Characterization of the key intermediates of carbene-catalyzed umpolung by nmr spectroscopy and X-Ray diffraction: Breslow intermediates, homoenolates, and azolium enolates

Article abstract of DOI:10.1002/anie.201303107

Caught in the act: Diamino enols, diamino dienols, azolium enolates, and azolium enols are postulated intermediates of the N-heterocyclic carbene catalyzed umpolung of aldehydes and enals. Several of these elusive reaction intermediates were generated wit

Full text of DOI:10.1002/anie.201303107

.
Angewandte
Communications
DOI: 10.1002/anie.201303107
Organocatalysis
Characterization of the Key Intermediates of Carbene-Catalyzed
Umpolung by NMR Spectroscopy and X-Ray Diffraction: Breslow
Intermediates, Homoenolates, and Azolium Enolates**
Albrecht Berkessel,* Veera Reddy Yatham, Silvia Elfert, and Jçrg-M. Neudçrfl
[
12]
[7]
In enzymes as well as in organocatalytic umpolung, catalysis
by N-heterocyclic carbenes hinges on the reversible formation
amino enols by Mayr et al.
and ourselves. With our
method for the generation of 2,2-diamino enols in hand, we
were able to characterize their solution structures and
reactivity by in situ NMR spectroscopy. Most importantly,
we could show that 2,2-diamino enols I (Scheme 1a) indeed
act as acyl anion equivalents, and selectively form cross-
[1,2]
of Breslow intermediates
(chemically: (di)amino enols) I
(
Scheme 1a) in which the innate polarity of, for example, an
[
7]
benzoin adducts with additional aldehyde. Whereas our
NMR study provided proof for the existence and d reactivity
1
of 2,2-diamino enols for the first time, NMR spectroscopy
could neither provide information on the geometric param-
eters of the crucial 2,2-diamino enol C=C double bond, nor on
its subtle responses to changes in electron density.
In contemporary carbene organocatalysis, probably the
3
3
most fascinating and proliferative area is a -d umpolung
“conjugate umpolung”), that is, reaction sequences originat-
ing from the interaction of an a,b-unsaturated aldehyde with
(
[
13,14]
a carbene (Scheme 1b).
Again, a Breslow-type inter-
mediate is assumed to be pivotal, namely the diamino dienol
II. In the latter, the g-position (b-position of the former enal)
carries a partial negative charge, and the diamino dienols II
consequently react as homoenolate equivalents. A subse-
[
15]
Scheme 1. Carbene-catalyzed umpolung of a) simple aldehydes and
b) a,b-unsaturated aldehydes.
quent OH–Cg proton shift in the diamino dienol II leads to
the azolium enolate III. In the latter, the b-position (a-
position of the former enal) carries a partial negative charge,
and the azolium enolates III are therefore considered as
enolate equivalents. Note that the formation of the inter-
mediate II and/or III is assumed to be the decisive branching
point for product formation in enal conjugate umpolung, for
example, for formation of g,d-unsaturated d-lactones vs.
formation of cyclopentenes (Scheme 1b). Quite substantial
effort has been invested to optimize reaction parameters
aldehyde substrate is inverted from electrophilic to nucleo-
[
3–5]
philic.
In Seebachꢀs terminology, the aldehyde undergoes
1
1
[6]
[1,2]
a -d umpolung. Postulated in 1958,
the first successful
generation of diamino enols from aldehydes and carbenes,
and their characterization by in situ NMR spectroscopy was
[7]
reported by us in 2012. Earlier attempts to isolate inter-
mediates resulted in the characterization of C2-hydroxy-
[8]
methyl azolium salts by Teles et al. and hydroxybenzyl
(solvent, base etc.) such that one pathway is favored over the
[
9]
[16–18]
azolium salts by OꢀDonoghue, Smith et al., the keto form of
Breslow intermediates and spiro-dioxolanes (i.e. 1:2 carbene–
other.
In sharp contrast to their pivotal character in
conjugate umpolung, no homoenolate II appears to have ever
been characterized. Azolium enolates III have been accessed
[
10]
aldehyde adducts) by our group,
aza analogues of the
[11]
[19,20]
Breslow intermediate by Rovis et al., and O-methylated
by addition of carbenes to ketenes.
Herein we now report the crystallization of three 2,2-
diamino enols I, their X-ray crystal structures, and the effects
of electron density modulation on their structural parameters,
in particular around the reactive enol C=C bond. Further-
[
*] Prof. Dr. A. Berkessel, V. R. Yatham, S. Elfert, Dr. J.-M. Neudçrfl
Cologne University, Department of Chemistry
Greinstrasse 4, 50939 Cologne (Germany)
E-mail: berkessel@uni-koeln.de
more we disclose the first characterization, by X-ray structure
analysis and NMR spectroscopy, of two diamino dienols II
and two azolium enolates III, together with NMR monitoring
of the tautomerization of a diamino dienol (II) to an azolium
enolate (III). Finally, we show that the protonation of
a diamino dienol (II) affords a cationic azolium enol.
Homepage: http://www.berkessel.de
[
**] Support by the Fonds der Chemischen Industrie and by BASF SE is
gratefully acknowledged. A.B. acknowledges COST membership
(
CM0905, Organocatalysis).
Supporting information for this article (detailed descriptions of the
crystallization experiments, DFT calculations, and NMR measure-
ments) is available on the WWW under http://dx.doi.org/10.1002/
anie.201303107.
[
7]
As we reported earlier, the saturated imidazolidinyli-
dene SIPr (1) reacts smoothly with a variety of aldehydes 2 in
1
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Chemie
have been expected, there is neither inter- nor intramolecular
hydrogen bonding to or from the enol OH group (see
Figure 1, top row, for a combined ball-and-stick/space-filling
representation of one (left) and two (right) molecules of the
2
,2-diamino enol 3a, as observed in the crystal). The crystal
Scheme 2. Saturated imidazolidinylidenes such as SIPr (1) react with
aldehydes 2 to afford 2,2-diamino enols 3.
packing of the 2,2-diamino enols 3a, 3c, and 3e appears to be
[
21]
based solely on dispersive hydrocarbon interactions. Note
that our DFT calculations on the diamino enol 3a (and the
diamino dienol 3g) reproduce their molecular structure in the
crystal quite well (see the Supporting Information).
THF at room temperature, affording 2,2-diamino enols 3
(
Scheme 2). Attempts to crystallize the 2,2-diamino enols 3
from THF solutions met, with no exception, with frustration.
To our delight, when we changed the solvent to benzene and
maintained strictly anaerobic conditions, we obtained crys-
talline samples of 3a, 3c, and 3e suitable for X-ray structural
analysis. The crystal structures of the 2,2-diamino enols 3a,
In the series 3c–3a–3e, there is increasing electron density
in the enol double bond due to the nature of the substituents
on the benzene ring. As a consequence, the enol C=C bond
(C2=C6) lengths decrease in the same order, with a simulta-
neous increase in the length of the 2,2-diamino enol C–N
bond (C2–N1 and C2–N3). For the 2,2-diamino enol 3c with
the weakest C2–C6 double bond, the deviation of the enol
moiety from planarity is greatest (dihedral angles N1-C2-C6-
C21 = 25.88 and N3-C2-C6-O1 = 22.28). The strengthening of
the C2=C6 bond, effected by higher electron density, is
3
c, and 3e are shown in Figure 1, together with selected bond
lengths and dihedral angles relevant to the 2,2-diamino enol
moiety (see the Supporting Information for ORTEP and
further space-filling representations, as well as crystal packing
diagrams). Note that the OH hydrogen atom could be located
in all three X-ray diffraction studies. Contrary to what may
accompanied by diminished deviation from planarity, as
evidenced by the lower N1-C2-C6-C21 and N3-C2-C6-O1
dihedral angles, for example, for the phenyl derivative 3a
(14.38 and 16.48), and in particular for the p-methoxyphenyl
2
,2-diamino enol 3e (10.88 and 11.58). Similar effects of the
substituent pattern on the length of the C=C bond have
been observed for stilbenes. For example, in E-stilbene itself,
[
[
22]
23]
dC
dC
=
= 1.338 ꢁ,
= 1.316 ꢁ.
whereas for 4,4’-dimethoxy-E-stilbene
Di[2,4-bis(trifluoromethyl)]stilbene ap-
C
=
C
pears to be an unknown compound.
In the course of our in situ NMR studies on the interaction
of aldehydes with imidazolidinylidenes such as SIPr (1), we
already noted the clean formation of the diamino dienol 3g
[7]
from E-cinnamic aldehyde 2g (Scheme 2 and Scheme 3a).
The reaction of this aldehyde with carbene 1 in benzene
provided crystals of 3g suitable for X-ray structural analysis,
and the crystal structure of 3g is shown in Figure 2a. The
diene moiety is virtually planar, with a torsion angle C2-C6-
C21-C22 of 177.4(3)8. The lengths of the two C=C bonds are
1
.362(4) ꢁ (C2=C6) and 1.356(3) ꢁ (C21=C22), whereas the
length of the connecting C–C single bond (C6–C21) amounts
to 1.424(3) ꢁ. As in the case of the 2,2-diamino enols 3a, 3c,
and 3e, the crystal packing of the diamino dienol 3g did not
indicate the involvement of the hydroxy group in hydrogen
bonding (see the Supporting Information). When a-methyl E-
cinnamic aldehyde (2h) was reacted with SIPr (1) at room
temperature in benzene, crystallization did not yield the
analogous diamino dienol 3h, but the azolium enolate 4a. As
illustrated in Scheme 3b, the formation of the azolium
enolate 4a can be explained by O-to-Cg proton shift in the
initially formed diamino dienol 3h. The X-ray crystal
structure of the azolium enolate 4a is shown in Figure 2b.
The enolate moiety is almost planar, with a dihedral angle O1-
C6-C21-C22 of À173.0(2)8. At the same time, the enolate
moiety is arranged almost perpendicular to the imidazolidi-
nium ring, as evidenced by dihedral angles O1-C6-C2-N3 of
Figure 1. X-ray crystal structures of the 2,2-diamino enols 3a, 3c, and
5
1.3(3)8 and O1-C6-C2-N1 of À122.7(2)8. The saturation of
3
e [R=2,6-bis(2-propyl)phenyl]; top row: combined ball-and-stick/
space-filling representation of one (left) and two (right) molecules of
,2-diamino enol 3a, as observed in the crystal.
the g-carbon atom is clearly revealed by its tetrahedral
2
geometry. As mentioned in the introduction, azolium enolates
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À1
Figure 3. Relative free energies [kJmol ] of the stationary points
involved in the formation of the azolium enolate 4a from the diamino
dienol 3h by means of an intramolecular 1,4-proton shift (gas phase,
298 K). For clarity, only the ipso C atoms of the 2,6-bis(2-propyl)phenyl
(Dipp) groups are drawn. Selected bond lengths and distances are
given in [ꢀ].
Scheme 3. SIPr (1) reacts with aldehydes 2g and 2j to afford the
diamino di/trienols 3g and 3j (a and d); SIPr (1) reacts with aldehydes
2
h and 2i to give the azolium enolates 4a and 4b (b and c); diamino
transfer appears feasible at room temperature. Note that the
diene moiety in the computed optimized structure of the
diamino dienol 3h is only slightly distorted, as evidenced by
dienol 3g is protonated by TFA to give the azolium enol 5 (e).
[24]
the dihedral angle C2-C6-C21-C23 of 1648 (cf. 177.48 in 3g).
As an example of a nonaromatic enal, (E)-crotonic
aldehyde (2i) was subjected to the same reaction and
crystallization conditions. In [D ]THF, the product displayed
8
1
a characteristic triplet in its H NMR spectrum at d =
3
13
3
.46 ppm ( J = 6.6 Hz, 1H, H21) and C NMR resonances
HH
at d = 172.4 (C2), 102.9 (C21), 18.0 (C22) ppm, features
consistent with the azolium enolate 4b (Scheme 3c; see the
Supporting Information for the complete NMR character-
ization). Again, crystallization from benzene was successful,
and the X-ray crystal structure of the azolium enolate 4b is
shown in Figure 2d. As in the case of the azolium enolate 4a,
the enolate partial structure is almost planar, with a dihedral
angle O1-C6-C21-C22 of 3.4(9)8 and 1.0(10)8, respectively
(two independent molecules per unit cell). We were delighted
to see that the Z configuration of the enolate typically
assumed (based on calculations) for reaction intermediates of
[
15,17,18]
this type
is indeed present in the crystal structures of
both 4a and 4b, and thus proven experimentally for the first
time. Close inspection of the crystal structure of 4b further-
more indicates that the enolate C–H hydrogen atom is
oriented towards the plane of the “left-hand” benzene ring.
For the two independent molecules of 4b present in the unit
cell, the distances of the enolate C–H hydrogen atom from the
centroid of the benzene ring are 2.864 ꢁ and 3.365 ꢁ,
Figure 2. X-ray crystal structures of the diamino dienol 3g, the azolium
enolates 4a and 4b, and the azolium enol 5 (TFA anion not shown);
R=2,6-bis(2-propyl)phenyl.
[
26]
respectively. This type of CH–p interaction has recently
been postulated to be crucial for high (stereo)selectivity in
a number of N-heterocyclic carbene catalyzed hetero-Diels–
[
19,20]
have been observed upon addition of NHCs to ketenes,
but never before from the reaction of a,b-unsaturated
aldehydes, as in our case.
[
18]
Alder reactions.
We also undertook a DFT study of the tautomerization of
As an example for the interaction of a dienal with SIPr
(1), (E,E)-sorbic aldehyde 2j (Scheme 3d) was employed. No
crystalline material could be obtained, but NMR monitoring
3
h to 4a, and the reaction path is shown in Figure 3. In line
[
15]
with results obtained by Sunoj et al., intramolecular proton
1
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of the reaction ([D ]THF, room temperature) clearly revealed
8
the quantitative formation of the diamino trienol 3j. In this
1
context, the H NMR resonance at d = 3.68 ppm (OH) is
1
3
indicative, together with C NMR signals at d = 142.7 (C2)
and 112.8 (C6) ppm (see the Supporting Information for full
NMR characterization of 3j). In the absence of oxygen, the
diamino trienol 3j was stable and did not show any tendency
towards further reaction, for example, tautomerization to the
corresponding azolium enolate.
Besides the neutral diamino dienol 3g (Figure 2a) and the
two neutral, but zwitterionic azolium enolates 4a and 4b
(
Figure 2b and d), we were also able to generate and
crystallize the g-protonation product of the diamino dienol
g, that is, the cationic azolium enol 5 (Scheme 3e). Upon
treatment of the diamino dienol 3g with trifluoroacetic acid
TFA), we observed the instantaneous disappearance of the
3
(
1
signals characteristic of 3g (e.g. H NMR doublets at d =
6
3
3
.08 ppm, J = 15.2 Hz, 1H, H21, and d = 5.53 ppm, J
=
HH
HH
1
5.2 Hz, 1H, H22), with concomitant formation of a new
3
product characterized by a triplet at d = 4.81 ppm ( J
7
7
the C NMR spectrum with the C2 and C22 signals moving
from 145.0 to 166.1 ppm, and 111.0 to 30.8 ppm, respectively
=
HH
3
.2 Hz, 1H, H21) and a doublet at d = 3.37 ppm ( J
=
HH
.2 Hz, 2H, H22). Similarly indicative were the changes in
1
3
(
see the Supporting Information for full NMR character-
ization of 5). Crystallization of the azolium enol 5 could be
achieved from benzene/n-hexane (1:1), and its X-ray crystal
structure is shown in Figure 2c. The enol is Z-configurated
and, as indicated by the dihedral angle N1-C2-C6-O1 of
À60.5(2)8, the enol substructure is almost orthogonal to the
[
27]
heterocyclic ring.
Note that the above g-protonation of the diamino dienol
g to the azolium enol 5, in an intermolecular fashion by an
3
external proton source (here TFA), corresponds to the
decisive step in the mechanism of redox esterification of
[
17,25]
a,b-enals formulated by Bode et al.
and by Scheidt
et al. However, at room temperature and throughout the
2 h of our measurement, the azolium enol 5 did not show any
[
28]
1
tendency to tautomerize to the corresponding acyl azolium
cation, the subsequent step typically formulated for redox
esterification.
Figure 4. a) Time course of the tautomerization of the diamino dienol
h to the azolium enolate 4a. b) Time course of the same reaction as
in (a), but with addition of methanol after a reaction time of 10.5 h;
3
[
17,25,28]
In our crystallization experiments, the azolium enolate 4a
was identified as the product resulting from the interaction of
the carbene SIPr (1) with a-methyl E-cinnamic aldehyde 2h
R=2,6-bis(2-propyl)phenyl.
(
X-ray structure of 4a: Figure 2b). NMR monitoring of this
data of 4a). Note that the diamino dienol 3g, derived from the
same carbene SIPr (1) and E-cinnamic aldehyde (2g), that is,
just lacking the a-methyl group, did not undergo tautomeri-
zation to an azolium enolate in solution under the same
conditions.
When 2 equiv of methanol was added to the 3h/4a
mixture present after 10.5 h reaction time, the concentration
of the diamino dienol 3h dropped sharply, with concomitant
regeneration of a-methyl E-cinnamic aldehyde (2h) and
formation of the methanol adduct of SIPr (6, Figure 4b). In
contrast, the concentration profile of the azolium enolate 4a
was basically unaffected by the addition of methanol; that is,
the concentration increased steadily with time. Figure 4b
summarizes our mechanistic interpretation: The formation of
diamino dienol 3h from aldehyde 2h and carbene 1 is
reaction ([D ]THF) revealed that the formation of the
azolium enolate 4a is preceded by that of the diamino
dienol 3h (Figure 4a, Scheme 3b). Indicative NMR features
of the diamino dienol 3h are H NMR singlets at d = 6.14 ppm
(
d = 144.5 (C2), 118.8 (C23), 116.5 (C6) ppm (see the
Supporting Information for COSY, HMQC, HMBC, and
NOESY data). At 258C, the maximum concentration of 3h
was reached at about 6 h. After roughly 2 h, the formation of
the azolium enolate 4a was noticeable and its concentration
increased over time. Characteristic resonances of the azolium
8
1
1
3
H23) and d = 4.18 ppm (OH), and C NMR resonances at
1
enolate 4a are the H NMR singlet at d = 3.12 ppm (2H,
1
3
H23), and the C NMR signals at d = 173.1 (C2) and 35.7
C23) ppm (see the Supporting Information for further NMR
(
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reversible, and addition of methanol results in a shift to the
left, presumbly driven by the formation of the carbeneꢀs
methanol adduct 6. In contrast, the tautomerization of 3h to
the azolium enolate 4a appears to be largely irreversible.
Note that alcohols such as methanol may additionally
accelerate the tautomerization of 3h to 4a, by proton
shuttling, within a seven-membered cyclic enol–alcohol
aggregate. According to our DFT calculations on this process
[10] A. Berkessel, S. Elfert, K. Etzenbach-Effers, J. H. Teles, Angew.
Chem. 2010, 122, 7275 – 7279; Angew. Chem. Int. Ed. 2010, 49,
7
120 – 7124.
11] a) D. A. DiRocco, K. M. Oberg, T. Rovis, J. Am. Chem. Soc.
012, 134, 6143 – 6145; b) D. A. DiRocco, T. Rovis, Angew.
Chem. 2012, 124, 6006 – 6008; Angew. Chem. Int. Ed. 2012, 51,
904 – 5906.
[
2
5
[12] B. Maji, H. Mayr, Angew. Chem. 2012, 124, 10554 – 10558;
Angew. Chem. Int. Ed. 2012, 51, 10408 – 10412.
[
13] Reviews: a) P.-C. Chiang, J. W. Bode, Science of Synthesis:
Asymmetric Organocatalysis, Vol. 1 (Ed.: B. List), Thieme,
Stuttgart, 2012, pp. 639 – 672; b) V. Nair, R. S. Menon, A. T.
Biju, C. R. Sinu, R. R. Paul, A. Jose, V. Sreekumar, Chem. Soc.
Rev. 2011, 40, 5336 – 5346.
(
see the Supporting Information), the transition state of the
tautomerization of 3h to 4a is lowered in energy by
À1
approximately 18 kJmol when one molecule of methanol
is involved (see Figure 3 for the noncatalyzed proton shift).
Thus, even with a decreasing concentration of diamino dienol
[14] a) C. Burstein, F. Glorius, Angew. Chem. 2004, 116, 6331 – 6334;
Angew. Chem. Int. Ed. 2004, 43, 6205 – 6208; b) S. S. Sohn, E. L.
Rosen, J. W. Bode, J. Am. Chem. Soc. 2004, 126, 14370 – 14371.
3
h, the concentration–time profile of azolium enolate 4a is
virtually unchanged.
[
[
15] Y. Reddi, R. B. Sunoj, Org. Lett. 2012, 14, 2810 – 2813.
16] Z. Fu, H. Sun, S. Chen, B. Tiwari, G. Li, Y. R. Chi, Chem.
Commun. 2013, 49, 261 – 263.
As mentioned earlier, the addition of TFA to the diamino
dienol 3g derived from E-cinnamic aldehyde (2g) did not
result in retro-cleavage to E-cinnamic aldehyde (2g) and
azolium trifluoroacetate, but in protonation to the azolium
[
17] J. Kaeobamrung, M. C. Kozlowski, J. W. Bode, Proc. Natl. Acad.
Sci. USA 2010, 107, 20661 – 20665.
[
29]
enol 5. Taken together, these observations nicely parallel
conclusions from recent studies by Bode et al. on N-mesityl
triazolylidenes, namely that the formation of the diamino
dienol from a-methyl E-cinnamic aldehyde (2h) is reversible,
whereas the analogous transformation of E-cinnamic alde-
[18] S. E. Allen, J. Mahatthananchai, J. W. Bode, M. C. Kozlowski, J.
Am. Chem. Soc. 2012, 134, 12098 – 12103.
[
19] a) M. Regitz, J. Hocker, B. Weber, Angew. Chem. 1970, 82, 394 –
95; Angew. Chem. Int. Ed. Engl. 1970, 9, 375 – 375; b) L. Weber,
U. Lassahn, H.-G. Stammler, B. Neumann, Eur. J. Inorg. Chem.
005, 4590 – 4597; c) Y.-G. Lee, J. P. Moerdyk, C. W. Bielawski, J.
3
2
[
30]
hyde (2g) is irreversible.
In summary, the selective generation and characteriza-
Phys. Org. Chem. 2012, 25, 1027 – 1032; d) M. Hans, J. Wouters,
A. Demonceau, L. Delaude, Chem. Eur. J. 2013, 19, 9668 – 9676.
[20] Upon acceptance of this manuscript, we were notified of
a related publication by Mayr and Maji discussing the synthesis
and ambident reactivity of azolium enolates derived from NHCs
and methyl phenyl ketene: B. Maji, H. Mayr, Angew. Chem.
[31]
tion of diamino enols, diamino dienols, azolium enolates,
and an azolium enol reported herein for the first time puts the
1
1
3
3
mechanistic analysis of NHC-catalyzed a -d and a -d umpo-
lung on a solid structural basis. Thus, a number of hitherto
postulated reaction intermediates were prepared and charac-
terized. The wealth of structure and reactivity data disclosed
will surely aid in the design of novel NHC-catalyzed
umpolung reactions, and in the in-depth understanding of
the existing ones.
2
2
013, DOI: 10.1002/ange.201303524; Angew. Chem. Int. Ed.
013, DOI: 10.1002/anie.201303524.
[
21] I. G. Kaplan, Intermolecular Interactions: Physical Picture,
Computational Methods and Model Potentials, Wiley, Hoboken,
2006.
[
[
[
22] A. Hoekstra, P. Meertens, A. Vos, Acta Crystallogr. Sect. B 1975,
31, 2813 – 2817.
23] C. R. Theocharis, W. Jones, C. N. R. Rao, J. Chem. Soc. Chem.
Commun. 1984, 1291 – 1293.
24] A non-coplanar arrangement of the dieneꢀs (3h) double bonds
had been proposed for this species before: ref. [25].
Received: April 13, 2013
Published online: August 28, 2013
Keywords: carbenes · NMR spectroscopy ·
[
[
25] S. S. Sohn, J. W. Bode, Org. Lett. 2005, 7, 3873 – 3876.
26] J. F. Malone, C. M. Murray, M. H. Charlton, R. Docherty, A. J.
Lavery, J. Chem. Soc. Faraday Trans. 1997, 93, 3429 – 3436.
27] It is worth noting that in the cases studied, the N-C-N bond angle
of the imidazolidine ring is indicative of the imidazolidinylidene
.
reaction mechanisms · umpolung · X-ray diffraction
[
(
3
4
as in the diamino enols 3a, 3c, and 3e and the diamino dienol
g) versus the imidazolidinium state (as in the azolium enolates
a and 4b or in the azolium enol 5). In the former case, the N1-
[
[
[
1] R. Breslow, J. Am. Chem. Soc. 1957, 79, 1762 – 1763.
2] R. Breslow, J. Am. Chem. Soc. 1958, 80, 3719 – 3726.
3] A. Grossmann, D. Enders, Angew. Chem. 2012, 124, 320 – 332;
Angew. Chem. Int. Ed. 2012, 51, 314 – 325.
C2-N3 angle typically amounts to 1088, whereas the same angle
experiences a small but significant widening to 110–1118 in the
cationic state.
[
4] D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 2007, 107,
5606 – 5655.
[
28] a) B. E. Maki, E. V. Patterson, C. J. Cramer, K. A. Scheidt, Org.
Lett. 2009, 11, 3942 – 3945; b) A. Chan, K. A. Scheidt, Org. Lett.
[
[
5] X. Bugaut, F. Glorius, Chem. Soc. Rev. 2012, 41, 3511 – 3522.
6] D. Seebach, Angew. Chem. 1979, 91, 259 – 278; Angew. Chem.
Int. Ed. Engl. 1979, 18, 239 – 258.
7] A. Berkessel, S. Elfert, V. R. Yatham, J.-M. Neudçrfl, N. E.
Schlçrer, J. H. Teles, Angew. Chem. 2012, 124, 12537 – 12541;
Angew. Chem. Int. Ed. 2012, 51, 12370 – 12374.
8] J. H. Teles, J.-P. Melder, K. Ebel, R. Schneider, E. Gehrer, W.
Harder, S. Brode, D. Enders, K. Breuer, G. Raabe, Helv. Chim.
Acta 1996, 79, 61 – 83.
9] C. J. Collett, R. S. Massey, O. R. Maguire, A. S. Batsanov, A. C.
OꢀDonoghue, A. D. Smith, Chem. Sci. 2013, 4, 1514 – 1522.
2
005, 7, 905 – 908.
[
29] No reaction was observed within ca. 12 h at room temperature
when methanol was added to the diamino dienol 3g.
30] J. Mahatthananchai, J. W. Bode, Chem. Sci. 2012, 3, 192 – 197.
31] CCDC 911917 (3a), 911918 (3c), 911919 (3e), 911921 (3g),
[
[
[
[
[
9
11920 (4a), 911922 (4b), and 925514 (5) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
1
1162 www.angewandte.org
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