Resolution and Diels - Alder catalysis with planar chiral arene-tethered ruthenium complexes

Article abstract of DOI:10.1021/om0501226

Planar chiral arene-tethered ruthenium complexes were applied to the Diels-Alder reaction of methacrolein and cyclopentadiene, with enantiomeric excesses up to 70%. The influence of a chiral phosphoramidite ligand on the catalytic selectivity was examined, along with counterion effects. The potential of asymmetric activation using the mixture of diastereomers formed from a racemic tethered complex and an enantiopure phosphine directly in the catalysis was investigated.

Full text of DOI:10.1021/om0501226

4132
Organometallics 2005, 24, 4132-4138
Resolution and Diels-Alder Catalysis with Planar Chiral
Arene-Tethered Ruthenium Complexes
J. W. Faller* and Philip P. Fontaine
Department of Chemistry, Yale University, New Haven, Connecticut 06520
Received February 20, 2005
Planar chiral arene-tethered ruthenium complexes were applied to the Diels-Alder
reaction of methacrolein and cyclopentadiene, with enantiomeric excesses up to 70%. The
influence of a chiral phosphoramidite ligand on the catalytic selectivity was examined, along
with counterion effects. The potential of asymmetric activation using the mixture of
diastereomers formed from a racemic tethered complex and an enantiopure phosphine directly
in the catalysis was investigated.
Introduction
Chiral pseudo-tetrahedral transition metal half-
to a pendant donor group. There have been many
cyclopentadienyl-derived versions of these compounds
17-20
reported;
the arene analogues, however, are some-
sandwich complexes have been used as asymmetric
21-37
what less common.
The use of tethered half-
Lewis acid catalysts in Diels-Alder and Mukaiyama
sandwich compounds in asymmetric catalysis has been
1
-8
reactions
and in hydrogen transfer reactions for the
38-40
limited to only a few cases so far.
9
-11
reduction of ketones and aldehydes.
compounds of the type [(η -CnHn)ML L L ] are chiral-
Half-sandwich
Recently, we reported the synthesis of a tethered
n
1
2 3
6
1
ruthenium half-sandwich complex, [Ru(η :η -2-dicyclo-
hexylphosphino-2′-(N,N-dimethylamino)biphenyl)Cl2], 1
at-metal, and pure enantiomers have been resolved in
1
2-15
several cases.
A potential problem when such
complexes are used as Lewis acid catalysts is that the
metal center of the active catalyst can racemize. This
is a process that can potentially have a detrimental
effect on the enantioselectivity of the catalyses, as the
computed and experimentally determined inversion
barriers for the 16-electron Lewis acids are less than
(
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1 16
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(
23) Bennett, M. A.; Edwards, A. J.; Harper, J. R.; Khimyak, T.;
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ligands, which are a class of mixed donor ligands in
which an arene or cyclopentadienyl (Cp) ring is linked
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Gilardoni, F.; Weber, J. Organometallics 1998, 17, 330.
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118, 4984.
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Bruneau, C.; Dixneuf, P. H. New J. Chem. 2001, 25, 519.
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R.; Bruneau, C.; Touchard, D.; Dixneuf, P. H. Chem. Eur. J. 2000, 6,
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9) Carmona, D.; Lamata, M. P.; Oro, L. A. Eur. J. Inorg. Chem.
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6, 7931.
2
(
6
(
11) Petra, D. G. I.; Reek, J. N. H.; Handgraaf, J. W.; Meijer, E. J.;
Dierkes, P.; Kamer, P. C. J.; Brussee, J.; Schoemaker, H. E.; van
Leeuwen, P. Chem. Eur. J. 2000, 6, 2818.
(12) Faller, J. W.; Mazzieri, M. R.; Nguyen, J. T.; Parr, J.; Tokunaga,
M. Pure Appl. Chem. 1994, 66, 1463.
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1
0.1021/om0501226 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 07/09/2005

Planar Chiral Arene-Tethered Ru Complexes
Organometallics, Vol. 24, No. 17, 2005 4133
Figure 2. BINAM-based resolution.
Figure 1. anti-Selective sythesis of racemic 2a-d from
racemic 1.
(
Figure 1),41 which forms as a pair of planar chiral
enantiomers. The tethered binding mode effectively
locks the ligand chirality by halting the atropisomeric
interconversion of the biphenyl moiety. Abstraction of
a chloride ion from 1 with AgSbF6 and treatment with
phosphine (L) resulted in the highly selective formation
6
1
of the chiral-at-metal complex [anti-Ru(η :η -2-dicyclo-
hexylphosphino-2′-(N,N-dimethylamino)biphenyl)(L)Cl]-
SbF6, 2a-d (Figure 1), as a pair of enantiomers. The
high anti selectivity was attributed to the blocking effect
of the NMe2 group. It was shown that the anti isomer
was the favored product both kinetically and thermo-
dynamically. The NMe2 group therefore can control the
chirality of the metal center. This being the case, the
dicationic complexes derived from enantiopure 2a-d,
formed by chloride abstraction with AgSbF6, were used
as Lewis acid catalysts for the Diels-Alder reaction of
methacrolein and cyclopentadiene. To improve the
selectivity and investigate a possible chiral poisoning/
asymmetric activation approach to this system, the
chiral phosphoramidite (S)-MonoPhos was also inves-
tigated as a ligand.
Figure 3. ORTEP drawing of the dication in (R,aR)-3a.
phine can readily displace the PPh3 ligand. It was in
this manner, for example, that single enantiomers of
2
b and 2c could be obtained.
A more convenient resolution of the neutral complex
was developed utilizing the ligand (R)-BINAM. Ab-
1
straction of both chlorides with AgSbF6 and treatment
with this chiral ligand resulted in the formation of two
diastereomeric complexes, 3a and 3b, which were
separable by fractional crystallization. Specifically,
complex 3a was found to be more insoluble and could
be obtained as a single diastereomer in up to 88% yield,
leaving a supernatant that was enriched in 3b (Figure
Results and Discussion
Resolution and Catalysis with 2a-d. It was
observed that 2a and 2d underwent spontaneous reso-
lution upon crystallization and could be obtained in
enantiomerically pure form via mechanical separation
of the crystals. Samples of 2a that were suitable for use
in catalytic reactions were collected (the enantiopurity
was verified with the specific rotation of 2a, which was
obtained from optical rotation measurements on a single
2
). Though 3b could not be isolated as a single diaste-
reomer, repeated crystallizations from the supernatant
solutions resulted in a product in up to 80% de. The
absolute stereochemistry of (R,aR)-3a was confirmed by
X-ray crystallography (Figure 3). The chirality descrip-
tor references the planar chirality in the ring defined
by the dimethylamino-substituted carbon atom in the
ring and the axial chirality (aR) of the BINAM ligand.
Once separated, 3a could be easily converted to
enantiopure 1 by stirring in a solution of HCl(aq) and
CH2Cl2. The ability to obtain these complexes in enan-
tiopure form allowed investigation of their potential as
precursors for asymmetric Lewis acid catalysts of the
4
2
crystal of the complex ). The spontaneous resolution
of 2a provides an effective starting point for the
development of other catalysts, as treatment of enan-
tiopure 2a with an excess (>5 equiv) of another phos-
(
41) Faller, J. W.; D’Alliessi, D. G. Organometallics 2003, 22, 2749.
42) For 2a: [R] (c ) 0.00175, CH Cl ): 143°.
(
D
2
2

4134 Organometallics, Vol. 24, No. 17, 2005
Faller and Fontaine
Table 1. Diels-Alder Catalysis with Dications
a
Derived from 2a-d
entry
ligand (L)
exo/endo
ee (%)
1b
2
PPh3
98/2
96/4
97/3
94/6
20
26
13
40
PPhMe2
P(OPh)3
PPh2Cy
3
Figure 4. Two diastereomers, (SRu,S,aS)-4a and
Ru
Phos.
4
(R ,R,aS)-4b, resulting from coordination of (S)-Mono-
a
All reactions were carried out at 10% catalyst loading and at
-
25 °C. Greater than 95% conversions were observed in all cases.
b
This result was initially reported in ref 41.
Diels-Alder reaction of methacrolein and cyclopenta-
diene.The most selective of these was derived from 2d,
which catalyzed the reaction with 89% de (exo) and 40%
ee. PCyPh2 is a comparatively large and donating
phosphine; however, bulkier phosphines (those with
cone angles larger than 157°) do not readily coordinate.
Although the selectivity is modest, these reactions
illustrate that the chiral-at-metal complexes are capable
of catalytic asymmetric induction.
Figure 5. Without the controlling effect of the NMe
2
group, (SRu,aS)-5a and (RRu,aS)-5b form in a 42:58 ratio.
Tethered Complexes with (S)-MonoPhos. The use
of the chiral phosphoramidite (S)-MonoPhos in place of
the achiral phosphines was also examined to evaluate
the improved enantioselectivity that would be expected
from a matched pair wherein the ligand chirality would
enhance that of the metal-centered chirality. Starting
from rac-1, the addition of 1 molar equiv each of (S)-
MonoPhos and AgSbF6 results in the formation of two
diastereomers in equal amounts (4a, 4b). Each diaste-
reomer can be obtained individually by starting from
resolved 1 or 2a.
We were unable to obtain crystals of either 4a or 4b
that were suitable for X-ray analysis; however, given
the propensity for other phosphines to bind in the anti
position, and the high diastereoselectivity observed in
this case (over 90% de for both complexes), it is likely
that (S)-MonoPhos also occupies the anti binding site.
Given this, and knowing the absolute configuration of
the planar chiral starting material, we can reasonably
assign absolute configurations for 4a and 4b.
Figure 6. 31P{ H} NMR in CD
1
of (A) 4a/4b, (B) 5a/5b
2
Cl
2
at room temperature, and (C) 5a/5b at 55 °C.
of the cyclohexyl groups (Figure 6). Each isomer con-
tains two inequivalent phosphorus atoms coupled through
ruthenium; one of the doublets is downfield (>140 ppm,
corresponding to the phosphoramidite), and the other
is in the region of 50-70 ppm. The spectra of 4a and
4
b individually show that in one case (4a) it is the
upfield resonance that is broad, while for the other
isomer it is the more downfield resonance that is broad.
The same can be said for 5, as ascertained from
integration of the resonances, as well as from the
coupling constants. In addition, heating 5 to 55 °C
sharpened the doublets, and it was apparent that the
resonances of the minor isomer 5a were sharpened to a
greater extent. The spectrum for the major isomer 5b
is similar to that of 4b, in that the broad resonance is
the one further downfield, which could suggest that the
metal chirality is the same for the two complexes.
To investigate the diastereomeric preference elicited
by (S)-MonoPhos in the absence of the NMe2 directing
6
1
group, [Ru(η :η -2-dicyclohexylphosphino-2′-biphenyl)-
(S)-MonoPhos)Cl](SbF6) was synthesized as two dia-
(
stereomers (5a, 5b). The ratio of the two diastereomers
was 58:42, and this ratio was unchanged in solution over
extended periods.
The absence of planar chirality in the tethering ligand
in this case means that it cannot influence the chirality
at the metal center. Therefore, in this case the diaste-
reoselectivity can be attributed solely to the binding
arrangement preferred by (S)-MonoPhos. The modest
diastereoselectivity exhibited by MonoPhos ensures that
The effect of the NMe group in complexes derived
from 1 is to direct the bulky phosphine ligand to the
2
anti binding site. Consequently, the chirality of the
tethered ligand controls the metal chirality to a large
extent (greater than 95% de for both isomers of 4).
However, in the absence of the directing group, the
chiral phosphoramidite (S)-MonoPhos also influences
the metal chirality, though to a lesser extent (16% de
for 5). Therefore, the chirality of the tethered ligand has
the greatest influence in determining the metal chiral-
6
the NMe2 group on the η arene ring in 4 will determine
the diasteromeric preference at the metal center.
3
1
1
An interesting feature in the P{ H} NMR spectra
that is common to each of the phosphoramidite-contain-
ing complexes (4, 5) is the appearance of one broad
resonance per diastereomer, indicative of some dynamic
behavior, perhaps the interconversion of conformations

Planar Chiral Arene-Tethered Ru Complexes
Organometallics, Vol. 24, No. 17, 2005 4135
Table 2. Diels-Alder Catalysis with Dications
a
Derived from 4a/4b
entry
catalyst precursor
exo/endo
ee (%) [config]
1
2
3
mix 4a,4bb
4b
4a
92/8
93/7
92/8
27 [R]
47 [R]
19 [R]
a
All reactions were carried out at 10% catalyst loading and at
25 °C. Greater than 95% conversion was observed in all cases.
-
b
A 1:1 mixture of both isomers.
ity, and so the anti preference is likely retained, even
in the case of 4a, where it is in opposition to the
preference of (S)-MonoPhos. In the case of 4b, the
controlling effects of both the tethered ligand and (S)-
MonoPhos on the metal chirality are in conjunction and
favor formation of the same diastereomer.
Catalysis with 4a/4b. The 1:1 mixture of both
isomers catalyzed the reaction with modest enantiose-
lectivity, indicating that the chiral phosphoramidite was
affecting the asymmetric induction. When each pure
diastereomer was used individually, it was observed
that both yielded an enantiomerically enriched product
with the same sense of chirality, although 4b was more
selective in doing so. It would appear that the selectivity
induced by MonoPhos overrides the induction by the
metal-centered chirality in 4a, whereas in 4b (the
matched diastereomer) the induction by the ligand and
metal center are acting in concert to produce a higher
ee.
Figure 7. Plot of conversion as a function of time for the
reaction of methacrolein and cyclopentadiene catalyzed by
6
1
[
Ru(η :η -2-dicyclohexylphosphino-2′-(N,N-dimethylamino)-
biphenyl)((S)-MonoPhos)](X)(Y). All reactions were done
with a 1:1 mixture of the two anti isomers at 5% catalyst
loading, with 0.025 mmol of catalyst in 10 mL of CH
at -25 °C. Aliquots of 0.25 mL, taken at the specified time
intervals, were quenched in precooled Et O and analyzed
by H NMR to determine conversion. (A) X ) Y ) SbF
B) X ) SbF , Y ) BF ; (C) X ) Y ) BF
2 2
Cl
2
1
6
;
(
6
4
4
.
when BF4- is used in place of SbF6 , which can be
-
-
attributed to the more coordinating nature of BF4 . The
variation in the reaction rate with the different anion
combinations is potentially related to the differences in
selectivity. In contrast to the previous examples, how-
ever, in this case the catalytic selectivity was greater
If each diastereomer in the 1:1 mixture of diastereo-
mers catalyzed the reaction at the same rate, then the
ee’s found for pure 4a and 4b would suggest an ee of
∼
33% for the mixture. Since the mixture gives an ee of
7%, the reaction rate with 4a is substantially faster
∼3×) than that with 4b. Since in this case the matched
diastereomer is more slowly reacting, this is not a good
-
-
with BF4 than SbF6 . K u¨ ndig has postulated that
C-H‚‚‚F hydrogen bonding interactions between a
ruthenium Lewis acid and various counterions were
responsible for the observed differences in turnover
2
(
4
3
case for asymmetric activation by forming the 1:1
mixture by adding the chiral phosphine to the racemic
precursor. If, on the other hand, the matched diastere-
omer reacted much faster than the mismatched dias-
tereomer, then the mixture would yield a high ee and
could obviate the need to separate the diastereomers.
Thus, one could use the asymmetric activation ap-
8
frequency. This type of hydrogen bonding, which is
sensitive to the nature of the counterion, could also
potentially be related to the differences in selectivity
observed in our case. As an alternative explanation,
recent PGSE-NMR measurements have shown that the
-
BF4 anion forms relatively strong ion pairs with
45
-
ruthenium Lewis acids, suggesting that the BF4
4
3
proach successfully if a chiral ligand was developed
in which the matched diastereomer reacted faster.
Counterion Effects. The counterion was also seen
to play a role in the enantioselectivity of the catalysis.
Different counterions could be introduced by using
different silver salts when abstracting the chloride
anion could more effectively affect the approach of the
diene. This could account for the observed slowing of
the reaction rate, as well as the difference in selectivity,
if, for example, the anion were slowing the rates of
various diastereomeric intermediates to different ex-
tents.
Epimerization of Metal Chirality. The potential
of epimerization at the various centers of chirality that
exist in these catalysts needs to be considered if one
wishes to understand the selectivity. The tethered
binding mode effectively halts the atropisomeric inter-
conversion of the ligand; therefore the resulting planar
chirality is fixed upon the initial formation of 1. Inver-
sion of the planar chirality has not been observed in any
of the complexes 1-4 during any of the manipulations
performed in this study.
-
ligands. The optimal anion combination was SbF6 /
-
BF4 , for which, when implemented along with the
matched isomer, an ee of 70% was attained. The same
catalyst at lower loading and at room temperature was
less selective (59% ee). The use of triflate as a counterion
at -25 °C slowed the reaction rate to such an extent
that less than 5% conversion took place after 18 h. At
room temperature, the reaction proceeded sluggishly
and with modest selectivity.
To examine the possible cause for the observed
counterion dependence, the catalyzed reaction rates
were monitored with the different “noncoordinating”
anions (Figure 7). In accordance with what has been
The cationic phosphine-containing complexes, such as
a-d and 4a-b, have varying degrees of lability in
2
8
,44
previously observed,
the reaction rates are slowed
(
44) Evans, D. A.; Murry, J. A.; von Matt, P.; Norcross, R. D.; Miller,
S. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 798.
(43) Faller, J. W.; Lavoie, A. R.; Parr, J. Chem. Rev. 2003, 103, 3345.
(45) Kumar, P. G. A.; Pregosin, P. S. Organometallics 2004, 23, 5410.

4136 Organometallics, Vol. 24, No. 17, 2005
Faller and Fontaine
Table 3. Diels-Alder Catalysis with
6
1
[
anti-Ru(η :η -2-dicyclohexylphosphino-2′-(N,N-dimethylamino)biphenyl)((S)-MonoPhos)](A)(B)
entry
catalyst precursor
anion (A)
anion (B)
temp (°C)
% loading
exo/endo
ee (%) [config]
-
-
1
2
3
4
5
6
7
mix 4a,4b
mix 4a,4b
mix 4a,4b
4b
SbF6
SbF6
-25
-25
-25
-25
-25
25
10
10
10
10
10
2
92/8
93/7
93/7
93/7
93/7
90/10
89/11
27 [R]
44 [R]
46 [R]
65 [R]
70 [R]
59 [R]
27 [R]
-
-
BF4
BF4
-
-
SbF6
BF4
-
-
BF4
BF4
-
-
4b
SbF6
BF4
-
-
4b
mix 4a,4b
SbF6
BF4
-
-
SbF6
CF3SO3
25
10
solution depending on the nature of the phosphine.41
The complexes with larger phosphines, such as 2a, are
relatively labile in solution. This was evidenced by the
generation of a nonequilibrium mixture of anti/syn
isomers by extraction of the chloride with AgSbF6 and
subsequent attack by Cl . It was observed that equi-
librium for 2a was reestablished in solution (t1/2 ≈ 25
h), while no equilibration was observed for 2b, which
has a smaller and more donating phosphine ligand.41
Further evidence of the lability of 2a is the ability to
exchange PPh3 with other phosphines (e.g., MonoPhos,
P(OPh)3) by heating a CH2Cl2 solution under reflux with
an excess of the other phosphine. These reactions yield
anti isomers as the product presumably owing to the
steric effect of the NMe2 group. It would appear that
more weakly bound ligands allow equilibration to the
more stable anti isomer at faster rates.
4a could account for the lower selectivity for this
catalyst with respect to that derived from 4b.
Conclusion
-
41
We have demonstrated that arene-tethered ligands
can provide an effective means for controlling metal-
centered chirality. This approach can be extended to the
design of asymmetric catalysts; the tethered complexes
reported herein were used to catalyze the Diels-Alder
reaction between methacrolein and cyclopentadiene,
giving products with high exo selectivity and with ee’s
up to 70%.
Experimental Section
All manipulations were carried out under a nitrogen atmo-
sphere using standard Schlenk techniques. All solvents were
distilled under nitrogen using standard desiccating agents.
The active catalyst for the Diels-Alder reaction is a
dicationic species generated by extraction of the chloride
ligand with a silver salt and then subsequent addition
AgSbF
6
, AgBF
PCy , C
Ph, P(iPr)Ph , methacrolein, dicyclopentadiene, and (+)-Eu-
hfc) (Aldrich) were used as received. The preparation of
4
2
2
-
2
C H C
6 4 6
H
4
2
31
of an excess (10 equiv) of methacrolein. P NMR studies
were performed on the species formed from the treat-
ment of either 4a or 4b with AgSbF6 and methacrolein.
In the case of 4a, two isomers in a ratio of 89:11 are
formed, and this ratio did not change over time.
Likewise, for 4b, two isomers were again observed, this
time in a ratio of 98:2, and once again this ratio
remained constant over time. Lowering the temperature
to -25 °C did not cause an observable change in either
of these ratios. To determine whether the ratios were
kinetically controlled or equilibrium values, THF was
added to the solution so as to change the polarity of the
solvent environment. Indeed, the ratios were changed
immediately (<5 min) upon the addition of THF; the
isomers corresponding to 4a now had a ratio of 68:32,
whereas in the case of 4b a new ratio of 95:5 was
observed. These ratios did not change over extended
periods of time in solution at room temperature, show-
ing that the change in polarity had perturbed the
equilibrium isomer ratios and that equilibration was
achieved in less than 5 min.
2
(
3
[Ru(benzene)Cl2], 1, 2a, and 2b was done according to
respective literature procedures. NMR spectra were recorded
on a Bruker 400 MHz (operating at 162 MHz for P), a Bruker
31
3
1
500 MHz (operating at 202 MHz for P), or a GE Omega 300
3
1
MHz (operating at 122 MHz for P) spectrometer. Chemical
shifts are reported in ppm relative to solvent peaks ( H) or an
1
H
3
PO
by Atlantic Microlabs.
General Procedure for Synthesis of [Ru(η :η -NMe
PCy )(L)(Cl)]SbF . Method A. A flame-dried flask
was charged with 1 molar equiv each of 1, AgSbF , and a
phosphine (L) under a stream of nitrogen. The flask was
evacuated and refilled with nitrogen, and then CH Cl (5 mL)
was added. The resulting mixture was stirred in the dark until
4
external standard. Elemental analyses were carried out
6
1
2
-
C
6
H
4
C H
6 4
2
6
6
2
2
3
1
1
the reaction was complete, as monitored by P{ H} NMR, at
which point it was filtered through Celite and dried under
vacuum. Crystals were obtained by slow diffusion of Et
a CH Cl solution.
Method B. A flame-dried flask was charged with 1 equiv
of 2a, 5 equiv of ligand (L), and CH Cl (5 mL) under a stream
2
O into
2
2
2
2
of nitrogen. The resulting solution was subjected to a freeze/
pump/thaw cycle and then was heated under reflux for 16 h.
The isomers are presumably a mixture of syn and anti
complexes with opposite chirality at the metal. The
equilibration would likely occur by dissociation of meth-
acrolein, generating a 16-electron species or one with a
weakly bound solvent ligand, followed by reassociation
of methacrolein. The observation of two isomers (for both
The volume of solvent was reduced to 1 mL, and Et
was added. A yellow solid formed, and the liquid was decanted
from it. An additional 5 mL of Et O was added, then decanted,
2
O (5 mL)
2
and the solid was dried under vacuum.
1
2c: 91% yield. H NMR (400 MHz, CDCl
3
): 7.81 (1H, d, J )
a and 4b) in the 3 P NMR spectrum indicates that the
1
6 4 6 4
7.2 Hz, C H P), 7.71-7.57 (3H, m, C H P), 7.38 (6H, dd, J )
.0 Hz, 7.6 Hz, POC
4
8
6
H
5
), 7.27 (3H, t, J ) 7.6 Hz, POC
6
H
5
),
interconversion is slow on the NMR time scale, and so
a barrier greater than 15 kcal/mol is expected for this
process. However, the equilibrium is reached quickly,
and so the slower catalyses reported herein were
performed where equilibration of isomers occurs rapidly
relative to the Diels-Alder reaction. The higher amount
of minor isomer in the case of the dication derived from
7
.19 (6H, d, J ) 8.0 Hz, POC
6
H
5
), 5.34 (1H, d, J ) 8.0 Hz,
6
6
6
η -C
6
H
4
N), 5.23 (1H, m, η -C
6
H
H
4
N), 5.04 (1H, m, η -C
6
H
4
N),
6
6
4
(
.89 (1H, t, J ) 6.0 Hz, η -C
6
4
N), 2.941 (3H, s, η -C
6
H
4
N-
6
3
CH )
2
), 2.936 (3H, s, η -C
6
H
4
N(CH ), 2.85 (1H, m, C
3
)
2
6
H
11),
(
46) Bennett, M. A.; Smith, A. K. J. Chem. Soc., Dalton Trans. 1974,
2, 233.

Planar Chiral Arene-Tethered Ru Complexes
Organometallics, Vol. 24, No. 17, 2005 4137
2
.57 (1H, m, C
.76-1.15 (13H, m, C
6
H
11), 2.15 (2H, m, C
6
H
11), 1.92 (4H, m, C
6
H
11),
7.59-7.19 (10H, m), (C
6
H
4
P and C20
12 2
H O P), 6.07 (1H, dd, J
3
1
6
1
6
H
11), 1.10 (1H, m, C
6
H
11). P NMR
) 6.8 Hz, 6.4 Hz, η -C H N), 5.89 (1H, d, J ) 8.8 Hz,
6 4
2
2
6
6
(
161.914 MHz, CDCl
3
): 123.8 (d, JPP ) 66 Hz), 67.6 (d, JPP
C
20
H
12
O
2
P), 5.78 (1H, m, η -C
6
H
4
N), 5.57 (1H, m, η -C
6
H
4
N),
N-
3 2
JPH ) 10.0 Hz, PN(CH ) ), 2.46-0.83
6
6
)
66 Hz). Anal. Calcd for C44
H
51ClF
6
NO
3
P
2
RuSb: C, 49.11;
6 4 6 4
5.15 (1H, d, J ) 6.8 Hz, η -C H N), 2.88 (6H, s, η -C H
(CH
(20H, m, C
3
H, 4.78; N, 1.30. Found: C, 49.16; H, 4.73; N, 1.39.
3
)
2
), 2.59 (6H, d,
11), 0.75 (1H, m, C
1
31
2
d: 94% yield. H NMR (400 MHz, CDCl
3
): 7.90-7.81 (2H,
6
H
6
H
11), 0.48 (1H, m, C
6
H
11).
P
2
m), 7.73 (1H, m), 7.70-7.52 (8H, m), 7.49-7.35 (3H, m), (C
and CyP(C
6
H
4
P
NMR (161.914 MHz, CD
2
Cl
2
): 154.0 (d, JPP ) 65 Hz), 51.6
6
2
6
H
5
)
2
), 5.92 (1H, m, η -C
6
H
4
N), 5.81 (1H, d, J ) 6.8
(d, JPP ) 65 Hz).
6
6
6
1
Hz, η -C
.07 (6H, s, η -C
1H, m), 2.15-0.96 (28H, m), 0.83 (1H, m), 0.69 (1H, m), (PPh
6
H
4
N), 4.83 (1H, m, η -C
6 4
H N), 3.55 (1H, m, η -C
6
H
4
N),
4b. H NMR (400 MHz, CD
2
Cl
2
): 8.07 (1H, d, J ) 8.8 Hz),
6
3
6
H
4
N(CH
)
3 2
), 2.76 (1H, m), 2.64 (1H, m), 2.49
8.02 (1H, d, J ) 8.4 Hz), 8.01 (2H, m), 7.81-7.20 (13H, m),
6
(
(
(
2
-
(C
6
H
4
P and C20
H
12
O
2
P), 6.23 (1H, t, J ) 6.0 Hz, η -C
6
H
H
4
N),
N),
3
1
6
6
C
6
H
11) and P(C
6
H
)
11 2
). P NMR (121.650 MHz, CDCl
3
): 51.1
d, JPP ) 38 Hz), 27.3 (d, JPP ) 38 Hz). Anal. Calcd for C44
NP RuSb: C, 51.10; H, 5.56; N, 1.35. Found: C, 51.09;
H, 5.55; N, 1.35.
Synthesis of Ru(η :η -NMe
NAM)](SbF . A flame-dried flask was charged with 1 (164.2
mg, 0.29 mmol), AgSbF (200 mg, 0.58 mmol), (R)-BINAM (84.5
mg, 0.30 mmol), and 5 mL of CH Cl under a stream of
5.67 (2H, m, η -C
2.983 (3H, s, η -C
6
H
4
N), 4.98 (1H, d, J ) 4.8 Hz, η -C
6
4
2
2
6
6
H
57
-
6
H
4
N(CH N(CH
3
)), 2.979 (3H, s, η -C
6
H
4
3
)),
3
ClF
6
2
2.40 (6H, d,
11), 2.23-1.09 (16H, m, C
P NMR (161.914 MHz, CD Cl
(br d).
Synthesis of [Ru(η :η -C
(Cl)]SbF (5). flame-dried flask was charged with
PCy )Cl (72.5 mg, 0.13 mmol), (S)-Mono-
Phos (50 mg, 0.14 mmol), and AgSbF (48 mg, 0.14 mmol)
under a stream of nitrogen. The flask was evacuated and
refilled with nitrogen, and the CH Cl (5 mL) was added. The
J
PH ) 10.0 Hz, PN(CH ) ), 2.75-2.46 (4H, m,
3 2
C
6
H
6
H
11), 0.96-0.83 (2H, m, C
6
H
11).
6
1
2
31
2
C
2 6
H
4 6
C H
4
PCy
2
)(η -(R)-BI-
2
2
): 154.8 (d, JPP ) 62 Hz), 58.5
6 2
)
6
1
6
6 4 6 4 2
H C H PCy )((S)-MonoPhos)-
2
2
6
A
H
6
1
nitrogen. The resulting solution was subjected to a freeze/
pump/thaw cycle and was then stirred at room temperature
for 1 h. After filtration through Celite, the product was dried
under vacuum, resulting in a red powder (344 mg, 95%). Anal.
Ru(η :η -C
6
H
4
C
6
4
2
2
6
2
2
Calcd for C46
H
52
F
6
N
3
PRuSb: C, 44.45; H, 4.19; N, 3.36.
resulting mixture was stirred in the dark for 6 h, at which
point it was filtered through Celite. The volume of solvent was
Found: C, 44.44; H, 4.58; N, 3.17. Crystallization was done
under a nitrogen atmosphere in a Schlenk tube, by slow
diffusion of an Et O layer (9 mL) into a solution of the product
2
reduced to 1 mL, and Et
formed, and the supernatant was decanted from it. An ad-
ditional 5 mL of Et O was added, then decanted. The solid
2
O (5 mL) was added. A yellow solid
in 9 mL of CH
2
Cl
2
.
2
1
3
a. H NMR (500 MHz, CDCl
3
): 8.30 (d, 1H, J ) 8.5 Hz),
2 2
was purified on silica gel eluting with CH Cl and dried under
vacuum (105 mg, 76%).
8
7
.04 (d, 1H, J ) 8.5 Hz), 7.95 (dd, 1H, J ) 6.5, 6.0 Hz), 7.25-
.77 (m, 11H), 7.05 (d, 1H, J ) 8.0 Hz), 6.82 (d, 1H, J ) 8.5
1
Minor Isomer (5a). H NMR (400 MHz, CDCl
3
): 8.26 (1H,
P and C20 P),
), 6.30 (1H, dd, J ) 6.8 Hz, 6.4 Hz, η -
Hz), (C20
H
12
N
2
H
4
and C
6
H
4
P); 6.62 (br d, 1H, J ) 10 Hz, NH),
d, J ) 9.2 Hz), 8.15-7.09 (15H, m), (C
6
H
4
12 2
H O
6
6
6
6
.32 (d, 1H, J ) 6.5 Hz, η -C
6
H
4
N), 6.12 (dd, 1H, J ) 5.0, 5.5
N), 5.96 (br d, 1H, J ) 10 Hz, NH), 5.75 (m, 2H,
6.66 (1H, m, η -C
6
H
5
6
6
6
Hz, η -C
6
H
4
C
6
H
5
), 6.08 (2H, m, η -C
6
H
5
), 5.40 (1H, d, J ) 5.6 Hz, η -C
6 5
H ),
6
6
3
NH and η -C
6
H
4
N), 5.15 (d, 1H, J ) 5.5 Hz, η -C
br d, 1H, J ) 10 Hz, NH), 2.92 (s, 6H, N(CH ), 0.29-2.54
11). P NMR (161.9 MHz,
(c 0.00169, CH Cl ): -192°.
): 8.18 (d, 1H, J ) 8.8 Hz),
6
H
4
N), 4.86
2.52 (6H, d,
H
J
PH ) 11.2 Hz, N(CH ), 2.83-0.58 (22H, m,
11). P NMR (161.914 MHz, CDCl
Hz), 65.6 (d, JPP ) 63 Hz).
3 2
)
3
1
2
(
(
3
)
2
C
6
3
): 146.1 (d, JPP ) 63
31
2
m, 21H, C
6
H11), -0.45 (br s, 1H, C
6
H
1
CDCl
3
): 58.4 (s). [R]
b. H NMR (400 MHz, CDCl
D
2
2
Major Isomer (5b). H NMR (400 MHz, CDCl ): 8.15-
3
1
3
3
7.09 (16H, m, C
6
H
4
P and C20
), 6.00 (1H, d, J ) 6.0 Hz, η -C
12 2
H O P), 6.34 (1H, dd, J ) 6.0
6
6
8
.11 (d, 1H, J ) 8.4 Hz), 7.98 (t, 1H, J ) 7.6 Hz), 7.90 (d, 1H,
Hz, 6.4 Hz, η -C
6
H
5
6
H
5
), 5.93
6
6
J ) 8.4 Hz), 7.25-7.77 (m, 10H), 6.96 (d, 1H, J ) 8.4 Hz),
6
2
(1H, br s, η -C H ), 5.64 (1H, br s, η -C H ), 5.47 (1H, d, J )
6 5 6 5
6
3
.82 (d, 1H, J ) 8.4 Hz), (C20
H
12
N
2
H
4
and C
6
H
4
P); 6.39 (m,
5.2 Hz, η -C
6
H
5
), 2.49 (6H, d, JPH ) 10.4 Hz, N(CH
3
)
2
), 2.83-
): 143.3
6
31
H, NH and η -C
6
H
4
N), 6.24 (br d, 1H, J ) 10 Hz, NH), 6.19
0.58 (22H, m, C
6
H
11). P NMR (161.914 MHz, CH
2
Cl
2
6
2
(
br d, 1H, J ) 11 Hz, NH), 5.78 (d, 1H, J ) 4.8 Hz, η -C
6
H
H
4
N),
N),
(d, JPP ) 69 Hz), 68.2 (br d).
6
6
5
4
2
.72 (m, 1H, η -C
6
H
4
N), 5.06 (d, 1H, J ) 6.4 Hz, η -C
6
4
General Procedure for Diels-Alder Catalysis. A flame-
dried flask was charged with 0.025 mmol of the cationic
ruthenium precatalyst and 0.023 mmol of AgX (X ) SbF , BF ,
.22 (br d, 1H, J ) 10 Hz, NH), 2.68 (br s, 6H, N(CH
3 2
) ), 0.29-
3
1
.54 (m, 21H, C
): 56.3 (s).
6
H11), -0.94 (br s, 1H, C H11). P NMR (161.9
6
6
4
MHz, CDCl
3
2 2
OTf) under a stream of nitrogen. CH Cl (3 mL) was added,
Conversion of 3a into (-)-1. A flask was charged with
and the mixture was stirred for 30 min, followed by filtration
through Celite to remove the precipitated AgCl. The filtrate
was added to a fresh flame-dried flask and was then subjected
to a freeze/pump/thaw cycle, and the flask was backfilled with
nitrogen. Methacrolein (20 uL, 0.25 mmol) was added at this
point, and the flask was then transferred to a freezer, where
it was allowed to cool to -25 °C for 1 h before the addition of
Cp (0.2 mL). The reactions were allowed to proceed at this
temperature for 16 h, at which point the volatiles were
removed under vacuum, and the resulting residue was passed
6
% HCl(aq) (10 mL), and to this was added a solution of 3a (78
2 2
mg in 5 mL of CH Cl ). The resulting mixture was stirred
vigorously for 30 min. The organic layer was separated and
washed with an additional 10 mL of 6% HCl(aq) and then
collected and dried under vacuum, resulting in quantitative
recovery of (-)-1. [R]
Synthesis of [Ru(η :η -NMe
Phos)(Cl)]SbF . A flame-dried flask was charged with 1 (75.7
mg, 0.13 mmol), AgSbF (46 mg, 0.13 mmol), and (S)-Mono-
Phos (49 mg, 0.14 mmol) under a stream of nitrogen. The flask
was evacuated and refilled with nitrogen, and then CH Cl (5
D
(c 0.00195, CH
2
Cl
2
): -986°.
6
1
C H
2 6 4
6
C H
4
PCy )((S)-Mono-
2
6
6
2
through a short plug of silica gel in Et O in order to remove
2
2
the inorganic material. Conversion and diastereoselectivity
1
mL) was added. The resulting mixture was stirred in the dark
for 2 h, at which point it was filtered through Celite. The
were determined by H NMR, and the enantioselectivity was
3
determined with the use of (+)-Eu(hfc) as a chiral shift agent.
volume was reduced to 1 mL, Et
precipitate the yellow powder, and the liquid was decanted.
An additional 5 mL of Et O was added, then decanted, and
the solid was dried under vacuum (135 mg, 90%). Anal. Calcd
for C48 RuSb: C, 51.24; H, 4.84; N, 2.49.
2
O (5 mL) was added to
Epimerization Studies on Dicationic Species. A flame-
dried flask was charged with 4a or 4b (36 mg, 0.032 mmol)
2
and AgSbF
followed by CH
6
(11 mg, 0.032 mmol) under a stream of nitrogen,
Cl (1.25 mL). The mixture was stirred for 30
2
2
H
54ClF
6
N
2
O
2
P
2
min, at which point methacrolein (26 uL, 0.32 mmol) was
added by syringe, and the resulting solution was syringed into
Found: C, 51.22; H, 4.91; N, 2.44.
1
31
4
a. H NMR (400 MHz, CD
2
Cl
2
): 8.08 (1H, d, J ) 9.2 Hz),
a sealed NMR tube under nitrogen. The P NMR studies were
31
8
.01 (1H, d, J ) 8.0 Hz), 7.90 (1H, d, J ) 8.0 Hz), 7.77 (1H,
dd, J ) 6.8 Hz, 8.0 Hz), 7.68 (1H, dd, J ) 8.0 Hz, 7.2 Hz),
performed directly. Species derived from 4a: P NMR (161.914
2
2 2
MHz, CH Cl ): major isomer, 149.6 (d, JPP ) 61 Hz), 49.8 (d,

4138 Organometallics, Vol. 24, No. 17, 2005
Faller and Fontaine
2
2
PP ) 61 Hz), minor isomer, 151.2 (d, 2JPP ) 63 Hz), 49.1 (d,
J
J
Supporting Information Available: A listing of X-ray
31
PP ) 63 Hz); ratio 89:11. Species derived from 4b: P NMR
crystallographic data, atomic positions, thermal parameters,
2
13
(161.914 MHz, CH
2
Cl
2
): major isomer, 151.5 (d, JPP ) 57 Hz),
bond distance and angles for 3a. Also C spectra for selected
2
2
4
4
9.6 (d, JPP ) 57 Hz), minor isomer, 153.5 (d, JPP ) 60 Hz),
8.7 (d, JPP ) 60 Hz); ratio 98:2.
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
2
These equilibrium values could be perturbed to new ones
over a time < 10 min by adding THF (0.05 mL) by syringe to
the NMR tube: 4a 68:32; 4b 95:5.
OM0501226