Identification of reaction intermediates in AlCl3-mediated cyclocondensation reactions by simultaneous in situ ATR-FTIR and UV-vis spectroscopy

Article abstract of DOI:10.1016/j.tet.2013.01.097

Functionalized trifluoromethyl-substituted salicylates have been synthesized based on AlCl₃-mediated cyclocondensation reactions using 4,4-dimethoxy-1,1,1-trifluorobut-3-en-2-one and 1,3-bis(silyloxy)-1,3- butadienes. Simultaneous in situ ATR-F

Full text of DOI:10.1016/j.tet.2013.01.097

Tetrahedron 69 (2013) 3338e3347
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Identification of reaction intermediates in AlCl3-mediated
cyclocondensation reactions by simultaneous in situ ATR-FTIR
and UVevis spectroscopy
,
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b
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Sebastian Reimann a , Leif R. Knopke , Anke Spannenberg , Angelika Bruckner ,
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*
*
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Oliver Kuhn , Peter Langer
Leibniz-Institut fur Katalyse e.V. an der Universitat Rostock (LIKAT), Albert Einstein Str. 29a, 18059 Rostock, Germany
Institut fur Chemie, Universitat Rostock, Albert-Einstein-Str. 3a, 18059 Rostock, Germany
Institut fur Physik, Universitat Rostock, Wismarsche Str. 43-45, 18057 Rostock, Germany
, Ursula Bentrup
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b
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a r t i c l e i n f o
a b s t r a c t
Article history:
Functionalized trifluoromethyl-substituted salicylates have been synthesized based on AlCl3-mediated
cyclocondensation reactions using 4,4-dimethoxy-1,1,1-trifluorobut-3-en-2-one and 1,3-bis(silyloxy)-1,3-
butadienes. Simultaneous in situ ATR-FTIR/UVevis spectroscopic measurements were carried out for
mechanistic studies. Real-time in situ spectroscopic investigations reveal the formation of an aluminum
trichloride bidentate-butenone adduct as intermediate. The observed band shifts in the respective
UVevis spectra in consequence of complex formation could be explained by theoretical calculations.
Lewis acidic solids (Al2O3 and TiO2) were also successfully utilized. Due to the strong substrate ad-
sorption on the solids detected by UVevis-DRS and FTIR transmission spectroscopy the obtained yields
were rather modest.
Received 25 October 2012
Received in revised form 9 January 2013
Accepted 16 January 2013
Available online 26 February 2013
Keywords:
Cyclization
Lewis acids
IR spectroscopy
UVevis spectroscopy
Reaction mechanisms
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
interaction of the Lewis acid with 1 played a decisive role in this
process. On the one hand salicylates 3 were observed using TiCl4
Lewis acid mediated formal [3þ3] cyclocondensation reactions
of 1,3-bis(silyloxy)-1,3-butadienes (Chan’s dienes) with 1,3-
dielectrophiles such as 3-alkoxy-2-en-1-ones, 1,3-ketoacetals, 1,3-
keto-S,O-acetals or 3-silyloxy-2-en-1-ones represent an important
method, which allows the one-pot synthesis of a great variety of
six-membered carbacycles, which are not readily available by other
methods.1,2 This includes, for example, the synthesis of various
functionalized salicylates, pyran-4-ones, phenols, and cyclo-
hexenones being important as core structures to many biologically
active small molecules.3
while on the other hand pyran-4-ones 4 were observed when
During the last years we intensively studied the scope of
cyclocondensation reactions of 1,3-bis(silyloxy)-1,3-butadienes
with electrophilic building blocks and found that the type of the
employed Lewis acid has an important influence on the product
distribution.4 In this context we recently reported mechanistic
studies of TiCl4 and TMSOTf (trimethylsilyl trifluoromethane-
sulfonate) mediated cyclization reactions of 1,3-bis(silyloxy)-1,3-
butadiene 2a with butenones 1a,b (Scheme 1) showing that the
Scheme 1. General reaction scheme. Influence of TiCl4 and TMSOTf on the product
* Corresponding authors. E-mail addresses: peter.langer@uni-rostock.de (P.
distribution in cyclocondensation reactions.
Langer), ursula.bentrup@catalysis.de (U. Bentrup).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.01.097

S. Reimann et al. / Tetrahedron 69 (2013) 3338e3347
3339
TMSOTf was employed. Results of ATR-FTIR and UVevis spectro-
scopic studies revealed that TiCl4 formed a bis-chelate complex A
while TMSOTf interacts with only one site of 1, forming an ionic
species B.5
compound 3a (Table 2), the structure of which was elucidated by
2D NMR spectroscopic methods (NOESY and COSY).
Against this background also other Lewis acids like AlCl3 should
be suitable to use in this kind of reaction having a high reactivity as
well and showing a different product selectivity than observed for
TiCl4 or TMSOTf.6 In this context, several examples have been re-
ported for TiCl4 or BF3$OEt2 promoted cyclization reaction of Chan’s
diene with 1,3-dielectrophiles.7,8 However, to the best of our
knowledge cyclocondensation reactions and their mechanistic in-
vestigation using AlCl3 as mediating reagent have not been re-
ported to date.
Scheme 2. Synthesis of 3aej. Reagents and conditions: (i) AlCl3 (0 ꢀC), CH2Cl2, ꢁ78 to
20 ꢀC, 12e14 h.
Table 2
Optimization of the synthesis of 3a
Comparing TiCl4 and AlCl3, the electronic properties should
differ from each other. Thus, the spectroscopic detection (UVevis)
of different transition states formed by interaction with the sub-
strates has to be assumed. However, AlCl3 is known to form chelate
complexes with bidentate ligands like acetyl acetonate.9 Therefore,
a similar intermediate as described in our previous study with
a
Ratio of 1a/2a [mmol]
AlCl3 [mmol]
CH2Cl2 [mL]
Yield of 3ab [%]
1:2
1:2
1:2
1:1
1:2
1:2
1:2
1:2
d
2
2
2
2
2
2
2
5
0
49
36
28
59
26
34
51
1 (25 ꢀC)
2 (25 ꢀC)
1 (0 ꢀC)
1 (0 ꢀC)
0.5 (0 ꢀC)
2 (0 ꢀC)
1 (0 ꢀC)
5
TiCl4 should be expected if AlCl3 interacts with 1.
Herein, we report the first AlCl3-mediated formal [3þ3] cyclo-
condensation reactions of 4,4-dimethoxy-1,1,1-trifluorobut-3-en-
2-one 1a and 1,3-bis(silyloxy)-1,3-butadienes 2 affording a variety
of functionalized 4-methoxy-6-(trifluoromethyl) salicylates with
yields up to 59%. The interaction of AlCl3 with both reactants was
studied by ATR-FTIR and UVevis spectroscopy. While UVevis
spectroscopy gives insight about changes in the electronic state of
the molecules, ATR-FTIR spectroscopy reveals information on the
molecular changes in terms of vibrations of characteristic bonds.
Simultaneously in situ ATR-FTIR/UVevis spectroscopic measure-
ments were applied to study the influence of the temperature
during the cyclization reaction. In this way comprehensive in-
formation about the reaction system is available because the same
sample is analyzed at the same time.
Furthermore, Al2O3 and TiO2 were used as mediating Lewis
acids in the described reaction system because of their easier
handling and separation after the reaction. Due to the strong ad-
sorption behavior of the substrates the yields were rather modest.
In this context UVevis-DRS and FTIR spectroscopy in transmission
mode were applied to analyze the adsorbed substrates.
a
Addition of AlCl3 at corresponding temperature.
Isolated yields.
b
Applied concentration, temperature (addition of AlCl3 and 2a),
and stoichiometry played important roles. The best yield could be
obtained when the butenone 1a was activated with AlCl3 at 0 ꢀC
and the diene 2a was added at ꢁ78 ꢀC, when the reaction was
carried out in a highly concentrated solution, and when an excess
(2.0 equiv) of 2a was employed. The reaction without AlCl3 was
unsuccessful. Products 3aej, containing the CF3 group in ortho
position to the ester group, were formed with excellent regio-se-
lectivity. The formation of the other regioisomer, containing the CF3
group located para to the ester group was not observed in these
examples. This finding is consistent with the results made by
Bunescu et al.4b who observed the same product- and regio-
selectivity using TiCl4 instead of AlCl3. However, the utilization of
AlCl3 could enhance the yields of 3 up to 22% (Table 1).
2.2. In situ studies: reaction monitoring by ATR-FTIR and
UVevis spectroscopy
2. Results and discussion
2.1. Synthetic studies
According the optimized synthesis protocol the butenone 1a
was activated with AlCl3 at 0 ꢀC while the diene 2a was added after
cooling at ꢁ78 ꢀC. Then, the reaction mixture was warmed up to
room temperature over a 12e14 h period. To investigate the acti-
vating effect and specific interaction of AlCl3 with both reactants in
dependence on temperature, simultaneous in situ ATR-FTIR/
UVevis spectroscopic measurements were exemplarily performed
using 1a and 2a as substrates. For the in situ experiments the
warm-up period of the reaction mixture was shortened to 4 h,
which has been found to be adequate for getting comparable re-
sults as found by normal synthesis protocol.
4,4-Dimethoxy-1,1,1-trifluorobut-3-en-2-one 1a and 1,3-
bis(silyloxy)-1,3-butadienes 2aej were prepared from the known
literature procedures.4b,10 The AlCl3-mediated reaction of 1a with
1,3-bis(silyloxy)-1,3-butadienes 2aej afforded 4-methoxy-6-(tri-
fluoromethyl)salicylates 3ej in 47e59% yield, respectively (Table 1,
Scheme 2). The reaction conditions were thoroughly optimized for
Table 1
Synthesis of 3aej
The in situ ATR-FTIR spectra recorded during the reaction of 1a
with 2a in the presence of AlCl3 are displayed in Fig. 1. The first
spectrum (Fig. 1a) shows 1a dissolved in CH2Cl2 at 20 ꢀC. Important
bands occur at 1683 cmꢁ1 with a shoulder at 1707 cmꢁ1, which
2, 3
R1
R2
Yield of 3aeja [%]
a
b
c
d
e
f
g
h
i
H
H
H
H
H
H
Me
Et
Me
59 (47)b
58 (36)b
46
53
46
i-Pr
i-Bu
n-Bu
i-Pent
n-Oct
Me
Me
Me
Me
belong to the
assigned to a
n
(C]O) vibration and at 1558 cmꢁ1, which can be
n
(C]C) vibration. When AlCl3 is added to 1a at 0 ꢀC
51
followed by cooling the reaction mixture to ꢁ60 ꢀC, the band of the
50 (34)b
58
n
(C]O) vibration is shifted to lower wavenumbers and appears at
1628 cmꢁ1 with increased intensity (Fig. 1b). This band shift in-
n-Non
n-Tetradec
52
47
j
dicates the interaction of 1a with AlCl3 via its carbonyl group.
Pasynkiewicz et al.11 reported also a strong
n
(C]O) band at
a
Isolated yields.
Reported yields in parantheses.4b
1628 cmꢁ1 for a monomeric aluminum methyl 2-hydroxybenzoate
b

3340
S. Reimann et al. / Tetrahedron 69 (2013) 3338e3347
Fig. 2. In situ UVevis transmission spectra of the AlCl3-mediated conversion of 1a and
2a to 3a: (a) 1 equiv 1a in CH2Cl2 (c¼0.5 mol Lꢁ1) at 20 ꢀC, (b) after addition of 1 equiv
AlCl3 at ꢁ60 ꢀC, (c)e(g) mixture of 1aþAlCl3þ2a (molar ratio 1:1:2) at (c) ꢁ60 ꢀC, (d)
20 min ꢁ60 ꢀC, (e) ꢁ40 ꢀC, (f) ꢁ30 ꢀC, (g) ꢁ20 ꢀC, and (h) after quenching with HCl
(aq).
Fig. 1. In situ ATR-FTIR spectra of the AlCl3-mediated reaction: (a) 1a in CH2Cl2
(c¼0.5 mol Lꢁ1) at 20 ꢀC without AlCl3, (b) 1aþAlCl3 at ꢁ60 ꢀC in 1:1-mixture (molar
ratio); and of 1aþAlCl3þ2a as 1:1:2-mixture (molar ratio) at ꢁ60, ꢁ50, ꢁ40, ꢁ30, ꢁ20,
ꢁ10, ꢁ5, and ꢁ1 ꢀC.
between 1a and AlCl3. The same effect was observed in the in situ
ATR-FTIR experiment indicating the favored complex formation at
lower temperatures within a continuous process.
chelate complex. Additional bands occur at 1348, 1201, 1164, and
1147 cmꢁ1. While the bands at 1348,1201, and 1147 cmꢁ1 result from
The addition of 2a at ꢁ60 ꢀC provokes immediately a strong shift
of the broad band to higher wavelengths (Fig. 2c), which is almost
completed after 20 min (Fig. 2d). With increasing temperature the
maximum is further shifted and reaches its maximum with 431 nm
(Fig. 2eeg). Finally, the quenching with hydrochloric acid (aq)
provokes a shift of the maximum to 357 nm (Fig. 2h).
shifted
1a, the band at 1164 cmꢁ1 is related to
multaneously, the intensity of the
n
(CeF) and
n
(CeO) modes of the methoxy and CF3 groups of
(CeOeAl) vibration.11 Si-
(C]C) band is decreased, which
n
n
was also observed when 1a forms a bis-chelate complex with TiCl4.5
The in situ ATR-FTIR spectra of the reaction mixture after dosing
the diene 2a to the 1:1-mixture of 1a and AlCl3 at ꢁ60 ꢀC are shown
in Fig. 1 as well. Starting from spectrum b in Fig. 1 the most sig-
The shift to lower wavelengths after mixing 1a with AlCl3 is
surprising because it is contrary to the observations with TiCl4. In
the TiCl4 route a strong shift to higher wavelengths could be ob-
served when 1a was mixed with TiCl4 due to the formation of a bis-
chelate complex. The Ti-atom is bonded to the oxygen of the car-
bonyl group as well as to the oxygen atom of one methoxy group.
nificant changes during the warming period from ꢁ60 ꢀC to ꢁ1 ꢀ
C
resulted in the appearance of new bands in the region of the n(C]
O), n(C]C), n(CeF), and n(CeO) vibrations at 1705, 1615/1577/1521,
1358/1323, and 1194/1145 cmꢁ1, respectively. These changes reflect
the formation of an intermediate complex, which cannot be
assigned in detail based on the existing data. For further elucidation
additional ex situ investigations have been carried out (cf. Chapter
2.3). However, the positions of the characteristic bands occurring
during reaction indicate that the reaction pathway is comparable to
Due to the delocalization of p-electrons an additional chromophore
was created, which caused the shift of the absorption band to
higher wavelengths in the UVevis spectrum of the formed chelate
complex. Hence, the shift to lower wavelengths in the UVevis
spectrum of the mixture of 1a and AlCl3 indicates a different elec-
tronic state of the formed species.
For a better assignment of the bands occurring in the UVevis
spectra of 1a and in the 1:1-mixture of 1a with AlCl3, diluted so-
lutions have also been measured (Fig. 3).
the TiCl4-mediated reaction as described in our previous work.5
A
comparison of the characteristic bands observed in AlCl3- and in
TiCl4-mediated reactions is shown in Table 3. The simultaneously
recorded in situ UVevis transmission spectra are displayed in Fig. 2.
Table 3
Positions and assignments of bands observed during reaction
5
1aþAlCl3þ2a (
n
/cmꢁ1
)
1aþTiCl4þ2a (
n
/cmꢁ1
)
Assignment
1705
1615
1577
1521
1358/1323
1194/1145
1706
1616
1581
1521
1354/1306
1194/1143
n
n
n
n
n
n
(C]O)
(C]C)
(C]C)
(C]C)
(CeF)
(CeO)/n(CeOeM)
(M¼Al, Ti)
Due to the relatively high concentration the UVevis spectra
have an unusual shape. The spectrum of 1a (Fig. 2a) shows at 20 ꢀC
a broad band with maximum at 335 nm and a weak one at 380 nm,
which might represent
p
/p
*
and n/
p transitions, respectively.
*
After the addition of AlCl3 at 0 ꢀC both signals are shifted to
shorter wavelengths. The band positions are shifted further with
decreasing temperature and finally appear at 319 nm and 361 nm at
ꢁ60 ꢀC (Fig. 2b), which might be caused by a stronger interaction
Fig. 3. Ex situ UVevis spectra of 1a in CH2Cl2 (dotted line) and 1aþAlCl3 in CH2Cl2
(solid line) (c¼5ꢂ10ꢁ4 mol Lꢁ1).

S. Reimann et al. / Tetrahedron 69 (2013) 3338e3347
3341
The spectrum of 1a shows one discrete band with a maximum at
278 nm while the 1:1-mixture of 1a and AlCl3 (CH2Cl2,
c¼5$10ꢁ4 mol Lꢁ1) shows a slightly more intense band at 275 nm. It
is not clear if this difference indicates a significant change in the
electronic structure or an interaction between 1a and AlCl3. On the
other hand, a band, which is characteristic for AlCl3 and would
occur at 240 nm12 cannot be observed. Hence, an interaction be-
tween 1a and AlCl3 is likely.
Starting from the molecular structure of the aluminum tris-
chelate complex 5 (Fig. 4a) we calculated the excitation spectra in
order to explain the shape of the UVevis spectrum of the 1:1-
mixture of 1a and AlCl3 theoretically (cf. Fig. 3). Therefore we
used the time-dependent density functional theory employing the
B3LYP functional (m3 grid) with a def2-TVZP basis set as imple-
mented in Turbomole.17 Likewise, the excitation spectra of the ti-
tanium bis-chelate complex (Fig. 4b) were investigated in
comparison, too.
One possible explanation for the shape of the UVevis spectrum
of the 1:1-mixture of 1a and AlCl3 might be the absence of d
electrons in the AlCl3-complex. Holm and Cotton13 as well as
Nakamoto et al.14,15 pointed out that d
ed and d ep in-
p
-
In the aluminum tris-chelate complex 5 the range between 197
and 285 nm was covered by the lowest 20 transitions. Here,
a peak at 246 nm, whose orbital decomposition is given in
Table S2 in Supplementary data, dominates the spectrum (Fig. S2).
The spectrum of complex 5 is shifted about 14 nm to lower
wavelengths in comparison to the spectrum of butenone 1a pos-
sessing the dominant contribution at 260 nm. These results con-
firm our conclusion from the UVevis spectra with regard to the
electronic situation in the complex formed between 1a and AlCl3
(Fig. 3). In fact, inspecting the participating orbitals of the alumi-
num tris-chelate complex 5 in Fig. S3 one notices that the tran-
p
p
p
p
teractions are important factors in the bond formation between
metal and donor system. These interactions lead to bonds with
resonance structures, which are also known as metalaromaticy.16
For example, the AleO bond in Al(acac)3 is characterized as
strongly covalent bond.14 The missing d
p-electrons cause that less
resonance structures appear and all bonds remain localized.
Nakamoto et al.14,15 described such complexes where the electrons
remain localized in the ligand skeleton of the chelate ring as ‘weak
complexes’. In contrast, the
p
-electrons and the d-electrons of the
sitions are of intra-ligand p/p type. The molecular orbitals of
*
metal atom in strong chelate complexes tend to be more or less
delocalized in the whole chelate ring. Consequently, the electronic
structure is less changed in weak chelate complex, and the UVevis
spectrum of the mixture of 1a and AlCl3 shows no significant dif-
ference at room temperature. Furthermore, the interaction is ob-
viously not strong enough to form a second chromophore as shown
for the 1:1-mixture of 1b and TiCl4 in CH2Cl2.5 Otherwise a second
band or a shoulder should be observed.
the aluminum-center are not involved due to the missing d
electrons causing a less resonated structure.
p-
On the contrary, the electrons of the titanium bis-chelate com-
plex are delocalized. The -orbitals of the ligands are located in the
manifold of the metal d-orbitals such that the lowest transitions are
of to d character (ligand-to-metal charge transfer type). Insofar,
p
p
the different observed band shifts in the UVevis spectra in conse-
quence of complex formation of 1a with TiCl4 and AlCl3, re-
spectively, could be explained by the theoretical calculations and
confirm the description of the electronic state made by Nakamoto
et al.14,15
Comparing the structures of both chelate complexes (Fig. 4) it
can be expected that the aluminum tris-chelate complex 5 should
be more stable than the chlorine-containing titanium bis-chelate
complex. Thus, the question arises if the isolated complex 5 is re-
ally an intermediate complex in the reaction of 1a with 2a.
Inspecting the low frequency areas of the ATR-FTIR spectra of
AlCl3 mixtures with 1a and 2a, respectively, significant bands can
be observed (Fig. 5).
2.3. On the nature of the intermediate complex of butenone
1a with AlCl3
Summarizing the spectroscopic findings described above a spe-
cific interaction of 1a with AlCl3 could be detected. However, these
findings do not allow to propose the structure of the formed in-
termediate complex.
For this reason it was tried to crystallize this complex from
a 1:1-mixture of the butenone 1a with AlCl3 in CH2Cl2 by slow
evaporation and cooling. Single crystals were isolated and studied
by X-ray structure analysis. The obtained molecular structure
shows that an aluminum chelate complex 5 has been formed under
the release of CH3Cl (Fig. S1, Table S1). The aluminum atom is
surrounded by three butenone moieties leading to a distorted oc-
tahedral coordination with oxygen, which is reached by the co-
ordination of two oxygen atoms of each chelate ligand (bond
lengths and angels are discussed in Supplementary data). This
finding is in contrast to the observed formation of a bis-chelate
complex between TiCl4 and 1a as important reaction in-
termediate,5 in which two chlorine atoms remain at the titanium
atom as ligand (Fig. 4).
Fig. 5. Ex situ ATR-FTIR spectra of 1a at 20 ꢀC, 1aþAlCl3 at ꢁ50 ꢀC, 2a at 20 ꢀC, and
2aþAlCl3 at 20 ꢀC in CH2Cl2 (c¼0.5 mol Lꢁ1).
The spectrum of 1a shows only bands at 621, 566, and 421 cmꢁ1
,
while no significant bands are observable in the spectrum of 2a
between 650 and 425 cmꢁ1. In the spectrum of the mixture of 1a
with AlCl3 the bands at 621, 566, and 421 cmꢁ1 are shifted to lower
Fig. 4. Structures of the aluminum tris-chelate complex 5 formed between AlCl3 and
1a (a) and the bis-chelate complex formed between TiCl4 and 1a (b), according to the
results of X-ray crystal structure analysis.

3342
S. Reimann et al. / Tetrahedron 69 (2013) 3338e3347
wavenumbers (606, 547, and 416 cmꢁ1). Strong new bands can be
seen at 519/501 cmꢁ1 and 457 cmꢁ1, which are obviously caused by
complex formation. Mixing of 2a with AlCl3 provokes the appear-
ance of new bands at 635, 527, 499, and 465 cmꢁ1. A solution of
AlCl3 in CH2Cl2 with the same concentration was also measured for
comparison (spectrum not shown). The observed bands at 622 and
441 cmꢁ1 can be assigned to the n8 and n13 vibration mode of
Al2Cl6.18 Such bands do not appear in the spectra of the AlCl3
mixtures with 1a and 2a.
loosing chlorine. At the end a stable chlorine-free aluminum tris-
chelate complex 5 is formed.
Based on the observed band positions in the ATR-FTIR spectrum
alone it is difficult to propose the structure of the formed first in-
termediate complex. However, it should be mentioned in this
context that Altwood et al.21 reported the structural characteriza-
tion of an aluminum trichloride-bidentate tertiary amine adduct 6
with trigonal bipyramidal coordination of the aluminum-center
(Scheme 3). Unfortunately no FTIR or Raman spectroscopic data of
this complex were given. Nevertheless, a similar structure 7 can be
proposed for the observed intermediate complex formed between
1a and AlCl3 (Scheme 3).
Despite different intensities the new bands occurring in the
spectra of both mixtures (Fig. 5) appear at comparable positions.
Thus, the type of interaction in terms of substrate activation
should be similar, which was found in the mixtures of 1a and 2a
with TiCl4, too.5
According to Weidlein et al.19 the band of
n(AleCl) mode in
methylmethoxyaluminumchloride CH3(OCH3)AlCl appears at 509/
495 cmꢁ1. Derouault et al.20 investigated AlCl3$THF complexes and
reported bands at 525 cmꢁ1 and 439 cmꢁ1 belonging to
n
(AleCl)
and
n(AleO) modes, respectively. Consequently, the observed
bands of the AlCl3 complexes with 1a and 2a at 519/501 cmꢁ1 and
527/499 cmꢁ1 (Fig. 5) can be assigned to
bands at 457 and 465 cmꢁ1 result from
n
(AleCl) modes, while the
n(AleO) vibrations.
Scheme 3. Structure of the aluminum trichloride-bidentate tertiary amine adduct 621
and postulated structure of the intermediate 7 formed between 1a and AlCl3.
In Fig. 6 the ATR-FTIR spectra of the crystallized aluminum tris-
chelate complex 5 and the intermediate complex formed by in-
teraction of 1a with AlCl3 in CH2Cl2 are compared. It is clearly seen
that the n
(AleCl) bands at 519/501 cmꢁ1 do not appear, as expected,
2.4. From single-phase to multi-phase systems: application of
Al2O3 and TiO2
in the spectrum of the aluminum tris-chelate complex 5 (Fig. 6a).
Considering the hash marked bands of the butanone moiety the
band at 452 cmꢁ1 can clearly be assigned to the
n
(AleO) mode. The
The utilization of insoluble inorganic solids such as zeolites, ti-
tanium silicates, aluminas or silica gels in organic reactions is of
considerable interest due to the easier catalyst separation, han-
dling, process efficiency, and reduction of side-products.22 Thus, we
selected sulfate-free TiO2 and Al2O3 as solid catalysts for elucidating
their potential for this type of cyclocondensation reaction. The se-
lection is based on the fact that the reaction works successfully in
liquid phase with TiCl4 as well as AlCl3 and both solid acids posses
Lewis acid centers. Furthermore, the adsorption of diketones on
TiO2 nano-particles has been studied already with FTIR spectros-
copy.23 Additionally, alumina has been applied successfully in or-
ganic synthesis as versatile mediating acid such as intra- and
intermolecular additions of OH groups, reductions, oxidations,
substitutions, and decarboxylation reactions.24
n(AleO) band and the bands of the butanone moiety appear slightly
shifted in the spectrum of the intermediate formed complex of 1a
with AlCl3, too (Fig. 6b). Thus, it can be concluded that the in-
termediate complex has another composition and structure as the
crystallized tris-chelate complex 5. Furthermore, the appearance of
strong
n(AleCl) vibrations in the ATR-FTIR spectrum of the in-
termediate complex formed during the reaction clearly indicates
that this complex does not accord with the crystallized aluminum
tris-chelate complex 5.
The utilization of Al2O3 in cyclocondensation reactions of 1a
with 1,3-bis(silyloxy)-1,3-butadienes 2a and 2b leaded to 4-
methoxy-6-(trifluoromethyl)salicylates 3a and 3b in low yields
(14e23%). No significantly improved yields were obtained by using
TiO2 instead of Al2O3 (Table 4). During the optimization it has been
proved that TiO2 and Al2O3 have to be activated under vacuum by
repeated heating for 2e3 h. The reaction with non-activated TiO2
Table 4
Synthesis of 3a,b mediated by Al2O3 and TiO2
2,3
R1
R2
CH2Cl2 [mL]
Mediating solid
Yield of 3aa [%]
a
a
a
b
a
b
H
H
H
H
H
H
OMe
OMe
OMe
Oi-Pr
OMe
Oi-Pr
3
1
0
1
1
1
Al2O3
Al2O3
Al2O3
Al2O3
TiO2
14
17
0
23
13
15
TiO2
Fig. 6. ATR-FTIR spectra of the crystallized Al tris-chelate complex 5 (a) and the re-
action intermediate of 1aþAlCl3 (b) in CH2Cl2 (c¼0.5 mol Lꢁ1) at 20 ꢀC. The hash
marked bands result from the butenone moiety.
a
Isolated yields.
and Al2O3 did not proceed. While Ranu et al.24b reported the
solvent-free MukaiyamaeMichael addition of silyl-enol ethers to
alkyl vinyl ketones on the surface of alumina, in our case a small
amount of CH2Cl2 seems to be important for the feasibility of the
reaction.
It has to be considered, that the latter complex 5 is formed by
longtime crystallization process. Obviously, the chlorine-
containing aluminum complex is the first intermediate formed by
interaction of AlCl3 with 1a, which is then slowly transformed by
a

S. Reimann et al. / Tetrahedron 69 (2013) 3338e3347
3343
Although the achieved yields were rather low, the utilization of
Al2O3 and TiO2 possessing Lewis acidity could be successfully ac-
complished. A possible reason for the comparable low yields might
be the strong adsorption of substrates on the surface of the solids.
Therefore, the interaction of 1a and 2a with Al2O3 and TiO2 was
studied by UVevis-DRS and FTIR transmission spectroscopy.
Fig. 7 shows the UVevis-DRS spectra of 1a adsorbed on Al2O3
and TiO2. The spectra of pure Al2O3 and TiO2 are included for
comparison. The spectrum of pure Al2O3 (Fig. 7a) shows two broad
weak bands at 272 nm and 368 nm. Because Al2O3 is known as
UVevis inactive, these bands result from impurities. The spectrum
of 1a adsorbed on Al2O3 (Fig. 7b) shows one broad intense band at
274 nm indicating nicely the presence of adsorbed 1a. The band
position is comparable with that observed for pure 1a and its 1:1-
mixture with AlCl3 in CH2Cl2 (cf. Fig. 3). The spectrum of the used
pure TiO2 shows a characteristic broad band between 230 and
400 nm (Fig. 7c) the intensity of which is increased by adsorption of
1a (Fig. 7d).
The spectrum of 1a adsorbed on Al2O3 shows an intense band at
1667 cmꢁ1 with shoulders at 1696, 1639, and 1629 cmꢁ1 being
prominent for different n(C]O) vibrations. The shift of the carbonyl
band of 1a to lower wavenumbers and splitting indicate an in-
teraction of 1a with the surface of Al2O3. Furthermore, the ap-
pearance of four different n(C]O) points to the fact that 1a adsorbs
in a different manner. On the one hand the band at 1667 cmꢁ1 with
shoulder at 1696 cmꢁ1 might be prominent for 1a physisorbed on
Al2O3 due to the weak interaction with the surface of the Al2O3. On
the other hand the bands at 1639 and 1629 cmꢁ1 are caused by 1a
chemisorbed on Al2O3. Hence, the carbonyl group is strongly in-
volved in the adsorption mechanism. As a result of this interaction
the
position of the
The
(C]C) band of 2a at 1647 cmꢁ1 shifts to lower wave-
n
(C]C) band is shifted from 1558 to 1572 cmꢁ1, while the
(CeF) band at 1328 cmꢁ1 remains unchanged.
n
n
numbers if adsorbed on Al2O3 while a new band appears at
1534 cmꢁ1, which cannot be surely assigned. Comparing the ad-
sorbate spectra of 1a and 2a on Al2O3 with those obtained from the
respective mixtures of 1a and 2a with AlCl3 some similarities
concerning characteristic band positions of n(C]O) and n(C]C) are
obvious. This suggests that the interaction of 1a and 2a with Al2O3
proceeds in a similar way as observed with AlCl3 in liquid phase.
3. Conclusions
Lewis acids like AlCl3, Al2O3, and TiO2 were successfully applied
in cyclocondensation reactions using 1,3-bis(silyloxy)-1,3-
butadienes and 4,4-dimethoxy-1,1,1-trifluorobut-3-en-2-one. The
obtained products are trifluoromethyl-substituted salicylates pos-
sessing the same regio-selectivity as observed for TiCl4-mediated
cyclocondensation reactions.
The AlCl3-mediated reaction proceeds via a comparable mech-
anism as found for TiCl4. Differences were observed in the mode of
activation of the reactants 1a and 2a with AlCl3. The determined
molecular structure of the crystallized intermediate complex
formed by interaction of 1a with AlCl3 indicates the formation of
a chlorine-free aluminum tris-chelate complex. In contrast, in situ
ATR-FTIR spectroscopic results revealed the appearance of strong
Fig. 7. UVevis-DRS spectra of pure Al2O3 (a), of 1a adsorbed on Al2O3 (b), of pure TiO2
(c), and of 1a adsorbed on TiO2 (d).
n
(AleCl) vibrations in the spectrum of the intermediate complex
Some more information concerning the nature of the adsorbates
can be obtained from FTIR transmission spectra of self-supporting
wafers of Al2O3 on which the reactant 1a or 2a has been sepa-
rately adsorbed (Fig. 8). Spectra of solutions of 1a and 2a in CH2Cl2
and their mixtures with AlCl3 are included for comparison.
formed during the interaction of 1a and AlCl3 indicating the for-
mation of a chlorine-containing complex. It is assumed that the
long crystallization process leads to a transformation of the first
formed chlorine-containing complex by loosing chlorine.
Otherwise this finding nicely demonstrates the benefit of in situ
spectroscopic studies enabling the real-time detection of in-
termediates formed in the respective reaction. In analogy to
a published structure of an aluminum trichloride-bidentate tertiary
amine adduct a similar aluminum trichloride bidentate-butenone
adduct with trigonal bipyramidal coordination of the aluminum-
center was proposed as possible structure of the formed in-
termediate complex.
Solid Lewis acidic Al2O3 and TiO2 have also been applied in the
cyclocondensation reaction. Due to the strong adsorption of the
reactants on the solids, which was detected by UVevis-DRS and
FTIR transmission spectroscopy, the yields were rather modest.
Obviously, the strong adsorption of the substrates hinders their
effective interaction which each other.
4. Experimental section
4.1. General comments
All glassware used were heated and dried for several times
under vacuum. All reactions were carried out under an inert at-
mosphere. AlCl3 was purchased from ACROS Organics (99.995%),
TiO2 was received from Sachtleben Chemie GmbH, and Al2O3 from
Fig. 8. ATR-FTIR spectra of 1a, 2a and their respective mixtures with AlCl3 in CH2Cl2
(c¼0.5 mol Lꢁ1) and FTIR transmission spectra of 1a and 2a adsorbed on Al2O3.

3344
S. Reimann et al. / Tetrahedron 69 (2013) 3338e3347
Sasol Germany GmbH. These solids were dried under vacuum by
repeated heating for 2e3 h before use. The solvent (CH2Cl2, 99.8%
extra anhydrous over molecular sieves, stabilized) was obtained
from ACROS Organics. 1,3-Bis(silyloxy)-1,3-butadienes (2) were
prepared according to a literature procedure from the corre-
(CeOH), 165.2 (CeOMe), 168.8 (C]O). 19F NMR (282.4 MHz, CDCl3):
d
¼ꢁ57.7 (CF3).
IR (ATR, cmꢁ1):
n
¼3125 (w), 3098 (w), 3013 (w), 2987 (w), 2945
~
(w), 2852 (w), 1663 (m), 1618 (m), 1577 (w), 1492 (w), 1432 (w),
1365 (s), 1232 (s), 1134 (s), 1097 (s), 1042 (s), 987 (s), 912 (s), 844 (s),
712 (s), 620 (s), 529 (m), 481 (w), 429 (m). GCeMS (EI, 70 eV): m/z
(%): 278 (Mþ, 15), 236 (17), 219 (31), 218 (100), 190 (28), 175 (15),
171 (8), 147 (6). HRMS (EI, 70 eV): calcd for C12H13O4F3 (Mþ):
278.07604; found: 278.076053. Anal. Calcd for C12H13O4F3 (278.22):
C, 51.80; H, 4.71. Found: C, 52.12; H, 4.672.
sponding b
-ketoesters in two steps and used within a few days.10
4,4-Dimethoxy-1,1,1-trifluorobut-3-en-2-one (1a) was synthesized
according to the protocol of Bunescu et al.4b
Thin layer chromatography (TLC) was run on Merck precoated
aluminum plates (Si 60 F254). Column chromatography was per-
formed using Merck Silicagel 60 (0.043e0.06 mm). NMR data were
recorded on a Bruker ARX 300 and Bruker ARX 400 spectrometers.
13C and 1H NMR spectra were referenced to signals of CDCl3, re-
spectively. Gas chromatographyemass analysis was carried out on
an Agilent HP-5890 instrument with an Agilent HP-5973 Mass
Selective Detector (EI) and HP-5 capillary column using helium
carrier gas. ESI HRMS measurements were performed on an Agilent
1969A TOF mass-spectrometer. Elemental analysis was performed
on a C/H/N/S-Microanalysator TruSpec CHNS (Leco).
4.3.3. Isobutyl
2-hydroxy-4-methoxy-6-(trifluoromethyl)benzoate
(3c). Starting with 1a (0.184 g, 1.0 mmol), AlCl3 (0.133 g,
1.0 mmol), 2c (0.605 g, 2.0 mmol) in CH2Cl2 (2.0 mL), 3c was
isolated as a slight yellow solid (0.134 g, 46%); mp¼40 ꢀC. 1H NMR
(400 MHz, CDCl3):
d
¼0.97 (d, 3JHeH¼6.8 Hz, 6H, CH3), 2.07 (m, 1H,
3
CH), 3.82 (s, 3H, MeO), 4.12 (d, JHeH¼6.8 Hz, 2H, CH2), 6.59 (d,
4JHeH¼2.4 Hz, 1H, AreH), 6.86 (d, JHeH¼2.7 Hz, 1H, AreH), 11.64
4
(s, 1H, OH). 13C NMR (100 MHz, CDCl3):
d¼19.1 (CH3), 19.1 (CH3),
27.5 (CH), 55.7 (MeO), 72.7 (CH2), 103.2 (C), 103.7 (CH), 109.2 (q,
3JCeF¼7.0 Hz, CH), 123.1 (q, JCeF¼271.0 Hz, CF3), 131.5 (q,
1
2JCeF¼32.0 Hz, CeCF3), 163.4 (CeOH), 165.3 (CeOMe), 169.6 (C]
4.2. General procedure for the synthesis of 3aej
O). 19F NMR (282.4 MHz, CDCl3):
d
¼ꢁ58.3 (CF3). IR (ATR, cmꢁ1):
~
n
¼2967 (m), 2915 (w), 2877 (w), 2728 (w), 1648 (m), 1614 (m),
To a CH2Cl2 solution (2.0 mL/1.0 mmol of 1a) of 1a (1.0 mmol)
was added AlCl3 (1.0 mmol) at 0 ꢀC. The reaction mixture was
cooled down to ꢁ78 ꢀC and 1,3-bis(silyloxy)-1,3-butadienes 2
(2.0 mmol) was added. The reaction mixture was allowed to warm
to 20 ꢀC for 12 h with stirring. HCl was added to the solution (10%,
15.0 mL), and the organic and aqueous layers were separated. The
latter was extracted with CH2Cl2 (3ꢂ15.0 mL). The combined or-
ganic layers were dried (Na2SO4), filtered, and the filtrate was
concentrated in vacuo. The residue was purified by flash chroma-
tography using n-heptane and ethyl acetate.
1594 (m), 1469 (m), 1427 (m), 1380 (m), 1331 (m), 1311 (m), 1272
(s), 1234 (m), 1211 (s), 1136 (s), 1041 (s), 989 (s), 856 (s), 628 (s),
540 (s), 445 (m), 387 (w). GCeMS (EI, 70 eV): m/z (%): 292 (Mþ,
18), 236 (18), 219 (37), 218 (100), 190 (21), 175 (9), 171 (10), 147 (6).
HRMS (EI, 70 eV): calcd for C13H15O4F3 (Mþ): 292.09170; found:
292.091940. Anal. Calcd for C13H15O4F3 (292.25): C, 53.43; H, 5.17.
Found: C, 53.40; H, 4.970.
4.3.4. n-Butyl
2-hydroxy-4-methoxy-6-(trifluoromethyl)benzoate
(3d). Starting with 1a (0.184 g, 1.0 mmol), AlCl3 (0.133 g,
1.0 mmol), 2d (0.605 g, 2.0 mmol) in CH2Cl2 (2.0 mL), 3d was
isolated as a yellowish oil (0.154 g, 53%). 1H NMR (400 MHz,
4.3. Characterization data of 3aej
CDCl3):
d
¼0.95 (t, 3JHeH¼6.7 Hz, 3H, CH3), 1.42e1.46 (m, 2H, CH2),
3
1.70e1.77 (m, 2H, CH2), 3.82 (s, 3H, MeO), 4.33 (t, JHeH¼6.8 Hz,
4
4.3.1. Methyl
2-hydroxy-4-methoxy-6-(trifluoromethyl)benzoate
2H, OCH2), 6.58 (d, JHeH¼2.8 Hz, 1H, AreH), 6.86 (d,
(3a). Data have been previously reported.4b Starting with 1a
(0.184 g, 1.0 mmol), AlCl3 (0.133 g, 1.0 mmol), 2a (0.521 g,
2.0 mmol) in CH2Cl2 (2.0 mL), 3a was isolated as a yellowish solid
4JHeH¼2.8 Hz, 1H, AreH), 11.61 (s, 1H, OH). 13C NMR (100 MHz,
CDCl3):
d
¼13.5 (CH3), 18.9 (CH2), 30.0 (CH2), 55.6 (MeO), 66.1
3
(OCH2), 103.2 (C), 103.5 (CH), 109.1 (q, JCeF¼7.0 Hz, CH), 123.0 (q,
(0.147 g, 59%); mp¼64e66 ꢀC. 1H NMR (300 MHz, CDCl3):
d¼3.83
1JCeF¼271.0 Hz, CF3), 131.6 (q, JCeF¼32.0 Hz, CeCF3), 163.3
2
4
(s, 3H, MeO), 3.93 (s, 3H, MeO), 6.59 (d, JHeH¼2.7 Hz, 1H, AreH),
(CeOH), 165.1 (CeOMe), 169.4 (C]O). 19F NMR (282.4 MHz,
4
6.87 (d, JHeH¼2.7 Hz, 1H, AreH), 11.41 (s, 1H, OH). 13C NMR
CDCl3):
d
¼ꢁ58.3 (CF3). IR (ATR, cmꢁ1):
~
n
¼2964 (m), 2938 (w),
(75 MHz, CDCl3):
d¼52.5 (MeO), 55.8 (MeO), 103.1 (C), 103.6 (CH),
2874 (w), 1652 (m), 1616 (m), 1592 (m), 1442 (m), 1425 (m), 1386
(m), 1334 (m), 1310 (m), 1276 (s), 1235 (m), 1206 (s), 1136 (s), 1040
(s), 988 (s), 863 (s), 715 (s), 627 (s), 534 (s), 445 (m), 380 (w).
GCeMS (EI, 70 eV): m/z (%): 292 (Mþ, 19), 219 (26), 218 (100), 190
(22). HRMS (EI, 70 eV): calcd for C13H15O4F3 (Mþ): 292.09170;
found: 292.091638. Anal. Calcd for C13H15 F3O4 (292.25): C, 53.43;
H, 5.17. Found: C, 53.34; H, 5.231.
109.3 (q, 3JCeF¼6.8 Hz, CH), 123.0 (q, 1JCeF¼271.5 Hz, CF3), 131.4 (q,
2JCeF¼31.5 Hz, CeCF3), 163.5 (CeOH), 165.1 (CeOMe), 169.7 (C]O).
19F NMR (282.4 MHz, CDCl3):
d
¼ꢁ58.9 (CF3). IR (ATR, cmꢁ1):
~
n
¼2957 (w), 2922 (w), 2853 (w), 1662 (m), 1614 (m), 1593 (m),
1440 (s), 1341 (m), 1312 (w), 1271 (m), 1136 (s), 1039 (m), 987 (m),
946 (m), 847 (m), 807 (m), 776 (w), 708 (m), 629 (s), 531 (s), 401
(s). GCeMS (EI, 70 eV): m/z (%): 250 (Mþ, 43), 219 (34), 218 (100),
190 (42), 175 (32), 171 (13), 147 (11). HRMS (EI, 70 eV): calcd for
C10H9O4F3 (Mþ): 250.04474; found: 250.044817.
4.3.5. Isopentyl 2-hydroxy-4-methoxy-6-(trifluoromethyl)benzoate
(3e). Starting with 1a (0.184 g, 1.0 mmol), AlCl3 (0.133 g,
1.0 mmol), 2e (0.633 g, 2.0 mmol) in CH2Cl2 (2.0 mL), 3e was
4.3.2. Isopropyl 2-hydroxy-4-methoxy-6-(trifluoromethyl)benzoate
(3b). Data have been previously reported.4b Starting with 1a
(0.184 g, 1.0 mmol), AlCl3 (0.133 g, 1.0 mmol), 2b (0.577 g, 2.0 mmol)
in CH2Cl2 (2.0 mL), 3b was isolated as a yellowish solid (0.161 g,
isolated as a slight yellow oil (0.140 g, 46%). 1H NMR (300 MHz,
3
CDCl3):
d
¼0.97 (d, JHeH¼6.8 Hz, 6H, CH3), 1.60e1.80 (m, 3H, CH
and CH2), 3.86 (s, 3H, MeO), 4.35 (t, 3JHeH¼6.9 Hz, 2H, OCH2), 6.58
4
4
(d, JHeH¼2.8 Hz, 1H, AreH), 6.86 (d, JHeH¼2.8 Hz, 1H, AreH),
58%); mp¼44e48 ꢀC. 1H NMR (300 MHz, CDCl3):
d
¼1.35 (d,
11.61 (s, 1H, OH). 13C NMR (75 MHz, CDCl3):
(CH3) 24.7 (CH), 36.7 (CH2), 55.7 (MeO), 64.9 (OCH2), 103.3 (C),
d¼22.3 (CH3), 22.3
3JHeH¼6.0 Hz, 6H, CH3), 3.82 (s, 3H, MeO), 5.26 (m, 1H, CH), 6.57 (d,
4JHeH¼2.7 Hz, 1H, AreH), 6.72 (d, 4JHeH¼2.7 Hz, 1H, AreH), 11.68 (s,
3
1
103.6 (CH), 109.2 (q, JCeF¼7.5 Hz, CH), 123.1 (q, JCeF¼271.5 Hz,
1H, OH). 13C NMR (75 MHz, CDCl3):
d
¼21.3 (CH3), 21.3 (CH3), 55.7
CF3), 131.7 (q, JCeF¼31.5 Hz, CeCF3), 163.4 (CeOH), 165.2
2
(MeO), 70.4 (CH), 103.5 (C), 103.7 (CH), 109.2 (q, 3JCeF¼7.5 Hz, CH),
(CeOMe), 169.5 (C]O). 19F NMR (282.4 MHz, CDCl3):
d¼ꢁ58.3
123.1 (q, 1JCeF¼274.5 Hz, CF3), 131.6 (q, 2JCeF¼33.8 Hz, CeCF3), 163.3
(CF3). IR (ATR, cmꢁ1):
n
¼2960 (m), 2872 (w), 1660 (w), 1619 (m),
~

S. Reimann et al. / Tetrahedron 69 (2013) 3338e3347
3345
1446 (w), 1275 (m), 1233 (s), 1137 (s), 1043 (s), 990 (s), 943 (m),
715 (s), 626 (m), 531 (m), 469 (m), 401 (m). GCeMS (EI, 70 eV): m/
z (%): 306 (Mþ, 14), 237 (8), 236 (16), 219 (33), 218 (100), 190 (16),
43 (10). HRMS (EI, 70 eV): calcd for C14H17O4F3 (Mþ): 306.10735;
found: 306.107777. Anal. Calcd for C14H17O4F3 (306.28): C, 54.90;
H, 5.59. Found: C, 54.87; H, 5.605.
for C12H13O4F3 (Mþ): 278.07604; found: 278.076396. Anal. Calcd
for C12H13O4F3 (278.22): C, 51.80; H, 4.71. Found: C, 51.89; H, 4.723.
4.3.9. Methyl
2-hydroxy-4-methoxy-3-nonyl-6-(trifluoromethyl)
benzoate (3i). Starting with 1a (0.184 g, 1.0 mmol), AlCl3 (0.133 g,
1.0 mmol), 2i (0.733 g, 2.0 mmol) in CH2Cl2 (2.0 mL), 3i was iso-
lated as a slight yellow solid (0.196 g, 52%); mp¼47 ꢀC. 1H NMR
4.3.6. n-Octyl
2-hydroxy-4-methoxy-6-(trifluoromethyl)benzoate
(400 MHz, CDCl3):
d
¼0.87 (t, 3JHeH¼8.0 Hz, 3H, CH3), 1.25e1.49 (m,
3
(3f). Starting with 1a (0.184 g, 1.0 mmol), AlCl3 (0.133 g, 1.0 mmol),
2f (0.717 g, 2.0 mmol) in CH2Cl2 (2.0 mL), 3f was isolated as a slight
yellow solid (0.177 g, 51%); mp¼37e39 ꢀC. 1H NMR (400 MHz,
14H, CH2), 2.66 (t, JHeH¼8.0 Hz, 3H, CH2), 3.88 (s, 3H, MeO), 3.93
(s, 3H, MeO), 6.85 (s, 1H, CH), 11.18 (s, 1H, OH). 13C NMR (75 MHz,
CDCl3):
d
¼12.6 (CH3), 21.2, 21.2, 21.6, 21.6, 26.0, 27.2, 27.8, 27.8,
CDCl3):
d
¼0.88 (t, 3JHeH¼7.0 Hz, 3H, CH3), 1.18e1.39 (m, 10H, CH2),
30.5, 30.8, 30.8, 31.6 (CH2), 50.4 (MeO), 55.3 (MeO), 101.4 (q,
1.72e1.76 (m, 2H, CH2), 3.82 (s, 3H, MeO), 4.31 (t, 3JHeH¼6.8 Hz, 2H,
OCH2), 6.58 (d, 4JHeH¼2.4 Hz, 1H, AreH), 6.86 (d, 4JHeH¼2.4 Hz, 1H,
3JCeF¼6.9 Hz, CH), 102.9 (C), 121.3 (C), 122.3 (q, JCeF¼271.0 Hz,
1
2
CF3), 127.6 (q, JCeF¼31.0 Hz, CeCF3), 159.4 (CeOH), 159.7
AreH), 11.61 (s, 1H, OH). 13C NMR (100 MHz, CDCl3):
d
¼13.8 (CH3),
(CeOMe), 168.9 (C]O). 19F NMR (282.4 MHz, CDCl3):
d
¼ꢁ58.3
(CF3). IR (ATR, cmꢁ1):
n
¼2955 (m), 2914 (s), 2850 (m), 1656 (m),
~
22.4 (CH2), 25.5 (CH2), 27.9 (CH2), 28.9 (CH2), 28.9 (CH2), 31.5 (CH2),
55.5 (MeO), 66.3 (OCH2), 103.1 (C), 103.4 (CH), 109.0 (q,
1583 (w), 1441 (s), 1404 (m), 1312 (s), 1250 (s), 1207 (s), 1127 (s),
1079 (m), 1003 (s), 932 (s), 864 (s), 811 (m), 716 (s), 692 (m), 572
(m), 487 (w), 425 (m), 406 (m). GCeMS (EI, 70 eV): m/z (%): 376
(Mþ, 32), 345 (17), 344 (13), 327 (16), 314 (14), 313 (79), 245 (18),
233 (13), 232 (100), 231 (87), 212 (15), 204 (21), 201 (10), 181 (20).
HRMS (EI, 70 eV): calcd for C19H27O4F3 (Mþ): 376.18560; found:
376.185024. Anal. Calcd for C19H27O4F3 (376.41): C, 60.63; H, 7.23.
Found: C, 60.87; H, 7.290.
3JCeF¼7.0 Hz, CH), 122.9 (q, JCeF¼271.0 Hz, CF3), 131.5 (q,
1
2JCeF¼32.0 Hz, CeCF3), 163.2 (CeOH), 165.1 (CeOMe), 169.3 (C]O).
19F NMR (282.4 MHz, CDCl3):
d
¼ꢁ58.3 (CF3). IR (ATR, cmꢁ1):
~
n
¼3087 (w), 2963 (w), 2925 (w), 2856 (w), 1656 (m), 1614 (m),
1448 (m), 1429 (m), 1388 (w), 1333 (m), 1312 (w), 1277 (m), 1235
(m), 1213 (m), 1144 (s), 1040 (s), 987 (s), 940 (s), 867 (m), 808 (s),
713 (s), 629 (m), 538 (m), 495 (m), 397 (m). GCeMS (EI, 70 eV): m/z
(%): 348 (Mþ, 13), 236 (25), 219 (27), 218 (100), 190 (10). HRMS (EI,
70 eV): calcd for C17H23O4F3 (Mþ): 348.15430; found: 348.154113.
Anal. Calcd for C17H23O4F3 (348.36): C, 58.61; H, 6.65. Found: C,
58.62; H, 6.669.
4.3.10. Methyl 2-hydroxy-4-methoxy-3-tetradecyl-6-(trifluoromethyl)
benzoate (3j). Starting with 1a (0.184 g, 1.0 mmol), AlCl3 (0.133 g,
1.0 mmol), 2j (0.914 g, 2.0 mmol) in CH2Cl2 (2.0 mL), 3j was isolated
as a slight yellow solid (0.209 g, 47%); mp¼67 ꢀC. 1H NMR (300 MHz,
4.3.7. Methyl 2-hydroxy-4-methoxy-3-methyl-6-(trifluoromethyl)
benzoate (3g). Data have been previously reported.4b Starting with
1a (0.184 g, 1.0 mmol), AlCl3 (0.133 g, 1.0 mmol), 2g (0.549 g,
2.0 mmol) in CH2Cl2 (2.0 mL), 3g was isolated as a slight yellow
CDCl3):
d
¼0.89 (t, 3JHeH¼7.6 Hz, 3H, CH3), 1.17e1.48 (m, 24H, CH2),
2.63 (t, 3JHeH¼7.6 Hz, 2H, CH2), 3.89 (s, 3H, MeO), 3.93 (s, 3H, MeO),
6.84 (s, 1H, CH), 11.18 (s, 1H, OH). 13C NMR (75 MHz, CDCl3):
d¼14.1
(CH3), 22.7, 23.0, 28.3, 29.4, 29.5, 29.5, 29.7, 29.7, 29.7, 29.7, 29.7,
solid (0.132 g, 50%); mp¼67 ꢀC. 1H NMR (300 MHz, CDCl3):
d
¼2.11
29.8, 31.9, 52.5 (MeO), 55.7 (MeO), 102.7 (q, 3JCeF¼7.5 Hz, CH), 104.2
1
(s, 3H, CH3), 3.89 (s, 3H, MeO), 3.93 (s, 3H, MeO), 6.84 (s, 1H, CH),
(C), 122.5 (CH), 123.5 (q, JCeF¼270.8 Hz, CF3), 129.0 (q,
11.25 (s, 1H, OH). 13C NMR (75 MHz, CDCl3):
d
¼8.3 (CH3), 52.6
2JCeF¼31.5 Hz, CeCF3), 160.7 (CeOH), 161.3 (CeOMe), 170.2 (C]O).
(MeO), 55.8 (MeO), 102.5 (q, 3JCeF¼6.75 Hz, CH), 104.0 (C), 117.7 (C),
123.5 (q, 1JCeF¼271.5 Hz, CF3), 128.9 (q, 2JCeF¼31.5 Hz, CeCF3), 160.7
(CeOH), 161.3 (CeOMe), 170.1 (C]O). 19F NMR (282.4 MHz, CDCl3):
~
19F NMR (282.4 MHz, CDCl3):
d
¼ꢁ58.2 (CF3). IR (ATR, cmꢁ1):
~
n
¼2953
(s), 2915 (s), 2848 (s), 1657 (s), 1612 (w), 1583 (w), 1464 (m), 1442
(m), 1405 (m), 1323 (m), 1251 (s), 1207 (s), 1127 (m), 1079 (m), 999
(m), 933 (m), 860 (m), 779 (w), 717 (s), 692 (m), 545 (m), 476 (w),
426 (w), 401 (w). GCeMS (EI, 70 eV): m/z (%): 446 (Mþ, 24), 415 (12),
414 (13), 397 (14), 384 (15), 383 (66), 245 (17), 233 (18), 232 (100),
231 (87), 212 (13), 204 (16), 181 (14). HRMS (EI, 70 eV): calcd for
C24H37O4F3 (Mþ): 446.26385; found: 446.263600.
d
¼ꢁ58.2 (CF3). IR (ATR, cmꢁ1):
¼2957 (w), 2929 (w), 2864 (w),
n
1656 (s), 1606 (s), 1586 (m), 1438 (m), 1401 (m), 1337 (m), 1315 (m),
1286 (s), 1247 (s), 1204 (s), 1122 (s), 998 (s), 927 (s), 841 (s), 805 (s),
712 (m), 677 (m), 545 (m), 418 (m), 396 (w). GCeMS (EI, 70 eV): m/
z (%): 264 (Mþ, 77), 233 (43), 232 (100), 231 (13), 214 (15), 213 (9),
212 (57), 204 (66), 203 (17), 202 (26), 185 (15), 182 (11). HRMS (EI,
70 eV): calcd for C11H21O4F3 (Mþ): 264.06039; found: 264.060573.
Anal. Calcd for C11H11O4F3 (264.20): C, 50.01; H, 4.20. Found: C,
50.41; H, 4.588.
4.4. X-ray crystal structure analysis of 5
The structure analysis was carried out by using a Bruker Kappa
APEX II duo diffractometer, using graphite-monochromated MoK
a
4.3.8. Methyl
2-hydroxy-4-methoxy-3-ethyl-6-(trifluoromethyl)
radiation. The structure was solved by direct methods and refined
by full-matrix least-squares procedures on F2 with the SHELXTL
software package.25 Crystal data: C15H12AlF9O9, M¼534.23, mono-
benzoate (3h). Starting with 1a (0.184 g, 1.0 mmol), AlCl3 (0.133 g,
1.0 mmol), 2h (0.577 g, 2.0 mmol) in CH2Cl2 (2.0 mL), 3h was
isolated as a yellowish solid (0.161 g, 58%); mp¼46 ꢀC. 1H NMR
clinic,
space
group
P21/n,
a¼7.0281(1),
b¼19.5037(3),
3
3
ꢀ
ꢀ
ꢀ
(300 MHz, CDCl3):
d
¼1.08 (t, JHeH¼7.8 Hz, 3H, CH3), 2.68 (q,
c¼15.0327(2) A,
b
¼98.170(1) , V¼2039.68(5) A , T¼150(2) K, Z¼4,
3JHeH¼7.2 Hz, 2H, CH2), 3.88 (s, 3H, MeO), 3.93 (s, 3H, MeO), 6.85
17,010 reflections measured, 4686 independent reflections
(s, 1H, CH), 11.18 (s, 1H, OH). 13C NMR (75 MHz, CDCl3):
d
¼12.8
(Rint¼0.0234), final R values (I>2 (I)): R1¼0.0343, wR2¼0.0822, final
s
3
(CH3), 16.4 (CH2), 52.5 (MeO), 55.7 (MeO), 102.7 (q, JCeF¼7.5 Hz,
R values (all data): R1¼0.0464, wR2¼0.0906, 310 refined parameters.
CCDC 896100 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
1
CH), 104.2 (C), 118.0 (C), 123.6 (q, JCeF¼271.5 Hz, CF3), 129.0 (q,
2JCeF¼31.5 Hz, CeCF3), 160.5 (CeOH), 161.1 (CeOMe), 170.2 (C]O).
19F NMR (282.4 MHz, CDCl3):
d
¼ꢁ58.2 (CF3). IR (ATR, cmꢁ1):
~
n
¼3026 (w), 2980 (w), 2957 (w), 2877 (w), 2857 (w), 1672 (s), 1599
(s), 1504 (s), 1438 (s), 1402 (m), 1316 (s), 1244 (s), 1204 (m), 1179 (s),
1120 (s), 1003 (w), 968 (m), 946 (m), 917 (m), 845 (s), 804 (m), 764
(m), 635 (m), 610 (m), 543 (m), 396 (s). GCeMS (EI, 70 eV): m/z (%):
278 (Mþ, 72), 247 (37), 246 (100), 231 (36), 226 (44), 218 (72), 208
(16), 203 (12), 200 (15), 181 (16), 175 (20). HRMS (EI, 70 eV): calcd
4.5. Coupled in situ ATR-FTIR/UVevis spectroscopy
The reaction was carried out in a homemade reaction cell5 under
inert atmosphere. The reaction cell was flushed with argon before

3346
S. Reimann et al. / Tetrahedron 69 (2013) 3338e3347
reaction and during dosing when the cell was open. The cooling has
been performed by Julabo cryostat. 4,4-Dimethoxy-1,1,1-
The adsorbed species were pressed to self-supporting wafers for
analysis by FTIR transmission spectroscopy. Samples in powder-
form were used for the analysis by UVevis-DRS spectroscopy. The
self-supporting wafers were made by pressing 50 mg material with
10 t to a disc with a diameter of 20 mm. The analysis was done with
a Bruker Tensor 27 spectrometer. The spectra were recorded with
32 scans at 2 cmꢁ1 resolution.
a
trifluorobut-3-en-2-one 1a (736 mg, 4.0 mmol) has been dis-
solved in CH2Cl2 (8.0 mL) at room temperature. The cooling started
and AlCl3 was added when a temperature of 0 ꢀC (533 mg,
4.0 mmol) was reached. The cooling was continued and 1,3-
bis(silyloxy)-1,3-butadienes 2 (2.080 g, 8.0 mmol) was added at
ꢁ60 ꢀC. Afterward the mixture was warmed up to 0 ꢀC during a 4 h
period with stirring. Finally the reaction was quenched by adding
hydrochloric acid (60.0 mL, 10%) to the reaction mixture. The in situ
ATR-FTIR spectra were measured with a fiber optical diamond ATR
probe (infrared fiber sensors, Aachen, Germany), which was con-
nected to a Nicolet Avatar 370 (Thermo Electron) FTIR spectrome-
ter. All spectra were recorded with 128 scans at 4 cmꢁ1 resolution.
The ex situ ATR-FTIR spectra were measured with a Bruker Alpha
spectrometer equipped with a Platinum ATR single reflection di-
amond ATR. The spectra were recorded with 32 scans at 4 cmꢁ1
resolution at room temperature.
Acknowledgements
S.R. would like to acknowledge gratefully the University of
Rostock (scholarship of the interdisciplinary faculty of the Univer-
sity of Rostock/Dept. LLM) and the State of Mecklenburg-Vorpom-
mern for financial support. Dr. L.R.K. would like to thank gratefully
the Leibniz Society for financial support.
Supplementary data
The UVevis spectra were sampled with an AvaSpec 2048 fiber
optical spectrometer (Avantes B.V., Eerbeek, Netherlands) equipped
with an AvaLight-DHS light source and an FCR-7xx200-2-45 re-
flection probe for the solid samples and an FDP-UV-micro-1
transmission probe for the liquid samples. All spectra of the solid
samples were recorded with 30 accumulations and an integration
time of 23 ms, and all spectra of liquid samples were recorded with
50 accumulations and an integration time of 10 ms.
Additional information concerning the molecular structure,
Raman- and ATR-FTIR spectra of the Al tris-chelate complex 5 as
well as calculation of the excitation spectra. 1H NMR and13C NMR
spectra of 3ae3j. Supplementary data associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.tet.2013.01.097.
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