New highly active heteroscorpionate-containing lutetium catalysts for the hydroamination of aminoalkenes: Isolation and structural characterization of a dipyrrolidinide-lutetium complex

Article abstract of DOI:10.1021/om2011672

The reactions of the hybrid scorpionate/cyclopentadiene compounds, as a mixture of regioisomers-1-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl]- 1,3-cyclopentadiene and 2-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl]- 1,3-cyclopentadiene (

Full text of DOI:10.1021/om2011672

Article
pubs.acs.org/Organometallics
New Highly Active Heteroscorpionate-Containing Lutetium Catalysts
for the Hydroamination of Aminoalkenes: Isolation and Structural
Characterization of a Dipyrrolidinide−Lutetium Complex
Antonio Otero,*,† Agustín Lara-Sanchez,*,† Carmen Najera,‡ Juan Fernandez-Baeza,†
́
́
́
Isabel Marquez-Segovia,† Jose Antonio Castro-Osma,† Javier Martínez,† Luis F. Sanchez-Barba,†
́
́
́
and Ana M. Rodríguez†
†
́
́
́
́
ica y Bioquımica, Universidad de Castilla-La Mancha, 13071-Ciudad Real, Spain
Departamento de Quımica Inorgan
́
ica, Organ
‡
́
Departamento de Quımica Organ
́
ica, Facultad de Ciencias, and Instituto de Sıntesis Organica (ISO), Universidad de Alicante,
́
Apdo 99, 03080-Alicante, Spain
S
* Supporting Information
ABSTRACT: The reactions of the hybrid scorpionate/cyclopentadiene compounds,
as a mixture of regioisomers1-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenyleth-
yl]-1,3-cyclopentadiene and 2-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl]-
1,3-cyclopentadiene (bpzcpH) and 1-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1-tert-buty-
lethyl]-1,3-cyclopentadiene and 2-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1-tert-butyleth-
yl]-1,3-cyclopentadiene (bpztcpH)with [Lu(CH2SiMe3)3(thf)2] proceed in very
high yields to give the free solvent neutral heteroscorpionate dialkyl lutetium
complexes [Lu(CH2SiMe3)2(bpzcp)] (1) and chiral [Lu(CH2SiMe3)2(bpztcp)] (2).
The structures in solution of 1 and 2 were investigated by VT NMR spectroscopy,
and a fluxional behavior corresponding to an exchange between the alkyl groups was
observed. The lutetium complex [Lu(CH2SiMe3)2(bpztcp)(thf)] (3) was isolated as
an enantiomerically enriched complex. Supramolecular CH−π interactions between
molecules in crystals of 3 have been identified in its X-ray molecular analysis, and they
explain the formation of a conglomerate among molecules of 3. Complexes 1−3 are efficient catalysts for the intramolecular
hydroamination of aminoalkenes, giving TOF values of up to 475 h−1 at 90 °C for 2,2-diphenyl-pent-4-enylamine (4) by using
complex 3 as catalyst. Enantioselectivities up tp 70% ee were achieved in the cyclization of the 1,2-disubstituted olefin 6 with the
high enantiopurity complex 3. The hydroamination reactions show apparently zero-order rate dependence on substrate
concentration and first-order rate dependence on catalyst concentration. Additionally, bicyclization of 2-allyl-2-methylpent-4-
enylamine (10) was achieved at 60 and 100 °C, giving exo,exo-2,4,6-trimethyl-1-azabicyclo[2.2.1]heptane (12). The protonolysis
reaction of complex [Lu(CH2SiMe3)2(bpztcp)] (2) with 2 equiv of 2,2-diphenyl-pent-4-enylamine (4) yielded a dipyrrolidinide
lutetium complex [Lu(NC4H5-2-Me-4,4-Ph2)2(bpztcp)] (13) as a mixture of two diastereoisomers. The structures of the
complexes were determined by spectroscopic methods, and the X-ray crystal structures of 3 and 13 were also established.
involves an intramolecular insertion of the unsaturated sub-
strate into a metal−amido bond,4i,6 via a four-centered transi-
tion state, followed by a rapid protonolysis of the intermediate
lanthanide−alkyl species formed by amine substrates to yield
the azacyclic product and regenerate the catalytically active
species (Figure 1).
The improvement of new transition-metal-based hydro-
amination catalysts with higher activity and selectivity, including
chiral catalysts for enantioselective hydroamination and systems
bearing ancillary ligands that permit facile tuning of catalytic
properties through adjustment of their steric and electronic
features, has remained a challenge.7 Heteroscorpionates are
among the most versatile types of tridentate ligands, and they
can coordinate to a wide variety of elements.8 Chemistry based
INTRODUCTION
■
The hydroamination of C−C multiple bonds is a prominent
and highly atom-efficient reaction for the synthesis of nitrogen-
containing compounds.1 In particular, the enantioselective
intramolecular cyclohydroamination of aminoalkenes or amino-
alkynes has applications in the pharmaceutical and fine chemical
industries.2 This catalytic addition of amine N−H function-
alities to unsaturated carbon−carbon bonds allows the pre-
paration of complex nitrogen-containing compounds in a waste-
free, highly atom-economical manner starting from simple and
inexpensive starting materials. A staggering number of catalytic
systems have been developed for this reaction, and these exhibit
a correspondingly high degree of mechanistic diversity.1−5 In
particular, the intramolecular hydroamination of a variety of
carbon−carbon unsaturations such as alkenes, alkynes, allenes,
and dienes using group 3 and rare-earth-metal complexes is
well documented.1−6 It has been established that this process
Received: November 23, 2011
Published: March 6, 2012
© 2012 American Chemical Society
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(2; bpztcp = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1-tert-butylethyl-
cyclopentadienyl) was achieved through a protonolysis reaction
involving alkane elimination. Thus, the reaction of bpzcpH and
rac-bpztcpH (see Figure S1 in the Supporting Information), as
a mixture of two regioisomers, bpzcpH = 1-[2,2-bis(3,5-
dimethylpyrazol-1-yl)-1,1-diphenylethyl]-1,3-cyclopentadiene
and 2-[2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-diphenylethyl]-
1,3-cyclopentadiene and bpztcpH = 1-[2,2-bis(3,5-dimethylpyr-
azol-1-yl)-1-tert-butylethyl]-1,3-cyclopentadiene and 2-[2,2-bis-
(3,5-dimethylpyrazol-1-yl)-1-tert-butylethyl]-1,3-cyclopentadi-
ene,10d with [Lu(CH2SiMe3)3(thf)2]12 in a 1:1 molar ratio in
toluene at 0 °C yielded the lutetium dialkyl complexes 1 and 2,
respectively, after the appropriate workup procedure, as white
solids in ca. 80% yield (Scheme 1). Complex 2 was obtained as
a racemic mixture.
Scheme 1. Synthesis of Lutetium Dialkyl Complexes 1 and 2
After the preparation of these complexes, we have considered
as a new challenge the preparation from rac-bpztcpH and
[Lu(CH2SiMe3)3(thf)2] of a related complex to 2 but enan-
tiomerically enriched. With this aim in mind, we are tried to
isolate an enantiopure alkyl lutetium complex by diffusion of
a toluene solution of racemic heteroscorpionate precursor
bpztcpH in hexane solution of [Lu(CH2SiMe3)3(thf)2] and
allowed the solution to crystallize slowly without stirring.13
Thus, by using this procedure we have able to isolate the dialkyl
complex [Lu(CH2SiMe3)2(bpztcp)(thf)] (3) enantiomerically
enriched by spontaneous resolution and preferential crystal-
lization of one enantiomer due to the preferential formation
of a conglomerate (see below their X-ray crystal structure
discussion). Complex 3 was obtained as a crystalline colorless
solid in ca. 35% yield from the reaction mixture (Scheme 2). It
is known that crystallization constitutes the most economical
procedure to obtain enantiopure compounds from racemic
mixtures, when the separation of the crystals of an enantiomer
due to the formation of a conglomerate is feasible.14 However,
in ordered three-dimensional crystals the formation of a con-
glomerate is not very frequent (and is even less predictable),
reflecting the tremendous preference (more than 90%) of
compounds to crystallize in centrosymetric space group.15
The alkyl complexes 1−3 were characterized spectro-
Figure 1. Proposed σ-bond insertion mechanism (diamido) for
intramolecular hydroamination.
on the design of this particular type of intriguing ligand has
been extended considerably in terms of coordination
chemistry9 and catalytic applications.10 In the past decade, a
number of research groups have contributed widely to this field,
designing new ligands related to the bis(pyrazol-1-yl)methane
system and incorporating several pendant donor arms.11
Bearing in mind both the versatility of this type of tridentate
monoanionic ligand and the efficiency of rare-earth-catalyzed
hydroamination, we decided to develop hydroamination cata-
lysts based on heteroscorpionate ligands. It was envisaged that
these compounds would have catalytic activity comparable to
that of the lanthanocene systems.
Herein, we describe in detail the synthesis and character-
ization of a series of achiral and chiral heteroscorpionate alkyl
lutetium complexes, one of them enantiomerically enriched,
and the first X-ray characterized dipyrrolidinide rare-earth complex.
The performance of those complexes in the hydroamination/
cyclization of aminoalkenes was explored under well-controlled
conditions.
RESULTS AND DISCUSSION
■
1
scopically. The H and 13C{1H} NMR spectra at room tem-
Syntheses and Structural Characterization. The syn-
thesis of the lutetium dialkyl complexes [Lu(CH2SiMe3)2-
(bpzcp)] (1; bpzcp = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1,1-
diphenylethylcyclopentadienyl) and [Lu(CH2SiMe3)2(bpztcp)]
perature of 1 (nonchiral compound) exhibit a singlet for each
of the H4, Me3, and Me5 pyrazole protons, indicating that the
pyrazole rings are equivalent, alongside two multiplets for the
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Scheme 2. Synthesis of Lutetium Dialkyl Complex 3
Enantiomerically Enriched
cyclopentadienyl protons and one set of signals for the alkyl
groups. A five-coordinate environment for this complex can be
proposed for which a symmetric plane exists: i.e., a square-
1
planar-pyramidal geometry (see Scheme 1). However, the H
and 13C{1H} NMR spectra at room temperature of complexes
2 and 3 (chiral compounds) show two singlets for each of the
H4, Me3, and Me5 pyrazole protons, four multiplets for the
cyclopentadienyl protons, and one set of signals for the alkyl
groups along with two multiplets for the THF protons in
complex 3. Additionally, the methylene protons of complexes 2
and 3, LuCH2SiMe3, are diastereotopic, giving rise to an AB
system (see Figures S3 and S4 in the Supporting Information),
due to the stereogenic carbon, Ca, from the heteroscorpionate
ligand. These results are consistent with a proposed five-
coordinate disposition for the complex 2 and an octahedral
structural disposition for the complex 3 (see Schemes 1 and 2).
Thus, these spectroscopic data indicate that the hybrid
scorpionate/cyclopentadienyl ligands are coordinated in a
facial, “tripodal” fashion with a κ2-NN-η5-Cp coordination
1
Figure 2. Variable-temperature H NMR spectra in the region of the
alkyl groups of complex 1.
of −40 °C and ΔG⧧ = 9.2 0.1 kcal/mol, whereas 2 has a
coalescence point of −60 °C and ΔG⧧ = 8.5 0.1 kcal/mol.
As we have previously commented, complex 3 was isolated
enantiomerically enriched by spontaneous resolution and
preferential crystallization of one enantiomer. We have tried
to confirm the presence in solution of the corresponding two
enantiomers in different proportions by adding different chiral
shift reagents, but unfortunately the results obtained are not
conclusive in order to establish the value of enantiomeric excess
(see Figures S3 and S4 in the Supporting Information).
Additionally, the value for the specific rotation of complex
3 (see the Experimental Section) is consistent with a
enantiomerically enriched complex.
The molecular structure of complex 3 was determined by an
X-ray diffraction study. Complex 3 forms a conglomerate that
crystallizes in the chiral P21 space group. This crystal structure
was determined by using Mo Kα radiation, and the asymmetric
unit contains one independent molecule with the following unit
cell dimensions: a = 10.015(5) Å, b = 19.622(3) Å, c =
10.476(2) Å, and β = 114.135(3)°. The crystals are enantiopure
with regard to the orientation of the stereogenic carbon, Ca,
since the molecule in the asymmetric unit has the S
configuration (four grains of crystals of the lutetium complex
3 were measured and only S complex 3 was detected). The
corresponding ORTEP drawing is depicted in Figure 4.
Crystallographic data and selected interatomic distances and
angles are given in Tables 1 and 2, respectively. The molecular
structure determined by X-ray diffraction is in good agreement
with the solution structure deduced from the NMR experi-
ments. The heteroscorpionate ligand is attached to the lutetium
atom through the two nitrogen atoms of pyrazole rings and the
cyclopentadienyl ring in a κ2-NN-η5-Cp coordination mode in
1
mode. The phase-sensitive H NOESY-1D NMR spectra were
also obtained in order to confirm the assignments of the signals
for the Me3, Me5, and H4 groups of each pyrazole ring. The
assignment of the 13C{1H} NMR signals was made on the basis
of 1H−13C heteronuclear correlation (g-HSQC) experiments. It
is worth noting that the room-temperature 1H NMR spectra of
complexes 2 and 3 show one set of resonances for the two
nonequivalent CH2SiMe3 ligands. This behavior indicates that
the two alkyl ligands exchange rapidly at room temperature.
The dynamic behavior of complexes 1 and 2 was studied by VT
NMR spectroscopy. In the case of complex 1, with an achiral
heteroscorpionate ligand, the VT NMR analysis showed that
the resonance of the methylene protons, Lu−CH2SiMe3,
directly bonded to the lutetium center broaden and become
resolved into two signals at low temperature, giving rise to an
AB system at −70 °C, probably owing to the lack of rotation of
the methylene protons at low temperature. A stacked plot of
the relevant sections of the variable-temperature 1H NMR
spectra of 1 and 2 are shown in Figures 2 and 3.
At room temperature the exchange of the two alkyl groups
for 2 occurs and, accordingly, one set of signals appears. When
the temperature was decreased to below −80 °C, two set of
signals (relative intensities 1:1) were observed for the alkyl
groups (see Figure 3). Free-energy values, ΔG⧧, for complexes
1 and 2 were calculated16 from VT NMR studies. Complex 1
has higher activation barriers, with a coalescence temperature
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1
Figure 3. Variable-temperature H NMR spectra in the region of the alkyl groups of complex 2.
literature.10,17 Supramolecular CH−π interactions (Table 3)
between single molecules in crystals of complex 3 that enable
stereochemical information to be mediated in three dimensions
have been identified and explain the formation of a con-
glomerate among molecules. A self-assembly of the molecules
in complex 3 gives rise to a chain formed along the b axis by
CH−π interactions between a Me unit (C25−H25B) of one
alkyl group and the pyrazole ring (pz2) of the other molecule
(see red lines in Figure 5). The H25B−(centroid of pyrazole)
distance has a value of 2.993 Å. The degree of displacement of
H25B from the center of the pyrazole ring (Doffset) is 1.12 Å.
The assembly of different chains takes place through another
intermolecular CH−π interaction established between the
C29−H29A methyl of the other alkyl group and the pyrazole
ring (pz1) of the other molecule from a neighbor chain (see
violet lines in Figure 5). The H29A−(centroid of pyrazole)
distance has a value of 3.078 Å, and the degree of displacement
of H29A from the center of the pyrazole ring (Doffset) is 1.18 Å.
These interactions give rise to a sheet extended to the c axis
(Figure 5). Self-assembly of the sheets extended to the bc plane
along the a axis by CH−π interactions between a Me group
(C28−H28C) of a alkyl group and the pyrazole ring of the
other molecule from a neighboring sheet (see blue lines in
Figure 6). The H28C−(centroid of pyrazole) distance has a
value of 3.046 Å. The degree of displacement of H28C from the
center of the pyrazole ring (Doffset) is 1.19 Å.
Figure 4. ORTEP view of 3. Ellipsoids are at the 30% probability level,
and hydrogen atoms have been omitted for clarity.
the expected fac coordination fashion. In addition, the lutetium
center is coordinated to two trimethylsilylmethylene groups
and one THF molecule, trans to the cyclopentadienyl ring. The
geometry around the metal center is distorted octahedral,
probably due to the constraints imposed by the chelating
ligand. This substantial distortion is manifested in the angles
N(3)−Lu(1)−N(1), C(22)−Lu(1)−N(3), and C(26)−
Lu(1)−N(1), with values for these angles being 69.9(4),
152.7(5), and 157.4(4)°, respectively. The Lu−C bond lengths
of 2.40(2) and 2.37(1) Å and Lu−N bond lengths of 2.60(1)
and 2.58(1) Å fall within the normal range reported in the
Catalytic Hydroamination Studies. Complexes 1−3 were
tested for catalytic intramolecular hydroamination processes.
They were active for the hydroamination/cyclization of various
aminopentene derivatives (see Tables 4 and 5). The catalyst
1
activity was monitored by H NMR spectroscopy by loading a
NMR tube with catalyst and substrate and heating the reaction
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Table 1. Crystal Data and Structure Refinement Details for 3
and 13
3
13
mol formula
C33H59LuN4OSi2
C55H65
LuN6·2.5(C7H8)
1215.44
formula wt
temp (K)
wavelength (Å)
cryst syst
space group
a (Å)
758.99
230(2)
230(2)
0.710 73
0.710 73
monoclinic
P21
monoclinic
P21/n
9.9720(6)
19.5503(10)
10.5346(6)
114.459(3)
1869.5(2)
2
15.199(2)
25.257(3)
15.904(2)
94.575(7)
6086(1)
b (Å)
c (Å)
β (deg)
V (Å3)
Z
4
calcd density (g/cm3)
1.348
1.326
abs coeff (mm−1
)
2.733
1.670
F(000)
784
2532
crystal size (mm3)
0.19 × 0.16 × 0.09
−11 ≤ h ≤ 11
−22 ≤ k ≤ 23
−12 ≤ l ≤ 12
12 069
0.19 × 0.18 × 0.09
−18 ≤ h ≤ 17
−30 ≤ k ≤ 28
−18 ≤ l ≤ 16
49 098
index ranges
Figure 5. Ball-and-stick view of the intermolecular interactions in
complex 3. The CH−π interactions that give rise to chains are shown
as red dashed lines, and the CH−π interactions that give rise to a sheet
extended to the bc plane are shown as violet dashed lines. The
hydrogen atoms are omitted for clarity.
no. of rflns collected
no. of indep rflns
6353 (R(int) =
0.0481)
10 525 (R(int) =
0.1400)
no. of data/restraints/
params
goodness of fit on F2
6353/14/382
10 525/0/568
1.057
0.859
final R indices (I > 2σ(I)) R1 = 0.0637
R1 = 0.0457
wR2 = 0.1228
wR2 = 0.1498
absolute structure param
0.016(19)
largest diff peak and hole, 2.509 and −1.564
0.762 and −0.703
e/Å3
Table 2. Bond Lengths (Å) and Angles (deg) for 3
Bond Lengths
a
Lu(1)−Ct(1)
Lu(1)−O(1)
Lu(1)−N(1)
2.351
Lu(1)−N(3)
Lu(1)−C(22)
Lu(1)−C(26)
2.58(1)
2.40(2)
2.37(1)
2.50(1)
2.60(1)
Bond Angles
N(3)−Lu(1)−N(1)
O(1)−Lu(1)−N(1)
O(1)−Lu(1)−N(3)
C(22)−Lu(1)−N(1)
69.9(4)
76.8(4)
73.0(3)
93.8(4)
C(22)−Lu(1)−O(1)
C(26)−Lu(1)−N(3)
C(26)−Lu(1)−O(1)
C(26)−Lu(1)−C(22)
82.3(5)
94.0(5)
83.4(5)
94.3(5)
a
Ct(1) is the centroid of Cp ring.
mixtures to different temperatures. Initially, to test their
reactivity the hydroamination of 2,2-diphenyl-4-pentenylamine
(4) was performed.18 The alkyl complexes 1−3 displayed high
catalytic activity in the hydroamination/cyclization of amino-
pentene (4) to give the cyclic product 7 under mild condi-
tions with a very low catalyst loading and short reaction times
(Table 4). The catalytic activities of complexes 1−3 in the hydro-
amination of substrate 4 were lower in comparison to highly
Figure 6. Self-assembly of the sheets extended to the bc plane along the
a axis. The CH−π interactions that give rise to explain the conglomerate
formation among molecules are shown as blue dashed lines.
active lanthanocene catalysts and are more comparable to the
activity found for other cyclopentadienyl and nonmetallocene
lanthanide-based complexes.4 Complex 3 promoted an
Table 3. CH−π Interactions for Complex 3 (Å, deg)
d(H···cent)
d(C−H···cent)
d(H···atom)
α(C−Hcent)
doffset
Figure 5 or 6 line color
H25B−pz2(N3N4)
H29A−pz1(N1N2)
H28C−pz1(N1N2)
2.993
3.078
3.046
3.819
3.748
3.787
2.782 (C6)
2.992 (C2)
2.550 (C2)
145.01
128.16
135.14
1.12
1.18
1.19
red
violet
blue
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Table 4. Hydroamination/Cyclization Reactions of Aminoalkenes Catalyzed by Complexes 1−3
a
b
entry
cat.
substrate
[cat.]/[substrate] (%)
T (°C)
t (h)
yield (%)
TOF (h‑1)
ee (%)
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
3
3
3
3
4
4
4
4
5
5
5
5
6
6
6
4
4
4
4
5
5
5
5
6
6
6
4
4
4
4
4
4
5
5
5
5
5
6
6
6
1
2
20
20
20
20
20
60
70
20
60
80
100
20
60
75
90
20
60
70
90
60
70
80
20
60
70
90
65
20
20
60
70
90
20
60
60
80
1.5
0.6
0.3
0.1
2.0
1.0
0.6
0.4
11.0
10.1
8.2
1.8
0.7
0.4
0.2
2.0
0.6
0.5
0.4
9.5
9.0
8.1
2.0
0.8
0.4
0.2
0.4
0.1
2.0
0.5
0.4
0.4
0.3
18.0
10.0
8.0
98
95
92
98
92
96
95
96
98
96
95
99
98
94
90
98
96
97
95
98
95
96
98
99
90
95
90
95
95
98
96
97
99
52
98
97
65.3
79.2
61.3
98.0
46.0
96.0
158.3
80.0
0.9
0
0
2
3
5
0
4
10
1
0
5
0
6
1
0
7
1
0
8
3
0
9
10
10
10
1
0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
0.9
0
1.2
0
55.0
140.0
235.0
450.0
49.0
160.0
194.0
237.5
1.0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
10
10
10
1
0
1.0
0
1.2
0
49.0
123.7
247.5
475.0
75.0
95.0
47.0
196.0
240.0
242.5
110.0
0.6
64
43
42
40
50
66
60
40
36
30
65
-
1
1
1
3
10
1
1
1
1
3
5
10
10
0.9
70
55
1.2
a
b
NMR yield determined relative to ferrocene internal standard. Enantiomeric excess determined by 19F NMR of Mosher amides.
asymmetric cyclization of substrate 4 up to 66% ee (Table 4,
entry 28). A decrease in the enantioselectivity values (Table 4,
entries 23−26) with the temperature was observed. A positive
effect of the temperature and catalyst loading on the catalyst
activity was observed (Table 4).
In order to determine the scope of complexes 1−3 as
precatalysts, a variety of aminoalkenes were tested at various
reaction conditions. Entries 5−8, 16−19, and 29−33 (Table 4)
correspond to the results obtained in the hydroamination/
cyclization of (1-allylcyclohexyl)methylamine (5), bearing a
cyclohexyl unit in the geminal position. For this substrate the
catalyst activity is lower than for 4, probably due to the
significant Thorpe−Ingold effect of the geminal phenyl units in
4 (see, for example, at 90 °C a comparison of TOF values,
entries 15 versus 19 and 26 versus 32). We have also
considered the hydroamination/cyclization of an internal 1,2-
disubstituted alkene, namely (E)-2,2-dimethyl-5-phenyl-4-pent-
en-1-amine (6), to give the cyclic product 9 (Table 4, entries
9−11, 20−22, and 34−36). For this cyclization reaction much
longer reaction times and higher catalyst loadings were required
to get high yields. Complex 3 gives the cyclic product 9 up to
70% ee (Table 4, entry 35).
Quantitative kinetic studies of the hydroamination/cycliza-
tion of aminoalkene 4 by using 2 as catalyst were carried out,
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Table 5. Hydroamination/Cyclization Reaction of 2-Allyl-2-methylpent-4-enylamine (10) Catalyzed by Complex 2
entry
cat.
substrate
product
[cat.]/[substrate] (%)
T (°C)
t (h)
yield (%)
TOF (h‑1)
dr (a:b)
a
1
2
3
4
2
2
2
2
10
10
11
11
11
11
12
12
5
5
5
60
100
60
0.8
0.4
99
24.7
49.0
0.5
1.2:1
a
98
1.2:1
b
c
c
17.5
9.1
40 (98)
exo,exo
exo,exo
b
5
100
41 (98)
0.9
a
b
c
NMR yield determined relative to ferrocene internal standard. Isolated yield respect to compound 10. Percent conversion of diastereoisomer 11a.
Figure 7. Linear regression fits for [2,2-diphenyl-4-penten-1-amine] versus time obtained for the first 30 min, indicating that the reactions are zero-
order in substrate concentration. Plot of [2,2-diphenyl-4-penten-1-amine] versus time, illustrating zero-order dependence on [2,2-diphenyl-4-penten-
1-amine]. The concentration of catalyst 2 is 0.024 M.
1
with the reaction monitored in situ by H NMR spectroscopy
(see the Supporting Information). Linear plots of substrate
concentration versus time were obtained for runs in the range
0.38−0.72 M, with the precatalyst concentration kept constant,
indicating that the reaction is zero order in substrate concen-
tration (Figure 7). Furthermore, a linear response was observed
for the reaction rate versus precatalyst concentration, with the
initial substrate concentration kept constant (Figures 8 and 9),
indicating that the reaction is first order in catalyst
concentration. This empirical rate law is typical of lanthanide-
mediated aminoalkene hydroamination/cyclization kinetics,
which shows zero-order dependence on substrate concentration
and first-order dependence on precatalyst concentration. The
most fruitful comparison is with Marks’ lanthanide systems, for
which a wealth of mechanistic information is available and for
which an imido mechanism is excluded.
Rare-earth catalysts, which catalyze the hydroamination reac-
tion through a σ-bond insertion mechanism, can also react with
secondary amine substrates. Thus, we explored the tandem
cyclization reaction of the substrate 2-allyl-2-methylpent-4-enyl-
amine (10) with compound 2 (Table 5). In the first step the
allyl-substituted secondary amine 11 was formed, and this
reacted further to give the tertiary amine 12. Cyclization of 10
(Table 5, entries 1 and 2) proceeded in quantitative yield, and
two diastereoisomers of pyrrolidine 11 were obtained (Figure
S9, Supporting Information). The diastereoselectivity for 11 is
low, and it is believed that the major isomer 11a has the methyl
group in the 2-position and the sterically slightly more demanding
allyl substituent in the 4-position with a cis orientation relative to
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Figure 8. Linear regression fits for ln [2,2-diphenyl-4-penten-1-amine] versus time obtained for the first 30 min. Plot of ln [2,2-diphenyl-4-penten-1-
amine] versus time for several catalyst 2 concentrations (0.017, 0.022, 0.026, 0.033 M). The concentration of 2-diphenyl-4-penten-1-amine is 0.534 M.
Figure 9. Plot of kobs versus concentration of [Lu(CH2SiMe3)2(bpztcp)] (2) for the cyclization of 2,2-diphenyl-4-penten-1-amine (4), showing first-
order dependence on catalyst concentration.
each other.19 This tandem cyclization reaction of 10 needed
longer reaction times, yielding the tertiary amine 12 with 40% and
41% conversion (Table 5, entries 3 and 4, respectively). The
tertiary amine 12 is isolated together with the minority isomer of
the secondary amine 11b (Figures S9 and S10, Supporting
Information) but with high diastereoselectivity to the exo,exo
product by preferential bicyclization of diastereoisomer 11a from
the secondary amine which has the methyl group in the 2-
position and the allyl subtituent in the 4-position with cis
orientation relative to each other.19
In order to gain an insight into the proposed general
mechanism for rare-earth-metal-catalyzed hydroamination,4,6
we decided to investigate the stoichiometric reaction of 2 with
the substrate 4 on the NMR-tube scale using toluene-d8 as
solvent at room temperature. The progress of the reaction was
monitored by NMR spectroscopy. The clean formation of the
dipyrrolidinide−lutetium complex [Lu(NC4H5-2-Me-4,4-
Ph2)2(bpztcp)] (13) (Scheme 3) was observed, and the
product was unambiguously characterized spectroscopically.5e
This dipyrrolidinide−lutetium complex 13 is also well observed
when the catalytic cyclization reaction of substrate 4 with 2 as
catalyst is carried out with higher catalyst loadings (see Figure
S5b, Supporting Information). The formation of 13 is con-
sistent with the mechanism proposed for the hydroamination/
cyclization of aminoalkenes with our precatalysts (Figure 1). In
fact, treatment of complex 2 with 2 equiv of 4 would lead to the
transient bis-amido species A through a well-documented
protonolysis process. This species could subsequently coor-
dinate the pendant alkene to give the insertion step through the
proposed four-membered transition states B and B′. However,
in the absence of more substrate 4, the proposed dialkyl-
lutetium intermediate C would undergo an intramolecular
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Scheme 3. Synthesis of Dipyrrolidinide Lutetium Complex 13 from the Stoichiometric Reaction of 2 with 4
proton transfer to give the final product 13. The NMR data
corroborate the formation of the 5-exo regioisomer versus 6-
endo, and this is consistent with the previously determined
regioselectivity of the intramolecular hydroamination/cycliza-
tion of aminopentenes catalyzed by rare-earth catalysts.4−6
When the reaction was carried out in a 1:1 (2:4) molar ratio,
the dipyrrolidinide−lutetium complex [Lu(NC4H5-2-Me-4,4-
Ph2)2(bpztcp)] (13) was formed together with the precursor
complex [Lu(CH2SiMe3)2(bpztcp)] (2). In the presence of an
excess of substrate, intermolecular proton transfer would lead
to the protonolysis of the proposed dialkyl−lutetium inter-
mediate to regenerate the catalytically active species A.20
Additionally, complex 13 exhibited catalytic activity in the
hydroamination/cyclization process; thus, treatment of com-
plex 13 with a large excess of substrate 4 at 70 °C led to
complete conversion into the pyrrolidine 7 in less than 2 h.
Attempts to isolate the proposed dialkyl−lutetium intermediate
C were unsuccessful, even when the stoichiometric reaction of
2 with the substrate 4 was carried out at lower temperatures.
Complex 13 was also synthesized on the preparative scale
and was isolated as pale yellow solid in good yield (90%) after
the appropriate workup procedure (see the Experimental
Section). Complex 13 has a chiral carbon in the heteroscor-
pionate scaffold and two chiral carbons in the N-heterocycles, a
situation that could generate four diastereoisomers. However,
the spectroscopic data of 13 show that there are only two
diastereoisomers in solution (Figure S5a, Supporting Informa-
tion). The molecular structure of complex 13 was confirmed by
X-ray crystallographic analysis. The corresponding ORTEP
drawing is depicted in Figure 10. Crystallographic data and
selected interatomic distances and angles are given in Tables 1
and 6, respectively. The molecular structure determined by
Table 6. Bond Lengths (Å) and Angles (deg) for 13
Bond Lengths
Lu(1)−N(1)
Lu(1)−N(3)
Lu(1)−N(5)
2.485(8)
2.710(8)
2.167(7)
Lu(1)−N(6)
Lu(1)−Ct(1)
2.193(7)
2.336
a
Bond Angles
98.9(3)
N(5)−Lu(1)−N(6)
N(5)−Lu(1)−N(1)
N(6)−Lu(1)−N(1)
N(5)−Lu(1)−N(3)
N(6)−Lu(1)−N(3)
N(1)−Lu(1)−N(3)
C(22)−N(5)−C(25)
C(42)−N(6)−C(39)
151.6(3)
66.6(2)
131.4(3)
87.4(3)
90.6(3)
104.1(7)
106.7(7)
a
Ct(1) is the centroid of the Cp ring.
X-ray diffraction is in good agreement with the solution structure
deduced from the NMR experiments. In the solid state, the
crystal contains a racemic mixture of a single diastereoisomer
(RSS/SRR), and the structure of the SRR enantiomer is
depicted in Figure 10. The heteroscorpionate ligand is attached
to the lutetium atom through the two nitrogen atoms of
pyrazole rings and the cyclopentadienyl ring in a κ2-NN-η5-Cp
coordination mode in the expected fac coordination fashion. In
addition, the lutetium center is coordinated to two pyr-
rolidinide ligands. The coordination environment of the
lutetium atom can be described as a distorted trigonal
bipyramid, probably due to the steric demand of pyrrolidinide
ligands and the constraints imposed by the chelating ligand.
This substantial distortion is manifested in the angles N(1)−
Lu(1)−N(3), N(6)−Lu(1)−N(3), and N(5)−Lu(1)−N(1),
with values for these angles being 66.6(2), 151.6(3), and
131.4(3)°, respectively. The Lu(1)−N(3) bond length of
2.710(8) Å is longer than the Lu(1)−N(1) bond length of
2.485(8) Å due to the trans effect from pyrrolidinide ligand.
The Lu−N5 and Lu−N6 bond lengths of 2.167(7) and
2.193(7) Å respectively confirm that the N atoms of the
heterocyclic groups are attached to the lutetium center in an
anionic fashion.21 The structure of 13 also indicates that the
two alkyl moieties of 2 are active sites for the catalyzed hydro-
amination/cyclization reaction, as proposed in Figure 1.
CONCLUSIONS
■
In conclusion, we report here a facile synthesis of a new family
of dialkyl heteroscorpionate lutetium compounds bearing
cyclopentadienyl groups as pendant donor arms. The lutetium
complex 3 was isolated enantiomerically enriched by a spon-
taneous resolution of a racemic mixture and the preferential
crystallization of one enantiomer due to the formation of a
conglomerate among molecules. These lutetium complexes
promoted efficiently the intramolecular hydroamination of
Figure 10. ORTEP representation of the molecular structure of 13
(ellipsoids drawn at 30% probability). H atoms and phenyl groups
from N-bound heterocycles are omitted for clarity.
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LuCH2SiMe3). 13C{1H} NMR (C6D6, 297 K): δ 67.1 (CH), 152.4,
150.9, 140.2, 139.6 (C3,3′ or 5,5′), 108.2, 107.5 (C4,4′), 15.7 (Me3), 15.4
(Me3′), 11.9 (Me5′), 10.9 (Me5), 57.8 (Ca), 35.2 (C(CH3)3)), 28.4
(C(CH3)3), 110.6 (Cb-Cp), 116.4 (Cc-Cp), 114.2 (Cc′-Cp), 110.9,
(Cd′-Cp), 109.9 (Cd-Cp), 37.2, 35.0 (LuCH2SiMe3), 5.0
(LuCH2SiMe3).
several aminopentenes. Complex 3 promoted the asymmetric
hydroamination reaction of the aminopentenes, and up to 70%
ee could be reached in the preparation of cyclic compound 9.
The dipyrrolidinide lutetium complex 13 has been isolated and
characterized by a stoichiometric reaction of 2 with the sub-
strate 4. To the best of our knowledge, this complex represents
the first example of an X-ray characterized dipyrrolidinide rare-
earth-metal complex. Their formation is consistent with the
general insertion mechanism for rare-earth metal catalysts, and
it must evolve from a bis-alkyl key catalytic intermediate result-
ing from an insertion step of a CC moiety into a metal−
amido bond. Furthermore, this amido complex 13 is itself
catalytically competent, as previously reported for other amido-
containing group 3 and rare-earth-metal complexes. Further
studies are being carried out to thoroughly examine the effect of
changes both to the rare-earth-metal center and the scorpionate
ligand framework on the catalytical activity of complexes in
hydroamination processes.
Synthesis of [Lu(CH2SiMe3)2(bpztcp)(thf)] (3). A solution of
bpzctpH (0.23 g, 0.69 mmol) in toluene (5 mL) was added dropwise
to a cooled (0 °C) solution of [Lu(CH2SiMe3)3(thf)2] (0.40 g, 0.69
mmol) in hexane (10 mL). The reaction mixture was stored without
stirring at −20 °C for 48 h. Colorless crystals of the highly enantiopure
25
complex 3 were obtained. Yield: 0.18 g (35%). [α]D = −21.3°
(c 0.10, toluene). Anal. Calcd for C33H59LuN4OSi2: C, 52.2; H, 7.8; N,
1
7.4. Found: C, 52.0; H, 7.8; N, 7.5. H NMR (C6D6, 297 K): δ 6.26
(s, 1 H, CH), 5.30 (s, 1 H, H4), 5.27 (s, 1 H, H4′), 2.39 (s, 3 H, Me3),
2.25 (s, 3 H, Me 3′), 1.81 (s, 3 H, Me5), 1.65 (s, 3 H, Me5′), 2.60 (s, 1
H, CHa), 0.80 (s, 9 H, C(CH3)3), 6.71, 6.69 (m, 2 H, Hd,d′-Cp), 6.12,
5.40 (m, 2 H, Hc,c′-Cp), 0.42 (s, 18 H, LuCH2SiMe3), A −0.41,
B −0.69 (AB, JAB = 11.2 Hz, 4 H, LuCH2SiMe3), 3.51 (m, 4 H, THF),
1.43 (m, 4 H, THF). 13C{1H} NMR (C6D6, 297 K): δ 67.6 (CH),
152.0, 151.0, 140.6, 139.5 (C3,3′ or 5,5′), 108.7, 108.0 (C4,4′), 15.9 (Me3),
15.5 (Me3′), 12.1 (Me5′), 11.1 (Me5), 57.6 (Ca), 35.7 (C(CH3)3), 28.6
(C(CH3)3), 110.0 (Cb-Cp), 116.7 (Cc-Cp), 114.6 (Cc′-Cp), 111.3,
(Cd′-Cp), 109.7 (Cd-Cp), 36.2, 35.5 (LuCH2SiMe3), 5.2
(LuCH2SiMe3), 68.1 (THF), 25.4 (THF).
EXPERIMENTAL SECTION
■
All manipulations were performed under nitrogen, using standard
Schlenk techniques. Solvents were predried over sodium wire (toluene,
n-hexane) and distilled under nitrogen from sodium (toluene) or
sodium−potassium alloy (n-hexane). Deuterated solvents were stored
over activated 4 Å molecular sieves and degassed by several freeze−
thaw cycles. Microanalyses were carried out with a Perkin-Elmer 2400
Synthesis of [Lu(NC4H5-2-Me-4,4-Ph2)2(bpztcp)] (13). A
solution of 1-amino-2,2-diphenyl-4-pentene (4; 0.15 g, 0.64 mmol)
in toluene (20 mL) was added to a solution of [Lu(CH2SiMe3)2
(bpztcp)] (2; 0.22 g, 0.32 mmol) in toluene (20 mL). The reaction
mixture was stirred for 1 h at room temperature. The solvent was
removed under vacuum and the remaining residue washed with hexane
(20 mL) to yield compound 13 as a pale yellow solid. The solid was
recrystallized from toluene/hexane (10/1, 20 mL at −20 °C) to give
pale yellow crystals of the compound 13. Yield: 0.52 g (90%). Anal.
Calcd for C55H65LuN6: C, 67.0; H, 6.6; N, 8.5. Found: C, 67.1; H, 6.9;
N, 8.3. 1H NMR (C6D6, 297 K; two diastereoisomers): δ 6.30, 6.24 (s,
2 H, CH), 5.31, 5.27, 5.25, 5.20 (s, 4 H, H4, H4′), 2.72, 2.51, 2.37, 2.31
(s, 12 H, Me3,3′), 1.73, 1.64, 1.60, 1.56 (s, 12 H, Me5,5′), 2.78 (s, 2 H,
CHa), 0.79, 0.77 (s, 18 H, C(CH3)3), 6.95, 6.66, 6.44, 6.21, 6.11, 6.06,
5.70, 5.57 (m, 8 H, Cp), 8.15−6.98 (m, 40 H, Ph NCHMeCH2
CPh2CH2), 4.40−4.10 (m, 12 H, H5 and H2 NCHMeCH2CPh2CH2),
2.41, 1.75 (m, 8 H, H4 NCHMeCH2CPh2CH2), 1.47, 1.13 (m, 12 H,
Me5 NCHMeCH2CPh2CH2). 13C{1H} NMR (C6D6, 297 K; two
diastereoisomers): δ 66.5 (CH), 154.4, 148.7, 148.1, 139.3, 139.0,
137.7 (C3,3′ or 5,5′), 107.5, 107.3, 107.2, 107.1 (C4,4′), 15.9, 15.6, 14.0,
12.9 (Me3,3′), 12.1, 11.8, 11.7, 11.2, 11.1 (Me5,5′), 58.7, 58.3 (Ca), 35.4,
35.2 (C(CH3)3), 28.4 (C(CH3)3), 125.3, 125.2, 114.7, 114.6, 114.2,
110.9, 110.8, 110.7, 110.2, 109.2 (Cp), 151.6, 151.4, 151.3, 151.2,
150.7, 150.6, 150.5, 150.3 (iC-Ph NCHMeCH2CPh2CH2), 129.3,
129.1, 128.6. 128.5, 128.4, 128.3, 128.1, (oC-Ph NCHMeCH2
CPh2CH2), 127.9, 127.8, 127.6, 127.5, 126.1, 126.0, 125.6, 125.4,
125.3 (mC- and pC-Ph NCHMeCH2CPh2CH2), 69.5, 66.6, 65.3
(NCHMeCH2CPh2CH2), 59.9, 59.4, 58.7, 58.4 (NCHMeCH2CPh2
CH2), 58.8, 58.6, 58.5 (NCHMeCH2CPh2CH2), 51.3, 49.8, 49.5, 47.5
(NCHMeCH2CPh2CH2), 25.6, 24.4, 22.4 (NCHMeCH2CPh2CH2).
General Procedure for Catalytic Intramolecular Hydro-
amination. In a typical small-scale experiment, 0.01 mmol (0.0068 g)
of catalyst [Lu(CH2SiMe3)2(bpztcp)] (2) and 1.00 mmol (0.2377 g)
of aminoalkene 2,2-diphenyl-4-penten-1-amine (4)23 were dissolved
in 0.75 mL of toluene-d8 and placed in a J. Young style NMR tube with
a resealable Teflon valve. The tube was closed and placed into an oil
bath that was preheated at the temperature desired. The reaction was
monitored at regular intervals by 1H NMR spectroscopy to determine
the optimum conversion.
1
CHN analyzer. H, 13C, and 19F NMR spectra were recorded on a
Varian Innova FT-500 (1H NMR 500 MHz, 13C NMR 125 MHz, and
19F NMR 470 MHz) spectrometer and referenced to the residual
deuterated solvent. The NOESY-1D spectra were recorded with the
following acquisition parameters: irradiation time 2 s and number of
scans 256, using standard VARIANT-FT software. Two-dimensional
NMR spectra were acquired using standard VARIANT-FT software
and processed using an IPC-Sun computer. The specific rotation
22
[α]D was measured at 22 °C on a Perkin-Elmer 241 polarimeter
equipped with a Na lamp operating at 589 nm with a light path length
of 10 cm. LuCl3 was used as purchased (Aldrich), and [LuCl3(thf)3],22
[Lu(CH2SiMe3)3(thf)2],12 bpzcpH,10d bpztcpH,10d 2,2-diphenyl-4-
penten-1-amine (4),23 (1-allylcyclohexyl)methylamine (5),23 (E)-2,2-
dimethyl-5-phenyl-4-penten-1-amine (6),23 and 2-allyl-2-methyl-4-
penten-1-amine (10)23 were prepared according to literature
procedures.
Synthesis of [Lu(CH2SiMe3)2(bpzcp)] (1). A solution of bpzcpH
(0.30 g, 0.69 mmol) in toluene (20 mL) was added dropwise to a
cooled (0 °C) solution of [Lu(CH2SiMe3)3(thf)2] (0.40 g, 0.69 mmol)
in toluene (20 mL). The reaction mixture was stirred for 2 h at 0 °C.
Evaporation of the solvent gave a white solid. The solid was
recrystallized from toluene/hexane (10/1, 20 mL at −20 °C) to give
colorless crystals of the compound 1. Yield: 0.40 g (75%). Anal. Calcd
for C37H51LuN4Si2: C, 56.8; H, 6.6; N, 7.2. Found: C, 56.9; H, 7.0; N,
1
7.0. H NMR (C6D6, 297 K): δ 6.93 (s, 1 H, CH), 5.25 (s, 2 H, H4),
2.42 (s, 6 H, Me3), 1.34 (s, 6 H, Me5), 7.42−6.90 (m, 10 H, Ph), 6.73
(m, 2 H, Hd-Cp), 5.70 (m, 2 H, Hc-Cp), 0.47 (s, 18 H, LuCH2SiMe3),
−0.49 (s, 4 H, LuCH2SiMe3). 13C{1H} NMR (C6D6, 297 K): δ 75.2
(CH), 145.3, 142.1 (C3 or 5), 108.3 (C4′), 15.6 (Me3), 11.4 (Me5),
151.7−128.3 (Ph), 61.0 (Ca), 110.0 (Cb-Cp), 118.1 (Cc-Cp), 110.7
(Cd-Cp), 38.7 (LuCH2SiMe3), 5.1 (LuCH2SiMe3).
Synthesis of [Lu(CH2SiMe3)2(bpztcp)] (2). The synthetic
procedure was the same as for complex 1, using bpztcpH (0.23 g,
0.69 mmol), [Lu(CH2SiMe3)3(thf)2] (0.40 g, 0.69 mmol), and toluene
(20 mL) to give 2 as a white solid. Yield: 0.38 g (80%). Anal. Calcd for
C29H51LuN4Si2: C, 50.7; H, 7.5; N, 8.2. Found: C, 50.9; H, 7.6; N, 8.5.
1H NMR (C6D6, 297 K): δ 6.25 (s, 1 H, CH), 5.26 (s, 1 H, H4), 5.24
X-ray Crystallographic Structure Determination. For X-ray
structure analyses the crystals of compounds 3 and 13·2.5C7H8 were
mounted on a glass fibers with Paratone-N oil and transferred to a
Bruker X8 APEX II CCD diffractometer using graphite-monochro-
mated Mo Kα radiation (λ = 0.710 73 Å). Data were integrated and
corrected for Lorentz−polarization effects using SAINT24 and were
3
(s, 1 H, H4′), 2.45 (s, 3 H, Me3), 2.30 (s, 3 H, Me ′), 1.75 (s, 3 H,
Me5), 1.61 (s, 3 H, Me5′), 2.60 (s, 1 H, CHa), 0.78 (s, 9 H, C(CH3)3),
6.75, 6.71 (m, 2 H, Hd,d′-Cp), 6.18, 5.58 (m, 2 H, Hc,c′-Cp), 0.50 (s,
18 H, LuCH2SiMe3), A −0.33, B −0.59 (AB, JAB = 11.2 Hz, 4 H,
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corrected for absorption effects using SADABS.25 Space group
assignments were based upon systematic absences, E statistics, and
successful refinement of the structures. Structures were solved by
direct methods with the aid of successive difference Fourier maps and
were refined against all data using the SHELXTL software package.26
In the case of 3 one SiMe3 and coordinated THF are disordered.
The groups were modeled over two static sites, and the occupancies of
the disorder components were refined initially and then fixed to a
value of 50:50. Least-squares restraints have been used to obtain a
better model. For 13, the compound crystallizes with 2.5 disordered
toluene molecules per asymmetric unit. The option squeeze27 was
used to eliminate the contribution of the electron density from the
intensity data. The derived quantities Mr, F(000), and Dx in the crystal
data were corrected with the contribution from this disordered solvent.
For both compounds, thermal parameters for all non-hydrogen atoms
were refined anisotropically, with the exception of the atoms of
disordered groups, which were assigned isotropic parameters due to a
convergence to NPD anisotropic parameters for some atoms.
Hydrogen atoms, added here, were assigned to ideal positions and
refined using a riding model with an isotropic thermal parameter.
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16571. (f) Kovacs, G.; Ujaque, G.; Lledos, A. J. Am. Chem. Soc. 2008,
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ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, text, tables, and CIF files giving experimental details,
procedures for catalytic intramolecular hydroamination, kinetic
measurements and representative kinetic plots, and X-ray
crystallographic data for complexes 3 and 13. This material is
available free of charge via the Internet at http://pubs.acs.org.
(e) Lauterwasser, F.; Hayes, P. G.; Brase, S.; Piers, W. E.; Schafer, L. L.
̈
Organometallics 2004, 23, 2234−2237.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: Antonio.Otero@uclm.es (A.O.). Tel: +34926295300.
Fax: +34926295318.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We gratefully acknowledge financial support from the Minister-
■
io de Ciencia e Innovacion
́
(MICINN) of Spain (Grant Nos.
CTQ2008-00318/BQU and Consolider-Ingenio 2010 ORFEO
CSD2007-00006) and the Junta de Comunidades de Castilla-
La Mancha, Spain (Grant No. PCI08-0010).
2005, 635−651. (e) Otero, A.; Fernan
́
dez-Baeza, J.; Antinolo, A.;
chez, A. Dalton Trans. 2004, 1499−1510.
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Tejeda, J.; Lara-San
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(f) Marques, N.; Sella, A.; Takats, J. Chem. Rev. 2002, 102, 2137−
2160.
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