The oxidation of hexahydrotriazines

Article abstract of DOI:10.1007/s00706-005-0414-8

N-Alkyl-2,4-dioxohexahydro-1,3,5-triazines oxidize readily with oxygen to the corresponding cyanuric acid derivates. The oxidation of 1,3,5-trimethyl-2,4- dioxohexahydro-1,3,5-triazine resulted in a stable form of hydroperoxide. During the oxidation of 1,3,5-trimethyl- 6-phenyl-2,4-dioxohexahydro-1,3,5-triazine this hydroperoxide could not be identified, however, the result was the stable reaction product bis-[6-phenyl-1,3,5-trimethyl-2,4-dioxohexahydro-1,3,5-triazin] peroxide. Springer-Verlag 2006.

Full text of DOI:10.1007/s00706-005-0414-8

Monatshefte f €u r Chemie 137, 185–190 (2006)
DOI 10.1007/s00706-005-0414-8
The Oxidation of Hexahydrotriazines
1
2
1
Roland Dorninger , Kurt Klepp , Reinhard Rametsteiner ,
Rudolf Schiffer , Harald Schmidt , and Clemens Schwarzinger1
1
1;ꢀ
1
Institute for Chemical Technology of Organic Materials, Johannes Kepler University Linz,
A-4040 Linz, Austria
2
Institute of Inorganic Chemistry, Johannes Kepler University Linz, A-4040 Linz, Austria
Received July 25, 2005; accepted (revised) October 4, 2005
Published online January 20, 2006 # Springer-Verlag 2006
Summary. N-Alkyl-2,4-dioxohexahydro-1,3,5-triazines oxidize readily with oxygen to the corres-
ponding cyanuric acid derivates. The oxidation of 1,3,5-trimethyl-2,4-dioxohexahydro-1,3,5-triazine
resulted in a stable form of hydroperoxide. During the oxidation of 1,3,5-trimethyl- 6-phenyl-2,4-
dioxohexahydro-1,3,5-triazine this hydroperoxide could not be identified, however, the result was the
stable reaction product bis-[6-phenyl-1,3,5-trimethyl-2,4-dioxohexahydro-1,3,5-triazin]peroxide.
Keywords. Cyanuric acid; Hexahydrotriazines; Hydroperoxides; Oxidation; Peroxides.
Introduction
N-Alkyl-2,4-dioxohexahydro-1,3,5-triazines are interesting solvents for the fluid–
fluid- extraction of aromatics from aromatics containing hydrocarbon mixtures.
Besides good selectivity and capacity their high resistance to hydrolysis is of
importance [1]. During stability investigations we observed the easy oxidation of
these compounds by air oxygen. There have been neither reports about the autox-
idation of hexahydrotriazines with oxygen nor any reports about their hydroper-
oxides or peroxides.
Results and Discussion
The oxidation of 2,4-dioxohexahydro-1,3,5-triazine (1a) to cyanuric acid (1b) is
successful only with strong oxidizing agents such as hydrogen peroxide or
potassium peroxydisulfate [2]. However, we found now that the N-alkyl com-
pounds 2a–5a oxidize surprisingly easily with oxygen to the corresponding cya-
nuric acid derivates 2b–5b. Since little can be found in scientific literature about
the oxidation of hexahydrotriazines, we examined the reaction of 2a with air using
ꢀ Corresponding author. E-mail: harald.schmidt@jku.at

1
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R. Dorninger et al.
Scheme 1
Table 1. Conversion based on oxygen consumption, rate constant, and composition of the reaction
3
solution at different temperatures and an initial concentration of 1 mol=dm 2a
ꢁ
ꢂ1
ꢂ3
Composition=mol dm
Temperature= C
Conversion=%
Rate constant=s
2
c
2b
ꢂ4
ꢂ4
ꢂ4
ꢂ4
7
8
9
0
0
0
73
90
90
90
1.76ꢃ10
0.364
0.411
0.178
0.107
0.374
0.511
0.748
0.824
5.24ꢃ10
6.22ꢃ10
1
00
7.35ꢃ10
ꢁ
chlorobenzene as solvent. Within the temperature range of 70–100 C a sponta-
neous absorption of oxygen of up to a volume of 90% occurred after an induction
period of only a few minutes. Oxygen consumption was observed by using a gas
burette and the composition of the solution was determined at the end of the
1
3
reaction by means of HPLC, HPLC-MS, and C NMR. The kinetic data of oxida-
tion of 2a and composition of the reaction solution at the end of the reaction are
illustrated in Table 1.
As indicated in Table 1, high concentrations of hydroperoxide 2c occur at
lower temperatures and disintegration to trimethylcyanuric acid (2b) begins with
increasing temperature. Conversions of this magnitude are not typical for
autoxidations. An activation energy of only 18.4 kJ=mol confirmed the ready
Fig. 1. Result of an X-ray structure analysis of 1,3,5-trimethyl-2,4-dioxo-6-hydroperoxohexa-
hydro-1,3,5- triazine (2c)

The Oxidation of Hexahydrotriazines
187
Table 2. Positional and equivalent isotropic displacement parameters for 1,3,5-trimethyl-2,4-
dioxo-6-hydroperoxohexahydro-1,3,5-triazine (2c), monoclinic, s.g. P2 =c (no. 14), a ¼ 8.494(2),
1
˚
ꢁ
b ¼ 8.985(2), c ¼ 11.372(5) A, ꢀ ¼ 94.61(2) , Z ¼ 4
Atom
x
y
z
Beqa
N(1)
N(2)
N(3)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
O(1)
O(2)
O(3)
O(4)
H(1)
H(2)
H5a
H5b
H5c
H4a
H4b
H4c
H6a
H6b
H6c
0.7189(2)
0.9731(2)
0.7583(2)
0.8797(3)
0.6543(3)
0.9167(2)
0.6131(3)
0.6964(3)
1.1452(3)
0.9304(2)
0.5127(2)
0.9344(2)
0.8334(2)
0.895(3)
0.986(2)
0.218
0.0984(2)
0.1856(2)
0.3513(2)
0.0708(3)
0.2389(3)
0.3248(3)
ꢂ0.0255(3)
1.0012(3)
0.1662(3)
ꢂ0.0522(2)
0.2599(2)
0.3412(2)
0.2317(2)
0.166(3)
0.088(2)
0.070
0.6345(2)
0.6205(2)
0.6036(2)
0.6448(2)
0.6172(2)
0.5762(2)
0.6582(3)
0.0765(2)
0.6279(2)
0.6745(2)
0.6169(2)
0.4526(1)
0.3904(1)
0.369(2)
0.112(2)
0.909
3.14(4)
3.01(4)
3.05(4)
2.93(4)
3.31(5)
2.97(4)
5.07(6)
4.32(6)
4.15(5)
3.99(4)
4.91(4)
3.66(3)
4.06(4)
ꢀ
5.6(6)
ꢀ
3.0(5)
ꢀ
5.5(6)
ꢀ
0.337
0.005
0.009
6.7(7)
ꢀ
0.394
0.036
0.878
6.9(8)
ꢀ
0.365
0.114
0.387
8.2(9)
9(1)ꢀ
0.493
0.992
0.368
ꢀ
0.369
0.037
0.259
8.2(9)
ꢀ
0.179
0.079
0.677
10(1)
ꢀ
9.1(9)
0.201
0.253
0.664
12(1)
ꢀ
0.172
0.126
0.559
8ꢁ2
3
P P
ꢀ ꢀ
Hydrogen atoms were treated as riding atoms and refined isotropically
a
Beq ¼
Uijai aj aiaj
j
i
ꢀ
oxidation of 2a. The isolated 1,3,5-trimethyl-2,4-dioxo-6-hydroperoxohexa-
hydro-1,3,5-triazine (2c) crystallized from chlorobenzene as monocline crystals
and proved itself exceptionally stable. The X-ray structure analysis (Fig. 1,
Table 2) of this compound confirms its structure. As in other 2,4-dioxohexa-
hydro-1,3,5-triazines [3–5], the triazine ring shows a weak boat configuration
in 2c. The molecules are connected in pairs by two almost linear hydrogen bonds,
C–O–O–Hꢄ ꢄ ꢄO¼C. In the crystal structure these pairs are stacked as hexagonal
packages.
Compounds 3a and 4a were oxidized with oxygen under similar conditions.
The alkylated cyanuric acids 3b and 4b were also verified in addition to the respec-
tive hydroperoxides 3c and 4c. Moreover, a considerable attack on the alkyl group
followed, leading to N-dealkylation as well as to numerous less significant oxida-
tion products. Of particular interest was that conversions of only about 30% were
reached, followed by no further oxygen consumption. The oxidation of 5a under
analogous conditions leads to a large number of oxidation products. In addition to
benzaldehyde and benzoic acid, tribenzylcyanuric acid (5b) was verified, which

1
88
R. Dorninger et al.
can be explained as a result of N-dealkylation and further oxidation. The corre-
sponding hydroperoxide could not to be identified.
Due to the ready oxidation of 2a it could be expected that a hexahydrotriazine
substituted in position 6 could also be easily oxidized with oxygen. All the more
ꢁ
surprising was the observation that there was no oxygen consumption at 100 C in
the case of 6-phenyl-1,3,5-trimethyl-2,4-dioxohexahydro-1,3,5-triazine (6a). Only
after addition of azobisisobutyronitrile as a radical starter, very rapid oxygen con-
sumption followed, which however broke off at a conversion of about 35%. At first
we assumed that this breaking off was due to the possible rearrangement and
splitting of 6c into phenol and trimethylcyanuric acid (2b). However, neither phe-
nol as possible inhibitor of oxidation nor 2b could be found in the reaction mixture.
During the oxidation in chlorobenzene as the solvent a white precipitate was
observed. The analysis of this reaction product showed a negative hydroperoxide
test with lead tetraacetate and a positive peroxide test with potassium iodide solu-
1
tion. In the H NMR spectrum of the reaction product the proton in position 6 of
the educt 6a is not present and the proton of the HOO- group clearly visible in the
1
3
compound 2c, is also not present. From the C NMR spectrum no characteristic
structures to discriminate between the compounds 6a and 6c can be recognized. By
means of ESI-MS (Na- and=or K-ionization) a molecular weight of 496 Da could
be determined corresponding to that of the peroxide 6b, and finally the data of an
elementary analysis confirmed the presence of 6b. The findings in short: The
compound in question is not hydroperoxide 6a but peroxide 6c.
Within the framework of these experiments, the oxidation of a CH -group
2
having the structure >N–CH –N< was of interest. Therefore oxidation of 1,3-
2
dimethylimidazolidine (7a), 1-methyl-2,3-dihydro-benzimidazole (8a), and 1,3-
dimethyl-2,3-dihydrobenzimidazole (9a) with oxygen was attempted as well as
the identification of the reaction products. Within the temperature range from
Scheme 2

The Oxidation of Hexahydrotriazines
189
Scheme 3
ꢁ
2
5 to 100 C using chlorobenzene as the solvent no oxygen consumption by 7a and
8
a was observed; in contrast however, 9a was very easily oxidized with oxygen to
ꢁ
yield 1,3-dimethyl-2-oxo-2,3-dihydrobenzimidazole (9b). Even at 25 C oxygen
consumption up to a conversion of 90% occurred spontaneously without an induc-
tion period. The corresponding hydroperoxide or peroxide could not be identified.
The water resulting from the oxidation does not lead to the hydrolysis of this
compound in the case of 9b because it is very stable with respect to water. Com-
pound 7a differs because slight amounts of water lead to hydrolysis resulting in
methylamine, which inhibits autoxidation. Compound 8a is also hydrolysis stable,
but the remaining NH-group acts as an inhibitor. This observation is in con-
cordance with investigations done by Drago and Riley [8], who have obtained
1-methyl-5-peroxi-2-pyrrolidone from the oxidation of 1-methyl-2-pyrrolidone
with oxygen. However, they did not observe oxygen consumption during oxidation
of 2-pyrrolidone.
Experimental
Preparation of 2,4-dioxohexahydro-1,3,5-triazine (1a) is described in Ref. [1] and the alkylation of 1a
in Ref. [2]. 6-Phenyl-2,4-dioxohexahydro-1,3,5-triazine was produced by condensation of benzalde-
ꢁ
ꢁ
hyde with urea at 100 C and subsequent elimination of ammonia at 180 C. The alkylation to the
-phenyl-1,3,5-trimethyl-2,4-dioxohexahydro-1,3,5-triazine (6a) takes place as described in Ref. [2].
6
0
0
The synthesis of N,N -dimethylimidazolidine (7a) is described in Ref. [6] and the synthesis of N,N -
dimethylbenzimidazoline (9a) in Ref. [7], N-methylbenzimidazoline (8a) is a resulting by-product. The
1
13
measurements of the H and C NMR were taken with a Bruker AC-200 NMR spectrometer and the
IR spectra using a Paragon 1000 FT=IR spectrometer (Perkin Elmer). The mass spectra were measured
using the ThermoFinnigan LCQ Deca XP plus ion trap MS.
X-Ray Structure Determination
The intensity data collection was performed at 294(1) K on a single crystal with the dimensions
3
˚
0
.3ꢃ0.3ꢃ0.125 mm using graphite monochromated MoKꢂ-radiation (ꢃ ¼ 0.71073 A). The crystal
structure was solved in P2 =c by direct methods (SIR92) and refined to a conventional R of 0.042
1
(
Rw¼ 0.042, GoF ¼ 0.57) for 1428 reflections (cutoff 3ꢄ(I) and 126 refined variables. Further details
on the structure determination and refinement have been deposited with the Cambridge Crystallo-
graphic Data Centre under CCDC 283669. The physical and spectroscopic data of compounds 2, 3, 4,
and 5 are published in Ref. [2].

1
90
R. Dorninger et al.: The Oxidation of Hexahydrotriazines
General Procedure
3
A solution of 0.78g 2a (5mmol) in 5 cm chlorbenzene was prepared and stirred at 70 C in a
ꢁ
microreactor. In all experiments quantitative O consumption was observed by using a gas burette.
2
The solution was directly analyzed by HPLC.
Trimethylcyanuric Acid (2b)
1
ꢁ
Mp 173–174 C; H NMR (200 MHz, CDCl ): ꢅ ¼ 3.35 (pp, 1,3,5-N-CH ) ppm.
3
3
1
,3,5-Trimethyl-2,4-dioxo-6-hydroperoxo-hexahydro-1,3,5-triazine (2c, C H N O )
6 11 3 4
ꢁ
1
Mp 130 C (dec); H NMR (200MHz, CDCl ): ꢅ ¼ 11.6 (s, CH–OOH) 5.94 (s, CH–OOH), 3.01
3
(s, 3-N–CH ), 2.87 (s, 2-N–CH ) ppm.
3 3
6
-Phenyl-1,3,5-trimethyl-2,4-dioxohexahydro-1,3,5-triazine (6, C H N O )
12 15 3 2
ꢁ
1
Mp 120 C; H NMR (200MHz, CDCl ): ꢅ ¼ 7.21–7.45 (m, phenyl), 5.14 (s, 6-C–H), 3.27
3
(s, 3-N–CH ), 2.87 (s, 1,5-N–CH ) ppm.
3 3
Bis-[6-phenyl-1,3,5-trimethyl-2,4-dioxohexahydro-1,3,5-triazin]peroxide (6c, C H N O )
2
4 28 6 6
1
H NMR (200MHz, CDCl ): ꢅ ¼ 7.25–7.45 (m, phenyl), 3.42 (s, 3-N–CH ), 2.69 (s, 1,5-N–CH )
3
3
3
þ
þ
þ
ppm; MS (ESI, positive mode): Na-adduct: m=z ¼ 519 (MNa ), 1015 (2MNa ); K-adduct: 535 (MK ),
þ
1
031 (2MK ); elemental analysis: calcd C 58.05, H 5.68, N 16.92, O 19.35; found C 57.9, H 5.8,
N 16.8%.
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[
[
[
[
[
[
[
[
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