1,3,5-Trialkyl-hexahydro-1,3,5-triazines-N-methylenealkylamines equilibria. 1H NMR studies in solutions

Article abstract of DOI:10.1016/j.molstruc.2008.05.019

The behavior of 1,3,5-trialkyl-1,3,5-hexahydrotriazines (A) in a variety of solvents was investigated by ¹H NMR. A are stable for linear alkyls in deuterated solvents such as chloroform, benzene, acetone, dioxane, dimethylsulfoxide and acetonitrile. In case of branched alkyls, A are in equilibrium with N-methylenealkylamines (B). The A/B ratio depends on solvent, concentration of the sample and temperature. A react easily with methanol and lead to the formation of N-(methoxymethyl)amines (C), which are in equilibrium with B. Both the quantitative evaluation of A-B equilibrium in solutions as well as the formation of C in methanol was described for the first time.

Full text of DOI:10.1016/j.molstruc.2008.05.019

Journal of Molecular Structure 892 (2008) 177–181
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Journal of Molecular Structure
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1,3,5-Trialkyl-hexahydro-1,3,5-triazines–N-methylenealkylamines equilibria.
1H NMR studies in solutions
*
´
´
Agnieszka Adamczyk-Wozniak , Krzysztof Bujnowski, Andrzej Sporzynski
Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3 Street, 00-664 Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
The behavior of 1,3,5-trialkyl-1,3,5-hexahydrotriazines (A) in a variety of solvents was investigated by 1
H
Article history:
Received 18 February 2008
Received in revised form 7 May 2008
Accepted 12 May 2008
NMR. A are stable for linear alkyls in deuterated solvents such as chloroform, benzene, acetone, dioxane,
dimethylsulfoxide and acetonitrile. In case of branched alkyls, A are in equilibrium with N-meth-
ylenealkylamines (B). The A/B ratio depends on solvent, concentration of the sample and temperature.
A react easily with methanol and lead to the formation of N-(methoxymethyl)amines (C), which are in
equilibrium with B. Both the quantitative evaluation of A–B equilibrium in solutions as well as the for-
mation of C in methanol was described for the first time.
Available online 20 May 2008
Keywords:
1,3,5-Triazines
N-methylenealkylamines
N-(methoxymethyl)amines
Equilibrium in solutions
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
has ever been done. 1,3,5-Trialkyl-hexahydro-1,3,5-triazines form
complexes with metal ions [19–22], some of the complexes have
1,3,5-Trialkyl-hexahydro-1,3,5-triazines (A) (Scheme 1) are re-
ported to be the stable products of primary aliphatic amine and
formaldehyde condensation [1–7]. The methods of their synthesis
have been extensively reviewed in 1959 by Smolin and Rapoport
[1]. A are reported to be stable in neutral or alkaline solutions
[8], but in the presence of aqueous acids readily decompose with
the formation of formaldehyde and the primary amine salt [4,5].
Under the action of HCl in anhydrous media they form imminium
salts [9]. Pyrolysis of A results in the formation of N-meth-
ylenealkylamines (B), stable at low temperature [10–12]. Some of
B were spectroscopically characterized at ꢀ60 °C [13]. At higher
temperatures they spontaneously trimerize. Due to their cyclic
structure, 1,3,5-trialkyl-hexahydro-1,3,5-triazines display temper-
ature-dependent 1H NMR spectra in the range of ring methylene
signals [14]. When heated with sodium methoxide in methanol
solution A give N-(methoxymethyl)amines (C). It is also possible
to perform the reverse reaction by heating C under reduced pres-
sure [15] (Scheme 1).
been applied as polymerization catalysts [20–22]. The most impor-
tant synthetic application of A is their use as precursors of N-meth-
ylenealkylamines (B) [9,18,23–32]. As such, A are applied in the
formal context of Mannich reaction [9,15,32] or other aminome-
thylation reactions [2,15]. The structures and concentrations of
the species in solutions should be crucial in their synthetic applica-
tions, so we found it interesting to investigate the behavior of A in
various solvents.
2. Experimental
1H NMR spectra were taken on Varian Gemini 200 MHz or
Varian Unity Plus 200 MHz (0 relaxation delay). IR spectra were
recorded on Perkin Elmer PE-577. Elemental analyses were carried
out on Perkin Elmer CHNS/O II Model 240. Several 1,3,5-trialkyl-
hexahydro-1,3,5-triazines were obtained by a known method of
formaldehyde and primary amine condensation [1,2,10,32] and
investigated as crude oily product. The IR spectra of the crude
1,3,5-trialkyl-hexahydro-1,3,5-triazines in condensed phase (thin
film or KBr pellet) revealed no bands in the range of 1685–
1580 cmꢀ1 corresponding to C@N bond in the monomeric struc-
The A–B equilibrium in solution has been deduced as early as in
1932 [4] and later on taken as a fact [16–18]. Nevertheless, no 1H
NMR spectra with signals corresponding to both A and B forms
have been described. The equilibrium was investigated by means
of IR in various solvents [3]. The monomeric form was reported
to be more stable in acidic media, but no quantitative assessment
ture.
A crystalline 1,3,5-tricyclohexyl-hexahydro-1,3,5-triazine
(TCH) and its liquid linear alkyl isomer (TH), purified by crystalli-
zation from acetone (TCH) and vacuum distillation (TH) were
investigated as model compounds. The purity of the compounds
was confirmed on the basis of elemental analysis (calculated
for C21H39N3 (TCH and TH) C: 75.62, N: 12.62, H: 11.76, measured
* Corresponding author. Tel.: +48 222345737; fax: +48 226282741.
E-mail address: agnieszka@ch.pw.edu.pl (A. Adamczyk-Woz´niak).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.05.019

´
178
A. Adamczyk-Wozniak et al. / Journal of Molecular Structure 892 (2008) 177–181
Table 1
R
N
1H chemical shifts observed in solutions of TH and TCH in various solvents at room
pyrolysis, Δt
temperature
N
3
3
H2C
Entry
Compound
Solvent
da (ppm)
db (ppm)
dc (ppm)
R
R
N
N
spontaneously
1
2
3
4
5
6
7
8
9
10
11
12
13
14
TCH
TCH
TCHb
TCHb
TCH
TCH
TCH
TH
TH
TH
TH
TH
CDCl3
3.60
3.53
3.32
3.49
3.66
3.39a
4.17
3.28
3.27
3.16
3.36
3.40
3.23
4.14
7.45, 7.05
7.43, 6.99
7.43, 7.02
–
–
–
–
–
–
4.32
–
–
–
–
–
–
R
R
(CD3)2CO
(CD3)2SO
CD3CN
C6D6
C4D8O2
CD3OD
CDCl3
(CD3)2CO
(CD3)2SO
CD3CN
C6D6
C4D8O2
CD3OD
A
B
7.42, 7.00
6.99, 6.82a
7.26, 6.86
7.45, 7.08
H
N
CH3ONa, CH3OH, reflux
–
–
–
–
–
–
–
O
Δt, reduced pressure
C
TH
TH
R = alkyl
4.26
a
Partly overlapped with a solvents signal.
Low solubility of TCH.
Scheme 1.
b
C: 75.59, N: 12.55, H: 11.62 (TCH) and C: 75.10, N: 12.89, H: 12.00
(TH) as well as IR spectra. Solutions of the compounds were pre-
pared in a variety of deuterated solvents and kept at room temper-
ature for 1 h.
3.1. The influence of the sample concentration on the A–B equilibrium
in inert solvents
The behavior of 1,3,5-trialkyl-hexahydro-1,3,5-triazines in inert
solvents depends on the alkyl substituent. In the case of linear alkyl
group, A are very stable and make the only form observed at the
investigated concentrations and temperatures. In the case of the
cyclohexyl substituents, a mixture of A and B was observed in all
the applied inert solvents. 13C NMR spectra of TH in CDCl3 also re-
vealed exclusively signals corresponding to A, whereas in case of
TCH, a signals of B (including the most characteristic one at
150 ppm corresponding to C@N group) could also be observed.
Due to good solubility of the investigated compounds and no
coincidence with solvent’s signals, chloroform and acetone were
chosen as solvents appropriate for further studies. Samples of var-
ious [A0] in acetone and chloroform were prepared. The relative A/
B molar ratio was estimated by integration of the corresponding
signals without internal standard. It was assumed that the A–B
equilibrium is the only process taking place in the sample and
a + b = 1, where a and b are molar fractions of A and B respectively.
The equilibrium molar concentrations of A and B were calculated
on the basis of the following Eqs. (1) and (2).
3. Results and discussion
The behavior of 1,3,5-trialkyl-hexahydro-1,3,5-triazines in chlo-
roform solutions is dependent on the alkyl substituents. For
branched alkyls such as t-Bu, i-Pr and cyclohexyl a mixture of A
and B was observed, whereas in case of linear alkyls such as n-
Bu, n-Pr and n-hexyl no signals characteristic of monomeric forms
were present. Moreover, considerable differences in reactivity of
linear and branched 1,3,5-trialkyl-hexahydro-1,3,5-triazines and
influence of the solvent and temperature applied could be ob-
served in the Mannich reaction of phenols with 1,3,5-tricy-
clohexyl-hexahydro-1,3,5-triazines [32].
A
crystalline 1,3,5-
tricyclohexyl-hexahydro-1,3,5-triazine (TCH) was chosen as
a
model substance due to its well defined structure, that was previ-
ously determined by X-ray [10]. The linear alkyl isomer (TH) was
investigated for comparison.
The characteristic signals corresponding to the methylene
groups of heterocyclic ring (a) in TH and TCH, the CH2@N group
(b) in N-methylenealkylamines B and O–CH2–N (c) in N-(methoxy-
methyl)amines C were observed in solutions (Fig. 1 and Table 1).
The equilibrium concentrations of the species were calculated on
the basis of the dissolved amount of A and the contents of the spe-
cies in solutions evaluated by means of integration of the corre-
sponding signals without internal standard.
½Aꢁ ¼ 3 ꢂ a ꢂ ½A0ꢁ=ð3 ꢂ a þ bÞ
½Bꢁ ¼ 3 ꢂ b ꢂ ½A0ꢁ=ð3 ꢂ a þ bÞ
ð1Þ
ð2Þ
On the basis of [A] and [B], the equilibrium constants have been cal-
culated following Eq. (3).
3
KA;B ¼ ½Bꢁ =½Aꢁ
ð3Þ
The calculated amounts of A and B depend on the solvent and
concentration of the sample (Table 2 and Fig. 2).
b
c
R
N
a
a
Table 2
H
H
Initial concentration of A, equilibrium concentration of A and B, equilibrium constants
in (CD3)2CO and CDCl3 solutions
D
N
C
N
R
N
N
R
Entry
(CD3)2CO
[A0]
CDCl3
[A]
R
R
D3C O
[A]
[B]
KA,B ꢂ 109
[B]
KA,B ꢂ 109
[ꢂ104/(mol dmꢀ3)]
[ꢂ104/(mol dmꢀ3)]
b
a
1
2
3
4
5
6
4.4
2.8
2.0
1.2
0.4
0.2
3.9
2.4
1.6
0.89
0.22
0.08
1.7
1.3
1.1
0.93
0.55
0.37
12.6
9.2
8.3
9.0
7.6
6.3
3.5
2.1
1.5
0.75
0.19
0.03
2.6
2.1
1.6
1.3
0.63
0.52
50.2
44.1
27.3
29.3
13.2a
46.9
A
C
B
R= cyclohexyl for TCH or n-hexyl for TH
a
The result was reproducibly obtained in several independent experiments.
Fig. 1. Characteristic groups of A, B and C species.

´
A. Adamczyk-Wozniak et al. / Journal of Molecular Structure 892 (2008) 177–181
179
Fig. 3. Equilibrium concentrations of A and B depending on temperature.
Fig. 2. Equilibrium concentrations of A and B in (CD3)2CO and CDCl3 solutions
depending on the initial concentration of A.
kept at the desired temperature and monitored by 1H NMR every
15 min, until no considerable changes in their compositions could
be detected. The equilibrium concentrations of A and B (Table 3
and Fig. 3) were evaluated according to the previously described
methodology.
A similar tendency in the changes of the equilibrium con-
centrations of A and B in (CD3)2CO and CDCl3 solutions was
observed. At low [A0], B is the predominant form in solution,
whereas at higher [A0] A is in majority. The observation ex-
plains some contradictory reports on the spectral properties
of 1,3,5-trialkyl-hexahydro-1,3,5-triazines. For example Jewett
et al. [33] report no B form in a variety of investigated solu-
tions. To the contrary, Müller et al. [34] gave exclusively shifts
Changes of the equilibrium concentrations depending on
temperature are very similar for acetone and chloroform. At
+10 °C and lower temperatures, A was the predominant form
in both of the solvents. At +30 °C and higher temperatures B
was in majority. The observed changes of the spectra with
the temperature confirm the existence of the expected cyclic
structure in solution. At ꢀ10 °C, broadening of signal corre-
sponding to methylene linkages (3.60 ppm), due to the slowing
of the chair to-chair interconversion of the cyclohexane back-
bone, was observed in both of the investigated solvents. At
ꢀ30 °C two broadened signals were observed instead of one.
The rate constant of the process at the coalescence tempera-
ture (ꢀ15 0.5 °C in CDCl3) was evaluated according to the fol-
lowing Eq. (4).
corresponding to
B to characterize 1,3,5-trialkyl-hexahydro-
1,3,5-triazines by 1H NMR. Similarly, the reported rapid decom-
position of the 1,3,5-tri-tert-butyl-hexahydro-1,3,5-triazine in
CDCl3 solution [14] might in fact be the result of low concen-
tration of the sample.
On the basis of the changes of the concentration of A it was pos-
sible to evaluate the average equilibrium constants of the observed
transformation which is about 9 ꢂ 10ꢀ9 in deuterated acetone and
about 4 ꢂ 10ꢀ8 in CDCl3. The shift of the equilibrium in CDCl3 can
be caused by acidic impurities resulting from partial decomposi-
tion of the solvent.
ke ¼ ð3:14 ꢂ 164 HzÞ=1:41 ¼ 365 sꢀ1
ð4Þ
3.2. The influence of temperature on the A–B equilibrium in inert
solvents
The value of free energy of activation at the coalescence tempera-
ture (50.35 kJ/mol = 12.02 kcal/mol), calculated following the Eyr-
ing equation is in agreement with the ones reported for analogous
compounds [33]. In the spectrum taken in CDCl3 at ꢀ50 °C, two
doublets (at 4.04 and 3.22 ppm) corresponding to equatorial and
axial hydrogens were present (Fig. 4).
The influence of the temperature on equilibrium concentrations
of A and B was also investigated. Solutions of TCH in CDCl3 and
(CD3)2CO were prepared (2 ꢂ 10ꢀ4 mol dmꢀ3). The spectra were ta-
ken at temperatures ranging from ꢀ30 to +50 °C. The samples were
Table 3
Calculated concentrations of A and B depending on temperature
Entry
Temperature
(CD3)2CO
CDCl3
[A]
[B]
KA,B
[A]
[B]
KA,B
[ꢂ104/(mol dmꢀ3)]
[ꢂ104/(mol dmꢀ3)]
1
2
3
4
5
–30
–10
10
30
50
1.97
1.91
1.76
1.33
0.76
0.08
0.26
0.72
2.00
3.72
2.60 ꢂ 10ꢀ12
9.20 ꢂ 10ꢀ11
2.12 ꢂ 10ꢀ9
6.02 ꢂ 10ꢀ8
6.77 ꢂ 10ꢀ7
1.86
1.82
1.67
1.43
0.72
0.40
0.52
0.98
1.68
3.82
3.44 ꢂ 10ꢀ10
7.73 ꢂ 10ꢀ10
5.64 ꢂ 10ꢀ9
3.32 ꢂ 10ꢀ8
7.74 ꢂ 10ꢀ7

´
180
A. Adamczyk-Wozniak et al. / Journal of Molecular Structure 892 (2008) 177–181
Fig. 4. 1H NMR of TCH in CDCl3 at various temperatures.
3.3. Behavior of 1,3,5-trialkyl-hexahydro-1,3,5-triazines in methanol
The solution of TCH [A0] = 2 ꢂ 10ꢀ4 mol dmꢀ3 was prepared at
room temperature and spectra taken at +50, +30, +10, ꢀ10 and
ꢀ30 °C according to the description given for analogous investiga-
tion in CDCl3 and (CD3)2CO. Due to the complexity of the system
the calculations were limited to the molar fractions of A, B and C.
The content of A in the mixture decreased with increasing temper-
ature (Table 5 and Fig. 6).
The molar fraction of C is reasonably high at ꢀ30 °C, rises with
increasing temperature reaching the highest recorded value of 0.59
at 30 °C, and then slightly decreases. The molar fraction of A de-
creases with increasing temperature from 0.54 at ꢀ30 °C to 0.24
at 50 °C. The molar fraction of B is lower than that of A and C at
investigated conditions, but it rises with increasing temperature
and at 50 °C is almost as high as the molar fraction of A.
Dissolving TCH and TH in deuterated methanol results in an
equilibrium mixture of A, B and C, which was confirmed on the ba-
sis of 1H, 13C, HSQC (Heteronuclear Single Quantum Coherence),
HMBC (Heteronuclear Multiple Bond Correlation) and DEPT 135
(Distortionless Enhancement by Polarization Transfer) spectra of
the TCH solution. The identification of the mixture components
was made by analysis of signals corresponding to 2, 4, 6, 7, 8, 10,
100 and 1000 H and C atoms (Fig. 5 and Table 4). The overlapping
20–60, 200–600 and 2000–6000 signals at 1–2 ppm in 1H and 20–40 ppm
in 13C spectra were not essential for the consideration.
The presence of A in the mixture is evident from the C-2,4,6
methylene (negative-phased in DEPT 135) signal at 81.1 ppm cor-
relating with 2,4,6-H at 4.17 ppm (HSQC) and 10-H signal at
2.71 ppm (HMBC). The C-7 signal of the CH2@N group in B
4. Conclusions
(154.5 ppm) is correlated with 7-H at 7.45 ppm and 7-Hb at
a
7.05 ppm (HSQC). 7-H correlates with C-100 signal at 74.6 ppm
a
On the basis of the results it can be stated that the reversible
depolymerization of 1,3,5-trialkyl-hexahydro-1,3,5-triazines (A)
with formation of N-methylenealkylamines (B) takes place in inert
solvents. The A/B ratio at equilibrium strongly depends on alkyl
substituent. For linear ones, here represented by n-propyl, n-butyl
and n-hexyl, the cyclic trimeric structures are highly stable and no
signals of B could be observed in 1H NMR spectra of the chloroform
solutions at room temperature. For branched alkyl isomers such as
iso-propyl, tert-butyl and cyclohexyl, considerable amount of form
B could be detected in those solutions. The results of more detailed
studies of the model compounds (TH and TCH) in a variety of inert
solvents (chloroform, benzene, dimethylsulfoxide, acetone, aceto-
nitrile) are in agreement with the above observation. Assuming
that B are the active species in methylaminoalkylating reactions,
the more branched isomers should react more easily. Further
investigation of A–B equilibrium for TCH in acetone and chloro-
form revealed that lowering the initial concentration of the sample
at room temperature results in B being the predominant form in
both solutions (Table 2). With increasing temperature the amount
of B in solution also rises (Table 3). 1,3,5-Trialkyl-hexahydro-1,3,5-
triazines in methanol solution are also in equilibrium with B,
however the latter readily transform into the corresponding
N-(methoxymethyl)amines (C) in the reversible reaction with the
(HMBC). The C-8 signal of compound C at 86.2 ppm (negatively-
phased in DEPT 135) is correlated with 8-H at 4.32 ppm (HSQC)
and 1000-H signal at 2.78 ppm (HMBC). The 1000-H signal, partly over-
lapped with 10-H, is correlated with C-1000 at 60.7 ppm (HSQC).
1H NMR spectra of the mixture recorded at various tempera-
tures confirmed the existence of a temperature-dependent A–B–
C equilibrium (Scheme 2).
4'
1'
4'''
1'''
5'
6'
5'''
6'''
3'
2'
3'''
2'''
4''
1''
5''
6''
3''
2''
N
ND
8
1
CH2
6
2
O
N 5 3N
7
N
4
6'
CH2
6'
3'
CD3
1'
2'
5'
4'
5'
4'
1'
2'
3'
A
B
C
Fig. 5. Components of the mixture resulting from dissolving TCH (A) in CD3OD.

´
A. Adamczyk-Wozniak et al. / Journal of Molecular Structure 892 (2008) 177–181
181
Table 4
1H and 13C NMR analytical signals and 1H, 13C correlations of the mixture components
Atom C
d (ppm)
Atom H
d (ppm)
Multiple
1H, 13C HSQC
1H, 13C HMBC
DEPT 135
A
C-2,4,6
C-10
C-7
81.1
53.8
154.5
2,4,6-H
10-H
4.17
2.71
7.45
7.05
s
+
+
+
+
C-10
C-2,4,6
C-100
;
"
;
ma
d
7-H
a
B
C
7-Hb
100-H
8-H
d
C-100
C-8
C-1000
74.6
86.2
60.7
"
;
"
4.32
2.78
s
+
+
C-1000
C-8
1000-H
ma
Abbreviations: s, singlet; d, doublet; m, multiplet. a, this signal overlapped with another signal; +, signal detected; ", positive-phased signal; ;, negative-phased signal.
Appendix A. Supplementary data
+ 3 CD3OD
3 C
A
3 B
The attribution of 1H NMR and 13C NMR signals of A and B
in CDCl3. 13C NMR spectra of TH and TCH in CDCl3. 13C NMR,
DEPT 135, HSQC, HMBC spectra of TCH in CD3OD. 1H NMR
spectra of TCH in CD3OD and (CD3)2CO at ꢀ30, ꢀ10, +10, +30
and +50 °C. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.molstruc.
2008.05.019.
Scheme 2.
Table 5
Molar fractions of A (a), B (b) and C (c) depending on temperature
Entry
Temperature
a
b
c
1
2
3
4
5
–30
–10
10
30
50
0.54
0.49
0.40
0.28
0.24
0.02
0.05
0.11
0.13
0.19
0.44
0.46
0.49
0.59
0.57
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This work was financially supported by Warsaw University of
Technology.