Allylation of aldehydes with potassium allyltrifluoroborate catalyzed by lanthanide-based metal-organic framework

Article abstract of DOI:10.1016/j.tetlet.2013.01.035

The use of the framework [Eu₂(fum)₃(H ₂O)₄]·(3H₂O), Eu-MOF, as catalyst for allylation of aldehydes by potassium allyltrifluoroborate is described. The method features the use of small catalyst loads and wet solvents, and the products were obtained in high yields, short reaction times, at room temperature with no further purification. The catalyst was recovered and reused up to six times in further allylation reactions without significant loss in the yields.

Full text of DOI:10.1016/j.tetlet.2013.01.035

Tetrahedron Letters 54 (2013) 1558–1561
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Allylation of aldehydes with potassium allyltrifluoroborate catalyzed
by lanthanide-based metal-organic framework
⇑
Juliano C. R. Freitas, Cristiane K. de Oliveira, Edilanio C. Cunha, Ivani Malvestiti , Severino Alves Jr.,
Ricardo L. Longo, Paulo H. Menezes
⇑
Departamento de Química Fundamental, Universidade Federal de Pernambuco, Recife, PE 50740-540, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
The use of the framework [Eu2(fum)3(H2O)4]Á(3H2O), Eu-MOF, as catalyst for allylation of aldehydes by
potassium allyltrifluoroborate is described. The method features the use of small catalyst loads and
wet solvents, and the products were obtained in high yields, short reaction times, at room temperature
with no further purification. The catalyst was recovered and reused up to six times in further allylation
reactions without significant loss in the yields.
Received 27 December 2012
Accepted 8 January 2013
Available online 16 January 2013
Keywords:
Potassium organotrifluoroborates
Allylation
Ó 2013 Elsevier Ltd. All rights reserved.
MOF catalysis
Metal-organic frameworks (MOFs) are crystalline materials
built with functionalized organic molecules that are linked by inor-
ganic units to form porous solids with a regular and sometimes
predictable structure.1
The reaction of allylic organometallic reagents with aldehydes
is synthetically analogous to the aldol addition of metal enolates,
because the resulting homoallyl alcohol can be easily converted
into aldol. Thus, the allylic organometallic reaction has attracted
attention and allylation has become one of the most useful meth-
ods for controlling the stereochemistry in acyclic systems.24
Notwithstanding the several available methods for the
allylation of carbonyl compounds in aqueous media based on, for
example, In,25 Sn,26 Zn,27 and Mg28 derived reagents, organoboron
compounds have proven to be very useful due to high yields and
excellent stereocontrol. In this context, methods for the allylation
of carbonyl compounds using allylic trifluoroborates promoted
by Lewis acids29 or palladium catalysts30 have been described.
The high specific surface area and porosity make the
metal-organic frameworks (MOFs) potential candidates for appli-
cations as both high-surface-area supports and intrinsic cata-
lysts.31 Syntheses and applications in catalysis of MOFs containing
transition-metal ions are well known, but few examples using lan-
thanide ions were described, particularly due to the predictability
of the coordination geometry of transition metal ions compared to
lanthanide ions.32
Because of their properties such as well-ordered microporosity,
distinct organic linkers, variety of inorganic moieties, high surface
area, and high absorption capacity, several applications in, for
instance, catalysis,2 gas storage,3 separations,4 and magnetism5
have been studied. In particular, the field of catalysis using
MOFs has experienced an impressive growth with a variety of
applications in organic reactions such as Knoevenagel6 and aldol
condensations,7 oxidation reactions,8 epoxide formation9 and
ring-opening,10 alkylation of amines,11 cyclopropanation reac-
tions,12 hydrogenation,13 Suzuki cross-coupling,14 transesterifica-
tion,15
cyclization reactions.18
Friedel–Crafts
reactions,16
cyanosilylation,17
and
The development of methods focusing on environmentally
benign reaction media has also been particularly prominent.19
Thus, despite the advances in the use of supercritical fluids,20 ionic
liquids,21 and fluorous media22 the use of water as a (co)-solvent
seems to be the best option due to its simplicity and very low cost.
In addition, catalytic methods that facilitate catalyst separation
and recycling are very important and relevant to the synthesis of
fine chemicals because they reduce the amount of waste products
and energy consumption.23
In this Letter, we report the use of a porous 3D open-framework
europium(III)-fumarate, [Eu2(fum)3(H2O)4]Á(3H2O), Eu-MOF, as a
catalyst to promote the addition of potassium allyltrifluoroborate
to aldehydes.33
In the course of developing milder reaction conditions, we first
examined the effect of the solvent to promote the reaction. Thus,
4-NO2-benzaldehyde, 1a (1 mmol) and potassium allyltrifluorobo-
rate, 2 (1.1 mmol) were treated at room temperature with catalyst
⇑
Corresponding author. Tel.: +55 81 2126 7473; fax: +55 81 2126 8442.
E-mail address: paulo.menezes@pq.cnpq.br (P.H. Menezes).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.01.035

J. C. R. Freitas et al. / Tetrahedron Letters 54 (2013) 1558–1561
1559
Table 2
(10 mol %) using different CH2Cl2/H2O ratios, and the progress of
the reaction was monitored by TLC. The results are presented in Ta-
ble 1, and all reported yields in this and other tables are isolated
yields.
Effect of the concentration of Eu-MOF on the allylation of 4-NO2-benzaldehyde 1a by
potassium allyltrifluoroborate 2 in 1:0.1 CH2Cl2/H2O at room temperature
Entry
MOF (mol %)
Time (min)
3a (%)
When only CH2Cl2 was used as the reaction solvent, a longer
reaction time was required to promote the allylation reaction,
probably due to the low solubility of potassium allyltrifluoroborate
(Table 1, entry 1). For a 1:1 mixture of CH2Cl2/H2O, a similar yield
was observed; however, the reaction time was significantly de-
creased (Table 1, entry 2). By decreasing the amount of water it
was observed that the reaction time also decreased with the yield
remaining at ca. 90% (Table 1, entries 3–6). When only water was
used as the reaction solvent 3a was obtained in a lower yield,
but at only 10 min (Table 1, entry 7).
Next we investigated the effect of the catalyst concentration on
the reaction yield. The catalytic load of Eu-MOF was varied from 1
to 25 mol % (Table 2). It was observed that the reaction yield did
not increase steadily with the amount of the catalyst. No signifi-
cant changes in yields and reaction times were observed by using
25 or 10 mol % of catalyst (Table 2, entries 1 and 2). However, with
further decreasing of the amount of catalyst the yield was slightly
decreased and longer reaction times were required (Table 2, en-
tries 3 and 4). In the absence of the catalyst, the product was ob-
tained only in 7% yield after 15 min (Table 2, entry 5).
1
2
3
4
5
25
10
5
1
—
12
15
25
30
15
94
93
90
87
7
After each run, the catalyst was separated from the reaction
mixture by centrifugation, washed with dichloromethane and re-
used. It was found that the Eu-MOF could be recovered and reused
in further allylation reactions with no significant loss in the yields.
However, the conversion of the aldehyde into the corresponding
homoallylic alcohol required longer reaction times after each run
(Table 4). Notice that no activation of the catalyst was performed
in each run.
Eu3+ luminescence spectroscopy was employed to investigate
the catalytic mechanism.34 Photoluminescence emission of the
pure Eu-MOF shows the typical transitions34 5D0 ? 7FJ (J = 0–4)
of Eu3+ observed in the 550–720 nm range as shown in Figure 1,
with 5D0 ? 7F0 being a very weak transition. Clearly, the emission
spectra of the Eu-MOF as prepared and after the first catalytic run
are very different. For instance, the lines were broadened and the
relative intensities of the 7F1 and 7F4 transitions increased signifi-
cantly in the spectrum of the solid (powder) after the first catalytic
run. For being magnetically allowed the 5D0 ? 7F1 transition can be
used as an internal standard of the Eu3+ coordination.34 Thus, the
differences in relative intensity ratios (see Table S1 in the
Supplementary data) suggest that the environment around the
Eu3+ changes when the catalyst is placed in the reaction medium.
A mixture of Eu-MOF and potassium allyltrifluoroborate has a very
similar luminescence spectrum as the reaction medium with the
catalyst, whereas a mixture of MOF and 4-NO2-benzaldehyde pre-
sents a spectrum practically identical to the pure catalyst. It might
thus be inferred that the allyltrifluoroborate ion or its (partially)
hydrolyzed species coordinates to the lanthanide center, possibly
by displacing water ligands, and significantly alters the chemical
environment.
With the optimized reaction conditions, namely, 1:0.1 CH2Cl2/
H2O and 10 mol % of Eu-MOF, we extended the scope of the
MOF-catalyzed allylation reaction to different aldehydes (Table
3). The method is robust to a wide range of functional groups with
aliphatic, aromatic,
a,b-unsaturated, and heterocyclic aldehydes
being efficiently allylated in very high yields.
The electronic nature of the substituents in the aromatic
aldehydes has little influence on the reaction. For example, the
allylation of 4-NO2- (Table 3, entry 1), 4-OCH3- (Table 3, entry 6),
4-F- (Table 3, entry 15), 4-Cl- (Table 3, entry 16), and 4-Br-benzal-
dehyde (Table 3, entry 17) gave the corresponding products in high
yields. Other aromatic aldehydes afforded good to high yields.
Given the disparate electronic effects (o-, m-, and p-substituents,
aromatic and aliphatic groups) associated with the aldehydes, the
small differences in the isolated yields are more likely to be due
to the solubilities of reactants and products. However, the differ-
ences in the reaction times might be related to the reactivity of
the aldehyde because electron-donating substituents usually re-
quire longer reaction times. Also, the reaction appears to be regio-
selective because only 1,2-addition was observed for
cinnamaldehyde (Table 3, entry 11). For aliphatic aldehydes, the
Eu-MOF-catalyzed allylation also exhibited high efficiency (Table
3, entries 12 and 13).
The hydrolysis of organotrifluoroborates35 is quite relevant to
their reactivity and Eu-MOF can in fact assist this hydrolysis, be-
cause allylboronic acid may have more affinity to lanthanide ions
by ligation through the hydroxyl groups.36 On the other hand, this
coordination could increase the reactivity of the allylborate spe-
cies, thus exerting the catalytic effect. Considering the small sizes
of the allyltrifluoroborate ion (molar volume of 133.7 Å3 molÀ1 37
)
or its (partially) hydrolyzed species (e.g., allyltrihydroxyborate mo-
lar volume of 166.2 Å3 molÀ1),37 they can fit within the cavities of
the Eu-MOF33 and coordinate to Eu3+ ion in the bulk of the solid
and not only on surface.
Table 1
Effect of CH2Cl2/H2O ratio on the allylation of 4-NO2-benzaldehyde 1a by potassium
allyltrifluoroborate 2 catalyzed by Eu-MOF (10 mol %)
These results suggest that Eu-MOF acts as a catalyst by activat-
ing the allyltrifluoroborate and/or its hydrolyzed products through
coordination, and the aldehyde is expected to interact with the
activated allylic species on the surface of the lanthanide-based
MOF. This behavior can account for the reaction times and yields
being practically independent of the size of the aldehyde (e.g., iso-
butyraldehyde (entry 12 in Table 3) and 2-naphthaldehyde (entry
BF3K
OH
2
O
Eu-MOF (10 mol%)
CH2Cl2:H2O, 25°C
O2N
O2N
3a
1a
Entry
CH2Cl2/H2O ratio (mL)
Time (min)
3a (%)
9 in Table 3) have molar volumes of 107.3 and 221.5 Å3 molÀ1
,
1
2
3
4
5
6
7
1:0
1:1
150
45
30
20
15
20
10
83
88
91
90
93
92
85
respectively).37 Alternatively, the Eu-MOF might catalyze the (par-
tial) hydrolysis of allyltrifluoroborate yielding allylboronic acid
derivatives, such as RB(F)OH and RB(OH)2 (R = CH2@CHCH2A),
which in solution are more reactive toward allylation and other
reactions.35 This also could explain the catalytic activity of the
Eu-MOF as well as the observed allylation properties. A detailed
1:0.5
1:0.25
1:0.1
1:0.01
0:1

1560
J. C. R. Freitas et al. / Tetrahedron Letters 54 (2013) 1558–1561
Table 3
Table 3 (continued)
Allylation of aldehydes 1 with potassium allyl trifluoroborate 2 catalyzed by Eu-MOF
(10 mol %)
RCHO
Product
OH
Time (min)
45
Yield (%)
85
BF3K
2
OH
O
R
O
F
R
MOF (10 mol%)
CH2Cl2:H2O (1:0.1), 25°C
14
15
16
17
1
3
1n
F
3n
OH
RCHO
Product
Time (min)
15
Yield (%)
93
O
OH
F
O
30
30
15
89
89
84
1o
1p
1q
F
O2N
1
2
3
3o
1a
1b
O2N
O2N
OH
OH
O
3a
3b
O2N
OH
Cl
Br
O
Cl
Br
15
10
84
83
3p
3q
O
OH
O
NO2
O
1c
NO2
3c
OH
4
5
10
15
90
86
1d
Table 4
3d
Catalyst Eu-MOF (10 mol %) recycling on the allylation of 4-NO2-benzaldehyde by
potassium allyltrifluoroborate
OH
O
BF3K
OH
1e
2
O
3e
MOF (10 mol%)
CH2Cl2:H2O (1:0.1)
25°C
O2N
OH
OH
O
O2N
3a
1a
MeO
MeO
6
7
8
15
25
20
93
91
91
1f
MeO
MeO
Run
Time (min)
3a (%)
3f
1
2
3
4
5
6
15
15
25
25
30
30
93
90
92
89
89
90
O
1g
3g
OH
O
OMe
1h
OMe
3h
mechanism of the Eu-MOF catalysis is beyond the scope of the
present contribution; however, controlled experiments and new
analyses are underway in our laboratories to clarify this
mechanism.
Additionally, we investigated the possibility of kinetic selectiv-
ity that could discriminate between two functional groups (alde-
hyde  ketone). Thus, a competitive experiment revealed that
such selectivity can be easily observed as demonstrated in Scheme
1.
In summary, we have shown that the framework [Eu2(fum)3(-
H2O)4]Á(3H2O), Eu-MOF, is an efficient catalyst for the allylation
of aldehydes by potassium allyltrifluoroborate. The method fea-
tures the use of small catalyst loads, and the products were ob-
tained in short reaction times with high yield and purity at room
temperature. The method is simple, fast, and efficient, uses water
as co-solvent, and is synthetically useful in that it could be applied
for the synthesis of more complex compounds. The lanthanide-
based Eu-MOF increases the reactivity of the allyltrifluoroborate
by coordination to the Eu3+ ion. The activated allylborate species
can react more efficiently toward aldehydes and/or undergo hydro-
lysis to yield more reactive allylboronic acid derivatives. These
more reactive species might also be released into the solution
and promptly perform the allylation of aldehydes.
OH
O
9
10
11
15
20
15
92
86
90
1i
3i
O
OH
O
O
1j
3j
OH
O
1k
3k
OH
O
12
13
15
15
71
91
1l
3l
OH
O
1m
3m

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1561
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Acknowledgments
We gratefully acknowledge CNPq (484778/2011-0), FACEPE
(PRONEX APQ-0859-1.06/08), CAPES, and inct-INAMI for financial
support. P. H. M., R. L. L., S. A. Jr., C. K. O., E. C. C., and J. C. R. F.
are thankful to CNPq for their fellowships.
Supplementary data
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Supplementary data (additional experimental procedures, ta-
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online version, at http://dx.doi.org/10.1016/j.tetlet.2013.01.035.
These data include MOL files and InChiKeys of the most important
compounds described in this article.
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References and notes
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