Zn(OTf)2-catalyzed addition of amines to carbodiimides: Efficient synthesis of guanidines and unpredicted formation of Zn-N amido species

Article abstract of DOI:10.1039/b923249b

Zn(OTf)₂ acts as an excellent catalyst precursor for addition of various amine N-H bonds to carbodiimides under an atmosphere of air, offering a convenient synthesis of substituted guanidines with high functional-group tolerance. A Zn-N amido species is shown to act as the active species.

Full text of DOI:10.1039/b923249b

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Zn(OTf)2-catalyzed addition of amines to carbodiimides: efficient synthesis of
guanidines and unpredicted formation of Zn–N amido species†
Dongzhen Li,a Jie Guang,a Wen-Xiong Zhang,*a,b Yang Wanga and Zhenfeng Xi*a
Received 5th November 2009, Accepted 21st January 2010
First published as an Advance Article on the web 8th February 2010
DOI: 10.1039/b923249b
Zn(OTf)2 acts as an excellent catalyst precursor for addition of various amine N–H bonds to
carbodiimides under an atmosphere of air, offering a convenient synthesis of substituted guanidines
with high functional-group tolerance. A Zn–N amido species is shown to act as the active species.
i
i
a
=
=
Table 1 Zn(OTf)2-catalyzed addition of aniline to PrN
C N Pr
Introduction
The search for readily available and cheap catalyst system for
more convenient and efficient chemical transformations is of great
interest and importance for academic and industrial research.
The general strategy is to explore the untouched elements with
the expectation superior to the known catalysts. Recently, cat-
alytic addition reaction of amine N–H bonds to carbodiimides
(catalytic guanylation reaction of amines or hydroamination of
carbodiimides) has received much current interest1,2 because it
provides a highly efficient and straightforward preparation of
multi-substituted guanidines serving as building blocks for many
biologically relevant compounds.3 The titanium imido complexes
were reported first in 2003 by Richeson and coworkers to effect
the catalytic addition of only primary aromatic amines to car-
Entry
cat. (mol%)
Solvent
T/◦C
Time/h
Yieldb(%)
1
2
3
4
5
6
7
8
—
AlCl3 (5)
FeCl3 (5)
ZnCl2 (5)
CuCl2 (5)
PdCl2 (5)
Pd(PPh3)4 (5)
Pd(OAc)2 (5)
Sc(OTf)3 (5)
La(OTf)3 (5)
Zn(OTf)2 (5)
Zn(OTf)2 (3)
Zn(OTf)2 (3)
Zn(OTf)2 (3)
C6D5Cl
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
THF-d8
140
80
80
80
80
80
80
80
80
80
80
80
80
80
24
3
3
3
3
10
24
2
3
3
3
5
6
6
1 (0)
1 (59)
1 (0)
1 (0)
1 (20)
1 (>95)
1 (60)
1 (95)
9
1 (25)
bodiimides because the catalytic process required the regeneration
10
11
12
13
14
1 (30)
4
=
of a “Ti N” double bond moiety. In 2006, a rare earth metal–
1 (quant.)
1 (quant.)
1 (quant.)
1 (quant.)
nitrogen single bond species generated in situ from a half-sandwich
rare earth metal alkyl complex and amines was reported by
Hou and coworkers to achieve the catalytic addition of both
primary and secondary amines to carbodiimides through the
nucleophilic addition mechanism of a metal–nitrogen single bond
to a carbodiimide.2h,i Montilla and coworkers reported that the
vanadium imido chloride Cl3V N(C6H3 Pr2-2,6) could also serve
Toluene-d8
a Conditions: aniline, 0.52 mmol; N,N¢-diisopropylcarbodiimide,
0.50 mmol. b Yields were determined by 1H NMR.
i
=
reported previously. We report here readily available Zn(OTf)2
acts as an excellent catalyst precursor for the addition of amine
N–H bonds to carbodiimides under a closed air atmosphere
yielding efficiently substituted guanidines with high functional-
group tolerance. The mechanistic studies show an unpredicted Zn–
N amido species is produced by release of a guanidinium triflate,
but acts as active intermediate in this process.
as a catalyst precursor for this addition, whose mechanism was
confirmed via the pathway of carbodiimide insertion into a V–N
amido bond combining the experimental and DFT calculation
methods.5 Other catalyst systems bearing M–N (M = Li, Ti, Ca,
Al, and lanthanides) amido moiety had been found to have high
catalytic activity.2 These reported catalyst systems were generally
sensitive to air and moisture and reactions were carried out under
the inert atmosphere. In contrast, the use of late transition metal
catalyst as well as the catalyst system open to air has not been
Results and discussion
Catalytic addition of primary aromatic amines to carbodiimides
aBeijing National Laboratory for Molecular Sciences (BNLMS), and Key
Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry
of Education, College of Chemistry, Peking University, Beijing 100871,
China
The neat aniline could not react with N,N¢-diisopropylcarbo-
i
i
= =
diimide PrN C N Pr at room temperature or higher tempera-
tures ◦(Table 1, entry 1). Gratifyingly, addition of 5 mol% of AlCl3
bState Key Laboratory of Elemento-Organic Chemistry, Nankai University,
Tianjin, 300071, China. E-mail: wx_zhang@pku.edu.cn, zfxi@pku.edu.cn;
Fax: +86 10 62751708; Tel: +86 10 62758294
i
i
= =
at 80 C led to rapid addition of aniline to PrN C N Pr to give
the N,N¢,N¢¢-trisubstituted guanidine 1 in 59% yield (entry 2). This
result encouraged us to screen various metal salts (entries 3–14).
Finally, Zn(OTf)2 was discovered the best choice for the reaction,
but the polarity of solvents did not show a significant influence on
the catalytic activity (entries 11–14).
† Electronic supplementary information (ESI) available: Experimental
details, X-ray data for 20 and 22, and scanned NMR spectra of all
new products. CCDC reference numbers 741355 + 741356. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b923249b
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Table 2 Catalytic addition of various amines to carbodiimidesa
A broad range of substituted aniline could be used for this
catalytic addition reaction, which was not affected by either
electron-withdrawing or -donating substituents or their positions
at the phenyl ring with high functional-group tolerance (Table 2,
entries 1–16). The aromatic C–X (X = F, Cl, Br, or I) bonds (entries
4–7), nitro NO2 (entry 12), and terminal alkyne unit (entry 9) can
survive the reaction conditions. It is noted that the reaction of
i
i
= =
4-acetylaniline and ethyl 4-aminobenzoate with PrN C N Pr
gave the corresponding products 15 and 16 in benzene at 80 C,
Product
(yield/%)b
◦
Entry Ar–NH2
1
R, R¢
Conditions
respectively (entries 14 and 15). This result is in striking contrast
with what was observed previously for the reported catalysts such
as a rare earth metal-alkyl or amido complexes1a,2h,i and AlMe3,2a
which failed to produce the corresponding guanidines because of
their sensitivity to carbonyl group and ester group. In addition,
Cy
C6H6, 80 ◦C, 5 h
2, 96
3, 96
4, 91
5, 97
6, 97
2
3
4
5
tBu
C6H6, 80 ◦C, 8 h
CH2Ph,Ph C6H6, 80 ◦C, 5 h
i
i
= =
when a diamine was used to react with 2 eq. of PrN C N Pr, the
corresponding biguanidine 17 was produced in high yield (entry
16). Heteroatom-containing amines such as amino-substituted
pyridine and thiazole were also applicable (entries 17 and 18).
Secondary amine, such as pyrrolidine in the presen◦ce of 3 mol%
iPr
iPr
C6H6, 80 ◦C, 3 h
C6H6, 80 ◦C, 3 h
i
i
= =
of Zn(OTf)2, could be added to PrN C N Pr at 80 C for 3 h, to
give the corresponding N,N¢,N¢¢,N¢¢-tetrasubstituted guanidines
19a in 94% yield (eqn (1)).
6
7
iPr
iPr
C6H6, 80 ◦C, 3 h
C6H6, 80 ◦C, 3 h
7, 97
8, 95
(1)
8
iPr
C6H6, 80 ◦C, 3 h
9, 96
Mechanistic studies
a) Reaction of 2- isopropylaniline, Zn(OTf)2 and carbodiimide
9
iPr
iPr
C6H6, 80 ◦C, 4 h
C6H6, 80 ◦C, 3 h
10, 94
11, 97
The reaction of Zn(OTf)2 with an excess amount of 2- isopropyl-
aniline at elevated temperatures was attempted to obtain the
active species in this catalytic process. Interestingly, the amine-
coordinated zinc complex 20 was isolated quantitatively from a
THF solution and confirmed by an X-ray diffraction analysis
(Fig. 1).‡ There is a crystallographic inversion centre in 20.
Complex 20 adopts a dimeric structure, in which the two Zn
atoms are bridged by two triflate units through the oxygen
atoms. Unexpectedly, a guanidinium triflate 22 was isolated from
10
11
iPr
C6H6, 80 ◦C, 3 h
12, 95c
12
13
14
iPr
iPr
iPr
CH2Cl2, reflux, 3 h 13, 85c
i
i
= =
the reaction of 20 with PrN C N Pr at room temperature.
The structure of 22 was unambiguously characterized by NMR
spectroscopy and an X-ray diffraction analysis (Fig. 2).‡ Further,
we isolated the Zn–N amido species 21 from the residue after the
C6H6, 80 ◦C, 3 h
C6H6, 80 ◦C, 3 h
14, 92c
15, 95c
guanidinium triflate 22 was isolated (Scheme 1). Although
a
single crystal of 21 suitable for X-ray crystallographic analysis
was not obtained, its 1H and 13C NMR spectra was rather
informative for the elucidation of the structure. The formation
15
16
17
iPr
iPr
iPr
C6H6, 80 ◦C, 3 h
16, 95c
CH2Cl2, reflux, 3 h 17, 94d
‡ Crystal data for 20: C48H68F12N4O14S4Zn2, Mw = 1412.04 g mol-1, T =
173(2) K, Tetragonal, sp◦ace group I4(1)/a, a = 35.199(5), b = 35.199(5),
C6H6, 80 ◦C, 5 h
THF, reflux, 5 h
18, 95
19, 93
3
-3
˚
˚
c = 11.268(2) A, b = 90 , V = 13960(4) A , Z = 8, rc = 1.344 Mg m ,
m = 0.894 mm-1, reflections collected: 62839, independent reflections: 6130
(Rint = 0.0623), Final R indices [I > 2sI]: R1 = 0.0644, wR2 = 0.1956, R
18
iPr
indices (all data): R1 = 0.0681, wR2 = 0.1999. 22: C17H28F3N3O3S, Mw
=
411.48 gmol-1, T = 173(2) K, Orthorhombic, space group Pna2(1)/a, a =
◦
3
˚
˚
16.419(3), b = 12.458(3), c = 10.520(2) A, b = 90 , V = 2151.8(7) A ,
Z = 4, rc = 1.270 Mg m-3, m = 0.196 mm-1, reflections collected: 16883,
independent reflections: 4604 (Rint = 0.0369), Final R indices [I > 2sI]:
a Conditions: amines, 2.06 mmol; carbodiimides, 2.00 mmol; Zn(OTf)2,
0.06 mmol; benzene, 5 mL. b Isolated yield. c Zn(OTf)2, 5 mol%. d The
biguanidine was isolated when a carbodiimide (4.00 mmol) was used.
R1 = 0.0441, wR2 = 0.0945, R indices (all data): R1 = 0.0482, wR2
=
0.0977.
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as an excellent catalyst active species for the addition of 2-
i
i
= =
isopropylaniline to PrN C N Pr to yield the guanidine 9 in
almost quantitative yield (eqn (2)).
(2)
b) Possible mechanism
Based on the above observations, a possible catalytic cycle for the
addition reaction of primary aromatic amines to carbodiimides
is proposed in Scheme 2.6 The reaction among primary amine,
Zn(OTf)2 and carbodiimide should yield straightforwardly an
amido species such as A by release of a guanidinium triflate.
Nucleophilic addition of the amido species A to a carbodiimide
would afford the guanidinate species B or C.7,8 Protonolysis of B
or C by another molecule of primary amine would regenerate the
amido A and release the more stable, final guanidine D.
Fig. 1 ORTEP drawing of 20 with 30% thermal ellipsoids. Hydrogen
atoms except those on the nitrogen atoms are omitted for clarity. Selected
˚
bond length (A): Zn(1)–N(1) 2.090(3), Zn(1)–N(2) 2.106(3), Zn(1)–O(1)
2.115(3), Zn(1)–O(4) 2.194(3), Zn(1)–O(7) 2.036(3), S(2)–O(4) 1.455(3),
S(2)–O(5) 1.442(3), S(2)–O(6) 1.441(3). ‘ = -x, -y + 1, -z + 2.
Fig. 2 ORTEP drawing of 22 with 30% thermal ellipsoids. Triflate and
hydrogen atoms except those on the nitrogen atoms are omitted for
◦
˚
clarity. Selected bond length (A) and angles ( ): N(1)–C(7) 1.357(3),
N(2)–C(7) 1.332(3), N(3)–C(7) 1.325(3), N(1)–C(7)–N(2) 119.94(19),
N(1)–C(7)–N(3) 118.93(19), N(2)–C(7)–N(3) 121.11(19).
Scheme 2 A possible mechanism of catalytic addition of amines to
carbodiimides.
Conclusions
We have demonstrated that Zn(OTf)2, under an atmosphere of air,
serves as an excellent catalyst precursor for the addition of various
primary and secondary amine N–H bonds to carbodiimides yield-
ing substituted guanidines. This work has the following features:
i) the use of cheap and readily available catalyst; ii) operational
simplicity carried out for the first time under an atmosphere of air;
iii) high functional-group tolerance, especially for carbonyl and
ester group surviving the present conditions which are sensitive to
previous catalyst systems. iv) an unpredicted Zn–N amido species,
which is produced by release of a guanidinium triflate, acts as the
active species in this catalytic process.
Experimental
Scheme 1 Formation of the Zn–N amido species 21 and guanidinium
triflate 22.
General
of 21 could be easily confirmed by the 1 : 1 reaction of Zn(OTf)2
with isopropylbenzenamido lithium which was in situ generated
from the corresponding amine and n-BuLi. 21 could also serve
Unless otherwise noted, all starting materials and solvents were
commercially available and were used without further purification.
C6D6, Toluene-d8, DMSO-d6 and THF-d8 (all 99+ atom% D) were
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obtained from Acros for NMR reactions. 1H and 13C NMR spectra
were recorded on a JEOL-AL400 spectrometer (FT, 400 MHz for
1H; 100 MHz for 13C) or a JEOL JNM-AL300 spectrometer (FT,
300 MHz for 1H; 75 MHz for 13C) at room temperature.
C6H4). 13C NMR (75 MHz, THF-d8): d 22.8, 28.1, 26.4, 68.4,
117.0, 119.2, 123.4, 125.8, 127.1, 133.5, 145.0.
22. Single crystals suitable for X-ray analysis were grown in
THF/Hexane at room temperature for 12 h. Colorless solid, yield
92%. 1H NMR (300 MHz, THF-d8): d 1.19–1.24 (m, 18H, CH3),
3.08–3.22 (m, 1H, CH), 3.92–4.02 (m, 2H, CH), 6.68 (br, 2H, NH),
7.18–7.43 (m, 4H, C6H4), 8.53 (br, 1H, NH). 13C NMR (75 MHz,
THF-d8): d 22.6, 23.7, 28.7, 45.9, 127.9, 128.2, 128.4, 129.5, 134.3,
146.4, 154.8.
Typical procedures for the catalytic reaction of primary aromatic
amines to carbodiimides
i) NMR tube reaction. Under an atmosphere of air, a J.
Young valve NMR tube was charged with Zn(OTf)2 (6 mg,
0.015 mmol), C6D6 (0.5 mL), aniline (48 mg, 0.52 mmol), N,N¢-
diisopropylcarbodiimide (63 mg, 0.50 mmol). The tube was sealed
and then heated at 80 ◦C. Formation of 1 was easily monitored by
1H NMR spectroscopy. The reaction was quantitative and finished
within 5 h.
Isolation of Zn–N amido species 21 from the reaction of the zinc
complex 20 with N,N¢-diisopropylcarbodiimide
N,N¢-Diisopropylcarbodiimide (252 mg, 2.0 mmol) was added to
a solution of complex 20 (706 mg, 1.0 mmol) in THF (5 mL).
The reaction was carried out at room temperature for 3 h. After
the solvent was removed under reduced pressure, the residue was
recrystallized in THF/Hexane to provide a colorless solid 22 after
filtering. Zn–N amido species 21 was obtained from the filtrate.
ii) Preparative scale reaction. Under an atmosphere of air, a
solution of aniline (192 mg, 2.06 mmol) in benzene (5 mL) was
added to Zn(OTf)2 (22 mg, 0.06 mmol) in a Schlenk tube. N,N¢-
Diisopropylcarbodiimide (252 mg, 2.00 mmol) was then added
to the above reaction mixture. The Schlenk tube was sealed, and
the reaction was carried out at 80 ◦C for 5 h. After the solvent
was removed under reduced pressure, the residue was extracted
with ether and filtered to give a clean solution. After removing the
solvent under vacuum, the residue was recrystallized in ether to
provide a colorless solid 1.
Preparation of the Zn–N amido species 21 by the 1 : 1 reaction of
Zn(OTf)2 with isopropylbenzenamido lithium
To a solution of 2-isopropylaniline (135 mg, 1.0 mmol) in THF was
1
◦
14. Colorless solid, yield 92%. H NMR (300 MHz, C6D6):
added n-BuLi (0.63 mL, 1.0 mmol) at -78 C. After the mixture
d 0.87 (d, J = 6.3 Hz, 12H, CH3), 3.50–3.61 (m, 4H, CH and
was stirred for 1 h at the same temperature, Zn(OTf)2 (364 mg,
1.0 mmol) was added and stirring was continued at -78 ◦C for
1 h. Then the reaction mixture was allowed to warm to room
temperature for 1 h. After the solvent was removed under reduced
pressure, the residue was washed with hexane. The residue was
recrystallized in ether to provide a colorless solid 21.
NH), 6.79–7.17 (m, 4H, C6H4). 13C NMR (75 MHz, C6D6): d
23.0, 43.3, 103.4, 120.2, 123.8, 133.5, 150.0, 156.1. IR (neat):
-1
=
v (C O) = 1574 cm ; HRMS calcd. for C15H24N3O: 262.1914,
found 262.1909.
15. Colorless solid, yield 95%, m. p. 115.4~115.9 ◦C. 1H NMR
(300 MHz, C6D6): d 1.01 (d, J = 6.3 Hz, 12H, CH3), 2.20 (s, 3H,
CH3), 3.79–3.83 (m, 2H, CH), 4.26 (br, 2H, NH), 7.06–7.89 (m,
21. Colorless solid, yield 90%. 1H NMR (300 MHz, DMSO-
d6): d 1.11 (d, J = 6.9 Hz, 6H, CH3), 1.72–1.76 (br m, 8H, CH2),
2.86–2.99 (m, 1H, CH), 3.56–3.60 (br m, 8H, CH2), 4.71 (br, 1H,
NH), 6.48–7.34 (m, 4H, C6H4). 13C NMR (75 MHz, DMSO-d6):
d 22.6, 25.4, 26.7, 67.4, 115.3, 116.9, 119.0, 123.3, 125.2, 126.4,
131.7.
4H, C6H4). 13C NMR (75 MHz, C6D6): d 23.1, 26.0, 43.4, 122.9,
-1
=
129.9, 130.6, 151.0, 157.4, 196.3. IR (neat): v (C O) = 1582 cm ;
HRMS calcd. for C16H26N3O2: 292.2019, found 292.2016.
Isolation and reaction of the zinc complex 20
To a solution of 2-isopropylaniline (608 mg, 4.5 mmol) in THF (1
mL) was added Zn(OTf)2 (727 mg, 2.0 mmol) in a Schlenk tube.
The reaction mixture was stirred at 100 ◦C for 3 h. After cooling to
room temperature, the reaction mixture was filtered and the solid
product was washed with hexane until the filtrate became colorless.
The filtrate was recrystallized in THF/Hexane to provide a
colorless zinc complex 20. Then N,N¢-Diisopropylcarbodiimide
(252 mg, 2.0 mmol) was added to a solution of complex 20 (706 mg,
1.0 mmol) in THF (5 mL). The reaction was carried out at room
temperature for 3 h. After the solvent was removed under reduced
pressure, the residue was recrystallized in THF/Hexane to provide
a colorless solid 22.
X-Ray crystallographic studies for 20 and 22
Crystals for X-ray analyses of 20 and 22 were obtained as
described in the preparations. Data collections were performed at
-100 ◦C on a Rigaku SATURN 724+ CCD diffractometer, using
graphite-monochromated Mo-Ka radiation (l = 0.71073 A). The
˚
determination of crystal class and unit cell parameters was carried
out by the CrystalClear (Rigaku Inc., 2008) program package.
The raw frame data were processed using the CrystalClear (Rigaku
Inc., 2008) to yield the reflection data file. The structure was solved
by use of SHELXTL program.9 Refinement was performed on F2
anisotropically for all the non-hydrogen atoms by the full-matrix
least-squares method. The hydrogen atoms were placed at the
calculated positions and were included in the structure calculation
without further refinement of the parameters. Crystal data, data
collection and processing parameters for 20 and 22 were given only
in Supporting Information.†
20. Single crystals suitable for X-ray analysis were grown in
THF/Hexane at room temperature for 12 h. Colorless solid, yield
95%. 1H NMR (300 MHz, THF-d8): d 1.20 (d, J = 6.9 Hz, 24H,
CH3), 1.72–1.18 (br m, 8H, CH2), 2.90–3.04 (m, 4H, CH), 3.61–
3.65 (br m, 8H, CH2), 4.45 (br, 8H, NH), 6.62–7.04 (m, 16H,
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Acknowledgements
This work was supported by the Natural Science Foundation of
China (Nos. 20702003, 20821062, and 20972006).
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ꢀ
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≡
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