Organoactinides promote the dimerization of aldehydes: Scope, kinetics, thermodynamics, and calculation studies

Article abstract of DOI:10.1021/ja105696p

Surprising catalytic activities have been found for the actinide complexes Cp*₂ThMe₂ (1), Th(NEtMe)₄ (2), and Me₂SiCp″₂Th(C₄H₉) ₂(3) toward oxygenated substrates. During the catalytic dimerization of benzaldehydes to their corresponding esters, complexes 1 and 2 gave 65 and 85% yield in 48 h, respectively, while the geometry-constrained complex 3 gave 96% yield in 24 h. Exploring the effect of substituents on benzaldehyde, it has been found that, in general, electron-withdrawing groups facilitate the reaction. Kinetic study with complexes 1 and 3 reveals that the rate of the reaction is first order in catalyst and substrate, which suggests the rate equation "rate = k[catalyst]¹[aldehyde]¹". The activation energy of the reaction was found to be 7.16 ± 0.40 and 3.47 ± 0.40 kcal/mol for complexes 1 and 3 respectively, which clearly indicates the advantage of the geometry-constrained complex. Astonishing are the reactivity of the organoactinide complexes with oxygen-containing substrates, and especially the reactivity of complex 3, toward the dimerization of substrates like p-methoxybenzaldehyde, m/p-nitrobenzaldehyde, and furanaldehyde and the reactivity toward the polymerization of terephthalaldehyde. Density functional theory mechanistic study reveals that the catalytic cycle proceeds via an initially four-centered transition state (+6 kcal/mol), followed by the rate-determining six-centered transition state (+13.5 kcal/mol), to yield thermodynamically stable products.

Full text of DOI:10.1021/ja105696p

ARTICLE
pubs.acs.org/JACS
Organoactinides Promote the Dimerization of Aldehydes:
Scope, Kinetics, Thermodynamics, and Calculation Studies
Manab Sharma,†,‡ Tamer Andrea,† Nigel J. Brookes,‡ Brian F. Yates,‡ and Moris S. Eisen*,†
†Schulich Faculty of Chemistry, Institute of Catalysis Science and Technology, Technion;Israel Institute of Technology,
Technion City, 32000 Haifa, Israel
‡School of Chemistry, University of Tasmania, Hobart, Tasmania, Australia
S Supporting Information
b
ABSTRACT: Surprising catalytic activities have been found for
the actinide complexes Cp*2ThMe2 (1), Th(NEtMe)4 (2), and
Me2SiCp00 Th(C4H9)2(3) toward oxygenated substrates. During
2
the catalytic dimerization of benzaldehydes to their correspond-
ing esters, complexes 1 and 2 gave 65 and 85% yield in 48 h,
respectively, while the geometry-constrained complex 3 gave 96%
yield in 24 h. Exploring the effect of substituents on benzaldehyde,
it has been found that, in general, electron-withdrawing groups
facilitate the reaction. Kinetic study with complexes 1 and 3
reveals that the rate of the reaction is first order in catalyst and
substrate, which suggests the rate equation “rate = k[catalyst]1-
[aldehyde]1”. The activation energy of the reaction was found to be 7.16 ( 0.40 and 3.47 ( 0.40 kcal/mol for complexes 1 and 3
respectively, which clearly indicates the advantage of the geometry-constrained complex. Astonishing are the reactivity of the
organoactinide complexes with oxygen-containing substrates, and especially the reactivity of complex 3, toward the dimerization of
substrates like p-methoxybenzaldehyde, m/p-nitrobenzaldehyde, and furanaldehyde and the reactivity toward the polymerization of
terephthalaldehyde. Density functional theory mechanistic study reveals that the catalytic cycle proceeds via an initially four-
centered transition state (þ6 kcal/mol), followed by the rate-determining six-centered transition state (þ13.5 kcal/mol), to yield
thermodynamically stable products.
’ INTRODUCTION
as Cp*2MH2 and Cp*2 M(Cl)H (M = Zr, Hf),6 were found to be
highly efficient in the dimerization of aliphatic aldehydes but
inactive toward aromatic aldehydes. Moreover, homologous
titanocene complexes were found to be inert under all
conditions.6
The catalytic dimerization of aldehydes to give the corre-
sponding esters (known as the Tishchenko reaction) is a century-
old process. Due to its atom efficiency, the reaction is still
drawing worldwide attention among industrial and academic
researchers.1 Since its first discovery in 1887, and a further
modification in 1906,2 aluminum alkoxides have been the tradi-
tional homogeneous catalysts to perform this reaction.3 During
the past decade, in addition to aluminum complexes, many other
catalysts have been screened, e.g., boron,4 iron,5 zirconium,6
ruthenium,7 rhodium,8 lanthanum,9 neodymium,9a ytterbium,9b
hafnium,6 osmium,10 and iridium,11 but interestingly, never
actinides. It is essential to indicate that, when aromatic aldehydes
are used, the majority of the catalysts show a sluggish activity for
the coupling reaction.
The lanthanide complexes Cp*2LnCH(SiMe3)2 (Ln = La, Nd;
Cp* = C5Me5) were found to catalyze the Tishchenko reaction of
aromatic aldehydes in moderate yields,9a,9b but the reaction usually
required heating for 3 days at 60 °C. The corresponding homo-
leptic lanthanides Ln[N(SiMe3)2]3 (Ln = La, Y, Sm)9b,9d were
found to be more reactive, with higher turnover frequencies, when
electron-withdrawing ligands were used. Group 4 complexes, such
In contrast to the rich chemistry that has been offered by
organolanthanide complexes during the past two decades, the
electrophilic d0/fn chemistry of organoactinide compounds has
been under a fervent investigation and has achieved a high level of
sophistication due to the unique structure-reactivity relationships
of the organoactinides12 and their outstanding performance in
various homogeneous catalytic processes.13 For example, catalytic
reactions involving neutral organoactinides were first limited to
C-H activations and hydrogenation of hydrocarbons.13 However,
recent applications of organoactinide complexes in challenging cataly-
tic organic chemical transformations like hydroamination,13n,o
oligomerization,13g,13h selective dimerization,14 hydrosilylation
of alkene and alkynes,15 coupling of silanes with amines,13t coupl-
ing of isonitriles with alkynes,16 disproportionation metathesis
of TMSCtCH, and cross-metathesis of TMSCtCH or
Received: June 28, 2010
Published: January 11, 2011
r
2011 American Chemical Society
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TMSCtCTMS with various terminal alkynes12m,17 have taken
this chemistry to a new level.
The stoichiometric and catalytic properties of organo-f-
element complexes are deeply influenced by the nature of the
π ancillary ligands.12g,12i,12p,12t,18 The justification for the com-
monly used pentamethylcyclopentadienyl ligand relates to the
fact that it allows researchers to obtain simple organoactinide
complexes with good solubility, thermal stability, and resistance
to ligand redistribution.19 Structurally, a significant opening of
the metal coordination sphere (frontier orbitals)20 at the σ-
ligand equatorial girdle is obtained by replacing the pentamethyl-
cyclopentadienyl ligation in Cp*2MR2 (M = f-element metal,
R = σ-bonded ligand) by a bridged ligand such as ansa-Me2-
Figure 1. Oxo-bridged uranium complex.
having an ansa-type ligand mimicking the model of reactive
M-O-M skeletons (M = metal), serving as oxide-supported
heterogeneous organometallic catalysts or metal-oxide-containing
catalysts.25 This complex was found to be extremely active for the
C-H and C-Si bond activations of aromatic silanes and
benzene (eq 3).17a,26
SiCp00 MR2 (Cp00 = C5Me4).13a,21 For organolanthanides, this
2
alteration allowed an increase of 10-100-fold in rates for the
olefin insertion into a M-R bond,22 whereas in organoactinides,
this modification has been shown to cause an increase of 103-fold
in their catalytic activity in the hydrogenation of 1-hexene and in
the hydrosilylation of alkynes and alkenes.21c,22a
n
fMe2SiCp00
Uð BuÞg ðμ -OÞ C6D5SiH3
2
2
PhSiH3 þ C6D6
f
In spite of the successful application of organoactinide com-
plexes toward various catalytic transformations, all oxygenated
substrates have been excluded on account of the high oxophilicity
of the organoactinides, which is expected to lower the activity due
to the predictably strong oxygen-actinide interactions. For
instance, it has been shown that the use of an alkoxy ligand in
place of an alkyl ligand resulted in a reduced catalytic activity in
the organoactinide-catalyzed hydrogenation of olefins.23 Despite
those findings, we have recently disclosed a new strategy for the
introduction of organoactinides as outstanding catalysts with
surprising activities toward the polymerization of cyclic mono-
and diesters.24a The strategy was designed with a metal-oxygen
bond24b-24e thermodynamically reacting/inserting with a sub-
strate to form a second metal-oxygen bond, with similar bond
energies, with entropy being the main parameter governing the
reaction (eqs 1 and 2).
þ H2=HD þ Ph2SiH2 þ SiH4
ð3Þ
On the basis of these results (eqs 1-3), we were interested to
study the scope of the insertion of a carbonyl substrate into a
metal-oxygen bond (eq 4) and to investigate if we can perform
this transformation catalytically.
Hence, we concentrate our research efforts on the reactivity of
actinide complexes, particularly for the dimerization of alde-
hydes, better known as the Tishchenko reaction. The goal of this
investigation was to examine the scope, chemoselectivity, ancil-
lary ligand sensitivity, kinetics, and thermodynamics of the
Tishchenko reaction catalyzed by the organothorium complexes
(Cp*)2ThMe2 (1), Th{N(CH3)CH2CH3}4 (2),27 and the ansa-
type bridged Me2SiCp00 Th(n-C4H9) (3),
2
which differ in their ancillary ligation as well as coordinative
unsaturation around the metal center, further enabling us to
compare the effect of oxophilicity versus their corresponding
coordinative unsaturation. Here, we also report the isotope and
substituents effects of a variety of aromatic aldehydes on the
catalytic reaction. Moreover, computational studies are becom-
ing a vital diagnostic tool to obtain insight into complex reaction
mechanisms as well as to reveal various facts about the electronic
states or bonding of transition metals,28 including f-block
elements.29 Therefore, the computational DFT method has been
employed to give an explicit account of the reaction mechanism
and bonding of various catalytic species and transition-state
structures. On the basis of kinetics and thermodynamics together
with density functional theory (DFT) studies, a suitable mecha-
nistic pathway is proposed for the studied reaction.
In view of this, we were intrigued by the reactivity of oxygen-
containing actinide complexes and wished to study the competition
between oxophilicity and coordinative unsaturation of ansa com-
plexes. Hence, we have synthesized the oxo-bridged organoactinide
complex {Me2SiCp00 U(nBu)}2(μ-O) (Cp00 = C5Me4) (Figure 1),
2
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’ RESULTS
Table 1. Coupling of Aromatic Aldehydes by Complexes
1-3a
We start by presenting results regarding the different reactiv-
ities of the organoactinide complexes with the studied aldehydes.
Following this, we disclose the kinetics and thermodynamics
studies, including the isotope effect and the effect of substituents,
and finally we present some stoichiometric reactions revealing
plausible active catalytic species.
Dimerization of Aldehydes by Complexes 1-3. The organo-
actinide complexes 1-3 were found to be active precatalysts for
the chemoselective dimerization of aldehydes (eq 5), giving the
corresponding esters in high to moderate yields, while no reaction
was observed in the absence of a catalyst. Reactions of these
complexes were performed with an excess of aldehyde, either in
toluene or in benzene (aldehyde:catalyst = 100:1), at room tem-
perature (25 °C), and results for the corresponding ester formation
are presented in Table 1. When the same dimerizations were
performed at a slightly higher temperature (35 °C), faster reactions
were observed (vide infra).
yield (%)
after 24 h
entry
cat.
RCHO
after 6 h
after 48 h
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
Ph
13
3.5
2.7
1.3
21
19
16
21
19
15
9
41
14
11
5
65
25
20
10
84
81
75
85
82
75
55
97
96
95
2
p-CH3-Ph
m-CH3-Ph
o-CH3-Ph
p-Cl-Ph
3
4
5
60
57
50
61
58
50
33
85
80
78
96
74
94
70
98
95
89
98
39
-
6
m-Cl-Ph
7
o-Cl-Ph
8
Ph
9
p-CH3-Ph
m-CH3-Ph
o-CH3-Ph
p-Cl-Ph
10
11
12
13
14
15
16
17
18
12
13
14
15
16
17
18
19
20
21
22
37
33
32
58
28
51
26
75
68
61
62
12
-
m-Cl-Ph
o-Cl-Ph
Ph
p-CH3-Ph
m-CH3-Ph
o-CH3-Ph
p-Cl-Ph
The catalytic reactions proceed, in most cases, to completion at
the corresponding temperature and were carefully followed by
either GC or GC/MS and H NMR spectroscopy. All products
m-Cl-Ph
1
o-Cl-Ph
were isolated and characterized by 1H, 13C, and 2D NMR
spectroscopy, GC/MS, high-resolution mass spectroscopy, and
when possible by comparison to literature spectra and to data from
commercially available analytically pure samples. Isolation proto-
cols for the products involved high-vacuum transfer of volatiles with
subsequent elimination of the solvent and final distillation of
products or purification by silica gel column chromatography.
GC/MS, MALDI-MS, and NMR analysis of the products demon-
strated that the esters were the exclusively obtained products.
The reactivity of the complexes with different aromatic alde-
hydes at various reaction times is presented in Table 1. It has been
shown that the coordinative unsaturation of the organoactinide
complexes used in this study follows the order 3 > 2 > 1, suggesting,
as can be observed in Table 1, that the higher the coordinative
unsaturation of the organoactinide complex, the faster the dimer-
ization (for example, compare entries 1, 8, and 15 or 2, 9, and
16).13a,20-23 In addition, a large difference in reactivity was
observed among the different aldehyde substituents (compare
entries 2, 3, and 4 with 5, 6, and 7). For each of the complexes,
electron-withdrawing substituents at the aromatic ring induce
better reactivities.
p-CN-Ph
p-MeO-Ph
o-NO2-Ph
m-NO2-Ph
p-NO2-Ph
2-furanaldehyde
terephthalaldehyde
62
51
98
42d
60b
49
Ph c
a Yields were determined from H NMR and GC/MS. b Conversion.
1
c Recycled catalyst. d At 60 °C.
the reaction with p-cyanobenzaldehyde precipitates cleanly out of
the solution, and the reaction was found to be reasonably fast, while
with p-methoxybenzaldehyde the reaction was rather slower
(compare entries 15 and 16 in Table 1). The slow conversion of
the p-methoxybenzaldehyde clearly supports the fact that the
presence of an electron-donatinggroup in the phenyl ringdecreases
the rate of the reaction.
It is important to point out that the activity of complex 3 was also
screened toward nonaromatic aldehydes, like cyclohexanaldehyde,
pivaldehyde, pentanaldehyde, and acetaldehyde, showing for all the
reactions only the stoichiometric insertion of two aldehyde mole-
cules into the two Th-C butyl bonds, producing the expected
corresponding alkoxides (along with some unidentified products).
Apart from these monoaldehydes, complex 3 was found also to
be very active toward a dialdehyde, viz. terephthalaldehyde, forming
dimeric as well as oligomeric and polymeric esters (eq 6). During a
reaction period of 6 h, at room temperature, a light yellow polymer
was formed, which starts precipitating out of the reaction solution.
After 20 h, the precipitate was collected by filtration, washed with
toluene, dried, and characterized by 1H NMR spectroscopy. Two
aldehyde signals, at δ 9.92 and 9.95 ppm, in addition to two sets of
Most interesting was the surprising activity of complex 3 toward
p-nitrobenzaldehyde, m-nitrobenzaldehyde, and furfuraldehyde,
improving the general scope of the reaction and introducing a
new concept regarding the group tolerance of organoactinides.
However, in the case of o-nitrobenzaldehyde, instead of the desired
product, an insoluble material was formed, presumably of a
polymeric nature, which we were unable to fully characterize.
The reaction of furfuraldehyde did not proceed at room
temperature; however, at 60 °C, 42% yield of the corresponding
ester was obtained after 48 h (Table 1, entry 20). To explore further
the potential scope of complex 3, reactions were also carried out
with p-methoxy- and p-cyanobenzaldehyde. The ester product of
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methylene signals, in the ranges δ 4.63-4.71 and 5.24-5.37 ppm,
clearly support the presence of two unsymmetrical end aldehydic
groups and two different types of methylene groups (m and n type,
eq 6) in the chain. The integration ratio between the aldehyde and
methylene signals allows us to calculate the degree of polymeriza-
tion and the number-average molecular weight (DP = 25; Mn =
3350 g/mol). Analysis of the soluble part of the reaction mixture,
even after 20 h, exhibits small amounts (<5%) of oligomers (up to
pentamers as exhibited by MALDI-TOF).
Table 2. Cross-Tishchenko Reaction of Benzaldehyde and
p-Tolualdehyde Catalyzed by Complex 1a
yield (%)
entry
cat.:RCHO:R0CHO
4
5
6
7
1
2
3
1:100:100
1:100:50
1:50:100
15
25
9
4
2
6
6
5
6
6
5
6
a 24 h. Yields were determined from the integration of 1H NMR spectra
and GC/MS.
One of the most important queries regarding catalytic processes
is the fate of the catalysts after a batch of reaction. Regarding
complex 3, we were astonished to find that, on completion of one
batch of the polymerization reaction (eq 6), when the solution was
charged again with the same amount of terephthalaldehyde, preci-
pitation of the polymer was again observed, producing almost the
same results as obtained after 6 h of the first batch of reaction. In
addition, during a typical recycling experiment with complex 3 for
the dimerization of benzaldehyde (eq 5), 85% of the initial activity
was found to be retained by the catalyst, which further supports the
longer durability as well as stability of the complex (Table 1,
entry 22).
The conceptual cross-coupling Tishchenko reaction between
two different aldehydes (benzaldehyde and p-tolualdehyde) to
afford four possible esters (eq 7) was also studied using complex 1,
and the results are presented in Table 2. As expected, the relative
reactivity of the corresponding aldehydes is the main factor toward
a chemoselective product. As benzaldehyde is more active than p-
tolualdehyde, product 4 is expected to be obtained in higher yields.
It must be taken into account that, since there are few kinetic steps
(vide infra), the reaction can be tailored by changing the ratio of the
two substrates. A simple illustration was made by using 1:1, 1:2, and
2:1 benzaldehyde:p-tolualdehyde ratios (Table 2).
Figure 2. Variation of reaction rate (kobs) with concentration of
precatalysts (a) 1 and (b) 3.
R to the carbonyl of the ester product, RCH2C(O)O (δ = 5.10
ppm) (see Supporting Information for typical spectra). When the
initial concentration of aldehyde was held constant and the
concentration of the catalytic precursor was varied over a ∼10-
fold concentration range, a plot of reaction rate vs precatalyst
concentration (Figure 2) indicates that the reaction is first-order
dependent on precatalyst for both complexes. These results
indicate that the best formulation of the active species in solution
should be of monomeric nature. Keeping the catalyst concentration
constant while varying the aldehyde over a ∼10-fold concentration
range, a plot of reaction rates vs aldehyde concentration (Figure 3)
indicates also a first-order kinetic dependence on aldehyde for both
complexes.
Thus, the apparent rate law for the dimerization of aldehydes
promoted by complexes 1 and 3 can be formulated as presented
in eq 8.
1
1
ν ¼ k½complexꢀ ½aldehydeꢀ
ð8Þ
Since complex 3 is a highly coordinative unsaturated complex, it
is expected that a limiting effective catalyst:substrate ratio will be
operative. Hence, by measuring the reaction rate over a wide range
of benzaldehyde concentrations while keeping the catalyst con-
centration constant, we have been able to determine that, as the
ratio of benzaldehyde to catalyst 3 increases from 100 to ∼330, the
rate of the reaction also increases; however, further increasing this
ratio results in a decrease of the reaction rate (Figure 4). This
suggests that, to obtain the fastest process, the preferential effective
substrate:catalyst ratio lies around 300:1.
Kinetic and Thermodynamic Studies of the Tishchenko
Reaction of Benzaldehyde Promoted by Complexes 1 and
3. In order to propose a plausible mechanism for the reaction, to
gain a better understanding of the effect of the ancillary ligands, and
to learn about the influence of the aldehyde and the catalyst on the
reaction rate, kinetics measurements were performed. The catalytic
reaction of 2 aldehyde f ester promoted by Cp*2ThMe2 and
Me2SiCp00 ThnBu2 was followed by in situ 1H NMR spectroscopy
2
in benzene-d6 or toluene-d8. The progress of the reaction was
monitored by the disappearance of the terminal aldehyde hydro-
gen, C(O)H (δ = 9.60 ppm), along with the appearance the proton
The influence of temperature on the reaction rate was
investigated by repeating the same reaction at different tempera-
tures while the concentrations of the catalyst and benzaldehyde
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Figure 5. Arrhenius plot of the dimerization of ([) p-chlorobenzalde-
hyde, (2) m-chlorobenzaldehyde, (9) m-methylbenzaldehyde, and (b)
p-methylbenzaldehyde by the complex 3.
Figure 3. Observed aldehyde concentration-dependent reaction rate
(kobs) for the precatalysts (a) 1 and (b) 3.
Figure 6. Kinetic isotopic effect on the Tishchenko reaction of benzal-
dehyde and R-d-benzaldehyde promoted by complex 1.
Table 3 shows, in general, that the presence of an electron-
withdrawing group at the aromatic ring induces a slight decrease
in the Ea as compared to that for the substituted benzaldehyde
with an electron-donating group. The difference in the enthalpy
of activation for the different groups gives an important insight
into the expected electron density in the metal at the RDS. The
thermodynamic parameters for ortho-methyl- or -chloride-
substituted benzaldehydes were not measured to avoid any steric
factors that are probably equally or more predominate as
compared to those observed in these cases (Table 1). For
example, the extremely low reactivity of the ortho-tolualdehyde
can be accounted for by the combined steric and electron effects
hindering the approach of an aldehyde molecule to the metal-
alkoxide bond. In the case of ortho-chlorobenzaldehyde, the
steric effect does not appear to be a dominant effect; however,
an additional electrostatic interaction between the metal and the
chlorine atom may be operative.
In addition to the kinetic and thermodynamic studies, a primary
isotopic effect was observed using R-deuterated benzaldehyde30
with kH/kD = 2.7 for complex 1, indicating that a hydride transfer is
involved in the RDS (Figure 6). Stoichiometric reactions between
the actinide complexes with benzaldehyde yield the expected
stoichiometric amounts of C6H5COOCH(CH3)C6H5, C6H5CO-
OCH(NEtMe)C6H5, and C6H5COOCH(C4H9)C6H5, as mea-
sured by GC and GC/MS, when complexes 1, 2, and 3 were used,
respectively. These results demonstrate that the aldehyde is able to
insert into the initial actinide complexes, producing the active
alkoxo species.
Figure 4. Variation of kobs with the ratio between concentration of
benzaldehyde and precatalyst 3 ([benzaldehyde] = 7.37 ꢁ 10-1, 1.47,
1.97, 2.45, 2.94, 3.43, 4.41, and 5.39 M; [precatalyst 3] = 7.37 ꢁ 10-3 M).
were kept constant. For both complexes 1 and 3, similar kinetic
dependences were observed on the aldehyde and the catalyst
over the range of temperatures studied (35-95 °C). The derived
activation parameters from Arrhenius and Eyring analysis are
(error values are in parentheses) Ea = 7.16(4) kcal/mol, ΔHq=
6.58(4) kcal/mol, ΔSq = -48.8(4) eu and Ea = 3.47(2) kcal/mol,
ΔHq= 2.80(4) kcal/mol, ΔSq = -65.2 (4)eu for complexes 1 and
3, respectively. These results indicate that, as expected, the
relatively more open coordinative complex 3 is more active than
its congener complex 1, and the significant negative ΔSq value
reveals a highly ordered metal-centered transition state for the
hydride shift from the alkoxy ethereal ligands to the incoming
benzaldehyde (vide infra, Figure 6 and Scheme 1). Moreover, the
higher negative ΔSq for complex 3 suggests a more organized
transition state, which further supports a larger degree of co-
ordinative unsaturation in complex 3 as compared to complex 1.
Effect of the Substituent on the Thermodynamics Param-
eters. To evaluate the effect of substituents in the benzene ring
on the dimerization reaction, the thermodynamic parameters (Ea,
ΔHq, and ΔSq) of the rate-determining step (RDS) were also
measured and calculated for the meta- and para-substituted ben-
zaldehyde with the electron-withdrawing chlorine and the electron-
donating methyl groups using complex 3 (Table 3 and Figure 5).
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Scheme 1. Plausible Mechanism for the Catalytic Dimerization of Aldehyde by Organoactinide Complexes, Taking Complex 3 as
a Representative Example
Stoichiometric Reactions. A few stoichiometric reactions
were performed to gain some insight into the plausible mecha-
nism of the reaction and to try to understand why only a dimer
product is obtained. When complex 3 was reacted with 2 equiv of
benzyl alcohol or n-butyl alcohol, the corresponding (bis)alkoxo
complexes 3a and 3b were obtained (eq 9).
Table 3. Thermodynamic Data of Substituted and Unsubsti-
tuted Benzaldehyde during the Tishchenko Reaction with
Precatalyst 3
substrate
Ea (kcal/mol)
ΔHq (kcal/mol)
ΔSq (eu)
benzaldehyde
3.47(4)
3.45(4)
3.41(4)
3.90(4)
4.36(4)
2.80(4)
2.79(4)
2.75(4)
3.24(4)
3.70(4)
-65.2(3)
-65.3(3)
-65.0(3)
-64.4(3)
-64.6(3)
m-chlorobenzaldehyde
p-chlorobenzaldehyde
m-methylbenzaldehyde
p-methylbenzaldehyde
lack of a plausible β-H elimination pathway in the mechanism.
Similar reactions with complex 1 produced the analogous com-
plexes 1a and 1b with the same trends in reactivity asfor complex 3.
In the stoichiometric reaction with complex 1a or 1b, the corre-
sponding dimeric dihydride complex ((Cp*2ThH2)2) was not
observed.13d Moreover, the reaction of complexes 1a and 1b with
H2 in pressure vessels did not produce the corresponding
(Cp*2ThH2)2, again corroborating a pathway not involving the
formation of a hydride intermediate.13d,13f,23
The reaction of complex 3a with benzaldehyde produced the
expected dimer product in a similar kinetic manner as with complex
3. In addition, almost no reactivity was observed, at room
temperature, for complex 3b with butyraldehyde, as shown above
for other aliphatic aldehydes. This result is indeed extremely
informative since it indicates the stability of complex 3b and the
’ DISCUSSION
Catalytic Dimerization of Aldehydes: Reaction Scope and
Mechanism. The catalytic results for the selective dimerization
of aldehydes promoted by complexes 1, 2, and 3 produced the
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corresponding esters with high chemoselectivity without any other
byproduct. It is worth mentioning here that, despite the fact that the
actual catalytic species is not well defined, the observed trend of
reactivity, 3 > 2 > 1, might arise due to differences in coordinative
unsaturation around the metal center. The presence of monoden-
tate N-donor ligands with bulkier substituents in 2 sterically
provides high shielding toward the metal center, while in 3 the
ansa-type Cp00-bridged ligands open up the frontier orbitals, with
the result that the Th center becomes more exposed to incoming
substrates. This relation between the coordinative unsaturation and
reactivity is consistent with previous observations for olefin inser-
tion into M-R bonds22 or the organoactinide catalytic hydro-
amination of alkynes.33b These results demonstrate the ability to
scope the catalytic use of organoactinide complexes with oxygen-
containing substrates and allow us to tailor the reactivity of these
complexes in the Tishchenko reaction by modulating the spectator
ancillary ligands. The lack of methane, EtMeNH, or butane using
complexes 1, 2, or 3, respectively, indicates that the aldehyde
proton is not acting as a protonating agent, producing neither an
acyl-type nor the dialkoxo metallacycle complex (eq 10).31
molecules into the thorium-alkoxide bond (Scheme 1, step 2)
results in the formation of complex 9. Complex 9 does not insert
any additional aldehyde molecules but rather eliminates the first
ester 10, in stoichiometric amounts, presumably either via a hydride
transfer to an incoming aldehyde through a six-centered transition
state (Scheme 1, step 3, TS1), with the concomitant production of
the active catalytic complex 11, or via a β-H elimination and rapid
insertion of an aldehyde to produce the active complex 11. The
catalytic cycle starts with the addition of an aldehyde molecule into
the thorium-alkoxide bond of complex 11 (Scheme 1, step 4, TS2)
in a similar manner to that in complex 3, via a four-centered
transition state, to produce complex 12, which performs a hydride
transfer to an additional aldehyde through a six-centered transition
state as the RDS (Scheme 1, step 5, TS3), regenerating the active
catalyst 11 and producing the final ester product 4. Alternatively, a
β-hydrogen elimination process from 12 may also generate the
same ester products 4, along with the formation of a Th-H
complex 13 (Scheme 1, Path B, TS4), which potentially may react
with an aldehyde to produce back the active catalytic species 11.
Interestingly, in step 2, β-hydrogen elimination is, in theory, also a
plausible process to yield the corresponding Th-H moiety and the
ketone, which indeed was not observed. Hence, it seems a rather
difficult and extremely challenging process to convert a Th-O
moiety into a Th-H motif. From a thermodynamic point of view,
we have calculated the enthalpy of the reaction including a β-
hydrogen elimination and found it to be energetically higher than
the reaction via the six-centered transition state32 (þ56 and -47
kcal/mol, respectively, eqs 11-13).32 Therefore, we suggest, on the
basis of the thermodynamic calculations, the lack of stoichiometric
methylbenzophenone, amide C6H5C(O)NEtMe, or butylbenzo-
phenone for complexes 1, 2, or 3, respectively, the observed
deuterium isotope effect, and the stability of the bis(alkoxo)
complexes, that if operative, β-hydrogen elimination should not
be the main termination pathway.
Moreover, in the reactions with benzaldehyde, the lack of
stoichiometric amounts of methylbenzophenone, the amide
C6H5C(O)NEtMe, or butylbenzophenone with complexes 1, 2,
or 3, respectively, indicates that, upon the insertion of the aldehyde
into the Th-C or the Th-N bond, no concomitant β-H elimina-
tion is operative. Similar reactivity was also observed using the
corresponding deuterium-benzaldehyde. Hence, it seems that a
Th-H motif is not the main active moiety/intermediate in the
reaction pathway with these organoactinides. When comparing the
activities of these complexes as a function of a specific substrate, two
majorconclusionscanbedrawn:electron-withdrawingsubstituents
at the aromatic ring induce better activities and the larger the
coordinative unsaturation at the metal center, the higher the reac-
tivity of the complex. Regarding the activity of the coordinatively
unsaturated complex 3, we were astonished to observe that it is able
to support incipient reactivity moieties such as nitriles, methoxide,
or even the nitro group.
It is important to point out that the lack of oligomeric
compounds implies that, upon formation of the dimer, the
termination (elimination of the ester) must be a thermodynami-
cally driven process, avoiding any further additional insertions.
Moreover, in the competition reaction, no statistical scrambling is
observed, suggesting that the process is governed at the metal
center and that no equilibrium or elimination of an already inserted
aldehyde, if operative, is efficient. A plausible mechanism for the
selective dimerization of aldehydes promoted by the organothori-
um complexes is presented in Scheme 1.
The mechanism consists of a sequence of well-known straight-
forward reactions such as an alkoxy formation via aldehyde insertion
into a metal-carbon or metal-nitrogen σ-bond and σ-bond
metathesis. The high oxophilicity of the thorium complexes initiates
the catalytic process via the addition of two equivalents of aldehyde
through a thermodynamically favored four-centered transition state
(ΔHcalc = -68 kcal/mol), to give the corresponding bis(alkoxo)
complex 8(Scheme 1, step 1). The subsequent addition of aldehyde
One important question that needs to be addressed regards
the fact that no trimer or oligomers were obtained; however, the
RDS is the elimination process via the six-centered transition
state. When complex 11 reacts with an aldehyde, there is only one
process that will yield complex 12. Any hydride transfer from
complex 11 to an incoming aldehyde, at this stage, will regenerate
the same starting materials and hence will have no net effect on
the reaction besides a kinetic delay. However, when an aldehyde
approaches complex 12, two possible pathways are feasible
(Scheme 2). In the first pathway (a) an additional insertion
may take place yielding complex 14, whereas in the second (b) a
hydride transfer will supply back the complex 11 and the ester
product 4. A substantiation indicating a greater preference for the
hydride transfer as compared to an additional aldehyde insertion
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Scheme 2. Two Possible Pathways for the Reaction of Benzaldehyde with Complex 12: (a) Addition to the Th-Alkoxo Bond via a
Four-Centered Transition State and (b) Elimination of Ester Product via Hydride Transfer from the Alkoxo Ligand to the
Incoming Benzaldehyde, Regenerating Catalytic Species 11
can be found in the entropy of activation. From the Eyring plots, the
ΔSq of activation for all studied aldehydes was calculated to
be ∼ -65 eu using complex 3 (Table 3). This result indicates a
highly ordered transition state with considerable bond-making to
compensate the bond-breaking. Thus, the process proceeds with a
high degree of entropic reorganization on approaching the transi-
tion state. In contrast, for reactions in which a four-centered
transition state is observed, a lower entropy of activation is acquired.
For example, when complex 3 was utilized in the hydrosilylation
of alkynes with PhSiH3, the entropy of activation was found
to be -22.06(5) eu.21e Moreover, stoichiometric reactions were
carried out to synthesize independently complex 14 from the
reaction of the corresponding bisalkoxo complex and the aldehyde;
however, complex 14 was never observed. Kinetically, the lack of
formation of complex 14 is very peculiar. This result might indicate
a plausible steric hindrance when an incoming aldehyde is
approaching the metal center. For pathway a, when a four-centered
transition state is forming, the benzylic carbon should reach a spatial
position close to the oxygen atom bonded to the metal, inducing a
tight transition state, whereas for pathway b, the benzylic carbon
reaches a position close to the benzylic hydrogen, creating less
hindrance between the incoming aldehyde and the alkoxo group. A
comparison of the calculated Ea for pathway a (8.5 kcal/mol, see
Figure 10) with the measured Ea (3.47 kcal/mol) for pathway b for
benzaldehyde corroborates our findings that complex 11 and the
corresponding dimer are the kinetic products when complex 12
reacts with an aldehyde and nooligomers are expected to be formed
when ΔEa > 5 kcal/mol.
For the reaction of R-deuterated benzaldehyde with complex 1,
the energy of activation was calculated as EaD = 7.74 kcal/mol, with
an enthalpy of activation of ΔHqD = 7.16 kcal/mol, as well as the
thermodynamic free energy of activations, ΔGq = 21.12(4)
H
kcal/mol and ΔGq = 21.70(4) kcal/mol for benzaldehyde and
D
deuterated benzaldehyde (kH/kD ≈ 2.7), respectively. This kinetic
difference between the deuterated and the nonlabeled benzalde-
hyde further supports that the RDS involves a hydride transfer to an
incoming monomer.
As we have described in Scheme 1, the most probable RDS of the
catalytic cycle is a hydride transfer from the organometallic moiety
to an incoming aldehyde. Therefore, we expect that substituents
with electron-withdrawing capabilities will enhance this process,
and indeed this is observed (Table 1). Moreover, the Ea values for
the different substituents exhibit the same trend, although there is
not a very high difference between the para and meta substituents
(Table 3). Hence, the reaction of two aldehydes competing for the
active site is interesting. When two different aldehydes were reacted
(benzaldehyde and p-Me-benzaldehyde) with different substrate
feeds, the homodimer 4, corresponding to benzaldehyde, was
obtained always in larger yields. Benzaldehyde is expected to be
more active than p-Me-benzaldehyde and hence will accept the
hydride faster. When a larger amount of the p-Me-benzaldehyde
was in the feed, the amounts of the esters were similar, indicating
that, in principle, tailoring a product might be possible. In the case
when benzaldehyde accepts the hydride transfer faster, the follow-
ing insertion will indicate the preference for the addition between
the two aldehydes (competing in a rapid step). Comparison of the
amounts of products 4 and 7 in Table 2 reveals that the insertion of
benzaldehyde is also partially preferred, since this competition
occurs via a four-centered transition state.
When terephthalaldehyde was used, a polymer was obtained
with DP = 25. Based on the mechanism proposed in Scheme 1, it is
very plausible that the polymer is obtained by the consecutive
insertions of esters having two aldehyde functional groups at the
chain ends. Hence, the two chemoselective types of insertions are
observed as depicted in eq 6.
At this point, DFT calculations were performed to find out the
thermodynamically as well as kinetically most favored pathway of
the reaction mechanism and to gain insight into the structure and
bonding of various probable key intermediates/transition states
that might be involved in the proposed catalytic cycle (Scheme 1).
Calculations were performed with a model of the highly reactive
Since the reaction of the R-deuterated benzaldehyde did not
yield another product besides the corresponding deuterated ester,
since no deuterium was scrambled at the substrate or at the
organometallic moiety, and since both reactions have the same
reaction profile (Figure 6), we can calculate the energy of activation
for the deuterated reaction using the probability kinetic equation
(eq 14) in addition to the free energy ΔGq for both processes.
kH
aD - EaH=RT
¼ eE
ð14Þ
kD
where kH and kD are the first-order rate constants for the
benzaldehyde and the R-deuterated benzaldehyde, respectively,
and EaD and EaH are the energies of activation for the reaction of
R-deuterated benzaldehyde and benzaldehyde, respectively.
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catalytic species 3, but to minimize computational expense, the Cp00
moiety was replaced by Cp (C5H5), so the final calculated complex
was Me2SiCp2Th(C4H9)2 (30), which should not have a major
effect on the overall reaction course. Before doing any intense
study, first we tried to validate the level of theory by comparing the
geometry-optimized parameters of complex 3 with available crystal
we were not concerned with the kinetic barriers for the various steps
because our prime interest lies in the actual catalytic cycle, and
indeed the formation of the actual catalytic species was found to be a
highly thermodynamically driven process (Figure 8).
The actual catalytic cycle (Scheme 1, steps 4 and 5) has been
studied explicitly considering the two possible termination path-
ways: (a) via a six-centered hydride-transfer transition state
(Scheme 1, TS3) or (b) through a β-hydride elimination process
(Scheme 1, TS4). For calculation efficiency, we have investigated at
any time only one aldehyde insertion into the Th-O bonds of the
various catalytic species, and the energy profile diagram is shown in
Figure 9. In the first step, insertion of an aldehyde molecule into the
Th-alkoxo bond of the catalytic species 110 takes place via a highly
ordered four-centered transition state (Figure 9, TS2), with a
kinetic barrier of þ6.0 kcal/mol, to form the unsymmetrical
alkoxo-ether intermediate 110a, which may undergo an addition
of another aldehyde via a similar four-centered transition state
(Figure 9, TS2a), to produce the symmetrical Th-bis(alkoxo)
intermediate 120, or hydride transfer may take place from the alkoxo
ligand to the incoming benzaldehyde via a six-centered transition
state (Figure 9, TS3), to regenerate the catalytic species 110 along
with the elimination of the dimeric ester product 4. At this stage, the
kinetic barriers of 6.3 and 13.5 kcal/mol for TS2a and TS3,
respectively, favor the formation of 120 over regeneration of 110.
However, the addition of another aldehyde to 120 may again lead to
the regeneration of complex 110 via the six-centered transition state
(Figure 9, TS3a, þ13.7 kcal/mol). Therefore, the calculated
activation barriers for the transition-state structures TS2a, TS3,
and TS3a (Figure 9) suggest that, during the course of the reaction,
there may be an equilibrium between species 110a and 120, and
considering the steric hindrance, regeneration of the active catalytic
species 110a along with elimination of ester product 4 might
preferably take place from 110a. Alternatively, β-hydrogen elimina-
tion from the alkoxo-ether ligand on 110a or 120 may also produce
the dimeric ester product 4, along with the formation of the
Th-hydride intermediates 130a and 130 (Figure 9, Path B), which
with additional aldehyde insertions will regenerate the catalytic
species 110. These pathways, as seen in Figure 9, are energetically
unfavorable, leading to the formation of thermodynamically highly
structure data for a closely related molecule, Me2SiCp00 Th-
2
(CH2SiMe3)2,13a and we found an excellent match (1.5-2.5%
deviation, see Supporting Information). For our calculations, the
proposed catalytic cycle is divided into two parts: (a) activation of
precatalyst 3 (Scheme 1, steps 1-3) and (b) the actual catalytic
cycle (Scheme 1, steps 4 and 5 and Path B). During the course of
the reaction, benzaldehyde molecules may approach the metal
center in two different ways: (i) frontal approach between the two
non-Cp ligands (Figure 7-i) or (ii) lateral approach betweenthe Cp
and non-Cp ligands (Figure 7-ii). We have found that the frontal
approach (Figure 7-i) is the less energetic as well as the sterically
more favored path.
The potential energy surface of the first part of the mechanism is
shown in Figure 8. Insertion of two aldehyde molecules into the
Th-C bond to form the Th-alkoxo complex 80 was found to be
very exothermic in nature, which is expected due to the high
oxophilicity of the thorium atom. Subsequent addition of aldehydes
to the Th-O bond is not highly exothermic but still is found to be
an energetically favored process to form complex 90, which further
reacts exothermically with another two moles of aldehyde to
produce the catalytic species 110 along with the elimination of the
first ester product 10. For this first part of the reaction mechanism,
Figure 7. Possible paths for benzaldehydes to approach the metal
center during the course of the reaction: (i) frontal approach and (ii)
lateral approach.
Figure 8. Potential energy surface for the formation of active catalytic species 110 (model of 11) from precatalyst 30 (model of 3).
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Figure 9. Potential energy surface for the possible pathways in the catalytic cycle.
Figure 10. Potential energy surface for the third addition of benzaldehyde across the Th-O bond of catalytic intermediates 110a and 120.
unstable Th-hydride intermediates, 130a and 130, and may have
even higher kinetic barriers. Therefore, in the catalytic cycle, the β-
hydrogen elimination step is quite unlikely. It is worth mentioning
here that, in each step of the reaction mechanism, prior to the
addition of aldehydes to the Th-O bonds in 110 and 120 a slightly
stable aldehyde adduct (Figure 9, 110i and 120i) has been observed,
evident of the high oxophilicity of the Th motif, which is probably
the main driving force of the reaction.
Interestingly, the calculated energy profile diagram
(Figure 10) for the third addition of benzaldehyde to the
intermediate species 120 or to 110a reveals that the kinetic barrier
for the process is not much higher (þ3.5 kcal/mol); however, the
intermediates formed, 140 or 140a (Figure 10), are thermodyna-
mically unstable. Therefore, this is more likely to be a thermo-
dynamically controlled process rather than kinetically controlled,
and hence addition of a third mole of benzaldehyde across the
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bond is found to form by the dative σTh-O interaction, which has
about 94% ligand orbital contribution due to the σ donation from
the O hybrid orbital of 64% s- and 36% p-character toward the
metal hybrid orbital having 91% d- and 7% f-character
(Figure 11b).
Similarly, in the complex 120, the σ-donation from the O hybrid
orbitals (61% s- and 39% p-character) to Th hybrid orbitals (90%
d- and 8% f-character) leads to the formation of the dative σTh-O
bond. In complexes 110 and 120, apart from the dative σ-donation, a
significant π-donation is also observed from the O lone pair (pure
p-character) to the symmetry-adapted vacant d-orbital of the metal
center (Figure 11c) and a weaker donation from the p-orbital of the
O atom to the vacant symmetry-adapted f-orbital of the metal
(Figure 11d). It is worth mentioning here that, in comparison to
complexes 110 and 120, precatalyst 30 shows a higher degree of
metal orbital participation (15%), with around 12% f-orbital
contribution in the Th-ligand (non-Cp) interaction, indicating
greater covalency.
Some very interesting subtleties can be found in support of the
formation of four- and six-centered transition states (Scheme 1,
TS2 and TS3). During the formation of the four-centered transi-
tion states, as the aldehyde molecule approaches the metal center
(2.47 Å), a bond-forming interaction has been observed, resulting
from significant donation of electron density from the O lone pair
(O(71), Figure 11e) of the sp-hybrdized orbital (54% s- and 46%
p-character) to the antibonding metal hybrid orbital (89% d- and
10% f-character) (Figure 11e). At the same time, weakening of the
dative σ-donation of the Th-alkoxo bond is observed; conse-
quently, the Th-O bond length (Th(20)-O(22), Figure 11e)
increases to 2.41 Å from 2.15 Å in 110. Another strong donation is
observed from the lone pair of the alkoxo oxygen (O(22),
Figure 11f) (84% p-character) to the lowest unoccupied molecular
orbital (LUMO) of the carbonyl carbon (p-orbital) (C(70),
Figure 11f) of the incoming benzaldehyde. It was interesting to
observe a significant drift of electron density from the C(70)-O-
(71) bond pair of the incoming benzaldehyde to the metal vacant
d-orbital, which further supports weakening of the C-O double
bond character with simultaneous bond-forming interaction be-
tween the O atom (O(71)) and the Th metal center. The NBO
population analysis of the transition state (TS2) indicates an
increase in electron density (natural) on the Th atom from
þ2.05 for 110 to þ1.88 in TS2 due to donation of more electron
density from the incoming benzaldehyde, while drift of electron
density from the alkoxo O atom (O22, Figure 11e,f) to the
aldehydic carbon (C(70)) results in the decreases of electron
density in O(22) from -0.874 for 110 to -0.772 in TS2. In TS2, an
increase in electron density on the aldehydic C atom (C(70)) was
expected, as it received electron density from the alkoxo O atom
(O(22)), but in contrast, it decreases from þ0.394 for pure
benzaldehyde to þ0.452, while on the O atom of the interacting
benzaldehyde it increases from -0.529 for pure benzaldehyde
to -0.677. This discrepancy may not be unusual, as there is a strong
net drift of electron density from the aldehydic unit (O lone pair as
well as C-O bond pair) to the Th metal center. These observa-
tions clearly indicate new bond-forming interactions between Th
and O of the incoming benzaldehyde and between the alkoxo
oxygen of the ligand and the benzaldehyde C atom, as well as
simultaneous bond-breaking/weakening interaction of the Th-
alkoxo bond in this four-centered transition state (Figure 11e,f).
During the formation of the six-centered transition states (TS3
and TS3a), the incoming benzaldehyde shows a similar type of
interaction toward the metal center as in the four-centered
Figure 11. Various molecular orbitals obtained during bond formation in
different spcies in the catalytic cycle: (a) dative σ-bond formation between
C of the butyl ligand and Th in the precatalyst 30; (b) dative σ-bond
formation between O of the alkoxo ligand and Th in the precatalyst 110;
(c) π-donation from O of the alkoxo ligand p-orbital to the Th d-orbital in
120 (similar donation also observed for 120); (d) weak π-donation from O
of the alkoxo ligand p-orbital to the Th f-orbital in 110 (similar donation
also observed for 120); (e) σ-donation from the sp hybrid orbital of O of
incoming benzaldehyde to the Th d-orbital in TS2; (f) lone-pair donation
from O of the alkoxo ligand to the LUMO of the carbonyl carbon atom of
incoming benzaldehyde in TS2; (g) σ-donation from the sp hybrid orbital
of O of incoming benzaldehyde to the Th d-orbital in TS3; (h) donation of
a C-H bond pair to the LUMO of the carbonyl carbon atom of incoming
benzaldehyde in TS3.
Th-O bond is unlikely, which is in agreement with the experi-
mental results.
To gain insight into the electronic behavior of various inter-
mediate complexes as well as formation of transition states
(Scheme 1), Natural Bond Orbital (NBO) analyses were carried
out, and in the following section we mainly discuss the bonding
interaction between non-Cp ligands and the Th center in various
catalytic species, skipping the well-established bonding of the
[(C5H5)2An]þ fragment. In complex 30, the σTh-C(butyl) bond is
formed by 85% contribution from the ligand C-hybrid orbital (72%
p-character) and 15% contribution of the metal hybrid orbital,
which consists of 69% d-character along with a significant contribu-
tion of 12% f-character (Figure 11a). In complex 110, ametal-ligand
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transition states. A strong donation for a bond-forming interac-
tion is observed from the O lone pair of the incoming benzalde-
hyde to the antibonding metal hybrid orbital (84% d- and 12%
f-character) (O(91)-Th(20), Figure 11g). It is interesting to
note that the O lone pair is in an sp-hybrdized orbital rather than
in a pure p-orbital, which further indicates weakening of the
C-O double-bond character in the incoming benzaldehye unit.
A significant decrease in the dative σ-donation and almost
absence of π-donation from the alkoxo O atom of the ligand
(involved in TS3 and TS3a formation) to the metal center is
observed, and consequently the Th-alkoxide bond length
increases to 2.42 Å in TS3 (2.41 Å in TS3a) from 2.17 Å in
complex 120. Apart from these, the expected hydride shift is well
evident from a strong donation of the C(53)-H(54) bond pair
toward the LUMO of the carbonyl carbon (C(90), Figure 11h)
of the incoming benzaldehyde.
published methods.13a,33 Benzaldehyde, o-tolualdehyde, m-tolualdehyde,
p-tolualdehyde, o-furaldehyde, acetaldehyde, phenylacetaldehyde, valeral-
dehyde, and cyclohexanaldehyde (Aldrich) were distilled over sodium
bicarbonate and stored inside a glovebox. p-Chlorobenzaldehyde, o-
nitrobenzaldehyde, m-nitrobenzaldehyde, and p-nitrobenzaldehyde
(Aldrich) were crystallized from a 1:3 mixture of EtOH/H2O, sublimed,
and stored in glovebox.
NMR spectra were recorded on Avance 500 and Avance 300 Bruker
spectrometers. Chemical shifts for 1H and 13C are referenced to internal
solvent resonances and are reported relative to tetramethylsilane. All
reactions monitored by NMR were performed in J. Young Teflon valve-
sealed NMR tubes. GC/MS experiments were conducted in a GC/MS
(Finnigan Magnum) spectrometer with a J&W DB5 column (30 m ꢁ
250 μm).
Synthesis of Me2SiCp002Th(OCH2C6H5)2 (3a). Under an argon
flush, 0.65 mL (6.2 mmol) of C6H5CH2OH was syringed into a 50-mL
Schlenck flask containing a magnetic stir bar and a solution of 2 g (3.10
mmol) of Me2SiCp002ThBu2 cooled at -78 °C. The mixture was stirred
for 1 h, and the solvent was replaced by 15 mL of hexane, keeping the
solution cold at all times. Cold filtration (-78 °C) afforded a white
precipitate. Yield: 1.82 g (0.82%). 1H NMR (300 MHz, C6D6): δ 0.67
(s, 6H), 1.78 (s, 12H, Cp00), 2.10 (s, 12H, Cp00), 3.64 (s, 4H), 6.45-722
(m, 10H) Anal. Calcd for C34H44O2SiTh: C, 54.83; H, 5.95. Found: C,
54.90;H, 5.91.
’ CONCLUSIONS
The results presented in this study have opened a new window in
the field of catalytic activities of organoactinide complexes contain-
ing oxygenated substrates. The main feature is the unexpected
activation of thorium-alkoxide bonds, which was believed to be a
dead-end in organoactinide chemistry. We have found that the
complexes 1-3 are active for the dimerization of aldehydes with
Synthesis of Me2SiCp002Th(OnBu)2 (3b). Under an argon flush,
0.68 mL (6.2 mmol) of nBuOH was syringed into a 50-mL Schlenck flask
containing a magnetic stir bar and a solution of 2 g (3.10 mmol) of
Me2SiCp002ThBu2 cooled at -78 °C. The mixture was stirred for an
additional hour, and the solvent was replaced by 15 mL of hexane,
keeping the solution cold at all times. Cold filtration (-78 °C) afforded
a white precipitate. Yield: 1.57 g (0.75%). 1H NMR (300 MHz, C6D6): δ
0.72 (s, 6H), 1.01 (t, J = 12 Hz, 6H, CH2CH3), 1.33 (m, 4H), 1.48 (m,
4H), 1.97 (s, 12H, Cp00), 2.17 (s, 12H, Cp00), 3.53 (t, J = 12 Hz, 4H)
Anal. Calcd for C28H48O2SiTh: C, 49.69; H, 7.15. Found: C, 49.20; H,
7.39.
Tishchenko Reaction: General Procedure. In a glovebox, a J.
Young NMR tube was charged with one of the complexes 1-3 (0.01
mmol), dissolved in 50-100 μL of NMR solvent (toluene-d8/benzene-
d6). To the solution was added 0.98 mmol of the corresponding aldehyde,
and the mixture was then immediately diluted to 500 μL with toluene-d8/
benzene-d6. The progress of the reaction was monitored continuously with
the help of 1H NMR. After 24-48 h, the tube was opened to air and the
reaction quenched with methanol. The products were identified by NMR
and GC/MS analysis of the crude reaction mixture.
Esterification of Benzaldehyde. Dimerization of benzaldehyde
to produce benzyl benzoate was carried out with 100 μL of benzalde-
hyde (0.98 mmol) and 0.01 mmol of complex 1, 2, or 3 following the
general procedure described above. The benzyl benzoate was obtained
exclusively in 65 and 85% yield for complexes 1 and 2, respectively, after
48 h, while for complex 3, a 96% yield was observed after 24 h of reaction.
Characterization Data for Benzyl Benzoate. 1H NMR (500 MHz,
C6D6): δ 5.14 (s, 2H, -CH2), 7.21-7.01 (m, 8H, Ph), 8.04 (d, J = 7.7 Hz,
2H, Ph). 13C NMR (125 MHz, C6D6): δ 68.4, 129.9, 130.1, 130.3, 130.5,
131.9, 132.6, 134.8, 138.5, 167.6. MS: m/z 212 (Mþ), 105 (PhCOþ), 91
(PhCH2þ).
the organoactinide complex Me2SiCp00 ThBu2 (3), having high
2
activity and, more surprisingly, allowing the dimerization of sub-
strates like furanaldehyde, cyanobenzaldehyde, and nitrobenzalde-
hyde. To our knowledge, this is the first example of a catalytic
process for actinide complexes in which such types of functional
groups have been part of a substrate.
The lower values of Ea and ΔHq, and the much lower negative
value of ΔSq, obtained for complex 3 as compared to those
obtained for complex 1 clearly indicate the positive impact of
obtaining a more coordinatively unsaturated complex by introdu-
cing the bridging ligand. However, at the same time, we were
surprised that the more active complex also allowed the more
reactive substrates to undergo chemoselective processes without
any decomposition or formation of different catalytic products.
Our vision to induce organoactinide complexes to be active
toward substrates containing a nucleophilic motif was based on
developing a thermoneutral reaction. Since actinides are ex-
pected to undergo metathesis-type reactions, following a four-
centered transition state, a reaction in which the same type of
metal-moiety bonds are made and broken is expected to exhibit
this requirement to a good extent. Hence, if this is achieved, the
reaction will be directed by entropy parameters, and taking into
account the enthalpy of the reaction, we can anticipate that a new
catalytic process will be afforded.
’ MATERIALS AND METHODS
All manipulations of air-sensitive materials were performed with the
rigorous exclusion of oxygen and moisture in flamed Schlenk-type glass-
ware on a dual-manifold Schlenk line, or interfaced to a high vacuum (10-5
Torr) line, or in a nitrogen-filled Vacuum Atmospheres glovebox with a
medium-capacity recirculator (1-2 ppm O2). Argon and nitrogen were
purified by passage through a MnO oxygen-removal column followed by a
4 Å molecular sieve column. All the solvents, benzene-d6, THF-d8, and
toluene-d8, were distilled under nitrogen from Na/K alloy. Hydrocarbon
solvents for vacuum line manipulations were stored in vacuo over Na/K
alloy in resealable bulbs. The syntheses of (Cp*)2ThMe2 (1), Th{N-
(CH3)CH2CH3}4 (2), and Me2SiCp002ThBu2 (3) were done according to
Esterification of p-Tolualdehyde. The esterification product
(40-methylbenzyl)-4-methylbenzoate was obtained by the dimerization
of 4-methylbenzaldehyde (0.98 mmol) in the presence of 0.01 mmol of
complex 1, 2 or 3 following the general procedure described above. The
product was obtained exclusively in 65 and 82% yield for complexes 1
and 2, respectively, after 48 h, while for complex 3, a 74% yield was
observed after 24 h of reaction.
Characterization Data for (40-Methylbenzyl)-4-methylbenzoate.
1H NMR (500 MHz, C6D6): δ 1.84 (s, 3H, -CH3), 1.93 (s, 3H, -CH3),
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5.08 (s, 2H, -CH2), 6.74 (d, J = 7.7 Hz, 2H, Ph), 6.83 (d, J = 7.7 Hz, 2H,
Ph), 7.08 (d, J = 7.7 Hz, 2H, Ph), 7.98 (d, J = 7.7 Hz, 2H, Ph). 13C NMR
(125 MHz, C6D6): δ 22.5, 22.8, 68.0, 130.2, 130.4, 130.8, 130.9, 131.5,
135.3, 139.3, 144.9, 167.7. MS: m/z 240 (Mþ), 119 (ArCOþ), 105
(ArCH2þ), 91 (MePhþ).
Esterification of m-Tolualdehyde. The esterification product
(30-methylbenzyl)-3-methylbenzoate was obtained by the dimerization of
3-methylbenzaldehyde (0.98 mmol) in the presence of 0.01 mmol of
complex 1, 2, or 3 following the general procedure described above. The
product was obtained exclusively in 20 and 75% yield for complexes 1 and
2, respectively, after 48 h, while for complex 3, 94% yield was observed after
24 h of reaction.
Hz, 1H, Ph), 7.07 (t, J = 8.4 Hz, 2H Ph), 6.86 (dd, 2H, Ph), 6.61-6.74
(m, 3H, Ph). MS: m/z 280 (Mþ).
Esterification of p-Methoxybenzaldehyde. The esterification
product (40-methoxybenzyl)-4-methoxybenzoate was obtained by the
dimerization of 0.98 mmol of 4-methoxybenzaldehyde in the presence of
6.5 mg (0.01 mmol) of complex 3 as catalyst following the general
procedure described above. The product was obtained exclusively in
22% yield (NMR) after 12 h of reaction.
Characterization Data for (40-Methoxybenzyl)-4-methoxybenzo-
ate. 1H NMR (500 MHz, C6D6): δ 5.20 (s, 2H, -CH2), 3.15 (s, 3H,
-CH3), 3.03 (s, 3H, -CH3) 6.51 (d, J = 8.7 Hz, 2H, Ph), 6.61 (d, J = 8.7
Hz, 2H, Ph), 7.08 (d, J = 7.8 Hz, 2H, Ph), 8.03 (d, J = 7.8 Hz, 2H, Ph).
13C NMR (125 MHz, C6D6): 57.2, 66.3, 115.2, 125.3, 129.7, 132.6,
136.1, 160.9, 168.0. MS: m/z 272 (Mþ).
Esterification of p-Cyanobenzaldehyde. The esterification
product (40-cyanobenzyl)-4-cyanobenzoate was obtained by the reaction
of 0.98 mmol of 4-cyanobenzaldehyde and 6.5 mg (0.01 mmol) of the
catalyst 3 following the general procedure described above. The product
was obtained exclusively in 86% yield (NMR) after 12 h of reaction.
Characterization Data for (40-Cyanobenzyl)-4-cyanobenzoate. 1H
NMR (500 MHz, CDCl3): δ 5.47 (s, 2H, -CH2), 8.22 (d, J = 6.0 Hz, 2H,
Ph), 7.80 (d, J = 9.0 Hz, 2H, Ph), 7.74 (d, J = 9.0 Hz, 2H, Ph), 7.58 (d, J =
6.0 Hz, 2H, Ph). 13C NMR (125 MHz, CDCl3): δ 66.2, 116.8, 117.8,
118.3, 128.4, 130.2, 132.3, 132.5, 134.3, 140.5, 165.2. MS: m/z 262
(Mþ), 131 (ArCOþ), 90 (ArCH2þ).
Characterization Data for (30-Methylbenzyl)-3-methylbenzoate.
1H NMR (500 MHz, C6D6): δ 1.96 (s, 3H, -CH3), 2.04 (s, 3H,
-CH3), 5.21 (s, 2H, -CH2), 6.90-7.05 (m, 6H, Ph), 8.00-8.01 (m,
2H, Ph). 13C NMR (125 MHz, C6D6): δ 20.9, 21.2, 66.9, 125.9, 127.4,
128.6, 128.8, 129.1, 129.5, 130.6, 130.8, 133.9, 136.7, 138.3, 138.4, 166.3.
MS: m/z 240 (Mþ), 119 (ArCOþ), 105 (ArCH2þ), 91 (MePhþ).
Esterification of o-Tolualdehyde. The esterification product
(20-methylbenzyl)-2-methylbenzoate was obtained by the catalytic
dimerization of 0.98 mmol of 2-methylbenzaldehyde and 0.01 mmol
of complex 1, 2, or 3 following the general procedure described above.
The product was obtained exclusively in 10 and 50% yield for complexes
1 and 2, respectively, after 48 h, while for complex 3, 70% yield was
observed after 24 h of reaction.
Characterization Data for (20-Methylbenzyl)-2-methylbenzoate.
1H NMR (500 MHz, C6D6): δ 2.07 (s, 3H, -CH3), 2.58 (s, 3H,
-CH3), 5.17 (s, 2H, -CH2), 6.88-7.02 (m, 6H, Ph), 7.24 (d, J = 7.5
Hz, 1H, Ph), 7.98 (d, J = 7.5 Hz, 1H, Ph). 13C NMR (125 MHz, C6D6):
δ 18.9, 22.0, 65.0, 126.0, 126.4, 126.5, 128.8, 129.9, 130.8, 131.6, 132.2,
132.7, 134.9, 137.5, 141.0, 167.1. MS: m/z 240 (Mþ), 119 (ArCOþ),
105 (ArCH2þ), 91 (MePhþ).
Esterification of p-Chlorobenzaldehyde. The esterification
product (40-chlorobenzyl)-4-chlorobenzoate was obtained by the reac-
tion of 0.98 mmol of 4-chlorobenzaldehyde and 0.01 mmol of complex
1, 2, or 3 following the general procedure described above. The product
was obtained exclusively in 60, 85, and 98% yield for complexes 1, 2, and
3, respectively, after 24 h of reaction.
Esterification of p-Nitrobenzaldehyde. The esterification
product (40-nitrobenzyl)-4-nitrobenzoate was obtained by the reaction
of 0.98 mmol of 4-nitrobenzaldehyde and 6.5 mg (0.01 mmol) of the
catalyst 3 following the general procedure described above. The product
was obtained exclusively in 51% yield (NMR) after 6 h of reaction.
Characterization Data for (40-Nitrobenzyl)-4-nitrobenzoate. 1H
NMR (500 MHz, C6D5CD3): δ 4.70 (s, 2H, -CH2), 7.70 (d, J = 9.0
Hz, 2H, Ph), 7.52 (d, J = 9.0 Hz, 2H, Ph), 7.28 (d, J = 9.0 Hz, 2H, Ph),
6.72 (d, J = 9.0 Hz, 2H, Ph). 13C NMR (125 MHz, C6D5CD3): δ 67.1,
117.7, 118.6, 130.6, 131.4, 166.2. MS: m/z 302 (Mþ), 151 (ArCOþ),
137 (ArCH2þ).
Esterification of m-Nitrobenzaldehyde. The esterification
product (30-nitrobenzyl)-3-nitrobenzoate was obtained by the reaction
of 0.98 mmol of 3-nitrobenzaldehyde and 6.5 mg (0.01 mmol) of the
catalyst 3 following the general procedure described above. The product
was obtained exclusively in 62% yield (NMR) after 6 h of reaction.
Characterization Data for (30-Nitrobenzyl)-3-nitrobenzoate. 1H
NMR (500 MHz, C6D5CD3): δ 4.78 (s, 2H, -CH2), 8.58 (s, 1H, Ph),
7.88 (s, 1H, Ph), 7.64 (dd, 2H, Ph), 7.58 (d, J = 6.9 Hz, 1H, Ph),
6.37-6.51 (m, 3H, Ph). 13C NMR (125 MHz, C6D5CD3): δ 68.5,
127.3, 129.1, 129.8, 131.4, 131.6, 133.6, 134.5, 136.4, 139.6, 140.2. MS:
m/z 302 (Mþ), 150 (ArCOþ), 136 (ArCH2þ).
Characterization Data for (40-Chlorobenzyl)-4-chlorobenzoate.
1H NMR (500 MHz, C6D6): δ 4.79 (s, 2H, -CH2), 7.65 (d, J = 9.2
Hz, 2H, Ph), 6.91 (d, J = 9.2 Hz, 2H, Ph), 6.85 (d, J = 9.0 Hz, 2H, Ph),
6.73 (d, J = 9.0 Hz, 2H, Ph). 13C NMR (125 MHz, C6D6): δ 65.1, 129.3,
128.6, 130.0, 130.2, 131.5, 136.1, 134.9, 139.5, 165.0. MS: m/z 280
(Mþ), 139 (ArCOþ), 125 (ArCH2þ).
Esterification of m-Chlorobenzaldehyde. Dimerization of
3-chlorobenzaldehyde to produce (30-chlorobenzyl)-3-chlorobenzoate
was carried out by mixing 0.98 mmol of the aldehyde with 0.01 mmol of
complex 1, 2, or 3 following the general procedure described above. The
product was obtained exclusively in 50, 80, and 95% yield for complexes 1,
2, and 3, respectively, after 24 h of reaction.
Esterification of o-Furanaldehyde. The esterification product
(20-furylmethyl)-2-furancarboxylate was obtained by the reaction of 0.98
mmol of o-furanaldehyde and 6.5 mg (0.01 mmol) of the catalyst 3 at
60 °C. The progress of the reaction was monitored continuously with the
help of 1H NMR. After 48 h, the products were identified by NMR and
GC/MS analysis. The ester product was obtained exclusively in 42% yield
(NMR) after 48 h of reaction at 60 °C.
Characterization Data for (30-Chlorobenzyl)-3-chlorobenzoate.
1H NMR (500 MHz, C6D6): δ 4.85 (s, 2H, -CH2), 8.06 (s, 1H, Ph),
7.73 (d, J = 9.0 Hz, 1H, Ph), 7.14 (s, 1H, Ph), 7.01 (t, J = 7.8 Hz, 1H, Ph),
6.75 (d, J = 9.0 Hz, 1H, Ph), 6.61-6.77 (m, 3H, Ph). 13C NMR (125
MHz, C6D6): δ 66.5, 128.3, 129.1, 131.0, 131.5, 130.6, 135.4, 136.7,
140.5, 166.0. MS: m/z 280 (Mþ), 139 (ArCOþ).
Characterization Data for (20-Furylmethyl)-2-furancarboxylate.
1H NMR (500 MHz, C6D5CD3): δ 5.02 (s, 2H, -CH2), 5.96-6.07
(m, 1H, aromatic), 5.96-6.04 (m, 2H), 6.11-6.15 (m, 1H, aromatic),
6.97-7.08 (dd, 2H, aromatic). 13C NMR (125 MHz, C6D5CD3): 58.0,
110.8, 111.2, 111.7, 118.2, 137.5, 142.2, 143.3, 146.3, 153.6. MS: m/z
190 (Mþ).
Esterification of o-Chlorobenzaldehyde. The esterification
product (20-chlorobenzyl)-2-chlorobenzoate was obtained by the reac-
tion of 0.98 mmol of 2-chlorobenzaldehyde and 0.01 mmol of complex
1, 2, or 3 following the general procedure described above. The product
was obtained exclusively in 57, 78, and 89% yield for complexes 1, 2, and
3, respectively, after 24 h of reaction.
Esterification of Terephthalaldehyde. The esterification pro-
duct poly[p-(carboxymethylene)phenylene] was obtained by the reac-
tion of 0.98 mmol of terephthalaldehyde and 6.5 mg (0.01 mmol) of the
catalyst 3 in toluene. The product was precipitated out of the solution,
Characterization Data for (20-Chlorobenzyl)-2-chlorobenzoate.
1H NMR (500 MHz, C6D6): δ 5.30 (s, 2H, -CH2), 7.64 (d, J = 8.4
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filtered off, and washed with toluene 3-4 times prior to analysis. The
product was obtained in 60% yield (NMR) after 6 h of reaction.
Characterization Data for Polyester of Poly[p-(carboxy-
methylene)phenylene]. 1H NMR (500 MHz, C2D2Cl4): δ 4.63-4.71
(m, 18H, -CH2), 5.29-5.37 (m, 28H, -CH2), 7.20-7.46 (m, 52H, Ph),
8.00-8.07 (m, 40H, Ph) 9.92 and 9.95 (s, 2H, -CHO). 13C NMR (125
MHz, C2D2Cl): δ 64.4, 64.8, 65.7, 66.2, 66.3, 126.4, 127.1, 127.5, 128.0,
128.3, 129.6, 129.8, 129.9.
Cross-Esterification. In a typical experiment, the NMR tube was
charged in the glovebox with 5 mg of Cp2*ThMe2 (0.0092 mmol) and
dissolved with benzene-d6, benzaldehyde (0.095 mL, 0.94 mmol; 0.047
mL, 0.47 mmol), and p-tolualdehyde (0.105 mL, 0.94 mmol; 0.052 mL,
0.47 mmol). The reaction was allowed to stand at room temperature for 2
days, and the products were identified by 1H NMR. The ratio between the
four products was calculated by integration of the four benzylic signals
appearing at δ 5.13 ppm for (40-methylbenzyl)-4-methylbenzoate (5), δ
5.12 ppm and δ 5.11 ppm for 6 and 7, and δ 5.09 ppm for benzyl benzoate
(4). MS: m/z 226 (Mþ).
Recycled Experiment. To determine the longevity of the catalyst,
typical recycled experiments were carried out with the catalyst 3. After
completion of one batch of dimerization reaction, the NMR tube was dried
overnight in a high-vacuum line, and a reddish brown mass was obtained.
The tube was then taken inside a glovebox, the reddish brown mass was
dissolved in C6D6, and a new batch of benzaldehyde (0.098 mmol) was
added. The reaction was again monitored by 1H NMR spectroscopy, and
the product was isolated as described in the general procedure.
Kinetic Study of the Esterification of Benzaldehyde. In a
glovebox, a J. Young NMR tube with a Teflon valve (previously flamed
in vacuum) was charged with complex Cp2*ThMe2 (5-40 mg,
0.0092-0.074 mmol as a solution in benzene-d6) or Me2SiCp002Th-
Bu2 (1.90 mg, from a 110.5 mM stock solution) and benzaldehyde
(0.03- 0.200 mL, 0.295-1.88 mmol). The mixture was then im-
mediately diluted to 400 μL with benzene-d6/toluene-d8. The tube
was frozen in liquid nitrogen until NMR measurements were initiated.
The tube was defrosted and quickly inserted into the NMR probe,
which had been maintained at 30 °C, and an initial spectrum was
recorded. Data were acquired using eight scans per time interval
sequence. The progress of the reaction was monitored by the
disappearance of the aldehyde signal around δ 9.56 ppm and the
appearance of the proton R to carbonyl around δ 5.06 ppm over three
half-lives. The concentration of the coupling product at time t was
determined from the area of the 1H integrals of the product (R
protons to carbonyl group), standardized to the total area of reso-
nances of the aldehyde proton of the starting material and the product.
Influence of the Catalyst Concentration. The influence of
catalyst concentration on the reaction rate at a specific temperature was
determined by using different catalysts (1 and 3) and concentrations
(7.37 ꢁ 10-3-2.95 ꢁ 10-2M), while the concentration of the aldehyde
(7.37 ꢁ 10-1 M) and temperature (30 °C) were kept constant. The rate
constant, kobs (M s-1), was calculated from the pseudo-linear part of the
plot ofthe product concentration vstime. Thekobs showed a linear increase
with increasing catalyst concentration. Hence, we deduce that the reaction
is first-order dependent on the catalyst.
concentration of benzaldehyde up to 5.39 M from 2.45 M while the
concentration of the catalyst 3 was kept constant (7.37 ꢁ 10-3 M).
Influence of the Temperature. The influence of temperature
on the reaction rate was investigated by carrying out the Tishchenko
reaction at different temperatures (35, 55, 75, and 95 °C) with constant
concentrations of the catalyst (7.37 ꢁ 10-3 M) and aldehyde (7.37 ꢁ
10-1 M).
A plot of product concentration vs time was made, and from the
pseudo-linear part of the plot, the rate constant, kobs (M s-1), was
calculated at each temperature. The energy of activation for the RDS was
calculated from the Arrhenius equation (ln k = ln A0 - Ea/RT, where R is
the gas constant, 1.987 cal mol-1 K-1). From the slope of the linear plot
of ln(kobs) as a function of 1/T, the value -Ea/R can be found, and the
activation energy for the rate-determining step can be calculated.
The enthalpy of activation (ΔHq) and entropy of activation (ΔSq) for
the RDS were calculated by using the Eyring equation,
q
q
k ¼ ðkT=hÞ e- ΔH =RT eΔS =R
f
lnðk=TÞ ¼ lnðk=hÞ - ΔHq=RT þ ΔSq=R
where κ is the Boltzmann constant, 1.38 ꢁ 10-23 J/K, and h is Planck's
constant, 6.626 ꢁ 10-34 J s.
3
Therefore, the slope of the plot ln(k/T) vs 1/T represents the
value -ΔHq/R, and the intersection with the Y-axis is the sum of
ln(κ/h) and ΔSq/R.
In a similar way, the thermodynamic parameters Ea, ΔHq, and ΔSq
were also determined for para- and meta-substituted chloro- and
methylbenzaldehydes.
Kinetic Isotopic Effect. In a typical experiment, a 5 mm NMR
tube was charged in a glovebox with 1 (5 mg, 0.0092 mmol as a solution
in benzene-d6), benzaldehyde and R-d-benzaldehyde (0.060 mL, 0.593
mmol). Benzene-d6 was then added to bring the total volume of the
solution to 0.5 mL. The tube was closed, quickly removed from the
glovebox, and maintained in liquid nitrogen until NMR measurements
were initiated. The sample was quickly warmed and inserted into the
probe, and an initial spectrum was recorded. Data were acquired using
eight scans per time interval sequence. The progress of the reaction was
monitored by the disappearance of the aromatic signal of the reactant at
δ 7.8 ppm and the appearance of the product at δ 8.1 ppm over three
half-lives. The concentration of the coupling product at time t was
determined from the area of the 1H integrals of the product standardized
to the total area of resonances of the aldehyde proton of the starting
material and the product. The value kH/kD was calculated from the
pseudo-linear part of each reaction.
Computational Details. The equilibrium molecular structures as
well as transition-state geometries were optimized using the hybrid
density functional B3LYP.34 The Stuttgart basis set in combination with
the 60-core-electron relativistic effective core potential (SDD)35 was
used for thorium; 6-31G*36 basis sets were used for carbon, hydrogen,
and oxygen. The effects of polarization and diffusion functions were
tested on C, H, and O atoms in single-point calculations and found to be
negligible (see Supporting Information). Spin-orbit interactions have
not been considered explicitly. After each successful optimization,
frequency calculations were performed to confirm that the structures
were minima or saddle points. All transition structures contained exactly
one imaginary frequency and were linked to reactant, products, or
intermediates using intrinsic reaction coordinate (IRC)37 calculations.
All thermodynamic data were calculated at the standard state (298.15 K
and 1 atm). The solvent effect was also tested by the PCM (SCRF)
model38 using toluene as solvent and the effect was found to be
negligible (see Supporting Information), which is quite expected due
to the low dielectric constant of toluene. All calculations were carried out
with the Gaussian03 suite of codes.39 All M€ulliken, NBO electron
densities, and bonding analyses were done using the NBO technique.40
Influence of the Benzaldehyde Concentration. To deter-
mine the aldehyde concentration dependence on reaction rate, the reaction
was carried out with a varying concentration of benzaldehyde (7.37 ꢁ
10-1-2.45 M), while the catalyst (1 and 3), concentration (7.37 ꢁ 10-3
M), and temperature (30 °C) were kept constant. The rate constant, kobs
(M s-1), was calculated from the pseudo-linear part of the plot of the
product concentration vs time. The plot of kobs against the concentration
showeda good linear fit, revealing that the reaction is first-orderin aldehyde
concentration.
Further, to determine the limiting catalyst:substrate ratio, in a similar
way as mentioned above, kobs was determined by increasing the
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a second-order perturbation theory.
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’ ASSOCIATED CONTENT
S
Supporting Information. Typical NMR spectra showing
b
concomitant formation of dimeric ester product from benzalde-
hyde, computational details of all structures (Cartesian coordi-
nates, Gibbs free energies), and complete ref 39. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
chmoris@tx.technion.ac.il
’ ACKNOWLEDGMENT
This research was supported by the Israel Science Foundation
Administered by the Israel Academy of Science and Humanities,
under Contract No. 518/09. M.S. thanks the Lady Davis Trust
for a postdoctoral Fellowship. We are also indebted to the
Australian Research Council and the Australian Partnership for
Advanced Computing (APAC) for all computing facilities. Author
M.S. is also thankful to Prof. Peter Boyd (at the University of
Auckland) for valuable suggestions and discussion.
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