Use of hemilabile N,N ligands in nitrogen-directed iridium-catalyzed borylations of arenes

Article abstract of DOI:10.1002/anie.201104544

The hemilabile character of 2-pyridyl carbaldehyde hydrazones as N,N bidentate ligands is key to performing regioselective Ir^III-catalyzed ortho borylations of 2-aryl pyridines(isoquinolines) and aromatic N,N-dimethylhydrazones (see scheme; pin=pinacol, Bn=benzyl). Internal "ate" complexes or products free from N-B interactions are formed depending on the steric properties of the substrates. Copyright

Full text of DOI:10.1002/anie.201104544

Communications
DOI: 10.1002/anie.201104544
Synthetic Methods
Use of Hemilabile N,N Ligands in Nitrogen-Directed Iridium-
Catalyzed Borylations of Arenes**
Abel Ros, Beatriz Estepa, Rocꢀo Lꢁpez-Rodrꢀguez, Eleuterio ꢂlvarez, Rosario Fernꢃndez,* and
Josꢄ M. Lassaletta*
ꢀ
The direct activation of unreactive C H bonds has emerged
as a very active field in organic synthesis.[1] One of the best
established reactions in this area is the direct borylation of
arenes with bis(pinacolato)diboron (B2pin2) or pinacolborane
(HBpin). After the seminal work by Smith, Hartwig, Miyaura,
and their respective co-workers, this method has reached an
impressive level of chemical efficiency,[2] mainly because of
the introduction of the [Ir(m-X)(cod)]2/di-tert-butylbipyridine
(dtbpy; X = Cl, OMe; cod = 1,5-cyclooctadiene) precatalyst
system. The regioselectivity of this reaction is typically driven
by steric factors,[3] thus making the method complementary to
directed ortho-metalation (DoM) reactions.[4] Very recently,
however, directed ortho borylations have been accomplished
by using directing groups such as siloxide or silylamine[5] and
carbonyl functionalities,[6] but the absence of any nitrogen-
directed borylation is noteworthy. Herein, we present our
Scheme 1. Regioselectivity in IrIII/dtbpy-catalyzed arene borylations.
Square represents a vacant coordination site.
ꢀ
results on the regioselective ortho C H borylation of 2-
arylpyridines (isoquinolines) and aromatic hydrazones.
The regioselectivity of IrIII/dtbpy-catalyzed reactions can
be arguably attributed to the lack of a coordination site in
complex B, which results from coordination of the directing
atom Y to the established catalytic species A[2c] (Scheme 1).
coordination site. It was foreseen that an ideal ligand should
still be a good donor in order to maintain a high level of
reactivity as in the reference dtbpy-based system.
Based on our own experience with related ligands,[8]
pyridine hydrazones 2–4, which maintain one of the pyridine
units in their structures, and bishydrazone 5 were considered
as candidates (Scheme 2). Additionally, pyridine imine 6[9]
and pyridine amine 7 were included for comparison in the
preliminary screenings. The relative activity of in situ formed
[IrCl(cod)]2/N,N-ligand catalysts (ligand = 1–7) was investi-
gated by using the borylation of 1-(1-naphthyl)isoquinoline
8a with B2pin2 in THF at 508C as a model reaction (Table 1).
The catalyst based on dtbpy ligand 1 is highly active, but, as
expected, affords a complex mixture of borylated products
that was not analyzed further (Table 1, entry 1). To our
delight, an excellent selectivity was observed in the reactions
performed with ligands 2–4, which resulted in a clean
formation of product 9a in good yields (Table 1, entries 2–
4). Bishydrazone 5 and pyridine amine 7 were ineffective at
508C (Table 1, entries 5 and 7), however, raising the reaction
temperature to 808C also led to the desired result in these
cases (Table 1, entries 8 and 9). Finally, pyridine imine 6
exhibited an intermediate behavior as its activity at 508C was
ꢀ
In this scenario, the direct activation of arene C H bonds can
only proceed through intermediate C and is therefore mainly
controlled by steric factors. On the other hand, the ability of
ꢀ
transition metals to selectively activate arene C H bonds in 2-
aryl pyridines[1d] made us speculate that such a reaction would
only require the generation of an additional coordination site
at an appropriate stage. Therefore, we decided to investigate
the behavior of potentially hemilabile N,N ligands,[7] in which
one of the nitrogen atoms is a weaker donor that is prone to
dissociation and therefore able to generate the required
[*] Dr. A. Ros, R. Lꢀpez-Rodrꢁguez, Dr. E. ꢂlvarez, Dr. J. M. Lassaletta
Instituto de Investigaciones Quꢁmicas (CSIC-US)
Amꢃrico Vespucio 49, 41092 Seville (Spain)
E-mail: jmlassa@iiq.csic.es
B. Estepa, Prof. R. Fernꢄndez
Departamento de Quꢁmica Orgꢄnica
Universidad de Sevilla
C/Prof. Garcꢁa Gonzꢄlez, 1, 41012 Sevilla (Spain)
[**] We thank the Spanish “Ministerio de Ciencia e Innovaciꢀn” (grants
CTQ2010-15297 and CTQ2010-14974, and fellowship to B.E.), the
European FEDER funds and the Junta de Andalucꢁa (grants 2008/
FQM-3833 and 2009/FQM-4537) for financial support. A.R. thanks
the EU for a Marie Curie Reintegration Grant (FP7-PEOPLE-2009-
RG-256461) and R.L.-R. thanks CSIC for a JAE predoctoral fellow-
ship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104544.
Scheme 2. N,N ligands with potential hemilabile character.
11724
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 11724 –11728

Table 1: Preliminary screening of the borylation of 8a.
Table 2: Directed ortho borylations of 2-aryl pyridines (isoquinolines).[a]
Entry
Ligand
X
T [8C]
t [h]
Yield[a] [%]
Entry Substrate
T
t
dB
dH(Me-pin)
Product Yield
(type)
n.d.[b]
67
78
[8C] [h] [ppm] [ppm]
[%][b]
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
5
7
–
3
3
1
3
5
6
7
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
OMe
OMe
OMe
OMe
OMe
OMe
50
50
50
50
50
50
50
80
80
80
80
50
RT
RT
RT
RT
RT
14
20
20
20
20
20
14
20
20
20
72
7
24
24
24
24
24
1
2
3
4
5
6
7
8
8a
8b
8c
8d
8e
8 f
8g
8h
8i
50
50
50
50
50
80
80
50
50
80
80
50
50
50
7
31.1
0.75, 0.95 9a (A)
0.75, 0.94 9b (A)
0.86
84
77
79
62
84
58
69
70
88
64[c]
73[c]
84
85
10 31.1
79
7
7
7
30.3
30.5
30.2
9c (A)
[c]
–
0.83, 1.01 9d (A)
60
1.11
1.14
1.15
1.15
0.85
1.43
1.43
1.44
1.44
1.27
9e (A)
9 f (A)
9g (A)
9h (A)
9i (A)
9j (B)
9k (B)
9l (B)
9m (B)
[c]
–
12 29.9
12 30.4
7
7
48 13.3
20 15.2
7
7
7
75
80
0
9
29.5
30.0
10
11
12
13
14
15
16
17
9
50[d,e]
84
10
11
12
13
14
8j
8k
8l
8m
8n
n.d.[b]
70[e]
10[e]
50[e]
5[e]
13.9
15.2
22.1
9n (–)[d] 78
[a] Reactions performed on a 0.5 mmol scale with 0.5 mmol of B2pin2.
[b] Yields of isolated products after purification by column chromato-
graphy on silica gel or precipitation with hexane. [c] 0.25 mmol of B2pin2
used. [d] A fast equilibrium between product types A and B is proposed.
[a] Yield of isolated product unless stated otherwise. [b] Complex mixture
of products, yield was not determined (n.d.). [c] No reaction. [d] HBpin
(1.05 equiv) was used as borylation agent. [e] Incomplete reaction, yield
was estimated from the 1H NMR spectrum of the crude reaction mixture.
maintained (Table 1, entry 6), but at slower rates than that of
pyridine hydrazones. A control experiment showed that no
reaction takes place in the absence of a ligand (Table 1,
entry 10). Further experiments with ligand 3 showed a
reduced reactivity of HBpin (Table 1, entry 11) and a better
rate for the reaction performed with [Ir(m-OMe)(cod)]2 as the
precatalyst (Table 1, entry 12). Additional reactivity tests
performed at room temperature with ligands 1, 3, and 5–7 and
precatalyst [Ir(m-OMe)(cod)]2 (Table 1, entries 13–17) con-
firmed 3 as the best choice of ligand.
Successive studies were aimed at analyzing the scope of
the reaction (Table 2, Scheme 3) and were therefore con-
ducted with ligand 3.[10] The reactions worked well with a
variety of 2-pyridyl and 2-isoquinolyl directing groups (8a–n),
and afforded the desired ortho-borylation products 9a–n in
good to excellent yields. Electron-donating (8e,g,h) and
withdrawing (8 f) groups were introduced onto the pyridine
ring; reactions of 8e,g,h were more effective, which was
expected because of the better coordination ability of the
nitrogen atom (Table 2, entries 5–8). Additional steric crowd-
ing around the nitrogen atom as in 3-methyl-2-(1-naphthyl)-
isoquinoline (8o) completely inhibited the reaction, even at
808C. This result highlights the selectivity that is reached with
Scheme 3. Borylation products 9a–i (type A), 9j–m (type B), 9n (equi-
librium), and unreactive starting material 8o.
ꢀ
ligand 3, which is inactive for the activation of regular Ar H
bonds, unless a directing effect facilitates the reaction. The
reaction proved to be highly selective in most cases. Only the
simplest pyridine derivatives 8j and 8k produced some
polyborylation products,[11] but by reducing the amount of
B2pin2 to 0.5 equivalents, this effect was minimized. Thus, the
desired products 9j and 9k could be isolated in 64% and 73%
yield, respectively, though longer reaction times were
required in these cases (Table 2, entries 10 and 11). The
structure of products 9 proved to be dependent on the
substitution pattern: more hindered products 9a–i (Table 2,
Angew. Chem. Int. Ed. 2011, 50, 11724 –11728
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 11725

Communications
entries 1–9) exhibited a much lower polarity, thus suggesting
1H NMR signals of the pinacol methyl groups, which appear
at 0.75–1.15 ppm for free pinacolates 9a–i (type A), and are
deshielded and appear at 1.40–1.45 ppm in internal N–B
complexes 9j–m (type B). The assignment was further con-
firmed by 11B NMR spectra of the products: the signal for the
first class of products regularly appears in the 29–31 ppm
range, while the N–B interaction results in a strong shielding
and the signals appear at 13–15 ppm, which are in agreement
with reported data for both types of boron atoms.[14] Finally,
9n appears to be an intermediate case, as its 11B NMR
spectrum shows a signal at 22.1 ppm and its 1H NMR
spectrum shows a signal at 1.27 ppm, which correspond to
the pinacolate methyl groups. In this case, a fast equilibrium
between both types of product is tentatively proposed.
Several benzaldehyde-derived imines were used as sub-
strates under the optimized reaction conditions in order to
identify a broader synthetic utility of the methodology. These
experiments revealed that electron-rich N,N-dimethylhydra-
zones 10 are also suitable substrates that afford the desired
ortho-borylated products 11 in good yields (Table 3).[15] The
method worked well for derivatives carrying electron-with-
drawing (10d,e,g,h; Table 3, entries 4,5,7, and 8) or electron-
donating (10b,c,f; Table 3, entries 2,3, and 6) substituents on
the aromatic ring, and allowed monoborylation of C-6
unsubstituted substrates 10a,d,e.
the absence of N–B interactions (type A), while unhindered
derivatives 8j–m (Table 2, entries 10–13) afforded products
9j–m with a higher polarity, a trend that was attributed to an
internal N–B dative bond (type B). This assignment was
confirmed by studies in the solid state and in solution.
Representative crystalline products 9a (Figure 1) and 9j
(Figure 2) were analyzed by X-ray crystallography.[12] Deriv-
ative 9a, with higher steric inhibition to a coplanar geometry,
shows a nearly perpendicular arrangement of the isoquinoline
and the 2-naphthyl rings (dihedral angle N1-C11-C1-C2 =
82.38), while the boron atom adopts a nearly perfect
trigonal-planar geometry that results from its sp2 hybrid-
ization (virtual dihedral angle C2-B1-O1-O2 = 179.968).[13]
On the other hand, the simplest product 9j shows a clear
N–B coordination (distance N1–B1 = 1.6622(16) ꢀ), thus
forcing coplanarity of the pyridine and phenyl rings (dihedral
angle N1-C1-C6-C7 = 0.68) and the pyramidalization of the
boron atom (virtual dihedral angle C7-B1-O1-O2 = 133.388).
In solution, both types of structures can be identified by the
Table 3: Borylation of aromatic N,N-dimethylhydrazones 10.[a]
Entry
Product
R
t [h]
Yield[b] [%]
1
2
3
4
5
6
7
8
9
11a
11b
11c
11d
11e
11 f
11g
11h
11i
H
16
24
24
24
17
10
8
80]
88
80
72
84
77
73
70
95
o-Me
o-OMe
o-F
m-Cl
p-OMe
p-Cl
p-F
Figure 1. X-ray structure of 9a. Hydrogen atoms are omitted for clarity.
Selected bond lengths [ꢅ] and bond angles [8]: O1–B1 1.374(2), O2–B1
1.362(2), C2–B1 1.556(2); O2-B1-O1 113.55(15). Thermal ellipsoids
drawn at 30% probability.
7
24
–
[a] Reactions performed on a 0.5 mmol scale. [b] Yields of isolated
products after purification by column chromatography on neutral
alumina.
According to the initial hypothesis, our results can be
explained by a mechanism initiated by formation of a
substrate–catalyst complex II, which might suffer a temporary
dissociation of the weaker nitrogen donor to lead to
intermediate III (Scheme 4). It is worth noting that the
proposed intermediate III would have the same configuration
as the starting catalyst I, with two cis-N ligands, three Bpin
ligands, and a vacant coordination site (square), the only
difference being that the two N ligands are not bridged as in
the original configuration. Once the equilibrium IÐIIÐIII is
Figure 2. X-Ray structure of 9j. Hydrogen atoms and a crystallization
H2O molecule are omitted for clarity. Selected bond lengths [ꢅ] and
bond angles [8]: N1–B1 1.6622(16), C7–B1 1.6366(16), O1–B1
1.4443(14), O2–B1 1.4445(14); O1-B1-O2 106.67(9). Thermal ellip-
soids drawn at 30% probability.
ꢀ
established, the activation of the ortho Ar H bonds in III
should be favored, thus leading to IV and then closing the
11726 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 11724 –11728

In conclusion, the hemilability of pyridine hydrazone
ligands is the key for iridium(III)-catalyzed nitrogen-directed
ortho borylations of several aromatic substrates under mild
conditions.
Experimental Section
General procedure for iridium-catalyzed selective borylations: A
dried Schlenk tube was charged with the substrate 8a–n (0.5 mmol) or
10a–f and B2pin2 (127 mg, 0.5 mmol). The catalyst stock solution[23]
(1 mL) and HBpin (3.7 mL, 5% mol) were added, and the reaction
mixture was stirred at the desired temperature until the starting
material was consumed (as shown by TLC). The mixture was cooled
to room temperature, concentrated to dryness, and purified by
column chromatography on silica gel (products 9a–h,j,k) or neutral
alumina (product 11; eluents: mixtures of n-hexane/EtOAc) or by
precipitation with hexane (products 9i,l–n).
Received: July 1, 2011
Revised: September 9, 2011
Published online: October 11, 2011
Scheme 4. Proposed mechanism and catalytic cycle.
ꢀ
Keywords: borylation · C H activation ·
homogeneous catalysis · iridium · N,N ligands
.
catalytic cycle after reductive elimination (!V), transligation
with release of the product (V!VII), and regeneration of I
according to the established mechanism.[16]
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One evident application for the borylated products 9 and
11 is their use as substrates in Suzuki–Miyaura (SM) cross-
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9a!12). After completion of the borylation reaction (8a!
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(Scheme 5). The overall process can be viewed as a direct
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Scheme 5. Synthetic applications of this methodology.
Angew. Chem. Int. Ed. 2011, 50, 11724 –11728
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 11727

Communications
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products in almost quantitative yields. Although chromatogra-
phy on silica gel led to extensive decomposition, purification on
alumina allowed the removal of remaining HBpin with a
moderate loss of product.
not
a substrate for the borylation reaction and remains
untouched after 48 h at 808C, even when the most active [Ir(m-
OMe)(cod)]2 precatalyst was used.
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7.0271(5), b = 22.3665(16), c = 10.2909(8) ꢀ, b = 94.708(2)8,
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[22] R. Fernꢂndez, C. Gasch, J. M. Lassaletta, J. M. Llera, J. Vꢂzquez,
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[23] Prepared by dissolving 3 (37.6 mg, 0.125 mmol) and [Ir(m-
[13] The virtual angle reaches a value of ꢁ 1808 for a planar atom and
ꢁ 1208 for a tetrahedral atom: K. N. Rankin, R. J. Boyd, J. Phys.
Chem. A 2002, 106, 11168.
OMe)(cod)]2 (41 mg, 0.063 mmol) in dry THF to
a total
volume of 25 mL (the solution was sonicated for 1 h). The
stock solution was kept under Ar.
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Angew. Chem. Int. Ed. 2011, 50, 11724 –11728