Crystalline 1H-l,2,3-Triazol-5-ylidenes: New stable mesoionic carbenes (MICs)

Article abstract of DOI:10.1002/anie.201001864

Figure Persentation Click makes MICs: A short modular synthesis allows the preparation of novel stable heterocycles that feature a lone pair of electrons on a carbon center. The donor properties of these mesoionic com-pounds are greater than those of classical N-heterocyclic carbenes; they are accessible by deprotonation of the corresponding conjugate acids using comparatively mild bases.

Full text of DOI:10.1002/anie.201001864

Angewandte
Chemie
DOI: 10.1002/anie.201001864
Stable Carbenes
Crystalline 1H-1,2,3-Triazol-5-ylidenes: New Stable Mesoionic
Carbenes (MICs)**
Gregorio Guisado-Barrios, Jean Bouffard, Bruno Donnadieu, and Guy Bertrand*
Until recently, the availability of neutral carbon-based k1C
ligands was limited to carbon monoxide, isocyanides, and
carbenes. Compared to phosphorus-based ligands, carbenes
tend to bind more strongly to metal centers, avoiding the
necessity for the use of excess ligand in catalytic reactions.
The corresponding complexes are often less sensitive to air
and moisture, and are remarkably resistant to oxidation.[1] As
the robustness of carbene complexes is largely due to the
presence of strong carbon–metal bonds, other types of
carbon-based ligands are highly desirable. It is noteworthy
that, although complexes between a carbene and a transition
metal have been known for a long time,[2] the recent
developments in their application in catalysis[3] have been
greatly facilitated by the availability of carbenes that are
stable enough to be bottled.[4,5] Moreover, carbenes, espe-
cially imidazol-2-ylidenes I[4c] and 1,2,4-triazol-5-ylidenes
II,[4e] are also excellent organocatalysts (Scheme 1).[6]
In 2001, Crabtree and co-workers first reported complex
A, which features an imidazole ring bound at the C5 position
(III), and not at C2 as commonly observed.[7] More recently,
Huynh and co-workers[8] and Albrecht and co-workers[9a]
showed that pyrazolium and 1,2,3-triazolium salts can serve
as precursors to metal complexes of type B and C, which
feature pyrazolin-4-ylidenes IV and 1,2,3-triazol-5-ylidenes V
as the ligand, respectively. As a consequence of their lineage,
these have also been referred to as N-heterocyclic carbenes
(NHCs). However, as no reasonable canonical resonance
forms containing a carbene can be drawn for free ligands III–
V without additional charges (see V’), these ligands have been
described as abnormal or remote carbenes (aNHCs or
rNHCs, respectively).[10] As they are, in fact, mesoionic
compounds,[11] we suggest naming this family of compounds
mesoionic carbenes (MICs). There have been no reported
dimerizations of MICs III and IV, which suggests that the
Wanzlick equilibrium pathway for classical carbenes is
disfavored;[12] this observation should lead to relaxed steric
requirements for their isolation. Moreover, experimental and
theoretical data suggest that MICs III–V are even stronger
electron-donating species than NHCs I and II, which opens up
interesting perspectives for their applications.[10]
Our recent success in the isolation of a free imidazol-5-
ylidene III[13] and pyrazolin-4-ylidenes IV (cyclic bent
allenes),[14,15] prompted us to investigate the possibility of
preparing new types of stable neutral compounds that feature
a lone pair of electrons on the carbon atom.[16] Preliminary
calculations (B3LYP, 6-311G(d,p); for details, see the Sup-
porting Information) predicted that the parent MIC V is
located at an energy minimum, about 32 kcalmolÀ1 above the
regioisomeric parent 1,2,4-triazol-5-ylidene II. Furthermore,
parent V is predicted to exhibit an appreciably large singlet–
triplet band gap (56 kcalmolÀ1), which is a good predictor of
carbene stability and thus of possible isolation. Herein, we
report the preparation, isolation, and characterization of two
free 1,2,3-triazol-5-ylidenes of type V.
Scheme 1. Classical NHCs I and II, their mesoionic carbene isomers
III–V, and the first complexes featuring the latter.
By analogy with the synthetic route used for preparing
NHCs and the related species III and IV, 1,2,3-triazolium salts
(2a,b) were targeted as precursors for the desired 1,2,3-
triazol-5-ylidenes (Va,b). A sterically hindered flanking aryl
substituent (2,6-diisopropylphenyl, Dipp) was selected to
provide kinetic stabilization to the ensuing free ligand. 1,2,3-
Triazole 1 was obtained in 83% yield from the copper-
catalyzed azide–alkyne cycloaddition (CuAAC, click chemis-
try) of 2,6-diisopropylphenyl azide and phenylacetylene.[17]
The one-pot conversion of aniline into the desired aryl azide,
followed in situ by CuAAC as reported by Moses and co-
workers[18] was found to be especially convenient for the
synthesis of 1. Alkylation of 1 with methyl or isopropyl
[*] Dr. G. Guisado-Barrios, Dr. J. Bouffard, B. Donnadieu,
Prof. G. Bertrand
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957)
Department of Chemistry, University of California
Riverside, CA 92521-0403 (USA)
Fax: (+1)951-827-2725
E-mail: guy.bertrand@ucr.edu
Homepage: http://research.chem.ucr.edu/groups/bertrand/
guybertrandwebpage/
[**] We are grateful to the NIH (R01 GM 68825) and the DOE (DE-FG02-
09ER16069) for financial support of this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001864.
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trifluoromethanesulfonate afforded the corresponding tri-
azolium salts in moderate to excellent yields (2a and 2b,
respectively; Scheme 2).
Figure 1. Molecular views (thermal ellipsoids set at 50% probability)
of 2a (top) and Va (bottom) in the solid state. For clarity, counter
ions, solvent molecules, and H atoms are omitted, except for the ring
hydrogen of 2a. Selected bond lengths [ꢀ] and angles [8] for 2a:
N1–N2 1.3208(16), N2–N3 1.3183(16), N3–C4 1.3559(17), C4–C5
1.3647(19), C5–N1 1.3446(17); N1-C5-C4 105.85(12). Va: N1–N2
1.3439(12), N2–N3 1.3216(13), N3–C4 1.3682(13), C4–C5 1.4053(14),
C5–N1 1.3662(13); N1-C5-C4 99.70(8).
Scheme 2. Synthesis of the free 1,2,3-triazol-5-ylidenes Va,b.
Potassium bases have been identified as the reagents of
choice for the depronation of carbene precursors, as they
avoid the formation of stable carbene–alkali-metal adducts
that are commonly encountered when lithium bases are
used.[12,13,14a,19] Gratifyingly, triazolium salts 2a,b were cleanly
deprotonated with either potassium bis(trimethylsilyl)amide
or potassium tert-butoxide in ethereal solvents to afford the
corresponding MICs Va and Vb in 55 and 39% yield,
respectively. Deprotonation was evidenced by the disappear-
1
ance of the triazolium CH signal in their H NMR spectra
(2a: d = 8.62 ppm; 2b: d = 8.85 ppm) and the appearance of a
signal at low field in the 13C NMR spectrum (Va: d =
202.1 ppm; Vb: d = 198.3 ppm). The structure of Va was
unambiguously confirmed by X-ray crystallography
(Figure 1).[20] In the solid state, Va contains a planar hetero-
cycle, characterized by bond lengths that are intermediate
between those of single and double bonds; both of these
features are indicative of electronic delocalization. Upon
deprotonation, the C5 carbon bond angle becomes more
acute (2a: 1068; Va: 1008), which is consistent with an
increased s character in the s lone pair orbital of Va
Scheme 3. Degradation of free 1,2,3-triazol-5-ylidene Va, and analogy
with the rearrangement of III into I.
À
compared to the C H bonding orbital of the precursor 2a.
This is in agreement with the generally observed trend for
apparent rearrangement is reminiscent of that recently
observed in the formation of imidazol-2-ylidenes of type I
from imidazol-5-ylidenes of type III that contain an electro-
philic Y group.[21] In agreement with this hypothesis, MIC Vb,
which contains the less-electrophilic isopropyl group at the
N3 position, appears much more robust with respect to this
decomposition pathway.
carbenes and their conjugate acids.[5]
In the solid state, with the exclusion of oxygen and
moisture, free 1,2,3-triazol-5-ylidene Va (m.p. 50–528C
decomp.) remained stable for several days at À308C and for
a few hours at room temperature. By contrast, Vb (m.p. 110–
1128C) was significantly more stable, showing no sign of
decomposition after three days at room temperature in the
solid state. Upon heating in a benzene solution for 12 hours at
508C, Va decomposed to give, among other products, triazole
3 (Scheme 3; for details, see the Supporting Information). We
surmise that the latter product results from a nucleophilic
attack of the carbon lone pair of Va on the methyl group of a
second molecule of Va, giving rise to heterocycles 4 and 5,
which react together to afford the observed product 3. This
To evaluate the donor properties of 1,2,3-triazol-5-yli-
denes, the [(Va)Ir(CO)2Cl] complex was prepared by addi-
tion of Va to [{Ir(cod)Cl}2] (cod = 1,5-cyclooctadiene), fol-
lowed by treatment with an excess of carbon monoxide. The
CO vibration frequencies (n = 2061 and 1977 cmÀ1; navg
=
2019 cmÀ1) are in line with those of the analogous iridium
complex, previously reported by Albrecht and co-workers
(navg = 2021 cmÀ1),[9a] and are indicative of donor properties
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Ed. Engl. 1989, 28, 621 – 622; c) A. J. Arduengo III, R. L.
that are superior to those of NHCs I and II (navg = 2022-
2031 cmÀ1),[22] but inferior to those of MICs III (navg = 2003-
2006 cmÀ1)[23] and IV (navg = 2002 cmÀ1).[14b]
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Free 1H-1,2,3-triazol-5-ylidenes, as exemplified by com-
pounds Va,b, possess an ensemble of properties that portend
to their utility. The synthesis of their precursors is short and
efficient, from readily available starting materials, yet is
modular and thus amenable to a wide variety of potential
analogues. As with other mesoionic carbenes III and IV, the
dimerization of MICs of type V has not been observed;
therefore, the preparation of comparatively unhindered MICs
is predicted to be viable. Their donor properties are greater
than those of NHCs of type I and II, but they are nonetheless
available by deprotonation using mild bases (e.g. alkoxides),
thus signaling their potential for applications, such as
nucleophilic organocatalysis. Free triazolylidenes V comple-
ment the rapidly growing numbers of neutral carbon-based
k1C ligands that are now available. We predict that many
other classes of MICs, that are derived from a variety of
heteroaromatic scaffolds, can be isolated. This endeavor is
currently the object of ongoing efforts in our laboratory.
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Experimental Section
1H-1,2,3-triazol-5-ylidenes (Va,b): Anhydrous Et2O (7 mL) was
added to a Schlenk flask containing triazolium salt 2a (0.40 g,
0.85 mmol) and KN(SiMe3)2 (0.18 g, 0.9 mmol) that had been cooled
to À788C. The reaction mixture was stirred at À788C for 10 min, and
allowed to warm to room temperature for 50 min before the solvent
was evaporated under reduced pressure. The residue was extracted
twice by trituration in anhydrous hexanes (2 ꢀ 5 mL). After filtration
and transfer into a second Schlenk flask using a filter cannula, the
solvent was evaporated to give Va as a pale purplish-white solid
(0.149 g, 55%). Using a similar procedure, Vb was obtained as
pinkish-white solid (0.053 g, 39%).
Received: March 29, 2010
Published online: May 27, 2010
Keywords: carbenes · carbene ligands · click chemistry ·
.
mesoionic compounds · triazoles
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