An unprecedented recyclable catalyst system for asymmetric hydroboration

Article abstract of DOI:10.1039/b105247k

Immobilised preformed chiral homogeneous catalysts were subjected to catalytic hydroboration of styrene with catecholborane? and the activity, regio- and enantioselectivity observed were similar to those found when the corresponding homogeneous catalyst was used, remaining constant for several consecutive runs.

Full text of DOI:10.1039/b105247k

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An unprecedented recyclable catalyst system for asymmetric
hydroboration
a
b
a
a
Anna M. Segarra, Ramon Guerrero, Carmen Claver and Elena Fernandez*
a
Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, Pça, Imperial Tàrraco 1,
3005 Tarragona, Spain. E-mail: elenaf@quimica.urv.es; Fax: +34 977 559563; Tel: +34 977 558046
Servei de Recursos Científico Tècnics, Universitat Rovira i Virgili, Pça, Imperial Tàrraco 1, 43005
Tarragona, Spain
4
b
Received (in Cambridge, UK) 14th June 2001, Accepted 27th July 2001
First published as an Advance Article on the web 3rd September 2001
Immobilised preformed chiral homogeneous catalysts were
subjected to catalytic hydroboration of styrene with cate-
cholborane† and the activity, regio- and enantioselectivity
observed were similar to those found when the correspond-
ing homogeneous catalyst was used, remaining constant for
several consecutive runs.
We started by examining the catalytic properties of the model
rhodium complex [Rh(cod)(R)-(BINAP)]BF 1 adsorbed on
4
commercial montmorillonite K10, MK10, (BET surface area =
2
21
221 m g ). In the homogeneous hydroboration–oxidation of
vinylarenes, the catalyst precursor 1 provides high yields and
regioselectivities on 1-arylethanol derivatives but only moder-
2
ate enantiomeric excesses, (entry 1, Table 1). This catalytic
Recently there has been considerable interest in the hetero-
geneisation of chiral homogeneous catalytic systems. Many
reports have shown that it is possible to design and produce new
chiral catalytic systems which have obvious advantages over
their soluble counterparts: they can induce asymmetry, they can
be removed from the reaction mixture by simple filtration and,
behaviour justifies our choice of catalytic system as it reveals
any increase8 or decrease in the stereoselectivity induced by
the supported catalyst.
,9
2 2
Orange solutions of 1 in CH Cl decolorised when clay was
added and they were stirred for 24 h under nitrogen. The
rhodium complex was believed to be mainly adsorbed on the
external surface on the basis of the insignificant shift of the
(001) diffraction line on the powder X-ray diffractogram
between MK10 and 1–MK10. The amount of metal complex
adsorbed by the clay (31 mg of 1 in 0.5 g of MK10) was
determined by gravimetric analysis which measured the
difference between the weights of the complex before and after
immobilisation.
Once the solid-supported catalyst was prepared, it was tested
for (a) activity, (b) regioselectivity, (c) enantioselectivity, (d)
resistance to degradation and (e) reusability. As Table 1 shows,
the activity and selectivity of catalyst 1–MK10 were lower than
those of the homogeneous catalytic system under the same
reaction conditions (entry 2, Table 1). The presence of
significant amounts of interlamellar water in montmorillonite
could favour degradation of catecholborane and/or the transi-
tion metal complex. 11B NMR experiments carried out between
MK10 and catecholborane over the timescale of the catalytic
experiment, showed that catecholboronic acid had formed.
However when the montmorillonite was heated to at least
100 °C, MK10T, before the immobilization of 1, the resulting
supported catalytic system 1–MK10T (with 40 mg of 1 in 0.5 g
of MK10T) exhibited comparable activitiy and selectivity to the
homogeneous system. Removal of the supported catalyst by
filtration and repeated hydroboration recycling is demonstrated
with no loss of activity or selectivity, (entry 3, Table 1).
1
in many cases, they can be recycled and used again.
One of the significant advantages of the optically enriched
organoboron adduct (*C–B) that is derived from the catalytic
asymmetric reaction of vinylarenes and catecholborane as the
borane source, is that the required stereocentre can be installed.
2
–4
5
This can be followed by a consecutive *C–O, *C–N, and
6
*
C–C bond-forming reaction, with stereochemical integrity
(
Scheme 1).
Although soluble rhodium complexes are often used to
promote this reaction, the inherent problems involved in
regenerating the catalytic species have yet to be resolved. In a
recent attempt to address these problems, rhodium complexes
modified with achiral polycyclic phosphiranes were used to
perform homogeneous hydroboration–oxidation of styrene. The
catalytic activity did not decrease after five consecutive
catalytic runs, when styrene and catecholborane were re-added
to the reaction mixture.7 However, although the rhodium
complexes were stable under O
2
, the phosphiranes’ ligands
. To prevent the catalyst
proved to be sensitive to alkaline H
2 2
O
from being destroyed, it has to be separated from the oxidative
work-up media. These problems could be minimised to a large
extent if the catalyst is separated from the reaction media before
oxidation but, to the best of our knowledge this has not yet been
attempted. The non-covalent immobilisation of homogeneous
chiral cationic catalyst on solid supports could be an interesting
alternative, because it is efficient and easy to prepare.
Here we report, for the first time, that rhodium complexes
immobilised on solid supports are highly active, selective and
reusable catalysts towards the hydroboration reaction of
styrene.
Table 1 Asymmetric hydroboration–oxidation of styrene towards (R)-
(
+)-1-phenylethanol catalysed by the immobilised [Rh(cod)(R)-(BI-
a
4
NAP)]BF
Catalytic
system
Yield
(%)
Branched
(%)
b
Entry
Run
Ee (%)
1c
2
3
1
1
1
1
2
1
2
92
41
96
99
26
94
99
92
97
97
35
63
57
47
55
60
5
1–MK10
1–MK10T
+
4
1–Na MT
35
a
Standard conditions: styrene–catecholborane–Rh complex = 1+1.1+0.02.
Solvent: THF. T: 25 °C. Time: 2 h. b (R) Configuration determined by GC
c
with chiral column FS-Cyclodex B-IP, 50 m 3 0.25 mm. Ref. 2 and 1
mol% precursor of catalyst.
Scheme 1
1808
Chem. Commun., 2001, 1808–1809
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b105247k

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Remarkable difference was observed in the hydroboration of
styrene when the supported catalytic system was prepared from
Table 2 Asymmetric hydroboration–oxidation of styrene towards (S)-
(2)-1-phenylethanol catalysed by the immobilised [Rh(cod)(S)-(QUI-
a
4
+
NAP)]BF
the preheated clay bentonite Na MT and the complex 1 (entry
4
, Table 1). Following the same impregantion procedure, the
Catalytic
system
Yield
(%)
Branched
(%)
amount of metal complex immobilised in bentonite was only
2.35 mg in 0.5 g of the clay. These data, together with the
b
Entry
Run
Ee (%)
2
+
greater basal distance (from 11.8 Å in Na MT to 16 Å in
2c
1
2
1
1
2
3
4
99
98
98
92
87
95
97
97
97
98
88
88.5
89
86
88
+
1
–Na MT), are consistent with the fact that the rhodium
2–MK10T
complexes are principally immobilised in bentonite (BET
2
21
surface area = 53 m
g ), by ion exchange and not
8
adsorption. The environment of complex 1 immobilised on the
internal surface of the bentonite might be different, thus
providing unexpected low conversion and selectivity on the
a
Standard conditions: styrene–catecholborane–Rh complex = 1+1.1+0.02.
Solvent: THF. T: 25 °C. Time: 1 h. b (S) Configuration determined by GC
c
with chiral column FS-Cyclodex B-IP, 50 m 3 0.25 mm. Ref. 3 and 1
branched product, which are improved on the second run.
The 1 F NMR spectra of 1 shows two singlets at d
9
mol% precursor of catalyst.
F
2154.2
and d
with the isotopic effect between F and B and B re-
spectively, Fig. 1(a), while the 31P spectra show a doublet at d
6.2 ppm (JP–Rh = 145.4 Hz), Fig. 2(a). The intensity of these
F
2154.3 ppm with an intensity ratio of 1+4, according
19 10 11
P
We only examined four consecutive runs in the hydro-
boration–oxidation of styrene with 2, (Table 2, entry 2) and in
all of them the regio- and stereoselectivity were constant and
comparable to the homogeneous version (Table 2, entry 1).
Only the conversion decreases slightly on reuse. This was not
due to leaching of the rhodium complex because no product was
formed when styrene and catecholborane were added to the
filtrate of the first run. The lower conversion was mainly due to
the loss of solid when the catalytic system was manipulated
between the runs.
The results show that the heterogeneised process is an
efficient one, and further work on the characterisation of 2
tethered to montmorillonite and its applicability to the hydro-
boration–oxidation of other substrates such as electron-rich,
electron-poor styrenes and b-substituted vinylarenes, are in
progress.
2
19
31
signals decreases significantly on F and P NMR locked
spectra of a slurry in CDCl when MK10T is added little by
3
little to complex 1. This is because 1 decreases in solution
during the immobilisation process. In addition, new broad
signals emerge from the baseline. Their coupling constant is
identical but they are shifted to lower fields, Fig. 1 and 2.
The new signals may be broad because the montmorillonite
restricts the mobility of the organometallic complex and they
may shift to lower fields due to the different chemical
environment provided by the solid. The fact that both nuclei (F
and P) have been affected in the immobilisation process, lets us
suggest that the two ionic parts of the metal complex could
interact with montmorillonite through weak forces such as
electrostatic and/or hydrogen bonding interactions with the
surface hydroxy groups. In contrast, supported hydrogen
bonded catalysts have been previously reported to take place
principally in a monodentate way between the teminal silanols
of different types of silica and the oxygen atom of sulfonate
groups from phosphine ligands contained in the zwitterionic
Notes and references
† The IUPAC name for catecholborane is hydro[pyrocatecholato(22)-
O,OA]boran.
1
0,11
Rh(
I
) complexes
or from triflate counter-anions of cationic
12
)11,13 complexes.
Ru(II
)
and Rh(
I
Since the recovered catalytic system 1–MK10T could be
reused with no loss of activity and selectivity, we extended the
study to the cationic rhodium complex [Rh(cod)(S)-(QUI-
1
2
P. A. Jacobs, D. E. Devos and I. F. J. Vankelecom, in Chiral Catalyst
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NAP)]BF
1-[2-(diphenylphosphino)-1-naphthyl]isoquinoline}, which
provides a higher asymmetric induction than 1.
4
2, modified with the P,N auxiliary ligand QUINAP
3
{
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Fig. 2 31P NMR of (a) 20 mg of 1 in CDCl
, (b) slurry of (a) + 50 mg of
3
MK10T, (c) slurry of (b) + 50 mg of MK10T and (d) slurry of (c) + 25 mg
of MK10T.
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Chem. Commun., 2001, 1808–1809
1809