Synthesis of diazafluorene- and diazafluorenone-N,N′-dioxides using HOF·CH3CN

Article abstract of DOI:10.1016/j.tet.2010.03.011

A variety of diazafluorenes and diazafluorenones were oxidized using the HOF·CH₃CN complex to form the corresponding N,N′-dioxide derivatives under mild conditions. The products exhibit red-shift absorptions in the UV/visible spectrum relative to the parent compounds. Many such oxidations could not be achieved with any other oxygen-transfer agent.

Full text of DOI:10.1016/j.tet.2010.03.011

Tetrahedron 66 (2010) 3297–3300
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Synthesis of diazafluorene- and diazafluorenone-N,N0-dioxides using HOF$CH3CN
*
Tal Harel, Neta Shefer, Youlia Hagooly, Shlomo Rozen
School of Chemistry, Tel-Aviv University, Tel-Aviv 69978, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
A variety of diazafluorenes and diazafluorenones were oxidized using the HOF$CH3CN complex to form
the corresponding N,N0-dioxide derivatives under mild conditions. The products exhibit red-shift ab-
sorptions in the UV/visible spectrum relative to the parent compounds. Many such oxidations could not
be achieved with any other oxygen-transfer agent.
Received 10 December 2009
Received in revised form 6 February 2010
Accepted 1 March 2010
Available online 6 March 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Fluorine
HOF$CH3CN
N-oxide
Diazafluorene
1. Introduction
this reagent are summarized in two reviews describing many first
or difficult to achieve transformations.11,12 Other unique reactions
Recently, diazafluorene derivatives are receiving extensive at-
tention as high performance electron transporting compounds,
hole-blocking materials, good candidates for flat panel displays
and more.1,2 In order to succeed in these fields it is necessary,
among other requirements, to introduce electron-withdrawing
of this reagent involve synthesis of episulfones13 and quinoxaline
N,N0-dioxides,14 transforming aldehydes to nitriles,15 amino acids to
a
-alkyl ones16 and oxidizing thiols and disulfides to either sulfonic
or sulfinic acids at will.17
groups into the
p
-conjugated systems3 and add electron de-
2. Results and discussion
ficient units such as oxadiazole.4,5 Previous works showed that
oxidation of the heteroatom in the relevant heterocycles narrowed
the HOMO–LUMO gap significantly thus increasing their electron
delocalization and affinity. Promising results were demonstrated
when thiazoles were transferred to the respective N-oxides6 and
oligothiophenes to their S,S0-dioxides.7,8 These results prompted us
to explore the oxidation of the diazafluorene and diazafluorenone
systems, by construction their respective N,N0-dioxide derivatives,
which in many cases could not be achieved because of lack of
suitable and powerful enough oxygen-transfer agents. We present
here a novel route for the preparation of these bis-oxidized het-
erocycles by using the acetonitrile complex of the hypofluorous
acid – HOF$CH3CN. Indeed, the electron affinity of the aforemen-
tioned N,N0-dioxides was improved, as expected, and the HOMO–
LUMO gap reduced.
Because of its potential importance, attempts to fully oxidize
4,5-diazafluoren-9-one (1a) have been made in the past including
the use of concentrated H2O2/Na2WO4 system, but only partial
oxidation was achieved and 4,5-diazafluoren-9-on-4-oxide (1c)
was isolated in 7% yield only.18 No traces of the target 4,5-diaza-
fluoren-9-on-4,5-dioxide (1b) were found. In order to check if other
oxygen-transfer agents are up to the challenge, we reacted 1a with
large excess of both dimethyl dioxirane (DMDO) and MCPBA, but
even after prolonged reaction times only minute traces of the de-
sired 1b were formed. We turned our attention to HOF$CH3CN, and
when using stochiometric amount or small excess of this reagent
only the starting material was recovered. However, using a large
excess (10 mol equiv) changed the picture completely and the
previously unknown 1b was formed in almost quantitative yield in
less then a minute (Scheme 1).
Another interesting substrate was 1,8-diazafluoren-9-one (DFO)
(2a). This compound is used extensively for fingerprint visualiza-
tion especially on paper. It seems that this compound is easier to
oxidize than 1a, and indeed it has been described once in the
literature where it was reported that 1,8-diazafluoren-9-on-1,8-
dioxide (2b) was formed in 47% yield after about 20 h heating with
very large excess of hydrogen peroxide.18 The facile formation of 2b
The HOF$CH3CN complex, easily prepared from diluted fluorine9
and aqueous acetonitrile, was developed some years ago.10 It has
established itself as one of the best oxygen-transfer agents chem-
istry has in its arsenal. Earlier processes developed with the aid of
* Corresponding author. Tel.: þ972 3 640 8378; fax: þ972 3 640 9293; e-mail
address: rozens@post.tau.ac.il.
0040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.03.011

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T. Harel et al. / Tetrahedron 66 (2010) 3297–3300
The preparation of 5a seemed to be a worthy challenge since, in
O
addition to the fluorine’s electron withdrawing ability, in many
cases it also contributes to the molecular stability, an important
issue in organic electronic devices.21 In several cases we have used
bromine trifluoride (BrF3) as an efficient tool for converting car-
bonyls to the CF2 group.22 Thus, we have prepared the corre-
sponding 9-dimethyl-hydrazone-4,5-diazafluorene (7)23 from 1a
and reacted it with 2.5 mol equiv of BrF3. In a few seconds the
difluoro derivative 5a was obtained in higher than 95% yield and
60% conversion. If an excess of BrF3 (3.5 mol equiv) was applied,
a full conversion was achieved and the yield of 5a reached 70%, but
was also accompanied by 30% of the unknown 2-bromo-9,9-
difluoro-4,5-diazafluorene (6a), a typical result of aromatic bro-
minations with BrF3 (Scheme 3).24
H2O2/Na2WO4
O
(Ref 18)
O
N
N
1c
O
N
N
N
N
O
F2 + H2O + CH3CN
HOF•CH3CN
O
1a
1b
95%
O
O
O
O
N
N
N
N
O
NNMe2
H2NNMe2
2a
90%
2b
N
N
N
N
Scheme 1. Oxygenation of diazafluoren-9-ones.
1a
7
BrF3
5a (and if desired + 6a)
Scheme 3. Preparation of 9,9-difluoro diazafluorenes.
relative to 1b is also evident from its reaction with HOF$CH3CN
where only 3 mol equiv were needed to produce it practically
instantaneously and in 90% yield (Scheme 1).
Indeed, the HOF$CH3CN complex demonstrated its unique ox-
ygen transfer abilities when reacted with both electron deficient 5a
and 6a. Applying 5 mol equiv of the reagent at room temperature
on 5a gave the unknown 9,9-difluoro-4,5-diazaflouren-4,5-dioxide
(5b) in 90% yield in a few seconds. The same procedure was
repeated with 6a and the new 2-bromo-9,9-difluoro-4,5-diaza-
fluoren-4,5-dioxide (6b) was formed quantitatively in 10 s (Scheme
2). For comparison, we have reacted both 5a and 6a with large
excess of either DMDO or MCPBA for several hours, but this gave
mostly the starting materials with less then 5% of the desired
products.
It is worth noting that, when 1b was qualitatively compared to
2a in regard to their affinity toward amino acids, in particular
alanine (i.e., a preliminary test for the efficiency of fingerprint
detection), no substantial differences were noticed.19 This indicates
that further studies on 1b should be conducted in order to evaluate
its possible role in this field.
Very few 4,5-diazafluorene N,N-dioxide derivatives such as
9,9-dimethyl-4,5-diazafluoren-4,5-dioxide (3b) were synthesized
in the past with orthodox oxygen-transfer agents.20 They were
used as precursors for building oligomeric diazafluorenes and
published mainly as patents. However, a prerequisite for a success
in the oxidation of diazafluorenes is the presence of electron
donating groups at C-9. Obviously, HOF$CH3CN could transfer
oxygen to the nucleophilic sites of such compounds as well, as
seen, for example, when 9,9-dimethyl-4,5-diazafluorene (3a) was
converted to the appropriate N,N0-dioxide (3b) in a few seconds
and in 90% yield. Similarly, 9,9-bis(4-methoxyphenyl)-4,5-diaza-
fluorene (4a)1 was oxidized to provide the new 9,9-bis
(4-methoxyphenyl)-4,5-diazafluoren-4,5-dioxide (4b) using 3 equiv
of HOF$CH3CN in 95% yield. The situation, however, was radically
different when electron-withdrawing groups are located at the
9-position. We have chosen to demonstrate this point by using
the previously unknown 9,9-difluoro-4,5-diazafluorene (5a)
(Scheme 2).
The spectral UV/vis properties of these easily obtained diaza-
fluorenes and diazafluorenones N,N0-dioxides are summarized in
Table 1. One can clearly see that a substantial narrowing of the
HOMO–LUMO energy gap, relative to the starting materials (
did take place.
DEg),
Table 1
Absorption (lmax, nm) and HOMO–LUMO energy gap (DEg, eV) from UV/vis, HOMO–
LUMO energy gap change (DDEg, eV) from UV/vis
Compd
lmax [nm]
DEg[eV]
DDEg[eV]
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
303
324
380
430
309
330
310
320
309
340
309
319
4.10
3.83
3.27
2.89
4.02
3.76
4.00
3.88
4.02
3.65
4.02
3.89
0.27
0.38
0.26
0.12
0.37
0.13
R2
R2
R1
R3
R1
R3
F2 + H2O + CH3CN
HOF•CH3CN
N
N
N
N
O
O
D
Eg(HOMO–LUMO gap)¼h
n/l.
DDEg¼ Eg(starting material)ꢀ
D
DEg(product).
R1 = R2 = Me; R3 = H
4a R1 = R2 = 4-MeOC6H4; R3 = H
3a
3b 90%
4b 95%
5b 90%
3. Conclusion
R1 = R2 = F; R3 = H
R1 = R2 = F; R3 = Br
5a
All the above-mentioned results show initial promising char-
acteristics suitable for compounds serving as electron transporting,
hole blocking and electroluminescent devices, and in all cases
6a
6b 95%
Scheme 2. Oxygenation of 4,5 diazafluorenes.

T. Harel et al. / Tetrahedron 66 (2010) 3297–3300
3299
better than the parent compounds. The oxygen transfer accom-
plished by the HOF$CH3CN complex is conducted under very mild
conditions and in very good yields. Considering the commercial
availability of premixed gases of fluorine/nitrogen, this method of
transferring oxygen may become a method of choice for many cases
were the alternatives are not potent enough.
petroleum ether cannot serve as solvents either since they too react
fast with BrF3. Solvents such as CHCl3, CH2Cl2, CFCl3 or, if solubility is
not an issue, any perfluoroalkane or perfluoroether may be used. Any
use of BrF3 should be conducted in a well-ventilated area, and caution
and common sense should be exercised.
At this point we would like to clarify that when dealing with
BrF3 all mol-equivalent numbers stated in this work are of ap-
proximate values since the reagent usually contains some bromine.
What is more, no matter what the solvent is, it will slowly react
with the reagent, effectively reducing the amount of bromine tri-
fluoride reaching the substrate.
4. Experimental section
4.1. General procedures
1H NMR spectra were recorded using 400 and 200 MHz spec-
trometers. The proton broadband decoupled 13C NMR spectra were
recorded at 100.5 MHz. MeOH-d4, DMSO-d6, D2O, and CDCl3 (Me4Si
as an internal standard) served as solvents. IR spectra were recor-
ded in KBr or CHCl3 on an FTIR spectrometer. MS were measured
under FAB, MALDI-TOF, DCI-CH4 or ESI-QqTOF conditions. UV
spectra were recorded in CH2Cl2 or MeOH.
4.6. General procedure for the preparation of the 9,9-
difluorodiazafluorene derivatives with BrF3
22c
9-Dimethylhydrazone-4,5-diazafluorene (7) (0.5 g, 2.2 mmol)
was dissolved in CHCl3 (20 mL) in a glass flask and cooled to 0 ꢁC.
The best results were achieved when the reagent (3.5 mol equiv)
BrF3 was dissolved in a few milliliter of CFCl3, cooled to 0 ꢁC, and
added drop wise at the same temperature using a glass dropping
funnel. The reaction mixture was then washed with aqueous
Na2SO3 till colorless. The aqueous layer was extracted three times
with CH2Cl2 and the combined organic layers dried over MgSO4.
Evaporation of the solvent followed by flash chromatography
yielded the desired fluorinated compounds.
4.2. General procedure for working with fluorine
Fluorine is a strong oxidant and a corrosive material. It should be
used with an appropriate vacuum line.25 For the occasional user,
however, various premixed mixtures of F2 in inert gases are com-
mercially available, thereby simplifying the process. Unreacted
fluorine should be captured by a simple trap containing a solid base
such as sodalime located at the outlet of the glass reactor. If ele-
mentary precautions are taken, work with fluorine is relatively
simple and we have never experienced any difficulties or un-
pleasant situations.
4.7. Experimental procedures and characterization data
4.7.1. 4,5-Diazafluoren-9-on-4,5-dioxide (1b). Compound 1b was
prepared from commercially available 1a (0.5 g, 2.75 mmol) as
described above, using 10 equiv of the oxidizing agent and recrys-
tallized from acetonitrile. A crystalline yellow solid (0.56 g, 95%
yield) was obtained; mp¼277–279 ꢁC decomp.; lmax(MeOH)
4.3. General procedure for producing HOF$CH3CN
A mixture of 10–20% F2 in nitrogen was used throughout this
work. The gas mixture was prepared in a secondary container prior
to the reaction and passed at a rate of about 400 mL per minute
through a cold (ꢀ15 ꢁC) mixture of 100 mL CH3CN and 10 mL H2O in
a regular glass reactor. The development of the oxidizing power was
monitored by reacting aliquots with an acidic aqueous solution of KI.
The liberated iodine was then titrated with thiosulfate. The typical
concentrations of the oxidizing reagent were around 0.4–0.6 M.
324 nm; IR(KBr) 1226, 1741 cmꢀ1; 1H NMR(DMSO-d6)
d 9.10 (2H, d,
J¼6.6 Hz), 8.52 (2H, d, J¼7.6 Hz), 8.16 ppm (2H, t, J¼7.0 Hz); 13C
NMR
d 182.31, 145.95, 143.77, 132.87, 132.37, 131.88 ppm; HRMS
(ESI-QqTOF) (m/z): calcd for C11H6N2O3 237.0270 (MNa)þ, found
237.0280 (3.95 ppm error).
4.7.2. 1,8-Diazafluoren-9-on-1,8-dioxide (2b)18. Compound 2b was
prepared from commercially available 2a (0.5 g, 2.75 mmol) as de-
scribed above, using 3 equivof the oxidizing agent and recrystallized
from methanol. A crystalline yellow solid (0.53 g, 90% yield) was
obtained; mp¼274 ꢁC decomp.; lmax(MeOH) 430 nm; IR 1270,
4.4. General procedure for working with HOF$CH3CN
The diazafluorene or diazafluorenone derivative was dissolved
in CH2Cl2, and the mixture was either cooled to 0 ꢁC or left in rt. The
oxidizing agent was then added in one portion to the reaction
vessel. The reaction was stopped after a few seconds and the excess
of HOF$CH3CN and the solvents evaporated. The crude product was
usually purified either by vacuum flash chromatography using sil-
ica gel aminopropyl (Merck) with increasing portion of MeOH in
EtOAc or by recrystallization.
1713 cmꢀ1; 1H NMR (DMSO-d6þD2O)
d
8.23 (2H, d, J¼6.6 Hz), 7.99
(2H, d, J¼7.7 Hz), 7.72 ppm (2H, t, J¼7.2 Hz); 13C NMR (DMSO-
d6þD2O)
d 188.89,144.18,139.78,133.79,130.33,124.98 ppm; HRMS
(ESI-QqTOF) (m/z) calcd for C11H6N2O3 237.0270 (MNa)þ, found
237.0262 (3.64 ppm error).
4.7.3. 9,9-Dimethyl-4,5-diazafluoren-4,5-dioxide (3b)20. Compound
3b was prepared from 3a20 (0.5 g, 2.5 mmol) as described above,
using 3 equiv of the oxidizing agent and chromatographed on ami-
nopropyl silica gel using ethyl acetate: methanol 70:30 as eluent. A
crystalline cream solid (0.52 g, 90% yield) was obtained; mp¼85–
4.5. Preparing and handling of BrF3
Although commercially available, we prepare BrF3 simply by
passing 0.6 mol commercial fluorine (ca. 95%) through 0.2 mol of
bromine placed in a copper reactor and held at temperatures between
4 and þ10 ꢁC. Under these conditions, the higher oxidation state of
bromine, BrF5, does not form in any appreciable amount.26 Since
practicallyallfluorineisconsumedduringthereactionasevidentfrom
the verysmall amountof F2 found at the outlet of the reactor (could be
determined by any iodometric method) it could be concluded that the
reaction is complete. The reagent can be stored in TeflonÒ containers
indefinitely. BrF3 tends to react very exothermically with water and
oxygenated organic solvents such as acetone or THF. Alkanes, like
86 ꢁC; lmax(MeOH) 330 nm; IR 1293 cmꢀ1; 1H NMR(200 MHz)
d 8.35
(2H, dd, 1J¼6.4 Hz, 2J¼0.8 Hz), 7.76 (2H, dd, 1J¼7.6 Hz, 2J¼0.8 Hz),
7.58 (2H, dt, 1J¼7.6 Hz, 1J¼6.4 Hz), 1.61 (6H, s); 13C NMR
d 155.12,
143.16, 128.80 (two carbons), 124.13, 46.90, 26.96 ppm; MS (FAB)
(m/z) calcd for C13H12N2O2 228.25, found 229 (MH)þ and 251 (MNa)þ.
4.7.4. 9,9-Bis(4-methoxyphenyl)-4,5-diazafluoren-4,5-dioxide (4b)
was prepared from 4a27. Compound 4b (0.5 g, 1.3 mmol) as de-
scribed above, using 3 equiv of the oxidizing agent and chromato-
graphed on aminopropyl silica gel using acetonitrile/ethyl acetate
30:70 as eluent. A crystalline cream solid (0.51 g, 95% yield) was

3300
T. Harel et al. / Tetrahedron 66 (2010) 3297–3300
obtained; mp¼236.5–238 ꢁC decomp.; lmax(MeOH) 320 nm; IR 1254,
chromatographed on aminopropyl silica gel using acetonitrile/ethyl
acetate 50:50 as eluent. A crystalline cream solid (0.53 g, 95% yield)
2835 cmꢀ1; 1H NMR(MeOH-d4)
d
8.35 (2H, dd, 1J¼6.5 Hz, 2J¼0.9 Hz),
7.49–7.57 (4H, m), 7.17 (4H, d, J¼8.8 Hz) 6.9 (4H, d, J¼8.8 Hz), 3.79(6H,
was obtained; mp¼180–182 ꢁC; lmax(CHCl3) 319 nm; IR 1208 cmꢀ1
;
s); 13C NMR
d
161.95, 153.56, 143.94, 143.09, 135.38, 131.15, 130.92,
1H NMR
d
8.43 (1H, s), 8.27 (1H, d, J¼6.7 Hz), 7.59 (1H, d, 3J¼1.3 Hz),
128.82, 128.38, 127.13, 116.26, 63.98, 56.67 ppm; HRMS (ESI-QqTOF)
(m/z): calcd for C25H20N2O4 413.1484 (MH)þ, found 413.1495
(2.86 ppm error). Anal. Calcd for C25H20N2O4/H2O:C, 69.76;H, 5.15; N,
6.51. Found: C, 70.16; H, 5.21; N, 6.35.
7.45 (1H, d, J¼7.4 Hz), 7.34 (1H, d, J¼7.1 Hz) ppm; 13C NMR
d 146.98,
146.31, 141.71, 141.41, 136.56 (2J¼27 Hz), 136.21 (2J¼26.3 Hz),
126.85, 123.15. 121.96, 119.55, 116.78 (t, 1J¼247 Hz) ppm; 19F NMR
d
ꢀ108.62 ppm; HRMS (DCIþCH4) (m/z) calcd for C11H5BrF2N2O2
316.9590 (MH)þ, found. 316.9600 (3.3 ppm error). Anal. Calcd for
C11H5BrF2N2O2: C, 41.93; H, 1.60; N, 8.89; Br, 25.36; F, 12.06. Found:
C, 41.68; H, 1.62; N, 8.42; Br, 25.43; F, 11.58.
4.7.5. 9-Dimethylhydrazone-4,5-diazafluorene (7)23. Compound 7
was prepared from 1a (1 g, 5.5 mmol), but not analytically purified.
Dimethyl hydrazine, 1a and acetic acid (1:1:1) in 50 mL of MeOH
were refluxed for 4 h resulting in the formation of 1c, which was
chromatographed on silica gel using ethyl acetate/petroleum ether
Acknowledgements
50:50 as eluent (1.1 g, 90% yield). 1H NMR (CDCl3)
d 8.70–8.73 (2H,
This work was supported by the Israel Science Foundation.
m), 8.32 (1H, dd, 1J¼7.8 Hz, 2J¼1.5 Hz), 8.13 (1H, dd, 1J¼7.8 Hz,
2J¼1.5 Hz) 7.37 (1H, m), 7.30 (1H, m), 2.99 (6H, s); 13C NMR
d 151.10,
Supplementary data
151.02, 134.54, 129.00, 123.75, 123.65, 48.85 ppm.
Complete 1H NMR, 13C NMR, 19F NMR, and IR data for all new
compounds. This material is available free of charge via the Internet
at http://www.Sciencedirect.com. Supplementary data associated
with this article can be found in online version at doi:10.1016/
j.tet.2010.03.011.
4.7.6. 9,9-Difluoro-4,5-diazafluorene (5a). Compound 5a was pre-
pared from 7 (0.5 g, 2.2 mmol) as described above using 3.5 equiv of
BrF3 and was chromatographed on silica gel using ethyl acetate/
petroleum ether 20:80 as eluent. A crystalline cream-brown solid
(0.32 g, 70% yield) was obtained; lmax(MeOH) 309 nm. 1H NMR
(200 MHz)
d 8.62–8.69 (2H, m), 7.67–7.91 (2H, m), 7.17–7.24 (2H,
m); 13C NMR
d 157.76, 153.74, 133.31, 131.38, 124.287, 119.89 (t,
References and notes
1J¼245.1 Hz) ppm; 19F NMR
d
ꢀ115.43 ppm. HRMS (MALDI-TOF)
(m/z) calcd for C11H6N2F2 227.0397 (MNa)þ, found 227.0391
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(2.53 ppm error).
4.7.7. 2-Bromo-9,9-difluoro-4,5-diazafluorene (6a). Compound 6a
was prepared from 7 (0.5 g, 2.2 mmol) as described above using
3.5 equiv of BrF3 and was chromatographed on silica gel using ethyl
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309 nm. 1H NMR
d
8.78 (1H, d, 2J¼2 Hz), 8.74 (1H, dd, 1J¼4.8,
2J¼1.2 Hz), 8.048–8.052 (1H, m), 7.92 (1H, dd, 1J¼7.6 Hz, 2J¼1.2 Hz),
7.33 (1H, dd, 1J¼7.6 Hz, 1J¼4.8 Hz); 13C NMR
d 156.79, 156.100,
154.78, 154.33,134.40, 133.04 (2J¼25.5 Hz), 131.48, 124.432, 121.47,
116.57(1J¼246.3 Hz) ppm; 19F NMR
ꢀ115.03 ppm. HRMS (CI) (m/z)
d
calcd for C11H5N2F2Br 282.9682 (MH)þ, found 282.9690 (2.8 ppm
error).
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16. Harel, T.; Rozen, S. J. Org. Chem. 2007, 72, 6500.
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18. Kloc, K.; Mlochowski, J.; Szulc, Z. Can. J. Chem. 1979, 57, 1506.
19. Pounds, C. A.; Phil, M.; Grigg, R.; Mongkolaussavaratana, T. J. Forensic Sci. 1990,
35, 169; See also: Almog, J.; Springer, E.; Weisner, S.; Frank, A.; Khodzhaev, O.;
Lidor, R.; Bahar, E.; Varkony, H.; Dayan, S.; Rozen, S. J. Forensic Sci. 1999, 44, 114.
20. Ohrui, H.; Senoo, A.; Kosuge, T. U.S. Patent 0161574 A1, 2008.
21. Le, Y.; Nitani, M.; Ishikawa, M.; Nakayama, K.-I.; Tada, H.; Kaneda, T.; Aso, Y. Org.
Lett. 2007, 9, 2115.
4.7.8. 9,9-Difluoro-4,5-diazafluoren-4,5-dioxide (5b). Compound 5b
was prepared from 5a (0.5 g, 2.4 mmol) as described above, using
5 equiv of the oxidizing agent and chromatographed on amino-
propyl silica gel using acetonitrile/ethyl acetate 50:50 as eluent. A
crystalline cream solid (0.52 g, 90% yield) was obtained; mp¼186–
187 ꢁC decomp.; lmax (CHCl3) 340 nm; IR 1222 cmꢀ1; 1H NMR
d 8.28
(2H,d, J¼6.8 Hz), 7.46 (2H, d, J¼6.8 Hz), 7.33 ppm (2H, t, J¼6.8 Hz);
13C NMR
d
146.19, 142.3, 136.72 (t, 2J¼27.16 Hz), 126.70, 119.46,
117.09 (1J¼244.7 Hz) ppm; 19F NMR
d
ꢀ109.08 ppm. HRMS
(DCIþCH4) (m/z) calcd for C11H6F2N2O2 237.0521 (MH)þ, found
237.0516 (2.2 ppm error). Anal. Calcd for C11H6F2N2O2: C, 55.94; H,
2.56. Found: C, 56.00; H, 3.01.
22. (a) Rozen, S.; Zamir, D.; Brand, M.; Hebel, D. J. Am. Chem. Soc. 1987, 109, 896; (b)
Sasson, R.; Hagooly, A.; Rozen, S. Org. Lett. 2003, 5, 769; (c) Rozen, S.; Mishani,
E.; Bar-Haim, A. J. Org. Chem. 1994, 59, 2918.
23. Deshpande, M. S.; Kumbhar, A. S. J. Chem. Sci. 2005, 117, 153.
24. Rozen, S.; Lerman, O. J. Org. Chem. 1993, 58, 239.
25. Dayan, S.; Kol, M.; Rozen, S. Synthesis 1999, 1427.
26. Stein, L. J. Am. Chem. Soc. 1959, 81, 1269.
27. Yamada, M.; Sun, J.; Suda, Y.; Nakaya, T. Chem. Lett. 1998, 1055.
4.7.9. 2-Bromo-9,9-difluoro-4,5-diazafluoren-4,5-dioxide
(6b). Compound 6b was prepared from 6a (0.5 g, 1.8 mmol) as
described above, using 5 equiv of the oxidizing agent and