Synthesis and characterization of bis(amidate) rare-earth metal amides and their application in catalytic addition of amines to carbodiimides

Article abstract of DOI:10.1039/c5nj00506j

Two new bis(amidate) lanthanide amides {LLn[N(SiMe₃)₂]·THF}₂ (H₂L = N,N′-(cyclohexane-1,2-diyl)-bis(4-tert-butylbenzamide); Ln = Sm(4), Yb(5)), which were prepared by the treatment of the bridged amide proligand H₂L with Ln[N(SiMe₃)₂]₃ in tetrahydrofuran, had been characterized by single-crystal X-ray diffraction and elemental analyses. Both complexes 4 and 5 and the three known isomorphs {LRE[N(SiMe₃)₂]·THF}₂ (RE = La(1), Nd(2), Y(3)) were successfully employed in the addition of amines to carbodiimides for the first time and were found to be efficient catalysts in the transformation at 60°C under solvent-free conditions. The Nd-based catalyst 2 showed the highest reactivity and provided various guanidines with good functional group tolerance in high to excellent yields.

Full text of DOI:10.1039/c5nj00506j

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Synthesis and characterization of bis(amidate)
rare-earth metal amides and their application in
catalytic addition of amines to carbodiimides†
Cite this:DOI: 10.1039/c5nj00506j
Hao Cheng, Yang Xiao, Chengrong Lu, Bei Zhao,* Yaorong Wang and Yingming Yao*
Two new bis(amidate) lanthanide amides {LLn[N(SiMe3)2]ꢀTHF}2 (H2L = N,N0-(cyclohexane-1,2-diyl)-bis(4-tert-butyl-
benzamide); Ln = Sm(4), Yb(5)), which were prepared by the treatment of the bridged amide proligand H2L with
Ln[N(SiMe3)2]3 in tetrahydrofuran, had been characterized by single-crystal X-ray diffraction and elemental analyses.
Both complexes 4 and 5 and the three known isomorphs {LRE[N(SiMe3)2]ꢀTHF}2 (RE = La(1), Nd(2), Y(3)) were
successfully employed in the addition of amines to carbodiimides for the first time and were found to be efficient
catalysts in the transformation at 60 1C under solvent-free conditions. The Nd-based catalyst 2 showed the highest
reactivity and provided various guanidines with good functional group tolerance in high to excellent yields.
Received (in Montpellier, France)
28th February 2015,
Accepted 16th June 2015
DOI: 10.1039/c5nj00506j
www.rsc.org/njc
the requirements for some reaction conditions, for example, the
relatively high temperature (80–100 1C), the use of organic solvents
Introduction
Guanidines have an important molecular framework, which shares (benzene, THF, toluene, etc.) and a slightly large dosage of the
a Y-shaped CN3 functional group,1 and are found in many bio- catalysts (0.5–3 mol%), were dissatisfactory and in need of improve-
logically and pharmaceutically active compounds.2 They are also ment. Thus, seeking new kinds of high efficiency catalysts to realize
widely used as ligands in the design of a variety of high efficiency the addition of amines to carbodiimides, which performed well
catalysts in many reactions.3 Meanwhile guanidines themselves as under mild reaction conditions, is of importance.
bases could directly catalyze many organic synthesis reactions, such
Very recently, a bisamide proligand H2L (H2L = N,N0-(cyclo-
as the Michael reaction, the Henry reaction, the Wittig reaction and hexane-1,2-diyl) bis(4-tert-butylbenzamide)) was successfully
the Strecker reaction.4 Metal catalytic addition of amines to carbo- designed for the preparation of three bridged bis(amidate)
diimides is a well-known methodology in guanidine synthesis, rare-earth metal amides {LRE[N(SiMe3)2]ꢀTHF}2 (RE = La(1),
since it is a highly atom-economic reaction, and the commonly Nd(2), Y(3)). The complexes 1–3 had proved to be excellent
used catalysts include main group metal-based catalysts,5 transition catalysts for the direct carboxylation by introduction of CO2
metal-based catalysts6 and rare-earth metal catalysts.7
into terminal alkynes at ambient pressure.8 To further explore
Rare-earth metal amides have shown highly efficient reactivities the application of these kinds of amides, we prepared two more
in the addition reaction of amines to carbodiimides. Wang firstly bis(amidate) rare-earth metal amides {LLn[N(SiMe3)2]ꢀTHF}2
employed a simple lanthanide amide [(Me3Si)2N]3Yb(m-Cl)Li(THF)3 (Ln = Sm(4), Yb(5)) via the treatment of the bridged amide proligand
to catalyze the reaction using both primary and secondary amines H2L with Ln[N(SiMe3)2]3. Then, five bridged bis(amidate) rare-earth
as the substrates and brought out good yields.7b This finding metal amides were screened for the addition of amines to
aroused several chemists’ enthusiasm and many kinds of rare-earth carbodiimides, and it was found that all the complexes are
metal amides with different ancillary ligands had been screened for efficient catalysts for the transformation. Fortunately, various
this reaction subsequently, such as indenyl, cyclopentadienyl, guanidines with good functional group tolerance were obtained
pyrrolyl, N-heterocyclic carbene and salen groups.7c–e,i–l However, in high to excellent yields using the highest reactive neodymium
catalyst 2 at 60 1C under solvent-free conditions.
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Dushu Lake Campus,
Soochow University, Suzhou 215123, People’s Republic of China.
Results and discussion
E-mail: zhaobei@suda.edu.cn, yaoym@suda.edu.cn; Fax: +86-512-65880305;
Tel: +86-512-65880305
Synthesis and characterization of bridged bis(amidate)
† Electronic supplementary information (ESI) available: General procedures for the
preparation of complexes, detailed crystallographic data for complexes 4 and 5, and
figures depicting solid state structures. CCDC 1017172 (4) and 1017175 (5). For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c5nj00506j
rare-earth metal amides
Firstly, three known bridged bis(amidate) lanthanide amides
1–3 {LRE[N(SiMe3)2]ꢀTHF}2 (RE = La(1), Nd(2), Y(3)) were prepared
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each samarium atom. It is the bridged oxygen atom in the
k2-(N,O):m-O bound amido group that coordinates to the other
samarium atom in the cell to connect two subunits. Each
cyclohexane-1,2-diyl group adopts a low-energy state chair conforma-
tion to stabilize the whole skeleton of crystals. Fig. 1 shows the
structure of the bridged bis(amidate) samarium amide 4 and some
selected bond lengths and bond angles. Because of the similarity of
the structural features of complexes 4 and 5, the detailed bond
parameters of complex 5 are omitted here.
Scheme 1 Synthesis of the bridged bis(amidate) lanthanide amides 4 and 5.
according to our previous work.8 In order to see about the effects of
central metals on the addition of amines to carbodiimides, the
bridged bis(amidate) lanthanide amides 4 and 5 were subsequently
prepared by simple metathesis reactions of the proligand H2L with
appropriate lanthanide precursors Ln[N(SiMe3)2]3 in THF at room
temperature overnight. The outcome of the common silylamine
elimination reaction was two desired bridged bis(amidate)
lanthanide amides {LLn[N(SiMe3)2]ꢀTHF}2 (Ln = Sm(4), Yb(5))
in good yields (Scheme 1).
Suitable single crystals of new complexes 4 and 5 were
obtained from tetrahydrofuran solution and the solid state
structures were determined by X-ray diffraction analysis. As
expected, both of them are crystallized in the triclinic crystal
Synthesis of guanidines by the reaction of amines with
carbodiimides catalyzed by bridged bis(amidate) rare-earth
metal amides
It is known that there are many successful cases of rare-earth
metal amides published regarding the catalytic addition reactions
of amines to carbodiimides.7b–e,i–l Curiosity evoked us to try bridged
bis(amidate) rare-earth metal amides 1–5 in the template reaction
of phenylamine (6a) with N,N0-diisopropylcarbodiimide and make
clear whether they could perform better. To our delight, an
excellent yield of 97% of the target product 1,3-diisopropyl-2-
phenylguanidine (7a) was obtained using complex 1 as the catalyst
(Table 1, entry 1). Immediately, the effects of the central metals
on different complexes were tested, and complexes 3 and 5 with
smaller ionic radii had the obvious tendency of decreasing
reactivities in this transformation (Table 1, entries 1–5). When
the dosage of the catalyst was reduced to half of the original, still
there was no significant gap between the catalytic reactivities of
complexes 1, 2 and 4 (Table 1, entries 6–8). Until the catalyst
loading was cut down to 0.05 mol%, different reactivities
between complexes 1, 2 and 4 emerged (Table 1, entries 9–11).
%
system, P1 space group, and have binuclear centrosymmetric
structures with three THF molecules in each unit cell. Since
complexes 4 and 5 are isomorphous, only the molecular structure
of the Sm-based complex 4 is depicted in Fig. 1 as a representation.
It shows that complex 4 is composed of two equivalent subunits
with one Sm(III) center for each moiety. Each bridged bis(amidate)
group adopts two similar bonding modes, namely k2-(N,O):m-O
bonding and monodentate O-bonding modes, to coordinate to
Table 1 Effects of catalysts, temperature and solvents on the preparation
of guanidines
Entry
Cat/mol%
Solvent
T/1C
t/min
Yielda/%
1
2
3
4
5
6
7
8
1/0.25
2/0.25
3/0.25
4/0.25
5/0.25
1/0.125
2/0.125
4/0.125
1/0.05
2/0.05
4/0.05
2/0.15
2/0.2
2/0.2
2/0.2
2/0.2
2/0.2
2/0.2
Nd[N(SiMe3)2]3/0.2
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
Tol
THF
—
60
60
60
60
60
60
60
60
60
60
60
60
60
25
40
80
60
60
60
15
15
15
15
15
30
30
30
30
30
30
15
15
15
15
15
15
15
15
97
96
65
96
70
87
90
89
22
34
30
71
98
17
50
88
33
36
67
9
10
11
12
13
14
15
16
17
18
19
Fig. 1 Structure of bridged bis(amidate) samarium amide 4, all hydrogen
atoms and THF molecules in the unit cell are omitted for clarity. Selected
bond lengths (Å) and bond angles (deg): Sm(1)–O(1) 2.196(3), Sm(1)–N(3)
2.308(3), Sm(1)–O(2A) 2.364(3), Sm(1)–O(3) 2.494(3), Sm(1)–N(2) 2.500(3),
Sm(1)–O(2) 2.550(3), O(1)–Sm(1)–O(3) 79.44(11), O(3)–Sm(1)–O(2A)
76.75(11), O(2A)–Sm(1)–N(2) 117.02(10), N(2)–Sm(1)–O(1) 77.53(11), N(3)–Sm(1)–
O(2) 134.62(11), O(2A)–Sm(1)–O(2) 66.13(10).
a
Isolated yields.
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Finally, complex 2 of the central Nd metal with a medium ionic good to excellent yields of the corresponding guanidines were still
radius was selected to be an ideal catalyst to give us an excellent gained as expected (Table 2, 7n–q). For the same substrate of
catalytic outcome in consideration of the efficiency and handy p-nitroaniline, the comparative outcome can be achieved at
operation, while only 67% of the product was produced in the 110 1C for 36 h with 2 mol% catalyst loading of Wang’s catalyst
7c
presence of the corresponding precursor Nd[N(SiMe3)2]3 [Z5-(CH2)2(C9H6)2]YN(TMS)2 or at 60 1C for 12 h with 2 mol%
(Table 1, entries 13 and 19). Different temperatures and the catalyst {(m-Z5:Z1):Z1-2-[(2,4,6-Me3C6H2)NCH2][C4H3N]SmN(SiMe3)2}2,7i
addition of polar or nonpolar solvents were also investigated, while Shen got a better result at 60 1C for 12 h with 0.5 mol%
and the optimal reaction condition was proved to be solvent-free catalyst [4-OMe-C6H4COCH(CNCHCHNiPr)]2 YbN(TMS)2.7k This
at 60 1C (Table 1, entries 13–18).
shows that there are a few electronic effects of substituents on
Exploration of the substrate scope was carried out soon. the reaction of amines with carbodiimides, however the steric
Various amines were screened at 60 1C under solvent-free hindrance has a little effect on the reactivities. For the relatively
conditions for an appropriate reaction time in the presence of large sterically hindered 2,6-diisopropyl aniline, the catalytic
0.2 mol% catalyst 2 and the results are shown in Table 2. When the efficiency markedly decreased to a yield of 64% after 24 h, while
substrates on aromatic amines are electron donating groups, aniline and o-methyl aniline underwent reaction for 15–20 min to
excellent yields of guanidines that varied from 90–98% were give the corresponding products in 98% and 90% yields, respec-
obtained in short time (Table 2, 7c–f). The outcome seems much tively (Table 2, 7a, 7d and 7s). The high efficiency catalyst system
better than the results of previous work.7c,j The aromatic amines also performed excellent in the case of heteroaromatic amines,
bearing electron withdrawing groups (EWGs) also gave us excellent secondary aromatic amines and alicyclic amines (Table 2, 7t–v).
yields of 92–98% with high efficiency (Table 2, 7g–l). It is worthy of No obvious effects on the reaction of the substituents of
note that although the stronger EWGs, the nitro group and the carbodiimide were observed and excellent yields were obtained
trifluoro group, on phenyl rings retarded the reaction process, in all cases of N,N0-dicyclohexylcarbodiimide (Table 2, 7b, 7e,
7h, and 7l).
Table 2 Substrate scope for the synthesis of guanidines via the treatment
Possible mechanism of the synthesis of guanidines by the
of amines with carbodiimidesa
reaction of amines with carbodiimides catalyzed by bridged
bis(amidate) rare-earth metal amides
Based on some literatures,7b–e,m we proposed a possible mechanism
for our catalytic transformation in Scheme 2. In the first step, in
the reaction of substrate amines with the rare-earth metal
amides, a new amido intermediate A was generated via aminolysis.
With the introduction of carbodiimide into the RE–N bond, a metal
guanidinate species B was produced. In the presence of a large
amount of amines, the regeneration of A was instantly completed
and the desired product C was achieved. If R2 is hydrogen atom,
generated C followed by the 1,3-proton migration released the
target guanylated product.
Scheme 2 A possible mechanism for the synthesis of guanidines by the
reaction of amines with carbodiimides catalyzed by bridged bis(amidate)
rare-earth metal amides.
a
Isolated yields.
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Conclusion
(1674.75): C 54.39, H 7.57, N 5.01, Yb 20.62; Found: C 54.51,
H 7.50, N 5.20, Yb 20.54. IR: (KBr, cmꢁ1): 2963(w), 2361(m),
2337(m), 1634(m), 1541(s), 1507(w), 1384(s), 1236(s), 1219(s),
1157(vs), 505(s).
In summary, totally five bridged bisamidate rare earth metal
amides {LRE[N(SiMe3)2]ꢀTHF}2 (RE = La(1), Nd(2), Y(3), Sm(4),
and Yb(5); H2L = N,N0-(cyclohexane-1,2-diyl)-bis(4-tert-butyl-
benzamide)) had been prepared by the reaction of the bridged
amide proligand H2L with RE[N(SiMe3)2]3 in good yields. X-ray
structural analysis shows that two new compounds 4 and 5 are
isomorphic as the known complexes 1–3 with binuclear centro-
symmetric structures. This is the first time that the bridged
bisamidate rare earth metal amides were successfully employed
in the preparation of valuable guanidines by the reaction of
amines with carbodiimides. Complex 2 with the central metal
Nd performed the best among those amides and gave us high to
excellent yields of guanidines with good tolerance of substituents. It
is a convenient access to synthesize important 1,3-disubstituted-2-
phenylguanidines and their derivatives. Much effort to make clear
the catalytic mechanism and explore new efficient catalysts is still in
process in our lab.
X-ray structure determination of complexes 4 and 5
Owing to their air and moisture sensitivity, suitable single
crystals of complexes 4 and 5 were each sealed in thin-walled
glass capillaries. Intensity data were collected on a Rigaku
Mercury CCD equipped with graphite-monochromatized
Mo Ka (l = 0.71075 Å) radiation. Details of the intensity data
collection and crystal data are given in Tables S1 and S2 in the
ESI.† The crystal structures of these complexes were solved by
direct methods, expanded by Fourier techniques and refined by
using the SHELXL-2013 program.10 Atomic coordinates and
thermal parameters were refined by full matrix least-square
analysis on F2. All the non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were all generated geometrically
with appropriate isotropic thermal parameters assigned.
General procedure for the synthesis of guanidines by the
reaction of amines with carbodiimides
Experimental
General procedure
Take complex 2, for example. A 10.0 mL Schlenk tube under
dried argon was charged with complex 2 (0.002 equiv.) and
aniline (1.0 equiv.) in turns. After stirring for 10 min, carbodiimide
(1.0 equiv.) was added. The resulting mixture was stirred at 60 1C for
an appropriate time, then hydrolyzed by water (1 mL), extracted with
dichloromethane (3 ꢂ 10 mL), and the organic layer was dried over
anhydrous Na2SO4 and filtered. The solvent was removed under
vacuum to afford the target product.
All manipulations and reactions were conducted under a
purified argon atmosphere by using vacuum line techniques
or in a glovebox. Rare-earth metal precursors RE[N(SiMe3)2]3
(RE = La, Nd, Sm, Yb, Y) and the proligand N,N0-(cyclohexane-
1,2-diyl)bis(4-tert-butylbenzamide) were prepared according to
literature methods.9 Solvents were distilled from sodium benzo-
phenone ketyl under argon prior to use. Solid amines were
degassed before use, and liquid amines were distilled from
CaH2 prior to use. The single crystal X-ray diffraction data were
recorded on a Rigaku Mercury CCD X-ray diffractometer. 1H and
13C NMR spectra were obtained on a Bruker-400 spectrometer in
CDCl3. Metal analyses were carried out by a complexometric
titration. Carbon, hydrogen, and nitrogen analyses were per-
formed by direct combustion on a Carlo-Erba EA-1110 instru-
ment. Melting points of the complexes were determined in
sealed Ar-filled capillary tubes and are uncorrected.
Acknowledgements
We gratefully acknowledge financial support from the National
Natural Science Foundation of China (Grant No. 21172165,
21132002 and 21372172), PAPD, the Major Research Project of
the Natural Science Foundation of the Jiangsu Higher Education
Institutions (Project 14KJA150007), and the Qing Lan Project.
General procedure for the synthesis of {LRE[N(SiMe3)2]ꢀTHF}2
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