The first tridentate phosphine ligand combining planar, phosphorus and carbon chirality

Article abstract of DOI:10.1039/b208384a

Two new diastereomerically pure tridentate phosphine ligands combining planar, phosphorus and carbon chirality have been conveniently synthesized via resolution of their phosphineoxide-diborane adducts and structurally characterized by X-ray analyses.

Full text of DOI:10.1039/b208384a

The first tridentate phosphine ligand combining planar, phosphorus and
carbon chirality†
Pierluigi Barbaro,*a Claudio Bianchini,a Giuliano Giambastiani*a and Antonio Tognib
a Istituto di Chimica dei Composti Organo Metallici - CNR, Via Jacopo Nardi 39, 50132 Firenze, Italy.
E-mail: pigi@fi.cnr.it; Fax: +39 055 2478366
b Department of Chemistry, Swiss Federal Institute of Technology, ETH Hönggerberg, CH-8093 Zürich,
Switzerland
Received (in Cambridge, UK) 29th August 2002, Accepted 1st October 2002
First published as an Advance Article on the web 16th October 2002
Two new diastereomerically pure tridentate phosphine
ligands combining planar, phosphorus and carbon chirality
have been conveniently synthesized via resolution of their
phosphineoxideUdiborane adducts and structurally charac-
terized by X-ray analyses.
new tridentate phosphine ligands P3Chir (Fig. 1) in which the
planar chirality of the ferrocenyl moiety is combined with
phosphorus and carbon stereocenters in an unprecedented
molecular assembly.‡ The reaction of (R)-(S)-PPFA with
racemic C6H5P(H)CH2CH2P(O)(C6H5)2 in acetic acid gave a
1+1 mixture of diastereomers 1 having opposite absolute
configuration at phosphorus (Fig. 1, step c).7 Treatment of this
mixture with borane dimethyl sulfide, followed by column flash
chromatography afforded the pure phosphine oxideUdiborane
adducts (R)C-(S)Fe-(S)P-2 and (R)C-(S)Fe-(R)P-2 in good yields,
respectively. All products could be easily identified by 31P{1H}
Tridentate phosphines are recognized as one of the most
important classes of auxiliaries in homogeneous catalysis by
transition metal complexes.1 Triphosphine ligands are able to
stabilize metal ions in a variety of oxidation states and
coordination geometries that can provide strong steric control
and, ultimately, fostering reactions that would otherwise be
inaccessible. In comparison with stereohomogeneous chelating
diphosphines,2 however, chiral triphosphines have been
scarcely investigated for the inherent difficulty in their
preparation, yet their potential in asymmetric catalysis remains
very high.3
In recent papers, we reported the synthesis of a series of bis-
ferrocenyl triphosphine ligands (Scheme 1) and described their
use in Ru(II), Ni(II) and Rh(III)-catalyzed enantioselective
reactions.4 This project was motivated by perspective of
combining optically pure 1,2-disubstituted ferrocenyl units, that
are excellent carriers of chiral information in asymmetric
catalysis,5 with a backbone containing three phosphorus atoms.
A most important feature of ferrocenyl ligands is their synthetic
versatility, allowing a great variety of derivatives to be
assembled into a single common fragment.6 Herein we report
the preparation, characterization and crystal structure of the two
1
and H NMR spectroscopy (Table 1 and ESI†). The absolute
configurations were determined by single-crystal X-ray diffrac-
tion analysis (Fig. 2).§ One-pot reduction (CeCl3, NaBH4,
LiAlH4)8/deboronation reactions of the two stereoisomers gave
pure 3 and 4, respectively (Fig. 1, step f and g).9
The X-ray crystal structure of 3 proved the reduction step to
proceed with retention of configuration at all stereocenters (Fig.
3).§ Most importantly, the synthetic procedure illustrated in
† Electronic supplementary information (ESI) available: experimental
section. See http://www.rsc.org/suppdata/cc/b2/b208384a/
Scheme 1
Fig. 1 Reaction scheme for the synthesis of (R)C-(S)Fe-(S)P-P3Chir (3) and (R)C-(S)Fe-(R)P-P3Chir (4). Reagents and conditions: a) H2O2, CHCl3; b)
tBuO2K+, THF, reflux; c) AcOH, 75 °C; d) BH3·(CH3)2S, THF; e) flash chromatography; f) CeCl3, NaBH4, LiAlH4, THF; g) morpholine, 70 °C.
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This journal is © The Royal Society of Chemistry 2002

Table 1 31P{1H} NMR data for the ligands
quite rigid ligand framework. Preliminary results confirm the
excellent ligand properties of the P3Chir molecules. The Ni(II
)
4JP1P2
Hz
/
3JP2P3
/
Hz
complexes [((R)C-(S)Fe-(S)P-P3Chir)Ni(CH3CN)]BF4 (5) and
[((R)C-(S)Fe-(R)P-P3Chir)Ni(CH3CN)]BF4 (6) were readily iso-
lated in the solid state and characterized in solution by NMR
spectroscopy. Sharp resonances were observed in both 31P{1H}
Entry
d P(1)
d P(2)
d P(3)
1
1
2
2
3
4
epi-A
epi-B
224.50 (d) 20.16 (dd) 33.42 (d) 18.3 50.2
225.57 (d) 21.24 (dd) 33.37 (d) 37.7 52.0
1
and H NMR spectra. The 31P{1H} NMR parameters indicate
(R)C-(S)Fe-(S)P 11.7 (br s) 32.0 (br s) 33.2 (d)
(R)C-(S)Fe-(R)P 11.8 (br s) 32.8 (br s) 32.8 (br s)
(R)C-(S)Fe-(S)P 225.89 (d) 22.02 (t) 212.07 (d) 36.0 32.7
(R)C-(S)Fe-(R)P 225.70 (d) 20.27 (dd) 213.32 (d) 12.9 35.3
43.8
the complexes to adopt a square-planar arrangement with trans
P(1) and P(3) phosphorus atoms.¶ The use of P3Chir complexes
with Ni(II), Rh(
I
), Ru(II) and Ir( ) metal ions in enantioselective
I
reactions is currently under way in our laboratories.
In conclusion, we have shown that triphosphine ligands
combining ferrocenyl, carbon and phosphorus chirality can be
obtained in very good yield by resolution of diborane protected-
phosphineoxide precursors, followed by stereoconservative
reduction.
In CDCl3, 161.98 MHz, 294 K. Chemical shifts in ppm, J in Hz.
Thanks are due to D. Masi for technical assistance and to
COST D24 chemistry action for support.
Notes and references
‡ (R)C-(S)Fe-(S)P-P3Chir = (R)-1-[(S)-2-diphenylphosphino)ferrocenyl]e-
thyl-(S)-[phenylphosphin-2-(diphenylphosphino)ethane].
§
Crystal data for (R)C-(S)Fe-(R)P-2: single crystals of (R)C-(S)Fe-(R)P-2
were obtained by recrystallization from CH2Cl2UCH3OH. C44H47FeB2OP3,
M = 762.24, orthorhombic, space group P212121 (no. 19), a = 11.436(15),
b = 31.504(5), c = 11.270(3) Å, V = 4060(5) Å3, T = 293(2) K, Z = 4,
m(Mo-Ka) = 0.522 mm21, 3202 measured reflections, 3202 independent
reflections, R indices (all data) R = 0.1055 and wR = 0.1368, Flack
parameter = 20.04(5).
Crystal data for 3: single crystals of 3 were obtained by recrystallization
from CH3OH. C44H41FeP3, M
= 718.53, orthorhombic, space group
P212121 (no. 19), a = 12.117(2), b = 15.381(9), c = 19.922(5) Å, V =
3713(2) Å3, T = 293(2) K, Z = 4, m(Mo-Ka) = 0.566 mm21, 2729
measured reflections, 2729 independent reflections, R indices (all data) R =
0.1179 and wR = 0.1423, Flack parameter = 0.01(5).
CCDC 184405 and 184406. See http://www.rsc.org/suppdata/cc/b2/
b208384a/ for crystallographic data in CIF or other electronic format.
Fig. 2 ORTEP drawing of (R)C-(S)Fe-(R)P-2. Selected bond lengths (Å) and
angles (°): P(1)UB(1) 1.923(12), P(2)UB(2) 1.910(11), P(3)UO(1) 1.470(6),
P(2)UC(11) 1.848(9); C(2)UC(11)UP(2)UC(13) 40.36(2), C(3)UC(2)U
C(11)UP(2) 45.79(2).
¶
31P{1H} NMR data (CD3CN, 161.98 MHz, 294 K). 5: d 6.68 (dd, P(1)),
84.20 (dd, P(2)), 65.68 (dd, P(3)), 2JP1P2 80.2, 2JP1P3 212.5, 2JP2P3 46.7 Hz.
6: d 12.50 (dd, P(1)), 74.12 (dd, P(2)), 62.40 (dd, P(3)), 2JP1P2 82.9, 2JP1P3
213.0, 2JP2P3 44.0 Hz.
1 F. A. Cotton and B. Hong, Prog. Inorg. Chem., 1992, 40, 179.
2 (a) H. Brunner and W. Zettlmeier, Handbook of Enantioselective
Catalysis with Transition Metal Compounds, VCH, Weinheim, 1993,
vol. 1 and 2; (b) R. Noyori, Asymmetric Catalysis in Organic Synthesis,
John Wiley & Sons, New York, 1994.
3 (a) M. J. Burk and R. L. Harlow, Angew. Chem., Int. Ed. Engl., 1990, 29,
1462; (b) M. J. Burk, J. E. Feaster and R. L. Harlow, Tetrahedron:
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Acta, 1991, 74, 983; (d) G. Jia, H. M. Lee and I. D. Williams,
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(g) C. R. Johnson and T. Imamoto, J. Org. Chem., 1987, 52, 2170; (h) H.
M. Lee, C. Bianchini, G. Jia and P. Barbaro, Organometallics, 1999, 18,
1961.
Fig. 3 ORTEP drawing of 3. Selected bond lengths (Å) and angles (°):
P(2)UC(11) 1.873(10); C(2)UC(11)UP(2)UC(13) 263.82(2), C(3)UC(2)U
C(11)UP(2) 101.87(2).
4 (a) P. Barbaro and A. Togni, Organometallics, 1995, 14, 3570; (b) P.
Barbaro, C. Bianchini and A. Togni, Organometallics, 1997, 16, 3004;
(c) P. Barbaro, C. Bianchini, W. Oberhauser and A. Togni, J. Mol. Catal.
A, Chemical, 1999, 145, 139.
Fig. 1 enables the large-scale preparation of the P3Chir ligands
in excellent overall yields using cheap and commercially
available reagents through simple manipulations. It is antici-
pated that as many as four stereoisomers can be prepared
starting from either (R)-(S)-PPFA (as described here) or (S)-(R)-
PPFA hence allowing a fine tuning of stereochemical control. It
is also worth noting that the P3Chir ligands can form metal
complexes displaying both five- and six-membered chelate
rings. Chelation to a metal is espected to significantly reduce the
number of available competing conformations, resulting in a
5 For a review, see: T. Hayashi, in Ferrocenes: Homogeneous Catalysis,
Organic Synthesis, Materials Science, ed. A. Togni and T. Hayashi,
VCH, Weinheim, 1995, pp. 105U142.
6 A. Togni, Chimia, 1996, 50, 86.
7 No kinetic resolution of the racemic secondary phosphine was observed.
See also: A. Togni, C. Breutel, M. C. Soares, N. Zanetti, T. Gerfin, V.
Gramlich, F. Spindler and G. Rihs, Inorg. Chim. Acta, 1994, 222, 213.
8 T. Imamoto, T. Kusumoto, N. Suzuki and K. Sato, J. Am. Chem. Soc.,
1985, 107, 5301.
9 Reductions using silane derivatives were unsucessful due to noticeable
epimerization at the P(2) center.
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