A cyclic diaminocarbene with a pyramidalized nitrogen atom: A stable N-heterocyclic carbene with enhanced electrophilicity

Article abstract of DOI:10.1002/anie.201202137

An anti-Bredt NHC! Placing one of the adjacent nitrogen atoms of an NHC in the bridgehead position of a bicyclic scaffold prevents the donation of its lone pair. Thus, the π-electron-accepting properties of the carbene center are enhanced, while the strong -electron-donating properties of classical NHCs are retained. Copyright

Full text of DOI:10.1002/anie.201202137

Angewandte
Chemie
DOI: 10.1002/anie.201202137
Stable Carbenes
A Cyclic Diaminocarbene with a Pyramidalized Nitrogen Atom:
A Stable N-Heterocyclic Carbene with Enhanced Electrophilicity**
David Martin, Nicolas Lassauque, Bruno Donnadieu, and Guy Bertrand*
Since their discovery,[1,2] thousands of reports have dealt with
the preparation and use of stable carbenes. Arguably, among
them,[3–6] the N-heterocyclic carbenes (NHCs) A have been
the most studied.[7–12] Despite their widespread applications,
NHCs still suffer from low modularity, which confines them to
a narrow range of strong s-donor ligands and nucleophiles. In
recent years, several strategies have successfully been devel-
oped for tuning their s-donor properties, which are mainly
linked to the s effect of the carbene substituents.[8] However,
the modulation of the p-acceptor properties of NHCs is more
challenging, because the high-energy LUMO is ruled by the
strong p donation of the two amino groups to the carbene
vacant orbital.[13] The replacement of a p-electron-donating
and s-withdrawing amino substituent by a s-donating alkyl
group (B) leads to an increase of electrophilicity but also
nucleophilicity (Figure 1).[14,15] Carbonyl groups can be intro-
duced into the backbone in order to compete with the carbene
center for the p donation of the nitrogen atom. The so-called
Figure 1. Energy (eV) of frontier orbitals of classical NHC A, cyclic
diamidocarbenes C are very electrophilic, but at the expense
(alkyl)(amino)carbene B, diamidocarbene C, and N-pyramidalized
of their nucleophilicity.[16–19] Herein we report the synthesis
(sum of angles around N=3368) diaminocarbene Apyr at the
and single-crystal X-ray diffraction study of a cyclic diamino-
carbene, which retains the nucleophilicity of classical NHCs,
but shows enhanced electrophilicity.
B3LYP/6-31G* level. Targeted carbene Apyra with its nonrelevant and
more representative resonance structures Apyra’ and Apyra’’, respectively.
We reasoned that for obtaining a carbene that retains the
nucleophilicity of classical NHCs A, amino groups that do not
bear electron-withdrawing substituents should be used. To
concomitantly enhance the electrophilicity, one of these
amino substituents should be restricted from donating its
lone pair to the carbene center. To support our hypothesis, we
performed ab initio calculations, which predicted that the
pyramidalization of one nitrogen atom of the parent NHC A,
as shown in Apyr, significantly decreases the energy of the
LUMO, without lowering the energy of the HOMO
(Figure 1).
=
which features a bridgehead C N bond, becomes irrelevant.
As a consequence, Apyra can be described as a “push–pull”
carbene,[13] in which only one amino group is p-donating, the
second one is acting as a s-withdrawing group (resonance
form Apyra’’).
The N-bridgehead amidinium salt 2H+ was prepared in
two steps (Scheme 1). The commercially available 3-piper-
idinemethanol was heated at 1608C with neat ethyl N-2,6-
diisopropylphenylformamidinate to quantitatively afford
The general reluctance, and sometimes the impossibility,
to form bridgehead double bonds in a sufficiently strained
polycyclic structure is well known as Bredtꢀs rule.[20] In the
anti-Bredt diaminocarbene Apyra, the resonance form Apyra’,
[*] Dr. D. Martin, Dr. N. Lassauque, B. Donnadieu, Prof. Dr. G. Bertrand
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957)
Department of Chemistry, University of California
Riverside, CA, 92521-0403 (USA)
E-mail: guy.bertrand@ucr.edu
Homepage: http://research.chem.ucr.edu/groups/bertrand/guy-
bertrandwebpage/
[**] The authors gratefully acknowledge financial support from the NSF
(CHE-1112133), DOE (DE-FG02-09ER16069), and NIH (R01 GM
68825).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202137.
Scheme 1. Synthesis of amidinium 2H+, carbene 2, and hexamethyldi-
silazane adduct 3.
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formamidine 1. The latter was reacted with trifluorometh-
anesulfonic anhydride in the presence of a base, and after
work-up, the amidinium 2H+ was isolated in 70% yield as
colorless crystals. Compound 2H+ is air and moisture stable in
the absence of acidic impurities. The 13C and 1H NMR signals
for the carbenium center are shifted strongly downfield (168
and 8.6 ppm, respectively) compared with those of known
amidinium salts containing six-membered rings ( ꢀ 152 and
ꢀ 7.5 ppm, respectively).[21–23] An X-ray diffraction analysis
for monocyclic diaminocarbenes containing five- and six-
membered rings, such as A (210–245 ppm),[8,26] but close to
those of the more electrophilic cyclic (alkyl)(amino)carbenes
B (300–320 ppm)[6] and diamidocarbenes C (262–280 ppm).[16]
A single-crystal X-ray diffraction analysis of 2 showed that
the environment around N1 and N2 remains almost
unchanged compared to 2H+ (N1 is planar and the sum of
À
the angles around N2 is 3428). The N1 C1 bond length
(1.349(2) ꢁ) and the N1-C1-N2 angle (112.9(1)8) compare
(Figure 2, left)[24] shows that the C1 N2 bond is longer
well with the values known for NHCs containing six-
À
(1.341(2) ꢁ) than the C1 N1 bond (1.310(2) ꢁ). As expected,
membered rings,[27] and the long N2 C1 bond distance
À
À
(1.407(2) ꢁ) clearly shows the weak or nonexistent p dona-
tion of N2 (Figure 1, right). Interestingly, carbene 2 proved to
be stable at room temperature both in solution and in the
solid state (mp: 138–1418C).
Similarly to classical NHCs, carbene 2 is a strong nucle-
ophile, shown by the instantaneous and quantitative reaction
at À788C with carbon dioxide, affording the betaine adduct 4
(Scheme 2).[28,29] To assess the basicity of 2, its protonated
form 2H+ was reacted at À788C with a stoichiometric amount
Figure 2. X-ray structures of 2H+ and 2 with thermal ellipsoids drawn
at 50% probability level. For 2H+, H1 is indicated, other hydrogen
atoms and the triflate anion are omitted for clarity. Selected bond
lengths [ꢀ] and angles [deg]: 2H+, C1–N1 1.310(2), C1–N2 1.341(2),
N1–C2 1.490(2), N1–C8 1.451(2), N2–C6 1.497(2), N2–C7 1.475(2);
N1-C1-N2 122.73(14), C1-N1-C2 120.49(14), C1-N1-C8 120.78(13), C2-
N1-C8 118.43(13), C1-N2-C7 115.45(14), C7-N2-C6 111.22(14), C6-N2-
C1 112.17(13). 2, C1–N1 1.349(2), C1–N2 1.407(2), N1–C2 1.489(2),
N1–C8 1.461(1), N2–C6 1.488(2), N2–C7 1.464(2); N1-C1-N2
112.9(1), C1-N1-C2 126.0(1), C1-N1-C8 118.7(1), C2-N1-C8 114.3(1),
C1-N2-C7 118.2(1), C7-N2-C6 110.2(1), C6-N2-C1 113.5(1).
N1 is in a planar environment, while the bridgehead nitrogen
atom N2 is pyramidalized; the sum of the bond angles around
N2 is 3398, a typical value for sp3-hybridized nitrogen atoms in
1-aza[3.3.1]bicyclononane derivatives.[25] In addition to this
pyramidalization, there is also a significant torsion around the
À
C1 N2 bond. Indeed, a geometric analysis indicates that the
Scheme 2. Reactivity of carbene 2.
dihedral angle between the lone pair of N2 and the formal
vacant p orbital of the carbene is approximately 348. Of note,
one of the six-membered rings can have an optimal chair
conformation, thus limiting the ring strain.
of the classical NHC 5[21] containing a six-membered ring
(pKa = 28),[30] and the instantaneous formation of the N-
bridgehead carbene dimer 6 was observed. Many diamino-
carbenes are also known to dimerize upon “proton cataly-
sis”.[31] Importantly, when 2 was reacted with 0.2 equivalents
of 5H+, carbene dimer 6 was also obtained instantaneously,
thus demonstrating that the basicity of 2 and 5 is very similar.
The Tolman electronic parameter (TEP) is a well recog-
nized measure of the net donation of a ligand (L) to transition
metals.[32] Addition of half an equivalent of a chloro(1,5-
cyclooctadiene)iridium dimer to 2, followed by treatment
with carbon monoxide afforded the cis-chlorodicarbonyliri-
dium complex 7, which was fully characterized by NMR
spectroscopy and X-ray crystallography. From the stretching
frequencies of the carbonyl groups in IR measurements, we
inferred[33] a TEP value of 2047 cmÀ1, which is higher than that
Monitoring the deprotonation of 2H+ by 13C NMR
spectroscopy
with
potassium
hexamethyldisilazide
(KHMDS) in THF at À788C allowed the observation of
a 13C NMR signal at a low field, thus suggesting the formation
of carbene 2.[26] However, when the solution was warmed to
À208C, this signal disappeared and after work-up, derivative
À
3, which results from the formal carbene insertion into the N
H bond of hexamethyldisilazane, was isolated in 78% yield as
a single diastereomer with the N(SiMe3)2 group located on the
less-hindered exo face.
In order to prevent the addition of the amine, the
deprotonation of 2H+ with KHMDS was carried out at
À788C. The solution was cooled to À908C, and addition of
pentane induced the precipitation of carbene 2, which was
isolated as a white powder in 62% yield. The 13C NMR
chemical shift of 2 (282 ppm) is at much lower field than those
of classical NHC
5 containing a six-membered ring
2
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À79.0 ppm; MS (m/z): [M+H]+ calcd for C19H30N2, 287.2435; found,
(2044 cmÀ1, see the Supporting Information), and consistent
with a ligand as strongly s-donating as 5, but significantly
more p-accepting. DFT calculations at the B3LYP/6-311g**
level on the parent N-methyl-substituted carbenes 2’ and 5’
(R = Me) are in line with our experimental findings. The
energy of the HOMO (À4.8 eV) and the proton affinity
(272 kcalmolÀ1) of 2’ are similar to those of carbene 5’, which
contains a six-membered ring (À5.0 eV, 270 kcalmolÀ1).
Conversely, carbene 2’ has a smaller singlet–triplet gap
(43 kcalmolÀ1) than 5’ (62 kcalmolÀ1), which confirms the
enhanced electrophilicity of 2’. To further prove this point
experimentally, 2 was reacted with cyclohexylisocyanide at
À788C, and we observed the clean formation of keteneimine
8. This coupling reaction does not occur with classical NHCs,
and is only known for electrophilic carbenes.[34–37]
These results, as a whole, demonstrate that the placement
of one of the two nitrogen atoms of an NHC in a bridgehead
position considerably increases the electrophilic character of
the carbene center, without diminishing its nucleophilicity.
Because a variety of bicyclic scaffolds are available, the
pyramidalization of the bridgehead nitrogen atom can be
modified at will. This simple topological modification should
allow the fine tuning of the p-accepting properties of NHCs,
and consequently of the catalytic properties of the corre-
sponding metal complexes.[38–40]
287.2436.
Isolation of carbene 2: A solution of the triflate salt 2H+ (1.46 g,
3.4 mmol) in THF (20 mL) was added at À788C to a solution of
KHMDS (758 mg, 3.8 mmol) in THF (3 mL). The solution was cooled
to À958C and pentane (15 mL) was slowly added and induced the
precipitation of a white solid. After stirring for 10 min at this
temperature, the supernatant was removed by a dry canula equipped
with a filter. The resulting solid was warmed to room temperature,
and dried under vacuum. Extraction with toluene (2 ꢂ 10 mL),
removal of the solvent, and drying under vacuum gave 2 (0.6 g,
62%) as a white powder. Crystals suitable for an X-ray crystallo-
graphic study were grown from a pentane/toluene (1:1) solution at
À158C. mp: 138–1418C. 1H NMR (C6D6, 300 MHz): d = 7.3 (m, 1H),
7.2 (m, 2H), 4.30 (broad d, J = 12 Hz, 1H), 3.8 (m, 1H), 3.4 (m, 2H),
3.3 (m, 2H), 3.1 (m, 2H), 2.91 (d, J = 12 Hz, 1H), 2.8 (m, 3H), 1.9
(broad s, 1H), 1.56 (d, J = 7 Hz, 3H), 1.39 (d, J = 8 Hz, 3H), 1.35 (d,
J = 8 Hz, 3H), 1.31 ppm (d, J = 7 Hz, 3H); d 13C NMR (C6D6,
125 MHz): d = 23.5 (CH3), 24.0 (CH3), 25.4 (CH3), 25.5 (CH3), 26.8
(CH2), 27.4 (CH2), 28.6 (CH), 28.7 (CH), 30.0 (CH), 47.4 (CH2), 49.7
(CH2), 56.8 (CH2), 123.6 (CHaro), 124.8 (CHaro), 127.1 (CHaro), 143.8
(Caro), 145.4 (Caro), 146.2 (Caro), 282 ppm (NCN).
Received: March 17, 2012
Published online: && &&, &&&&
Keywords: anti-Bredt · carbenes · ligand desing · NHC
.
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N), 1461, 1278, 1252, 1162, 1031 cmÀ1; H NMR (CDCl3, 300 MHz):
1
d = 8.46 (s, 1H), 7.46 (pseudo t, J = 9 Hz, 1H), 7.28 (d, J = 9 Hz, 1H),
7.24 (d, J = 9 Hz, 1H), 4.3 (m, 1H), 3.85 (dd, J = 6 and 15 Hz, 1H),
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143.8 (Caro), 133.8 (Caro), 131.5 (CHaro), 125.6 (CHaro), 124.9 (CHaro),
120.9 (q, J = 320 Hz, CF3), 60.1 (CH2), 50.3 (CH2), 49.2 (CH2), 30.1
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Stable Carbenes
An anti-Bredt NHC! Placing one of the
adjacent nitrogen atoms of an NHC in the
bridgehead position of a bicyclic scaffold
prevents the donation of its lone pair.
Thus, the p-electron-accepting properties
of the carbene center are enhanced, while
the strong s-electron-donating properties
of classical NHCs are retained.
D. Martin, N. Lassauque, B. Donnadieu,
G. Bertrand*
&&&& — &&&&
A Cyclic Diaminocarbene with
a Pyramidalized Nitrogen Atom: A Stable
N-Heterocyclic Carbene with Enhanced
Electrophilicity
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