Nonionic Superbasic Phosphazenes
A R T I C L E S
The free base was prepared according to the general procedure
described below. To 15.9 g (33.4 mmol) of 8a‚HBF4 was added 33.4
mL of a 1 M solution of t-BuOK. The raw product s a yellow oil s
was recrystallized from hexane at -30 °C to give 11.3 g (87%) of
colorless crystals, mp 101-102 °C (in a sealed capillary).
31P NMR (81.00 MHz, C6D6): δ 7.4. 1H NMR (200.13 MHz,
C6D6): δ 2.74 (s, 36H), 1.42 (1H). 13C NMR (50.32 MHz, C6D6): δ
40.2, 157.7. HRMS (EI): calcd for C15H37N10P, 388.29404; found,
388.29273.
General Procedure for the Liberation of Phosphazene Bases from
Their HBF4 or HBPh4 Salts. The corresponding salts of 8d, 9, 10,
and 31-33 bases were dissolved in as small an amount as possible of
dry MeOH (or in a mixture of MeOH with MeCN or THF), and the
calculated amount of 30% MeOK in MeOH was added. In the cases of
8a, 8b, 8c, and 11, the corresponding salts were dissolved in
dimethoxyethane (DME) or suspended in THF and the calculated
amount of t-BuOK in DME was added at reduced temperature. The
mixture was stirred, and the temperature was allowed to rise to room
temperature. The solid (KBF4 or KBPh4, respectively) was filtered off
in a glovebox or by use of a Schlenk-type filter. The volatile
components were removed under reduced pressure, and the residue was
extracted with dry hexane. The extract was filtered, hexane was removed
in vacuo, and the residue was dried under high vacuum.
or phosphonium centers and three tmg substituents to a
sulfonium center provided cations with enhanced stability under
extreme conditions (e.g., high temperature, strongly basic media,
powerful nucleophiles).17,18 This led us to expect similar
hydrolytic stability also for the tmg-substituted carbimines and
phosphazenes.
Incorporation of amino groups into rings (e.g., pyrrolidinyl)
has been demonstrated to enhance the basicity of phosphazene
bases.7b Thus, the guanidinophosphazenes derived from imida-
zolidine-2-imine (e.g., 12, 13, 14, and 22; see Scheme 2) or
1,3-dimethylimidazolidine-2-imine (e.g., 18-20) could be even
more potent (bases) compared to acyclic guanidinophos-
phazenes. The second reason for introduction of these subunits
into the phosphazene structure is the low toxicity of 1,3-
dimethylimidazolidinone, which is a potential final product of
hydrolysis of 1,3-dimethylimidazolidine-2-imine or may be
derived from guanidinophosphazenes.
Experimental Section
Synthesis. The Kirsanov and Staudinger reaction schemes were used
for the synthesis of the guanidinophosphazene bases. A representative
synthesis procedure is presented for (tmg)3PdNH (8a). The syntheses
of the rest of the new compounds are described in the Supporting
Information.
Spectra. The NMR spectra were recorded on a Bruker AC-200 NMR
spectrometer. Standard 5-mm NMR tubes were used for the measure-
ments, except for those of the samples of the free phosphazene bases,
which were prepared in the glovebox under an Ar atmosphere in special
NMR tubes with screwable stoppers. TMS was added as the internal
standard.
(tmg)3PdNH (8a). In a 1- L three-neck flask, equipped with a
mechanical stirrer, a dropping funnel with a N2 outlet, and a
thermometer, were placed 40.1 g (192.8 mmol) of phosphorus penta-
chloride and 400 mL of dried chlorobenzene. The mixture was cooled
to -30 °C and vigorously stirred. Then 135.0 g (1173.9 mmol) of
tetramethylguanidine was added dropwise during 0.5 h at such a rate
as to prevent the temperature from rising above -10 °C. The resulting
slurry was kept at -30 °C for 1 h, allowed to warm to ambient
temperature during 1 h, and then stirred for an additional 1 h at this
temperature. The dropping funnel was replaced with a gas inlet, the
mixture was cooled to -30 °C again, and an excess of NH3 (10 g,
588.2 mmol) was added into the mixture with vigorous stirring at a
rate that kept the temperature of the solution at -20 to -10 °C. The
mixture was kept at this temperature for an additional 4 h. The solid
(tetramethylguanidine hydrochloride and ammonium chloride) was
filtered off and washed two times with chlorobenzene (2 × 50 mL),
and the solvent was evaporated at reduced pressure (10-2 Torr) to
dryness to leave a light yellow solid (imino-tris(tetramethylguanidino)-
phosphonium chloride). This solid was dissolved in water (200 mL),
and a solution of 23.0 g (209.1 mmol) of NaBF4 was added in one
portion. The resulting (tmg)3PdNH‚HBF4 (8a‚HBF4) was extracted with
methylene chloride (3 × 100 mL). The solvent was removed in vacuo
to give 90.1 g (98.2%) of colorless solid, mp 114-115 °C (from
acetone/water).
The mass spectra were recorded on an Agilent 1100 Series LC-
MSD Trap-XCT instrument.
X-ray Crystallography. X-ray-quality crystals of 8c and 8c‚HBF4
were grown from hexane at -30 °C and from water, respectively. They
were mounted on a glass fiber with KEL-F oil, and data were collected
on a Siemens P4 diffractometer at a temperature of 173 K. Structures
were determined using direct methods with the SHELX program
package. The full crystallographic tables are included in the Supporting
Information. The most important pieces of evidence for the increasing
extent of delocalization of tmg groups with phosphorus atom on going
from 8c to 8c‚HBF4 are given below.
(tmg)3PdN-t-Bu (8c). All three guanidine centers, C(1), C(6), and
C(11), are almost trigonal planar (∑ e 360°). The average P-N-C-N
torsion angle (30.2°) for the evaluation of planarity at the C-N double
bond indicates a deviation of the C-N double bond from the expected
planar geometry. In agreement with this trend are the angles P1-N1-
C1, 134.5°, P1-N7-C11, 127.8°, and P1-N10-C16, 129.13° (average
P-N-C angle of 130.5°), which are greater than the angle expected
for sp2-hybridized nitrogen. Meanwhile, there is still a distinct difference
in bond lengths between guanidine C-N double bonds on one hand
and the distances from carbon atoms of the guanidine imino group,
C(1), C(6), and C(11), to the nitrogen atoms on the periphery, N(2),
N(3), N(5), N(6), N(8), and N(9) (average 138.3 pm), on the other
hand. An ORTEP diagram of the molecular structure of 8c is shown
in Figure 1.
1H NMR (200.13 MHz, CDCl3): δ 2.88 (s, 36H), 3.0 (s, 36H), 4.18
(s, 2H). 31P NMR (81.00 MHz, CDCl3): δ -8.7 (s). 13C NMR (50.32
MHz, THF-d8): δ 40.5, 160.5. 19F NMR (188.31 MHz, CDCl3): δ
-151.1. HRMS (FAB+): calcd for C15H38N10P, 389.30185; found,
389.30028.
(tmg)3PdN-t-Bu‚HBF4 (8c‚HBF4). The position of the proton was
taken from the difference Fourier map and refined isotropically. Its
location on the former phosphoimino nitrogen is in accordance with
Anal. Calcd for C15BF4H38N10P: C, 37.83; H, 8.04; N, 29.41.
Found: C, 37.38; H, 7.93; N, 28.62.
2
1
the observation of the JPH constant in the H and 31P NMR spectra.
For the protonated 8c, only a slight deviation from tetrahedral
arrangement of the phosphorus atom was revealed. On protonation of
the free base 8c, the bonds between C1, C6, C11 and N1, N4, N7,
respectively, of the imino groups became elongated. In contrast to the
neutral form, the average C-N guanidine double bond lengths (131.2
pm) in the protonated form became more similar to the average bond
length (136.5 pm) between the C1, C6, and C11 carbons and the
nitrogens of the NMe2 groups. These observations imply significant
(17) Tris(tetramethylguanidino)sulfonium chloride, [(dma)2CdN]3S+Cl-, was
synthesized straightforwardly from sulfur tetrachloride and tetrameth-
ylguanidine, pregenerated at -78 °C: Henrich, M.; Marhold, M.; Kolo-
meitsev, A. A.; Kalinovich, N.; Ro¨schenthaler, G.-V. Tetrahedron Lett.
2003, 44, 5795-5798.
(18) For application of TMG-derived phase-transfer catalysts, see: (a) Marhold,
M.; Pleschke, A.; Schneider, M.; Kolomeitsev, A. A.; Ro¨schenthaler, G.-
V. J. Fluorine Chem. 2004, 125, 1031-1038. (b) Henrich, M.; Marhold,
A.; Kolomeitsev, A. A.; Ro¨schenthaler, G.-V. (to Bayer AG). Patents DE
10129057/EP 1266904/US 2003036667, December 18, 2002.
9
J. AM. CHEM. SOC. VOL. 127, NO. 50, 2005 17659
Kolomeitsev, Alexander A.
