Reversible P-C bond formation for saturated α-aminophosphine ligands in solution: Stabilization by coordination to Cu(I)

Article abstract of DOI:10.1039/a902783a

The saturated α-aminophosphine ligands containing a secondary amine function Ph₂PCH(R')NHR'' establish a solution equilibrium witb Pb₂PH and R'CH=NR'' and are stabilized by electron-withdrawing substituents R' and R'' and by coordination of the phosphorous donor to Cu(I).

Full text of DOI:10.1039/a902783a

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Reversible P–C bond formation for saturated a-aminophosphine
ligands in solution: stabilization by coordination to Cu(I)
Jacques Andrieu,* Jochen Dietz, Rinaldo Poli* and Philippe Richard
L aboratoire de Synthese et dÏElectrosynthese Organometalliques, Faculte des Sciences ““GabrielÏÏ,
Universite de Bourgogne, 6 Boulevard Gabriel, 21100 Dijon, France.
E-mail: Rinaldo.poli=u-bourgogne.fr
L e t t e r
Received 3rd February 1999, Accepted 7th April 1999
The saturated a-aminophosphine ligands containing a secondary
redissolved crystallised product show the presence of the
diphenylphosphine and imine starting materials in amounts
that increase with time, indicating a reversible PÈC bond
amine function Ph PCH(Rº)NHRA establish a solution equi-
2
librium with Ph PH and RºCH2NRA and are stabilized by
2
electron-withdrawing substituents Rº and RA and by coordi-
cleavage. NMR monitoring of this process in CDCl
3
nation of the phosphorous donor to Cu(I).
(integration of the 1H NMR imine CH2N and product
PÈCHÈN signals) reveals the establishment of a stable equi-
librium position (t \ 10 min; K \ 50 after [2 d), which
Since the discovery that bifunctional P,N ligands increase
considerably the activity and/or the selectivity of palladium,
ruthenium or rhodium catalysts1h4 the preparation of this
type of ligand has been the subject of extensive investigations.
Amongst these ligands, a-aminophosphines crucially control
the activity and selectivity of alkyne methoxycarbonylation
catalysts.1 Unsaturated ligands of this type (having an
1@2
2
was independently conÐrmed by NMR monitoring of the
reaction between Ph PH and PhCH2NPh. A closer exami-
2
nation shows that an analogous equilibrium is also estab-
lished for the anionic species, although this is shifted to a
weaker extent toward the PCN product 1 relative to the
neutral system (K \ 10 by 31P NMR integration in
1
THFÈC D ). Thus, our NMR experiments show that the
sp2-hybridized
N donor) are exempliÐed by 2-PyPPh
(Py \ C H N). Saturated versions of this class, i.e.
6
6
2
instability of the a-P,N ligand and of its related anionic form
in solution is due to a reversible PÈC bond formation, with
proton migration for the neutral species.
5
4
Ph PCH(R)NR@RA have received little attention. While the
2
Mannich reaction between Ph PH, CH O and the Me NH
2
2
2
The generalization to a wider group of imines establishes
the substituent e†ect on the equilibrium position (see Scheme
2). In particular, the presence of an electron-withdrawing
group has a beneÐcial e†ect on the ligand formation when this
is located either on the nitrogen atom (d [ a [ b [ c) or on
the carbon atom (e [ b). This study shows that the careful
choice of substituents may allow the synthesis of stable (in
solution) a-P,N saturated ligands.
compounds leads to the stable (both in the solid state and in
solution) Ph PCH NMe ligand having a tertiary amine func-
2
2
2
tion,5 aminophosphines prepared by addition of Et PH to
2
CH 2NtBu5 or Ph PH to PhCH2NPh6 without solvent have
2
2
only been described in the solid state. The subsequent reaction
with iodomethane leads to an unexpected PÈC bond cleav-
age.6 Anionic a-P,N ligands prepared by nucleophilic addition
of Ph P~Li` to PhCH2NPh or RLi to 2-PyPPh have also
2
2
As we are interested in the exploration of the coordinating
properties of these P,N ligands, we wished to examine whether
coordination to a suitable metal could also a†ect the stability
showed instability in solution.7,8 We now wish to present
NMR and synthetic studies that enable us to rationalise the
solution instability of a-P,N ligands with secondary amine
functions and to understand the electronic factors that favor
their stabilization.
of the PÈC bond. The above mentioned 2-PyPPh ligand is
2
known to act as an assembling ligand (adopting a l-g2 coordi-
nation mode) in dinuclear copper(I) and in numerous other
polynuclear complexes.4,9 Addition of one equivalent of
[Cu(NCMe) ][BF ]10 to the equilibrium mixture containing
The reaction of Ph P~Li` with one equivalent of N-
2
benzylideneaniline, followed by quenching with water and
extraction in toluene a†ords the desired product 2a, see
Scheme 1.
The 31P and 1H NMR spectroscopic properties conÐrm the
nature of the product. However, the NMR spectra of the
4
4
2, Ph PH and PhCH2NPh in CDCl produces a solu-
2
3
tion with a major 31P NMR resonance at d [ 3.90 and a
minor one at d [ 35.2 corresponding to the copper
complexes [Cu(NCMe) (Ph PCHPhNHPh)][BF ] 3, and
x
2
4
Scheme 1
Scheme 2
New J. Chem., 1999, 23, 581È583
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starting materials. 1H NMR (CDCl ): d 7.97È6.68 (m, 20 H,
3
aromatics), 5.16 [dd, 1H, PCH, 2J(P,H) \ 3.2, 3J(H,H) \ 6.5],
4.41 [dd, 1H, NCH, 3J(P,H) \ 3.8, 3J(H,H) \ 6.5 Hz,
exchange readily with D O]. 31P NMR (CDCl ): d 3.70 (s).
2
3
13C NMR (CDCl ): d 140.00È113.72 (m, 24C, aromatics),
3
56.92 [d, 1C, PCH, 1J(PC) \ 14.0 Hz].
By analogous reactions, equilibrium mixtures of com-
pounds 2bÈe with the corresponding starting materials were
obtained (equilibrium constants as indicated in Scheme 2). 1H
NMR (CDCl ). PCHN proton: d 4.12 and 4.10 (2b, d, 2J
\
3
PH
9.8, and 2J \ 5.6 respectively), 4.22 (2c, d, 2J \ 9), 5.20
PH
PH
PH
(2d, dd, 2J \ 7.0, 3J \ 2.9), 4.63 and 4.27 (2e, d, 2J \ 9.8
HH
PH
and 2J \ 5.6 Hz respectively); NH proton: d 2.35 (2b, s, br),
PH
4.28 (2c, d, 3J \ 5.8), 5.05 (2d, dd, 3J \ 6.7, 3J \ 2.9
PH
PH
HH
Hz), 2.03 (2e, s, br). 31P NMR (CDCl ): d 1.49 and 0.89 (2b),
3
1.20 (2c), 5.78 (2d), 4.85 and 4.61 (2e). Compounds 2b and 2e
are present as both possible diastereoisomers.
Preparation of compound 4
Fig. 1 An ORTEP14 view of the cation of compound 4. Selected
bond distances (Ó) and angles (¡): CuÈN1 2.073(6), CuÈN2 2.104(6),
CuÈP 2.153(2), PÈC 1.878(6), CÈN3 1.466(8); N1ÈCuÈN2 87.7(2),
N1ÈCuÈP 133.9(2), N2ÈCuÈP 126.8(2), N3ÈCÈP 104.8(4).
The equilibrium solution of ligand 2a obtained as described
above was transferred to
a Schlenk tube containing
[Cu(NCMe) ][BF ] (0.540 g, 1.72 mmol) and the mixture was
4
4
stirred for 1 h. 1H NMR (CDCl ): d 7.46È6.46 (m, 20 H,
[Cu(NCMe) (PPh H)][BF ], respectively (Scheme 1). Only
3
x
2
4
aromatics), 5.23 (s, br, 1H, PCH), 4.61 (s, br, 1H, NCH), 2.00
(s, br, 12H, CH CN, free and coordinated were not separated).
traces of coordinated Ph PH and free PhCH2NPh have been
2
detected in the 1H NMR spectrum (K \ 800), indicating an
3
3
31P NMR (CDCl ):
d
[3.90 (s). 13C NMR (CDCl ):
almost quantitative shift of the equilibrium. This result shows
3
3
d 145.57È113.57 (m, 48C, aromatics), 56.95 (s, br, 2C, PCH).
that removal of electron density from the phosphine end of
the molecule also stabilizes the PÈC bond.
In
a
control experiment, coordination of PHPh to
2
[Cu(MeCN) ]` yields a 31P NMR resonance at d [3.47. To
Compound 3 is highly unstable, even in neat MeCN, leading
to the slow precipitation of metallic copper. Addition of
Me NCH CH NMe (tmeda), on the other hand, yields stable
4
this solution was added tmeda (0.26 ml, 1.72 mmol). The
resulting solution was stirred for 2 h, Ðltered and concentrated
to half volume. Complex 4 was isolated by precipitation with
Et O and washed twice with this solvent. Colorless crystals
2
2
2
2
[Cu(tmeda)(Ph PCHPhNHPh)][BF ] 4, with NMR proper-
2
4
ties quite similar to those of 3. Colourless single crystals of 4
were obtained from CHCl . The geometry of the cation,
shown in Fig. 1, reveals a rare example of three-coordination
2
were obtained from CHCl (0.742 g, 68%). 1H NMR (CDCl ):
3
3
3
d 7.58È6.60 (m, 20H, aromatics), 5.36 (m, 2H, PCH ] NCH),
2.61 (s, br, 4H, NCH coordinated), 2.43 (s, br, 12H, NCH
for a Cu(I) complex.11,12 The a-P,N ligand is only P-
coordinated and the amino function remains dangling,
2
3
coordinated). 31P NMR (CDCl ): d 13.77 (s). 13C NMR
3
(CDCl ): d 134.32È114.27 (m, 24C, aromatics), 58.00 (s, br, 1C,
whereas the analogous 2-PyPPh ligand always adopts a l-P,
3
2
PCH), 48.36 (s, br, 6C, NCH ] NCH from tmeda
N coordination mode in dinuclear copper complexes.9,11 Note
3 2
N PF Cu: C, 60.55; H, 6.04; N
coordinated) (Calc. for C
H
that
a Ñuxional behaviour interconverting P- and N-
32 38
3
4
6.62; Found: C, 60.33; H, 5.92; N 6.43%).
coordinated ligands in solution is inconsistent with the
observed variation of the 31P chemical shift from the free to
the coordinated ligand, which is similar to that observed for
Crystal structure analysis of compound 4
the 2-PyPPh system.
2
Crystal dimensions: 0.3 ] 0.3 ] 0.2 (mounted in capillary).
The data collection was carried out on a CAD4 Enraf-Nonius
goniometer. The structure was solved by interpretation of
the Patterson map and subsequent di†erence Fourier tech-
niques. All non-hydrogen atoms were reÐned anisotropi-
In conclusion, we have shown that the instability of a-P,N
ligands with a secondary amine function and their corre-
sponding anions in solution is due to reversible PÈC bond
formation, which can be reduced or suppressed by a decrease
of electron density. This can be accomplished by either elec-
tron withdrawing substituents on the nitrogen or carbon
atoms, or by coordination of the phosphorus donor. These
results allow an easy and rapid access to new complexes with
saturated a-aminophosphine ligands. Further studies are in
progress in order to control the chirality of the central carbon
atom (PÈC*ÈN) and to explore the coordination properties of
a-P,N ligands toward early and late transition metals.
cally.13 Crystal and reÐnement data: C
H
N PF Cu, M \
32 38
3
4
r
634.20, orthorhombic, space group Pna2 , a \ 19.528(1),
1
b \ 16.812(1), c \ 9.719(1) Ó, Z \ 4, V \ 3190.8(4) Ó3, o
\
calc
1.320 g cm~3, Mo-Ka radiation (j \ 0.71073 Ó), k(Mo-Ka) \
0.782 mm~1, T \ 298 K, F(000) \ 1320, 2369 independent
reÑections measured up to sin(h)/j \ 0.616. Final residual
indices: R (F2) \ 0.096 for all data and R(F) \ 0.035 for 1867
w
reÑections with I [ 2p(I), S \ 1.072. Crystallographic data for
the structure reported in this paper have been deposited with
the Cambridge Crystallographic Data Center as supplemen-
tary publication no. CCDC-105288.
Experimental
All manipulations were carried out under an atmosphere of
puriÐed nitrogen using standard Schlenk techniques. All sol-
vents were dried and deoxygenated prior to use. All chemical
shifts are given in ppm.
Acknowledgements
We acknowledge the Conseil Regional de Bourgogne and the
CNRS for support of this work and the European Commis-
sion for an Eramus exchange undergraduate student program
concerning Jochen Dietz from the University of Kaiserslau-
tern, Germany.
Reversible formation of compounds 2a–e
To a solution of PhCH2NPh (0.312 g, 1.72 mmol) in CDCl
3
(10 ml) was added Ph PH (0.300 ml, 1.72 mmol). The mixture
2
was stirred for 2 h, yielding an equilibrium of 2a with the
582
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