Triphenylmethyldifluoramine: A stable reagent for the synthesis of gem-bis(difluoramines)

Article abstract of DOI:10.1039/b203811k

The conversion of ketones into geminal bis(difluoramines) can be achieved under mild two-phase reaction conditions by employing triphenylmethyldifluoramine as an in situ source of difluoramine.

Full text of DOI:10.1039/b203811k

Triphenylmethyldifluoramine: a stable reagent for the synthesis of
gem-bis(difluoramines)†
G. K. Surya Prakash,*a Markus Etzkorn,a George A. Olah,*a Karl O. Christe,ab Stefan Schneidera
and Ashwani Vijb
a Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California,
University Park, Los Angeles, CA 90089-1661, USA. E-mail: gprakash@usc.edu; Fax: +1 213 740 6270;
Tel: +1 213 740 5984
b Air Force Research Laboratory, Edwards Air Force Base, CA, 93524, USA
Received (in Corvallis, OR, USA) 16th April 2002, Accepted 13th June 2002
First published as an Advance Article on the web 10th July 2002
The conversion of ketones into geminal bis(difluoramines)
can be achieved under mild two-phase reaction conditions by
employing triphenylmethyldifluoramine as an in situ source
of difluoramine.
The structural features of 3 are similar to those reported for p-
bromophenyldiphenyl)methyldifluoramine.10 The C–N dis-
tance of 1.523(2) Å found in 3 is longer than the corresponding
distance of 1.481 Å in triphenylmethylamine.11 The presence of
two highly electronegative fluorines on the nitrogen atom
reduces back-donation from the nitrogen lone pair, thereby
decreasing the C–N bond order. The torsion angles from the
difluoramine group to the ipso-carbon atoms of the phenyl
rings, namely F1–N1–C1–C8 and F2–N1–C1–C2 are 56.4(1)°
and 39.1(1)°, respectively, and reflect the deviation of these
atoms from perfect staggering.
The introduction of a difluoramine functionality into organic
moieties and the properties of the obtained target compounds
have been investigated intensively.1 More recently a new class
of high-energy containing materials, gem-bis(difluoramine)-
substituted heterocyclic nitramines has gained much attention
as high-energy oxidizers: HNFX (1)2 and TNFX (2)3 have been
successfully synthesized from their corresponding ketone
derivatives. Usually this transformation is achieved under
strongly acidic conditions with an excess of condensed
difluoramine (HNF2),4 an unpredictably shock-sensitive and
thermally unstable, gaseous compound.5 HNF2 can be gen-
erated from different precursors, e.g. tetrafluorohydrazine,6
N,N-difluorourea7 or N,N-difluorocarbamates.8 A short report
on the synthesis of triphenylmethyldifluoramine (3) mentions
its hydrolysis to HNF2.9 Except for 3, a stable crystalline
compound, all of the other HNF2 precursors have only limited
stability.
Compound 3 proved to be a convenient reagent for the
conversion of ketones (A) into gem-bis(difluoramines) (B) thus
circumventing the potential hazards associated with previous
methodologies.
The closest intramolecular H…F contacts found in the crystal
structure of 3 result from H3…F1 and F2…F15 at 2.281(16)
and 2.334(16) Å, respectively. The closest intermolecular H…N
contacts result from H15…N1 and H9…N1 at 2.408(16) and
2.553(16) Å, respectively. All these contacts are shorter than the
sum of the van der Waals radii of nitrogen and hydrogen (1.55
+ 1.20 = 2.75 Å) or fluorine and hydrogen (1.47 + 1.20 = 2.67
Å). A plot of the unit cell is provided with the ESI.†‡
We have investigated two-phase organic solvent/oleum
media bis(difluoraminations) with 3 under mild reaction
conditions for some model ketones. Typically a solution of the
ketone (A) and 2.2 equivalents of 3 in CH2Cl2, CHCl3 or CFCl3
were added dropwise to the same volume of oleum (30%) at 0
°C. Due to the potential hazards of the intermediate HNF2 and
any labile products, all reactions were carried out on a small
scale ( ~ 1 mmol). The completion of the reactions was
Triphenylmethyldifluoramine (3) is accessible from chloro-
triphenylmethane and N2F4 in multi-gram quantities as stable,
non-hygroscopic, colorless crystals.9† It can be stored without
decomposition or any other hazards at ambient conditions. The
complete characterization of 3 was carried out by IR, Raman,
1H, 13C and 19F NMR spectroscopy. Additionally, we obtained
single crystals suitable for a X-ray crystal structure determi-
nation.‡
1
confirmed by NMR spectroscopy (19F, H, 13C). Thus, they
were usually worked up after 15–20 min by pouring into ice/
water, followed by extraction (CH2Cl2) and evaporation of the
solvent from the dried organic phase.
The crude product contained only triphenylmethanol and the
gem-bis(difluoramine) that (on a larger scale) can be purified by
repeated crystallization, chromatography on neutral aluminum
† Electronic supplementary information (ESI) available: optimized prepara-
tion of 3, analytical data for 3, 18–21 and a plot of the unit cell of 3. See
http://www.rsc.org/suppdata/cc/b2/b203811k/
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CHEM. COMMUN., 2002, 1712–1713
This journal is © The Royal Society of Chemistry 2002

Table 1 Conversion of ketones to gem-bis(difluoramines)
Entry
Ketone (A)
gem-Bis(difluoramine) (B)
Yield (%)
Ref.
1
2
3
4
5
6
7
8
Acetone (4)
2,2-Bis(difluoramino)propane (12)
1,1-Bis(difluoramino)cyclopentane (13)
1,1-Bis(difluoramino)cyclohexane (14)
1,1,4,4-Tetrakis(difluoramino)cyclohexane (15)
90
80
80
85
4a
4a
4a
4a
14
—
—
—
—
—
—
Cyclopentanone (5)
Cyclohexanone (6)
Cyclohexa-1,4-dione (7)
N-Acetylpiperidinone (8)
9a (n = 1)
9b (n = 9)
9c (n = 10)
4,4-Bis(difluoramino)-N-acetylpiperidine (16)
85
ab
17a
17b
17c
18
20a
20b
80b
85b
90b
0c
9
10
11
10
11a (R = H)
11b (R = CH3)
0c
a Decomposition upon work-up; b Analytical data provided in the ESI†; c Only formation of the cage-systems 19a (19b) in ca. 80% yield was observed.
oxide or distillation, respectively. The given yields are based on
1H NMR data of the crude product containing only the gem-
bis(difluoramine) (B) and triphenylmethanol. A variety of
known (entries 1–5) as well as new (entries 6–11) difluoramine
derivatives was prepared in good to excellent yields (Table
1).
nism, find alternative preparations of the reagent, and synthe-
size potential high-energy materials.
We gratefully acknowledge the financial support by the
Office of Naval Research (Program Manager: Dr Judah
Goldwasser), the Air Force Office of Scientific Research and
the National Science Foundation.
CAUTION: gem-Bis(difluoramines) are potentially shock-
sensitive and thermolabile. They should be handled with care!
Notes and references
‡ Crystal data: the data were collected on a Bruker 3-circle platform
diffractometer equipped with a SMART CCD detector with the c-axis fixed
at 54.74° from a fine-focus tube and an LT-3 apparatus for low temperature
data collection: 3, C19H15F2N, Mr = 295.32, monoclinic, a = 10.3882(12),
b = 9.8259(11), c = 14.6065(16) Å, b = 92.402(2)°, V = 1489.6(3) Å3,
T = 213(2) K, space group P21/n, Z = 4, Dc = 1.317 Mg m23, m(Mo-Ka)
= 0.094 mm21, 9649 reflections mesuared, 3035 unique (Rint = 0.0321).
The final wR2 was 0.1098 (all data). CCDC reference number 188416. See
http://www.rsc.org/suppdata/cc/b2/b203811k/ for crystallographic data in
CIF or other electronic format.
1 W. Hanefeld, N,N-Dihalogen-amine, ed. D. Klaman in Methoden der
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Although 17a could not yet be isolated on this small scale, it
represents the smallest alicyclic geminal bis(difluoramine) so
far characterized. The bisketone 11a (11b) could only be
converted to the bridged hemiaminal 19a (19b); neither a larger
excess of 3 nor extended reaction time furnished the tetrakis(di-
fluoramine) 20a (20b).
Mechanistically, the reactions involve cleavage of 3 to the
triphenylmethyl cation and difluoramine. The latter or the in situ
product, difluorsulfamic acid, reacts with the ketone A to
provide gem-bis(difluoramine) B. It is noteworthy that our
reaction did not proceed in conc. H2SO4, although acidolysis of
3 was observed spectroscopically: the 19F NMR signal of 3
disappeared and the trityl cation was observed in the 13C NMR
spectrum. Also employing other strong acids such as FSO3H,
CF3CO2H, CF3SO3H or CF3SO3H/(CF3SO2)2O resulted in
similar behavior. In the case of FSO3H/SbF5 only decomposi-
tion of 3 was observed. We have never been able to observe free
or protonated difluoramine.12 It is also possible that under the
reaction conditions SO3 in oleum—besides being a water
scavanger – might react with 3 to form F2NSO3H as the ultimate
difluoraminating reagent (vide supra). The triphenylmethyl
cation might also act like a Lewis acid catalyst, polarizing the
carbonyl group (Mukayama type activation13).
In conclusion, the readily preparable, stable triphenylme-
thyldifluoramine (3) can be conveniently employed for the
efficient conversion of ketones (A) under mild reaction
conditions to the respective geminal bis(difluoramines) (B).
This is the first difluoramination methodology that avoids the
use of neat HNF2 and therefore minimizes its potential hazards.
Work is underway to further investigate the reaction mecha-
11 C. Gidwell and G. Furgeson, Acta Crystallogr., Sect. C, 1994, 50,
924.
12 K. O. Christe, Inorg. Chem., 1975, 14, 2821.
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