Synthesis and reactivity of P-chiral tethered (η1: η6-phosphinoarene)ruthenium complexes

Article abstract of DOI:10.1021/om800076x

A new group of tethered phosphinoarene ruthenium complexes containing a stereogenic center at the phosphine atom is presented. The introduction of other elements of chirality, either as a plane or at the ruthenium center, is explored.

Full text of DOI:10.1021/om800076x

Copyright 2008
Volume 27, Number 9, May 12, 2008
American Chemical Society
Communications
Synthesis and Reactivity of P-Chiral Tethered
(η1:η6-phosphinoarene)ruthenium Complexes
Rosario Aznar,† Guillermo Muller,*,† Daniel Sainz,† Merce` Font-Bardia,‡ and
Xavier Solans‡
Departament de Qu´ımica Inorga`nica and Departament de Cristal · lografia, Mineralogia i Dipo`sits
Minerals, UniVersitat de Barcelona, Mart´ı i Franque`s s/n, E-08028 Barcelona, Spain
ReceiVed January 28, 2008
Scheme 1
Summary: A new group of tethered phosphinoarene ruthenium
complexes containing a stereogenic center at the phosphine
atom is presented. The introduction of other elements of
chirality, either as a plane or at the ruthenium center, is
explored.
The chemistry of Ru(II) complexes stabilized by phos-
phorus donor ligands has received considerable attention in
recent years, since the discovery of important catalytic
systems such as Noyori’s BINAP and Grubbs’ carbene
catalyst.1,2 A rigid chiral environment around the metal center
in transition-metal complexes is an interesting goal when
these compounds are used for the discrimination of prochiral
substrates in catalytic organic synthesis.3,4 With this purpose
a new family of chiral tethered ruthenium phosphinoarene
complexes have been prepared. Recent examples of chiral
ruthenium complexes of this type have been described, in
which the phosphinoarene chelates have a stereogenic center
located in the bridge,5 planar chirality,6 or a stereogenic
center in the phosphine substituents.7 Other η6:η1-arene
complexes containing nitrogen, oxygen, sulfur, or carbene
coordination arms are also known8 (Scheme 1).
The new complexes are built up on the chiral framework of
an enantiopure P-stereogenic phosphine. The starting materials
were obtained by the enantioselective deprotonation methodo-
(5) Pinto, P.; Marconi, G.; Heinemann, F. W.; Zenneck, U. Organo-
metallics 2004, 23, 374.
(6) (a) Faller, J. W.; D’Alliessi, D. G. Organometallics 2003, 22, 2749.
(b) Faller, J. W.; Fontaine, P. P. Organometallics 2005, 24, 4132. (c) Faller,
J. W.; Fontaine, P. P. Organometallics 2007, 26, 1738.
* To whom correspondence should be addressed. Fax: (+34)934907725.
E-mail: guillermo.muller@qi.ub.es.
† Departament de Qu´ımica Inorga`nica.
(7) Pinto, P.; Go¨tz, A. W.; Marconi, G.; Hess, B. A.; Marinetti, A.;
Heinemann, F. W.; Zenneck, U. Organometallics 2006, 25, 2607.
(8) (a) Morris, D. J.; Hayes, A. M.; Wills, M. J. Org. Chem. 2006, 71,
7035. (b) Geldbach, T. J.; Laurenczy, G.; Scopellini, R.; Dyson, P. J.
Organometallics 2006, 25, 733. (c) Miyaki, Y.; Onishi, T.; Kurosawa, H.
Inorg. Chim. Acta 2000, 300-302, 369. (d) Therrien, B.; Ward, T. R.
Angew. Chem., Int. Ed. 1999, 38, 405.
‡ Departament de Cristal · lografia, Mineralogia i Dipo`sits Minerals.
(1) Noyori, R. Angew. Chem., Int. Ed. 2002, 41, 2008.
(2) Grubbs, R. H. Angew. Chem., Int. Ed. 2006, 45, 3760.
(3) Trost, B. M.; Fredericksen, M. U.; Rudd, M. T. Angew. Chem., Int.
Ed. 2006, 45, 6630.
(4) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. ReV. 2002, 102, 1731.
10.1021/om800076x CCC: $40.75
2008 American Chemical Society
Publication on Web 04/04/2008

1968 Organometallics, Vol. 27, No. 9, 2008
Communications
Scheme 2. Preparation of the Phosphines a-f a
a
Legend: (a) s-BuLi/(-)-sparteine; (b) XCH2R.
logy proposed by Evans,9 using sparteine as chiral auxiliary.
By this method the first recrystallization of the borane-protected
phosphine gave the pure S enantiomer with yields between 35
and 68%10 (Scheme 2).
The ruthenium-tethered complexes were obtained in two
successive steps: initial coordination of pure deprotected phos-
phine over ruthenium arene halide dimers, followed by intramo-
lecular arene substitution (eqs 1 and 2).11
Figure 1. ORTEP drawing of the molecular structure of 3b, shown
at the 50% probability level. Some hydrogen atoms are omitted
for clarity. Selected bond distances (Å) and angles (deg): Ru-Cl(2),
2.3925(13); Ru-Cl(1), 2.4267(13); Ru-P, 2.3299(10); P-Ru-Cl(2),
88.93(4); P-Ru-Cl(1), 96.95(3); Cl(2)-Ru-Cl(1), 85.61(4); Ru-
C(6), 2.165(4); Ru-C(3), 2.244(3); P-C(8)-C(7)-C(6), 43.540.
Decomposition of 2f was observed when the same intramo-
lecular substitution was carried out under the same conditions,
even starting from the dinuclear ruthenium complex [RuCl(µ-
Cl)(C6H6)]2. In 3d the NOE contacts between the three arene
protons and the tert-butyl or methyl phosphine fragments
showed that the terminal phenyl ring of the naphthyl substituent
is distal to the tert-butyl group.
(1)
(2)
The proton NMR of 3c in d8-toluene showed the signals of the
five different methyl substituents of the arene group. Only the
signal of the 4-methyl group appears significantly coupled to
1
the phosphorus atom, as could be confirmed by the H{31P}
NMR spectrum. When the spectrum was obtained at 220 K,
the signal at 1.08 ppm of the tert-butyl group collapsed,
revealing hindered rotation about the P-CMe3 bond.12
The molecular structure of the tethered complex 3b was
determined by single-crystal X-ray analysis (Figure 1).13 The
molecular structure corresponds to a distorted pseudo-octahedral
geometry around the ruthenium center. Bond distances and
angles are similar to those previously reported for related
tethered complexes.14
The coplanarity of the benzylic carbon with the mean plane
defined by the arene atoms, observed when the bridging P-arene
When the phosphine contains nonsymmetric aryl groups as
in the case of 2d,e, a stereogenic plane is created in this process.
Complete diastereoselectivity was observed in the intramolecular
arene substitution for 3d, but mixtures of both diastereoisomers
were obtained in the arene ligand displacement reaction for 3e.
(11) Typical procedure for the synthesis of complexes 2: dichloro(η6-
p-cymene)((R)-tert-butylmethyl(2-phenylethyl)phosphine)ruthenium(II) (2a).
A 2.00 mL portion of tetrafluoroboric acid diethyl ether complex (2.38 g,
14.6 mmol) was slowly added to a stirred, cooled (0 °C) solution of 0.80 g
(3.6 mmol) of a in 18 mL of CH2Cl2. After 1 h, the ice bath was removed
and conversion was monitored by 31P{1H} NMR. To the reaction mixture
were added 30 mL of CH2Cl2 and 80 mL of degassed, aqueous saturated
NaHCO3 solution. The organic layer was separated, and the aqueous layer
was extracted with CH2Cl2 (3 × 20 mL). The volume was reduced to 20
mL, and 0.60 g of [Ru(µ-Cl)Cl(η6-p-cymene)]2 (0.99 mmol) was added.
After 30 min the conversion was monitored by 31P{1H} NMR and the rest
of the [Ru(µ-Cl)Cl(η6-p-cymene)]2 was added (0.275 g, 0.45 mmol). After
30 min 20 mL of water was added. The resulting mixture was extracted
with CH2Cl2 (3 × 10 mL) and then evaporated to dryness, affording a crude
oil. The pure orange solid (1.46 g, 98%) was obtained after washing
repeatedly with acetone and pentane and evaporating to dryness. 31P{1H}
NMR (101.2 MHz, C6H5Cl, 298 K; δ (ppm)): 28.1 (s). Typical procedure
for the synthesis of complexes 3: dichloro(κP-η6-(R)-tert-butylmethyl(2-
phenylethyl)phosphine)ruthenium(II) (3a). A 1.49 g portion of 2a (2.90
mmol) was disolved in chlorobenzene (200 mL). The resulting solution
was stirred at 120 °C for 4 h. The product was precipitated by reducing the
volume to ca. 100 mL and adding 20 mL of hexane. The precipitate was
filtered and washed with pentane. Yield: 0.72 g, 66%. 31P{1H} NMR (101.2
MHz, C6H5Cl, 298 K; δ (ppm)): 61.9 (s). 1H NMR, 13C NMR, IR, and
elemental analysis data are given in the Supporting Information.
(9) Muci, A. R.; Campos, K. R.; Evans, D. A. J. Am. Chem. Soc. 1995,
117, 9075.
(10) Typical procedure for the synthesis of phosphines a-f: (S)-tert-
butyl(2-phenylethyl)methylphosphine-borane (a). (-)-Sparteine (5.53 g,
23.6 mmol) was dissolved in 50 mL of diethyl ether, and the solution was
cooled to -78 °C with stirring. A sec-butyllithium solution (17.5 mL of
1.3 M solution in hexanes) was added by syringe. After 10 min, a solution
of tert-butyldimethylphosphine-borane (2.50 g, 18.9 mmol) in diethyl ether
(45 mL) was added by syringe and the solution was stirred at -78 °C.
Three hours later benzyl bromide (3.40 mL, 28.3 mmol) was added. The
reaction mixture was warmed to room temperature overnight and then was
quenched by the addition of 50 mL of water. The suspension was extracted
with diethyl ether (3 × 40 mL). The combined organic layers were dried
with sodium sulfate and filtered, and the solvent was removed in vacuo.
The resulting residue was purified by chromatography on silica gel
(hexane-ethyl acetate, increasing polarity), furnishing the product (2.80 g,
67%). Optically pure product was obtained by recrystallization from hexane.
31P{1H} NMR (101.2 MHz, C6H5Cl, 298 K; δ (ppm)): 25.0 (q, JPB ≈ 60.0
Hz). 1H NMR, 13C NMR, IR, and elemental analysis data are given in the
Supporting Information.

Communications
Organometallics, Vol. 27, No. 9, 2008 1969
consists of three sp3 carbon atoms, is not observed here.5,15 The
benzylic carbon is displaced 0.164 Å from that plane, the
phosphorus atom lies at 2.791 Å from the plane, and the chlorine
atoms are displaced by 3.201 and 3.176 Å. This shows the
distortion that the bidentate ligand generates in the nonregular
trigonal geometry of the RuCl2P fragment.
The molecular structure in the solid state is consistent with
the spectroscopic results in solution for 3b. Thus, the shorter
H-H distances between H(1) and H(5) and the hydrogen atoms
of the ethylene bridge measured in the solid state are H(7a)-H(5)
and H(8b)-H(1) at 2.368 and 2.500 Å, respectively. The
combined NOESY and HSQC 2D spectra of 3d in solution
permit the complete assignation of the proton NMR spectrum
and confirm that the contacts observed connecting arene and
ethylene bridge protons involve the same pairs of hydrogen
atoms, H(7a)-H(5) and H(8b)-H(1), which showed shorter
H-H distances in the solid state. Therefore, the [Ru{η1:η6-
PMe(t-Bu)(CH2CH2(3,5-Me2C6H3))}] group has a rigid confor-
mation.
the reaction. The major diastereoisomer of 4a,b precipitated
from the concentrated solution. The NOESY spectra showed
contacts between protons of the PPh3 ligand and t-Bu fragment
according to an RPRRu configuration of the cation complex. In
nonpolar solvents such as CH2Cl2 and CHCl3 the epimerization
of 4a was not observed after 20 days. However, after 7 days in
acetone the amounts of both diastereoisomers of 4b were similar.
The substitution reaction is kinetically controlled, since the more
crowded diastereoisomer was obtained in preference.16 Theoreti-
cal calculations are in progress to provide an explanation of
the substitution pattern.
The highest ion mass observed in the ESI mass spectra of
the ruthenium complexes usually corresponded to the [M - Cl]+
and [M + CH3CN - Cl]+ fragments when acetonitrile was used
as solvent.
To evaluate the discrimination power of the P-stereogenic
center in the two remaining coordination positions around the
Ru atom, the substitution of one chloro ligand by PPh3 was
studied with 3a,b (eq 3). The stoichiometric reaction is very
slow in nonpolar or polar solvents such as CH2Cl2 and acetone:
30 h was needed to complete reaction of 3a,b under irradiation
with visible light. In the dark the substitution was not observed.
Both diasteroisomers were obtained in a constant 3/1 ratio during
(3)
The 1H-19F correlation of a solution of 4a-RPRRu in CDCl3
(HOESY experiment) showed contacts of fluorine with the
protons of both the PPh3 and the tert-butyl groups. This is
-
consistent with a position of the PF6 anion on the opposite
side of the remaining chloro ligand in the ionic pair.
In conclusion, we have obtained a group of P-chiral ligands
that allows the synthesis of new tethered phosphinoarene
ruthenium complexes. Furthermore, it is possible to introduce
elements of planar chirality with complete diastereoselectivity.
Work currently in progress is focusing on the preparation of
ruthenium complexes containing labile ligands in the coordina-
tion sphere and their use as precursors in catalytic processes.
(12) Ma, L.; Woloszynek, R. A.; Chen, W.; Ren, T.; Protasiewicz, J. D.
Organometallics 2006, 25, 3301.
(13) Structure determination of 3b: molecular weight, 408.29 g/mol;
crystal size, 0.2 × 0.1 × 0.1 mm; crystal system, orthorhombic; space group,
P212121; a ) 7.467(3) Å, b ) 13.096(4) Å, c ) 17.923(4) Å, R ) ꢀ ) γ
) 90.00°; V ) 1752.7(10) Å3; Z ) 4; Fcalcd ) 0.946 g cm3, µlin ) 1.277
mm-1; λ ) 0.710 73 Å; T ) 293 K; 17 326 reflections collected (R(int) )
0.0390); final R indices (I > 2σ(I)) R1 ) 0.0316, wR2 ) 0.0864, GOF )
1.178. CCDC 652572 contains the supplementary crystallographic data for
this structure. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
(14) (a) Umezawa-Vizzini, K.; Guzman-Jimenez, I. Y.; Whitmire, K. H.;
Lee, T. R. Organometallics 2003, 22, 3059. (b) Ghebreyessus, K. Y.; Nelson,
J. H. Inorg. Chim. Acta 2003, 350, 12. (c) Therrien, B.; Ward, T. R.;
Pilkington, M.; Hoffmann, C.; Gilardoni, F.; Weber, J. Organometallics
1998, 17, 330. (d) Abele, A.; Wursche, R.; Klinga, M.; Rieger, B. J. Mol.
Catal. A: Chem. 2000, 160, 23.
Acknowledgment. This study was supported by the Spanish
Ministerio de Educacio´n y Ciencia (MEC, project CTQ2007-
61058/BQU). R.A. thanks the DURSI of the Autonomous
Government of Catalonia (Generalitat de Catalunya) for the
award of a Ph.D. grant.
Supporting Information Available: Text and figures giving
experimental information, including details of the syntheses and
NMR spectra, and a CIF file giving X-ray structural data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(15) (a) Ghebreyessus, K. Y.; Nelson, J. H. Organometallics 2000, 19,
3387. (b) Furstner, A.; Liebl, M.; Lehmann, C. W.; Picquet, M.; Kunz, R.;
Bruneau, C.; Touchard, D.; Dixneuf, P. H. Chem. Eur. J. 2000, 6, 1847.
(c) Bennett, M. A.; Edwards, A. J.; Harper, J. R.; Khimyak, T.; Willis,
A. C. J. Organomet. Chem. 2001, 629, 7. (d) Smith, P.-D.; Wright, A. H.
J. Organomet. Chem. 1998, 559, 141.
OM800076X
(16) Recent mechanistic studies: Chaplin, A. B.; Fellay, C.; Laurenczy,
G.; Dyson, P. J. Organometallics 2007, 26, 586.