Pronounced effects of substituents on the iridium-catalyzed borylation of aryl C-H bonds

Article abstract of DOI:10.1039/b913949d

Iridium trisboryl complexes containing bisphosphine and bipyridine ligands and pinacolate and catecholate substituents on boron are reported. A large difference in reactivity towards the borylation of C-H bonds is observed for this series of trisboryl complexes, and this difference is attributed to the electron-donating properties of the pinacolate vs. catecholate groups, and the steric and electronic properties of bipyridine vs. bisphosphine ligands.

Full text of DOI:10.1039/b913949d

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Pronounced effects of substituents on the iridium-catalyzed borylation
of aryl C–H bondswz
Carl W. Liskey, Carolyn S. Wei, Dale R. Pahls and John F. Hartwig*
Received (in College Park, MD, USA) 14th July 2009, Accepted 7th August 2009
First published as an Advance Article on the web 20th August 2009
DOI: 10.1039/b913949d
Iridium trisboryl complexes containing bisphosphine and
bipyridine ligands and pinacolate and catecholate substituents
on boron are reported. A large difference in reactivity towards
the borylation of C–H bonds is observed for this series of
trisboryl complexes, and this difference is attributed to the
electron-donating properties of the pinacolate vs. catecholate
groups, and the steric and electronic properties of bipyridine vs.
bisphosphine ligands.
to the analogous trisboryl complexes. Thus, we sought an
alternative route to complexes of the type [Ir(dtbpy)[B(OR)2]3-
(cyclooctene)], and we found that the reaction of dtbpy with
[Ir(p-xylene)(Bcat*)3] (Bcat* = 4-tert-butylcatecholboryl),4 and
10 equiv. of COE formed [Ir(dtbpy)(Bcat*)3(cyclooctene)] (1b) in
86% isolated yield (eqn (1)). This complex contained resonances
for the bound COE at 5.11 and 1.51 ppm, and one set of
catecholate groups (vide infra). The 11B NMR spectrum consisted
of a single broad resonance at 34 ppm.
Metal-catalyzed borylations of aryl C–H bonds1–8 have become a
selective and convenient method to generate arylboronate esters
from arenes and either pinacolborane or bis-pinacolato diboron
reagents.9–11 The most active catalyst for this process is generated
from the combination of an iridium(I) precursor and di-tert-
butylbipyridine (dtbpy) as a ligand,10 and the most effective
reagents for Ir-catalyzed C–H borylation contain pinacolate
groups (pin) on boron. Detailed mechanistic studies have shown
that the reactions catalyzed by Ir(I) and dtbpy occur by
generation of iridium-trisboryl complexes and reaction of these
trisboryl complexes with arenes to form arylboronate esters in
the turnover-limiting step.12,13
ð1Þ
Free COE exchanges with the bound COE in 1b on the NMR
timescale. A solution of 1b and 3 equiv. of COE in methyl-
cyclohexane-d14 contained a single set of resonances for the free
and bound olefin. At ꢀ25 1C, resonances for both free (d 5.61)
and bound COE (d 4.78) were observed. This exchange process
occurs on a timescale similar to that of the exchange of free COE
with the bound COE in pinacolboryl complex 1a.13 This process
with 1a was shown to be dissociative,13 and a similar dissociation
from 1b, followed by site exchanges within the resulting
five-coordinate intermediates, would account for the equivalence
of the Bcat groups cis and trans to the COE ligand in 1b.
We have begun to assess the origins of the effect of the
substituents at boron and the dative ligands on the metal on
the composition, structure and reactivity of the catalytic inter-
mediates in this borylation process. We report our initial results
from these studies on intermediates containing a substituted
catecholboryl group, rather than a pinacolboryl group, and
di-tert-butylbipyridine or an alkylbisphosphine as the dative
ligand. These changes in boron substituents and dative
ligands, relative to those in the previously characterized
[Ir(dtbpy)(Bpin)3(cyclooctene)], create remarkable changes in
structure, reactivity and energetics, which are reported here.
We previously isolated the trisboryl complex [Ir(dtbpy)(Bpin)3-
(cyclooctene)] (1a), which readily dissociates cyclooctene (COE)
to form the 16-electron species [Ir(dtbpy)(Bpin)3], which is an
intermediate in the catalytic borylation of arenes.13 Although the
reaction of [Ir(COD)(OMe)]2 (COD = 1,5-cyclooctadiene),
dtbpy, HBpin and COE to form 1a occurred in high yield,
analogous reactions conducted with catecholboranes did not lead
An ORTEP diagram of complex 1b is shown in Fig. 1. The
structural parameters about the metal center in both 1a and 1b
are provided in Table 1. The structures of 1a and 1b are closely
related. Both complexes are pseudooctahedral with a facial
orientation of the three boronate groups. The Ir–N bond lengths
and the Ir–B bond lengths for the boryl group trans to COE in 1a
and 1b are within 0.005 A of each other. The Ir–B bond lengths
to the two boronate groups trans to the bipyridine ligand in 1b
are only 0.021 and 0.028 A shorter and the two Ir–olefin bonds in
1b are, on average, only 0.028 A longer than those in 1a.
To begin to assess the origin of the differences in reactivity
of the bipyridine and bisphosphine catalysts, we prepared
trisboryl complexes containing bisphosphine ligands. The
reaction of [Ir(p-xylene)(Bcat*)3] with 1,2-bis(diisopropyl-
phosphino)ethane (dippe) or 1,2-bis(dicyclohexylphosphino)-
ethane (dcpe) in the presence or absence of COE formed
the bisphosphine-ligated trisboryl complexes 3a and 3b
(eqn (2)). In contrast to 1a and 1b, these complexes were
16-electron, five-coordinate species [Ir(L2)(Bcat*)3] lacking a
COE ligand. Thus, these complexes are phosphine analogs of
the unsaturated bipyridine intermediates in the borylation of
arenes catalyzed by [Ir(COD)(OMe)]2 and dtbpy. Complexes
3a and 3b do not bind COE to form stable complexes.
Department of Chemistry, University of Illinois, 600 South Mathews Av,
Urbana, IL 61801, USA. E-mail: Jhartwig@illinois.edu
w This article is part of a ChemComm ‘Catalysis in Organic Synthesis’
web-theme issue showcasing high quality research in organic
chemistry. Please see our website (http://www.rsc.org/chemcomm/
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z Electronic supplementary information (ESI) available: Experimental
procedures, CIF files for structure determinations, and coordinates for
optimized structures. CCDC 740059 and 740060. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b913949d
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Fig. 2 ORTEP drawing of complex 3a with atomic numbering.
Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown
at the 35% probability level.
Fig. 1 ORTEP drawing of complex 1b with atomic numbering.
Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown
at the 35% probability level.
100 1C for 16 h led to the formation of just one equivalent
of PhBcat* per iridium.
Table 1 Ir–X bond distances in 1a, 1b, and 3a
L1/A
L2/A
B1/A
B2/A
B3/A
C1/A
C2/A
Consistent with the lack of published borylations of arenes
with the common catecholate-substituted diboron reagent
B2cat2, the reaction of B2cat2 with benzene in the presence
of 2.5 mol% [Ir(COD)OMe]2 and 5 mol% dtbpy required
120 1C for 24 h, even in neat arene, to occur to completion, as
summarised in eqn (3).
1a 2.177(4) 2.221(4) 2.055(7) 2.057(6) 2.027(6) 2.318(5) 2.318(5)
1b 2.173(4) 2.222(4) 2.051(6) 2.029(6) 2.006(6) 2.348(5) 2.344(5)
3a 2.368(1) 2.365(1) 1.979(5) 2.075(5) 2.070(5) —
—
Addition of 5 equiv. of COE to these species did not affect the
1
shape or position of the olefinic resonances in the H NMR
spectrum.
ð3Þ
This lower reactivity of the catecholate complexes could
result from faster decomposition of the catalyst in the presence
of by-products15 formed from these reagents, or reduced
reactivity of the trisboryl complex with the C–H bonds
of arenes. To distinguish between these possibilities, we
compared the reactions of benzene with isolated, dtbpy-ligated
tris-Bpin and tris-Bcat* complexes 1a and 1b.
ð2Þ
An ORTEP diagram showing the structure of five-coordinate,
bisphosphine complex 3a is shown in Fig. 2, and the structural
parameters are included in Table 1. Complex 3a adopts a
square pyramidal structure with a facial orientation of the
three boronate groups. The bond lengths of the boryl groups
trans to the ligand are 0.058 and 0.053 A longer than the
average of these bond lengths in complex 1b. Most striking,
the Ir–B distance for the boryl group trans to the open
coordination site in 3a is 0.072 A shorter than that for the
boryl group trans to the COE ligand in 1b and about 0.1 A
shorter than the Ir–B distance for the boryl groups trans to the
phosphine groups.
The tris-Bpin complex 1a was previously shown to react
with benzene solvent to form three equivalents of PhBpin at
room temperature.13 In contrast, the tris-Bcat* complex 1b is
stable for several hours in benzene at room temperature. At
50 1C over 2 h, 1.95 mol (65% yield per Bcat* group on the
iridium complex) of PhBcat* was formed per mol of trisboryl
complex (eqn (4)). Moreover, the profile for this reaction was
far from first order and suggests that the reaction occurs with
an induction period or with autocatalysis that will be the
subject of future studies.
The effect of these structural changes on the catalytic
reactions was evaluated. The presence of the open coordination
site on 3a and 3b might lead one to expect that complexes
generated from the dippe and dcpe ligands in 3a and 3b would
be active catalysts. However, consistent with the prior lower
reactivity of complexes of other bisphosphines as catalysts,11,14
the reaction of HBpin with benzene catalyzed by the combination
of [Ir(COD)(OMe)]2 and dcpe formed PhBpin in a low 24%
yield, even after heating at 120 1C for 24 h. Moreover, complex
3b was stable in benzene. Even heating of 3b in benzene
at 80 1C for 4 h led to little conversion, and heating at
ð4Þ
This difference in reactivity between Bpin and Bcat*
complexes 1a and 1b can be explained by differences in electron
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density at the metal center. The chemical shift of the bound
olefinic carbons in Bcat* complex 1b is 4.78 ppm, which is
0.85 ppm downfield of those in the tris-Bpin complex 1a. This
difference is consistent with a less electron-rich metal center in the
tris-Bcat* complex. In addition, the two carbonyl complexes
[Ir(dtbpy)(Bpin)3(CO)] (4a) and [Ir(dtbpy)(Bcat*)3(CO)] (4b)
were prepared, and, consistent with the two alkene chemical
shifts, the nCO value of the tris-Bcat* complex 4b (2017 cmꢀ1) is
30 wavenumbers higher than that (1987 cmꢀ1) for the tris-Bpin
complex 4a. This difference in electron density at the metal also
agrees with calculations recently reported by Marder et al.16 on
the s-donating properties of Bpin and Bcat ligands.
determination of the origin of the lack of reactivity of the
unsaturated phosphine complexes awaits further studies, the
computed energetics imply that this lack of reactivity results
from a combination of increased steric hindrance in the
absence of stronger electron donation by the phosphine ligand
vs. the bipyridine ligand that would favor oxidative addition of
the arene C–H bond.18,19
We thank the NSF for support of this work, Johnson
Matthey for iridium complexes, and Abbott Laboratories for
a graduate fellowship to CWL. This research was supported in
part by the National Science Foundation through TeraGrid
resources provided by NCSA.20
To understand how the differences in structures and
electronic properties between complexes 1a, 1b, and 3b affect
C–H bond cleavage, we calculated the enthalpies for oxidative
addition of benzene to the dtbpy-ligated Bpin compound 1a
(previously studied theoretically by Sakaki et al.17) and the
Bcat compounds 1b and 3b (eqn (5)). The pinacolate group
was truncated to an ethylene glycolate group (eg), and the
tert-butyl groups on both the bipyridine ligand and the
substituted catechols were replaced by hydrogen (1a0 and 1b0).
The bisphosphine was truncated to 1,2-bis(dimethylphosphino)-
ethane (dmpe).
Notes and references
1 For initial reports of the borylation of C–H bonds by isolated
boryl complexes see ref. 2.
2 (a) K. M. Waltz, X. He, C. Muhoro and J. F. Hartwig, J. Am.
Chem. Soc., 1995, 117, 11357–11358; (b) K. M. Waltz and
J. F. Hartwig, Science, 1997, 277, 211–213; (c) K. M. Waltz,
C. N. Muhoro and J. F. Hartwig, Organometallics, 1999, 18,
3383–3393; (d) K. M. Waltz and J. F. Hartwig, J. Am. Chem.
Soc., 2000, 122, 11358–11369.
3 For an initial observation of the borylation of arenes in low yields,
see a GC-MS trace in the supporting information of ref. 4.
4 P. Nguyen, H. P. Blom, S. A. Westcott, N. J. Taylor and
T. B. Marder, J. Am. Chem. Soc., 1993, 115, 9329–9330.
5 For initial reports of this process see ref. 6.
6 (a) C. N. Iverson and M. R. Smith, J. Am. Chem. Soc., 1999, 121,
7696–7697; (b) H. Y. Chen, S. Schlecht, T. C. Semple and
J. F. Hartwig, Science, 2000, 287, 1995–1997.
ð5Þ
7 For reviews of this process, see ref. 8.
8 (a) T. Ishiyama and N. Miyaura, J. Organomet. Chem., 2000, 611,
392–402; (b) T. Ishiyama and N. Miyaura, J. Organomet. Chem.,
2003, 680, 3–11.
The DH for the oxidative addition of benzene to catecholboryl
complex 1b0 was computed to be 14.3 kcal molꢀ1, which is
2.6 kcal molꢀ1 higher than the 11.7 kcal molꢀ1 value computed
for oxidative addition to the Beg complex 1a0. This less
favorable enthalpy for the addition of benzene to 1b0 is
consistent with the lower reactivity of the catecholboryl com-
plex 1b observed experimentally. The computed enthalpies for
9 For leading initial references, see ref. 10.
10 (a) T. Ishiyama, J. Takagi, K. Ishida, N. Miyaura, N. R. Anastasi
and J. F. Hartwig, J. Am. Chem. Soc., 2002, 124, 390–391;
(b) T. Ishiyama, J. Takagi, J. F. Hartwig and N. Miyaura, Angew.
Chem., Int. Ed., 2002, 41, 3056–3058; (c) T. Ishiyama, J. Takagi,
Y. Yonekawa, J. F. Hartwig and N. Miyaura, Adv. Synth. Catal.,
2003, 345, 1103–1106; (d) T. Ishiyama, Y. Nobuta, J. F. Hartwig
and N. Miyaura, Chem. Commun., 2003, 2924–2925.
11 J. Y. Cho, M. K. Tse, D. Holmes, R. E. Maleczka and
M. R. Smith, Science, 2002, 295, 305–308.
12 For a preliminary report of the isolation of the iridium trisboryl
intermediate, see ref. 10a.
13 T. M. Boller, J. M. Murphy, M. Hapke, T. Ishiyama, N. Miyaura
and J. F. Hartwig, J. Am. Chem. Soc., 2005, 127, 14263–14278.
14 R. E. Maleczka, Jr, F. Shi, D. Holmes and M. R. Smith, J. Am.
Chem. Soc., 2003, 125, 7792–7793.
addition to the dmpe-ligated Bpin complex (10.9 kcal molꢀ1
)
and to the dmpe-ligated Bcat complex (14.6 kcal molꢀ1) were
similar to those for addition to the analogous bpy complexes.
However, the dippe and dcpe ligands in the experimental work
are much more sterically demanding than dmpe; thus, the
actual enthalpies for reactions of the dippe and dcpe
complexes are certainly much larger. Thus, these computed
enthalpies are also consistent with the observed trends in
reactivity.
15 (a) S. A. Westcott, H. P. Blom, T. B. Marder, R. T. Baker and
J. C. Calabrese, Inorg. Chem., 1993, 32, 2175–2182; (b) Boronic
Acids: Preparation Applications in Organic Synthesis and Medicine,
ed. G. Hall, Wiley-VCH, Weinheim, Germany, 2005.
16 X. W. Zhan, S. Barlow and S. R. Marder, Chem. Commun., 2009,
1948–1955.
In conclusion, we have shown that the substituents on the
oxygen of dioxaborolane ligands have a strong effect on
the reactivity of trisboryl complexes that are intermediates in
the catalytic borylation of arenes, and that complexes of
hindered bisphosphines are much less reactive toward arenes
than complexes of bipyridine, despite the presence of an open
coordination site in the bisphosphine complex. We propose
that the effect of the substituents on boron results from
differences in electron density of the metal center, which affects
the enthalpy for the oxidative addition of benzene. Although a
17 H. Tamura, H. Yamazaki, H. Sato and S. Sakaki, J. Am. Chem.
Soc., 2003, 125, 16114–16126.
18 For values of bisphosphine and bpy complexes see ref. 19.
19 (a) P. A. Christensen, A. Hamnett, S. J. Higgins and J. A. Timney,
J. Electroanal. Chem., 1995, 395, 195–209; (b) C. A. Tolman,
J. Am. Chem. Soc., 1970, 92, 2956–2965.
20 TeraGrid: Analysis of Organization, System Architecture, and
Middleware Enabling New Types of Applications, HPC and Grids
in Action, ed. Lucio Grandinetti, IOS Press, Amsterdam, 2007.
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