A Purely Organic Tricarbanion

Article abstract of DOI:10.1002/anie.201803647

How many carbanions can an organic molecule accommodate? The formation of purely organic carbanions with multiple charges is challenging as charge stabilization cannot be achieved through metal coordination. Previously, only quaternary ammonium dicarbanion salts had been reported. By using highly electron-deficient trifluoromethanesulfonyl (triflyl or Tf) groups, the formation of a purely organic tricarbanion has been realized for the first time.

Full text of DOI:10.1002/anie.201803647

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Title: A Purely Organic Tricarbanion
Authors: Benjamin List, Denis Höfler, Richard Goddard, Julia B.
Lingnau, and Nils Nöthling
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803647
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10.1002/anie.201803647
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A Purely Organic Tricarbanion
Denis Höfler, Richard Goddard, Julia B. Lingnau, Nils Nöthling, and Benjamin List*
Abstract: How many carbanions can an organic molecule
accommodate? The formation of purely organic carbanions with
multiple charges is challenging as charge stabilization cannot be
achieved via metal coordination. So far only quaternary ammonium
dicarbanion salts have been reported.[1] Using highly electron-
deficient trifluoromethanesulfonyl (triflyl or Tf) groups, the formation
of a purely organic tricarbanion could be realized for the first time.
instead of the expected ammonium salt of 2, we obtained
tetraalkylammonium tricarbanion salt (Scheme 1).
Remarkably, compound 1 is not only the first purely organic
1
tricarbanion, but also contains an aldehyde functional group.
Formally, carbanions can be regarded as conjugate bases of C–
H acids.[2] They play prominent roles as reactive intermediates in
a variety of transformations. Classic examples are the Grignard,
aldol, Wittig and Knoevenagel reactions.[3] Carbanions can be
primarily stabilized through electron-withdrawing groups and
metal coordination. Lithiated polycyclic aromatic compounds
have been reported with up to four negative charges while
lanthanum salts of fullerenes have been observed with formally
up to six negative charges.[4] Replacing metals with sterically
hindered cations such as organic, quaternary ammonium
cations, greatly reduces the degree of ion pairing.[5] As a result,
almost “naked” counteranions[6] with associated enhanced
nucleophilicity and reactivity are obtained.[6-7] This concept has
been leveraged for example in the alkylation of quaternary
ammonium enolates.[5,8]
Scheme 1. Route to tricarbanion 1. Tf = SO2CF3.
In the absence of ion pairing, strong electron withdrawing
groups, such as nitrile and triflyl (Tf, trifluoromethanesulfonyl)
groups have to be employed to increase the stability of organic
carbanionic species.[9] This strategy proved to be successful for
the synthesis of several quaternary ammonium dicarbanion
salts.[1] However, the formation of higher charged purely organic
multi-carbanions has so far, to the best of our knowledge, not
been realized. Here we report the synthesis and characterization
of the first organic tricarbanion.
Extending our work on strong, allylic C-H acids,[9b] we
recently became attracted to C3 symmetric acid 2. However,
when we reacted triformylmethane[10] with bistriflylmethane,
trimethylorthoacetate, and acetic anhydride, followed by
subsequent addition of tetraethylammonium bicarbonate,
[*]
Prof. Dr. B. List
Max-Planck Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany)
E-mail: list@kofo.mpg.de
Figure 1. Crystal structure of tricarbanion 1. Selected atoms are labelled.
Hydrogen atoms were calculated. For the carbanionic carbons C1, C6, and C7
the sum of all (surrounding) angles is 358.6°, 359.7°, and 359.9° respectively.
Selected bond lengths (Å): C4–O1 1.217(4), C4–C3 1.476(4), C3–C2
1.331(4), C2–C1 1.476(4).
Supporting information for this article is given via a link at the end of
the document.
CCDC1824208 contains the supplementary crystallographic data for
this paper. This data can be obtained free of charge from The
Cambridge Crystallographic Data Centre.
The structure of the tricarbanion 1 was elucidated by X-ray
crystallography (Figure 1).[11] All three carbanionic carbon atoms
(C1, C6, and C7) have trigonal planar geometries. Interestingly,
the bond lengths of the α,β-unsaturated aldehyde moiety are
very similar to those of acrolein,[12] ruling out charge
delocalization in this group. Rather, these observations suggest
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that the three negative charges are mainly delocalized in the
triflyl moieties.
The fact that the aldehyde is stable against further
nucleophilic attack indicates anionic charge stabilization
amongst the triflyl groups. Theoretical calculations on the model
species dimethyl sulfone, trifluoromethyl(methyl)-sulfone and the
carbanion 1,1-ditriflylethan-1-ide (see Supporting Information)
reproduce the tendency of the triflyl O atoms to adopt a position
close to the plane of the carbanion. Further, a statistical
evaluation of ditriflylcarbanion crystal structures from the
Cambridge Structure Database also supports this observation
(see Supporting Information).
Remarkably, for all three carbanions in 1 the triflyl groups
adopt similar conformations despite different crystal
environments. For all six CF3 groups the S-C bonds lie almost
perpendicular to the S,C,S planes of the carbanions (Figure 2).
In addition, for each carbanion the CF3 groups of neighboring
triflyl groups lie trans to one another.
The notion of a build-up of negative charge on the triflyl
oxygen atoms is also supported by the fact that several of the O
atoms exhibit short O···H-C(N) hydrogen bonding interactions
with the α-hydrogens of the tetraethyl ammonium cation
(Figure 4). The shortest O···C distance is 3.219(4) Å and is
comparable to the O···H-C(N) distances observed in the crystal
structures of the tetrabutylammonium salts of malonic acid
dialkyl esters.[6] This is further supported by a previously
2-
reported charge density study of the SO4 anion in which the
oxygen atoms carry considerable negative charge.[14]
Figure 2. A For all six trifyl groups, the S-CF3 bond lies almost perpendicular
to the mean plane of the adjacent carbanion. B On each carbanion the CF3
groups point in opposite directions relative to the mean plane of the carbanion.
Shown are the respective (F3)C-S···S-C(F3) torsion angles.
Figure 4. Crystal structure of the tricarbanion showing the interaction of the
molecules with one tetraethyl ammonium cation. A (N)C(-H)···O hydrogen
bonding interaction with a distance of 3.219(4) Å between the sulfonyl oxygen
of the tricarbanion and the α-carbon atom of the cation is observed.
Figure 3. A Displacements of the O atoms in the triflyl groups from the mean
plane of the neighboring carbanion. B All six triflyl groups exhibit an increased
angle between the carbanion C atom and the midpoint between the O atoms
of each triflyl group about the sulfur atom, compared with the neutral dimethyl
sulfone.[13]
Moreover, the oxygen atoms of neighboring triflyl groups lie
close to the planes of the respective carbanionic centers
(Figure 3A). In each of the six triflyl groups, the O atoms are
bent away from the carbanion C atom in the direction of the CF3
group. This is most clearly seen by comparing the C-S-O' angles
in each triflyl group, where O' is the midpoint of the two O atoms
(Figure 3B). The average difference in the C-S-O' angles in 1 is
27° compared with 0° in the crystal structure of dimethyl sulfone,
where no negative charge is present. The tendency of both
oxygen atoms to lie in the plane of the carbanion may improve
the negative charge delocalization between the carbanionic
center and its neighboring four triflyl oxygen atoms.
Scheme 2. Plausible pathway to tricarbanion 1.
At the moment, we have no evidence on how the
tricarbanion is formed under these reaction conditions. However,
a likely pathway can be proposed (Scheme 2). Accordingly,
triformylmethane could react with two equivalents of
bistriflylmethane to give aldehyde
3 in a Knoevenagel
condensation.[15] This is followed by a Michael addition of
bistriflylmethane, rather than a third Knoevenagel condensation,
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10.1002/anie.201803647
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to give the desired tricarbanion after deprotonation. Studies to
clarify this are currently underway.
137, 232-238; d) L. Feng, et al., J. Am. Chem. Soc. 2006, 128, 5990-
5991.
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In conclusion, although tetraalkylammonium ions are only
weakly coordinating cations and as such provide limited
electrostatic stabilization, the formation of a purely organic
tricarbanion salt has been observed for the first time. Our results
showcase the exceptional anion stabilizing ability of triflyl groups
which we consider essential for the formation of stable highly
charged anionic species, opening the possibility of the formation
of organic polycarbanions.
M. T. Reetz, S. Hütte, R. Goddard, J. Am. Chem. Soc. 1993, 115, 9339-
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Acknowledgements
Generous support by the Max-Planck-Society, the Deutsche
Forschungsgemeinschaft (Leibniz Award to B.L. and Cluster of
Excellence RESOLV, EXC 1069), and the European Research
Council (Advanced Grant “C–H Acids for Organic Synthesis,
CHAOS”) is gratefully acknowledged. R.G. thanks J. Breidung
for helpful discussions. We thank the members of our analytical
departments for their excellent service.
[10] Z. Arnold, M. Budesinsky, J. Org. Chem. 1988, 53, 5352-5353.
[11] Please see reference [16] for crystal data.
[12] a) K. Kuchitsu, T. Fukuyama, Y. Morino, J. Mol. Struct. 1968, 1, 463-
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8472.
Keywords: tetraalkylammonium cation
•
purely organic
tricarbanion • highly charged anion • triflyl stabilization
[14] M. S. Schmøkel, S. Cenedese, J. Overgaard, M. R. V. Jørgensen, Y.-S.
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[16] The crystal structure of 1: C38H65Cl2F18N3O13S6, Mr = 1377.19 g.mol-1,
colourless prism, monoclinic, P21/c (no. 14), a = 11.372(2) Å, b =
45.769(8) Å, c = 12.482(2) Å, β = 115.053(3)°, V = 5885.3(17) Å3, T =
100(2) K, Z = 4, dcalcd = 1.554 Mgm-3, Mo-Kα radiation, λ = 0.71073 Å.
Further details of the crystal structure analysis are given in the
Supporting Information. CCDC-1824208.
[2]
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Denis Höfler, Richard Goddard, Julia B.
Lingnau, Nils Nöthling, and Benjamin
List*
1 – 3
A Purely Organic Tricarbanion
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