MeO-Biphep as an eight-electron donor to ruthenium(II). Chiral dialkyl complexes with an unprecedented binding mode

Article abstract of DOI:10.1021/om970323c

The chiral bis(3,5-di-tert-butylphenyl)phosphino MeO-Biphep complex Ru(OAc)₂(1a), with two complexed P donors, reacts with MeLi to give Ru(Me)₂(1a), in which only one P atom is coordinated together with an arene η⁶-C₆

Full text of DOI:10.1021/om970323c

Organometallics 1997, 16, 3735-3736
3735
MeO-Bip h ep a s a n Eigh t-Electr on Don or to
Ru th en iu m (II). Ch ir a l Dia lk yl Com p lexes w ith a n
Un p r eced en ted Bin d in g Mod e
Nantko Feiken, Paul S. Pregosin,* and Gerald Trabesinger
Inorganic Chemistry ETH Zu¨rich, Universita¨tstrasse 6, CH-8092 Zu¨rich, Switzerland
Received April 18, 1997X
Summary: The chiral bis(3,5-di-tert-butylphenyl)phos-
phino MeO-Biphep complex Ru(OAc)2(1a ), with two
complexed P donors, reacts with MeLi to give Ru(Me)2-
(1a ), in which only one P atom is coordinated together
with an arene η6-C6H3 moiety derived from one of the
biaryl rings. This is an unprecedented binding mode
for this type of chiral complex.
Chiral bidentate phosphines are amongst the most
important auxiliaries in enantioselective homogeneous
catalysis.1 Within this group, the atropisomeric com-
pounds (e.g., the Binap2 and Biphep3 classes) have been
widely employed with impressive success, especially in
the hydrogenation chemistry associated with Ru(II).2-4
We have recently shown5 that the chiral MeO-Biphep
(ligands 1) afford the novel pentadienyl cationic com-
Reaction of Ru(OAc)2(1a ) with 3 equiv of methyl-
lithium affords the dimethyl complex RuMe2(1a ) (3a )
in good yield.6 The 31P NMR spectrum of 3a reveals
Ru(OAc)2(1a ) + 3MeLi sssf 3a
toluene/ether
two widely separated resonances, δ 65.8 and -17.5 ppm
(!), with the latter appearing at even lower frequency
than the uncomplexed ligand, (δ -12.7 ppm). The
observed J (P,P) value of ca. 4 Hz (see Figure 1) is much
too small for two nonequivalent 31P spins to both be
coordinated to Ru. In addition to the two nonequivalent
13C methyl resonances, (δ -5.3 and -8.7 ppm), the H
1
and 13C spectra show three aryl resonances shifted to
unexpectedly low frequency (see the C,H correlation in
Figure 2). These three CH biaryl 13C signals appear
between 70 and 100 ppm, in agreement with the
literature for π-arene complexes.7 Together with the
mass spectral and microanalytical data, these NMR
data8 indicate that the complex has the structure
plexes Ru(η5-C8H11)(1)+ (2), in which these bidentate
ligands function as six-electron donors, with the two
additional electrons arising from the coordination of a
biaryl double bond. This is an unexpected coordination
mode and allows this ligand class additional electronic
flexibility.
We show here that the same ligand can distort even
further, by dissociating one of the two P atoms and
complexing one of the ligand biaryl moieties; i.e., a 1,2,6-
trisubstituted η6-arene compound is formed. Ligand 1a
now functions as an eight-electron donor, in which only
a single tertiary phosphine atom is coordinated to
ruthenium.
X Abstract published in Advance ACS Abstracts, August 1, 1997.
(1) Genet, J . P. Acros Org. Acta 1995, 1. Brown, J . M. Chem. Soc.
Rev. 1993, 25(1), 4. Kitamura, M.; Noyori, R. In Modern Synthetic
Methods; Scheffold, R., Ed.; Springer-Verlag: Berlin, 1989; Vol. 5, pp
116-199.
(2) Ohta, T.; Tonomura, Y.; Nozaki, K.; Takaya, H.; Mashima,
K.Organometallics 1996, 15, 1521. Zhang, X.; Uemura, T.; Matsumura,
K.; Sayo, N.; Kumobayashi, H.; Takaya, H. Synlett. 1994, 501.
Kitamura, M.; Tokunaga, M.; Noyori, R. J . Am. Chem. Soc. 1993, 115,
144. Ohta, T.; Miyake, N.; Seido, H.; Kumobayashi, H.; Akutagawa,
S.; Takaya, H.Tetrahedron Lett. 1992, 33, 635. Mashima, K.; Hino, T.;
Takaya, H. J . Chem. Soc., Dalton Trans. 1992, 2099. Ohta, T.; Takaya,
H.; Noyori, R. Inorg. Chem. 1988, 27, 566.
3a : R1 ) CH3, R ) 3,5-di-tert-butylphenyl
3b: R1 ) CH2Si(CH3)3, R ) 3,5-di-tert-butylphenyl
There are examples of phosphine-arene donors in the
literature;9 however, these are of a more conventional
type9
(3) Schmid, R.; Broger, E. A.; Cereghetti, M.; Crameri, Y.; Foricher,
J .; Lalonde, M.; Mueller, R. K.; Scalone, M.; Schoettel, G.; Zutter; U.
Pure Appl. Chem. 1996, 68, 131. Heiser, B.; Broger, E. A.; Crameri, Y.
Tetrahedron: Asymmetry 1991, 2, 51.
(4) Mezzetti, A.; Tschumper, A.; Consiglio, G. J . Chem. Soc., Dalton
Trans. 1995, 49. Mezzetti, A.; Costella, L.; Del Zotto, A.; Rigo, P.;
Consiglio, G. Gazz. Chim. Ital. 1993, 123, 155.
(5) Feiken, N.; Pregosin, P. S.; Trabesinger, G. Organometallics
1997, 16, 537.
in which arene complexation is preplanned via the use
S0276-7333(97)00323-3 CCC: $14.00 © 1997 American Chemical Society

3736 Organometallics, Vol. 16, No. 17, 1997
Communications
chelate. Compounds 3a ,b are also the first reported
stable Ru(II) dialkyl complexes containing a chiral
auxiliary.
The methyl complex 3a reacts smoothly, in high yield,
with 1, or even 2 equiv of HBF4 in the presence of excess
benzene to yield the somewhat more conventional 18e
monomethyl η6-C6H6 derivative 4, in which both phos-
phorus donors are again coordinated.11 Obviously, if
given a choice, the metal favors the classical mode of
arene coordination, which is presumably less strained.
F igu r e 1. 31P NMR spectrum for 3a . Note (a) the low-
frequency position of the uncomplexed P donor and (b) the
very small J (P,P) value (162 MHz, CD2Cl2).
4: R ) 3,5-di-tert-butylphenyl
It is interesting that 1a can readily take up so many
novel coordination modes, and studies relating these
new complexes to catalytic reactions are in progress.
Ack n ow led gm en t. P.S.P. thanks the Swiss Na-
tional Science Foundation, the ETH Zurich, and F.
Hoffmann-La Roche AG for financial support. We also
thank F. Hoffmann-La Roche AG for a gift of Biphep
ligands and the complex Ru(OAc)2(1a ), as well as
J ohnson Matthey for the loan of precious metals.
OM970323C
F igu r e 2. Section of the phase-sensitive HMQC, 1H-
detected 13C,1H correlation for 3a showing the relatively
low frequency positions of the three biaryl CH resonances,
indicated as C3-C5, from the coordinated η6-biaryl ring.
The three remaining resonances appear at 102.9 (C1), 134.2
(C2, carrying the OMe group), and 92.3 (C6) ppm, respec-
tively, with the last signal visible in the section shown. The
open and closed cross-peaks reflect the phases; the hori-
zontal and vertical 1-D spectra are shown for clarity. The
unlabeled signal in the proton direction arises from CHDCl2
(8) The NMR assignments were made on the basis of COSY,
NOESY, P,H, C,H (both one-bond and long-range HMBC two- and
three-bond) 2-D correlations. The P,H correlations are important in
that they pinpoint the various ortho aromatic protons, which are useful
NOE probes. The aromatic protons of the 3,5-di-tert-butylphenyl groups
show the usual 3J (P,H) values of ca. 7-8 Hz. The J (P,H) values to the
complexed arene are much smaller, presumably due to the complex-
ation. The resolutions in the 1H and 13C spectra are (0.3 and (0.6
Hz, respectively.
1
(13C at 100.6 MHz, H at 400 MHz, CD2Cl2).
(9) Singewald, E. T.; Shi, X.; Mirkin, C. A.; Schofer, S. J .; Stern, C.
L. Organometallics 1996, 15, 3062.
of spacer methylene groups. Moreover, these compounds
are not involved in enantioselective hydrogenation.
A related reaction using ((trimethylsilyl)methyl)-
lithium gave 3b, and apart from the (trimethylsilyl)-
methyl ligands, the NMR spectroscopic details10 for this
analog are similar to those for 3a . Both 3a and 3b are
stable 18e complexes of Ru(II) containing an unprec-
edented coordination mode for the chiral MeO-Biphep
(10) Synthesis of Ru((trimethylsilyl)methyl)2{(R)-(1a )} (3b): To a
solution of 48.0 mg (0.003 83 mmol) of Ru((R)-(1a ))(OAc)2 in 2 mL of
toluene was added 3.9 equiv of Me3SiCH2Li (150 µL, 0.150 mmol of a
1.0 M solution in pentane). After 24 h the orange solution was taken
to dryness. The residue was redissolved in 2 mL of CH2Cl2 and stirred
for another 24 h. After evaporation to dryness, 2 mL of hexane was
added. This mixture was then transferred into an NMR tube and stored
for 1 day at -30 °C in order to let the lithium salts precipitate. After
decantation and evaporation to dryness 38.1 mg (76%) of a yellow
powder was isolated. MS (FAB): m/ z 1250.6 ([M - tert-butyl]+), 1234
([M - SiMe3] +). 31P{1H} NMR: δ 60.8, -18.4 [J (P,P) ) 6 Hz]. 1H (13C)
NMR: δ first CH2SiMe3, -0.63 (-7.05), 1 H, 2J (H,H) ) 13.3 Hz,
3J (P,H) ) 13.2 Hz; -0.81 (-7.05), 1 H, 2J (H,H) ) 13.3 Hz, 3J (P,H) )
3.5 Hz; second CH2SiMe3, -0.70 (-5.82), 2 H, tightly coupled AB spin
system, 3J (H,H) ) ca. 14-15 Hz, 3J (P,H) ) 7.0 Hz; 2 SiMe3, -0.23
(3.56), 9H, -0.50 (3.92), 9H; complexed biaryl positions 1-6, respec-
tively (103.6), (132.3), 6.15 (88.0), 5.32 (71.5), 5.33 (94.6), (91.4); normal
biaryl 1′-6′, (134.0), (158.3), 6.66 (110.0), 7.34 (129.5), 6.80 (126.6),
C6′ not assigned; MeO, 2.94 (54.2), 3.59(62.2); ortho H for the 3,5-di-
tert-butylphenyl, 6.88, 7.15 (two sets of two which overlap), 7.61; t-Bu,
1.11, 1.19, 1.34, 1.43.
(6) Synthesis of Ru(Me)2(1) (3a ): To a solution of 57.2 mg (0.004 57
mmol) of Ru((R)-(3,5-t-Bu2)4-MeO-BIPHEP)(OAc)2 in 2 mL of toluene
was added 3 equiv of methyllithium (1.6 M solution in diethyl ether;
85 µL, 0.14 mmol). After 1 h the orange solution was stirred for ca. 15
s in air before it was evaporated to dryness. The residue was extracted
with 2 × 3 mL of hexane. After filtration of the extracts through Celite,
the pale yellow solution was evaporated to dryness, to afford 48.4 mg
(91%) of a yellow powder. Anal. Calcd for C72H102O2P2Ru: C, 74.38;
H, 8.84. Found: C, 74.16; H, 8.73. IR (CsI): 595, 483 cm -1. MS
(FAB): m/ z 1148.5 ([M - Me]+), 1132.4 ([M - 2Me]2+). 31P{1H} NMR
(11) Synthesis of [Ru(Me)((R)-(1a ))(benzene)]BF4 (4): To a solution
of Ru(Me)2((R)-(1a )), (3a ; 34.0 mg, 0.0292 mmol) in 2 mL of i-PrOH
was added 20 µL of benzene, followed by the addition of 2 equiv of
HBF4‚H2O (c 8.2 M; 2.0 equiv, 7.2 µL, 0.058 mmol). The reaction
(CD2Cl2): δ ) 65.8, -17.5 ppm [J (P,P) ) 3.7 Hz]. 1H (13C) NMR:
δ
Me, 0.10 (-8.7); Me, -0.48 (-5.3); for the complexed biaryl positions
1-6, respectively (102.9), (134.2), 5.69 (87.1), 4.87 (74.8), 4.65 (99.9),
(92.3); for the normal biaryl positions 1′-6′ (133.2), (158.7), 6.60 (109.7),
7.28 (129.7), 6.80 (124.8), (123.1); MeO, 2.79 (53.9), 3.58 (59.7); ortho
H for the 3,5-di-tert-butylphenyl groups, 6.63, 6.84, 7.15, 7.30; t-Bu
1.05, 1.24, 1.32, 1.35. There are five resolved 13C t-Bu methyl signals
between 31.5 and 31.8 ppm, in the ratio 1:1:2:2:2.
mixture was refluxed shortly and then stirred for
3 h at room
temperature. Subsequently, the solvent was distilled, (in vacuo) and
the crude product redissolved in 4 mL of CH2Cl2. This solution was
dried over MgSO4. Evaporation to dryness gave 35.7 mg (93%) of a
yellow powder. Anal. Calcd for C77H105BF4O2P2Ru: C, 70.46; H, 8.06.
Found: C, 69.48; H, 7.62. MS (FAB): m/ z 1225.7 ([M+]). 31P{1H}
NMR: δ 46.5, 45.6 [J (P,P) ) 57 Hz]. 1H (13C) NMR: δ Me, 0.42 (-5.7),
3J (P,H) ) 6 Hz (both P atoms exhibit the same 3J value), 2J (P,C) )
12.5 Hz; η6 C6H6, 5.28 (96.2), 2J (P,C) ) 2.4 Hz.
(7) Mann, B. E.; Taylor, B. F. 13C NMR Data for Organometallic
Compounds; Academic Press: London, 1981, p 254. Bennett, M. A.;
McMahon, I. J .; Pelling, S.; Brookhart, M.; Lincoln, D. M. Organome-
tallics 1992, 11, 127.