Chiral diiminophosphoranes: A new class of ligands for enantioselective transition metal catalysis

Article abstract of DOI:10.1039/a709023d

Chiral diamines such as (1R,2R)-1,2-diaminocyclohexane or R-2,2′-diamino-1,1′-binaphthalene can be converted into the corresponding diimino(triphenyl)phosphoranes, which serve as C₂-symmetric ligands for enantioselective transition metal cataly

Full text of DOI:10.1039/a709023d

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Chiral diiminophosphoranes: a new class of ligands for enantioselective
transition metal catalysis
Manfred T. Reetz,*† Edward Bohres and Richard Goddard
Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mu¨lheim/Ruhr, Germany
Chiral diamines such as (1R,2R)-1,2-diaminocyclohexane or
R-2,2A-diamino-1,1A-binaphthalene can be converted into the
corresponding diimino(triphenyl)phosphoranes, which
serve as C2-symmetric ligands for enantioselective transition
metal catalyzed reactions such as copper-mediated
cyclopropanations.
P
P(1)
N(1)
N
Chiral nitrogen-containing compounds are gaining increasing
importance as ligands in enantioselective transition metal
catalyzed reactions.1 Although iminophosphoranes2 of the
N*
P*
N(2)
general structure R3PNNRA possess a donor position at the
P(2)
nitrogen function capable of metal complexation,3 chiral
versions have not been prepared to date. Since iminophos-
phoranes are readily available from the corresponding primary
amines RANH2, we envisioned a simple route to chiral chelating
diiminophosphoranes and their possible use as ligands in
enantioselective transition metal catalyzed reactions. Here we
Fig. 1 Molecular structures of 2 (left) and 4 (in 4 the CD2Cl2 solvent of
crystallization has been removed for clarity)
report the first examples of this concept.
Upon reacting the commercially available diamines 1 and 3
with Ph3PBr2 followed by base treatment according to a
literature procedure (modified by working in the presence of
molecular sieves),4 compounds 2 and 4 were obtained in yields
of 82 and 75%, respectively.
ane groups in the free ligand (2) occupy diaxial positions, in the
complexed ligand both groups are equatorial, presumably
enabling the
N
atoms to accommodate the metals
[2·Rh(cod)BF4 N···N 2.665(6) Å, N–Rh–N 77.3(3)°; 2·CoCl2
N···N 2.75(2) Å, N–Co–N 85.6(6)°]. In addition, the extent of
the C2 symmetry induced by the ligand strongly depends on the
coordination sphere of the metal. In 2·Rh(cod)BF4 the approx-
imately square-planar coordinated Rh atom deviates strongly
(2.818 Å) from the local C2 symmetry of the iminocyclohexane
ring (dashed line, Fig. 2, left). In 2·CoCl2, on the other hand, the
loss of symmetry is much smaller and distorted tetrahedral Co
lies approximately (±0.2 Å) on the local C2 axis of symmetry
(dashed line, Fig. 2, right).
Preliminary studies using these chiral metal complexes for
enantioselective catalysis have also been undertaken. Particul-
arly worthy of note is the copper catalyzed cyclopropanation5 of
styrene 5 by ethyl diazoacetate 6 using the complex 4·CuOTf
(1.5 mol%) as the catalyst (Scheme 2). Adducts 7 and 8 were
NH2
N
PPh3
NH2
N
PPh3
1
2
NH2
NH2
N
PPh3
PPh3
N
3
4
Scheme 1
Ligands 2 and 4 are crystalline compounds, which were
characterized by standard methods, including 31P NMR spec-
troscopy (d 20.5 and 24.6, respectively, in CD2Cl2) and X-ray
crystallography‡ (Fig. 1). The effect of the crystal environment
appears to have a more significant effect on 2 than 4, since
whereas 4 exhibits exact C2 symmetry in the crystal [N···N*
3.617(8) Å], even in spite of the presence of solvent of
crystallization, in 2 the local C2 symmetry is restricted to the
cyclohexane ring and the two nitrogen atoms [N(1)···N(2)
3.770(2) Å].
P(2)
P(2)
N(2)
Co(1)
N(2)
Rh
N(1)
Cl(2)
Cl(1)
N(1)
These two chiral ligands are suitable for preparing a wide
variety of new types of transition metal complexes. Indeed, the
complexes 2·Rh(cod)BF4, 2·CoCl2 and 4·CuOTf were readily
synthesized in yields of ca. 80% by reacting the corresponding
ligands with [RhCl2(cod)]2 (followed by NaBF4), CoCl2(thf)x or
CuOTf, as appropriate. The crystal structures of 2·Rh(cod)BF4
and 2·CoCl2 confirm the flexibility of the diimino(triphenyl)-
phosphorane ligand (Fig. 2). Thus, whereas the iminophosphor-
P(1)
P(1)
Fig. 2 Molecular structures of the [2·Rh(cod)]+ cation in 2·Rh(cod)BF4
(left) and 2·CoCl2. Dashed lines indicate local C2 symmetry.
Chem. Commun., 1998
935

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Å, U = 4465(1) Å3, T = 293 K, orthorhombic, space group P212121 (no.
19), Z = 4, Dc = 1.39 g cm23, m = 0.51 mm21. Enraf-Nonius CAD4
Ph
Ph
CO2Et
CO2Et
+
+
N2 CHCO2Et
Ph
diffractometer, Mo-Ka X-radiation, l
=
0.71069 Å. 5746 measured
5
6
7
8
reflections, 5619 unique, 5114 observed [I
>
2.0s(Fo2)]. Analytical
absorption correction (Tmin 0.7769, Tmax 0.9805). The structure was solved
by direct methods (SHELXS-86) and refined by full-matrix least squares
(SHELXL-97) on F2 for all data with Chebyshev weights to R = 0.059
(obs.), wR = 0.151 (all data), S = 1.03, H atoms riding, max shift/error
Scheme 2
formed in equal amounts, each having an enantiomeric excess
(ee) of 90 and 74% in favour of the absolute configurations
(S,R) and (R,R), respectively.§
In summary, we have developed a new and readily accessible
class of chiral nitrogen containing ligands for asymmetric
transition metal catalysis. The possibility of varying the
substituents of phosphorus and/or utilizing other chiral dia-
mines or functionalized amines is currently being explored.
0.001, residual rmax = 1.681 e Å23
.
2·CoCl2·3CH2Cl2: C45H46Cl8CoN2P2, Mr = 1019.3, blue prism, crystal
size 0.35 3 0.42 3 0.62 mm, a = 24.447(1), b = 9.8142(4), c = 40.257(2)
Å, b = 99.710(2)°, U = 9520.1(8) Å3, T = 100 K, monoclinic, space group
C2 (no. 5), Z = 8, Dc = 1.42 g cm23, m = 0.91 mm21. Siemens SMART
diffractometer, Mo-Ka X-radiation, l = 0.71073 Å. 48840 measured
reflections, spherical absorption correction, 20400 unique, 13253 observed
[I > 2.0s(Fo2)]. The structure was solved by direct methods (SHELXS-86)
and refined by full-matrix least squares (SHELXL-97) on F2 for all data
(Co, Cl, P anisotropic) with Chebyshev weights to R = 0.086 (obs.),
wR = 0.199 (all data), S = 1.07, H atoms riding, max. shift/error 0.001,
residual rmax = 0.998 e Å23. CCDC 182/796.
Notes and References
† E-mail: reetz@mpi-muelheim.mpg.de
‡ Crystal data: 2: C42H40N2P2, Mr = 634.7, colourless plate, crystal size
0.32 3 0.60 3 0.88 mm, a = 11.3778(2), b = 8.7847(2), c = 17.5062(3)
Å, b = 103.296(1)°, U = 1702.85(6) Å3, T = 100 K, monoclinic, space
group P21 (no. 4), Z = 2, Dc = 1.24 g cm23, m = 0.16 mm21. Siemens
§ Procedure: a catalyst solution of 4·CuOTf was prepared by mixing 1
equiv. of a copper(i) triflate benzene complex with 1.2 equiv. of ligand 4 in
dry CH2Cl2 under an atmosphere of argon. A Schlenk tube was charged with
1.0 ml of a 0.015 m catalyst solution, freed from the solvent in vacuo and
charged with styrene (1.0 ml of a 1.0 m solution in 1,2-dichloroethane). The
mixture was cooled to 220 °C and treated with 6 (3 mmol in 3 ml of
1,2-dichloroethane) within 3 h using a syringe pump. The mixture was
analyzed by GC (80% conversion).
SMART diffractometer, Mo-Ka X-radiation, l
= 0.71073 Å. 19124
measured reflections, 9573 unique, 9288 observed [I > 2.0s(Fo2)]. The
structure was solved by direct methods (SHELXS-86) and refined by full-
matrix least squares (SHELXL-97) on F2 for all data with Chebyshev
weights to R = 0.059 (obs.), wR = 0.160 (all data), S = 1.05, H atoms
riding, max. shift/error 0.001, residual rmax = 0.968 e Å23
.
1 A. Togni and L. M. Venanzi, Angew. Chem., 1994, 106, 517; Angew.
Chem., Int. Ed. Engl., 1994, 33, 497; A. Pfaltz, Acc. Chem. Res., 1993, 26,
339.
2 A. W. Johnson and W. C. Kaska, Ylides and Imines of Phosphorus,
Wiley, New York, 1993.
3 E. W. Abel and S. A. Mucklejohn, Inorg. Chim. Acta, 1979, 37, 107;
P. Imhoff, C. J. Elsevier and C. H. Stam, Inorg. Chim. Acta, 1990, 175,
209; R. W. Reed, B. Santarsiero and R. G. Cavell, Inorg. Chem., 1996,
35, 4292; R. Appel and P. Volz, Z. Anorg. Allg. Chem., 1975, 413, 45.
4 K.-W. Lee and L. A. Singer, J. Org. Chem., 1974, 39, 3780.
5 Recent review: V. K. Singh, A. DattaGupta and G. Sekar, Synthesis,
1997, 137.
4·CD2Cl2: C57H42Cl2D2N2P2: Mr = 889.8, yellow prism, crystal size
0.28 3 0.39 3 0.46 mm, a = 10.703(1), b = 18.868(2), c = 22.522(2) Å,
U = 4547.9(8) Å3, T = 100 K, orthorhombic, space group C2221 (no. 20),
Z
=
4, Dc
=
1.30 g cm23, m
=
0.26 mm21. Enraf-Nonius CAD4
0.71069 Å. 2568 measured
diffractometer, Mo-Ka X-radiation, l
=
reflections, 2051 observed [I > 2.0s(Fo2)]. The structure was solved by
direct methods (SHELXS-86) and refined by full-matrix least squares
(SHELXL-97) on F2 for all data with Chebyshev weights to R = 0.074
(obs.), wR = 0.231 (all data), S = 1.04, H atoms riding, solvent CD2Cl2
disordered over two positions, max. shift/error 0.001, residual rmax = 0.633
e Å23
.
2·Rh(cod)BF4: C50H52BF4N2P2Rh, Mr = 932.6, orange prism, crystal
size 0.35 3 0.52 3 0.59 mm, a = 13.732(2), b = 16.915(2), c = 19.222(3)
Received in Cambridge, UK, 16th December 1997; 7/09023D
936
Chem. Commun., 1998