Carbazolyl nitrenium cations: Generation and reactions

Article abstract of DOI:10.3987/COM-00-9027

Carbazolyl nitrenium cations were generated during the thermolysis and photolysis of 1-carbazol-9-yl-2,4,6-triphenylpyridinium (Py⁺-Cz) tetrafluoroborate in 2,2,2-trifluoroethanol- and TFA - mesitylene mixtures. The thermolysis of Py⁺-Cz in TFA-mesitylene mixture gave 9-mesitylcarbazole in good yield along with carbazole, non-symmetrical mesitylene dimer, 2,4,6-triphenylpyridine, and a small amount of 9-(3,5-dimethylbenzyl)carbazole, whereas the thermolysis in trifluoroethanol - mesitylene mixture gave 9-mesitylcarbazole as the main product along with carbazole and 2,4,6-triphenylpyridine. The product distributions lead to two major conclusions. First, photolysis and thermolysis of Py⁺-Cz salt produce nitrenium ions that can be in singlet or triplet state dependant on the reaction conditions. Second, the carbazolyl nitrenium ion exhibits singlet state in the ground state.

Full text of DOI:10.3987/COM-00-9027

HETEROCYCLES, Vol. 53, No12, 2000, pp. 2679 - 2688, Received, 1st August, 2000
CARBAZOLYL
REACTIONS
NITRENIUM
CATIONS:
GENERATION
AND
Dariusz Bogdal
Department of Chemistry, Politechnika Krakowska ul. Warszawska 24, 31-155
Krakow, Poland; fax: +48 12 628-20-38; e-mail: pcbogdal@cyf-kr.edu.pl
Abstract - Carbazolyl nitrenium cations were generated during the thermolysis and
photolysis of 1-carbazol-9-yl-2,4,6-triphenylpyridinium (Py+-Cz) tetrafluoroborate
in 2,2,2-trifluoroethanol- and TFA - mesitylene mixtures. The thermolysis of
Py+-Cz in TFA-mesitylene mixture gave 9-mesitylcarbazole in good yield along
with carbazole, non-symmetrical mesitylene dimer, 2,4,6-triphenylpyridine, and
a small amount of 9-(3,5-dimethylbenzyl)carbazole, whereas the thermolysis in
trifluoroethanol - mesitylene mixture gave 9-mesitylcarbazole as the main product
along with carbazole and 2,4,6-triphenylpyridine. The product distributions lead to
two major conclusions. First, photolysis and thermolysis of Py+-Cz salt produce
nitrenium ions that can be in singlet or triplet state dependant on the reaction
conditions. Second, the carbazolyl nitrenium ion exhibits singlet state in the ground
state.
INTRODUCTION
Nitrenium ions (R2N+) are reactive intermediates which contain a divalent, positively charged nitrogen
atom and can possess either the singlet (S=0) (I) or triplet (S=1) (II) as the lowest energy configuration.1
At the moment of reaction, the singlet or triplet nature of the nitrenium ions determines the type of
products which will be formed; moreover, they may react in either the ground state or exited state. The
singlet is capable of rearrangement,2 addition to alkenes,3 C-H insertion and electrophilic aromatic
substitution4,5 while the triplet usually abstracts hydrogen to form the parent amine.6
+
N
+
+
N
N
I
III
II
The parent nitrenium (H2N+) and simple alkyl derivatives (R2N+) are predicted to have a triplet ground
state7,8, and a photoionization study8 shows the lowest singlet to be 3.0 kcal/mol above the triplet ground
state of H2N+. The triplet-singlet energy gap is calculated to decrease with increasing substitution and

indeed the nitrenium ion (III) is predicted to have a singlet ground state.7,9 Further distortion of the C-N-C
angles would stabilise the singlets.10
Arylnitrenium ions are expected to be more stable than alkyl substituted ions owing to the possible
delocalization of the positive charge onto the ring. In general, benzene rings act as electron donors and
stabilise the singlet relative to the triplet state.9 The available ab initio calculation on phenyl nitrenium
predicts the singlet is favoured by ca. 10 kcal/mol.11 More recent semi-empirical calculations12 show that
nitrenium ions with aromatic substituents are ground-state singlets with substantial energy gaps. However,
electron-withdrawing groups on the phenyl ring and sterically demanding substituents are both predicted
to stabilize the triplet relative to the singlet state.12
In contrast to the other nitrenium ions, there is virtually nothing known about nitrenium ions derived from
five-membered heteroaromatic nitrogen compounds e.g., pyrrole, indole, carbazole, imidazole, and
triazoles. A great deal has been published on studies, both experimental and computational, of
corresponding radicals and anions.13 On the other hand, the only two papers about a possible generation
and reactions of 4-(1,2,4-triazolyl) cation in the thermolysis of the 1-(1,2,4-triazol-4-yl)-2,4,6-trimethyl-
pyridinium salt reported the formation of a variety of products, but none derived from attack of the free
nitrenium ion. According to the authors, a possible explanation is a SET from trimethylpyridine to the
nitrenium ion within the solvent cage.14,15
In the present article there are reported the preliminary studies on the generation and reactions of the
nitrenium ions derived from carbazole. The experiments reported herein were undertaken with the aim of
further characterising the generation of carbazolyl nitrenium ions. Of special interest is the behaviour of
carbazolyl nitrenium ions and whether they exhibit the triple or singlet state in the ground state.
RESULTS AND DISCUSSION
The reaction of 9-aminocarbazole and 2,4,6-triphenylpyrylium tetrafluoroborate under the standard
conditions produced 1-carbazol-9-yl-2,4,6-triphenylpyridinium tetrafluoroborate (Py+-Cz). It was expected
that the steric effect upon the phenyl groups at C-2 and C-6 of the pyridine ring and its elimination by N-N
cleavage would provide an appropriate driving force for the desired generation of carbazolyl nitrenium ion.
A degassed solution of the pyridinium salt in a TFA - mesitylene or alternatively 2,2,2-trifluoroethanol -
mesitylene mixture was heated or irradiated under the conditions described in Table 1, generating at least 5

compounds. After the work-up procedure, the products were isolated by preparative TLC chroma-
tography, while yields were determined by gas chromatography.
Table 1: Thermolysis and Photolysis of 1-carbazol-9-yl-2,4,6-triphenylpyridinium tetrafluoroborate
(Py+-Cz) in TFA-mesitylene and 2,2,2-trifluoroethanol (TFE) -mesitylene solutions.
Py+-Cz
1
[%]
3
2
[%]
22
26
3
3
4
[%]
45
5
TFA
[mL]
1.5
3.0
-
Mesitylene
TFE
[mL]
-
[mg]
[mL]
[%]
71
64
82
74
58
62
67
[%]
100 a)
140 a)
100 a)
140 a)
100 b)
140 b)
100 b)
6.0
1
2
-
7.0
-
5
40
6.0
1.5
3.0
-
40
37
47
52
63
35
-
6.0
6
39
-
2.0
3.0
-
7.0
19
16
6
traces
traces
3
-
7.0
-
-
7.0
2.0
-
benzene [mL]
100 b)
140 b)
100 b)
140 b)
1.5
3.0
-
7.0
7.0
7.0
7.0
-
54
62
58
65
-
-
-
-
70
75
70
64
-
-
-
-
-
-
-
-
-
3.0
4.5
-
a) - thermolysis reaction; b) - photolysis reaction
The thermolysis in a TFA - mesitylene mixture at 162oC gave 9-mesitylcarbazole (4) in good yield along
with carbazole (1), non-symmetrical mesitylene dimer (2), 2,4,6-triphenylpyridine (3), and 9-(3,5dimethyl-
benzyl)carbazole (5), whereas the thermolysis in a trifluoroethanol-mesitylene mixture gave 9-mesityl-
carbazole (4) as the main substitution product along with carbazole (1) and 2,4,6-triphenylpyridine (3);
non-symmetrical mesitylene dimer (2) was detected in low yield.
The photolysis of the pyridinium salt in a TFA - mesitylene mixture gave 2,4,6-triphenylpyridine (3) and
carbazole (1) in high yield along with the non-symmetrical mesitylene dimer (2); traces of 9-mesityl-
carbazole (4) were detected. The photolysis of the pyridinium salt in a 2,2,2-trifluoroethanol - mesitylene
mixture gave carbazole (1) as the main product along with the triphenylpyridine (3). Small amount of 9-
mesitylcarbazole (4) was formed; the non-symmetrical mesitylene dimer (2) was detected in very low yield.
The results are indicated in Table 1.

The formation of triphenylpyridine (3), to the exclusion of the salt from a radical cation of 2,4,6-tri-
phenylpyridine, argues against initial homolysis of the pyridinium salt;16 if the homolysis had occurred, a
salt from radical cation of triphenylpyridine would be formed without contamination by triphenylpyridine.
Moreover, the absence of 3,3',5,5'-tetramethyldiphenylethane makes a radical process unlikely.17,18
In contrast to the SN2'reactions of 1-diphenylamino- and 1-carbazol-9-ylpyridinium cations with pyridine at
120oC, in which the nucleophile was capable of attacking only the para position of phenyl groups,19 in the
present case the mesitylene derived products, 9-mesitylcarbazole (4) and 9-(3,5-dimethylbenzyl)carbazole
(5), arise here from the attack on mesitylene by an electrophilic intermediate, while the formation of the
non-symmetrical mesitylene dimer (2) may be explained by invoking a mesitylene cation or radical cation
intermediates.
As it is proposed in the reaction scheme (Figure 1), direct photolysis of 1-carbazol-9-yltriphenylpyridinium
tetrafluoroborate forms the excited singlet of the pyridinium salt. The latter partitions between heterolytic
decomposition of the pyridinium salt to give singlet nitrenium carbazolyl ions and intersystem crossing to
give triplet pyridinium salt. The triplet undergoes heterolytic decomposition, giving triplet nitrenium
carbazolyl ions. The difference in the product of photolysis and thermolysis may be attributed to the
difference in the mechanism of reaction of the singlet and triplet nitrenium ions.
The singlet nitrenium ion either reacts with mesitylene to form the products like 9-mesitylcarbazole (4) and
9-(3,5-dimethylbenzyl)carbazole (5) or the mesitylene dimer by SET mechanism from the nitrenium ion.
The formation of the non-symmetrical mesitylene dimer (2) may be explained by invoking a mesitylene
radical cation intermediate.16 The compound, 9-(3,5-dimethylbenzyl)carbazole (5), is the result of C-H
insertion process of the singlet carbazolyl nitrenium ion to the methyl group in mesitylene. In order to
prove the structure of the compound (5) as well as to compare it against the structure of the compound
(4), it was prepared by alternative method (e.g., the direct reaction of α-chloromesitylene with carbazole
under PTC conditions).
On the other hand, the triplet nitrenium ions either reacts by H atom transfer to form the parent amine,
carbazole (1), or suffer intersystem crossing to singlet nitrenium ion. In both cases (e.g., photolysis in the
presence of TFA or trifluoroethanol) 9-mesitylcarbazole (4) was formed in low yield, what indicates that
intersystem crossing of triplet carbazolyl nitrenium ion to the singlet ground state is slower than H atom-
transfer and formation of the parent amine, carbazole (1).
Both the singlet and triplet channels are possible in direct irradiation experiments, whereas only the singlet
channel is operative in all the thermal experiments, and both the singlet and triplet products can be
expected in photolytic reactions. Therefore, the simplest and most probable explanation is that the singlet

is the ground state for the carbazolyl nitrenium cation. The relative yields of the products depend on the
rate of intersystem crossing relative to heterolytic decomposition. The singlet ground state would agree
with the analysis of our experiments as well as experiments and quantum-mechanical calculations on
structurally similar arylnitrenium ions,20,21
Me
N
N
H
N
Me
Me
Me
Me
CH2+
Me
Me
+
electrophilic
substitution
(1)
(5)
Me
Me
(4)
Me
mesitylene
CH
insertion
Me
Me
hydride
abstraction
electrophilic
substitution
(2)
mesitylene
+
+
N
heat
S0
Py+
Cz
Ph
Py+
Cz
UV
+
+
N
Ph
Ph
N
S1
S1
S0
(3)
ISC
ISC
Ph
N
Py+
Cz
+
+
N
Ph
Ph
T1
T1
(3)
mesitylene
hydrogen
abstraction
Me
CH2+
Me
mesitylene
+
Me
Me
N
H
Me
Me
Me
(2)
(1)
+
: 1-carbazol-9-yl-2,4,6-triphenylpyridinium tetrafluoroborate
Py -Cz
Figure 1: Thermolysis and photolysis of 1-carbazol-9-yl-2,4,6-triphenylpyridinium tetrafluoroborate.

Thermal reactions are expected to initially generate the singlet nitrenium ion according to the view of
mechanistic difference between thermolysis and photolysis. The yield of 9-mesitylcarbazole (4) in the
thermolysis in the presence of TFA was higher than in the presence of trifluoroethanol. Possible reason for
the lack of the formation of non-symmetrical mesitylene dimer (2) during the reaction in a trifluoroethanol
- mesitylene mixture is that the hydride transfer is from trifluoroethanol rather faster than from mesitylene.
For the photolysis reaction, the yield of parent amine, carbazole (1), was higher in the presence of
trifluoroethanol and it is likely that in the thermolysis experiments the yield of non-symmetrical mesitylene
dimer (2) was much lower than for the reactions in a TFA - mesitylene mixtures
The photolysis of the pyridinium salt in benzene solutions produced only carbazole (4), and
triphenylpyridine (3). Due to lower electron density on benzene than mesitylene ring, none of products
arised from the attack of nitrenium ions on benzene was detected (Table 1).
CONCLUSIONS
Two major conclusions can derived from the present work. First, it is provided that photolysis and
thermolysis of the 1-carbazol-9-ylpyridium salt produced nitrenium ions which can be in singlet or triplet
state. The singlet carbazolyl nitrenium ion is produced by thermolysis of the pyridium salt, while the triplet
state is generated by direct photolysis of the salt. The difference in the product distributions of photolysis
and thermolysis reactions may be ascribe to different natures of the singlet and triplet nitrenium ions.
Second, the carbazolyl nitrenium ion exhibits singlet state in the ground state. Since the state of nitrenium
ion controls its chemical reactivity, before quantitative spectroscopic and kinetic experiments
(measurements) on nitrenium carbazolyl ions can be interpreted, the mechanism of photo- and thermal
generation as well as subsequent chemical reactions must be well understood.
EXPERIMENTAL
IR spectra were recorded using a Bio-Rad FTS 165 spectrophotometer. 1H-NMR spectra were taken with
a Tesla 487 C instrument, TMS being used as an internal standard; the chemical shifts are expressed in
δ-values downfield from TMS. MS spectra were recorded with a Hewlett-Packard GC/MS 5985
spectrometer. Elemental analyses were performed on a Perkin-Elmer 2400 microanalyser. Melting points

measured on a Boetius-PHMK 05 microscope plates are uncorrected. Preparative TLC was performed
with Aluminiumoxid 60F 254 Typ E (5713) plates (Merck).
Dichloromethane, ethyl acetate, cyclohexane, acetone, toluene, and benzene were purified by standard
methods before use. Mesitylene, trifluoroacetic acid and 2,2,2-trifluoroethanol were purified by distillation.
The following compounds were reagent grade and used without further purification: carbazole (Fluka),
2,4,6-triphenylpyrylium tetrafluoroborate (Aldrich).
9-Nitrosocarbazole: A mixture of glacial acetic acid (100 mL), THF (100 mL), and carbazole (10 g,
60 mmol) was stirred at rt while sodium nitrite (8 g, 120 mmol) was added in small portions to the solution
within 1 h. Stirring was continued for additional 3 h; the reaction flask was put a couple of times into hot
water-bath to accelerate the reaction. Then THF was evaporated and water (100 mL) was added to
precipitate a solid material. The recrystalisation of the solid from ethyl ether gave golden-yellow needles of
9-nitrosocarbazole (10 g, 85%). mp 80-82oC (lit.,22 80-82oC). Anal. Calcd for C12H8N2O: C,
73.54; H, 4.14; N, 14.30. Found: C, 73.23; H, 3.98; N, 14.07.
9-Aminocarbazole: A mixture of glacial acetic acid (100 mL), THF (100 mL), and 9-nitrosocarbazole
(5.0 g, 26 mmol) was stirred at rt while zinc dust (9 g, 138 mmol) was added in two portions to the
solution within 30 min. Stirring was continued for additional 3 h. Then solid material was filtrated off and
the solution was neutralised with 20% aqueous solution of potassium hydroxide. After evaporation of
THF, the solid precipitated from the solution was filtered off and recrystalised from isooctane to afford
4.0 g (85%) of 9-aminocarbazole as long white needles. mp 147-9oC (lit.,22 151-152.5oC). Anal. Calcd for
C12H10N2: C, 79.14; H, 5.52; N, 15.44. Found: C, 79.39; H, 5.40; N, 15.32.
1-Carbazol-9-yl-2,4,6-triphenylpyridinium tetrafluoroborate (Py-Cz+): A solution of 9-amino-
carbazole (0.42 g, 2.5 mmol) and 2,4,6-triphenylpyrylium tetrafluoroborate (0.92 g, 2.5 mmol) in ethanol
(30 mL) was heated under reflux for 4 h. The product crystalized as yellow needles (1.2 g) while the
solution was cooling down. The recrystalisation from ethanol gave 1-carbazol-9-yl-2,4,6-triphenyl-
pyridinium tetrafluoroborate as yellow plates (0.97 g, 70%). mp 204-206oC (lit.,19 210-212oC). Anal.
Calcd for C35H25N2BF4: C, 75.03; H, 4.51; N, 5.02. Found: C, 75.30; H, 4.67; N, 4.71.

Thermolysis of 1-carbazol-9-yl-2,4,6-triphenylpyridinium tetrafluoroborate (Py-Cz+): Thermolysis of
1-carbazol-9-yl-2,4,6-triphenylpyridinium tetrafluoroborate was carried out in a sealed tube at 162oC.
Detail conditions are given in Table 1. After the reaction, 10% solution of potassium carbonate was added
to the mixture till the pH of the solution exceeded 7. The organic layer was extracted with ethyl acetate.
After that the products were isolated by preparative TLC while yields were determined by GC/MS
analysis. Carbazole and 2,4,6-triphenylpyridine were identified by comparison with original samples,
whereas the structures of following compounds were confirmed by spectroscopic methods.
1
9-Mesitylcarbazole (4): mp 152-154oC; H-NMR (CDCl3): 1.81 (s, 6H, 2x(-CH3)), 2.41(s, 3H, -CH3),
6.89-7.40 (m, 6H, aromatic rings), 8.11-8.22 ppm (m, 2H, aromatic rings of carbazole); IR (KBr):
3050(w), 3016w), 2918(m), 2852(m), 1626(m), 1594(m), 1495(s), 1476(s), 1451(s), 1314(s), 1234(s),
751(s), 721(s) cm-1. MS (m/z) 286 (22)(M+1) 285 (100)(M+). Anal. Calcd for C21H19N: C, 88.40; H, 6.74;
N, 4.91. Found: C, 88.32; H, 6.99; N, 4.60.
2-(3,5-Dimethylbenzyl)-1,3,5-trimethylbenzene (non-symmetrical mesitylene dimer) (2): mp 69-71oC
1
(lit.,23 67-68oC). H-NMR (CDCl3): 2.20 (s, 6H, 2x(-CH3), 3',5'-), 2.23 (s, 6H, 2x(-CH3), 2,6-), 2.29 (S,
3H, -CH3, 4-), 3.93 (s, 2H, -CH2-), 6.63 (s, 2H, ortho-aromatic ring), 6.73 ppm (s, 1H, para-aromatic
ring), 6.88 (s, 2H, meta-aromatic ring); IR (KBr): 3020(m), 2975(s), 2915(m), 2880(m), 1609(m),
1505(s), 1458(s), 1385(m), 1020(m), 880(s), 820(s), 815(m) cm-1. MS (m/z): 239(17) (M+1), 238(86)
(M+), 223(57) (M-(CH3)), 208(12) (M-2x(CH3)), 193(14) (M-3x(CH3)), 132(65) (mesitylyl-CH+),119(26)
(mesitylyl+), 91(11) (phenyl-CH+). The structure of the compounds was confirmed by the comparison
against the physical and spectra data for the symmetrical mesitylene dimer (2,4,6,2',4',6'-hexamethyl-
biphenyl).17
Photolysis of 1-carbazol-9-yl-2,4,6-triphenylpyridinium tetrafluoroborate (Py-Cz+): Photolysis of
1-carbazol-9-yl-2,4,6-triphenylpyridinium tetrafluoroborate was conducted externally in a quartz tube
(25 x 2 cm) with a high pressure mercury lamp (400 W) after purging with dry argon for 15 min. During
the reaction the solution was stirred with a small magnetic bar. Details conditions are given in Table 1.
Analysis and characterisation of the products were performed as for the thermal reactions.
9-(3,5-Dimethylbenzyl)carbazole (5): A solution of benzoyl peroxide (0.12 g, 0.50 mmol), freshly
distilled sulfuryl chloride (5 mL, 50 mmol), and mesitylene (35 mL, 250 mmol) was stirred at 100oC for

1 h. After cooling down traces of sulfuryl chloride were evaporated under reduced pressure to afford
1.4 M solution (35 mL) of α-chloromesitylene in mesitylene. The concentration of the solution was
determined by means of gas chromatography. In the next step, the solution of the chloromesitylene (7 mL,
10 mmol) was added dropwise within 1 h to a solution of carbazole (1.67 g, 10 mmol) in a mixture of
water (10 mL), NaOH (10 g), heptane (40 mL), and tetrabutylammonium chloride (0.2 g) at 90oC. The
reaction was continued for additional 2 h while all the time the mixture was vigorously stirred. Then
aqueous phase was separated and washed with methylene chloride (2 x 25 mL). The extract was combined
with the organic phase and solvents were evaporated under reduced pressure to give a heavy oil.
Recrystallisation from heptane affords 9-(2,4-dimethylbenzyl)carbazole as long white needles (2.45 g,
1
86%); mp 119-121oC. H-NMR (CDCl3): (s, 6H, 2x(-CH3)), 5.41 (s, 2H, -CH2-), 6.77 (s, 2H, ortho-
aromatic ring), 6.84 (s, 1H, para-aromatic ring), 7.12-7.45 (m, 6H, aromatic rings of carbazole), 8.06-8.14
ppm (m, 2H, aromatic rings of carbazole). IR (KBr): 3050(m), 3019(m), 2915((m), 2857(m), 1627(m),
1595(m), 1484(s), 1459(s), 1324(s), 1211(s), 750(s), 722(s) cm-1; MS (m/z): 286(20) (M+1), 285(70)
(M+), 119(100) (mesitylyl+). Anal. Calcd for C21H19N: C, 88.40; H, 6.72; N, 4.94. Found: C, 88.45; H,
6.84; N, 4.70.
ACKNOWLEDGEMENTS
Professor R. A. Abramovitch is greatfully acknowledged for his helpful discussion before the preparation
of the manuscript. The work was supported by the Polish State Committee for Scientific Affairs (Grant
3 P03 005 07).
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