMR spectrum of crude reaction mixture.
Value in parenthesis refers to isolated yield.

V. Gudla, R. Balamurugan / Tetrahedron Letters 53 (2012) 5243–5247
5245
Table 2
Applicability of gold catalyst in epoxide rearrangementa (see below-mentioned references for further information)
Entry Epoxide (1)
Time
(min)
Product (2)
Yieldb
(%)
Selected literature data for comparison and comments
O
Ph
O
OMe
InCl3 (1.0 equiv, 65%b)1e
Ph
Ph
OMe
1
2
15
5
83
98
2a (ref 1e)
CHO
2b
1a
O
Cu(BF4)2ÁnH2O (25 mol %, 90%b);1o,1p Bi(OTf)3ÁxH2O (0.1 mol %, 73%b);1h InCl3
(0.6 equiv, 85%b);1e VO(OR)Cl2 (0.5 equiv, 90%b);1f IrCl3ÁxH2O (1 mol %, 100%
conversion)1i; LPDE (1.0 equiv, 85%b);1q mesoporous aluminosilicates (100%
conversion, 5% polymeric material)1s
Ph
1b
O
CHO
2c (ref. 1e)
3
10
89
InCl3 (0.6 equiv, 90%b);1e VO(OR)Cl2 (0.5 equiv, 90%b)1f
MeO
MeO
Ph
1c
Cu(BF4)2ÁnH2O (25 mol %, 95%b);1o,1p Bi(OTf)3ÁxH2O (0.1 mol %, 92%b);1h
BiOClO4ÁxH2O (20 mol %, 90%b);1g IrCl3ÁxH2O (1 mol %, 100% conversion);1i InCl3
(0.6 equiv, 90%b);1e Mesoporous aluminosilicates (65% conversion);1s LPDE
(1.0 equiv, 60%b);1q Mixed-valent iron (2 mol %, 86%b)1m
O
Ph
4
5
5
5
98
Ph CHO
Ph
2d
Ph
trans-1d
O
Bi(OTf)3ÁxH2O (0.1 mol %, 89%b);1h BiOClO4ÁxH2O (20 mol %, 90%b);1g IrCl3 xH2O
(1 mol %, 100% conversion);1i Cu(BF4)2ÁnH2O (25 mol %, 95%b)1p
98c
Ph CHO
Ph
Ph
cis-1d
2d
O
H
OH
O
Ph
Ph
Bi(OTf)3ÁxH2O (0.1 mol %, 89%b);1h 5 mol dmÀ3 LiClO4-diethylether (78%b);1r InCl3
6
7
10
5
89
95
Ph
(1.0 equiv, 72%b)1e
1e
O
2e (ref. 1h)
H
O
O
OH
O
Bi(OTf)3ÁxH2O (0.1 mol %, 89%b);1h No reaction with 5 mol dmÀ3 LPDE and along with
Ph
Ph
Ph
10% Yb(OTf)3 resulted in 73% yield1r
1f
O
2f (ref. 1h)
O
O
Ph
Ph
OMe
Ph
OMe
8
10
15
5
76d
93
InCl3 (1.0 equiv, No rearrangement; resulted in chlorohydrin)1e
Fe(tpp)ClO4 (2 mol %, 99%b)1j
1g
OH
4 (ref. 15)
O
Ph OTBS
9
OTBS
O
2h (ref. 1j)
1h
O
O
10
96
Pd(OAc)2/3PBu3 (5 mol %, 98%b):1d InCl3 (0.6 equiv, 88%b)1e
2i (ref. 1d)
1i
O
n-Pr
n-Pr
11
12
13
5
5
5
92
83
88
O
Cl
Cl
2j
1j
O
Ph
Ph
O
1k
2k (ref. 16)
O
Ph
Ph
Ph
Ph
1l
O
2l (ref. 17)
O
Ph
95e
89
’
14
15
5
1m
O
2m (ref. 17)
O
Ph
1d
10
No reaction with Pd(OAc)2/PPh3 or PBu3
1n
O
2n (ref. 18)
(continued on next page)

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V. Gudla, R. Balamurugan / Tetrahedron Letters 53 (2012) 5243–5247
Table 2 (continued)
Entry Epoxide (1)
Time
(min)
Product (2)
Yieldb
(%)
Selected literature data for comparison and comments
O
Ph
OH
16
10
76
Pd(PPh3)4 (4 mol %, 62%b)1b
Ph
OH
O
1o
2o (ref. 1b)
IrCl3 (1 mol %, 4% of cyclopentane carboxaldehyde);1i BiOClO4ÁxH2O (no reaction);1g
Bi(OTf)3ÁxH2O (no reaction);1h InCl3 (1.6 equiv, no rearrangement: resulted in
cholohydrin);1e LiClO4 (no reaction);1q Mixed-valent iron (2 mol %,
polymerization)1m
O
O
17
75f
88
73
1p
2p
CHO
Ph
Ph
18
180f
Fe(tpp)OTf (2 mol %, 94%b)1k
O
2q
1q
All the reactions were carried out with 0.1 mol % AuCl3 and 0.3 mol % AgSbF6 in dioxane at rt unless otherwise stated.19
Yields refer to pure isolated products.
a
b
c
d
e
f
ꢀ11% of hydrogen migrated product also present as revealed from 1H NMR of the column purified material.
After NaBH4 reduction of crude reaction mixture.
ꢀ3% tert-butyl migrated product was also observed in 1H NMR of crude reaction mixture.
2 mol % of AuCl3 and 6 mol % of AgSbF6 were used.
This study reveals that the gold-catalyzed rearrangement of
the lone pair on oxygen as depicted in oxonium ion IV generates
substituted epoxides is regioselective through C–O bond cleavage
which will lead to the formation of more stable carbocation. Also
the rearrangement follows hydrogen migration mainly, that is, if
there are two groups at b-position, hydrogen migration is preferred
over methyl or substituted methyl group. However, when there is a
competition between phenyl/acetyl/benzoyl and hydrogen the
migration of phenyl/acetyl/benzoyl is preferred.
Based on these results a plausible mechanism for this epoxide
rearrangement could be drawn as shown in Scheme 1. Cationic
gold species coordinates to oxygen of oxirane 1, which leads to
the C–O bond cleavage to generate stable carbocation intermediate
II regioselectively. Migration of R3 generates carbocation III. When
phenyl is in the b-position as in stilbene oxide, it will migrate pref-
erentially through well-established phenonium ion. The interme-
diate III is stabilized when R4 is alkyl by hyperconjugation effect.
In the case of substrates 1e and 1f which bear, respectively, acetyl
and benzoyl groups at the b-position, intermediate III is destabi-
lized if hydrogen migration takes place. This could be the reason
for the preference of acetyl or benzoyl migration over hydrogen
migration. In the reaction of substrate 1g, the ester group might
not destabilize the intermediate III to the extent that a carbonyl
group of acetyl and benzoyl could do. Eventually hydrogen migra-
tion is preferred in 1g. The intermediate III which is stabilized by
final carbonyl compound 2 by releasing gold catalyst back into
the catalytic cycle.
AuCl3/3AgSbF6 catalyzed epoxide to carbonyl rearrangement
takes place very efficiently in dioxane to result carbonyl com-
pounds in high yields in a short span of time. Unlike other promot-
ers used for the epoxide to carbonyl rearrangement, this catalytic
system has wider substrate scope. The migrating aptitude of differ-
ent substituents has been evaluated. Since gold catalysts can per-
form a variety of conversions, this rearrangement can be coupled
with such reactions to find new tandem reactions. Studies in this
direction are currently under process.
Acknowledgments
The authors thank the Department of Science and Technology
(DST), India for financial support. VG thanks the Council of Scien-
tific and Industrial Research (CSIR), India for a fellowship.
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
07.056.
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19. Representive procedure for the rearrangement of 1a: To a solution of the epoxide
1a (100 mg, 0.61 mmol) in dry dioxane (3 mL) freshly prepared AuCl3 (0.2 mg,
0.1 mol %) and AgSbF6 (0.6 mg, 0.3 mol %) solutions in dry dioxane were added
and the reaction mixture was allowed to stir at room temperature. The reaction
was monitored by TLC and found to complete in 20 min. Dioxane was
evaporated. The residue was loaded on a silica gel column and was eluted with
EtOAc/hexanes (7/93) mixtures to obtain pure compound 2a in 83% yield as
colorless liquid. Rf = 0.27 in 1:10 EtOAc/hexanes. 1H NMR (400 MHz, CDCl3): d
3.38 (s, 3H), 3.75 (s, 2H), 4.06 (s, 2H), 7.22–7.35 (m, 5H). 13C NMR (100 MHz,
CDCl3): d 46.0, 59.1, 76.8, 127.0, 128.6, 129.3, 133.3, 205.7.
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