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
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solid washed with Et2O (3×10 mL) to afford 2 as a pale orange solid. Yield: 80 mg
(68%); m.p.: 210–215 °C (decomp.). Anal. Calc. for 2 (C40H36BIrO4P2, 845.79): C, 56.80;
H, 4.30. Found: C, 56.45; H, 4.37. 1H NMR (270 MHz, THF-d8)(ppm):δ7.63 (m, 8H, Ar),
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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).