An efficient entry to monoiminophosphoranes derived from 1,2-bis(diphenylphosphino)ethane: New bidentate P,N ligands

Article abstract of DOI:10.1016/S0040-4039(00)02017-7

Base-catalyzed Michael-type addition of diphenylphosphane to P,P,P-diphenylvinyl iminophosphoranes yielded monoiminophosphoranes derived from 1,2-bis(diphenylphosphino)ethane, heteroditopic P,N-donor ligands which are not easily available by the diract monoimination of the parent bis(phosphane).

Full text of DOI:10.1016/S0040-4039(00)02017-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 605–607
An efficient entry to monoiminophosphoranes derived from
1,2-bis(diphenylphosphino)ethane: new bidentate P,N ligands
Mateo Alajar´ın,* Carmen Lo´pez-Leonardo and Pilar Llamas-Lorente
Departamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, Universidad de Murcia, Campus de Espinardo, 30100 Murcia, Spain
Received 30 September 2000; accepted 1 November 2000
Abstract—Base-catalyzed Michael-type addition of diphenylphosphane to P,P,P-diphenylvinyl iminophosphoranes yielded
monoiminophosphoranes derived from 1,2-bis(diphenylphosphino)ethane, heteroditopic P,N-donor ligands which are not easily
available by the direct monoimination of the parent bis(phosphane). © 2001 Elsevier Science Ltd. All rights reserved.
Oxidation at only one phosphorus atom of a bis(phos-
phane) creates heteroditopic phosphane ligands with
contrasting (e.g. hard/soft) reactivities of their two
donor centers. A number of mono-oxidized bis(phos-
phane) derivatives have proven to be efficient ligands in
coordination and organometallic chemistry,1 and their
complexes are appreciated as valuable catalysts.2 The
heteroatom at the phosphorus(V) center in these
P(III),P(V) bidentate ligands is typically O, S, Se or N.
In the course of a program devoted to the preparation
of new heteroditopic bis(phosphane)-based ligands6 we
directed our attention to the monoiminophosphoranes
derived from dppe. When we tested the selective
monoimination of dppe with p-tolylazide, under the
reaction conditions we had established as optimal for
dppm (1 equiv of azide, slow addition over the phos-
phane, in acetonitrile, rt),6 we obtained low levels of
selectivity (2:1, mono:bis iminophosphoranes, as mea-
sured by 1H and 31P NMR analysis of the crude).
Moreover, the tedious separation of the components of
the final mixture by column chromatography resulted
in poor isolated yield of the desired monoiminophos-
phorane (always lower than 30%, in several runs) due
to extensive hydrolysis of its PꢀN function to give the
phosphane oxide dppeO and p-toluidine.
Despite its great potential and anticipated diversity, the
coordination and catalytic chemistry of mono-oxidized
bis(phosphanes) still remains scarcely explored due to
the lack of good general methods for their synthesis.
Mono-oxidation of bis(phosphanes) is not easily con-
trolled. Most reagents readily oxidize both phosphorus
atoms, a result which generally prevails with identically
substituted phosphane units.
As an alternative to the not practicable monoimination
of dppe, we here report a general and efficient method
for the preparation of monoiminophosphoranes derived
from that bis(phosphane), in two high-yielding steps,
starting from the commercially available diphenyl-
vinylphosphane.
In this context, the selective monoimination of bis-
(diphenylphosphino)alkanes with azides via the
Staudinger reaction3 has been successfully applied to
the preparation of several monoiminophosphoranes
derived from bis(diphenylphosphino)methane (dppm).4
The high selectivity of these reactions probably lies on
the proximity of the two phosphane groups, which
results in the failure of the imination at the second
phosphorus atom by steric reasons. With other bis-
(diphenylphosphino)alkanes of longer chain, such as
bis(diphenylphosphino)ethane (dppe), propane (dppp)
and butane (dppb), the monoimination only could be
achieved in good yield by the particularly unreactive
trimethylsilylazide.5
It has been reported that the CꢀC bond of
diphenylvinylphosphane and its chalcogenides (oxide
and sulfide) is reactive in Michael-type additions of a
variety of nucleophilic reagents, such as phosphanes,7
alcohols8 and amines.9 By contrary to the oxide and
sulfide Ph2P(ꢀX)CHꢀCH2 (X=O, S), the P,P,P-
diphenylvinyl iminophosphoranes Ph2P(ꢀNR)CHꢀCH2
are poorly known species whose chemistry remains
almost unexplored.10 We envisaged that the Michael-
type additions of secondary phosphanes (e.g. Ph2PH) to
these species could be a potential source of the elusive
dppe monoiminophosphoranes.
* Corresponding author. Fax: +34.968.364149; e-mail: alajarin@
um.es
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02017-7

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M. Alajar´ın et al. / Tetrahedron Letters 42 (2001) 605–607
To this end we prepared a number of iminophospho-
ranes 1 by reacting diphenylvinylphosphane with a
range of aryl, acyl and phosphoryl azides under the
standard conditions of the Staudinger reaction (Scheme
1).
pounds 2 could be further purified by flash chromatog-
raphy on deactivated (5% Et3N) silica gel column,
eluting with ethyl acetate/hexane (1:1, v:v). They could
be also crystallized from CH2Cl2/Et2O providing this
operation was carried out rapidly (in less than 3 h),
otherwise partial hydrolysis of the PꢀN function
occurred to yield crystals contaminated by dppeO.
Compounds 1 were isolated after a simple work-up and
obtained as crystalline solids in good yields (76–94%).
The characterization of 1 was straightforward following
their analytical and spectral data. The 31P NMR (121.4
MHz, CDCl3) spectra of 1a–d showed a singlet in the
range −0.77 to −1.03 ppm, whereas that of 1e appeared
at 18.06 ppm. Two doublets (2JPP=32.5 Hz) at −7.31
Analytical and spectral data of 2 were consistent with
their presumed structures.12 The 31P NMR spectra of
the N-aryl derivatives 2a–d in CDCl3 showed two well-
separated doublets (3JPP=46.5–46.7 Hz) near −12 and 7
ppm, attributed to the phosphane and iminophospho-
rane functions respectively, the P(III) nucleus lying
typically upfield from the P(V) one. The presence of the
1
and 9.80 ppm were observed for 1f. In their H NMR
spectra, the signals attributed to the vinylic protons
appeared as the ABM portion of an ABMX system,
due to their coupling with the phosphorus nucleus.
1
ethylene fragment in 2 was ascertained by their H and
13C NMR spectra.
We were pleased to find that the base-catalyzed
Michael addition of diphenylphosphane to the P,P,P-
diphenylvinyl iminophosphoranes 1 led to the dppe
monoiminophosphoranes 2 in excellent yields (Table 1).
With ligands 2 in hand, we carried out a first test
of their ability to coordinate metal ions. Reaction
of 2c with dichlorobis(benzonitrile)palladium(II) in
dichloromethane at room temperature afforded com-
1
plex 3 in 90% yield. Elemental analysis, H and 13C
The catalytic activity of three different bases (phos-
phazene base P4-t-Bu,11 KN(TMS)2 and t-BuOK) in
combination with two reaction solvents was checked,
and we found no relevant differences in terms of
efficiency. These reactions were completed in less than
30 min at room temperature and their yields, estimated
by NMR analysis, were nearly quantitative. Com-
NMR, and FAB-MS data of 3 are in accord with the
proposed structure. The 31P NMR spectrum of 3 in
CDCl3 showed two doublets (3JPP=11.4 Hz) at 24.72
and 21.93 ppm, notably shifted downfield when com-
pared with those of the free ligand 2c (−12.61 and 6.62
ppm), as expected for the coordination in a s-N and
Scheme 1.
Table 1. Compounds 2 prepared by Michael-type addition of Ph2PH to 1
R
Basea
Solvent
Yield (%)b
R
Basea
Solvent
Yield (%)b
C6H5
C6H5
C6H5
4-H3C-C6H4
4-H3CO-C6H4
4-H3CO-C6H4
4-H3CO-C6H4
P4-t-Bu
Toluene
Toluene
THF
90
89
93
91
90
93
93
4-Br-C6H4
4-Br-C6H4
4-Br-C6H4
(E)-4-H3C-C6H4-CHꢀCHCO
(E)-4-H3C-C6H4-CHꢀCHCO
(PhO)2P(O)
P4-t-Bu
KN(TMS)2
t-BuOK
KN(TMS)2
t-BuOK
t-BuOK
Toluene
Toluene
THF
Toluene
THF
86
92
93
78
80
79
KN(TMS)2
t-BuOK
t-BuOK
P4-t-Bu
KN(TMS)2
t-BuOK
THF
Toluene
Toluene
THF
THF
a 0.1 equivalents.
b Isolated pure products after flash column chromatography.

M. Alajar´ın et al. / Tetrahedron Letters 42 (2001) 605–607
607
s-P chelate mode. We are currently engaged in further
studies of ligands 2, in an attempt to systematize their
coordinating abilities.
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Acknowledgements
6. Alajar´ın, M.; Lo´pez-Leonardo, C.; Llamas-Lorente, P.;
Bautista, D. Synthesis, in press.
This work was supported by the Direccio´n General de
Ensen˜anza Superior (Project PB95-1019), Fundacio´n
Se´neca-CARM (Project PB/2/FS/99) and Acedesa (a
division of Takasago). One of us (P. L.-L.) also thanks
Fundacio´n Se´neca-CARM for a fellowship.
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11. Available as 1 M n-hexane solution from Fluka Chemie
AG.
12. 2c: yellow needles (dichloromethane/diethyl ether). Mp
;
125–127°C; IR (Nujol) 1500, 1456 cm−1 1H NMR
(CDCl3)
l 2.18 [m, 2H, CH2P(III)], 2.47 [m, 2H,
CH2P(V)], 3.71 (s, 3H, OCH3), 6.62 (d, 2H, 3JHH=9.3
3
Hz, Harom), 6.63 (d, 2H, JHH=9.3 Hz, Harom), 7.23–7.25
(m, 10H, Ph), 7.39–7.51 (m, 6H, Ph), 7.65–7.72 (m, 4H,
Ph); 13C NMR (CDCl3) l 19.33 [dd, 1JCP(III)=14.8,
2JCP(V)=3.8 Hz, CH2P(III)], 24.31 [dd, 1JCP(V)=64.9,
2JCP(III)=16.8 Hz, CH2P(V)], 55.68 (OCH3), 114.49 (C-3),
123.60 (d, 3JCP=17.4 Hz, C-2), 128.60 (d, 3JCP(III)=6.4
Hz, Cm), 128.82 (d, 3JCP(V)=11.0 Hz, Cm), 128.90 (Cp),
1
4
129.78 (d, JCP(V)=109.5 Hz, Ci), 131.64 (d, JCP(V)=2.3
2
2
Hz), 131.65 (d, JCP(V)=9.2 Hz, Co), 132.75 (d, JCP(III)
=
18.6 Hz, Co), 137.39 (d, 1JCP(III)=13.3 Hz, Ci), 144.70
(C-1), 152.01 (C-4); 31P NMR (CDCl3) l −12.61 [d,
3JPP=46.7 Hz, P(III)], 6.62 [d, JPP=46.7 Hz, P(V)]; MS
3
m/e (EI) 519 (90, M+), 182 (100). Anal. calcd for
C33H31NOP2: C, 76.29; H, 6.01; N, 2.70. Found: C,
76.35; H, 5.92; N, 2.65.
.
.