N,N′-disubstituted ureas: Influence of substituents on the formation of supramolecular polymers

Article abstract of DOI:10.1002/chem.200304801

Symmetrical N,N′-disubstituted ureas have been synthesized and characterized. Among them, the branched dialkylureas prepared are highly soluble in organic media. Moreover, the solutions obtained are very viscous in heptane, if the branched alkyl groups are not too bulky (i.e. a methyl group on the α carbon, or an ethyl group on the β carbon). Due to the strong, bifurcated hydrogen bonds between the urea moieties, linear supramolecular polymers are formed. The degree of association of these supramolecular polymers has been determined by FTIR spectroscopy.

Full text of DOI:10.1002/chem.200304801

FULL PAPER
N,N'-Disubstituted Ureas: Influence of Substituents on the Formation of
Supramolecular Polymers
Fre¬de¬ric Lortie,[a] Sylvie Boileau,[a] and Laurent Bouteiller*[b]
Abstract: Symmetrical N,N'-disubstituted ureas have been synthesized and charac-
terized. Among them, the branched dialkylureas prepared are highly soluble in
organic media. Moreover, the solutions obtained are very viscous in heptane, if the
branched alkyl groups are not too bulky (i.e. a methyl group on the a carbon, or an
ethyl group on the b carbon). Due to the strong, bifurcated hydrogen bonds between
the urea moieties, linear supramolecular polymers are formed. The degree of
association of these supramolecular polymers has been determined by FTIR
spectroscopy.
Keywords:
polymers ¥ self-assembly ¥ supra-
molecular chemistry
IR spectroscopy
¥
state,[11 14] but surprisingly fewquantitative studies in solution
have been published.[4a, 15, 16] This apparent lack of interest is
probably due to the fact that, in solution, the connection
between repeat units should involve more than two hydrogen
bonds if very long chains are sought. However, we think that a
good knowledge of the behavior of mono-ureas in solution
can help understand more complex systems such as bis-ureas.
Accordingly, we present here results on the self-association of
several disubstituted mono-ureas in nonpolar solvents. It is
shown that there is a large influence of the substituents on the
strength of the association.
Introduction
Supramolecular polymers are chains of small molecules held
together through reversible noncovalent interactions.[1 3] This
reversibility is responsible for the appearance of newproper-
ties, as compared to those of usual covalent polymers. For
instance, the molecular-weight dependence of supramolecular
polymers on concentration, solvent polarity, and temperature
leads to unusual rheological properties.
In the case of supramolecular polymers held together by
hydrogen bonds, the N,N'-disubstituted urea moiety is often
used, for example in compounds such as bis-ureas[4] or tetra-
ureas.[5] The same chemical function is also used in the closely
related field of organogelators.[6 10] This ubiquity of the N,N'-
disubstituted urea moiety can be ascribed to its relatively
strong bifurcated hydrogen bonds, which lead to the forma-
tion of linear chains (Scheme 1). This association pattern of
mono-ureas has been thoroughly demonstrated in the solid
Results and Discussion
Description of aliphatic ureas: Investigations of N,N'-dialkyl-
urea solutions have established the association pattern shown
in Scheme 1,[15, 16] but they have been limited to dilute
solutions in carbon tetrachloride and benzene because the
ureas used, such as diethylurea (EU), are poorly soluble in
lowpolarity solvents. However, to study the formation of long
supramolecular chains, solutions at higher concentrations and
in solvents of the lowest possible polarity are required.
Consequently, a range of dialkylureas was synthesized
(Scheme 2) in order to obtain heptane-soluble compounds.
Three classes of substituents were investigated: linear alkyls
(MU, EU, PU, OU, ODU) and branched alkyls with branch-
ing either on the a carbon (MHU, DMHU) or on the b carbon
(EHU, MPHU, DPHU). Both the size of the substituents and
the distance between the urea function and the branching
point were varied because they were expected to have an
influence on the strength of the association. Moreover, no
heteroatom was included in order to avoid intramolecular
Scheme 1. Association pattern of N,N'-disubstituted ureas.
[a] Dr. F. Lortie, Dr. S. Boileau
¡
Laboratoire de Recherche sur les Polymeres, UMR 7581, CNRS
2 rue Henri Dunant, B.P. 28, 94320 Thiais (France)
[b] Dr. L. Bouteiller
¡
Laboratoire de Chimie des Polymeres, UMR 7610
¬
Universite Pierre et Marie Curie
4 place Jussieu, 75252 Paris Cedex 05 (France)
Fax : (‡33)1-44-27-70-89
E-mail: bouteil@ccr.jussieu.fr
3008
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/chem.200304801
Chem. Eur. J. 2003, 9, 3008 3014

3008 3014
hydrogen bonding with the urea function. Computer simu-
lations showed that for all these disubstituted ureas, the most
stable conformation of the urea moiety in the isolated
molecule is the trans trans conformation, which allows self-
assembly according to Scheme 1.
increases the solubility: while none of the linear-alkyl-
substituted ureas is significantly soluble in heptane, all the
branched-alkyl-substituted ureas are highly soluble in this
solvent.
Association of aliphatic ureas: The supramolecular associa-
tion of the substituted ureas was investigated by capillary
viscosimetry. Figure 1a shows the relative viscosity of heptane
Scheme 2. Structures of N,N'-dialkylureas studied.
The ureas were obtained in high yields by treating primary
amines with triphosgene (Scheme 3), which is a safe analogue
of phosgene.[17] Only two amines were not commercially
available and were thus synthesized according to Scheme 4.
In the first step, a branched nitrile was obtained by base-
catalyzed substitution of acetonitrile or propionitrile. In the
second step, the nitrile was reduced to the corresponding
amine.
*
^
^
Figure 1. Relative viscosity of a) EHU ( ), MPHU ( ) and DPHU (
)
Scheme 3. Synthesis of disubstituted ureas from primary amines.
~
~
*
and b) EHU ( ), MHU ( ) and DMHU ( ) versus concentration in
heptane at 258C. The curves are a guide for the eye only.
solutions of the three ureas branched on the b carbon. Among
them, EHU displays the highest viscosity: at 40 gLÀ1, the
viscosity of the solution is six times that of the pure solvent.
This viscosity value is remarkable, bearing in mind the
relatively lowconcentration and the simplicity of the com-
pound. Increasing the number and the size of the branches on
the b carbon is responsible for a strongly reduced viscosity in
the case of MPHU and DPHU. The relative viscosity values
of the ureas branched on the a carbon are shown in Figure 1b
and are compared to those of EHU. Belowa concentration of
25 gLÀ1, solutions of the three
Scheme 4. Synthesis of two primary amines.
The present set of substituted ureas was then tested for
solubility (at 10 gLÀ1 and at room temperature) in heptane
and carbon tetrachloride (Table 1). As expected, branching
ureas EHU, MHU, and DMHU
have very similar viscosities.
Table 1. Solubility of N,N'-dialkylureas in carbon tetrachloride and heptane.[a]
MU
EU
PU
OU
ODU
MHU
DMHU
EHU
MPHU
DPHU
However, at higher concentra-
tions significant differences ap-
pear, and the viscosity increases
in the order DMHU < MHU <
EHU at a given concentration.
carbon
I
S
I
I
I
S
S
S
S
S
tetrachloride
heptane
I
I
I
I
I
S
S
S
S
S
[a] I: insoluble, S: soluble. Solubility was tested at a concentration of 10 gLÀ1 and at room temperature.
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L. Bouteiller et al.
FULL PAPER
Correlation of these results with the molecular structure is not
straightforward, because the viscosity of the solutions can be
expected to depend not only on the length of the supra-
molecular chains, but also on their flexibility and their
interactions with the solvent. Indeed, by analogy with
covalent polymers, the viscosity should be higher in a good
solvent than in a theta solvent.
To decouple these different contributions, other techniques
must be used. In the case of dialkylureas, it has been shown
that FTIR spectroscopy is particularly well suited to deter-
mining the association constants of the equilibria involved,[16]
and thus the length of the associated species. Unfortunately,
heptane is not transparent enough in the relevant IR region,
consequently carbon tetrachloride was used as a solvent.
Figure 2 shows the FTIR spectra of a 10À2 molLÀ1 solution of
EHU in carbon tetrachloride at different temperatures: two
À
ˆ
N H and two C O stretching vibrations are observed, and
can be attributed to the free and hydrogen-bonded groups.[18]
As shown by their relative evolution, an increase in temper-
ature results in dissociation of the supramolecular polymer.
After cooling down the solution, an identical spectrum is
obtained, thus proving the reversibility of the association. The
values of the frequencies of the four vibrations for the
different substituted ureas are reported in Table 2. First of all,
À
ˆ
the values of free N H and C O stretching vibrations show
that for all compounds considered here, the conformation of
the urea moiety is trans trans, thus allowing self-assembly
according to Scheme 1.[19] This proves that the steric bulk due
to the branched alkyls does not change the conformation of
the monomer, even in the case of DPHU. This point is worth
mentioning because N,N'-di-tbutylurea has been shown to be
partly in an out trans conformation,[20, 21] that is, one of the
À
N H groups is out of the plane of the urea function, due to
repulsion between the tert-butyl substituents and the carbonyl
À
ˆ
group. Secondly, the displacement of the N H and C O
stretching vibrations due to hydrogen bonding is similar in the
cases of EU, MHU, DMHU, EHU, and MPHU; this shows
qualitatively that the strengths of the hydrogen bonds formed
are comparable. In the case of DPHU however, the displace-
À
ˆ
ment of the N H and C O stretching vibrations due to
hydrogen bonding is much smaller; this proves that hydrogen
bonds are formed, but that they are of lower stability than for
the other compounds.
A quantitative analysis of the spectra was performed
according to Jadzyn et al.,[16] and was based on the measure-
À
ment of the intensity of the free N H stretching vibration
(nNf ÀH†. The molar extinction coefficient is obtained at very
lowconcentration (about 10 À4 molLÀ1, Table 2), and then
used at higher concentrations to derive the fraction of free
À
N H groups (Figure 3). This Figure shows that at a given
concentration, the length of the supramolecular polymers
increases in the order DPHU ꢀ MPHU < DMHU ꢁ MHU ꢁ
EHU < EU. Moreover, the association can be modeled by
considering an infinite set of equilibria (Scheme 5). The
simplest (isodesmic) model (i.e. Kn ˆ K, for n ꢂ 2)[22] does not
yield a good description of the data, as already reported for
EU,[16] but the second simplest model (K2=K ˆ Kn, for n ꢂ 3)
yields an excellent fit for all the data sets. The values of the
constants derived are reported in Table 2. In the case of EHU
K2 ꢀ K, this shows that the association is cooperative in the
sense that the formation of higher oligomers is favored
relative to the formation of dimers. The origin of this
Figure 2. FTIR spectra of a 2.34 gLÀ1 (10À2 molLÀ1) solution of EHU in
carbon tetrachloride versus temperature (15, 30, 45, and 608C). Spectral
À
ˆ
regions characteristic of a) N H and b) C O. Arrows indicate the direction
of change with increasing temperature.
Table 2. Characteristics of solutions of N,N'-dialkylureas in carbon tetrachloride at room temperature.
EU
MHU
DMHU
EHU
MPHU
DPHU
nNf ÀH [a]
nNb ÀH [a]
nCf ˆO [a]
nCb ˆO [a]
eNf ÀH [b]
3448
3300 3400
1689
3437
3300 3390
1684
3437
3300 3390
1684
3455
3300 3400
1689
3456
3320 3410
1691
3454
3360 3410
1691
1633
1630
1630
1632
1633
1655
98 Æ 3
17 Æ 7
467 Æ 22
80 Æ 2
10 Æ 4
330 Æ 17
82 Æ 2
82 Æ 2
11 Æ 4
350 Æ 14
107 Æ 2
23 Æ 6
156 Æ 6
102 Æ 2
30 Æ 11
26 Æ 1
[c]
K2
9 Æ 4
K[c]
325 Æ 19
À1
À
ˆ
À
[a] N H and C O stretching frequencies (nb: hydrogen bonded; nf: free) in cm . [b] Molar extinction coefficient of the free N H stretching vibration, in
LmolÀ1 cmÀ1. [c] Dimerization (K2) and multimerization (K) constants in LmolÀ1
.
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N,N'-Disubstituted Ureas
3008 3014
~
*
*
Figure 3. Fraction of free NH groups of EU ( ), EHU ( ), MHU ( ),
Figure 4. Calculated n-mer distribution in a 10À2 molLÀ1 solution of EHU
~
^
^
DMHU ( ), MPHU ( ), and DPHU ( ) versus concentration in carbon
tetrachloride at room temperature. The curves are calculated according to
ref. [16], with the constants reported in Table 2.
*
^
(
) and DPHU ( ). (The values of the constants used are in Table 2.)
Scheme 5. Definition of the association constant Kn, n ꢂ 2.
cooperativity is the polarization of the urea function subse-
quent to dimerization.[23] All other compounds investigated
here showthis behavior, except DPHU. The multimerization
constants (K) increase in the order DPHU ꢀ MPHU <
DMHU ꢁ MHU ꢁ EHU < EU; this is obviously related to
the steric bulk of the substituents. More surprisingly, the
dimerization constants (K2) increase in the order DMHU ꢁ
MHU ꢁ EHU ꢃ EU < MPHU < DPHU; this means that the
substituents of DPHU are not large enough to inhibit
dimerization, but are bulky enough to destabilize higher
oligomers. Moreover, the increased stability of the dimer of
DPHU compared with the other dimers could be due to a
micropolarity effect, because the large dipentylheptyl group
reduces the number of solvent molecules that can approach
the urea function.
This two-constant model (K2=K ˆ Kn, for n ꢂ 3) is an
approximation because the association constants for the
formation of trimers and higher oligomers are probably not
strictly equal to each other. However, knowledge of the
association constants of this simple model makes it possible to
compute an approximation of the whole distribution of
oligomers, as a function of concentration. For example,
Figure 4 shows the distribution of oligomers for EHU and
DPHU at a concentration of 10À2 molLÀ1. At this concen-
tration, a DPHU solution mainly contains monomers, dimers,
and trimers. The behavior of EHU is quite different: together
with a significant monomer fraction, there is a broad oligomer
fraction centered around the decamer. The bimodality of this
distribution is a consequence of the cooperativity of the
association, which imposes a low dimer concentration. From
the distributions, the average degree of polymerization can be
calculated (Figure 5). Coming back to the interpretation of
Figure 1a, it is nowclear that the strong decrease in viscosity
Figure 5. Calculated number-average degree of polymerization versus
concentration, in a solution of EU (––), EHU (––) MPHU (
DPHU ( ¥¥¥ ). (The values of the constants used are in Table 2.)
), and
going from EHU to MPHU and to DPHU is due to a decrease
in the degree of association.
Description of aromatic ureas: N,N'-diarylureas were also
investigated because the electron-withdrawing effect of the
aromatic ring is expected to strengthen the association.[15] Due
to the very lowsolubility of diphenylurea ( DPU) in nonpolar
solvents, several compounds were synthesized to improve
solubility (Scheme 6). The para position of the benzene ring
was substituted with linear (BPU, OPU) or branched (sBPU)
alkyl groups. Alternatively, one (oBPU) or both (TMPU,
DEPU, DIPPU) ortho positions were substituted with alkyl
groups of increasing size. Despite these substitutions, none of
the present N,N'-diarylureas is significantly soluble in hep-
tane. Moreover, only DIPPU is soluble in carbon tetrachlor-
ide.
Association of DIPPU: Computer simulations showed that
the trans trans conformation of an isolated molecule of
DIPPU is, in fact, less stable than the cis trans conformation
by 8.3 kJmolÀ1. The association of DIPPU was nevertheless
investigated (Figure 6) and compared to the association of
EHU and DPHU. The lowviscosity is in agreement with the
absence of supramolecular chains in solution. Moreover,
FTIR spectroscopy (Figure 7) definitely proves that the
association does not proceed according to Scheme 1. At the
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L. Bouteiller et al.
FULL PAPER
lowest concentration investigated (10À4 molLÀ1), two free
À1
À
N H vibrations are detected at 3428 and 3408 cm and are
probably due to two different conformations. We tentatively
assign them to the out and trans conformations, respective-
ly.[24] When the concentration is increased, the intensity of the
À
À
free N H out band decreases and a hydrogen-bonded N H
band appears at 3170 cmÀ1. The fact that the maximum of the
À
hydrogen-bonded N H band does not depend on concen-
tration indicates that finite aggregates are formed and not
indefinite chains, because in this case, the value of the
maximum should increase with concentration.[25] In the same
À
concentration range, the free N H trans band is not affected
À1
ˆ
and the free C O vibration (1690 cm ) is progressively
replaced by a hydrogen-bonded band (1677 cmÀ1). These
different results are in agreement with DIPPU being in the
out trans conformation and with the formation of dimers
À
through hydrogen bonding of the N H groups in the out
conformation (Scheme 7).
Scheme 6. Structures of N,N'-diarylureas studied.
Scheme 7. Proposed association pattern for DIPPU.
Conclusion
Symmetrical N,N'-disubstituted ureas have been synthesized
and characterized. Among the eight diarylureas prepared,
only DIPPU is soluble in carbon tetrachloride. Moreover, this
compound does not form a supramolecular polymer. All the
branched dialkylureas synthesized are highly soluble in
heptane and carbon tetrachloride. If the branched alkyl
groups are not too bulky, then the corresponding dialkylureas
(MHU, DMHU, EHU) self-assemble to form supramolecular
polymers in solution.
~
*
^
)
Figure 6. Relative viscosity of EHU ( ), DIPPU ( ) and DPHU (
versus concentration in carbon tetrachloride at 258C.
Experimental Section
Synthesis: N,N'-Dimethylurea (MU), N,N'-dipropylurea (PU), N,N'-di-
octadecylurea (ODU), and N,N'-diphenylurea (DPU) (Aldrich) were used
as received. N,N'-Diethylurea (EU) (Aldrich) was recrystallized in
heptane. The synthesis of N,N'-di(2-ethylhexyl)urea (EHU) has been
previously reported.[4a] Unless specified, all reagents were from Aldrich and
were used as received. Yields are not optimized.
6-cyano-6-methylundecane: The synthesis was adapted from previous
literature.[26] A mixture of 1-propionitrile (10.5 g, 0.19 mol), 1-bromopen-
tane (66.5 g, 0.44 mol), and sodium-dried toluene (70 mL, SDS) was placed
in a 500 mL flask fitted with a condenser and a 500 mL dropping funnel. A
suspension of sodium amide (50%wt in toluene, 29.5 g, 0.43 mol) was
placed in the dropping funnel and diluted with more toluene (50 mL). The
flask was placed under nitrogen and warmed to 808C. The sodium amide
suspension was added at such a rate that gentle refluxing resulted (the
dropping funnel must be large enough to avoid plugging by the suspension).
After the addition was completed, the reaction mixture was stirred and
heated under reflux for two additional hours. The flask was cooled in an ice
bath, and water (200 mL) was added. The organic phase was separated,
Figure 7. Normalized FTIR spectra of solutions of DIPPU in carbon
tetrachloride versus concentration (10À4, 10À3, 10À2, and 10À1 molLÀ1).
À
ˆ
Spectral regions characteristic of a) N H and b) C O. Arrows indicate the
direction of change with increasing concentration.
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N,N'-Disubstituted Ureas
3008 3014
then washed with water until neutral pH was reached, dried over
magnesium sulfate and concentrated, leading to a crude oil. Purification
was performed by silica gel column chromatography with cyclohexane/
toluene (50:50) as the eluent (yield: 60%). 1H NMR (200 MHz, CDCl3):
d ˆ 1.5 (m, 7H), 1.3 (m, 12H), 0.9 (t, 6H); 13C NMR (50 MHz, CDCl3): d ˆ
(s, 2H), 7.3 (d, 4H), 7.0 (d, 4H), 2.4 (m, 4H), 1.5 (m, 4H), 1.2 (m, 4H), 0.8 (t,
6H); 13C NMR (50 MHz, [D6]DMSO): d ˆ 152.5, 137.3, 135.5, 128.3, 118.1,
34.0, 33.1, 21.5, 13.6; IR (KBr): nÄ ˆ 3310, 1640 cmÀ1; elemental analysis
calcd (%) for C21H28N2O: C 77.74, H 8.70, N 8.63, O 4.93; found C 77.76, H
8.75, N 8.77, O 4.73.
124.8, 39.4, 36.7, 31.8, 24.5, 24.0, 22.5, 14.0; IR (neat): nÄ ˆ 2230 cmÀ1
.
N,N'-di(4-octylphenyl)urea (OPU): OPU was synthesized analogously to
MPHU. Purification was performed by recrystallization in ethanol. Yield:
73%; m.p. 1798C; no suitable solvent for NMR analysis has been
identified; IR (KBr): nÄ ˆ 3310, 1635 cmÀ1; elemental analysis calcd (%)
for C29H44N2O: C 79.77, H 10.16, N 6.42, O 3.66; found C 79.66, H 10.15, N
6.60, O 3.58.
6-cyano-6-pentylundecane: The synthesis was analogous to the previous
one, starting with
a mixture of acetonitrile (6 mL, 0.115 mol) and
1-bromopentane (60 g, 0.40 mol). Purification was performed by silica gel
column chromatography with toluene as the eluent (yield: 40%). 1H NMR
(200 MHz, CDCl3): d ˆ 1.5 (m, 6H), 1.3 (m, 18H), 0.9 (t, 9H). 13C NMR
(50 MHz, CDCl3): d ˆ 125.2, 40.5, 36.0, 31.8, 23.8, 22.3, 13.8; IR (neat): nÄ ˆ
N,N'-di(4-sbutylphenyl)urea (sBPU): sBPU was synthesized analogously
to MPHU. Purification was performed by recrystallization in heptane.
Yield: 55%; m.p. 2388C; 1H NMR (200 MHz, [D6]DMSO): d ˆ 7.4 (s, 2H),
7.2 (d, 4H), 7.0 (d, 4H), 2.5 (m, 2H), 1.5 (m, 4H), 1.2 (d, 6H), 0.8 (t, 6H);
13C NMR (50 MHz, [D6]DMSO): d ˆ 154.2, 143.1, 136.0, 127.8, 121.3, 41.2,
31.3, 21.9, 12.3; IR (KBr): nÄ ˆ 3310, 1640 cmÀ1; elemental analysis calcd (%)
for C21H28N2O: C 77.74, H 8.70, N 8.63, O 4.93; found C 77.39, H 8.71, N
8.73, O 5.16.
2230 cmÀ1
.
2-methyl-2-pentylheptylamine: The synthesis was adapted from previous
literature.[27] A solution of 6-cyano-6-methylundecane (18.3 g, 94 mmol) in
diethyl ether (100 mL) was slowly added under nitrogen to a cooled (ice
bath) solution of lithium aluminium hydride (100 mL, 1 molLÀ1 in diethyl
ether) diluted in sodium-dried diethyl ether (100 mL). After the mixture
had been stirred for 2 h, water (4 mL), 20% sodium hydroxide (3 mL), and
water (4 mL) were added successively, with continued cooling and vigorous
stirring. The inorganic residue was filtered, and the ether fraction was
concentrated. Purification was performed by distillation (958C, 3 mm Hg).
Yield: 79%; 1H NMR (200 MHz, CDCl3): d ˆ 2.4 (s, 2H), 1.2 (m, 16H), 1.0
(brs, 2H), 0.9 (t, 6H), 0.8 (t, 3H); 13C NMR (50 MHz, CDCl3): d ˆ 50.4,
37.1, 36.6, 32.9, 23.1, 22.7, 22.6, 14.1.
N,N'-di(2-butylphenyl)urea (oBPU): oBPU was synthesized analogously
to MPHU. Purification was performed by recrystallization in ethanol.
Yield: 77%; m.p. 1968C; 1H NMR (200 MHz, [D6]DMSO): d ˆ 8.1 (s, 2H),
7.6 (d, 2H), 7.1 (m, 4H), 6.9 (t, 2H), 2.5 (t, 4H), 1.5 (m, 4H), 1.5 (m, 4H), 1.3
(m, 4H), 0.9 (t, 6H); 13C NMR (50 MHz, [D6]DMSO): d ˆ 153.5, 138.9,
133.5, 129.2, 125.9, 123.5, 123.4, 31.6, 30.5, 21.9, 13.6; IR (KBr): nÄ ˆ 3310,
1640 cmÀ1; elemental analysis calcd (%) for C21H28N2O: C 77.74, H 8.70, N
8.63, O 4.93; found C 77.02, H 8.62, N 8.53, O 5.84.
N,N'-di(2,4,6-trimethylphenyl)urea[31] (TMPU): TMPU was synthesized
analogously to MPHU. Purification was performed by recrystallization in
ethanol. Yield: 78%; m.p. 2018C; no suitable solvent for NMR analysis has
been identified; IR (KBr): nÄ ˆ 3290, 1640 cmÀ1; elemental analysis calcd
(%) for C19H24N2O: C 76.99, H 8.16, N 9.45, O 5.40; found C 76.73, H 8.14,
N 9.27, O 5.86.
2,2-dipentylheptylamine: The synthesis was analogous to the previous one.
Purification was performed by distillation (758C, 2 mm Hg). Yield: 55%;
1H NMR (200 MHz, CDCl3): d ˆ 2.4 (s, 2H), 1.0 (m, 26H), 0.8 (t, 9H);
13C NMR (50 MHz, CDCl3): d ˆ 47.3, 38.8, 34.5, 32.8, 22.6, 22.5, 14.0.
N,N'-di(2-methyl-2-pentylheptyl)urea (MPHU): MPHU was prepared
according to
a slightly modified version of a procedure described
previously.[28] Trisphosgene (1.6 g, 5.4 mmol) in dichloromethane (20 mL)
was slowly added under nitrogen to a stirred solution of 2-methyl-2-
pentylheptylamine (6 g, 30 mmol) and triethylamine (Merck, 4.5 mL,
32 mmol) in dichloromethane (50 mL; SDS, distilled over phosphorus
pentoxide) in an ice-cooled flask. The ice bath was removed, and the
mixture was stirred for 2 h before aqueous HCl (60 mL, 0.1n) was added.
The organic phase was separated, then washed with water until neutral pH
was reached, dried over magnesium sulfate, and concentrated, leading to a
white powder. Purification was performed by recrystallization in acetoni-
trile. Yield: 82%; m.p. 1608C; 1H NMR (200 MHz, CDCl3): d ˆ 5.0 (brs,
2H), 2.9 (d, 4H), 1.2 (m, 32H), 0.8 (m, 18H); 13C NMR (50 MHz, CDCl3):
d ˆ 159.0, 48.3, 37.0, 36.5, 32.5, 22.4, 21.5, 14.1; IR (KBr): nÄ ˆ 3345,
1625 cmÀ1; elemental analysis calcd (%) for C27H56N2O: C 76.35, H 13.29, N
6.60, O 3.77; found C 76.06, H 13.43, N 6.75, O 3.76.
N,N'-di(2,6-diethylphenyl)urea (DEPU): DEPU was synthesized analo-
gously to MPHU. Purification was performed by recrystallization in
ethanol. Yield: 39%; m.p. 2908C (lit. 2888C);[32] 1H NMR (200 MHz,
[D6]DMSO): d ˆ 7.7 (s, 2H), 7.1 (m, 6H), 2.6 (m, 8H), 1.1 (t, 12H); IR
(KBr): nÄ ˆ 3270, 1630 cmÀ1; elemental analysis calcd (%) for C21H28N2O: C
77.74, H 8.70, N 8.63, O 4.93; found C 77.63, H 8.81, N 8.51, O 5.05.
N,N'-di(2,6-diisopropylphenyl)urea (DIPPU): DIPPU was synthesized
analogously to MPHU. Purification was performed by recrystallization in
heptane. Yield: 69%; m.p. 2308C (lit. 229.58C);[33] 1H NMR (200 MHz,
[D6]DMSO): d ˆ 7.3 7.1 (m, 6H), 5.9 (s, 1H), 5.4 (s, 1H), 3.4 (t, 2H), 3.0 (t,
2H), 1.3 (d, 6H), 1.1 (s, 18H); 13C NMR (50 MHz, [D6]DMSO): d ˆ 156.5,
148.2, 146.6, 131.2, 131.0, 129.2, 128.0, 124.0, 123.3, 28.5, 25.5, 23.8, 22.1;
elemental analysis calcd (%) for C25H36N2O: C 78.90, H 9.53, N 7.36, O 4.20;
found C 78.92, H 9.61, N 7.32, O 4.15.
N,N'-di(2,2-dipentylheptyl)urea (DPHU): DPHU was synthesized analo-
gously to MPHU. Purification was performed by recrystallization in
ethanol/water (5:1). Yield: 75%; m.p. 1308C;1H NMR (200 MHz, CDCl3):
d ˆ 4.4 (t, 2H), 2.9 (d, 4H), 1.2 (m, 48H), 0.8 (t, 18H); 13C NMR (50 MHz,
CDCl3): d ˆ 158.5, 45.6, 38.3, 35.0, 32.8, 22.6, 22.3, 14.0; IR (KBr): nÄ ˆ 3365,
1633 cmÀ1; elemental analysis calcd (%) for C35H72N2O: C 78.29, H 13.51, N
5.22, O 2.98; found C 78.36, H 13.45, N 5.15, O 3.05.
Capillary viscosimetry: Measurements were performed at 25 Æ 0.18C with a
Cannon Manning semimicro viscometer. Solutions in heptane (SDS) or
carbon tetrachloride (SDS) were prepared 1 day prior to the measurements
and filtered through Millex membranes (F ˆ 0.45 mm). Solvents were used
as received.
N,N'-di(1-methylheptyl)urea (MHU): MHU was synthesized analogously
to MPHU. Purification was performed by recrystallization in acetonitrile.
Yield: 77%; m.p. 708C (lit. 488C);[29] 1H NMR (200 MHz, CDCl3): d ˆ 4.1
(d, 2H), 3.6 (m, 2H), 1.2 (m, 20H), 1.1 (d, 6H), 0.8 (t, 6H); 13C NMR
(50 MHz, CDCl3): d ˆ 154.0, 42.8, 34.3, 28.5, 25.9, 22.7, 19.2, 18.3, 10.7; IR
(KBr): nÄ ˆ 3330, 1630 cmÀ1; elemental analysis calcd (%) for C17H36N2O: C
71.77, H 12.75, N 9.85, O 5.62; found C 71.53, H 12.86, N 9.86, O 5.74.
IR spectroscopy: Infrared spectra were recorded at room temperature on a
Perkin Elmer 1760 spectrometer in KBr cells of 0.05 to 2.5 cm path length.
The temperature-controlled measurements were performed with a heating
device (P/N21525) from Specac. Solutions in carbon tetrachloride (SDS)
were prepared 1 day prior to the measurements. Preliminary tests showed
that solutions obtained from carbon tetrachloride that had been saturated
with water or dried on molecular sieves yielded the same spectrum.
Consequently, carbon tetrachloride was used as received. Quantitative data
N,N'-di(1,5-dimethylhexyl)urea (DMHU): DMHU was synthesized anal-
ogously to MPHU. Purification was performed by silica gel column
chromatography with dichloromethane/ethyl acetate (90:10) as the eluent.
Yield: 65%; m.p. 518C;1H NMR (200 MHz, CDCl3): d ˆ 4.2 (d, 2H), 3.6
(m, 2H), 1.4 (m, 2H), 1.2 (m, 12H), 1.0 (d, 6H), 0.8 (d, 12H); 13C NMR
(50 MHz, CDCl3): d ˆ 157.4, 46.1, 38.9, 37.9, 27.9, 23.8, 22.6, 21.7; IR (KBr):
nÄ ˆ 3330, 1630 cmÀ1; elemental analysis calcd (%) for C17H36N2O: C 71.77,
H 12.75, N 9.85, O 5.62; found C 71.59, H 12.82, N 9.82, O 5.77.
À
ˆ
analysis was based on the N H vibration, because the intensity of the C O
vibration was not precise enough, due to high solvent absorption at this
À
wavelength. The molar extinction coefficient of the free N H stretching
vibration was determined on sufficiently dilute solutions, such that only the
free component was detected. At higher concentrations, after deconvolu-
À
tion of the N H vibration curve, the height of the free component was used
[16]
À
to derive the fraction of free N H groups.
Then, the association
N,N'-di(4-butylphenyl)urea (BPU): BPU was synthesized analogously to
MPHU. Purification was performed by recrystallization in ethanol. Yield:
77%; m.p. 1888C (lit. 2008C);[30] 1H NMR (200 MHz, [D6]DMSO): d ˆ 8.5
constants were determined by nonlinear curve fitting. The main source of
uncertainty (which is mentioned in Table 2 and Figure 5) is due to the
uncertainty of the deconvolution, because the shape of the hydrogen-
Chem. Eur. J. 2003, 9, 3008 3014
www.chemeurj.org
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3013

L. Bouteiller et al.
FULL PAPER
À
bonded N H band is ill-defined. The values of the association constants
Science 1999, 286, 1540 1543; c) L. A. Estroff, A. D. Hamilton,
Angew. Chem. 2000, 112, 3589 3592; Angew. Chem. Int. Ed. 2000, 39,
3447 3450; G. Wang, A. D. Hamilton, Chem. Eur. J. 2002, 8, 1954
1961.
obtained for EU (Table 2) are significantly different from the values
reported previously (K2 ˆ 110 LmolÀ1, K ˆ 780 LmolÀ1),[16] but these latter
values are certainly less precise because the molar extinction coefficient of
À
the free N H band was determined by extrapolation from solutions
[9] J. H. Jung, Y. Ono, S. Shinkai, Chem. Eur. J. 2000, 6, 4552 4557.
[10] J. J. E. Moreau, L. Vellutini, M. Wong Chi Man, C. Bied, J. Am. Chem.
Soc. 2001, 123, 1509 1510.
[11] M. C. Etter, Z. Urbanczyk-Lipkowska, M. Zia-Ebrahimi, T. W.
Panunto, J. Am. Chem. Soc. 1990, 112, 8415 8426.
[12] a) X. Zhao, Y.-L. Chang, F. W. Fowler, J. W. Lauher, J. Am. Chem.
Soc. 1990, 112, 6627 6634; b) Y.-L. Chang, M.-A. West, F. W. Fowler,
J. W. Lauher, J. Am. Chem. Soc. 1993, 115, 5991 6000.
[13] D. Ranganathan, C. Lakshmi, I. L. Karle, J. Am. Chem. Soc. 1999, 121,
6103 6107.
À
containing a fraction of hydrogen bonded N H groups.
Computer modeling: Molecular-modeling calculations were done using the
Consistent Valence Force Field as implemented in the software InsightII
from Molecular Simulations Incorporated. All calculations were done in
the gas phase with a dielectric constant of 1 and with no cut-off.
Acknowledgement
[14] L. S. Shimizu, M. D. Smith, A. D. Hughes, K. D. Shimizu, Chem.
Commun. 2001, 1592 1593.
[15] T. V. Kozlova, V. V. Zharkov, Zh. Prikl. Spektrosk. 1981, 35, 303 311.
[16] J. Jadzyn, M. Stockhausen, B. Zywucki, J. Phys. Chem. 1987, 91, 754
757.
We thank Prof. J. Jadzyn (Institute of Molecular Physics, Poznan, Poland)
for insightful discussions.
[17] H. Eckert, B. Forster, Angew. Chem. 1987, 99, 922 923; Angew.
Chem. Int. Ed. Engl. 1987, 26, 894 895.
[1] N. Zimmerman, J. S. Moore, S. C. Zimmerman, Chem. Ind. 1998, 604
610.
[18] Y. Mido, Bull. Chem. Soc. Jpn. 1974, 47, 1833 1837.
[19] Y. Mido, Spectrochim. Acta 1973, 29A, 1 6.
[2] Supramolecular Polymers (Ed.: A. Ciferri), Marcel Dekker, New
York, 2000.
[20] Y. Mido, T. Gohda, Bull. Chem. Soc. Jpn. 1975, 48, 2704 2707.
[21] B. Galabov, S. Ilieva, B. Hadjieva, T. Dudev, J. Mol. Struct. 1997, 407,
47 51.
[3] L. Brunsveld, B. J. B. Folmer, E. W. Meijer, R. P. Sijbesma, Chem. Rev.
2001, 101, 4071 4097.
√
[4] a) S. Boileau, L. Bouteiller, F. Laupretre, F. Lortie, New J. Chem. 2000,
[22] R. B. Martin, Chem. Rev. 1996, 96, 3043 3064.
[23] A. Masunov, J. J. Dannenberg, J. Phys. Chem. B 2000, 104, 806 810.
[24] The other possibility, in agreement with previous data,[21] is to assign
24, 845 848; b) F. Lortie, S. Boileau, L. Bouteiller, C. Chassenieux, B.
¬
√
Deme, G. Ducouret, M. Jalabert, F. Laupretre, P. Terech, Langmuir
2002, 18, 7218 7222.
the N H vibrations detected at 3428 and 3408 cmÀ1 to the trans and cis
À
[5] R. K. Castellano, R. Clark, S. L. Craig, C. Nuckolls, J. Rebek, Jr.,
Proc. Natl. Acad. Sci. USA 2000, 97, 12418 12421.
conformations, respectively. However, computer simulations of an
isolated DIPPU molecule showthat, in the cis trans conformation,
[6] K. Hanabusa, K. Shimura, K. Hirose, M. Kimura, H. Shirai, Chem.
Lett. 1996, 885 886.
À
the trans N H group is completely buried within the molecule and is
not accessible for hydrogen bonding. Consequently, this alternative
assignment is in conflict with the evolution of the FTIR spectrum at
higher concentrations.
[7] a) J. van Esch, R. M. Kellogg, B. L. Feringa, Tetrahedron Lett. 1997, 38,
281 284; b) J. van Esch, S. De Feyter, R. M. Kellogg, F. De Schryver,
B. L. Feringa, Chem. Eur. J. 1997, 3, 1238 1243; c) J. van Esch, F.
Schoonbeek, M. de Loos, H. Kooijman, A. L. Spek, R. M. Kellogg,
B. L. Feringa, Chem. Eur. J. 1999, 5, 937 950; d) F. Schoonbeek, J.
van Esch, B. Wegewijs, D. B. A. Rep, M. P. de Haas, T. M. Klapwijk,
R. M. Kellogg, B. L. Feringa, Angew. Chem. 1999, 111, 1486 1490;
Angew. Chem. Int. Ed. 1999, 38, 1393 1397; e) F. S. Schoonbeek, J. H.
van Esch, R. Hulst, R. M. Kellogg, B. L. Feringa, Chem. Eur. J. 2000, 6,
2633 2643; f) J. Brinksma, B. L. Feringa, R. M. Kellogg, J. van Esch,
Langmuir 2000, 16, 9249 9255; g) M. de Loos, J. van Esch, R. M.
Kellogg, B. L. Feringa, Angew. Chem. 2001, 113, 633 636; Angew.
Chem. Int. Ed. 2001, 40, 613 616.
[25] See, for example, Figure 2 in ref. [4a].
[26] N. Sperber, D. Papa, E. Schwenk, J. Am. Chem. Soc. 1948, 70, 3091
3094.
[27] L. H. Amundsen, L. S. Nelson, J. Am. Chem. Soc. 1951, 73, 242 244.
[28] D. B. Grotjahn, C. Joubran, Tetrahedron: Asymmetry 1995, 6, 745
752.
[29] G. Cainelli, F. Manescalchi, Synthesis 1979, 141 144.
[30] L. Verbit, G. A. Lorenzo, Mol. Cryst. Liq. Cryst. 1975, 30, 87 99.
[31] H. J. Knˆlker, T. Braxmeier, G. Schlechtingen, Synlett 1996, 502 504.
[32] I. A. Atanassova, J. S. Petrov, N. M. Mollov, Synthesis 1987, 734 736.
[33] G. S. Skuches, P. S. Carleton, J. Appl. Polym. Sci. 1984, 29, 3431 3443.
[8] a) A. J. Carr, R. Melendez, S. J. Geib, A. D. Hamilton, Tetrahedron
Lett. 1998, 39, 7447 7450; b) C. Shi, Z. Huang, S. Kilic, J. Xu, R. M.
Enick, E. J. Beckman, A. J. Carr, R. E. Melendez, A. D. Hamilton,
Received: February 3, 2003 [F4801]
3014
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2003, 9, 3008 3014