Nickel-Catalyzed Intramolecular 1,2-Aryl Migration of Mesoionic Carbenes (iMICs)

Article abstract of DOI:10.1002/anie.202014328

Intramolecular 1,2-Dipp migration of seven mesoionic carbenes (iMIC^Ar) 2 a–g (iMIC^Ar=ArC{N(Dipp)}₂CHC; Ar=aryl; Dipp=2,6-iPr₂C₆H₃) under nickel catalysis to give 1,3-imidazoles (IMD^Ar) 3 a–g (IMD^Ar=ArC{N(Dipp)CHC(Dipp)N}) has been reported. The formation of 3 indicates the cleavage of an N?C_Dipp bond and the subsequent formation of a C?C_Dipp bond in 2, which is unprecedented in NHC chemistry. The use of 3 in accessing super-iMICs (5) (S-iMIC=ArC{N(Dipp)N(Me)C(Dipp)}C) has been shown with selenium (6), gold (7), and palladium (8) compounds. The quantification of the stereoelectronic properties reveals the superior σ-donor strength of 5 compared to that of classical NHCs. Remarkably, the percentage buried volume of 5 (%V_bur=45) is the largest known amongst thus far reported iMICs. Catalytic studies show a remarkable activity of 5, which is consistent with their auspicious stereoelectronic features.

Full text of DOI:10.1002/anie.202014328

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Accepted Article
Title: Nickel Catalyzed Intramolecular 1,2-Aryl Migration of Mesoionic
Carbenes (iMICs)
Authors: Rajendra Ghadwal, Arne Merschel, Timo Glodde, Beate
Neumann, and Hans-Georg Stammler
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10.1002/anie.202014328
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Nickel Catalyzed Intramolecular 1,2-Aryl Migration of Mesoionic
Carbenes (iMICs)
Arne Merschel, Timo Glodde, Beate Neumann, Hans-Georg Stammler, and Rajendra S. Ghadwal*[a]
Dedication ((optional))
[a]
A. Merschel, T. Glodde, B. Neumann, Dr. H.-G. Stammler, Priv.-Doz. Dr. R. S. Ghadwal
Molecular Inorganic Chemistry and Catalysis, Inorganic and Structural Chemistry
Center for Molecular Materials, Faculty of Chemistry, Universität Bielefeld
Universitätsstrasse 25, D-33615, Bielefeld, Germany
E-mail: rghadwal@uni-bielefeld.de; Homepage: www.ghadwalgroup.de
Supporting information for this article is given via a link at the end of the document. CCDC 2004392–2004403 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk
Abstract: Intramolecular 1,2-Dipp migration of seven mesoionic
carbenes (iMICAr) 2a-g (iMICAr = ArC{N(Dipp)}2CHC; Ar = aryl; Dipp =
2,6-iPr2C6H3) under nickel catalysis to give 1,3-imidazoles (IMDAr) 3a-
g (IMDAr = ArC{N(Dipp)CHC(Dipp)N}) has been reported. The
formation of 3 indicates the cleavage of an N‒CDipp bond and the
subsequent formation of a C‒CDipp bond in 2, which is unprecedented
in NHC chemistry. The use of 3 in accessing super-iMICs (5) (S-iMIC
= ArC{N(Dipp)N(Me)C(Dipp)}C) has been shown with selenium (6),
gold (7), and palladium (8) compounds. The quantification of the
stereoelectronic properties reveals the superior σ-donor strength of 5
compared to that of classical NHCs. Remarkably, the percentage
buried volume of 5 (%Vbur = 45) is the largest known amongst thus far
reported iMICs. Catalytic studies show a remarkable activity of 5,
which is consistent with their auspicious stereoelectronic features.
N-Heterocyclic carbenes (NHCs) (I, Figure 1) are most versatile
carbon-donor neutral ligands in synthesis and catalysis[1] as well
as in materials science.[2] This is largely attributed to the
auspicious stereoelectronic properties of NHCs.[3] A clear insight
into the stereoelectronic properties of NHCs[3-4] facilitates the
rational choice of an NHC for a particular application.[5] Mesoionic
carbenes (iMICs) (II, “i” refers to 1,3-imidazole derived) are a
subclass of the family of NHCs with the carbenic carbon atom at
the unusual C4 (or C5) position.[6] iMICs (II) (also known as
abnormal NHCs)[7] are stronger σ-donors than classical NHCs (I),
Figure 1. Schematic representations of classical NHCs (I) and iMICs (II). First
examples of structurally characterized iMIC-metal complex (III), free iMIC (IV),
and C5-protonated iMIC (V).
iMICs (V) under nickel catalysis to afford 1,2,4-triaryl-1,3-
imidazoles (IMDAr) and reveal their utility in accessing super bulky
S-iMICs (VI) (Figure 1). Besides the quantification of the
stereoelectronic properties, the catalytic activity of VI has been
probed and compared with those of V and IPr (IPr = 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene).
and hence have
a significant potential in synthesis and
catalysis.[6-7] However, rational synthetic methods for iMIC-
compounds remain extremely scarce compared to those of
NHCs.[7a] In 2001, Crabtree et al. reported the first iMIC-complex
(III).[8] The first crystalline NHC was isolated by Arduengo et al. in
1991,[9] but it was only in 2009 when Bertrand et al. prepared the
first unmasked iMIC (IV).[10]
The deprotonation of C2-arylated 1,3-imidazolium salts (1a-gBr
affords the desired iMICs 2a-g (Scheme 1a).[12] Treatment of a
toluene solution of 2a with 1 eq. of Ni(cod)2 (cod = 1,5-
cyclooctadiene) led to the formation of an unexpected product 3a
in 35% yield (Scheme 1a), which indicates the migration of a Dipp
substituent of 2a to the carbene carbon atom to give 3a. This type
of reactivity is unprecedented in NHC chemistry.[15] The exact
mechanism for the formation of 3a is currently unknown. The
stronger σ-donor property of iMICPh (2a)[4c] probably makes the
Ni(0) center in the putative complex A electron-rich (Scheme
1b).[16] Oxidative addition of Ni(0) into the adjacent C‒N bond via
B is likely to form a Ni(II) species C, which eventually undergoes
reductive elimination to yield 3a. Note, (iMICAr)Ni(CO)3 complexes
comprising π-acidic CO ligands are stable solids.[4c] The
stoichiometric conversion of 2a into 3a can be translated into a
Like IV, the use of an aryl substituent at the C2-position is shown
to be the reliable strategy for accessing iMIC-compounds.[6-7] We
previously reported two catalytic protocols for the direct C2-
arylation of NHCs to C2-arylated 1,3-imidazoli(ni)um salts[11] and
revealed their suitability in preparing C5-protonated free iMICs
(V),[12] metal complexes,[13] and stable radicals.[14] In addition to
the σ-donor properties, the steric profile of ligands also plays a
key role in the stability and catalytic activity of derived
complexes.[1] However, experimental tools to improve the
properties of iMICs remained virtually unknown.[7] Herein, we
report an unprecedented 1,2-Dipp migration in C5-protonated
1
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Table 1. Nickel catalyzed 1,2-Dipp shift of 2a to 3a.[a]
catalytic process that is also applicable to others iMICs (2b-g).
Treatment of a freshly prepared toluene solution of iMICs (2a-g)
with 5 mol% of Ni(cod)2 yields the anticipated compounds 3a-g in
54-97% yields (Scheme 1c). Interestingly, no conversion of 2a
into 3a was observed in THF (Table 1, entries 1, 2), instead an
intractable mixture of products was formed. In addition to nBuLi,
KN(SiMe3)2 can also be used as a base (entry 4). Addition of a
small amount of cod (10 mol%) to the reaction mixture improves
the yield (entries 6, 7). 3a-g have been characterized by NMR
spectroscopy and mass spectrometry. Solid-state molecular
structures of some selected compounds have been determined
by X-ray diffraction (see Supporting Information). Consistent with
the molecular structure (Scheme 1c), the 1H NMR spectrum of 3a
exhibits two septets and four doublets for the CHMe2 groups. The
backbone proton of 3a (6.82 ppm) resonates at a higher field
compared to that of (1a)Br (8.38 ppm).[11]
Compounds 3a-g featuring bulky Dipp substituents at the 1,4-
positions are promising starting materials of new sterically
demanding iMICs. The direct N-methylation of 3a-g affords 1,3-
imidazolium salts (4a-g)X (X = PF6, I, OTf or BF4) in good to
excellent yields (Scheme 2). (4a-f)X were characterized by NMR
spectroscopy, mass spectrometry, and X-ray diffraction studies
(see the Supporting Information). Selenium compounds have
proven appropriate candidates to analyze the electronic
properties of singlet carbenes.[3-4, 17] 6a and 6b were prepared by
the treatment of (4a)PF6 and (4b)PF6) with Se powder and
Entry
(1a)Br (g)
Solvent
Base
t
Yield[b]
1
0.5
0.5
1.0
1.0
2.5
1.0
2.5
THF
THF
Tol
nBuLi
KN(SiMe3)2
nBuLi
19
19
19
21
20
17
17
-
2
-
3
54
93
63
92
86
4
Tol
KN(SiMe3)2
nBuLi
5
Tol
6[c]
7[c]
Tol
nBuLi
Tol
nBuLi
[a]All reactions to prepare 2a were performed at −40 °C and after 30 minutes
stirring at room temperature was added Ni(cod)2 at −20 °C. [b]Yields refer to
isolated compounds. [c]With 10 mol% of 1,5-cyclooctadiene (cod) as an additive.
KN(SiMe3)2, respectively (Scheme 2). Compound 7 was isolated
as a white solid using a freshly prepared toluene solution of 5b
and (Me2S)AuCl. Similarly, 8-H and 8-Ph were obtained by
treating 5b with 0.5 eq. of [(allyl)PdCl]2 and [(cin)PdCl]2,
respectively. The 77Se{1H} NMR spectra of 6a (+9.0 ppm) and 6b
(+2.0 ppm) exhibit a lower-field signal compared to that of (2a)Se
(‒3.0 ppm) and (2b)Se (‒15.1ppm) containing a C5-protonated
iMIC (2a,b) (Table 2).[4c] The C2‒Se1 bond length and N1‒C2‒
C3 bond angle of 6a (1.856(2) Å, 104.8(1)°) (Figure 2) are
comparable with that of(2a)Se (1.859(2) Å, 104.3(1)°).[3c] The C2‒
Au1 bond lengths and the N1‒C2‒C3 bond angles of 7 (1.983(2)
Å/ 104.3(1)°) and (2g)AuCl (2.008(4) Å)/ (102.5(1)°)[12a] are fully
consistent. The C2‒Pd1 bond length of 8-H (2.042(2) Å) is slightly
larger compared to those of other iMIC-Pd complexes (1.955(2)‒
2.030(2) Å),[13c] which may be due to steric reason.[18]
Scheme 1. (a) Catalytic conversion of 2a-g into 3a-g. (b) Mechanistic proposal
for 1,2-Dipp shift of 2. (c) Isolated examples of 3. Inset: Solid-state molecular
structure of 3a.
Scheme 2. Synthesis of (4a-f)X, 6a, 6b, 7, and 8-R (R = H or Ph). Only 5a,b
were used to prepare 6-8. Inset: Solid-state molecular structure of (4b)PF6.
2
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6a
7
8-H
Figure 2. Solid state molecular structures of 6a, 7, and 8-H.
well with that of IPr (44.5%),[19] also containing two flanking Dipp
substituents.
Table 2. Stereoelectronic properties and calculated energy parameters of iMICs
(2a, 2b), S-iMICs (5a, 5b), and classical NHC (IPr).
Having analyzed the stereoelectronic properties, we decided to
probe the scope of 5a,b in palladium catalyzed standard C‒C and
C‒N cross coupling reactions and compare the results with
classical NHC (IPr) and iMICs (2a,b). The corresponding Pd(II)
precatalysts were generated in-situ using appropriate ligand
precursors [(4a,b)PF6, (1a,b)Br, or (IPr)HCl], Pd(OAc)2, and a
base (see the Supporting Information). For Suzuki-Miyaura C‒C
coupling reactions (Table S1), the yields were clearly higher with
5a,b (entries 4 and 5) than those with 2a,b (entries 2 and 3) and
IPr (entry 1). Strikingly, the amount of homocoupling byproduct
was also smaller with 5a,b (2-4%) compared to that with IPr (6%)
and 2a,b (8-14%). The use of isolated compounds (IPr)(cin)PdCl
and 8-Ph (cin = cinamyl) gave comparable results (Table S2). For
the C–N cross coupling of an aryl bromide and morpholine at room
temperature, the use of either 5b, 2a or IPr precursor gave the
product in 65-99% yield (Table S3). However, the coupling
reaction of aryl chloride and morpholine with IPr (entry 2) or 2a
(entry 3) was rather sluggish (Scheme 3). In strong contrast, the
use of 5b gave 78-99% conversion under similar experimental
conditions (entries 4 and 5), which clearly emphasizes the
superior activity of 5b over iMIC 2a and IPr.
[a]
77Se[a]
(ppm)
JC‒Se
%Vbur
HOMO
(eV)[b]
LUMO
(eV)[b]
1st PA[c]
(kcal/mol)
(Hz)
2a
2b
5a
5b
IPr
‒3.0
‒15.1
9.0
214
211
210
208
234
32.6[d]
33.1[d]
-
‒5.065
‒4.824
‒5.046
‒4.824
‒5.911
‒1.417
‒0.954
‒1.384
‒0.881
‒0.480
290.2
293.8
291.5
297.1
270.4
2.0
45.0[e]
44.5[e]
87
[a]Measured for the corresponding selenium compounds. [b]Calculated at the
B3LYP/def2-TZVPP level of theory. [c]The first proton affinity (1st PA) calculated
at the B3LYP/def2-TZVPP//def2-SVP level of theory.[d]Calculated for (2a)CuI
and (2b)CuBr.[4c] [e]Calculated for the corresponding AuCl adducts.[19]
The Tolman electronic parameter (TEP)[20] is the most commonly
used method for evaluating the electronic properties of ligands,
but like other IR methods, it is not very precise and has some
limitations.[21] In contrast, NMR methods usually give consistent
results, which are broadly applicable.[3-4] The values of 31P{1H}
and 77Se{1H} NMR signals of the corresponding (NHC)PPh and
(NHC)Se species are equally significant but the synthesis of
(NHC)Se is rather straightforward (see above).[3-4, 22] The 77Se{1H}
NMR data (Table 2) indicate weaker π-accepting properties of
2a,b,[4c] and 5a,b compared to that of IPr. Interestingly, 5a,b are
found to be slightly more π-acidic with respect to C5-protonated
iMICs 2a,b.[4c] This may be attributed to the presence of an
additional electron-withdrawing aryl (Dipp) group next to the
carbene carbon atom in 5a,b. The presence of a para-NMe2 group
in 2b and 5b enhances the electron density at the carbene center.
In summary, an unprecedented 1,2-aryl migration of a series of
C5-protonated iMICs (2a-g) under nickel catalysis to afford 1,2,4-
triayl-1,3-imidazoles (3a-g) has been reported. The direct N-
methylation of 3a-g has been shown to afford the corresponding
imidazolium salts (4)X, which are precursors of the super-iMICs
(5). S-iMIC-compounds 6-8 have been isolated and characterized.
Thus, 2b/5b are weaker π-acceptors but stronger σ-donors than
Entry
Base
Precursor
Carbene
-
Yield (%)[a]
1
2a/5a, respectively. This trend is fully consistent with the JC Se
‒
coupling constants.[4a, 23] Smaller the value of coupling constant,
larger the σ-donor strength. Thus, a clear trend in the π-acceptor
(IPr>5a>5b>2a>2b) and σ-donor (5b>5a>2b>2a>IPr) properties
of carbenes can be concluded. The σ-donor properties of 2a,b,
5a,b, and IPr can also be nicely correlated with the calculated first
proton affinity (1st PA) as well as with the energy of HOMO (Table
2).[24] The percentage buried volume (%Vbur) calculated by
employing Cavallo’s online program SambVca[4f, 5a] for 2a,b, 5b,
and IPr is shown in Table 2. The %Vbur for 5b (45.0%) is
considerably larger than those of 2a,b (~33%)[4c] but compares
1
2
3
4
5
LiN(SiMe3)2
LiN(SiMe3)2
LiN(SiMe3)2
LiN(SiMe3)2
KOtBu
none
(IPr)HCl
-
IPr
7
(IPrPh)Br (1a)Br
(4b)PF6
iMICPh (2a)
S-iMICDMP (5b)
S-iMICDMP (5b)
9
78
99
(4b)PF6
Scheme 3. Room temperature Buchwald-Hartwig C‒N cross couplings. [a]GC-
MS yield as an average of two independent runs.
3
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Acknowledgements
The authors gratefully acknowledge support from the Deutsche
Forschungsgemeinschaft (DFG GH 129/7-1) and thank Professor
Norbert W. Mitzel for his continuous encouragement. The support
by computing time provided by the Paderborn Center for Parallel
Computing (PC2) is acknowledged.
Keywords: mesoionic carbenes • ligand design • aryl migration •
C‒N bond activation • nickel catalysis
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10.1002/anie.202014328
Angewandte Chemie International Edition
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Entry for the Table of Contents
Super-iMICs (5) with the largest buried volume (Vbur. = 45%) known thus far for iMICs are accessible via unprecedented Ni-catalyzed
intramolecular 1,2-aryl migration of C5-protonated iMICs (2) to 3, subsequent N-alkylation of 3 to 4, and the deprotonation of 4. S-iMICs
(5) are stronger σ-donors and superior π-acceptors than 2. A comparative catalytic study reveals superior activity of S-iMICs (5) over
iMICs (2) and NHC (IPr).
Institute and/or researcher Twitter usernames: @Ghadwal_Group @unibielefeld
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