Unexpected and then rational synthesis of a new class of anionic phosphino-oxazoline ligands; structure and catalytic properties of a ruthenium complex

Article abstract of DOI:10.1039/b002515l

A new class of anionic, hybrid chelating ligands is reported which contain a phosphine moiety covalently linked to an oxazoline of which the nitrogen atom formally carries a partly delocalized negative charge; the first metal complex, a ruthenium derivative, has been characterized by X-ray diffraction and shown to be active in catalytic transfer hydrogenation.

Full text of DOI:10.1039/b002515l

Unexpected and then rational synthesis of a new class of anionic
phosphino–oxazoline ligands; structure and catalytic properties of a ruthenium
complex†
Pierre Braunstein,*a Frédéric Naud,a Claudia Graiffb and Antonio Tiripicchiob
a Laboratoire de Chimie de Coordination (UMR 7513 CNRS), Université Louis Pasteur, 4, rue Blaise Pascal,
F-67070 Strasbourg Cedex, France. E-mail: braunst@chimie.u-strasbg.fr
b Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Università di Parma, Centro
di Studio per la Strutturistica Diffrattometrica del CNR, Parco Area delle Scineze 17A, I-43100 Parma, Italy
Received (in Basel, Switzerland) 29th March 2000, Accepted 11th April 2000
Published on the Web 9th May 2000
A new class of anionic, hybrid chelating ligands is reported
which contain a phosphine moiety covalently linked to an
oxazoline of which the nitrogen atom formally carries a
partly delocalized negative charge; the first metal complex, a
ruthenium derivative, has been characterized by X-ray
diffraction and shown to be active in catalytic transfer
hydrogenation.
2.102(3) Å in (A) and 2.111(3) Å in (B), involves the N(1) atom
bearing the partial negative charge. Whereas the double bond is
mainly localized on the C–N bond in the PCH2oxMe2 ligand
[bond distance 1.281(4) Å in both A and B], electronic
delocalization occurs within the N(1)C(14)C(13)P(1) moiety of
PCHoxMe2, as indicated by the values of the C–C [1.370(5) Å in
(A) and 1.371(5) Å in (B)], C–N [1.328(4) Å in (A) and
1.317(4) Å in (B)] and P–C [1.753(3) Å in (A) and 1.754(4) Å
in (B)] bond distances. Consistently, the IR spectrum of 2
contains two bands at 1618 and 1535 cm21 assigned to the CNN
and CÌN vibrations of PCH2oxMe2 and PCHoxMe2, re-
spectively. In the 1H NMR spectrum, each proton in PCH2oxMe2
and PCHoxMe2 appears as a single resonance in agreement with
the lack of any symmetry element in the molecule.
The selective synthesis of 2 was unexpected and found to
depend on the nature of the cation associated with the fluoride
anion. Thus, reaction of 1 with LiF only afforded a low yield of
2. A more rational and general access to the anionic chelate in
2 consisted in the reaction of 1 with ButOK in THF and selective
deprotonation of only one PCH2 group was observed [eqn.
(1)].
The numerous synthetic pathways to oxazolines make them
versatile synthons for ligand design1 and their availability in
enantiomeric pure forms accounts for the increasing use over
the last 10 years of oxazoline-based ligands for asymmetric
reactions catalyzed by transition metal complexes.2 We have
recently investigated the synthesis of Pd and Ru complexes with
the neutral P,N-type ligand (oxazolinylmethyl)diphenylphos-
phine I (abreviated PCH2oxMe2) and studied their catalytic
properties for ethylene/CO copolymerization and asymmetric
transfer hydrogenation of ketones, respectively.3
We then felt it would be of interest to isolate Ru hydrido
complexes of potential catalytic relevance but reactions of
[Ru(H)Cl(PPh3)3] or [Ru(H)(OAc)(PPh3)3] with I were not
1
selective and although hydrido species were observed by H
NMR spectroscopy, no pure complexes could be isolated. We
then reacted [Ru(OAc)(PCH2oxMe2)2]Cl 1,† prepared from
[RuCl2(PCH2oxMe2)2]3b and Tl(OAc), with CsF in acetone in
order to prepare a monofluoride derivative that could be
subsequently cleanly transformed into a hydride complex.4 The
31P{1H} NMR spectrum of the only product formed, 2,
2
exhibited an AB spin system at d 46.8 and 58.3 with J(PP)
35.8 Hz which, surprinsingly, showed no coupling with a 19F
nucleus. The reaction of [RuCl2(PCH2oxMe2)2] itself with CsF
was found to be less selective and not further investigated. An
X-ray diffraction study‡ established the ligand arrangement in 2
and the fact that one of the P,N-ligands had unexpectedly been
deprotonated. The crystals contain two crystallographically
independent, but almost identical, molecules (A and B). A view
of the structure of one of them (A) is shown in Fig. 1. Both
ligands (excepting for the substituents) are roughly planar, but
in the deprotonated one the N(1)C(14)C(13)P(1) moiety is
practically planar. The two Ru–N bond distances in the
octahedral complexes differ remarkably and the shorter,
Fig. 1 View of the molecular structure of one (A) of the two crystallographic
independent (A and B) molecules [Ru(OAc)(PCHoxMe2)(PCH2oxMe2)] 3
together with the atomic numbering system. Selected bond distances (Å)
and angles (°) in molecule A (values in square brackets refer to molecule B):
Ru–P(1) 2.278(2) [2.270(1)], Ru–P(2) 2.226(1) [2.231(1)], Ru–N(1)
2.102(3) [2.111(3)], Ru–N(2) 2.166(3) [2.150(3)], Ru–O(3) 2.236(2)
[2.228(2)], Ru–O(4) 2.209(2) [2.222(2)], P(1)–C(13) 1.753(3) [1.754(4)],
C(13)–C(14) 1.370(5) [1.371(5)], C(14)–N(1) 1.328(4) [1.317(4)], C(16)–
N(1) 1.495(4) [1.479(4)], P(2)–C(31) 1.857(4) [1.851(4)], C(31)–C(32)
1.474(5) [1.481(5)], C(32)–N(2) 1.281(4) [1.281(4)], C(34)–N(2) 1.515(4)
[1.499(4)]; Ru–N(1)–C(14) 116.8(2) [116.5(2)], N(1)–C(14)–C(13)
126.3(3) [126.4(3)], C(14)–C(13)–P(1) 111.9(3) [111.7(3)], Ru–N(2)–
C(32) 119.8(2) [119.6(2)], N(2)–C(32)–C(31) 124.8(3) [124.7(3)], C(32)–
C(31)–P(2) 105.9(2) [106.7(3)].
† Electronic supplementary information (ESI) available: synthesis and
spectral characterisation of 1–3, See http://www.rsc.org/suppdata/cc/b0/
b002515l/
DOI: 10.1039/b002515l
Chem. Commun., 2000, 897–898
This journal is © The Royal Society of Chemistry 2000
897

(1)
= 0.569 mm21, a = 18.256(4), b = 20.590(5), c = 20.454(5) Å, b =
111.28(6)°, V = 7164(3) Å3, Z = 8, Dc = 1.398 g cm23. Philips PW 1100
diffractometer, q–2q scan technique, room temperature. 20871 Unique
reflections measured (3 < q < 30°) and used in the refinement. All hydrogen
atoms placed at their geometrically calculated positions and refined riding
on their parent atoms, excepting those bonded to C(31) and C(13), which
were localized in the final DF map and refined isotropically, wR2 0.0876
and R1 [for 8473 reflections with I > 2s(I)] 0.0369. All calculations were
carried out on the DIGITAL Alpha Station 255 computers of the “Centro di
Studio per la Strutturistica Diffrattometrica “ del CNR, Parma, using the
SHELX-97 systems of crystallographic computer programs.18 CCDC
182/1603. See http://wwww.rsc.org/suppdata/cc/b0/b002515l/ for crys-
tallographic files in .cif format.
Anionic ligands incorporating an oxazoline ring are few
compared to their neutral analogs and they belong to two main
families. In class A, the anionic charge is carried by an
exocyclic atom (C2,5 O2,6 S2 7) and the nitrogen atom of the
oxazoline formally remains a neutral donor.
§ Catalytic transfer hydrogenation: Typical procedure for the catalytic
transfer hydrogenation of acetophenone: 2 (0.0052 g, 0.0068 mmol) was
dissolved in 13 mL PriOH in a 50 mL two-neck round bottom flask fitted
with a reflux condenser. Acetophenone (0.158 mL, 1.36 mmol) was added
and the yellow solution was brought to reflux. The solution was stirred for
10 min and 0.36 mL (0.034 mmol) of a solution of PriONa in PriOH (0.1 M)
was added. The volume of PriOH was adjusted so that all catalytic runs were
performed with an initial concentration in acetophenone of 0.1 M. The
addition of PriONa was considered as the starting time of the reaction.
Conversion was determined by gas chromatography using a Lipodex A 25
m 3 0.25 mm column.
Some of their complexes are catalysts for, e.g. ethylene
polymerization,6a sulfide oxidation,8 alkyl- and phenyl-zinca-
tion of aldehydes6b,c and conjugate addition to enones.7 In the
second class of ligands, B, the anionic charge is partly localized
on the nitrogen atom of the oxazoline but only one representa-
tive is known which has led to efficient Ti,9 Cu,10 Zn11 or Mg12
catalysts for several asymmetric reactions, e.g. reduction of
ketones,9b cyclopropanation of olefins,10 allylzincation,11 and
hydrocyanation of aldehydes.12 Although there are two reported
X-ray crystal structures of Cu13 and Rh complexes,14 the
precatalysts were usually prepared in situ and not isolated (Zn,
Mg, Ti).
1 T. G. Gant and A. I. Meyers, Tetrahedron, 1994, 50, 2297.
2 C. Bolm, Angew. Chem., Int. Ed. Engl., 1991, 30, 542; A. K. Ghosh, P.
Mathivanan and J. Cappiello, Tetrahedron: Asymmetry, 1998, 9, 1; A.
Pfaltz, Synlett, 1999, 51, 835.
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F. Speiser, J. Chem. Soc., Dalton Trans., 2000, 1067; (b) P. Braunstein,
C. Graiff, F. Naud, A. Pfaltz and A. Tiripicchio, manuscript in
preparation.
4 D. Huang and K. G. Caulton, J. Am. Chem. Soc., 1997, 119, 3185.
5 S. E. Denmark, R. A. Stavenger, A.-M. Faucher and J. P. Edwards,
J. Org. Chem., 1997, 62, 3375; Y. Motoyama, Y. Mikami, H.
Kawakami, K. Aoki and H. Nishiyama, Organometallics, 1999, 18,
3584.
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Organometallics, 1995, 14, 4994; (b) C. Bolm, K. Muñiz-Fernández, A.
Seger, G. Raabe and K. Günther, J. Org. Chem., 1998, 63, 7860; (c) C.
Bolm and K. Muñiz, Chem. Commun., 1999, 1295; (d) C. J. Fahrni and
A. Pfaltz, Helv. Chim. Acta, 1998, 81, 491.
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8 C. Bolm, T. K. K. Luong and K. Harms, Chem. Ber., 1997, 130, 887.
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M. Bandini, P. G. Cozzi, L. Negro and A. Umani-Ronchi, Chem.
Commun., 1999, 39.
10 R. E. Lowenthal, A. Abiko and S. Masamune, Tetrahedron Lett., 1990,
31, 6005; D. A. Evans, K. A. Woerpel, M. M. Hinman and M. M. Faul,
J. Am. Chem. Soc., 1991, 113, 726.
11 M. Nakamura, A. Hirai and E. Nakamura, J. Am. Chem. Soc., 1996, 118,
8489.
12 E. J. Corey and Z. Wang, Tetrahedron Lett., 1993, 34, 4001.
13 J. Hall, J.-M. Lehn, A. DeCian and J. Fischer, Helv. Chim. Acta, 1991,
74, 1.
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Karalulov, Tetrahedron: Asymmetry, 1994, 5, 561.
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Rabu, A. Tiripicchio and F. Ugozzoli, Inorg. Chem., 1996, 35, 5986.
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The new ligand system in II provides an interesting extension
to anionic four-electron donor phosphino enolate ligands of
type III currently much investigated. These and related
phosphorus/oxygen chelates confer special reactivity to their
complexes, as found, e.g. in reactions with various organic or
inorganic electrophiles15 or in the highly selective convertion of
ethylene into linear a-olefins.16 The anionic ligand PCHoxMe2
found in 2 is therefore the first representative of a new class of
hybrid ligands which combine some of the aspects of systems B
and III and its chemistry therefore offers considerable potential.
Preliminary catalytic studies with complex 2 have been
performed for the transfer hydrogenation of aryl alkyl and
dialkyl ketones by propan-2-ol, a reaction of current interest.17
Under standard conditions ([Ru]/[ketone]/[PriONa]
=
1+200+5, [ketone] = 0.1 M, T = 82 °C), 2 exhibits very high
to high activity for hydrogenation of acetophenone (98% yield
in 5 min) and cyclohexylmethyl ketone (91% in 5 h)
respectively.§
Access to complexes containing this new anionic P,N-ligand
can be generalized, as shown with Pd since reaction of
[Pd(dmba)Cl(PCH2ox)] with ButOK in THF afforded
[Pd(dmba)(PCHox)] 3 in 84% yield.
Notes and references
‡ Crystal data for 2: C38H42N2O4P2Ru, M = 753.75 monoclinic, space
group P21/c, graphite monochromated Mo-Ka radiation, l = 0.71073 Å, m
898
Chem. Commun., 2000, 897–898