Active versus passive substituent participation in the auxiliary-mediated asymmetric synthesis of an octahedral metal complex

Article abstract of DOI:10.1002/asia.201200532

Ambiguous chirality transfer: A surprising reversal of chirality transfer from carbon to metal is reported in the asymmetric synthesis of a chiral ruthenium complex, affording the metal-centered configuration or δ depending only on the chemical compositio

Full text of DOI:10.1002/asia.201200532

DOI: 10.1002/asia.201200532
Active versus Passive Substituent Participation in the Auxiliary-Mediated
Asymmetric Synthesis of an Octahedral Metal Complex
Liang-An Chen,[a] Jiajia Ma,[a] Mehmet Ali Celik,[b] Hong-Lang Yu,[a] Zexing Cao,[a]
Gernot Frenking,[b] Lei Gong,*[a] and Eric Meggers*[a, b]
Over the last few decades organic chemistry has devel-
oped highly sophisticated synthetic tools for the stereocon-
trol of chemical transformations.[1] However, most of the
synthetic strategies can be traced back to just two basic ef-
fects that often lead to opposite stereochemical outcomes:
On the one hand steric hindrance based on repulsive non-
bonding interactions and on the other hand stereochemical
reactions that proceed through a preassociation by attractive
forces (electrostatic interaction, hydrogen bonding, Lewis
acid-base adducts, coordinative bonds, or covalent bonds).[2]
We here wish to report that these general concepts of pas-
sive and active substituent control in stereochemical trans-
formations can be applied to the asymmetric synthesis of co-
ordination complexes.[3] In the herein discussed example,
a surprising reversal of chirality transfer from carbon to
Scheme 1. Controling chirality-at-metal by the substituent of a salicyloxa-
zoline auxiliary.
metal occurs—leading to either the L- or D-enantiomer of
[RuACHTUNGTRENNUNG
(bpy)3]2+ (bpy=2,2’-bipyri-
dine)—solely depending on the
chemical composition of the
side chain of a salicyloxazoline
auxiliary (Scheme 1).[4]
In the quest for straightfor-
ward and economical asymmet-
ric syntheses of chiral octahe-
dral metal complexes, we found
that the reaction of the readily
accessible ruthenium precursor
Scheme 2. Diastereoselective one-pot formation of salicyloxazolinate complexes 2a–2e. See Table 1 for more
details.
complex
with (S)-5-phenyl-2-(2’-hydroxy
[RuACHTUNGTRENNUNG
(DMSO)4Cl2][5]
phenyl)oxazoline (1a) in the presence of 2,2’-bipyridine
(8 equiv) and K2CO3 (10 equiv) in C6H5Cl/DMF (8:1) at
1408C for 2 hours afforded, in a one-pot procedure, complex
L-2a (59%) with high diastereocontrol (52:1 d.r.) (Table 1,
entry 1; Scheme 2). However, to our surprise, when we em-
ployed the oxazoline derivative 1b instead, in which the
phenyl substituent is replaced by a thioether, the stereo-
chemical outcome of this reaction was completely switched,
providing the D-diastereomer (D-2b) smoothly in 83% yield
with 66:1 d.r. under optimized conditions with EtOH as the
solvent, Et3N (10 equiv) as the base, and reflux overnight
(Table 1, entry 2). Additional methyl groups at the 4-posi-
tion of the oxazoline moiety (auxiliary 1c, 66% yield, 31:1
d.r.) somewhat reduced yields and diastereoselectivity
(Table 1, entry 3). Most interestingly, replacing the sulfur
atom of 1b by a methylene group (1d) caused the complete
loss of diastereoselectivity. Even just changing the position
of the thioether (1e) strongly abated the outcome of the re-
[a] L.-A. Chen, J. Ma, H.-L. Yu, Prof. Z. Cao, Prof. L. Gong,
Prof. Dr. E. Meggers
College of Chemistry and Chemical Engineering
Xiamen University
Xiamen 361005 (P. R. China)
Fax : (+86)592-2185821
E-mail: meggers@xmu.edu.cn
gongl@xmu.edu.cn
[b] Dr. M. A. Celik, Prof. Dr. G. Frenking, Prof. Dr. E. Meggers
Fachbereich Chemie
Hans-Meerwein-Straße
Philipps-Universitꢀt Marburg
35043 Marburg (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200532.
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COMMUNICATION
Table 1. Diastereoselectivity of the formation of complexes 2a–e as a function of the
salicyloxazoline substituents.
However, thioethers are formidable coordinating
ligands for transition metals and it can be expected
that the thioether substituent of auxiliaries 1b and
1c is transiently coordinated to the ruthenium
center and thereby controling the stereochemical
course of the reaction by temporarily blocking a co-
ordination side.[8–10] In fact, we noticed that reac-
tions with thioether-containing auxilaries 1b and 1c
proceeded significantly faster compared to auxiliary
1a, suggesting that the highly nucleophilic thioether
attacks the ruthenium first, followed by the forma-
tion of a tridentate chelate. Upon coordination to
an additional 2,2’-bipyridine ligand, the two inter-
mediates I and II would be generated (Scheme 3).
Entry Auxiliaries 1a–e
Conditions[a]
Product Yield L:D[b]
1
2
3
4
5
R=C6H5, R’=H (1a)
PhCl/DMF 8:1[c] 2a
59% 52:1
83% 1:66
66% 1:31
44% 1:1.3
R=CH2SCH3, R’=H (1b)
R=CH2SCH3, R’=CH3 (1c)
R=CH2CH2CH3, R’=H (1d)
EtOH[d]
EtOH
EtOH
2b
2c
2d
R=CH2CH2SCH3, R’=H (1e) EtOH
many products
n.d.[e]
[a] Reaction conditions: [RuACTHUNGTR(ENUNNG DMSO)4Cl2], auxiliary, bpy, together with a base were
heated in the indicated solvent under argon atmosphere in the dark. See the Experi-
mental Section and Supporting Information for more details. [b] Diastereomeric ratios
determined by 1H NMR. [c] Optimized solvent mixture. In EtOH, yields and d.r.
value are reduced. [d] Ethanol is the solvent of choice for this reaction. For example,
no main product is formed using the solvent PhCl/DMF (8:1). [e] not determined.
action (Table 1, entry 5), thus revealing that the thioether
and its position are essential for the course and stereoselec-
tivity of this reaction.
A crystal structure of the complex L-2a is shown in
Figure 1 and reveals a stacking of the phenyl substituent on
Figure 2. X-ray crystal structure of the racemic salicyloxazolinate com-
plex 2b. The hexafluorophosphate counterion is omitted for clarity and
only the enantiomer D-2b is shown.
Figure 1. X-ray crystal structure of the salicyloxazolinate complex L-2a.
The hexafluorophoshate counterion and an ether solvent molecule are
omitted for clarity. The absolute configuration was determined.
The subsequent substitution of the remaining monodentate
ligand (chloride in Scheme 3) and the hemilabile[11,12] thio-
ether in favor of a second 2,2’-bipyridine ligand would pro-
vide the L-diastereomer from intermediate I and the ob-
served D-diastereomer from intermediate II. DFT calcula-
tions at the M05-2X/def2-SVP level reveal that the inter-
mediate I is by 2.7 kcalmolꢀ1 more stable compared the in-
termediate II. Considering an equilibrium between I and II
through a pentacoordinated intermediate, it is apparently
the more reactive intermediate II that reacts faster to the
observed thermodynamically favored D-diastereomer,
whereas the more stable tridentate chelate in intermediate I
blocks a conversion into the unobserved L-enantiomer. The
fast preequilibrium between the intermediates I and II,
which subsequently react irreversibly to the diastereomers
L-2c and D-2c, respectively, presumably realizes a Curtin–
Hammett-situation where the relative transition state ener-
gies for the conversions I!L-2c versus II!D-2c and not
the ground state energy differences between the intermedi-
ates I and II determine the product distribution. The pro-
posed mechanism revolving around a fast equilibrium be-
tween two intermediates I and II is consistent with the ex-
perimentally observed preference of the solvent ethanol for
this reaction (Table 1), as a previous study revealed the es-
tablishment of a complete equilibrium of related intermedi-
ates in EtOH but not in other solvents such as C6H5Cl.[7]
a coordinated 2,2’-bipyridine ligand and the metal-centered
configuration L (left-handed propeller).[6] The stereochemi-
cal outcome of this reaction was expected and consistent
with previous results from our group regarding the metal-
centered stereocontrol by salicyloxazoline auxiliaries bear-
ing aliphatic or aromatic substituents.[7] The previous reac-
tions were demonstrated to be under thermodynamic con-
trol and, in fact, density functional theory (DFT) calcula-
tions (M06-2X/6-311+GACTHUNRGTNE(NUG 3d,p)) reveal that the experimen-
tally observed main diastereomer L-2a (R=Ph) with its
stacked phenyl group is by 1.43 kcalmolꢀ1 more stable com-
pared to the trace diastereomer D-2a.
A crystal structure of the thioether complex D-2b (R=
CH2SCH3) is displayed in Figure 2.[6] It is noteworthy that
this structure was obtained from the crystal structure of the
racemic salicyloxazolinate complex 2b since the pure enan-
tiomer D-2b did not crystallize in our hands. The structure
confirms an opposite stereochemistry with a D-configuration
at the metal (right-handed propeller), unambiguously dem-
onstrating that the thioether ligand, although essential for
the stereochemical outcome of the reaction, is indeed not
coordinated to the ruthenium in the product.
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Auxiliary-Mediated Asymmetric Synthesis of an Octahedral Metal Complex
Scheme 3. Proposed mechanism for the thioether-assisted diastereoselective formation of D-isomers. Shown are also the optimized geometries of the pro-
posed intermediates I and II as well as the reaction products L-2c and D-2c, which were all calculated at the M05-2X/def2-SVP level. The Gibbs free
energy of intermediate I is by 2.7 kcalmolꢀ1 lower compared to intermediate II, whereas the Gibbs free energies of the products D-2c and L-2c differ
only by 0.7 kcalmolꢀ1 in favor of the observed major reaction product D-2c.
Furthermore, although we could not detect the proposed re-
active intermediates I or II, we succeeded in demonstrating
the capability of the thioether
trolled synthesis of octahedral metal complexes will be cru-
cial to fully exploit the opportunities provided by the rich
stereochemistry of octahedral coordination geometries for
applications in the life sciences.[16]
oxazoline 1b to coordinate
transition metals in a meridional
tridentate fashion, as shown in
Figure 3 with a crystal structure
Experimental Section
of [PdClACHTUNGTRNEUNG
(1b-H)].[13,14]
Stereoselective Synthesis of L-2a.
Finally, in order to demon-
strate that the salicyloxazoli-
nate ligands serve as true auxil-
iaries, complexes L-2a and D-
2b were subjected to TFA
treatment in the presence of
2,2’-bipyridine to provide [Ru-
[RuACHTUNTRGNEUNG(DMSO)4Cl2] (20.0 mg, 0.041 mmol) together with oxazoline 1a
(9.9 mg, 0.041 mmol), 2,2’- bipyridine (51.2 mg, 0.33 mmol), and K2CO3
(56.7 mg, 0.41 mmol) was heated in PhCl/DMF (8:1, 2.2 mL) at 1408C
under argon atmosphere for two hours. The crude material was purified
by silica gel column chromatography using CH3CN/H2O/sat.KNO3
(150:3:1). Eluents were concentrated to dryness, the resulting material
was dissolved in minimal amounts of ethanol/water, and the product pre-
cipitated upon addition of excess solid NH4PF6. The precipitate was cen-
trifuged, washed twice with water, and dried under high vacuum to
afford a purple solid (19.7 mg, 59%) with 52:1 d.r. 1H NMR (400 MHz,
CD3CN): d=8.94 (d, J=5.7 Hz, 1H), 8.76 (d, J=5.6, 1H), 8.40 (d, J=
8.1 Hz, 1H), 8.29 (d, J=8.1 Hz, 1H), 7.99 (td, J=7.7, 1.4 Hz, 1H), 7.89
(d, J=5.6 Hz, 1H), 7.76 (m, 3H), 7.70 (m, 2H), 7.61 (m, 2H), 7.52 (m,
1H), 7.12 (m, 2H), 7.06 (m, 2H) 6.98 (tt, J=7.5, 1.2 Hz, 1H), 6.71 (t, J=
7.9 Hz, 2H), 6.41 (m, 2H), 6.10 (d, J=7.6 Hz, 2H), 5.01 (dd, J=9.6,
4.1 Hz, 1H), 4.90 (t, J=8.8 Hz, 1H), 4.16 ppm (dd, J=8.7, 4.1 Hz, 1H);
13C NMR (100 MHz, CD3CN): d=171.5, 164.7, 160.7, 159.3, 159.1, 158.3,
153.9, 152.2, 151.5, 150.5, 141.4, 136.8, 136.5, 136.4, 135.4, 134.0, 131.1,
129.5, 128.5, 127.3, 127.2, 127.0, 126.6, 125.9, 124.6, 124.3, 124.2, 124.1,
123.8, 114.0, 110.3, 76.3, 72.5 ppm; IR (film): n˜ =3364, 2956, 2924, 2853,
Figure 3. X-ray crystal struc-
ture of the complex [PdCl(1b-
H). Only one of two independ-
ent Pd complexes is shown.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
98.5:1.5 e.r.), respectively, from
substitution of the auxiliaries
by 2,2’-bipyridine under complete retention of configura-
tion.
In conclusion, we here reported a surprising sulfur-effect
for the chirality transfer from carbon to metal, most likely
relying on the active participation of a transiently coordinat-
ing thioether substituent, thus causing a reversal of the ste-
reochemical outcome compared to the commonly applied
passive steric control of chiral substituents. This work there-
fore reveals a new mechanism for the stereocontrolled syn-
thesis of octahedral metal complexes. From a purely practi-
cal perspective, the here reported reaction sequence pro-
vides a highly convenient access to nonracemic ruthenium
polypyridyl complexes starting from the common precursor
1659, 1632, 1608, 1461, 1378, 1260, 1092, 1019, 843, 802, 753, 559 cmꢀ1
;
HRMS calcd for C35H28N5O2Ru (M-PF6)+ 652.1286, found: 652.1294; CD
(MeCN): l, nm (De, Mꢀ1 cmꢀ1) 243 (ꢀ27), 301 (+105).
Stereoselective Synthesis of D-2b.
[RuACHTUNTRGNEUNG(DMSO)4Cl2] (18.0 mg, 0.037 mmol) together with oxazoline 1b
(9.1 mg, 0.041 mmol), 2,2’-bipyridine (12.7 mg, 0.081 mmol), and triethyl-
amine (53.4 mL, 0.37 mmol) were heated in ethanol (15 mL) under argon
atmosphere at reflux overnight. The solvent was removed under reduced
pressure and the residue purified by silica gel column chromatography
[Ru
ACHTUNGTRENNUNG(DMSO)4Cl2], which can be synthesized from RuCl3 in
a single step.[15] A better understanding of the stereocon-
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Eric Meggers et al.
COMMUNICATION
using CH3CN/H2O/sat.KNO3 (100:3:1). The eluents were concentrated to
dryness, the resulting material dissolved in minimal amounts of ethanol/
water, and the product precipitated upon addition of excess solid
NH4PF6. The precipitate was centrifuged, washed twice with water, and
dried under high vacuum to afford 23.9 mg (83%) of a brownish solid
with 66:1 d.r. 1H NMR (500 MHz, CD3CN): d=8.98 (d, J=5.3 Hz, 1H),
8.79 (d, J=5.1 Hz, 1H), 8.49 (d, J=8.2 Hz, 1H), 8.43 (d, J=8.2 Hz, 1H),
8.35 (dd, J=8.1, 4.2 Hz, 2H), 8.09 (td, J=7.9, 1.5 Hz, 1H), 7.97 (m, 2H),
7.82 (td, J=8.3, 1.4 Hz, 1H), 7.74 (td, J=8.0, 1.4 Hz, 1H), 7.63 (m, 1H),
7.54 (dd, J=8.2, 1.9 Hz, 1H), 7.50 (d, J=5.3 Hz, 1H), 7.44 (m, 1H), 7.11
(m, 2H), 6.99 (m, 1H), 6.35 (m, 2H), 4.39 (dd, J=9.1, 4.4 Hz, 1H), 4.06
(t, J=8.4 Hz, 1H), 3.14 (m, 1H), 2.58 (dd, J=13.1, 10.9 Hz, 1H), 2.42
(dd, J=13.1, 2.2 Hz, 1H), 1.46 ppm (s, 3H); 13C NMR (125 MHz,
CD3CN) d=170.3, 163.5, 159.7, 158.2, 157.9, 157.6, 154.2, 152.5, 151.3,
150.6, 136.0, 135.9, 135.7, 134.8, 132.7, 129.6, 126.9, 126.2, 125.7, 125.6,
123.8, 123.34, 123.32, 123.2, 122.9, 112.9, 110.1, 71.6, 65.0, 37.2, 13.9 ppm;
IR (film): n˜ =3076, 3019, 2964, 2915, 1606, 1536, 1461, 1442, 1418, 1387,
1347, 1260, 1235, 1155, 1070, 1035, 1019, 972, 928, 877, 830, 756, 728, 685,
656, 575, 555 cmꢀ1; HRMS calcd for C31H28O2N5RuS (M-PF6)+ 636.1002,
found: 636.1003; CD (MeCN): l, nm (De, Mꢀ1 cmꢀ1) 300 (ꢀ108), 286 (+
21).
[6] CCDC 883768 and CCDC 883769 contain the supplementary crystal-
lographic data for L-2a and for the racemic complex 2b. These data
can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[7] a) L. Gong, S. P. Mulcahy, K. Harms, E. Meggers, J. Am. Chem. Soc.
2009, 131, 9602–9603; b) L. Gong, S. P. Mulcahy, D. Devarajan, K.
Harms, G. Frenking, E. Meggers, Inorg. Chem. 2010, 49, 7692–7699.
[8] For somewhat related ONS-ligands which coordinate in a tridentate
fashion, see for example: a) G. Rajsekhar, C. P. Rao, P. K. Saarenke-
to, E. Kolehmainen, K. Rissanen, Inorg. Chem. Commun. 2002, 5,
649–652; b) S. Dhar, D. Senapati, P. K. Das, P. Chattopadhyay, M.
Nethaji, A. R. Chakravarty, J. Am. Chem. Soc. 2003, 125, 12118–
12124; c) S. Dhar, M. Nethaji, A. R. Chakravarty, Inorg. Chem.
2006, 45, 11043–11050; d) I. D. Kostas, B. R. Steele, A. Terzis, S. V.
Amosova, A. V. Martynov, N. A. Makhaeva, Eur. J. Inorg. Chem.
2006, 2642–2646.
[9] The related ligand 2-(2’-hydroxyphenyl)oxazoline-4-carboxylate is
capable of serving as a coordinating ON-bidentate or ONO-triden-
tate ligand. See, for example: M. D. Godbole, M. P. Puig, S. Tanase,
H. Kooijman, A. L. Spek, E. Bouwman, Inorg. Chim. Acta 2007,
360, 1954–1960.
[10] The related natural chiral siderophore desferriferrithiocin has been
demonstrated to coordinate metal ions in a tridentate fashion. See:
a) K. Langemann, D. Heineke, S. Rupprecht, K. N. Raymond, Inorg.
Chem. 1996, 35, 5663–5673; b) F. E. Hahn, T. J. McMurry, A. Hugi,
K. N. Raymond, J. Am. Chem. Soc. 1990, 112, 1854–1860.
Acknowledgements
[11] For the concept of hemilability of multidentate ligands, see: a) C. S.
Slone, D. A. Weinberger, C. A. Mirkin, Prog. Inorg. Chem. 1999, 48,
233–355; b) P. Braunstein, F. Naud, Angew. Chem. 2001, 113, 702–
722; Angew. Chem. Int. Ed. 2001, 40, 680–699.
We are grateful for support from the “National Thousand Plan” Founda-
tion of P. R. China and the “985 Program” of the Chemistry and Chemi-
cal Engineering disciplines of Xiamen University.
[12] The lability of the thioether coordination site results from the strain
caused by the stereogenic carbon within the oxazoline moiety.
[13] PdClACHTNUTRGENNUG(1b-H)] was synthesized from PdCl2 in methanol (56 mm) by
Keywords: asymmetric synthesis
·
chiral auxiliary
·
coordination chemistry · stereocontrol · sulfur
addition of LiCl (3 equiv) followed by addition of oxazoline 1b
(1 equiv).
[14] CCDC 885320 contains the supplementary crystallographic data for
[PdClACTHUNRTGNEUNG(1b-H)]. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
[1] a) Y. Gnas, F. Glorius, Synthesis 2006, 1899–1930; b) M. A. Rizzac-
casa, M. Perkins, Stoichiometric Asymmetric Synthesis Blackwell,
2000; c) P. J. Walsh, M. C. Kozlowski, Fundamentals of Asymmetric
Catalysis, University Science Books, 2009.
[2] a) A. H. Hoveyda, D. A. Evans, G. C. Fu, Chem. Rev. 1993, 93,
1307–1370; b) M. T. Reetz, Acc. Chem. Res. 1993, 26, 462–468;
c) G. Rousseau, B. Breit, Angew. Chem. 2011, 123, 2498–2543;
Angew. Chem. Int. Ed. 2011, 50, 2450–2494.
[3] For reviews on asymmetric coordination chemistry, see: a) J.-L.
Pierre, Coord. Chem. Rev. 1998, 178–180, 1183–1192; b) U. Knof,
A. von Zelewsky, Angew. Chem. 1999, 111, 312–333; Angew. Chem.
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1248–1263; Angew. Chem. Int. Ed. 1999, 38, 1194–1208; d) P. D.
Knight, P. Scott, Coord. Chem. Rev. 2003, 242, 125–143; e) C.
Ganter, Chem. Soc. Rev. 2003, 32, 130–138; f) J. Lacour, V. Hebbe-
Viton, Chem. Soc. Rev. 2003, 32, 373–382; g) H. Amouri, M. Gru-
selle, Chirality in Transition Metal Chemistry, Wiley, Chichester,
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[4] For auxiliary-mediated asymmetric synthesis of metal complexes,
see: E. Meggers, Chem. Eur. J. 2010, 16, 752–758.
[15] This thioether-assisted asymmetric synthesis can even be executed
starting directly from RuCl3·3H2O, albeit in somewhat lower yields.
A typical procedure: RuCl3·3H2O (15.0 mg, 0.057 mmol) together
with oxazoline 1b (14.1 mg, 0.063 mmol), 2,2’-bipyridine (19.6 mg,
0.13 mmol), and triethylamine (82 mL, 0.57 mmol) were heated in
ethanol (22 mL) under argon atmosphere at reflux overnight. The
solvent was removed under reduced pressure and the residue puri-
fied by silica gel column chromatography using CH3CN/H2O/
sat.KNO3 (100:3:1). The eluents were concentrated to dryness, the
resulting material dissolved in minimal amounts of ethanol/water,
and the product precipitated upon addition of excess solid NH4PF6.
The precipitate was centrifuged, washed twice with water, and dried
under high vacuum to afford 23.5 mg of D-2b (53%) with 36:1 d.r.
[16] E. Meggers, Chem. Commun. 2009, 1001–1010.
Received: June 14, 2012
Published online: && &&, 0000
[5] I. P. Evans, A. Spencer, G. Wilkinson, J. Chem. Soc. Dalton Trans.
1973, 204–209.
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COMMUNICATION
Asymmetric Coordination Chemistry
Liang-An Chen, Jiajia Ma, Mehmet
Ali Celik, Hong-Lang Yu, Zexing Cao,
Gernot Frenking, Lei Gong,*
Eric Meggers*
&&&& — &&&&
Active versus Passive Substituent
Participation in the Auxiliary-Medi-
ated Asymmetric Synthesis of an Octa-
hedral Metal Complex
Ambiguous chirality transfer: A sur-
prising reversal of chirality transfer
from carbon to metal is reported in the
asymmetric synthesis of a chiral ruthe-
nium complex, affording the metal-
centered configuration L or D depend-
ing only on the chemical composition
of the side chain of a chiral salicyloxa-
zoline auxiliary.
Chem. Asian J. 2012, 00, 0 – 0
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