Copper(II) tetrafluroborate-promoted Meinwald rearrangement reactions of epoxides

Article abstract of DOI:10.1016/j.tet.2010.08.078

Epoxides undergo a highly efficient Meinwald rearrangement in the presence of catalytic quantities of copper(II) tetrafluoroborate to give carbonyl compounds in high yields and with excellent selectivity. The low toxicity and ease of handling of this reagent make it an attractive alternative to the more corrosive or costly Lewis acids frequently employed.

Full text of DOI:10.1016/j.tet.2010.08.078

Tetrahedron 66 (2010) 8377e8382
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Copper(II) tetrafluroborate-promoted Meinwald rearrangement reactions
of epoxides
Mathew W.C. Robinson, Kathryn S. Pillinger, Ian Mabbett, David A. Timms, Andrew E. Graham *
Sustainable Environment Research Centre (SERC), Department of Science and Sport, University of Glamorgan, Glyntaff, Pontypridd CF37 4AT, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 May 2010
Received in revised form 6 August 2010
Accepted 31 August 2010
Available online 9 September 2010
Epoxides undergo a highly efficient Meinwald rearrangement in the presence of catalytic quantities of
copper(II) tetrafluoroborate to give carbonyl compounds in high yields and with excellent selectivity. The
low toxicity and ease of handling of this reagent make it an attractive alternative to the more corrosive or
costly Lewis acids frequently employed.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Copper(II) tetrafluoroborate
Nanoporous aluminosilicates
Lewis acid catalyst
Meinwald rearrangement
1. Introduction
protocol, with the most important factors being identified as the
migratory aptitude of the various epoxide substituents and the
3
Epoxides are readily available and highly versatile in-
termediates, which due to their high reactivity, undergo either
stereospecific nucleophilic ring-opening reactions to yield valu-
able bifunctional compounds or rearrangement reactions to give
Lewis acid catalyst employed. Typically, if there is no structural
bias, a mixture of products is produced due to the lack of regio-
selectivity in the initial ring-opening process. Hence, there has
been considerable interest in the development of Lewis acids that
display high selectivity and which give highly efficient conver-
sions employing catalytic quantities.
1
cyclic or carbonyl compounds. In particular, the rearrangement
reaction of epoxides to produce carbonyl compounds, the Mein-
wald rearrangement, has attracted considerable recent interest
given the high efficiency of this protocol and its potential for
The original rearrangement protocols employed either boron
trifluoride diethyletherate, lithium salts, magnesium bromide or
methylaluminium bis(4-bromo-2,6-di-tert-butylphenoxide) (MABR),
2
4
application in synthetic and industrial processes (Scheme 1).
These developments have concentrated on investigating and
improving the regiochemical outcome of the rearrangement
which all proved to be highly useful procedures; however, these
reagents are toxic or require the catalyst to be employed in stoi-
chiometric quantities. More recent protocols, employing catalytic
quantities of Lewis acids, such as palladium salts, indium tri-
chloride, iridium trichloride and both bismuth and gallium salts,
O
1
R
R
5
H
have all proven to be more efficient. There remain, however,
2
LA
O
R3
H
limitations to these protocols, which need to be addressed, whether
it be moderate product selectivity, the toxic or corrosive nature of
the catalysts employed, the production of addition or polymeric
products or the requirement to employ expensive rare metals. Re-
cently, while investigating addition reactions to epoxides catalyzed
R1
O
R3
R1
R3
O
R2
H
R2
H
1
2
R
R
R3
4 2 2
by copper(II) tetrafluoroborate (Cu(BF ) $nH O) under microwave
Scheme 1.
irradiation, we noted that in some cases, the epoxides substrates
underwent a competing Meinwald rearrangement process to pro-
duce carbonyl products in moderate yields. We were somewhat
6
surprised to discover that there had only been one previously re-
port in the literature of the use of Cu(II) salts to catalyze epoxide
rearrangement reactions and this prompted us to initiate studies
*
Corresponding author. Tel.: þ44 1443482460; e-mail address: AEGraham@
7,8
glamorgan.ac.uk (A.E. Graham).
0
040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.08.078

8378
M.W.C. Robinson et al. / Tetrahedron 66 (2010) 8377e8382
to assess their application as catalysts for the Meinwald rear-
rangement. In this paper we report our recent progress in the de-
Terminal epoxides underwent an extremely facile rearrange-
ment process at room temperature to rapidly give the aldehyde
product in excellent yields (entries 1e4). The rearrangement of
styrene oxide proceeded readily to give phenylacetaldehyde con-
taminated with trace quantities of the suspected aldol product
velopment of Cu(BF
4 2 2
) $nH O as an efficient and highly selective
9
catalyst for epoxide rearrangement.
(Table 2, entry 1). This example is noteworthy given the sensitivity
2
. Results and discussion
of phenylacetaldehyde to acidic conditions and the difficulties en-
5
d
countered in previously reported procedures, and clearly dem-
onstrates the benign nature of this catalyst. We next probed the
chemoselectivity of the catalyst employing a range of substituted
stilbene oxides (entries 5e8). These substrates proved to be less
reactive than the terminal epoxides, and while the rearrangement
did occur at room temperature, these reactions were sluggish and
were therefore carried out under reflux conditions. The rear-
rangement of these materials has previously been used to dem-
onstrate the selectivity of rearrangement processes, as the reaction
can occur with either phenyl migration to produce diphenylace-
4 2 2
We initially investigated the use of both Cu(BF ) $nH O and
2
copper(II) triflate (Cu(OTf) ) under reaction conditions similar to
7
those previously reported. The rearrangement of trans-stilbene
oxide catalyzed by Cu(OTf) proved disappointing with low isolated
2
yields produced, even on extending reaction times and increasing
the quantity of catalyst to 10 mol % (Table 1, entry 1). Reactions
employing styrene oxide and a-methylstyrene under these modi-
fied conditions, however, did proceed to give aldehyde products in
high yield and with excellent regioselectivity (entries 2 and 3).
4
b
Under similar reaction conditions, Cu(BF
4
)
2
$nH
2
O proved to be
taldehyde, or with hydrogen migration to give deoxybenzoin.
a highly efficient catalyst (entries 4e6), providing high conversion
Under reflux conditions, trans-stilbene oxide underwent regiose-
of the epoxide to the aldehyde products with excellent selectivity.
lective rearrangement to produce diphenylacetaldehyde in excel-
lent yield as the only product observed by H NMR spectroscopy of
1
On increasing the quantity of Cu(BF
4
)
2
$nH
2
O to 25 mol %, reaction
times for the rearrangement reaction were significantly reduced
and provided near quantitative conversions to the aldehyde (en-
the crude reaction mixture (entry 5). cis-Stilbene oxide produced
diphenylacetaldehyde as the major product, in addition to small
quantities (approximately 10%) of the deoxybenzoin product (entry
6). Interestingly, the ratio of these products was unchanged when
the reaction was carried out at room temperature over extended
tries 7e9). Given that Cu(BF
4 2 2
) $nH O is significantly cheaper than
Cu(OTf) , we chose this catalyst for further study employing a range
2
of structurally diverse epoxides (Table 2).
Table 1
Optimisation of the Meinwald rearrangement reaction using Cu(II) salts
Entry
1
Epoxide
Cu(II) salt
Quantity (mol %)
10
Time (h)
6
Productsa
Conversionb (%)
11
Ph
O
Ph
H
Cu(OTf)
2
Ph
Ph
O
H
H
O
O
Ph
Ph
2
3
Cu(OTf)
Cu(OTf)
2
2
10
10
3
3
90
90
Ph
Ph
O
O
Ph
O
Ph
H
4
4
Cu(BF )
2 2
$nH O
10
6
62
Ph
Ph
O
H
H
O
O
Ph
Ph
5
6
Cu(BF
4
)
)
2
$nH
2
O
O
10
10
3
3
45
90
Ph
Ph
O
O
Cu(BF
4
2
$nH
2
Ph
O
Ph
H
H
7
8
Cu(BF
4
)
)
2
$nH
2
O
O
25
25
6
95
98
Ph
Ph
Ph
O
O
O
O
Cu(BF
4
2
$nH
2
0.5
Ph
H
9
Cu(BF
4
)
2
$nH
2
O
25
1
Ph
95
Ph
O
a
All reactions were carried out in 10 mL of anhydrous CH
2
Cl
2
at room temperature.
b
Determined by 1H NMR analysis of the crude reaction mixture.

M.W.C. Robinson et al. / Tetrahedron 66 (2010) 8377e8382
8379
Table 2
4 2 2
Rearrangements of epoxides catalyzed by Cu(BF ) $nH O
Entry
1
Epoxidea
Time (h)
0.3
Productb
Yieldc (%)
H
O
Ph
90
90
Ph
O
O
H
2
3
0.25
1
O
O
H
H
85
90
Ph
Ph
Ph
O
O
Ph
Ph
O
4
5
6
7
2
Ph
Ph
O
O
Ph
Ph
H
95d
95d,e
93d
1.5
Ph
H
Ph
Ph
O
Ph
O
4
Ph
Ph
Ph
O
OMe
NO2
O
O
H
O
2.5
OMe
NO2
H
O
8
9
6
10d
O
O
5
2
89
OH
10
100d
O
OH
a
b
c
All epoxides were used as supplied or were synthesized using published procedures.
Reactions were carried out using 25 mol % of catalyst in 10 mL CH
All products gave satisfactory spectroscopic data.
Carried out at reflux.
2 2
Cl at room temperature.
d
e
Isolated as a 90:10 mixture of diphenylacetaldehyde: deoxybenzoin.
reaction times. para-Methoxy stilbene oxide underwent efficient
rearrangement to give the substituted diphenylacetaldehyde (entry
hydration of the epoxide to give the diol as the major product in
high yield (entry 10).
7
), whereas the para-nitro analogue gave only low conversions
We further investigated the selectivity of Cu(II) salts employing
even after extended reaction times at reflux temperatures (entry 8).
Disappointingly, the rearrangement of cyclohexene oxide provided
only trace amounts of cyclohexanone even after prolonged heating
in chloroform, although the starting material was recovered un-
changed in contrast to previous reports where halohydrins are
produced.5b Dihydronaphthalene oxide did, however, produce the
a more challenging substrate, in this case, a-pinene oxide. This
rearrangement reaction is a well documented transformation in the
literature due to its industrial importance and is particularly de-
manding in terms of selectivity given that a number of potential
products can be formed during the rearrangement process
(Scheme 2). The initially formed cation, produced on ring-opening
of the epoxide, can undergo several competing rearrangement
processes typical of terpenoids with Lewis acid catalysts typically
favouring the formation of cyclopentenic aldehydes and Br o€ nsted
expected b-tetralone in high yield when extended reaction times
were employed (entry 9). Finally, we investigated the rearrange-
ment of alkyl substituted epoxides, which gave low conversions of
the aldehyde product (w10%) due to the efficient catalysis of the
10
acids, typically favouring the formation of menthyl derivatives.

8380
M.W.C. Robinson et al. / Tetrahedron 66 (2010) 8377e8382
campholenic aldehyde in excellent overall yield and with excellent
selectivity as determined by H NMR and GCeMS analysis of the
1
OH
crude reaction mixture.
OH
With an efficient protocol developed for the solution phase
rearrangement of epoxides, we next studied the possibility of
supporting Cu(II) salts on a heterogeneous support material. There
has been considerable interest in the development of such het-
OH
trans-carveol
OH
trans-sorberol
erogeneous systems, which catalyze the rearrangement of
a-pinene
iso-pinocamphol
oxide with high selectivity, given the importance of campholenic
11
aldehyde as an intermediate for the fragrance industry. We re-
cently reported the synthesis and efficient rearrangement of ep-
oxides catalyzed by nanoporous aluminosilicate materials, which
gave campholenic aldehyde in good yield and with good selectiv-
O
O
12
ity, and were intrigued as to the possibility of employing such
a nanoporous support in combination with Cu(II) salts to further
improve selectivity. With this aim in mind, we synthesized a small
series of nanoporous silicates (S-0) and aluminosilicates (AS-(250)
and AS-(150)) using our evaporation-induced supramolecular as-
iso-pinocamphone
para-cymene
12
sembly (EISA) approach. We have previously demonstrated that
materials with moderate aluminium loadings perform as excellent
catalysts for the rearrangement process, while materials with very
low aluminium loadings and the unmodified silica material display
O
O
12
low Lewis acidity and display no catalytic activity. The corre-
sponding Cu(II) modified nanoporous materials (S-0Cu, AS(250)Cu
and AS-(150)Cu), which were prepared using an post-synthetic in-
cipient wetness impregnation approach. In this protocol, the
nanoporous silica or aluminosilicate was stirred in an aqueous so-
fenclochenal
campholenic aldehyde
Scheme 2. Major products arising from the acid catalyzed rearrangement of a-pinene
oxide.
We assessed the ability of both Cu(OTf)
catalyze the rearrangement of -pinene oxide (Table 3), and de-
termined the selectivity of the rearrangement process using
NMR and GCeMS analysis. We were gratified to observe that while
Cu(OTf) gave approximately equal quantities of trans-carveol and
campholenic aldehyde (entry 1), Cu(BF $nH O rapidly catalyzed
2
and Cu(BF
4
)
2
$nH
2
O to
4 2 2
lution of Cu(BF ) $nH O followed by drying and calcination to af-
a
ford the Cu(II) supported materials as green powders, which were
characterized using a range of standard techniques to assess surface
1
H
13
area and metal content (Table 4). All of the materials produced
displayed the expected large surface areas typical of nanoporous
silicas and absorbed Cu(II) ions in similar quantities to previous
2
4
)
2
2
14
an efficient and highly selective rearrangement reaction to give
literature reports.
Table 3
Product distribution from the Cu(II) catalyzed rearrangement of
a
-pinene oxide
Catalysta
% Compositionb
Yield (%)
OH
OH
OH
OH
O
O
c
Cu(OTf)
2
d
5
2
38
5
2
d
d
d
8
d
10
5
1
43
85
95
95
65
88
c
Cu(BF
Cu(BF
4
)
)
2
d
4
2
d
a
2 2
Reactions were carried out using 1 mmol epoxide in 10 mL CH Cl at room temperature.
b
c
Determined by 1H NMR and/or GCeMS analysis of the crude reaction mixture.
Catalyst (10 mol %), 3 h.
d
Catalyst (25 mol %), 20 min.
Table 4
Physical characteristics of nanoprous catalysts
Catalyst
Si/Al
nominal)
Si/Al
(EDX)
Copper concentration
Surface area
Conversion of styrene oxide
to phenyacetaldehyde (%)
a
ꢀ1
a
2
ꢀ1
b
c,d
(
(mmol g
)
(m
g
)
S-0
S-0Cu
AS-(250)
AS-(250)Cu
AS-(150)
AS-(150)Cu
d
d
d
697
704
646
672
606
598
0
0
10
7
25
20
d
d
0.1
d
135
135
81
171
210
91
0.09
d
81
84
0.07
a
Determined by EDX analysis.
Surface areas were obtained by the BET method.
b
c
Reactions were carried out using 1 mmol of epoxide in 10 mL CH
2 2
Cl under reflux conditions for 2 h.
d
Determined by 1H NMR analysis of the crude reaction mixture.

M.W.C. Robinson et al. / Tetrahedron 66 (2010) 8377e8382
8381
Our initial studies assessed the ability of these materials to
catalyze the rearrangement of styrene epoxide as a model reaction,
and we were disappointed to observe that none of these modified
materials functioned as effective catalysts, giving poor conversions
to phenylacetaldehyde (Table 4). Only the AS-(150) and AS-(150)Cu
materials provided significant conversions to the aldehyde product
although, in all cases, styrene oxide was recovered unchanged.
NMR analysis. Surface area measurements were obtained from BET
experiments.
4.3. Typical procedure for the preparation of nanoporous
aluminosilicate catalysts by an EISA approach: AS-(150)
catalyst
Similarly, reactions involving a-pinene oxide also gave poor con-
Cetyltrimethylammonium bromide (4.0 g, 11.0 mmol) was dis-
solved in an aqueous solution of hydrochloric acid (2.5 mL, 0.1 M)
and ethanol (17.5 mL). Tetraethylorthosilicate (25 mL, 112.0 mmol)
versions to campholenic aldehdye with the epoxide again being
recovered unchanged. This lack of reactivity may reflect the rela-
tively low loadings of Cu(II) present in these catalysts, that the
metal exists in a chemically inert state, or indeed an inability of the
substrate to access the active catalytic sites. Our studies are cur-
rently on-going to understand and to develop efficient Cu(II) doped
nanoporous silicate materials.
ꢂ
was then added and the mixture stirred for 10 min at 40 C. The
solution was cooled to room temperature and aluminium nitrate
nonahydrate (0.31 g, 0.83 mmol) was added in one portion. The
mixture was stirred for 20 min and then left to age at room tem-
perature for 1 week. The resultant orange solid was crushed into
ꢂ
a fine powder, dried overnight at 90 C and then calcined in air at
3
. Conclusions
ꢂ
5
50 C for 12 h to remove the organic template. The resulting white
mesoporous aluminosilicate catalyst was characterized by EDX and
MAS NMR ( Al and Si) analysis. Surface area measurements were
obtained from BET experiments.
27
29
4 2 2
In conclusion, we have demonstrated that Cu(BF ) $nH O is
a highly efficient and reagent, which catalyzes the Meinwald rear-
rangement of a range of epoxides to produce carbonyl compounds
in high yields under mild reaction conditions. The reagent displays
excellent regioselectivity, as demonstrated by the rearrangement of
4.4. Preparation of Cu(II) modified nanoporous catalysts
stilbene oxide and
a-pinene oxide, which gave high yields of
The nanoporous material (500 mg) was added to a stirred so-
campholenic aldehyde with excellent selectivity. The material is
commercially available, and its benign nature, in addition to its low
cost and ease of use, offers a highly attractive alternative to
established methodologies. Disappointingly, attempts to develop
novel, selective catalysts by supporting this reagent onto a high
surface area silicate support generated materials that gave only
disappointing conversions of epoxides.
lution of copper tetrafluoroborate (1.0 g) in water (10 mL) at room
temperature. After this time the catalyst was filtered from the so-
lution using a Büchner funnel with minimal washing and dried for
ꢂ
2
4 h at 70 C. The dry powder was then calcined for 5 h in a furnace
ꢂ
at 550 C.
4.5. Typical procedure for the supported copper nanoporous
silicate catalyzed Meinwald rearrangement of epoxides
4
4
. Experimental
The nanoporous catalyst (50 mg) was added to a solution of
styrene oxide (1 mmol) in dichloromethane (10 mL) and the mix-
ture stirred for 2 h under reflux conditions. After this time, the
reaction mixture was cooled to room temperature and the catalyst
removed by filtration through a Celite plug, which was washed
with further quantities of dichloromethane (2ꢁ5 mL). The com-
bined solvents were removed under reduced pressure and the
.1. Typical procedure for the copper(II) tetrafluoroborate
catalyzed Meinwald rearrangement of epoxides: 2-
phenylpropionaldehyde
1
2
Copper tetrafluoroborate (59 mg, 0.25 mmol, 25 mol %) was
added to a solution of -methylstyrene oxide (127 mg, 0.95 mmol)
a
in dry dichloromethane (10 mL) at room temperature. Upon com-
pletion of the reaction (TLC) the mixture was diluted with
dichloromethane (30 mL) and washed with water (3ꢁ40 mL). The
organic phase was dried over magnesium sulfate, and the solvent
removed to give an oil that was purified by chromatography (1:10
ethyl acetate/hexane) to give 2-phenylpropionaldehyde (108 mg,
1
resulting oil was analyzed by H NMR and/or GCeMS.
Acknowledgements
The authors thank the Engineering and Physical Sciences Re-
search Council for funding (MWCR) and the EPSRC National Mass
Spectrometry Service, Swansea University. The authors are in-
debted to Michael Kean and Valma Szymanski for their invaluable
help in the preparation of this manuscript.
ꢀ1
8
5%) as a colourless oil;
n
max/cm (neat) 1718, 1493, 1452, 1267,
1
1020, 759, 697; H NMR (400 MHz; CDCl
3
) d
¼9.62 (1H, d, J¼1.5 Hz),
7
.40e7.20 (5H, m), 3.55 (1H, qd, J¼7 and 1.5 Hz), 1.45 (3H, d,
13
J¼7 Hz); C NMR (100 MHz; CDCl
3
)
d
¼201.6, 138.1, 129.5, 128.7,
þ
127.5, 53.4, 15.0; MS (EI) m/z, 134 (M) ; HRMS calculated for
þ
4
) 152.1070, found 152.1070.
Supplementary data
C
9
H14NO (MþNH
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.tet.2010.08.078. These data in-
clude MOL files and InChIKeys of the most important compounds
described in this article.
4
.2. Preparation of nanoporous silicate catalyst S-0 by an EISA
approach
Cetyltrimethylammonium bromide (4.0 g, 11.0 mmol) was dis-
solved in an aqueous solution of hydrochloric acid (2.5 mL, 0.1 M)
and ethanol (17.5 mL) and tetraethylorthosilicate (25 mL,
References and notes
1
4
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0 C. The solution was then cooled to room temperature and
ꢂ
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