Cyclic (amino)(aryl)carbenes (CAArCs) as strong σ-donating and π-accepting ligands for transition metals

Article abstract of DOI:10.1002/anie.201507844

Cyclic (amino)(aryl)carbenes (CAArCs) result from the replacement of the alkyl substituent of cyclic (alkyl)(amino) carbenes (CAACs) by an aryl group. This structural modification leads to enhanced electrophilicity of the carbene center with retention of

Full text of DOI:10.1002/anie.201507844

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201507844
German Edition: DOI: 10.1002/ange.201507844
N-Heterocyclic Carbenes
Cyclic (Amino)(aryl)carbenes (CAArCs) as Strong s-Donating and
p-Accepting Ligands for Transition Metals
Bin Rao, Huarong Tang, Xiaoming Zeng,* Liu Liu, Mohand Melaimi, and Guy Bertrand*
Dedicated to Professor Manfred Scheer on the occasion of his 60th birthday
Abstract: Cyclic (amino)(aryl)carbenes (CAArCs) result from
the replacement of the alkyl substituent of cyclic (alkyl)-
(amino) carbenes (CAACs) by an aryl group. This structural
modification leads to enhanced electrophilicity of the carbene
center with retention of the high nucleophilicity of CAACs, and
therefore CAArCs feature a small singlet–triplet gap. The
isoindolium precursors are readily prepared in good yields,
and deprotonation at low temperature, in the presence of
[RhCl(cod)]2 and [(Me2S)AuCl] lead to air-stable rhodium
and gold CAArC-supported complexes, respectively. The
rhodium complexes promote the [3+2] cycloaddition of
diphenylcyclopropenone with ethyl phenylpropiolate, and
induce the addition of 2-vinylpyridine to alkenes by CH
activation. The gold complexes allow for the catalytic three-
component preparation of 1,2-dihydroquinolines from aniline
and phenyl acetylene. These preliminary results illustrate the
potential of CAArC ligands in transition-metal catalysis.
stabilization of transition-metal centers and main-group
elements in different oxidation states,[9] including paramag-
netic species,[10] and for the activation of small molecules and
enthalpically strong bonds.[11]
Herein we report that the replacement of the alkyl
substituent of CAACs by an aryl group[12] results in enhanced
electrophilicity of the carbene center, with retention of the
high nucleophilicity of CAACs. Thus, cyclic (amino)-
(aryl)carbenes (CAArCs) E (Figure 1) feature an even
smaller singlet–triplet gap than CAACs. To illustrate the
utility of this novel type of singlet carbene, we show that air
stable [(CAArC)RhCl(COD)] and [(CAArC)AuCl] com-
plexes are efficient catalysts for several chemical processes.
The enhanced s-donor and p-acceptor properties of
CAACs B compared to classical NHCs A is due to replace-
ment of one of the s-withdrawing and p-electron donating
amino substituent by a s-donating alkyl group. Note that
other strategies have been developed to lower the energy of
the LUMO, as exemplified by the anti-Bredt NHCs C[13] and
diamidocarbenes D (Figure 1).[14] Carbenes C are very com-
parable to CAACs, whereas in the case of D, the increased
electrophilicity is accompanied by a significant decrease of
the energy of the HOMO, and thus D are weak s-donating
ligands. We hypothesized that the simple replacement of the
alkyl group of CAACs B by an aryl group will further enhance
the electrophilicity of the carbene center, without disrupting
the energy of the HOMO, which is ruled by the s-effects. To
test our hypothesis, we first carried out a computational study
using density functional theory (DFT; B3LYP/TZVP). The
results show that CAArC E displays a remarkably low-energy
LUMO compared to that observed for CAAC B and
concomitantly, it features a high energy HOMO as observed
for CAACs. As a result, CAArCs have the smallest singlet–
triplet gap of the series (41.5 kcalmolÀ1) (Figure 1).
Encouraged by these results, we attempted the synthesis
of CAArCs (Scheme 1). Starting from readily available
primary amines 1 and 2-bromobenzaldehydes 2, imines 3
are accessible in almost quantitative yields. Then, lithiation
with n-butyl lithium, addition of benzophenone, followed by
treatment with trifluoromethanesulfonic anhydride, afford
the desired isoindolium salts 4 in a one-pot procedure. A wide
range of CAArC precursors 4a–4i were prepared in moder-
ate to good yields by variation of the nitrogen substituent and
aryl scaffold, including chiral isoindolium salt 4 f from an
enantiomerically pure amine. Note that the synthesis can be
conducted on a multiple-gram scale (4a), and the salts can be
isolated by filtration without further purification.
T
he introduction of N-heterocyclic carbenes (NHCs) as
ligands for transition metals has generated many break-
throughs in catalysis.[1–3] This is mostly the result of their
strong s-donor properties, which are due to the lower
electronegativity of carbon compared to Group 15 and 16
elements. Several types of carbenes have been prepared,[4] but
a wider diversity is still needed to match their phosphorus-
based counterparts. It has been shown that cyclic (alkyl)-
(amino)carbenes (CAACs) B[5,6] are both stronger s-donors
and p-acceptors than classical imidazolin-2-ylidene A[7]
(Figure 1). Consequently, they feature a smaller singlet–
triplet gap, and their complexes are more thermally robust, as
a result of stronger metal–carbon bonds.[8] In addition, the
peculiar electronic properties of CAACs B allow for the
[*] B. Rao, H. Tang, Prof. Dr. X. Zeng
Frontier Institute of Science and Technology
Xi’an Jiaotong University
Xi’an, Shaanxi, 710054 (P.R. China)
E-mail: zengxiaoming@mail.xjtu.edu.cn
L. Liu, Dr. M. Melaimi, Prof. Dr. G. Bertrand
UCSD-CNRS Joint Research Chemistry Laboratory (UMI 3555)
Department of Chemistry and Biochemistry
University of California, San Diego
La Jolla, CA 92093-0358 (USA)
E-mail: guybertrand@ucsd.edu
Prof. Dr. X. Zeng
State Key Laboratory of Elemento-organic Chemistry
Nankai University, Tianjin 300071 (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201507844.
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The optimized structure of Ea (Figure 2b) shows
a planar isoindoline-type core. The N-Ccarbene-Caryl
angle (105.08C) and the Ccarbene–Caryl bond length
(1.473 Š) are smaller and shorter, respectively, than
those of CAACs (106.58C and 1.516 Š).[6a] The
HOMO is a lone-pair of electrons that is mainly
localized on the carbene center (Figure 2c). The
LUMO is a p-type orbital that is distributed
predominantly on the isoindolium motif, confirming
that the aryl substituent serves as a p-acceptor,
increasing the electrophilicity of the carbene center
(Figure 2d).
Importantly, despite the instability of free
CAArCs, treatment of isoindolium salts 4a–e and
4i with LiN(SiMe3)2 at À788C, in the presence of
[RhCl(cod)]2, leads to air-stable [(CAArC)RhCl-
(cod)] complexes 6a–e and 6i, which were isolated
in moderate to good yields (43–82%; Scheme 2).[16]
The structure of 6a was ascertained by a single-
crystal X-ray diffraction study (Figure 3). The
13C NMR signal for the carbene carbon of com-
plexes 6 appeared as a doublet around d = 250 ppm
(1JCRh ꢀ 45 Hz), which is significantly downfield and
upfield shifted compared to those observed for
rhodium complexes of NHCs (d ꢀ 190 ppm)[17] and
CAACs (d ꢀ 278 ppm), respectively.[18] Bubbling
carbon monoxide through dichloromethane solu-
tions of 6a,b,d,e gave the cis-chlorodicarbonylrho-
dium complexes 7a,b,d,e in almost quantitative
yield (Scheme 2). The average IR stretching fre-
quencies for the carbonyl ligands (2032–2036 cmÀ1)
indicate that the overall donor properties of
CAArCs are superior to those of NHCs (2039–
2041 cmÀ1), and similar to those of CAACs
(2036 cmÀ1).[19] Interestingly, the 77Se NMR chem-
ical shift of derivatives 8c and 8d (d 77Se: 601 and
616 ppm, respectively), prepared following the
same procedure as for the sulfur adduct 5 but
employing selenium, indicate that CAArCs are
even more p-accepting than the bis(diisopropyla-
mino)carbene (d 77Se: 593 ppm)[7c,d] and therefore
than CAACs and of course NHCs[7a] (Scheme 3).
CAArC supported gold complexes can also be
prepared by treatment of isoindolium salts with
LiN(SiMe3)2 at À788C in the presence of
[(Me2S)AuCl]. [(CAArC)AuCl] complexes 9a–9d
were isolated in 37–88% yields (Scheme 4). The
signals for the carbene carbon are upfield shifted by
20 ppm compared to those of [(CAAC)AuCl], but
again greatly downfield shifted compared to those
Figure 1. Energy (eV) of the HOMO and LUMO of carbenes A–E, and singlet–
triple gap in parentheses (kcalmolÀ1) of carbenes A–C and E calculated at the
B3LYP/TZVP level of theory. [a] The triplet state of D is a biradical involving the
carbonyl groups, and thus the singlet–triplet gap cannot be compared with those
of the other carbenes.
Scheme 1. Synthesis of a variety of substituted isoindolium salts 4.
With the isoindolium salts in hand, we attempted to
prepare the free CAArCs by low-temperature deprotonation
using different bases such as MN(SiMe3)2 (M = K, Na, Li),
KOtBu, and NaH, but in all cases a complex mixture was
obtained. However, addition of S8 at À788C to a THF solution
of 4a and LiN(SiMe3)2 led to the corresponding thioamide 5
in 62% yield, suggesting that the free CAArC Ea is formed
but has only a short lifetime even at low temperature
(Figure 2a).
of NHCs. A single-crystal X-ray diffraction study of 9d shows
that gold is in a nearly linear environment (Ccarbene-Au-Cl
angle 177.58) with a Ccarbene–Au bond length of 1.998(38) Š
(Figure 3, right).
As a proof of principle, we briefly tested the influence of
the CAArC ligands on the catalytic activity of rhodium and
gold centers. By using 2 mol% of rhodium complex 6a, the
[3+2] cycloaddition of diphenylcyclopropenone and ethyl
phenylpropiolate proceeded at 1108C, giving the cyclopenta-
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Scheme 4. Synthesis of CAArC-supported gold complexes.
2,4-dienone derivative 10 in 85% yield after 16 h
(Scheme 5).[20] Note that this result compares favorably with
those obtained with NHC-Rh complexes (74% yield after
Figure 2. a) Trapping reaction with elemental sulfur and solid-state
structure of 5 with thermal ellipsoids set at 30% probability (all
hydrogen atoms are omitted for clarity).[15] b) Optimized structure of
carbene Ea at the B3LYP/Def2SVP level of theory. c) HOMO of carbene
Ea (isovalue=0.03). d) LUMO of carbene Ea (isovalue=0.03).
18 h at 1108C).[20b] Recently, C H activation reactions have
À
emerged as one of the most powerful tools in the preparation
of synthetically useful molecules.[21] To our delight, the N-
adamantyl-substituted [(CAArC)RhCl(cod)] (6d) activates
the terminal vinyl-CH bond of 2-vinylpyridine,
which allows for the addition to an alkene as
shown by the formation of 11.[22] Lastly, the
CAArC-ligated gold complex 9d allows for the
catalytic three-component preparation of 1,2-dihy-
droquinoline 12 from aniline and phenyl acety-
lene.[23]
In summary, isoindolium salts, which can be
decorated at will on the nitrogen and on the aryl
scaffold, are readily available in large quantities.
They are precursors of five-membered carbenes
that we name CAArCs. These species feature both
strong s-donating and p-accepting properties, and
consequently a small singlet–triplet gap. Despite
their instability, they can be used as ligands for transition
metals, and our preliminary studies indicate that their
peculiar electronic properties give rise to catalytically active
metal complexes.
Scheme 2. Synthesis of CAArC-ligated rhodium complexes 6 and 7.
Figure 3. X-ray structures of complexes 6a and 9d with thermal
ellipsoids set at 30% probability. Hydrogen atoms are omitted for
clarity. Selected bond lengths [Š] and angles [8]: 6a, N1-C28 1.334(42),
C28-C21 1.473(32), C28-Rh1 2.008(31), Rh1-Cl1 2.397(1); N1-C28-C21
105.5(2), N1-C28-Rh1 129.6(2), C21-C28-Rh1 124.1(2); 9d, N1-C20
1.324(45), C20-C1 1.455(51), C20-Au1 1.998(38), Au1-Cl1 2.290(10);
N1-C20-C1 108.8(3), N1-C20-Au1 129.8(3), C1-C20-Au1 121.3(3), C20-
Au1-Cl1 177.5(11).
Scheme 5. The application of CAArC-ligated rhodium and gold com-
À
plexes as active catalysts for the promotion of [3+2] cycloaddition, C
H bond activation, and three-component preparation of 1,2-dihydro-
quinoline.
Scheme 3. Synthesis of CAArC-ligated seleno-amides.
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[2] For thematic issues on NHCs for catalysis, see: a) T. Rovis, S. P.
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Experimental Section
General procedure for the preparation of 4: nBuLi (5.25 mmol,
1.05 equiv) was added dropwise at À788C to an ether solution
(14 mL) of mines 3 (5 mmol), and the resulting mixture was stirred at
the same temperature for 1 h. Subsequently, a solution of diphenyl
ketone (5.25 mmol, 1.05 equiv) in ether (3 mL) was added and the
mixture warmed to room temperature and stirred for another 30 min.
Then trifluoromethanesulfonic anhydride (5.25 mmol, 1.05 equiv)
was added dropwise at À788C. The reaction mixture was stirred at
room temperature for 1 h. After filtration, the solid was washed with
Et2O (10 ” 3 mL), and extracted with CH2Cl2 (30 mL). The organic
solvent was removed by evaporation to give the corresponding
isoindolium salts 4 as white or pale yellow solids.
5: THF (2 mL) was added at À788C to a dry Schlenk flask
containing 4a (0.2 mmol, 116 mg), LiN(SiMe3)2 (0.2 mmol, 37 mg),
and S8 (0.2 mmol, 6.4 mg). The reaction mixture was warmed to room
temperature and stirred overnight. The volatiles were removed under
reduced pressure and thioamide 5 was purified by column chroma-
tography on silica gel (petroleum ether/ethyl acetate 50:1) (57 mg,
62%).
General procedure for the synthesis of CAArC-Rh complexes 6:
THF (4 mL) was added at À788C to a dry Schlenk flask containing
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3492 – 3499.
isoindolium triflate
4
(0.42 mmol, 1.05 equiv), LiN(SiMe3)2
(0.44 mmol, 1.1 equiv, 73 mg), and [Rh(COD)Cl]2 (0.2 mmol,
98.6 mg). After stirring at room temperature for 6 h, the volatiles
were removed under vacuum, and CAArC-Rh complexes 6 obtained
after purification by column chromatography (petroleum ether/ethyl
acetate) as orange powders.
General procedure for the synthesis of CAArC-Rh complexes 7:
Carbon monoxide was bubbled (60 min) at room temperature into
a solution of rhodium complex 6 (50 mg) in dichloromethane (5 mL).
The color of the solution changed from red to yellow. After removing
the volatiles under vacuum, Rh-CO complexes 7 were obtained as
yellow powders in almost quantitative yields.
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General procedure for the synthesis of CAArC-Au complexes 9:
THF (2 mL) was added at À788C to a dry Schlenk flask containing
isoindolium triflate
4
(0.21 mmol, 1.05 equiv), LiN(SiMe3)2
(0.22 mmol, 1.1 equiv, 37 mg), and [AuCl(SMe2)] (0.2 mmol,
58.6 mg). The mixture was warmed to room temperature and stirred
for 6 h. After removing the volatiles under vacuum, CAArC-Au
complexes 9 were obtained after purification by column chromatog-
raphy (petroleum ether/ethyl acetate) as white or gray powders.
Single crystals of 9d were obtained from a THF/hexane solution at
room temperature.
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Acknowlegements
Thanks are due to the NSFC (21202128, 21572175) and the
DOE (DEFG02-13ER16370) for financial support, and the
China Scholarship Council for a graduate fellowship (L.L.).
We thank Prof. Yanzhen Zheng (XJTU) for X-ray diffraction
analysis.
Keywords: electronic properties · gold ·
homogeneous catalysis · N-heterocyclic carbenes · rhodium
How to cite: Angew. Chem. Int. Ed. 2015, 54, 14915–14919
Angew. Chem. 2015, 127, 15128–15132
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Received: August 21, 2015
Published online: October 12, 2015
Angew. Chem. Int. Ed. 2015, 54, 14915 –14919
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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