Synthesis of Chiral Phosphazene Bases

Article abstract of DOI:10.1007/s10593-016-1927-2

[Figure not available: see fulltext.] Air-stable and crystalline tetraaminophosphonium tetrafluoroborates possessing chiral, enantiomerically pure 1,2-diamine moiety have been synthesized by a three-step sequential one-pot approach. The tetrafluoroborate

Full text of DOI:10.1007/s10593-016-1927-2

DOI 10.1007/s10593-016-1927-2
Chemistry of Heterocyclic Compounds 2016, 52(8), 541–545
Synthesis of chiral phosphazene bases
Martins Priede1, Elina Priede1, Jaan Saame2, Ivo Leito2, Edgars Suna1*
1 Latvian Institute of Organic Synthesis,
21 Aizkraukles St., Riga LV-1006, Latvia; е-mail: edgars@osi.lv
2 Institute of Chemistry, University of Tartu,
14a Ravila St., Tartu 50411, Estonia; е-mail: ivo.leito@ut.ee
Published in Khimiya Geterotsiklicheskikh Soedinenii,
2016, 52(8), 541–545
Submitted July 28, 2016
Accepted August 19, 2016
Air-stable and crystalline tetraaminophosphonium tetrafluoroborates possessing chiral, enantiomerically pure 1,2-diamine moiety have
been synthesized by a three-step sequential one-pot approach. The tetrafluoroborate salts can be purified by recrystallization or chroma-
tography and subsequently converted to the phosphazene bases by treatment with t-BuOK. Basicity values in tetrahydrofuran have been
measured for the obtained phosphazene bases by means of spectrophotometric titration.
Keywords: phosphazene bases, tetraaminophosphonium salts, basicity, one-pot synthesis, pKip value.
Phosphazene bases (PBs) have been widely employed in
stoichiometric amounts as strong non-nucleophilic bases in
a range of organic transformations,1 including stereoselec-
tive transformations, such as Mukaiyama aldol reaction,
asymmetric alkylation, Henry reaction, and Julia–
Kocienski olefination.1b,2 However, information regarding
the use of chiral PBs to catalyze enantioselective reactions
is rather scarce.3 Thus, chiral PBs have been used as
ligands in a copper-catalyzed enantioselective cyclo-
propanation reaction3a and also as catalysts in enantio-
selective amination of 2-alkyltetralone derivatives and their
analogs.3b Ooi has successfully utilized chiral PBs in
stereoselective hydrophosphonylation of aldehydes,3c
Michael addition,3d and Payne-type oxidation of N-sulfonyl
imines.3e
There are a number of synthetic methods for the pre-
paration of PBs.1b,3,4,5 Among them, Alexandrova et al.
demonstrated a synthesis of PBs by the Staudinger reaction
between triaminophosphanes and alkyl azides.4c The
advantages of this approach are readily available starting
materials, simple product isolation procedure, and good
yields. Therefore we decided to adapt this methodology for
the synthesis of new PBs with backbone chirality. In the
meantime, we were also aware of difficulties with
purification of PBs. Thus, most of the known PBs are oil-
like materials, and relatively high basicity of PBs compli-
cates their purification by chromatographic methods.
Furthermore, distillation under high vacuum, a commonly
used method for the purification of PBs, apparently is not
suitable for the small-scale synthesis of higher molecular
weight PBs possessing structurally complex chiral moieties.
Herein, we report that chiral 1,2-diphenyl diamine-based PBs
can be conveniently purified by preparation of the cor-
responding tetrafluoroborate salts. Previously unreported
PBs with chiral backbone have been obtained in a multistep
sequential one-pot synthesis from chiral, enantiomerically
pure 1,2-diamine.
The synthesis of chiral PBs commenced with the
preparation of triaminophosphane 3 from hexamethyl-
phosphorous triamide (2) and the chiral 1,2-diamine 1
(Scheme 1). Incomplete conversion of the chiral 1,2-diamine 1
(93%) after prolonged heating (>12 h) was accompanied by
concomitant decomposition of the reaction product 3
(31P NMR assay). The triaminophosphane 3 was also found
to be unstable in air, as well as in the presence of moisture
and protic solvents. The poor stability of the non-crystal-
line phosphane 3 complicated its isolation and separation
from the unreacted starting materials 1 and 2, so we looked
for other approaches, such as in situ derivatization to obtain
a stable, crystalline, and easy-to-purify material.
Tertiary phosphine–borane complexes are relatively
stable and usually appear to be crystalline. They can be
easily prepared in reactions with borane and subsequently
cleaved to release free phosphines.6 We envisaged that the
preparation of crystalline complex between borane and
triaminophosphane 3 would allow for efficient purification
of compound 3. Indeed, the desired complex 7 was readily
formed in the reaction between borane and phosphane 3
0009-3122/16/52(8)-0541©2016 Springer Science+Business Media New York
541

Chemistry of Heterocyclic Compounds 2016, 52(8), 541–545
Scheme 1
and was recrystallized from EtOAc to >95% purity (1H and
31P NMR). The structure of complex 7 was confirmed by
X-ray crystallographic analysis (Fig. 1). With the purified
triaminophosphane–borane complex 7 in hand, cleavage of
the complex 7 to triaminophosphane 3 by reaction with
excess of Et2NH was attempted. The desired compound 3
was not obtained at room temperature, whereas at elevated
temperatures (100°C) decomposition of the formed triamino-
phosphane 3 was observed by 31P NMR method. Apparently,
the required harsh reaction conditions for cleaving the
remarkably stable Lewis acid–base complex 77 were not
compatible with the relatively sensitive triaminophosphane
3. Therefore we decided to avoid the isolation and purifi-
cation of intermediate 3 during the multistep synthesis of
PBs 6.
Instead, the phosphane 3 was converted without isola-
tion to the target phosphazenes 6a,b in reactions with tert-
butyl azide (4a) or phenyl azide (4b), respectively. As
anticipated, both PBs 6a,b turned out to be oily materials,
which complicated their isolation from the reaction mixture
and subsequent purification. Therefore we sought a suitable
acid to form crystalline salts of PBs 6a,b that could be
more convenient to handle. We chose HBF4 as the most
suitable acid because tetrafluoroborate salts of several PBs
have been used for the preparation of crystals for X-ray
analysis.4b,c Hence, treatment of the reaction mixture
containing phosphazenes 6a,b with aqueous HBF4 solution
followed by extraction into the organic phase provided
crystalline and stable tetrafluoroborates 5a,b (50% and
63% yields, respectively, calculated for three steps starting
from the 1,2-diamine 1). The purity of these salts could be
increased to >95% by crystallization from EtOAc or
reversed-phase chromatography. The purified salts 5a,b
were converted back into the free bases 6a,b in 99% yield
by treatment with t-BuOK in t-BuOH (Scheme 1).
Basicity of the obtained chiral PBs 6a,b in tetra-
hydrofuran was measured using UV-vis spectrophotometric
titration method (Table 1, entries 1 and 6)8 and the obtained
values were compared to those of other representative
organic bases. Basicity of the phosphazene 6a was equal or
higher than for DBU, 1,1,3,3-tetramethylguanidine,
pyrrolidine, and PhP1(dma) (entry 1 vs entries 2–5).
Notably, the PB 6a containing a tert-butylimino moiety was
approximately 4 pKip units more basic than the analog 6b
containing a phenylimino group. The marked difference in
basicity apparently resulted from the greater electron-
donating ability of tert-butyl group compared to that of the
phenyl group, and especially from the lack of strong
resonance stabilization of the neutral base (as opposed to
the protonated form) occurring via the benzene ring in
compound 6b. Determination of the basicity for com-
pounds 6a,b is a crucial step toward the application of
these new chiral PBs in enantioselective synthesis.
Figure 1. The molecular structure of triaminophosphane–borane
complex 7 with atoms represented by thermal vibration ellipsoids
of 50% probability.
542

Chemistry of Heterocyclic Compounds 2016, 52(8), 541–545
Triphenylphosphine oxide (27.0 ppm for 31P nuclei) was
used as internal standard for 31P NMR spectra. Optical
rotation measurements were performed on a PerkinElmer
141 polarimeter. Melting points were determined on an
OptiMelt apparatus and were not corrected. All reagents
and solvents were purchased from commercial suppliers
and used without further purification. Toluene was distilled
from P2O5 prior to use.
Table 1. pKip values of phosphazene bases 6a,b
and several other representative bases in THF
Entry
1
Base
pKip
18.0
(4R,5R)-N,N,1,3-Tetramethyl-4,5-diphenyl-1,3,2-diaza-
phospholidin-2-amine (3). Compound 3 was synthesized
according to a modified procedure reported previously.9
Diamine 110 (721 mg, 3.00 mmol) and hexamethyl-
phosphorous triamide (2)11 (0.6 ml, 3.00 mmol) were
placed in a pressure vial, dissolved in anhydrous toluene
(1.5 ml) under argon, and heated at 110°C. After 4 h, the
reaction mixture was cooled to room temperature and
flushed with argon to remove the formed dimethylamine.
The heating and flushing procedure was repeated 2 times
(overall time of heating 12 h). The conversion of the
starting material was monitored by 31P NMR spectroscopy.
The reaction mixture was cooled to room temperature and
flushed with argon. The obtained solution of crude product
3 in toluene was diluted with anhydrous toluene to 5 ml
volume and the resulting 0.6 M solution was directly used
in the synthesis of PBs 6a,b without further purification.
31P NMR spectrum, δ, ppm: 124.2.9
2
18.08b
3
4
17.08b
15.38b
5
6
15.38b
13.9
Me
NMe2
N
P
(4R,5R)-2-tert-Butylamino-2-dimethylamino-1,3-dimethyl-
N
N
4,5-diphenyl[1,3,2]diazaphospholidin-2-ium
tetrafluoro-
borate (5a). A solution of tert-butyl azide (4a)12 in toluene
(1 M, 2.0 ml, 1.96 mmol) was added to the previously
obtained solution of crude triaminophosphane 3 (0.6 M, 3.0 ml,
1.78 mmol) under argon. The reaction mixture was heated
at 60°C for 5 h and the conversion of the starting material
was monitored by 31P NMR spectroscopy. After cooling to
0°C, a solution of HBF4 in water (50% (v/v), 0.33 ml,
2.67 mmol) was added. The reaction mixture was allowed
to warm up and was well stirred at room temperature for
5 min. Water (25 ml) was added and the mixture was
extracted with EtOAc (2×25 ml). The combined organic
extracts were washed with brine (25 ml), dried over Na2SO4,
and concentrated under reduced pressure. The residue was
purified by crystallization from EtOAc. Yield 420 mg
Me
6b
In summary, the synthesis of previously unreported
chiral PBs possessing an enantiomerically pure 1,2-diamine
moiety has been achieved. The synthesis relies on a three-
step sequential one-pot preparation of air-stable and
crystalline tetraaminophosphonium tetrafluoroborates, which
can be purified by recrystallization or reversed-phase
chromatography. Subsequent treatment of tetrafluoroborate
salts with t-BuOK afforded the desired PBs. The triamino-
phosphane intermediate en route to chiral PBs was also
isolated as a complex with borane, and its structure was
confirmed by X-ray crystallographic analysis. Basicity
values in THF for the synthesized PBs have been deter-
mined by spectrophotometric titration method. The PB
containing a tert-butylimino moiety had the same or higher
20
(50%), white solid, mp 151–152°C (EtOAc). [α]D 14.4
1
(c 1.00, CHCl3). H NMR spectrum, δ, ppm (J, Hz): 1.52
(9H, s, C(CH3)3); 2.50 (6H, dd, J = 11.2, J = 5.0, N(CH3)2);
3.05 (6H, d, J = 9.7, H3CNPNCH3); 4.18 (2H, s,
PhCHCHPh); 4.87 (1H, d, J = 9.0, NH); 7.05–7.14 (4H, m,
H Ar); 7.31–7.41 (6H, m, H Ar). 13C NMR spectrum,
δ, ppm (J, Hz): 29.4 (d, J = 4); 30.9 (d, J = 2); 31.8 (d,
J = 5); 38.9 (d, J = 4); 70.9 (d, J = 12); 72.7 (d, J = 9);
128.5 (s); 128.7 (s); 129.6 (d, J = 6); 129.8 (d, J = 13);
135.0 (d, J = 11); 135.2 (d, J = 8). 31P NMR spectrum,
δ, ppm: 38.0. Found, m/z: 385.2521 [M]+. C22H34N4P.
Calculated, m/z: 385.2521.
basicity
than
DBU,
1,1,3,3-tetramethylguanidine,
pyrrolidine, and PhP1(dma). The applications of 1,2-
diamine-derived chiral PBs will be reported elsewhere.
Experimental
IR spectrum of borane complex 7 was recorded on a
Shimadzu IR Prestige-21 FT-IR spectrometer. H, 13C, and
1
31P NMR spectra were acquired on a Varian Inova 400 MR
spectrometer (400, 101, and 162 MHz, respectively). The
solvents were CDCl3 (compound 3) and CD2Cl2 (the rest of
(4R,5R)-2-(Dimethylamino)-1,3-dimethyl-4,5-diphenyl-
2-(phenylamino)-1,3,2-diazaphospholidin-2-ium tetra-
fluoroborate (5b). A solution of phenyl azide (4b)13 in
toluene (1 M, 0.6 ml, 0.59 mmol) was added to the
previously obtained solution of crude triaminophosphane 3
1
the compounds). H NMR chemical shifts were reported
relative to the residual solvent protons as internal standard
(CD2Cl2: 5.32 ppm). Solvent carbon atoms served as
internal standard for 13C NMR spectra (CD2Cl2: 54.0 ppm).
543

Chemistry of Heterocyclic Compounds 2016, 52(8), 541–545
(0.6 M, 0.9 ml, 0.54 mmol) under argon. The reaction
139.1 (d, J = 5); 151.7 (d, J = 1). 31P NMR spectrum,
δ, ppm: 21.4.
(4R,5R)-N,N,1,3-Tetramethyl-4,5-diphenyl-1,3,2-di-
mixture was stirred at 21°C for 14 h and the conversion of
the starting material was monitored by 31P NMR spectro-
scopy. After cooling to 0°C, a solution of HBF4 in water
(50% (v/v), 0.1 ml, 0.81 mmol) was added. The reaction
mixture was allowed to warm up and was stirred at room
temperature for 5 min. The residue was dissolved in water
(15 ml) and extracted with EtOAc (2×25 ml). The
combined organic extracts were washed with brine (25 ml),
dried over Na2SO4, and concentrated under reduced pressure.
The residue was purified by reversed-phase chromato-
graphy (gradient 10:100–100:0 MeCN/H2O). Yield 168 mg
(63%), white solid, mp 83–85°C (EtOAc). [α]D20 27.9 (c 1,
azaphospholidin-2-amine–borane complex (7). A solu-
tion of triaminophosphane 3 in dry toluene (0.6 M, 2.5 ml,
1.5 mmol) was placed in a pressure vial under argon.
A solution of BH3 in THF (1 M, 3.1 ml, 3.1 mmol) was
slowly added at room temperature, and the reaction mixture
was stirred for 14 h. The conversion of the starting material
was monitored by 31P NMR spectroscopy. Solvent was
evaporated under reduced pressure and the residue was
purified by column chromatography (silica gel, gradient
elution from 5% to 10% EtOAc–petroleum ether). Yield
244 mg (49%), white solid, mp 117–118°C (EtOAc).
IR spectrum (KBr), ν, cm–1: 2368 (BH). 1H NMR
spectrum, δ, ppm (J, Hz): 0.25–1.11 (3H, br. m, BH3); 2.35
(6H, ddd, J = 17.6, J = 10.9, J = 1.2, N(CH3)2); 2.87 (6H,
dd, J = 9.0, J = 1.3, H3CNPNCH3); 3.82 (1H, d, J = 8.7,
PhCHCHPh); 4.03 (1H, d, J = 8.7, PhCHCHPh); 7.03–7.10
(4H, m, H Ar); 7.23–7.31 (6H, m, H Ar).13C NMR spectrum,
δ, ppm (J, Hz): 29.4 (d, J = 6); 31.2 (d, J = 10); 36.7 (d,
J = 4); 73.3 (s); 73.7 (d, J = 4); 77.4 (s); 127.8 (s); 128.2 (d,
J = 1); 128.2 (s); 128.5 (s); 128.6 (s); 138.1 (d, J = 5);
138.6 (d, J = 8). 31P NMR spectrum, δ, ppm (J, Hz): 109.2
(dd, J = 204, J = 95). Found, %: C 65.62; H 8.36; N 12.65.
C18H27N3BP. Calculated, %: C 66.07; H 8.32; N 12.84.
X-ray structural study of compound 7. Monocrystals
of compound 7, obtained by crystallization from EtOAc
(C18H27BN3P, M 327.21) were orthorhombic, space group
1
CHCl3). H NMR spectrum, δ, ppm (J, Hz): 2.49 (3H, d,
J = 10.8, NCH3); 2.62 (3H, d, J = 10.9, NCH3); 3.13 (6H,
d, J = 10.3, H3CNPNCH3); 4.02 (1H, d, J = 9.2,
PhCHCHPh); 4.17 (1H, d, J = 9.2, PhCHCHPh); 6.55–6.59
(2H, m, H Ar); 7.02–7.08 (2H, m, H Ar); 7.14–7.20 (2H,
m, H Ar); 7.24–7.29 (1H, m, H Ar); 7.31–7.43 (6H, m, H Ar);
7.47–7.53 (2H, m, H Ar). 13C NMR spectrum, δ, ppm
(J, Hz): 29.5 (d, J = 4); 29.7 (d, J = 4); 38.4 (d, J = 4); 71.5 (d,
J = 12); 71.7 (d, J = 11); 126.5 (d, J = 5); 127.3 (d, J = 2);
128.3 (d, J = 8); 126.5 (d, J = 5); 129.4 (s); 129.6 (s); 129.7 (s);
129.9 (s); 130.5 (d, J = 1); 135.3 (d, J = 8); 135.4 (d, J = 9);
136.6 (d, J = 1).31P NMR spectrum, δ, ppm: 39.1. Found, m/z:
405.2209 [M]+. C24H30N4P. Calculated, m/z: 405.2208.
Synthesis of phosphazene bases 6a,b (General method).
The appropriate PB salt 5 (0.85 mmol) was placed in a
pressure vial, and a solution of t-BuOK in t-BuOH (1 M,
1.7 ml, 1.69 mmol) was added under argon. The reaction
mixture was heated at 80°C for 1 h and the conversion of
the starting material was monitored by 31P NMR spectro-
scopy. After cooling to room temperature, the reaction
mixture was extracted with anhydrous pentane (3×10 ml)
and concentrated under reduced pressure.
P212121, at 173 K:
a
10.1143(2),
b
13.6442(3),
c 13.8352(3) Å; V 1909.28(7) Å3; Z 4; dcalc 1.138 g·cm–3;
F(000) 704. The intensities of 5318 reflections were deter-
mined on a Nonius Kappa CCD diffractometer (MoKα radia-
tion, λ 0.71073 Å, 2θ < 60o), 5313 independent reflections
(Rint 0.0076) were used in further refinement. The structure
was solved directly and refined by least squares method in
anisotropic full matrix approximation by F2hkl. The hydro-
gen atom positions were calculated geometrically and
refined in isotropic approximation by the "rider'' model.
The final probability factor values for compound 7:
wR2 0.135 and GOF 1.008 for all independent reflections
(R1 0.050 calculated by F for 4326 observed reflections
with I>2σ(I)). All calculations were performed with the
SHELXS97 software suite.14 The complete crystallographic
data set was deposited at the Cambridge Crystallographic
Data Center (deposit CCDC 1500119).
(4R,5R)-2-(tert-Butylimino)-N,N,1,3-tetramethyl-4,5-
diphenyl-1,3,2λ5-diazaphospholidin-2-amine (6a). Yield
1
327 mg (>99%), slightly yellow oil. H NMR spectrum,
δ, ppm (J, Hz): 1.32 (9H, s, C(CH3)3); 2.26 (3H, d, J = 9.5,
NCH3); 2.29 (3H, d, J = 9.6, NCH3); 2.88 (6H, d, J = 9.8,
H3CNPNCH3); 3.90 (1H, d, J = 9.4, PhCHCHPh); 3.98
(1H, d, J = 9.3, PhCHCHPh); 7.05–7.14 (4H, m, H Ar);
7.23–7.28 (6H, m, H Ar). 13C NMR spectrum, δ, ppm
(J, Hz): 30.3 (d, J = 4); 30.5 (d, J = 3); 34.5 (d, J = 9); 38.7
(d, J = 3); 71.3 (d, J = 10); 73.1 (d, J = 6); 128.6 (s); 128.7
(s); 128.7 (s); 129.1 (d, J = 2); 137.9 (s). 31P NMR
spectrum, δ, ppm: 13.5.
(4R,5R)-N,N,1,3-Tetramethyl-4,5-diphenyl-2-(phenyl-
imino)-1,3,2λ5-diazaphospholidin-2-amine (6b). Yield
344 mg (>99%), off-white solid. 1H NMR spectrum,
δ, ppm (J, Hz): 2.37 (6H, dd, J = 9.7, J = 7.2, N(CH3)2);
2.98 (6H, d, J = 10.0, H3CNPNCH3); 3.99 (2H, AB, q,
J = 14.5, PhCHCHPh); 6.73–6.79 (1H, m, H Ar); 6.90–
6.95 (2H, m, H Ar); 6.95–7.00 (2H, m, H Ar); 7.06–7.12
(2H, m, H Ar); 7.14–7.26 (5H, m, H Ar); 7.27–7.31 (3H,
m, H Ar). 13C NMR spectrum, δ, ppm (J, Hz): 29.8 (d, J = 4);
30.5 (d, J = 4); 37.9 (d, J = 3); 72.0 (d, J = 10); 72.6 (d,
J = 8); 118.2 (d, J = 2); 125.0 (s); 125.1 (s); 128.5 (s);
128.6 (s); 128.9 (s); 129.0 (s); 129.1 (s); 139.0 (d, J = 4);
The Supplementary information file containing results
of X-ray crystallographic analysis is available at http://
link.springer.com/journal/10593.
Authors thank Dr. S. Belyakov (Latvian Institute of
Organic Synthesis) for X-ray crystallographic analysis.
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