Metal-free access of bulky: N, N ′-diarylcarbodiimides and their reduction: Bulky N, N ′-diarylformamidines

Article abstract of DOI:10.1039/c6nj00907g

A metal-free synthesis of symmetrical and unsymmetrical bulky N,N′-diaryl carbodiimides from the dehydrosulfurisation of their corresponding N,N′-diarylthiourea with 4-dimethylaminopyridine (DMAP) and iodine under mild reaction conditions with moderate to excellent yields has been established. In the literature, the classical method of dehydrosulfurisation of bulky N,N′-diarylthiourea to N,N′-diarylcarbodiimide has been reported using toxic metal oxide (HgO) and magnesium sulphate (MgSO₄) under harsh reaction conditions. Furthermore, easy access to 1,3-disubstituted symmetric and unsymmetrical N,N′-diaryl formamidines involving the reaction of symmetrical and unsymmetrical N,N′-diaryl carbodiimides with sodium borohydride is described. The widely used method for the preparation of bulky N,N′-diaryl formamidines is the treatment of primary amines with triethylorthoformate in the presence of acid under high temperature reaction conditions.

Full text of DOI:10.1039/c6nj00907g

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Metal-free access of bulky N,N0-diarylcarbodiimides
and their reduction: bulky N,N0-diarylformamidines†
Cite this:DOI: 10.1039/c6nj00907g
Thota Peddarao, Ashim Baishya, Milan Kr. Barman, Ajay Kumar and
Sharanappa Nembenna*
A metal-free synthesis of symmetrical and unsymmetrical bulky N,N0-diaryl carbodiimides from the
dehydrosulfurisation of their corresponding N,N0-diarylthiourea with 4-dimethylaminopyridine (DMAP)
and iodine under mild reaction conditions with moderate to excellent yields has been established. In the
literature, the classical method of dehydrosulfurisation of bulky N,N0-diarylthiourea to N,N0-diaryl-
carbodiimide has been reported using toxic metal oxide (HgO) and magnesium sulphate (MgSO4) under
harsh reaction conditions. Furthermore, easy access to 1,3-disubstituted symmetric and unsymmetrical
N,N0-diaryl formamidines involving the reaction of symmetrical and unsymmetrical N,N0-diaryl
carbodiimides with sodium borohydride is described. The widely used method for the preparation of
bulky N,N0-diaryl formamidines is the treatment of primary amines with triethylorthoformate in the
presence of acid under high temperature reaction conditions.
Received (in Montpellier, France)
21st March 2016,
Accepted 1st July 2016
DOI: 10.1039/c6nj00907g
www.rsc.org/njc
N,N0-diaryl CDIs.5 Thus, it is very clear that bulky N,N0-diaryl
CDIs are academically as well as industrially important organic
Introduction
Carbodiimides (CDIs) are a special class of reactive organic molecules.
compounds containing a NQCQN core. In the structure of
The preparation of carbodiimides can be broadly classified
CDIs, i.e., R–NQCQN–R, the substituent R can be alkyl, aryl, into (a) the dehydrosulfurisation of 1,3-di-substituted thioureas,
and acyl,1 many alkyl CDIs, such as dicyclohexylCDI (DCC) and (b) preparation from isocyanates or isothiocyanates, (c) the
diisopropylCDI (DICD), are commercially available and widely dehydration of ureas, (d) preparation from cyanamides, (e) the
used in organic synthesis, in particular, as dehydrating agents nitrene rearrangement and (f) preparation from haloformates
in the synthesis of lactams, antibiotics, nucleotides and and thermolysis reactions.1 Among all the methods mentioned
peptides.2 On the other hand, the commercial availability of above, the classical method used to prepare aryl CDIs is the
bulky N,N0-diarylCDIs is very limited. Both alkyl and aryl CDIs, dehydrosulfurisation of thiourea. In addition, in recent years
particularly bulky N,N0-diarylCDIs are the most important pre- there have been reports on the metal-catalysed synthesis of CDIs6
cursors used for the preparation of a variety of N-donor ligands, and this method of synthesis is restricted to unsymmetrical CDIs
metal complexes and other applications.3 For instance, com- (unsymmetrical; R a R and symmetrical; R = R, in the structure
pound 1a (vide infra) (1a = DippNQCQNDipp; (Dipp = 2,6-iPr2- of R–NQCQN–R). However, very recently Ji and Wang et al.
C6H3) (vide infra) has been utilized by several research groups reported a metal-free synthesis of CDIs using a I2/CHP (cumene
for the synthesis of ligands, metal complexes and other appli- hydroperoxide) mediated cross coupling of isocyanides with
cations.4 In addition, N,N0-di(2,6-diethylphenyl)CDI is used as a amines.7 This is a very attractive method for the synthesis of
stabilizer for polyester based polyurethanes.1 In this context, CDIs but shows poor yields for N,N0-diaryl CDIs. Actually, we
we have previously reported a metal-free synthesis of a library of were inspired by the study of Patel and co-workers,8 i.e., a
bulky guanidines upon the reaction of cyclic secondary amines with greener protocol used for the preparation of CDIs, in which
bulky aryl CDIs and air stable ‘‘NHC-CDI’’ adducts (zwitterions) the authors reported 15 examples of aryl CDIs prepared via
via the direct addition of N-heterocyclic carbenes (NHCs) to dehydrosulfurisation of thiourea in the presence of triethylamine/
iodine under mild reaction conditions. This synthetic route was
School of Chemical Sciences, National Institute of Science Education and Research
(NISER), Bhubaneswar, 751005, India. E-mail: snembenna@niser.ac.in;
Fax: +91-674-2302436; Tel: +91-674-2494174
† Electronic supplementary information (ESI) available: 1H and 13C{1H} NMR
spectra for compounds 1a–15a and 1b–10b; X-ray crystal data for compound 4a.
CCDC 982795. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c6nj00907g
effectively employed to prepare both symmetrical (R–NQCQN–R;
R = p-tolylphenyl) and unsymmetrical (N-(2,6-dimethylphenyl)-N0-
(phenyl))CDIs. It would also be quite attractive for the syntheses of
bulky aromatic CDIs with ortho-substituted aromatic substituent’s,
but in these cases the reported yields are lower (vide infra). In 2010,
Akamanchi and co-workers reported an o-iodoxybenzoic acid (IBX)
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Scheme 1 The synthesis of bulky N,N0-diaryl CDIs.
mediated oxidative desulfurisation of 1,3-disubstituted thioureas
Herein, we report a metal-free and facile synthesis of 15
to prepare carbodiimides.9 Recently, Nakazava and co-workers examples of symmetrical and unsymmetrical bulky N,N0-diaryl
reported the dehydrogenative desulfurisation of thiourea CDIs in good to excellent yields on a multi-gram scale. This was
derivatives to produce CDIs using hydrosilane and an iron achieved via the dehydrosulfurisation of their corresponding
complex.10 In 2007, Cowley et al. reported the synthesis of thioureas using dimethylaminopyridine (DMAP) as a base and
two examples of bulky aryl CDIs, including DippNQCQNDipp iodine as a thiophilic agent in THF under mild reaction conditions.
and MesNQCQNMes (Mes = 2,4,6-Me3C6H2), via the dehydro- Furthermore, reduction of these CDIs with the commercially
sulfurisation of thioureas in the presence toxic HgO and available reducing agent NaBH4 in ethanol afforded bulky
anhydrous MgSO4 in refluxing toluene (see Scheme 1).11 Very N,N0-diaryl formamidines in excellent yields.
recently, Fortier and co-workers reported ‘‘super bulky’’ CDIs
from the dehydration of urea derivatives in the presence of
P2O5/Al2O3 in pyridine at 115 1C.12 To the best of our knowl-
edge, there have been no reports on the metal-free synthesis of
bulky N,N0-diaryl CDIs.
Results and discussion
The synthesis of bulky N,N0-diaryl carbodiimides
In recent years, there has been a growing interest in the use Various symmetrical and unsymmetrical bulky N,N0-diaryl thiourea
of 1,3-di-substituted N,N0-diarylformamidines (FormHs) as ligands precursors have been utilized for the preparation of bulky N,N0-
in organometallic chemistry, as well as catalysis due to their diaryl CDIs and synthesized using the following literature
unique properties.13 More importantly, N,N0-diaryl FormHs are methods. Symmetrical N,N0-diaryl thioureas were synthesized
air and moisture stable and ideal precursors for the preparation upon the reaction of the dropwise addition of carbon disulphide
cyclic formamidiniums;14 these salts can be converted into neutral to a solution of aromatic amine and triethylamine in water/
NHCs upon treatment with base. Thus, N,N0-diaryl FormHs are acetonitrile,11,20 whereas unsymmetrical N,N0-diaryl thioureas
very important organic molecules, which are less sensitive towards were synthesized via the treatment of aryl isothiocyanate with an
hydrolysis than their alkyl substituted congeners. The steric and aromatic amine in acetonitrile.21
electronic properties of 1,3-di-substituted diaryl FormHs can be
Our investigation began with dehydrosulfurisation of bulky
tuned over a wide range by changing the substituent’s on the aryl N,N0-diaryl thiourea, i.e., DippN(H)CQSN(H)Dipp (Dipp = 2,6-
rings. The general formula RNQC(R0)NHR R0 = H formamidines, iPr2-C6H3) using two equivalents of triethylamine as base and
CH3 amidines and NR2 guanidine type of ligands of which bulky iodine as a thiophilic agent in THF at room temperature. In this
amidine and guanidine type ligand systems can be prepared case, after work up, we were able to isolate a 52% yield of the
by insertion of metal reagents into the CDIs followed by an corresponding CDI (DippNQCQNDipp) product. The formation
aqueous work up.15 In contrast, the general method used for the of the desired product in the lower yield may be due to the steric
synthesis of bulky aryl symmetric FormHs is the treatment of crowding revealed by the two bulky isopropyl substituents at the
aryl amines with triethylorthoformate in the presence of a adjacent ortho-positions of the aryl groups, which may hamper
catalytic amount of glacial acetic acid at reflux. This simple the deprotonation step by the hindered triethylamine base.
method of synthesis was first reported by Roberts in 1949.16 Furthermore, we chose pyridine (pKa 5.2) as a base, because it
Unsymmetrical FormHs can also be accessed using a slight is less steric in nature in comparison to triethylamine and
modification of the procedure.14k Moreover, there have been performed the reaction under the same reaction conditions
several reports on the preparation of formamidines in the along with iodine. We isolated the desired CDI product in only
literature.17 However, the widely employed method used for 37%; this was due to the lower basicity of pyridine in comparison
the preparation for bulky N,N0-diaryl FormH is the treatment of to triethylamine (pKa 10.8). Furthermore, dehydrosulfurisation
aryl amines with triethylorthoformate in the presence of a of the DippN(H)CQSN(H)Dipp thiourea derivative was performed
catalytic amount of glacial acetic acid at reflux temperature.18 using two equivalents of DMAP (pKa 9.7) as a base and one
To the best of our knowledge, there have been no reports on the equivalent of iodine under identical reaction conditions; DMAP
hydrogenation or reduction of symmetrical and unsymmetrical is a stronger base in comparison to pyridine. To our delight, we
bulky N,N0-diaryl CDIs to produce symmetrical and unsymmetrical successfully isolated the desired CDI product in 90% yield
bulky aryl FormHs. Although, in 1978, Yamada and co-workers (Table 1, entry 3). More importantly, we scaled up the reaction
reported the reduction of alkyl carbodiimides using sodium using 8 grams of the bulky thiourea precursor, in which a very
borohydride to produce FormHs.19
marginal difference in the isolated yield was observed.
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Table 1 Screening of the base and thiophilic agent for the dehydrosulfurisation of symmetrical and unsymmetrical thioureaa,b
Entry
Thiophilic agent/base
Substrate
Product
Isolate yieldb (%)
1
2
3
4
5
I2/NEt3
I2/Py
I2/DMAP
ICH2I/DMAP
ICH2CH2I/DMAP
52
37
90
27
80
6
7
8
9
I2/NEt3
I2/Py
I2/DMAP
ICH2I/DMAP
ICH2CH2I/DMAP
50
54
89
25
77
10
a
All reactions were performed using 1 equivalent of the corresponding thiourea (1.0 g), 1 equivalent of the thiophilic agent and 2 equivalents of
b
base in THF, temperature: r.t. and reaction time: 1 h. Isolated yield.
Furthermore, instead of iodine, diiodomethane and diiodo- methyl substituents in the adjacent meta positions of the aryl
ethane were utilized as thiophilic agents for the dehydrosul- groups yielded only a 50% yield. In addition, CDIs bearing
furisation of DippN(H)CQSN(H)Dipp using DMAP as the base bulky substituents, such as tert-butyl (6a) and isopropyl (7a)
under the identical reaction conditions described above. We noticed groups, in the para positions were formed in 65% and 49%
the formation of DippNQCQNDipp in 27% (Table 1, entry 4) and yield, respectively, wherein unsymmetrical CDIs are concerned,
80% (Table 1, entry 5) yield, when we employed diiodomethane and except compound (2,6-diisopropylphenyl)-(p-tert-butylphenyl)-
diiodoethane as the thiophilic agents, respectively.
carbodiimide 11a, which is isolated in 51% yield; all the
Furthermore, we extended this screening of base and thiophilic remaining compounds (2,6-diisopropylphenyl)-(2,4,6-trimethyl-
agent for the dehydrosulfurisation of unsymmetrical thiourea phenyl)carbodiimide (8a; 89%), 2,6-diisopropylphenyl)-(2,6-
derivatives. For this, we chose DippN(H)CQSN(H)Mes as a pre- diethylphenyl)carbodiimide (9a; 88%), (2,6-diisopropylphenyl)-
cursor and followed the same procedures as described above. (2,6-di-methylphenyl)carbodiimide, (10a, 83%), (2,6-diisopro-
From Table 1 (entries 1–10), it is very clear that dehydrosul- pylphenyl)phenyl carbodiimide, (12a; 80%), (2,4,6-trimethyl-
furisation of both symmetrical and unsymmetrical thioureas phenyl)-(2,6-diethylphenyl)carbodiimide, (13a; 73%), (2,4,6-
can be effectively performed using two equivalents of DMAP as trimethylphenyl)-(2,6-dimethylphenyl)carbodiimide, (14a; 74%)
base and one equivalent of iodine as a thiophilic agent in THF and (2,4,6-trimethylphenyl)phenyl carbodiimide (15a; 86%) were
at room temperature (Table 1, entries 3 and 8). Dehydrosul- isolated in very good yield.
furisation of DippN(H)CQSN(H)Dipp using DMAP/I2 as the
All these symmetrical and unsymmetrical bulky N,N0-diaryl
1
base and thiophilic agent was also carried out in different CDIs were characterized by H, 13C, IR and mass spectrometry
1
solvents such as ethyl acetate, dichloromethane, and acetonitrile. analyses. The H NMR spectra for compounds 1a–15a exhibit
DippNQCQNDipp was isolated in each case in a less than 90% the expected resonances and the complete disappearance of the
yield (Table 1, entry 3), which indicates that THF is a better solvent N–H proton signals was observed. The 13C NMR spectra show
for the dehydrosulfurisation of bulky N,N0-diarylthiourea. This was that the sp hybridized carbon atom of the NQCQN core
attributed to the improved solubility of the bulky thiourea in THF resonates in the range of 134–148 ppm for the symmetrical
in comparison to the other solvents studied. Furthermore, we CDIs and 136–147 ppm for the unsymmetrical CDIs. These
extended our studies to the dehydrosulfurisation of various less bulky aryl unsymmetrical CDIs are colorless liquids, while the
to more bulky symmetrical and unsymmetrical thioureas using symmetrical CDIs are low melting solids that melt in the range
DMAP/I2 to obtain their corresponding symmetrical and unsym- of 32–64 1C. Furthermore, compound 4a was characterized
metrical diarylCDIs (see Table 2). It is worth mentioning that the using single crystal X-ray structural analysis. It crystallizes in
dehydrosulfurisation reaction of [CyN(H)CQSN(H)Cy; (Cy = the monoclinic space group P21/c. The molecular structure is
cyclohexyl)] using DMAP/I2 in THF to obtain dicyclohexyl carbo- depicted in Fig. 1. The N2–C1–N1 bond angle is 170.1(2)1,
diimide (DCC) was attempted. However, we noticed only trace which deviates from the linearity value 1801. The average N–C
formation of the desired product.
bond distance in the NQCQN core is 1.201(2) Å (4a) and is
As can be observed from Table 2, symmetrical CDIs bearing in good agreement with the Mes and Dipp substituted CDIs
bulky substituent’s in the ortho positions of the aryl groups 1a, (average C–N bond distance 1.215 (10) Å).10 Attempts to prepare
3a, and 4a were isolated in excellent yields (482%), while extremely bulky N,N0-diaryl CDIs failed due to the unsuccessful
compound 2a was isolated in 67% yield. However, CDI bearing preparation of their corresponding thiourea precursors.
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Table 2 The synthesis of symmetrical and unsymmetrical N,N0-diarylCDIsa,b
a
All reactions were performed using 1 equivalent of the corresponding thiourea (4.0 g scale), 1 equivalent of I2 and 2 equivalents of DMAP in THF.
Isolated yield.
b
Fig. 1 The molecular structure of 4a with thermal ellipsoids drawn at a
30% probability. Hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å) and bond angles (1): N2–C1 1.197(2), N1–C1 1.208(2),
N1–C6 1.414(2), N2–C17 1.412(2); N2–C1–N1 170.1(2).
Fig. 2 The proposed reaction mechanism.
easily separated from the desired product by filtration, whereas
the DMAP salt was removed by column chromatography.
The proposed mechanism for the formation of the N,N0-
diaryl CDIs is depicted in Fig. 2. The dehydrosulfurisation of
thiourea was achieved using two equivalents of DMAP as base
Reduction of bulky N,N0-diaryl carbodiimides: N,N0-diaryl
formamidines
and one equivalent of iodine as the thiophilic agent. DMAPH+Iꢀ Bulky N,N0-diaryl CDIs are important precursors used for the
and solid sulfur are side products in this reaction. Sulfur can be preparation of a variety of bulky N-donor ligand systems such
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unsymmetrical ‘‘NHC-CDI’’ adducts, i.e., zwitterions via the direct
addition of NHC to CDIs.5b Surprisingly, there have been no
reports on the hydrogenation of bulky N,N0-diaryl CDIs used to
produce their corresponding FormHs. In view of this, herein we
report, 10 examples of symmetrical and unsymmetrical FormHs via
the reduction of their corresponding CDIs. Several symmetrical and
unsymmetrical bulky aryl FormHs were isolated in excellent yield
(76–98%) by employing simple, straightforward and facile reaction
conditions. This was achieved by the reduction of the corresponding
CDIs using commercially available sodium borohydride in ethanol
under ambient reaction conditions. All compounds (1b–10b) were
1
characterized by H and 13C NMR spectroscopy. For compounds
1
1b–10b, they display complicated H and 13C NMR spectra. As we
described in the Introduction section, N,N0-diaryl formamidines are
widely used in the coordination chemistry; however, their accurate
NMR spectra rarely appear in the literature, due to their complex
Scheme 2 The four possible isomers of formamidines in solution.
as amidines and guanidine. Bulky amidines and guanidines are NMR spectra.14d This complexity arises from the presence of more
used as ligands to construct metal complexes across the periodic than one isomer in the solution state (see Scheme 2).5a However, the
table. We have previously shown that these CDIs are ideal purity of these compounds was confirmed by high performance
precursors for the metal-free synthesis of bulky guanidines liquid chromatography (HPLC) analysis.
achieved by the treatment CDIs with secondary cyclic amines.5a
As can be observed from Table 3, four symmetrical (1b–4b)
Very recently, we reported a library of air stable symmetrical and and six unsymmetrical (5b–10b) bulky aryl formamidines
Table 3 The reduction of symmetrical and unsymmetrical N,N0-diarylCDIsa,b
a
b
All reactions were performed using 1 equivalent of the CDI (1.0 g scale) and 1.1 equivalents of NaBH4 in ethanol at room temperature. Isolated
yield.
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were synthesized. However, compounds 1,3-di( p-tert-butyl-phenyl)- X-ray crystallographic details
carbodiimide (6a) and 1,3-di(p-isopropyl-phenyl)carbodiimide (7a)
The crystal data for compound 4a was collected on a Bruker
SMART CCD diffractometer (MoKa radiation, l = 0.71073 Å).
The structure was solved by direct methods using the SHELXS-97
program and refined using full-matrix least-squares methods
against F2 with SHELXL-97.22 Hydrogen atoms were fixed at
calculated positions and their positions were refined using a
riding model. All non-hydrogen atoms were refined with anisotropic
displacement parameters. The crystal data and the cell parameters
for compound 4a are summarized in Table S1 (see ESI†).
were treated with sodium borohydride in ethanol, independently
to obtain corresponding FormHs. Unfortunately, we did not isolate
the desired products, such as N,N0-bis(4-tert-butyl-phenyl)-
formamidine and N,N0-bis(4-isopropyl-phenyl)formamidine, because
several spots were observed in thin layer chromatography.
Conclusions
In conclusion, we have shown the facile synthesis of various
symmetrical and unsymmetrical bulky N,N0-diaryl carbodiimides
via the dehydrosulfurisation of their corresponding thioureas
using a base and iodine in THF under mild reaction conditions
in moderate to high yields. Bulky aryl carbodiimides have very
broad applications in polymer, inorganic and organic syntheses.
More importantly, these bulky carbodiimides are ideal precursors
to prepare various bulky N-donor ligands such as formamidines,
amidines, guanidines and NHC-CDI adducts. These bulky N-donor
ligands are ideal precursors used for the isolation of unusual metal
complexes, in particular, low valent and/or low oxidation state
metal complexes. Furthermore, we produced 1,3-disubstituted
symmetrical and unsymmetrical bulky aryl formamidines under
mild reaction conditions in excellent yields upon the reduction
of their corresponding carbodiimides with readily available
sodium borohydride. Bulky aryl formamidines are important
precursors used for the preparation of imidazolinium salts,
these salts upon further treatment with base can produce s-donor
N-heterocyclic carbene ligands.
General procedure for the synthesis of symmetrical and
unsymmetrical N,N0-diarylcarbodiimides
All reactions were performed using 4 grams of the corres-
ponding 1,3-disubstituted aryl thiourea. The corresponding
thiourea 1.0 (equiv.) was dissolved in THF (around 50–80 mL)
and to this solution dimethylaminopyridine (DMAP) 2.0 (equiv.)
was added. The reaction mixture was cooled to 0 1C and iodine
1.1 (equiv.) was added portionwise. Once the addition of iodine
was completed, the reaction mixture was allowed to warm to
room temperature and stirring continued for two hours. During
this period, the progress of the reaction was monitored by
thin layer chromatography, once complete consumption of the
thiourea was occurred, the reaction mixture was filtered and
washed with n-hexane (150 mL), and the solvent was removed in
vacuum. The compound was purified by column chromato-
graphy on silica gel using 100% n-hexane as the eluent.
Characterization of the CDIs (1a–15a)
1,3-Bis(2,6-di-isopropylphenyl)carbodiimide (1a)11
3.29 g, 9.07 mmol, 90%); m. p. 45–50 1C; H NMR (400 MHz,
CDCl3, 25 1C): d 7.14 (s, 6H, ArH), 3.50 (m, 4H, CH(CH3)2), 1.29
(d, J = 8 Hz, 24H, CH(CH3)2). 13C NMR (101 MHz, CDCl3, 25 1C):
. (Yield:
1
Experimental section
General information
All reactions were performed in an open atmosphere. Chemicals d 142.9 (NCN), 133.3 (Ar–C), 124.9 (Ar–C), 123.3 (Ar–C), 29.2
were purchased from Sigma-Aldrich, Alfa-Aesar, and Spectro- ArCH(CH3)2, 23.3 ArCH(CH3)2. IR (KBr): n (cmꢀ1): 3224 s, 2920
chem and used as received unless otherwise stated. Column m, 2732 m, 2162 (CQN) m, 1731 s, 1583 m, 1470 m, 1376 m,
chromatography and TLC were performed on silica gel (100–200) 1209 m, 1146 m, 1032 m, 956 m, 936 m, 853 m, 769 s, 725 m,
and using UV light. 1H and 13C{1H} NMR spectra were obtained 602 s, 537 m. HRMS (ESI-TOF-Q) m/z: [M + H]+ calcd for
on a Bruker AV-400 spectrometer (1H: 400 MHz, 13C{1H}:
100.6 MHz) and were referenced to the resonances of the
C
25H35N2 363.2795; found: 363.2806.
1,3-Bis(2,4,6-trimethylphenyl)carbodiimide (2a)11. (Yield:
1
solvent used. IR spectra were obtained on a Perkin-Elmer 2.38 g, 8.576 mmol, 67%); m. p. 40–48 1C; H NMR (400 MHz,
FT-IR spectrometer. Mass spectra were obtained on a Bruker CDCl3, 25 1C): d 6.84 (s, 4H, ArH), 2.34 (s, 12H, o-CH3), 2.25 (s,
micrOTOF-Q II spectrometer. Melting points were taken on an 6H, p-CH3). 13C NMR (101 MHz, CDCl3, 25 1C): d 134.0 (NCN),
electrothermal apparatus and are uncorrected. All symmetrical 133.4 (Ar–C), 132.5 (Ar–C), 129.0 (Ar–C), 20.8 (CH3), 18.9 (CH3).
and unsymmetrical 1,3-disubstituted thioureas were synthe- IR (KBr): n (cmꢀ1): 3224 s, 2920 m, 2732 m, 2162 (CQN) m, 1731
sized according to literature procedures.11,20,21 The purity of s, 1583 m, 1470 m, 1376 m, 1209 m, 1146 m, 1032 m, 956 m, 936
the formamidines was determined by high pressure liquid m, 853 m, 769 s, 725 m, 602 s, 537 m. HRMS (ESI-TOF-Q) m/z:
chromatography (HPLC) using a C18 Agilent ZORBAX GF-450 [M + H]+ calcd for C19H24N2 279.1856; found: 279.1861.
Eclipse XDB column. Analytical HPLC was performed on an
1,3-Bis(2,6-diethylphenyl)carbodiimide (3a). (Yield: 2.95 g,
1
Agilent Technologies 1200 Series (Quaternary Pump) system 9.63 mmol, 82%); colorless liquid; H NMR (400 MHz, CDCl3,
equipped with a UV detector set at 254 nm. The compounds 25 1C): d 7.09–7.05 (m, 6H, ArH), 2.80 (q, J = 7.5 Hz, 8H,
were dissolved in acetonitrile. The following eluent system was CH2(CH3)), 1.27 (t, J = 7.5 Hz, 12H, CH2(CH3)). 13C NMR (101
used: 70% n-hexane/30% isopropanol. HPLC retention times MHz, CDCl3, 25 1C): d 138.8 (NCN), 134.5 (Ar–C), 126.6 (Ar–C),
(HPLC tR) were obtained at flow rates of 1 mL minꢀ1 using an 124.8 (Ar–C), 25.8 (CH2CH3), 14.7 (CH2CH3). IR (KBr): n (cmꢀ1):
isocratic run.
2966 m, 2932 m, 2163 (CQN) m, 1588 m, 1450 m, 1284 m, 1267 m,
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1215 m, 1180 m, 1116 m, 1101 m, 1016 m, 837 m, 661 m, 594 m. 3H, ArH), 3.47 (m, 2H, (CH(CH3)2)), 2.81 (q, J = 7.5 Hz, 4H,
HRMS (ESI-TOF-Q) m/z: [M + H]+ calcd for C21H27N2 307.2169; CH2(CH3)), 1.28 (dd, J = 7.2, 2.1 Hz, 18H, (CH3)2). 13C NMR (101
found: 307.2185.
MHz, CDCl3, 25 1C): d 142.9 (NCN), 138.7 (Ar–C), 134.6 (Ar–C),
1,3-Bis(2,6-dimethylphenyl)carbodiimide (4a)20. (Yield: 3.13 g, 133.2 (Ar–C), 126.6 (Ar–C), 125.9 (Ar–C), 125.1 (Ar–C), 124.7
1
12.516 mmol, 89%); m. p. 44–53 1C; H NMR (400 MHz, CDCl3, (Ar–C), 123.4 (Ar–C), 29.2 (CH(CH3)2), 25.8 CH2(CH3), 23.3
25 1C): d 7.03 (d, J = 8 Hz, 4H, ArH), 6.94–6.97 (m, 2H, ArH), 2.4 (s, (CH(CH3)2), 14.6 (CH2(CH3)). IR (KBr): n (cmꢀ1): 3234 m, 3063
12H, CH3). 13C NMR (101 MHz, CDCl3, 25 1C): d 135.9 (NCN), m, 2923 m, 2167 (CQN) m, 1921 m, 1857 m, 1587 m, 1454 m,
132.8 (Ar–C), 128.3 (Ar–C), 124.5 (Ar–C), 19.0 (CH3). IR (KBr): 1363 m, 1324 m, 1256 m, 1182 m, 1101 m, 1060 m, 1041 m, 934 m,
n (cmꢀ1): 2942 s, 2914 m, 2205 m, 2164 (CQN) m, 2106 s, 1588 872 m, 795 m, 749 m, 593 m. HRMS (ESI-TOF-Q) m/z: [M + H]+
m, 1466 m, 1262 m, 1192 m, 1090 m, 917 m, 769 s, 729 m, 664 s, calcd for C23H31N2 335.2409; found: 335.240.
582 m. HRMS (ESI/TOF-Q) m/z: [M + H]+ calcd for C17H19N2
251.1543; found: 251.1544.
(2,6-Diisopropylphenyl)-(2,6-dimethylphenyl)carbodiimide (10a).
(Yield: 2.987 g, 9.749 mmol, 83%); colorless liquid; 1H NMR
1,3-Bis(3,5-dimethylphenyl)carbodiimide (5a)6i. (Yield: 1.760 g, (400 MHz, CDCl3, 25 1C): d 7.13 (s, 3H, ArH), 7.04 (d, J = 7.4 Hz,
7.03 mmol, 50%); m. p. 45–49 1C; 1H NMR (400 MHz, CDCl3, 2H, ArH), 6.97–6.95 (m, 1H, ArH), 3.45 (m, 2H, CH(CH3)2), 2.40
25 1C): d 6.81 (s, 6H, ArH), 2.30 (s, 12H, CH3). 13C NMR (101 MHz, (s, 6H, (CH3)), 1.27 (d, J = 6.9 Hz, 12H, CH(CH3)2). 13C NMR (101
CDCl3, 25 1C): d 139.3 (NCN), 138.4 (Ar–C), 135.9 (Ar–C), 127.4 MHz, CDCl3, 25 1C): d 142.9 (NCN), 136.2 (Ar–C), 133.2 (Ar–C),
(Ar–C), 121.9 (Ar–C), 21.2 (CH3). IR (KBr): n (cmꢀ1): 2942 s, 2914 m, 132.6 (Ar–C), 128.3 (Ar–C), 127.1 (Ar–C), 125.2 (Ar–C), 124.3
2205 m, 2164 (CQN) m, 2106 s, 1588 m, 1466 m, 1262 m, 1192 m, (Ar–C), 123.4 (Ar–C), 29.2 (CH(CH3)2), 23.4 (CH(CH3)2), 19.0
1090 m, 917 m, 769 s, 729 m, 664 s, 582 m. HRMS (ESI-TOF-Q) m/z: (CH3). IR (KBr): n (cmꢀ1): 3226 m, 2962 m, 2158 (CQN) m,
[M + H]+ calcd for C17H19N2 251.1543; found: 251.1567.
1590 m, 1454 m, 1323 m, 1258 m, 1184 m, 1095 m, 1060 m, 934
1,3-Di(p-tert-butylphenyl)carbodiimide (6a). (Yield: 2.34 g, m, 795 m, 766 m, 749 m, 666 m, 594 m. HRMS (ESI-TOF-Q) m/z:
1
7.635 mmol, 65%); m. p. 64–69 1C; H NMR (400 MHz, CDCl3, [M + H]+ calcd for C21H27N2 307.2169; found: 307.2203.
25 1C): d 7.11 (d, J = 8 Hz, 4H, ArH), 7.4 (d, J = 8 Hz, 4H, ArH),
(2,6-Diisopropylphenyl)(p-tert-butylphenyl)carbodiimide (11a).
1.32 (s, 18H, C(CH3)3). 13C NMR (101 MHz, CDCl3, 25 1C): d (Yield: 1.85 g, 5.534 mmol, 51%); colorless liquid; 1H NMR
148.8 (NCN), 135.9 (Ar–C), 126.5 (Ar–C), 123.8 (Ar–C), (400 MHz, CDCl3, 25 1C): d 7.35–7.33 (m, 2H, ArH), 7.14 (d, J =
34.6 (C(CH3)3), 31.4 (C(CH3)3). IR (KBr): n (cmꢀ1): 2965 m, 4 Hz, 3H, ArH), 7.13–7.07 (m, 2H, ArH), 3.41 (m 2H, CH(CH3)2),
2128 (CQN) m, 1601 m, 1508 m, 1363 m, 1284 m, 1267 m, 1.32 (s, 9H, C(CH3)3), 1.27 (d, J = 6.8 Hz, 12H, CH(CH3)2). 13C
1215 m, 1180 m, 1116 m, 1101 m, 1016 m, 837 m, 661 m, 594 m. NMR (101 MHz, CDCl3, 25 1C): d 147.7 (NCN), 142.8 (Ar–C),
HRMS (ESI-TOF-Q) m/z: [M + H]+ calcd for C21H27N2 307.2169; 137.2 (Ar–C), 132.6 (Ar–C), 130.2 (Ar–C), 126.5 (Ar–C), 125.7 (Ar–C),
found: 307.2202.
123.4 (Ar–C), 123.3 (Ar–C), 34.5 (C(CH3)3), 31.5 (C(CH3)3), 29.4
1,3-Di(p-isopropylphenyl)carbodiimide (7a). (Yield: 1.74 g, CH(CH3)2, 23.3 CH(CH3)2. IR (KBr): n (cmꢀ1): 3226 m, 3068 m,
1
6.27 mmol, 49%); m. p. 50–58 1C; H NMR (400 MHz, CDCl3, 3030 m, 2961 m, 2166 (CQN) m, 1589 m, 1574 m, 1470 m, 1362 s,
25 1C): d 7.18 (d, J = 8.4 Hz, 4H, ArH), 7.12–7.09 (m, 4H, ArH), 2.9 1324 s, 1257 m, 1244 m, 1203 s, 1179 m, 1113 m, 1073 m, 1041 m,
(m, 2H, CH(CH3)2), 1.24 (d, J = 8 Hz, 12H, CH(CH3)2). 13C NMR 934 m, 834 m, 795 m, 749 m, 666 m, 594 m, 551 s. HRMS
(101 MHz, CDCl3, 25 1C): d 146.5 (NCN), 136.2 (Ar–C), 127.5 (ESI-TOF-Q) m/z: [M + H]+ calcd for C23H31N2 335.2482; found:
(Ar–C), 124.1 (Ar–C), 33.8 (CH(CH3)2), 24.1 (CH(CH3)2). IR (KBr): 335.2503.
n (cmꢀ1): 3219 s, 2961 m, 2147 (CQN) m, 1895 m, 1603 m, 1575
(2,6-Diisopropylphenyl)phenyl carbodiimide (12a)21. (Yield:
1
m, 1505 m, 1275 m, 1208 m, 1174 m, 1101 m, 1054 m, 1017 m, 2.85 g, 10.24 mmol, 80%); colorless liquid; H NMR (400 MHz,
833 m, 729 m, 643 m, 597 m. HRMS (ESI-TOF-Q) m/z: [M + H]+ CDCl3, 25 1C): d 7.37–7.33 (m, 2H, ArH), 7.20–7.17 (m, 5H, ArH),
calcd for C19H23N2 279.1856; found: 279.1867.
7.14 (d, J = 8 Hz, 1H, ArH), 3.44 (m, 2H, CH(CH3)2), 1.31 (d, J =
(2,6-Diisopropylphenyl)-(2,4,6-trimethylphenyl)carbodiimide 8 Hz, 12H, CH(CH3)2). 13C NMR (101 MHz, CDCl3, 25 1C): d
(8a). (Yield: 3.21 g, 10.04 mmol, 89%); colorless liquid; 1H NMR 142.9 (NCN), 140.2 (Ar–C), 132.3 (Ar–C), 129.6 (Ar–C), 125.8
(400 MHz, CDCl3, 25 1C): d 7.16 (s, 3H, ArH), 6.89 (s, 2H, ArH), (Ar–C), 124.6 (Ar–C), 123.8 (Ar–C), 123.4 (Ar–C), 29.4 CH(CH3)2,
3.5 (m, 2H, CH(CH3)2), 2.41 (s, 6H, (CH3)), 2.31 (s, 3H, (CH3)), 23.2 CH(CH3)2. IR (KBr): n (cmꢀ1): 3746 m, 2963 m, 2360 m,
1.31 (d, J = 8 Hz, 12H, CH(CH3)2). 13C NMR (101 MHz, CDCl3, 2158 (CQN) m, 1472 m, 1244 m, 1194 m, 1072 m, 749 m, 689 m,
25 1C): d 142.8 (NCN), 133.9 (Ar–C), 133.5 (Ar–C), 133.4 (Ar–C), 593 m. HRMS (ESI-TOF-Q) m/z: [M + H]+ calcd for C19H23N2
132.4 (Ar–C), 128.9 (Ar–C), 127.4 (Ar–C), 125.0 (Ar–C), 123.4 279.1856; found: 279.1881.
(Ar–C), 29.1 (CH(CH3)2), 23.4 (CH(CH3)2), 20.8 (CH(CH3)2), 18.9
(2,4,6-Trimethylphenyl)-(2,6-diethylphenyl)carbodiimide (13a).
CH3. IR (KBr): n (cmꢀ1): 3227 m, 2963 m, 2169 (CQN) m, 1921 (Yield: 2.615 g, 8.94 mmol, 73%); colorless liquid; 1H NMR
m, 1858 m, 1589 m, 1454 m, 1363 m, 1323 m, 1257 m, 1186 m, (400 MHz, CDCl3, 25 1C): d 7.08 (s, 3H, ArH), 6.88 (s, 2H, ArH),
1159 m, 1096 m, 1059 m, 1040 m, 934 m, 853 m, 794 m, 767 m, 2.83 (q, J = 7.5 Hz, 4H, CH2(CH3)), 2.40 (s, 6H, o-(CH3)), 2.29 (s,
749 m, 730 m, 666 m, 595 m. HRMS (ESI-TOF-Q) m/z: [M + H]+ 3H, p-(CH3)), 1.29 (t, J = 8 Hz, 6H, CH2(CH3)). 13C NMR (101
calcd for C22H29N2 321.2325; found: 321.2367.
MHz, CDCl3, 25 1C): d 138.7 (NCN), 134.8 (Ar–C), 134.0 (Ar–C),
(2,6-Diisopropylphenyl)-(2,6-diethylphenyl)carbodiimide (9a). 133.2 (Ar–C), 132.5 (Ar–C), 129.0 (Ar–C), 128.2 (Ar–C), 126.6 (Ar–C),
(Yield: 3.194 g, 9.55 mmol, 88%); colorless liquid; 1H NMR 124.8 (Ar–C), 25.8 o-(CH3), 20.8 (CH(CH3)2), 18.9 p-(CH3), 14.7
(400 MHz, CDCl3, 25 1C): d 7.13 (s, 3H, ArH), 7.06 (q, J = 4.2 Hz, CH2(CH3). IR (KBr): n (cmꢀ1): 3226 m, 2923 m, 2159 (CQN) m,
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1590 m, 1470 m, 1377 m, 1253 m, 1198 m, 1159 m, 1111 m,
N,N0-Bis(2,6-dimethylphenyl)formamidine (1b)23. (Yield: 0.796 g,
1
1134 m, 852 m, 750 m, 595 m. HRMS (ESI-TOF-Q) m/z: [M + H]+ 3.155 mmol, 79%); m. p. 180–183 1C; H NMR (400 MHz, C6D6,
calcd for C20H25N2 293.2012; found: 293.2050.
25 1C): d 7.10–6.92 (m, 4H), 6.91 (dd, 2H), 6.75 (d, 1H), 5.00 (d, 1H),
(2,4,6-Trimethylphenyl)-(2,6-dimethylphenyl)carbodiimide 2.31 (s, 3H), 2.16 (s, 6H), 1.85 (s, 3H). 13C NMR (101 MHz, C6D6,
(14a)6d. (Yield: 2.62 g, 9.92 mmol, 74%); colorless liquid; 25 1C): d 146.6, 145.8, 137.0, 134.2, 128.7, 128.4, 128.3 128.0, 127.8,
1H NMR (400 MHz, CDCl3, 25 1C): d 7.05 (d, J = 8 Hz, 2H, 126.3, 123.1, 30.2, 18.8, 18.5, 18.2. IR (KBr): n (cmꢀ1): 3449 m, 2940
ArH), 6.99–6.96 (m, 1H, ArH), 6.88 (s, 2H, ArH), 2.42 (s, 6H, m, 2921 m, 1651 s, 1638 s, 1632 s, 1611 s, 1589 m, 1466 m, 1202 m,
o-(CH3)), 2.39 (s, 6H, o-(CH3)), 2.29 (s, 3H, p-(CH3)). 13C NMR (101 821 s, 771 s, 759 m. HRMS (ESI-TOF-Q) m/z: [M + H]+ calcd for
MHz, CDCl3, 25 1C): d 136.2 (NCN), 134.1 (Ar–C), 133.1 (Ar–C), C17H21N2 253.1699; found: 253.1666. HPLC: PHPLC 100%, tR 13.28.
132.7 (Ar–C), 132.6 (Ar–C), 129.0 (Ar–C), 128.3 (Ar–C), 124.4
N,N0-Bis(2,6-diisopropylphenyl)formamidine (2b)14i,23b. (Yield:
(Ar–C), 20.8 o-(CH3), 19.0 o-(CH3), 18.9 p-(CH3). IR (KBr): n 0.764 g, 2.09 mmol, 76%); m. p. 200–205 1C; 1H NMR (400 MHz,
(cmꢀ1): 3563 m, 3220 m, 2920 m, 2732 m, 2147 (CQN) m, 1921 C6D6, 25 1C): d 10.95 (br, 1H), 7.07–7.05 (m, 7H), 3.43 (m, 4H),
m, 1731 m, 1592 m, 1470 m, 1377 m, 1258 m, 1201 m, 1161 m, 1.35 (dd, 1H), 1.12 (d, 22H), 1.05 (d, 1H). 13C NMR (101 MHz,
1104 m, 1063 m, 1032 m, 853 m, 766 m, 733 m, 595 m. HRMS C6D6, 25 1C): d 155.5, 146.9, 146.1, 143.8, 140.8, 139.6, 138.9,
(ESI-TOF-Q) m/z: [M + H]+ calcd for C18H21N2 265.1749; found: 134.1, 125.8, 123.7, 123.4, 28.3, 23.7. IR (KBr): n (cmꢀ1): 3131 m,
265.1699.
3030 m, 2962 m, 2929 s, 1664 s, 1587 m, 1462 m, 1453 m, 1288 m,
(2,4,6-Trimethylphenyl)phenyl carbodiimide (15a). (Yield: 822 s, 799 s, 754 m. HRMS (ESI-TOF-Q) m/z: [M + H]+ calcd
1
3.0 g, 12.72 mmol, 86%); colorless liquid; H NMR (400 MHz, for C25H37N2 365.2951; found: 365.2967. HPLC: PHPLC 100%,
CDCl3, 25 1C): d 7.34–7.30 (m, 2H, ArH), 7.18–7.11 (m, 3H, ArH), tR 12.93.
6.87 (s, 2H, ArH), 2.35 (s, 6H, o-(CH3)), 2.27 (s, 3H, p-(CH3)). 13
C
N,N0-Bis(2,4,6-trimethylphenyl)formamidine (3b)24. (Yield:
NMR (101 MHz, CDCl3, 25 1C): d 140.3 (NCN), 135.0 (Ar–C), 0.977 g, 3.484 mmol, 97%); m. p. 198–200 1C; 1H NMR
132.8 (Ar–C), 132.2 (Ar–C), 131.9 (Ar–C), 129.6 (Ar–C), 129.5 (400 MHz, C6D6, 25 1C): d 6.93–6.99 (dd, 2H), 6.78 (s, 2H),
(Ar–C), 129.0 (Ar–C), 128.9 (Ar–C), 124.7 (Ar–C), 123.7 (Ar–C), 6.59 (s, 1H), 5.04 (d, 1H), 2.34 (s, 3H), 2.26 (s, 1H), 2.20 (s, 6H),
20.9 p-(CH3), 19.0 o-(CH3). IR (KBr): n (cmꢀ1): 3543 m, 3221 m, 2.16 (s, 3H), 2.06 (s, 2H), 1.89 (s, 3H). 13C NMR (101 MHz, C6D6,
3063 m, 2920 m, 2148 (CQN) m, 1594 m, 1485 m, 1283 m, 25 1C): d 146.4, 144.1, 135.6, 134.5, 134.1, 131.7, 129.4, 129.4,
1206 m, 1161 m, 1073 m, 1027 m, 899 m, 853 m, 790 m, 755 m, 129.1, 20.9, 20.8, 18.8, 18.5, 18.2. IR (KBr): n (cmꢀ1): 3434 m,
724 m, 689 m, 618 m, 547 m. HRMS (ESI-TOF-Q) m/z: [M + H]+ 3152 m, 2965 m, 2944 m, 2916 m, 1642 s, 1614 s, 1608 s,
calcd for C16H17N2 237.1386; found: 237.1401.
1215 m, 849 s, 776 m. HRMS (ESI-TOF-Q) m/z: [M + H]+ calcd
for C19H25N2 281.2012; found: 281.2034. HPLC: PHPLC 100%,
tR 12.57.
General procedure for the synthesis of symmetrical and
unsymmetrical N,N0-diaryl formamidines
N,N0-Bis(2,6-diethylphenyl)formamidine (4b)23b. (Yield: 0.956 g,
1
All reactions were performed using 1 gram of the corresponding 3.10 mmol, 95%); m. p. 108–110 1C; H NMR (400 MHz, C6D6,
symmetrical or unsymmetrical N,N0-diaryl carbodiimide. The corres- 25 1C): d 7.0–6.90 (m, 7H), 5.21 (d, 1H), 2.62 (q, 8H), 1.15 (t, 12H).
ponding N,N0-diaryl carbodiimide 1.0 (equiv.) was dissolved in 13C NMR (101 MHz, C6D6, 25 1C): d 154.8, 146.7, 145.3, 140.8,
ethanol (30 mL) and to this solution was added sodium borohydride 138.9, 135.6, 134.4, 126.8, 126.6, 125.2, 123.6, 117.9, 116.7, 25.2,
1.1 (equiv.) and the reaction mixture stirred at room temperature for 25.0, 24.9, 14.9, 14.8, 14.5. IR (KBr): n (cmꢀ1): 3158 m, 3067 m,
10 hours. The progress of the reaction was monitored by thin layer 2963 m, 2934 s, 1647 s, 1630 s, 1586 m, 1541 m, 1453 m, 1284 m,
chromatography (TLC). When the starting material was completely 864 s, 808 m, 774 s, 755 m. HRMS (ESI-TOF-Q) m/z: [M + H]+ calcd
consumed, the excess sodium borohydride was quenched with an for C21H29N2 309.2325; found: 309.2348. HPLC: PHPLC 100%,
aqueous ammonium chloride solution and the mixture extracted tR 12.43.
with diethyl ether. Then, the organic portion was dried over
N-(2,6-Diisopropylphenyl)-N0-phenylformamidine (5b)25. (Yield:
anhydrous sodium sulfate. The solvent was removed in vacuum 0.916 g, 3.268 mmol, 91%); m. p. 115–120 1C; 1H NMR (400 MHz,
and the compound was dried under high vacuum, and then CDCl3, 25 1C): d 8.70 (br, 1H), 7.99 (br, 1H), 7.25–6.71 (m, 8H),
purified by column chromatography with silica gel (100–200 mesh) 3.43–3.25 (m, 2H), 1.27 (d, 12H). 13C NMR (101 MHz,
using 5% ethyl acetate in n-hexane as the eluent to yield colorless CDCl3, 25 1C): d 150.5, 149.9, 143.9, 141.6, 141.2, 129.6, 124.8,
solids in good to excellent (76–98%) yield.
123.5, 122.4, 116.4, 48.6, 28.1, 23.7. IR (KBr): n (cmꢀ1): 3437 m,
Note: N,N0-disubstituted formamidines exhibit a mixture of 3376 m, 3123 m, 3176 m, 2962 s, 1673 s, 1601 m, 1589 m, 1495 m,
isomers in solution therefore their solution NMR spectra are 1458 m, 1362 m, 1307 m, 885 s, 800 s, 782 s, 752 m. HRMS
always complicated, thus it is difficult to assign the accurate (ESI-TOF-Q) m/z: [M + H]+ calcd for C19H25N2 281.2012; found:
peaks. In particular, a large number of peaks than expected 281.2020.
were observed for unsymmetrical N,N0-disubstituted formami-
dines in comparison to the symmetrical FormHs.
N-(2,6-Diisopropylphenyl)-N0-(2,6-dimethylphenyl)formamidine
(6b). (Yield: 0.926 g, 3.0 mmol, 91%); m. p. 113–115 1C; H NMR
1
(400 MHz, CDCl3, 25 1C): d 7.29–7.06 (m, 7H), 5.59 (t, 1H), 3.31–
Characterization of formamidines (1b–10b)
3.21 (m, 2H), 2.31 (s, 3H), 2.27 (s, 3H), 1.32 (d, 1H), 1.21 (dd,
Note: Isolated yields of formamidines are reported with respect 11H). 13C NMR (101 MHz, CDCl3, 25 1C): d 147.8, 146.9, 146.1,
to the carbodiimide.
145.2, 142.4, 139.2, 136.4, 133.3, 129.0, 128.9, 128.8, 128.4,
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126.3, 126.2, 123.9, 123.7, 123.5, 123.2, 116.3, 28.3, 28.1, 28.1, References
24.2, 23.7, 23.2, 18.8, 18.7, 17.8. IR (KBr): n (cmꢀ1): 3376 m,
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3330 m, 3184 m, 3138 m, 3063 m, 3029 m, 2964 m, 1645 s, 1463 m,
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[M + H]+ calcd for C21H29N2 309.2325; found: 309.2309.
N-(2,6-Diisopropylphenyl)-N0-(2,4,6-trimethylphenyl)formamidine
1
(7b)14c. (Yield: 0.925 g, 2.87 mmol, 90%); m. p. 80–82 1C; H NMR
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(400 MHz, CDCl3, 25 1C): d 7.21–6.89 (m, 6H), 5.56 (s, 1H), 3.20–3.27
(m, 2H), 2.27 (s, 6H), 2.22 (s, 3H), 1.20 (t, 12H). 13C NMR (101 MHz,
CDCl3, 25 1C): d 150.0, 148.2, 147.4, 146.2, 142.4, 139.3, 136.1, 133.8,
133.6, 129.5, 129.1, 123.9, 123.7, 123.4, 123.1, 116.3, 28.1, 28.0, 24.2,
23.7, 23.2, 20.9, 18.7, 18.5, 17.8. IR (KBr): n (cmꢀ1): 3374 m, 3243 m,
2959 m, 2923 s, 1644 s, 1609 s, 1515 m, 1460 m, 1233 m, 850 s, 828 s,
783 m, 762 m. HRMS (ESI-TOF-Q) m/z: [M + H]+ calcd for C22H31N2
323.2482; found: 323.2467. HPLC: PHPLC 100%, tR 12.32.
N-(2,6-Diethylphenyl)-N0-(2,6-diisopropylphenyl)formamidine
(8b). (Yield: 0.985 g, 2.93 mmol, 98%); m. p. 118–120 1C;
1H NMR (400 MHz, CDCl3, 25 1C): d 7.21 (dd, 1H), 7.12 (s,
3H), 7.08 (s, 3H), 5.58 (t, 1H), 3.33 (d, 2H), 2.68–2.62 (m, 6H),
1.33 (dd, 4H), 1.19 (dd, 12H). 13C NMR (101 MHz, CDCl3, 25 1C):
d 147.6, 147.3, 145.8, 144.1, 142.6, 140.6, 139.1, 135.0, 134.5,
133.4, 127.8, 127.2, 126.9, 126.5, 126.3, 123.8, 123.7, 123.5,123.4,
123.2, 28.3, 28.2, 28.1, 24.8, 24.4, 24.1, 23.8, 23.7, 22.8, 15.1, 14.7,
14.3. IR (KBr): n (cmꢀ1): 3415 m, 3296 m, 3189 m, 2931 m, 2868
m, 1664 s, 1586 m, 885 s, 876 s, 826 m, 800 m, 768 s, 757 m.
HRMS (ESI-TOF-Q) m/z: [M + H]+ calcd for C23H33N2 337.2638;
found: 337.2654. HPLC: PHPLC 100%, tR 11.70.
N-(2,6-Dimethylphenyl)-N0-(2,4,6-trimethylphenyl)formamidine
(9b). (Yield: 0.906 g, 3.40 mmol, 90%); m. p. 158–160 1C; 1H NMR
(400 MHz, CDCl3, 25 1C): d 7.05–6.89 (m, 6H), 2.26 (s, 15H).
13C NMR (101 MHz, CDCl3, 25 1C): d 147.2, 133.9, 130.7, 129.1,
128.4, 117.8, 116.3, 113.4, 29.8, 20.9, 18.8, 18.7. IR (KBr): n
(cmꢀ1): 3362 s, 3195 m, 2969 s, 2916 m, 1642 s, 1615 s, 1608 m,
1592 m, 850 s, 793 m, 758 m. HRMS (ESI-TOF-Q) m/z: [M + H]+
calcd for C18H23N2 267.1856; found: 267.1820.
N-(2,6-Diethylphenyl)-N0-(2,4,6-trimethylphenyl)formamidine
(10b). (Yield: 0.966 g, 3.28 mmol, 96%); m. p. 128–130 1C;
1H NMR (400 MHz, CDCl3, 25 1C): d 7.29–6.62 (br, 6H), 5.54 (s,
1H), 2.65 (s, 4H), 2.29 (d, 9H), 1.21 (s, 6H). 13C NMR (101 MHz,
CDCl3, 25 1C): d 136.0, 133.3, 129.5, 129.3, 126.7, 126.4, 123.7,
116.3, 25.0, 20.9, 18.6, 15.0. IR (KBr): n (cmꢀ1): 3379 m, 3187 m,
3060 m, 2932 m, 2874 m, 1677 s, 1644 m, 1609 m, 1546 m, 863 s,
850 s, 827 m, 810 m, 792 m, 781 m. HRMS (ESI-TOF-Q) m/z:
[M + H]+ calcd for C20H27N2 295.2169; found: 295.2192. HPLC:
PHPLC 100%, tR 12.71.
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Acknowledgements
The authors would like to thank the National Institute of Science
Education and Research (NISER), Bhubaneswar for supporting this
study. A. B. and T. P. acknowledge the University Grant Commission
(UGC) Govt of India for their fellowships. S. N. acknowledges the
Science and Engineering Research Board (SERB), Department of
Science and Technology (DST) for the research grant No. SR/S1/
IC-52/2012.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016
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