Synthesis and electronic spectroscopy of bromocarbazoles. Direct bromination of N- and C-substituted carbazoles by N-bromosuccinimide or a N-bromosuccinimide/silica gel system

Article abstract of DOI:10.1002/hlca.200690110

The preparation, isolation and characterization by elemental analysis and ¹H-NMR, ¹³C-NMR, and MS data of the bromo derivatives of N-substituted carbazoles, i.e., of 9-methyl-9H-carbazole (1), 9-phenyl-9H-carbazole (2), 9-benzyl-9H-carbazole (3), 2-methoxy-9-methyl-9H- carbazole (4), and of C-substituted carbazoles, i.e., of 2-(acetyloxy)-9H- carbazole (5) and 3-nitro-9H-carbazole (6), are reported, in part for the first time. As brominating reagents, N-bromosuccinimide (NBS) or NBS/silica gel in CH₂Cl₂, NBS in AcOH, KBrO₃/KBr in EtOH doped with a catalytic amount of H₂SO₄, or KBrO₃/KBr in AcOH were employed, and their uses were compared. Semi-empirical PM3 calculations were performed to predict the reactivity of the N-substituted and C-substituted carbazoles and of their bromo derivatives and found to verify the experimental results. The UV-absorption and fluorescence and phosphorescence emission spectra of the bromocarbazole derivatives in MeCN solution at 298 K and in a solid matrix at 77 K were compared with those of the corresponding carbazoles 1-6. The dynamic properties of the lowest excited singlet and triplet states (τ_f, τ_p, φ_f, and φ_p) were measured under the same experimental conditions. The intramolecular spin-orbital-coupling effect of the Br-atom and NO₂ group on the spectroscopic data, photophysical parameters, and on the photo reactivity were also briefly analyzed.

Full text of DOI:10.1002/hlca.200690110

Helvetica Chimica Acta – Vol. 89 (2006)
1123
Synthesis and Electronic Spectroscopy of Bromocarbazoles. Direct
Bromination of N- and C-Substituted Carbazoles by N-Bromosuccinimide or a
N-Bromosuccinimide/Silica Gel System
by María B. Ponce, Franco M. Cabrerizo, Sergio M. Bonesi* and Rosa Erra-Balsells
CIHIDECAR-CONICET, Departamento de Química Orgánica, Facultad de Ciencias Exactas y
Naturales, Universidad de Buenos Aires, Pabellón II, 3er Piso, Ciudad Universitaria, 1428 – Buenos Aires,
Argentina
(phone: +54-11-4576-3346; fax: +54-11-4576-3346; e-mail: smbonesi@qo.fcen.uba.ar)
The preparation, isolation and characterization by elemental analysis and 1H-NMR, 13C-NMR, and
MS data of the bromo derivatives of N-substituted carbazoles, i.e., of 9-methyl-9H-carbazole (1), 9-phen-
yl-9H-carbazole (2), 9-benzyl-9H-carbazole (3), 2-methoxy-9-methyl-9H-carbazole (4), and of C-substi-
tuted carbazoles, i.e., of 2-(acetyloxy)-9H-carbazole (5) and 3-nitro-9H-carbazole (6), are reported, in
part for the first time. As brominating reagents, N-bromosuccinimide (NBS) or NBS/silica gel in
CH2Cl2, NBS in AcOH, KBrO3/KBr in EtOH doped with a catalytic amount of H2SO4, or KBrO3/KBr
in AcOH were employed, and their uses were compared. Semi-empirical PM3 calculations were per-
formed to predict the reactivity of the N-substituted and C-substituted carbazoles and of their bromo
derivatives and found to verify the experimental results. The UV-absorption and fluorescence and phos-
phorescence emission spectra of the bromocarbazole derivatives in MeCN solution at 298 K and in a solid
matrix at 77 K were compared with those of the corresponding carbazoles 1–6. The dynamic properties
of the lowest excited singlet and triplet states (tf, tp, ff, and fp) were measured under the same experi-
mental conditions. The intramolecular spin–orbital-coupling effect of the Br-atom and NO2 group on the
spectroscopic data, photophysical parameters, and on the photo reactivity were also briefly analyzed.
Introduction. – As part of our ongoing program on the photochemistry of azacarba-
zoles [1–6], carbazoles [7–8], N-acyl- and N-alkylcarbazoles [9–11], and nitrocarba-
zoles [12] and especially on the photoinduced electron-transfer process in which they
can be involved [12b], we decided to go on with the study of the photophysical proper-
ties of bromo derivatives of several N- and C-substituted carbazoles. Furthermore, we
are interested in the photoinduced heterolytic cleavage of the CꢀBr bond in polycyclic
and heterocyclic bromo-substituted aromatic compounds. In these cases, the aryl (or
heteroaryl) cation formed from the triplet excited state can react with different nucle-
ophiles, such as olefines, arenes, or nucleophilic solvents forming new CꢀC or Cꢀhet-
eroatom bonds in a clean and one-pot photoreaction step [13]. Thus, to begin with, we
now report the synthesis, characterization, and electronic spectra of several N- and C-
substituted bromocarbazoles prepared by direct bromination of 9-methyl-9H-carbazole
(1), 9-phenyl-9H-carbazole (2), and for the first time, of 9-benzyl-9H-carbazole (3), 2-
methoxy-9-methyl-9H-carbazole (4), 2-(acetyloxy)-9H-carbazole (5), and 3-nitro-9H-
carbazole (6) (see 3a,b, 4a–c, 5a–c, and 6a in Scheme 1).
Traditionally, the direct bromination reaction of arenes and heteroarenes, were car-
ried out with Br2 in the presence of a catalyst, most often iron. Ferric chloride and other
© 2006 Verlag Helvetica Chimica Acta AG, Zürich

1124
Helvetica Chimica Acta – Vol. 89 (2006)
Scheme 1
Lewis acids are often directly used as catalysts, as is I2 [14]. Bromination of the aromatic
moiety can be achieved in the presence of thallium(III) acetate which induces a high
regioselectivity para to an ortho–para-directing group [15]. For activated aromatic
compounds, including amines, phenols, naphthalene, and polyalkylbenzenes, no cata-
lyst is needed. Indeed, the bromination is so rapid that it is carried out with a dilute sol-
ution of Br2 in H2O at room temperature. But even under these experimental condi-
tions, dibromination and tribromination take place when the aromatic moiety is acti-
vated for electrophilic aromatic substitution. Thus, the chemical yield of monobromo
derivatives is considerably diminished. A mixture of KBr and KBrO3 in EtOH in the
presence of catalytic amounts of sulfuric acid has been used as a direct brominating
agent, but regioselectivity is difficult to improve [16]. Other brominating methods
have been used, among them HOBr and N-bromoamides, such as N-bromosuccinimide
and tetraalkylammonium tribromide [17]. In general, these reactions are acid-cata-
lyzed. Dibromoisocyanuric acid in the presence of sulfuric acid is a very good brominat-
ing agent for substrates with strongly deactivating substituents [18]. Also, a mixture of
KBr and KNO3 in AcOH at 808 is used as a brominating agent [19]. Nevertheless,
bromo and nitro derivatives are obtained in the reaction mixture, and when the temper-
ature is raised, only the nitration reaction takes place. As a brominating agent, also HBr
in dimethylsulfoxide has been employed, but fails as a general brominating agent
because the oxidation power of the reaction mixture is important, and alkyl or alkenyl
substituent groups present in the aromatic compound are easily oxidized [20]. These
brominating methods are less commonly used because aromatic compounds with
acid-sensitive functions or oxidizeable substituent groups such as esters, amides, acyl-
ACHTREUNG

Helvetica Chimica Acta – Vol. 89 (2006)
1125
The synthesis of 3-bromo-9-methyl-9H-carbazole and of 3,6-dibromo-9-methyl-9H-
carbazole has been described a long time ago [16]. This previously described bromina-
tion reaction of 9-methyl-9H-carbazole (1) with Br2 in AcOH at room temperature pro-
vided 3,6-dibromo-9-methyl-9H-carbazole (1d), after recrystallization from AcOH.
When KBrO3/KBr in AcOH was used as the brominating agent, a mixture of 3-
bromo- (1b) and 3,6-dibromo-9-methyl-9H-carbazole (1d) was obtained. These com-
1
pounds were characterized by their melting point and H-NMR and IR spectra but
no UV-absorption and emission data were published [21]. Although Ambrose et al.
[21] pointed out that the purity and the structure of obtained 1d was confirmed by
1H-NMR spectroscopy, no spectroscopic data were reported. The 3,6-dibromo-9-phen-
yl-9H-carbazole (2b) was prepared from 9-phenyl-9H-carbazole (2) with 2 equiv. of Br2
in AcOH at room temperature and characterized only by its melting point and elemen-
tal analysis [21].
To the best of our knowledge, the direct bromination of 9-benzyl-9H-carbazole (3),
2-methoxy-9-methyl-9H-carbazole (4), 2-(acetyloxy)-9H-carbazole (5), and 3-nitro-
9H-carbazole (6) has not been described. Furthermore, several bromo derivatives of
these carbazoles and some of carbazoles 1 and 2 are not known. Recently, we have
described N-chlorosuccinimide [22] and N-iodosuccinimide [23] in CH2Cl2 or CHCl3
as successful, convenient, and mild chlorinating and iodinating agents providing
mono-, di-, and polychloro and -iodo compounds, respectively, depending on the stoi-
chiometry used. Smith et al. [24] have shown that N-bromosuccinimide (NBS) in CHCl3
is a versatile brominating agent for heterocyclic compounds, and in the case of carba-
zole, mono-, di-, tri-, or tetrabromo derivatives were obtained depending on the stoichi-
ometry used.
In the present paper, we describe the preparation, isolation, and characterization of
several bromocarbazoles obtained from the N-substituted and C-substituted carbazoles
1–6 (Scheme 1) by direct bromination with two types of brominating agents, i.e.,
KBrO3/KBr in the presence of the acid catalysts sulfuric acid or AcOH, and NBS in
CH2Cl2 in the presence or absence of silica gel and in AcOH. These brominating sys-
tems are mild reagents for the selective preparation of N-substituted and C-substituted
mono- and dibromocarbazoles, providing for the first time the bromocarbazoles 3a,b,
4a–c, 5a–c, and 6a (see Scheme 1). Additionally, atomic-charge densities were calcu-
lated and used to discuss the reactivity of carbazoles 1–6. Finally, the UV-absorption
and fluorescence and phosphorescence emission data of the bromocarbazole deriva-
tives were compared with those of the corresponding carbazoles 1–6, and the dynamic
properties of the lowest excited singlet and triplet states (tf, tp, ff, and fp) were meas-
ured. The intramolecular heavy-atom effects of the Br-atom and the NO2 group on the
spectroscopic and photophysical properties are discussed briefly.
Results and Discussion. – Bromination of Carbazoles. To investigate the bromina-
tion of the carbazoles 1–6, we proceeded systematically and comparatively using differ-
ent brominating reagents. Thus, five different methods (i)–(v) were employed: (i) NBS
in CH2Cl2, (ii) NBS in CH2Cl2 in the presence of SiO2, (iii) NBS in AcOH, (iv) KBrO3/
KBr in MeOH in the presence of catalytic amounts of conc. sulfuric acid and (v)
KBrO3/KBr in AcOH as solvent and acid catalyst. These brominations were carried
out at room temperature to get the bromocarbazole derivatives in high yield.

1126
Helvetica Chimica Acta – Vol. 89 (2006)
Thus, 9-methyl-9H-carbazole (1) was treated according to the general procedure
described in Exper. Part with 1–2 equiv. of NBS or NBS/SiO2 in CH2Cl2 or NBS in
AcOH. The yields of the bromocarbazole derivatives 1a–d were determined by GC
after an appropriate time, and the results are presented in Table 1.
Table 1. Bromination of 9-Methyl-9H-carbazole (1) by Different Methods
Entry Methoda) Carbazole 1/BAb) [molar ratio] Conversion [%] Products [% yield]c)
1a
1b
1c
1d
1
2
3
4
5
6
7
8
9
(i)
1:1
1:1
1:1
1:1
1:1
1:2
1:2
1:2
1:2
1:2
92.1
94.7
100
100
100
100
100
100
100
100
0.7
1.9
1.5
–
–
0.5
–
–
–
–
91.4
79.4
67.7
50.1
57.8
29.3
–
–
7.1
–
–
–
–
–
3.6
1.0
–
3.1
1.9
–
–
(ii)
(iii)
(iv)
(v)
(i)
(ii)
(iii)
(iv)
(v)
13.4
30.8
49.9
32.8
69.2
92.3
96.9
90.5
10
100
a) Reaction time: 20 min; all experiments were carried out at room temperature. (i) NBS/CH2Cl2, (ii)
NBS/CH2Cl2/SiO2, (iii) NBS/AcOH, (iv) KBr/KBrO3/MeOH/H2SO4 (cat.), and (v) KBr/KBrO3/AcOH.
b) BA=brominating agent. c) Quantitative GC (Ultra 2) analysis.
When 1 equiv. of NBS in CH2Cl2 was used, 3-bromocarbazole 1b was obtained in
91% yield besides traces of 1-bromocarbazole 1a (Table 1, Entry 1). With 1 equiv. of
NBS in the presence of an acid catalyst such as SiO2 or AcOH, 3-bromocarbazole 1b
was still the main product but 3,6-dibromocarbazole 1d was formed in a significant
amount (Entries 2 and 3, resp.), besides traces of 1a. These results can be rationalized
by the presence of acetyl hypobromite (MeCOOBr) as attacking agent when AcOH is
the acid catalyst, this brominating agent being formed according to the equilibrium
reaction shown in Scheme 2. When silica gel is used as a catalyst agent, one possible
attacking agent present in the reaction mixture is hypobromite acid (HBrO). Thus,
comparing the results of Entries 1–3 led us to conclude that the release of the electro-
phile species, the bromonium ion (Br+), from NBS in CH2Cl2 under neutral conditions
is slower than the formation of MeCOOBr or HBrO under acidic conditions. There-
fore, the reaction with NBS (1 equiv.) is regioselective under neutral conditions yielding
the monobromo product 1b. Under acidic conditions, the regioselectivity decreases
with concomitant increase of dibromination.
When 9-methyl-9H-carbazole (1) was treated with 1 equiv. of the classical bromi-
nating agent, a mixture of KBrO3/KBr in the presence of a catalytic amount of a min-
Scheme 2

Helvetica Chimica Acta – Vol. 89 (2006)
1127
eral acid (conc. sulfuric acid) in MeOH at 258, 3-bromocarbazole 1b and 3,6-dibromo-
carbazole 1d were formed in a 1:1 molar ratio (Table 1, Entry 4). A similar trend was
observed in the classical bromination of 1 in the presence of AcOH, 1b and 1d being
formed in a 2 :1 molar ratio (Entry 5). These results confirm our conclusions about
the loose of regioselectivity on monobromination of 1 under acidic conditions due to
the possible presence of the attacking agents MeCOOBr or HBrO in the reaction mix-
ture. However, when the classical brominating method is used, other brominating spe-
cies derived from hypobromous acid such as H2
which are indistinguishable at the present time. Additionally, the brominating species
H2
BrO+ may be operative in the bromination of 1 in the presence of a catalytic amount
BrO+, Br2O, or Br+ might be present
AHCTREUNG
of conc. sulfuric acid (H0 =ꢀ1.87).
Treatment of 9-methyl-9H-carbazole (1) with 2 equiv. of NBS under neutral condi-
tions yielded compounds 1b and 1d in a 3 :7 molar ratio (Table 1, Entry 6). In the same
reaction under acidic conditions (SiO2 or AcOH), 3,6-dibromocarbazole 1d was formed
in >90% yield (Entries 7 and 8). Similar results were obtained with the classical bromi-
nating agents KBr/KBrO3/MeOH/H2SO4 (cat.) or KBr/KBrO3/AcOH, 1d being again
the main product (Entries 9 and 10). Thus, regioselective dibromination of 1 can be
achieved in an easy and clean way when acidic conditions are used.
The 9-phenyl-9H-carbazole (2) and 9-benzyl-9H-carbazole (3) are less reactive than
9-methyl-9H-carbazole (1). Thus, the bromination with NBS (1 equiv.) in CH2Cl2 under
neutral or acidic conditions required longer periods of time. Monobromination of 2 and
3 produced the expected products, i.e., 3-bromocarbazoles 2a and 3a, respectively, in
good yields (Entries 1–3 in Tables 2 and 3). In some of these reactions, significant
amounts of the corresponding 3,6-dibromocarbazoles 2b or 3b were also obtained
(Entry 2 in Table 2 and Entries 1 and 2 in Table 3). Nevertheless, the monobromo com-
pounds 2a and 3a were obtained in high yield with NBS (1 equiv.) under neutral con-
ditions. The bromination of 2 and 3 with 2 equiv. of NBS under neutral and acidic con-
ditions gave the 3,6-dibromocarbazoles 2b and 3b, respectively, in fairly good yield,
besides a significant amount of the monobromo compounds 2a and 3a, except for
one case (Entries 6–8 in Tables 2 and 3). Thus dibromination of 2 and 3 with NBS is
best conducted under neutral conditions. In particular, 3b was obtained in almost
100% yield under neutral conditions. The bromination of 2 and 3 with 1 equiv. of
KBr/KBrO3 in MeOH under acidic conditions produced the 3-bromocarbazoles 2a
and 3a besides the 3,6-dibromocarbazoles 2b and 3b, respectively (Entries 4 and 5 in
Tables 2 and 3), monobromination being favored in the case of KBr/KBrO3 in the pres-
ence of a catalytic amount of conc. sulfuric acid. With 2 equiv. of the classical brominat-
ing agent, dibromination of 2 and 3 took place preferentially, yielding 2b and 3b as the
main product, respectively, except for one case, besides 2a and 3a in a significant
amount, except for one case (Entries 9 and 10 in Tables 2 and 3).
It is noteworthy to mention that in the bromination reaction of 9-methyl-9H-carba-
zole (1), 1-bromocarbazole 1a and 1,6-dibromocarbazole 1c were formed in very low
yield (<1%) while, in the bromination of 9-phenyl- (2) and 9-benzyl-9H-carbazole
(3), neither 1-bromo- nor 1,6-dibromocarbazoles were detected in the reaction mix-
tures. Comparing these results with those obtained in the chlorination [22b] and iodi-
nation [23b] of N-substituted carbazoles, we conclude that the reactivity of the incom-
ing electrophile Br+ at the ortho-position of the carbazole moiety is dramatically dimin-

1128
Helvetica Chimica Acta – Vol. 89 (2006)
Table 2. Bromination of 9-Phenyl-9H-carbazole (2) by Different Methods
Entry
Methoda)
Carbazole 2/BAb) [molar ratio]
Conversion [%]
Products [% yield]c)
2a
2b
1
2
3
4
5
6
7
8
9
(i)
1:1
1:1
1:1
1:1
1:1
1:2
1:2
1:2
1:2
1:2
95.5
96.3
85.0
89.0
100
100
100
100
100
91.5
89.6
83.7
86.2
79.0
12.5
25.1
27.1
21.4
59.8
–
6.8
–
–
18.0
87.5
74.9
72.9
78.6
38.4
(ii)
(iii)
(iv)
(v)
(i)
(ii)
(iii)
(iv)
(v)
10
100
a) Reaction time: 60 min; all experiments were carried out at room temperature. (i) NBS/CH2Cl2, (ii)
NBS/CH2Cl2/SiO2, (iii) NBS/AcOH, (iv) KBr/KBrO3/MeOH/H2SO4 (cat.), and (v) KBr/KBrO3/AcOH.
b) BA=brominating agent. c) Quantitative GC (Ultra 2) analysis.
Table 3. Bromination of 9-Benzyl-9H-carbazole (2) by Different Methods
Entry
Methoda)
Carbazole 3/BAb) [molar ratio]
Conversion [%]
Products [% yield]c)
3a
3b
5.2
3.7
–
13.0
60.5
98.5
68.7
67.2
90.0
98.3
1
2
3
4
5
6
7
8
9
(i)
1:1
1:1
1:1
1:1
1:1
1:2
1:2
1:2
1:2
1:2
90.0
82.3
82.0
100
65.0
100
100
94.1
100
84.7
78.8
79.6
85.9
4.0
(ii)
(iii)
(iv)
(v)
(i)
(ii)
(iii)
(iv)
(v)
–
31.3
26.9
6.3
10
100
–
a) Reaction time: 50 min; all experiments were carried out at room temperature. (i) NBS/CH2Cl2, (ii)
NBS/CH2Cl2/SiO2, (iii) NBS/AcOH, (iv) KBr/KBrO3/MeOH/H2SO4 (cat.), and (v) KBr/KBrO3/AcOH.
b) BA=brominating agent. c) Quantitative GC (Ultra 2) analysis.
ished when a substituent group is attached at the N-atom. This behavior is due to the
presence of a bulky substituent such as Me, Ph, or PhCH2 at the N-atom of the carba-
zole moiety and, at the same time, to the high molecular volume of the electrophile
(Br+).
We also studied the bromination of 2-methoxy-9-methyl-9H-carbazole (4). Thus,
monobromination of 4 with 1 equiv. of NBS under neutral or acidic conditions gave
the 3-bromocarbazole 4a, which was the main product in ca. 80–90% yield, besides
the 3,6-dibromocarbazole 4b (Table 4, Entries 1–3). With 2 equiv. of NBS under neutral
conditions, the 3,6-dibromocarbazole 4b was obtained in 95% yield (Entry 6), whereas
under acidic conditions, a significant amount of 1,3,6-tribromocarbazole 4c was also

Helvetica Chimica Acta – Vol. 89 (2006)
1129
Table 4. Bromination of 2-Methoxy-9-methyl-9H-carbazole (4) by Different Methods
Entry
Methoda)
Carbazole 4/BAb) [molar ratio]
Conversion [%]
Products [% yield]c)
4a
4b
4c
1
2
3
4
5
6
7
8
9
(i)
1:1
1:1
1:1
1:1
1:1
1:2
1:2
1:2
1:2
1:2
1:1d)
1:2d)
100
100
100
100
100
100
100
100
100
100
90
88.0
81.8
79.6
–
–
5.0
–
–
–
–
89.5
54.3
12.0
10.9
20.4
74.1
89.3
95.0
89.3
63.7
51.9
58.6
–
–
–
–
(ii)
(iii)
(iv)
(v)
(i)
(ii)
(iii)
(iv)
(v)
(i)
23.9
8.3
–
8.3
34.2
39.5
35.4
–
10
11
12
(i)
100
45.4
–
a) Reaction time: 15 min; all experiments were carried out at room temperature, unless indicated other-
wise. (i) NBS/CH2Cl2, (ii) NBS/CH2Cl2/SiO2, (iii) NBS/AcOH, (iv) KBr/KBrO3/MeOH/H2SO4 (cat.), and
(v) KBr/KBrO3/AcOH. b) BA=brominating agent. c) Quantitative GC (Ultra 2) analysis. d) Method (i) at
08.
formed (Entries 7 and 8). In all these reactions, a total consumption of the starting
material was achieved. With 1 equiv. of the classical brominating agent, 4b and 4c
were formed in ca. 3 :1 and ca. 11:1 molar ratios for methods (iv) and (v), respectively
(Entries 4 and 5). However, the corresponding 4b/4c molar ratios with 2 equiv. are 4 :3
and ca. 8 :5, respectively (Entries 9 and 10).
These results encouraged us to perform the bromination at temperatures lower than
room temperature (258) to enhance the regioselectivity of the mono- and dibromina-
tion of compound 4. Therefore, monobromination of 4 with 1 equiv. of NBS at 08
gave the 3-bromocarbazole 4a in 90% yield as the only brominated product (Table 4,
Entry 11), whereas with 2 equiv. of NBS, no improvement in the yield of 4b was
observed, 4a/4b being obtained in a molar ratio close to 1:1 (Entry 12). Similar experi-
ments were carried out at 08 with 4 and 1 or 2 equiv. of the traditional brominating
agent, but no improvement was observed (results not shown).
The 2-(acetyloxy)-9H-carbazole (5), treated with 1 equiv. of NBS in CH2Cl2 under
neutral or acidic conditions at room temperature, provided 5a in ca. 50–65% yield,
besides significant amounts of 6-bromocarbazole 5b and 3,6-dibromocarbazole 5c
(Table 5, Entries 1–3). Monobromination of 5 could not be improved even if the reac-
tions were carried out at 08, which yielded a similar product distribution as at room tem-
perature. With 2 equiv. of NBS in AcOH 5 provided 3,6-dibromocarbazole 5c in a sim-
ple and clean reaction (Entry 7). However, the bromination of 5 with 2 equiv. of NBS in
CH2Cl2 under neutral and acidic (SiO2) conditions was not regioselective furnishing a
mixture of 3-bromocarbazole 5a and 3,6-dibromocarbazole 5c, the latter being the
main product (Entries 5 and 6). Bromination of 5 with KBr/KBrO3 in MeOH doped
with conc. sulfuric acid was not possible due to a rapid hydrolysis of the acetyloxy
group of 5. But 3,6-dibromocarbazole 5c was obtained in 100% yield with 2 equiv. of

1130
Helvetica Chimica Acta – Vol. 89 (2006)
Table 5. Bromination of 2-(Acetyloxy)-9H-carbazole (5) by Different Methods
Entry
Methoda)
Carbazole 5/BAb) [molar ratio]
Conversion [%]
Products [% yield]c)
5a
5b
5c
13.1
8.4
21.5
78.3
78.9
61.5
1
2
3
4
5
6
7
8
(i)
1:1
1:1
1:1
1:1
1:2
1:2
1:2
1:2
97.2
93.1
99.0
100
100
100
53.8
65.5
47.7
12.1
15.2
38.5
–
30.3
17.6
29.0
–
1.1
–
(ii)
(iii)
(v)
(i)
(ii)
(iii)
(v)
100
100
–
–
100
100
–
a) Reaction time: 15 min; all experiments were carried out at room temperature. (i) NBS/CH2Cl2, (ii)
NBS/CH2Cl2/SiO2, (iii) NBS/AcOH, and (v) KBr/KBrO3/AcOH. b) BA=brominating agent. c) Quanti-
tative GC (Ultra 2) analysis.
KBr/KBO3 in AcOH (Entry 8), while when 1 equiv. of the brominating agent was used,
a mixture of 5a and 5c was formed (Entry 4).
The bromination of 3-nitro-9H-carbazole (6) with NBS was carried out in CH2Cl2/
THF 3 :1 owing to the low solubility of 6 in CH2Cl2. When 6 was treated with 1 equiv. of
NBS at room temperature under neutral and acidic conditions, 6-bromocarbazole 6a
was formed in 70% yield as the only product (Table 6, Entries 1–3). With 2 equiv. of
NBS, 6a was obtained selectively in 89–100% yield both under neutral and acidic con-
ditions (Entries 6–8). Similar results were obtained with 1 or 2 equiv. of KBr/KBrO3
(Entries 4, 5, 9, and 10).
Table 6. Bromination of 3-Nitro-9H-carbazole (6) by Different Methods
Entry
Methoda)
Carbazole 6/BAb) [molar ratio]
Conversion [%]
6a [% yield]c)
1
2
3
4
5
6
7
8
9
(i)
1:1
1:1
1:1
1:1
1:1
1:2
1:2
1:2
1:2
1:2
74.4
73.9
67.9
100
33.2
99
100
100
100
73.4
71.2
70.6
62.7
80.2
31.1
95.9
100
89.3
100
70.8
(ii)
(iii)
(iv)
(v)
(i)
(ii)
(iii)
(iv)
(v)
10
a) Reaction time: 45 min; all experiments were carried out at room temperature. (i) NBS/CH2Cl2/THF,
(ii) NBS/CH
N
N
AcOH. b) BA=brominating agent. c) Quantitative GC (Ultra 2) analysis.
Semiempirical Calculations. As known, the high reactivity of the aromatic ring of
carbazole in electrophilic substitution makes it highly probable that these reactions
reflect the established Br+ activity of the different brominating systems. Also, the
experimental results obtained in the bromination of carbazoles show a preferential sub-

Helvetica Chimica Acta – Vol. 89 (2006)
1131
stitution pattern which is the C(3) and/or C(6) para-substitution with respect to the N-
atom. This was confirmed by the net atomic-charge values calculated by using the semi-
empirical PM3 method, as implemented in the version of the HyperChem 7.0 Suite pro-
gram [25] to obtain the optimized geometry of carbazoles 1–6 and bromocarbazole
derivatives. As can be seen from the static charge-distribution pattern in carbazoles
1–6 and their bromocarbazole derivatives (Table 7), the calculated preferential electro-
philic substitution positions (kinetic attack) are in agreement with the experimental
results obtained.
Table 7. Calculated Static Charge Distributiona)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
1
ꢀ0.108
ꢀ0.103
ꢀ0.116
ꢀ0.109
ꢀ0.104
ꢀ0.104
ꢀ0.115
ꢀ0.116
ꢀ0.108
ꢀ0.195
ꢀ0.190
ꢀ0.191
ꢀ0.198
ꢀ0.120
ꢀ0.171
ꢀ0.120
ꢀ0.172
ꢀ0.137
ꢀ0.135
ꢀ0.082
ꢀ0.065
ꢀ0.056
ꢀ0.083
ꢀ0.066
ꢀ0.063
ꢀ0.081
ꢀ0.078
ꢀ0.061
0.111
0.128
0.132
0.166
0.060
0.104
0.065
0.109
0.010
ꢀ0.126
ꢀ0.131
ꢀ0.141
ꢀ0.124
ꢀ0.131
ꢀ0.132
ꢀ0.129
ꢀ0.128
ꢀ0.134
ꢀ0.165
ꢀ0.186
ꢀ0.187
ꢀ0.197
ꢀ0.153
ꢀ0.174
ꢀ0.152
ꢀ0.175
ꢀ0.447
ꢀ0.444
ꢀ0.050
ꢀ0.030
ꢀ0.017
ꢀ0.050
ꢀ0.029
ꢀ0.026
ꢀ0.048
ꢀ0.044
ꢀ0.024
ꢀ0.016
0.008
1b
1d
2
2a
2b
3
3a
3b
4
4a
4b
4c
5
ꢀ0.047
ꢀ0.018
ꢀ0.049
ꢀ0.045
ꢀ0.024
ꢀ0.125
ꢀ0.140
ꢀ0.125
ꢀ0.124
ꢀ0.133
ꢀ0.079
ꢀ0.056
ꢀ0.079
ꢀ0.076
ꢀ0.059
ꢀ0.108
ꢀ0.113
ꢀ0.116
ꢀ0.116
ꢀ0.110
ꢀ0.028
ꢀ0.133
ꢀ0.064
ꢀ0.108
ꢀ0.056
ꢀ0.053
ꢀ0.033
ꢀ0.030
ꢀ0.047
ꢀ0.047
ꢀ0.027
ꢀ0.027
ꢀ0.065
ꢀ0.024
ꢀ0.122
ꢀ0.121
ꢀ0.128
ꢀ0.130
ꢀ0.124
ꢀ0.121
ꢀ0.132
ꢀ0.131
ꢀ0.116
ꢀ0.131
ꢀ0.087
ꢀ0.085
ꢀ0.068
ꢀ0.065
ꢀ0.078
ꢀ0.079
ꢀ0.061
ꢀ0.061
ꢀ0.076
ꢀ0.058
ꢀ0.105
ꢀ0.104
ꢀ0.099
ꢀ0.100
ꢀ0.105
ꢀ0.104
ꢀ0.100
ꢀ0.098
ꢀ0.101
ꢀ0.095
0.012
0.021
ꢀ0.032
5a
5b
5c
6
0.000
ꢀ0.028
0.004
0.055
6a
0.012
0.059
a) Calculations were performed by using the PM3 method [25].
It is interesting to mention that, even though the highest negative-charge densities
are localized at C(1) and C(3) and also at C(6) and C(8), the experimental results show
an almost exclusive tendency for para-bromination with respect to the substituted N-
atom of the carbazoles (see the product distribution obtained from 1 (R1 =Me), 2
(R1 =Ph), 3 (R1 =PhCH2), and 4 (R1 =Me)). These results can be explained by the
large volume of the Br+ ion hindering its approach to the ortho-position, particularly
in case of compounds 2–4. A similar steric effect is observed for 2-(acetyloxy)-9H-car-
bazole (5) where the AcO group, which is bulky enough, hinders the Br+ approach to
the ortho-position C(1), and where the incoming electrophile attacks exclusively the
unhindered para-positions C(3) and C(6). In conclusion, it seems certain that electro-
philic attack of carbazole under a variety of conditions occurs preferentially at the
C-atoms with charge densities higher than ꢀ0.095 when no steric hindrance is present.
Spectroscopy of Carbazoles and Bromocarbazole Derivatives. The absorption spec-
tra of carbazoles and bromocarbazole derivatives in MeCN at 298 K generally exhibit
three bands in the 255–280, 275–305, and 305–370 nm region, as shown in the Figure

1132
Helvetica Chimica Acta – Vol. 89 (2006)
for 9-methyl-9H-carbazole (1) and its bromo derivatives 1b and 1d. Previous spectro-
scopic studies and theoretical calculations have been carried out [26] allowing for
unambiguous assignment of the lowest-lying excited electronic states of carbazoles as
1
1La and Lb in the C2v symmetry point group with the short (z) axis and long (y) axis
respectively in the plane of the molecule. Comparing the position and oscillator
strength of the bands located in the 300- and 350-nm region of the bromocarbazoles
with that of carbazoles, it is possible to assign them to 1La (S2 ꢀS0) and 1Lb (S1 ꢀS0) elec-
tronic transitions too (see Fig.).
Figure. Electronic absorption and fluorescence emission spectra (MeCN; 298 K) and phosphorescence
emission spectra (solid matrix; 77 K) of 9-methyl-9H-carbazole (1; —–), 3-bromo-9-methyl-9H-carba-
zole (1b; –––), and 3,6-dibromo-9-methyl-9H-carbazole (1d; ·····). Concentration: 1.0·10ꢀ5 M; lexc 320
nm.
The absorption spectra of the bromocarbazole derivatives exhibit a noticeable bath-
ochromic shift of the 1Lb band ranging from 10 to 19 nm compared to that of the carba-
zoles 1–6 (see Table 8). In general, the absorption spectra of bromocarbazoles show a
small bathochromic shift of the other two electronic transition bands with respect to
carbazoles 1–6. In the Figure, these smaller bathochromic shifts can be seen for 9-meth-
yl-9H-carbazole (1) and its bromo derivatives 1b and 1d, which range from 6 to 10 nm,
as the number of Br-atoms attached to the carbazole moiety increases. This significant
red shift observed when the Br-atom is located at C(3) and/or C(6) of the carbazole
moiety can be interpreted in terms of mesomeric and/or inductive effects due to the
higher electronic density of the Br-atom as compared to the H-atom at C(3) and/or
C(6) [27].
The carbazoles 1–5 are good fluorescent chromophores with ff values ranging from
0.35 to 0.45 and tf values in the order of 11.4–14.7 ns (see Table 8). These compounds
display mirror symmetry when the fluorescent emission and absorption spectra are
compared, as shown for the spectra of 9-methyl-9H-carbazole (1) in the Figure. This

Helvetica Chimica Acta – Vol. 89 (2006)
1133
indicates that the geometries of the molecules are quite similar in their ground and
excited states. For 3-nitro-9H-carbazole (6), no fluorescence emission was detected
due to a high spin–orbital coupling between the nitro group and the carbazole moiety
[27][28]. A similar behavior is expected for the bromocarbazole derivatives 1b,d, 2a,b,
3a,b, 4b,c, and 5a,c where a higher spin–orbital-coupling mechanism operates as the
number of Br-atoms attached to the carbazole moiety is increased. Thus, no fluores-
cence emission was detected for these compounds in MeCN solutions at 298 K, and
it can be concluded that these molecules dissipate efficiently the energy from the elec-
tronic singlet excited state (S1) through radiationless processes such as internal conver-
sion and/or intersystem crossing processes.
We also measured the phosphorescence emission spectra of carbazoles 1–6 and
their bromo derivatives in a solid matrix at 77 K, and compounds 1b,d, 2a,b, 3a,b,
4b,c, and 5a,c showed to be good phosphorescent chromophores. The maxima wave-
length values of phosphorescence emission, lmax
and, in general, these values are located between 408 and 428 nm. The Figure shows
the phosphorescence emission spectra of compounds 1, 1b, and 1d. The lmax(phosph)
A
AHCTREUNG
values of the bromocarbazole derivatives exhibit a bathochromic shift ranging between
9 and 17 nm compared to that of the carbazoles 1–5. This behavior accounts for the
increase of the charge density in the carbazole moiety due to the presence of the Br-
substituent at C(3) and/or C(6) and can be interpreted by inductive and/or mesomeric
effects [27]. In this connection, it is worthy to mention that the shapes of the phospho-
A
are similar to that of the carbazoles 1–5 (see Fig. for 1, 1b, and 1d). Taking into account
Table 8. Spectroscopic Data (lmax), Fluorescence Quantum Yield (ff) and Phosphorescence Quantum
Yield (fp) of Carbazoles and Bromocarbazoles in MeCN at 298 K and in a Solid Matrix at 77 K
lmax
A
lmax
(fluo) [nm]a)
lmax
T
ffa)
tf [ns]c)
fpb)
1
345
355
362
342
352
361
342
351
357
330
342
349
332
343
349
365
365
356
408
414
417
407
415
424
410
415
425
412
426
428
409
420
424
480
481
0.43
14.7
–
–
11.4
–
–
12.3
–
–
14.3
–
–
13.2
–
–
–
–
0.24
d
d
1b
1d
2
2a
2b
3
3a
3b
4
4b
4c
5
)
)
)
)
0.56
0.74
0.35
0.69
0.88
0.47
0.70
0.99
0.23
0.77
0.94
0.24
0.71
0.96
0.04
>0.001
d
d
358
0.34
d
d
)
)
)
d
d
)
355
0.40
d
d
)
)
)
d
d
)
350
0.45
d
d
)
)
)
d
d
)
340
0.45
d
d
5a
5c
6
)
)
)
)
)
d
d
)
d
d
)
d
d
6a
)
a) Measured in MeCN at 298 K. b) Measured in the solid matrix iPrOH/Et2
A
from [26a]. d) No fluorescence detected.

1134
Helvetica Chimica Acta – Vol. 89 (2006)
that the shape of the phosphorescence emission spectra of these compounds are ‘naph-
thalene-like’ and considering that the phosphorescence lifetime (tp) is in the time scale
of seconds, we concluded that the electronic excited triplet state is most likely due to a
p,p* electronic transition [28]. A similar conclusion was previously reported for 9H-
carbazole, 3-bromo-9H-carbazole, and 3,6-dibromo-9H-carbazole [26a]. It is notewor-
thy to mention that the bromocarbazole derivatives show no fluorescence emission in a
solid matrix at 77 K, while carbazoles 1–6 are good fluorescent and phosphorescent
chromophores under the same experimental conditions [26a]. These results confirm
that the spin–orbital-coupling mechanism operates to a great extent in bromocarba-
zoles in a solid matrix at 77 K, and the deactivation of the singlet excted state (S1)
through radiationless processes takes place efficiently. Also, under these experimental
conditions, population of the triplet excited state (T1) of bromocarbazoles 1b,d, 2a,b,
3a,b, 4b,c, and 5a,c seems to be very efficient because these compounds show fp values
ranging between 0.56 to 0.99 (Table 8).
In the case of 6-bromo-3-nitro-9H-carbazole (6a), no red shift of lmax(phosph) was
G
observed in comparison with 3-nitrocarbazole (6). The fp values of 6 and 6a are lower
than 0.05, and we conclude that the deactivation of the triplet excited state (T1) takes
place efficiently through a radiationless process (internal conversion) where the radia-
tive process (phosphorescence emission) does not compete to a greater extent. Taking
into account that the phosphorescene emission spectra of compounds 6 and 6a are quite
similar and are ‘benzophenone-like’, we conclude that the electronic triplet excited
state is most likely to be due to an n,p* electronic transition [28].
In conclusion, the spectroscopic study shows that bromocarbazoles 1b,d, 2a,b, 3a,b,
4b,c, 5a,c, and 6a are not fluorescent chromophores and populate efficiently the lowest
triplet excited state. In this regard, these compounds could be considered as possible
samples for studying the photoinduced heterolytic cleavage of the CꢀBr bond taking
into account that the photoreaction takes place efficiently from the lowest triplet
excited state.
Conclusions. – The regioselectivity of the bromination reaction of N- and C-substi-
tuted carbazoles 1–6 was improved when 1 equiv. of NBS/CH2Cl2 at room temperature
was used as brominating agent. Thus, monobromocarbazole derivatives were obtained
as the main product with chemical yields higher than 75%. Acidic conditions (NBS/
SiO2 and NBS/AcOH) diminished significantly the regioselectivity of the monobromi-
nation. Dibromocarbazole derivatives were obtained with high regioselectivity when 2
equiv. of NBS/CH2Cl2 were used as brominating agent, as well as when 2 equiv. of KBr/
KBrO3 under acidic condition were employed. Theoretical calculations of the static
charge distributions in carbazoles 1–6 correlated with the experimental results, and
the preferential sites of kinetic attack by the incoming electrophile Br+ were nicely pre-
dicted.
The carbazoles 1–5 and their bromo derivatives 1b,d, 2a,b, 3a,b, 4b,c, and 5a,c,
showed a noticeable bathochromic shift in the absorption and phosphorescence emis-
sion spectra at 298 and 77 K as the number of Br-atoms attached to the carbazole moi-
ety increased. This red shift is due to mesomeric and/or inductive effects arising from
the higher electronic density of the Br-atom as compared to a H-atom at C(3) and/or
C(6). Also, these bromocarbazole derivates are good phosphorescent chromophores

Helvetica Chimica Acta – Vol. 89 (2006)
1135
in solid matrix at 77 K due to a high spin–orbital-coupling mechanism between the Br-
atoms and the carbazole moiety with fp values in the range 0.23–0.99. The shape of the
phosphorescence emission spectra of these compounds are ‘naphthalene-like’, and tak-
ing into account the phosphorescence lifetime values, the lowest triplet excited state is
most likely due to a p,p* electronic transition. In the case of compounds 6 and 6a, the
‘benzophenone-like’ shape of the phosphorescent emission spectra indicates that the
lowest triplet excited state is due to an n,p* electronic transition. The effect of a Br-
atom on the n,p* transitions is very minor because compounds 6 and 6a showed similar
absorption and phosphorescence spectra (Table 8).
In conclusion, the regioselectivity of the brominaton reaction provides an alterna-
tive method for the synthesis of selected bromo-substituted heterocyclic compounds
which are not commercially available. These compounds are interesting species to be
tested as substrates for the study of the photoinduced heterolytic cleavage reaction
of the CꢀBr bond. In this regard, spectroscopic and photophysical analyses of bromo-
carbazole derivatives are required to assess which lowest excited state (singlet or triplet
state) is efficiently populated. Thus, we could determine that compounds 1b,d, 2a,b,
3a,b, 4b,c, 5a,c, and 6a populate their lowest triplet excited states efficiently and,
from these results, we expect that the photoinduced heterolytic cleavage reaction
will take place as well. Further studies regarding this photochemical reaction is in prog-
ress in our laboratory.
The authors thank the Universidad de Buenos Aires (X022), CONICET (PIP05/5443), and ANPCyT
(PICT 6-12312) for partial financial support. We are grateful to Dr. H. Nonami (Ehime University, Japan)
for providing the HyperChem Suite 7.0. R. E.-B., F. M. C., and S. M. B. are research members of CONI-
CET.
Experimental Part
General. CH2Cl2, CHCl3, MeOH, AcOH, hexane, AcOEt, and other reagents used were of anal.
i
grade. Solvents were freshly distilled and dried before use. MeCN, PrOH, and Et2
A
trograde and were purchased from J. T. Baker. The 9-methyl-9H-carbazole (1), 9-phenyl-9H-carbazole
(2), 9H-carbazole, and N-bromosuccinimide (NBS) were purchased from Aldrich. The 9H-carbazol-2-
ole was purchased from Aldrich and recrystallized from EtOH before used. The 2-methoxy-9-methyl-
9H-carbazole (4) [29], 9-benzyl-9H-carbazole (3) [30], 2-(acetyloxy)-9H-carbazole (5) [31], and 3-
nitro-9H-carbazole (6) [32] were prepared according to the procedures described in the literature.
TLC: Merck aluminium silica gel sheets (0.2 mm layer thickness, silica gel 60 F254). Column chromatog-
raphy (CC): Merck silica gel 60 (0.040–0.063 mm); FC=flash chromatography. Gas chromatography
(GC): Hewlett Packard 5890 with an Ultra-2-Hewlett-Packard capillary column (crosslinked 5% phenyl-
polysiloxane; 30 m×0.25 mm×0.50 mm); gas carrier N2; FID detector. Melting points: Fisher-Jones appa-
ratus; not corrected. UV/VIS Spectra: UV-1203-UV-VIS Shimadzu spectrophotometer. Fluorescence
and phosphorescence spectra: Perkin-Elmer-LS-50 spectrofluorometer; fluorescence quantum yield
determination in MeCN at 298 K in the same apparatus relative to that of quinine sulfate in 0.1M
i
HClO4 [26]; phosphorescence quantum yield determination in a solid matrix of PrOH/Et2
A
K in the same apparatus relative to that of carbazole [26]. H- and 13C-NMR Spectra: Bruker-AC-200
and Bruker-500 spectrometer; chemical shifts d in ppm rel. to internal SiMe4, J in Hz; standard pulse
sequences. EI-MS: VG-ZAB-BEQ instrument; in m/z (rel %).
1
Theoretical Calculations. The ground-state geometry and static charge distribution of carbazoles 1–6
and bromocarbazole derivatives were calculated by using the semi-empirical parameterized PM3 method

1136
Helvetica Chimica Acta – Vol. 89 (2006)
as implemented in version 7.0 of the HyperChem Program [25] which is established to be effective in
studies of molecules containing heteroatoms, compared with other methods such as MINDO/3 or
MNDO.
Bromination Reaction of Carbazoles 1–6 with NBS and NBS/Silica Gel in CH2Cl2. General Proce-
dure 1 (G.P. 1).To a stirred soln. of each carbazole 1–6 (0.55 mmol) in CH2Cl2 (10 ml) containing silica
gel (6 g) (or not depending on the brominating method used), a soln. of NBS (0.55 to 1.10 mmol depend-
ing on the stoichiometry used) in CH2Cl2 (5 ml) was added dropwise. The mixture was stirred for an
appropriate time in the absence of light at r.t. under normal (air) atmosphere (TLC silica gel 60 F245, hex-
ane/AcOEt mixtures) and GC (Ultra-2-HP) monitoring. The mixture was then filtered and the silica gel
washed with CH2Cl2 (3×15 ml). The combined filtrate was washed with H2O (2×50 ml), dried (Na2SO4),
and evaporated. The brownish solid residue was dissolved in the minimum volume of CH2Cl2, this soln.
mixed with silica gel 60 and evaporated, and the brownish residue put on top of a prep. column and sub-
jected to FC (silica gel, hexane/AcOEt mixtures). The isolated products were characterized by m.p., 1H-
and 13C-NMR, MS and microanalysis.
The same procedure was applied to the brominations with NBS/AcOH replacing the solvent CH2Cl2
by AcOH.
Bromination Reaction of Carbazoles 1–6 With KBrO3/KBr/Catalytic Acid Systems. General Proce-
dure 2 (G.P. 2). To a stirred soln. of each carbazole 1–6 (0.55 mmol) in MeOH (20 ml), solid KBr
(0.15 to 0.30 mmol depending on the stoichiometry used) and KBrO3 (0.28 to 0.56 mmol depending on
the stoichiometry used) were added. Then, conc. sulfuric acid (42 ml to 0.84 ml depending on the stoichi-
ometry used; d=1.85 g/ml) was added dropwise. The mixture was stirred for an appropriate time in the
absence of light at r.t. under normal (air) atmosphere (TLC silica gel 60 F245, hexane/AcOEt mixtures)
and GC (Ultra-2-HP) monitoring). The mixture was then neutralized with 10% aq. Na2CO3 soln. and
extracted with CH2Cl2 (3×15 ml), the org. phase washed with H2O (3×20 ml), dried (Na2SO4) and evapo-
rated, the brownish solid residue dissolved in the minimum volume of CH2Cl2, this soln. mixed with silica
gel 60 and evaporated, and the brownish solid residue put on top of a prep. column and subjected to FC
(silica gel, hexane/AcOEt mixtures). The isolated products were characterized by m.p., 1H- and 13C-
NMR, MS and microanalysis.
The same procedure was applied to the brominations with KBr/KBrO3/AcOH replacing the solvent
MeOH by AcOH.
1-Bromo-9-methyl-9H-carbazole (1a). White needles (hexane). M.p. 608. 1H-NMR (CDCl3, 500
MHz): 8.25 (dd, J=2.0, 8.2, HꢀC(4)); 8.09 (d, J=8.0, HꢀC(5)); 7.54 (dd, J=2.0, 7.9, HꢀC(2)); 7.50
(t, J=8.0, HꢀC(6)); 7.39–7.23 (m, HꢀC(7), HꢀC(8)); 7.17 (t, J=8.2, HꢀC(3)); 3.82 (s, MeꢀN(9)).
EI-MS (70 eV): 261 (98.7), 259 (100, M+), 246 (5.6), 244 (5.0, [MꢀMe]+), 181 (16.3), 179 (20.2,
[MꢀBr]+), 165 (10), 164 (9.4), 139 (6.0). Anal. calc. for C13H10BrN (260.13): C 60.02, H 3.87, Br 30.72,
N 5.38; found: C 59.93, H 3.85, Br 30.81, N 5.41.
3-Bromo-9-methyl-9H-carbazole (1b). White needles (hexane). M.p. 78–798. 1H-NMR (CDCl3, 200
MHz): 8.18 (d, J=2.1, HꢀC(4)); 8.01 (d, J=8.1, HꢀC(5)); 7.54 (dd, J=2.1, 7.9, HꢀC(2)); 7.50 (t, J=8.11,
HꢀC(6)); 7.38–7.24 (m, HꢀC(7), HꢀC(8)); 7.17 (d, J=7.9, HꢀC(1)); 3.83 (s, MeꢀN(9)). 13C-NMR
(CDCl3, 200 MHz): 141.2 (C(9a)); 139.5 (C(8a)); 128.2 (C(4)); 126.4 (C(5)); 124.4 (C(4a)); 123.0
(C(6)); 121.7 (C(4b)); 120.5 (C(2)); 119.3 (C(7)); 111.6 (C(3)); 109.9 (C(1)); 108.7 (C(8)); 29.1 (Meꢀ
N(9)). EI-MS (70 eV): 261 (98.9), 259 (100, M+), 246 (4.8), 244 (4.0, [MꢀMe]+), 181 (12.6), 179 (19.2,
[MꢀBr]+), 165 (9), 164 (8.3), 139 (4.0). Anal. calc. for C13H10BrN (260.13): C 60.02, H 3.87, Br 30.72,
N 5.38; found: C 59.95, H 3.85, Br 30.84, N 5.40.
1,6-Dibromo-9-methyl-9H-carbazole (1c). White needles (hexane). M.p. 1238. 1H-NMR (CDCl3, 500
MHz): 8.25 (dd, J=2.0, 8.2, HꢀC(4)); 8.13 (d, J=2.3, HꢀC(5)); 7.60 (dd, J=2.3, 8.5, HꢀC(7)); 7.44 (dd,
J=2.0, 8.0, HꢀC(2)); 7.28 (d, J=8.5, HꢀC(8)); 7.18 (t, J=8.0, HꢀC(3)); 3.82 (s, MeꢀN(9)). EI-MS (70
eV): 341 (39), 339 (91, M+), 337 (42.2), 324 (6), 323 (10, [MꢀMe]+), 321 (5.2), 261 (35.2), 259 (43.1,
[MꢀBr]+), 179 (21), 168 (18.3), 167 (37), 164 (17.1), 138 (5). Anal. calc. for C13H9Br2N (339.03): C
46.06, H 2.68, Br 47.14, N 4.13; found: C 45.95, H 2.67, Br 47.31, N 4.15.
1
3,6-Dibromo-9-methyl-9H-carbazole (1d). White plates (hexane). M.p. 1418. H-NMR (CDCl3, 200
MHz): 8.13 (d, J=2.0, HꢀC(4), HꢀC(5)); 7.57 (dd, J=2.0, 8.5, HꢀC(2), HꢀC(7)); 7.27 (d, J=8.5, Hꢀ
C(1), HꢀC(8)); 3.85 (s, MeꢀN(9)). 13C-NMR (CDCl3, 200 MHz): 139.8 (C(8a), C(9a)); 129.0 (C(4),

Helvetica Chimica Acta – Vol. 89 (2006)
1137
C(5)); 123.3 (C(4a), C(4b)); 123.2 (C(2), C(7)); 112.0 (C(3), C(6)); 110.1 (C(8), C(1)); 29.2 (MeꢀN(9)).
EI-MS (70 eV): 341 (38.8), 339 (90.3, M+), 337 (43.8), 324 (5), 323 (10, [MꢀMe]+), 321 (4.8), 261 (36.4),
259 (42.1, [MꢀBr]+), 179 (20.5), 168 (18.2), 167 (39.5), 164 (16.7), 138 (5). Anal. calc. for C13H9Br2N
(339.03): C 46.06, H 2.68, Br 47.14, N 4.13; found: C 45.93, H 2.67, Br 47.30, N 4.16.
3-Bromo-9-phenyl-9H-carbazole (2a). White needles (hexane). M.p. 988. 1H-NMR (CDCl3, 200
MHz): 8.22 (d, J=2.1, HꢀC(4)); 8.12 (d, J=8.2, HꢀC(5)); 7.67–7.54 (m, HꢀC(2), HꢀC(7)); 7.53 (m,
2 Ho); 7.40 (m, 2 Hm, Hp); 7.28 (dd, J=8, HꢀC(6)); 7.30 (d, J=7.9, HꢀC(1)); 7.29 (d, J=8.1, Hꢀ
C(8)). 13C-NMR (CDCl3, 200 MHz): 141.3 (C(9a)); 139.6 (C(8a)); 137.3 (Cispo); 130.1 (Co); 128.6 (Cm);
127.8 (C(4)); 127.1 (Cp); 126.7 (C(5)); 125.2 (C(4a)); 123.1 (C(6)); 122.4 (C(4a)); 120.5 (C(2)); 120.4
(C(7)); 112.8 (C(3)); 111.3 (C(1)); 110.0 (C(8)). EI-MS (70 eV): 323 (100), 321 (90, M+), 243 (23.5,
[MꢀBr]+), 241 (53.7), 166 (30.4, [MꢀBr – Ph]+), 139 (3.4), 138 (5), 137 (4). Anal. calc. for C18H12BrN
(322.20): C 67.10, H 3.75, Br 24.80, N 4.35; found: C 66.23, H 3.77, Br 24.75, N 4.33.
3,6-Dibromo-9-phenyl-9H-carbazole (2b). White plates (hexane). M.p. 159–1608. 1H-NMR (CDCl3,
200 MHz): 8.20 (d, J=8.2, HꢀC(4), HꢀC(5)); 7.60 (t, J=8.0, 2 Ho); 7.55–7.45 (m, HꢀC(2), HꢀC(7), 2
Hm, Hp); 7.24 (d, J=8.0, HꢀC(1), HꢀC(8)). 13C-NMR (CDCl3, 200 MHz): 139.8 (C(8a), C(9a)); 136.8
(Cipso); 130.1 (Co); 129.4 (Cm); 128.1 (C(4), C(5)); 126.9 (Cp); 123.8 (C(4a), C(4b)); 123.2 (C(2), C(7));
113.1 (C(3), C(6)); 111.5 (C(1), C(8)). EI-MS (70 eV): 403 (43), 401 (76.5, M+), 399 (41.2), 323 (6.5),
321 (7.2, [MꢀBr]+), 241 (40), 239 (22), 168 (4), 167 (38), 164 (9), 139 (2), 137 (5). Anal. calc. for
C18H11Br2N (401.10): C 53.90, H 2.76, Br 39.84, N 3.49; found: C 53.86, H 2.75, Br 39.91, N 3.48.
1
3-Bromo-9-benzyl-9H-carbazole (3a). White needles (hexane). M.p. 111–1128. H-NMR (CDCl3,
200 MHz): 8.25 (d, J=2.1, HꢀC(4)); 8.09 (dd, J=2.4, 8.1, HꢀC(5)); 7.51 (dd, J=2.1, 7.9, HꢀC(2));
7.44 (dd, J=1.9, 7.5, HꢀC(7)); 7.40 (m, 2 Ho); 7.31–7.21 (m, HꢀC(6), 2 Hm, Hp); 7.22 (d, J=7.5, Hꢀ
C(8)); 7.12 (d, J=7.9, HꢀC(1)); 5.5 (s, PhCH2). 13C-NMR (CDCl3, 200 MHz): 141.0 (C(9a)); 139.3
(C(8a)); 136.7 (Cipso); 128.9 (Co); 128.5 (Cm); 127.6 (C(4)); 126.6 (C(5)); 126.3 (Cp); 124.8 (C(4b));
123.2 (C(6)); 122.0 (C(4a)); 120.6 (C(2)); 119.6 (C(7)); 112.1 (C(3)); 110.4 (C(1)); 109.2 (C(8)); 46.7
(PhCH2). EI-MS (70 eV): 337 (17), 335 (17, M+), 257 (22, [MꢀBr]+), 246 (2, [MꢀC6H5CH2]+), 244
(2), 167 (8), 166 (4), 165 (5), 91 (100, C
N
N
Br 23.77, N 4.17; found: C 67.81, H 4.18, Br 23.82, N 4.18.
3,6-Dibromo-9-benzyl-9H-carbazole (3b). White plates (hexane). M.p. 156–1578. 1H-NMR (CDCl3,
200 MHz): 8.16 (d, J=2.1, HꢀC(4), HꢀC(5)); 7.52 (dd, J=2.1, 8.3, HꢀC(2), HꢀC(7)); 7.30–7.25 (m, 2
Hm, Hp); 7.21 (d, J=8.3, HꢀC(1), HꢀC(8)); 7.06 (dd, J=1.9, 7.9, 2 Ho); 5.45 (s, PhCH2). 13C-NMR
(CDCl3, 200 MHz): 139.4 (C(9a), C(8a)); 136.2 (Cipso); 129.3 (Co); 128.9 (Cm); 127.8 (C(4), C(5));
126.3 (Cp); 123.6 (C(4a), C(4b)); 123.3 (C(2), C(7)); 112.5 (C(3), C(6)); 110.7 (C(1), C(8)); 46.8
(PhCH2). EI-MS (70 eV): 417 (25), 415 (48, M+), 413 (26), 335 (3, [MꢀBr]+), 333 (2), 255 (3), 164
(13), 91 (100, C
N
N
C 54.85, H 3.14, Br 38.63, N 3.39.
3-Bromo-2-methoxy-9-methyl-9H-carbazole (4a). White needles (hexane). M.p. 1408. 1H-NMR
(CDCl3, 200 MHz): 8.20 (s, HꢀC(4)); 7.96 (dd, J=2.1, 8.2, HꢀC(5)); 7.47–7.33 (m, J=2.2, 7.9, Hꢀ
C(6), HꢀC(7)); 7.25 (dd, HꢀC(8)); 6.83 (s, HꢀC(1)); 4.02 (s, MeO); 3.80 (s, MeN). 13C-NMR
(CDCl3, 200 MHz): 154.5 (C(2)); 141.1 (C(8a)); 141.0 (C(9a)); 125.0 (C(7)); 124.5 (C(4)); 121.9
(C(4b)); 119.4 (C(5)); 119.3 (C(6)); 117.3 (C(4a)); 108.4 (C(8)); 102.8 (C(3)); 92.0 (C(1)); 56.5
(MeO); 29.2 (MeN). EI-MS (70 eV): 291 (22), 289 (24, M+), 276 (7), 274 (7, [MꢀMe]+), 261 (23), 259
(23, [MꢀMeO]+), 248 (9.3), 246 (12.5), 195 (8.4), 179 (8.7, [MꢀMeOꢀBr]+), 167 (47.2), 166 (4.3),
164 (5.1). Anal. calc. for C14H12BrNO (290.16): C 57.95, H 4.17, Br 27.54, N 4.83; found: C 57.82, H
4.15, Br 27.64, N 4.86.
3,6-Dibromo-2-methoxy-9-methyl-9H-carbazole (4b). White plates (hexane). M.p. 1818. 1H-NMR
(CDCl3, 200 MHz): 8.09 (s, HꢀC(4)); 7.99 (d, J=2.3, HꢀC(5)); 7.47 (dd, J=2.3, 8.1, HꢀC(7)); 7.17
(d, J=8.1, HꢀC(8)); 6.76 (s, HꢀC(1)); 4.01 (s, MeO); 3.73 (s, MeN). 13C-NMR (CDCl3, 200 MHz):
155.1 (C(2)); 141.5 (C(8a)); 139.5 (C(9a)); 127.5 (C(7)); 124.7 (C(4)); 123.6 (C(4b)); 122.3 (C(5));
116.1 (C(4a)); 112.2 (C(6)); 109.9 (C(8)); 103.4 (C(3)); 92.0 (C(1)); 56.5 (MeO); 29.3 (MeN). EI-MS
(70 eV): 371 (54), 369 (100, M+), 367 (53), 356 (12.5), 354 (24.1, [MꢀMe]+), 352 (11.6), 327 (16.5), 325
(28.9, [MꢀMeO]+), 324 (15.4), 247 (11.2), 245 (10.3), 166 (12), 165 (4). Anal. calc. for C14H11Br2NO
(369.05): C 45.56, H 3.00, Br 43.30, N 3.80; found: C 45.43, H 2.98, Br 43.41, N 3.82.

1138
Helvetica Chimica Acta – Vol. 89 (2006)
1
1,3,6-Tribromo-2-methoxy-9-methyl-9H-carbazole (4c). White plates (hexane). M.p. 1988. H-NMR
(CDCl3, 200 MHz): 8.10 (s, HꢀC(4)); 7.99 (d, J=2.0, HꢀC(5)); 7.45 (dd, J=2.0, 8.0, HꢀC(7)); 7.15
(d, J=8.0, HꢀC(8)); 4.01 (s, MeO), 3.72 (s, MeN). 13C-NMR (CDCl3, 200 MHz): 155.1 (C(2)); 141.5
(C(8a)); 139.5 (C(9a)); 127.6 (C(7)); 124.8 (C(4)); 123.6 (C(4b)); 122.3 (C(5)); 116.2 (C(4a)); 112.1
(C(6)); 109.9 (C(8)); 103.4 (C(3)); 104.3 (C(1)); 56.5 (MeO); 29.3 (MeN). EI-MS (70 eV): 453 (2.2),
451 (9.3), 449 (12.1, M+), 447 (8.5), 445 (2.1), 371 (43), 369 (100, [MꢀBr]+), 367 (45), 356 (12.5), 354
(24.1, [MꢀMe]+), 352 (11.6), 327 (17), 325 (28.9, [MꢀMeO]+), 324 (15.4), 247 (11.2), 245 (13), 166
(12), 165 (4). Anal. calc. for C14H10Br3NO (447.95): C 37.54, H 2.25, Br 53.51, N 3.13; found: C 37.62,
H 2.27, Br 53.38, N 3.12.
2-(Acetyloxy)-3-bromo-9H-carbazole (=3-Bromo-9H-carbazol-2-ol Acetate; 5a). White needles
(hexane). M.p. 138–1398. 1H-NMR (CDCl3, 200 MHz): 8.30 (br. s, HꢀN(9)); 7.82 (s, HꢀC(4)); 7.71
(d, J=8.3, HꢀC(5)); 7.38 (dd, J=1.9, 8.6, HꢀC(7)); 7.26 (t, J=8.3, HꢀC(6)); 7.25 (d, J=8.3, Hꢀ
C(8)); 7.02 (s, HꢀC(1)); 2.47 (s, Me). 13C-NMR (CDCl3, 200 MHz): 170.6 (CO); 149.3 (C(2)); 140.5
(C(9a)); 140.0 (C(8a)); 128.1 (C(7)); 122.1 (C(4)); 122.8 (C(5)); 120.1 (C(4b)); 119.5 (C(4a)); 113.1
(C(8)); 112.2 (C(6)); 105.5 (C(3)); 104.0 (C(1)); 21.2 (Me). EI-MS (70 eV): 305 (12.5), 303 (10.3, M+),
263 (67.3), 261 (67, [MꢀCH2=CO]+), 182 (100, [MꢀCH2=COꢀBr]+), 154 (25.4), 127 (21.9). Anal.
calc. for C14H10BrNO2 (304.14): C 55.29, H 3.31, Br 26.27, N 4.61; found: C 55.19, H 3.29, Br 26.37, N 4.63.
2-(Acetyloxy)-6-bromo-9H-carbazole (=6-Bromo-9H-carbazol-2-ol Acetate; 5b). White plates (hex-
ane). M.p. 160–1618. 1H-NMR (CDCl3, 200 MHz): 8.21 (br. s, HꢀN(9)); 7.95 (d, J=2.1, HꢀC(5)); 7.75
(d, J=8.8, HꢀC(4)); 7.45 (dd, J=2.1, 8.3, HꢀC(7)); 7.17 (d, J=8.3, HꢀC(8)); 7.04 (d, J=2.4, HꢀC(1));
6.87 (dd, J=2.4, 8.8, HꢀC(3)); 2.40 (s, Me). 13C-NMR (CDCl3, 200 MHz): 170.6 (CO); 149.3 (C(2));
140.5 (C(9a)); 139.0 (C(8a)); 128.1 (C(7)); 122.6 (C(4)); 120.8 (C(5)); 120.1 (C(4a)); 119.5 (C(4b));
113.1 (C(8)); 112.2 (C(6)); 112.1 (C(1)); 103.9 (C(3)); 21.2 (Me). EI-MS (70 eV): 305 (8.5), 303 (9.3,
M+), 263 (57.3), 261 (56, [MꢀCH2=CO]+), 182 (100, [MꢀCH2=COꢀBr]+), 154 (27.4), 127 (17.9).
Anal. calc. for C14H10BrNO2 (304.14): C 55.29, H 3.31, Br 26.27, N 4.61; found: C 55.25, H 3.28, Br
26.36, N 4.64.
2-(Acetyloxy)-3,6-dibromo-9H-carbazole (=3,6-Dibromo-9H-carbazol-2-ol Acetate; 5c). White nee-
dles (hexane/AcOEt 9 :1). M.p. 1708. 1H-NMR (CDCl3, 200 MHz): 8.56 (br. s, HꢀN(9)); 7.71 (d, J=1.8,
HꢀC(5)); 7.60 (s, HꢀC(4)); 7.48 (dd, J=1.8, 8.5, HꢀC(7)); 7.11 (d, J=8.5, HꢀC(8)); 6.94 (s, HꢀC(1));
2.47 (s, Me). 13C-NMR (CDCl3, 200 MHz): 168.6 (CO), 145.7 (C(2)); 139.4 (C(9a)); 139.2 (C(8a)); 128.5
(C(7)); 124.4 (C(4)); 123.1 (C(5)); 123.0 (C(4b)); 120.9 (C(4a)); 113.1 (C(8)); 111.2 (C(6)); 106.5 (C(1));
105.7 (C(3)); 20.6 (Me). EI-MS (70 eV): 385 (13), 383 (26.5, M+), 381 (12.5), 343 (32.4), 341 (65,
[MꢀCH2=CO]+), 339 (31.7), 262 (95.3), 260 (100, MꢀCH2=COꢀBr]+), 181 (25.3), 153 (12.3), 126
(9.8). Anal. calc. for C14H9Br2NO2 (383.04): C 43.90, H 2.37, Br 41.72, N 3.66; found: C 43.86, H 2.35,
Br 41.82, N 3.69.
6-Bromo-3-nitro-9H-carbazole (6a). Yellow plates (hexane). M.p. 1758. 1H-NMR (CDCl3, 200
MHz): 11.1 (br. s, HꢀN(9)); 8.83 (d, J=2.1, HꢀC(4)); 8.22 (dd, J=2.1, 8.4, HꢀC(2)); 8.13 (d, J=2.3,
HꢀC(5)); 7.46 (dd, J=2.3, 8.1, HꢀC(6), HꢀC(7)); 7.37 (d, J=8.4, HꢀC(1)); 7.28 (d, J=8.1, Hꢀ
C(8)). 13C-NMR (CDCl3, 200 MHz): 143.4 (C(3)); 140.1 (C(8a)); 139.6 (C(9a)); 129.6 (C(7)); 123.9
(C(5); 121.9 (C(4b)); 121.7 (C(2)); 121.1 (C(4b)); 118.0 (C(4)); 113.7 (C(8)); 112.3 (C(6)); 111.4
(C(1)). EI-MS (70 eV): 292 (100), 290 (76, M+), 262 (20.3), 260 (19.0, [MꢀNO]+), 246 (24.7), 244
(29.3, [MꢀNO2]+), 165 (46), 164 (31.9, [MꢀNO2 ꢀBr]+). Anal. calc. for C12H7BrN2O2 (291.10): C
49.51, H 2.42, Br 27.45, N 9.62; found: C 49.61, H 2.45, Br 27.38, N 9.58.
REFERENCES
[1] R. Erra-Balsells, A. R. Frasca, Tetrahedron 1983, 39, 33.
[2] M. C. Biondic, R. Erra-Balsells, J. Photochem. Photobiol. A: Chem. 1990, 51, 341.
[3] M. C. Biondic, R. Erra-Balsells, J. Chem. Soc., Perkin Trans. 2 1992, 1049.
[4] M. C. Biondic, R. Erra-Balsells, J. Chem. Soc., Perkin Trans. 2 1993, 887.
[5] M. C. Biondic, R. Erra-Balsells, An. Asoc. Quim. Argent. 1993, 81, 403.
[6] M. C. Biondic, R. Erra-Balsells, J. Photochem. Photobiol. A: Chem. 1994, 77, 149.
[7] R. Erra-Balsells, A. R. Frasca, Tetrahedron 1984, 25, 5363.

Helvetica Chimica Acta – Vol. 89 (2006)
1139
[8] R. Erra-Balsells, A. R. Frasca, An. Asoc. Quim. Argent. 1985, 73, 201.
[9] S. M. Bonesi, R. Erra-Balsells, J. Photochem. Photobiol. A: Chem. 1991, 56, 55.
[10] S. M. Bonesi, R. Erra-Balsells, J. Heterocycl. Chem. 1991, 28, 1035.
[11] S. M. Bonesi, R. Erra-Balsells, J. Photochem. Photobiol. A: Chem. 1997, 110, 271.
[12] a) M. Mesaros, S. M. Bonesi, M. A. Ponce, R. Erra-Balsells, G. M. Bilmes, Photochem. Photobiol.
Sci. 2003, 2, 808; b) A. Cors, S. M. Bonesi, R. Erra-Balsells, ‘Photoreduction of Nitrocompounds’,
unpublished results.
[13] S. Protti, M. Fagnoni, M. Mella, A. Albini, J. Org. Chem. 2004, 69, 3465; A. Fraboni, M. Fagnoni, A.
Albini, J. Org. Chem. 2003, 68, 4886; M. Mella, P. Coppo, B. Guizzardi, M. Fagnoni, M. Freccero, A.
Albini, J. Org. Chem. 2001, 66, 6344; B. Guizzardi, M. Mella, M. Fagnoni, M. Freccero, A. Albini, J.
Org. Chem. 2001, 66, 6353; B. Guizzardi, M. Mella, M. Fagnoni, A. Albini, Tetrahedron 2000, 56,
9383.
[14] J. March, ‘Advanced Organic Chemistry. Reactions, Mechanism and Structure’, Ed. John Wiley,
New York, 1987, p. 530.
[15] A. McKillop, D. Bromley, J. Org. Chem. 1972, 37, 88.
[16] N. P. Buu-Hoi, R. Royer, Recl.Trav. Chim. Pays-Bas 1947, 66, 533.
[17] S. Kajigaeshi, M. Moriwaki, T. Tanaka, S. Fujisaki, T. Kakinami, T. Okamoto, J. Chem. Soc., Perkin
Trans. 1 1990, 897.
[18] R. Gottardi, Monatsh. Chem. 1968, 99, 815; R. Gottardi, Monatsh. Chem. 1969, 100, 42.
[19] J. Y. Kulmanakova, M. S. Yusubov, I. A. Perederina, K.-W. Chi, Proceedings – KORUS 2000, 1, 222.
[20] M. S. Yusov, E. A. Krasnakutskaya, V. D. Filimonov, L. F. Kovaleva, Khim. Geterots. Soedin. 1992,
11, 1477.
[21] J. F. Ambrose, L. L. Carpenter, R. F. Nelson, J. Electrochem. Soc. 1975, 122, 876.
[22] a) S. M. Bonesi, R. Erra-Balsells, J. Heterocycl. Chem. 1997, 34, 877; b) S. M. Bonesi, R. Erra-Bal-
sells, J. Heterocycl. Chem. 1997, 34, 891; M. A. Ponce, O. I. Tarzi, R. Erra-Balsells, J. Heterocycl.
Chem. 2003, 40, 419.
[23] a) S. M. Bonesi, R. Erra-Balsells, J. Heterocycl. Chem. 2001, 38, 77; b) M. E. Monge, S. M. Bonesi, R.
Erra-Balsells, J. Heterocycl. Chem. 2002, 39, 933.
[24] K. Smith, D. M. James, A. G. Mistry, M. R. Bye, D. J. Faulkner, Tetrahedron 1992, 36, 7479.
[25] HyperChem TM 7.0 Suite, Ontario, 2002.
[26] a) S. M. Bonesi, R. Erra-Balsells, J. Lumin. 2001, 93, 51; b) S. M. Bonesi, R. Erra-Balsells, J. Lumin.
2002, 97, 83.
[27] O. I. Tarzi, R. Erra-Balsells, Photochem. Photobiol. B: Biol. 2005, 80, 29.
[28] N. J. Turro, ‘Modern Molecular Photochemistry’, Benjamin, Menlo Park, 1978.
[29] Y. Oikawa, J. Org. Chem. 1976, 41, 1118.
[30] H. Heany, S. V. Levy, J. Chem. Soc., Perkin Trans. 1 1973, 499.
[31] H. Erdtman, F. Haglid, N. E. Stjernstrom, Acta Chim. Scand. 1961, 15, 1761.
[32] R. Erra-Balsells, A. R. Frasca, An. Asoc. Quim. Argent. 1985, 73, 207.
Received February 14, 2006