A facile and efficient method for the rearrangement of aryl-substituted epoxides to aldehydes and ketones using bismuth triflate

Article abstract of DOI:10.1016/S0040-4039(01)01519-2

Aryl-substituted epoxides undergo smooth rearrangement in the presence of 0.01-0.1 mol% Bi(OTf)₃·xH₂O. The rearrangement is regioselective with aryl-substituted epoxides, and products arise from cleavage of the benzylic C-O bond. The highly catalytic nature of this method coupled with the fact that the reagent is relatively non-toxic, easy to handle and inexpensive make it an attractive alternative to more corrosive and toxic Lewis acids, such as BF₃·Et₂O, currently used to effect epoxide rearrangements.

Full text of DOI:10.1016/S0040-4039(01)01519-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 8129–8132
A facile and efficient method for the rearrangement of
aryl-substituted epoxides to aldehydes and ketones using
bismuth triflate
Kaushik A. Bhatia, Kyle J. Eash, Nicholas M. Leonard, Matthew C. Oswald and Ram S. Mohan*
Department of Chemistry, Illinois Wesleyan University, Bloomington, IL 61701, USA
Received 17 July 2001; revised 3 August 2001; accepted 10 August 2001
Abstract—Aryl-substituted epoxides undergo smooth rearrangement in the presence of 0.01–0.1 mol% Bi(OTf) ·xH O. The
3
2
rearrangement is regioselective with aryl-substituted epoxides, and products arise from cleavage of the benzylic CꢀO bond. The
highly catalytic nature of this method coupled with the fact that the reagent is relatively non-toxic, easy to handle and inexpensive
make it an attractive alternative to more corrosive and toxic Lewis acids, such as BF ·Et O, currently used to effect epoxide
3
2
rearrangements. © 2001 Elsevier Science Ltd. All rights reserved.
The rearrangement of epoxides to carbonyl compounds
is a useful synthetic transformation and hence several
is an efficient reagent for the rearrangement of epoxides
to carbonyl compounds. A continued search for a
6
1,2
reagents have been utilized for this purpose.
The
more efficient bismuth-based catalyst for epoxide rear-
rangement formed the basis of this study. We now wish
to report that bismuth triflate is a highly efficient
catalyst for rearrangement of aromatic epoxides to
carbonyl compounds (Eq. (1)). Bismuth triflate has
constitution of the rearrangement product is deter-
mined by the identity of the Lewis acid, the migratory
aptitude of the epoxide substituents, and the solvent
(
Scheme 1).
7
been used as a catalyst for Friedel–Crafts acylations,
8
9
Despite the number of methods that have been devel-
oped for epoxide rearrangement, only a few are both
regioselective and catalytic in nature. With increasing
environmental concerns, it is imperative that new ‘envi-
sulfonylation of arenes, 0Diels–Alder reactions and
1
aza-Diels–Alder reactions.
3
ronment friendly’ reagents be developed. Recently,
(1)
bismuth compounds have become attractive candidates
for use as reagents in organic synthesis due to their low
4
,5
toxicity. In an earlier study, we discovered that bis-
muth(III) oxide perchlorate, BiOClO ·xH O (20 mol%)
The results of this study are summarized in Table 1.
Bismuth triflate is not commercially available but can
4
2
Scheme 1.
Keywords: bismuth and compounds; epoxides; rearrangements.
*
Corresponding author. Fax: (309)-556-3864; e-mail: rmohan@titan.iwu.edu
0
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01519-2

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K. A. Bhatia et al. / Tetrahedron Letters 42 (2001) 8129–8132
Table 1. Rearrangement of epoxides with Bi(OTf) ·xH O in CH Cl
3
2
2
2
Entrya
1
Epoxide
Timeb
Product
Yield (%)
92c
20 min
30 min
2
89c
3
4
5
6
1 h
77d
75d
73d
74d
1 h
5 min
5 min
714
20 min
89c
815
20 min
20 min
20 min
89c
90c
92c
916
1
1
016
1
5 h, reflux
NR

K. A. Bhatia et al. / Tetrahedron Letters 42 (2001) 8129–8132
8131
Table 1. (Continued)
Entrya
Epoxide
Timeb
Product
NR
Yield (%)
1
2
12 h, reflux
1
3
30 min
NR
a
b
c
Superscript against entry number refers to literature reference for the rearrangement product.
All reactions were run at room temperature unless otherwise mentioned.
Based on 1H and C NMR spectral analysis, the crude product was estimated to be >98% and hence it was not purified further.
13
d
e
Refers to yield of isolated product after purification by Kugelrohr distillation or flash column chromatography.
1
Ratio of products was determined by H NMR analysis of the crude product.
be easily synthesized in the lab following a literature
both cis- and trans-b-methylstyrene oxides (entries 3
and 4) gave only phenylacetone upon rearrangement.
Rearrangement of styrene oxide (entry 5) occurred
readily and gave moderate yields of the acid sensitive
compound phenylacetaldehyde. Aliphatic epoxides
bearing a tertiary epoxide carbon also underwent rear-
rangement readily. For example, 1-methylcyclohexene
oxide underwent ready rearrangement to give an 89:11
mixture of 2-methylcyclohexanone (migration of methyl
group) and 1-methyl-1-cyclopentanecarboxaldehyde
1
1
procedure. The rearrangement does not require the
use of an inert atmosphere or anhydrous solvent. The
reagent is insoluble in common organic solvents and is
used as a suspension. The catalyst is highly efficient and
0.1 mol% was sufficient to promote smooth rearrange-
ment of most epoxides. For example, only 7 mg of the
catalyst was needed to catalyze the rearrangement of 2
12
g of trans-stilbene oxide to diphenylacetaldehyde. The
catalytic efficiency of bismuth triflate was tested with
styrene oxide as a model substrate. Remarkably, rear-
rangement of styrene oxide occurred even with as little
as 0.01 mol% of bismuth triflate (15 mmol of styrene
(
migration of CꢀC bond), respectively. In contrast,
rearrangement of 1-methylcyclohexene oxide with
LiBr–HMPA in benzene gave 1-methyl-1-cyclopentane-
2b
−
3
carboxaldehyde as the major product (95%). Aliphatic
epoxides lacking a tertiary epoxide carbon did not
undergo rearrangement in the presence of bismuth tri-
flate. Both cyclohexene oxide and 1,2-epoxyhexane
were recovered unchanged, even when heated at reflux
for 12 h. When cyclohexene oxide is heated with InCl3,
while no rearrangement occurs, the corresponding
oxide, 1.5×10 mmol bismuth triflate). This fact makes
this procedure especially attractive for large-scale syn-
thesis. Dichloromethane was found to be the best
solvent for the rearrangement. Rearrangement of
trans-stilbene oxide using bismuth triflate was very slow
in diethyl ether. In tetrahydrofuran, in addition to
diphenylacetaldehyde, several unidentifiable byproducts
were formed. When a solution of trans-stilbene oxide in
dichloromethane was treated with 0.1 mol% of tri-
fluoromethanesulfonic acid, the solution turned red and
the resulting diphenylacetaldehyde was found to be
considerably impure. This observation suggests that
bismuth triflate is acting as a Lewis acid and not simply
releasing triflic acid into the solution.
2d
chlorohydrin is obtained in a good yield.
The rearrangement of epoxides bearing an electron-
withdrawing group was also studied. Acyl-substituted
epoxides (entries 7–10) underwent smooth rearrange-
ment with exclusive migration of the acyl group. Thus,
this method provides easy access to b-oxoaldehydes via
rearrangement of epoxides. Epoxides bearing a cyano
group (entries 11 and 12) or nitro group (entry 13)
proved resistant to the reaction conditions and the
starting epoxide was recovered in good yield in all
cases. In contrast, when trans-b-methyl-b-nitrostyrene
oxide is treated with BF ·Et O, a mixture of five differ-
The rearrangement of trans-stilbene oxide (entry 1)
proceeded with exclusive phenyl group migration to
give diphenylacetaldehyde as the only product. Thus,
this procedure is a good example of an epoxide rear-
rangement method that is both highly catalytic and
regioselective in nature. In contrast, the rearrangement
3
2
13
ent products is obtained. In the presence of InCl3,
rearrangement accompanied by loss of the nitro group
2d
of trans-stilbene oxide by MgBr in benzene gave a 3:1
2
to give 1-chloro-1-phenyl acetone is observed.
mixture of diphenylacetaldehyde (phenyl migration)
2
a
and deoxybenzoin (hydrogen migration).
With
In summary, this work demonstrates a new method for
high-yielding, selective rearrangement of aromatic
epoxides to carbonyl compounds using Bi(OTf)3.
Advantages of this method include the highly catalytic
nature of the reagent, low toxicity and low cost of the
Lewis acid catalyst, fast reaction rates and insensitivity
of the Lewis acid to air and moisture.
BF ·Et O, trans-stilbene oxide gives only diphenylac-
3
2
etaldehyde while the cis isomer gives a mixture of
2
a
diphenylacetaldehyde and deoxybenzoin. Rearrange-
ment of cis-stilbene oxide (entry 2) with bismuth triflate
gave a similar product mixture. It was found that
hydrogen migrates in preference to a methyl group:

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K. A. Bhatia et al. / Tetrahedron Letters 42 (2001) 8129–8132
6. Anderson, A. M.; Blazek, J. M.; Garg, P.; Payne, B. J.;
Acknowledgements
Mohan, R. S. Tetrahedron Lett. 2000, 41, 2527.
. (a) Labrouill e` re, M.; Le Roux, C.; Gaspard, H.; Lapor-
terie, A.; Dubac, J. Tetrahedron Lett. 1997, 38, 8871; (b)
R e´ pichet, S.; Le Roux, C.; Dubac, J.; Desmurs, J. R. Eur.
J. Org. Chem. 1998, 2743.
7
The authors wish to acknowledge funding by the
National Science Foundation (RUI grant). We also
wish to thank Professor C. Le Roux (CNRS, Universite´
Paul-Sabatier) for useful comments on the synthesis of
bismuth triflate.
8
. R e´ pichet, S.; Le Roux, C.; Hernandez, P.; Dubac, J. J.
Org. Chem. 1999, 64, 6479.
9
. (a) Garrigues, B.; Gonzaga, F.; Robert, H.; Dubac, J. J.
Org. Chem. 1997, 62, 4880; (b) Robert, H.; Garrigues, B.;
Dubac, J. Tetrahedron Lett. 1998, 39, 1161.
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