(Arene)ruthenium complexes with bis(oxazolines): Synthesis and applications as asymmetric catalysts for Diels-Alder reactions

Article abstract of DOI:10.1021/om010092g

Reaction of the dimers [RuCl₂(arene)]₂ (arene = benzene, p-cymene, mesitylene) with bis(oxazolines) (N-N = bis(2-oxazoline) (box), 2,2-bis(2-oxazolinyl)propane (bop), 1,2-bis(2-oxazolinyl)benzene (benbox)) in the presence of NaSbF

Full text of DOI:10.1021/om010092g

Organometallics 2001, 20, 3029-3034
3029
(Ar en e)r u th en iu m Com p lexes w ith Bis(oxa zolin es):
Syn th esis a n d Ap p lica tion s a s Asym m etr ic Ca ta lysts for
Diels-Ald er Rea ction s
David L. Davies,* J ohn Fawcett, Shaun A. Garratt, and David R. Russell
Department of Chemistry, University of Leicester, Leicester LE1 7RH, U.K.
Received February 7, 2001
Reaction of the dimers [RuCl2(arene)]2 (arene ) benzene, p-cymene, mesitylene) with bis-
(oxazolines) (N-N ) bis(2-oxazoline) (box), 2,2-bis(2-oxazolinyl)propane (bop), 1,2-bis(2-
oxazolinyl)benzene (benbox)) in the presence of NaSbF6 gives the complexes [RuCl(N-
N)(arene)][SbF6] (1-8), which have been fully characterized. Treatment of these cations with
AgSbF6 generates dications which in some cases are enantioselective catalysts for Diels-
Alder reaction of methacrolein and cyclopentadiene. Two complexes, [RuCl(iPr-benbox)(p-
cymene)][SbF6] (5) and [Ru(OH2)(iPr-bop)(mes)][SbF6]2 (10; mes ) mesitylene), have been
characterized by X-ray crystallography.
In tr od u ction
lysts. With C1-symmetry ligands the complexes can in
principle exist as two diastereomers. The presence of
two diastereomeric catalysts could lead to a reduction
in enantioselectivty in a catalytic process. Very high
diastereoselectivity has been found for some (arene)-
ruthenium complexes; indeed, in favorable cases only
one isomer is observed.9 To avoid the possibility of
diastereomers, C2-symmetry chiral ligands can be used,
in which case only one isomer is possible when coordi-
nated to the metal. This approach has been very
successful with chiral C2-symmetric bis-phosphines, for
example the use of [RuCl(binap)(arene)]+ in asymmetric
hydrogenation;10 it should be noted, though, that during
catalysis the arene dissociates from the metal, and
hence, the active catalyst no longer has a half-sandwich
structure. More recently, Ku¨ndig has reported a cyclo-
pentadienylruthenium complex of a C2-symmetry bis-
phosphonite which retains the half-sandwich structure
during catalysis of Diels-Alder reactions.11
(Arene)ruthenium complexes with chiral bidentate
ligands have been known since 1977;1 however, their
great potential as chiral catalysts has only recently been
demonstrated. The best example so far is the asym-
metric transfer hydrogenation of ketones, with enan-
tiomeric excesses (ee) of >99%, using (arene)ruthenium
complexes with a chiral ligand derived from a chiral
diamine as catalyst.2 (Arene)ruthenium complexes with
chiral bidentate ligands containing at least one hard
donor atom have attracted much study recently,3-9
particularly their potential applications as chiral cata-
(1) Dersnah, D. F.; Baird, M. C. J . Organomet. Chem. 1977, 127,
C55.
(2) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97.
(3) For amino acidate complexes see for example: (a) Carmona, D.;
Vega, C.; Lahoz, F. J .; Atencio, R.; Oro, L. A.; Lamata, P. M.; Viguri,
F.; San J ose, E. Organometallics 2000, 19, 2273. (b) Sheldrick, W. S.;
Heeb, S. Inorg. Chim. Acta 1990, 168, 93. (c) Sheldrick, W. S.; Heeb,
S. J . Organomet. Chem. 1989, 377, 357. (d) Carmona, D.; Lahoz, F. J .;
Atencio, R.; Oro, L. A.; Lamata, M. P.; Viguri, F.; San J ose, E.; Vega,
C.; Reyes, J .; J oo, F.; Katho, A. Chem. Eur. J . 1999, 5, 1544. (e) Kramer,
R.; Polborn, K.; Wanjek, H.; Zahn, I.; Beck, W. Chem. Ber. 1990, 123,
767.
(4) For salicylaldimine comlexes see for example: (a) Brunner, H.;
Oeschey, R.; Nuber, B. J . Chem. Soc., Dalton Trans. 1996, 1499. (b)
Brunner, H.; Oeschey, R.; Nuber, B. J . Organomet. Chem. 1996, 518,
47. (c) Brunner, H.; Oeschey, R.; Nuber, B. Inorg. Chem. 1995, 34, 3349.
(d) Brunner, H.; Neuhierl, T.; Nuber, B. Eur. J . Inorg. Chem. 1998,
1877. (e) Mandal, S. K.; Chakravarty, A. R. J . Chem. Soc., Dalton
Trans. 1992, 1627. (f) Mandal, S. K.; Chakravarty, A. R. Inorg. Chem.
1993, 32, 3851.
(5) For pyridineimine comlexes, see for example: (a) Carmona, D.;
Vega, C.; Lahoz, F. J .; Elipe, S.; Oro, L. A.; Lamata, P. M.; Viguri, F.;
Garcia-Correas, R.; Cativiela, C.; Lopez-Ram deViu, M. P. Organome-
tallics 1999, 18, 3364. (b) Davies, D. L.; Fawcett, J .; Krafczyk, R.;
Russell, D. R. J . Organomet. Chem. 1997, 545-546, 581.
(6) For ortho-metalated arylethylamine comlexes, see for example:
(a) Gul, N.; Nelson, J . H. Polyhedron 1999, 18, 1835. (b) Hansen, H.
D.; Maitra, K.; Nelson, J . H. Inorg. Chem. 1999, 38, 2150. (c) Gul, N.;
Nelson, J . H. Organometallics 1999, 18, 709. (d) Attar, S.; Nelson, J .
H.; Fischer, J .; Decian, A.; Sutter, J . P.; Pfeffer, M. Organometallics
1995, 14, 4559. (e) Attar, S.; Catalano, V. J .; Nelson, J . H. Organo-
metallics 1996, 15, 2932. (f) Brunner, H.; Zwack, T. Organometallics
2000, 19, 2423.
In the past decade there has been a surge of interest
in chiral nitrogen-donor ligands for use in asymmetric
catalysis and bidentate, C2-symmetry ligands such as
semicorrins and particularly bis(oxazolines) have been
very successful.12,13 The use of C2-symmetry nitrogen
donor ligands in (arene)ruthenium complexes has been
studied much less well. Kurosawa et al. have studied
(8) For complexes with PN donor ligands, see for example: (a)
Carmona, D.; Cativiela, C.; Elipe, S.; Lahoz, F. J .; Lamata, M. P.;
Lopez-Ram de Viu, M. P.; Oro, L. A.; Vega, C.; Viguri, F. Chem.
Commun. 1997, 2351. (b) Arena, C. G.; Calamia, S.; Faraone, F.; Graiff,
C.; Tiripicchio, A. J . Chem. Soc., Dalton Trans. 2000, 3149.
(9) For pyridineoxazoline complexes see: (a) Davenport, A. J .;
Davies, D. L.; Fawcett, J .; Garratt, S. A.; Russell, D. R. J . Chem. Soc.,
Dalton Trans. 2000, 4432. (b) Davenport, A. J .; Davies, D. L.; Fawcett,
J .; Garratt, S. A.; Russell, D. R. Chem. Commun. 1999, 2331. (c) Davies,
D. L.; Fawcett, J .; Garratt, S. A.; Russell, D. R. Chem. Commun. 1997,
1351.
(10) Mashima, K.; Kusano, K.; Ohta, T.; Noyori, R.; Takaya, H. J .
Chem. Soc., Chem. Commun. 1989, 1208.
(11) Ku¨ndig, E. P.; Saudan, C. M.; Bernardinelli, G. Angew. Chem.,
Int. Ed. 1999, 38, 1220.
(12) Pfaltz, A. Acta Chem. Scand. 1996, 50, 189.
(13) Ghosh, A. K.; Mathivanan, P.; Cappiello, J . Tetrahedron:
Asymmetry 1998, 9, 1.
(7) For complexes with PO donor ligands see: (a) Faller, J .; Parr,
J . Organometallics 2000, 19, 1829. (b) Faller, J .; Patel, B. P.; Albrizzio,
M. A.; Curtis, M. Organometallics 1999, 18, 3096. (c) Faller, J .;
Grimmond, B. J .; Curtis, M. Organometallics 2000, 19, 5174.
10.1021/om010092g CCC: $20.00 © 2001 American Chemical Society
Publication on Web 06/01/2001

3030 Organometallics, Vol. 20, No. 14, 2001
Davies et al.
Ta ble 1. Com p lex Nu m ber in g Sch em e
complex
arene
N-N
complex
arene
N-N
1
2
3
4
mes
mes
iPr -box
5
6
7
8
p-cymene iPr-benbox
iPr-bop
mes
mes
mes
Et-benbox
iPr-benbox
Ph-benbox
benzene
p-cymene Et-benbox
iPr-benbox
bis(phenyloxazolinyl)propane complexes,14 and a di-
aminobiphenyl complex in which the ligand has axial
chirality has also been reported.15 In this paper we
report the synthesis of several (arene)ruthenium com-
plexes with three different types of bis(oxazolines) (N-
N), namely bis(2-oxazoline) (box), 2,2-bis(2-oxazolinyl)-
propane (bop), and 1,2-bis(2-oxazolinyl)benzene (benbox).
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion of [Ru Cl(N-N)-
(a r en e)][EF 6] (1-8; E ) P , Sb). The complexes [RuCl-
(N-N)(arene)][EF6] (1-8; E ) P, Sb) (Table 1) were
synthesized in high yield by treatment of the dimers
[RuCl2(arene)]2 with 2 equiv of the bis(oxazoline) ligand
(N-N) and NaSbF6 (or KPF6) in methanol at reflux. The
complexes were characterized by 1H NMR and mass
spectroscopy, elemental analysis, and X-ray diffraction
for 4.
F igu r e 1. Structure drawing of the cation of 4, showing
the atom-labeling scheme. Displacement ellipsoids are
shown at the 30% level. H atoms are omitted for clarity.
Selected bond distances (Å) and angles (deg): Ru(1)-N(1)
)
2.127(8), Ru(1)-N(2) ) 2.119(9), Ru(1)-Cl(1) )
2.403(2); N(2)-Ru(1)-N(1) ) 80.2(4), N(2)-Ru(1)-Cl(1) )
88.6(3), N(1)-Ru(1)-Cl(1) ) 88.1(2).
As explained above, only one isomer is possible, since
the complexes are only chiral at the ligand, not at the
metal. The free ligands have C2 symmetry; however, the
symmetry is lost on complexation and the 1H NMR
spectra of 1-8 are more complex as a result. In all of
the complexes the oxazoline protons shift downfield on
The ruthenium atom has a pseudooctahedral geometry
with the arene occupying three adjacent sites of the
octahedron. The most striking feature of the structure
is that the oxazoline rings are rotated out of the plane
of the benzene ring 48.3° (N1-O1) and 45.2° (N2-O2),
allowing coordination to the ruthenium with a chelate
bite angle of 80.2(4)°. A similar situation was found in
the complex [Zn(iPr-benbox)Cl2], which had torsion
angles of 46.4 and 46.5°.16 These rotations will tend to
relieve steric interactions between the ethyl on C(14)
and the η6-arene while putting the ethyl on C(3) closer
to the chloride. The benzene ring lies rather close to the
p-cymene, with the C(6)-C(23) and C(11)-C(17) dis-
tances being 3.33 Å, consistent with the NOESY NMR
data for 7 described above. The Ru-N(1) and Ru-N(2)
distances, 2.127(8) and 2.119(9) Å, respectively, are
statistically the same and are very similar to the Ru-
N(oxazoline) distances found in (arene)ruthenium
pyridyloxazoline complexes, 2.103(5)-2.128(5) Å.9a
i
coordination; in complexes with Pr substituents the CH
proton shifts downfield compared to free ligand and the
Me groups give rise to four doublets, one of which is
often shifted upfield. The major ion in the mass spec-
trum of each complex corresponds to [RuCl(N-N)-
(arene)].
1
In the H NMR spectra of 1 and 2 the signals for the
η6-mesitylene were observed at δ 2.32 and 5.67 for 1
and δ 2.25 and 5.71 for 2, while for 6 and 7 the
corresponding signals were considerably upfield (at δ
1.96 and 4.55 and at δ 1.95 and 4.77 for 6 and 7,
respectively). Such a substantial shift, almost 1 ppm in
the case of the aromatic protons, is unlikely to be due
to a difference in the donor properties of the ligands
since they are all bis(oxazolines). The upfield shift in 7
is most likely due to a ring current effect from the
benzene backbone of the benbox ligand. To test this
hypothesis, a NOESY spectrum of 7 was run. This
showed cross-peaks between both mesitylene signals (δ
1.95 and 4.77) and all three multiplets (total integration
4H) assigned to the benzene backbone of the ligand. In
complex 8 the mesitylene signals were even more
shielded (δ 1.74 and 4.07); in this case there may also
be a ring current from one of the phenyl substituents
on the oxazolines. Similar â-phenyl effects have been
found in (arene)ruthenium salicylaldimine complexes.4a
Crystals of [RuCl(Et-benbox)(p-cymene)]PF6 (4) were
obtained that were suitable for X-ray crystallography,
and the structure of the cation is shown in Figure 1.
Syn th esis a n d Ch a r a cter iza tion of [Ru (L)(N-N)-
(a r en e)][SbF 6]2 (9-12; L ) OH2, Aceton e). To acti-
vate 1-8 for use in asymmetric catalysis, it is necessary
to remove the strongly bound chloride ligand. This was
done by treatment with AgSbF6 in CH2Cl2/acetone to
give a precipitate of AgCl and the solvent complexes
[Ru(L)(N-N)(arene)][SbF6]2 (L ) acetone, OH2). The
water ligand is obtained from the acetone solvent and/
or during the workup, which is carried out in air. A few
of these dicationic complexes (9-12), derived from 1-3
and 7 respectively, were isolated and characterized by
1H NMR spectroscopy and mass spectrometry and in
some cases elemental analysis; characterization of 10
1
was also carried out by X-ray diffraction. The H NMR
spectra of 9-12 provide evidence for a number of
different processes: (i) exchange between acetone co-
ordination and water coordination, (ii) exchange be-
(14) (a) Asano, H.; Katayama, K.; Kurosawa, H. Inorg. Chem. 1996,
35, 5760. (b) Kurosawa, H.; Asano, H.; Miyaki, Y. Inorg. Chim. Acta
1998, 270, 87.
(15) Alguindigue, S. S.; Khan, M. A.; Ashby, M. T. Organometallics
1999, 18, 5112.
(16) Bolm, C.; Weickhardt, K.; Zender, M.; Ranff, T. Chem. Ber.
1991, 124, 1173.

(Arene)ruthenium Complexes with Bis(oxazolines)
Organometallics, Vol. 20, No. 14, 2001 3031
Sch em e 1. P r op osed F lu xion a lity of Solven t
Com p lexes [Ru (L)(N-N)(a r en e)][SbF 6]2 (L )
Aceton e, OH2)
tween free and coordinated water, (iii) proton/deuterium
exchange on the coordinated water, and (iv) a fluxional
process which can reestablish C2 symmetry for the bis-
(oxazoline) (Scheme 1).
1
The H NMR spectra of 9, 11, and 12 in d6-acetone
show evidence for acetone- and water-coordinated spe-
cies. Addition of water shifts the position of the equi-
librium and hence allows assignment of these species.17
In the case of 9 the signals for the acetone-coordinated
species are rather broad, suggesting that the fluxional
process (iv) above (Scheme 1) is occurring at a rate
similar to the NMR time scale and is faster for the
acetone-coordinated complex than for the water-coor-
dinated species. We have observed similar equilibria
between acetone- and water-coordinated species, with
faster exchange of acetone in related pyridyloxazoline
complexes.9a The complexes 9, 11, and 12 each show a
broad signal at ca. δ 3, due to free water, and two
separate signals at ca. δ 7, due to coordinated H2O and
HOD. Addition of small aliquots of either H2O or D2O
allowed assignment of these signals. Surprisingly, the
signal for coordinated HOD is at higher frequency than
that of H2O. High-frequency deuterium isotope shifts,
though uncommon, have been observed previously in
systems where hydrogen bonding occurs.18 Kurosawa et
al. suggested, on the basis of kinetic measurements, that
acetone may hydrogen bond to the coordinated water
in [Ru(OH2)(Ph-bop)(C6H6)][BF4]2 (13).14 The observa-
tion of separate signals for coordinated H2O and HOD
shows that proton exchange is slow on the NMR time
scale.
F igu r e 2. Structure drawing of the cation of 10, showing
the atom-labeling scheme. Displacement ellipsoids are
shown at the 30% level. H atoms are omitted for clarity.
Selected bond distances (Å) and angles (deg): Ru(1)-N(1)
) 2.135(11), Ru(1)-N(2) ) 2.153(12), Ru(1)-O(3) )
2.160(10); N(2)-Ru(1)-N(1) ) 83.5(5), N(2)-Ru(1)-O(3)
) 81.1(4), N(1)-Ru(1)-O(3) ) 85.1(4).
of acetone does not occur to any extent in this case.
Similar fluxional behavior has been observed for the
analogous complex [Ru(OH2)(Ph-bop)(C6H6)][BF4]2 (13)
reported by Kurosawa et al.;14 in that case the exchange
process was frozen out at 223 K, with free and coordi-
1
nated water signals observed by H NMR at δ 6.60 and
3.57, respectively (i.e. consistent with the chemical shifts
reported here).
Recrystallization of 10 from acetone/ether gave crys-
tals that were suitable for an X-ray structure determi-
nation. The structure of the dication is shown in Figure
2 with selected bond distances and angles. The bond
distances and angles are similar to those for the known
ruthenium analogue 13.14 Indeed, the Ru-N(1), Ru-
N(2), and Ru-O(3) bond distances of 10 and 13 are
statistically the same, and similar N(1)-Ru-N(2) che-
late angles are found in each complex (83.5(5)° for 10
and 82.6(4)° for 13). There is a significant difference (4°)
between the angles N(2)-Ru(1)-O(3) (81.1(4)°) and
N(1)-Ru(1)-O(3) (85.1(4)°), suggesting that the ligand
is rotated slightly to reduce interaction between the
isopropyl next to N(1) and the coordinated water. We
noted a similar rotation to relieve steric interactions in
pyridyloxazoline complexes.9a Examination of the struc-
ture of the related complex 1314 reveals an almost
identical rotation with the larger N-Ru-O angle to the
oxazoline bearing the substituent oriented toward the
water, as seen in 10.
Ca ta lysis. The Diels-Alder reaction is one of the
most important in organic chemistry, and great progress
has been made in developing enantioselective versions.
Recently there has been particular interest in using
chiral transition-metal-based Lewis acid catalysts.19
Chiral (arene)ruthenium complexes of bis-phosphine
monoxides,7a phosphinooxazolines,8a pyridylimines,5a
and pyridyloxazolines9a have been used to catalyze
Diels-Alder reactions.
The 1H NMR spectrum in d6-acetone of [Ru(L)(iPr-
bop)(mes)]2+ (10), derived from 2, showed evidence of
exchange process (iv) above (Scheme 1) occurring at a
rate comparable with the NMR time scale (400 MHz,
300 K). Thus, the CMe2 group, which is expected to give
rise to two inequivalent singlets, was observed as a
broad signal at δ 1.7, while only two (of four) methyls
of the isopropyl substituents and two of the six oxazoline
protons were easily visible as a sharp doublet at δ 1.2
and a sharp doublet of doublets at δ 4.9, respectively.
In addition, there was a broad hump at δ 2.9 due to
water, with no signal visible due to coordinated water.
1
At 233 K, all of the H NMR signals were well-resolved,
i
those due to the Pr-bop ligand being much like those
of the precursor 2, though with the majority of the
signals more deshielded, due to formation of a dication.
Free water was observed as a broad singlet at δ 3.61
with singlets at δ 7.06 and 7.10 (ratio 7:3) in the region
expected for coordinated water. Even at 233 K only one
complex was observed, which suggests that coordination
(17) No attempt was made to dry the d6-acetone; hence, no detailed
assessment of the relative stabilities of water- and acetone-coordinated
complexes was made.
(19) Carmona, D.; Lamata, M. P.; Oro, L. A. Coord. Chem. Rev. 2000,
200-202, 717. J ohnson, J . S.; Evans, D. A. Acc. Chem. Res. 2000, 33,
325. Yamamoto, H. Lewis Acids in Organic Synthesis; Wiley-VCH:
Weinheim, Germany, 2001.
(18) Hansen, P. E. Prog. NMR Spectrosc. 1988, 20, 207.

3032 Organometallics, Vol. 20, No. 14, 2001
Davies et al.
pentanediol with literature values.20 This is consistent
with the isopropyl shielding the Si face of an S-trans-
coordinated methacrolein leading to attack of cyclopen-
tadiene at the Re face, as we have described previously
for (arene)ruthenium pyridyloxazoline complexes.9a
In conclusion, we have synthesized a number of
(arene)ruthenium complexes [Ru(Cl)(N-N)(arene)][SbF6]
with C2-symmetry bis(oxazoline) ligands. Treatment of
these with AgCl gave the dications [Ru(L)(N-N)(are-
ne)]2+ (L ) acetone, OH2), which in some cases were
capable of catalyzing the Diels-Alder reaction of meth-
acrolein and cyclopentadiene with moderate enantio-
selectivity. The best ligand in terms of the catalysis
(benbox) has a flexible connection between the two
oxazolines, allowing the ligand to adopt a nonplanar
conformation. However, the enantioselectivity with the
benbox catalyst derived from 7 is still less than that
with the C1-symmetry pyridyloxazoline complexes, which
are the most enantioselective (arene)ruthenium cata-
lysts for this reaction reported so far.9a,26
Ta ble 2. En a n tioselective Diels-Ald er Rea ction of
Meth a cr olein w ith Cyclop en ta d ien e Ca ta lyzed by
[Ru (L)(N-N)(a r en e)][SbF 6]2 (L ) Aceton e, OH2), in
Dich lor om eth a n e
cat. pre- cat. temp
entry cursor
t
yielda exo:
(%)
ee (%)
endo (abs confign)
(%) (°C)d (h)
1
2
3
4
5
6
7
8
9
1
2
7
3
5
7
6
7
8
5
5
5
2
2
2
2
2
2
rt
rt
rt
rt
rt
rt
0
0
0
rt
24
48
47
94:6
10 (S)
0
15b 88:12
0.5 >95* 94:6
2
3
0.5 >95* 94:6
24
7
66 (S)
31 (S)
6 (S)
65 (S)
45 (R)c
67 (S)
5 (S)
0
90
87
88:12
88:12
94
88
13
15
94:6
94:6
88:12
85:15
48
48
10
a
The isolated yield is quoted, except for those marked with an
asterisk, which were experiments done in an NMR tube, where
b
the yield is based on relative integrations. Yields were somewhat
variable with this catalyst, but the ee was always very low,
suggesting an achiral catalytic impurity may have been present.
Increasing the amount of hindered base24 to 3 mol equiv led to
the low yield reported. c The opposite configuration amino alcohol
d
(i.e., RC) was used in the ligand synthesis. rt ) room tempera-
ture.
Exp er im en ta l Section
As mentioned above, treatment of [RuCl(pymox)-
(arene)][SbF6] (1-8) with AgSbF6 gives the solvent
complexes [Ru(L)(N-N)(arene)][SbF6]2 (L ) acetone,
OH2); these dications are good Lewis acids and can
catalyze the Diels-Alder reaction of methacrolein with
cyclopentadiene. The catalysis can be carried out on
freshly prepared solutions of these dications, either after
filtration to remove the AgCl or after isolation, similar
catalytic results being observed in either case. The
results (Table 2) will be described in terms of the
chloride precursor complexes 1-8. Entries 1-3 show the
effect of changing the type of oxazoline ligand with the
same substituents and same arene. In this case the
benbox complex 7 is the most successful catalyst in
terms of activity and enantioselectivity. The bop com-
plex 2 is inactive (the yield is comparable to the thermal
reaction, entry 10), which is somewhat surprising, since
this is the most frequently used bis-oxazoline in general.
However, it is consistent with the fact that dication 10
showed no evidence of acetone coordination in d6-
acetone, whereas both 9 and 12 did.
The effect of changing the arene has been examined
for the benbox catalysts (entries 4-6). All three com-
plexes are quite active, giving high yields in a few hours.
There is a large variation in enantioselectivity, with the
mesitylene complex 7 being best, as found with related
(arene)ruthenium pyridyloxazoline complexes,9a but
with benzene complex 3 being rather better than the
p-cymene complex 5; notably, though, in both these
cases the exo:endo selectivity is also poorer. The effect
of changing the oxazoline substituents has been studied
(entries 7-9): the alkyl substituents (Et, iPr) are much
better than phenyl, which has very low activity. Chang-
ing the catalyst loading from 5 to 2% (entries 3 and 6)
has almost no effect, while lowering the temperature
(entries 6 and 8) has surprisingly little effect on the
enantioselectivity, though the rate of reaction is reduced
as expected.
Petroleum ether and diethyl ether were dried by refluxing
over purple sodium/benzophenone under nitrogen, while dichlo-
romethane was purified by refluxing over calcium hydride and
acetone from calcium sulfate. The reactions described were
carried out under nitrogen; however, once isolated as pure
solids, the compounds are air-stable and precautions for their
storage are unnecessary. 1H NMR spectra were obtained using
a Bruker spectrometer at 300 MHz in CDCl3 unless stated
otherwise; chemical shifts were recorded in ppm (referenced
to tetramethylsilane or residual protons in the NMR solvent).
FAB mass spectra were obtained on a Kratos Concept mass
spectrometer using an NOBA matrix. Microanalyses were
performed by Butterworth Laboratories Ltd., Middlesex, U.K.
The bis(oxazolines) were prepared by literature procedures
(box,21 bop,22 benbox16) from the relevant amino alcohol, which
in turn was prepared by reduction of the amino acid23 (99%
optical purity), except for phenylglycinol, which was a gift from
NSC Technologies.
P r ep a r a tion s of [Ru Cl(bis(oxa zolin e))(a r en e)][SbF 6]
(1-8). A solution of bis(oxazoline) ligand (2 equiv) and NaSbF6
(2 equiv) in MeOH (10 cm3) was added to [MCl2(ring)]2 (1
equiv), and the resulting suspension was heated to reflux for
2 h. A yellow-brown solution was obtained, which was then
evaporated and the crude residue dissolved in CH2Cl2. Filtra-
tion through Celite (to remove NaCl and any black decomposi-
tion product), gave a red-orange solution, which was evapo-
rated, and the crude complex was recrystallized from CH2Cl2/
ether. The scale and yields for individual complexes are shown
below. The PF6 salts could be prepared similarly using KPF6;
the spectroscopic propeties were identical.
[Ru Cl(iP r -box)(m es)][SbF 6] (1). Complex 1 was prepared
i
from [RuCl2(mes)]2 (80 mg, 0.137 mmol), Pr-box (68 mg, 0.31
mmol), and NaSbF6 (75 mg, 0.29 mmol) in 184 mg yield (94%).
Anal. Calcd for C21H34ClF6N2O2RuSb‚H2O: C, 34.33; H, 4.66;
1
N, 3.81. Found: C, 33.82; H, 4.22; N, 3.56. H NMR: δ 0.74,
1.03, 1.04, 1.10 (4 × d, 3H, J ) 7 Hz, CHMe2), 2.14 (m, 1H,
CHMe2), 2.32 (s, 9H, C6Me3), 2.54 (m, 1H, CHMe2) 4.26 (m,
1H, NCH), 4.66 (t, 1H, J ) 10 Hz, OCH), 4.81 (m, 2H, OCH),
(20) Evans, D. A.; Murry, J . A.; Matt, P. v.; Norcross, R. D.; Miller,
S. J . Angew. Chem., Int. Ed. Engl. 1995, 34, 798.
(21) Mu¨ller, D.; Umbricht, G.; Weber, B.; Pfaltz, A. Helv. Chim. Acta
1991, 74, 232.
(22) Denmark, S. E.; Nakajima, N.; Nicaise, O. J .-C.; Faucher, A.-
M.; Edwards, J . P. J . Org. Chem. 1995, 60, 4884.
(23) McKennon, M. J .; Meyers, A. I.; Drauz, K.; Schwarm, M. J . Org.
Chem. 1993, 58, 3568.
Using SC ligands the major product was identified as
(1R,2S,4R)-2-methylbicyclo[2.2.1]hept-5-ene-2-carbalde-
hyde, by comparison of the sign of the optical rotation
and the GC behavior of the acetal formed from (2R,4R)-

(Arene)ruthenium Complexes with Bis(oxazolines)
Organometallics, Vol. 20, No. 14, 2001 3033
4.95 (m, 1H, NCH), 5.15 (t, 1H, J ) 9 Hz, OCH), 5.67 (s, 3H,
C6H3Me3) (water of solvation observed at δ 1.61). MS (FAB+):
m/z 481, [M]+; 446, [M - Cl]+.
(t, 1H, J ) 9 Hz, OCH), 7.82 (m, 2H, C6H4), 7.93 (m, 1H, C6H4),
8.22 (d, 1H, J ) 8 Hz, C6H4). MS (FAB+): m/z 558, [M + H]+.
[Ru Cl(P h -ben box)(m es)][SbF 6] (8). Complex 8 was pre-
pared from [RuCl2(mes)]2 (70 mg, 0.12 mmol), Ph-benbox (92
mg, 0.25 mmol), and NaSbF6 (65 mg, 0.25 mmol) in 178 mg
yield (86%). Anal. Calcd for C33H32ClF6N2O2RuSb: C, 46.04;
H, 3.75; N, 3.25. Found: C, 45.79; H, 3.63; N, 3.18%. 1H
NMR: δ 1.74 (s, 9H, C6Me3), 4.07 (s, 3H, C6H3Me3), 4.55 (dd,
1H, OCH), 4.71 (t, 1H, J ) 9 Hz, OCH), 5.06 (m, 2H, OCH +
NCH), 5.43 (m, 2H, OCH + NCH), 6.84 (m, 2H, Ph), 7.27 (m,
4H, Ph), 7.52 (m, 4H, Ph), 7.97 (m, 2H, C6H4), 8.17 (d, 1H, J
) 8 Hz, C6H4), 8.29 (d, 1H, J ) 8 Hz, C6H4). MS (FAB+): m/z
626, [M + H]+.
P r ep a r a tion s of [Ru (L)(bis(oxa zolin e))(a r en e)][SbF 6]2
(L ) OH2, a ceton e) (9-12). A solution of AgSbF6 (1 equiv)
in acetone (0.5 cm3) was added to a solution of [RuCl(bis-
(oxazoline))(arene)][SbF6] (1 equiv) in CH2Cl2 (4 cm3), giving
a yellow-orange solution and an immediate precipitate of AgCl.
The solution was stirred for 30 min at room temperature
(protected from light) and then was filtered through Celite to
remove AgCl. Evaporation, followed by washing with CH2Cl2,
afforded solvent complexes as orange oils. In some cases the
products could be recrystallized from acetone/ether, affording
a crop of fine needles. The scale and yields for individual
complexes are shown below.
[Ru Cl(iP r -bop )(m es)][SbF 6] (2). Complex 2 was prepared
i
from [RuCl2(mes)]2 (80 mg, 0.137 mmol), Pr-bop (74 mg, 0.28
mmol), and NaSbF6 (72 mg, 0.28 mmol) in 150 mg yield (72%).
Anal. Calcd for C24H38ClF6N2O2PRu: C, 43.15; H, 4.19; N, 5.73.
Found: C, 43.15; H, 4.21; N, 5.73 (analysis on PF6 salt). 1H
NMR: δ 0.55, 0.92, 0.95, 1.04 (4 × d, 3H, J ) 7 Hz, CHMe2),
1.38 (s, 3H, CMe2), 1.61 (s, 3H, CMe2), 2.25 (s, 9H, C6Me3),
2.47 (m, 2H, CHMe2), 4.18 (t, 1H, J ) 9 Hz, OCH), 4.37 (m,
1H, NCH), 4.45 (dd, 1H, J ) 9, 1 Hz, OCH), 4.48 (m, 1H, NCH),
4.67 (m, 2H, NCH + OCH), 5.71 (s, 3H, C6H3Me3). MS
(FAB+): m/z 523, [M]+; 488, [M - Cl]+.
[Ru Cl(iP r -ben box)(C6H6)][SbF 6] (3). Complex 3 was pre-
pared from [RuCl2(C6H6)]2 (60 mg, 0.12 mmol), iPr-benbox (75
mg, 0.25 mmol), and NaSbF6 (63 mg, 0.24 mmol) in 138 mg
yield (77%). Anal. Calcd for C24H30ClF6N2O2RuSb: C, 38.40;
1
H, 4.03; N, 3.73. Found: C, 38.17; H, 3.92; N, 3.62. H NMR:
δ 0.53, 0.84, 0.96, 1.16 (4 × d, 3H, J ) 7 Hz, CHMe2), 2.59 (m,
1H, CHMe2), 2.76 (m, 1H, CHMe2), 4.42 (m, 1H, OCH), 4.58
(t, 1H, J ) 9 Hz, OCH), 4.71 (m, 1H, NCH), 4.90 (m, 3H, 2
OCH + NCH), 5.68 (s, 6H, C6H6), 8.03 (m, 3H, C6H4), 8.25 (m,
1H, C6H4). MS (FAB+): m/z 516, [M + H]+.
[Ru Cl(Et-ben box)(p-MeC6H4P r i)][SbF 6] (4). Complex 4
was prepared from [RuCl2(p-MeC6H4Pri)]2 (80 mg, 0.13 mmol),
Et-benbox (76 mg, 0.28 mmol), and NaSbF6 (72 mg, 0.28 mmol)
in 161 mg yield (79%). Anal. Calcd for C26H34ClF6N2O2PRu:
C, 45.39; H, 4.98; N, 4.07. Found: C, 45.22; H, 4.87; N, 3.85
(analysis on PF6 salt). 1H NMR (250 MHz): δ 0.71 and 0.98 (2
× t, 3H, J ) 7 Hz, CH2Me), 1.27 and 1.28 (2 × d, 3H, J ) 7 Hz
CHMe2), 1.49 (m, 2H, CH2Me), 1.64 (s, 3H, ArMe), 1.74 (m,
1H, CH2Me), 2.42 (m, 1H, CH2Me), 2.76 (sept, 1H, J ) 7 Hz,
CHMe2), 4.10 (m, 1H, NCH), 4.35 (d, 1H, J ) 7 Hz, p-Cy), 4.44
(m, 1H, NCH), 4.51 (m, 2H, OCH), 4.67 (dd, 1H, J ) 9, 8 Hz,
OCH), 4.84 (dd, 1H, J ) 9.5, 8 Hz, OCH), 5.18 (d, 1H, J ) 7
Hz, p-Cy), 5.54 (m, 2H, p-Cy), 7.87 (m, 3H, Ar H), 8.20 (m,
1H, Ar H). MS (FAB+): m/z 543, [M]+; 507, [M - HCl]+.
[Ru Cl(iP r -ben box)(p-MeC6H4P r i)][SbF 6] (5). Complex 5
was prepared from [RuCl2(p-MeC6H4Pri)]2 (80 mg, 0.13 mmol),
iPr-benbox (84 mg, 0.28 mmol), and NaSbF6 (72 mg, 0.28 mmol)
in 164 mg yield (78%). Anal. Calcd for C28H38ClF6N2O2PRu:
C, 41.68; H, 4.75; N, 3.47. Found: C, 41.45; H, 4.47; N, 3.58
[Ru (OH2)(iP r -box)(m es)][SbF 6]2 (9). Complex 9 was pre-
pared from 1 (50 mg, 0.070 mmol) and AgSbF6 (25 mg, 0.073
mmol) in 63 mg yield (97%). The compound was only charac-
terized in solution. 1H NMR (d6-acetone): δ 0.71, 1.05, 1.12,
and 1.13 (4 × d, 3H, J ) 7 Hz, CHMe2), 2.42 (s, 9H, C6Me3),
2.46 (m, 1H, CHMe2), 2.91 (m, 1H, CHMe2), 4.72 (m, 1H, NCH),
4.87 (t, 1H, J ) 10 Hz, OCH), 5.12 (m, 2H, OCH), 5.26 (m,
2H, OCH + NCH), 6.34 (s, 3H, C6H3Me3), 7.15 (s, 2H, H2O).
MS (FAB+): m/z 682, [M - H2O + SbF6]+.
[Ru (OH2)(iP r -bop )(m es)][SbF 6]2 (10). Complex 10 was
prepared from 2 (27 mg, 0.038 mmol) and AgSbF6 (13 mg,
0.038 mmol) in 34 mg yield (91%). Anal. Calcd for C24H40F12
-
N2O3RuSb2‚Et2O: C, 31.99; H, 4.79; N, 2.66. Found: C, 31.90;
H, 4.74; N, 2.61. 1H NMR (400 MHz, 233 K, d6-acetone): δ
0.54, 1.06, 1.08, 1.12 (4 × d, 3H, J ) 7 Hz, CHMe2), 1.53 and
1.77 (2 × s, 3H, CMe2), 2.42 (m, 1H, CHMe2), 2.42 (s, 9H,
C6Me3), 2.96 (m, 1H, CHMe2), 4.36 (t, 1H, J ) 10 Hz, OCH),
4.52 (m, 1H, NCH), 4.88 (t, 1H, J ) 10 Hz, OCH), 4.94 (m,
2H, 2 × OCH), 5.25 (m, 1H, NCH), 6.41 (s, 3H, C6H3Me3), 7.06
(s, 2H, H2O). MS (FAB+): m/z 682, [M - H2O + SbF6]+.
[Ru (OH2)(iP r -ben box)(C6H6)][SbF 6]2 (11). Complex 11
was prepared from 3 (100 mg, 0.133 mol) and AgSbF6 (48 mg,
1
(analysis on PF6 salt). H NMR: δ 0.44, 0.82, 1.00, and 1.12
(4 × d, 3H, J ) 7 Hz, CHMe2), 1.27 (s, 3H, ArMe), 1.30 and
1.35 (2 × d, 3H, J ) 7 Hz, CHMe2), 2.47 and 2.67 (2 × m, 1H,
CHMe2), 3.02 (sept, 1H, J ) 7 Hz, CHMe2), 4.20 (d, 1H, J ) 7
Hz, p-Cy), 4.34 (t, 1H, J ) 9 Hz, OCH), 4.57 (m, 2H, NCH +
OCH), 4.78 (m, 1H, NCH), 4.91 (m, 2H, 2 × OCH), 5.19, 5.40,
and 5.83 (3 × d, 1H, J ) 7 Hz, p-Cy), 7.80 (m, 3H, C6H4), 8.29
(m, 1H, C6H4). MS (FAB+): m/z 571, [M]+; 535, [M - HCl]+.
[Ru Cl(Et-ben box)(m es)][SbF 6] (6). Complex 6 was pre-
pared from [RuCl2(mes)]2 (80 mg, 0.137 mmol), Et-benbox (76
mg, 0.28 mmol), and NaSbF6 (72 mg, 0.28 mmol) in 168 mg
yield (80%). Anal. Calcd for C25H32ClF6N2O2PRu: C, 44.55; H,
4.79; N, 4.16. Found: C, 44.06; H, 4.76; N, 4.10 (analysis on
0.14 mmol) in 107 mg yield (83%). Anal. Calcd for C24H32F12
-
N2O3RuSb2: C, 29.74; H, 3.33; N, 2.89. Found: C, 29.89; H,
3.59; N, 2.80. 1H NMR (d6-acetone): δ 0.53, 0.84, 0.96, 1.16 (4
× d, 3H, J ) 7 Hz, CHMe2), 2.00 (m, 1H, CHMe2), 2.78 (m,
1H, CHMe2), 4.45 (ddd, 1H, J ) 10, 2, 3 Hz, NCH), 4.81 (t,
1H, J ) 10 Hz, OCH), 4.95 (m, 1H, NCH), 5.13 (m, 1H, OCH),
5.24 (m, 2H, OCH), 6.02 (s, 6H, C6H6), 7.65 (s, 2H, H2O), 8.06
(m, 3H, C6H4), 8.31 (m, 1H, C6H4). MS (FAB+): m/z 716, [M -
H2O + SbF6]+.
1
PF6 salt). H NMR (250 MHz): δ 0.75 and 0.99 (2 × t, 3H, J
[Ru (OH2)(iP r -ben box)(m es)][SbF 6]2 (12). Complex 12
was prepared from 7 (100 mg, 0.126 mmol) and AgSbF6 (44
mg, 0.128 mmol) in 117 mg yield (92%). Anal. Calcd for
) 7 Hz, CH2Me), 1.56 (m, 2H, CH2Me), 1.84 (m, 1H, CH2Me),
1.96 (s, 9H, C6Me3), 2.40 (m, 1H, CH2Me), 4.25 (t, 1H, J ) 9
Hz, OCH), 4.43 (m, 1H, NCH), 4.55 (s, 3H, C6H3Me3), 4.58 (m,
2H, OCH), 4.60 (m, 1H, OCH), 4.73 (m, 1H, NCH), 7.82 (m,
3H, C6H4), 7.94 (m, 1H, C6H4). MS (FAB+): m/z 529, [M]+.
[Ru Cl(iP r -ben box)(m es)][SbF 6] (7). Complex 7 was pre-
pared from [RuCl2(mes)]2 (80 mg, 0.137 mmol), iPr-benbox (90
mg, 0.30 mmol), and NaSbF6 (72 mg, 0.28 mmol) in 196 mg
yield (90%). Anal. Calcd for C27H36ClF6N2O2RuSb: C, 40.90;
C
27H38F12N2O3RuSb2‚Me2CO: C, 33.70; H, 4.15; N, 2.62.
Found: C, 33.75; H, 4.40; N, 2.61. 1H NMR (d6-acetone): δ
0.60, 0.93, 1.05, and 1.35 (4 × d, 3H, J ) 7 Hz, CHMe2), 2.1
(m, 1H, CHMe2), 2.14 (s, 9H, C6Me3), 2.75 (m, 1H, CHMe2),
4.23 (ddd, 1H, J ) 10, 5, 3 Hz, NCH), 4.65 (t, 1H, J ) 10 Hz,
OCH), 4.83 (m, 1H, NCH), 5.08 (m, 3H, 3 × OCH), 5.30 (s,
3H, C6H3Me3), 7.23 (s, 2H, H2O), 8.12 (m, 2H, C6H4), 8.18 (m,
1H, C6H4), 8.43 (m, 1H, C6H4). MS (FAB+): m/z 758 [M - H2O
+ SbF6]+.
1
H, 4.58; N, 3.53. Found: C, 40.76; H, 4.51; N, 3.46. H NMR
(400 MHz): δ 0.47, 0.86, 0.97, and 1.13 (4 × d, 3H, J ) 7 Hz,
CHMe2), 1.95 (s, 9H, C6Me3), 2.42 (m, 1H, CHMe2), 2.75 (m,
1H, CHMe2), 4.25 (m, 1H, NCH), 4.34 (dd, 1H, J ) 9, 4 Hz,
OCH), 4.68 (m, 3H, 2 OCH, NCH), 4.77 (s, 3H, C6H3Me3), 4.84
Ca t a lysis R ea ct ion s. Sch len k R ea ct ion s (u n d er N2).
The catalyst was prepared in situ from the chloride complex
and 1 equiv of AgSbF6 in CH2Cl2; the solution was filtered

3034 Organometallics, Vol. 20, No. 14, 2001
Davies et al.
through Celite into a Schlenk tube to remove AgCl. Alterna-
tively, the isolated water-coordinated complexes could be used.
Methacrolein (1 mmol) and 2,6-di-tert-butylpyridine (1 equiv/
mol of catalyst)24 were added to the catalyst solution (0.2 or
0.5 mmol) in CH2Cl2 (2 cm3). The resulting yellow solution was
equilibrated at the appropriate temperature before addition
of cyclopentadiene (2 mmol). At the end of the reactions, the
mixture was passed through a plug of silica, the solvent was
removed, and the product was obtained as a colorless oil. The
(Rint ) 0.0110), R1 ) 0.0535 (for data I > 2σ(I)) and wR2 )
0.1502 (for all data), for 442 parameters, GOF ) 1.115, Flack
parameter 0.02(7).
Data for both 4 and 10 were collected on a Siemens P4
diffractometer using graphite-monochromated Mo KR radia-
tion; λ ) 0.7107 Å, The data were corrected for Lorentz and
polarization effects, and semiempirical absorption corrections
based on ψ scans were applied. The structures were solved by
Patterson methods and refined by full-matrix least squares
on F2 using the program SHELXTL-PC.25 All hydrogen atoms
bonded to carbon were included in calculated positions (C-H
) 0.96 Å) using a riding model. The hydrogen atoms of the
water molecule in 10 were not located and therefore are not
included in refinement cycles. All non-hydrogen atoms were
refined with anisotropic displacement parameters. High re-
sidual peaks (2.35 and -2.14 e Å-3) in the final difference
Fourier maps of 10 were located on the line x, 0.25, 0.75, which
also passes through the Ru atom. We have no explanation for
these high residual peaks; however, the data set is satisfactory
in all other respects.
1
exo:endo ratio was determined by H NMR spectroscopy, and
the enantiomeric excess was determined by 1H NMR or GC
after conversion to the acetal with (2R,4R)-pentanediol.20
NMR Tu be Exp er im en ts (in Air ). Methacrolein (0.25
mmol) was added to a suspension of catalyst (5 µmol) in CD2-
Cl2 (0.5 cm3), which led to rapid dissolution of catalyst to give
a yellow solution. The solution was transferred to an NMR
tube, and 2,6-di-tert-butylpyridine (1 equiv/mol of catalyst) and
cyclopentadiene (0.5 mmol) were added. The 1H NMR spectrum
was run immediately and then repeated after suitable time
intervals. The exo:endo ratio and enantiomeric excess were
determined as described above.
X-r a y Str u ctu r e Deter m in a tion s. Crystal data for 4:
Ack n ow led gm en t. We thank the University of
Leicester for a studentship (S.A.G.), J ohnson Matthey
for the loan of RuCl3, and NSC Technologies for a gift
of phenylglycinol.
orange block, crystal size 0.36 × 0.32 × 0.26 mm, C26H34
-
ClF6N2OPRu, Mr ) 688.04, orthorhombic, space group P212121,
a ) 11.569(1) Å, b ) 13.546(2) Å, c ) 18.480(2) Å, V ) 2896.1-
(6) Å3, Z ) 4, Dc ) 1.578 g cm-3, T ) 293(2) K, µ ) 0.754 mm-1
,
F(000) ) 1400, 3640 reflections collected with 5 < θ < 25°,
3448 unique (Rint ) 0.0285), R1 ) 0.0527 (for data I > 2σ(I))
and wR2 ) 0.2416 (for all data), for 352 parameters, GOF )
1.135, Flack parameter -0.12(10).
Crystal data for 10: orange block, crystal size 0.54 × 0.23
× 0.21 mm, C24H40F12N2O3RuSb2‚C4H10O, Mr ) 1051.27,
orthorhombic, space group P212121, a ) 13.0350(13) Å, b )
16.5521(13) Å, c ) 17.945(2) Å, V ) 3871.7(6) Å3, Z ) 4, Dc )
1.804 g cm-3, T ) 190(2) K, µ ) 1.862 mm-1, F(000) ) 2072,
4500 reflections collected with 2.6 < θ < 25.0°, 4344 unique
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates, bond lengths and angles, and thermal parameters.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM010092G
(25) Sheldrick, G. M. SHELXTL-PC; Siemens Analytical X-ray
Instruments, Madison, WI, 1990.
(26) Note added in proof: A higher enantioselectivity with an
(arene)ruthenium catalyst has recently been reported. Faller, J . W.;
Grimmond, B. J .; D’Alliessi, D. G. J . Am. Chem. Soc. 2001, 123, 2525.
(24) Bonnesen, P. V.; Puckett, C. L.; Honeychuck, R. V.; Hersh, W.
H. J . Am. Chem. Soc. 1989, 111, 6070.