Hydroboration of vinyl arenes using SiO2-supported rhodium catalysts

Article abstract of DOI:10.1055/s-0028-1087542

The metal-catalyzed hydroboration of vinyl arenes using catecholborane (HBcat) and pinacolborane (HBpin) has been examined with SiO₂- supported rhodium catalysts. Reactions with simple vinyl arenes (ArCH=CH ₂) and HBcat using Rh(acac

Full text of DOI:10.1055/s-0028-1087542

LETTER
477
Hydroboration of Vinyl Arenes Using SiO2-Supported Rhodium Catalysts
S
iO -Supported Rh-Ca
i
talyzed
c
H
ydrobora
h
tions ael J. Geier,a Stephen J. Geier,a Christopher M. Vogels,a François Béland,b Stephen A. Westcott*a
2
a
b
Received 21 July 2008
this important reaction.2d,8 An elegant study by Fernández
and co-workers8a has shown that asymmetric hydrobora-
tions of vinyl arenes can be carried out effectively when
the metal is immobilized on clay bentonite. The use of
Abstract: The metal-catalyzed hydroboration of vinyl arenes using
catecholborane (HBcat) and pinacolborane (HBpin) has been exam-
ined with SiO2-supported rhodium catalysts. Reactions with simple
vinyl arenes (ArCH=CH2) and HBcat using Rh(acac)(coe)2
(coe = cyclooctene) gave selective formation of the corresponding supported rhodium catalysts has also received consider-
able attention in other areas of organic synthesis.9
branched isomers [ArCH(Bcat)Me]. Catalyst systems could be re-
used with no appreciable loss in activity or selectivity.
As part of our ongoing program designing new catalyst
Key words: catalysis, heterogeneous, hydroborations, rhodium,
systems for borylation reactions,10 we have examined the
silica support
catalyzed hydroboration of a variety of vinyl arenes using
rhodium complexes bound to sila-bond diphenylphos-
phine (Si-DPP, Figure 1).11 Reactions with the precursor
[RhCl(coe)2]2 (1, coe = cis-cyclooctene) were conducted
The hydroboration of alkenes and alkynes, which consti-
tutes the addition of a B–H bond across a carbon–carbon
as a heterogeneous analogue of commonly used
multiple bond, is an extremely important reaction in or-
RhCl(PPh3)3, where the phosphine ligands of Si-DPP
ganic synthesis.1 Although simple boron hydride reagents
would displace the labile cyclooctene ligands.5a
such as borane (H3B·X, where X is a Lewis base) and 9-
borabicyclo[3.3.1]nonane react readily with alkenes at
room temperature, hydroborations with catecholborane
PPh2
(HBcat, cat = 1,2-O2C6H4) generally require elevated
temperatures. The discovery that transition metals can be
Figure 1 Si-DPP
used to catalyze the addition of HBcat to alkenes and
We have found that hydroborations of 4-vinylanisole (4-
MeOC6H4CH=CH2) with two equivalents of HBcat in
THF using precursor 1 (1 mol%) and two equivalents of
Si-DPP gave a mixture of products (Table 1, entry 1). Al-
though the branched product 4-MeOC6H4CH(Bcat)Me
(Scheme 1, i)12 was the major boron-containing product, it
is somewhat surprising that the linear product 4-
MeOC6H4CH2CH2Bcat (ii) was not observed to any ap-
preciable extent. These results are somewhat surprising as
hydroborations of 4-MeOC6H4CH=CH2 using the homo-
geneous analogue, 1 + 2 Ph3P or 2 MePh2P, affords the
linear product ii with high selectivity (>95%). Minor
amounts of the alkenylboronate ester trans-4-
MeOC6H4CH=CHBcat (iii) and the diborated product 4-
MeOC6H4CH2CH(Bcat)2 (iv) were also observed in solu-
tion. The formation of the alkenylboronate ester product
presumably arises from a dehydrogenative borylation
reaction, and the diborated product iv is likely generated
by a subsequent and selective hydroboration of iii.5d,13
These competing reactions are believed to arise when the
alkene initially inserts into the Rh–B bond, followed by a
selective b-H elimination step to give the alkenylboronate
ester along with a molecule of dihydrogen (Scheme 2).
The hydrogenation product (4-MeOC6H4Et) is always
generated to some extent in these reactions (usually 5%).
Attempts to improve selectivities by increasing the
amount of Si-DPP to six equivalents (entry 2) were unsuc-
cessful.
alkynes has become an attractive and well-established
technique in organic synthesis. These reactions can have
regio-, chemo-, or stereoselectivities, complementary, or
more remarkably, opposite to those from products ob-
tained via the uncatalyzed variant. For example, hydrobo-
rations of styrenes (ArCH=CH2) with HBcat proceed to
give selectively either the expected linear product
(ArCH2CH2Bcat)
or
the
branched
product
[ArCH(Bcat)Me], depending upon the choice of catalyst
used to affect this transformation.2
The majority of metal-catalyzed hydroborations have em-
ployed rhodium complexes, such as RhCl(PPh3)3, and the
reaction has been postulated to proceed via loss of phos-
phine to give the transient species ‘RhCl(PPh3)2’ followed
by oxidative addition of HBcat to the metal center,3 with
subsequent insertion of the alkene into either the Rh–H2a,4
or the Rh–B5 bond. Reductive elimination of the B–C or
C–H bond, respectively, would then give the correspond-
ing alkylboronate ester. The unusual branched product is
believed to arise when the rhodium center can best stabi-
lize a benzylic intermediate during the catalytic cycle.6
Although a considerable amount of work in this area has
focused on catalyst development,7 much less studied is the
heterogenization or other phase-separation approaches to
SYNLETT 2009, No. 3, pp 0477–0481
x
x.
x
x.
2
0
0
9
Advanced online publication: 21.01.2009
DOI: 10.1055/s-0028-1087542; Art ID: S06708ST
© Georg Thieme Verlag Stuttgart · New York

478
M. J. Geier et al.
LETTER
Bcat
fluorine group did not seem to effect product distributions
(entry 3).
Bcat
Bcat
+
Table 2 Si-DPP Rhodium-Catalyzed Recycling of 4-Vinylanisole
R
R
HBcat
i
ii
Entry
Activity
>99
97
Selectivitya
>98
Rh (5 mol%)
R
Bcat
+
1
2
3
R = OMe
R = F
R = H
Bcat
>98
R
R
iii
iv
95
>96
a Selectivities were determined by 1H NMR spectroscopy and con-
firmed by GC-MS.
Scheme 1 The hydroboration of vinylarenes with HBcat using
Si-DPP rhodium catalysts
Table 1 Si-DPP Rhodium-Catalyzed Hydroboration of Vinyl
Arenes
We then decided to investigate reactions with
Rh(acac)(coe)2 (2, acac = acetylacetonato),14 as the
chelating diphosphine derivatives of this complex are well
known to be active and selective hydroboration cata-
lysts.5a Indeed, Rh(acac)(dippe) [dippe = 1,2-bis(diiso-
propylphosphino)ethane] was the first catalyst precursor
to be used successfully in the hydroboration of a tetrasub-
stituted alkene.15 In this study, we have found that 2 (1
mol%) with more than two equivalents of Si-DPP can
effectively catalyze the selective formation of 4-
MeOC6H4CH(Bcat)Me and 4-FC6H4CH(Bcat)Me for
hydroborations of 4-vinylanisole and 4-fluorostyrene,
(entries 4 and 5, respectively), using HBcat. This excel-
lent selectivity was also observed for other simple vinyl
arenes, including styrene and 2-vinylnaphthalene. Re-
moval of the immobilized catalyst was achieved by simple
filtration, affording essentially pure organoboronate ester.
This methodology therefore provides a clean and efficient
route to these valuable synthetic precursors. Repeated hy-
droboration recycling could be achieved (Table 2) with no
significant loss of activity or selectivity. When excess
Si-DPP was employed (up to 36 equiv per rhodium), how-
ever, significant amounts of the hydrogenation product
and B2cat3 were observed, arising from the degradation of
the HBcat.5a
Entry Catalyst, boraneR
Solvent Si-DPP/Rh i/ii/iii/iva
1
2
1, HBcat
1, HBcat
1, HBcat
2, HBcat
2, HBcat
2, HBcat
2, HBcat
2, HBpin
2, HBpin
2, HBpin
MeO
MeO
F
THF
THF
THF
THF
THF
THF
toluene
THF
THF
THF
2
6
2
2
2
1
2
2
2
2
80:0:15:5
80:0:20:0
85:0:10:5
100:0:0:0
100:0:0:0
80:20:0:0
100:0:0:0
93:6:1:0
3
4
MeO
F
5
6
MeO
MeO
MeO
F
7
8
9
80:13:5:2
70:20:5:5
10
H
a Product ratios were determined by 1H NMR spectroscopy and con-
firmed by GC-MS.
Predominant formation of the corresponding branched
product was also observed in reactions with 4-fluoro-
styrene, where the effect of the electron-withdrawing
PR3
PR3
Bcat
Bcat
R
Rh
Cl
+
Rh
coordination
H
R
Cl
H
PR3
PR3
insertion into the
Rh–B bond
R
PR3
Rh
PR3
R
H
H
H
+
Cl
Rh
Cl
β-H elimination
H
Bcat
PR3
PR3
Bcat
H
Scheme 2 Dehydrogenative borylation of vinyl arenes
Synlett 2009, No. 3, 477–481 © Thieme Stuttgart · New York

LETTER
SiO2-Supported Rh-Catalyzed Hydroborations
479
Bcat
Attempts to catalyze this reaction using only one equiva-
lent of Si-DPP gave the branched isomer i (80%) along
with minor amounts (20%) of the linear product ii (entry
6). Similar selectivities are observed in reactions using 2
in the absence of added phosphine. Conversely, hydro-
borations catalyzed using one equivalent of MePh2P or
Ph3P and 2 gave exclusive formation of i. It is therefore
reasonable to assume that greater than one equivalent of
Si-DPP is required to displace one of the coe ligands in 2
as not all the phosphines are surface accessible for Rh
binding. Reactions conducted in toluene (entry 7) gave
similar selectivities, however, reactions performed in
CDCl3 gave a complex mixture of products, arising from
a competing dehydrogenative borylation reaction, as well
as a significant amount of borane degradation.
Bcat
+
HBcat
v
vi
Rh (5 mol%)
Bcat
+
Bcat
Bcat
vii
viii
Scheme 3 The hydroboration of 2,4,6-trimethylstyrene with HBcat
using Rh/Si-DPP
In summary, these results suggest that reactions using Si-
DPP are remarkably different from their homogeneous
counterparts and further work in this area is needed to gain
an understanding of the selectivities observed in these
reactions. Reactions using simple vinyl arenes with HBcat
and catalytic amounts of Rh(acac)(coe)2 (2)/>2Si-DPP
gave essentially pure organoboronate ester upon removal
of the immobilized catalyst. As well, tethered diphos-
phines will be used as chelating ligands for rhodium in an
effort to improve selectivities.
Unfortunately, reactions with pinacolborane (HBpin,
pin = 1,2-O2C2Me4) lacked the regioselectivity observed
with HBcat. Reactions using two equivalents of Si-DPP
and 1 mol% of 2 with HBpin and 4-vinylanisole, 4-fluoro-
styrene, or styrene (entries 8–10) gave a mixture of boron-
containing products arising from competing hydrobora-
tion and dehydrogenative borylation pathways.16 Reac-
tions with HBpin have garnered significant attention
recently owing to the inherent air and chromatographic
stability of the resulting products.17 A recent study has
shown that novel homogenous zwitterionic rhodium com-
plexes also gave a mixture of products in hydroborations
of vinyl arenes using HBpin.17 Interestingly, Crudden and
co-workers have reported excellent selectivities in favor
of the branched product for similar hydroborations using
cationic rhodium systems.17l
Acknowledgment
Thanks are extended to the Natural Science and Engineering Re-
search Council of Canada, the Canada Research Chairs/Canadian
Foundation for Innovation/Atlantic Innovation Foundation pro-
grams, and Mount Allison University for financial support. We also
thank Dan Durant and Roger Smith (MtA) for expert technical assi-
stance and anonymous reviewers for helpful comments.
Although reactions of Si-DPP and 2 could be used to se-
lectively generate branched products in the hydroboration
of simple vinyl arenes using HBcat, complex product dis-
tributions were observed in reactions using more hindered
substrates. For instance, addition of HBcat to 2,4,6-tri-
methylstyrene (Scheme 3) using Rh/Si-DPP (1:2) gave
only 10% of the corresponding branched isomer (v) along
with 40% of the linear isomer (vi).18 Products arising from
a dehydrogenative borylation pathway are also observed
to a significant extent as 5% of the vinyl boronate ester
(vii) is observed along with 40% of the related bisborylat-
ed product (viii) and the hydrogenation product (5%).
Likewise, reactions of a-methyl styrene [PhC(Me)=CH2]
with HBcat gave only the linear product
PhC(Me)HCH2Bcat (35%) and the bisborylated product
PhC(Me)HCH(Bcat)2 (65%). These products presumably
once again arise from initial formation of the vinyl boro-
nate ester PhC(Me)=CH(Bcat) followed by either hydro-
genation to give PhC(Me)HCH2Bcat, or hydroboration to
give PhC(Me)HCH(Bcat)2. The formation of the vinyl
boronate ester PhC(Me)=CH(Bcat) was observed in reac-
tions using excess Si-DPP. Similar selectivities were ob-
served with reactions of HBpin and these bulkier styrenes.
Attempts to improve selectivities by altering the order and
rate of addition, as well as solvent, all proved unsuccess-
ful.
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4-MeOC6H4CH2CH2 (Bcat) (ii; R = OMe; Bcat): 1H NMR
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4-MeOC6H4CH=CH(Bcat) (iii; R = OMe; Bcat): 1H NMR
(270 MHz, C6D6): d = 7.82 [d, J = 18.3 Hz, CH=CH(Bcat)],
6.34 [d, J = 18.3 Hz, CH=CH(Bcat)].
4-MeOC6H4CH2CH(Bcat)2 (iv; R = OMe; Bcat): 1H NMR
(270 MHz, C6D6): d = 3.36 [d, J = 8.2 Hz, CH2CH(Bcat)2],
2.11 [t, J = 8.2 Hz, CH2CH(Bcat)2].
4-FC6H4CH(Bcat)Me (i; R = F; Bcat): 1H NMR (270 MHz,
C6D6): d = 2.60 [q, J = 7.7 Hz, CH(Bcat)CH3), 1.36 [d,
J = 7.7 Hz, CH(Bcat)CH3].
4-FC6H4CH=CH(Bcat) (iii; R = F; Bcat): 1H NMR (270
MHz, C6D6): d = 7.62 [d, J = 18.3 Hz, CH=CH(Bcat)], 6.23
[d, J = 18.3 Hz, CH=CH(Bcat)].
4-FC6H4CH2CH(Bcat)2 (iv; R = F; Bcat): 1H NMR (270
MHz, C6D6): d = 3.22 [d, J = 8.2 Hz, CH2CH(Bcat)2], 1.99
[t, J = 8.2 Hz, CH2CH(Bcat)2].
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(11) Si-DPP (R39030B) is available at SiliCycle
(www.silicycle.com).
(12) Experimental Procedure
(16) Selected NMR Spectroscopic Data
4-MeOC6H4CH(Bpin)Me (i; R = OMe; Bpin): 1H NMR (270
MHz, C6D6): d = 2.56 [q, J = 7.7 Hz, CH(Bpin)CH3], 1.50
[d, J = 7.7 Hz, CH(Bpin)CH3].
In a typical experiment, a THF (0.5 mL) solution of
Rh(acac)(coe)2 (8 mg, 0.019 mmol) was added to a THF (2
mL) slurry of Si-DPP (60 mg, 0.056 mmol), and the mixture
was stirred for 5 h. To this mixture was added a THF (0.5
mL) solution of 2-vinylnaphthalene (135 mg, 0.87 mmol)
followed by a THF (0.5 mL) solution of HBcat (126 mg,
1.05 mmol). The reaction was allowed to proceed for 18 h at
which point the mixture was filtered through a small plug of
Celite before solvent was removed under vacuum. The
residual oil was dissolved in C6D6 (1 mL) and analyzed by
multinuclear NMR spectroscopy.5a,10 Confirmation of
product formation was carried out using GC-MS on products
derived from a basic, oxidative workup.
4-MeOC6H4CH2CH2(Bpin) (ii; R = OMe; Bpin): 1H NMR
(270 MHz, C6D6): d = 2.87 [t, J = 7.9 Hz, CH2CH2 (Bpin)],
1.12 [t, J = 7.9 Hz, CH2CH2(Bpin)].
4-MeOC6H4CH=CH(Bpin) (iii; R = OMe; Bpin): 1H NMR
(270 MHz, C6D6): d = 7.80 [d, J = 18.3 Hz, CH=CH(Bpin)],
6.40 [d, J = 18.3 Hz, CH=CH(Bpin)].
4-FC6H4CH(Bpin)Me (i; R = F; Bpin): 1H NMR (270 MHz,
C6D6): d = 2.45 [q, J = 7.7 Hz, CH(Bpin)CH3], 1.38 [d,
J = 7.7 Hz, CH(Bpin)CH3].
4-FC6H4CH2CH2(Bpin) (ii; R = F; Bpin): 1H NMR (270
MHz, C6D6): d = 2.70 [t, J = 8.1 Hz, CH2CH2 (Bpin)], 1.12
[t, J = 8.1 Hz, CH2CH2(Bpin)].
Selected NMR Spectroscopic Data
4-FC6H4CH=CH(Bpin) (iii; R = F; Bpin): 1H NMR (270
MHz, C6D6): d = 7.60 [d, J = 18.5 Hz, CH=CH(Bpin)], 6.27
4-MeOC6H4CH(Bcat)Me (i; R = OMe; Bcat): 1H NMR (270
Synlett 2009, No. 3, 477–481 © Thieme Stuttgart · New York

LETTER
[d, J = 18.5 Hz, CH=CH(Bpin)].
SiO2-Supported Rh-Catalyzed Hydroborations
481
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4-FC6H4CH2CH(Bpin)2 (iv; R = F; Bpin): 1H NMR (270
MHz, C6D6): d = 3.11 [d, J = 8.2 Hz, CH2CH(Bpin)2], 1.50
[t, J = 8.2 Hz, CH2CH(Bpin)2].
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PhCH(Bpin)Me (i; R = H; Bpin): 1H NMR (270 MHz,
C6D6): d = 2.51 [q, J = 7.4 Hz, CH(Bpin)CH3], 1.42 [d,
J = 7.4 Hz, CH(Bpin)CH3].
PhCH2CH2(Bpin) (ii; R = H; Bpin): 1H NMR (270 MHz,
C6D6): d = 2.80 [t, J = 8.0 Hz, CH2CH2 (Bpin)], 1.03 [t,
J = 8.0 Hz, CH2CH2 (Bpin)].
PhCH=CH(Bpin) (iii; R = H; Bpin): 1H NMR (270 MHz,
C6D6): d =7.70 [d, J = 18.5 Hz, CH=CH(Bpin)], 6.40 [d,
J = 18.5 Hz, CH=CH(Bpin)].
PhCH2CH(Bpin)2 (iv; R = H; Bpin): 1H NMR (270 MHz,
C6D6): d = 3.20 [d, J = 8.0 Hz, CH2CH(Bpin)2], 1.30 [t,
J = 8.0 Hz, CH2CH(Bpin)2].
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(18) Selected NMR Spectroscopic Data
2,4,6-Me3C6H2CH(Bcat)Me (v): 1H NMR (270 MHz, C6D6):
d = 2.90 [q, J = 7.7 Hz, CH(Bcat)CH3], 1.42 [d, J = 7.7 Hz,
CH(Bcat)CH3].
2,4,6-Me3C6H2CH2CH2(Bcat) (vi): 1H NMR (270 MHz,
C6D6): d = 2.81 [t, J = 7.9 Hz, CH2CH2 (Bcat)], 1.31 [t,
J = 7.9 Hz, CH2CH2 (Bcat)].
2,4,6-Me3C6H2CH=CH(Bcat) (vii): 1H NMR (270 MHz,
C6D6): d = 7.90 [d, J = 18.3 Hz, CH=CH(Bcat)], 6.10 [d,
J = 18.3 Hz, CH=CH(Bcat)].
2,4,6-Me3C6H2CH2CH(Bcat)2 (viii): 1H NMR (270 MHz,
C6D6): d = 3.41 [d, J = 8.2 Hz, CH2CH(Bcat)2], 2.11 [t,
J = 8.2 Hz, CH2CH(Bcat)2].
Synlett 2009, No. 3, 477–481 © Thieme Stuttgart · New York

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