Synthesis and isolation of an acyclic tridentate bis(pyridine)carbodicarbene and studies on its structural implications and reactivities

Article abstract of DOI:10.1002/anie.201406481

The simple synthetic development of acyclic pincer bis(pyridine)carbodicarbene is depicted herein. Presented is the first isolated structural pincer carbodicarbene with a C-C-C angle of 143°, larger than the monodentate framework. More importantly, theore

Full text of DOI:10.1002/anie.201406481

Angewandte
Chemie
DOI: 10.1002/anie.201406481
Carbodicarbenes
Synthesis and Isolation of an Acyclic Tridentate Bis(pyridine)carbo-
dicarbene and Studies on Its Structural Implications and Reactivities**
Yu-Chen Hsu, Jiun-Shian Shen, Bo-Chao Lin, Wen-Ching Chen, Yi-Tsu Chan, Wei-Min Ching,
Glenn P. A. Yap, Chao-Ping Hsu,* and Tiow-Gan Ong*
Abstract: The simple synthetic development of acyclic pincer
bis(pyridine)carbodicarbene is depicted herein. Presented is
the first isolated structural pincer carbodicarbene with a C-C-C
angle of 1438, larger than the monodentate framework. More
importantly, theoretical analysis showed that this carbodicar-
bene embodies a more allene-like character. Palladium com-
plexes supported by this pincer ligand are active catalysts for
Heck–Mizoroki and Suzuki–Miyaura coupling reactions.
Scheme 1. Known and new carbodicarbenes.
S
teady research efforts toward the development of unique
bonding environments for carbon species, with the aim of
defying the “octet rule”, have been a fundamental challenge
in organic chemistry for centuries. This curiosity-driven work
eventually led to the isolation of stable free six-electron
carbon species, namely phosphino–silyl carbenes[1a,b] and N-
heterocyclic carbenes (NHC).[1c,d] More importantly, the
discovery of NHCs also generated numerous breakthroughs
in organometallic chemistry and catalysis.[2]
metal complexes supported by pincer ligands are gaining
increasing momentum in contemporary science, owing to
their ability to activate strong bonds[7] and their utility in
catalysis.[8] We envisioned that a rationally designed pincer
bis(pyridine)carbodicarbene with rigid tridentate chelation,
such as that shown in C, would be desirable, as such a ligand
design will lead to the accumulation of new chemical knowl-
edge as well as new potential catalytic applications.[9] Herein,
we report the synthesis and characterization of bis-
(pyridine)carbodicarbene and its pincer palladium com-
plexes. At this stage, these captodative, carbogenic com-
pounds are still largely regarded as a laboratory curiosity,
rarely used for supporting metal-mediated catalytic reactions.
In order to evaluate the catalytic activity of the palladium
complexes supported by pincer carbodicarbene, we have also
A few years ago, the groups of Bertrand[3] and Fꢀrstner[4]
pioneered a rather peculiar carbon(0)-based class of com-
pounds, coined as “carbodicarbenes” or “bent allenes”, by
synthesizing the corresponding compounds A and B, respec-
tively (Scheme 1). Subsequently, our laboratory has under-
taken the synthetic endeavor of increasing the structural
diversity of carbodicarbene A, with the hopes of extending its
utility in organometallic chemistry.[5] To date, the reported
carbodicarbenes and their metal complexes are limited
essentially to a monodentate framework.[6] Therefore, the
need exists to develop a pincer or multidentate-type carbo-
dicarbene to explore new chemical reactivity. Moreover,
À
reported their utility in catalyzing C C cross-couplings such
as Heck–Mizoroki and Suzuki–Miyaura reactions.
The preliminary synthesis of the dicationic salt 4, the
precursor of an acyclic pincer-type carbodicarbene, was
initiated with a commercially available 1-iodo-2-nitrobenzene
(Scheme 2), which undergoes palladium-mediated amination
with 2-aminopyridine to give 1, from which diamine 2 is
furnished in excellent yield (92%) through hydrogenation.
Compound 2 was then heated at reflux with diethyl malonate
to afford 3 in high yield. Bismethylation of 3 with iodo-
methane in acetonitrile results in the dicationic salt 4,
confirmed by single-crystal X-ray diffraction (Figure 1,
[*] J. Shen,[+] Dr. B. Lin, Dr. W. Chen, Dr. W. Ching, Dr. C. Hsu, Dr. T. Ong
Institute of Chemistry, Academia Sinica
Taipei, Taiwan (Republic of China)
E-mail: tgong@gate.sinica.edu.tw
Dr. T. Ong
Department of Applied Chemistry, National Chiao Tung University
Taiwan (Republic of China)
Y. Hsu,[+] Prof. Y. Chan
Department of Chemistry, National Taiwan University
Taipei, Taiwan (Republic of China)
left).[10] Double deprotonation of
4 with NaN(SiMe3)2
afforded the air-sensitive carbodicarbene 5 in 50% yield. In
1
the H NMR spectrum of 5, the methylene resonance of 4 at
d = 5.91 ppm has disappeared and the singlet methyl reso-
nance with an integration of six protons has shifted from
4.14 ppm to 2.68 ppm, thus suggesting that 5 is a free
carbodicarbene. The molecular structure of 5 was further
elucidated by single-crystal X-ray diffraction (Figure 1, right).
To the best of our knowledge, compound 5 represents the first
isolated free form of a pincer-type, acyclic carbodicarbene.
The connectivity of the bent allenic moiety, C6-C14-C16, was
confirmed, with bond distances of 1.333(2) and 1.324(2) ꢁ,
Prof. G. Yap
Department of Chemistry & Biochemistry
University of Delaware (USA)
[+] These authors contributed equally to this work.
[**] This work was financially supported by the Ministry of Science &
Technology of Taiwan. (NSC-101-2628M-001-002-MY grant) and
Academia Sinica Funding.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201406481.
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in the presence of K2CO3 as a base at 1008C, affording
complex 6 in good yield. In 6 (Figure 2), the cationic
palladium center has a square planar environment with
À
a tridentate NCN coordination. The palladium carbodicar-
Figure 2. Crystal structure of complex 6 with thermal ellipsoids
depicted at 30% probability. Hydrogen atoms and the iodine counter-
ion have been omitted for clarity. Selected bond lengths [ꢀ] and angles
[deg]: Pd1–Cl1 2.4090(8), Pd1–N1 2.035(3), Pd1–N6 2.033(3), Pd1–
C14 1.973(3), C14–C6 1.369(5), C14–C15 1.398(5); C6-C14-C15
126.5(3), Cl1-Pd1-C14 177.24(10).
bene bond in 6 (1.973(3) ꢁ) is shorter than that in our
À
previously reported non-pincer NHC Pd complexes
(2.101(4) ꢁ).[5a] The rather short Pd C bond is attributed to
À
a tight pincer effect. Not surprisingly, the C center of the
Scheme 2. Synthetic routes toward carbodicarbene 5 and its palladium
complex 6.
carbodicarbene ligand contributes a strong trans influence on
À
the chloride ligand opposite to it, thus lengthening the Pd Cl
distance to 2.4090(8) ꢁ.[11] This result is consistent with
previous reports of related tridentate pincer-type structures
of LCL complexes.[12] Interestingly, the geometry around Pd is
close to idealized square-planar, but both benzimidazolium
(or pyridine) rings are puckered with respect to each other to
avoid unfavorable steric interactions invoked by the methyl
side arms in the framework.
To further understand the molecular structures and their
implication in molecular properties, we further employed
DFT calculation. At the level of B3LYP/Lan2Ldz/6-31G*, the
optimized structures of 5 and 6 were obtained, which are in
good agreement with the X-ray crystal structures (see
Figure 3 and the Supporting Information for details). Addi-
tionally, we have modeled the molecular structure of 5’, based
on structure 6 excluding the palladium atom, to examine the
stereoelectronic interaction of the lone pairs. Both lone pairs
at the pyridyl fragments of 5’ are directed toward the central
carbodicarbene region (Figure 3b). The model structure of 5’
is 23.0 kcalmolÀ1 higher in energy than 5, thus indicating
unfavorable stereoelectronic interactions among those lone
pairs. Figure 3a and b shows the two highest occupied
molecular orbitals HOMO and HOMO-1 of 5 and 5’. The
HOMO-1 of 5 is clearly a s-type orbital and localized at the
central carbon atom C14. The HOMO of 5 is a p-type lone-
pair orbital that has also the largest coefficient at C14 with
respect to the C-C-C plane. We note that these results for 5
are similar to previous computational results for a number of
carbodicarbenes.[13] Interestingly, the frontier orbital arrange-
ment for 5’ is reverse with respect to 5.
Figure 1. Crystal structures of compounds a) 4 and b) 5 with thermal
ellipsoids depicted at 30% probability. Solvent molecules and hydro-
gen atoms (except for those on the methylene group of 4) have been
omitted for clarity. Selected bond lengths [ꢀ] and angles [deg]: 4: C6–
C14 1.496(6), C14–C15 1.493(6), N2–C6 1.352(5), N6–C6 1.330(5),
N3–C15 1.333(6), N4–C15 1.346(5); C15-C14-C6 115.8(3). 5: C6–14
1.333(2), C14–C16 1.324(2), N2–C6 1.418(2), N3–C6 1.389(2), N4–C16
1.3993(18), N5–C16 1.4242(18); C6-C14-C16 143.61(15).
which are comparable to the previous reports by our group[5a]
and Bertrand and co-workers.[3] However, the bond angle of
143.68 in the allenic moiety of 5 is larger than that reported for
other acyclic carbodicarbenes (134.88), despite the relative
lack of steric hindrance. More interestingly, the two N4-C16-
N5 and N2-C6-N3 planes are twisted by 848, a feature that
bears resemblance to an allene (908). Finally, the lone pairs
residing at pyridine and those of the carbodicarbene are
oriented in an anti manner, presumably to avoid unfavorable
stereoelectronic interactions.
To investigate the coordination chemistry of the carbodi-
carbene, dicationic salt 4 in CH3CN was introduced to PdCl2
One of intriguing questions worth considering at this
juncture is the nature of the C-C-C moiety of 5. A larger C6-
2
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distribution of 5 and 6.[15] NPA showed that the NHC
fragments in 6 carry a significant amount of positive charge,
but not as much in 5,[16] suggesting a lack of a strong electron-
donating contribution from C6 and C15 in 5. Again, the NPA
is consistently in line with the ELF interpretation that
carbodicarbene 5 has embodied a more allene-like character
compared to previously known carbodicarbenes A and B. We
speculate that the pyridine moieties, which are an electron-
withdrawing entity in 5, may play a role in that respect.
Recent years have seen increasing interest in the develop-
ment of NHC-promoted metal-mediated homogeneous cat-
alysis. Interestingly, endeavors using carbodicarbene-based
catalysts are still lacking. Thus, the development of carbodi-
carbenes as an NHC alternative for catalysis is definitely
worthwhile to pursue. To evaluate the catalytic activity of the
palladium complexes supported by pincer carbodicarbene, we
studied these compounds as potential catalysts for the
Mizoroki–Heck cross-coupling reaction. To our delight,
even during the preliminary studies for optimization of
reaction conditions (Table 1), the coupling product 20a was
obtained in good yield (entry 1).[17]
Figure 3. Plots of the HOMO and HOMO-1 of a) 5 and b) 5’ based on
calculations at B3LYP/Lan2Ldz/6-31G* level.
C14-C16 angle with almost orthogonal torsional angles
compared to A and C would conceivably mean some allenic
character in 5. To examine the extend of the allene-like
character of bis(pyridine)carbodicarbene, we used the pro-
gram “TopMod” to perform the electron localization function
(ELF) analysis[14] on 5 and 5’. The electron localization
function (ELF) is a useful method to study the disposition of
electron density of bonding and lone pairs. The ELF[14]
analyses of 5 and 5ꢀ with their contour plots are displayed in
Figure 4. A lone pair close to the central C14 is observed for
both 5 and 5’. The valence basin near this central atom
(denoted as V(C14)) has electronic populations of 1.54eÀ and
2.10eÀ for 5 and 5’, respectively. This result supports
a pronounced carbene character at the central C14 atom in
both conformers. Yet, the lower electron population in
compound 5 suggests that it possesses significant allene-like
character. Similar ELF results were reported previously for
a bent allene of 1,2-cyclopentadiene.[14d] We also performed
a natural population analysis (NPA) to map out the charge
Table 1: Optimization of the Mizoroki–Heck reaction catalyzed by
complex 6.[a]
Entry
Ammonium salt
Yield[b] [%] (20a:21a)
1
2
3
4
5
TBA-Br
97 (92:5)
20 (20:1)
54 (49:5)
88 (85:3)
no reaction
TBA-BF4
TBA-HSO3
TBA-Cl
TBA-PF6
[a] Reactions were carried out with aryl bromide (1.0 mmol), styrene
(1.45 mmol), NaOAc (1.18 mmol), Pd catalyst 6 (0.5 mol%), ammoni-
um salt (2 g) at 1208C for 2 h. [b] Yield and regioselectivity determined
by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as internal
standard. TBA=tetrabutylammonium.
With the optimized reaction conditions in hand, the scope
of the Mizoroki-Heck reaction mediated by catalyst 6 was
studied further (Table 2). Excellent yields (entries 1–3) were
observed for aryl bromide bearing ketone (10a), cyanide
(10b), and aldehyde (10c). Similarly, derivatives (10d–f) with
more electron-donating groups did not seem to affect the
yield of the reaction. The amino substituent (10g) is also
compatible with the reaction conditions, giving a high yield of
product (82%). Encouragingly, the less expensive aryl
chloride can be used for the coupling process, giving good
yields (entries 8–10). It is noteworthy, that all reactions
mediated by complex 6 were performed under organic-
solvent-free conditions using aqueous media with ammonium
salts. The scope of this protocol was further expanded to butyl
acrylate with various aryl bromides (Table 3). Remarkably,
levels of the coupling products 22a–d were consistently high
Figure 4. Electron localization function (ELF) analysis of a) 5 and b) 5’,
with the magnification of the central carbodicarbene (or bent allenic)
moieties on the right. Basin populations for the valence lone pair at
the central C14 atom are also given.
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Table 2: Scope of Mizoroki–Heck reactions with styrene.[a]
Table 4: Suzuki–Miyaura cross-coupling reaction.[a]
Entry
10
X
R1
20
Yield [%] (20:21)
1
10a
10b
10c
10d
10e
10 f
10g
10h
10i
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
COCH3
CN
COH
H
20a
20b
20c
20d
20e
20 f
20g
20a
20b
20c
97 (92:5)
99 (95:4)
94 (89:5)
90 (90:0)
91 (85:6)
93 (84:9)
82 (75:7)
94 (90:4)
90 (85:5)
97 (91:6)
2[d]
3[d]
4[d]
5[c]
6[b]
7
CH3
OMe
NH2
COCH3
CN
8[e]
9[e]
10[d]
10j
COH
[a] Reactions were carried out with aryl halide (1.0 mmol), styrene
(1.45 mmol), NaOAc (1.18 mmol), 6 (0.5 mol%), TBA-Br (2 g) at 1208C
for 2 h. [b] 3-Chloropyridine (2.65 mol%) was added. [c] 2,6-Dimethyl-
pyridine (2.65 mol%) was added. [d] 3-Chloropyridine (2.65 mol%) was
added, 4 h, 1408C. [e] Performed at 1408C. Yields and regioselectivity
determined by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as
internal standard.
Table 3: Scope of Mizoroki–Heck reactions with butyl acrylate.[a]
[a] Reactions were carried out with aryl bromide (1 mmol), aryl boronic
acid (1.3 mmol), 6 (3.5 mol%), K3PO4 (2.67 mmol), 3-chloropyridine
(2.65 mol%) in toluene (2 mL) at 1208C for 4 h. [b] Yields were
R1
22
Yield [%]
1
Entry
10
determined by H NMR using 1,3,5-trimethoxybenzene as internal
standard. [c] Yields of isolated products. [d] Aryl chloride was used
instead of aryl bromide. [e] H2O (0.3 mL) was added.
1
2
3
4
5
6
10a
10b
10c
10d
10e
10 f
COCH3
CN
COH
H
CH3
OMe
22a
22b
22c
22d
22e
22 f
92
94
92
90
78
79
suitable substrates, but gave the products (23ab, 23ae, and
23af; Table 4, entries 4–6) in moderate yields typical for
electron-donating derivatives. Relatively hindered 2-bromo-
1,3,5-trimethyl benzene (10g) was also a capable coupling
partner for different boronic reagents (Table 4, entries 7–9),
but the sterically bulky mesitylboronic acid (13d) gave the
product 23da only in a low yield (entry 11). Finally, less active
4’-chloroacetophenone is also a suitable reagent for this
reaction (Table 4, entry 10).
[a] Reactions were carried out with aryl halide (1.0 mmol), butyl acrylate
(1.45 mmol), NaOAc (1.18 mmol), 6 (0.5 mol%), TBA-Br (2 g) at 1208C
for 2 h. Yields determined by 1H NMR spectroscopy using 1,3,5-
trimethoxybenzene as internal standard.
(> 90%), while the methyl and methoxy analogues (10e–10 f)
gave satisfactory conversion.
To further demonstrate the versatility of the catalyst, 6
was tested for the Suzuki–Miyaura coupling reaction
(Table 4) between aryl boronic acids and aryl halides. To
our delight, a highly efficient cross coupling of boronic
reagent 13a and acetophenyl bromide 10a to product 23aa
can be achieved with 3.5 mol% of Pd catalyst 6 in the
presence of around three equivalents of K3PO4 as base and 3-
chloropyridine as additive (Table 4, entry 1). High yields were
observed with several boronic reagents, such as 13b and 13c
(products 23ba and 23ca, respectively; Table 4, entries 2 and
3). Likewise, aryl bromides that contain electron-withdrawing
(10b) and electron-donating groups (10e and 10 f) are
In summary, a new type of pincer bis(pyridine)-
carbodicarbene framework has been prepared, isolated, and
fully characterized. The pincer carbodicarbene 5 embodies
a more allene-like character compared to previously known
monodentate carbodicarbenes A and B. The corresponding
tridentate palladium complexes have also been successfully
prepared and shown to be active catalysts in Heck and Suzuki
À
C C cross-coupling reactions. This work represents a proof-
of-concept of carbodicarbene-promoted metal catalysis. Fur-
ther studies of the mechanistic pathways of cross-coupling
reactions catalyzed by carbodicarbene–Pd complexes are
under way.
4
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Preparation of compound 5: In a 100 mL, one-necked, round-
bottomed flask, 4 (3.44 g, 5 mmol) was placed in anhydrous benzene
(10 mL). A solution of NaN(SiMe3)2 (NaHDMS) (1.85 g, 10 mmol) in
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1H NMR (400 MHz, C6D6): d = 8.36 (br, 2H), 8.31 (br, 1H), 8.29 (br,
3H), 7.03 (t, J = 7.6 Hz, 2H), 6.97 (br, 4H), 6.43 (s, 4H), 2.69 ppm (s,
6H); 13C NMR (100 MHz, C6D6): d = 153.8, 148.5, 139.1, 137.5, 136.3,
133.3, 122.7, 120.5, 119.9, 119.1, 117.3, 113.4, 105.9, 29.6 ppm; HR-
MS(FAB): calculated for [C27H22N6]+: 431.1984; Found: 431.1982.
CCDC 1005971 (4), 1005972 (5), and 1005973 (6) contain the
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Received: June 23, 2014
Revised: July 29, 2014
Published online: && &&, &&&&
À
Keywords: carbenes · carbodicarbenes · C C coupling ·
À
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[16] See the Supporting Information for further details.
[17] We also performed the solvent-free Heck cross-coupling reac-
tion with various Pd catalysts supported by NHCs and PPh3. The
survey of the experiments under similar conditions suggested
that catalyst 5 was much more effective than IMes, IPr, and PPh3
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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.
Angewandte
Communications
Communications
Carbodicarbenes
Y. Hsu, J. Shen, B. Lin, W. Chen, Y. Chan,
W. Ching, G. Yap, C. Hsu,*
T. Ong*
&&&& — &&&&
Synthesis and Isolation of an Acyclic
Tridentate Bis(pyridine)carbodicarbene
and Studies on Its Structural Implications
and Reactivities
The acyclic pincer ligand bis(pyridine)-
carbodicarbene was synthesized, iso-
lated, and characterized. It features a C-C-
C angle of 1438, which is larger than that
in the monodentate framework. Palla-
dium complexes supported by this ligand
are active catalysts in Heck–Mizoroki and
Suzuki–Miyaura coupling reactions.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!