Expanding the Ligand Framework Diversity of Carbodicarbenes and Direct Detection of Boron Activation in the Methylation of Amines with CO2

Article abstract of DOI:10.1002/anie.201507921

A simple and convergent synthetic strategy used to increase the diversity of the carbodicarbene ligand framework through incorporation of unsymmetrical pendant groups is reported. Structural analysis and spectroscopic studies of ligands and their Rh complexes are reported. Reactivity studies reveal carbodicarbenes as competent organocatalysts for amine methylation using CO₂ as a synthon. A unique B-H-activated boron-carbodicarbene complex was isolated as a reaction intermediate, providing mechanistic insight into the CO₂ functionalization process.

Full text of DOI:10.1002/anie.201507921

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201507921
German Edition: DOI: 10.1002/ange.201507921
Carbene Ligands
Expanding the Ligand Framework Diversity of Carbodicarbenes and
Direct Detection of Boron Activation in the Methylation of Amines
with CO2
Wen-Ching Chen, Jiun-Shian Shen, Titel Jurca, Chun-Jung Peng, Yen-Hsu Lin, Yi-Ping Wang,
Wei-Chih Shih, Glenn P. A. Yap, and Tiow-Gan Ong*
Abstract: A simple and convergent synthetic strategy used to
increase the diversity of the carbodicarbene ligand framework
through incorporation of unsymmetrical pendant groups is
reported. Structural analysis and spectroscopic studies of
ligands and their Rh complexes are reported. Reactivity
studies reveal carbodicarbenes as competent organocatalysts
for amine methylation using CO as a synthon. A unique
2
BÀH-activated boron–carbodicarbene complex was isolated as
[
6]
a reaction intermediate, providing mechanistic insight into the
elements and transition metals in catalytic applications. For
example, Stephan et al. reported that ruthenium complexes
bearing cyclic bent allene ligands are highly active and
functional-group-tolerant catalysts for the hydrogenation of
CO functionalization process.
2
T
he first example of a ligand-stabilized, lone-pair-containing
[
1]
[6a]
carbon derivative in the zero oxidation state, CL2, was
reported by Ramirez and co-workers with the isolation of
inert olefins. Both our group and Meek and co-workers
have reported the first examples of CDC-based pincer ligand
scaffolds, which were successfully utilized for a range of
[2]
carbodiphosphoranes A. However, the unique chemical
nature of A was not fully realized at that time, as organic
chemistry was strictly based on the doctrine of four valence
electrons in the bonding of carbon. However, this paradigm
began to shift with the landmark theoretical report of a species
coined as a “carbodicarbene” composed of a divalent carbon-
[6b]
catalytic transformations, such as CÀC cross-coupling,
[6c]
intermolecular hydroamination,
and hydro-heteroaryla-
[6d]
tion. Moreover, aided by the strong s basicity and peculiar
bonding topology embedded in the CDC framework, our
group has successfully isolated the elusive dicationic borane
species, which is, to date, unattainable through the use of
other common s-donating ligands. As a consequence, this
family of captodative carbogenic compounds has a promising
future as a complementary surrogate to the well-established
NHC and cyclic(alkyl)(amino)carbene (CAAC) ligands,
with the possibility of accessing new modes of chemical
reactivity. Regrettably however, interest in these CDC
frameworks has remained low, largely as a result of the lack
of diverse architectural scaffolds for possible organometallic
[
7]
(
0) center with two N-heterocyclic carbene (NHC) ligands,
!
[3]
NHC!C(0) NHC, from Frenking and co-workers. They
postulated that the central carbon atom would possess two
lone pairs of electrons and would reside within a nonlinear
CÀCÀC moiety. Less than a year later, Bertrand et al. and
[
8]
[9]
Fürstner and co-workers took up the conceptual challenge
and successfully synthesized the corresponding carbodicar-
[
4]
[5]
benes B and C, respectively.
Carbodicarbenes (CDCs) or bent allenes have been
proven to be stronger donor ligands for both main group
[10,11]
chemistry and catalysis applications.
Unquestionably, the development of synthetic strategies
for increasing the structural diversity of CDCs would not only
dramatically expand the utility of the ligand, but would also
facilitate the discovery of new and unconventional reaction
[
+]
[+]
[
*] Dr. W. Chen, J. Shen, C. Peng, Y. Lin, Y. Wang, Dr. W. Shih,
Prof. T. Ong
Institute of Chemistry, Academia Sinica
Taipei, Taiwan (Republic of China)
E-mail: tgong@gate.sinica.edu.tw
pathways. Our group has previously successfully prepared
[6b, 11]
several CDC scaffolds.
However, as structural diversity
Dr. T. Jurca
has mostly been introduced by varying the substituents
around the nitrogen centers (Scheme 1a), the overall scope
School of Chemistry, University of Bristol
Cantock’s Close, Bristol, BS8 1TS (UK)
[6b,11,12]
of this ligand topology has remained relatively narrow.
Prof. G. P. A. Yap
Department of Chemistry and Biochemistry
University of Delaware (USA)
To broaden the scope of the CDC ligand framework with
a view to accessing new reaction pathways, we sought to
assemble CDCs bearing unsymmetrical units.
Prof. T. Ong
Based on retrosynthetic analysis, we postulated that
Department of Applied Chemistry
National Chiao Tung University, Taiwan (Republic of China)
a modular S 2-type reaction between a nucleophile and an
N
+
[
] These authors contributed equally to this work.
electrophile (Scheme 1b), akin to that initially reported by
Kuhn and co-workers for the synthesis of bis(1,3,4,5-tetra-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201507921.
[13]
methyl-2-imidazolyl)methylium iodide, would be suitable
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Scheme 1. Synthetic strategies for carbodicarbenes (CDCs).
for constructing an array of unsymmetrical CDCs. As N-
heterocyclic olefin (NHO) adducts possess a strong nucleo-
philicity and can be easily synthetically modified, they are
[14]
ideal nucleophiles for this synthetic approach. Herein, we
describe the development of a simple, modular synthetic
procedure for the preparation of unsymmetrical CDCs
bearing different ligands. Furthermore, we were able to
employ CDCs to perform reductive N-methylation of amines
with CO2 in the presence of borane. Additionally, the
isolation of a carbodicarbene-activated borane provides key
Scheme 2. Synthesis of carbodicarbene 7a and related carbodicarbenes
b–d.
7
information to determine the mechanism of the CO seques-
2
tration process by carbenoid-based organocatalysts, an area of
intense recent interest (see below).
Encouraged by our success in constructing CDCs 7a–d,
To our knowledge, there are currently no examples of
unsymmetrically substituted CDCs and thus we set out to
synthesize ligands with this challenging topology. The NHO
molecule ene-1,1-diamine 3a was chosen as our target
nucleophilic fragment. The multi-step synthetic approach
began with the copper-promoted CÀN cross-coupling reaction
we were curious if our facile S 2 strategy could be applied
N
towards the synthesis of more sterically challenging ana-
logues, the success of which would represent significant
growth in the scope of CDC ligand topology. With this in
mind, we sought to transform the commonly used NHC 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr; 2e), to the
corresponding methylene imidazole 3e using a two-step
of commercially available 2-methylbenzoimidazole and o-
bromopyridine in the presence of tetrabutylammonium
fluoride (TBAF) and 2-aminopyrimidine-4,6-diol to afford
sequential olefination process with CH I, followed by treat-
3
ment with NaN(SiMe3)2 (Scheme 3a). The resulting com-
pound 3e was then reacted with 5a, resulting in the formation
of cationic 6e, which was subsequently converted into the
target CDC 7e by deprotonation. We were also able to
construct phosphine-stabilized carbone 7 f using a similar
method (Scheme 3b), further reinforcing the general utility of
[
15]
1
a.
Subsequent methylation of 1a with iodomethane
furnished imidazolium 2a which was deprotonated by potas-
sium hydride to afford the desired product 3a in nearly
quantitative yield (96%; Scheme 2). The complementary
electrophilic fragment 5a, containing a thioether moiety, was
synthesized by a one-step reaction of 1-methyl-2-(methyl-
13
our approach. Interestingly, the C NMR spectrum of 7 f
showed a signal attributable to a carbenic carbon at d =
64.4 ppm, which was more upfield than the corresponding
signal in the spectra of compounds 7a–e (d ꢀ 110 ppm). It is
noteworthy to mention that the isolation of non-benzanne-
lated carbones like 7e and 7 f further reinforces the theoret-
ical prediction by Frenking and co-workers about the stability
[
16]
thio)benzoimidazole (4a) with MeOTf in acetonitrile. With
compounds 3a and 5a in hand, we set out to attempt the
fusion of these two distinct fragments to form the unsym-
metrical carbodicarbene 7a. Reaction of 3a with 5a in THF
solution furnished cationic salt 6a in 56% yield by recrystal-
lization, whose structure was confirmed by single-crystal X-
ray diffraction and other conventional spectroscopic meth-
[
3c]
of such species in the condensed state.
[17]
ods.
Deprotonation of 6a with the base KN(SiMe3)2
The molecular structures of 7c and 7e were confirmed by
single-crystal X-ray diffraction studies (Figure 1a,b). Struc-
ture 7c featured a bent C11ÀC1ÀC2 moiety with CÀC bond
afforded the air-sensitive, free, unsymmetrical carbodicar-
bene 7a in 73% yield (Scheme 2). Although we were unable
to obtain single crystals of 7a suitable for X-ray diffraction,
NMR analysis was able to definitively confirm its formation;
disappearance of the signature resonance signal for the
methine CH of 6a at d = 5.04 ppm confirmed the formation
of free carbodicarbene 7a. Utilizing this synthetic method, we
lengths of 1.3455(16) and 1.3401(16) Š and a bond angle of
137.55(12)8, which were comparable to previously reported
[
4,11]
examples.
In contrast to 7c, the C2ÀC1ÀC29 moiety in 7e
featured unequal electronic interactions resulting in CÀC
bond lengths of 1.318(3) and 1.344(3) Š, respectively. The
were able to access C -symmetric CDC 7b, as well as the other
unsymmetrically substituted CDCs 7c and 7d (Scheme 2).
shorter C1ÀC29 bond in 7e, close to the benzimidazole
2
fragment, was more planar (torsional angle ꢀ 1698) than the
1
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[17]
plex 8a in good yield (71%).
Similar
conditions were employed with 7e to
furnish complex 8e. Single-crystal X-ray
diffraction studies revealed the structures
of both 8a and 8e (Figure 2). Both species
featured the bent allenic moieties coordi-
1
nated in a h monodentate fashion to
a rhodium center with the complex having
an overall square-planar configuration. The
RhÀCDC bond lengths of molecules 8a
and 8e (2.117(2) and 2.116(2) Š, respec-
tively) were longer than those previously
reported for molecules from both our
Scheme 3. Synthesis of carbodicarbenes a) 7e and b) 7 f. Dipp=2,6-diisopropylphenyl.
[11]
group (2.109(2) Š) and that of Bertrand
[
4]
et al. (2.089(7) Š), highlighting the steri-
cally encumbering environment within this
Figure 2. Solid-state structures of a) 8a and b) 8e with thermal ellip-
[29]
soids set at 30% probability. Solvent molecules and hydrogen atoms
have been omitted for clarity.
CDC framework. The strongly electron-donating CDC
moiety for both 8a and 8e exerted a trans influence on the
RhÀC bond of the opposite carbonyl ligand, with a RhÀCO
Figure 1. Solid-state structures of a) 7c, b) 7e, and c) 7 f with thermal
ellipsoids set at 30% probability. Hydrogen atoms have been
omitted for clarity.
[29]
bond length approximately 0.040 Š longer compared to the
RhÀCO bond of the cis carbonyl ligand (1.840(4) Š) detected
imidazole fragment (IPr), signifying a more effective p con-
jugation along the C1ÀC29 bond with the aromatic ring.
in 8a. As a result of the strong push–push effect caused by the
IPr moiety on the carbon center, the electron-donating
capability of carbodicarbene 7e was enhanced compared to
its pyridinyl counterpart 7a. This was confirmed by means of
IR spectroscopy, with the IR stretching modes of the CO
2
Consequently, the shift towards greater sp character for the
C1ÀC29 bond would decrease the electronic repulsion, in turn
rendering the C-C-C bond angle more linear. It was thus
unsurprising that the allenic C-C-C angle of 7e (146.11(19)8)
was larger than that of 7c (137.55(12)8). The molecular
À1
groups of 8e (2045, 1964 cm ) occurring at lower wave-
À1
numbers than those of 8a (2054, 1977 cm ).
structure of 7 f stabilized by PPh was also confirmed by
As a result of fluctuating costs and the finite resources of
petroleum-based carbon feedstocks as well as the implications
3
single-crystal X-ray diffraction (Figure 1c). The P1ÀC1 bond
length (1.6435(12) Š) in 7 f was shorter than typical PÀC
single bonds (1.8 Š), indicating a partial double-bond char-
acter. This was comparable with a recent carbodiphosphorane
structure reported by Alcarazo, Fürstner, and co-workers
of CO in climate change, the production of organic chemicals
2
using carbon dioxide as a C1 source has become an area of
[19,20]
intense research.
As a consequence, many systems
demonstrating the recycling of CO have been reported; for
2
[
18]
[21]
(
PÀC bond length ꢀ 1.6398–1.6416 Š). However, the P-C-C
example the generation of methanol by hydroboration,
carboxylation by CÀC bond formation,
[22]
bond angle (133.25(10)8) was smaller than the P-C-C bond
angle for 7 f (143.04(10)8).
formylation
and synthesis of cyclic
[
23]
through CÀN bond formation,
[24]
The comparison of solid-state structures of transition
metal centers supported by CDCs can shed light on various
aspects of ligand structure and bonding behavior. We chose to
investigate the coordination of CDCs 7a and 7e to rhodium
centers. Carbodicarbene 7a in benzene was allowed to react
with [{Rh(CO) Cl} ] at ambient temperature affording com-
carbonate products. Recently, an elegant procedure for the
CO -based methylation of NÀH moieties mediated by
2
[25]
ruthenium catalysts has been developed. Shortly thereafter,
this proof of principle was successfully adapted for more
benign organocatalysts, namely NHC- and proazaphospha-
trane-promoted reductive functionalization of CO for meth-
2
2
2
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[26]
ylation of amines. We have thus explored the analogous
utility of CDCs as organocatalysts for the methylation of
amines with CO as substrate. The initial optimization study
2
was performed in THF solution with diphenylamine (10a)
and 9-borabicyclo[3.3.1]nonane (9-BBN; 11) under 1 atmos-
phere of CO using 10 mol% CDC catalyst. It was found that
2
catalysts 7b and 7e (at a reaction temperature of 808C)
effectively mediated the reaction to afford N-methyldi-
phenylamine 12a in excellent yields of 84–86% (Table 1;
entry 4 and 8). Thus both CDCs were selected as benchmark
catalysts for subsequent exploration. Further optimization
revealed that the activity of these catalysts could be improved
further to obtain the desired products in yields of circa 90%
by changing the solvent to toluene and increasing the reaction
temperature to 1008C (entry 9,10).
Table 1: Optimization process for the methylation of amine using CO2.
Scheme 4. Methylation of various amines using CO . Standard con-
2
ditions: amine (1 mmol), 9-BBN (4 equiv), toluene (0.5 mL), 7e
(
10 mol%), CO
product. [a] Yield determined by GC. [b] In the presence of 9-BBN
4.5 equiv).
2
(1 atm), 1008C, 1.5 h. Yields are of the isolated
(
(10 f), or N-methylaniline (10h) as the reagent generally led
to good yields of product. Even the sterically encumbered N-
isopropylaniline (10i) resulted in a yield of 52% for the
isolated product, highlighting reasonable tolerance for steric
bulk within this system. N-methylaniline bearing an electron-
donating group, such as methyl (10j) or methoxy (10k),
resulted in lower yields. The CDC-promoted double methyl-
ation was also readily extended to primary amines with
sterically demanding substituents, such as 10l and 10m, to
furnish dimethylated products. Finally, cyclic alkylamines,
such as piperidine (10n) and morpholine (10o), were also
readily methylated, with 10n furnishing near quantitative
yields (97%).
[
a]
Entry
Catalyst
Solvent
Temp [8C]
Yield [%]
[
[
b]
b]
1
2
3
4
5
6
7
8
9
7b
7b
7b
7b
7a
7c
7d
7e
7b
7e
7 f
THF
THF
THF
THF
THF
THF
THF
THF
90
50
90
80
80
80
80
80
67
13
83
86
80
80
25
84
89
90
86
toluene
toluene
toluene
100
100
100
1
1
0
1
To obtain a better understanding of the reaction mecha-
nism, we performed a series of experiments. First, an
equimolar mixture of 7e and 9-BBN (11) was allowed to
react to afford adduct 9e [Eq. (1)]. The solid-state structure
Reaction conditions: amine (1 mmol), 9-BBN (4 equiv), solvent
0.5 mL), catalyst (10 mol%), CO (1 atm), 1.5 h. [a] Yield determined by
(
2
GC. [b] 5 mol% of 7b.
With the optimized reaction conditions in hand, the scope
of the CDC-catalyzed amine methylation reaction with CO2
as carbon source was expanded, using in particular 7e as the
catalyst (Scheme 4). Excellent yields (circa 90%) were
obtained for diphenylamine (10a) and bromo-substituted
diphenylamine 10b. A lower yield was obtained using nitro-
substituted derivative 10c, whereas the yield with the
diphenylamine derivative containing electron-donating
methoxy groups 10d was moderate (78%). Using N-benzyl-
aniline (10e), N-cyclohexylaniline (10g), dibenzylamine
of 9e was confirmed by single-crystal X-ray diffraction studies
(Figure 3). The structure was composed of a trivalent boron
complex bearing a CDC ligand with a planar geometry (sum
of angles = 3608). Surprisingly, the original hydride (H29) on
1
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Scheme 5. Proposed mechanism for the catalytic methylation of 10a
Figure 3. Solid state-structure of 9e with thermal ellipsoids set at 30%
probability. Hydrogen atoms, with the exception of the C(29)H, have
been omitted for clarity.
[
29]
with CO and 9-BBN.
2
studies, we have identified a unique BÀH-activated species
which we believe to be a key intermediate in the reaction
process. This has important implications for the mechanistic
understanding of the reaction as promoted not only by CDCs,
but also by analogous NHCs, an area of intense recent
interest.
the boron center of 9-BBN has migrated to the C29 position
of the CDC. In the H NMR spectrum of 9e, a typical singlet
resonance signal for a tertiary C-H moiety at d = 5.26 ppm
further confirmed the presence of the hydrogen atom at the
C29 position. Performing a similar methylation reaction of
1
amine 10a with CO using 9e as catalyst generated product
The potential utility of CDCs as catalysts in the synthesis
of valuable platform chemicals from renewable carbon feed-
stocks, or as ligands for the isolation of transition metal, or
main group based coordination complexes, further underlines
the importance of expanding the diversity of this framework.
Further expansion of both ligand structure and reactivity, as
well as a detailed mechanistic investigation of the CDC-
2
1
2a in yield of 90% [Eq. (2)].
mediated reductive N-methylation of amines with CO , is
2
currently under investigation.
Acknowledgements
To our knowledge, compound 9e is the first isolated
reaction intermediate for the reduction of CO to methyl-
This work was financially supported by the Ministry of
Science and Technology of Taiwan (MOST-104-2628-M-001-
005-MY4 grant) and an Academia Sinica Career Develop-
ment Award (104-CDA-M08). Finally, this lab is grateful to
Dr. Yuh-Sheng Wen for assistance with X-ray diffraction data
collection and to Dr. Mei-Chun Tseng for mass spectrometry.
2
amines in the presence of borane. Moreover, it shows
a unique borane activation mode, whereby the C29 atom of
the benzoimidazolylidene ligand unit appears to work syn-
ergistically with the carbon center (C1) as a facile hydrogen-
transfer site. This is in contrast to the activation of the BÀH
fragment by other carbenoid species which typically involve
[27]
oxidative addition at a single carbenic site. Additionally,
this result stands in contrast to other proposed types of
Keywords: carbene ligands · carbodicarbenes · CO activation ·
2
ligand design · organocatalysis
activation modes for the analogous silane activation for CO
reductive functionalization, as predicted by DFT studies.
2
28]
[
How to cite: Angew. Chem. Int. Ed. 2015, 54, 15207–15212
Angew. Chem. 2015, 127, 15422–15427
With the above findings, we believe that we have further
validated the reaction pathway previously proposed by
[26a]
[
1] The use of arrows provides only one possible bonding descrip-
tion for the compounds discussed herein. Alternative structures
are provided in the Supporting Information. For a thorough
discussion, please see: a) D. Himmel, I. Krossing, A. Schnepf,
Angew. Chem. Int. Ed. 2014, 53, 370; Angew. Chem. 2014, 126,
Cantat and co-workers
as illustrated in Scheme 5.
In summary, a straightforward synthetic method for
unsymmetrical carbodicarbenes has been reported. Employ-
ing this method, we have prepared what is, to our knowledge,
the first example of such a species. These results are an
important development in the diversification of this promis-
ing ligand framework. Unsymmetrical CDCs were success-
fully ligated to Rh centers, and analysis revealed that the
resulting coordinated CDCs featured strong electron-donat-
ing characteristics. Finally, CDCs were shown to be compe-
tent catalysts for the reductive N-methylation of amines with
3
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1419542 (8a),1419543 (7 f), 1419544 (8e), and 1419545 (9e) con-
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These data are provided free of charge by The Cambridge
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Received: August 24, 2015
Published online: October 22, 2015
1
5212 www.angewandte.org
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