Isomerization-induced asymmetric coordination chemistry: From auxiliary control to asymmetric catalysis

Article abstract of DOI:10.1002/anie.201003139

Asymmetric generator: Chiral (S)-2-(isopropylsulfinyl)phenol and a more electron-rich methoxy derivative (R=OCH₃) are capable of inducing and even catalyzing a chirality-generating trans-cis isomerization of two 2,2′-bipyridine ligands in an oc

Full text of DOI:10.1002/anie.201003139

Angewandte
Chemie
DOI: 10.1002/anie.201003139
Asymmetric Catalysis
Isomerization-Induced Asymmetric Coordination Chemistry: From
Auxiliary Control to Asymmetric Catalysis**
Lei Gong, Zhijie Lin, Klaus Harms, and Eric Meggers*
Dedicated to Professor Bernd Giese on the occasion of his 70th birthday
Methods for the synthesis of enantiopure chiral-at-metal
octahedral coordination compounds are strongly in demand
to fully exploit the structural opportunities of octahedral
coordination spheres in the areas of catalysis, materials
sciences, and life sciences. However, compared to the avail-
ability of highly sophisticated methods for the asymmetric
synthesis of organic molecules, the synthetic control of metal-
centered chirality is still in its infancy.[1] For example, to the
best of our knowledge, not a single report exists of the
catalytic asymmetric synthesis of a chiral octahedral metal
complex.
Herein we show that tailored chiral sulfoxide ligands are
capable of inducing a stereocontrolled ligand isomerization
within an octahedral coordination sphere, which can be
exploited for the auxiliary-mediated[2] and even the catalytic
asymmetric synthesis of chiral ruthenium complexes
(Scheme 1).
Scheme 2. Highly diastereoselective conversion of prochiral trans-[Ru-
(bpy)2(MeCN)2]2+ (1) into chiral cis-D-[Ru(bpy)2{(S)-SO}]+ (D-(R)-2).
According to the Cahn–Ingold–Prelog priority rules, the assignment of
the absolute stereochemistry at the sulfur changes upon coordination
from S to R.
to the cis complex in a bidentate fashion but not the trans
starting compound. Furthermore, if chiral, such a ligand might
lead to the asymmetric formation of predominately one
metal-centered stereoisomer (Scheme 1).
Surprisingly, although initial efforts with our previously
reported chiral salicyloxazoline auxiliary[4] were not success-
ful at all, we discovered that the reaction of the trans complex
1 with (S)-(isopropylsulfinyl)phenol, (S)-SO,[5] in DMF or
ethylene glycol in the presence of 10 equiv Et3N, led to the
completely diastereoselective formation of D-(R)-2 without
any detectable L-(R)-2 (Scheme 2 and Table 1).[6]
Table 1: Asymmetric trans–cis isomerization of achiral complex 1 into
D-(R)-2.[a]
Entry Solvent
[1]
Yield
d.r.[b]
Scheme 1. Asymmetric coordination chemistry by asymmetric cis–trans
isomerization induced by a chiral auxiliary or catalyst.
1
2
3
4
MeCN, acetone, or C6H5Cl 200 mm no product
–
DMF
200 mm
200 mm
25 mm
80%
81%
72%
only D-(R)
only D-(R)
50:1
[3]
HOCH2CH2OH
HOCH2CH2OH
We selected trans-[Ru(bpy)2(MeCN)2](CF3SO3)2
(1,
Scheme 2) as our achiral model complex in which a trans–
cis isomerization of the two 2,2’-bipyridine (bpy) ligands
would afford a chiral coordination sphere. It was envisioned
that certain bidentate ligands might be capable of inducing
such an isomerization as they will only be able to coordinate
[a] Conditions: Reaction of 1 with 2 equiv of (S)-SO and 10 equiv Et3N at
958C in a sealed brown glass vial. [b] Diastereomeric ratios determined
by 1H NMR spectroscopy.
It is noteworthy that yields and diastereoselectivities of
this conversion are very sensitive to reaction conditions. For
example, no product is formed with the solvents MeCN,
acetone, or C6H5Cl, whereas in the preferred solvents DMF
and ethylene glycol the d.r. value decreases slightly when the
reaction is executed at lower concentrations (Table 1). This
latter observation can be rationalized with a competition
between a thermal background isomerization of the trans
complex 1 into its thermodynamically more stable cis isomer
and the (S)-SO-induced reaction to D-(R)-2.
[*] Dr. L. Gong, Z. Lin, Dr. K. Harms, Prof. Dr. E. Meggers
Fachbereich Chemie, Philipps-Universitꢀt Marburg
Hans-Meerwein-Strasse, 35032 Marburg (Germany)
Fax: (+49)6421-282-2189
E-mail: meggers@chemie.uni-marburg.de
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(DFG).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003139.
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Communications
A crystal structure of the monocation D-(R)-2 (Figure 1)
confirms the D-configuration at the ruthenium center. The
chiral bidentate (S)-(isopropylsulfinyl)phenol auxiliary was
initially chosen because it positions the sulfur-centered
chirality in direct vicinity to the metal center and is thus
Scheme 3. Acid-induced substitution of the chiral sulfoxide ligand (S)-
SO for bpy under retention of configuration.
isomerization-induced asymmetric synthesis. Revealingly,
after some optimization we found that the reaction of 1 at a
high concentration in ethylene glycol (300 mm) with 0.2 equiv
(S)-SO in the presence of TFA and bpy afforded D-3 with a
yield of 96% and an e.r. = 5.8:1 (Scheme 4). In the same
system, the more electron-rich methoxy derivative (S)-SO’
even afforded an e.r. of 8.0:1 with a yield of 93%.[12] This
Figure 1. Crystal structure of D-(R)-2 (ellipsoids set at 50% probability;
the PF6ꢀ counterion and a water molecule are omitted for clarity).
Selected bond lengths [ꢁ] and angles [8]: Ru1—S1 2.237(3), Ru1—O2
2.071(7), Ru1—N1 2.049(8), Ru1—N2 2.047(10), Ru1—N3 2.070(10),
Ru1—N4 2.084(10); N1-Ru1-O2 173.4(3), N2-Ru1-N3 179.8(3), N4-
Ru1-S1 170.5(2), O2-Ru1-N4 86.9(3), N3-Ru1-O2 85.2(3).
Scheme 4. Catalytic synthesis of enantiomerically enriched D-[Ru-
(bpy)3]2+ (D-3) with the chiral ligands (S)-SO or (S)-SO’ as catalysts.
expected to have an especially strong influence on controlling
the absolute metal-centered configuration. Indeed, the crystal
structure demonstrates that the isopropyl group comes in very
close proximity to one of the bpy ligands.[7] The complex
appears to release some strain by distorting the five-mem-
bered sulfoxide chelate ligand out of planarity. It is apparent
that in the disfavored opposite diastereomer the isopropyl
substituent would sterically clash with the CH group in the
6-position of one bpy ligand, thus explaining why L-(R)-2 is
not observed under optimized reaction conditions.[8] Interest-
corresponds to turnover numbers of more than 3 and
demonstrates that these sulfinylphenols constitute true cata-
lysts for the asymmetric conversion 1!D-3. Apparently, (S)-
SO and even more so the more nucleophilic (S)-SO’ react
significantly faster with 1 than bpy, which will be monoproto-
nated and thus less reactive under the acidic reaction
conditions, and be subsequently recycled through an acid-
promoted replacement by bpy, thus allowing a full catalytic
cycle (Scheme 1).[13] It is noteworthy that along with the right
amount of TFA, the nature of the solvent is crucial for a
successful catalysis in this system. We recognized that in
ethylene glycol the trans complex 1 forms a suspension and is
dissolved only to about 10% at 300 mm. This is apparently an
important requirement for observing turnover, probably
because it allows the catalysts (S)-SO or (S)-SO’ to be in
excess of the substrate 1 at all times during the catalysis.[14]
In conclusion, we have presented an example of highly
efficient isomerization-induced asymmetric coordination
chemistry. Chiral (S)-(isopropylsulfinyl)phenol is capable of
converting achiral trans-[Ru(bpy)2(MeCN)2]2+ into chiral cis-
D-[Ru(bpy)2{(S)-(isopropylsulfinyl)phenolato}]+ under sub-
stitution of two acetonitrile ligands and accompanied by a
chirality-generating trans–cis isomerization of the bpy ligands.
The ligand (S)-(isopropylsulfinyl)phenol constitutes a chiral
auxiliary as it can be replaced by bpy under complete
retention of configuration in an acid-induced manner. Fur-
thermore, this study culminated in what is probably the first
example of catalytic asymmetric coordination chemistry, with
a small organic molecule serving as an asymmetric catalyst for
ꢀ
ingly, the Ru N bond in trans position to the sulfoxide ligand
is elongated to 2.084 ꢀ. It can be speculated that this
structural trans effect of the sulfoxide ligand might go along
with a kinetic trans effect, thus helping to release the second
acetonitrile ligand after the initial coordination of (S)-SO in
the course of the reaction 1!D-(R)-2.[9]
We then investigated the removal of the chiral sulfoxide
ligand (S)-SO from D-(R)-2. A protonation of the phenolate
ligand should decrease the chelate strength,[4] and indeed,
when we treated D-(R)-2 with 5 equiv of trifluoroacetic acid
(TFA) in the presence of 15 equiv of bpy in freshly distilled
dry acetonitrile at 1108C (sealed vial) for 2 h, (S)-SO was
replaced smoothly by bpy under retention of configuration,
affording D-[Ru(bpy)3]2+ (D-3) in a yield of 63% with an
enantiomeric ratio e.r. = 99.4:0.6, as determined by chiral
HPLC (Scheme 3).[10,11] Thus, it can be concluded that (S)-
(isopropylsulfinyl)phenol serves as a powerful chiral auxiliary
for converting the achiral trans complex 1 into a virtually
enantiomerically pure ruthenium polypyridyl complex.
Beyond its function as a chiral auxiliary, we envisioned
that (S)-SO might even be able to serve as a catalyst for
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hydrogen bond. For an analogous interaction, see: D. Hesek, Y.
Inoue, S. R. L. Everitt, H. Ishida, M. Kunieda, M. G. B. Drew,
J. Chem. Soc. Dalton Trans. 1999, 3701 – 3709.
the enantioselective, organocatalytic synthesis of an octahe-
dral metal complex.
[8] This is consistent with the observed high chemical instability of
the minor diastereomer L-(R)-2.
[9] For structural and kinetic trans effects in octahedral complexes,
see: B. J. Coe, S. J. Glenwright, Coord. Chem. Rev. 2000, 203, 5 –
80.
Received: May 24, 2010
Published online: July 29, 2010
Keywords: asymmetric catalysis · auxiliary control · chirality ·
.
coordination chemistry · ruthenium
[10] Optimized reaction conditions: 50 mm D-(S)-2, 15 equiv bpy,
5 equiv TFA, distilled and dry MeCN, 1108C in a sealed brown-
glass vial, 2 h. See the Supporting Information for more details.
[11] For ligand substitutions under retention of the metal-centered
configuration, see: a) T. J. Rutherford, M. G. Quagliotto, F. R.
Keene, Inorg. Chem. 1995, 34, 3857 – 3858; b) X. Hua, A.
von Zelewsky, Inorg. Chem. 1995, 34, 5791 – 5797; c) D. Hesek,
Y. Inoue, S. R. L. Everitt, H. Ishida, M. Kunieda, M. G. B. Drew,
Chem. Commun. 1999, 403 – 404; d) H. Chao, J.-G. Liu, C.-W.
Jiang, L.-N. Ji, X.-Y. Li, C.-L. Feng, Inorg. Chem. Commun.
2001, 4, 45 – 48; e) M. Brissard, O. Convert, M. Gruselle, C.
Guyard-Duhayon, R. Thouvenot, Inorg. Chem. 2003, 42, 1378 –
1385.
[1] For asymmetric coordination chemistry, see: a) U. Knof, A.
von Zelewsky, Angew. Chem. 1999, 111, 312 – 333; Angew.
Chem. Int. Ed. 1999, 38, 302 – 322; b) P. D. Knight, P. Scott,
Coord. Chem. Rev. 2003, 242, 125 – 143; c) J. Lacour, V. Hebbe-
Viton, Chem. Soc. Rev. 2003, 32, 373 – 382.
[2] For a recent review on chiral-auxiliary-mediated asymmetric
coordination chemistry, see: E. Meggers, Chem. Eur. J. 2010, 16,
752 – 758.
[3] Synthesized in analogy to: J. L. Walsh, B. Durham, Inorg. Chem.
1982, 21, 329 – 332.
[4] L. Gong, S. P. Mulcahy, K. Harms, E. Meggers, J. Am. Chem. Soc.
2009, 131, 9602 – 9603.
[5] The ee of (S)-SO was ꢁ 99%. Obtained conveniently from
optically pure sulfinates of diacetone glucose. For the method,
see: I. Fernꢁndez, N. Khiar, J. M. Llera, F. Alcudia, J. Org. Chem.
1992, 57, 6789 – 6796.
[6] The reaction of a chiral monodentate sulfoxide with a related
trans complex leads to only very modest diastereoselectivities;
see: a) D. Hesek, Y. Inoue, S. R. L. Everitt, H. Ishida, M.
Kunieda, M. G. B. Drew, Inorg. Chem. 2000, 39, 317 – 324; b) F.
Pezet, J.-C. Daran, I. Sasaki, H. Aꢂt-Haddou, G. G. A. Balavoine,
Organometallics 2000, 19, 4008 – 4015.
[12] Optimized reaction conditions for the asymmetric catalysis with
chiral sulfoxides: 300 mm 1, 0.2 equiv (S)-SO or (S)-SO’,
1.0 equiv bpy, 6.0 equiv TFA, ethylene glycol, 958C in a sealed
brown-glass vial under air, 48 h; thereafter addition of 10 equiv
bpy and another 2 h at 958C to replace the ruthenium-bound
catalyst. See the Supporting Information for more details.
[13] The chiral sulfoxides are configurationally stable under the
acidic reaction conditions.
[14] The ruthenium complex intermediates and products were tested
to be configurationally stable and soluble under the applied
reactions conditions, which excludes mechanistic explanations
for the observed enantiomeric excess involving chiral ion-pairing
interactions or solubility differences of the intermediate ruthe-
nium diastereomers.
[7] The structure also reveals
a close intramolecular contact
(3.12 ꢀ) between the sulfoxide oxygen and the CH group next
ꢀ
ꢀ
to N3, which could be interpreted with a weak S O···H C
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