Alkali metal tert-butoxides, hydrides and bis(trimethylsilyl)amides as efficient homogeneous catalysts for Claisen-Tishchenko reaction

Article abstract of DOI:10.1002/adsc.201200970

Shelf-available alkali metal tert-butoxides, hydrides and bis(trimethylsilyl)amides were shown to be highly efficient homogeneous precatalysts for the disproportionation of aldehydes to the corresponding carboxylic esters. Potassium compounds in combination with 18-crown-6 ether could drastically increase the rate of reaction in a few cases. Alternatively, efficient aldol condensations were found for aldehydes possessing an enolizable methylene group at the α-position to the aldehyde functionality. The active species involved in this esterification using any of these alkali metal catalysts is expected to be the metal alkoxide. Potassium compounds were found to be much more efficient when compared to analogous sodium compounds and kinetic studies revealed the rate-determining step to be a second order concerted hydride transfer from a potassium hemiacetal species to another molecule of aldehyde. Copyright

Full text of DOI:10.1002/adsc.201200970

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DOI: 10.1002/adsc.201200970
Alkali Metal tert-Butoxides, Hydrides and
Bis(trimethylsilyl)amides as Efficient Homogeneous Catalysts
for Claisen–Tishchenko Reaction
Kunjanpillai Rajesha and Heinz Berkea,*
a
Institute of Inorganic Chemistry, University of Zꢀrich, Winterthurerstrasse 190, CH-8057 Zꢀrich, Switzerland
FaxACHTUNGTRENNUNG(+41)-1-635-6802
E-mail: hberke@aci.uzh.ch
Received: November 3, 2012; Published online: March 15, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200970.
Abstract: Shelf-available alkali metal tert-butoxides,
hydrides and bis(trimethylsilyl)amides were shown
to be highly efficient homogeneous precatalysts for
the disproportionation of aldehydes to the corre-
sponding carboxylic esters. Potassium compounds in
combination with 18-crown-6 ether could drastically
increase the rate of reaction in a few cases. Alterna-
tively, efficient aldol condensations were found for
aldehydes possessing an enolizable methylene
group at the a-position to the aldehyde functionali-
ty. The active species involved in this esterification
using any of these alkali metal catalysts is expected
to be the metal alkoxide. Potassium compounds
were found to be much more efficient when com-
pared to analogous sodium compounds and kinetic
studies revealed the rate-determining step to be
a second order concerted hydride transfer from
a potassium hemiacetal species to another molecule
of aldehyde.
attracted wide attention for the last century and is ap-
plied in the food, polymer, dye and perfume industry
as well as for the production of ethyl acetate.[2]
A
number of compounds of the main group elements,[3]
transition metal[4] and rare earth[5] compounds, as well
as N-heterocyclic carbenes[6] are found to be active to
catalyze this reaction. Traditional catalysts for this
process include mainly sodium[3a,b] and in particularly
aluminium[3c–g] alkoxides. Boric acid,[3h] (i-Bu)2AlH,[3i]
alkaline earth metal amides,[3j,k] LiBr/Et3N,[3l]
NaH,[3b,m,n] Grignard reagents in combination with thi-
olates,[3o], selenide ions,[3p] transition metal complexes
based on Fe,[4a,b] Ru,[4c–h] Rh,[4i–k] Os,[4l] Ir,[4m,n] Ni,[4o,p]
Zr,[4q] Hf[4q] and lanthanide complexes,[5a–g] particularly
lanthanide amides,[5b–e] and organoactinide com-
AHCTUNGTRENNUNG
plexes[5h,i] have also been employed for this reaction.
Quite recently, this disproportionation reaction be-
tween two different selected aldehydes could be ac-
complished to a certain extent.[1f,4p] However, many of
these auxiliaries are toxic, not commercially available
or expensive, suffer from low reaction rates, require
high catalyst loadings, are air- and moisture-sensitive,
undergo side reactions, react sluggishly and are ineffi-
cient for heteroaromatic aldehydes, which are known
to be difficult to disproportionate to esters.[3n,7]
Keywords: aldehydes; alkali metal compounds;
Claisen–Tishchenko reaction; crown ethers; esters
Recently, we have reported rhenium complexes as
Simple, efficient and environmentally friendly trans- precatalysts for the hydrogenation of various olefins[8]
formations are challenging goals in chemical synthe- and nitriles.[9] Later, when we started to study the ac-
sis. The greatest achievements in these respects have tivity of one of these complexes in combination with
evoked from the concept of catalysis. Featuring high t-BuOK for the catalytic hydrogenation of benzalde-
atom economy, the dimerization or disproportionation hyde using H2, we came across the formation of
of aldehydes to the corresponding carboxylic esters, benzyl benzoate as a by-product. Although the rheni-
named Claisen–Tishchenko[1] reaction (Scheme 1), has um hydrides generating during this reaction are capa-
ble of catalyzing this disproportionative esterification,
we found that the simple, inexpensive and commer-
cially available t-BuOK itself is an efficient catalyst
for this reaction. Much to our surprise and to the best
of knowledge, this reaction using t-BuOK as catalyst
Scheme 1. The Claisen–Tishchenko reaction.
has not been recognized or documented so far.
Adv. Synth. Catal. 2013, 355, 901 – 906
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Table 1. Claisen–Tishchenko reaction catalyzed by t-BuOK.
Entry
Product Ester [R][a]
Cat. [mol%]
Temp. [8C]
TOF [1st h][b] [hÀ1]
Time [h]
Yield[c] [%]
1
2
3
4
5
6
7
8
9
phenyl
1
2
2
2
2
5
2
2
5
23
23
80
23
23
80
23
23
80
81
29
48
6
42
17
8
1.5
18
1
24
1.5
1
24
24
1.5
96
94
94
92
93
80
92
88
79
4-chlorophenyl
4-chlorophenyl
2-chlorophenyl
4-bromophenyl
4-fluorophenyl
4-methoxyphenyl
2-thienyl
7
15
2-furanyl
10[d]
5
80
14
1.5
77
[a]
R for entries 1–9.
By GC/MS based on the consumption of aldehyde.
Isolated yield.
[b]
[c]
[d]
o-Phthalaldehyde was used as the starting material.
Table 2. Claisen–Tishchenko reaction catalyzed by alkali metal catalysts.
Entry
Product Ester [R]
Cat./
[mol%]
Temp. [8C]
TOF [1st h][a] [hÀ1]
Time [h]
Yield[b] [%]
1
2
3
4
5
6
7
8
t-BuONa/5
23
23
23
23
60
60
23
23
80
60
60
80
23
6
–
4
9
86
Li[N(SiMe3)2]/5
Na[N(SiMe3)2]/5
K[N(SiMe3)2]/5
Na[N(SiMe3)2]/2.5
K[N(SiMe3)2]/2.5
T
90
60
7
12
2
1.3
1
1
2
2
1.2
2
<5[a]
85
E
R
15
22
31
84
98
17
13
21
16
14
86
88
90
phenyl
N
E
KH/1
KH/1
KH/5
97
97[c]
84
9
4-fluorophenyl
4-chlorophenyl
4-bromophenyl
2-furanyl
10
11
12
13
K[N
K[N
U
86
85
KH/5
KH/5
83
tert-butyl
93[d]
[a]
By GC/MS based on the consumption of aldehyde.
[b]
[c]
[d]
Unless mentioned, all reactions were carried out in toluene; isolated yield.
The reaction was carried out without a solvent.
1
Solvent: benzene; yield by H NMR spectroscopy using mesitylene as internal standard.
Using 1 mol% of t-BuOK, benzaldehyde could be benzaldehyde, reaction with 5 mol% of t-BuONa at
disproportionated to benzyl benzoate in 1.5 h at room room temperature showed a TOF of 6 hÀ1 in the first
temperature in toluene with a TOF of 81 hÀ1 in the hour giving rise to 86% yield of benzyl benzoate in
first hour giving 98% GC yield of benzyl benzoate 9 h (Table 2, entry 1). The scope of the t-BuOK-cata-
along with 1% benzyl alcohol and 1% tert-butyl ben- lyzed reaction has been tested with various aromatic
zoate. This gave an isolated yield of 96% of benzyl and heteroaromatic substrates (Table 1). The TOF of
benzoate (Table 1, entry 1). To test whether the catal- the reaction could be increased by carrying it out at
ysis is due to other metal contaminants, we tested elevated temperatures without affecting the yield
99.99% sublimed grade (trace metals basis) of t- (Table 1, entries 2 and 3). Unlike 2-thiophenecarbox-
BuOK which showed almost the same results. t- aldehyde, a comparatively higher loading of 5 mol%
BuONa was also found to be active for this reaction, of the catalyst had to be adopted at 808C in the case
but was far less efficient. In the disproportionation of of 2-furfuraldehyde giving rise to a yield of 79% of
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Alkali Metal tert-Butoxides, Hydrides and Bis(trimethylsilyl)amides as Efficient Homogeneous Catalysts
the ester in 1.5 h (Table 1, entries 8 and 9). An intra-
molecular Claisen–Tishchenko reaction was observed
when o-phthalaldehyde was used, which with 5 mol%
of the catalyst at 808C gave a yield of 77% of the
phthalide cyclic ester in 1.5 h (Table 1, entry 10).
Furthermore, we tested alkali metal bis(trimethylsi-
lyl)amides for this Claisen–Tishchenko reaction and
found them to be efficient (Table 2). Much to our sur-
prise, these readily available, simple alkali amides
also catalyzed these reactions, but were as yet not
documented in literature although alkaline earth am-
ACHTUNGTRENNUNG
ides[3j,k] and lanthanide amides[5b] as catalysts for such
reactions were reported recently. Heavier amides are
usually prepared from the alkali metal amides.[10] The
activities of the alkali amides for the Claisen–Tish-
chenko reactions were found to increase from the
Scheme 2. Proposed mechanism for the Claisen–Tishchenko
reaction catalyzed by KH/t-BuOK/K[NACHTUNTGRNEUNG(SiMe3)2] with or
without 18-crown-6 ether as a co-catalyst.
almost inefficient Li[NACHTUNGTERN(NUNG SiMe3)2] to the quite efficient
K[N(SiMe3)2]. Although the reaction of benzaldehyde
ACHTUNGTRENNUNG
with 5 mol% of catalyst loading gave the ester at
room temperature, a lower loading of 2.5 mol% was
sufficient to give even better yield of the ester at
608C (Table 2, entries 3–6).
facts, a mechanism similar to one proposed for the
The t-BuOK-catalyzed disproportionation of ben- lanthanide amide catalyses is suggested (Scheme 2).[5b]
zaldehyde showed the formation of tert-butyl ben- It worth mentioning that the KH-catalyzed dispropor-
zoate and benzyl alcohol, the latter is expected to be tionation of benzaldehyde could be carried out even
formed when potassium benzyloxide is quenched with without any solvent giving rise to 97% yield of benzyl
water. A kinetic study of this reaction with benzalde- benzoate (Table 2, entry 8). When compared to t-
hyde showed a linear relationship between 1/ACTHUNRTGNEUNG[reac- BuOK as a catalyst and since the butyl ester cannot
tant] and time indicating a second-order reaction with be formed, a higher amount of the catalyst appears to
respect to the reactant, benzaldehyde (Figure 1). The be present causing more of the product in the KH-
metal alkoxide is assumed to be the active species, for catalyzed reaction (cf. Table 1, entries 6 and 9, respec-
which further evidence could be obtained from the tively, to Table 2, entries 9 and 12). Using 5 mol% of
ability of KH to catalyze this reaction with the same KH, the aliphatic tertiary aldehyde, trimethyl acetal-
order, but appeared in comparison to be a little faster dehyde could be disproportionated to the correspond-
which is attributed to the direct formation of the po- ing ester in 93% yield (Table 2, entry 13).
tassium alkoxide (Table 2, entry 7). Based on these
However, in the case of t-BuONa similar observa-
tions were made as in the case of t-BuOK except that
the reaction revealed a first order kinetic (Figure 2).
Since tert-butyl benzoate is initially formed quantita-
tively with respect to the precatalyst t-BuONa, the
formation of the sodium benzyloxide could not be the
rate-determining step. So, it is assumed that either the
insertion of aldehyde into the sodium alkoxide (B, Na
instead of K) or the non- concerted hydride transfer
from the formed sodium hemiacetal species (C, Na in-
stead of K) to another molecule of aldehyde would
be the rate-determining step. If the hydride transfer
occurs in a concerted manner (D, Na instead of K),
then the former would be the rate-determining step.
However, the latter pathway, which would involve
a b-hydride elimination from the sodium hemiacetal
species to form NaH, seems not to be plausible.
Roesky and co-workers have rationalized a mecha-
nism involving a concerted hydride transfer for homo-
leptic lanthanide amide-catalyzed Claisen–Tishchenko
reaction,[5b] like the one suggested here for t-BuOK.
A similar mechanism can be proposed for the
Figure 1. Linear plot of 1/[benzaldehyde] vs. time in the t-
BuOK-catalyzed Claisen–Tishchenko reaction indicating
a second order kinetics.
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ed by Hill and co-workers in the case of alkaline
earth metal amides catalyzing the Claisen–Tishchenko
reaction.[3j]
In order to improve the rate of this disproportiona-
tion reaction catalyzed by potassium compounds, we
added 18-crown-6 (1 equiv. with respect to the cata-
lyst) (Table 3). The reaction rates were drastically im-
proved in a few cases. With 1 mol% of t-BuOK and
18-crown-6 ether, the disproportionation of benzalde-
hyde in benzene was completed in 10 min at room
temperature giving rise to 96% yield of benzyl ben-
zoate with a TOF of 582 hÀ1 (Table 3, entry 1). Under
the same reaction conditions and with the same load-
ings, KH showed a TOF of 1176 hÀ1 completing the
reaction in 5 min with 97% yield of benzyl benzoate
(Table 3, entry 2). With loadings of 2 mol%
K[NACTHNURTGNE(UNG SiMe3)2] and 18-crown-6 ether, the reaction
Figure 2. Linear plot of ln[benzaldehyde] vs. time in the t-
BuONa-catalyzed Claisen–Tishchenko reaction indicating
a first order kinetics.
showed a TOF of 6 hÀ1 giving rise to 91% yield of
benzyl benzoate in 8 h. This strategy was adopted for
the disproportionation of a few more aromatic alde-
hydes (Table 3, entries 3–7). However, when com-
pared, addition of this crown ether decreased the effi-
K[N
of the reaction of benzaldehyde with 0.5 equiv. of carboxaldehyde and was almost inactive for the dis-
K[N(SiMe3)2], GC/MS analysis showed benzyl ben- proportionation of 4-fluorobenzaldehyde. Since the
ACHTUNGTRENNUNG(SiMe3)2]-catalyzed disproportionation. At 30 min ciency in the case of 2-furfuraldehyde and 2-thienyl-
ACHTUNGTRENNUNG
zoate, benzyl alcohol – which is expected to be active species involved is expected to be the 18-
formed when potassium benzyloxide is quenched with crown-6 ether potassium benzyloxide when any of
water, the hydrosilylated product benzyl trimethylsilyl these potassium compounds are used, we studied the
ether and traces of other unidentified products. How- kinetics of this reaction with benzaldehyde using the
ever, the major oxidized species could not be detect- K[NACHTNUTRGNEUNG(SiMe3)2]/18-crown-6 ether system and found it to
ed. Some of the above observations were also report- be second order with respect to benzaldehyde, like
Table 3. Claisen–Tishchenko reaction catalyzed by potassium compounds along with 1 equiv. of 18-crown-6 ether with re-
spect to the catalysts.
Entry
Product Ester [R]
phenyl
Cat./
[mol%]
TOF[a] [hÀ1]
Time [h]
Yield[b] [%]
1
2
3
t-BuOK/1
KH/1
582
1176
6
0.17
0.083
8
96
97
91
K[N
N
4
5
6
7
8
9
10
4-chlorophenyl
2-chlorophenyl
4-bromophenyl
4-methoxyphenyl
tert-butyl
KH/2
KH/2
KH/1
KH/1
KH/5
KH/5
KH/3
32
1.5
92
144
291
39
74
–
0.33
0.33
2.5
0.25
10
95
95[c]
97
93[d]
cyclohexyl
n-pentyl
45[c,e]
[f]
–
3
–
[a]
TOF by GC/MS based on the consumption of the aldehyde.
[b]
[c]
[d]
[e]
[f]
Unless mentioned, solvent: benzene; temperature 238C; isolated yield.
Solvent: toluene
Yield by 1H NMR spectroscopy using mesitylene as internal standard.
Temperature 808C.
Aldol condensation products in 90% (E/Z=97/3); traces of other isomers were also formed; yield by GC/MS based on
the consumption of aldehyde.
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Alkali Metal tert-Butoxides, Hydrides and Bis(trimethylsilyl)amides as Efficient Homogeneous Catalysts
vide the product with acceptable rates and yields.
Higher efficiencies of the catalysts are expected to be
achieved by utilizing the opportunity of ligand sphere
tuning, particularly with transition metals as well as
lanthanides and actinides, which has acquired suffi-
cient interest in recent times and as mentioned, we
are also involved in developing this transformation
using suitable rhenium complexes. This would also
impart help to overcome the limitations of any pri-
mary aliphatic aldehydes, as well as to effect cross-
Claisen–Tishchenko disproportionation.
Experimental Section
Alkali Metal Compounds-Catalyzed Claisen–
Tishchenko Reaction
t-BuOK (10 mg, 0.089 mmol) was charged in a 10-mL glass
vial (for room temperature reactions) or a 25-mL Young
Schlenk tube (for heating reactions). Toluene (1 mL) was
added to it. To this, benzaldehyde (945.8 mg, 8.9 mmol) was
added followed by another portion of toluene (1 mL). The
vessel was closed and the mixture was stirred (in the glove
box itself for room temperature reactions or kept outside in
an oil bath for heating reactions). The reaction was moni-
tored by GC/MS for which the samples were taken in the
glove box, quenched immediately with water outside. When
the reaction was completed, the mass was concentrated to
dryness. It was purified by silica gel flash column chroma-
tography (eluent: hexane/ethyl acetate) to afford benzyl
Figure 3. Linear plot of 1/[benzaldehyde] vs. time in the
K[N(SiMe3)2]/18-crown-6 ether-catalyzed Claisen–Tishchen-
ko reaction indicating a second order kinetics.
ACHTUNGTRENNUNG
the one without 18-crown-6 ether, indicating a similar
mechanism (Figure 3). We suppose that the increased
activity by adding the crown ether is caused by an in-
creased solubility of the potassium (crown ether) salt.
Then, we tested this strategy also for aliphatic pri-
mary, secondary and tertiary aldehydes as reaction
substrates. The aliphatic tertiary aldehyde, trimethyla-
cetaldehyde could be disproportionated to the corre-
sponding ester in 93% yield within 15 min at room
temperature using KH and 18-crown-6 ether loadings
of 5 mol% (Table 3, entry 8). Though the reaction was
not uniform, addition of 5 mol% of KH and 18-
crown-6 ether at a temperature of 808C was adopted
for the disproportionation of the aliphatic aldehyde,
cyclohexanecarboxaldehyde, which gave 45% yield of
the desired ester in 10 h (Table 3, entry 9). However,
aldehydes possessing a methylene group at the a-posi-
tion to the aldehyde functionality, hexanal for in-
stance, revealed under the conditions of the addition
of 3 mol% of KH and 18-crown-6 ether at room tem-
perature, formation of aldol condensation products in
>90% yield (E/Z=97/3) (Table 3, entry 10).
In summary, the simple, cheap, shelf-available t-
BuOK and KH, as well as t-BuONa are efficient cata-
lysts for the Claisen–Tishchenko disproportionation
of aromatic and even heteroaromatic aldehydes, as
well as of aliphatic secondary and tertiary aldehydes.
Alternatively, aldol condensation products were
formed when primary aldehydes were used. Addition
of a catalytic quantity of 18-crown-6 ether could in-
crease the rate of reaction in some cases. The simple
potassium and sodium metal amides, although less ef-
ficient for the disproportionation reaction when com-
pared to the corresponding homoleptic lanthanide
benzoate as
(4.274 mmol, 96%).
The KH, t-BuONa, K[NACTHNUGRTNEN(UGN SiMe3)2] and Na[NHCAUTNTGREN(NUNG SiMe3)2] cata-
a
pale yellow liquid; yield: 907.1 mg
lyzed Claisen–Tishchenko reactions were also performed in
a similar manner.
Potassium Compounds/18-Crown-6 Ether-Catalyzed
Claisen–Tishchenko Reaction
KH (3 mg, 0.0748 mmol) and 18-crown-6 ether (19.75 mg,
0.0748 mmol) were charged in a Young NMR tube. Ben-
zene-d6 (0.3 mL) was added to it. To this, benzaldehyde
(793.7 mg, 7.48 mmol) was added followed by another por-
tion of benzene-d6 (0.3 mL). The tube was closed and the re-
action mass was shaken well. The reaction was monitored
1
by H NMR spectroscopy for the absence of benzaldehyde.
When the reaction was completed, the mass was concentrat-
ed to dryness. It was purified by silica gel flash column chro-
matography (eluent: hexane/ethyl acetate) to afford benzyl
benzoate as
(3.615 mmol, 97%).
t-BuOK/18-crown-6 ether and K[NCATHNUGRTNE(NGU SiMe3)2]/18-crown-6
a pale yellow liquid; yield: 767.35 mg
ether catalyzed Claisen–Tishchenko reactions were also per-
formed in a similar manner.
Acknowledgements
Funding from the Swiss National Science Foundation and the
amides, as well as alkaline earth amides, could pro- University of Zurich is gratefully acknowledged.
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