A well-defined NHC-Ir(III) catalyst for the silylation of aromatic C-H bonds: Substrate survey and mechanistic insights

Article abstract of DOI:10.1039/c6sc04899d

A well-defined NHC-Ir(iii) catalyst, [Ir(H)₂(IPr)(py)₃][BF₄] (IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene), that provides access to a wide range of aryl- and heteroaryl-silanes by intermolecular dehydrogenative C-H bond silylation has been prepared and fully characterized. The directed and non-directed functionalisation of C-H bonds has been accomplished successfully using an arene as the limiting reagent and a variety of hydrosilanes in excess, including Et₃SiH, Ph₂MeSiH, PhMe₂SiH, Ph₃SiH and (EtO)₃SiH. Examples that show unexpected selectivity patterns that stem from the presence of aromatic substituents in hydrosilanes are also presented. The selective bisarylation of bis(hydrosilane)s by directed or non-directed silylation of C-H bonds is also reported herein. Theoretical calculations at the DFT level shed light on the intermediate species in the catalytic cycle and the role played by the ligand system on the Ir(iii)/Ir(i) mechanism.

Full text of DOI:10.1039/c6sc04899d

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Received 00th January 20xx,
A Well-Defined NHC-Ir(III) Catalyst for the Silylation of Aromatic
C–H Bonds: Substrate Survey and Mechanistic Insights
Laura Rubio-Pérez,a Manuel Iglesias,*,a Julen Munárriz,b Victor Polo,b Vincenzo Passarelli,a,c Jesús J.
Pérez-Torrente,a and Luis A. Oro*,a,d
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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A well-defined NHC-Ir(III) catalyst, [Ir(H)2(IPr)(py)3][BF4] (IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene), that
provides access to a wide range of aryl- and heteroarylsilanes by intermolecular dehydrogenative C–H bond silylation has
been prepared and fully characterized. The directed and non-directed functionalisation of C–H bonds has been
accomplished successfully using the arene as the limiting reagent and a variety of hydrosilanes in excess, including Et3SiH,
Ph2MeSiH, PhMe2SiH, Ph3SiH and (EtO)3SiH. Examples that show unexpected selectivity patterns that stem from the
presence of aromatic substituents in hydrosilanes are also presented. The selective bisarylation of bis(hydrosilane)s by
directed or non-directed silylation of C–H bonds is also reported herein. Theoretical calculations at the DFT level shed light
on the intermediate species of the catalytic cycle and the role played by the ligand system on this Ir(III)/Ir(I) mechanism.
di(hydro)silanes.6 Intermolecular silylations may be classified
into directed and undirected reactions. Directed silylations
require the presence of a coordinating group in the substrate
Introduction
Organosilicon compounds are key building blocks in modern
organic synthesis, often used as intermediates for complex
molecules or monomers for silicone polymers. The synthetic
versatility of organosilanes can be attributed to their
straightforward functionalisation by various organic
transformations, together with the low cost and non-toxic
nature of silicon reagents.1 Moreover, conjugated
organosilicon materials are attractive targets per se owing to
that reversibly binds the catalyst. This interaction leaves a C–H
bond in the proximity of the active site, which facilitates its
activation and defines the selectivity of the process. These
reactions mostly use disilanes7 or hydrosilanes as silicon
sources. The latter usually requires the presence of a hydrogen
acceptor,8 although acceptorless reactions have also been
described.9 Undirected silylation reactions, on the other hand,
make use of substrates that lack a coordinating group able to
direct the reaction. These are more challenging substrates due
to the ensuing selectivity issues and low reactivity; however,
the scope of this reaction has experienced significant
progress10 since the early reports by Curtis and Berry.11
their unique properties, which permit
a
widespread
applicability in the field of organic electronics and photonics.2,3
The preparation of organosilanes by catalytic silylation of C–H
bonds represents a more atom- and step-efficient alternative
than stoichiometric processes4 and cross-coupling reactions.5
The silylation of arenes and heteroarenes, in particular, are
important reactions due to the ubiquitous presence of these
moieties in natural products and materials. These reactions are
typically divided into two big groups: intermolecular and
intramolecular. The former requires the prefunctionalisation of
the (hetero)arene with a hidrosilane moiety, which may be
achieved by hydrosilylation or dehydrogenative silylation using
In spite of the prodigious advances that the C–H silylation
methodology has experienced in recent years,12 there is still
much room for further development. On the lookout for
expanding the synthetic reach of this catalytic process, various
improvements may be envisaged: (1) the use of more
synthetically useful hydrosilane partners is an unresolved
problem.12a
organotrialkoxysilanes by catalytic C–H bond silylation remains
widely unexplored.13,14,15 (2)
comprehensive survey of
For
instance,
the
preparation
of
A
hydro(aryl)silanes would be of interest owing to the potential
applicability of these reactions in the synthesis of new
materials. Only a limited number of examples has been
hitherto reported on this topic.16,17 (3) The use of the arene as
the limiting reagent is of remarkable importance for the
synthetic applicability of this reaction since the arene is
frequently the most valuable component in these
transformations. Examples of the non-directed silylation of
arenes under this stoichiometry are scarce and a wider
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substrate scope, especially regarding unactivated substrates, is
highly desirable.18
Most of the literature on catalytic dehydrogenative silylation
of C–H bonds has focused on the use of “in situ” generated
catalysts from commercial metal precursors and ligands.
However, somewhat less attention has been paid to the
development of well-defined organometallic complexes.16c,19
In this regard, the design of catalysts featuring N-heterocyclic
carbenes (NHCs) as ancillary ligands has been surprisingly
overlooked,19b especially when taking into account their
success story in homogeneous catalysis.20
Scheme 1 Synthesis of complex 1.
We report herein on the synthesis and characterisation of a
well-defined Ir(III)-NHC complex that behaves as an efficient
and versatile catalyst for the dehydrogenative silylation of
aromatic C–H bonds for a wide range of hydrosilanes using the
arene as limiting reagent. By means of this catalytic process we
have prepared
a broad variety of arylsilanes, including
examples of the elusive triarylsilanes and trialkoxysilanes. In
addition, an experimental and theoretical study on the
mechanism that controls this process is discussed here.
In the search for new catalysts for the silylation of C–H bonds,
we envisaged an NHC-Ir species featuring labile ligands that
would allow for the coordination of substrates and
additives,21,19b while facilitating the C–H and Si–H activation
processes thanks to the unique properties of the NHC ligand.22
On these grounds, complex [Ir(H)2(IPr)(py)3][BF4] (IPr = 1,3-bis-
Fig. 1 ORTEP view of the cation [Ir(H)2(IPr)(py)3]+ in 1 with the numbering scheme
adopted. Most hydrogens are omitted for clarity and thermal ellipsoids are at 50%
probability. Selected bond lengths (Å) and angles (°): C(1)-N(2) 1.378(4), C(1)-N(5)
1.379(4), C(1)-Ir(1) 1.996(3), N(30)-Ir(1) 2.180(3), N(36)-Ir(1) 2.231(3), N(42)-Ir(1)
2.126(3), N(2)-C(1)-N(5) 102.2(3), C(1)-Ir(1)-N(42) 172.88(13), C(1)-Ir(1)-N(30)
95.39(13), N(42)-Ir(1)-N(30) 89.24(12), C(1)-Ir(1)-N(36) 103.04(13), N(42)-Ir(1)-N(36)
81.93(11), N(30)-Ir(1)-N(36) 94.44(11).
(2,6-diisopropylphenyl)imidazol-2-ylidene) (1) would be an
excellent candidate for this study since the pyridine ligands can
be straightforwardly substituted23 and the two hydrides may
be removed with a hydrogen acceptor or expelled as molecular
hydrogen.24
Variable temperature NMR analysis shows no line broadening
at 193 K, which is consistent with a free energy rotation barrier
lower than ca. 30 kJ·mol-1. Two different types of pyridine
ligands are observed at δ 8.14 and 7.84 ppm in a 2:1 ratio,
respectively, which are situated in a facial disposition. The
distinct environments observed for the pyridine ligands
(labeled “a” and “b” in Scheme 1) are consistent with pyridine
dissociation being slow on the NMR time scale.
Results and discussion
Synthesis and characterisation of the pre-catalyst.
Complex
1 was prepared in good yield in acetone from
[Ir(acetone)(COD)(IPr)][BF4] (COD = 1,5-cyclooctadiene) in the
presence of excess pyridine (py) under a hydrogen atmosphere
(Scheme 1).25
Crystals of complex
1 were obtained by slow diffusion of
The most representative resonance in the 13C NMR is that
corresponding to the carbene carbon at δ 154.7 ppm. The 19F
diethylether into a saturated dichloromethane solution. Its
global connectivity pattern was confirmed by single crystal X-
ray diffraction (Fig. 1). The molecular structure of
1 shows that
NMR spectrum confirms the cationic nature of 1 with a peak at
δ –155.2 ppm that was assigned to the BF4– counterion.
the iridium centre adopts a slightly distorted octahedral
geometry, with the two pyridines cis to the IPr ligand visibly
displaced from the equatorial plane, probably due to the steric
interference of the bulky wingtip groups of the NHC.
Remarkably, the two pyridines in the equatorial plane, trans to
the hydrides, feature longer Ir–N bond lengths compared to
that situated in trans position to the IPr ligand.
Catalysis.
Initial catalytic tests using
1 as pre-catalyst and 2-(2-
thienyl)pyridine as substrate focused on the optimisation of
the reaction conditions and the assessment of whether a
hydrogen acceptor would be required. When norbornene was
employed as a hydrogen acceptor, nearly quantitative yields
The 1H NMR spectrum of
1 shows a singlet in the highfield
were obtained after 24
h at 110 °C; however, under
region at δ –22.48 ppm for both hydride ligands. The IPr ligand
presents one singlet for the NCH protons at δ 7.07 ppm and a
septuplet for the CHMe2 protons of the isopropyl groups at δ
2.87 ppm, which suggests a fast rotation of the NHC ligand
about the Ir–C bond at room temperature.
acceptorless conditions only a 45% yield was achieved. Other
hydrogen acceptors such as cyclohexene or 3,3-dimethyl-1-
butene were tested, although somewhat lower yields were
obtained.
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the directing group, i.e. the pyridine moiety, undergoes
exclusively the silylation of its C5–H bond. This rare selectivity
has also been reported recently by Oestreich and co-
workers.26
The intermolecular non-directed silylation of aromatic and
heteroaromatic molecules was also achieved employing the
arene as limiting reactant (3 equivalents of silane), including
among these reactions the regioselective silylation of
naphthalene at the C2-position. This is, to the extent of our
knowledge, the first example of naphthalene functionalisation
by catalytic C–H bond silylation. The silylation of m-xylene,
thiophene, benzothiophene and 2-methylthiophene was also
regioselective, which contrasts with the mixture of
regioisomers obtained for fluoro-, chloro-, ethylbenzene and
sec-butylbenzene with triethylsilane (Scheme 3). To our
delight, the selective silylation of chlorobenzene to afford
exclusively the para isomer was accomplished with PhMe2SiH.
The relative reactivity of the different silanes may be
estimated from the results presented in Schemes 2 and 3. The
least reactive silane is (EtO)3SiH since it only works for the
most reactive substrate, 2-thienylpyridine, and requires a
reaction time of 96 h. The following hydrosilane in an
ascending order of reactivity would be Ph3SiH, as longer
reaction times are required compared to Et3SiH, Ph2MeSiH,
PhMe2SiH, which usually show similar reactivity.
A competitive experiment was performed using 1 equivalent of
2-phenylpyridine and 1 equivalent of ethylbenzene with Et3SiH
under the reaction conditions described in Scheme 2 in order
to assess the relative reactivity of directed and non-directed
reactions. Exclusive silylation of 2-phenylpyridine was
observed, which supports the expected reactivity boost that
stems from the presence of a directing group.
Scheme 2 Directed dehydrogenative silylation of aromatic and heteroaromatic rings.
Reaction conditions: Yield determined by 1H NMR using THF-d8. cat 1 (5 mol%),
norbornene (0.40 mmol), arene (0.13 mmol), HSiR3 (0.40 mmol) in THF (2 mL) at 110 oC.
c Isolated yields.
a
b
In order to assess the scope of
1 as a pre-catalyst for the
silylation of C–H bonds with different hydrosilanes, a variety of
aromatic substrates with and without a directing group
(Scheme 2 and Scheme 3, respectively) were examined. The
catalytic reactions were performed in THF at 110 °C in a sealed
flask using a 5 mol % catalyst loading and a hydrosilane/arene
ratio of 3:1.
The use of
1 as pre-catalysts permits the silylation of 2-(2-
thienyl)pyridine with a wide range of hydrosilanes, namely,
Et3SiH, Ph2MeSiH, PhMe2SiH, Ph3SiH and (EtO)3SiH.
Remarkably, to the best of our knowledge, these are the only
examples of the intermolecular catalytic silylation of aryl C–H
bonds that successfully employ triaryl-17 or trialkoxysilanes
(excluding the boron catalysed silylation of N,N-
dimethylaniline reported by Hou et al.10a and the silatranes
reported by Miyaura et al.13). However, in the case of the
latter, no product was recovered when purification of the
crude mixture was attempted by column chromatography.
Other substrates featuring nitrogen-containing directing
groups, namely, 1-phenylpyrazole, 2-phenylpyridine, and 2-(p-
tolyl)pyridine, were also successfully converted to the silylated
products, except for triethoxysilane (Scheme 2). To our
surprise, the silylation of 2-(p-tolyl)pyridine showed an
unexpected selectivity shift when aromatic silanes were used
instead of triethylsilane. At variance with previous examples,
Scheme 3 Non-directed dehydrogenative silylation of aromatic and heteroaromatic
a
b
rings. Reaction conditions: Disilylated product was identified in 7% yield. cat 1 (5
mol%), norbornene (0.40 mmol), arene (0.13 mmol), HSiR3 (0.40 mmol) in THF (2 mL) at
110 oC. c Isolated yields.
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al.,18b however, no kinetic or theoretical support for this
postulation has been presented so far.
In order to attain a better understanding of the catalytic cycle
that operates in these reactions, a computational study at the
DFT
D3(PCM)/def2TZVP//B3LYP-D3/def2SVP
considering pre-catalyst
level
was
performed
using
the
B3LYP-
level
theoretical
1, 2-phenylpyridine, HSiMe3 as model
for hydrosilane and NBE (norbornene) as hydrogen acceptor.
The energetic profiles for the directed silylation of 2-
phenylpyridine, with and without NBE as hydrogen acceptor,
are shown in Fig. 2 and Fig. 3.
The first part of the mechanism involves the dehydrogenation
of
species capable of undergoing cyclometallation of 2-
phenylpyridine. The dehydrogenation of with NBE requires
1 by the hydrogen acceptor to give a square planar Ir(I)
Scheme
4 Directed and non-directed dehydrogenative silylation of aromatic and
a
heteroaromatic rings with bis(hydrosilane)s. Reaction conditions: cat 1 (5 mol %),
norbornene (0.40 mmol), arene (0.13 mmol), bis(hydrosilane)s (0.40 mmol) in THF (2
mL) at 110 oC. b Isolated yields.
1
the exchange of the pyridine ligand by the olefin followed by
the migratory insertion of the double bond into one Ir–H bond
via 3‡ (“‡” denotes a transition state) surmounting an energy
barrier of 19.0 kcal·mol–1. The alkyl intermediate thus formed
and the remaining hydride ligand undergo reductive
elimination through 5‡ to give norbornane (NBA) and the Ir(I)
The selective synthesis of bisarylated bis(silanes) was achieved
by reaction of arenes with the bis(hydrosilane)s, employing
1
as pre-catalysts (Scheme 4). It is worth mentioning that, in
contrast with other examples in the literature, no formation of
monoarylated product10b was observed in spite of using excess
bis(hydrosilane)s. Due to its unique selectivity this reaction
may find application as a methodology for the chemoselective
synthesis of new conjugated organosilicon materials, which
have been hitherto prepared by means of stoichiometric
reactions3b,e,f,27 or catalytic silylation from aryl halides.28
square-planar intermediate 6. The overall dehydrogenation
process is exergonic (–15.2 kcal·mol–1) and features an
activation energy of 21.0 kcal·mol–1. Coordination of 2-
phenylpyridine (Phpy) and dissociation of pyridine affords 7,
which subsequently releases a second py ligand and undergoes
oxidative addition of the C–H bond adjacent to the pyridine
moiety through a barrierless process (S.I.) to yield
8 (–27.0
Mechanistic Insights.
kcal·mol–1). Alternatively, the non-directed o-, m- and p-
activations of the Ph ring present remarkably higher activation
barriers, and a certain amount of para or meta product would be
expected due to the similar energies of their transition states (see
The mechanistic knowledge on this type of reactions is mainly
restricted to the experimental study by Hartwig et al.21 on the
Rh(I)-catalyzed silylation of arenes, and the theoretical
calculations reported by Murata and co-workers on a Ru-
catalysed process.29 A plausible mechanism for an Ir(III)-
catalysed silylation reaction was proposed by Mashima et
S.I.)
. Hence, N-coordination of Phpy is required to explain the
selectivity of the reaction, similarly to Morokuma’s study 30
.
Fig. 2 DFT calculated Gibbs free energy profile at 110 °C and concentration 1M (in kcal·mol–1and relative to 1 and isolated molecules) for the Ir-catalysed silylation of 2-
phenylpyridine with a hydrogen acceptor.
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Fig. 3 DFT calculated Gibbs free energy profile at 110 °C and concentration 1M (in kcal·mol–1 and relative to 1 and isolated molecules) for the Ir-catalysed silylation of 2-
phenylpyridine without hydrogen acceptor.
At this point, coordination of py affords the resting state
which can be isolated by reacting with Phpy (vide infra). for the transformation of 7’ into
Coordination of the silane to yields 10, which undergoes a
complex assisted metathesis ( -CAM) between the Ir–C bond
9
,
equilibrium with 7 under the reaction conditions, thus allowing
1
8.
8
σ-
Reactivity Studies.
σ
Reactivity of 1. In the search for experimental evidence that
would support the mechanism proposed above, several
stoichiometric experiments were performed. The reaction of
of the phenyl moiety and the Si–H bond of the silane via
transition state 11‡, thus yielding the dihydride intermediate
12.
complex
1 at room temperature with 1 equivalent of 2-
An alternative Ir(V) pathway has been discarded since no
stationary point on the potential energy surface could be
found for the hypothetically conceivable Ir(V) intermediate
resulting from the oxidative addition of the silane to the
cyclometallated species, which agrees with the mechanism
proposed by Mashima and co-workers.18b Finally, the
substitution of the silylated substrate by a pyridine molecule
phenylpyridine (Phpy), 2-thienylpyridine (Thpy), 2-(p-
tolyl)pyridine (p-tolylpy) or 1-phenylimidazole (Phpz), with and
without
cyclometallated derivatives, complexes
5). In this regard, the sluggish formation of complexes
13 15 in the presence of norbornene at room temperature,
norbornene,
afforded
the
corresponding
15 (Scheme
and
9
and 13
-
9
-
and the concomitant generation of norbornane, agrees with
the calculated energy barrier (21.0 kcal·mol–1) for the
releases the reaction product and regenerates 1, this process
being neatly exergonic by –23.1 kcal·mol–1. The effective
activation energy for the catalytic cycle is 27.2 kcal·mol–1 based
on the energy span concept,31 defined in this case by off-cycle
formation of intermediate 9.
All the complexes were isolated as air stable solids and fully
characterised by multinuclear NMR spectroscopy. In addition,
species
Alternatively, the thermic dehydrogenation of
9 .
and transition state 11‡
the molecular structures of complexes
9 and 13 have been
1
to afford 6 is
determined by X-ray diffraction analysis on suitable crystals
obtained by slow diffusion of diethyl ether into a solution of
the corresponding complex in CH2Cl2 (Fig. 4 and 5).
also affordable under the reaction conditions but the overall
process is thermodynamically much less favourable (Fig. 3).
Noteworthy, no transition structures could be found for the
1
The most representative resonances in the H NMR are those
reductive elimination of H2 from 1 to form 6 plus hydrogen
(See S.I.).32 The thermodynamics for the acceptor and
acceptorless reaction profiles differ by 34.5 kcal/mol, which is
in the highfield region, corresponding to the hydrido ligands,
which shift upon cyclometallation of the substrate from δ –
22.48 ppm in
1 to δ –18.14, –19.30, –18.10 and –19.70 ppm
approximately the
ꢀH° for the hydrogenation of norbornene
(33.2 kcal/mol).33 In addition, the higher energy span found for
this process explains the lower reactivity observed for the
acceptorless reaction (27.2 kcal·mol–1 and 34.6 kcal·mol–1 for
the acceptor and acceptorless processes, respectively). The
possibility of oxidative addition of the silane over the NHC-Ir(I)
in 13, 14 and 15, respectively.
9,
intermediate
species (7’) is 11.4 kcal·mol–1 less stable than that resulting
from the oxidative addition of the C–H bond ( ) and only 6.0
kcal·mol–1 more stable than (
). Therefore, 7’ may be in
7 was also studied; however, the resulting
8
7
Scheme 5 Synthesis of complexes 9 and 13-15.
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Besides, APT plus HSQC and HMBC NMR experiments support
the metallation of the corresponding substrates thereby
confirming the directed C–H activation process.
The X-ray diffraction analysis provides valuable information
that may shed light into the selectivity patterns observed in
directed silylation. In both compounds the Ir(III) centre shows
a distorted octahedral geometry with the cyclometallated
ligand accommodated in the equatorial plane, cis to the IPr
ligand. The pyridine moiety of the Phpy-1H and Thpy-1H
ligands is situated trans to the hydride, thus allowing the two
py ligands to seat trans to the IPr ligand and the metallated
carbon atom.
The distorted geometry of
9 and 13 is attributable to the steric
repulsion between the cumbersome side arms of IPr and the
cyclometallated ligand. This causes the NHC ligand to move
away from Phpy-1H (
168.91(12) and C(1)-Ir(1)-N(37) 101.11(13) for
N(41) 170.95(15) and C(1)-Ir(1)-N(36) 100.97(15) for 13) and
closer to the apical py ligand (C(30)-Ir(1)-N(42) 82.93(13) for
9
) or Thpy-1H (13) (C(1)-Ir(1)-N(42)
9
or (C(1)-Ir(1)-
Fig. 5 ORTEP view of the cation [Ir(H)(IPr)(Thpy-1H)(py)2]+ in 13·1.5·CH2Cl2 with the
numbering scheme adopted. Most hydrogens are omitted for clarity and thermal
ellipsoids are at 50% probability. Selected bond lengths (Å) and angles (°): C(1)-N(2)
1.375(5), C(1)-N(5) 1.382(5), C(1)-Ir(1) 2.007(4), C(30)-Ir(1) 2.013(4), N(36)-Ir(1)
2.195(4), N(41)-Ir(1) 2.146(4), N(47)-Ir(1) 2.176(4), N(2)-C(1)-N(5) 102.8(3), C(1)-Ir(1)-
C(30) 97.21(17), C(1)-Ir(1)-N(41) 170.95(15), C(30)-Ir(1)-N(41) 83.69(16), C(1)-Ir(1)-
N(47) 92.20(15), C(30)-Ir(1)-N(47) 170.58(16), N(41)-Ir(1)-N(47) 86.97(14), C(1)-Ir(1)-
N(36) 100.97(15), C(30)-Ir(1)-N(36) 79.16(17), N(41)-Ir(1)-N(36) 88.05(13), N(47)-Ir(1)-
N(36) 99.36(15).
9
or C(30)-Ir(1)-N(41) 83.69(16) for 13). Moreover, the geometry
of the NHC is also affected: i) the yaw angle (in plane tilting of
the NHC) is ca. 10° for
9
and 13; ii) The methyl (iPr) group
situated above the py moiety of the cyclometallated ligand
shows a dihedral angle CAr(C–H)···Cipso(C-iPr)···CCH(iPr)···CMe(iPr) of ca.
26°, while the other iPr groups feature dihedral angles
between 40 and 57°. Both structural parameters are indicative
of the steric constrains originated upon cyclometallation. On
these grounds, an increase of steric hindrance in the system,
as is the case of p-tolylpy, which would be exacerbated
Scheme 6 Synthesis of complex 16.
by the use of aromatic silanes, may lead to the decoordination
of the py moiety, reductive elimination and, eventually,
oxidative addition of the C–H that affords the least
encumbered species. The metallated intermediate originated
from the oxidative addition of the C5–H bond is the one that
situates the methyl group furthest from de IPr ligand (see py-
silylation products described in Scheme 2).
The reaction of
1
with 1 equivalent of 2,2’-bipyridine (bipy) at
in CH2Cl2 affords complex
room temperature
[Ir(bipy)(H)2(IPr)(py)][BF4] (16) (Scheme 6), which shows no
catalytic activity. This suggests that the presence of the
chelating ligand, bipy, thwarts the activation of the arene,
which consequently inhibits the catalytic activity of the
complex. Moreover, the addition of pyridine (10 equivalents)
to the reaction of Phpy with Et3SiH, under the conditions
described in table 2, resulted in a significant decrease of the
Fig.
4
ORTEP view of the cation [Ir(H)(IPr)(Phpy-1H)(py)2]+ in 9·CH2Cl2 with the
numbering scheme adopted. Most hydrogens are omitted for clarity and thermal
ellipsoids are at 50% probability. Selected bond lengths (Å) and angles (°): C(1)-N(2)
1.375(4), C(1)-N(5) 1.376(4), C(1)-Ir(1) 2.019(4), C(30)-Ir(1) 2.017(3), N(37)-Ir(1)
2.186(3), N(42)-Ir(1) 2.138(3), N(48)-Ir(1) 2.194(3), N(2)-C(1)-N(5) 102.5(3), C(30)-Ir(1)-
C(1) 99.60(14), C(30)-Ir(1)-N(42) 82.93(13), C(1)-Ir(1)-N(42) 168.91(12), C(30)-Ir(1)-
N(37) 79.42(13), C(1)-Ir(1)-N(37) 101.11(13), N(42)-Ir(1)-N(37) 89.96(12), C(30)-Ir(1)-
N(48) 164.74(12), C(1)-Ir(1)-N(48) 95.37(12), N(42)-Ir(1)-N(48) 81.81(11), N(37)-Ir(1)-
N(48) 100.63(12).
1
catalytic activity. In this case, the H MNR of the crude shows
only a 57% conversion, which contrast with the example
reported in table 2 (without added py) were total conversion
was obtained from the crude mixture.
6 | J. Name., 2012, 00, 1-3
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Reactivity of the cyclometallated complexes. The addition of Furthermore, these species become inactive if the position
3 equivalents of triethylsilane to a solution of in CH2Cl2 at trans to the NHC, where silane coordination should take place,
9
room temperature renders the starting complex unaltered, is blocked with a phosphane ligand; (ii) the reaction rates are
which is consistent with the higher temperatures required for significantly reduced in the presence of excess py; moreover,
the formation of the organosilane and the calculated energy when the two coordination sites trans to the hydrides in
barrier for this process (27.2 kcal·mol–1 from to 11‡). blocked with bipy, the resulting complex, 16
is not
Attempts to identify reaction intermediates in situ by NMR competent catalyst for the silylation of Phpy with Et3SiH; (iii)
spectroscopy in 1,1,2,2-tetrachloroethane-d2 showed that no complex only reacts with the silane at high temperatures to
reaction takes place up to 100 °C. afford directly the silylated product, which is in agreement
With the intention of finding support for the calculated with the -CAM reaction being the rate limiting step followed
mechanism, cyclometallated complexes and 14 were by a downslope process toward the organosilane
1 are
9
,
a
2
σ
9
1.
employed as pre-catalysts under the reaction conditions Finally, an experiment employing PhMe2SiD and Phpy showed
described in Scheme 2. The reaction of Phpy with Et3SiH no deuterium incorporation into the silylated product, which
catalysed by
catalysed by 14 gave the silylated products in 81% and 54%
yield, respectively (almost identical yields compared to ).
These experiments, together with the DFT calculations, seem
to suggest that may be a resting state that enters the
catalytic cycle upon loss of a pyridine ligand.
Moreover, complex related to
[Ir(CH3CN)(H)(IPr)(Phpy-1H)(PPhMe2)][BF4]
9 and the reaction of p-tolylpy with Ph2MeSiH also agrees with the proposed mechanism.
1
Conclusions
9
We have prepared a well-defined Ir(III) complex that acts as an
efficient pre-catalyst for the intermolecular silylation of a wide
variety of arenes and heteroarenes with and without a
directing group. Moreover, in view of expanding the synthetic
applicability of this reaction the (hetero)arene was successfully
employed in all cases as limiting reagent. This process is
compatible with the use of several hydrosilanes, including
examples with Et3SiH, Ph2MeSiH, PhMe2SiH, Ph3SiH and
(EtO)3SiH. Noteworthy, in certain cases, the presence of
aromatic substituents in the hydrosilanes triggers
unprecedented selectivity patterns worthy of a more in-depth
a
1
,
namely
which
(
17),
presents a PPhMe2 ligand trans to the NHC and an acetonitrile
cis to the hydride, instead of the apical and equatorial pyridine
ligands in 1, was prepared (Scheme 7). When complex 17 was
used as catalyst for the reaction of Phpy with Et3SiH, under the
reaction conditions described in Scheme 2, no silylated
product is obtained. The fact that 17 is inactive toward the
silylation of Phpy agrees with the proposed mechanism, since
a labile position trans to the IPr ligand is required for the end-
on coordination of the silane. Complex 17 features a strongly
coordinating ligand trans to the NHC that blocks this
coordination site, while the availability of an easily accessible
position cis to the hydride does not seem to play any role in
the reaction, which further supports the calculated
mechanism. In this regard, the use of the IPr ligand probably
facilitates the dissociation of the trans positioned py (NHCs
feature stronger trans effects than the ligands usually
employed for these transformations),34 thus generating an
available coordination site that may account for the
unexpected activity of this system towards less reactive
silanes, e.g. (EtO)3SiH.
study in the future. The use of
1 as pre-catalyst also permits
the efficient bisarylation of bis(hydrosilane)s by directed or
non-directed silylation of C–H bonds, which may provide a new
tool for the synthesis of conjugated organosilicon materials.
The mechanistic studies performed in this work point toward
an Ir(III)/Ir(I) mechanism where the dehydrogenation of the
Ir(III) species
intermediate (
1
generates
a very electron-rich NHC-Ir(I)
6) that allows for the facile activation of the
arene’s C–H bond.
Experimental
General Considerations.
In summary, the reactivity shown by complex
cyclometallated complexes and 13 15 is in accordance with
with the calculated reaction profile since: (i) the addition of
the arene to gives the corresponding resting states of the
catalytic cycle (complexes and 13 15), which exhibit virtually
1 and the
All experiments were carried out under an inert atmosphere
using standard Schlenk techniques. The solvents were dried by
known procedures and distilled under argon prior to use or
obtained oxygen- and water-free from a Solvent Purification
System (Innovative Technologies). The starting complex was
9
-
1
9
-
identical catalytic activity compared to 1.
prepared
according
to
a
literature
procedure
[Ir(COD)(IPr)(acetone)][BF4].[25f] All other commercially
available starting materials were purchased from Sigma-
Aldrich, Merck and J. T. Baker and were used without further
purification. H2 gas (>99.5 %) was obtained from Infra.
1H, 13C{1H}, 19F, H-29Si HMBC, H-13C HMBC, H-13C HSQC and
1H-1H COSY NMR spectra were recorded either on a Bruker
ARX 300 MHz or a Bruker Avance 400 MHz instruments.
Chemical shifts (expressed in parts per million) are referenced
to residual solvent peaks for 1H and 13C{1H}, and to an external
1
1
1
Scheme 7 Synthesis of complex 17.
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reference of CFCl3 for 19F. Coupling constants, J, are given in CD2Cl2, 298 K): δ 165.1 (s, C2-py), 153.5 (s, Co-py-a), 152.7 (s, Co-py-
Hz. Spectral assignments were achieved by combination of 1H- b), 150.8 (s, Ir-CIPr), 148.2 (s, C6-py), 146.6 and 146.4 (both s, Cq-
1H COSY, 13C APT and 1H-13C HSQC/HMBC experiments. C, H, IPr), 145.3 (s, Ir-CPh), 143.6 (s, Cq-Ph), 143.4 (s, Cm2-Ph), 138.0 (s,
and N analyses were carried out in a Perkin-Elmer 2400 Cp-py-a), 137.1 (s, CqN), 137.0 (s, Cp-py-b), 136.8 (s, C4-py), 130.2 (s,
CHNS/O analyser. GC-MS spectra were recorded on a Hewlett- Cp-IPr), 129.5 (s, Cp-Ph), 126.2 (s, Cm-py-a), 125.7 (s, Cm-py-b), 125.6
Packard GC-MS system. Column chromatography was (s, =CHN), 124.4 and 123.7 (both s, Cm-IPr), 123.6 (s, Co-Ph), 123.0
performed using silica gel (70-230 mesh).
(s, C5-py), 121.4 (s, Cm1-Ph), 119.9 (s, C3-py), 29.0 and 28.9 (s,
CHMeIPr), 26.9, 26.2, 21.3, and 20.8 (all, s, CHMeIPr). 19F NMR
(400 NMR, CD2Cl2, 298 K): δ –152.5 (s, BF4). Anal. Calcd. for
C48H55IrN5BF4 (981.41): C, 58.77; H, 5.65; N, 7.14. Found: C,
58.70; H, 5.66; N, 7.16.
Synthesis and Characterisation of Complexes 9 and 13-17.‡
[Ir(H)2(IPr)(py)3][BF4]
(1)
.
A
solution
of
[Ir(COD)(IPr)(acetone)][BF4] (300 mg, 0.36 mmol) in acetone
(10 mL) was reacted with pyridine (0.5 mL) and stirred under a
hydrogen atmosphere (1 bar) for 1 h. The resulting pale yellow
solution was concentrated to ca. 0.5 mL, and treated with
diethyl ether to afford a white solid. The solid was separated
by decantation, washed with diethyl ether, and dried in vacuo.
A CH2Cl2 solution (0.4 mL) of this solid (12 mg) was layered
with diethyl ether (5 mL) and stored into a glove box at room
temperature to afford crystals suitable for X-ray diffraction.
Yield: 72% (234 mg, 0.25 mmol). 1H NMR (300 MHz, CD2Cl2,
263 K): δ 8.14 (d, JH-H = 5.0, 4H, Ho-py-a), 7.84 (d, JH-H = 5.7, 2H,
Ho-py-b), 7.71 (t, JH-H = 7.6, 2H, Hp-py-a), 7.66 (t, JH-H = 7.5, 1H, Hp-
py-b), 7.30 (t, JH-H = 7.7, 2H, Hp-IPr), 7.11 (d, JH-H = 7.7, 4H, Hm-IPr),
7.09 (dd, JH-H = 7.6, 5.0, 4H, Hm-py-a), 7.07 (s, 2H, =CHN), 6.96
(dd, JH-H = 7.5, 5.7, 4H, Hm-py-b), 2.87 (sept, JH-H = 6.9, 4H,
CHMeIPr), 1.16 and 1.11 (both d, JH-H = 6.9, 24H, CHMeIPr), –
22.48 (s, 2H, Ir-H). 13C {1H}-APT NMR plus HSQC and HMBC (75
MHz, CD2Cl2, 298 K): δ 155.3 (s, Co-py-b), 154.7 (s, Ir-CIPr), 153.5
(s, Co-py-a), 145.6 (s, Cq-IPr), 138.1 (s, CqN), 136.8 (s, Cp-py-b), 136.6
(s, Cp-py-a), 129.9 (s, Cp-IPr), 125.9 (s, Cm-py-a), 125.7 (s, Cm-py-b),
123.8 (s, Cm-IPr), 28.9 (s, CHMeIPr), 25.9 and 21.6 (both s,
CHMeIPr). 19F NMR (400 NMR, CD2Cl2, 298 K): δ –155.2 (s, BF4).
Anal. Calcd. for C42H54BF4IrN5 (908.40 + CH2Cl2): C, 52.02; H,
5.69; N, 7.05. Found: C, 51.95; H, 5.85; N, 6.85.
[Ir(H)(IPr)(Thpy-1H)(py)2][BF4] (13). 2-Thienylpyridine (21 mg,
0.13 mmol) was added to a solution of
1 (120 mg, 0.13 mmol)
in 5 mL of dichloromethane and the resulting solution was
stirred for 40 min at room temperature. After this time, the
resulting light yellow solution was concentrated to ca. 0.5 mL
and diethyl ether was added to give a white solid. The solid
thus formed was separated by decantation, washed with
diethyl ether and dried in vacuum. Yield: 67% (87 mg, 0.09
1
mmol). H NMR (400 MHz, CD2Cl2, 283 K): δ 8.27 (d, JH-H = 5.3,
2H, Ho-py-a), 7.92 (t, JH-H = 7.8, 1H, Hp-py-a), 7.63 (dd, JH-H = 7.9,
6.9, 1H, H4-py), 7.49 (t, JH-H = 7.6, 2H, Hp-IPr), 7.48 (d, JH-H = 7.9,
1H, H3-py), 7.47 (t, JH-H = 7.1, 1H, Hp-py-b), 7.30 (dd, JH-H = 7.8, 5.3,
2H, Hm-py-a), 7.29 (d, JH-H = 6.2, 1H, Ho-py-b), 7.25 and 7.13 (both
d, JH-H = 7.6, 4H, Hm-IPr), 7.20 (d, JH-H = 5.5, 1H, H6-py), 7.11 (s, 2H,
=CHN), 6.94 and 5.48 (both d, JH-H = 4.7, 2H, HTh), 6.78 (dd, JH-H
= 7.1, 6.2, 2H, Hm-py-b), 6.66 (dd, JH-H = 6.9, 5.5, 1H, H5-py), 2.86
and 2.28 (both sept, JH-H = 6.9, 4H, CHMeIPr), 1.13, 1.06, 1.05,
and 0.55 (all d, JH-H = 6.9, 24H, CHMeIPr), –19.30 (s, 1H, Ir-H).
13C {1H}-APT NMR plus HSQC and HMBC (100 MHz, CD2Cl2, 298
K): δ 160.9 (s, C2-py), 154.0 (s, Co-py-a), 152.4 (s, Co-py-b), 150.1 (s,
Ir-CIPr), 148.5 (s, Ir-CTh), 148.4 (s, C6-py), 146.6 and 146.2 (both s,
Cq-IPr), 140.0 and 128.0 (both s, CTh), 138.1 (s, Cp-py-a), 137.3 (s,
C4-py), 137.1 (s, CqN), 137.0 (s, Cp-py-b), 136.8 (s, Cq-Th), 130.3 (s,
Cp-IPr), 126.3 (s, Cm-py-a), 125.5 (s, Cm-py-b), 125.4 and 124.3 (both
s, Cm-IPr), 123.8 (s, =CHN), 120.4 (s, C5-py), 119.2 (s, C3-py), 29.1
and 28.9 (s, CHMeIPr), 27.0, 26.3, 21.4, and 20.7 (all, s,
CHMeIPr). 19F NMR (400 NMR, CD2Cl2, 298 K): δ –153.0 (s, BF4).
Anal. Calcd. for C46H53IrN5BF4 (995.40): C, 55.98; H, 5.41; N,
7.10. Found: C, 55.93; H, 5.46; N 7.10.
[Ir(H)(IPr)(Phpy-1H)(py)2][BF4] (9). 2-Phenylpyridine (19 µL,
0.13 mmol) was added to a solution of
1 (120 mg, 0.13 mmol)
in 5 mL of dichloromethane and the resulting solution was
stirred for 40 min at room temperature. After this time, the
resulting light yellow solution was concentrated to ca. 0.5 mL
and diethyl ether was added to give a white solid. The solid
thus formed was separated by decantation, washed with
diethyl ether and dried in vacuum. Yield: 63% (82 mg, 0.08
mmol). A CH2Cl2 solution (0.3 mL) of this solid (10 mg) was
layered with diethyl ether (5 mL) and stored into a glove box at
[Ir(H)(IPr)(py)2(p-tolylpy-1H)][BF4] (14). 2-(p-Tolyl)pyridine (22
mg, 0.13 mmol) was added to a solution of
1 (120 mg, 0.13
mmol) in 5 mL of dichloromethane and the resulting solution
was stirred for 40 min at room temperature. After this time,
the resulting light yellow solution was concentrated to ca. 0.5
mL and diethyl ether was added to give a light yellow solid.
The solid thus formed was separated by decantation, washed
with diethyl ether and dried in vacuum. Yield: 67% (88 mg,
room temperature to afford crystals suitable for X-ray
1
diffraction. H NMR (300 MHz, CD2Cl2, 298 K): δ 8.24 (d, JH-H
=
5.1, 2H, Ho-py-a), 7.92 (t, JH-H = 6.6, 1H, Hp-py-a), 7.91 (d, JH-H = 7.9,
1H, H3-py), 7.72 (dd, JH-H = 7.9, 6.7, 1H, H4-py), 7.55 (d, JH-H = 7.9,
1H, Ho-Ph), 7.49 (t, JH-H = 6.7, 1H, Hp-py-b), 7.47 (t, JH-H = 6.6, 2H,
Hp-IPr), 7.40 (d, JH-H = 5.6, 1H, H6-py), 7.37 (d, JH-H = 6.0, 2H, Ho-py-
b), 7.31 (dd, JH-H = 6.6, 5.1, 2H, Hm-py-a), 7.11 (d, JH-H = 6.6, 4H,
Hm-IPr), 7.09 (s, 2H, =CHN), 6.78 (dd, JH-H = 6.7, 5.6, 1H, H5-py),
6.77 (dd, JH-H = 6.7, 6.0, 2H, Hm-py-b), 6.75 (dd, JH-H = 7.9, 7.7, 1H,
Hm1-Ph), 6.46 (dd, JH-H = 8.2, 7.7, 1H, Hp-Ph), 5.97 (d, JH-H = 8.2,
1H, Hm2-Ph), 2.87 and 2.25 (both sept, JH-H = 6.6, 4H, CHMeIPr),
1.11, 1.05, 1.02, and 0.43 (all d, JH-H = 6.6, CHMeIPr), –18.14 (s,
1H, Ir-H). 13C {1H}-APT NMR plus HSQC and HMBC (75 MHz,
0.09 mmol). 1H NMR (400 MHz, CD2Cl2, 298 K): δ 8.20 (d, JH-H
=
5.1, 2H, Ho-py-a), 7.93 (t, JH-H = 6.9, 1H, Hp-py-a), 7.90 (d, JH-H = 7.6,
1H, H3-py), 7.69 (dd, JH-H = 7.6, 6.9, 1H, H4-py), 7.49 (t, JH-H = 6.9,
1H, Hp-py-b), 7.48 (both t, JH-H = 7.9, 2H, Hp-IPr), 7.46 (d, JH-H = 8.2,
1H, Ho-Ph), 7.36 (br, 2H, Ho-py-b), 7.33 (d, JH-H = 5.8, 1H, H6-py),
7.30 (dd, JH-H = 6.9, 5.1, 2H, Hm-py-a), 7.23 and 7.09 (both d, JH-H
= 7.9, 4H, Hm-IPr), 7.22 (s, 2H, =CHN), 6.79 (dd, JH-H = 6.9, 5.3,
2H, Hm-py-b), 6.71 (dd, JH-H = 6.9, 5.8, 1H, H5-py), 6.58 (d, JH-H
=
8.2, 1H, Hm1-Ph), 5.75 (s, 1H, Hm2-Ph), 2.91 and 2.20 (both br, 4H,
8 | J. Name., 2012, 00, 1-3
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CHMeIPr), 1.95 (s, 3H, Me), 1.15, 1.04, 1.03, and 0.37 (all d, JH-H 6.9, 24H, CHMeIPr), –19.80 (s, 2H, Ir-H). 13C {1H}-APT NMR plus
= 6.2, 24H, CHMeIPr), –18.10 (s, Ir-H). 13C {1H}-APT NMR plus HSQC and HMBC (75 MHz, CD2Cl2, 298 K): δ 156.3 (s, Cq-dipy),
HSQC and HMBC (100 MHz, CD2Cl2, 298 K): δ 165.0 (s, C2-py), 155.4 (s, Co-py), 153.0 (s, Co-dipy), 150.5 (s, Ir-CIPr), 147.4 (s, Cq-IPr),
153.5 (s, Co-py-a), 152.4 (s, Co-py-b), 151.2 (s, CIPr-Ir), 147.9 (s, C6- 137.0 (s, Cp-dipy), 136.9 (s, CqN), 136.7 (s, Cp-py), 130.1 (s, Cp-IPr),
py), 146.5 and 146.4 (both s, Cq-IPr), 145.2 (s, Ir-CPh), 143.9 (s, 127.0 (s, Cm1-dipy), 125.1 (s, Cm-py), 124.5 (s, Cm-IPr), 123.5 (s,
Cm2-Ph), 141.1 (s, Cq-Ph), 139.3 (s, Cq-Me), 138.1 (s, Cp-py-a), 137.0 =CHN), 123.1 (s, Cm2-dipy), 28.8 (s, CHMeIPr), 25.5 and 21.3 (s,
(s, Cp-py-b), 136.9 (s, CqN), 136.6 (s, C4-py), 130.3 (s, Cp-IPr), 126.1 CHMeIPr). 19F NMR (400 NMR, CD2Cl2, 298 K): δ –153.1 (s, BF4).
(s, Cm-py-a), 125.7 (s, Cm-py-b), 125.2 and 124.2 (both s, Cm-IPr), Anal. Calcd. for C42H51BF4IrN5 (905.38): C, 55.75; H, 5.68; N,
123.7 (s, =CHN), 123.6 (s, Co-Ph), 122.8 (s, Cm1-Ph), 122.5 (s, C5- 7.74. Found: C, 56.24; H, 5.73; N 7.59.
py), 119.7 (s, C3-py), 29.0 and 28.9 (both s, CHMeIPr), 26.9, 26.2, [Ir(CH3CN)(H)(IPr)(Phpy-1H)(PPhMe2)][BF4] (17). A solution of
21.3, and 20.3 (all s, CHMeIPr), 21.4 (s, Me). 19F NMR (400 NMR, [Ir(COD)(IPr)(acetone)][BF4] (150 mg, 0.18 mmol) in
CD2Cl2, 298 K): δ –152.9 (s, BF4). Anal. Calcd. for C49H57IrN5BF4 acetonitrilee (5 mL) was stirred under a hydrogen atmosphere
(995.43): C, 59.15; H, 5.77; N 7.04. Found: C, 59.15; H, 5.76; N, (1 bar) for 1 h. The solvent was removed under reduced
7.10.
[Ir(H)(IPr)(Phpz-1H)(py)2][BF4] (15). 1-Phenylpyrazole (17 µL, redissolved in dichloromethane (5 mL). Subsequently, the
0.13 mmol) was added to a solution of (120 mg, 0.13 mmol) resulting solution was treated with dimethylphenylphosphine
pressure and the remaining pale yellow residue was
1
in 5 mL of dichloromethane and the resulting solution was (0.19 mmol, 27 μL) and allowed to react at room temperature
stirred for 1 h at room temperature. After this time, the for 30 min. Then, 1-phenylpyrazole (25 µL, 0.19 mmol) was
resulting light yellow solution was concentrated to ca. 0.5 mL added to the solution and stirred at room temperature for 1 h.
and diethyl ether was added to give a white solid. The solid The resulting pale yellow solution was filtered through Celite,
thus formed was separated by decantation, washed with concentrated to ca. 0.5 mL, and treated with diethyl ether to
diethyl ether and dried in vacuum. Yield: 65% (77 mg, 0.09 afford a white solid. The solid was separated by decantation,
1
mmol). H NMR (400 MHz, CD2Cl2, 283 K): δ 8.30 (d, JH-H = 5.1, washed with diethyl ether, and dried in vacuo. Yield: 64% (115
1
2H, Ho-py-a), 7.81 (d, JH-H = 2.9, 1H, H5-pz), 7.80 (t, JH-H = 6.8, 1H, mg, 0.11 mmol). H NMR (400 MHz, CD2Cl2, 298 K): δ 7.86 (d,
Hp-py-a), 7.44 (t, JH-H = 8.0, 2H, Hp-IPr), 7.36 (t, JH-H = 6.9, 1H, Hp-py- JH-H = 5.4, 1H, H6-py), 7.53 (d, JH-H = 7.9, 1H, H3-py), 7.47 (dd, JH-H
b), 7.35 (d, JH-H = 5.4, 2H, Ho-py-b), 7.21 (dd, JH-H = 6.8, 5.1, 2H, = 7.9, 6.8, 1H, H4-py), 7.46 (d, JH-H = 7.6, 1H, Ho-Phpy), 7.45 (t, JH-H
Hm-py-a), 7.14 (d, JH-H = 1.9, 1H, H3-pz), 7.08 (s, 2H, =CHN), 7.07 = 7.7, 2H, Hp-IPr), 7.38 and 7.04 (both d, JH-H = 7.7, 4H, Hm-IPr),
(d, JH-H = 8.0, 4H, Hm-IPr), 6.92 (d, JH-H = 7.6, 1H, Ho-Ph), 6.79 (dd, 7.25 (t, JH-H = 7.2, 1H, Hp-Ph), 7.16 (ddd, JH-H = 7.8, 7.2, JH-P = 2.1,
JH-H = 7.6, 7.1, 1H, Hm1-Ph), 6.76 (d, JH-H = 7.4, 1H, Hm2-Ph), 6.71 2H, Hm-Ph), 7.07 (s, 2H, =CHN), 6.92 (dd, JH-H = 7.6, 7.1, Hm1-phpy),
(dd, JH-H = 6.9, 2H, Hm-py-b), 6.69 (dd, JH-H = 7.4, 7.1, 1H, Hp-Ph), 6.81 (dd, JH-H = 7.4, 7.1, 1H, Hp-Phpy), 6.72 (dd, JH-P = 9.7, JH-H
6.45 (dd, JH-H = 2.9, 1.9, 1H, H4-pz), 2.85 and 2.49 (both sept, JH-H 7.8, 2H, Ho-Ph), 6.69 (d, JH-H = 7.4, 1H, Ho-Phpy), 6.62 (dd, JH-H
=
=
= 6.9, 4H, CHMeIPr), 1.12, 1.09, 1.02, and 0.76 (all d, JH-H = 6.9, 6.8, 5.4, 1H, H5-py), 2.55 and 2.41 (both sept, JH-H = 6.8, 4H,
CHMeIPr), –19.70 (s, 1H, Ir-H). 13C {1H}-APT NMR plus HSQC and CHMeIPr), 2.09 (s, 3H, MeCN), 1.35, 1.14, 1.02, and 0.86 (all d,
HMBC (100 MHz, CD2Cl2, 283 K): δ 154.6 (s, Co-py-a), 152.0 (s, Co- JH-H = 6.8, 24H, CHMeIPr), 1.23 and 0.57 (both d, JH-P = 9.7, 6H,
py-b), 149.1 (s, Ir-CIPr), 146.4 and 146.0 (both s, Cq-IPr), 143.9 (s, PMe), -18.1 (d, JH-P = 17.9, IrH). 13C {1H}-APT NMR plus HSQC
Ir-CPh), 143.0 (s, Cm2-Ph), 138.7 (s, C3-pz), 137.6 (s, Cp-py-a), 136.9 and HMBC (75 MHz, CD2Cl2, 298 K): δ 163.5 (s, C2-py), 163.3 (d,
(s, Cp-py-b), 130.0 (s, Cp-IPr), 128.5 (s, Cq-Ph), 126.4 (s, C5-pz), 126.3 JH-P = 118.7, Ir-CIPr), 149.4 (s, C6-py), 145.9 and 145.2 (both s, Cq-
(s, Cm-py-a), 126.2 (s, Cp-Ph), 125.3 (s, Cm-py-b), 125.2 (s, =CHN), IPr), 143.9 (d, JH-P = 2.8, Cq-Phpy), 143.0 (s, Cm2-Phpy), 142.8 (d, JH-P
123.8 and 123.6 (both s, Cm-IPr), 122.6 (s, Cm1-Ph), 111.1 (s, Co-Ph), = 11.7, Ir-CPh), 137.4 (s, CqN), 135.5 (s, C4-py), 132.8 (d, JH-P
=
107.6 (s, C4-pz), 29.1 and 28.9 (both s, CHMeIPr), 26.8, 26.3, 48.8, Cq-Ph), 130.3 (s, Cp-IPr), 129.9 (s, Cp-Phpy), 129.1 (d, JH-P = 2.5,
21.2, and 21.2 (all s, CHMeIPr). Anal. Calcd. for C46H55BF4IrN6 Cp-Ph), 129.0 (d, JH-P = 8.5, Cm-Ph), 128.1 (d, JH-P = 9.2, Co-Ph),
(971.41 + 0.5·CH2Cl2): C, 55.11; H, 5.57; N, 8.29. Found: C, 125.2 and 125.1 (both s, =CHN), 123.9 (s, Co-Phpy), 123.9 and
55.08; H, 5.85; N, 8.61.
[Ir(bipy)(H)2(IPr)(py)]BF4 (16). 2,2´-bipyridine (16 mg, 0.10 MeCN), 118.6 (s, C3-py), 28.5 and 28.4 (both s, CHMeIPr), 26.8,
mmol) was added to a solution of (80 mg, 0.10 mmol) in 5 25.3, 22.8, and 21.5 (all s, CHMeIPr), 13.8 and 9.3 (both d, JH-P
123.4 (both s, Cm-IPr), 122.4 (s, C5-py), 120.7 (s, Cm1-Phpy), 118.7 (s,
1
=
mL of CH2Cl2. The resulting solution was stirred for 30 min at 41.5, PMe), 3.4 (s, MeCN). 31P NMR (100 NMR, CD2Cl2, 298 K):
room temperature. After this time, the resulting yellow δ –28.0. 19F NMR (400 NMR, CD2Cl2, 298 K): δ –152.5 (s, BF4).
solution was concentrated to ca. 0.5 mL and diethyl ether was Anal. Calcd. for C48H60BF4IrN4P (1003.42 + CH2Cl2): C, 54.10; H,
added to give a yellow solid. This solid was separated by 5.74; N, 5.15. Found: C, 54.89; H, 6.08; N, 5.62.
decantation, washed with diethyl ether and dried in vacuum.
General Procedure for the Catalytic Silylation of C-H Bonds.
1
Yield: 79% (69 mg, 0.0764 mmol). H NMR (400 MHz, CD2Cl2,
A sealed flask was charged with complex
1 (5 mol%), THF (2.0
298 K): δ 8.67 (d, JH-H = 8.1, 2H, Hm2-dipy), 8.56 (d, JH-H = 6.1, 2H,
o-py), 8.42 (dd, JH-H = 8.1, 7.8, 2H, Hp-dipy), 8.29 (t, JH-H = 7.6, 2H,
mL), the arene (1 eq., 0.13 mmol), norbornene (3 eq., 0.40
mmol) and the hydrosilane (3 eq., 0.40 mmol). The solution
was kept at 110 °C in a thermostatic bath for the reaction time
described in the article. The progress of the reactions was
monitored by 1H NMR spectroscopy and the conversion was
H
Hp-IPr), 8.05 (t, JH-H = 7.6, 1H, Hp-py), 7.97 (d, JH-H = 7.6, 4H, Hm-
IPr), 7.80 (d, JH-H = 5.1, 2H, Ho-dipy), 7.67 (dd, JH-H = 7.8, 5.1, 2H,
Hm1-dipy), 7.54 (s, 2H, =CHN), 7.43 (dd, JH-H = 7.6, 6.1, 2H, Hm-py),
3.28 (sept, JH-H = 6.9, 4H, CHMeIPr), 1.69 and 1.50 (both d, JH-H
=
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Journal Name
and T. Hiyama, Bull. Chem. Soc. Jpn., 2000, 73, 1409–1417; d)
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determined by integration of the starting material with the
products. At the end of the reaction, the solution was
concentrated under reduced pressure to afford the crude
residue, which was purified by column chromatography on
silica gel using mixtures of hexane/ethyl acetate to isolate the
corresponding product.
2
5
6
4
e) L. J. Gooßen and A.-R. S. Ferwanah, Synlett, 2000, 12
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Acknowledgements
For examples of intramolecular silylation see: a) E. M.
Simmons and J. F. Hartwig, J. Am. Chem. Soc. 2010, 132
,
This work was supported by the Spanish Ministry of Economy
and Competitiveness (MINECO/FEDER) (CONSOLIDER INGENIO
CSD2009-0050, CTQ2015-67366-P and CTQ2013-42532-P
projects) and the DGA/FSE-E07. The support from KFUPM-
University of Zaragoza research agreement and the Centre of
17092–17095; b) Q. Li, M. Driess and J. F. Hartwig, Angew.
Chem., Int. Ed. 2014, 53, 8471–8474; c) A. Kuznetsov and V.
Gevorgyan, Org. Lett. 2012, 14, 914–917; d) A. Kuznetsov, Y.
Onishi, Y. Inamoto and V. Gevorgyan, Org. Lett. 2013, 15
2498–2501.
,
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For examples of directed silylation with disilanes see: a) M.
Tobisu, Y. Ano and N. Chatani, Chem. Asian J. 2008, , 1585–
Research Excellence in Petroleum Refining
& KFUPM is
3
gratefully acknowledged. V. P. thankfully acknowledges the
resources from the supercomputer "Memento", technical
expertise and assistance provided by BIFI-ZCAM (Universidad
de Zaragoza). L.R.-P thanks CONACyT for a postdoctoral
fellowship (204033). J. M. acknowledges financial support from
the Ministry of Education Culture and Sports (FPU14/06003).
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