Barsilowsky's base and Perkin's base: Two products from the oxidation of p-toluidine

Article abstract of DOI:10.3184/174751914X13995700264041

The oxidation of p-toluidine in H⁺/H₂O can give mainly a tetramer, a trimer, or a mixture of both. The outcome is dependent upon the oxidant, the pH and the concentration of p-toluidine. Both the trimer and tetramer are definitively

Full text of DOI:10.3184/174751914X13995700264041

JOURNAL OF CHEMICAL RESEARCH 2014
VOL. 38 JUNE, 351–355
RESEARCH PAPER 351
Barsilowsky’s base and Perkin’s base: two products from the oxidation
of p‑toluidine
M. John Plater* and William T.A. Harrison
Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen AB24 3UE, UK
The oxidation of p‑toluidine in H+/H2O can give mainly a tetramer, a trimer, or a mixture of both. The outcome is dependent upon
the oxidant, the pH and the concentration of p‑toluidine. Both the trimer and tetramer are definitively characterised by X‑ray
single crystal structure determination.
Keywords: Barsilowsky’s base, Perkin’s base, p‑toluidine, potassium dichromate, potassium permanganate
A number of studies have focussed on unravelling the products
formed from the oxidation of p‑toluidine. In 1873, Barsilowsky
first reported the oxidation of p‑toluidine with KMnO4 or
K3Fe(CN)6 to give a tetramer of molecular formula C28H28N4.1–6
In 1879, Perkin studied the oxidation of p‑toluidine hydrosulfate
with K2Cr2O7 and reported a trimer of molecular formula
C21H21N3 and some tetramer of molecular formula C28H27N3.7–9
In 1884, the oxidation was examined by Klinger and Pitschke
who followed Barsilowsky’s method.10 They obtained a tetramer
of molecular formula C28H28N4. In 1893, Green studied the
oxidation of p‑toluidine with K2Cr2O7 in dilute sulfuric acid,
characterised the product with the molecular formula C21H21N3
and drew the correct structure.11 The empirical formula of
C7H7N was suggested from previous studies12 and a molecular
weight determination showed that the substance was formed
from 3 moles of p‑toluidine. In 1901 and 1910, Börnstein studied
the oxidation of p‑toluidine in dilute sulfuric acid and reported
a trimer and a tetramer analogous to Perkin’s work.13,14 In 1901,
Börnstein drew the structure of the trimer and tetramer13 and
suggested the names for these compounds as Barsilowsky’s
base and Perkin’s base respectively.13 Reported melting
points for these compounds vary considerably suggesting the
two compounds were difficult to separate.11 The trimer and
tetramer and other by‑products were also isolated by Saunders
from the action of the enzyme peroxidase upon p‑toluidine.15–17
The oxidation of p‑toluidine with K3Fe(CN)6 in liquid NH3
gave the trimer.18 The above studies were done before X‑ray
crystallography and NMR spectroscopy became available,
apart from work by Kametani,18 so we have carried out further
experiments to confirm the structure of these products and to
be sure of the identity of Barsilowsky’s base. The trimer, as
proposed by Green,11 was recently reported as the product of
the oxidation of p‑toluidine with FeCl3.19
K2Cr2O7 at different pHs, concentrations and temperatures. The
oxidation of p‑toluidine with aqueous alkaline K3Fe(CN)6 was
unsatisfactory in our hands.
In summary, the oxidation of either p‑toluidine in dilute
sulfuricacidat60 °CwithKMnO4 ortheoxidationofp‑toluidine
hydrochloride in H2O at 60 °C with KMnO4 gave exclusively
the
trimer
2‑amino‑5‑methyl‑1,4‑benzoquinone‑1,4‑(4‑
methylphenylimine) 1 (entries 1 and 2). TLC analysis showed
that the trimer was also formed exclusively by performing both
these reactions at room temperature and a concentration of
10 mg mL–1. The trimer was characterised spectroscopically
and by X‑ray crystallography and the data correspond to the
correct structure suggested by Green (Fig. 1).11 This suggests
that Barsilowsky’s compound is indeed a trimer and not a
tetramer because he worked with KMnO4 as oxidant.1 However,
the oxidation of p‑toluidine in dilute sulfuric acid with K2Cr2O7
at 60 °C gives a different less polar product (entry 3). This was
characterised spectroscopically and by X‑ray crystallography
as the tetramer 2‑(4‑methylphenylamino)‑5‑methyl‑1,4‑
Table 1 Yields of trimer 1 and tetramer 2 isolated by oxidising p‑toluidine
with either KMnO4 or K2Cr2O7 under different conditions
Entry
Conditions
Trimer 1/%a Tetramer 2/%a
p‑Toluidine (10 mg/mL)
KMnO4/dil H2SO4/60 °C
p‑Toluidine HCl (7.2 mg mL–1)
KMnO4/60 °C
p‑Toluidine (10 mg mL–1)
K2Cr2O7/dil H2SO4/60 °C
p‑Toluidine (10 mg mL–1)
K2Cr2O7/dil H2SO4/rt
p‑Toluidine (5 mg mL–1)
K2Cr2O7/dil H2SO4/60 °C
p‑Toluidine HCl (3.4 mg mL–1)
K2Cr2O7/60 °C
1
9
11
2
0
0
2
3
4
5
6
12
6
1
14
6
Discussion
16
0
Scheme 1 shows the structure of the trimer 1 and tetramer 2
correctly drawn by Bornstein in 1901. Table 1 summarises our
results from the oxidation of p‑toluidine with either KMnO4 or
aYields calculated from the mass of isolated material purified by flash
column chromatography.
N
K2Cr2O7
or
N
+
KMnO4
N
N
NH3
NH2
NH2
HN
1
2
Scheme 1 Formation of either a trimer 1, a tetramer 2 or both from the oxidation of p‑toluidine in dilute H2SO4 or p‑toluidine hydrochloride.
* Correspondent. E‑mail: m.j.plater@abdn.ac.uk
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352 JOURNAL OF CHEMICAL RESEARCH 2014
Fig. 1 The molecular structure of 1 showing 50% displacement ellipsoids
[symmetry code (i)=1–x, 1–y, 1–z]. The intramolecular N2–H2aN1i
hydrogen bond [N–H=0.89 Å, HN=2.32 Å; N–HN=109°] is shown as a
double‑dashed line. Only one disorder component for C4 and N2 is shown.
Selected geometrical data (Å,°): C1–C2=1.337 (4); C2–C3=1.456 (4); C1–
C3i =1.483 (4); C3–N1=1.289 (4); C3–N1–C5=118.8 (3). The dihedral angle
between the central ring and the pendant ring is 87.33 (12)°. In the crystal,
an N2–H2bN1ii (ii=x–y+1/3, x–1/3, 2/3–z) hydrogen bond occurs, but
there are no aromatic π–π stacking interactions. For the results of, and our
comments on, on a previous crystallographic study see the experimental
section.19
Fig. 2 The molecular structure of 2 showing 50% displacement ellipsoids.
The intramolecular N2–H1N3 hydrogen bond [N–H=0.88 (3) Å, HN=2.19
(2) Å; N–HN=110.6 (19)°] is shown as a double‑dashed line. Selected
geometrical data (Å,°): C1–C2=1.481 (3); C2–C3=1.453 (3); C3–C4=1.353
(3); C4–C5=1.480 (3); C5–C6=1.461 (3); C6–C1=1.345 (3); C2–N1=1.306
(3); C4–N2=1.377 (3); C5–N3=1.295 (3); C2–N1–C8=120.7 (2); C4–N2–
C15=126.3 (2); C5–N3–C22=122.1 (2). The dihedral angles between the
central ring and the C8, C15 and C22 rings are 58.02 (8), 52.03 (9) and
67.62 (8)°, respectively. There are no π–π stacking interactions in the
crystal and only possible directional interactions are very weak C–Hπ
bonds with Hπ>2.75 Å.
benzoquinone‑1,4‑(4‑methylphenylimine) 2 (Fig. 2). A small
amount of the trimer 1 was also isolated in this reaction.
The contrast of these two reactions using different oxidants
at approximately the same pH is striking. Both solutions,
p‑toluidine in dil H2SO4 and p‑toluidine hydrochloride, turn
blue litmus red. Using pH indicator paper the first is estimated
to have a pH of 4.0–5.0 and the second 6.0. If the same reaction
with K2Cr2O7 is performed at room temperature, rather than at
60 °C, after work‑up a mixture of the trimer 1 and tetramer
2 was present but in lower yield (entry 4). The product ratio
was not significantly altered by a change in temperature, but
halving the concentration of p‑toluidine favoured the trimer 1
(entry 5). This may explain Green’s result. He carried out the
oxidation of p‑toluidine in dilute sulfuric acid with K2Cr2O7 at
5 °C and obtained the trimer 1.11 Perkin performed this reaction
at room temperature and obtained trimer 1 and some tetramer
2.8 Saunders also showed, using peroxidase and hydrogen
peroxide as oxidant, that the tetramer is only produced when
the concentration of p‑toluidine is greater than 1 g L–1.16 The
oxidation of p‑toluidine hydrochloride in H2O at 60 °C with
K2Cr2O7 also gave exclusively the trimer 1 (entry 6). The
milder acidic conditions change the selectivity from tetramer
2 to trimer 1. In summary, using K2Cr2O7 as oxidant a higher
pH (6.0) and a lower concentration of p‑toluidine favour the
trimer 1 or a mixture of trimer 1 and tetramer 2 rather than
the tetramer 2. Likewise a lower pH (4.0–5.0) and a greater
concentration of p‑toluidine favours the tetramer 2.
Some experiments were performed to investigate the
mechanistic pathways to these two products (Scheme 2).
Treatment of the trimer 1 with p‑toluidine hydrochloride in
refluxing MeOH for 2 h or with p‑toluidine hydrochloride and
K2Cr2O7 in H2O gave none of the tetramer 2. This showed that
the trimer 1 is not an intermediate on route to the tetramer 2.
These were small scale reactions. Hydrolysis and attempted
oxidative degration of the tetramer 2 does not give the dimer 3.
The tetramer 2 is therefore not an intermediate on route to the
trimer 1. Hydrolysis of the trimer 1 to the dimer 3 has already
been reported by Green11 and we verified this. The dimer 3
has previously been made by us at low pH and by independent
synthesis.15
Proposed mechanistic pathways
Scheme 3 summarises some possible mechanistic pathways that
might lead to the trimer 1 and tetramer 2. The selectivity of the
reaction with different oxidants, concentrations of reagents and
solution pHs is unusual. The diarylamine linkage in tetramer
2, formed in relatively high yield, is also unprecedented in this
chemistry. Since the product mixtures formed using either
KMnO4 or K2Cr2O7 are different their pathways may also be
p-toluidine HCl
K2Cr2O7
p-toluidine HCl/MeOH
N
2
2
∆
∆
N
1
NH2
dil H2SO4
N
O
3
NH2
Scheme 2 The reactivity of trimer 1.
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JOURNAL OF CHEMICAL RESEARCH 2014 353
NH2
H
N
HN
NH2
4
N
H
6
5
NH2
N
N
7
H
H
H
N
N
N
N
N
N
9
H
NH2
N
1
H
H
8
H
N
N
N
N
N
O
N
NH2
HN
H
3
2
10
Scheme 3 Proposed routes leading to the trimer 1, tetramer 2 and dimer 3.
different. For example, the oxidation of p‑toluidine with KMnO4
may proceed through intermediate 4 to trimer 1 or through
intermediates 5, 7 and 8 to trimer 1. Intermediates 5, 7 and 8
should however also give tetramer 2, which does not form so
intermediate 5 is less likely with KMnO4 as oxidant. Oxidation
with K2Cr2O7 could proceed via the same intermediates 5, 7 and
8 to trimer 1, through intermediates 5 and 7, then 8 or 9 and 10
to tetramer 2, or through intermediates 6, 9 and 10 to tetramer
2. To form trimer 1, intermediate 7 would firstly require
condensation with p‑toluidine by a Michael addition reaction
to give intermediate 8, then oxidation, to give trimer 1. To form
tetramer 2 intermediate 7 could react in two ways. It could
add p‑toluidine by a Michael addition to give intermediate 8,
followed by an imine exchange reaction with p‑toluidine to give
intermediate 10, or it could undergo an imine exchange reaction
with p‑toluidine to give intermediate 9, followed by a Michael
addition of p‑toluidine to give intermediate 10. These reactions
explain how the unusual diarylamine structure might form.
At higher pH (6.0) the Michael addition reaction alone and
oxidation is favoured leading to trimer 1, but at a lower pH (4.0–
5.0) and with a higher concentration of p‑toluidine the imine
exchange reaction on either compound 7 or 8 is favoured, so
the tetramer 2 forms. A change of temperature did not change
the product ratio so increasing the temperature may accelerate
both Michael addition, oxidation and imine exchange reactions.
Imminium salt 8 should be more electrophilic than imminium
salt 7, so may condense more readily with p‑toluidine, but it
would also be easily oxidised. Both imminium salts would
also be easily hydrolysed. We considered ditolylamine 6 as
another intermediate that might follow a new reaction pathway
leading to tetramer 2, although we have no evidence for this
intermediate. However, Saunders has previously shown that
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354 JOURNAL OF CHEMICAL RESEARCH 2014
disorder of the methyl and amine groups, but resulted in unrealistic
anisotropic displacement and geometrical parameters and our own
attempts at modelling the structure in R3 led to the same problems.
We therefore prefer the centrosymmetric space group, but the non‑
centrosymmetric space group (possibly with merohedral twinning)
cannot be ruled out. The C‑bound hydrogen atoms for 1 and 2 were
geometrically placed (C–H=0.95–0.98 Å) and refined as riding atoms;
the methyl groups were allowed to rotate, but not to tip, to best fit the
electron density. The N‑bound hydrogen atoms in 1 were located in a
difference map and refined as riding atoms. The N‑bound hydrogen
atom in 2 was located in a difference map and its position was freely
refined.
ditolylamine 6 with p‑toluidine can be oxidised to tetramer 2 by
hydrogen peroxide and the enzyme peroxidase.16 The evidence
was based on an improved yield of tetramer 2 with ditolylamine
6 present. The oxidation of p‑toluidine and ditolylamine 6 with
K2Cr2O7 gave recovered ditolylamine 6 (67%) and a yield of
tetramer 2 of 14%. The high yield of recovered starting material
suggests that ditolylamine 6 is not an intermediate, although
at the p‑toluidine concentration of 2.14 mg/mL the trimer had
been expected to form spontaneously and not the tetramer. Since
the coupling of intermediate 4 with p‑toluidine should give
exclusively the trimer 1 the tetramer 2 must arise exclusively
via intermediate 5 from the oxidation of p‑toluidine.
1: C21H21N3, Mr =315.41, red block, 0.36×0.18×0.08 mm, trigonal,
space group R3 (no. 148), Z= 9, a= 20.9896 (8), c= 10.0985 (4)
Å, V= 3853.0 (3) Å 3 at 100 K. Number of measured and unique
reflections=12736 and 1686, respectively (–25≤ꢀh≤ꢀ22, –25≤ꢀk≤ꢀ25,
The exclusive formation of dimer 3 at lower pH20 could result
from the hydrolysis of trimer 111 arising from a more facile
oxidation of intermediate 8 at a lower pH. The conversion of
intermediate 5 into trimer 1 with K2Cr2O7 was reported by Green
who typically worked with dilute H2SO4 in the cold.11 Aromatic
amine oxidations are complex, as is mauveine synthesis, and we
cannot be sure if the amines couple by standard reactions, such
as Michael addition and imine exchange, or by the coupling of
aminyl radicals. p‑Toluidine works well and the methyl group
can stabilise aminyl radicals which may facilitate the reactions
indicated in Scheme 3.
–12≤ꢀl≤ꢀ12;
2θmax = 52.0°;
Rint = 0.054).
Fi nal
R(F) = 0.089,
wR(F2)=0.167 for 112 parameters and 1620 reflections with I> 2σ(I)
(corresponding R‑values based on all 1686 reflections=0.092 and
0.168, respectively), CCDC reference number 988298.
2: C28H27N3, Mr =405.53, ruby‑red block, 0.08×0.06×0.02 mm,
monoclinic, space group P21/n (No. 14), Z= 4, a= 13.5019 (7), b= 6.3512
(3), c= 25.918 (2) Å, β= 95.425 (7)°, V= 2212.6 (2) Å 3 at 100 K. Number
of measured and unique reflections=20455 and 3902, respectively
(–16≤ꢀh≤ꢀ16, –7≤ꢀk≤ꢀ7, –30≤ꢀl≤ꢀ30; 2θmax = 50.2°; Rint = 0.097).
Final R(F) = 0.052, wR(F2)=0.096 for 288 parameters and 2375
reflections with I> 2σ(I) (corresponding R‑values based on all 3902
reflections=0.115 and 0.119, respectively), CCDC reference number
982643.
Entry 1: 2-Amino-5-methyl-1,4-benzoquinone-1,4-(4-methyl
phenylimine) (1): p‑Toluidine (1.0 g, 0.009 mol) and KMnO4 (2.95 g,
0.019 mol) in dilute sulfuric acid (10 drops, 0.5 mL of cH2SO4 in
200 mL of H2O) were heated for 45 min at 60 °C. After cooling the
reaction was filtered through a fine pore sinter and washed with H2O
(100 mL) then extracted with MeOH (50 mL) and CH2Cl2 (50 mL×4).
The combined fractions were evaporated to dryness and purified
by chromatography on silica gel. Dichloromethane eluted the title
compound (89 mg, 9%) as an orange solid, m.p.>225 °C from
dichloromethane/light petroleum ether 40–60. λmax (CH2Cl2)/nm 440
(log ε 3.99) and 295 (4.37); νmax (KBr disc) 3306br, 3022w, 2914w,
1646s, 1603vs, 1572vs, 1501vs, 1433s, 1369s, 1310s, 1258s, 1230s,
1208s, 1168w, 1017w, 844s, 807s, 736w, 715w and 480s; δH(400 MHz;
CDCl3) 2.18 (s, 3H), 2.32 (s, 3H), 2.37 (s, 3H), 4.80–5.20 (s, br, 2H),
5.86 (1H, s), 6.58 (s, 1H), 6.78–6.84 (m, 4H), 7.14 (d, 2H, J= 8.0) and
7.20 (d, 2H, J= 7.6); δC(100.1 MHz; CDCl3) 18.4, 20.9, 21.0, 95.5,
120.6, 120.8, 129.4, 129.6, 133.5, 134.7, 144.0, 146.7, 153.3 and 159.6 (3
resonances are missing), m/z (orbitrap ASAP) 316.1808 (M+ +H, 100%)
C21H22N3 requires 316.1808. The same reaction at room temperature
gave exclusively the trimer by TLC analysis (25:75 Et2O/light petrol
40–60).
Conclusion
X‑ray crystal structure determinations have verified the
structure of trimer 1 formed from the oxidation of p‑toluidine
with KMnO4 in hot, dilute H2SO4 and tetramer 2 formed from
the oxidation of p‑toluidine with K2Cr2O7 in hot, dilute H2SO4.
Spectroscopic methods have also been used to confirm the
structures of both trimer 1 and tetramer 2. The oxidation of
p‑toluidine with either K2Cr2O7 or KMnO4 shows unusual
selectivity enabling predominantly trimer 1, tetramer 2, or a
mixture of both to form depending upon the oxidant, the pH
and the concentration. In summary, proposed mechanistic
pathways suggest that trimer 1 can form via intermediates 4 or
5 depending upon the choice of oxidant, but tetramer 2 can only
form via intermediate 5. Barsilowsky’s base is believed to be
the trimer 11 and not the tetramer 2 although the two syntheses
are closely related and the product ratio is easily affected. Some
possible mechanistic pathways have been discussed.
Experimental
IR spectra were recorded on an ATI Mattson FTIR spectrometer using
potassium bromide discs. UV spectra were recorded using a Perkin‑
Elmer Lambda 25 UV‑VIS spectrometer with CH2Cl2 as the solvent.
1H and 13C NMR spectra were recorded at 400 MHz and 100.5 MHz
respectively using a Varian 400 spectrometer. Chemical shifts, δ are
given in ppm relative to the residual solvent and coupling constants, J
are given in Hz. Low resolution and high resolution mass spectra were
obtained at the University of Wales, Swansea using electron impact
ionisation and chemical ionisation. Melting points were determined on
a Kofler hot‑stage microscope. Solutions were evaporated to dryness
with gentle warming or in vacuo. Dichloromethane can bump when
heated in beakers.
Entry 2: 2-Amino-5-methyl-1,4-benzoquinone-1,4-(4-methyl
phenylimine) (1): p‑Toluidine hydrochloride (1.43 g, 0.01 moles) and
KMnO4 (2.95 g, 0.019 moles) in H2O (200 mL) were heated at 60 °C
for 45 min. After cooling the reaction was filtered, washed with H2O,
then extracted with MeOH (50 mL) followed by dichloromethane
(50 mL×4). The combined fractions were evaporated to dryness and
purified by chromatography on silica gel. Dichloromethane eluted
the title compound (114 mg, 11%) as an orange solid with identical
spectroscopic properties to previous material.
X-ray crystallographic characterisation of trimer 1 tetramer 2
The X‑ray intensity data were collected on
a Rigaku CCD
Entry 3: 2-(4-Methylphenylamino)-5-methyl-1,4-benzoquinone-1,4-
(4-methylphenylimine) (2) and 2-amino-5-methyl-1,4-benzoquinone-
1,4-(4-methylphenylimine) (1): p‑Toluidine (2.0 g, 0.0187 mol) in
dilute H2SO4 (1.0 mL of cH2SO4 in 200 mL H2O) was treated with
K2Cr2O7 (5.49 g, 0.0187 mol) and heated at 60–70 °C for 1.5 h. The
reaction was allowed to cool and a brown precipitate was filtered
off. This was extracted with MeOH (50 mL) and CH2Cl2 (50 mL×4)
and the combined extracts evaporated to dryness in a beaker. The
product was purified by chromatography on silica gel. Elution with
diffractometer (graphite monochromated MoKα radiation,
λ=0.71073 Å, T= 10 0 K). 21 The structures were solved by direct
methods with SHELXS‑9722 and the atomic models were refined
against |F|2 with SHELXL‑97.22 We modelled the structure of 1 in
space group R3 with half a molecule in the asymmetric unit, which
necessitates statistical disorder of the methyl (C4) and amine (N2)
groups attached to the central benzene ring. The recent report19 of
the structure of 1 modelled the space group as R3. This resolved the
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dichloromethane gave the title compound 2 (232 mg, 12%) as a red
solid, m.p. 201–202 °C (lit.11 175, 227, 244–245, 216–220, 220–225 °C)
from dichloromethane/light petroleum ether 40–60. λmax (CH2Cl2)/
nm 453 (log ε 3.7) and 301 (4.1); νmax (KBr disc) 3340vs, 3109–2819vs,
1636vs, 1507vs, 1500vs, 1346vs, 1306vs, 1282vs, 1254vs, 1168s,
1109s, 1017s, 936w, 875vs, 850vs, 822s, 810s, 671w, 585w and 560w;
δH(400 MHz; CDCl3) 2.24 (3H, s), 2.30 (3H, s), 2.36 (3H, s), 2.41 (3H,
s), 6.36 (1H, s), 6.68 (1H, d, J=1.6), 6.83 (2H, d, J=8.0), 6.86 (2H, d,
J=8.4), 7.00 (2H, d, J=8.4), 7.07 (2H, d, J=8.0), 7.16 (2H, d, J= 8.0),
7.23 (2H, d, J=8.0) and 7.76 (1H, s); δC(100.1 MHz; CDCl3) 18.5, 20.8,
20.9, 21.0, 94.3, 120.6, 120.7, 120.8, 120.9, 129.3, 129.7, 129.8, 132.8,
133.5, 134.6, 137.0, 144.8, 146.6, 153.6 and 159.4 (two resonances are
overlapping); m/z (orbitrap ASAP) 406.2270 (M+ +H, 100%) C28H28N3
requires 406.2278. Elution with Et2O/light petrol (25:75) gave title
compound 1 (32 mg, 2%) with identical spectroscopic properties to
previous material.
Entry 4: 2-(4-Methylphenylamino)-5-methyl-1,4-benzoquinone-1,4-
(4-methylphenylimine) (2) and 2-amino-5-methyl-1,4-benzoquinone-
1,4-(4-methylphenylimine) (1): p‑Toluidine (2.0 g, 0.0187 mol) in
dilute H2SO4 (1.0 mL of cH2SO4 in 200 mL H2O) was treated with
K2Cr2O7 (5.49 g, 0.0187 mol) at room temperature for 1.5 h. The
reaction was allowed to cool and a brown precipitate was filtered
off. This was extracted with MeOH (50 mL) and CH2Cl2 (50 mL×4)
and the combined extracts evaporated to dryness in a beaker. The
product was purified by chromatography on silica gel. Elution with
dichloromethane gave the title compound 2 (112 mg, 6%). Elution
with Et2O/light petrol (25:75) gave title compound 1 (20 mg, 1%) with
identical spectroscopic properties to previous material.
Entry 5: 2-(4-Methylphenylamino)-5-methyl-1,4-benzoquinone-1,4-
(4-methylphenylimine) (2) and 2-amino-5-methyl-1,4-benzoquinone-
1,4-(4-methylphenylimine) (1): p‑Toluidine (1.0 g, 0.00935 mol) in
dilute H2SO4 (0.5 mL of cH2SO4 in 200 mL H2O) was treated with
K2Cr2O7 (2.75 g, 0.0094 mol) and heated at 60–70 °C for 1.5 h. The
reaction was allowed to cool and a brown precipitate was filtered
off. This was extracted in the sinter with MeOH (50 mL) and CH2Cl2
(50 mL×4) and the combined extracts evaporated to dryness in a
beaker. The product was purified by chromatography on silica gel.
Elution with dichloromethane gave the title compound 2 (54 mg,
6%). Elution with Et2O/light petrol (25:75) gave the title compound 1
(136 mg, 14%). Both products had identical spectroscopic properties to
previous material.
Entry 6: 2-Amino-5-methyl-1,4-benzoquinone-1,4-(4-methyl
phenylimine) (1): p‑Toluidine hydrochloride (0.67 g, 0.0047 mol) and
K2Cr2O7 (1.37 g, 0.0047 mol) in H2O (200 mL) were heated at 60 °C
for 1.5 h. After cooling, the reaction was filtered, washed with H2O,
then extracted with MeOH (50 mL) followed by CH2Cl2 (50 mL×4).
The combined extracts were evaporated to dryness and purified
by chromatography on silica gel. Dichloromethane eluted the title
compound (78 mg, 16%) with identical spectroscopic properties to
previous material.
benzoquinone‑1,4‑(4‑methylphenylimine) 1 (10 mg, 0.032 mmol) and
p‑toluidine hydrochloride (20 mg, 0.14 mmol) in MeOH (30 mL) were
refluxed for 2 h. TLC analysis (25:75 Et2O/light petrol) showed some
baseline decomposition and only starting material.
Attempted synthesis of 2-(4-methylphenylamino)-5-methyl-1,4-
benzoquinone-1,4-(4-methylphenylimine) (2): 2‑Amino‑5‑methyl‑
1,4‑benzoquinone‑1,4‑(4‑methylphenylimine) 1 (10 mg, 0.032 mmol),
p‑toluidine hydrochloride (20 mg, 0.14 mmol) and K2Cr2O7 (41 mg,
0.14 mmol) were heated in H2O (20 mL) at 60 °C for 1 h. The reaction
was cooled, neutralised with KOH, then extracted with CH2Cl2. TLC
analysis (25:75 Et2O/light petrol) showed only the starting material to
be present.
Attempted synthesis of 2-amino-5-methyl-1,4-benzoquinone-4-(4-
methyl)phenylimine (3): 2‑(4‑Methylphenylamino)‑5‑methyl‑1,4‑
benzoquinone‑1,4‑(4‑methylphenylimine) 2 (30 mg, 0.074 mmol) in
dilute sulfuric acid (2 mL of cH2SO4 in 20 mL of H2O) was treated with
K2Cr2O7 (50 mg, 0.17 mmol) and heated at 60 °C for 1 h. The reaction
was cooled, neutralised with KOH, then extracted with CH2Cl2. TLC
analysis (25:75 Et2O/light petrol) showed no starting material and
none of the dimer 3 to be present.
Attempted oxidation of p-toluidine and ditolylamine 6 with K2Cr2O7:
p‑Toluidine (214 mg, 2.0 mmol) and ditolylamine 6 (200 mg, 1.0 mmol)
were heated in water (100 mL) at 60 °C acidified with cH2SO4 (0.1 mL,
2 drops) and treated with K2Cr2O7 (298 mg, 1.0 mmol). After 1.5 h the
mixture was cooled, filtered and extracted with MeOH (50 mL) then
CH2Cl2 (50 mL×4). The products were purified by chromatography
on silica gel. Dichloromethane eluted the recovered starting material
6 (134 mg, 67%) then Et2O/light petrol (25:75) eluted the tetramer
2 (28 mg, 14%) with identical spectroscopic properties to previous
material.
We are grateful to the UK EPSRC national mass spectrometry
service centre for mass spectra.
Received 23 February 2014; accepted 3 April 2014
Paper 1402486 doi: 10.3184/174751914X13995700264041
Published online: 10 June 2014
References
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2-Amino-5-methyl-1,4-benzoquinone-4-(4-methyl)phenylimine (3):
2‑Amino‑5‑methyl‑1,4‑benzoquinone‑1,4‑(4‑methylphenylimine)
1 (50 mg, 0.16 mmol) in dilute sulfuric acid (2 mL of cH2SO4 in
20 mL H2O) was heated at 60 °C for 30 min. The reaction was cooled,
neutralised with KOH, then extracted with CH2Cl2. The product was
purified by chromatography on silica gel. Elution with Et2O/light
petrol (25:75) gave the title compound as an orange/red solid with
identical spectroscopic properties to material prepared previously by
the K2Cr2O7 oxidation of p‑toluidine at a similar pH.15
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Attempted synthesis of 2-(4-methylphenylamino)-5-methyl-1,4-
benzoquinone-1,4-(4-methylphenylimine) (2): 2‑Amino‑5‑methyl‑1,4‑
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