A versatile synthesis of a new bisiminophosphorane

Article abstract of DOI:10.1016/j.tetlet.2009.07.112

The iminophosphorane CH₂CH₂[P{{double bond, long}NP({double bond, long}O)(OPh)₂}Ph₂]₂ is synthesized in high yields (80-97%) via a very convenient procedure using diphenylphosphoryl azide (DPPA) and 1

Full text of DOI:10.1016/j.tetlet.2009.07.112

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Tetrahedron Letters 50 (2009) 5622–5624
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Tetrahedron Letters
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A versatile synthesis of a new bisiminophosphorane
a
Andreea Micle a, Natalia Miklášova b, Richard A. Varga c, Aurelia Pascariu a, , Nicoleta Plesßu ,
*
Mihaela Petric a, Gheorghe Ilia a,d,
*
a Institute of Chemistry, 24 M. Viteazul Blv., 300223 Timisoara, Romania
b Comenius University in Bratislava, Faculty of Pharmacy, Department of Chemical Theory of Drugs, Odbojarov 10, 83232 Bratislava, Slovakia
c National Centre for X-ray Diffraction, Babes-Bolyai University, Faculty of Chemistry and Chemical Engineering, Department of Inorganic Chemistry,
Arany Janos Nr. 11, 400028 Cluj-Napoca, Romania
d The West University of Timisoara, Faculty of Chemistry-Biology-Geography, 16 Pestalozzi Blv., 300115 Timisoara, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
The iminophosphorane CH2CH2[P{@NP(@O)(OPh)2}Ph2]2 is synthesized in high yields (80–97%) via a very
convenient procedure using diphenylphosphoryl azide (DPPA) and 1,2-bis(diphenylphosphino)ethane.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 12 June 2009
Revised 8 July 2009
Accepted 17 July 2009
Available online 24 July 2009
Keywords:
Diphenylphosphoryl azide
Bis(iminophosphorane)ethane
Since the discovery of the imination reaction of phosphines
with azides, the Staudinger reaction,1 a large number of mono-
and bis-phosphines have been converted effectively into the corre-
sponding iminophosphoranes or bis(iminophosphoranes).2
It is known that iminophosphoranes are important reagents and
intermediates in organic synthesis.3,4 Recently, their use for the
introductionof imine units5 inthesynthesisof nitrogenheterocycles
via inter- and intramolecular aza-Wittig reactions has been devel-
oped.3a,4a Successful methods include N-thiophosphorylated and
N-phosphorylated iminophosphoranes as models for dendrimers.6
Organic azides have assumed an important position at the inter-
face between chemistry, biology, medicine, and materials science.7
Diphenylphosphoryl azide has been used extensively as a peptide
coupling reagent,8 in the transformation of alcohols into amines,9
in the transformation of the oxo-function of quinoline/pyridine/qui-
nazolin-4-ones to form 4-azido-derivatives,10 in the transformation
of acids into acyl azide intermediates,11,12 and in the synthesis of
ruthenium complexes containing iminophosphorane ligands.13 To
our knowledge, the reactivity of DPPA toward 1,2-bis(diphenylphos-
phino)ethane has not been investigated.
Monoimination and bisimination of bis(diphenylphosphino)
methane (DPPM) with azides have been successfully applied for
the preparation of several iminophosphorane-phosphine ligands
[Ph2PCH2P(@NR)Ph2].14,15
The bisimination of 1,2-bis(diphenylphosphino)ethane (DPPE)16
(Scheme 1) has been studied recently in our laboratory. The reac-
tion proceeds in the presence of excess diphenylphosphoryl azide
in toluene at room temperature. In this case, the final product
was purified by slow diffusion of hexane in which the correspond-
ing iminophosphorane 2 is not soluble.
The thermal behavior of the solid iminophosphorane 2 was
studied by differential scanning calorimetry (DSC) and thermo-
gravimetric (TG) analysis. The TG measurements proved that no
solvent or water remained with the crystals. The change in weight
was complete in two steps, but the phases were highly delimited,
maybe because of the symmetry of the molecule. This compound
began to lose weight at 270 °C and reached the first maximum rate
of weight loss at 355 °C when 30% of the compound had decom-
posed. The second maximum rate of weight loss was at 390 °C
when 58% of the compound had decomposed. The total weight loss
We were interested in the preparation of iminophosphoranes
starting from diphenylphosphoryl azide (DPPA) and tertiary phos-
phines because these species seem to have been less studied and
they are important as intermediates in organic synthesis and as li-
gands in organometallic chemistry, especially with gold or silver.
* Corresponding authors. Tel.: +40 256 491818; fax: +40 256 491824 (A.P.).
E-mail address: ilia@acad-icht.tm.edu.ro (A. Pascariu).
Scheme 1. Synthesis of bis(iminophosphorane)ethane {CH2CH2[P{@NP(@O)(OPh)2}
Ph2]2}.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.07.112

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A. Micle et al. / Tetrahedron Letters 50 (2009) 5622–5624
5623
between 0 and 600 °C was 88%. The DSC shows that two endother-
mic and one exothermic processes occur. The melting point of the
sample was attributed to the endothermic processes at 133 °C.
The structure of compound 2 was determined by X-ray diffrac-
tion measurements (CCDC 694483), representative bond lengths
and angles are presented in Table 1.
Figure 1 shows the molecular structure of compound 2, where
the P–N single bond length is 1.58(3) Å and the P–N double bond
distance is 1.57(3) Å, being typical values for iminophosphorane
moieties. The lengths of the P–O bonds are 1.54(5) Å for the double
bond and 1.56(5) Å and 1.50(5) Å, respectively, for the single
bonds. The angle C14–O2–P2 is 117.60(4)° and that of P2–O3–
C20 is 116.50(4)°, while the torsion angles are as follows: C14–
O2–P2–O3, 162.1(4)°, and C20–O3–P2–O2, 52.0(5)° (probably due
to weak OÁÁÁH intermolecular interactions between the O3 atom
and the hydrogen atoms of the neighboring aromatic rings). The
angle C7–P1–N1 has a value of 114.77(2)° and that of the torsion
angle C7–P1–N1–P2 is 65.5(3)°.
The molecular structure of 2 is directed by weak OÁÁÁH intermo-
lecular interactions (2.557 Å, 2.635 Å, and 2.595 Å which are close
to the sum of the van der Waals radii) due to the packing effect.
The structure contains a disordered toluene solvent molecule and
edge-face interactions between one of the phenyl rings and the sol-
vent (d1 = 5.324 Å and d2 = 3.189 Å) are present, Figure 2. How-
Figure 2. Intramolecular aromatic edge-to-face interactions with
toluene solvent molecule (d1 = 5.324 Å and d2 = 3.189 Å).
a
disordered
ever, no p-p stacking was observed.
In conclusion, an efficient method for the synthesis of the novel
iminophosphorane {CH2CH2[P{@NP(@O)(OPh)2}Ph2]2} has been
developed. The product was isolated in high yield and character-
ized from melting point, IR, 1H NMR, 13C NMR, 31P NMR, UV, and
X-ray data. The advantages of the present method are that the
reaction can be monitored by TLC and the evolution of nitrogen
is easily observed. Compound 2 is an air-stable white solid, soluble
in chlorinated solvents, acetonitrile, and ethyl acetate, and is insol-
uble in non-polar solvents such as hexane or pentane. Further
studies on the reactivity of this iminophosphorane are in progress.
Table 1
Acknowledgments
Selected bond lengths (Å) and angles (°) for compound 2
Bond length (Å)
Angle (°)
The authors would like to thank Prof. Dr. Eurico Cabrita, New
University of Lisbon, for providing NMR facilities, Assoc. Prof. Dr.
Geza Bandur, Politehnica University of Timisoara, for DSC and TG
measurements and Dr. Monika Simon, Polytechnic University of
Timisoara, for FT-IR measurements.
C1–P1
C7–P1
C13–P1
C14–O2
C20–O3
N1–P1
N1–P2
O3–P2
O1–P2
O2–P2
1.79(4)
1.80(4)
1.80(4)
1.52(7)
1.58(9)
1.57(3)
1.58(3)
1.50(5)
1.58(3)
1.54(5)
C1–P1–C7
C1–P1–C13
C14–O2–P2
P2–O3–C20
O3–P2–O1
O3–P2–O2
O1–P2–O2
O3–P2–N1
O1–P2–N1
O2–P2–N1
P1–N1–P2
106.87(2)
107.43(2)
117.60(4)
116.50(4)
107.30(3)
102.70(3)
109.90(3)
115.70(2)
114.30(2)
106.30(2)
129.90(2)
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.07.112.
References and notes
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Figure 1. Molecular structure of compound 2; hydrogen atoms are omitted for
clarity.

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A. Micle et al. / Tetrahedron Letters 50 (2009) 5622–5624
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treated dropwise, at room temperature, with a solution of diphenylphosphoryl
azide (1 ml, 4.62 mmol) in toluene (5 ml). After 1 h (TLC), the solvent was
removed under reduced pressure to give a colorless oil. A microcrystalline
white solid was obtained by slow diffusion of hexane. Characterization data for
2: yield: 97% (1.53 mg); mp = 135–137 °C (CH3CN) (DSC 133 °C); IR (KBr, cmÀ1
)
m
3050 (m), 3048 (m), 2956 (m), 2912 (m), 1590 (m), 1487 (m), 1438 (m), 1327
(s), 1202 (vs), 915 (vs), 821 (m), 746 (s), 692 (s), 512 (s); 1H NMR (400 MHz,
CDCl3, 25 °C): d = 2.96 (s, 4H, CH2CH2), 7.05–7.73 (m, 40H, Ph); 13C NMR
(100 MHz, CDCl3, 25 °C): d = 21.6, 22.0, 120.7, 123.8, 128.3, 128.9, 129.3, 131.2,
132.3, 152.4; 31P NMR (162 MHz, CDCl3, 25 °C): d = À6.7 (dd, JPP = 36.5 Hz,
2
3JPP = 12.9 Hz, 2P, (PhO)2P@O), 18.4 (dd, JPP = 36.5 Hz, JPP = 12.9 Hz, 2P,
Ph2P@N); UV (CH2Cl2) k1 243 nm, k2 277 nm, % of P content: theoretical
13.88; experimental17 13.79.
2
3
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Polym. 2003, 55, 151; Popa, A.; Ilia, G.; Iliescu, S.; Davidescu, C. M.; Pascariu, A.;
Bora, A. Rev. Chim. 2003, 54, 834.
16. Synthesis of CH2CH2[P{@NP(@O)(OPh)2}Ph2]2.
A
solution of 1,2-
bis(diphenylphosphino)ethane (0.700 g, 1.77 mmol) in toluene (15 ml) was