Nucleophilic addition of enols and enamines to α,β-unsaturated acyl azoliums: Mechanistic studies

Article abstract of DOI:10.1002/anie.201109042

1,4 but not 1,2! The reactivity of 1 towards different nucleophiles (deprotonated β-diketones, enamines, and malonodinitrile) was investigated by NMR and kinetic experiments. These investigations proved that C-C bond formation occurs by a Michael-type 1,4-addition and not by a 1,2-addition and subsequent [3,3]-sigmatropic rearrangement. The first X-ray structure of an α,β-unsaturated acyl azolium salt (1) is also presented. Copyright

Full text of DOI:10.1002/anie.201109042

Angewandte
Chemie
DOI: 10.1002/anie.201109042
Organocatalysis
Nucleophilic Addition of Enols and Enamines to a,b-Unsaturated Acyl
Azoliums: Mechanistic Studies**
Ramesh C. Samanta, Biplab Maji, Suman De Sarkar, Klaus Bergander, Roland Frçhlich,
Christian Mꢀck-Lichtenfeld,* Herbert Mayr,* and Armido Studer*
N-Heterocyclic carbenes (NHCs) have been successfully used
[1]
ꢀ
as catalysts in various C C bond-forming reactions. Along
this line, the chemistry of a,b-unsaturated acyl azolium ions
has recently gained great attention. These reactive inter-
mediates can be generated by three different routes: a) Pro-
tonation of the Breslow intermediate formed in the reaction
of an ynal with an NHC;[2,3] b) reaction of an a,b-unsaturated
acyl fluoride with an NHC;[4] and c) oxidation of the Breslow
intermediate formed in the reaction of an enal with an NHC
(Scheme 1).[5]
Scheme 2. Possible mechanisms for reaction of enols or enamines
=
with acyl azolium ions A (NHC N,N’-dimethylimidazoline, X=O,
Scheme 1. Different routes for the generation of acyl azolium ions.
NH).
a,b-Unsaturated acyl azolium ions turned out to be
reactive and synthetically highly useful electrophiles in
intermolecular reactions with a-ketoenols,[6] b-diketones,[7]
and enamines[8] for the preparation of dihydropyranones
and dihydropyridinones. Two different mechanisms have
been suggested for such transformations (Scheme 2). The
deprotonated enol or enamine B can react with A by
a Michael-type 1,4-addition to give enolate D.[7] Alternatively,
the same intermediate D can be generated by 1,2-addition to
generate C, which further reacts in a [3,3]-sigmatropic
rearrangement to give D.[6,8] Proton transfer provides E and
intramolecular acylation eventually leads to dihydropyra-
nones or dihydropyridinones F. Herein we give experimental
and theoretical support that an isolated acylazolium salt
reacts with various nucleophiles by means of the 1,4-addition
pathway. Moreover, we present the first X-ray structure of an
a,b-unsaturated acyl azolium A.[9]
To properly investigate the reactivity of acyl azolium ions,
we decided to prepare and fully characterize such an
intermediate A and to study its reactivity towards different
nucleophiles. The synthesis of A (R1 = Ph) turned out to be
surprisingly straightforward. N-Methylimidazole was depro-
tonated with nBuLi in THF at ꢀ788C in the presence of
TMEDA. Subsequent addition of methyl cinnamate at that
temperature afforded ketone 1 in 54% yield (Scheme 3).[10]
Reaction of 1 with MeOTf in Et2O at room temperature and
[*] R. C. Samanta, Dr. S. De Sarkar, Dr. K. Bergander, Dr. R. Frçhlich,
Dr. C. Mꢀck-Lichtenfeld, Prof. Dr. A. Studer
Organisch-Chemisches Institut
Westfꢁlische Wilhelms-Universitꢁt Mꢀnster
Corrensstrasse 40, 48149 Mꢀnster (Germany)
E-mail: cml@uni-muenster.de
studer@uni-muenster.de
B. Maji, Prof. Dr. H. Mayr
Department Chemie, Ludwig-Maximilians-Universitꢁt Mꢀnchen
Butenandtstrasse 5–13 (Haus F), 81377 Mꢀnchen (Germany)
E-mail: herbert.mayr@cup.uni-muenchen.de
[**] This work was financially supported by the NRW International
Graduate School of Chemistry (stipend to R.C.S.) and the Deutsche
Forschungsgemeinschaft (SFB 749). We thank Prof. Stefan Grimme
for providing the infrastructure for this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201109042.
Scheme 3. Synthesis of acyl azolium 2. TMEDA= tetramethylethylene-
diamine.
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lmax = 333 nm (in DMSO) or at lmax = 326 nm (in CH3CN)
using the previously described equipment.[12] All kinetic
experiments were performed under pseudo-first-order con-
ditions using an excess of nucleophiles. From the exponential
decay of the UV/Vis absorbance of 2, the pseudo-first-order
rate constants kobs (sꢀ1) were obtained. Plots of kobs versus the
concentrations of the nucleophiles were linear with negligible
intercepts (Tables S1–S6 in the Supporting Information). The
slopes of these linear plots gave the second-order rate
constants k (Mꢀ1 sꢀ1), which are listed in Table 1. We found
Table 1: Second-order rate constants for the reactions of 2 with the
nucleophiles 4, 5, and 6 at 208C.
Figure 1. Molecular structure of 2 representing an example of a com-
pound of type A.
[a]
Nucleophile
N/sN
k/Mꢀ1 sꢀ1
4a
4b
19.36/0.67
17.64/0.73
2.29ꢂ105[b]
5.84ꢂ104[b]
purification by recrystallization provided acyl azolium salt 2
in 82% yield.
X-ray structure analysis (Figure 1)[11] showed that the
4c
4d
16.27/0.77
13.91/0.86
9.03ꢂ103[b]
2.75ꢂ102[b]
=
C(7) C(8) bond and the carbonyl group are nearly perfectly
coplanar (torsion angle = 6.2(9)8). In this conformation, the
carbonyl moiety ideally activates the olefin for a 1,4-addition
reaction. The plane of the five-membered ring is twisted with
respect to the carbonyl group (torsion angle 35.58).
We first studied the reaction of the acyl azolium salt 2 with
various nucleophiles in CH3CN or DMSO at room temper-
ature (RT). b-Diketones and b-ketoesters are well known to
react with acceptors A to afford compounds of type F. Indeed,
reaction of 2 with deprotonated acetylacetone delivered 3
(Scheme 4). In the reactions with deprotonated malono-
dinitrile 4a, enamine 5, and ketene acetal 6 the corresponding
5
6
16.42/0.70
10.52/0.78
4.96ꢂ102[c]
6.19ꢂ10ꢀ2[c]
[a] Nucleophile-specific parameters N and sN from Ref. [13]. [b] Solvent
DMSO. [c] Solvent CH3CN.
that switching the counteranion in 2 from OTfꢀ to Iꢀ did not
greatly affect the k value of the addition reaction with acacꢀ.
Addition of acacꢀ to the iodide salt at 208C in DMSO
occurred with a rate constant of 6.14 ꢀ 104 mꢀ1 sꢀ1 (Table S7 in
the Supporting Information) which compares well with the
value measured for triflate 2 (5.84 ꢀ 104 mꢀ1 sꢀ1).
Figure 2 shows that (lgk)/sN correlates linearly with N as
required by Equation (1),[12] which calculates second-order
Scheme 4. Reactions of 2 with various nucleophiles. DMSO=dimethyl
sulfoxide, 4-DMAP=dimethylaminopyridine.
product acyl azolium salts were difficult to isolate. The
intermediate azolium salts obtained after nucleophilic addi-
tion were therefore first treated with 1 equiv HBF4 (for 4a
and 5 for enolate protonation) followed by MeOH and N,N-
dimethylaminopyridine (DMAP, 2 equiv) to provide the
corresponding methyl esters in moderate to good yields
(57–88%).
The kinetics of the addition reaction of 4–6 to 2 were
followed by photometric monitoring of the decay of the
absorbance of 2 at 208C in anhydrous DMSO or CH3CN at
Figure 2. Plot of (lgk)/sN versus N for the reactions of 2 with the
nucleophiles 4, 5, and 6 (solvents are specified in Table 1). Slope of
the correlation line is fixed to 1.0 as required by Equation (1).
2
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rate constants from the electrophile-specific parameter E and
two solvent-dependent nucleophile-specific parameters N
and sN. One can therefore conclude that the rate-determining
step of the reactions of 2 with 4–6 is mechanistically analogous
to the reactions of 4–6 with benzhydrylium ions which react
ene, 1.5 equiv) was added. The NMR tube was quickly
transferred into the NMR probe, which was precooled to
ꢀ408C. The resonances of the acyl azolium 2 fully disap-
peared and new broad signals belonging to an intermediate of
type D appeared (see Figure S1 in the Supporting Informa-
tion). In agreement with the kinetic experiment using 4b, this
NMR study revealed that the reaction of 2 with deprotonated
acetylacetone is very fast at low temperatures (< 30 s, time
necessary for mixing and transfer to the NMR probe). An
intermediate of type C resulting from 1,2-addition was not
detectable, either because it was not formed or because the
sigmatropic rearrangement was very fast. We noted that when
the probe was warmed to ꢀ208C the NMR signals remained
unchanged. However, further warming to room temperature
led to formation of 3 (see Scheme 4) which represents
a compound of the general structure F. Hence, this NMR
experiment showed that intramolecular acylation from E to F
under liberation of the carbene requires temperatures above
ꢀ208C. We performed additional NMR experiments with
deprotonated malonodinitrile and 3-aminocrotononitrile as
nucleophiles and unambiguously identified the corresponding
1,4-addition products (see the Supporting Information). No
indication for the formation of an intermediate derived from
a 1,2-addition reaction was obtained (3-aminocrotononitrile
ꢀ
by C C bond formation (see reactions on p. S43 in the
Supporting Information). These adduct formations have
previously been used to derive the N and sN parameters
listed in Table 1.[12]
lg k20 ꢁ ¼ sNðE þ NÞ
ð1Þ
C
The kinetic experiments do not exclude attack of these
nucleophiles at the carbonyl group of 2 in a rapid preequili-
brium step, which is kinetically irrelevant. Since initial
carbonyl attack followed by [3,3]-sigmatropic rearrangement
cannot be formulated for all the nucleophiles listed in Table 1,
however, one would have to postulate that this stepwise
process can only lead to the observed products if it proceeds
with the same rate as the direct attack of these nucleophiles at
the conjugate position of 2, a rather unlikely construction.
According to Equation (1), the negative intercept on the
abscissa of Figure 2 yields E = ꢀ11.52 for 2 (by minimization
of D2 = S[lgkꢀsN(N+E)]2), which is compared with other
electrophiles in Figure 3. This plot shows that owing to the
ꢀ
could react in a 1,2-addition by C N bond formation). In all
NMR experiments we could identify intermediates of type D
(for acacꢀ after O-silylation) but not the corresponding
proton-transfer intermediates of type E (see the Supporting
Information). Apparently the proton transfer from D to E is
reversible with D dominating in the equilibrium.
Since neither the kinetic nor NMR experiments allowed
us to exclude a rapid preequilibrium step in the reaction of 2
with deprotonated acetylacetone, we finally decided to study
this particular transformation using high-level DFT calcula-
tions. We investigated the addition of deprotonated acetyl-
acetone to 2 with a meta GGA functional (TPSS) including
a correction for dispersion interactions using a triple zeta
quality basis set (TPSS-D3(BJ)/def2-TZVP).[14] Furthermore,
a continuum solvation model (COSMO)[15] was used in all
calculations, simulating the solvent properties of THF (e =
7.58) (for details see the Supporting Information).
The formation of a close ion pair (2/acacꢀ) in THF was
found to be exothermic by ꢀ23.5 kcalmolꢀ1 (Scheme 2, R1 =
Ph, R2 = COCH3, R3 = CH3, X = O). Attempts to calculate
the energy of the corresponding intermediate C were
unsuccessful, because it was not a local minimum in the
solvent, but relaxed to the ion pair 2/acacꢀ showing that C
cannot be an intermediate. On the other hand, 1,4-addition
leads to an intermediate of type D, which has a relative energy
of ꢀ25.8 kcalmolꢀ1 with respect to the isolated reactants. We
Figure 3. Comparison of electrophilicity parameter E of 2 and other
electrophiles. E parameters from Ref. [13]:
strong electron-withdrawing nature of the imidazolium ring, 2
is seven orders of magnitude more electrophilic than a struc-
turally analogous chalcone, comparable to highly electron-
deficient neutral Michael acceptors such as benzylidene
malononitriles and 2-benzylidene indan-1,3-diones.[13] How-
ever, its electrophilicity is 1000 times lower than that of the
structurally related iminium ion derived from Hayashi–
Jorgensen pyrrolidine and 104 to 106 lower than those derived
from MacMillanꢁs imidazolidinones.[13]
ꢀ
have located a transition-state structure for the C C bond
formation in the addition step (Figure 4). This structure also
reveals that the 1,4-addition leads to the cis enolate, in
agreement with the NMR studies (see the Supporting
Information). The relative energy of this structure is
To get further information on the reaction, we followed
the transformation of 2 with acetylacetone in the presence of
1
¼
base by low-temperature H NMR spectroscopy. We found
ꢀ17.9 kcalmolꢀ1. The very low barrier (DE = 5.6 kcalmolꢀ1
that acetylacetone did not react with 2 at room temperature in
[D8]THF in the absence of base. The NMR sample was then
cooled to ꢀ788C and DBU (1,8-diazabicyclo[5.4.0]undec-7-
with respect to 2/acacꢀ, without ZPE correction) allows
a facile and fast 1,4-addition even at low temperatures, as
observed in the NMR experiment.
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Keywords: acylazolium ions · kinetics · organocatalysis ·
.
N-heterocyclic carbenes · ucleophilic additions
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[2] Pioneering studies on homoenolate chemistry using NHC
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[3] K. Zeitler, Org. Lett. 2006, 8, 637. See also Refs. [6] and [7b,c].
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Figure 4. DFT-D3(BJ)+COSMO-optimized transition-state structure
ꢀ
for the 1,4-addition of deprotonated acac to acyl azolium 2. The C C
distance is given in ꢃ.
In order to address solvent effects we also calculated the
energy difference DE between the ion pair (2/acacꢀ) and the
¼
1,4-adduct, and the 1,4-addition barrier DE in DMSO (e =
46.7) and CH3CN (e = 37.5). We obtained very similar values
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K. A. Scheidt, Org. Lett. 2007, 9, 371; b) B. E. Maki, K. A.
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indicating that solvent effects are weak for this reaction
¼
(DMSO:
DE = 6.3 kcalmolꢀ1,
DE = ꢀ1.4 kcalmolꢀ1
;
¼
CH3CN: DE = 6.2 kcalmolꢀ1, DE = ꢀ1.5 kcalmolꢀ1). The
calculated barrier fits very well with the activation enthalpy
¼
¼
DH = 4.9 kcalmolꢀ1 calculated from DG = 10.7 kcalmolꢀ1
derived from the kinetic experiment (see Table 1) assuming
¼
DS = ꢀ20 e.u.
Since many NHC-catalyzed reactions have been con-
ducted with triazole-derived carbenes, we conducted addi-
tional calculations for the reaction of acacꢀ with the triazole
congener of 2 (see Scheme 1, R1 = Ph, R2 = R3 = Me, X = N)
and found the addition reaction starting with the ion pair to be
¼
slightly more exothermic (THF: DE = ꢀ4.7 kcalmolꢀ1, DE =
¼
4.0 kcalmolꢀ1; CH3CN: DE = ꢀ4.2 kcalmolꢀ1, DE = 4.4 kcal
molꢀ1). Consequently, reaction barrier was even lower.
Interestingly, for the triazole system we found an intermedi-
ate of type C for the 1,2-addition pathway. However, the
energy difference with respect to the ion pair was + 13.3 kcal
molꢀ1 (THF) and therefore the 1,2-addition pathway at least
with deprotonated acetylacetone can also be excluded for the
triazole derivative.
In conclusion, we have reported the synthesis and full
characterization of the acyl azolium ion 2. High level DFT-
calculations and kinetic experiments clearly showed that
acetylacetone, malonodinitrile, enamine 5, and silylenol ether
6 react with the a,b-unsaturated acyl azolium 2 by means of
a 1,4-Michael-type addition reaction. Based on the kinetic
experiments and on the calculations the two-step mechanism
by 1,2-addition followed by [3,3]-sigmatropic rearrangement
[11] CCDC 857879 contains the supplementary 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.
ꢀ
for the key C C-bond formation can be excluded.
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Received: December 21, 2011
Revised: February 10, 2012
Published online: && &&, &&&&
4
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[15] A. Klamt, G. Schꢄꢄrmann, J. Chem. Soc. Perkin Trans. 2 1993,
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Communications
Organocatalysis
R. C. Samanta, B. Maji, S. De Sarkar,
K. Bergander, R. Frçhlich,
C. Mꢁck-Lichtenfeld,* H. Mayr,*
A. Studer*
&&&& — &&&&
1,4 but not 1,2! The reactivity of 1 towards
different nucleophiles (deprotonated b-
diketones, enamines, and malono-
dinitrile) was investigated by NMR and
kinetic experiments. These investigations
by a Michael-type 1,4-addition and not by
a 1,2-addition and subsequent [3,3]-sig-
matropic rearrangement. The first X-ray
structure of an a,b-unsaturated acyl azo-
lium salt (1) is also presented.
Nucleophilic Addition of Enols and
Enamines to a,b-Unsaturated Acyl
Azoliums: Mechanistic Studies
ꢀ
proved that C C bond formation occurs
6
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