2-Aminopyrrolyl dilithium compounds: Synthesis, structural diversity, and catalytic activity for amidation of aldehydes with amines

Article abstract of DOI:10.1021/om4006609

Five dilithium compounds containing bidentate dianionic pyrrolyl ligands, [{2-(CH₃NCH₂)C₄H₃N}Li ₂(TMEDA)₃] (1), {[μ-η⁵-2-[CH ₃CH₂NCH₂]C₄H₃N]Li ₂(TMEDA)}₂ (2), {[μ-η⁵: η¹-2-[(CH₃)₃CNCH₂]C ₄H₃N]Li₂(TMEDA)}₂ (3), {[η⁵-2-[(CH₃)₂CHNCH₂]C ₄H₃N]Li₂ (TMEDA)}₂ (4), and {[η⁵-2-[(CH₂)₅CHNCH₂]C ₄H₃N]Li₂(TMEDA)}₂ (5), were synthesized, and their structural features were provided. Compounds 1-5 were proved to be a series of efficient catalysts for amidation reactions of aldehydes with amines in good to excellent yields under mild conditions.

Full text of DOI:10.1021/om4006609

Article
pubs.acs.org/Organometallics
2‑Aminopyrrolyl Dilithium Compounds: Synthesis, Structural
Diversity, and Catalytic Activity for Amidation of Aldehydes with
Amines
Zhiqiang Guo,† Qiao Liu,‡ Xuehong Wei,*,†,‡ Yongbin Zhang,† Hongbo Tong,† Jianbin Chao,†
Jianping Guo,§ and Diansheng Liu*,†
†Institute of Applied Chemistry, Shanxi University, Taiyuan, 030006, People’s Republic of China
‡School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan, 030006, People’s Republic of China
§Solid Wastes Resource Utilization and Energy Saving Building Materials State Key Laboratory, Beijing, 100041, People’s Republic of
China
S
* Supporting Information
ABSTRACT: Five dilithium compounds containing bidentate
dianionic pyrrolyl ligands, [{2-(CH3NCH2)C4H3N}Li2(TMEDA)3]
(1), {[μ-η5-2-[CH3CH2NCH2]C4H3N]Li2(TMEDA)}2 (2), {[μ-
η5:η1-2-[(CH3)3CNCH2]C4H3N]Li2(TMEDA)}2 (3), {[η5-2-
[(CH3)2CHNCH2]C4H3N]Li2 (TMEDA)}2 (4), and {[η5-2-
[(CH2)5CHNCH2]C4H3N]Li2(TMEDA)}2 (5), were synthesized,
and their structural features were provided. Compounds 1−5 were
proved to be a series of efficient catalysts for amidation reactions of
aldehydes with amines in good to excellent yields under mild
conditions.
corresponding transition or rare-earth metal salts. The
intermediate iminopyrrolyl alkali-metal complexes are generally
prepared and employed in situ.6 Hence, they have rarely been
isolated from reaction solution and are poorly characterized in
the solid state, let alone the characterization of their structural
features. Structure elucidation is a crucial part of the
understanding of reactivities and reaction mechanisms and for
selective and specific applications in organolithium chemistry.7
Recently, we reported the synthesis, characterization, and
application of lithium compounds containing substituted
pyrrolyl ligands for the cyclotrimerization of isocyanate to
corresponding isocyanurate.8 Unlike monolithium reagents,
organo-dilithium reagents provide access to interesting and
useful compounds that are not available by other means in
terms of reactivity and synthetic application.9 In our continuing
interest in developing alkali metal compounds containing
substituted pyrrolyl ligands as catalysts, here we report the
synthesis and structural features of five dilithium compounds
containing bidentate dianionic pyrrolyl ligands with η1, η5, or
both bonding modes and their catalytic activity for amidation of
aldehydes with amines.
INTRODUCTION
■
Over the past few years, substituted pyrrolyl as a supporting
ligand with various metals has attracted increasing attention by
virtue of its strong metal−ligand bonds and exceptional tunable
steric and electronic features required for compensating
coordinative unsaturation of metal centers.1 Especially, they
hold more flexible coordination modes ranging from N-η1 (σ)
to -η5 (π). It was demonstrated that donor-substituted pyrrolyl
ligands can bind to the metal atoms in either a η1 or a η5
fashion,2 but only a few examples exist in which the pyrrolyl
units bind to metal atoms in both ways simultaneously.3
Furthermore, the coordination mode (η1 or η5) of the pyrrolyl
ligands may show a dramatic influence on the catalytic activity
in many cases. For example, Hou and co-workers reported that
rare earth metal aminobenzyl complexes with η5-coordinated
pyrrolide showed a high catalytic activity and stereocontrol for
the polymerization of styrene; however, η1-coordinated pyrrolyl
rare earth metal aminobenzyl complexes had no catalytic
activity.4 In addition, Cui’s group reported that bidentate
pyrrole-imino lanthanide complexes with mixed η5/η1 coordi-
nation modes exhibited moderate to high catalytic activities
toward polymerizations of lactide or isoprene; however, the
polymerization of isoprene is less controlled.5
Usually, these pyrrolyl metal complexes are synthesized
based on the deprotonation of iminopyrrolyl ligand precursors
with nBuLi, LDA, or NaH. Then the mixture is reacted with the
Received: July 4, 2013
Published: August 12, 2013
© 2013 American Chemical Society
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Scheme 1. Synthetic Routes to Compounds 1−5
C4H3N), 6.67 (s, 2H, C4H3N), 4.42 (s, 4H, CH2NCH3), 3.09 (s, 6H,
CH2NCH3), 2.22(s, 12H, CH2), 1.90 (s, 36H, N(CH3)2). 13C NMR
(C6D6): 136.39 (C4H3N), 105.73 (C4H3N), 102.20 (C4H3N), 100.73
(C4H3N), 60.12 (CH2NCH3), 57.79 (TMEDA), 50.20 (CH2NCH3),
45.73 (TMEDA). Li NMR (C6D6): −1.47, −2.26. Anal. Calcd for
C30H64Li4N10: C, 60.80; H, 10.88; N, 23.63. Found: C, 60.49; H,
11.13; N, 23.34.
EXPERIMENTAL SECTION
■
General Remarks. Unless otherwise noted, all syntheses and
manipulations of air-sensitive materials were performed under a
purified nitrogen atmosphere using standard Schlenk techniques.
Tetrahydrofuran and diethyl ether were distilled from sodium-
benzophenone under nitrogen. Hexane and toluene were dried using
sodium potassium alloy and distilled under nitrogen prior to use. All
aldehydes and amines were sublimed, recrystallized, or distilled before
7
{[2-(CH3CH2NCH2)C4H3N]Li2(TMEDA)}2 (2). The synthesis of
compound 2 was carried out following procedures similar to those
used for the preparation of 1. The white solid was recrystallized from a
saturated diethyl ether and hexane solution to yield colorless crystals of
1
7
use. H NMR (300 MHz), 13C NMR (75.5 MHz), and Li NMR
(116.6 MHz) spectra of the compounds were recorded on a Bruker
DRX 300 instrument in C6D6 at 298 K and referenced internally to the
residual solvent resonances (1H, 13C) or externally (7Li). Elemental
analyses were performed on a Vario EL-III instrument. Melting points
were determined on a Stuart SMP10 melting point apparatus and
uncorrected. All aminopyrrolyl ligands were synthesized according to
the literature procedure.10
1
2 (0.583 g, 77%). Mp: 73 °C (dec). H NMR (C6D6): 6.95 (s, 2H,
C4H3N), 6.64 (s, 2H, C4H3N), 6.37 (s, 2H, C4H3N), 4.60 (s, 4H,
CH2NC2H5), 3.52 (s, 4H, CH2CH3), 2.00 (s, 24H, N(CH3)2), 1.83 (s,
8H, CH2) 1.65 (s, 6H, CH2CH3). 13C NMR (C6D6): 146.76 (C4H3N),
123.38 (C4H3N), 107.43 (C4H3N), 100.34 (C4H3N), 57.43
(CH2NC2H5), 55.77 (TMEDA), 52.28 (CH2CH3), 44.35
Preparation. [{2-(CH3NCH2)C4H3N}Li2(TMEDA)3] (1). A nBuLi
hexane solution (6.0 mmol, 2.2 M) was added dropwise to a hexane
(20 mL) solution of C4H3NH(CH2NHCH3)-2 (0.330 g, 3.0 mmol) at
0 °C under nitrogen. After the reaction mixture was stirred at room
temperature for 2 h, TMEDA (0.679 mL, 4.5 mmol) was stepwise
added, and the mixture was then stirred for another 6 h. The white
solid was isolated by filtration and recrystallized from a saturated
hexane solution to yield colorless crystals of 1 (0.413 g, 53%). Mp: 84
°C (dec). 1H NMR (C6D6): 7.13 (s, 2H, C4H3N), 6.84 (s, 2H,
7
(TMEDA), 19.96 (CH2CH3). Li NMR (C6D6): 2.48, −3.99. Anal.
Calcd for C26H52Li4N8: C, 61.90; H, 10.39; N, 22.21. Found: C, 61.73;
H, 10.21; N, 22.08.
[{μ-η5:η1-2-[(CH3)3CNCH2]C4H3N}Li2(TMEDA)]2 (3). The synthesis of
compound 3 was carried out following procedures similar to those
used for the preparation of 1. The white solid was recrystallized from a
saturated hexane solution to yield colorless crystals of 3 (0.563 g,
67%). Mp: 71 °C (dec). 1H NMR (C6D6): 6.92 (s, 2H, C4H3N), 6.75
(s, 2H, C4H3N), 6.48 (s, 2H, C4H3N), 4.56 (s, 4H, CH2NBut), 1.90 (s,
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24H, N(CH3)2), 1.73(s, 8H, CH2), 1.55 (s, 18H, CH2NBut). 13C
NMR (C6D6): 138.86 (C4H3N), 115.64 (C4H3N), 99.70 (C4H3N),
93.87 (C4H3N), 48.35 (TMEDA), 43.40 (C(CH3)3), 39.07
(CH2NBut), 37.39 (TMEDA), 24.17(C(CH3)3). 7Li NMR (C6D6):
n
(CH2)5)-2 with 2 equiv of BuLi in the presence of TMEDA
in hexane, respectively. As shown in Scheme 1, an equal molar
ratio of the substituted pyrrole and TMEDA was employed,
except for the synthesis of compound 1 (the substituted pyrrole
and TMEDA in a molar ratio of 1:1.5). Each of 1, 2, 3, 4, and 5
was easily purified by crystallization from hexane, diethyl ether,
or a mixture of hexane and diethyl ether and was characterized
by satisfactory C, H, and N microanalysis, 1H, 13C{1H}, and 7Li
NMR spectra in C6D6 at ambient temperature, and single-
crystal X-ray structural data.
1
0.56, −4.55. H NMR (d8-THF): 6.49 (s, 2H, C4H3N), 5.94 (s, 2H,
C4H3N), 5.64 (s, 2H, C4H3N), 4.15 (s, 4H, CH2NBut), 2.27 (s, 8H,
CH2), 2.11 (s, 24H, N(CH3)2), 1.08 (s, 18H, CH2NBut). 13C NMR
(d8-THF): 145.23 (C4H3N), 123.99 (C4H3N), 108.28 (C4H3N),
101.39 (C4H3N), 58.51 (TMEDA), 52.14 (C(CH3)3), 48.68
(CH2NBut), 46.14 (TMEDA), 32.79 (C(CH3)3). Anal. Calcd for
C30H60Li4N8: C, 64.27; H, 10.79; N, 19.99. Found: C, 64.01; H, 10.83;
N, 19.73.
X-ray Single-Crystal Structures of 1−5. Crystals of 1
were obtained from a saturated hexane solution at −5 °C. As
shown in Figure 1, each of the lithium atoms is four-
[{η5-2-[(CH3)2CHNCH2]C4H3N}Li2(TMEDA)]2 (4). The synthesis of
compound 4 was carried out following procedures similar to those
used for the preparation of 1. The white solid was recrystallized from a
saturated diethyl ether and hexane solution to yield colorless crystals of
1
4 (0.567 g, 71%). Mp: 93 °C (dec). H NMR (C6D6): 6.99 (s, 2H,
C4H3N), 6.71 (s, 2H, C4H3N), 6.39 (s, 2H, C4H3N), 4.63 (s, 4H,
CH2N iPr), 3.28 (s, 2H, CH(CH3)2), 1.96 (s, 24H, N(CH3)2), 1.70 (s,
i
8H, CH2) 1.32−1.44 (d, 6H, CH2N Pr). 13C NMR (C6D6): 147.92
(C4H3N), 124.27 (C4H3N), 108.34 (C4H3N), 101.58 (C4H3N), 56.83
i
(TMEDA), 55.75 (CH2N Pr), 55.29 (CH(CH3)2), 45.85 (TMEDA),
7
28.59 (CH(CH3)2). Li NMR (C6D6): −1.47, −8.69. Anal. Calcd for
C28H56Li4N8: C, 63.15; H, 10.60; N, 21.04. Found: C, 63.19; H, 10.30;
N, 21.19.
{[η5-2-[(CH2)5CHNCH2]C4H3N]Li2(TMEDA)}2 (5). The synthesis of
compound 5 was carried out following procedures similar to those
used for the preparation of 1. The white solid was recrystallized from a
saturated diethyl ether solution to yield colorless crystals of 5 (0.597 g,
65%). Mp: 77 °C (dec). 1H NMR (C6D6): 7.22 (s, 2H, C4H3N), 6.83
(s, 2H, C4H3N), 6.70 (s, 2H, C4H3N), 4.03 (s, 4H, CH2NCy), 2.50
(m, 2H, CH), 1.88 (s, 24H, N(CH3)2), 1.65(s, 8H, CH2), 1.60 (d, 4H,
Cy), 1.54 (d, 4H, Cy), 1.19 (d, 4H, Cy), 1.04 (d, 4H, Cy), 0.79 (d, 4H,
Cy). 13C NMR (C6D6): 135.93 (C4H3N), 125.84 (C4H3N), 105.04
(C4H3N), 104.24 (C4H3N), 54.05 (TMEDA), 45.46 (TMEDA),42.64
(CH2NCy), 42.00 (Cy), 30.92 (Cy), 23.50 (Cy), 22.66 (Cy). 7Li
NMR (C6D6): 6.04, 5.83. Anal. Calcd for C34H64Li4N8: C, 66.65; H,
10.53; N, 18.29. Found: C, 66.27; H, 10.27; N, 18.47.
Typical Procedure for Amidation Reaction. A 30 mL Schlenk
flask was charged with the ether solution of dilithium compound
(10.00 mL, 0.1 mmol). Aniline was added (0.09 mL, 1.00 mmol); after
stirring for 0.5 h, benzaldehyde was then added (0.31 mL, 3.00 mmol).
The resulting mixture was stirred at room temperature for 3 h. The
reaction was monitored by TLC until complete consumption of amine.
All the volatiles were removed under vacuum. Purification of the crude
product by column chromatography (ethyl acetate/petroleum ether,
1:5) afforded amides. All the amides were identified by spectral
comparison with literature data.
Figure 1. ORTEP diagram of compound 1. Thermal ellipsoids are
drawn at the 20% probability level. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and bond angles (deg): Li1−N1
2.104(7), Li1−N4 2.112(7), Li2−N1 2.026(7), Li2−N4 2.025(7),
Li2−N2 2.193(7), Li3−N3 2.042(7), Li3−N2 2.042(7), Li3−N4
2.190(7), Li4−N3 2.060(7), Li4−N2 2.139(7), N1−C4 1.367(5),
N1−C1 1.374(5), N2−C6 1.451(5), N2−C5 1.474(5), C4−C5
1.504(6), C10−C11 1.500(5); N1−Li1−N4 101.2(3), N1−Li2−N4
107.1(3), N1−Li2−N2 97.0(3), N4−Li2−N2 98.7(3), N3−Li3−N2
107.4(3), N3−Li3−N4 97.4(3), N2−Li3−N4 98.3(3), N3−Li4−N2
103.2(3), N9−Li4−N10 82.3(2), Li2−N1−Li1 75.4(3), Li3−N2−Li4
73.1(3), Li3−N2−Li2 64.3(3), Li3−N3−Li4 74.8(3), Li2−N4−Li1
75.2(3), Li2−N4−Li3 64.6(3).
X-ray Crystallography. Single-crystal X-ray diffraction data of the
compounds were collected on a Bruker Smart Apex CCD
diffractometer using monochromated Mo Kα radiation, λ = 0.71073
Å. A total of N reflections were collected by using the ω scan mode.
Corrections were applied for Lorentz and polarization effects as well as
absorption using multiscans (SADABS).11 Each structure was solved
by the direct method and refined on F2 by full matrix least-squares
(SHELX97)12 using all unique data. Then the remaining non-
hydrogen atoms were obtained from the successive difference Fourier
map. All non-hydrogen atoms were refined with anisotropic
displacement parameters, whereas the hydrogen atoms were con-
strained to parent sites, using a riding mode (SHELXTL).13 Details of
the modeling of disorder in the crystals can be found in their CIF files.
coordinated by a dianionic bidentate pyrrolyl ligand and
TMEDA. One of the TMEDA molecules acts as a bridged
ligand, and its two nitrogen atoms are coordinated to Li2 and
Li3, respectively. The bond distances of N1−Li1, N1−Li2,
N3−Li3, and N3−Li4 are 2.104(7), 2.026(7), 2.042(7), and
2.060(7) Å, respectively, which are close to those in the
monoanionic substituted pyrrolyl lithium compounds.7
Compound 2 was crystallized from a mixed solution of
hexane and diethyl ether. As illustrated in Figure 2, the two
ligand moieties are linked by two lithium atoms (Li1, Li3),
where Li1 is coordinated by two pyrrolyl rings in η5 mode to
form a distorted sandwich geometry. The bond distances of
Li1−N1 2.195(10) Å, Li1−C1 2.213(11) Å, Li−C2 2.303(11)
Å, Li1−C3 2.322(12) Å, and Li1−C4 2.247(11) Å are shorter
than Li1−N3 2.280(10) Å, Li1−C8 2.240(10) Å, Li−C9
2.277(10) Å, Li1−C10 2.321(11) Å, and Li1−C11 2.322(11)
Å, respectively, and the distances of Li1 to the N1C1C2C3C4
plane and the N3C8C9C10C11 plane are 1.925 and 1.961 Å,
respectively. The dihedral angle between the N1C1C2C3C4
plane and the N3C8C9C10C11 plane is 17.01°.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Dilithium Com-
pounds. Each of the dilithium compounds 1−5 was readily
prepared in good yield from the reaction of substituted pyrrole
precursor C4 H3 NH(CH2 NHCH3 )-2, C4 H3 NH-
(CH2NHCH2CH3)-2, C4H3NH(CH2NHCH(CH3)2)-2,
C4H3NH(CH2NHCH(CH3)3)-2, or C4H3NH(CH2NHCH-
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Figure 2. ORTEP diagram of compound 2. Thermal ellipsoids are drawn at the 20% probability level. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and bond angles (deg): Li1−N1 2.195(10), Li1−C1 2.213(11), Li−C2 2.303(11), Li1−C3 2.322(12), Li1−C4
2.247(11), Li1−N3 2.280(10), Li1−C8 2.240(10), Li−C9 2.277(10), Li1−C10 2.321(11), Li1−C11 2.322(11), Li2−N2 1.967(10), Li2−N1
2.051(9), Li3−N4 1.966(10), Li3−N2 1.968(10), Li3−N3 2.312(10), Li4−N4 1.978(9), Li4−N3 2.086(9); N2−Li2−N1 84.5(4), N4−Li3−N3
76.8(3), N2−Li3−N1 69.7(3), N4−Li4−N3 82.1(3), Li2−N2−Li3 87.1(4), Li3−N4−Li4 83.2(4), N2−C5−C4 112.0(5), N4−C12−C11 110.6(4).
Compound 3 was crystallized from a saturated solution of
hexane as a dimer (Figure 3). Li1 is coordinated by two
membered rings; the bond angles of N1−Li2−N2 and N3−
Li2−N4 are 81.2(2)° and 86.6(3)°.
The molecular structures of the dimeric compounds 4 and 5
are very similar and are illustrated in Figure 4 and Figure 5,
respectively. The half-sandwich geometries of 4 and 5 are close
to that of 3. For compound 4, the bond lengths of Li2−N2′,
Li2−C1′, Li2−C2′, Li2−C3′, and Li2−C4′ are 2.187(4),
2.203(4), 2.293(4), 2.297(4), and 2.234(4) Å, respectively,
and for compound 5, the bond lengths of Li2−N1, Li2−C1,
Figure 3. ORTEP diagram of compound 3. Thermal ellipsoids are
drawn at the 20% probability level. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and bond angles (deg): Li1−N1′
2.397(6), Li1−C1′ 2.390(7), Li1−C2′ 2.384(7), Li1−C3′ 2.363(8),
Li1−C4′ 2.360(7), Li1−N2 1.986(7), Li1−N1 2.144(7), Li2−N2
1.979(7), Li2−N1 2.088(6), N2−C5 1.458(4), N2−C6 1.458(4),
C4−C5 1.498(5); N2−Li1−N1 79.7(2), N2−Li2−N1 81.2(2), C4−
N1−Li2 96.0(3), C4−N1−Li1 95.2(3), C5−N2−C6 113.4(3), C5−
N2−Li2 97.9(3), N1−C4−C5 118.8(3), N2−C5−C4 109.3(3).
Symmetry elements for 3: ′ −x, y, −z+1/2.
Figure 4. ORTEP diagram of compound 4. Thermal ellipsoids are
drawn at the 20% probability level. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and bond angles (deg): Li2−N2′
2.187(4), Li2−C1′ 2.203(4), Li2−C2′ 2.293(4), Li2−C3′ 2.297(4),
Li2−C4′ 2.234(4), Li1−N2 2.014(4), Li1−N1 2.029(4), Li2−N1
1.923(4), N1−C5 1.454(3), N1−C6 1.458(3), N2−Li2′ 2.187(4),
C4−C5 1.495(3); N2−Li1−N1 88.79(16), C5−N1−C6 110.34(18),
C5−N1−Li1 98.54(16), C4−N2−Li1 103.58(17), N2−C4−C5
119.8(2), N1−C5−C4 112.38(18). Symmetry elements for 4: ′ −x
+2, −y+2, −z.
nitrogen atoms in one ligand and a pyrrolyl ring of the other
ligand in η5 mode to form a half-sandwich geometry. The bond
lengths of Li1−N1′, Li1−C1′, Li1−C2′, Li1−C3′, and Li1−C4′
are 2.397(6), 2.390(7), 2.384(7), 2.363(8), 2.360(7) Å,
respectively. The distance of Li1 to the N1′C1′C2′C3′C4′ is
2.067 Å, which is much longer than those (1.925, 1.961 Å) in
compound 2. Both of the nitrogen atoms of the bidentate
ligands and TMEDA bond to Li2, forming two spiral five-
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compounds containing bidentate dianionic pyrrolyl ligands in
the amidation reaction of aldehydes with amines was tested in
this paper.
The initial study of the amidation reaction was carried out
using benzaldehyde with aniline as a model substrate catalyzed
by dilithium compounds 1−5. The results are listed in Table 1.
Table 1. Optimized Reaction Conditions Using Compounds
a
1−5 for the Amidation of Aldehydes with Amines
b
entry
catalyst
mol % of catalyst
solvent
yield (%)
1
1
2
3
4
5
3
3
3
1
2
4
5
3
3
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
5%
10%
15%
THF
THF
THF
THF
THF
hexane
toluene
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
37.4
39.6
44.7
40.3
35.9
49.4
48.3
73.7
61.3
68.8
72.6
70.7
94.9
95.2
2
3
4
5
6
Figure 5. ORTEP diagram of compound 5. Thermal ellipsoids are
drawn at the 20% probability level. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and bond angles (deg): Li2−N1
2.223(11), Li2−C1 2.207(12), Li2−C2 2.261(12), Li2−C3 2.286(12),
Li2−C4 2.250(11), N1−Li1 1.979(10), Li1−N2 2.005(10), Li2−N2′
1.916(11), N2−C6 1.458(6), N2−C5 1.469(6), C4−C5 1.508(7);
C4−N1−Li1 104.4(4), N2−Li1−N1 88.2(4), C6−N2−C5 110.1(4),
C5−N2−Li1 99.9(4), N1−C4−C5 119.9(5), N2−C5−C4 110.9(5).
Symmetry elements for 3: ′ −x+1, −y+2, −z.
7
8
9
10
11
12
13
14
a
The molar ratio of amine and aldehyde is 3:1, and amine was first
Li2−C2, Li2−C3, and Li2−C4 are 2.223(11), 2.207(12),
2.261(12), 2.286(12), and 2.250(11) Å, respectively. The
distances of lithium to the pyrrolyl ring plane are 1.910(4) and
1.831(11) Å in 4 and 5, respectively, which are much shorter
than those in 2 or 3. [The dimeric compound names (3, 4, and
5) discussed here are still used, but their solution states should
be monomeric; the 13CNMR data are consistent with the
monomeric solution structure.]
added to the catalyst solution; then aldehyde was added after 30 min.
Isolated yield based on amine.
b
We were pleased to find that in the presence of 5% mol 1−5
the amidation can proceed smoothly at room temperature in
THF to afford corresponding amide in moderate yields. It can
be seen that the steric effects of ligand is minimal (Table 1,
entries 1−5), although compound 3 gave the highest yield
under the same reaction conditions. Among the different
solvents used for optimization (Table 1, entries 3, 6−8), the
best results was obtained when diethyl ether was employed as
solvent in the presence of 5% mol of compound 3. This
indicates that solvent effect plays a key role in this reaction
system. Hence, the catalytic behavior of compound 1, 2, 4, or 5
for this reaction in Et2O was also investigated for comparison
(Table 1, entries 3, 9−12), and the results show that compound
3 still gave the highest yield. It is worth noting that the
amidation reaction can afford 94.9% isolated yield when the
loading of catalyst 3 is increased to 10% (Table 1, entry 13).
The yield increases slightly when catalyst loading increases to
15% (Table 1, entry 14).
Using the optimized reaction conditions and preferred
catalyst identified, the generality and scope of the amidation
reaction catalyzed by compound 3 were investigated, as
summarized in Table 2. A variety of aldehydes and amines
can be smoothly transformed to the corresponding amides in
good to excellent isolated yields at room temperature in 3 h,
revealing the practicability of the present method. The aromatic
amines with an electron-withdrawing group at the p-position on
the phenyl ring give high yields relative to the amines with an
electron-donating group, but there is no explicit trend in the
yields. The strongly withdrawing nitro substitutent dramatically
reduces reactivity (Table 2, entries 7). The steric hindrance of
Catalytic Activities of Compounds 1, 2, 3, 4, and 5 for
Amidation of Aldehydes with Amines. In recent years, the
direct amidation of aldehydes with amines to produce amides
has garnered significant attention because of easy availability of
starting materials. The various reaction systems have been
reported, including metal-free conditions,14 metal-based cata-
lysts,15 nontransition metals,16 and a number of rare earth
systems. In view of substantial waste generation, reduced
toxicity profile, and requiring either stoichiometric additives or
long reaction times, the alkali metals and rare-earth metals are
recognized as an attractive choice for this desirable trans-
formation. For example, Marks has carried out this trans-
formation by simple, commercially available rare-earth amido
complexes.17 Shen’s group has reported a number of rare-earth
guanidinate,18 amidinate,19 and heterobimetallic complexes20
for this transformation. Wang21 and Schafer22 also showed that
rare-earth metal amido and amidate complexes are effective
catalysts, respectively. However, to the best of our knowledge,
only a few papers about alkali metal complexes for this
transformation have been published. In those studies, the
oxidative mixture of potassium iodide/TBHP (tert-butyl
hydroperoxide) and sodium hydride was reported for this
transformation in succession.23 In the meantime, Ishihara also
reported the use of lithium diisopropylamide in a Cannizzaro-
type reaction.16 Thus, to further explore the application of alkali
metal compounds, the catalytic behavior of the five dilithium
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Organometallics
Article
consistent with the previous reports. Attempts to use alkyl
aldehydes or alkyl amines as substrates, such as C6H11CHO,
(CH3)2CHCHO, C2H5CHO, C3H7CHO, (CH3)3CNH2,
C4H8NH, and C5H10NH in catalytic amidation were
unsuccessful due to side reactions. Based on the above
experimental results and related literature, a mechanism similar
to those proposed for the lanthanide amide catalysts is
suggested.17−19,20b,22
Table 2. Amidation of Aldehydes with Amines Catalyzed by
Compound 3
a
CONCLUSION
■
In summary, a series of dilithium compounds containing
bidentate dianionic pyrrolyl ligands were prepared from the
reaction of corresponding aminopyrrolyl ligand precursors with
n
two equivalents of BuLi in the presence of TMEDA. The
resulting corresponding compounds 1−5 were obtained in
moderate to good yields. The X-ray diffraction analysis
indicated that the compounds exhibited structural diversity
with different substituents, i.e., with the pyrrolyl ring
coordinated to lithium metal in η1 mode, η5 mode, or both
ways simultaneously. These dilithium compounds are efficient
catalysts for the amidation reactions of aldehydes with amines
under mild reaction conditions.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Copies of the H NMR and 13C NMR spectra for amidation
products (S1), crystal data details of data collection and
refinements (S2), and crystallographic data for compounds 1−
5 in CIF format (S3) are available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: xhwei@sxu.edu.cn; dsliu@sxu.edu.cn. Tel: 0086-
15536566161. Fax: 0086-351-7011688.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science
Foundation of China (No. 20572065) and the Natural Science
Foundation of Shanxi Province (Nos. 2008011021,
2011021011-1) is gratefully acknowledged.
REFERENCES
■
(1) (a) Gade, L. H. Chem. Commun. 2000, 173. (b) Mutsui, S.;
Spaniol, T. P.; Takagi, Y.; Yoshida, Y.; Okuda, J. J. Chem. Soc., Dalton
Trans. 2002, 4529. (c) Yoshida, Y.; Matsui, S.; Fujita, T. J. Organomet.
Chem. 2005, 690, 4382. (d) Mashima, K.; Tsurugi, H. J. Organomet.
Chem. 2005, 690, 4414. (e) Yang, Y.; Li, S. H.; Cui, D. M.; Chen, X. S.;
Jing, X. B. Organometallics 2007, 26, 671.
a
Amine was first added to the catalyst solution, and after 30 min,
b
aldehyde was added. Isolated yield based on amine.
(2) (a) DuBois, M. R. Coord. Chem. Rev. 1998, 174, 191.
(b) Schumann, H.; Gottfriedsen, J.; Demtschuk, J. Chem. Commun.
1999, 2091. (c) Ascenso, J. R.; Dias, A. R.; Ferreira, A. P.; Galvao, A.
C.; Salema, M. S.; Veiros, L. F. Inorg. Chim. Acta 2003, 356, 249.
(3) (a) Hock, A. S.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc.
2006, 128, 16373. (b) Chen, Y. C.; Lin, C. Y.; Li, C. Y.; Huang, J. H.;
Chang, L. C.; Lee, T. Y. Chem.Eur. J. 2008, 14, 9747. (c) Liu, C.;
Zhou, S. L.; Wang, S. W.; Zhang, L. J.; Yang, G. S. Dalton Trans. 2010,
39, 8994.
(4) Nishiura, M.; Mashiko, T.; Hou, Z. M. Chem. Commun. 2008,
2019.
(5) (a) Yang, Y.; Liu, B.; Lv, K.; Gao, W.; Cui, D. M.; Chen, X. S.;
Jing, X. B. Organometallics 2007, 26, 4575. (b) Yang, Y.; Li, S. H.; Cui,
anilines also exerted some effect on this reaction, as indicated
from the reaction of aniline substituted at the 2-, 3-, or 4-
position (Table 2, entries 2−4). In the case of alkyl amines, the
reaction with benzylamine afforded the product in 78.7%
isolated yield (Table 2, entry 8). The properties of the aromatic
aldehydes also affect the amidation reaction, in which the
strongly electron-withdrawing nitro, bromo, and chloro
substituents enhance reactivity (Table 2, entries 10−12),
while the strongly donating methoxy substitutent dramatically
reduces reactivity (Table 2, entry 13). These general trends are
4682
dx.doi.org/10.1021/om4006609 | Organometallics 2013, 32, 4677−4683

Organometallics
Article
D. M.; Chen, X. S.; Jing, X. B. Organometallics 2007, 26, 671. (c) Yang,
Y.; Cui, D. M.; Chen, X. S. Dalton Trans. 2010, 39, 3959. (d) Wang, L.
F.; Liu, D. T.; Cui, D. M. Organometallics 2012, 31, 6014.
(6) (a) Bellabarba, R. M.; Gomes, P. T.; Pascu, S. I. Dalton Trans.
2003, 4431. (b) Puente, P. P.; Jesus, E. D.; Flores, J. C.; Sal, P. G. J.
Organomet. Chem. 2008, 693, 3902. (c) Gomes, C. S. B.; Suresh, D.;
Gomes, P. T.; Veiros, L. F.; Duarte, M. T.; Nunes, T. G.; Oliveira, M.
C. Dalton Trans. 2010, 39, 736.
́
(7) (a) Setzer, W. N.; Rague Schleyer, P. V. Adv. Organomet. Chem.
1985, 24, 353. (b) Weiss, E. Angew. Chem., Int. Ed. Engl. 1993, 32,
1501. (c) Gessner, V. H.; Daschlein, C.; Strohmann, C. Chem.Eur. J.
̈
2009, 15, 3320. (d) Daschlein, C.; Gessner, V. H.; Strohmann, C.
̈
Chem.Eur. J. 2010, 16, 4048. (e) Granitzka, M.; Poppler, A. C.;
̈
Schwarze, E. K.; Stern, D.; Schulz, T.; John, M.; Herbst-Irmer, R.;
Pandey, S. K.; Stalke, D. J. Am. Chem. Soc. 2012, 134, 1344.
(8) Guo, Z. Q.; Wang, S.; Tong, H. B.; Chao, J. B.; Wei, X. H. Inorg.
Chem. Commun. 2013, 33, 68.
(9) (a) Xi, Z. F. Acc. Chem. Res. 2010, 43, 1342. (b) Liu, L. T.; Zhang,
W. X.; Luo, Q.; Li, H.; Xi, Z. F. Organometallics 2010, 29, 278.
(c) Zhang, S. G.; Wei, J. N.; Zhan, M.; Luo, Q.; Wang, C.; Zhang, W.
X.; Xi, Z. F. J. Am. Chem. Soc. 2012, 134, 11964. (d) Zhou, Y.; Zhang,
W. X.; Xi, Z. F. Organometallics 2012, 31, 5546. (e) Zhang, S. G.;
Zhan, M.; Zhang, W. X.; Xi, Z. F. Organometallics 2013, 32, 4020.
(10) (a) Huang, J. H.; Chen, H. J.; Chang, J. C.; Zhou, C. C.; Lee, G.
H.; Peng, S. M. Organometallics 2001, 20, 2647−2650. (b) Hsieh, C.
C.; Chao, W. J.; Horng, Y. C.; Lee, H. M. J. Chin. Chem. Soc. 2009, 56,
435−442. (c) Tsai, C. F.; Chen, H. J.; Chang, J. C.; Hung, C. H.; Lai,
C. C.; Hu, C. H.; Huang, J. H. Inorg. Chem. 2004, 43, 2183−2188.
(11) Sheldrick, G. M. SADABS Correction Software; University of
Gottingen: Gottingen, Germany, 1996.
̈
̈
(12) Sheldrick, G. M. SHELX97, Program for Crystal Structure
Refinement; University of Gottingen: Gottingen, Germany, 1997.
̈
̈
(13) SHELXTL, Program for Crystal Structure Refinement; Bruker
Analytical X-ray Instruments Inc.: Madison, WI, USA, 1998.
(14) (a) Liang, J.; Lv, J.; Shang, Z. C. Tetrahedron 2011, 67, 8532.
(b) Bode, J. W.; Sohn, S. S. J. Am. Chem. Soc. 2007, 129, 13798.
(c) Liu, Z. J.; Zhang, J.; Chen, S. L.; Shi, E. B.; Xu, Y.; Wan, X. B.
Angew. Chem., Int. Ed. 2012, 51, 3231.
(15) (a) Ghosh, S. C.; Ngiam, J. S. Y.; Seayad, A. M.; Tuan, D. T.;
Chai, C. L. L.; Chen, A. Q. J. Org. Chem. 2012, 77, 8007. (b) Cadoni,
R.; Porcheddu, A.; Giacomelli, G.; Luca, L. D. Org. Lett. 2012, 14,
5014. (c) Zhu, M. W.; Fujita, K. I.; Yamaguchi, R. J. Org. Chem. 2012,
77, 9102.
(16) Ishihara, K.; Yano, T. Org. Lett. 2004, 6, 1983.
(17) Seo, S.; Marks, T. J. Org. Lett. 2008, 10, 317.
(18) (a) Qian, C. W.; Zhang, X. M.; Li, J. M.; Xu, F.; Zhang, Y.; Shen,
Q. Organometallics 2009, 28, 3856. (b) Qian, C. W.; Zhang, X. M.;
Zhang, Y.; Shen, Q. J. Organomet. Chem. 2010, 695, 747.
(19) Wang, J. F.; Li, J. M.; Xu, F.; Shen, Q. Adv. Synth. Catal. 2009,
351, 1363.
(20) (a) Xu, B.; Huang, L. L.; Yang, Z. J.; Yao, Y. M.; Zhang, Y.;
Shen, Q. Organometallics 2011, 30, 3588. (b) Li, J. M.; Xu, F.; Zhang,
Y.; Shen, Q. J. Org. Chem. 2009, 74, 2575. (c) Cai, C. Y.; Li, L.; Xu, F.;
Shen, Q. Chin. Sci. Bull. 2010, 55, 3641.
(21) (a) Wu, Y. J.; Wang, S. W.; Zhang, L. J.; Yang, G. S.; Zhu, X. C.;
Zhou, Z. H.; Zhu, H.; Wu, S. H. Eur. J. Org. Chem. 2010, 326.
(b) Zhang, L. J.; Su, S. P.; Wu, H. P.; Wang, S. W. Tetrahedron 2009,
65, 10022. (c) Zhang, L. J.; Wang, S. W.; Zhou, S. L.; Yang, G. S.;
Sheng, E. H. J. Org. Chem. 2006, 71, 3149.
(22) Thomson, J. A.; Schafer, L. L. Dalton Trans. 2012, 41, 7897.
(23) (a) Reddy, K. R.; Maheswari, C. U.; Venkateshwar, M.; Kantam,
M. L. Eur. J. Org. Chem. 2008, 3619. (b) Wang, X. B.; Wang, D. Z.
Tetrahedron 2011, 67, 3406.
4683
dx.doi.org/10.1021/om4006609 | Organometallics 2013, 32, 4677−4683