Reaction of formamides with fluorotris(pentafluorophenyl)silane

Full text of DOI:10.1007/s11178-008-3028-x

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2008, Vol. 44, No. 3, pp. 472–473. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © V.V. Levin, A.D. Dil’man, P.A. Belyakov, M.I. Struchkova, 2008, published in Zhurnal Organicheskoi Khimii, 2008, Vol. 44, No. 3,
pp. 472–473.
SHORT
COMMUNICATIONS
Reaction of Formamides
with Fluorotris(pentafluorophenyl)silane
V. V. Levin, A. D. Dil’man, P. A. Belyakov, and M. I. Struchkova
Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Leninskii pr. 47, Moscow, 119991 Russia
e-mail: dilman@ioc.ac.ru
Received July 4, 2007
DOI: 10.1134/S1070428008030287
Carboxylic acid amides are weakly reactive toward
nucleophiles. In particular, carboxamides are generally
inert with respect to organosilicon compounds due to
both low electrophilicity of the amide carbonyl group
and low polarity of the silicon–carbon bond [1]. How-
ever, we can presume that organosilicon reagents with
enhanced Lewis acidity of the silicon atom should
form complexes with carboxamides in which both
electrophilic and nucleophilic components should be
activated.
Unfortunately, we failed to isolate and identify prod-
ucts in the reactions of (C6F5)3SiF with N,N-dimethyl-
acetamide and N-methylpyrrolidin-2-one; as a result,
intractable mixtures were obtained.
Scheme 2 shows a possible reaction mechanism.
Initially, amide I reacts with (C6F5)3SiF to give a com-
plex with five-coordinate silicon atom; this complex
contains activated carbonyl group and polarized Si–C
bond. The subsequent transfer of one pentafluoro-
phenyl group gives hemiaminal intermediate which is
capable of undergoing ionization in the presence of the
second silane molecule to form iminium cation. In the
final step, pentafluorophenyl group is transferred from
the five-coordinate silicon complex to iminium cation.
We recently showed that the Lewis acidity of
silanes can be enhanced to an appreciable extent via
introduction of three electron-withdrawing penta-
fluorophenyl groups to the silicon atom [2–6]. The
most effective reagent for building up new C–C bonds
was fluorotris(pentafluorophenyl)silane [5, 6]. We be-
lieved it to be interesting to examine the behavior of
carboxamides in the presence of (C6F5)3SiF.
Scheme 2.
F
R2N
O
R3SiF
Si
R
R2N
O
R = C6F5
R
R
Fluorotris(pentafluorophenyl)silane reacted with
N,N-disubstituted formamides Ia–Id to give the corre-
sponding N,N-dialkylbis(pentafluorophenyl)methan-
amines IIa–IId (Scheme 1). The reactions were carried
out under solvent-free conditions by heating the reac-
tants (formamide–silane ratio 10:1) for several hours
at 100°C. Both N,N-dialkylformamides and cyclic
amides can be involved in the process; however, the
reaction with morpholine-4-carbaldehyde (Id) required
longer time, and the yield of compound IId was poor.
I
F
F
Si
R
F
Si
R
R
R2N
O
R3SiF
R
O
Si
R
R
R
R
R
R2N
R
R2N
R
II
Scheme 1.
Thus we have revealed a new reaction of amides,
which is based on the ability of silicon atom to form
five-coordinate intermediates. The key specific feature
of the process is activation of the amide carbonyl
group due to reaction with fluorotris(pentafluoro-
phenyl)silane.
C6F5
100°C
R2NCHO
+ (C6F5)3SiF
R2N
C6F5
Ia–Id
IIa–IId
R = Me (a), Et (b); R2N = pyrrolidin-1-yl (c), morpholino (d).
472

REACTION OF FORMAMIDES WITH FLUOROTRIS(PENTAFLUOROPHENYL)SILANE
473
Bis(pentafluorophenyl)methanamines IIa–IId
(general procedure). A mixture of 548 mg (1 mmol) of
(C6F5)3SiF and 10 mmol of formamide Ia–Id was
heated on a boiling water bath over a period indicated
below. The mixture was cooled, 1 ml of a saturated
aqueous solution of sodium carbonate and 1 ml of
water were added, and the resulting suspension was
stirred for 2 min and treated with diethyl ether–hexane
(1 :1; IIa, IIc, IId) or hexane (IIb) (3 ×7 ml). The
extracts were combined, washed with water (2×2 ml),
filtered through a layer of Na2SO4, and evaporated.
13C NMR spectrum (75 MHz, CDCl3), δC, ppm: 23.7,
53.5, 55.7 q (J = 1.2 Hz), 113.6 m, 137.8 d.m (J =
243.1 Hz), 141.2 d.m (J = 243.9 Hz), 145.3 d.m (J =
253.0 Hz). 19F NMR spectrum (188 MHz, CDCl3),
δF, ppm: –162.8 m (4F, m-F), –154.7 t (2F, p-F, J =
20.8 Hz), –140.1 m (4F, o-F). Found, %: C 48.54;
H 2.46; N 3.31. C15H7F10N. Calculated, %: C 48.94;
H 2.17; N 3.36.
4-[Bis(pentafluorophenyl)methyl]morpholine
(IId). Yield 27% (according to the NMR data using
trichloroethylene as reference). By chromatography
(hexane–EtOAc, 12:1) we isolated 190 mg of com-
pound IId which contained (according to the NMR
data) ~10% of an unidentified product which could not
be separated by vacuum distillation. bp 132–136°C
N,N-Dimethylbis(pentafluorophenyl)methan-
amine (IIa). Reaction time 2 h. The product was pur-
ified by chromatography using hexane–ethyl acetate
(40:1) as eluent. Yield 510 mg (87%),* Rf 0.21 (hex-
1
1
ane–EtOAc, 25:1), mp 42–43°C. H NMR spectrum
(bath temperature, 0.36 mm). H NMR spectrum
(200 MHz, CDCl3), δ, ppm: 2.33 s (6H, CH3), 5.13 s
(1H, CH). 13C NMR spectrum (50 MHz, CDCl3), δC,
ppm: 44.5 q and 58.1 q (J = 1.6 Hz), 112.9 m,
137.8 d.m (J = 256.6 Hz), 141.1 d.m (J = 255.1 Hz),
145.3 d.m (J = 250.6 Hz). 19F NMR spectrum
(188 MHz, CDCl3), δF, ppm: –162.3 m (4F, m-F);
–154.7 t (2F, p-F, J = 21.5 Hz), –140.5 d.m (4F, o-F,
J = 14.5 Hz). Found, %: C 45.87; H 1.81; N 3.49.
C15H7F10N. Calculated, %: C 46.05; H 1.80; N 3.58.
(200 MHz, CDCl3), δ, ppm: 2.48 t [4H, N(CH2)2, J =
4.0 Hz], 3.76 t [4H, O(CH2)2, J = 4.2 Hz], 5.28 s (1H,
CH). 13C NMR spectrum (75 MHz, CDCl3), δC, ppm:
52.6, 57.2, 66.8, 112.0 m, 137.8 d.m (J = 253.6 Hz),
140.9 d.m (J = 255.1 Hz), 145.3 d.m (J = 246.1 Hz).
19F NMR spectrum (282 MHz, CDCl3), δF, ppm:
–162.1 m (4F, m-F), –154.3 t (2F, p-F, J = 20.8 Hz),
–140.1 d.m (4F, o-F, J = 15.6 Hz).
1
The H, 13C, and 19F NMR spectra were measured
N,N-Diethylbis(pentafluorophenyl)methanamine
(IIb). Reaction time 2 h. The product was purified by
chromatography using hexane as eluent, followed by
distillation. Yield yield 335 mg (45%), Rf 0.2 (hexane),
on a Bruker AC-200 spectrometer. All syntheses were
performed under dry argon. Kieselgel 60 silica gel
(40–63 μm; Merck) was used for chromatography.
This study was performed under financial support
by the Ministry of Science of the Russian Federation
(project no. MK-4483.2007.3) and by the program of
the Presidium of the Russian Academy of Sciences and
Foundation for Support of Russian Sciences.
1
bp 110–115°C (bath temperature, 0.43 mm). H NMR
spectrum (200 MHz, CDCl3), δ, ppm: 1.02 t (6H, CH3,
J = 13.9 Hz), 2.65 q (4H, CH2, J = 13.9 Hz), 5.77 s
(1H, CH). 13C NMR spectrum (50 MHz, CDCl3),
δC, ppm: 12.5, 44.6, 52.8 q (J = 1.6 Hz), 113.6 m,
137.8 d.m (J = 253.6 Hz), 140.9 d.m (J = 255.1 Hz),
145.3 d.m (J = 246.1 Hz). 19F NMR spectrum
(282 MHz, CDCl3), δF, ppm: –162.6 t (4F, m-F, J =
17.8 Hz), –155.4 t (2F, p-F, J = 21.7 Hz), –141.1 d.m
(4F, o-F, J = 19.2 Hz). Found, %: C 48.70; H 2.31;
N 3.22. C15H7F10N. Calculated, %: C 48.70; H 2.64;
N 3.34.
REFERENCES
1. Weber, W.P., Silicon Reagents for Organic Synthesis,
Berlin: Springer, 1983.
2. Dilman, A.D., Belyakov, P.A., Korlyukov, A.A., Struch-
kova, M.I., and Tartakovsky, V.A., Org. Lett., 2005,
vol. 7, p. 2913.
1-[Bis(pentafluorophenyl)methyl]pyrrolidine
(IIc). Reaction time 1 h. The product was purified by
chromatography using hexane–ethyl acetate (40:1) as
eluent, followed by distillation. Yield 326 mg (52%),
Rf 0.31 (hexane–EtOAc, 40:1), bp 112–116°C (bath
temperature, 0.42 mm), mp 51–52°C. H NMR spec-
trum (200 MHz, CDCl3), δ, ppm: 1.81–1.93 m [4H,
(CH2)2], 2.48–2.61 m [4H, N(CH2)2], 5.28 s (1H, CH).
3. Levin, V.V., Dilman, A.D., Belyakov, P.A., Korlyu-
kov, A.A., Struchkova, M.I., Antipin, M.Y., and Tartakov-
sky, V.A., Synthesis, 2006, p. 489.
4. Dil’man, A.D., Arkhipov, D.E., Belyakov, P.A., Struch-
kova, M.I., and Tartakovskii, V.A., Russ. Chem. Bull.,
2006, vol. 55, p. 517.
1
5. Dilman, A.D., Levin, V.V., Karni, M., and Apeloig, Y.,
J. Org. Chem., 2006, vol. 71, p. 7214.
6. Levin, V.V., Dilman, A.D., Belyakov, P.A., Struchko-
va, M.I., and Tartakovsky, V.A., Tetrahedron Lett., 2006,
vol. 47, p. 8959.
* Hereinafter, the given yields were calculated assuming transfer
of three pentafluorophenyl groups from (C6F5)3SiF molecule.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 3 2008