Anionic phenoxy-amido rare-earth complexes as efficient catalysts for amidation of aldehydes with amines

Article abstract of DOI:10.1039/c5ra20285j

A series of anionic organo-rare-earth amido complexes stabilized by dianionic phenoxy-amido ligands were prepared and their catalytic behavior for amidation reactions of aldehydes with amines was elucidated. Amine elimination reaction of Ln[N(SiMe₃)₂]₃(μ-Cl)Li(THF)₃ with an equimolar of lithium aminophenoxy {[HNO]¹Li(THF)}₂, which was prepared by the reaction of [HNOH]¹ {[HNOH]¹ = N-p-fluoro-phenyl(2-hydroxy-3,5-di-tert-butyl)benzylamine} with one equivalent of n-BuLi in tetrahydrofuran (THF) in situ, gave the anionic phenoxy-amido rare earth amido complexes [NO]¹₂Ln[N(SiMe₃)₂][Li(THF)]₂ [Ln = Y (1), Yb (2), Sm (3), Nd (4)] in high isolated yields. Similar reactions of Ln[N(SiMe₃)₂]₃(μ-Cl)Li(THF)₃ with {[HNO]²Li(THF)}₂, and {[HNO]³Li(THF)}₂ in THF gave the anionic rare-earth amides [NO]²₂Ln[N(SiMe₃)₂][Li(THF)]₂ [Ln = Sm (5), Nd (6)] and [NO]³₂Ln[N(SiMe₃)₂][Li(THF)]₂ [Ln = Sm (7), Nd (8)] {[HNOH]² = N-p-chloro-phenyl(2-hydroxy-3,5-di-tert-butyl)benzylamine; [HNOH]³ = N-p-bromo-phenyl(2-hydroxy-3,5-di-tert-butyl)benzylamine}, respectively. All of these complexes were fully characterized. X-ray structural determination revealed that these complexes are isostructural, and have solvated monomeric structures. Each of the rare-earth ions is coordinated by two phenoxy-amido ligands and one N(SiMe₃)₂ group, and the coordination geometry can be described as a distorted trigonal bipyramid. Each of the lithium atoms is surrounded by one aryloxo group, one amido group and one THF molecule, and the coordination geometry can be described as a trigonal plane. The catalytic behavior of these rare-earth amides for the amidation reaction of aldehyde with amine was elucidated. It was found that these complexes are efficient catalysts for this transformation to produce amides in good to excellent yields under mild reaction conditions, and in some cases, diacylamide compounds can be prepared conveniently.

Full text of DOI:10.1039/c5ra20285j

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Anionic phenoxy-amido rare-earth complexes as
efficient catalysts for amidation of aldehydes with
amines†
Cite this: RSC Adv., 2015, 5, 94768
*
Chao Wang, Lingling Huang, Min Lu, Bei Zhao, Yaorong Wang, Yong Zhang, Qi Shen
*
and Yingming Yao
A series of anionic organo-rare-earth amido complexes stabilized by dianionic phenoxy-amido ligands
were prepared and their catalytic behavior for amidation reactions of aldehydes with amines was
elucidated. Amine elimination reaction of Ln[N(SiMe3)2]3(m-Cl)Li(THF)3 with an equimolar of lithium
aminophenoxy {[HNO]1Li(THF)}2, which was prepared by the reaction of [HNOH]1 {[HNOH]1 ¼ N-p-
fluoro-phenyl(2-hydroxy-3,5-di-tert-butyl)benzylamine} with one equivalent of n-BuLi in tetrahydrofuran
(THF) in situ, gave the anionic phenoxy-amido rare earth amido complexes [NO]12Ln[N(SiMe3)2][Li(THF)]2
[Ln ¼ Y (1), Yb (2), Sm (3), Nd (4)] in high isolated yields. Similar reactions of Ln[N(SiMe3)2]3(m-Cl)Li(THF)3
with {[HNO]2Li(THF)}2, and {[HNO]3Li(THF)}2 in THF gave the anionic rare-earth amides [NO]22Ln
[N(SiMe3)2][Li(THF)]2 [Ln ¼ Sm (5), Nd (6)] and [NO]32Ln[N(SiMe3)2][Li(THF)]2 [Ln ¼ Sm (7), Nd (8)]
{[HNOH]2
¼
N-p-chloro-phenyl(2-hydroxy-3,5-di-tert-butyl)benzylamine; [HNOH]3
¼
N-p-bromo-
phenyl(2-hydroxy-3,5-di-tert-butyl)benzylamine}, respectively. All of these complexes were fully
characterized. X-ray structural determination revealed that these complexes are isostructural, and have
solvated monomeric structures. Each of the rare-earth ions is coordinated by two phenoxy-amido
ligands and one N(SiMe3)2 group, and the coordination geometry can be described as a distorted trigonal
bipyramid. Each of the lithium atoms is surrounded by one aryloxo group, one amido group and one
THF molecule, and the coordination geometry can be described as a trigonal plane. The catalytic
behavior of these rare-earth amides for the amidation reaction of aldehyde with amine was elucidated. It
was found that these complexes are efficient catalysts for this transformation to produce amides in good
to excellent yields under mild reaction conditions, and in some cases, diacylamide compounds can be
prepared conveniently.
Received 1st October 2015
Accepted 27th October 2015
DOI: 10.1039/c5ra20285j
www.rsc.org/advances
achieved is that organo-rare-earth complexes can promote or
catalyze the amidation reactions of aldehydes with amines with
Introduction
high efficiency, under mild reaction conditions, without the
need for oxidants and strong bases, in comparison with the
transformation of aldehydes to amides catalyzed by transition
metal complexes.8–17 Wang et al. found that aromatic aldehydes
can react with rare-earth amides Ln[N(SiMe3)2]3(m-Cl)Li(THF)3
to give acylamides via the Cannizzaro-type disproportionation
reaction.8 Marks et al. reported that homoleptic rare-earth
complexes Ln[N(SiMe3)2]3 can catalyze the transformation of
aldehydes with amines to acylamides under mild reaction
conditions.9 Further studies revealed that anionic hetero-
bimetallic rare-earth–alkali complexes are more efficient cata-
lysts for the above transformation with a wide range of scope for
the substrates.10,11 These results encouraged us to further
elucidate the effect of the structures of heterobimetallic rare-
earth/alkali metal complexes on their catalytic activity for the
amidation reactions of aldehydes with amines.
The synthesis of aromatic and aliphatic acylamides has received
considerable attention for a long time in organic synthesis,
because the amide group is a basic functional group in many
molecules, including polymers, nature products, pharmaceuti-
cals and proteins.1 Generally, these compounds can be prepared
by the reactions of carboxylic acids or acyl halides with
amines.2–4 However, the direct amidation of aldehydes with
amines is one of the most desired methods to acylamides
because of the atom-economic character and easy availability of
the starting materials.5–7 In recent years, an important progress
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Dushu Lake
Campus, Suzhou 215123, People's Republic of China. E-mail: yrwang@suda.edu.cn;
yaoym@suda.edu.cn; Fax: +86 512 65880305
† Electronic supplementary information (ESI) available. CCDC 1412810–1412817
(complexes 1–8). For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c5ra20285j
Phenoxy-amido groups, as dianionic N, O chelate ligands,
have several attractive features, such as possessing the dual
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these complexes were determined in sealed capillaries under
argon.
Synthesis of [NO]1 Y[N(SiMe3)2][Li(THF)]2 (1)
2
A THF solution of [HNOH]1 (30 mL, 2.00 g, 6.09 mmol) was
cooled to ꢀ10 ꢁC, and then nBuLi in hexane (2.36 mL, 6.09
mmol) was added dropwise. The mixture was stirred for about 1
h at ꢀ10 ꢁC, and then warmed to room temperature gradually. A
THF solution of Y[N(SiMe3)2]3(m-Cl)Li(THF)3 (2.52 g, 3.05 mmol)
was added to this solution, and the solution was stirred over-
night at 50 ꢁC. The solvent was removed under vacuum, and
toluene (30 mL) was added to extract the residue. The suspen-
sion was stirred at 50 ꢁC for about half an hour, and the
precipitate was separated by centrifugation. The ltrate was
concentrated to about 12 mL, and then 3 mL of hexane was
added. Yellow crystals were obtained at room temperature in
Chart 1 Aminophenoxo ligands and complexes 1–10.
characteristics of oxygen- and nitrogen-based ligands, being
easily available, and their steric and/or electronic properties are
easily tunable.18–20 Recently, we introduced this ligand system to
organo-rare-earth chemistry, and a series of neutral and anionic
phenoxy-amido rare-earth complexes were prepared conve-
niently by the direct amine elimination reaction between rare-
earth amide and aminophenols.21–24 It was found that many
phenoxy-amido organo-rare-earth complexes were efficient
initiators for the ring-opening polymerization of lactides and
cyclic carbonate.21–23 To further expand the application of
phenoxy-amido rare-earth complexes on homogeneous catal-
ysis, a series of heterobimetallic rare-earth–lithium amido
complexes stabilized by different aminophenoxo ligands
(Chart 1) were synthesized. It was found that these rare-earth
amido complexes are efficient catalysts for the amidation
reaction of aldehydes with amines to produce amides in good to
excellent yields under mild conditions, and in some cases,
diacylamide compounds can be prepared conveniently. Here we
report these results.
ꢁ
a few days (2.73 g, 83%). Mp: 237–239 C (dec.) anal. calcd for
C
56H86F2Li2N3O4Si2Y (1062.26): C, 63.32; H, 8.16; N, 3.58; Y,
1
8.37. Found: C, 62.83; H, 8.09; N, 3.53; Y, 8.38. H NMR (300
MHz, C6D6, 25 ꢁC, ppm): d 7.52 (d, 2J (H, H) ¼ 8.7 Hz, 4H, ArH),
2
7.16 (s, 8H, ArH), 4.66 (d, J (H, H) ¼ 14.0 Hz, 2H, CH2), 4.29
(d, 2J (H, H) ¼ 14.0 Hz, 2H, CH2), 2.79 (s, 8H, a-CH2 THF), 1.72
(s, 18H, But), 1.45 (s, 18H, But), 0.88 (s, 8H, b-CH2 THF), 0.14
(t, 18H, N(SiMe3)2). 13C NMR (75 MHz, C6D6, 25 ꢁC, ppm):
d 159.6 (Ph), 155.2 (Ph), 138.8 (Ph), 136.6 (Ph), 129.6 (Ph), 125.2
(Ph), 123.1 (Ph), 117.9 (Ph), 117.6 (Ph), 68.3 (a-CH2 THF), 51.1
(CH2Ar), 35.5 (C(CH3)3), 34.4 (C(CH3)3), 32.1 (C(CH3)3), 31.2
(C(CH3)3), 25.0 (b-CH2 THF), 4.61 (SiMe3). IR (KBr pellet, cmꢀ1):
3390(s), 2958(s), 2866(s), 1612(s), 1510(s), 1470(s), 1438(m),
1302(s), 1215(m), 1224(s), 1130(w), 1165(w), 932(m), 833(s).
Crystals suitable for an X-ray diffraction analysis were obtained
by the slow cooling of a hot toluene/hexane solution.
Experimental section
General methods
Synthesis of [NO]1 Yb[N(SiMe3)2][Li(THF)]2 (2)
2
Complex 2 was prepared by the same procedure as that
described for complex 1, but Yb[N(SiMe3)2]3(m-Cl)Li(THF)3 (2.78
g, 3.05 mmol) was used instead of Y[N(SiMe3)2]3(m-Cl)Li(THF)3.
Pale-yellow microcrystals were isolated from a concentrated
toluene–hexane solution at room temperature (2.93 g, 84%).
Mp: 200–202 ꢁC (dec.) anal. calcd for C56H86F2Li2N3O4Si2Yb
(1146.38): C, 58.67; H, 7.56; N, 3.67; Yb, 15.09. Found: C, 59.01;
H, 7.48; N, 3.58; Yb, 15.13. IR (KBr pellet, cmꢀ1): 3450(s),
2960(s), 2866(s), 1616(s), 1510(s), 1469(s), 1416(s), 1358(m),
1306(m), 1215(m), 1215(m), 1130(w), 1049(w), 983(w), 833(m).
The microcrystals were recrystallized from toluene–hexane
solution to give the crystals suitable for an X-ray diffraction
analysis.
All of these organo-rare-earth complexes are extremely sensitive
to oxygen and moisture, thus, the manipulations were performed
under a nitrogen atmosphere in glovebox. HN(SiMe3)2, tetrahy-
drofuran (THF), toluene, and hexane were dried according to the
methods reported in the literature, and distilled prior to use.
Aminophenol [HNOH] {[HNOH]1 ¼ N-p-uoro-phenyl(2-hydroxy-
3,5-di-tert-butyl)benzylamine, [HNOH]2 ¼ N-p-chloro-phenyl(2-
hydroxy-3,5-di-tert-butyl)benzylamine, [HNOH]3 ¼ N-p-bromo-
phenyl(2-hydroxy-3,5-di-tert-butyl)benzylamine}, {[HNOH] ¼ N-p-
methylphenyl(2-hydroxy-3,5-di-tert-butyl)-benzylamine},25
and
the organo-rare-earth amides Ln[N(SiMe3)2]3(m-Cl)Li(THF)3,26
[NO]Sm[N(SiMe3)2][Li(THF)]2 (9), and [NO]Nd[N(SiMe3)2]
[Li(THF)]2 (10)21 were prepared according to the published
procedures, respectively. Rare-earth metal analyses were per-
formed according to the reported method.27 Carbon, hydrogen,
Synthesis of [NO]1 Sm[N(SiMe3)2][Li(THF)]2 (3)
2
and nitrogen analyses were performed by direct combustion with Complex 3 was synthesized by the same procedure as that
a Carlo-Erba EA-1110 instrument. The IR spectra were recorded described for complex 1, but Sm[N(SiMe3)2]3(m-Cl)Li(THF)3 (2.71
with a Nicolet-550 FTIR spectrometer as KBr pellets. The 1H and g, 3.05 mmol) was used instead of Y[N(SiMe3)2]3(m-Cl)Li(THF)3.
13C NMR analyses for complex 1 were recorded in a C6D6 solution Yellow crystals were obtained from a concentrated toluene–
with a Unity Varian spectrometer. The NMR spectra for the other hexane solution at room temperature (2.70 g, 79%). Mp: 199–
complexes were not determined, because of the paramagnetism 201 ꢁC (dec.) anal. calcd for C56H86F2Li2N3O4Si2Sm (1123.69): C,
of the Nd, Sm and Yb ions. The uncorrected melting points of 59.86; H, 7.71; N, 3.74; Sm, 13.38. Found: C, 59.40; H, 7.68; N,
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3.62; Sm, 13.37. IR (KBr pellet, cmꢀ1): 3445(s), 2957(s), 2868(s), 82%). Mp: 202–204 C (dec.) anal. calcd for C56H86Br2Li2N3O4-
ꢁ
1616(w), 1510(s), 1477(s), 1360(w), 1227(s), 1165(s), 1049(w), Si2Nd (1239.40): C, 54.27; H, 6.99; N, 3.39; Nd, 11.64. Found: C,
933(m), 833(s).
54.62; H, 6.53; N, 3.69; Nd, 11.86. IR (KBr pellet, cmꢀ1): 3329(s),
2956(s), 2866(s), 1616(m), 1469(s), 1300(m), 1240(s), 1176(s),
1051(m), 931(w), 827(s). The microcrystals were recrystallized
from toluene–hexane solution to give the crystals suitable for an
X-ray diffraction analysis.
Synthesis of [NO]1 Nd[N(SiMe3)2][Li(THF)]2 (4)
2
Complex 4 was synthesized by the same procedure as that
described for complex 1, but Nd[N(SiMe3)2]3(m-Cl)Li(THF)3 (2.69
g, 3.05 mmol) was used instead of Y[N(SiMe3)2]3(m-Cl)Li(THF)3.
Pale blue crystals were obtained at room temperature in several
days (2.76 g, 81%). Mp: 197–200 ꢁC (dec.) anal. calcd for C56-
H86F2Li2N3O4Si2Nd (1117.58): C, 60.18; H, 7.76; N, 3.76; Nd,
12.91. Found: C, 61.02; H, 7.53; N, 3.69; Nd, 12.86. IR (KBr
pellet, cmꢀ1): 3327(s), 2959(s), 2866(s), 1616(s), 1510(s), 1469(s),
1358(m), 1231(s), 1163(m), 988(w), 931(m), 833(s).
Typical procedure for amidation reaction
The procedures for the reactions of aldehydes with amines
catalyzed by 1 to 10 are similar, and a typical procedure is given
below. A solution of catalyst (0.80 mL, 0.02 mmol) was added to
a 20 mL ask by syringe, and then N-methyl-benzylamine was
added (0.13 mL, 1 mmol). The solution was stirred for half an
hour at room temperature, and then benzaldehyde was added
(0.30 mL, 3.00 mmol). The mixed solution was stirred for 3 h,
and the reaction was terminated by adding ethyl acetate. The
pure product was obtained by purication of the crude product
via column chromatography using ethyl acetate/petroleum
ether (1 : 7) as an eluent.
Synthesis of [NO]2 Sm[N(SiMe3)2][Li(THF)]2 (5)
2
Complex 5 was synthesized by the same procedure as that
described for complex 3, but [HNOH]2 (30 mL, 2.50 g, 7.25
mmol) was used instead of [HNOH]1. Pale yellow crystals were
obtained at room temperature in several days (3.39 g, 81%). Mp:
200–202 ꢁC (dec.) anal. calcd for C56H86Cl2Li2N3O4Si2Sm
(1156.59): C, 58.15; H, 7.49; N, 3.63; Sm, 13.00. Found: C, 59.02;
H, 7.53; N, 3.69; Sm 12.86. IR (KBr pellet, cmꢀ1): 3332(m),
2959(s), 2866(s), 1664(m), 1469(s), 1357(m), 1236(s), 1167(m),
931(m), 823(s).
X-ray crystallography
Suitable single crystals of complexes 1 to 8 were sealed in a thin-
walled glass capillary for determining the single-crystal struc-
tures. Intensity data were collected with a Rigaku Mercury CCD
area detector in u scan mode using Mo-Ka radiation (l ¼
Synthesis of [NO]2 Nd[N(SiMe3)2][Li(THF)]2 (6)
˚
0.71070 A). The diffracted intensities were corrected for Lorentz/
2
polarization effects and empirical absorption corrections.
Details of the intensity data collection and crystal data are given
in Tables S1 and S2.†
The structures were solved by direct methods and rened by
full-matrix least-squares procedures based on |F|2. All the non-
hydrogen atoms were rened anisotropically. All the H atoms
were held stationary and included in the structure factor
calculation in the nal stage of full-matrix least-squares
renement. The structures were solved and rened using
SHELEXL-97 programs.
Complex 6 was prepared by the same procedure as that
described for complex 4, but [HNOH]2 (30 mL, 2.50 g, 7.25
mmol) was used instead of [HNOH]1. Pale blue microcrystals
were obtained at room temperature in a few days (3.41 g, 82%).
Mp: 250–252 ꢁC (dec.) anal. calcd for C56H86Cl2Li2N3O4Si2Nd
(1150.50): C, 58.46; H, 7.53; N, 3.65; Nd, 12.54. Found: C, 58.31;
H, 7.48; N, 3.58; Nd, 12.39. IR (KBr pellet, cmꢀ1): 3361(s),
2955(s), 2866(s), 1604(m), 1487(s), 1359(m), 1242(s), 1049(m),
932(s), 831(s). The microcrystals were recrystallized from
toluene–hexane solution to give the crystals suitable for an X-ray
diffraction analysis.
Results and discussion
Synthesis and characterization of the phenoxy-amido rare-
Synthesis of [NO]3 Sm[N(SiMe3)2][Li(THF)]2 (7)
2
Complex 7 was prepared by the same procedure as that
described for complex 3, but [HNOH]3 (30 mL, 2.50 g, 6.42
mmol) was used instead of [HNOH]1. Pale yellow crystals were
obtained at room temperature in several days (3.32 g, 82%). Mp:
172–174 ꢁC (dec.) anal. calcd for C56H86Br2Li2N3O4Si2Sm
(1245.51): C, 54.00; H, 6.96; N, 6.96; Sm, 12.07. Found: C, 53.60;
H, 6.69; N, 6.92; Sm, 12.27. IR (KBr pellet, cmꢀ1): 3329(s),
2956(s), 2867(s), 1596(m), 1469(s), 1355(m), 1299(s), 1241(s),
1178(s), 1047(m), 932(w), 831(s).
earth metal complexes
We previously reported that the reaction of Ln[N(SiMe3)2]3(m-Cl)
Li(THF)3 with {[HNO]Li(THF)}2, which can be conveniently
prepared by the reaction of aminophenol [HNOH] {[HNOH] ¼
N-p-methylphenyl(2-hydroxy-3,5-di-tert-butyl)benzylamine} with
1 equivalent of BunLi in THF, is an efficient method for the
synthesis of the anionic phenoxy-amido rare-earth metal
amides [NO]2Ln[N(SiMe3)2][Li(THF)]2.21 Thus, a series of rare-
earth metal complexes stabilized by phenoxy-amido groups
with different substituents were prepared by the reaction of Ln
[N(SiMe3)2]3(m-Cl)Li(THF)3 with lithium aminophenolates
Synthesis of [NO]3 Nd[N(SiMe3)2][Li(THF)]2 (8)
2
Complex 8 was prepared by the same procedure as that {[HNO]1Li(THF)}2–{[HNO]3Li(THF)}2 as shown in Scheme 1. All
described for complex 4, but [HNOH]3 (30 mL, 2.00 g, 5.14 the anionic phenoxy-amido rare-earth metal amides 1–8 were
mmol) was used instead of [HNOH]1. Pale blue microcrystals obtained in good isolated yields, which were characterized by
were obtained at room temperature in several days (2.61 g, elemental analysis, and IR spectra. Complex 1 was further
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Scheme 1 Synthesis of complexes 1–8.
characterized by NMR spectroscopy. In the 1H NMR spectrum of
complex 1, the signals of –N(SiMe3)2 group and coordinated
THF molecules were observed. Furthermore, two sets of reso-
nances for the four tert-butyl substituents on the phenyl rings
indicated that the chemical environments of the two phenoxy-
amido groups are identical in solution. However, the reso-
nances for the four protons of the CH2 bridges were observed as
two AB doublets at d ¼ 4.66 and 4.29 ppm with the 2JHH value of
14.0 Hz, which means magnetic nonequivalence of the two
protons on the bridging methylene groups. Obviously, this
nonequivalence resulted from the coordination of the phenoxy-
amido group to the yttrium ion, which constrained the
conformation of the bridging methylene group. Similar
phenomenon was also observed in the carbon-bridged
bis(phenolate) titanium and aluminum complexes.28 All of
these complexes are extremely sensitive to air and moisture.
They have good solubility in THF, and moderate solubility in
toluene and hexane.
Fig.
1
Molecular structure of complex
1
showing the atom-
numbering scheme. Thermal ellipsoids are drawn at the 20% proba-
bility level, and hydrogen atoms are omitted for clarity. Complex 2–8
are isomorphous with complex 1.
˚
(1.917(7) and 2.193(7) A), but are in accordance with the corre-
sponding bond parameters in complexes [NO]2Ln[N(SiMe3)2]
[Li(THF)]2 [Ln ¼ Sm (9), Nd (10)].21 It is worthy to note that the p
interaction between the lithium atom and the carbon atom of
the phenoxy-amido ligand exists in complexes 1–8. The distance
between Li1 and C16A in complexes ranges from 2.49(1) to
2.56(2) A, which falls in the range of p contacts between lithium
ions and carbon atoms of arene rings reported in the literature
(2.31–2.57 A).
˚
29–31
˚
The denitive molecular structures of complexes 1–8 were
determined by single-crystal X-ray structure analysis. These
complexes have anionic rare earth–lithium heterobimetallic
structures crystallized in the tetragonal system. The overall
molecular structures of these complexes are nearly the same,
except one different R substituent group on the phenyl ring.
Thus, only the molecular diagram of complex 1 is provided in
Fig. 1. Their selected bond parameters are listed in Table 1. In
these complexes, the rare-earth ion is coordinated by two
oxygen atoms, two nitrogen atoms from two phenoxy-amido
ligands, and one nitrogen atom from the amido group. The
coordination geometry around the rare-earth ion can be best
described as a distorted trigonal bipyramid, in which O1, O1A
are considered to occupy the apical positions. The lithium atom
in complexes 1–8 is coordinated by one nitrogen atom and one
oxygen atom from two different phenoxy-amido ligands, and
one oxygen atom of the THF molecule to form a trigonal plane.
The bond lengths of Ln–O(Ar) in complexes 1–8 range from
Catalytic activity for amidation of aldehydes with amines
The catalytic behavior of these heterobimetallic rare-earth/
alkali metal complexes for the amidation reactions of alde-
hydes with amines was examined. The amidation of benzalde-
hyde 11a with N-methyl-benzylamine 12a was rst examined as
a model reaction catalyzed by the heterobimetallic lanthanide
complexes 1–10, and the results are summarized in Table 2. It
can be seen that all of these complexes can catalyze this trans-
formation to yield the amide 13aa in good to excellent yields at
25 ꢁC with 3 mol% catalyst loading (Table 2, entries 1–10) aer 3
h. The ionic radii of lanthanide metals have obvious effect on
the catalytic activity, and the bimetallic samarium–lithium
complexes 3 gave the highest yield under the same reaction
conditions. The kinetics of the amidation of benzaldehyde with
N-methyl-benzylamine catalyzed by complexes 1–4 have been
measured in d8-THF. In each case, the yields increased as the
reaction time was prolonged (Fig. S1†), and a rst-order
dependence on the concentration of N-methyl-benzylamine
was observed in the presence of excess benzaldehyde
(Fig. S2†). The activity for these rare-earth complexes follows the
trend Yb < Y < Nd < Sm as evidenced by respective kapp values,
which may be ascribed to the fact that rare-earth metals of
larger ionic radii are advantageous to the coordination of
substrates. The electron effect of the aminophenoxo ligands has
slight inuence on the catalytic activity of these hetero-
bimetallic complexes. The complexes with both electron-
withdrawing and electron-donating groups at the p-position of
˚
2.194(4) to 2.337(6) A and the bond lengths of Ln–N(Ar) range
˚
from 2.206(8) to 2.454(6) A (see Table 1), which are in agreement
with the Ln–O(Ar) and Ln–N(Ar) bond lengths in anionic
phenoxy-amido rare-earth metal complexes (THF) Ln[3,5-But2-2-
O-C6H2CH(NPri2)-N-C5H4N]2Li20
and
[NO]2Ln[N(SiMe3)2]
[Li(THF)]2,21 when the difference in ionic radii is considered.
˚
The Li–O(Ar) bond lengths (range from 1.83(1) to 1.87(1) A) and
˚
the Li–N bond lengths (range from 2.05(1) to 2.08(2) A) in
complexes 1–8 (Table 1) are slightly shorter than the corre-
sponding bond lengths in complex {[HNO]Li(THF)}2 (ref. 21)
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Table 1 Selected bond lengths (A˚) and bond angles (deg) for complexes 1–8
Compound
1
2
3
4
5
6
7
8
Ln1–O1
Ln1–N1
Ln1–N2
Li1–O1
Li1–O2
Li1–N1A
Li1–C16A
O1A–Ln1–N1
O1–Ln1–N2
O1–Ln1–N1
N1–Ln1–N2
O1–Ln1–O1A
N1–Ln1–N1A
2.229(3)
2.344(5)
2.244(8)
1.85(1)
1.85(1)
2.05(1)
2.49(1)
79.3(1)
106.1(1)
82.4(1)
125.1(1)
147.9(2)
109.7(3)
2.194(4)
2.300(5)
2.206(8)
1.85(1)
1.85(1)
2.07(1)
2.49(2)
79.8(2)
105.2(1)
82.8(2)
125.2(2)
149.5(3)
109.6(3)
2.294(4)
2.407(5)
2.291(8)
1.83(1)
1.86(1)
2.06(1)
2.52(1)
77.7(1)
108.3(1)
80.6(1)
126.9(1)
143.4(2)
106.3(3)
2.337(6)
2.422(8)
2.335(10)
1.85(2)
1.92(3)
2.08(2)
2.289(5)
2.406(7)
2.295(10)
1.86(2)
1.86(2)
2.06(2)
2.318(5)
2.454(6)
2.332(7)
1.85(2)
1.83(2)
2.07(2)
2.56(2)
76.8(2)
109.8(1)
79.6(2)
127.2(2)
140.4(2)
105.6(3)
2.285(4)
2.412(5)
2.291(9)
1.87(1)
1.80(1)
2.07(1)
2.53(1)
77.4(2)
108.7(1)
80.2(2)
127.3(2)
142.7(2)
105.3(3)
2.313(4)
2.438(5)
2.337(8)
1.85(1)
1.85(1)
2.05(1)
2.52(2)
77.1(2)
109.4(1)
79.6(2)
127.4(2)
141.2(2)
105.3(3)
2.53(2)
77.4(3)
2.52(2)
77.9(2)
109.7(2)
79.5(3)
126.7(2)
140.7(3)
106.7(4)
108.6(2)
80.2(2)
126.8(2)
142.9(3)
106.5(4)
the phenyl ring can catalyze the amidation reaction efficiently. loading is decreased from 2 mol% to 1 mol% (Table 3, entries 4
Using the Sm complexes with different substituted group on the and 5). The amidation reaction catalyzed by complex 3
aminophenoxo ligand as catalysts, the isolated yields ranged completed aer 3 h. Shortening reaction time resulted in
from 88 to 96% (Table 2, entries 3, 5, 7 and 9).
a decrease in yields (Table 3, entries 5–7). Reaction media also
Optimization experiments were then conducted using have somewhat inuence on the reaction. It was found that THF
complex 3 as the catalyst, and the results were provided in is a better solvent than toluene (Table 3, entries 5 and 10), and
Table 3. It can be seen that an excess of aldehyde (3 equiv.) is the reaction was conducted in solvent-free condition to give
required for reaching high yield (Table 3, entries 1–3), because product 13aa in a relatively lower yield (Table 3, entries 5 and 9)
aldehyde acts as not only a reactant but also an oxidant, in comparison with that in THF. It is worthy to note that the
according to the reaction mechanism suggested by Marks et al.9 high yield was also obtained using complex 3 prepared in situ as
Besides, a possibly occurred side reaction of dimerization of catalyst for this transformation (Table 3, entry 11), reecting
aldehyde, namely the Tishchenko reaction, also consumes that the anionic phenoxy-amido samarium complex can be
aldehyde. In order to suppress the Tishchenko reaction, amine easily prepared quantitatively.
was added to the reaction mixture at rst and the mixture was
Under the optimal reaction conditions (Table 3, entry 5),
stirred for 30 min, and then aldehyde was added. Catalyst the scopes of aldehydes and amines were explored,
loading has obvious inuence on the reaction. The yield
decreases dramatically from 96% to only 22% when the catalyst
Table
3 Amidation of benzaldehyde with N-methyl-benzylamine
catalyzed by complex 3a
Table
2 Amidation of benzaldehyde with N-methyl-benzylamine
catalyzed by complexes 1–10a
Mol% of Molar ratio
Yieldb
(%)
Entry Cat.
11a : 12a
Time (h) T (ꢁC) Solvent
1
2
3
4
5
6
7
8
9
3
3
3
1
2
3
2
2
2
1 : 1
2 : 1
3 : 1
3 : 1
3 : 1
3 : 1
3 : 1
3 : 1
3 : 1
3
3
3
3
3
2
1
2
3
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
THF
THF
THF
THF
THF
THF
THF
THF
Solvent-
free
Toluene
THF
54
80
96
22
96
90
67
88
83
Entry
Cat.
Molar ratio 11a : 12a
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
9
10
3 : 1
3 : 1
3 : 1
3 : 1
3 : 1
3 : 1
3 : 1
3 : 1
3 : 1
3 : 1
86
79
96
94
91
93
90
89
88
89
10
2
2
3 : 1
3 : 1
3
3
r.t.
r.t.
75
90
11c
a
a
Amine was rst added to the catalyst solution, aer 30 minutes,
Amine was rst added to the catalyst solution, aer 30 minutes,
h. aldehyde was added. b Isolated yield based on amine. c Complex 3 was
prepared in situ.
aldehyde was added, THF as solvent, room temperature,
3
b
Isolated yield based on amine.
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respectively, and the results were listed in Table 4. It can be in 65% yield under the same reaction conditions (Table 4,
seen that the amidation reaction of aromatic aldehyde pro- 13ba and 13ea). This inuence of the substituent on the
ceeded smoothly to generate the expected amides in good to aromatic aldehyde is consistent with that reported in the
excellent isolated yields. Electronic effect of the aldehyde has literature.9–11 The bulkiness of the amine has obvious effect on
an obvious inuence on the transformation. The aromatic the amidation reaction. The reactions of aldehydes with cyclic
aldehyde with electron-withdrawing substituent at the p- secondary aliphatic amines, such as pyrrolidine, piperidine
position of the phenyl ring showed apparently higher reactivity and morpholine, proceeded smoothly in this system to
in comparison with the aldehyde with electron-donating generate the desired acylamides in excellent isolated yields
substituent. For example, the reaction of p-uoroaldehyde (Table 4, 13ab–13ed), whereas the reaction of diethylamine
with N-methyl-benzylamine gave the corresponding amide in with benzaldehyde give the amide in dramatically low yield,
98% isolated yield, whereas the similar reaction of p-methoxy- and the yield can be improved to moderate by raising the
aldehyde with N-methyl-benzylamine gave the nal product reaction temperature or prolonging the reaction time (Table 4,
13ae). The properties of the amines also showed signicant
effect on the reaction. The yields for the reactions of benzal-
dehyde with both linear and cyclic secondary alkyl amines
range from 89 to 96%, whereas the yield is in trace amount for
Table 4 Amidation of aldehydes and amines catalyzed by complex 3a
the reaction of benzaldehyde with primary alkyl amine, such as
benzylamine (Table 4, 13af) and butylamine (Table 4, 13ag).
This difference can be attributed to the fact that the primary
alkyl amine can react easily with aldehyde to form imine and
water, and the latter can decompose the catalyst. However, the
reactions of aniline with aldehydes give the amide in good
isolated yield (Table 4, 13ah–13dh), which reecting the rela-
tively lower reactivity of primary aromatic amine with aldehyde
in comparison with primary aliphatic amine. The presence of
electron-withdrawing group on the aniline has no obvious
inuence on the transformation, but the presence of electron-
donating group on the aniline is disadvantageous for the
amidation reaction. For example, the yield of the reaction of p-
uoro-aniline with benzaldehyde is similar to that of unsub-
stituted aniline, whereas the yield decreases obviously for the
reaction of p-methoxy aniline (Table 4, 13ai and 13aj). Methyl-
phenylamine is also a suitable substrate for this transformation
and a 72% yield was obtained under the same reaction condi-
tions (Table 4, 13ak).
To our knowledge, there is no report concerning the
synthesis of diamides from the amidation of aldehydes with
diamines. Therefore, we also explored the amidation of alde-
hydes and diamines catalyzed by complex 3 under the same
reaction conditions (Scheme 2). This is the rst report that
a rare-earth complex can catalyze the direct amidation of
a
Amine was rst added to the catalyst solution, aer 30 minutes, 3
b
equimolar of aldehyde was added. Isolated yield based on amine.
c
Reaction temperature was 60 ꢁC. d Reaction time was 24 h.
Scheme
2
Amidation of aldehydes with diamines catalyzed by
complex 3.
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diamine with aldehyde. The reaction of benzaldehyde 11a with
piperazidine 12m was rst examined to optimize the reaction
conditions. It was found that the molar ratio of aldehyde and
diamine has signicant inuence on the reaction, and 5
equivalent of aldehyde is required to generate the diacylamide
in a high yield, because aldehyde acts as not only a reactant, but
also an oxidant. In accordance with that observed above, the
aromatic aldehyde with electron-withdrawing group showed
higher activity in comparison with the aldehyde with electron-
donating group for the reaction of aldehyde with diamine. For
instance, the reaction of p-uoroaldehyde 11b with piperazidine
12m gave the expected diamide 13bm in 90% isolated yield,
whereas the similar reaction of p-methoxyaldehyde 11e with
piperazidine 12m gave the diamide 13em in only 45% yield
under the same reaction conditions. The bulkiness of the
diamine has a profound effect on the reaction. The reaction of
benzaldehyde 11a with tetrahydroquinoxaline 12n generated
diacylamide 13an in only 5% yield and generated mono-
acylamide in 39% yield under the same reaction conditions.
However, the bis-amidation reaction is not applicable for the
reaction of aromatic aldehyde with primary diamine or the
reaction of aliphatic bisaldehyde with primary and secondary
amine.
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Conclusions
In summary, a series of anionic organo-rare-earth amido
complexes stabilized by dianionic phenoxy-amido ligands were
synthesized, and well characterized. The application of the rare-
earth complex stabilized by phenoxy-amido ligand on organic
transformation was explored for the rst time. It was found that
these heterobimetallic organo-rare-earth complexes are efficient
catalysts for the amidation reactions of aldehydes with amines
to produce amides in good to excellent yields under mild
reaction conditions. The new catalyst system has a wide range of
scope for aromatic aldehyde and amine, and in some cases,
diacylamide compounds can be prepared conveniently by the
reaction of aromatic aldehyde with cyclic secondary diamine in
this system.
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Acknowledgements
Financial support from the National Natural Science Founda-
tion of China (Grants 21172165, 21132002, and 21372172), the
PAPD, the Major Research Project of the Natural Science
Foundation of the Jiangsu Higher Education Institutions
(Project 14KJA150007) and the Qing Lan Project is gratefully
acknowledged.
18 D. J. H. Emslie, W. E. Piers, M. Parvez and R. McDonald,
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