Chiral (Mercaptophenyl)oxazolines as auxiliaries for asymmetric coordination chemistry

Article abstract of DOI:10.1002/ejic.201200203

(4S)-2-(4-Isopropyl-4,5-dihydrooxazol-2-yl)-4-nitrobenzenethiol {(S)-TS} and its tert-butyl derivative {(S)-TS'} were developed as chiral auxiliaries for the asymmetric synthesis of polypyridyl ruthenium complexes. In their deprotonated form, these (merca

Full text of DOI:10.1002/ejic.201200203

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FULL PAPER
DOI: 10.1002/ejic.201200203
Chiral (Mercaptophenyl)oxazolines as Auxiliaries for Asymmetric Coordination
Chemistry
Marianne Wenzel[a] and Eric Meggers*[a,b]
Keywords: Asymmetric synthesis / Chiral auxiliaries / Ruthenium
(4S)-2-(4-Isopropyl-4,5-dihydrooxazol-2-yl)-4-nitrobenzene-
thiol {(S)-TS} and its tert-butyl derivative {(S)-TSЈ} were de-
veloped as chiral auxiliaries for the asymmetric synthesis of
polypyridyl ruthenium complexes. In their deprotonated
form, these (mercaptophenyl)oxazolines were used as biden-
tate ligands and allowed the efficient transfer of chirality
from the oxazoline moiety to the ruthenium stereocenter. Af-
ter the induction of the absolute metal-centered configura-
tion, the auxiliaries were labilized by converting the coordi-
nated thiolate into a thioether ligand upon methylation with
Meerwein salt, followed by the thermal replacement with
2,2Ј-bipyridine or 1,10-phenanthroline ligands under reten-
tion of configuration to afford octahedral polypyridyl ruthe-
nium complexes with high enantiomeric excesses. These
thiol-based auxiliaries complement our previously developed
acid-labile chiral auxiliaries and thus expand the toolbox for
the asymmetric synthesis of chiral ruthenium complexes.
asymmetric coordination chemistry using chelating ligands.
The formation of a bidentate chelate prevents the auxiliary
from rotation around the metal-ligand bond resulting in an
improved stereoinduction. We applied carefully tailored chi-
ral bidentate auxiliaries such as salicyloxazolines (Salox),[11]
sulfinylphenols (SO and SOЈ),[12] a binaphthyl ligand (HO-
MOP),[13] and a simple sulfinamide (ASA)[14] which all
transfer their chiral information efficiently to an octahedral
metal center and are removed afterwards in a traceless fash-
ion under complete retention of the configuration at the
metal (Figure 1). However, all current reaction sequences
are based on an acid-induced auxiliary substitution. To in-
Introduction
Chiral octahedral metal complexes attract increasing at-
tention in the field of catalysis,[1] materials sciences,[2] and
life sciences.[3] Typically, their chemical, physical and bio-
logical characteristics are closely related to the stereochemi-
cal arrangement of the ligands around the metal center with
only one stereoisomer exhibiting the desired properties.[4]
To circumvent a cost- and time-consuming separation by
chiral resolution techniques of stereoisomers formed during
synthesis, a stereoselective synthetic route, i.e. an asymmet-
ric synthesis, is often the strategy of choice, but general and
convenient methods to obtain stereochemically pure octa-
hedral metal complexes are still rare.[5] Regarding the auxil-
iary-mediated asymmetric synthesis of octahedral coordina-
tion compounds, a few methods were developed over the
last decades using for example tartrate,[6] monodentate chi-
ral sulfoxides,[7] chiral cleavable linkers,[8] or chiral counteri-
ons[9] as auxiliaries.[10] Unfortunately, these methods either
provide only moderate stereochemical induction or their ap-
plication is limited with respect to their scope. Hence, more
broadly applicable methods for the stereoselective synthesis
of enantiopure compounds with metal-centered chirality
are demanded.
A few years ago, our group started investigations in this
field and established the concept of auxiliary-mediated
[a] Fachbereich Chemie, Philipps-Universität Marburg,
Hans-Meerwein-Straße, 35043 Marburg, Germany
Fax: +49-6421-2822189
E-mail: meggers@chemie.uni-marburg.de
[b] Department of Chemical Biology, College of Chemistry and
Chemical Engineering, Xiamen University,
Figure 1. Comparison of previously developed and in this study
introduced chelating chiral auxiliaries for the asymmetric synthesis
of polypyridyl ruthenium complexes.
Xiamen 361005, P. R. China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200203.
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M. Wenzel, E. Meggers
FULL PAPER
crease the scope of available methods for asymmetric coor- ion in an overall yield of Ն 76% over three steps. The auxil-
dination chemistry, we were therefore seeking new strategies iary is storable at –20 °C under argon for months but de-
with respect to the induction of the auxiliary replacement. composes when stored in solution under air.
We here wish to report our progress into this direction and
present (4S)-2-(4-isopropyl-4,5-dihydrooxazol-2-yl)-4-nitro-
Synthesis of Precursor Complexes
benzenethiol {(S)-TS} and its tert-butyl derivative {(S)-
TSЈ} as chiral auxiliaries for the asymmetric synthesis of
chiral polypyridyl ruthenium complexes (Figure 1).
To provide a suitable precursor complex that acts as a
starting point for the asymmetric coordination chemistry,
auxiliary ligand (S)-TS was treated with [{Ru(η6-C6H6)-
Cl(μ-Cl)}2] in acetonitrile in the presence of triethylamine at
room temperature for 4 h (Scheme 2). The resulting benzene
half-sandwich complex was stable enough to be purified by
column chromatography to afford (S)-4a in a yield of 80%.
In case of the tert-butyl derivative (S)-TSЈ, the reaction time
had to be extended to 5 h. Subsequent silica gel column
chromatography allowed isolation of (S)-4b only in a mod-
erate yield of 63%. The increased reaction time and limited
stability during column chromatography are most likely an
effect of the sterically more demanding tert-butyl group. It
hinders the coordination at the metal center and favors the
decomposition of the formed half-sandwich complex (S)-
4b. Alternatively, it was also possible to utilize the crude
auxiliary ligand in the synthesis of (S)-4a, b without lower-
ing the yield. Remaining salts from the ligand synthesis
were removed subsequently by column chromatography.
The pseudo-tetrahedral complexes (S)-4a and (S)-4b can ex-
ist as two different diastereomers. However, it was not pos-
sible to make a statement with respect to diastereoselectiv-
ity of this reaction as (S)-4a, b underwent a solvent depend-
ent epimerization at the ruthenium center. This is in accord-
ance with observations made by Brunner and Davies for
structurally similar arene half-sandwich complexes.[17] Sub-
sequently, the benzene ligand in (S)-4a, b was exchanged
against substitutionally labile ligands which are crucial for
further diastereoselective introduction of polypyridyl li-
gands. Accordingly, treatment of (S)-4a, b with UV-irradia-
tion in acetonitrile followed by evaporation of solvents af-
forded (S)-5a, b without the need of further purification.
The precursor complexes (S)-5a, b are sensitive to air and
should be prepared freshly when used in diastereoselective
synthesis experiments.
Results and Discussion
Synthesis of (Mercaptophenyl)oxazolines
We selected the chiral (mercaptophenyl)oxazolines (S)-
TS (R = iPr) and (S)-TSЈ (R = tBu) as our new generation
of chiral auxiliaries (Figure 1). In comparison to the pre-
viously established salicyloxazolines, the hydroxy group is
replaced by a thiol and an additional nitro group at the
aromatic backbone is supposed to stabilize the aromatic
thiol towards air oxidation.[15] The synthesis of these auxil-
iary ligands started from the commercially available 2-
bromo-5-nitrobenzoic acid (1) following a procedure re-
ported by Bauer et al. for the first two reaction steps.[16]
Accordingly, 1 was converted quantitatively into 2-bromo-
5-nitrobenzoyl chloride using oxalyl chloride and catalytic
amounts of DMF (Scheme 1). By reacting the crude acid
chloride with a chiral amino alcohol in the presence of tri-
ethylamine the stereochemical information was introduced
to obtain hydroxybenzamide 2a, b. 4-Toluenesulfonyl chlor-
ide, triethylamine, and catalytic amounts of DMAP effected
the cyclization reaction and the oxazoline moiety in 3a, b
was formed. Finally the bromo substituent in 3a, b was re-
placed in a nucleophilic aromatic substitution reaction with
sodium sulfide and acidic aqueous work-up afforded the
corresponding (mercaptophenyl)oxazoline (S)-TS (R = iPr)
or (S)-TSЈ (R = tBu). Further purification by suspending
the crude product in anhydrous toluene and subsequent fil-
tration allowed isolation of the auxiliary in a salt-free fash-
Scheme 2. Synthesis of the precursor complexes (S)-5a, b as start-
ing points for asymmetric coordination chemistry.
Diastereoselective Coordination Chemistry
The precursor complex (S)-5a was treated with 2.5 equiv.
of 2,2Ј-bipyridine (bpy), 5,5Ј-dimethyl-2,2Ј-bipyridine
(dmbpy), or 1,10-phenanthroline (phen) to afford the com-
Scheme 1. Synthesis of the chiral (mercaptophenyl)oxazolines (S)-
TS and (S)-TSЈ. TsCl = 4-toluenesulfonyl chloride, DMAP = 4-
(dimethylamino)pyridine.
2
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Chiral Auxiliaries for Asymmetric Coordination Chemistry
plexes Λ-(S)-6a–8a diastereoselectively, favoring the forma- the absolute configuration at the ruthenium center was
tion of the Λ-configuration at the ruthenium center in anal- achieved by means of CD spectroscopy for diastereomeric
ogy to our previous salicyloxazoline system (Scheme 3).[11] complexes synthesized in this work relative to CD data of
The diastereoselectivity of this conversion originates from (S)-Salox containing diastereomers as Λ.[11] The CD spec-
the steric demand of the alkyl group at the stereocenter lo- trum of Λ-(S)-6a is shown in Figure 2 in comparison with
cated in close proximity to the coordinating nitrogen atom the structurally similar (S)-Salox bearing diastereomeric
of the oxazoline moiety. Thus, incoming bidentate ligands complex Λ-[Ru(bpy)2{(S)-Salox-H}]PF6.
arrange in a fashion resulting in minimal steric interaction
Determination of Diastereomeric Ratios
with the alkyl chain. Crystal structures published by Meg-
gers et al. for the related (S)-Salox system support this ex-
The ratios of formed Λ-(S)- and Δ-(S)-diastereomers
were determined from 1H NMR spectra of the dia-
stereomerically enriched purified product by peak integra-
tion and comparison of the integral values. Signals chosen
for this purpose were those of the protons in ortho position
to a coordinating nitrogen atom of one polypyridyl ligand.
For example, the reaction of (S)-5a with bpy (2.5 equiv.) in
chlorobenzene at a concentration of 5 mm at 110 °C for 3 h
afforded Λ-(S)-6a with a diastereoselectivity of 59:1 (Fig-
ure 3, a). A 1:1 mixture of Λ/Δ-(S)-6a served as the refer-
ence for peak identification (Figure 3, b).
planation.[11]
Scheme 3. Diastereoselective coordination chemistry with (mer-
captophenyl)oxazoline complexes (S)-5a, b. NN = 2,2Ј-bipyridine
[Λ-(S)-6a, b], 5,5Ј-dimethyl-2,2Ј-bipyridine [Λ-(S)-7a], or 1,10-
phenanthroline [Λ-(S)-8a, b]. See Table 1 for reaction conditions.
Determination of Absolute Metal-Centered Configuration
Unfortunately, it was not possible to obtain single crys-
tals suitable for X-ray analysis. The auxiliaries (S)-TS and
(S)-TSЈ exhibit a structure very similar to (S)-Salox and the
same configuration at the chiral carbon atom in 4-position
of the oxazoline moiety which determines the metal-cen-
tered chirality emerged upon coordination of two polypyr-
idines to the ruthenium center. As the configuration of pre-
viously published (S)-Salox bearing bis(polypyridyl) ruthe-
nium complexes was identified by crystal structure analysis
as Λ,[11] the resulting metal-centered configuration of dia-
stereomers formed by the reaction of (S)-5a with polypyr-
idines was expected to be the same. Hence, assignment of
Figure 3. 1H NMR spectra excerpts (500 MHz, CD3CN) of (a) the
reaction that afforded Λ-(S)-6a with a high diastereoselectivity (dr
= 59:1) and (b) a 1:1 mixture Λ/Δ-(S)-6a. Depicted are the signals
of the protons in ortho position to a coordinating nitrogen atom of
one bipyridyl ligand.
Solvent Dependence of Diastereoselectivity
The extent of the stereoselectivity strongly depends on
the solvent chosen for this reaction. For the reaction of (S)-
5a with bpy, the best solvent regarding diastereoselectivity
turned out to be chlorobenzene as a slightly polar nonco-
ordinating solvent affording Λ-(S)-6a with a high dr (59:1,
62% yield) (Table 1, entry 1). This observation is in accord-
ance with the results obtained for the (S)-Salox system.[11]
A polar aprotic solvent like DMF led to a modest dia-
stereoselectivity of 16:1 but a significantly increased yield
(79%) (Table 1, entry 4). In ethanol as an example for a
polar protic solvent direct conversion into [Ru(bpy)3]2+ was
observed additionally to the formation of diastereomeric
product complex, thus excluding ethanol as a suitable sol-
vent. Other reaction parameters like concentration, tem-
perature, and the number of equivalents of introduced poly-
pyridine were also investigated but did not show a signifi-
cant effect on the diastereoselectivity. In general, the trends
observed in this work are comparable to results obtained
for the (S)-Salox system studied previously.[11]
Figure 2. CD spectrum of the diastereomeric complex Λ-(S)-6a (dr
= 59:1) (black line) compared with data of the structurally analo-
gous complex Λ-[Ru(bpy)2{(S)-Salox-H}]PF6 (dr = 120:1) (grey
line) according to ref.[11b] Spectra were recorded in acetonitrile at
concentrations of 0.2 mm.
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Table 1. Diastereoselective coordination chemistry with (mercapto- Replacement of the Auxiliary Ligand Under Retention of
phenyl)oxazoline complexes (S)-5a, b.[a]
Configuration
Entry Precursor NN
Solvent Time
C6H5Cl 3 h
Product
Yield dr[b]
The final step of the stereoselective synthesis of tris(poly-
pyridyl) ruthenium complexes includes the replacement of
the auxiliary against a third polypyridyl ligand under reten-
tion of the configuration at the metal center, requiring the
weakening of the auxiliary-metal bond. In this work, this
was achieved by a methylation of the thiolate complexes Λ-
(S)-6a–8a and Λ-(S)-6b, 8b. For example, the reaction of Λ-
(S)-6a with trimethyloxonium tetrafluoroborate (2 equiv.)
in dichloromethane at room temperature for 3 h and subse-
quent reaction of the intermediate complex with an excess
of bpy (15 equiv.) in acetonitrile at 110 °C for 24 h in a
sealed vessel afforded Λ-12 in a yield of 68% (Scheme 4 and
Table 2, entry 1).
1
2
3
4
5
6
7
(S)-5a
(S)-5a
(S)-5a
(S)-5a
(S)-5a
(S)-5b
(S)-5b
bpy
Λ-(S)-6a 62% 59:1
Λ-(S)-7a 64% 40:1
Λ-(S)-8a 68% 56:1
Λ-(S)-6a 79% 16:1
Λ-(S)-8a 80% 23:1
Λ-(S)-6b 72% 34:1
Λ-(S)-8b 64% 93:1
dmbpy C6H5Cl 3 h
phen C6H5Cl 1 h
bpy
phen DMF 45 min
bpy DMF 90 min
phen DMF 45 min
DMF 90 min
[a] Conditions: 5 mm precursor, 2.5 equiv. of NN = 2,2Ј-bipyridine
(bpy), 5,5Ј-dimethyl-2,2Ј-bipyridine (dmbpy), or 1,10-phenanthrol-
1
ine (phen), 110 °C. [b] Determined by H NMR spectroscopy.
Ligand Dependence
In addition to bpy, dmbpy and phen were introduced as
bidentate ligands. Contrasting to the observations made for
the (S)-Salox system, the reaction of (S)-5a with dmbpy as
a sterically more demanding bipyridine derivative did not
result in higher selectivities for Λ-(S)-7a (dr = 40:1, 64%
yield) (Table 1, entry 2).[11] In the case of phen, the dia-
stereomeric ratio obtained for the resulting ruthenium com-
plex Λ-(S)-8a was 56:1 (68% yield) (Table 1, entry 3), sim-
ilar to the dr found for Λ-(S)-6a. However, the reaction time
could be reduced to 1 h as also observed for the related
(S)-Salox system.[11] Overall, for all investigated polypyridyl
ligands, diastereoselectivities and yields for reactions with
the precursor (S)-5a were satisfactory and in the same
range.
Scheme 4. Replacement of the thiophenolato ligand under reten-
tion of configuration induced by methylation with the Meerwein
salt. NЈNЈ = bpy (Λ-12), dmbpy (Λ-13), phen (Λ-14), or 4,4Ј-di-
methoxy-2,2Ј-bipyridine (dmobpy) (Λ-15–17). See Table 2 for de-
tails.
Table 2. Ligand activation and substitution under retention of con-
figuration.
Influence of Alkyl Substituent in the 4-Position of the
Oxazoline
Starting
mat.
Reaction
cond.
Entry
dr[a]
NЈNЈ
Product Yield er[ g]
[b]
[b]
[b]
[c]
[c]
[c]
[c]
[c]
Due to its close proximity to the metal center, the steric
demand of the alkyl chain in 4-position of the oxazoline
moiety should affect the diastereoselectivity of complex for-
mation significantly.[11] We found that an increase of the
bulkiness of the auxiliary ligand by introduction of a steri-
cally more demanding alkyl chain (tert-butyl vs. isopropyl)
in 4-position of the oxazoline moiety led to a higher dia-
stereoselectivity. But – carried out in the same solvent
chlorobenzene – this was achieved at the expense of the
yield of the reaction. As the solvent screening gave high
yields but low selectivities in DMF when (S)-5a was used
as a precursor, we decided to react (S)-5b with bpy in DMF.
1
2
3
4
5
6
7
8
Λ-(S)-6a 59:1 Me3OBF4
Λ-(S)-7a 40:1 Me3OBF4
Λ-(S)-8a 56:1 Me3OBF4
Λ-(S)-6a 59:1 Me3OBF4
Λ-(S)-7a 40:1 Me3OBF4
Λ-(S)-8a 56:1 Me3OBF4
Λ-(S)-6b 34:1 Me3OBF4
Λ-(S)-8b 93:1 Me3OBF4
Λ-(S)-6a 59:1 MeI[d]
bpy
dmbpy Λ-13
phen Λ-14
dmobpy Λ-15
dmobpy Λ-16
dmobpy Λ-17
dmobpy Λ-15
dmobpy Λ-17
dmobpy Λ-15
dmobpy Λ-15
Λ-12
68%
74%
61%
74%
79%
76%
85%
86%
43:1
26:1
36:1
35:1
27:1
34:1
23:1
42:1
9
10
11
Ͻ 7% n.d.
39%
n.d.
[e]
Λ-(S)-6a 59:1 MeI/AgPF6
48:1
n.d.
Λ-(S)-6a 59:1 TFA[f]
bpy
Λ-12
[a] Determined by 1H NMR spectroscopy. [b] Conditions: (i)
CH2Cl2, 50 mm, 2 equiv. Me3OBF4, room temp., 3 h. (ii) MeCN,
100 mm, 15 equiv. NЈNЈ, 110 °C, sealed vial, 24 h. [c] Conditions:
After a reaction time of 90 min, Λ-(S)-6b was isolated with (i) CH2Cl2, 50 mm, 2 equiv. Me3OBF4, room temp., 3 h. (ii) MeCN,
100 mm, 15 equiv. dmobpy, 110 °C, sealed vial, 20 h. [d] Condi-
a dr of 34:1 in a yield of 72% (Table 1, entry 6). For phen
tions: (i) MeCN, 50 mm, 5 equiv. MeI, room temp., 5 h. (ii) MeCN,
as polypyridyl ligand, quantitative conversion of starting
100 mm, 15 equiv. dmobpy, 110 °C, sealed vial, 20 h. [e] Conditions:
material was reached already after 45 min to afford Λ-(S)-
(i) MeCN, 50 mm, 5 equiv. MeI, 10 equiv. AgPF6, room temp., 5 h.
8b with a high dr of 93:1 and an acceptable yield of 64%
(Table 1, entry 7). As expected, diastereomeric complexes
Λ-(S)-6b and Λ-(S)-8b were formed with an improved selec-
tivity and yields even higher than obtained for the forma-
tion of Λ-(S)-6a and Λ-(S)-8a in DMF. Unfortunately, as a
trade-off, the stability of complexes bearing auxiliary ligand
(ii) MeCN, 100 mm, 15 equiv. dmobpy, 110 °C, sealed vial, 20 h. [f]
Conditions: MeCN, 50 mm, 5 equiv. TFA, 15 equiv. bpy, 110 °C,
sealed vial, 12 h. Traces of product formed. [g] Determined by chi-
ral HPLC (see experimental section and Supporting Information
for conditions).
The enantiomeric ratio of the product complex Λ-12 was
(S)-TSЈ is significantly lower than (S)-TS with the result determined by chiral HPLC analysis to be 43:1. The HPLC
that the diastereomers decomposed more easily when ex- trace of the enantioenriched complex is displayed in part a
posed to air.
of Figure 4 compared with the trace for the racemic mixture
4
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Chiral Auxiliaries for Asymmetric Coordination Chemistry
Λ/Δ-12 as a reference (Figure 4, b). Comparison of the dr atom in 1-position of the aromatic system of the coordi-
(59:1) before replacement of the auxiliary ligand and the er nated auxiliary ligand. Furthermore, the comparison of the
(43:1) of the enantiomerically enriched product Λ-12 dem- NMR spectrum with data of a 1:1 diastereomeric mixture
onstrates that the substitution occurred within experimental of complexes obtained from the reaction of racemic cis-
errors without any significant loss of stereochemical infor- [Ru(bpy)2Cl2] with the corresponding thioether showed a
mation at the metal center. The absolute stereochemistry of match in chemical shifts for one set of signals. Additionally,
Λ-12 was assigned relative to published CD data as the Λ- HRMS confirms the presence of the sulfido complex Λ-(S)-
configuration at the ruthenium center.[11]
9a.
It is worth mentioning that other methylating agents
were also tested. Initially the diastereomerically enriched
complex Λ-(S)-6a was treated with methyl iodide (5 equiv.)
in acetonitrile at room temperature for 5 h. Subsequent ad-
dition of an excess of dmobpy (15 equiv.) and heating to
110 °C in a sealed vessel for 20 h resulted in formation of
product as analyzed by TLC (Table 2, entry 9). However,
the isolated yield was very low so no chiral HPLC analysis
was performed to determine the enantiopurity of the prod-
uct complex Λ-15. To increase the electrophilicity of methyl
iodide (5 equiv.), silver hexafluorophosphate (10 equiv.) was
added as an activating agent and the reaction was repeated
under the same conditions. This time Λ-15 was isolated in
a yield of 39% and an er of 48:1 (Table 2, entry 10). To
achieve higher conversion, trimethyloxonium tetrafluoro-
borate as an even stronger methylation electrophile was re-
quired. For comparison with the previously reported (S)-
Salox system, we also tried to activate the auxiliary ligand
Figure 4. HPLC traces of (a) enantioenriched Λ-[Ru(bpy)3](PF6)2
(Λ-12) (er = 43:1) synthesized from Λ-(S)-6a and (b) the racemic
mixture Λ/Δ-12. HPLC conditions: Daicel Chiralpak IA
(250ϫ4.6 mm); solvent A = 0.1% TFA (aq), solvent B = acetoni-
trile (flow rate 0.5 mL/min, column temperature 40 °C, UV-absorp-
tion detected at 254 nm, 15–30% B in 20 min).
Diastereomeric complexes Λ-(S)-7a and Λ-(S)-8a were using acid.[11] Accordingly, heating of complex Λ-(S)-6a in
also converted into their corresponding tris-homoleptic poly- presence of trifluoroacetic acid (5 equiv.) and bpy
pyridyl ruthenium complexes Λ-13 (er = 26:1, 74% yield) (15 equiv.) in acetonitrile at 110 °C in a sealed vial (stan-
and Λ-14 (er = 36:1, 61% yield) under reaction conditions dard conditions for auxiliary cleavage) resulted in poor con-
applied for the synthesis of Λ-12 (Table 2, entries 2 and 3). version (Table 2, entry 11). Even if the reaction time was
The er values and yields were in the same range as for Λ- increased to 12 h, Λ-12 was isolated only in traces (TLC,
12. This method is also applicable for the introduction of HRMS).
different bidentate ligands like 4,4Ј-dimethoxy-2,2Ј-bipyr-
idine (dmobpy) to obtain heteroleptic polypyridyl ruthe-
nium complexes. By methylation of the diastereomeric com-
plexes Λ-(S)-6a–8a and subsequent reaction with dmobpy
(15 equiv.) the corresponding complexes Λ-15–17 were
formed. Due to the electron-rich character of the dmobpy
Conclusions
In summary, we here introduced the (mercaptophenyl)-
oxazolines (S)-TS and (S)-TSЈ which provide high stereo-
chemical control of the metal-centered configuration of
ligand the reaction time for the substitution step was
polypyridyl ruthenium complexes. Contrasting to previously
slightly reduced to 20 h. In comparison with reactions that
reported acid-mediated methods, labilization of the auxil-
afforded homoleptic complexes, the yields marginally in-
iary ligand was achieved by chemoselective methylation at
the sulfur atom, thus converting it from a strong coordinat-
ing thiolate to a more labile thioether. Hence, in the pres-
creased (74–79%) whereas the er was not affected (Table 2,
entries 4–6). The auxiliary (S)-TSЈ in diastereomeric com-
plexes Λ-(S)-6b, 8b was also replaced against dmobpy under
ence of polypyridine ligands, the methylated auxiliary was
standard reaction conditions (Table 2, footnote [c]). The
succeptible to substitution under retention of configuration,
yields were negligibly higher than for complexes prepared
resulting in the formation of tris(polypyridyl) ruthenium
by reaction of Λ-(S)-6a, 8a (Table 2, entry 7 and 8). This
complexes with high enantiomeric purities. Thus, these
might be an effect of the sterically more demanding alkyl
thiol-based auxiliaries complement our previously devel-
substituent in 4-position of the oxazoline moiety in the aux-
oped chiral auxiliaries and expand the toolbox for the
iliary which makes the ligand cleaved more easily from the
asymmetric synthesis of chiral ruthenium complexes.
diastereomer.
Finally, to verify the regioselectivity of the methylation
step, we isolated the product of the methylation reaction
with Λ-(S)-9a by column chromatography and subsequent
Experimental Section
anion metathesis as the PF6 salt. Two-dimensional NMR
General Methods and Materials: All reactions were carried out un-
der argon atmosphere using dried glassware. Stereoselective coordi-
3
experiments (HMBC) revealed a J coupling between the
methyl protons and the quaternary sp2-hybridized carbon nation chemistry was performed in the dark to avoid light-induced
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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/KAP1
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M. Wenzel, E. Meggers
FULL PAPER
decomposition or racemization. Chemicals were purchased from
Sigma Aldrich, Alfa Aesar, or Acros Organics and used without
further purification. l-Valinol[18] and [{Ru(η6-C6H6)Cl(μ-Cl)}2][19]
were prepared according to published procedures. Solvents were
dried with calcium hydride (acetonitrile, dichloromethane, DMF),
sodium (toluene), or sodium/benzophenone (THF) and distilled
freshly prior to use. Chlorobenzene (HPLC grade) was used with-
out further drying. Column chromatography was performed on sil-
ica gel (230–400 mesh). Photolytic reactions were performed using
a Heraeus UV Reactor System equipped with a mercury medium
pressure lamp (UV immersion lamp TQ 150, 150 W), quartz cool-
ing jacket and uranium filter. NMR spectra were recorded on a
Bruker DPX 250 (250 MHz), Bruker Avance 300 (300 MHz),
(4S)-2-(2-Bromo-5-nitrophenyl)-4-isopropyl-4,5-dihydrooxazole (3a):
To a suspension of benzamide 2a (1.80 g, 5.44 mmol) in dichloro-
methane (55 mL) were added triethylamine (2.3 mL, 16.3 mmol),
4-toluenesulfonyl chloride (2.07 g, 10.9 mmol), and DMAP (66 mg,
0.54 mmol), and it was heated to reflux for 20 h. After addition of
water (2 mL), reflux continued for 1 h. The yellowish solution was
cooled down to room temp. and diluted with dichloromethane
(20 mL). The organic layer was washed with water, 0.2 m NaHSO4
(aq.), water, NaHCO3 (satd. aq.), water (30 mL each), dried with
Na2SO4, and solvents were evaporated in vacuo. Subsequent purifi-
cation by column chromatography (hexane/ethyl acetate, 10:1) af-
forded 3a (1.59 g, 5.06 mmol, 93%) as a slightly yellow solid. 1H
NMR (300 MHz, CDCl3): δ = 8.55 (d, J = 2.7 Hz, 1 H), 8.12 (dd,
Bruker DRX 400 (400 MHz), or Bruker Avance 500 (500 MHz) J = 8.8, 2.8 Hz, 1 H), 7.84 (d, J = 8.8 Hz, 1 H), 4.54–4.45 (m, 1
spectrometer at 298 K. Infrared spectra were recorded on a Bruker
Alpha FTIR instrument. High-resolution mass spectra were ob-
tained with a Finnigan LTQ-FT instrument using electrospray
mass ionization. CD spectra were recorded on a JASCO J-810 CD
spectropolarimeter (200–600 nm, band width 1 nm, scanning speed
50 nm/min, accumulation of 5 scans, cell length 1 mm). Chiral
HPLC chromatograms were obtained on an Agilent 1200 Series
HPLC System using a Daicel Chiralpak IA or IB column
(250ϫ4.6 mm); solvent A: 0.1% TFA (aq), solvent B: acetonitrile
(flow rate 0.5 mL/min, column temperature 40 °C, UV-absorption
detected at 254 nm). Diastereomeric ratios were determined by
1H NMR spectroscopy (c ≈ 35 mm, NS = 1024) and enantiomeric
ratios by chiral HPLC. See the Supporting Information for the syn-
thesis and reactions of (S)-TSЈ.
H), 4.26–4.17 (m, 2 H), 1.92 (sept, J = 6.7 Hz, 1 H), 1.07 (d, J =
6.8 Hz, 3 H), 1.00 (d, J = 6.8 Hz, 3 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 160.8, 146.7, 135.1, 131.4, 129.3, 126.3, 125.6, 73.3,
70.8, 32.7, 18.7, 18.3 ppm. IR (film): ν = 3103, 3083, 2359, 2929,
˜
2903, 2872, 1656, 1607, 1569, 1526, 1465, 1415, 1386, 1341, 1307,
1275, 1243, 1111, 1026, 960, 913, 859, 833, 803, 737, 663, 574, 498,
450 cm–1. HRMS: m/z calcd. for C12H14BrN2O3 [M + H]+
313.0182, found 313.0184.
(4S)-2-(4-Isopropyl-4,5-dihydrooxazol-2-yl)-4-nitrobenzenethiol
[(S)-TS]: (Bromoaryl)oxazoline 3a (500 mg, 1.60 mmol) was dis-
solved in acetonitrile (20 mL), purged with argon for 30 min, and
cooled down to 0 °C. Sodium sulfide trihydrate (254 mg,
1.92 mmol) was added in one portion and purging with argon con-
tinued for 15 min. The resulting suspension turned red slowly. The
reaction mixture was stirred at 0 °C for 2 h and then at room temp.
for 18 h. Hydrochloric acid (1 m) was added until pH = 1, the yel-
low suspension was poured into a dropping funnel attached to
Schlenk-flask, and diluted with dichloromethane (250 mL). The or-
ganic layer was collected and solvents were evaporated in vacuo.
The crude product was suspended in anhydrous toluene (30 mL)
and filtered through a Schlenk frit (D4). The residue was washed
with anhydrous toluene (4ϫ5 mL) to afford a clear yellow solu-
tion. Evaporation of solvents at 40 °C yielded (S)-TS (383 mg,
1.44 mmol, 90%) as an orange solid. 1H NMR (300 MHz, CDCl3):
δ = 15.97 [s(br), 1 H], 8.64 (d, J = 2.6 Hz, 1 H), 7.88 (dd, J = 9.2,
2.7 Hz, 1 H), 7.71 (d, J = 9.2 Hz, 1 H), 4.92 [t(dd), J = 9.5 Hz, 1
H], 4.61 (dd, J = 9.3, 7.3 Hz, 1 H), 4.37 [dt(ddd), J = 9.7, 7.3 Hz,
1 H], 2.00 (sept, J = 6.8 Hz, 1 H), 1.12 (d, J = 6.7 Hz, 3 H), 1.06
(d, J = 6.8 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 176.1,
171.2, 140.4, 137.7, 126.6, 124.2, 116.0, 72.9, 65.7, 32.4, 18.3 (2 C)
2-Bromo-5-nitrobenzoyl Chloride: 2-Bromo-5-nitrobenzoic acid (1)
(4.92 g, 20.0 mmol) was suspended in dichloromethane (100 mL).
After addition of DMF (77 μL, 1.00 mmol) the suspension was co-
oled to 0 °C, oxalyl chloride (2.62 mL, 30.0 mmol) was added drop-
wise and stirring at 0 °C continued for another 30 min. The reac-
tion mixture was warmed up to room temp. upon which the reac-
tion mixture became clear and evolution of gas (HCl) was ob-
served. The solution was stirred at room temp. for 4 h and evapora-
tion of solvents afforded a pale yellow solid that was used directly
for the synthesis of benzamide 2a, b without further purification.
1H NMR (300 MHz, CDCl3): δ = 8.86 (d, J = 2.6 Hz, 1 H), 8.27
(dd, J = 8.8, 2.6 Hz, 1 H), 7.95 (d, J = 8.8 Hz, 1 H) ppm. IR (film):
ν = 3101, 1788, 1758, 1602, 1572, 1526, 1453, 1343, 1294, 1253,
˜
1189, 1106, 1043, 994, 931, 912, 836, 736, 671, 641, 571, 474 cm–1.
(2S)-2-Bromo-N-(1-hydroxy-3-methylbutan-2-yl)-5-nitrobenzamide
(2a): To a solution of l-valinol (2.27 g, 22.0 mmol) in dichloro-
methane (75 mL), triethylamine (5.6 mL, 40.0 mmol) was added
and cooled to 0 °C. 2-Bromo-5-nitrobenzoyl chloride (5.29 g,
20.0 mmol) in dichloromethane (60 mL) was added dropwise yield-
ing a suspension. After warming up to room temp. stirring contin-
ued for another 4 h at room temp. The reaction mixture was diluted
with dichloromethane (100 mL), washed with water, 0.2 m NaHSO4
ppm. IR (film): ν = 3095, 2962, 2876, 2615, 1628, 1591, 1543, 1460,
˜
1321, 1290, 1211, 1119, 1052, 920, 837, 776, 733, 674, 604, 535,
487, 438 cm–1. HRMS: m/z calcd. for C12H13N2O3S (M – H)–
265.0652, found 265.0653.
Benzene Half-Sandwich Complex (S)-4a: [{Ru(η6-C6H6)Cl(μ-Cl)}2]
(150 mg, 300 μmol) and (S)-TS (192 mg, 720 μmol) were suspended
in acetonitrile (15 mL) and purged with argon for 30 min. Triethyl-
(aq.), water, NaHCO3 (satd. aq.), and water (100 mL each). Sol- amine (105 μL, 750 μmol) was added and the red suspension was
vents were evaporated in vacuo and recrystallization from ethanol/ stirred at room temp. for 4 h. The reaction mixture was centrifuged
hexane afforded 2a (6.05 g, 18.3 mmol, 92%) as colorless needles. and the remaining red residue was washed with diethyl ether until
1H NMR (300 MHz, [D6]DMSO): δ = 8.35 (d, J = 9.0 Hz, 1 H),
the solution remained colorless (3ϫ8 mL). The crude product was
8.19–8.14 (m, 2 H), 7.96 (dd, J = 8.5, 0.5 Hz, 1 H), 4.70 [t(dd), J purified over a short silica gel column (10 cm, DCM/MeOH,
= 5.7 Hz, 1 H], 3.81–3.72 (m, 1 H), 3.51–3.47 (m, 2 H), 1.91 (sept, 50:1 Ǟ 20:1). Solvents were evaporated in vacuo to afford complex
J = 6.7 Hz, 1 H), 0.95 (d, J = 6.8 Hz, 3 H), 0.90 (d, J = 6.8 Hz, 3 (S)-4a (230 mg, 479 μmol, 80%) as a dark red solid. The reaction
H) ppm. 13C NMR (75 MHz, [D6]DMSO): δ = 165.3, 146.3, 140.7, was performed in the dark and subsequent work-up and purifica-
134.2, 126.7, 124.8, 123.3, 61.1, 56.6, 28.2, 19.7, 18.1 ppm. IR tion procedure under reduced light to avoid light-induced decom-
1
(neat): ν = 3395, 3254, 3089, 2963, 2876, 1644, 1607, 1555, 1526, position. H NMR (300 MHz, CD3CN): δ = 8.36 (d, J = 2.2 Hz,
˜
1462, 1391, 1345, 1066, 1033, 921, 831, 738, 494 cm–1. HRMS: m/z
1 H), 7.76 (dd, J = 9.1, 2.2 Hz, 1 H), 7.59 (d, J = 9.1 Hz, 1 H),
5.61 (s, 6 H), 4.72 [dt(ddd), J = 9.2, 3.1 Hz, 1 H], 4.62 (dd, J = 9.0,
calcd. for C12H14BrN2O4 (M – H)– 329.0142, found 329.0145.
6
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20203
/KAP1
Date: 10-05-12 08:59:25
Pages: 9
Chiral Auxiliaries for Asymmetric Coordination Chemistry
3.3 Hz, 1 H), 4.43 [t(dd), J = 9.1 Hz, 1 H], 3.07–2.98 (m, 1 H), 1.06
(+104), 391 (–10), 465 (+9), 514 (–4). HRMS: m/z calcd. for
(d, J = 7.2 Hz, 3 H), 0.77 (d, J = 6.6 Hz, 3 H) ppm. IR (neat): ν
C32H29N6O3RuS (M – PF6)+ 679.1066, found 679.1059.
˜
= 3067, 2958, 2870, 1613, 1591, 1551, 1494, 1452, 1380, 1320, 1241,
1124, 1098, 1051, 1004, 967, 912, 863, 824, 737, 674, 604, 530,
468 cm–1. HRMS: m/z calcd. for C18H19N2O3RuS (M – Cl)+
445.0159, found 445.0156.
Auxiliary Removal. General Procedure: An oven dried argon-
flushed brown glass vial was charged with diastereomeric complex
(1.0 equiv.) in dichloromethane (50 mm). Trimethyloxonium tetra-
fluoroborate (2.0 equiv.) was added, the vessel was sealed, and the
suspension was stirred at room temp. for 3 h. Solvents were evapo-
rated in vacuo. The residue was redissolved in acetonitrile (100 mm)
and polypyridine ligand (15 equiv.) was added. The atmosphere in
the vessel was exchanged by purging with argon and the sealed vial
was heated to 110 °C (oil bath temperature). Evaporation of sol-
vents followed and the sample was washed with small portions of
diethyl ether (15 mL total). Purification by column chromatog-
raphy (MeCN Ǟ MeCN/H2O/satd. aq. KNO3 50:3:1) and evapora-
tion of solvents followed. The orange residue was redissolved in a
minimum amount of ethanol/water (Λ-12–14) or water (Λ-15–17)
and precipitated by addition of solid NH4PF6. Water was added to
10 mL and the suspension was centrifuged. The orange precipitate
was washed with water (3 mL and 1 mL) and dried under high
vacuum to afford the enantiomerically enriched polypyridyl ruthe-
nium complex as an orange solid.
Precursor Complex (S)-5a: A photolysis reactor equipped with a
cooling jacket and an uranium filter was charged with acetonitrile
(210 mL). The solvent was purged with nitrogen for 30 min, com-
plex (S)-4a (225 mg, 470 μmol) was added, and the red solution
was irradiated with a mercury medium-pressure lamp for 1 h while
purging with nitrogen continued. Solvents were evaporated in
vacuo to provide precursor complex (S)-5a (243 mg, 463 μmol,
98%) as a purple black solid. (S)-5a was used in diastereoselective
synthesis experiments without further purification and should not
be stored for longer than approximately two weeks (–20 °C under
1
argon atmosphere) as decomposition occurs. H NMR (300 MHz,
CDCl3): δ = 8.68 (dd, J = 2.4, 0.4 Hz, 1 H), 7.72 (dd, J = 9.1,
2.3 Hz, 1 H), 7.67 (dd, J = 9.1, 0.4 Hz, 1 H), 5.30 [dt(ddd), J = 9.1,
3.9 Hz, 1 H], 4.42 (dd, J = 8.8, 4.1 Hz, 1 H), 4.29 [t(dd), J = 9.0 Hz,
1 H], 2.85 (dsept, J = 7.0, 3.1 Hz, 1 H), 2.46 (s, 3 H), 2.38 (s, 3 H),
2.29 (s, 3 H), 0.96 (d, J = 7.0 Hz, 3 H), 0.69 (d, J = 7.0 Hz, 3 H)
ppm. 13C NMR (100 MHz, CDCl3): δ = 167.5, 160.2, 140.6, 134.1,
127.6, 121.7, 121.6, 121.3, 120.8, 120.6, 73.0, 66.2, 28.8, 18.8, 13.8,
Λ-12: According to the general procedure for auxiliary removal,
Λ-(S)-6a (20.0 mg, 24.3 μmol) was treated with trimethyloxonium
tetrafluoroborate (7.2 mg, 48.6 μmol) in dichloromethane (490 μL).
After evaporation of solvents, the residue was redissolved in aceto-
nitrile (245 μL), 2,2Ј-bipyridine (57 mg, 365 μmol) was added, and
heating for 24 h followed. Subsequent purification afforded Λ-12
(14.2 mg, 16.5 μmol, 68%) with an er of 43:1. 1H NMR (300 MHz,
CD3CN): δ = 8.50 (d, J = 8.2 Hz, 6 H), 8.05 (dt, J = 7.9, 1.4 Hz,
6 H), 7.73 (d, J = 5.3 Hz, 6 H), 7.39 (ddd, J = 7.2, 5.7, 1.2 Hz, 6
H) ppm. 13C NMR (75 MHz, CD3CN): δ = 157.9, 152.6, 138.7,
4.8, 4.6, 4.5 ppm. IR (film): ν = 3452, 2964, 2919, 2872, 2275, 2212,
˜
1591, 1542, 1482, 1378, 1305, 1274, 1236, 1176, 1125, 1093, 1050,
969, 916, 865, 807, 730, 672, 644, 613, 538, 485 cm–1. HRMS: m/z
calcd. for C18H22N5O3RuS (M – Cl)+ 490.0485, found 490.0478.
Diastereoselective Synthesis. General Procedure: To a 5 mm solution
of precursor complex (S)-5a, b (1.0 equiv.) was added polypyridine
ligand (2.5 equiv.). The dark purple solution was purged with argon
for 30 min and heated to 110 °C. The reaction mixture was allowed
to cool down to room temp., solvents were evaporated in vacuo,
and the crude product was purified by column chromatography
(MeCN Ǟ MeCN/H2O/satd. aq. KNO3 300:3:1). After evapora-
tion of solvents, the residue was redissolved in a minimum amount
of ethanol/water, precipitated by addition of NH4PF6 (satd. aq.),
and water was added to 12 mL. The suspension was centrifuged
and the black precipitate was washed with water (2ϫ10 mL) and
dried under high vacuum to yield the diastereomeric complex as a
black solid.
128.5, 125.2 ppm. IR (film): ν = 1604, 1445, 1312, 1268, 1165, 880,
˜
833, 756, 731, 555, 418 cm–1. CD (MeCN, 0.1 mm): λ/nm (Δε/
m
–1 cm–1) 219 (–38), 230 (–7), 239 (–27), 256 (+11), 277 (–146), 291
(+331), 321 (–20), 466 (+16). HRMS: m/z calcd. for C30H24N6Ru
(M
2PF6)2+ 285.0551, found 285.0547; m/z calcd. for
C30H24F6N6PRu (M – PF6)+ 715.0750, found 715.0731. HPLC:
Daicel Chiralpak IA column, 15–30% B in 20 min: tR(Δ-12)
–
=
17.13 min (AΔ-12 = 50.19), tR(Λ-12) = 17.79 min (AΛ-12 = 2150.04).
Supporting Information (see footnote on the first page of this arti-
cle): Additional experimental procedures and analytical data in-
cluding the synthesis and reactions with (S)-TSЈ.
Λ-(S)-6a: According to the general procedure for diastereoselective
synthesis, (S)-5a (150 mg, 286 μmol) was treated with 2,2Ј-bipyr-
idine (112 mg, 714 μmol) in chlorobenzene (57 mL) for 3 h to af-
ford Λ-(S)-6a (146 mg, 177 μmol, 62%) with a dr of 59:1. 1H NMR
(500 MHz, CD3CN) (signals of major diastereomer listed only): δ
= 9.57 (d, J = 5.4 Hz, 1 H), 8.69 (d, J = 5.5 Hz, 1 H), 8.66 (d, J =
2.5 Hz, 1 H), 8.51 (d, J = 8.1 Hz, 1 H), 8.41 (d, J = 8.2 Hz, 1 H),
8.38 (d, J = 8.1 Hz, 1 H), 8.26 (d, J = 8.1 Hz, 1 H), 8.12 [dt(ddd),
J = 8.0, 1.4 Hz, 1 H], 8.01 [t(dd), J = 7.8 Hz, 1 H], 7.92 [dt(ddd),
J = 8.0, 1.4 Hz, 1 H], 7.82 (d, J = 5.3 Hz, 1 H), 7.79 [dt(ddd), J =
8.2, 1.2 Hz, 1 H], 7.72 (dd, J = 9.0, 1.9 Hz, 1 H), 7.61–7.58 (m, 2
H), 7.52 (d, J = 9.0 Hz, 1 H), 7.30–7.27 (m, 2 H), 7.14–7.11 (m, 1
H), 4.52–4.48 (m, 2 H), 4.02–4.00 (m, 1 H), 0.23–0.18 (m, 4 H),
0.08–0.07 (m, 3 H) ppm. 13C NMR (125 MHz, CD3CN) (signals
of major diastereomer listed only): δ = 163.3, 159.0, 158.3, 153.2,
152.4, 151.9, 151.5, 138.0, 137.9, 137.8, 137.2, 137.1, 136.3, 136.3,
128.0, 127.8, 127.5, 127.0, 125.0, 124.7, 124.6, 124.2, 123.9, 122.0,
Acknowledgments
This work was supported by Deutsche Forschungsgemeinschaft
(DFG) (grant number ME 1805/4-1).
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M. Wenzel, E. Meggers
FULL PAPER
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Job/Unit: I20203
/KAP1
Date: 10-05-12 08:59:25
Pages: 9
Chiral Auxiliaries for Asymmetric Coordination Chemistry
Asymmetric Coordination Chemistry
M. Wenzel, E. Meggers* .................. 1–9
(Mercaptophenyl)oxazolines were used as
chiral auxiliaries for the asymmetric syn-
thesis of polypyridyl ruthenium complexes.
Key step was the conversion of a coordi-
nated thiolate into a labilized thioether
upon methylation with Meerwein salt, fol-
lowed by a thermal substitution with an
achiral ligand under retention of configur-
ation.
Chiral (Mercaptophenyl)oxazolines as
Auxiliaries for Asymmetric Coordination
Chemistry
Keywords: Asymmetric synthesis / Chiral
auxiliaries / Ruthenium
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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