Functionalized 3 H -1,2,3,4-triazaphosphole derivatives: Synthesis and structural characterization of novel low-coordinate phosphorus heterocycles

Article abstract of DOI:10.1080/10426507.2015.1119144

The synthesis of novel functionalized 3H-1,2,3,4-triazaphosphole derivatives is presented, making use of the modular [3+2] dipolar cycloaddition reaction between an azide and a phosphaalkyne. These low-coordinate phosphorus heterocycles were prepared in high yield and could be characterized crystallographically. This strategy provides the possibility to access hitherto unknown chelating and potentially chiral P^O ligands based on triazaphospholes.

Full text of DOI:10.1080/10426507.2015.1119144

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Functionalized 3H-1,2,3,4-triazaphosphole
derivatives: Synthesis and structural
characterization of novel low-coordinate
phosphorus heterocycles
Julian A. W. Sklorz, Maike Schnucklake, Matthias Kirste, Manuela Weber,
Jelena Wiecko & Christian Müller
To cite this article: Julian A. W. Sklorz, Maike Schnucklake, Matthias Kirste, Manuela
Weber, Jelena Wiecko & Christian Müller (2016) Functionalized 3H-1,2,3,4-triazaphosphole
derivatives: Synthesis and structural characterization of novel low-coordinate phosphorus
heterocycles, Phosphorus, Sulfur, and Silicon and the Related Elements, 191:3, 558-562, DOI:
10.1080/10426507.2015.1119144
To link to this article: http://dx.doi.org/10.1080/10426507.2015.1119144
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Date: 22 March 2016, At: 11:01

PHOSPHORUS, SULFUR, AND SILICON
, VOL. , NO. , –
http://dx.doi.org/./..
Functionalized H-,,,-triazaphosphole derivatives: Synthesis and structural
characterization of novel low-coordinate phosphorus heterocycles
Julian A. W. Sklorz, Maike Schnucklake, Matthias Kirste, Manuela Weber, Jelena Wiecko, and Christian Müller
Institut für Chemie und Biochemie, Freie Universität Berlin, Berlin, Germany
ABSTRACT
ARTICLE HISTORY
Received  September 
Accepted  November 
The synthesis of novel functionalized 3H-1,2,3,4-triazaphosphole derivatives is presented, making use of the
modular[3+2]dipolarcycloadditionreactionbetweenanazideandaphosphaalkyne. Theselow-coordinate
phosphorus heterocycles were prepared in high yield and could be characterized crystallographically. This
strategy provides the possibility to access hitherto unknown chelating and potentially chiral P^O ligands
based on triazaphospholes.
KEYWORDS
Phosphorus heterocycles;
triazaphospholes; P-ligands;
cycloaddition; ligand design
GRAPHICAL ABSTRACT
triazole-derivatives in a rather facile manner, depending on the
availability of the phosphaalkyne and the azide.
Introduction
3,5-Disubstituted 3H-1,2,3,4-triazaphosphole derivatives of
type 1 are the phosphorus analogs of the well-studied 1,4-
disubstituted 1,2,3-triazoles (2, Figure 1).1 According to
Nyulászi and Regitz, triazaphospholes have a conjugated π-
systems with a high degree of aromaticity and possess a rather
high π-density at the phosphorus atom due to a significant
N-C = P ↔ N+ = C-P− conjugation.2 These low-coordinate
λ3σ2-phosphorus heterocycles can be prepared by a [3+2]
cycloaddition of phosphaalkynes with azides, as demonstrated
by Regitz et al. in 1984.3,4 In contrast to the Huisgen [3+2]
reaction and the copper-catalyzed azide-alkyne cycloaddi-
tion (CuAAC, “click”-reaction), only one single regioisomer
is thermally formed without the need of a catalyst.5 This
provides the possibility to construct phosphorus containing
The incorporation of additional donor-functionalities within
the heterocyclic framework is an important aspect for the
design of triazaphosphole derivatives and their use in more
applied research fields. As a matter of fact, coordination com-
pounds containing chelating N^N ligands or different donor-
combinations, such as N^O, P^O, or P^N, have been exploited to
a large extend as functional components in electronic and lumi-
nescent materials and as precursor for homogeneous catalytic
reactions.6 Consequently it could be interesting to create novel
ligand systems based on polydentate triazaphospholes, in order
to develop these interesting low-coordinate phosphorus hete-
rocycles to their full potential. However, donor-functionalized
triazaphospholes are very rare. Jones and co-worker described
CONTACT Christian Müller
c.mueller@fu-berlin.de
In memory of Prof. Dr. Reinhard Schmutzler
Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstr. /,  Berlin, Germany.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gpss.
©  Taylor & Francis Group, LLC

PHOSPHORUS, SULFUR, AND SILICON
559
Scheme . Synthesis of functionalized triazaphospholes 13 and 14.
Reduction with excess NaBH4 in ethanol led to the correspond-
ing alcohol 9 in 88% isolated yield.12 In future investigations,
this reduction step could be performed with a chiral reagent,
in order to obtain the entantiomerically pure alcohol.13 We fur-
ther protected the OH-functionality with a TMS-group because
phosphaalkynes are prone to nucleophilic attack. The racemic
mixture of the TMS-protected compound 10 could be obtained
in 85% isolated yield as a coloress oil.
Figure . Selected H-,,,-triazaphospholes 1, 3-6 and triazole 2.
All compounds were characterized by means of NMR spec-
troscopy as well as mass spectrometry.
recently the access to tripodal P3-ligands, such as compound 3.7
The same group also reported on the facile assembly of tris-
and poly-(triazaphospholes), starting from the corresponding
polyazides.8 Our group could recently demonstrate the access to
new classes of chelating P,N as well as pincer-type P,N,P-hybrid
ligands.9,10 The pyridyl-functionalized compounds 4 and 5 were
easily prepared starting from (pyridylmethyl)azide, respectively,
pyridotetrazole and 1-phospha-2-tert-butyl-acetylene. Similar-
ily, starting from 2,6-bis(azidomethyl)pyridine, the pincer-type
neutral P,N,P-ligand 6 could be obtained in high yields.
Inspired by these recent results and by our long-standing
interest in low-coordinate phosphorus heterocycles, we set out
to develop potentially chelating and also chiral triazaphospholes
with different donor-combinations and our first achievements in
this direction will be reported here.
10 was further converted in a [3 + 2] cycloaddition reac-
tion with 1-phospha-2-tert-butyl-acetylene (tBuCࣕP, 11) as
well as with 1-phospha-2-trimethylsilyl-acetylene (Me3SiCࣕP,
12) to give regioselectively the corresponding 3H-1,2,3,4-
triazaphosphole derivatives 13 and 14, according to Scheme 2.
13 and 14 were isolated as colorless solids in 86% and 97% yield,
respectively.
Both compounds show the typical downfield shift of the
phosphorus resonance in the 31P{1H} NMR spectrum. While
t
for the Bu-substituted compound 13 the signal is observed
at δ(ppm) = 174.0 (CD2Cl2), the SiMe3-group in 14 has a
rather large influence on the chemical shift, as the phosphorus-
resonance is detected at δ(ppm) = 218.3 (CD2Cl2).
Because structural information on triazaphosphole deriva-
tives are rare, we attempted a crystallographic characterization
of the products and single crystals, suitable for X-ray crystal
structure analysis, were obtained from concentrated solutions of
13 and 14 in pentane. Triazaphospholes 13 and 14 crystallized
in the space group P21/c and P21/n, respectively, and the molec-
ular structures in the crystal, along with selected bond lengths
and angles, are depicted in Figures 2 and 3. The graphical repre-
sentation of 13 and 14 confirm the successful synthesis of these
novel functionalized triazaphosphole-derivatives.
Results and discussion
Derived from our recently reported pyridyl-functionalized P,N-
hybrid ligand 4, we aimed for the development of a func-
tionalized triazaphosphole derivative containing a P^O donor-
combination. At the same time, we were wondering whether
we could implement a stereogenic center in the ligand back-
bone, in order to have the possibility to access also chiral triaza-
phospholes in the future. Therefore, we started from the com-
mercially available 2-bromo-1-(p-toyl)ethane-1-one (7), which
gave 8 after reaction with excess NaN3 according to Scheme 1.11
Figure. Molecularstructureof 13 inthecrystal. Displacement ellipsoidsareshown
˚
at the % probability level. Selected bond lengths (A) and angles (°): P()-C():
.(), P()-N(): .(), N()-N(): .(), N()-N(): .(), C()-C(): .(),
N()-C(): .(), C()-O(): .(), O()-Si(): .(). N()-P()-C(): .().
Scheme . Synthesis of (-azido--(-methylphenyl)ethoxy)trimethylsilane (10).

560
J. A. W. SKLORZ ET AL.
chemical shifts are reported relative to the residual resonance
in the deuterated solvents. Infra-Red-spectra were obtained
from Nicolet iS10 MIR FT-IR Spectrometer. ESI-TOF-Mass
Spectrometry was performed on Agilent 6210 ESI-TOF, Agi-
lent Technologies, Santa Clara, CA, USA, (5 μL/min, 4 kV,
15 psi.). The voltage was optimized during the measurements
for the max. abundance of [M+X]1 signal (X: H, Na, K). The
EI measurement was performed on a MAT 711, Varian Bre-
t
men, using an electron energy of 80eV. Me3SiCࣕP, BuCࣕP,
2-Azido-1-(4-methylphenyl)ethanone (8) and 2-Azido-1-(4-
methylphenyl)ethanol (9) were prepared according to the liter-
ature.11,12,18,19
(2-Azido-1-(4-methylphenyl)ethoxy)trimethylsilane (10)
Compound 9 was dissolved in penante (65 mL) and 1,4-
diazabicyclo[2.2.2]octane (1.47 g, 13 mmol, 1.3 eq.) were added
under stirring. Subsequently, trimethylsilylchloride (1.73 g,
16 mmol, 1.6 eq.) was added and the reaction mixture was
stirred for overnight at room temperature. Diluted hydrochlo-
ric acid (1 N, 35 mL) was added and the mixture was
neutralized with NaHCO3. The organic phase was extracted
three times with diethylether and the collected solutions were
washed with water, brine and dried over MgSO4. The sol-
vent was removed in vacuo, yielding the product as a bright
Figure. Molecularstructureof14 inthecrystal. Displacementellipsoidsareshown
˚
at the % probability level. Selected bond lengths (A) and angles (°): P()-C():
.(), P()-N(): .(), N()-N(): .(), N()-N(): .(), C()-Si(): .(),
N()-C(): .(), C()-O(): .(), O()-Si(): . (). N()-P()-C(): .().
Although known for many years, only a few 3H-1,2,3,4-
triazaphospholes have so far been characterized crystallograph-
ically by Jones et al. and our group.7–10 All available structural
parameters indicate significant conjugation within the phos-
phorus heterocycles. In case of 13 and 14 the P(1)-C(1) dis-
˚
˚
˜
yellow oil (2.1 g, 8.4 mmol, 84%). IR (ATR, neat): v =
tances (1.723(2) A and 1.713(2) A) are slightly shorter than the
1
2958 (N3); 2095 (N3); 1509; 1436; 1251; 812 cm−1. H-NMR
14
˚
P-C bond distances in 2,4,6-triarylphosphinines (∼1.75 A).
(400 MHz, CDCl3): δ = 0.03–0.11 (m, 9H, Si(CH3)3), 2.34
(s, 3H, CH3), 3.05–3.12 (m, 1H, CH2N), 3.32–3.41 (m, 1H,
CH2N), 4.81 (dd, 1H, J = 8.5; 3.0 Hz, CHO(Si(CH3)3), 7.11–
7.23 (m, 4H, Ar-H) ppm. 13C{1H}-NMR (101 MHz, CDCl3):
δ = 0.03 (Si(CH3)3), 21.25 (CH3), 58.69 (C-N3), 74.79 (C-
OSi(CH3)3), 125.96 (Ar), 129.18 (Ar), 137.69 (Ar), 138.63
(Ar) ppm. MS (EI, 80 eV) m/z: 221.1 [M-N2]+• (calc.: 221.
1230).
It thus lies between the values of a localized P = C double
bond [(diphenylmethylene)-(mesityl)phosphine, MesP = CPh2:
15
˚
˚
1.692 A] and a P-C single bond (PPh3: 1.83 A ). The N-
P-C angles are with 86.21(11) ° and 87.48(11)° almost close
to 90º and differ significantly from the C-P-C angle in the
aromatic six-membered phosphinines (∼100º).16 The near 90º
bond angle, which is characteristic for these phosphorus com-
pounds, can apparently more easily be achieved in a planar
pentagon (ideally 107º), than in a hexagon (ideally 120º). This
can be explained by the fact that the lighter sp2-hybridized
ring atoms strive for the trigonal planar arrangement. A simi-
lar effect contributes to the planarization of the σ3-P atom in
polyphosphaphospholes.17 As suggested by Nyulászi and Regitz,
all these observations are in line with a significant aromaticity
within 1,2,3,4-triazaphospholes.
5-(tert-Butyl)-3-(2-(p-tolyl)-2-[(trimethylsilyl)oxy]ethyl]-
3H-1,2,3,4-triazaphosphole (13)
Azide 10 (0.895 g, 3.59 mmol, 1.0 eq.) was dissolved in toluene
t
and an excess of BuCࣕP in toluene was added. Stirring was
continued at room temperature for 5 h. Subsequently, the sol-
vent and the residual phosphaalkyne were removed in vacuo and
the crude product was recrystallized from pentane. The product
Experimental
could be obtained as white crystals (1.0 g, 2.9 mmol, 87%). IR
General
1
(ATR, neat): v = 2956; 1252; 1088; 939; 841 cm−1. H-NMR
˜
All reactions were performed in an argon atmosphere by using (400 MHz, C6D6): δ = −0.13–0.16 (m, 9H, Si(CH3)3), 1.50
Schlenk techniques, unless stated otherwise. All the glassware (d, J = 1.4 Hz, 9H, tBu), 2.07 (s, 3H, CH3), 4.28–4.39 (m, 1H,
was dried prior to use by heating under vacuum. All the com- CH2N), 4.46–4.55 (m, 1H, CH2N), 5.04 (dd, 1H, J = 9.1; 3.4 Hz,
mon chemicals were commercially available and purchased 1H, CHO(Si(CH3)3), 6.94 (d, J = 7.8 Hz, 2H, Ar-H), 7.10 (d, J =
from AldrichChemical Co., ABCR, Alfa Aesar or Acros as well 8.1 Hz, 2H, Ar-H) ppm. 13C{1H}-NMR (101 MHz, C6D6): δ
as Eurisol and were used as received. All solvents were dried = 0.29 (Si(CH3)3), 21.09 (CH3), 31.84 (tBu), 35.34 (tBu), 60.09
and degased using standard techniques or used from Braun Sol- (C-N3), 75.24 (C-OSi(CH3)3), 126.10 (Ar), 129.46 (Ar), 137.74
ventsystems. The 1H, 13C{1H}, and 31P{1H} NMR spectra were (Ar), 138.81 (C = P) ppm. 31P{1H}-NMR (162 MHz): δ =
recorded on a Fa. JEOL ECX400 (400 MHz), Fa. JEOL ECP500 174.03 ppm. Elemental analysis calcd (%) for C17H28N3OPSi:
(500 MHz) or on Fa. BRUKER AVANCE500 (500 MHz), Fa. C 57.29, N 11.88, H 7.81. Found: C 57.23, N 11.72,
BRUKER AVANCE700 (700 MHz) spectrometers, and all the H 8.089.

PHOSPHORUS, SULFUR, AND SILICON
561
5-(Trimethylsilyl)-3-(2-(p-tolyl)-2-
[(trimethylsilyl)oxy]ethyl]-3H-1,2,3,4-triazaphosphole (14)
Stoe IPDS 2T diffractometer with a rotating anode (MoKα radi-
˚
ation; λ = 0.71073 A) up to a resolution of (sinθ/λ)max = 0.69
−1
˚
A
at a temperature of 210.0 K. 2991 reflections were unique
(Rint = 0.102). The structures were solved with SHELXS-201321
by using direct methods and refined with SHELXL-201321 on
F2 for all reflections. Non-hydrogen atoms were refined by
using anisotropic displacement parameters. The positions of
the hydrogen atoms were calculated for idealized positions. 215
parameter were refined with one restraint. R1 = 0.050 for 2736
reflections with I > 2σ(I) and wR2 = 0.102 for 5783 reflections,
S = 0.881, residual electron density was between −0.20 and 0.21
Prior to use, the concentration of Me3SiCࣕP in toluene
was determined by means of 31P{1H} NMR spectroscopy
(105.7 mmol/L). This solution (25 mL, 1.0 eq.) was added to
azide 10 (0.344 g, 1.38 mmol, 1.0 eq.) and stirring was continued
for 1 h at room temperature, while the course of the reaction was
monitored by means of 31P{1H} NMR spectroscopy. The sol-
vent and all volatiles were removed in vacuo and the product
could be obtained as white solid (479.8 mg, 1.31 mmol, 97%).
eA−3. Geometry calculations and checks for higher symmetry
˚
1
IR (ATR, neat): v = 2962; 1259; 1091; 1016; 943 cm−1. H-
˜
were performed with the PLATON program22
NMR (400 MHz, CD2Cl2): δ = −0.18—0.28 (m, 9H, Si(CH3)3),
0.35–0.41 (m, 9H, O-Si(CH3)3), 2.32 (s, 3H, CH3), 4.56–4.64 (m,
1H, CH2N), 4.72–4.81 (m, 1H, CH2N), 4.97 (dd, 1H, J = 9.0;
3.4 Hz, 1H, CHO(Si(CH3)3), 7.15 (d, J = 7.8 Hz, 2H, Ar-H),
7.25 (d, J = 7.9 Hz, 2H, Ar-H) ppm. 13C{1H}-NMR (101 MHz,
CD2Cl2): δ = 0.76 (O-Si(CH3)3), 0.80 (Si(CH3)3), 20.89 (CH3),
59.65 (C-N3), 74.57 (C-OSi(CH3)3), 126.00 (Ar), 129.13 (Ar),
137.85 (Ar), 138.28 (C = P) ppm. 31P{1H}-NMR (162 MHz):
δ = 218.32 ppm.
CCDC- 1424046 (14) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Conclusions
We have presented the access to novel functionalized 3H-
1,2,3,4-triazaphosphole derivatives making use of the modu-
lar [3 + 2] dipolar cycloaddition reaction between an azide
and a phosphaalkyne. The TMS-protected 3(1-methyl-4-[1-
[trimethylsilyl)oxy]ethyl]phenyl-1,2,3,4-triazaphospholes have
been prepared in high yield and were characterized crystallo-
graphically. These compounds can be used as a starting point
for the preparation of chelating and chiral P^O ligands based
on low-coordinate phosphorus heterocycles for applications in
the field of asymmetric homogeneous catalysis. Experiments to
deprotect the OH-functionality and to start from enantiomeri-
cally pure building blocks are currently performed in our labo-
ratories and will be reported elsewhere.
X-ray crystal structure determination of 13
Crystals suitable for X-ray diffraction were obtained by cooling
slowly down a hot saturated solution of 13 in pentane. Crys-
tallographic data: C17H28N3OPSi; Fw = 349.48; 0.35 mm ×
0.04 mm × 0.02 mm; colorless needle, monoclinic; P21/c; a =
˚
6.2652(2), b = 16.0712(5), c = 20.0483(6) A; α = 90°, β = 96.058
3
−3
˚
(2), γ = 90°; V = 2007.38(11) A ; Z = 4; Dx = 1.156 gcm
;
μ = 1.837 mm−1. 12965 reflections were measured by using
a SAINT v8.34A (Bruker, 2013)20 diffractometer (CuKα radi-
˚
ation; λ = 1.54178 A) up to a resolution of (sinθ/λ)max = 0.60
A
−1 at a temperature of 100.07 K. 2991 reflections were unique
˚
(Rint = 0.049). The structures were solved with SHELXS-201321
by using direct methods and refined with SHELXL-201321 on
F2 for all reflections. Non-hydrogen atoms were refined by
using anisotropic displacement parameters. The positions of
the hydrogen atoms were calculated for idealized positions. 215
parameter were refined with one restraint. R1 = 0.050 for 2991
reflections with I > 2σ(I) and wR2 = 0.125 for 3499 reflections,
S = 1.112, residual electron density was between −0.30 and 0.45
Funding
The authors thank the Deutsche Forschungsgemeinschaft (DFG), the
European Initial Training Network SusPhos (317404), and the Free Uni-
versity of Berlin, Institute of Chemistry and Biochemistry, for financial
support.
References
eA−3. Geometry calculations and checks for higher symmetry
˚
were performed with the PLATON program22
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charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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Crystals suitable for X-ray diffraction were obtained by cool-
ing slowly down a hot saturated solution of 14 in pentane.
Crystallographic data: C16H28N3OPSi2; Fw = 365.56; 0.35 mm
× 0.04 mm × 0.02 mm; colorless needle, monoclinic; P21/n;
˚
a = 6.1929(5), b = 3.452(3), c = 15.0751(12)A; α = 90°, β =
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Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew.
3
−3
˚
97.705(3), γ = 90°; V = 2169.7(3) A ; Z = 4; Dx = 1.119 gcm
;
μ = 0.244 mm−1. 14570 reflections were measured by using a

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Organometallics, Top. Organomet. Chem. Vol. 29, 2010;(c) Takeda, H.;
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