Homoleptic lanthanide amides as homogeneous catalysts for alkyne hydroamination and the Tishchenko reaction

Article abstract of DOI:10.1002/1521-3765(20010716)7:14<3078::AID-CHEM3078>3.0.CO;2-E

The homoleptic bis(trimethylsilyl)amides of Group 3 metals and lanthanides of the general type [Ln{N(SiMe₃)₂}₃] (1) (Ln = Y, lanthanide) represent a new class of Tishchenko precatalysts and, to a limited extent, precatalysts for the hydroamination/cyclization of aminoalkynes. It is shown that 1 is the most active catalyst for the Tishchenko reaction. This contribution presents investigations on the scope of the reaction, substrate selectivity, lanthanide-ion size-effect, and kinetic/ mechanistic aspects of the Tishchenko reaction catalyzed by 1. The turnover frequency is increased by the use of large-center metals and electron-with-drawing substrates. The reaction rate is second order with respect to the substrate. While donor atoms, such as nitrogen, oxygen, or sulfur, on the substrate decrease the turnover frequency, 1 shows a tolerance for a large number of functional groups. For the hydroamination/cyclization of aminoalkynes, 1 is less active than the well-known metallocene catalysts. On the other hand, 1 is much more readily accessible (one-step synthesis or commercially available), than the metallocenes and might therefore be an attractive alternative catalyst.

Full text of DOI:10.1002/1521-3765(20010716)7:14<3078::AID-CHEM3078>3.0.CO;2-E

FULL PAPER
Homoleptic Lanthanide Amides as Homogeneous Catalysts for Alkyne
Hydroamination and the Tishchenko Reaction**
Markus R. Bürgstein, Helga Berberich, and Peter W. Roesky*[a]
Dedicated to Professor Hansgeorg Schnöckel on the occasion of his 60th birthday
Abstract: The homoleptic bis(trimeth-
ylsilyl)amides of Group 3 metals and
lanthanides of the general type
[Ln{N(SiMe3)2}3] (1) (Ln ˆ Y, lantha-
nide) represent a new class of Tishchen-
ko precatalysts and, to a limited extent,
precatalysts for the hydroamination/cy-
clization of aminoalkynes. It is shown
that 1 is the most active catalyst for the
Tishchenko reaction. This contribution
presents investigations on the scope of
the reaction, substrate selectivity, lan-
thanide-ion size-effect, and kinetic/
mechanistic aspects of the Tishchenko
reaction catalyzed by 1. The turnover
frequency is increased by the use of
large-center metals and electron-with-
drawing substrates. The reaction rate is
second order with respect to the sub-
strate. While donor atoms, such as nitro-
gen, oxygen, or sulfur, on the substrate
decrease the turnover frequency,
1
shows a tolerance for a large number
of functional groups. For the hydroami-
nation/cyclization of aminoalkynes, 1 is
less active than the well-known metal-
locene catalysts. On the other hand, 1 is
much more readily accessible (one-step
synthesis or commercially available),
than the metallocenes and might there-
fore be an attractive alternative catalyst.
Keywords: aldehydes
´
dimeriza-
tion ´ homogeneous catalysis ´ lan-
thanides ´ N ligands ´ rare-earth
compounds
catalysts for the Meerwein ± Ponndorf ± Verley reduction[17]
and the hydrocyanation,[18] whereas lanthanide shift-reagents,
such as [Eu(fod)3] (fod ˆ 6,6,7,7,8,8,8-heptafluoro-2,2-dimeth-
yl-3,5-octanedionato) can be used as a catalyst for the Diels ±
Alder[19] and hetero-Diels ± Alder reactions.[20]
Introduction
Recently, there has been a significant research effort to
establish lanthanide(iii) compounds as catalysts for various
organic transformations. Lanthanide catalysts are active in
two fields of application. One involves organolanthanide
compounds as catalysts for the transformation of olefins,
dienes, and, to a lesser extent, alkynes. Especially, metal-
locenes, such as [(C5Me5)2LnR] (R ˆ CH(SiMe3)2, N(SiMe3)2,
H) have proven to be highly efficient catalysts[2] for a variety
of olefin transformations, including hydrogenation,[3] poly-
merization,[4] hydroamination,[5] hydrosilylation,[6] hydrobora-
tion,[7] and hydrophosphination.[8] Unfortunately, the metal-
locenes are not readily accessible. On the other hand,
lanthanide alkoxides, triflates, and halogenides were used
for Lewis acid catalyzed organic reactions.[9] Thus, lanthanide
triflates [Ln(OTf)3] are very active catalysts for the aldol,[10]
Michael,[11] allylation,[12] Diels ± Alder,[13] and glycosylation
reactions[14] as well as for Friedel ± Crafts acylations.[15, 16]
Lanthanide alkoxides [Ln(OR)3] have proven to be useful
In contrast to these well-established catalysts, homoleptic
lanthanide(iii) amides were not expected to be active as
homogeneous catalysts. Recently, we communicated that the
homoleptic bis(trimethylsilyl)amides of Group 3 metals and
lanthanides, ([Ln{N(SiMe3)2}3], 1)[21] (Ln ˆ Y, lanthanide), are
the most active catalysts for the Tishchenko reaction.[22]
Meanwhile, other groups reported the use of 1 for the ring-
opening polymerization of e-caprolactone and d-valerolac-
tone.[23] The ternary system [Nd{N(SiMe3)2}3]/(iBu)3Al/
Et2AlCl was used to polymerize butadiene to highly cis-1,4-
polybutadiene.[24] Compound 1 belongs to a class of materials
that has been known for the last 25 years. Recently, in
particular, it has proven to be a valuable starting material in
lanthanide chemistry because of the facile cleavage of the
silylamide group.[25] Compound 1 is very readily accessible. It
can either be prepared from a simple one-step synthesis or it
can be bought (Ln ˆ Y). Therefore, it is even more surprising
that, prior to our investigations, there are almost no reports of
1 as a catalyst.
[a] Priv.-Doz. Dr. P. W. Roesky, Dr. M. R. Bürgstein, H. Berberich
Institut für Anorganische Chemie der Universität Karlsruhe
Engesserstrasse 15, 76128 Karlsruhe (Germany)
Fax : (‡49)721-661921
E-mail: roesky@achibm6.chemie.uni-karlsruhe.de
The Tishchenko reaction (or Claisen ± Tishchenko reac-
tion), which is the dimerization of aldehydes to form the
[**] Already communicated in part. See Ref. [1].
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3078±3085
Table 1. Tishchenko reaction of benzaldehyde to benzyl benzoate with
different catalysts.
corresponding carboxylic ester [Eq. (1)], has been known for
about a century.[26]
1
Entry Catalyst
Nt [
]
Yield[%]
T[8C] Ref.
1
2
3
4
5
6
1a
1b
1c
63
87
80
70
98
98
88
21[a]
21[a]
21[a]
(1)
[(C5Me5)2LaCH(SiMe3)2]
SmI2
La(OiPr)3
RT
[32]
±
±
no reaction 21[a]
no reaction 21[a] [32]
Its industrial importance is reflected in the great number of
patents. Thus, the Tishchenko ester of 3-cyclohexenecarbal-
dehyde is the precursor for the formation of epoxy resins,
which are durable against environmental influences. Benzyl-
benzoate is used as a dye carrier. Other uses include solvents
for cellulose derivatives, plasticizers, and the food industry.[27]
Traditionally, aluminum alkoxides[28, 29, 30] have been used as
homogeneous catalysts for the Tishchenko reaction. More
recently, other catalysts, such as boric acid[31] and a few
transition metal complexes, have been used. However, these
alternative catalysts are either only reactive under extreme
reaction conditions (e.g., boric acid), are difficult to prepare
(e.g., [(C5Me5)2LaCH(SiMe3)2]),[32] slow (e.g., [(C5H5)2-
ZrH2]),[33] expensive (e.g., [H2Ru(PPh3)2]),[34] or give small
yields (e.g., K2[Fe(CO)4]).[35] In this contribution, we present a
full account of the reaction scope, substrate selectivity,
lanthanide-ion size-effect, and kinetic/mechanistic aspects of
the Tishchenko reaction catalyzed by 1. Furthermore, we
report that 1 can be used to a limited extent as a precatalyst
for the hydroamination/cyclization of aminoalkynes.
7
1.9
8
70
21[a]
8
9
Al(OiPr)3
51
67
21[a]
21[a] [40]
10
11
76
21[a] [40]
[(C5H5)2ZrH2]
[(C5H5)2HfH2]
[H2Ru(PPh3)2]
K2[Fe(CO)4]
B(OH)3
7
9
23
71
34
17
17
20
60
250
[33]
[33]
[34]
[35]
[31]
12
13
14
15
[a] Reaction conditions (this work): 1 mol% catalyst in C6D6.
reaction to show that R N(SiMe3)2 is formed in a stoichio-
metric ratio. A comparison of [Y{N(SiMe3)2}3] (1a), [La-
{N(SiMe3)2}3] (1b), and [Sm{N(SiMe3)2}3] (1c) (entries 1 ± 3)
shows that, for an almost quantitative turnover, 1b has a
higher activity than the corresponding Sm catalyst 1c, where-
as the Y catalyst 1a produces smaller yields and is less active.
The dependence of the TOF on the ionic radius of the central
metal atom has already been observed.[32] A comparison with
other readily accessible lanthanide compounds, such as
Results and Discussion
The goal of this study was to explore the scope of 1 for its use
in two areas of lanthanide-catalyzed reactions: C ± C multiple
bond transformations and organic reactions catalyzed by
Lewis acids. This section begins with the discussion of the
Tishchenko reaction, for which 1 is a very active catalyst,
followed by a short examination of 1 as a precatalyst for the
hydroamination/cyclization of aminoalkynes. In the case of
the latter reaction, 1 is less effective than the known systems.
[37]
SmI2 or [La(OiPr)3][32] (entries 5 and 6), which are used as
Tishchenko catalysts, showed that these are inactive for the
case of benzaldehyde. In contrast, the homoleptic alkoxide
[Sm{O-2,6-(tBu)2-C6H3}3], which was recently introduced as a
catalyst, shows some activity (entry 7).[38] However, the TOFs
and the yields are significantly lower than those observed for
The Tishchenko reaction: To compare the reaction rates of 1
with other catalysts, the standard reaction of benzaldehyde to
benzyl benzoate was chosen. Benzyl benzoate is used, among
others, as solvent for artificial musk, as a perfume fixative, in
confectionery, and in chewing-gum flavors. For these uses, it is
necessary to prepare the benzoate without any contamination
by irritants and or odoriferous materials, such as benzyl
chlorides or acids.[27, 35] The reaction rate and the yield were
determined by NMR spectroscopy in C6D6 with ꢀl mol%
catalyst at 218C (Table l). The turnover frequencies (TOFs)
were determined from a turnover of 50%.[36] The kinetic data
1b and 1c. From the reaction with the standard aluminum
[29, 30]
catalyst Al(OiPr)3
under the reaction conditions de-
scribed above, the yield was not quantitative. The reaction rate
observed with Al(OiPr)3, which agrees with earlier measured
values,[29, 39] is roughly an order of magnitude smaller than that
observed with 1b (entry 8). Even the recently reported high-
speed Tishchenko catalyst, (2,7-dimethyl-1,8-biphenylene-
dioxy)bis(diisopropoxyaluminum) (entry 9) and (2,7-dimeth-
yl-1,8-biphenylenedioxy)bis(dibenzoyloxyaluminum) (entry 10),
do not reach the activity of 1b.[40] Although we do not know of
a more active catalyst for the Tishchenko reaction, two
compounds are described in the literature ([(C5Me5)2LaCH-
(SiMe3)2] (entry 4),[32] and [Al(OCH2Ph)3]) that could com-
pete with 1b. [(C5Me5)2LaCH(SiMe3)2] would probably form
1
obtained in this study were acquired by H NMR spectro-
scopic monitoring of the reactions. The decrease of the
characteristic aldehyde proton signal, concomitant with the
increase in the proton signal of the benzyl group, was
normalized to the proton resonances of the stoichiometrically
generated R N(SiMe3)2 reaction byproducts. Tetramethyl-
benzene was used as an independent standard in a test
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P. W. Roesky et al.
FULL PAPER
the same catalytically active species as 1b (see below);
however, it is evidently more difficult to prepare.
[Al(OCH2Ph)3] reaches almost the same TOF as 1b, but is
exclusively optimized for the dimerization of benzaldehyde[39]
(Table 1).
without solvent or in hydrocarbons in order to determine the
isolated yields. The products were characterized by elemental
1
analysis as well as by H and 13C NMR spectroscopy. The
workup of the reaction was very simple: in the case of the
solvent-free reaction, the product can usually be easily
transferred by vacuum. In the case of the reaction with o-
phthalaldehyde, the ester precipitates cleanly out of the
solution (entry 9).
The absence of a useful transition metal catalyst for the
Tishchenko reaction is demonstrated by a comparison of 1b
and K2[Fe(CO)4], which in the presence of crown ethers is one
of the fastest known transition metal catalysts. The yields and
activities were compared for two selected substrates (Table 2).
With 1b, benzaldehyde is dimerized in almost quantitative
yields, whereas the iron(ii) catalyst gives only 46% conversion
(entries 1 and 2). Furthermore, the suitability of 1b for the
dimerization of furfural should be emphasized, because when
this reaction is carried out with aluminum alkoxide catalysts
or K2[Fe(CO)4]/crown ether catalysts, very low yields are
observed.[29, 35, 41] Thus, only 3.4% yield was obtained by the
use of the transition metal catalyst (entry 3), which is in
contrast to 40% conversion for the lanthanide-catalyzed
reaction (entry 4).
To investigate the tolerance of 1b for functional groups,
various substituted benzaldehydes were used as substrates
(Table 3). To avoid a significant steric influence of the
functional group, only para-substituted benzaldehydes were
used. The reactions were carried out at 218C in C6D6, with
ꢀ5 mol% of catalyst in order to determine the TOF. All the
reactions were repeated on a preparative scale (5 g reactant)
Most of the para-substituted benzaldehydes are converted
to the corresponding carboxylic esters in quantitative or
almost quantitative yields. The only exception is 4-(dimethyl-
amino)benzaldehyde. Thus can it be shown, that a large
number of functional groups are tolerated by 1b. These results
are remarkable, since functional groups often show the
tendency to block lanthanide catalysts. As seen by the TOFs,
the Tishchenko process is faster when the aromatic ring has an
electron-withdrawing group. A slight exception are 4-fluoro-
and 4-chlorobenzaldehyde (entries 1 and 2), which are a bit
slower than one would expect from the electronic influence of
the substituents. A similar dependence has been observed
with other catalysts, such as K2[Fe(CO)4]/crown ether[35] and
KO2/crown ether.[42] Therefore, substrates that contain elec-
tron-releasing groups on the aromatic ring have significantly
lower TOFs. The lowest TOFs were observed if the substrate
has an electron-releasing group with a donor atom, such as
nitrogen, oxygen, or sulfur (entries 6 ± 8). These atoms form
hemilabile Lewis acid/base adducts with the catalyst and thus
Table 2. A Comparison of 1b and K2[Fe(CO)4]/crown ether[35] as catalysts for the Tishchenko reaction.
1
Entry
Substrate
Product
Catalyst
Mol% of catalyst
Nt [h
]
Yield[%]
98
T[8C]
1
1b
1
87
21
2
3
4
K2[Fe(CO)4]/crown ether
1b
20
1
ꢁ 1
2
46
40
3.4
20
21
33
K2[Fe(CO)4]/crown ether
33
ꢁ 1
Table 3. Results for the 1b-catalyzed dimerization of aromatic aldehydes.[a]
1
Entry
Substrate
Product
Nt [h
]
Yield NMR-scale[b, c] [%]
Isolated yield[b,d] [%]
1
2
3
4
5
6
7
8
R ˆ
F
Cl
Br
CN
CH3
SCH3
OCH3
N(CH3)2
44
38
106
94
11
2
87
98
66
47
71
80
78
84
86
35
quant.
quant.
quant.
97
quant.
26
2.1
±
9
> 1500
90[e]
85[e, f]
[a] Reaction temperature: 218C. [b] 5 mol% catalyst. [c] Solvent C6D6. [d] No solvent, 5 g reactant. [e] 1 mol% catalyst. [f] 50 mL pentane/hexane.
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Lanthanide Amides as Catalysts
3078±3085
hamper the Tishchenko process. This effect is best seen in the
case of 4-(dimethylamino)benzaldehyde (entry 8). Not sur-
prisingly, the intramolecular Tishchenko reaction of o-phthal-
aldehyde to the corresponding lactone gives high yields and is
this, we choose a few heterocyclic aldehydes (that contain
oxygen and nitrogen as substrates, Table 5). Compared to all
other substrates, furfural and methylfurfural react very slowly
and the turnovers are low (entries 1 and 2). Furthermore,
methylfurfural does not give a clean conversion and no clean
product could be isolated. The rate for furfural is more than
an order of magnitude lower than that with benzaldehyde.
Nevertheless, in comparison to aluminum alkoxide or K2-
[Fe(CO)4]/crown ether cata-
1
very fast (TOF > 1500 h ) (entry 9).
In addition to aromatic aldehydes as substrates for the
Tishchenko reaction, we also investigated the use of aliphatic
aldehydes (Table 4). We were interested in cyclic and non-
lysts, much higher turnovers are
obtained by the use of 1b
Table 4. Results for the 1b-catalyzed dimerization of aliphatic aldehydes.
1
Entry Substrate
Product
Nt [h
]
Yield NMR-scale[a] [%] Isolated yield[b] [%]
as the catalyst (see above;
Table 2). The low turnovers
for furfural and methylfurfural
compared with benzaldehyde
and even pyridine-3-carbalde-
hyde can be explained by the
nature of the substrate. Upon
reaction, furfural and meth-
ylfurfural can coordinate in
1
> 1500 quant.
> 1500 quant.
75
80
2
3
4
5
> 1500 quant.
> 1500 quant.
80
a
chelating fashion through
the carboxylic group and the
ether oxygen atom. Thus, a five-
membered metallacycle is
formed that partly blocks the
catalyst.
84
±
50[c]
45[c]
[a] Reaction conditions: 218C; 1 mol% catalyst in C6D6. [b] Reaction conditions: 218C; 1 mol% catalyst, no
solvent, 5 g reactant. [c] Reaction temperature: 788C ! RT; the trimer 2-ethyl-1,3-hexanediol monobutyrate
was found as a byproduct.
Kinetics and reaction mecha-
nism of the Tishchenko reac-
tion: In order to elucidate a
cyclic reactants. Again, the reactions were carried out at 218C
in C6D6 with ꢀ1 mol% of catalyst in the first step in order to
determine the TOF. Subsequently, all the reactions were
repeated on a preparative scale (5 g reactant) without solvent.
The extremely high TOFs for all substrates should be noted.
The reactions are usually so rapid, that the product can be
only detected in NMR-scale reactions. Thus, on the NMR
timescale, quantitative yields were observed for all substrates.
Moreover, the catalyst in the reaction solution is still active
after several days so that a reaction which has been completed
for some time can be restarted by the addition of new
reactant. Although 1b catalyzes the dimerization of alde-
hydes, with or without one a-H atom, quickly and in high
yields, the reaction of butanal at
provisional reaction mechanism, the dimerization of benz-
aldehyde was studied in more detail. 1H NMR spectros-
copy and GC/MS studies show that, at the beginning of
the reaction, bis(trimethylsilyl)amine and benzylic acid
bis(trimethylsilyl)amide are cleaved off from 1 to give a
lanthanide alkoxide A that is, most probably, the catalyti-
cally active species (Scheme 1). Investigations of the para-
magnetic compound 1c on catalytic and stoichiometric
scales show that all three amide groups are cleaved with-
out a significant induction time. Signals appear in the
1H NMR spectra that may be attributed to SmOCH2 groups.
It can be assumed that the catalytically active species is
either the same or very similar to the compound that is formed
218C only gives higher coupling
Table 5. Results for the 1b-catalyzed dimerization of heterocyclic aldehydes.
products (entry 5).[30, 43, 44] If the
reaction is started at 788C,
then butyl butyrate and 2-ethyl-
1,3-hexanediol monobutyrate
are produced as dimeric and
trimeric products in almost the
same ratio.[44] The trimeric
product is formed by a tandem
aldol-Tishchenko reaction.[45]
We were also interested in
the reactivity of heterocyclic
substrates in the Tishchenko
reaction catalyzed by 1b. For
1
Entry Substrate
Product
Nt [h
]
Yield NMR-scale[a] [%] Isolated yield[b] [%]
1
2
40[b]
36[b]
2
3
±
±
28[c,d]
±
68[c]
37[c]
[a] 218C; 1 mol% catalyst in C6D6. [b] 218C; 1 mol% catalyst, no solvent, 5 g reactant. [c] 5 mol% catalyst.
[d] Unknown sideproduct.
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P. W. Roesky et al.
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catalytic activity of 1 compared with the well-established
metallocenes, such as [(C5Me5)2LnR] (R ˆ N(SiMe3)2,
CH(SiMe3)2), in C ± C multiple-bond transformations, the
catalytic hydroamination/cyclization[5, 46] of aminoolefins and
aminoalkynes was investigated. The rigorously anaerobic
reaction of the catalysts with dry, degassed aminoolefin
and aminoalkynes (3 ± 5 mol% catalyst) proceeds regio-
specifically in toluene as shown in Table 6. The TOFs
obtained in this study were acquired by 1H NMR moni-
toring of the reactions. The decrease of the substrate proton
signal concomitant with the increase of product proton
signal was normalized to the proton resonances of the
stoichiometrically generated NH(SiMe3)2 reaction by-
products.
The catalytic activity of 1a and 1b for the catalytic
hydroamination/cyclization of aminoalkynes at 608C is lower
than that of the established metallocene [(C5Me5)2YCH-
(SiMe3)2] (2) (entries 1 ± 6) at 218C. Since it was shown
previously that the hydroamination of aminoalkynes is faster
than that of aminoolefins,[5] it might be expected that no
conversion is observed for aminoolefins with 1a and 1b as
catalysts (entries 8 and 9). The low TOFs and yields of 1a and
1b relative to 2 can be explained by the absence of any
ancillary ligands in the homoleptic amines. As seen in the
1H NMR spectra of catalytic reactions, all N(SiMe3)2 groups
are cleaved off from the central metal because of the large
excess of substrate. Thus, for each substrate a very different
ligand environment is generated on the metal center. This
might be an explanation for the differences in the observed
rates.
Scheme 1. Proposed mechanism for the catalysis of the Tishchenko
reaction by 1.
in the Tishchenko reaction with [(C5Me5)2LaCH(SiMe3)2].[32]
This hypothesis is supported by the following observations:
i) with 1b similar yields are obtained as for [(C5Me5)2LaCH-
(SiMe3)2], and ii) during the reaction, both catalysts can
completely interchange the whole of their original ligand
shells. 1H NMR spectroscopic investigations of the reaction of
1c with stoichiometric amounts of benzyl alcohol or benzal-
dehyde show that different compounds are formed. Kinetic
investigations indicate that 1/[reactant] is related linearly to
the reaction time; that is, there
is a second-order reaction with
respect to the reactant. Such
Table 6. Catalytic hydroamination/cyclization results.[a]
reaction kinetics have already
been established for the alu-
minum-catalyzed reaction for
the dimerization of benz-
aldehyde[39] and the aldol-
Tishchenko reaction catalyzed
by the lithium enolate of p-
(phenyl-sulfonyl)isobutyrophe-
none.[45a] Presumably, a mole-
cule of the reactant coordi-
nates to A (!B), which in
turn undergoes an alkoxide
transfer (!C; Scheme 1). A
second molecule of the reac-
tant attaches itself to C fol-
lowed by a hydride transfer,
which is probably the rate-
determining step (!D).[39]
1
Entry
Substrate
Product
Catalyst
Nt [h
]
Yield NMR-scale[a] [%]
quant.
1
2[b]
3.3
2
3
1a
1b
0.3
±
71
±
4
2[b]
8.6
quant.
5
6
1a
1b
0.1
0.09
75
quant.
7
2[b]
0.5
quant.
8
9
1a
1b
±
±
±
±
[a] 608C; 3 ± 5 mol% catalyst in [D8]toluene. [b] 218C.
Even 1 is less active for the hydroamination/cyclization
of aminoalkynes then the metallocenes, the persuasive
argument for 1 is its ready accessibility. For the metallo-
cenes, even access to C5Me5H is laborious or expensive. In
contrast, 1 can either be prepared from a simple one-step
synthesis or it can be bought (Ln ˆ Y). In certain cases it
might be advantageous to spend more time on the catalytic
experiments by the use of a commercially available catalyst
instead of spending time and money on the synthesis of the
catalyst.
Hydroamination/cyclization of aminoalkynes: Organolantha-
nide complexes exhibit unique reactivity characteristics for
the activation of unsaturated organic substrates. This is a
result of the high electrophilicity of f-element centers,
relatively large ionic radii, an absence of conventional
oxidative-addition/reductive-elimination mechanistic path-
ways, and high kinetic lability.[46] In order to understand the
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Lanthanide Amides as Catalysts
3078±3085
8.6 Hz, 2H; aromatic), 7.76 (d, J(H,H) ˆ 8.6 Hz, 2H; aromatic); elemental
Conclusion
analysis calcd (%) for C14H10Cl2O2: C 59.81, H 3.59; found C 58.10, H 3.25.
4-Bromobenzyl 4-bromobenzoate:[32] 1H NMR (C6D6, 250 MHz, 258C):
d ˆ 4.84 (s, 2H; CH2O), 6.74 (d, J(H,H) ˆ 8.5 Hz, 2H; aromatic), 7.15 (d,
J(H,H) ˆ 6.5 Hz, 2H; aromatic), 7.31 (d, J(H,H) ˆ 6.6 Hz, 2H; aromatic),
7.67 (d, J(H,H) ˆ 8.7 Hz, 2H; aromatic); elemental analysis calcd (%) for
C14H10Br2O2: C 45.44, H 2.72; found C 45.01, H 2.90.
4-Cyanobenzyl 4-cyanobenzoate:[32] 1H NMR (C6D6, 250 MHz, 258C): d ˆ
4.80 (s, 2H; CH2O), 6.75 (m, J(H,H) ˆ 8.1 Hz, 2H; aromatic), 6.96 (m, 2H;
aromatic) 7.23 (d, J(H,H) ˆ 8.4 Hz, 2H; aromatic), 7.67 (d, J(H,H) ˆ
8.6 Hz, 2H; aromatic); elemental analysis calcd (%) for C16H10N2O2: C
73.27, H 3.84; found C 72.71, H 3.82.
4-Methylbenzyl 4-methylbenzoate:[32] 1H NMR (C6D6, 250 MHz, 258C):
d ˆ 1.96 (s, 3H; CH3), 2.07 (s, 3H; CH3), 5.22 (s, 2H; CH2O), 6.72 (m,
J(H,H) ˆ 8.6 Hz, 2H; aromatic), 6.84 ± 6.95 (m, 4H; aromatic), 7.17 (m,
2H; aromatic), 7.54 (d, J(H,H) ˆ 8.8 Hz, 2H; aromatic), 8.11 (d, J(H,H) ˆ
6.5 Hz, 2H; aromatic); elemental analysis calcd (%) for C16H16O2: C 79.97,
H 6.71; found C 79.28, H 6.76.
In summary, it should be emphasized that the bis(trimethylsil-
yl)amide compounds of the lanthanides represent a new class
of Tishchenko catalysts and, to a limited extent, precatalysts
for the hydroamination/cyclization of aminoalkynes. The
catalytic activity is a result of the high Lewis acidity and the
facile interchangeability of the ligand sphere. These com-
pounds are distinguished by a number of practical advantages,
such as the ease of accessibility, inexpensive metals, extremely
high activities for the Tishchenko reaction (to our knowledge
there are no catalysts that are more active), and a high
durability of the catalysts. These advantages lead us to hope
that 1, which is already a standard reagent in organolantha-
nide chemistry, will find further application in catalysis.
4-Methylthiobenzyl 4-methylthiobenzoate:[32] 1H NMR (C6D6, 250 MHz,
258C): d ˆ 1.88 (s, 3H; SCH3), 1.94 (s, 3H; SCH3), 5.12 (s, 2H; CH2O), 7.59
(m, 4H; aromatic), 8.00 (m, 4H; aromatic); elemental analysis calcd (%)
for C16H16S2O2: C 63.13, H 5.30; found C 62.78, H 5.61.
4-Methoxybenzyl 4-methoxybenzoate:[33] 1H NMR (C6D6, 250 MHz,
258C): d ˆ 3.14 (s, 3H; OCH3), 3.26 (s, 3H; OCH3), 5.20 (s, 2H; CH2O),
6.72 (m, J(H,H) ˆ 8.7 Hz, 2H; aromatic), 7.21 (d, J(H,H) ˆ 8.7 Hz, 2H;
aromatic), 7.54 (d, J(H,H) ˆ 8.8 Hz, 2H; aromatic), 8.10 (d, J(H,H) ˆ
8.9 Hz, 2H; aromatic); elemental analysis calcd (%) for C16H16O4: C
70.57, H 5.92; found C 70.01, H 5.70.
4-Dimethylamino 4-dimethylaminobenzoate:[47] 1H NMR (C6D6, 250 MHz,
258C): d ˆ 2.42 (s, 6H; (CH3)2N), 2.53 (s, 6H; (CH3)2N), 4.64 (s, 2H;
CH2O), 6.72 (d, J(H,H) ˆ 8.7 Hz, 2H; aromatic), 7.67 ± 7.71 (m, 1H;
aromatic), 8.13 ± 8.18 (m, 2H; aromatic); elemental analysis calcd (%)
for C18H22N2O2: C 72.46, H 7.43, N 9.39; found C 72.70, H 7.85, N 9.30.
Phthalide:[48] 1H NMR (CDCl3, 250 MHz, 258C): d ˆ 5.21 (s, 2H; CH2O),
6.51 (d, J(H,H) ˆ 0.8 Hz, 1H; aromatic), 6.83 ± 6.98 (m, 8H; aromatic), 7.68
(d, J(H,H) ˆ 7.4 Hz, 1H; aromatic); elemental analysis calcd (%) for
C8H6O2: C 71.64, H 4.51; found C 69.35, H 4.59.
Cyclohexylmethyl cyclohexanecarboxylate:[33] 1H NMR (CDCl3, 250 MHz,
258C): d ˆ 0.87 ± 1.01 (m, 2H), 1.06 ± 1.30 (m, 6H), 1.35 ± 1.54 (m, 2H),
1.55 ± 1.74 (m, 9H), 1,86 ± 1.91 (m, 2H), 2.22 ± 2.33, (m, 1H), 3.85 (d,
J(H,H) ˆ 6.4 Hz, 2H; CH2O), 5.62 ± 6.67 (m, 4H); elemental analysis calcd
(%) for C14H24O2: C 74.95, H 10.78; found C 74.37, H 10.50.
3-Cyclohexenylmethyl 3-cyclohexenecarboxylate: 1H NMR (CDCl3,
250 MHz, 258C): d ˆ 1.23 ± 1.39 (m, 1H), 1.60 ± 2.21 (m, 10H), 2.22 ± 2.26
(m, 2H), 2.50 ± 2.62 (m, 1H), 3.98 (d, J(H,H) ˆ 6.4 Hz, 2H; CH2O), 5.62 ±
6.67 (m, 4H); elemental analysis calcd (%) for C14H20O2: C 76.33, H 9.15;
found C 76.28, H 9.29.
Neopentyl neopentanoate:[33] 1H NMR (C6D6, 250 MHz, 258C): d ˆ 0.80 (s,
6H; (CH3)3C), 1.16 (s, 9H; (CH3)3C), 3.74 (s, 2H; CH2O); elemental
analysis calcd (%) for C14H20O2: C 69.72, H 11.70; found C 68.95, H 11.50.
Isobutyl isobutanoate:[33] 1H NMR (C6D6, 250 MHz, 258C): d ˆ 0.75 (d,
J(H,H) ˆ 6.7 Hz, 6H; (CH3)2CH), 1.07 (d, J(H,H) ˆ 7.0 Hz, 6H;
(CH3)2CH), 1.73 (sept, J(H,H) ˆ 6.8 Hz, 1H; (CH3)2CH), 2.38 (sept,
J(H,H) ˆ 6.9 Hz, 1H; (CH3)2CH), 3.80 (d, J(H,H) ˆ 6.5 Hz, 2H; CH2O);
elemental analysis calcd (%) for C8H16O2: C 66.63, H 11.18; found C 66.53,
H 10.97.
Experimental Section
General: All manipulations of air-sensitive materials were performed with
the rigorous exclusion of oxygen and moisture in flamed Schlenk-type
glassware either on a dual manifold Schlenk line, or interfaced to a high
vacuum (10 4 torr) line, or in an argon-filled Braun glove box (Uni-
Lab1200/780). Hydrocarbon solvents were distilled under nitrogen from
Na wire, prior to use. Deuterated solvents were obtained from Aldrich (all
99 atom% D) and were degassed, dried, and stored in vacuo over Na/K
alloy in resealable flasks. NMR spectra were recorded on Bruker AC250.
Chemical shifts are referenced to internal solvent resonances and are
reported relative to tetramethylsilane. Elemental analyses were performed
at the microanalytical laboratory of the Institute of Inorganic Chemistry in
Karlsruhe (Germany). All aldehydes were obtained from Aldrich and were
degassed, dried over LiAlH4, and stored under nitrogen in resealable
flasks. 5-Phenyl-4-pentyne-1-amine,[5d] 4-pentyne-1-amine,[5d] and 4-pen-
tene-1-amine[5b] were prepared by literature procedures.
General procedure for the Tishchenko reaction (NMR scale): Compound 1
was weighed under argon into an NMR tube. C6D6 (ꢀ0.7 mL) was
condensed into the tube and the mixture was frozen to 1968C. The
reactant was injected onto the solid mixture, and the whole sample was
melted and mixed just before insertion into the core of the NMR machine
(t0). The ratio between the reactant (product) and the catalyst was exactly
calculated by comparison of the integration of all CHO (CH2O) signals
with the N(SiMe3)2 signals. The latter were used as an internal standard for
the kinetic measurements. Tetramethylbenzene was used as an indepen-
dent standard in a test reaction to show that R N(SiMe3)2 is formed in a
stoichiometric ratio.
General procedure for the Tishchenko reaction (preparative scale)
Method A, without solvent: Under protective gas, the catalyst was stirred in
a tempered reaction flask. The reactant (5 g) was added directly to the
catalyst. An exothermic reaction was generally observed. After 1 d, the
product was isolated by distillation. (The products with high TOFs can be
worked up after 1 h (see Tables 3 and 4)).
Method B, in solution: The catalyst and the reactant (5 g) were each
dissolved in pentane/hexane (1:1; 25 mL). The solution of the reactant was
added to the catalyst solution in a tempered flask. After 1 d, the product
was isolated by distillation or filtration.
Benzyl benzoate:[33] 1H NMR (C6D6, 250 MHz, 258C): d ˆ 5.21 (s, 2H;
CH2O), 6.98 ± 7.27 (m, 7H; aromatic), 7.67 ± 7.71 (m, 1H; aromatic), 8.13 ±
8.18 (m, 2H; aromatic); elemental analysis calcd (%) for C14H12O2: C 79.23,
H 5.70; found C 78.93, H 5.72.
4-Fluorobenzyl 4-fluorobenzoate:[32] 1H NMR (C6D6, 250 MHz, 258C): d ˆ
4.95 (s, 2H; CH2O), 6.57 ± 7.08 (m, 4H; aromatic), 7.49 (m, 2H; aromatic),
7.85 (m, 2H; aromatic); elemental analysis calcd (%) for C14H10F2O2: C
67.74, H 4.06; found C 67.52, H 3.98.
4-Chlorobenzyl 4-chlorobenzoate:[32] 1H NMR (C6D6, 250 MHz, 258C):
d ˆ 4.89 (s, 2H; CH2O), 6.81 ± 7.14 (m, 4H; aromatic), 7.39 (d, J(H,H) ˆ
2-Ethyl-1,3-hexanediol monobutyrate:[44] B.p. 81 8C (0.1 mm); 1H NMR
(CDCl3, 250 MHz, 258C): d ˆ 0.94 (m, 8H) 1.26 1 ± 71 (m, 8H), 2.31 (dt,
4H), 3.60 (br, 2H) 4.40 ± 4.30 (m, 2H).
2-Furylmethyl 2-furancarboxylate:[49] 1H NMR (CDCl3, 250 MHz, 258C):
d ˆ 5.29, (s, 2H; CH2O), 6.38 (dd, J(H,H) ˆ 1.8, 3.3 Hz, 1H), 6.49 (dd,
J(H,H) ˆ 1.9, 3.4 Hz, 2H), 7.20 (dd, J(H,H) ˆ 0.7, 3.5 Hz, 1H), 7.44 (dd,
J(H,H) ˆ 0.7, 1.8 Hz, 1H), 7.57 (dd, J(H,H) ˆ 0.7, 1.6 Hz, 1H); elemental
analysis calcd (%) for C10H8O4: C 62.50, H 4.20; found C 62.35, H 4.34.
4-Methyl-2-furylmethyl 4-methyl-2-furancarboxylate: 1H NMR (C6D6,
250 MHz, 258C): d ˆ 1.80 (s, 3H; CH3), 1.91 (s, 3H; CH3), 5.11, (s, 2H;
CH2O), 5.59 (m, 1H), 5.65 (m, 1H), 6.17 (m, 1H), 6.95 (m, 1H); no pure
product was obtained.
Chem. Eur. J. 2001, 7, No. 14
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P. W. Roesky et al.
FULL PAPER
3-Pyridinecarboxylic acid 3-pyridinylmethyl ester: 1H NMR (C6D6,
250 MHz, 258C): d ˆ 4.90 (s, 2H; CH2O), 6.73 (m, 2H; aromatic), 7.65
(m, 1H), 7.94 (m, 2H; aromatic), 8.52 (m, 2H; aromatic), 9.12 (s, 1H;
aromatic); elemental analysis calcd (%) for C12H10N2O2: C 67.28, H 4.71, N
13.08; found C 67.53, H 4.75, N 12.89.
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General procedure for the hydroamination reaction (NMR scale): Com-
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Received: February 7, 2001 [F3059]
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