NHC-catalyzed hydroacylation of styrenes

Article abstract of DOI:10.1002/anie.201209291

New hydroacylation catalysts: Highly electron-rich N-heterocyclic carbenes (NHCs) facilitate the intermolecular hydroacylation of unstrained olefins. This unprecedented organocatalytic coupling joins simple and abundant aldehydes and styrenes to yield valuable ketone products. EWG=electron-withdrawing group, EDG=electron-donating group. Copyright

Full text of DOI:10.1002/anie.201209291

Angewandte
Chemie
DOI: 10.1002/anie.201209291
Organocatalysis
NHC-Catalyzed Hydroacylation of Styrenes**
Michael Schedler, Duo-Sheng Wang, and Frank Glorius*
The hydroacylation of multiple bonds is a highly versatile
reaction that uses abundant starting materials such as alkenes
and aldehydes for the formation of valuable ketone struc-
tures.[1] However, the catalytic hydroacylation is an underex-
plored reaction and has only recently gained considerable
interest. Until now, rhodium complexes have been the most
common catalysts for olefin hydroacylation. A major problem
for these catalyst systems is an undesired decarbonylation
step, which often limits suitable substrates to aldehydes
bearing additional coordinating groups, such as ortho-
hydroxybenzaldehydes,[2] although certain substrate combi-
nations[3] or the use of ruthenium catalysts[4] avoid this
problem. An alternative approach is the intermediate trans-
formation of aldehydes to imines that bind more strongly to
the catalyst preventing decarbonylation.[5]
A promising alternative to metal-catalyzed hydroacyla-
tions is the use of N-heterocyclic carbenes (NHCs) as
organocatalysts.[6] NHCs are versatile catalysts for several
umpolung reactions, including the benzoin condensation and
the Stetter reaction.[7] While the classical Stetter reaction is
Herein we report the NHC-catalyzed hydroacylation of
styrenes [Eq. (3)], the first intermolecular NHC-catalyzed
hydroacylation of unstrained, rather electron-neutral alkenes.
This transformation is remarkable as the organocatalyzed
transformation of electron-neutral alkenes such as styrenes is
a long-standing challenge in organocatalysis; only some rare
examples of organocatalyzed epoxidations[11] and organo-
catalyzed brominations,[12] and a few examples relying on
radical mechanisms have been reported.[13]
À
limited to the addition of an aldehyde to a polarized C C
Recently, we designed a family of novel NHCs bearing
a 2,6-dimethoxyphenyl moiety.[14] We hypothesized that these
electron-rich NHCs would result in increased nucleophilicity
of the Breslow intermediate[15,16] and would thus be suitable
for the challenging hydroacylation of styrenes. Starting with
the slightly activated p-cyanostyrene (2a) under the condi-
tions employed for the hydroacylation of cyclopropenes,[9b] we
were pleased to obtain the linear and branched hydroacyla-
tion regioisomers l-3a and b-3a, respectively. In addition to
the four dimethoxy-NHCs 4a, 5a, 6a, and 7a, we also tested
the commonly used NHCs 4b and 5b (Table 1, entries 1–6).
As seen previously for intermolecular hydroacylations, the
triazolium-derived NHCs were superior to other NHC
scaffolds. We were especially happy to see our hypothesis of
the higher reactivity of the 2,6-dimethoxy moiety confirmed
as 4a was the best of the NHCs tested (Table 1, entry 1, 50%
yield). While a screening of bases and reaction temper-
atures[17] did not improve the yield, the solvent had a marked
influence on the reaction. Changing the solvent to THF
improved the yield to 72% (Table 1, entry 7). All attempts to
further improve the yield in this solvent proved to be futile.[17]
We realized that several different side reactions prevented
us from optimizing the reaction to full conversion. Two side
reactions were of particular importance: Firstly, the 2,6-
dimethoxy NHCs are not only highly reactive catalysts for the
desired hydroacylation, but they also catalyze redox reactions
of benzoin, an ubiquitous reaction intermediate in NHC-
catalyzed umpolung reactions of aldehydes. This side reaction
yields deoxybenzoin 8a and consumes the aldehyde irrever-
sibly (Scheme 1A).
double bond such as in nitroolefins, chalcones, and alkylidene
malonates, the hydroacylation of less activated multiple
bonds would broaden the scope of NHC organocatalysis
significantly. To date, the latter type of NHC-catalyzed
hydroacylation is limited to intramolecular reactions
[Eq. (1)][8] and to compounds with strained multiple bonds
[Eq. (2)].[9,10]
[*] M. Schedler, Dr. D.-S. Wang, Prof. Dr. F. Glorius
Organisch-Chemisches Institut
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Corrensstrasse 40, 48149 Mꢁnster (Germany)
E-mail: glorius@uni-muenster.de
[**] Generous financial support from the Deutsche Forschungsge-
meinschaft (SFB 858) and the Fonds der Chemischen Industrie
(M.S.) is gratefully acknowledged. The research of F.G. is supported
by the Alfried Krupp Prize for Young University Teachers of the
Alfried Krupp von Bohlen and Halbach Foundation. We thank
Nathalie E. Wurz for helpful discussions and Mirco Fleige and Karin
Gottschalk for skillful technical support. NHC=N-heterocyclic
carbene.
Secondly, the products 3a contain an acidic methylene
group in a-position to the carbonyl group. Under basic
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209291.
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Table 1: Optimization of the reaction conditions.[a]
Table 2: Variation of the aldehyde.
Entry NHC·HX Ratio 1a/2a Solvent
Yield [%][b] Ratio l/b[c]
1
2
3
4
5
6
7
4a
4b
5a
5b
6a
7a
4a
4a
4a
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1.5:1
1,4-dioxane 50
1,4-dioxane 21
84:16
75:25
[g]
1,4-dioxane
1,4-dioxane
2
0
–
–
[g]
1,4-dioxane 27
81:19
[g]
1,4-dioxane
THF
DMF
3
–
72
86:14
83:17
85:15
8
13[d]
9[e]
DMF
97 (89)[f]
[a] Standard conditions: 1 equiv=^ 0.1 mmol, 10 mol% NHC·HX,
1.5 equiv K3PO4, solvent (0.25m), 408C, 16 h. [b] Yield (l-3a+b-3a)
1
determined by H NMR spectroscopy with CH2Br2 as an internal
standard. [c] Ratio of linear and branched product determined by
1H NMR spectroscopy of the crude reaction mixture. [d] Byproduct 9a
(Scheme 1) was formed in 63% yield. [e] 1 equiv 2a (0.2 mmol) was
used. [f] Yield of isolated products (both products together), 1 equiv 2a
(0.5 mmol), 5 mol% 4a, DMSO as solvent. [g] Only the linear product
was detected.
Scheme 1. Undesired side reactions in the hydroacylation of styrenes;
Ar=p-CN-C6H4.
[a] Ratio of linear to branched product determined by 1H NMR
spectroscopy of the crude reaction mixture. [b] DMF, room temperature,
10 mol% 4a, 0.5 equiv K3PO4.[17]
conditions this methylene group can be deprotonated and the
resulting enolate can undergo further reaction. In highly polar
solvents as DMF this side reaction becomes the main pathway
and the desired product 3a is formed in only 13% yield; 3a
adds to another molecule of styrene, forming byproduct 9a
(Table 1, entry 8; Scheme 1B). By changing the stoichiometry
to an excess of aldehyde instead of an excess of styrene we
could suppress both problematic side reactions and the
product 3a was formed in 97% yield (NMR yield, Table 1,
entry 9). Using less reactive styrenes, we obtained even better
results in DMSO.[17] Furthermore, the catalyst loading could
be lowered to 5 mol% without decreasing the yield.
m-chlorobenzaldehyde were as good as those with p-chloro-
benzaldehyde, only substitution in ortho position was not
tolerated.[18] We were also pleased to see that several
heteroaromatic aldehydes (Table 2, 1 f–h) gave the hydro-
acylation products in good to excellent yields. Additionally,
we could show that electronically differently substituted
aromatic aldehydes were well tolerated in this reaction.
If one compares the regioselectivities observed for this
reaction, it can be noted that electron-rich aldehydes yield
only the linear product, while electron-poor aldehydes give
a mixture of the two products. Generally, the ratio of the two
products can be attributed to the electronic character of the
With these optimized conditions we tested a broad scope
of different aromatic aldehydes. Generally, the linear product
was obtained as the main product together with a small, easily
separable amount of the branched product. The results with
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acylation of styrene would be a big step forward for the field
of NHC-catalyzed intermolecular hydroacylations. Using
styrene under our optimized conditions yielded promising
traces of product; NHC-catalyzed redox chemistry caused the
undesired consumption of most of the aldehyde, thus
preventing better yields.
In order to accelerate the desired hydroacylation step, the
amount of styrene was increased and a 1:1 mixture of styrene
and DMSO was found to be optimal.[17] Using the triazolium
salt 4b and aldehyde 1k yielded the branched hydroacylation
product b-3u in 42% yield (Table 4). Under the same
conditions we could even obtain the branched hydroacylation
product of the electron-rich styrenes 3v and 3w in moderate
yields (Table 4).
Figure 1. Ratio of l-3 to b-3 as a function of the Hammett parameter
s of the aldehyde.[17]
Table 4: Variation of the styrene: electron-rich styrenes.
aldehyde, which can be measured by the Hammett parameter
s (Figure 1).[17,19]
We then focused on the variation of styrene 2 and were
able to show that several styrenes with electron-withdrawing
substituents including ester groups (Table 3, 3p) and
trifluoromethyl groups (Table 3, 3r, 3s) gave the targeted
ketones in good to excellent yields.
The hydroacylation of unsubstituted styrene is of partic-
ular interest not only because styrene is one of the most
important bulk chemicals, but also because the reaction is an
especially challenging transformation. The successful hydro-
Table 3: Variation of the styrene: electron-poor styrenes.
[a] Ratio of linear to branched product determined by 1H NMR spectros-
copy of the crude reaction mixture.
Based on all our results we propose a mechanism for the
hydroacylation of styrenes (Scheme 2). The sequence starts
with the attack of the NHC on the aldehyde, providing the
Breslow intermediate 12 after a formal 1,2 H-shift. This can
react in a fast, but reversible way with another aldehyde,
yielding benzoin. Hence, when we used 4,4’-dichlorobenzoin
instead of the corresponding aldehyde as the starting material
we obtained 3a in 96% yield (81% l-3a, 15% b-3a). Instead
of the formation of the Breslow intermediate 12, 10 can also
transfer a hydride yielding the poisoned catalyst 11 and
reduce benzoin to deoxybenzoin.[20]
The Breslow intermediate is then able to attack the
styrene in three possible ways, which is manifested in the two
different regioisomers of the product that are formed.
Nucleophilic attack of the Breslow intermediate on the
styrene should give the linear product. In this case an anionic
charge is formed on the benzylic position and proton transfer
from the Breslow intermediate to the styrene follows as the
second step (Scheme 2, left). After the elimination of the
NHC the product is formed and the catalytic cycle is
complete. This mechanism resembles the mechanism of the
Stetter reaction and is favored by electron-rich aldehydes
[a] Ratio of linear/branched product determined by 1H NMR spectros-
copy of the crude reaction mixture.
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as esters, ketones, and nitrile groups are well tolerated and
valuable products are formed in good yields from simple and
ubiquitous starting materials.
Received: November 20, 2012
Published online: January 25, 2013
Keywords: hydroacylation · N-heterocyclic carbenes ·
.
organocatalysis · styrenes · synthetic methods
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Scheme 2. Proposed mechanism for the hydroacylation of styrenes.
(increased nucleophilicity of the Breslow intermediate) and
electron-poor styrenes (stabilization of the anionic charge).
Alternatively, the Breslow intermediate may first protonate
the styrene at the terminal carbon of the double bond leading
to a positive charge in the benzylic position (Scheme 2, right).
This full protonation mode would arguably correspond to an
unprecedented activation mode of the Breslow intermediate
and is favored by electron-rich styrenes and electron-poor
aldehydes (higher acidity of the OH group of the Breslow
intermediate by stabilization of the negative charge in 15).
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the formation of the branched products cannot be explained
by the classical mechanism of the Stetter reaction.
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extremes (cationic intermediate versus anionic intermediate),
a third intermediate mechanism is possible (Scheme 2,
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[18] Substitution with smaller groups was tolerated to some extent
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table).
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