Catalytic hydroboration of vinylarenes using a zwitterionic arylspiroboronate ester iridium complex

Article abstract of DOI:10.1016/j.inoche.2010.07.044

Addition of B₂cat₃ (cat = 1,2-O₂C ₆H₄) to Ir(acac)(dppb) (1; where dppb = 1,4-bis(diphenylphosphino)butane) gave the novel arylspiroboronate ester iridium complex Ir(η⁶-catBcat)(dppb) (2),

Full text of DOI:10.1016/j.inoche.2010.07.044

Inorganic Chemistry Communications 13 (2010) 1396–1398
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Catalytic hydroboration of vinylarenes using a zwitterionic arylspiroboronate ester
iridium complex
Jennifer A. Melanson a, Christopher M. Vogels a, Andreas Decken b, Stephen A. Westcott a,
⁎
a
Department of Chemistry and Biochemistry, Mount Allison University, Sackville, New Brunswick, E4L 1G8, Canada
b
Department of Chemistry, University of New Brunswick, Fredericton, New Brunswick, E3B 5A3, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Addition of B2cat3 (cat=1,2-O2C6H4) to Ir(acac)(dppb) (1; where dppb=1,4-bis(diphenylphosphino)
butane) gave the novel arylspiroboronate ester iridium complex Ir(η6-catBcat)(dppb) (2), the first example
of an iridium compound containing a coordinating [Bcat2]− anion. Complex 2 is a remarkably selective
catalyst precursor for the hydroboration of unhindered vinylarenes using pinacolborane.
© 2010 Elsevier B.V. All rights reserved.
Received 16 April 2010
Accepted 31 July 2010
Available online 7 August 2010
Keywords:
Arylspiroboronate ester
Catalysis
Hydroboration
Regioselectivity
Zwitterion
The addition of boranes to unsaturated organic molecules has become
a remarkably important synthetic methodology in organic synthesis [1].
Although H3B.X (X=THF, SMe2) adds readily to alkenes at –80 °C [2],
other hydroboration reagents, such as diorganyloxyboranes, are slow to
react even at room temperature. For instance, catecholborane (HBcat,
cat=1,2-O2C6H4) and pinacolborane (HBpin, pin=1,2-O2C2Me4) add to
alkenes and alkynes only at elevated temperatures [3] or in the presence of
a transition metal catalyst [4].
A considerable amount of research has focused on investigating the
mechanism and scope of the catalyzed hydroboration reaction [5].
Although a number of transition metals have been found to catalyze
hydroborations with HBcat and HBpin, reactions using rhodium
complexes are usually the most effective for reactions with alkenes.
For instance, phosphinorhodium acetylacetonato complexes are re-
markably active catalyst precursors for the hydroboration of a wide
range of vinylarenes (ArCH=CH2) using HBcat, giving selective
formation of the unusual branched products, ArCH(Bcat)CH3 [6]. The
catalyst resting states in thesesystemsare believed to be the zwitterionic
complexes of the type Rh(η6-catBcat)(P2) (P2=diphosphine), arising
from the redistribution of borane substituents [7]. The unusual
arylspiroboronate ester [Bcat2]− is bound to the rhodium fragment via
all six carbons of one of the catecholato rings.
sphino)butane) [17] gave the novel arylspiroboronate ester iridium
complex Ir(η6-catBcat)(dppb) (2) in 68% yield (Scheme 1) [18]. While
rhodium compounds containing arylspiroboronate esters are well
documented [19], and the ruthenium complex RuH(η6-catBcat)(PCy3)2
has been prepared by the addition of HBcat to RuH2(η2-H2)2(PCy3)2 [20],
complex 2 represents, to the best of our knowledge, the first example of an
iridium compound containing a coordinating [Bcat2]− anion. Complex 2
has been characterized by a number of physical methods, including
multinuclear NMR spectroscopy. The 11B{1H} NMR spectrum shows a
sharp singlet at δ 13.6 ppm for the four coordinate boron anion [21] and
the 31P{1H} NMR peak shifts from δ 10.8 ppm in 1 to 4.0 ppm for
complex 2. More diagnostic, however, are the 1H NMR data, as the
resonances for the hydrogens on the Bcat group bound directly to
iridium are observed at δ 5.20 and 4.68 ppm, while the uncoordinated
catecholato peaks remain downfield at δ 6.56–6.38 ppm. In comparison,
thearyl peaks in [NBu4][Bcat2] are observed atδ 6.57 ppm in the 1H NMR
data [22]. Significant upfield shifts are also observed in 2 by 13C{1H}
NMR spectroscopy, where carbon atoms of the Bcat group coordinated
to the iridium fragment are located at δ 140.3, 84.4, and 79.7 ppm. For
comparison, the related carbon resonances in [NBu4][Bcat2] are
observed at δ 151.8, 117.9, and 108.5 ppm.
Complex 2 has also been characterized by a single crystal x-ray
diffraction study (Fig. 1), confirming that the arylspiroboronate ester
is bound to the iridium fragment in a η6 fashion [23–26]. Within the
bound Bcat group, four of the iridium carbon distances Ir(1)–C(33)
2.270(3), Ir(1)–C(34) 2.297(3), Ir(1)–C(32) 2.302(3), and Ir(1)–C(31)
2.312(3)Å, are noticeably shorter than the other two at Ir(1)–C(30)
2.446(3) and Ir(1)–C(29) 2.456(3)Å. This slippage away from a true η6
bonding mode has been observed in the rhodium analogues where the
potential surface for such distortions is reputably quite shallow, and is
Recently, there has also been considerable interest in the use of iridium
complexes in an effort to expand the scope of these catalyzed
hydroborations [8–15]. With this is mind, we have found that addition
of B2cat3 [16] to Ir(acac)(dppb) (1; where dppb=1,4-bis(diphenylpho-
⁎
Corresponding author. Tel.: +1 506 364 2372; fax: +1 506 364 2313.
E-mail address: swestcott@mta.ca (S.A. Westcott).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.07.044

J.A. Melanson et al. / Inorganic Chemistry Communications 13 (2010) 1396–1398
1397
Scheme 2. Catalyzed hydroboration of vinylarenes using complex 2 and HBpin.
Scheme 1. Synthesis of arylspiroboronate ester iridium complex 2 by addition of B2cat3
to Ir(acac)(dppb) (1).
these hydroborations, the results of which will be reported in due
course.
presumably a ground-state phenomenon. Bond distances and angles
within the arylspiroboronate ester are similar to those reported
previously [19,27].
Acknowledgements
We then decided to examine complex 2 for its potential to catalyze
the addition of HBcat and HBpin to vinylarenes [28,29]. To our
surprise, and contrary to the results using the highly active and selective
rhodium analogues, no significant reaction was observed with HBcat
and 4-vinylanisole using 5 mol% of 2 at room temperature after 16 h.
Interestingly, however, reactionsof HBpin and 4-vinylanisole proceeded
smoothly at room temperature (T=18 h) to give selective formation of
the linear hydroboration product (Scheme 2) [30]. This observation is
unusual as HBcat is normally the more active of the diorganyloxy-
boranes in catalyzed hydroborations. Similar regioselectivities in
favour of the linear hydroboration product were observed in reactions
with 4-fluorostyrene and 4,4,5,5-tetramethyl-2-(4-vinylphenyl)-1,3,2-
dioxaborolane with HBpin. Unfortunately, attempts to add this borane
to the more hindered substrates α- and β-methylstyrene proved
unsuccessful. It is clear that hydroborations with this iridium complex
are proceeding via a different mechanism, as compared to the rhodium
analogues. In a recent report, Crudden et al. [31] suggested that cleavage
of the B–H bond in HBpin is facilitated by an additional Lewis acid. While
it is possible that a similar alternative pathway may be occurring in
reactions with 2, further work is needed to elucidate the mechanism in
We thank NSERC, ACS-PRF (Grant Number 50093-UR3), the
Canada Research Chair Programmes, and Mount Allison University
for financial support. Shrubby Durant and Roger Smith are thanked for
their valuable technical skills.
Appendix A. Supplementary material
CCDC 767792 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.inoche.2010.07.044.
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H, 4.30. Found: C, 56.45; H, 4.37. 1H NMR (270 MHz, THF-d8)(ppm):δ7.63 (m, 8H, Ar),
7.44–7.34 (ov m, 12H, Ar), 6.56–6.38 (ov m, 4H, C6H4O2), 5.20 (m, 2H, η6-C6H4O2), 4.68
(m, 2H, η6-C6H4O2), 2.61 (br m, 4H, PCH2), 1.61 (br m, 4H, PCH2CH2); 11B NMR
(87 MHz, THF-d8) (ppm): δ 13.6 (s); 13C{1H} NMR (67 MHz, THF-d8) (ppm): δ 152.2,
151.9, 140.3, 139.2 (m, CP), 132.9 (t, JCP=5.6 Hz, C6H5), 129.6, 127.9 (t, JCP =5.1 Hz,
C6H5), 117.5, 116.5, 108.0, 107.8, 84.4, 79.7, 31.3 (m, PCH2), 23.3 (s, PCH2CH2); 31P{1H}
NMR (109 MHz, THF-d8) (ppm): δ 4.0.
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Fig. 1. Molecular structure of 2 with ellipsoids drawn at 50% probability level. Hydrogen
atoms omitted for clarity. Selected bond distances (Å) and angles (deg): Ir(1)–P(1)
2.2252(7), Ir(1)–P(2) 2.2348(7), Ir(1)–C(33) 2.270(3), Ir(1)–C(34) 2.297(3), Ir(1)–C(32)
2.302(3), Ir(1)–C(31) 2.312(3), Ir(1)–C(30) 2.446(3), Ir(1)–C(29) 2.456(3), B(1)–O(4)
1.463(4), B(1)–O(3) 1.469(4), B(1)–O(1) 1.503(4), B(1)–O(2) 1.506(4); P(1)–Ir(1)–P(2)
95.77(3), P(1)–Ir(1)–C(33) 103.53(8), O(4)–B(1)–O(3) 106.4(2), O(4)–B(1)–O(1) 112.0
(2), O(3)–B(1)–O(1) 110.8(2), O(4)–B(1)–O(2) 111.9(2), O(3)–B(1)–O(2) 112.2(2), and
O(1)–B(1)–O(2) 103.7(2).

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J.A. Melanson et al. / Inorganic Chemistry Communications 13 (2010) 1396–1398
[23] Orange plate-like crystals of 2 were grown from a saturated Et2O solution at room
temperature. Single crystals were coated with Paratone-N oil, mounted using a
polyimide MicroMount and frozen in the cold nitrogen stream of the goniometer. A
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temperature of 173(1)K. The detector distance was 5 cm. The data were reduced
[SAINT, 24] and corrected for absorption [SADABS, 25]. The structure was solved by
direct methods and refined by full-matrix least squares on F2 [SHELXTL, 26]. All non-
hydrogen atoms were refined using anisotropic displacement parameters. Hydrogen
atoms were included in calculated positions and refined using a riding model.
Crystallographic data for 2: C40H36BIrO4P2, Mr=845.64, monoclinic, space group
P21/n with a=12.4711(16)Å, b=21.718(3)Å, c=12.7368(16)Å, α=90o,
[24] SAINT 7.23A, Bruker AXS Inc, Madison, WI, USA, 2006.
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[30] The appropriate borane (1.2 equivalents) in 0.5 mL of C6D6 was added to a 0.5 mL
C6D6 solution of 2 (5 mol%) and the vinylarene (1 equivalent). Reactions were
monitored over time by multinuclear NMR spectroscopy and products were
identified by comparison to previously reported compounds [D.A. Kanas, S.J.
Geier, C.M. Vogels, A. Decken, S.A. Westcott, Inorg. Chem. 47 (2008) 8727 as well
as by GC-MS analysis.
β=96.698(2)o, γ=90o, V=3426.2(7)Å3, Z=4, Dc=1.639 Mg m−3
1680, GoF=1.068, R1 =0.0205, and ωR2=0.0554 for all data.
, F(000)=
[31] C.J. Lata, C.M. Crudden, J. Am. Chem. Soc. 132 (2010) 131.