Divalent lanthanide complexes: Highly active precatalysts for the addition of N-H and C-H bonds to carbodiimides

Article abstract of DOI:10.1021/jo801693z

(Chemical Equation Presented) Various divalent lanthanide complexes with the formula LnL₂(sol)_x (L = N(TMS)₂, sol = THF, x = 3, Ln = Sm (I), Eu (II), Yb (III); L = MeC₅H₄, sol = THF, x = 2, Ln = Sm (IV); L = ArO(Ar = [2,6-(^tBu)₂-4- MeC₆H₂]), sol = THF, x = 2, Ln = Sm (V)), especially complexes I-III, serve as excellent catalyst precursors for catalytic addition of various primary and secondary amines to carbodiimides, efficiently providing the corresponding guanidine derivatives with a wide range of substrates under solvent-free condition. The reaction shows good functional groups tolerence. Complexes I-III are also excellent precatalysts for addition of terminal alkynes to carbodiimides yielding a series of propiolamidines. The active sequence of Yb < Eu < Sm for metal and MeC₅H₄ < ArO < N(TMS)₂ for ligand around the metal was observed for both reactions. The first step in both reactions was supposed to include the formation of a bimetallic bisamidinate samarium species originating from the reduction-coupling reaction of carbodiimide promoted by lanthanide(II) complex. The active species is proposed to be a lanthanide guanidinate and a lanthanide amidinate.

Full text of DOI:10.1021/jo801693z

Divalent Lanthanide Complexes: Highly Active Precatalysts for the
Addition of N-H and C-H Bonds to Carbodiimides
Zhu Du, Wenbo Li, Xuehua Zhu, Fan Xu, and Qi Shen*
Key Laboratory of Organic Synthesis of Jiangsu ProVince, Department of Chemistry and Chemical Engineering,
Suzhou UniVersity, Dushu Lake Campus, Suzhou UniVersity, Suzhou 215123, People’s Republic of China, and
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Academia Sinica,
Shanghai 200032, People’s Republic of China
qshen@suda.edu.cn
ReceiVed July 30, 2008
Various divalent lanthanide complexes with the formula LnL2(sol)x (L ) N(TMS)2, sol ) THF, x ) 3,
Ln ) Sm (I), Eu (II), Yb (III); L ) MeC5H4, sol ) THF, x ) 2, Ln ) Sm (IV); L ) ArO(Ar )
[2,6-(tBu)2-4-MeC6H2]), sol ) THF, x ) 2, Ln ) Sm (V)), especially complexes I-III, serve as excellent
catalyst precursors for catalytic addition of various primary and secondary amines to carbodiimides,
efficiently providing the corresponding guanidine derivatives with a wide range of substrates under solvent-
free condition. The reaction shows good functional groups tolerence. Complexes I-III are also excellent
precatalysts for addition of terminal alkynes to carbodiimides yielding a series of propiolamidines. The
active sequence of Yb < Eu < Sm for metal and MeC5H4 < ArO < N(TMS)2 for ligand around the
metal was observed for both reactions. The first step in both reactions was supposed to include the formation
of a bimetallic bisamidinate samarium species originating from the reduction-coupling reaction of
carbodiimide promoted by lanthanide(II) complex. The active species is proposed to be a lanthanide
guanidinate and a lanthanide amidinate.
Introduction
metals.3 Therefore, synthesis of guanidines and amidinates
has attracted increasing attention. The direct guanylation of
The formation of C-C and C-N bonds catalyzed by
lanthanide complexes is an active research area.1 Guanidines
and amidinates are important structural motifs found in many
biologically and pharmaceutically active compounds.2 Guanid-
inate and amidinate derivatives can also serve as ancillary
ligands in the preparation of a variety of metal complexes,
including those of the main, transition, and lanthanides
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8966 J. Org. Chem. 2008, 73, 8966–8972
10.1021/jo801693z CCC: $40.75 2008 American Chemical Society
Published on Web 10/21/2008

DiValent Lanthanide Complexes
SCHEME 1
amine with carbodiimides provides a convenient and atom-
economical approach to guanidines. However, the guanylation
of amines without catalyst requires harsh conditions.4
Recently, catalytic guanylation of amines with carbodiimides
has been reported by using imido complexes of titanium and
vanadium,5 and metal amide complexes as precatalysts,
including LiN(SiMe3)2,6 lanthanide amides,1g,h and titanac-
arborane amide.7 The former imido complexes are efficient
catalysts only for the guanylation of primary aromatic amines
with carbodiimides as the active species being an imido
intermediate. The metal amide systems can catalyze guany-
lation of amines with carbodiimides to afford the correspond-
ing substituted guanidines with a wide scope of amines
including primary aromatic amines and aliphatic secondary
amines. In those cases a metal guanidinate formed via
nucleophilic addition of metal amide to carbodiimide is
proved to be the active species. Half-sandwich lanthanide
alkyl complexes were also reported to serve as another kind
ofprecatalystforefficientsynthesisofsubstitutedguanidines.1b,c
The catalytic reaction proceeds through nucleophilic addition
of an amide moiety formed in situ by acid-base reaction
between a lanthanide alkyl bond and an amine N-H bond
to a carbodiimide. The addition of terminal alkynes across
carbodiimides has been first proven to be a straightforward
route to propiolamidines by Hou’s group using half-sandwich
lanthanide alkyl complexes as precatalysts.1a Then, lithium
amides and lanthanide amides were also found to be efficient
precatalysts.6,1g
TABLE 1. Catalytic Guanylation of an Aniline with an
N,N′-Diisopropylcarbodiimidea
catalyst
temp/ time/
°C
entry cat. loading (mol%)
R
min product yield (%)b
1
I
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1
0.5
0.5
0.5
0.5
3
F
rt
rt
3
4
1
3
3
1
3
1
3
3
3
3
3
3
1
1
3
98
98
96
80
85
87
91
97
82
90
87
88
83
94
95
2
I
I
II
H
H
F
H
F
H
H
H
H
H
H
F
3c
4
rt
rt
40
6
5
II
rt
4
6
III
III
III
IV
IV
V
V
VI
VII
rt
rt
60
rt
60
rt
60
60
rt
60
45
180
120
70
80
52
360
14
240
7
8c
9
10
11
12
13
14
0.5
1
F
H
15d VIII
60
Divalent lanthanide complexes are efficient single-electron
transfer reagents which have been widely used as precatalysts
in organic synthesis.8 Prompted by theses results, we considered
that divalent lanthanide complexes might be used as efficient
precatalysts in the addition of N-H and C-H bonds to
carbodiimides, because the reduction-coupling of carbodiimides
can be promoted by a divalent metallocene samarium complex
(MeC5H4)2Sm(THF)2,3k and a divalent amide complex Sm[N-
(TMS)2]2(THF)3,9 to produce the corresponding bimetallic
bisamidinate samarium complex. Herein, we report the high
catalytic activity of divalent lanthanide complexes for addition
of amines and terminal alkynes to carbodiimides. The lanthanide
guandinate and lanthanide amidinate were supposed to generate
in situ by insertion of carbodiimide into lanthanide amide or
lanthanide alkynyl intermediate formed through protonation of
the bisamidinate, which was produced through the reduction-
coupling reaction of carbodiimides mediated by a divalent
a 1 mmol of p-flouroaniline or aniline,
1
mmol of
N,N′-diisopropylcarbodiimide. b Isolated yields. c The reaction was run
in THF, Ccat. ) 0.019 mmol/mL. d Reference 7.
complex. The influence of ancillary ligand around the central
metals on reactivity is also discussed.
Results and Discussion
Aromatic amines do not react with diisopropylcarbodiimide
even with prolonged reaction time and raised reaction temper-
ature to 100 °C without a catalyst. An addition of 0.5 mol % of
Sm[N(TMS)2]2(THF)3 (I)10a led to efficient catalytic guanylation
of aniline with diisopropylcarbodiimide under solvent-free
condition to afford a quantitative yield of guanidine 3 at room
temperature in 4 min. The result prompted us to survey the
reactivity of various divalent complexes. Thus, a series of
divalent complexes were synthesized by metathesis reaction of
LnI2 with metal salts, including Ln[N(TMS)2]2(THF)3 (Ln )
Eu (II), Yb (III)),10b,c Sm(MeC5H4)2(THF)2 (IV),11 Sm(ArO)2-
(THF)2 (Ar ) [2,6-(tBu)2-4-MeC6H2]) (V),12 and SmI2 (VI)
(Scheme 1). For comparison, trivalent lanthanide amide com-
plexes Sm[N(TMS)2]3(µ-Cl)Li(THF)3 (VII)13 and Yb[N(T-
MS)2]3(µ-Cl)Li(THF)3 (VIII)1h were also prepared. The two
reactions of aniline and p-flouroaniline with diisopropylcarbo-
diimide, respectively, were then conducted using these com-
plexes. As shown in Table 1 all the complexes are effective.
The reactivity depends greatly on the central metals with the
active trend of Yb < Eu < Sm (Table 1 entries 1-7). The ligand
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Du et al.
TABLE 2. Catalytic Addition of Primary Aromatic Amines to Carbodiimidesa
a The reaction was performed by treating 1 equiv of amines with 1 equiv of carbodiimides at rt. b Isolated yields. c 60 °C.
around the central metal shows a significant influence. The active
sequence of -I < -MeC5H4 < -OAr < -N(TMS)2 was
observed, which indicated a combination of Lewis acidic metal
with a stronger basic ligand led to a more active complex (Table
1, entries 1, 4, 6, 9, 11, and 13). It was noticed that divalent
amide complex is even more active than the trivalent amide
Ln[N(TMS)2]3(µ-Cl)Li(THF)3 (Ln ) Sm (VII) and Yb7 (VIII))
(Table 1 entries 14 and 15). For example, the reaction of aniline
with diisopropylcarbodiimide catalyzed by III afforded the
product 3 in 97% yield for 3 h in THF solution at 60 °C, while
the same reaction with Yb[N(TMS)2]3(µ-Cl)Li(THF)3 gave 3
in 95% yield for 4 h (Table 1 entries 8 and 15). Besides, the
reaction of p-flouroaniline with diisopropylcarbodiimide by I
yielded the product 1 in 98% yield at room temperature in 3
min without solvent, while the same reaction with VII gave 1
in 22% for 3 min and 94% for 14 min, respectively. This may
be because of the presence of a coordinated LiCl.
The reaction went much faster under solvent-free condition
than that in THF solvent (Table 1, entries 2 and 3), Thus, I
was then chosen as the precatalyst for the guanylation of various
primary aromatic amines with carbodiimides under solvent-free
condition. Representative results are summarized in Table 2.
As can be seen from Table 2, I is a very robust and efficient
catalyst. All these reactions with primary aromatic amines
could take place at room temperature with use of only 0.5
mol % of I to yield the corresponding N,N′,N′′-trisubstituted
guanidines in excellent yields (Table 2, entries 1-17). The
reaction was not influenced by either electron-withdrawing
or electron-donating substituents at the phenyl group. The
catalyst system showed good functional group tolerance. The
8968 J. Org. Chem. Vol. 73, No. 22, 2008

DiValent Lanthanide Complexes
TABLE 3. Catalytic Addition of Aliphatic Amines to Carbodiimidesa
a The reaction was performed by treating 1 equiv of amines with 1 equiv of carbodiimides at 80 °C for 3 h. b Isolated yields.
TABLE 4. Catalytic Addition of a Phenylacetylene with an
group of Cl, Br, and even NO2 at the phenyl ring remained
unchanged in the present reactions (Table 2, entries 13, 14,
and 17). Notably, the reaction of diisopropylcarbodiimide
with an aromatic amine bearing two bulky substituents on
both ortho positions at the phenyl ring can also occur
smoothly to give the corresponding guanidine in excellent
yield, although the reaction requires a higher reaction
temperature (60 °C) and prolonged reaction time (12 h) due
to steric hindrance (Table 2, entry 16).
The reactions with secondary aliphatic amines also went
smoothly, even though secondary amines are generally less
active than primary amines. In the presence of 0.5 mol % of I,
various acyclic and cyclic secondary amines could react with
diisopropylcarbodiimide and dicyclohexylcarbodiimide at 80 °C
to afford the corresponding guanidines in excellent yields after
3 h (Table 3, entries 1-9). The reaction with 1-methylpiperazine
can still give good yield, even if it is less active than other cyclic
secondary amines (Table 3, entries 7 and 8). Examination of
the reactivity of n-BuNH2 with diisopropylcarbodiimide gave
satisfactory yield (94%, entry 10).
The excellent results obtained in addition of amines to
carbodiimides encouraged us to further survey the catalytic
reactivity of divalent lanthanide complexes for addition of
terminal alkynes to carbodiimides. The reactions of pheny-
lacetylene with 1,3-diisopropylcarbodiimide were first examined
at 80 °C with complexes I-V at 1 mol % loading under various
conditions. The results are listed in Table 4. All reactions went
smoothly providing propiolamidine 28 in good to excellent
yields depending both on the central metals and the ligand
around the metal. The same active sequences as those found
for guanylation were observed: Yb < Eu < Sm (Table 4, entries
1-3) and -MeC5H4 < -OAr < -N(TMS)2 (Table 4, entries
1, 4, and 5). In contrast to the addition of amines to carbodi-
imides, the present reaction in THF is more active than that in
solvent-free condition (Table 4, entries 1 and 6).
N,N′-Diisopropylcarbodiimidea
catalyst
loading (mol%)
entry
cat.
temp/°C
time/h
yield (%)b
1
2
3
4
I
1
1
1
1
1
1
80
80
80
80
80
80
3
3
3
3
3
3
94
90
86
81
83
82
II
III
IV
V
I
5
6c
a 1 mmol of phenylacetylene,
1 mmol of N,N-diisopropylcarbo-
diimide, in THF, Ccat. ) 0.013 mmol/mL. b Isolated yields. c The
reaction was conducted under solvent-free condition.
(Table 5). In general, the addition reaction proceeds efficiently
at 80 °C to provide the desired product in excellent yields,
regardless of electron-donating substituents on the phenyl ring.
The results can be compared with those of half-sandwich yttrium
alkyl complexes1a and are somewhat better than those found
by indenyl lanthanide amides.1g
To understand the real active species, stoichiometric
reactions of I and IV respectively with N,N′-diisopropylcar-
bodiimide were conducted. After workup a light yellow
powder was obtained for both reactions. The powder was
supposed to be A and A′ as shown in eq 1. An attempt to
obtained the crystals of A and A′ suitable for X-ray analysis
was unsuccessful. Moreover, the 1H NMR data for the Sm(III)
complex A and A′ are not valuable. Therefore, the further
examination of A and A′ was conducted by the in situ reaction
of A and A′ with aniline. A small amount of biamidine
together with aniline and a trace of diketone from the
hydrolysis of the biamidine by contaminated water in the
system was obtained. But purifying the biamidine is very
difficulty because it is more polar and sensitive. Therefore
the hydrolysis of the reaction mixture of A and aniline, and
With reaction conditions optimized (Table 4, entry 1), we
then examined the scope of this divalent lanthanide amide-
catalyzed addition with selected alkynes and carbodiimides
J. Org. Chem. Vol. 73, No. 22, 2008 8969

Du et al.
TABLE 5. Catalytic Addition of Terminal Alkynes to Carbodiimidesa
a The reaction was performed by treating 1 equiv of alkynes with 1 equiv of carbodiimides at 80 °C. b Isolated yields.
SCHEME 2. Proposed Mechanism for Addition of Amines and Terminal Alkynes to Carbodiimides
A′ and aniline was conducted and led to the isolation of pure
diketone derived from biamidine. The isolation of diketone
indicates the bimetallic bisamidinate complexes A and A′
were really the product of the first reaction step in the reaction
process of carbodiimide with amine by I and IV, respectively
(Scheme 2). To confirm the formation of A and A′ is essential
for the catalytic reactions by I and IV, the reactions of
p-fluoroaniline and o-methylaniline with N,N′-diisopropyl-
carbodiimide using A respectively, and the reaction of aniline
with N,N′-diisopropylcarbodiimide using A′ were further
examined. As shown in Table 6, A or A′ is really a more
efficient catalyst related to I or IV, as the reaction with I or
IV required longer time versus those with A or A′ for
obtaining the same yields. On the basis of the above results,
a possible catalytic cycle by I for addition of amines and
terminal alkynes to carbodiimides is proposed in Scheme 2.
8970 J. Org. Chem. Vol. 73, No. 22, 2008

DiValent Lanthanide Complexes
TABLE 6. Catalytic Addition of Amines to Carbodiimides with I (IV) and A (A′)a
a The reaction was performed by treating 1 equiv of amines with 1 equiv of carbodiimides at rt. b Isolated yields.
temperature for 3 min. The reaction mixture was then hydrolyzed
Reaction of I with RNdCdNR affords a bimetallic bisa-
midinate complex A via reduction-coupling of RNdCdNR.
The protonolysis of A by an amine and alkyne releases a
biamidine and two amide and alkynyl species B and B′,
respectively. Nucleophilic addition of B and B′ to a carbo-
diimide affords the corresponding species C and C′. The
protonolysis of C and C′ by another amine and alkyne
molecule releases the product and generates B and B′.
with water (0.5 mL), extracted with dichloromethane (3 × 10 mL),
dried over anhydrous Na2SO4, and filtered. After the solvent was
removed under reduced pressure, the residue was recrystallized in
1
hexane to provide a white solid 3 (98% yield). H NMR (CDCl3)
δ 7.22 (2H), 6.95-6.91 (1H), 6.86-6.84 (2H), 3.77 (2H), 3.61 (2H),
1.17-1.15 (12H). 13C NMR (CDCl3) δ 150.5, 150.4, 129.4, 123.7,
121.5, 43.4, 23.6.
General Procedure 2: For the Direct Synthesis of Guanidines
from Reaction of Secondary Amines with Carbodiimides
Catalyzed by I (Product 18 as an Example). A 30 mL Schlenk
tube was charged with I (0.011 g, 0.018 mmol). To the flask were
added N,N′-diisopropylcarbodiimide (0.441 g, 3.50 mmol) and
pyrrolidine (0.248 g, 3.50 mmol). The resulting mixture was stirred
at 80 °C for 3 h. The reaction mixture was then hydrolyzed with
water (0.5 mL), extracted with hexane (3 × 10 mL), dried over
anhydrous Na2SO4, and filtered. After the solvent was removed
under vacuum, the final product 18 was obtained as a white powder
(98% yield). 1H NMR (CDCl3) δ 3.38 (2H), 3.26 (4H), 1.80 (4H),
1.11-1.10 (12H). 13C NMR (CDCl3) δ 153.7, 47.9, 46.8, 26.3, 24.8.
Conclusions
Divalent lanthanide complexes I-V were first found to be
highly active precatalysts for catalytic guanylation of amines
with carbodiimides under solvent-free condition with a wide
scope of amines. Complexes I-V are also efficient catalyst
precursors for addition of terminal alkynes to carbodiimides.
The active sequences of -I < -MeC5H4 < -ArO < -N(T-
MS)2 for ligands and Yb < Eu < Sm for metals were observed
for both reactions. The first step for both reactions was supposed
to include the formation of a bimetallic bisamidinate samarium
species originating from the reduction-coupling reaction of
carbodiimide promoted by a samarium(II) complex. The active
species is proposed to be a lanthanide guanidinate for addition
of amines and a lanthanide amidinate complex for addition of
terminal alkynes. Further studies on the applications of divalent
complexes in C-N bond formation are now underway.
General Procedure 3: For the Direct Synthesis of Propiola-
midines from Reaction of Terminal Alkynes with Carbodiim-
ides Catalyzed by I (Product 28 as an Example). A 30.0 mL
Schlenk tube in a drybox under dried argon was charged with I
(0.010 g, 0.015 mmol), phenylacetylene (0.153 g, 1.500 mmol),
and THF (1.20 mL). To the mixture was added N,N′-diisopropy-
lcarbodiimide (0.189 g, 1.5 mmol). The Schlenk tube was taken
outside and the mixture was stirred at 80 °C for 3 h. After the
solvent was removed under reduced pressure, the residue was
extracted with hexane and filtered to give a clean solution. The
solvent was evaporated under vacuum; recrystallization of the crude
1
from n-hexane afforded the product as a yellow solid (94%). H
NMR (CDCl3) δ 7.51-7.48 (2H), 7.39-7.33 (3H), 3.99-3.93 (2H),
1.17-1.16 (12H). 13C NMR (CDCl3) δ 141.0, 131.7, 129.1, 128.2,
121.2, 91.6, 91.4,.79.1, 45.8, 23.7.
General Procedure 4: For Synthesis and Characterization
i
of A and A′ (A ) [{(Me3Si)2N}2Sm(µ-C2N4 Pr4)Sm{N(SiMe3)2}2]
as an Example). Synthesis: A solution of N,N′-diisopropylcarbo-
diimide (0.204 g, 2 mmol) in THF (20 mL) was added at ambient
temperature with stirring to a purple solution of [Sm{N-
(SiMe3)2}2(THF)3] (0.1 M, 20 mL, 2 mmol) in hexane. The color
turned gradually to light yellow, and the mixture was stirred
continuously for 1 h. The volume of solvent was reduced to 20
mL and stored at -10 °C for 2 days. Light-yellow powders of A
were collected (1.03 g, 43%).
Experimental Section
General Procedure 1: For the Direct Synthesis of Guanidines
from Reaction of Aromatic Amines with Carbodiimides Cata-
lyzed by I (Product 3 as an Example). A 30 mL Schlenk tube
was charged with I (0.006 g, 0.009 mmol). To the flask were added
N,N′-diisopropylcarbodiimide (0.239 g, 1.90 mmol) and aniline
(0.177 g, 1.90 mmol). The resulting mixture was stirred at room
J. Org. Chem. Vol. 73, No. 22, 2008 8971

Du et al.
Characterization: Element Analysis and IR of A ) [{(Me3Si)2N}2
Sm(µ-C2N4iPr4)Sm{N(SiMe3)2}2]. Found: C, 37.59; H, 8.26; N, 9.11.
Anal. Calcd for C38H102N8Si8Sm2: C, 38.14; H, 8.59; N, 9.36. IR
(KBr) 2966 (s), 1646 (s), 1630 (s), 1496 (m), 1384 (m), 1364 (m),
solid (0.160 g, 93%). 1H NMR (CDCl3) δ 1.12-1.13 (12H),
3.78-3.88 (2H), 4.07-4.08 (2H). 13C NMR (CDCl3) δ 157.4, 42.5,
24.0.
Acknowledgment. We are grateful to the National Natural
Science Foundation of China (Grant nos. 20632040 and
20572077) for financial support.
1248 (w), 1176 (m), 1128 (w), 843 (w) cm-1
.
i
Isolation of Diketone O2C2H2N2 Pr2 from the Mixture of A
and 2 equiv of Aniline. A solution of A (1.196 g, 1 mmol) in
THF (10 mL) was added to aniline (0.186 g, 2 mmol) in THF (5
mL) with stirring at room temperature. After stirring for 1 h, distilled
water (10 mL) was added to the system and extracted with
dichloromethane (3 × 10 mL), then the organic phase was dried
over anhydrous Na2SO4 and filtered. The solvent was removed
under reduced pressure, then washed with OEt2 to obtain a white
Supporting Information Available: Experimental proce-
1
dures and characterizations and copies of H NMR and 13C
NMR of new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
JO801693Z
8972 J. Org. Chem. Vol. 73, No. 22, 2008