Hydroamination of isocyanates and isothiocyanates by alkaline earth metal initiators supported by a bulky iminopyrrolyl ligand

Article abstract of DOI:10.1039/d0nj01509a

A series of new heteroleptic alkaline earth (Ae) metal complexes of general formula [{(Ph₂CHN-CH)₂C₄H₂N}AeI(THF)₃] {Ae = Ca (2), Sr (3), and Ba (4)} were synthesizedviasalt metathesis by reacting potassium salt of ligand1-K[{(Ph₂CHN-CH)₂C₄H₂N}K(THF)₂] with anhydrous alkaline earth metal diiodides (AeI₂). The homoleptic calcium and barium complexes [{(Ph₂CHN-CH)₂C₄H₂N}₂Ae] [Ae = Ca (5), Ba (6)] were prepared by treating metal bis-hexamethyldisilazide [Ae{N(SiMe₃)₂}₂(THF)₂] with the protic ligand1-H[(Ph₂CH-N-CH)₂C₄H₂NH] in a 1:2 molar ratio. Calcium complex5was used as an active pre-catalyst for the addition of N-H bond of arylamines across the heterocumulenes such as phenylisocyanate (PhNCO) and phenylisothiocyanate (PhNCS) under neat conditions, and up to 99% yields of the corresponding urea and thiourea derivatives were obtained.

Full text of DOI:10.1039/d0nj01509a

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Hydroamination of isocyanates and isothiocyanates by alkaline
earth metal initiators supported by bulky iminopyrrolyl ligand
Kulsum Bano,a Srinivas Anga,a Archana Jain,b Hari Pada Nayek,c and Tarun K. Panda*a
Received 00th January 20xx,
Accepted 00th January 20xx
Abstract:
A
series of new heteroleptic alkaline earth (Ae) metal complexes of general formula
DOI: 10.1039/x0xx00000x
[{(Ph2CHN=CH)2C4H2N}AeI(THF)3] {Ae = Ca (2), Sr (3), and Ba (4)} were synthesized via salt metathesis by reacting potassium
salt of ligand 1-K [{(Ph2CHN=CH)2C4H2N}K(THF)2] with anhydrous alkaline earth metal diiodides (AeI2). The homoleptic
calcium and barium complexes [{(Ph2CHN=CH)2C4H2N}2Ae] [Ae = Ca (5), Ba (6)] were prepared by treating metal bis-
hexamethyldisilazide [Ae{N(SiMe3)2}2(THF)2] with the protic ligand 1-H [(Ph2CH-N=CH)2C4H2NH] in 1:2 molar ratio. Calcium
complex 5 was employed as an active pre-catalyst for the addition of N-H bond of arylamines across the heterocumulenes
such as phenylisocyanate (PhNCO) and phenylisothiocyanate (PhNCS) under the neat condition and a yield of corresponding
urea and thiourea derivatives up to 99% was obtained.
Introduction
There is a growing interest in the syntheses and characterization
of group II metal-based complexes due to their utility as
Ureas and thioureas, also known as carbamides and versatile catalysts in various organic transformations and their
thiocarbamides, respectively, have a wide range of applications nontoxicity, biocompatibility, easy availability and low cost are
in organic, medicinal, supramolecular, and materials also attractive features.40-54 However, one major problem in
chemistry.1-7 Several methodologies including transition metal- studying the catalytic features of alkaline-earth (Ae) metal-
catalyzed oxidative carbonylation of amines,8-13 a reductive based complexes is that they easily undergo Schlenk
coupling of nitro compounds with amines and CO,14-17 the iron- equilibrium and show different molecular compositions in the
mediated dehydrogenative coupling of methanol and solution state. To overcome this problem, a wide variety of
amines,18,19 and coupling of silylamines and carbon dioxide sterically bulky nitrogen-based ancillary ligands, such as
using indium pre-catalyst20 have been adopted for the tris(pyrazolyl)borates,55-58
aminotroponiminates,59,60
1,4-diaza-1,3-
β-
preparation of urea derivatives. Most of these methods involve diketiminates,61-65 iminopyrroles,66-75
expensive metals, stoichiometric amounts of reagent and/or butadiene,76,77 and mixed nitrogen–oxygen based phenolate
harsh reaction conditions. Therefore, the development of an and alkoxo ligands78-82 have been introduced which resulted in
atom-efficient and environmentally benign process for urea an the formation of a variety of well-defined alkaline earth metal
d thiourea preparation remains an important target. Recently, complexes with the controlled catalytic activity and selectivity.
catalytic hydroamination of isocyanates and isothiocyanates Among the multi-dentate nitrogen-based ligands, bis-
have provided a facile route to obtain urea and thiourea iminopyrrolyl ligands played an important role as unique
derivatives. Catalytic hydroamination of alkenes, allenes or monoanionic ligands to stabilize the various oxophilic Ae-metal
alkyne has been widely exploited to prepare amines, enamines, complexes. Bochmann et al. were the first to introduce the
and imines.21-30 On the other hand, catalytic addition of N-H to tridentate 2,5-bis(aryliminomethyl)pyrrolyl ligand into
isocyanates and isothiocyanates is limited to very few metal transition metal chemistry.83 Mashima et al. also reported a
complexes including zinc,31 iron,32 titanium,33-35 actinides,36,37 series of homoleptic and heteroleptic yttrium complexes with
and alkaline earth metals,38,39 etc.
different bis-iminopyrroles and varying substituents at the
imine nitrogen atom.84,85 Roesky et al. reported the study of
alkali metal, alkaline earth and divalent lanthanide complexes
of the 2,5-bis{N-(2,6-diisopropylphenyl)iminomethyl}-pyrrole
ligand.86-93 Recently, our research group developed alkali metal
complexes bearing bulky substituted bis-iminopyrrolyl ligands
and explored their structural diversity in terms of their
coordination modes and nuclearity.94 Upon varying the
substituent bulkiness present on the imine nitrogen, these
ligands  and -coordination modes and produced interesting
structural motifs. As a continuation of this investigation, we are
prompted to learn about the effect of the introduction of the
aDepartment of Chemistry, Indian Institute of Technology, Hyderabad, Kandi,
Sangareddy, 502285, Telangana, India.
E-mail: tpanda@chy.iith.ac.in
bDepartment of Physics and Chemistry, Mahatma Gandhi Institute of Technology,
Gandipet, Hyderabad 500075, Telangana, India.
cDepartment of Applied Chemistry, Indian Institute of Technology (ISM), Dhanbad,
826004, Jharkhand, India.
Electronic supporting information available: Crystallographic data of complexes 2-4 and
6, and 1H, 13C NMR spectra and characterization data of 7a-7i, 8a-8i, and 9a-9i reported
in this article is given.
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sterically demanding bulky bis-iminopyrrolyl ligands on the
structure, stability, and reactivity of alkaline earth metal
complexes. Determining the structure and reactivity of the
alkaline earth metal species is an important step towards the
design and development of efficient catalysts; however, a full
realization of the catalytic potential of these elements still
requires substantial advances in terms of understanding their
basic coordination and organometallic chemistry.
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DOI: 10.1039/D0NJ01509A
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Herein, we describe a full account of the synthesis and
structural aspects of heteroleptic alkaline earth metal
complexes of general formula [{(Ph2CHN=CH)2C4H2N}AeI(THF)3]
[Ae = Ca (2), Sr (3) and Ba (4)] and homoleptic alkaline earth
metal complexes [{(Ph2CHN=CH)2C4H2N}2Ae] [Ae = Ca (5), Ba
(6)]. We also describe an efficient protocol for the catalytic
addition of N−H bonds to isocyanates and isothiocyanates with
a broader substrate scope using calcium complex 5 as an active
pre-catalyst to yield urea and thiourea derivatives.
Figure 1. The solid-state structure of calcium complex 2, with all hydrogen atoms
omitted for clarity. Selected bond lengths (Å) and bond angles (°): Ca1–N1
2.407(4), Ca1–N3 2.638(4), Ca1–I1 3.186(1), Ca1–O1 2.380(4), Ca1–O2 2.459(4),
Ca1–O3 2.367(3), O3–Ca1–O1 165.83(1), O3–Ca1–N1 83.59(1), O1–Ca1–N1
87.46(1), O3–Ca1–O2 84.13(2), O1–Ca1–O2 93.681(2), N1–Ca1–O2 131.43(1), O3–
Ca1–N3 89.36(1), O1–Ca1–N3 97.24(1), N1–Ca1–N3 66.97(1), N1-Ca1-N2
[37.89(1)°], O2–Ca1–N3 159.17(1), O3–Ca1–I1 98.35(9), O1–Ca1–I1 94.87(9), N1–
Ca1–I1 150.42(1), O2–Ca1–I1 77.94(9), N3–Ca1–I1 83.50(9).
Results and Discussion
Synthesis and structure details of heteroleptic alkaline-earth
metal complexes.
The mono-iodo Ae complexes [{(Ph2CHN=CH)2C4H2N}AeI(THF)3]
(2–4) were prepared in good yields (ca. 75–82%) via salt
metathesis reaction of the potassium salt of the ligand (1-K)
with anhydrous AeI2 in 1:1 equivalent ratio in THF at 60 °C for
12 hours (Scheme 1).
N
K
N
AeI2
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
N
N
N
N
Ae
THF,
60
°C,
12h
O
O
I
-KI
(THF)3
1-K
(
)
2
3
4
Ae = Ca ( ), Sr ( ), Ba ( )
Scheme 1. Synthesis of heteroleptic alkaline earth metal
complexes 2-4 from 1-K.
Figure 2. The solid-state structure of strontium complex 3, with all hydrogen
atoms omitted for clarity. Selected bond lengths (Å) and bond angles (°): Sr1–N1
2.506(2), Sr1–N2 3.106(3), Sr1–N3 2.846(3), Sr1–O1 2.543(2), Sr1–O2 2.601(2),
Sr1–O3 2.529(2), Sr1–I1 3.291(4), N1–Sr1–O3 81.39(8), N1–Sr1–O1 88.21(8), O3–
Sr1–O1 166.97(8), N1–Sr1–O2 139.16(8), O3–Sr1–O2 86.58(8), O1–Sr1–O2
96.33(8), N1–Sr1–N3 61.94(8), O3–Sr1–N3 87.70(8), O1–Sr1–N3 94.35(8), O2–
Sr1–N3 156.49(8), N1–Sr1–N2 59.01(8), O3–Sr1–N2 80.19(7), O1–Sr1–N2
87.73(8), O2–Sr1–N2 80.57(7), N3–Sr1–N2 120.81(7), N1–Sr1–I1 143.31(6), O3–
Sr1–I1 97.19(6), O1–Sr1–I1 95.84(6), O2–Sr1–I1 76.75(6), N3–Sr1–I1 81.38(5), N2–
Sr1–I1 157.29(5).
The air and moisture sensitive complexes 2-4 were fully
characterized by standard NMR spectroscopy and combustion
analysis. In the 1H NMR spectra, each complex displayed a sharp
singlet at δH 8.03 ppm (for 2 and 4) and 8.19 ppm (for 3) for the
resonance of corresponding imine (N=CH) proton, and a singlet
at δH 5.53 (for 2), 5.27 (for 3), and 5.52 (for 4) ppm for the
resonance of the corresponding N–CHPh2 proton of the ligand.
The resonance signal of imine proton of complex 3 is slightly
deshielded presumably due to the adjacent pyrrole ring and
exhibits modest low field shift compared to those of complexes
2 and 4. In the 13C{1H} NMR spectra, resonance signals for the
corresponding imine carbon (N=CH) were observed at δC 160.6
(for 2), 160.4 (for 3), and 160.6 ppm (for 4), which are
significantly shifted to the low-field region when compared to
the free ligand 1-H (151.2 ppm).94 The corresponding resonance
signal for the CH carbon appeared at δC 79.0 (for 2), 77.8 (for 3),
and 79.2 ppm (for 4).
The solid-state molecular structures of complexes 2–4 were
determined by single-crystal X-ray diffraction analysis. The
mono-iodo calcium complex 2 crystallizes in the triclinic space
group P-1, with two independent molecules in the unit cell. The
molecular structure of compound 2 is shown in Figure 1 and
details of the structural and refinement parameters are given in
Table TS1 in supporting information. As shown in Figure 1, the
2 | J. Name., 2012, 00, 1-3
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[159.17(1)°], and O2–Ca1–I1 (77.94°) indicate the
unsymmetrical coordination sphere around calcium ion.
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DOI: 10.1039/D0NJ01509A
The strontium and barium complexes 3 and 4 are
isostructural. Both the complexes crystallized in the triclinic
space group P-1. The solid-state structures of complex 3 and 4
are shown in Figures 2 and 3, respectively, and details of the
structural parameters are given in Table TS 1 in supporting
information. Similar to complex 2, the monoanionic ligand 1 is
chelated to the central metal ion (Sr and Ba) in a 3 fashion
through one pyrrolide nitrogen and two imine nitrogen atoms.
Both complexes 3 and 4 displayed two sets of long–short bond
distances, indicating coordination of amido and imine nitrogen
atoms to the metal ion. The metal–nitrogen bond lengths, Sr1–
N1 (2.506(2) Å), Sr1–N2 (3.106(3) Å), and Sr1–N3 (2.846(3) Å)
and Ba1–N1 (2.665(3) Å), Ba1–N2 (2.978(3) Å), and Ba1–N3
(3.119(3) Å) are good in agreement with reported literature
values and similar to [(Dipp2Pyr)MI(THF)n (M = Sr and Ba).86
Based on long-short bond lengths within the imine nitrogen
atoms, imine nitrogen atom which is trans to iodine exhibited a
weak  donor character (long bond); however, imine nitrogen
atom cis to iodine showed a strong  donor character (short
bond). The Sr1–I1 bond (3.291(4) Å) is quite longer than the
Ca1–I1 (3.078(1) Å) bond in complex 2, and shorter than the
Ba1-I1 (3.514(4) Å) due to increase in the metal ionic radius
from Ca2+ (1.00 Å), Sr2+ (1.18 Å) to Ba2+ (1.35 Å). The sevenfold
coordination number of both strontium and barium is observed
by the 3-coordination of the ligand, one iodine atom, and three
THF molecules adopting distorted pentagonal bipyramidal
geometry around the central metal ion. In the distorted
pentagonal bipyramidal arrangement, pyrrolide nitrogen, imine
nitrogen atoms, an iodine atom, and one oxygen atom from THF
[N1, N2, N3, I1, and O2 (for Sr) or O3 (for Ba)] occupy equatorial
positions and the remaining two oxygen atoms from two THF
molecules (O1 and O3 (for Sr) or O1 and O2 (for Ba) are in an
apical position. The bite angles, N1–Sr1–N2 [59.01(8)°], N1–
Sr1–N3 [61.94(8)°], N3–Sr1–O2 [156.49(8)°], O2–Sr1–I1
[76.75(6)°], N2–Sr1–I1 [157.29(5)°], and O1–Sr1–O3
[166.97(8)°] are in the expected range.86 The bite angles for the
complex 4 N1–Ba1–N2 [58.96(8)°], N1–Ba1–N3 [57.09(9)°], N2–
Ba1–I1 [82.96(6)°], N3–Ba1–O3 [84.81(8)°], O3–Ba1–I1
[76.70(6)°], and O1–Ba1–O2 [171.48(1)°] are greater than the
complex 3. It is noted that analogous strontium and samarium
derivatives tend to form isostructural complexes due to their
similar ionic radii, and in the present case, the complex 3 is
isostructural to the analogous samarium and europium
complexes [(Dipp2Pyr)LnI(THF)3] (Ln = Sm, Eu).88
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Figure 3. Solid-state structure of barium complex 4, with all hydrogen atoms
omitted for clarity. Selected bond lengths (Å) and bond angles (°): Ba1–N1
2.665(3), Ba1–N2 2.978(3), Ba1–N3 3.119(3), Ba1–O1 2.686(3), Ba1–O2 2.779(3),
Ba1–O3 2.767(3), Ba1–I1 3.514(4), N1–Ba1–O1 83.30(9), N1–Ba1–O3 141.39(8),
O1–Ba1–O3 101.32(10), N1–Ba1–O2 94.92(9), O1–Ba1–O2 171.48(10), O3–Ba1–
O2 85.29(8), N1–Ba1–N2 58.96(8), O1–Ba1–N2 82.45(10), O3–Ba1–N2 159.32(8),
O2–Ba1–N2 89.48(8), N1–Ba1–N3 57.09(9), O1–Ba1–N3 85.73(9), O3–Ba1–N3
84.81(8), O2–Ba1–N3 100.34(8), N2–Ba1–N3 115.84(8), N1–Ba1–I1 141.89(6),
O1–Ba1–I1 90.71(7), O3–Ba1–I1 76.70(6), O2–Ba1–I1 85.55(7), N2–Ba1–I1
82.96(6), N3–Ba1–I1 160.11(6).
monoanionic ligand is coordinated to the central calcium ion in
a 3 fashion through two imine nitrogen and one pyrrolyl
nitrogen atoms. The calcium–nitrogen bond length of N1–Ca1
2.407(4) Å indicates the formation of amido bond with calcium
ion through pyrrolyl nitrogen. The bond distance of N3–Ca1
2.638(4) Å represents the coordination of imine nitrogen N3 of
ligand 1 to the calcium ion to form a Ca1-N3 dative bond.
However, the second imine nitrogen (N2) atom is very weakly
coordinated to the calcium ion, as reflected by the longer
distance [3.320(4) Å] between Ca1 and N2 atoms. The bite angle
N1-Ca1-O2 of 131.43(1)° is also larger than that of N1-Ca1-N2
[37.89(1)°] indicating the nitrogen atom N2 as a neighbouring
atom to calcium ion rather than oxygen atom O2. It is also
observed that the Ca1–I1 bond distance of 3.186(1) Å is slightly
longer than that found in
a
similar complex
[(Dipp2Pyr)CaI(THF)3],86 3.078(1) Å reported by Roesky et al.
(where Dipp2Pyr = bis-2,6-diisopropylphenyl pyrrolyl). It may be
due to the presence of bulky CHPh2 groups in the ligand moiety
1 instead of 2,6-diisopropylphenyl groups. The Ca–O bond
distances are also well in agreement with the values reported in
the literature.86 The seven-fold coordination number of the
calcium ion is observed by the κ3-coordination of the ligand 1,
one iodine atom and three THF molecules adopting a distorted
pentagonal bipyramidal geometry around the central calcium
ion. The iodine, imine nitrogen atoms and one oxygen atom
(O2) are in the equatorial position and two oxygen atoms from
two other THF molecules are in the apical position. The bite
angle of imine nitrogen N1 and iodine atom is 83.504° (N1–Ca1–
I1). The other bite angles, N3–Ca1–N1 [66.97(1)°], N3–Ca1–O2
Table 1. Comparison of the bond lengths of complexes 2-4.
Bond length
(Å)
Ca (2)
Sr (3)
Ba (4)
M–N1pyrrolyl
M–N2imine
M–N3imine
M–I1
2.407(4)
3.320(4)
2.638(4)
3.186(1)
2.506(2)
3.106(3)
2.846(3)
3.291(4)
2.665(3)
2.978(3)
3.119(3)
3.514(4)
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The complexes 2-4 were also found to be isostructural with the structure of complex 6 is shown in Figure 4 and the details of
Ae metal complexes bearing [(Dipp2Pyr)]-ligand moiety (where the structural parameters are given in Table TS1.
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Dipp2Pyr = bis-2,6-diisopropyl phenyl pyrrolyl) reported by
Roesky et al.86 It was noted that all the complexes 2-4 are
monomeric and this is quite unusual since strontium and
barium ions tend to form dimeric complexes, bridging through
iodine atoms.86-93 There is a limited number of monomeric
heteroleptic mono-iodo complexes bearing very bulky ligands
reported in the literature.94 Furthermore, the solid-state
structures of complexes 2-4 reveal that there is a trend of
increase in metal–nitrogen and metal–iodine bond lengths from
calcium to barium (Table 1), due to increase in the ionic radius
of metal ions from calcium to barium: Ca2+ (1.00 Å), Sr2+ (1.18
Å), and Ba2+ (1.35 Å).
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Synthesis and structure details of homoleptic alkaline earth
amido complexes.
Figure 4. The solid-state structure of barium complex 6. All hydrogen atoms and selected
phenyl groups are omitted for clarity. Selected bond lengths (Å) and bond angles (°): Ba1–
The treatment of the protic ligand 1-H with
[Ae{N(SiMe3)2}2(THF)2] in a 1:2 ratio at ambient temperature
affords the homoleptic alkaline earth metal complexes of
molecular formula [{(Ph2CHN=CH)2C4H2N}2Ca] (5) and
[{(Ph2CHN=CH)2-C4H2N}2Ba] (6) in 72% and 80% yield
respectively (Scheme 2). We attempted to introduce the
[N(SiMe3)2] moiety as the leaving group into the metal
coordination sphere by treating the protic ligand 1-H with
[Ca{N(SiMe3)2}2(THF)2] in a 1:1 ratio at ambient temperature,
however, complex 5 was isolated as the stable major product
in this case too. This is due to the high reactivity of the calcium
bis-hexamethyldisilazide and the dynamic Schlenk equilibrium
of alkaline earth metal ions.
N1 2.641(2), Ba1–N2 3.057(2), Ba1–N3 3.017(2), Ba1–N4 2.664(2), Ba1–N5 2.837(6),
Ba1–N6 3.057(2), N1–Ba1–N5 126.06(13), N2–Ba1–N5 109.2(3), N2–Ba1–N1 58.50(7),
N4–Ba1–N5 58.50(14), N4–Ba1–N1 168.36(8), N4–Ba1–N2 110.35(7), N3–Ba1–N5
106.3(3), N3–Ba1–N1 58.17(7), N3–Ba1–N2 116.66(6), N3–Ba1–N4 132.90(7), N6–Ba1–
N5 113.34(12), N6–Ba1–N1 120.56(7), N6–Ba1–N2 106.19(7), N6–Ba1–N4 56.81(7), N6–
Ba1–N3 105.38(7).
The six-fold coordination number is observed by the 3-chelation
mode of two ligands to the central barium ion adopting a distorted
octahedral. Ba–N bond lengths, Ba1–N1 [2.643(3) Å], Ba1–N2
[3.061(3) Å], Ba1–N3 [3.018(3) Å], Ba1–N4 [2.663(4) Å], Ba1–N5
[3.063(3) Å], and Ba1–N6 [2.860(5) Å] are similar to those we
previously reported for the barium complex [Ba(2-
(Ph3CN=CH)C4H3N)2(THF)3]70 where Ba–N bond distances were Ba–
N1 [2.731(5) Å], Ba1–N2 [2.762(5) Å], Ba1–N3 [2.946(5) Å] and Ba1–
N4 [2.933(5) Å], and for [(ImpDipp)2Ba(THF)2] (ImpDipp = [(2,6–
iPr2C6H3–CN=CH)2–C4H3N]) where Ba–N bond distances were [Ba–N1
2.821(5) Å and Ba–N2 2.823(4) Å].69 The bite angle between N1–Ba1–
N4 is nearly linear and measures 168.28(1)°. A dihedral angle of
86.04° between two planes containing N1, N2, N3, and Ba1 atoms
and N4, N5, N6, and Ba1 atoms, indicates an orthogonal arrangement
of the two ligands with the central barium metal ion to avoid steric
crowding with each other.
Ph
Ph
[Ae{N(SiMe3)2}2(THF)2]
N
Ae
N
N
H
Ph
Ph
Ph
Ph
N
N
N
N
Ph
Ph
N
N
Ph
Ph
0.5
Toluene, rt
-2 HN(SiMe3)2
1-H
(
)
Ph
Ph
5
6
)
Ae = Ca ( ), Ba (
Scheme 2. Synthesis of alkaline earth metal complexes 5 and 6.
Both air and moisture sensitive complexes 5 and 6 were fully
characterized using standard NMR spectroscopy and analytical
techniques. The 1H NMR spectra of complex 5 and 6 were
measured in C6D6, displayed a sharp singlet at δH 7.85 ppm for
5 and 7.92 ppm for 6, which can be best assigned to the imine
CH proton (N=CH), slightly shifted to the high-field region, when
compared to the neutral ligand 1-H (δH 8.15 ppm).78-82 Also, the
resonance of the methine proton of the -CHPh2 group present
in ligand 1 appeared at δH 5.15 ppm for 5 and 5.20 ppm for 6.
The solid-state structure of complex 5 could not be established
due to the poor data set of the crystals. However, we have
established the molecular structure of complex 6 using single-
crystal X-ray diffraction analysis. The homoleptic barium
complex 6 crystallizes in the monoclinic space group P21/c, with
four independent molecules in the unit cell. The solid-state
Catalytic hydroamination of isocyanates or iso-thiocyanates
In the beginning, all Ae-metal complexes 2-6 synthesized were
screened as catalysts to meditate the hydroamination reaction of
isocyanate and isothiocyanates with aniline. Catalytic experiments
were performed using 5 mol % of the complex (i.e. 2-6) along with
equimolar amounts of phenylisothiocyanate and aniline under the
neat condition at 60 °C for 12 h. It was observed that only homoleptic
complexes 5 and 6 exhibited catalytic activity for the same reaction
and resulted in the formation of the corresponding thiourea
derivative in an excellent yield ~98 % (Table 2, Entries 2 & 3). As a
control experiment, when the hydroamination reaction of
isothiocyanate with aniline was performed in the absence of the
catalyst, it did not yield any product at room temperature as well as
4 | J. Name., 2012, 00, 1-3
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2
3
4
5
6
7
at elevated temperature (Table 2, Entries 10 & 11). Table 2 also in excellent yield of ≥95% (Table 3). In contrast, arVyileawmAirnticelesOwnliitnhe
DOI: 10.1039/D0NJ01509A
shows that temperature has a considerable effect on the reaction. electron-withdrawing groups such as nitro-, fluoro-, and bromo-
No formation of thiourea was observed even in the presence of showed slightly low reactivity, generating a slightly lower conversion
catalyst 5 when the hydroamination reaction of isothiocyanate with (~80-85%) under similar conditions, due to deactivation of aryl
aniline was carried out at room temperature for a prolonged time moiety.
(Table 2, Entry 1). It presumably due to the activation of catalysts 5
and 6 at an elevated temperature only and exhibited comparable
activity for the given reaction under a similar reaction condition
8
9
Table 3. Hydroamination of isocyanates and arylamines catalyzed by
calcium complex 5 as pre-catalyst.a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(Table 2, Entries 2 & 3). Besides, when the hydroamination of
O
R'
Complex 5 (3 mol%)
60 0C, 12 h
isothiocyanate and aniline was performed using 3 mol % of the
complex 5 or 6, almost the similar yield of thiourea was obtained as
that with 5 mol% of catalyst (Table 2, Entries 4 & 5). We also studied
the effect of various solvents such as THF, toluene, benzene, and
hexane, etc. on the product yield when the hydroamination of
isothiocyanate with aniline was performed using 3 mol % of the
complex 5 at 60 °C for 12 h (Table 2, Entries 6-9). It was observed that
the least yield ~70% of the thiourea product obtained when THF was
used as a solvent.
N
+
H2N
N
C
O
N
H
H
R
R
R'
7a 8f
-
O
O
O
N
N
H
N
N
H
N
N
H
H
H
H
H3C
MeO
7c, 99%
7a, 98%
7b, 99%
O
O
O
N
N
H
N
H
N
H
H
N
N
H
H
N
F
7e, 98%
7f, 98%
7d, 85%
O
Table 2. Screening results for hydroamination of phenyl
isothiocyanate with aniline
O
O
N
N
N
H
N
F
H
H
N
N
H
H
H
Br
7h, 99%
Entry Catalyst
mol
%
5
5
5
3
3
3
3
3
3
0
0
Solvent
Temp.,
Time
RT, 12 h
Isolated
7i, 85%
7g, 89%
Yield
O
O
O
1
2
5
5
6
5
6
5
5
5
5
5
5
Neat
Neat
Neat
Neat
Neat
Toluene
THF
Benzene
Hexane
Neat
-
N
H
N
N
N
H
Cl
N
N
Cl
Cl
Cl
60°C, 12 h
60°C, 12 h
60°C, 12 h
60°C, 12 h
60°C, 12 h
60°C, 12 h
60°C, 12 h
60°C, 12 h
RT, 12 h
98
98
98
98
98
70
95
86
-
H
H
H
H
3
4
5
6
7
8
MeO
8a,98%
O
8b, 99%
8c, 99%
O
O
N
H
N
N
H
N
Cl
H
H
N
N
H
Cl
H
Br
N
8e, 98%
8d, 85%
8f, 98%
9
10
11
O
O
O
Neat
60°C, 12 h
-
N
N
Cl
N
N
Cl
N
H
N
Cl
H
H
H
H
H
O2N
8i, 99%
8h, 98%
8g, 82%
OMe
In
a further study to analyze the substrate scope, the
aAll reactions were performed under neat conditions; Yields are calculated from isolated
pure products.
hydroamination reactions of several aryl amines with isocyanate and
isothiocyanate catalyzed by homoleptic calcium complex
[{(Ph2CHN=CH)2-C4H2N}2Ca] (5) were investigated under mild
conditions. Catalytic experiments were performed using 3 mol % of
calcium complex (5) and equimolar amounts of either isocyanate or
isothiocyanate and arylamines, which were stirred and heated at 60
The catalytic hydroamination of arylamines was further extended to
4-chlorophenylisocyanate (ClPhNCO), and similar types of results
were obtained as with phenyl isocyanate (Table 2, 8a-8i).
ͦ
The substrate scope was further expanded to phenyl
isothiocyanate (PhNCS). The reaction of phenyl isothiocyanate with
unsubstituted aniline and substituted arylamines, having o-/p-OMe,
atmosphere. The respective urea or thiourea derivatives were
isolated in each case and analyzed through 1H and 13C NMR
spectroscopy (see ESI). The yields were calculated after the isolation
of pure products. The reactions displayed a broad substrate scope.
The results of the catalytic hydroamination to isocyanates and
phenyl isothiocyanate are presented in Table 3 and 4, respectively.
In most of the cases, complete substrate conversion resulting in the
corresponding urea/thiourea with excellent chemoselectivity was
achieved. It was observed that the reaction of 4-methyl-phenyl
isocyanate with aniline and the substituted arylamines such as 4-
OMePhNH2, PhCH2NH2, 2-MePhNH2, and 2,6-(Me)2PhNH2, bearing
alkyl groups, halides proceeded conveniently at 60 ͦC temperature
and resulted in the selective production of the corresponding
thiourea with ~82-96% yield (Table 4, 9a-9i). The molecular structure
of 9f is shown in Figure 5. From this survey, we see that complex 5
displayed broad substrate scope and showed comparable activity
with previously reported precatalysts such as zinc (II) complex [2-
{(Ph2P-(=Se)}2NCH2(C5H4N)ZnCl2],31 low coordinate Fe (II) complex
(2,6-Mes2C6H3)2Fe (Mes
complex [Th(ImDippN){N(SiMe3)2}3].36,37
=
2,4,6-Me3C6H2),32 and thorium (IV)
electron-donating group, proceeded smoothly at 60 ͦC temperature
under neat conditions and afforded the respective urea derivatives
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4
5
6
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8
9
The most plausible mechanistic pathway for the insertion
View Article Online
DOI: 10.1039/D0NJ01509A
reaction between heterocumulenes and arylamines mediated by
homoleptic calcium complex, [{(Ph2CHN=CH)2-C4H2N}Ca] (5) as a
pre-catalyst is depicted in Scheme 3. Similar mechanistic cycles were
also recently proposed by Tamm and Eisen for the actinide complex
and titanium complexes by us.34,37,95-98 In the initial step of this
stepwise process, the calcium precatalyst 5 reacted with one
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 4. Substrate scope for hydroamination of isothiocyanate and
arylamines using complex 5 as pre-catalyst.a
S
R'
Complex 5 (3 mol%)
60 0C, 12 H
N
+
H2N
N
C
S
N
H
H
R'
Figure 5. The solid-state structure of 1-(2,6-dimethylphenyl)-3-phenylthiourea 9f. Some
hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (°):
S(1)-C(1) 1.681(2), N(1)-C(1) 1.340(3), N(1)-C(10) 1.425(3), N(1)-H(2) 0.8600, N(2)-C(1)
1.344(3), N(2)-C(2) 1.437(3), N(2)-H(1) 0.8600, C(1)-N(1)-C(10) 127.11(18), C(10)-N(1)-
H(2) 116.4, C(1)-N(2)-C(2) 123.04(17), C(1)-N(2)-H(1) 118.5, C(2)-N(2)-H(1) 118.5, N(1)-
C(1)-N(2) 117.54(18), N(1)-C(1)-S(1) 120.86(16), N(2)-C(1)-S(1) 121.58(16). CCDC No.
1993225.
9a - 9i
S
S
S
N
N
H
N
N
H
N
N
H
H
H
H
O
9a,98%
S
9b, 99%
9c, 99%
S
S
N
H
N
N
N
H
N
N
H
H
H
H
N
Experimental Section
9e, 98%
9d, 94%
9f, 96%
S
S
General Information
S
N
H
N
N
N
H
N
N
H
H
H
H
All manipulations of air-sensitive materials were performed with the
rigorous exclusion of oxygen and moisture in flame-dried Schlenk-
type glassware either on a dual manifold Schlenk line, interfaced to
a high vacuum (10−4 torr) line or in an argon-filled M. Braun glove
box. Tetrahydrofuran and diethyl ether were pre-dried over Na wire
and distilled under nitrogen from sodium and benzophenone ketyl
before use and n-pentane were distilled under nitrogen from LiAlH4
Br
9h, 98%
9g, 85%
9i, 98%
OMe
aAll reactions were performed under neat conditions; Yields are calculated from isolated
pure products.
equivalent of arylamine. The nucleophilic attack of the nitrogen atom
from the incoming arylamine onto the electrophilic calcium center
resulted in the formation of the intermediate complex (I) upon the
elimination of protic ligand 1-H. Complex (I), the active catalyst,
further reacted with two equivalents of isocyanate/isothiocynate to
give ureate/thioureate complex (II). In the final step to propagate the
catalytic cycle (Scheme 3), complex (II) reacted with one more
equivalent of arylamine to yield the corresponding urea/thiourea
product and the active catalyst (I).
1
and stored in the glove box. H NMR (400 MHz) and 13C{1H} NMR
spectra were recorded on a Bruker Avance III-400 spectrometer.
Elemental analyses were performed on a Bruker Euro EA at the
Indian Institute of Technology Hyderabad (IITH). Ligand 1-H, 1-K,
[Ca{N(SiMe3)2}2(THF)2], and [Ba{N(SiMe3)2}2(THF)2] were prepared
according to the procedure described in the literature.94,99,100 The
anhydrous CaI2, SrI2, BaI2 (99.999%), and NMR solvent C6D6 were
purchased from Sigma-Aldrich, and C6D6 was dried under Na/K alloy
before use.
Ph
Ph
N
Ph
Ph
N
Ca
N
ArNH2
N
N
N
N
Ph
Ph
Preparation of [{(Ph2CHN=CH)2C4H2N}CaI(THF)3] (2)
Ph
Ph
N
Ca NHAr
- 1-H
N
Ph
Ph
In a 50 mL dry Schlenk flask, the potassium salt of the ligand 1-K (200
mg, 0.31 mmol) and CaI2 (92 mg, 0.31 mmol) were mixed in 15 mL
THF at ambient temperature and the reaction mixture was stirred for
16 hours at 60 °C. The precipitate of KI was filtered through a glass
fiber filter paper and the filtrate was dried in vacuo. The resulting
white compound was further purified by washing with pentane and
crystals suitable for X-ray analysis were grown from THF/pentane at
–20 ˚C. Yield 201 mg (76%). 1H NMR (400 MHz C6D6, 25 °C): δH = 8.03
(s, 2H, CH=N), 7.50 (d, 8H, ArH), 7.23 (m, 8H, ArH), 7.04 (t, 4H, ArH),
6.73 (s, 2H, PyH), 5.53 (s, 2H, CH), 3.45 (br s, 12H, O–CH2–CH2), 1.17
(br s, 12H, O–CH2–CH2) ppm; 13C{1H} NMR (100 MHz, C6D6, 25 °C): δC
= 160.5 (N=C), 143.7 (Ar–C) 141.1 (Ar-C), 129.3 (Ar–C), 129.0 (Ar–C),
127.7 (py–C), 116.5 (Py–C), 79.0 (CH), 69.2 (O–CH2–CH2), 25.7 (O–
2 PhN=C=E
E = O, S
Ph
NHAr
E
Ph
(5)
Ph
(I)
N
H
Ph
Ph
Ph
N
ArNH2
N
N
Ph
Ca
E
N
Ph
(II)
NHAr
Scheme 3: A most plausible mechanism for the catalytic addition of
the N–H bond to isocyanate/isothiocyanate by homoleptic
calcium(II) precatalyst 5.
6 | J. Name., 2012, 00, 1-3
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CH2–CH2) ppm. Elemental analysis: C44H50CaIN3O3 (835.85): Calcd. C 119.2 (Py–C), 73.7 (CH) ppm. Elemental analysis: C64H52BaN6
View Article Online
DOI: 10.1039/D0NJ01509A
63.22, H 6.03, N 5.03. Found C 62.91, H 5.84, N 4.88.
(1042.33): Calcd. C 73.74, H 5.03, N 8.06. Found C 73.51, H 4.79, N
7.81.
Preparation of [{(Ph2CHN=CH)2C4H2N}SrI(THF)3] (3)
Typical Procedure for Hydroamination of isocyanates and
isothiocyanates with arylamines
Following a procedure similar to that described for complex 2, using
1–K (200 mg, 0.31 mmol) and SrI2 (108 mg, 0.31 mmol). Yield 227 mg
(82%). 1H NMR (400 MHz, C6D6, 25 °C): δH = 8.19 (s, 2H, CH=N), 7.12 Arylamine (0.352 mmol) was added under neat condition into the
(d, 8H, ArH), 7.03 (t, 8H, ArH), 6.98 (d, 4H, ArH), 6.90 (s, 2H, PyH), reaction mixture of isocyanate or isothiocyanate (0.352 mmol) and
5.27 (s, 2H, CH), 3.51 (m, 12H, O–CH2–CH2), 1.39 (m, 12H, O–CH2– calcium complex (5) (0.010 mmol). This was done in a 25 mL dry
CH2) ppm; 13C{1H} NMR (100 MHz, C6D6, 25 °C): δC = 160.4 (N=C), Schlenk flask inside the glovebox. The dark red reaction mixture was
145.1 (Ar–C), 129.1 (Ar–C), 127.3 (py–C), 119.9 (Py–C), 77.8 (CH), stirred for 12 h at 60˚C temperature. The solvent was thereafter
68.1 (O–CH2–CH2), 26.1 (O–CH2–CH2) ppm. Elemental analysis: evaporated in vacuo. A white solid compound was obtained in each
C44H50IN3O3Sr (883.39): Calcd. C 59.82, H 5.70, N 4.76. Found C 59.39, case which was purified by washing several times with n-hexane. The
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
H 5.43, N 4.45.
conversion of amines was calculated from isolated pure products.
Preparation of [{(Ph2CHN=CH)2C4H2N}BaI(THF)3] (4)
X-Ray Crystallographic Studies of Complexes 2, 3, 4, 6 and 9f
Following a procedure similar to that described for complex 2, using Single crystals of compounds 2–4 were grown from a mixture of THF
1-K (200 mg, 0.31 mmol) and BaI2 (123 mg, 0.31 mmol). Yield 220 mg and n-pentane, and complexes 6 was grown either from hexane or
(75%). 1H NMR (400 MHz, C6D6, 25 °C): δH = 8.03 (s, 2H, CH=N), 7.50 diethyl ether/hexane at –20 °C in an inert atmosphere. For
(d, 8H, ArH), 7.23 (t, 8H, ArH), 7.04 (t, 4H, ArH), 6.73 (s, 2H, PyH), 5.52 compounds, 2–6, a crystal of suitable dimensions was mounted on a
(s, 2H, CH), 3.45 (m, 12H, O–CH2–CH2), 1.20 (m, 12H, O–CH2–CH2) CryoLoop (Hampton Research Corp.) with a layer of light mineral oil
ppm; 13C{1H} NMR (100 MHz, C6D6, 25 °C): δC = 160.5 (N=C), 143.7 and placed in a nitrogen stream at 150(2) K and all measurements
(Ar–C), 141.1 (Ar–C), 129.4 (Ar–C), 129.0 (Ar–C), 127.7 (py–C), 116.5 were made on an Agilent Supernova X-Calibur Eos CCD detector with
(Py–C), 79.2 (CH), 69.0 (O–CH2–CH2), 25.8 (O–CH2–CH2) ppm. graphite monochromatic Cu-Kα (1.54184 Å) radiation. Crystal data
Elemental analysis: C44H50IN3O3Ba (933.11): Calcd. C 56.63, H 5.40, N and structure refinement parameters are summarized in Table TS1 in
4.50. Found C 56.34, H 5.19, N 4.17.
supporting information. The structures were solved by direct
methods (SIR92)101,102 and refined on F2 by full-matrix least-squares
methods; using SHELXL-97.103 Non-hydrogen atoms were
anisotropically refined. H atoms were included in the refinement in
calculated positions riding on their carrier atoms. The ORTEP-3
program was used to draw the molecules. Crystallographic data
(excluding structure factors) for the structures reported in this article
have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. CCDC 1518707 (2),
1518710 (3), 1518708 (4), 1518711(6), 1993225 (9f). Copies of the
data can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK (fax: +(44)1223-336-033; email:
deposit@ccdc.cam.ac.uk).
Preparation of [{(Ph2CHN=CH)2C4H2N}2Ca] (5)
In a 50 mL dry Schlenk flask, ligand 1-H (200 mg, 0.44 mmol) and
[Ca{N(SiMe3)2}2(THF)2] (110 mg, 0.22 mmol) were mixed together in
20 mL of toluene at an ambient temperature and the reaction
mixture was stirred for 6 hours. After evaporation of the toluene in
vacuo, the white residue yielded was recrystallized from hexane at –
1
20 °C and gave colorless crystals. Yield 150 mg (72%). H NMR (400
MHz, C6D6, 25 °C)  ppm 7.85 (s, 2H, CH=N) 6.93 - 7.01 (m, 20H, ArH),
6.59 (s, 2H, PyH), 5.15 (s, 2H, CH) ppm. 13C{1H} NMR (100 MHz,
C6D6,25 °C): C = 156.0 (N=C), 141.2 (Ar–C), 138.2 (Ar C), 129.3 (Ar–
C), 129.1 (Ar–C), 129.0 (Ar–C), 128.2 (Py–C), 119.2 (Py–C), 73.7 (CH)
ppm. Elemental analysis: C64H52CaN6 (944.39): Calcd. C 73.74, H 5.03,
N 8.06. Found C 73.51, H 4.79, N 7.81.
Conclusions
In summary, we successfully prepared a series of new alkaline earth
metal complexes 2-6 supported by the bulky bis-iminopyrrolyl ligand,
and the solid-state structures of heteroleptic complexes
[{(Ph2CHN=CH)2C4H2N}AeI(THF)3] Ae : Ca (2); Sr (3); Ba (4) and
homoleptic barium complex [{(Ph2CHN=CH)2C4H2N}2Ba] (6) were
established using X-ray crystallography. The solid-state structures of
2-4 and 6 reveal the flexibility and multidentate behavior of the bis-
iminopyrrolyl ligand 1-H. In the solid-state, the monoanionic ligand 1
exhibited κ3-chelation mode in complex 2 to adopt a distorted
octahedral geometry around the calcium ion, whereas κ3-chelation
mode of the ligand 1 was observed in complexes 3 and 4 to adopt a
pentagonal bipyramidal arrangement around the strontium and
Preparation of [(Ph2CHN=CH)2C4H2N}2Ba] (6)
In a 50 mL dry Schlenk flask, ligand 1-H (200 mg, 0.44 mmol) and
[Ba{N(SiMe3)2}2(THF)2] (132 mg, 0.22 mmol) were mixed together in
20 mL of toluene at an ambient temperature and the reaction
mixture was stirred for 6 hours. After evaporation of the toluene in
vacuo, the white residue yielded was recrystallized from diethyl
ether/hexane at –20 °C and gave colorless crystals. Yield 126 mg
1
(55%). H NMR (400 MHz, C6D6, 25 °C):  = 7.92 (s, 2x2H, CH=N)),
6.93-7.02 (d, 2x20H, ArH), 6.69 (s, 2x2H, PyH), 5.20 (s, 2x2H, CH) ppm.
13C{1H} NMR (100 MHz, C6D6,25 °C): C = 156.0 (N=C), 141.2 (Ar–C),
138.2 (Ar C), 129.3 (Ar–C), 129.1 (Ar–C), 129.0 (Ar–C), 128.2 (Py–C),
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barium ions. The homoleptic calcium and barium complexes 5 and 6 15. F. Ragaini and S. Cenini, J. Mol. Catal. A: Chem.,V1ie9w9A6rt,ic1le0O9n,lin1e-
25.
DOI: 10.1039/D0NJ01509A
were synthesized through the silylamine elimination route. The solid-
state structure of barium complex (6) adopts a distorted octahedral
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wide substrate scope.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgments
This work was supported by the Science and Engineering Research
Board (SERB), Department of Science and Technology (DST),
Government of India, under project no. (EMR/2016/005150). K.B and
S.A. thank CSIR, India for their Ph.D. fellowship
(09//1001(0028)/2017-EMR-I) and (09/1001(0010)/2011-EMR-I)
respectively.
Keywords: alkaline earth metals • bis-iminopyrrole • catalysis
hydroamination • isocyanates
28. E. Bernoud, P. Oulie, R. Guillot, M. Mellah and J. Hannedouche,
Angew. Chem., Int. Ed. 2014, 53, 4930-4934.
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DOI: 10.1039/D0NJ01509A
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Hydroamination of isocyanates and isothiocyanates by Alkaline Earth Metal
Initiators supported by Bulky Iminopyrrolyl Ligand
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Kulsum Bano,a Srinivas Anga,a Archana Jain,b Hari Pada Nayek,c and Tarun K. Panda*a
Synthesis and structural features of heteroleptic and homoleptic alkaline earth metal complexes
supported by bulky bis-iminopyrrolyl ligands are reported. The catalytic hydroamination of
isocyanates and isothiocyanates with aryl amines using calcium complex is presented.
Ph
Ph
N
Ca
N
N
N
N
N
Ph
Ph
Ph
Ph
Ph
Ph
E
Ca catalyst (3 mol%)
R'
N
N
+
N C E
H2N
H
H
60 0C, 12 h
R
R
R'
Up to 99% yield
E = O, S