H.-H. Limbach et al.
Compound [15N2]10: This compound was prepared by a modification of
the synthesis described for the unlabeled compound.[65] Compound
[15N2]5 (195 mg, 0.9 mmol) was dissolved in concentrated sulfuric acid
(0.9 mL, 96%) and cooled to ꢀ158C. A mixture of concentrated nitric
acid (56.7 mg, 100%) and sulfuric acid (0.18 mL, 96%) was added drop-
wise to the cold solution. After stirring for 5 min, the reaction mixture
was poured on ice (20 g) and was neutralized with aqueous ammonia.
The precipitate was filtered out and subjected to chromatography (eluent
hexane/ethyl acetate 1:1). The fractions containing the product were col-
lected and the solvent was removed under vacuum. The residue was re-
crystallized from methanol to give 10 (159 mg, 68%) as dark red crystals.
M.p. 135–1368C.
mately 2 d. The nonsoluble product 10H CHTUNGTERNNNUG
+A[BARF]ꢀ was then filtered and
dried under vacuum for another day. Then the weighted amount of the
substance was placed into the NMR spectroscopic sample tube and the
solvent, [D8]toluene, was added by means of vacuum transfer. Prior to
that, [D8]toluene was dried over fresh molecular sieves and basic Al2O3.
Synthesis of the deuterated freon mixture CDF3/CDF2Cl: A modification
of the procedure reported by Golubev et al. was used.[70] SbF3 (30 g,
168 mmol), CDCl3 (20 g, 168 mmol), and SbCl5 (1.5 mL) were mixed
inside a Teflon-coated autoclave and heated to 1008C for a few hours.
After the pressure had risen to 30–35 bar, heating was continued for an-
other hour. Afterwards, the autoclave was cooled to room temperature,
and the freon mixture was condensed through a small column filled with
KOH to a bulb filled with KOH on the high-vacuum line (10ꢀ6 mbar).
During this process it was degassed several times to remove air in the
system. The freon mixture was then carefully thawed to a temperature of
ꢀ508C, at which it was left for 45 min. The freon mixture was condensed
into a small steel lecture bottle filled with some dry tetrabutylammonium
fluoride, in which it was left overnight. The next day it could be con-
densed to another steel lecture bottle for long-term storage. It was dried
for 30 min with Al2O3 directly prior to use. The CDF3/CDF2Cl ratio was
3:1 as determined from the residual solvent 1H NMR spectroscopic sig-
nals.[7]
Compound [15N2]11: This compound was prepared by a modification of
the synthesis described for the unlabeled compound.[62] A solution of 1-
chlorobenzotriazole (180 mg, 1.17 mmol) in chloroform (10 mL) was
added dropwise under stirring to
a
solution of [15N2]5 (250 mg,
1.17 mmol) in chloroform (10 mL) cooled to ꢀ158C. After complete ad-
dition, the reaction mass was stirred for another 30 min. The solution was
concentrated (ꢅ2 mL) and subjected to chromatography. The yellow
fraction (Rf =0.73) was collected and the solvent was distilled, thus yield-
ing 11 (250 mg, 86%) as a pale yellow oil.
Compound [15N2]12: This compound was prepared by a modification of
the synthesis described for the unlabeled compound.[64] A solution of N-
bromosuccinimide (140 mg, 0.8 mmol) in chloroform (10 mL) was slowly
NMR spectroscopy experiments: The NMR spectroscopy measurements
were carried out using a Bruker AMX 500 instrument operating at
500.13 MHz for 1H, at 125.76 MHz for 13C, and at 50.70 MHz for 15N. A
Bruker AC 250 spectrometer operating at 250.13 MHz for 1H and at
62.90 MHz for 13C and a Bruker DRX 750 spectrometer operating at
(ꢅ30 min) added dropwise under stirring to
a
solution of [15N2]5
(165 mg, 0.75 mmol) in chloroform (8 mL) cooled to ꢀ208C. The brown
reaction mass was stirred at ꢀ158C for an additional 20 min, concentrat-
ed to a volume of 2 mL, and subjected to chromatography (eluent chloro-
form). The light yellow fraction was collected and the solvent was dis-
tilled to yield 12.
1
749.98 MHz for H and at 188.58 MHz for 13C were used for the measure-
ments at other magnetic fields. 1H and 13C chemical shifts are given
versus TMS, and 15N chemical shifts versus external nitromethane. For
the NMR spectroscopy experiments using CDF3/CDF2Cl as solvent, spe-
cial thick-wall NMR spectroscopy tubes (Wilmad, Buena) were em-
ployed, equipped with a Teflon-needle valve.
Compound [15N2]13: The synthesis of the unlabeled compound has been
described previously.[66] The labeled compound was prepared using the
following modified procedure.[67] A solution of 2-chloro-1,3,5-trinitroben-
zene in acetonitrile (5 mL; for synthesis, see the Supporting Information)
was added under occasional shaking to a solution of [15N2]5 (108 mg,
0.5 mmol) in acetonitrile (5 mL). After 15 min, water (20 mL) was added,
and the solution was extracted with diethyl ether (7ꢇ10 mL). The com-
bined extracts were washed with a saturated sodium bicarbonate solution
and with water. The ethereal solution was dried over magnesium sulfate
and distilled. The residue was recrystallized from acetonitrile to yield 13
(127 mg, 60%) as dark violet crystals.
Acknowledgements
This work has been supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie (Frankfurt).
Protonation of proton sponges: To prepare the protonated perchlorates,
a solution of the corresponding base in ethyl acetate (0.04 mmol in 2 mL)
was added to perchloric acid (60%, 0.1 mmol). After 2 min, the mixture
was diluted with diethyl ether (3 mL). The precipitate was collected,
washed with diethyl ether, and dried. Recrystallization from ethanol af-
forded colorless crystals of the perchlorates. The yields were nearly quan-
titative.
To prepare the protonated trifluoroacetate 10H+, trifluoroacetic acid
(99%, Aldrich) was added directly to an NMR spectroscopy sample tube
that contained the corresponding base dissolved in a deuterated solvent.
[1] Hydrogen Transfer Reactions, Vols. 1–4, Wiley-VCH, Weinheim,
2007.
[2] H. H. Limbach, Hydrogen Transfer Reactions (Eds.: J. T. Hynes, J.
Klinman, H. H. Limbach, R. L. Schowen), 2007, Chapter 6, pp. 135–
221.
[3] a) H. H. Limbach, J. Manz, Ber. Bunsenges. Phys. Chem. 1998, 102,
289–291; b) K. B. Schowen, H. H. Limbach, G. S. Denisov, R. L.
Schowen, Biochim. Biophys. Acta 2000, 1458, 43–62.
[4] Isotope Effects in the Biological and Chemical Sciences (Eds.: A.
Kohen, H. H. Limbach), Taylor & Francis, Boca Raton FL, 2005.
[5] H. H. Limbach, J. M. Lopez, A. Kohen, Phil. Trans. B 2006, 361,
1399–1415.
[6] N. S. Golubev, G. S. Denisov, S. N. Smirnov, D. N. Shchepkin, H. H.
Limbach, Z. Phys. Chem. (Muenchen Ger.) 1996, 196, 73–84.
[7] I. G. Shenderovich, A. P. Burtsev, G. S. Denisov, N. S. Golubev,
[8] M. Ramos, I. Alkorta, J. Elguero, N. S. Golubev, G. S. Denisov, H.
To prepare 10H CHTUNGTRENNUNG
+A[BARF]ꢀ, we proceeded as follows. Firstly, a synthesis
of HBARF was performed according to the procedure reported by Brook-
hart et al.[68] and modified by Grꢀndemann.[69] As HBARF is very hygro-
scopic and sensitive to oxygen, the whole preparation procedure was car-
ried out either in a dry box under an argon atmosphere or on a vacuum
line. Proton sponge 10 (0.01 mmol) was placed in an NMR spectroscopy
sample tube. HBARF was dissolved in purified and dried dichlorome-
thane. The solution that contained HBARF (about 0.008 mmol) was
added directly to the NMR spectroscopy sample tube. Afterwards, the
solvent was evaporated on a vacuum line, then CD2Cl2 or freon mixture
(0.4 mL) was condensed directly in the sample tube.
Samples of 10H CHTUNGTRENNUNG
+A[BARF]ꢀ in [D8]toluene were prepared in a different
way. The sodium salt of HBARF, NaBARF, which is stable in air under
normal conditions, was added to the flask containing water (the solubility
of NaBARF in H2O is very low, so it was not dissolved). Subsequently, a
stoichiometric amount of proton sponge 10 and HCl (1.5 equiv) were
added, and the solution was stirred at room temperature for approxi-
[11] a) C. L. Perrin, B. K. Ohta, J. Mol. Struct. 2001, 559–601, 1–12;
c) C. L. Perrin, J. S. Lau, B. K. Ohta, Pol. J. Chem. 2003, 77, 1693–
1702.
1688
ꢆ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 1679 – 1690
Pietrzak, Mariusz
