Iron-catalysed regio- And stereoselective head-to-tail dimerisation of styrenes

Article abstract of DOI:10.1002/adsc.201000096

Cationic iron salts and complexes catalyse the head-to-tail dimerisation of styrene derivatives to form the corresponding dimers with good to excellent yields and stereoselectivities. In particular, the use of a phosphine-iron(III) complex allows the recovery and reuse of the catalyst. The counteranion plays an important role in both the activity and selectivity. These catalysts constitute an alternative to those based on toxic and expensive late-transition metals.

Full text of DOI:10.1002/adsc.201000096

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DOI: 10.1002/adsc.201000096
Iron-Catalysed Regio- and Stereoselective Head-to-Tail
Dimerisation of Styrenes
a
a
a,
Jose R. Cabrero-Antonino, Antonio Leyva-Pꢀrez, and Avelino Corma *
a
Instituto de Tecnologꢁa Quꢁmica (UPV-CSIC), Universidad Politꢀcnica de Valencia, Consejo Superior de Investigaciones
Cientꢁficas, Avda. de los Naranjos s/n, 46022 Valencia, Spain
Fax : (+34)-9638-77809; phone: (+34)-9638-77800; e-mail: acorma@itq.upv.es
Received: February 5, 2010; Published online: June 30, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000096.
[
5]
Abstract: Cationic iron salts and complexes catalyse
the head-to-tail dimerisation of styrene derivatives
to form the corresponding dimers with good to ex-
cellent yields and stereoselectivities. In particular,
phosphines complexes/Zn metal and iron pyridine-
[6]
bisimine complex as catalytic systems, given access
to a-linear alkanes. Meanwhile, the tail-to-tail dimeri-
sation is less common, although a typical example of
this kind can be found in the Pd-catalysed cycloiso-
merisation of 1,6-dienes. Finally, the head-to-tail di-
merisation can be considered the more attractive
transformation since a new allylic carbon stereogenic
centre is formed. In fact, several natural products con-
the use of a phosphine-iron ACHUNTGRNENUG( III) complex allows the
[
7]
recovery and reuse of the catalyst. The counteran-
ion plays an important role in both the activity and
selectivity. These catalysts constitute an alternative
to those based on toxic and expensive late-transi-
tion metals.
[
8a]
taining this chemical structure can be found. For in-
stance, the chiral carbon of enantiomerically-enriched
non-steroidal anti-inflammatories such as Naproxen
TM
Keywords: dimerisation of styrenes; homogeneous
catalysis; iron; recoverable catalyst; triflates; trifli-
mide iron salts
has been accomplished by head-to-tail vinylation of
[
8b]
styrene derivatives.
Several catalytic systems have
been reported for the head-to-tail dimerisation of al-
kenes: the above-commented cobalt-phosphines/Zn
[
5]
metal mixture, a combination of palladium/indium
[
9]
triflate/phosphines
and
a
palladium complex/
[
2]
The dimerisation of alkenes is a 100% atom-economi- BF ·OEt , the three of them giving preferentially
3
2
[
1]
cal reaction that gives access to useful intermediates.
the E-isomer, cationic palladium complexes that lead
[
10]
Traditionally, this reaction has been performed in the to the Z-isomer and nickel-diphosphine complexes
[
11]
presence of Brønsted acids or late-transition metals as that give the E-isomer under basic conditions. Curi-
[2–4]
catalysts.
The regioselectivity of the reaction fol- ously, a Rh(I) catalyst activates instead the ortho-po-
[12]
lows three different pathways (Scheme 1).
sition of styrene.
Iron catalysis has attracted again the interest of
Both the regio- and stereoselectivity of the reaction
vary depending on the catalyst employed and the re- chemists mainly due to its beneficial environmental
action conditions. On the one hand, the head-to-head and economical aspects compared to late transition
[
13]
dimerisation of alkenes has been achieved with cobalt metals. As a consequence, new iron-catalyzed reac-
tions have appeared in the last years, opening new
avenues in the substitution of pollutant metals by iron
[
13]
salts and complexes.
Iron AHCTNUGTERNNNUG( III) is a harder Lewis
acid compared to late-transition metal cations, allow-
ing better activation of C=C double bonds. In fact,
several new iron-catalysed transformations of alkenes
[
14–16]
have been recently reported, including arylations,
[
17]
[18]
1,4 additions to dienes
and hydroaminations.
FeCl3 is the usual catalyst for these transforma-
[
14,15,18]
Scheme 1. Possible reaction pathways for the dimerisation of tions.
alkenes.
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Jose R. Cabrero-Antonino et al.
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As far as we know, the iron-catalysed head-to-tail
[
19]
dimerisation of alkenes has not been reported yet.
Considering the proposed mechanisms for this reac-
[20]
tion, it was envisaged that simple Fe ACHUTNGRNENUG( III) salts could
play the catalytic role of the toxic and expensive
metal complexes used previously. In the one hand, In-
1
0
5
ACHTUNGTRENNUNG( III) [4d , compare with Fe ACHTUNGERTNNUNG( III) 3d ] has been used to
[
20]
form stabilised benzylic carbocations of styrene de-
rivatives and, on the other hand, activation of the soft
C=C double bond nucleophile is also required.
Given that Fe(III) activates similarly styrene deriva-
tives and can add soft carbon nucleophiles,
[
2,5,11]
ACHTUNGTRENNUNG
[
14,21]
a cata-
lytic activity could be expected. Indeed, we will see
that the head-to-tail dimerisation of styrenes proceeds
regio- and stereoselectively under iron catalysis, lead- Figure 1. Comparison of the catalytic activity of different
ing to E-styryl derivatives. It can be anticipated that metal salts and complexes for the dimerisation of 1 to 2: A)
conversion of 1 (%) and B) yield of 2 (%).
FeCl does not present enough Lewis acidity to cata-
3
lyse this reaction, which explains why the dimerisation
of styrene derivatives was not detected in the previ-
ously reported FeCl -catalysed processes. Higher lysts were more active that the corresponding Fe(II)
3
acidic species such as Fe ACHTUNGETRNN(UNG OTf) or Fe ACHTUNGTRNENUG( NTf ) (n=2,3) counterparts (compare entries 18 and 21 and en-
n 2 n
[
22,23]
are needed.
To our surprise, Fe ACHNUTGTNERNUG( NTf ) and tries 15 and 20). The in-situ generated triflimide cata-
2 2
(NTf ) salts have rarely been employed as cata- lysts are as active as the isolated compounds (com-
[
23]
Fe
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
2
3
[
21,24]
lysts,
although their high Lewis acidities makes pare entries 18 and 19). The activity of the iron cata-
[
25]
them suitable catalytic entities.
lyst does not come from copper impurities,
since
The dimerisation of styrene 1 was chosen as a test the copper catalysts are less active that the corre-
[
9]
reaction (Scheme 2). In accordance with Bedford,
,4-dioxane was the best solvent found. A screening
sponding iron salts (compare entries 21 and 23).
From these results it can be claimed that Fe ACHTUNGTRENNUNG( III)
1
of possible catalysts including metal salts (Supporting salts are effective catalysts for the head-to-tail dimeri-
Information, Table S1) and metal complexes (Sup- sation of styrene. In general, the use of NTf as
porting Information, Table S2) was carried out. A counteranion instead of OTf increases the reaction
À
2
À
summary of the more relevant results obtained is de- rate but decreased slightly the selectivity for 2. This
picted in Figure 1.
Concerning the metal salts, the corresponding ments (Supporting Information, Figure S1 and
chlorides of Cu(II), Co(II), Pd(II), Fe(II) and Fe(III) Table S3). It could be observed that, indeed,
do not work as catalysts (Supporting Information, Fe(NTf ) shows the better catalytic activity and, as
counteranion effect was studied by kinetics experi-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
2
3
À
À
Table S1, entries 2–6). HCl (conc.) does not work the NTf groups are replaced by the OTf groups,
2
either (entry 7). However triflic and triflimidic acids the activity decreases and the selectivity increases.
worked well in catalytic amounts (entries 8–12). The Nevertheless, Fe
ACHTUNGTNRENUNG( NTf ) gives an optimum conversion
2 3
corresponding in situ generated triflate salts of Cu(II) with a good yield of 2. Moreover, triflimide salts are
(
entry 13) and Fe(II) (entry 14) and the Co(II) trifli- less prone to hydrolysis than triflates and can be re-
[26]
mide salt (entry 15) did not work, but the triflimidic covered by precipitation in hexane.
In fact, the
salt of Fe(II) (entries 18 and 19) and the triflate and Fe
triflimidic salts of Ag(I) (entries 16 and 17) and with hexane (20:1 respect to dioxane) although the
Fe(III) (entries 20–22) gave good to excellent conver- activity decreased from 85% conversion to 21% con-
sions and selectivities for 2. Compared with corrosive version in a second use.
and unstable organic acids, the in-situ generated iron- With this result in hand and taking in account our
(III) catalytic system is more convenient and easy-to- previous work on the use of highly cationic phosphine
handle, while effective. As expected, the Fe(III) cata- complexes as reusable catalyst in alkyne transforma-
ACHTUNGTNERNU(GN NTf ) catalyst could be recovered by precipitation
2 3
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Scheme 2. Metal-catalysed dimerisation of styrene 1.
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Adv. Synth. Catal. 2010, 352, 1571 – 1576

Iron-Catalysed Regio- and Stereoselective Head-to-Tail Dimerisation of Styrenes
[26]
tions, we envisaged phosphine triflimide iron com- catalytic activity, observing that the higher the
plexes as possible reusable active catalysts. For that number of phosphines, the lesser the activity is (com-
reason a second screening, where different triflate pare entries 4 and 8 and entry 21 in Table 1). Remark-
and triflimide phosphine iron (and cobalt) complexes ably, the triflate complex (Ph P) Fe CAHTUNGTERNNNU(G OTf) showed an
3
2
3
were used as catalysts for the head-to-tail dimerisa- excellent conversion (92%) and selectivity for 2
tion of styrene, was carried out (see Supporting Infor- (>98%, entry 5). Thus, we examined again the coun-
mation, Table S2). As previously, the triflate com- teranion effect for the Fe ACHUTNGRNENUG( III) bis(triphenylphosphine)
plexes were less active as catalyst that the correspond- complex (Supporting Information, Figure S2 and
ing triflimide complexes, regardless the metal em- Table S4). The counteranion exchange effect is even
ployed [compare entries 1 and 2 for Co and entries 3 more pronounced in this case than for the simple salt
and 4 for Fe
using Fe(III) and even Fe(II) complexes in catalytic Information), the activity decreases and the selectivity
amounts (entries 5–8). The number of PPh units in of 2 increases as NTf ions are replaced by OTf in
ACHTUNGTRENNUNG( III)]. Good yields of 2 were achieved by (compare with Figure S1 and Table S3, Supporting
ACHTUNGTRENNUNG
À
À
3
2
the coordination sphere has a moderate effect in the the complex. An abrupt loss of activity is observed
Table 1. Head-to-tail dimerisation of styrene derivatives catalysed by cationic iron ACHUTNGRNEGUN( III) salts.
[a]
Entry
1
Substrate
Catalyst (mol%)
Main Product
Yield [%]
[95]
FeCl +3AgOTf (5)
3
2
3
FeCl +3AgNTf (2)
94, [60]
3
2
FeCl +3AgNTf (5)
97
3
2
4
5
FeCl +3AgNTf (2)
51
61
3
2
FeCl +3AgNTf (5)
3
2
[
b]
6
7
FeCl +3AgNTf (10)
[75]
3
2
[c]
FeCl (2)
polymerisation
–
3
Adv. Synth. Catal. 2010, 352, 1571 – 1576
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Jose R. Cabrero-Antonino et al.
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Table 1. (Continued)
[a]
Entry
Substrate
Catalyst (mol%)
Main Product
polymerisation
Yield [%]
[c]
8
FeCl (2)
–
3
9
FeCl +3AgNTf (2)
54
3
2
[d]
1
0
1
FeCl +3AgNTf (10)
mixture of isomers
N.D.
>95
3
2
1
FeCl +3AgNTf (2)
–
3
2
1
1
2
3
FeCl +3AgNTf (2)
N.D.
N.D.
–
–
3
2
FeCl +3AgNTf (2)
3
2
[
[
[
[
a]
1
GC or H NMR yield. In square brackets, isolated yield.
7
b]
days reaction time.
Room temperature.
c]
d]
At least six different dimers were found (GC-MS m/z=236, intensity pattern 1:3:2:3:1:3).
for the full triflate complex. Nevertheless, (PPh ) Fe- yield after a prolonged reaction time (entry 6). In
3
2
ACHTUNGTRENNUNG( NTf ) could be used as a recoverable and reusable contrast, electron-rich substrates 13 and 14 did not
2 3
catalyst for the head-to-tail dimerisation of 1 to 2 and stop at the dimer but polymerised even when using
only a moderate decrease throughout the reuses was FeCl as catalyst at room temperature (entries 7 and
3
observed while a good selectivity was maintained 8). Substitution across the double bond also modifies
(
Figure 2).
the catalytic activity. a-Methylstyrene 15 reacted well
In view of the catalytic results obtained (see (entry 9). However, b-methylstyrene 17 needed 5
Figure 1 above), we selected the catalytic system times more catalyst to achieve complete conversion
FeCl +3AgNTf to explore the scope of the reaction (entry 10). When both a- and b-positions were substi-
3
2
(
Table 1).
tuted (entry 11) no reaction was observed. Stilbene
Styrene derivatives 1, 3 and 5 gave excellent yields did not react either (entry 12). Alkylalkenes were not
after isolation (entries 1–3). Substrates having a reactive (entry 13).
methyl substituent were reactive as well, although, A possible mechanism for the Fe ACHUTNGTNRENUNG( III)-catalyzed
partial polimerisation was observed (entries 4 and 5). head-to-tail dimerisation of styrenes is depicted in
Gratifyingly, the electron-poor styrene derivative 11 Scheme 3. This mechanistic proposal is based on the
could form the corresponding dimer in good isolated well-known capability of iron ACHUTNGRENNUG( III) to activate the ben-
[
14]
zylic position of double bonds and the also reported
[
21]
iron
A
H
U
G
E
N
N
Thus, the
cationic FeACHTUNGTRENNNUG
benzylic cation and would also activate the double
bond. After formation of the new CÀC bond, the cat-
alyst would be regenerated for a new cycle. The
marked counteranion effect observed on the reaction
À
rate and selectivity suggest that X plays an active
role, probably stabilising the different cationic species
during the catalytic cycle.
Different experiments were carried out to check
this reaction pathway (see Supporting Information).
Figure 2. Reusability study for the head-to-tail dimerisation For instance, ethyl triflate was added to the reaction
of 1 catalysed by (PPh ) Fe
A
H
U
G
R
N
U
G
mixture in order to trap any hypothetic nucleophilic
3
2
2 3
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Iron-Catalysed Regio- and Stereoselective Head-to-Tail Dimerisation of Styrenes
septum was fitted. Then, 1,4-dioxane (1,5 mL) was added.
The mixture was magnetically stirred at room temperature
for 30 min. Then, styrene (230 mL, 2.0 mmol) was added and
the mixture was magnetically stirred at 808C for 18 h. After
cooling, n-hexane (30 mL) was added, observing the precipi-
tation of the catalyst. The liquid was passed through a filter
containing a Celiteꢃ layer on top. The solid was washed
with n-hexane and filtered again. The filtrates were concen-
trated under reduced pressure and, after drying under
vacuum, the resulting residue 1,3-diphenyl-1-butene was ob-
tained as an clear oil; yield: 189 mg (95%). Further purifica-
tion was performed by flash column chromatography on
silica using neat n-hexane as eluent. The spectroscopic data
2
match those previously reported.
Typical Reaction Procedure for Reusing
(
PPh ) Fe ACHTUNGTRENNG(U NTf ) (Figure 2)
3 2 2 3
AHCTUNGTRNNEUG( PPh ) FeCl (68.6 mg, 10 mol%) and AgNTf2 (116.5 mg,
3 2 3
3
0 mol%) were placed in a 10-mL round-bottomed flask. A
Scheme 3. Plausible mechanism of the iron
head-to-tail dimerisation of styrene 1.
ACHTUNGTRENNU(GN III)-catalysed
rubber septum was fitted. Then, 1,4-dioxane (0.75 mL) was
added. The mixture was magnetically stirred at room tem-
perature for 30 min. Then, styrene (115 mL, 1.0 mmol) was
added and the mixture was magnetically stirred at 808C for
FeÀC species during the cycle [Supporting Informa-
tion, Eq. (S1)]. Unfortunately, no ethyl groups were
incorporated in the products but instead oxygen was
added on the benzylic positions, obtaining the corre-
sponding ketones, which confirms the presence of
benzylic carbocations. In a second experiment, the
styrene derivative 11 was added together with 1 [Sup-
porting Information, Eqs. (S2) and (S3)] and, curious-
ly, the dimerisation reaction was inhibited. This sug-
gests that the stabilisation of the benzylic carbocation
is the key step during the cycle, since the electron-
withdrawing nature of the ring in 11 retards and even
inhibits the reaction. Furthermore, this also explains
the high reactivity observed for electron-donor aryl
rings such as substrates 13 and 14, which stabilise
better the benzylic carbocation.
1
8 h. After cooling, n-hexane (30 mL) was added, observing
the precipitation of the catalyst. The liquid was decanted,
the remaining catalyst was washed with hexane and the
liquid was decanted again. The combined organic phases
were analysed by GC. The catalyst was dried and used in a
next run.
Supporting Information
Supporting information for this article including synthesis of
catalysts, reaction procedures, compound characterization,
additional reaction schemes and NMR spectra, is available.
Acknowledgements
In summary, the Fe-catalysed head-to-tail dimerisa-
tion of styrene derivatives has been accomplished.
Fe ACHTUNGTRENNUNG( III) is a better catalyst compared to late-transition
metals. The corresponding dimers of different substi-
tuted substrates have been obtained in good to excel-
lent yields by using simple cationic salts. The use of a
J. R. C.-A. thanks MCIINN for the concession of a doctoral
grant. A. L.-P. thanks CSIC for a contract under JAE pro-
gram. MAT 2006 project and PROMETEO from Generalitat
Valenciana are also acknowledged. The authors thank J.
Gaona for performing preliminary tests.
phosphine iron
ACHTUNGTRENNUNG( III) complex allows recovering and re-
using the catalyst. Since a new stereogenic center is
formed in the dimerisation, the use of a chiral phos- References
phine might allow enantioselective induction on the
product. Studies in this sense are currently underway.
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3
2876–2886.
3
1576
asc.wiley-vch.de
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2010, 352, 1571 – 1576