Titanium-salan-catalyzed asymmetric oxidation of sulfides and kinetic resolution of sulfoxides with H2O2 as the oxidant

Article abstract of DOI:10.1002/ejoc.200800277

Asymmetric oxidation of sufides to sulfoxides by aqueous hydrogen peroxide with catalysis by titanium-salan complexes is presented. Optically active sulfoxides have been obtained with good to high enantioselectivities (up to 97% ee) by a tandem enantioselective oxidation and kinetic resolution procedure, the catalyst performing over 500 turnovers with no loss of enantioselectivity. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800277

FULL PAPER
DOI: 10.1002/ejoc.200800277
Titanium-Salan-Catalyzed Asymmetric Oxidation of Sulfides and Kinetic
Resolution of Sulfoxides with H2O2 as the Oxidant
Konstantin P. Bryliakov*[a] and Evgenii P. Talsi[a]
Keywords: Asymmetric catalysis / Kinetic resolution / Hydrogen peroxide / Sulfoxides / Titanium / Oxidation
Asymmetric oxidation of sufides to sulfoxides by aqueous hy-
drogen peroxide with catalysis by titanium-salan complexes
is presented. Optically active sulfoxides have been obtained
with good to high enantioselectivities (up to 97% ee) by a
tandem enantioselective oxidation and kinetic resolution
procedure, the catalyst performing over 500 turnovers with
no loss of enantioselectivity.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
low turnover numbers (so that 4–16 mol-% of the catalyst
is required), complexity, and expensiveness.
Introduction
The importance of asymmetric sulfoxidations for the
pharmaceutical chemistry has stimulated the search for
other transition-metal-based catalytic systems. An ideal
catalytic system is expected to be cheap, simple and robust,
environmentally safe, efficient (in terms of turnover num-
bers), and highly enantioselective, and to use a readily avail-
able “green” oxidant. Accordingly, catalytic systems based
on nontoxic metals such as titanium and using hydrogen
peroxide as oxidant are of particular interest. Attempts to
find appropriate ligand systems for titanium(IV)-catalyzed
sulfoxidations with H2O2 date from 1986, when Pasini[5a]
reported titanium(IV) salen complexes capable of oxidizing
thioanisole with Ͻ20% ee. Fifteen years later, Katsuki ex-
amined a mononuclear “second-generation” titanium-salen
complex (bearing additional elements of chirality in the
3,3Ј-positions of the salen ligand) as a sulfoxidation catalyst
and found only poor enantioselectivity.[5b] Surprisingly,
though, when this complex was converted into its di-µ-oxo
binuclear titanium counterpart, this appeared to be a very
active and enantioselective catalyst for oxidation of alkyl
aryl sulfides with H2O2 or urea hydroperoxide (UHP; this
gave better results) in methanolic solution (up to 92–99% ee
values).[5b,5c] Soon after, this system was successfully ap-
plied to the asymmetric oxidation of cyclic dithioacetals
(with ee values of the resulting monosulfoxides ranging
from 39 to 99%).[5d] Later, Jackson and co-workers re-
ported solid-supported titanium catalysts featuring a β-
amino alcohol-derived chiral Schiff base as the chirality car-
rier, obtaining sulfoxidation enantioselectivities (for alkyl
aryl sulfoxides) between 45 and 72% ee.[5e] Very recently,
a family of titanium(IV) complexes with N-salicylidene--
amino alcohol-derived Schiff bases capable of asymmetric
oxidation of prochiral sulfides with H2O2 has been pub-
lished.[5f] The sulfoxidation reactions proceeded with high
Optically pure sulfoxides are valuable chiral auxiliaries in
organic synthesis, the sulfinyl group being one of the most
efficient and versatile chiral controllers in C–C and C–X
bond formation.[1] Some sulfoxides are important bioactive
ingredients in the pharmaceutical industry.[2] There are sev-
eral possible approaches to enantiomerically pure organic
compounds: (i) resolution of racemic mixtures, (ii) chemical
modification of chiral objects available from the “chiral
pool”, and (iii) asymmetric synthesis. The most challenging
is catalytic asymmetric synthesis, because one chiral catalyst
molecule can (in principle) create millions of chiral product
molecules. Asymmetric oxidations of prochiral sulfides to
sulfoxides mediated either by biological (e.g., isolated en-
zymes or whole cells, or antibodies) or by chemical (both
metal complexes and metal-free catalysts) agents are known
in the literature.[2a,3]
Historically, the first systems used for the asymmetric
oxidation of sulfides were those of Kagan[3a,3b] and Mod-
ena,[3c] who first applied the “Katsuki–Sharpless reagent”
Ti(OiPr4)/diethyl tartrate/alkyl hydroperoxide, with appro-
priate modifications, to asymmetric oxidations of prochiral
sulfides by alkyl hydroperoxides. Catalytic versions and
modifications of the titanium tartrate systems were later de-
veloped (with up to 90% yields and 90% ee values for cer-
tain sulfides) and remain the most widely applied so far
(including industrial applications; see, for example[3b,4d]).
However, these systems have certain disadvantages, such as
[a] Boreskov Institute of Catalysis, Siberian Branch of the Russian
Academy of Sciences,
Pr. Lavrentieva 5, 630090 Novosibirsk, Russian Federation
Fax: +7-3833-308056
E-mail: bryliako@catalysis.ru
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2008, 3369–3376
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3369

K. P. Bryliakov, E. P. Talsi
FULL PAPER
selectivity and conversion levels, but with only moderate the reaction products along with the starting sulfides. The
enantioselectivities (up to 65% ee).[5f,5g]
ligands containing 3-tert-butyl substituents in the salicylid-
In recent years, salan [N,NЈ-bis(salicylamine)] and salalen ene rings (1, 2) showed quite low enantioselectivities and
(N-salicylamine-NЈ-salicylimine) complexes of different required rather long times for the reactions to proceed to a
transition metals have been applied in asymmetric catalytic significant extent (Table 1, Entries 1, 2). The introduction
processes.[6] Iron(III)-salan complexes bearing additional of 5-nitro substituents decreased the oxidation enantio-
centers of chirality in the aromatic rings, for example, were selectivity, while similarly poor results were obtained with
found to be capable of catalyzing efficient oxidation of alkyl ligand 3. However, the use of ligands 4 and 5, bearing no
aryl sulfides with H2O2 in aqueous media (in 81– substituents in their 3-positions, led to much higher ee val-
96% ee).[6c] AlIII-salalen complexes with additional chiral ues (Table 1, Entries 4, 5), thus allowing greater optimism
centers, taking advantage of the synergistic combination of about the prospects of the titanium-salan complexes in
initial enantioselective oxidation and the subsequent oxidat- asymmetric sulfoxidation. The ee obtained with ligand 6,
ive kinetic resolution process, showed even more remark- with Ph substituents in the 3-positions of the salicylidene
able results in methanolic solutions: alkyl aryl sulfoxides rings and in the chiral diamine moiety was not as high;
were obtained in up to 94–99% ee values.[6d,6e] Notably, bis- however, the use of cyclohexane-1,2-diamine as the chirality
µ-oxo-bridged binuclear Ti-salan catalysts have been found carrier was more profitable. It was found that the sulfoxid-
to catalyze enantioselective epoxidation of olefins with ation enantioselectivity attained its maximum at a ligand/
H2O2 as the oxidant, demonstrating moderate to high titanium ratio of 1.0 (Table 1, Entries 7–9), thus indicating
enantioselectivities (55–98% ee values).[6f,6g]
that the catalytically active sites include one salan ligand
We have found that similar titanium-salan complexes are per titanium. When the catalytic system was tested in
capable of catalyzing enantioselective oxidation of sulfides MeOH with UHP as the catalyst (by Katsuki’s oxidation
to sulfoxides with H2O2, with simultaneous kinetic resolu- protocol[5b,5c]), the ee was much poorer (Entry 10). It was
tion of the sulfoxides (which increases the optical yield). also found that other bulky substrates [e.g., 2-(methylthio)-
Here we report the preparation of a family of various salan naphthalene] were oxidized with much lower enantio-
ligands and their corresponding titanium(IV) complexes selectivity with the 7/Ti(OiPr)4 system (Table 1, Entry 11).
and their application as enantioselective sulfoxidation cata- In all cases, the absolute sulfoxide configuration was re-
lysts.
tained [i.e., use of the (S,S) ligand led to (S) sulfoxides].
Significant amounts of sulfone were found in the reaction
products, thus suggesting that the second oxidation step
might noticeably affect the ee of the sulfoxide, through ki-
Results and Discussion
In the course of our studies, a number of tetradentate netic resolution (see below).
salan ligands of the type 1 were synthesized (Scheme 1) by
For the most successful ligands, the corresponding di-µ-
condensation of the corresponding substituted salicylalde- oxo titanium salan complexes were prepared (Scheme 1)
hydes with optically pure 1,2-diamines and subsequent re- and used as catalysts (Table 2). One can see that the gain in
duction of the resulting Schiff bases with NaBH4 (see the ee due to the replacement of the ligand/Ti(OiPr)4 by prelim-
Experimental Section). Preliminary screening of the tita- inarily prepared titanium complexes was about +9 to
nium-salan catalysts generated in situ was performed, with +24% ee at comparable oxidant-to-substrate ratios (cf.
use of a small excess of the oxidant (Table 1). In the oxi- Table 1 and Table 2). Complexes Ti-4 and Ti-5 showed
dation experiments, sulfoxides and sulfones were found as moderate to high enantioselectivities in the oxidation of the
Scheme 1. Salan ligands considered and the synthesis of the corresponding di-µ-oxo titanium complexes.
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Eur. J. Org. Chem. 2008, 3369–3376

Asymmetric Sulfide Oxidation and Sulfoxide Resolution
Table 1. Enantioselective oxidation of sulfides with the system 1–8/Ti(OiPr)4/H2O2.[a]
Entry
Ligand/Ti
RЈ
R
Reaction time
[h]
Sulfoxide/sulfone
yield [%][b]
Selectivity
[%][b]
Sulfoxide
Sulfoxide
config.[d]
ee [%][c]
1
2
3
4
5
6
7
8
9
1/Ti = 1.25
2/Ti = 1.0
3/Ti = 1.0
4/Ti = 1.0
5/Ti = 1.0
6/Ti = 1.0
7/Ti = 0.85
7/Ti = 1.0
7/Ti = 1.25
7/Ti = 1.0[e]
7/Ti = 1.0
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Me
CH2Ph
CH2Ph
CH2Ph
CH2Ph
CH2Ph
CH2Ph
CH2Ph
CH2Ph
CH2Ph
CH2Ph
2-Naph
24
14
1
5
5
24
24
2.5
15
15
1
41.5/8.5
58.5/9.5
32.5/7.0
62.5/29.5
64.5/26.5
62.5/8.5
67.5/20.0
62.5/14.5
68.8/17.0
60.0/3.0
72.0/16.0
83.0
86.0
82.5
68.0
70.5
88.0
77.0
81.0
80.0
95.0
82.0
13.0
5.0
2.0
R
R
S
R
R
R
S
S
S
S
S
62.5
62.0
43.0
59.0
64.0
60.0
14.0
22.0
10
11
[a] [Oxidant]/[substrate]/[titanium] ratio was 56:50:1. Reaction was carried out at room temperature (25 °C) in CH2Cl2. The sulfide initial
concentration was 0.05 . The preparation of the catalyst systems in situ is described in the Experimental Section. [b] Determination
1
based on H NMR measurements of the sulfide, sulfoxide, and sulfone relative concentrations in the reaction products. [c] The enantio-
meric excess values were measured by 1H NMR with Eu(hfc)3 chiral shift reagent in CCl4. [d] The absolute configuration was determined
as described in the Supporting Information of ref.[7] [e] Reaction was carried out in methanol, UHP was used as the oxidant.
selected sulfides (Table 2, Entries 1–8). As expected, poorer tries 13–15). Particularly high enantioselectivity was
results were obtained with complex Ti-6 (Entries 9, 10). Ti- achieved for the oxidation of PhSCH2Ph, a bulky substrate
8 demonstrated unexpectedly low ee values and reactivities, that can be regarded as a model for pyrmetazole, the pro-
thus demonstrating the particularly negative effect of the chiral precursor S-omeprazole, the efficient proton-pump
highly electron-withdrawing group in the 5-position[8] (see inhibitor sulfoxide.[3b,4d] In all cases, however, the oxidation
Entries 11, 12). In turn, complex Ti-7, although giving a selectivity (and hence sulfoxide yield) was not very high (in-
very modest ee in the oxidation of p-BrPhSMe, showed dicating pronounced overoxidation to the sulfones), so we
rather high enantioselectivities with other substrates (En- attempted to increase the chemoselectivity by using a
Table 2. Enantioselective oxidation of sulfides with the Ti-4 to Ti-8/H2O2 systems.[a]
Entry Complex
[O]/[S]
[mol/mol]
RЈ
R
Reaction time Sulfoxide/sulfone
Selectivity
[%][b]
Sulfoxide
Configuration[d]
[h]
yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Ti-4
Ti-4
Ti-4
Ti-4
Ti-5
Ti-5
Ti-5
Ti-5
Ti-6
Ti-6
Ti-8
Ti-8
Ti-7
Ti-7
Ti-7
Ti-7
Ti-7Ј
1.12
1.12
1.28
1.6
1.12
1.12
1.28
1.6
1.28
1.12
1.28
1.12
1.12
1.12
1.12
0.64
1.12
CH3
CH3 p-BrPh
iPr
Ph
CH3
Ph
2
2
3
3
2
2
3
3
5
77.5/12.0
70.5/10.5
52.2/40.0
51.0/47.6
76.0/16.5
68.3/10.7
60.5/21.0
62.0/34.0
55.5/19.5
67.5/11.0
23.0/3.5
87.0
87.0
56.6
51.7
82.0
86.5
73.5
64.5
74.5
86.0
85.0
90.0
84.0
63.5
80.0
92.5
87.0
45.0
42.0
69.5
86.0
46.7
39.0
64.0
74.5
42.5
52.0
4.5
R
R
R
R
R
R
R
R
R
R
R
R
S
Ph
CH2Ph
Ph
CH3 p-BrPh
iPr
Ph
iPr
Ph
iPr
Ph
Ph
CH2Ph
Ph
CH2Ph
Ph
5
2.5
2.5
16
16
2
16
4.5
CH2Ph
26.5/3.0
4.0
CH3 p-BrPh
72.5/13.5
47.5/27.0
75.0/19.0
48.0/4.0
16.0
69.0
88.0
82.0
0
iPr
Ph
Ph
Ph
Ph
S
S
S
–
CH2Ph
CH2Ph
CH2Ph
30.0/4.5
[a] [Substrate]/[titanium complex] ratio was 100:1 mol/mol unless otherwise stated. Reaction was carried out at room temperature (25 °C)
1
in CH2Cl2. The sulfide initial concentration was 0.05 . [b] Determination based on H NMR measurements of the sulfide, sulfoxide,
and sulfone relative concentrations in the reaction products. [c] The enantiomeric excess values were measured by 1H NMR with Eu-
(hfc)3 chiral shift reagent in CCl4. [d] The absolute configuration was determined as described in the Supporting Information of ref.[7]
Eur. J. Org. Chem. 2008, 3369–3376
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smaller quantity of the oxidant (Table 2, Entry 16). This re- Table 3. The ratios of the apparent rate constants for the model
kinetics scheme of the titanium-catalyzed tandem sulfoxidation/ki-
sulted in higher selectivity but lower ee, indicating that the
high enantioselectivity is attained both through the asym-
netic resolution process.[a]
metric oxidation and through the subsequent kinetic resolu-
tion.[9] Importantly, when 7Ј, containing N–CH3 functional
groups, was used as the chiral ligand, the resulting titanium
catalyst showed no oxidation enantioselectivity, together
with reduced reactivity, thus confirming the proposed cru-
cial role of the hydrogen bonding of the N–H hydrogen and
the coordinated peroxo group.[6f]
Entry Complex
Sulfide
kS/kR
kЈR/kЈS
kЈR/kR
To probe the kinetic resolution process in more detail,
we performed an oxidation of PhSOCH2Ph under the same
conditions as in Table 2, Entry 16. Complex Ti-7 was cho-
sen because of the optimum selectivity/enantioselectivity
combination. The reaction mixture contained 62% of the
sulfone and 38% of the sulfoxide in 77% ee (S configura-
tion), revealing a stereoselectivity factor[9,10] of 6.25. Thus,
one could expect that an excess of the oxidant could be
1
2
(S,S)-Ti-7
(S,S)-Ti-7 PhSCH2Ph
PhSiPr
4.1
8.1
1.8
7.0
4.0
6.25[b]
[a] Determination based on the experimental data shown in Fig-
ure 1. [b] Determination based on the results of a separate kinetic
resolution experiment (see the text).
exploited to improve the ee of the sulfoxide (at the expense tage of CH2Cl2 as solvent over others. A decrease in the
of the sulfoxide yield). To illustrate this, PhSOiPr and catalyst loading down to 0.2 mol% did not cause any deteri-
PhSOCH2Ph were obtained in up to 82.5 and 97.0% ee val- oration in enantioselectivity, thus demonstrating the high
ues, respectively, with a 1.6-fold excess of the oxidant. The efficiency of the catalytic system studied. However, further
dependences of the sulfoxide and sulfone overall yields, to- reduction of the catalyst concentration (to [S]/[Cat] = 1000
gether with the sulfoxide ee values vs. the oxidant/substrate and 2000) led to lower conversions and enantioselectivities
ratio, are shown in Figure 1. From these data, the kinetic after 24 and 48 h, respectively. Apparently, the catalyst
parameters of the sulfoxidation/kinetic resolution tandem turnover limit lies between the [S]/[Cat] ratios of 500 and
process could be evaluated. Indeed, the analysis of the 1000. A reduction in the reaction temperature to 0 °C re-
model reaction scheme (see the scheme for Table 3, in which sulted in somewhat better chemo- and enantioselectivities
S, SO, and SO2 stand for the sulfide, sulfoxide, and sulfone, (Table 4, Entries 8–10). Indeed, with complex Ti-7,
respectively; see Supporting Information for details) gave PhSOSCH2Ph was obtained in 77.5% yield and 92.5% ee
the following ratios of the apparent rate constants (Table 3). (cf. 75.0% yield and 88.0% ee at room temperature, Table 2,
The lower stereoselectivity of the sulfoxidation stage (kS/kR) Entry 15) at the same oxidant/substrate ratio. Ti-4- and Ti-
along with the relatively fast second oxidation stage (kЈR/ 5-catalyzed sulfoxidations gave essentially the same ee val-
kR) explain the lower sulfoxide yield and ee for the oxi- ues as at room temperature, but rather higher yields (cf.
dation of PhSiPr (in relation to the case of PhSCH2Ph).
Table 4, Entries 8–9 and Table 2, Entries 4 and 8). This re-
Further, the effects of solvent, temperature, and sub- sult is not surprising, because for the room-temperature oxi-
strate/catalyst ratio on the sulfoxidation were probed. Sev- dations we had to use higher excesses of H2O2 to achieve
eral reactions were performed in different solvents (Table 4, similar ee levels, so the systems demonstrate potential for
Entries 1–4; Table 1, Entry 10), demonstrating the advan- oxidation procedure improvement by use of lower oxidation
Figure 1. Sulfoxide and sulfone yields and the sulfoxide enantioselectivities vs. oxidant/substrate ratio in the oxidation of PhSiPr (a) and
PhSCH2Ph (b). Catalyst: Ti-7, CH2Cl2, 25 °C, sulfide/Ti-7 = 100:1.
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Asymmetric Sulfide Oxidation and Sulfoxide Resolution
Table 4. Effect of solvent, temperature, and the substrate/catalyst ratio on the PhSCH2Ph asymmetric oxidation by the systems Ti-4, Ti-
5, and Ti-7/H2O2.[a]
Entry
Cat.
Temp.
[°C]
Solvent
[O]/[S]
[mol/mol]
[S]/[Cat]
[mol/mol]
Reaction time
[h]
Sulfoxide/sulfone
yield [%][b]
Select.
[%][b]
Sulfoxide Config.[d]
ee [%][c]
1
2
3
4
5
6
7
8
9
Ti-7
Ti-7
Ti-7
Ti-7
Ti-7
Ti-7
Ti-7
Ti-4
Ti-5
Ti-7
Ti-4
Ti-4
25
25
25
25
25
25
25
0
CHCl3
CCl4
1.6
1.6
1.6
1.6
1.6
100
100
100
100
500
1000
2000[e]
100
100
100
4
4
4
4
4
24
48
5
5
5
58.0/27.5
8.0/7.0
68.0
53.0
47.0
65.5
64.0
77.5
91.5
82.0
87.0
85.5
66.0
85.5
88.0
57.5
66.5
97.0
97.0
77.0
60.0
81.0
75.5
92.5
72.5
59.0
S
S
S
S
S
S
S
R
R
S
R
R
toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
23.0/26.0
65.0/34.5
63.5/35.5
49.5/14.5
34.0/3.0
71.0/15.5
67.5/10.0
77.5/13.0
60.5/31.0
85.5/14.0
1.6
1.6
1.12
1.12
1.12
1.28
1.28
0
0
10
11
12
0[f]
0[g]
100
100
6
6
[a] [Substrate]/[titanium complex] ratio was 100:1 mol/mol and sulfide initial concentration was 0.05  unless otherwise stated. Reaction
1
was carried out in CH2Cl2. [b] Determination based on H NMR measurements of the sulfide, sulfoxide, and sulfone relative concentra-
1
tions in the reaction products. [c] The enantiomeric excess values were measured by H NMR with Eu(hfc)3 chiral shift reagent in CCl4.
[d] The absolute configuration was determined as described in the Supporting Information of ref.[7] [e] Sulfide initial concentration was
0.1 . [f] PhSiPr was used as the substrate. [g] 2-NaphSMe was used as the substrate.
temperatures. The fair to high ee values obtained for the However, the enantioselectivities observed were not as en-
oxidation of other substrates at 0 °C (Table 4, Entries 11, couraging as for the Ti-catalyzed sulfoxidations (Table 5).
12) support this conclusion. Importantly, cumyl hydroper- Zhu and co-workers proposed an oxovanadium(IV) salan
oxide used as the oxidant resulted in no conversion of the complex as the reactive intermediate.[6b] Our experience in
substrate (PhSCH2Ph, catalyst: Ti-4).
vanadium-catalyzed oxidations with hydroperoxides points
Inspired by the findings of Zhu,[6b,6i] who applied salan- in favor of formation of the vanadium(V) reactive interme-
type ligands to vanadium- and tungsten-catalyzed sulfoxid- diates upon interaction of the vanadium(IV) precursors
ations, we also examined our salan ligands in vanadium- with the terminal oxidants.[11] We have shown that vana-
and molybdenum-catalyzed reactions. The catalysts were dium-salan-catalyzed oxidations can also be performed
prepared in situ by mixing the metal precursors with the starting with vanadium(V) precursors [e.g., VO(OnBu)3; see
ligands. Interestingly, unlike in the case of the Ti-salan sys- Table 5, Entries 6–11], indicating that vanadium(V) rather
tems, reversal of the sulfoxide absolute configuration was than vanadium(IV) intermediates are the true oxidizing spe-
generally observed with the Mo- and V-salan counterparts. cies.
Table 5. Enantioselective oxidation of sulfides with the 1–7/metal/H2O2 system.[a]
Entry Ligand/Metal
Metal source
Sulfide
Reaction time Sulfoxide/sulfone
Sulfoxide
Sulfoxide
[h]
yield [%][b]
ee [%][c]
config.[d]
1
2
3
4
5
6
7
8
9
7/Mo = 1.0
7/Mo = 1.0
1/Mo = 1.5
7/V = 1.5
MoO2(acac)2
MoO2(acac)2
MoO2(acac)2
VO(acac)2
PhSCH2Ph
PhSCH3
p-CH3PhSCH3
PhSCH2Ph
p-CH3PhSCH3
p-BrPhSCH3
p-CH3PhSCH3
p-BrPhSCH3
p-CH3PhSCH3
p-CH3PhSCH3
p-BrPhSCH3
24
24
24
15
4
14
14
20
14
15
15
79.5/12.0
82.0/10.5
57.0/3.0
86.0/10.5
85.8/5.5
89.5/6.0
51.5/6.5
32.0/4.5
82.0/12.5
85.5/14.0
78.0/13.5
8.5
0
2.5
8.5
10.0
2.0
R
–
S
R
R
R
S
R
R
R
R
1/V = 1.5
VO(acac)2
7/Ti = 1.5
2/Ti = 1.5
4/Ti = 1.5[e]
4/Ti = 1.5
5/Ti = 1.0
5/Ti = 1.0
VO(OnBu)3
VO(OnBu)3
VO(OnBu)3
VO(OnBu)3
VO(OnBu)3
VO(OnBu)3
6.0
37.5
19.5
17.0
13.5
10
11
[a] [Oxidant]/[substrate]/[titanium] ratio was 56:50:1. Reaction was carried out at room temperature (25 °C) in CH2Cl2. The sulfide initial
concentration was 0.05 . [b] Determination based on 1H NMR measurements of the sulfide, sulfoxide, and sulfone relative concentrations
1
in the reaction products. [c] The enantiomeric excess values were measured by H NMR with Eu(hfc)3 chiral shift reagent in CCl4. [d]
The absolute configuration was determined as described in the Supporting Information of ref.[7] [e] Reaction carried out in CHCl3 at
–12 °C without stirring.
Eur. J. Org. Chem. 2008, 3369–3376
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3373

K. P. Bryliakov, E. P. Talsi
FULL PAPER
SnCl4 (1 mmol) was then added with stirring, followed by Et3N
(4 mmol). After the mixture had been stirred for 60 min at room
temperature, paraformaldehyde (33 mmol) was added, and the
Conclusions
For the first time, catalytic asymmetric oxidation of pro-
chiral sulfides with aqueous H2O2 has been performed in flask was heated to 100 °C for 8–12 h. The resulting mixture was
cooled to room temperature, poured into water (50 mL), acidified
to pH 2, and extracted with diethyl ether (2ϫ50 mL). The extract
was washed with aqueous NaCl and dried with CaSO4, and the
solvent was removed. The crude product was, if necessary, purified
by column chromatography on SiO2 (eluent: hexane/ethyl acetate)
and recrystallized from hexane.
the presence of a novel titanium-salan-based catalytic sys-
tem. Optically active sulfoxides have been obtained with
good to high enantioselectivities (up to 97% ee). The sys-
tem is particularly suitable for bulky thioethers, which could
be regarded as models for the syntheses of bioactive com-
pounds. The high asymmetric induction levels are attained
in a tandem enantioselective oxidation/kinetic resolution
procedure. The catalysts demonstrate high efficiency, being
capable of performing over 500 turnovers with no loss of
enantioselectivity. The catalytic system studied employs chi-
ral ligands prepared from readily available precursors, a
non-toxic metal, and a “green” oxidant; the reaction sol-
vent – dichloromethane – is a volatile liquid and can be
easily separated from the reaction mixture by distillation
and recycled. The chemo- and enantioselectivities of the
sulfoxidation can be increased by lowering the reaction tem-
perature. Detailed studies of the temperature effect on both
the oxidation and kinetic resolution stages and the detailed
reaction mechanism are currently underway and will be re-
ported in future publications.
3-Adamantyl-2-hydroxy-5-methylbenzaldehyde:
1H
NMR
(250 MHz, CCl4, 20 °C): δ = 11.67 (1 H, OH), 9.79 (1 H, CHO),
7.25 (1 H, Ar-H), 7.17 (1 H, Ar-H), 2.32 (3 H, Ar-CH3), 2.12, 2.09
(9 H, -CH2-, -CH-), 1.78 (6 H, -CH2-) ppm.
2-Hydroxy-3-phenylbenzaldehyde: 1H NMR (250 MHz, CDCl3,
20 °C): δ = 11.54 (1 H, OH), 9.97 (1 H, CHO), 7.59–7.01 (8 H, Ar-
H) ppm.
Some of the synthesized aldehydes were subjected to nitration ac-
cording to ref.[12b]
2-Hydroxy-5-nitro-3-phenylbenzaldehyde: 1H NMR (250 MHz,
CCl4, 20 °C): δ = 12.06 (1 H, OH), 10.00 (1 H, CHO), 8.44 (2 H,
Ar-H), 7.53–7.33 (5 H, Ph) ppm.
3-tert-Butyl-2-hydroxy-5-nitrobenzaldehyde: 1H NMR (250 MHz,
CCl4, 20 °C): δ = 12.47 (1 H, OH), 9.99 (1 H, CHO), 8.37 (2 H,
Ar-H), 1.49 (9 H, tBu) ppm.
General Method for the Preparation of Salen Precursors of the Salan
Ligands by Schiff Condensation of the Corresponding Salicylalde-
hydes with Chiral Diamines: The substituted salicylaldehyde
(4.5 mmol) and the chiral diamine (2 mmol) were heated at reflux
in EtOH (10–20 mL) for 3 h (if necessary, 10 vol.-% of CHCl3 was
added for better solubility). The mixture was cooled to room tem-
perature. If precipitation of the Schiff base was observed, the solid
was filtered off, washed with hexane, and dried in vacuo. Otherwise,
the reaction mixture was evaporated in a flow of air and the tetra-
dentate Schiff base was separated and purified by column
chromatography on SiO2 (eluent: hexane/ethyl acetate).
Experimental Section
General Remarks: CDCl3 and CCl4 (analytical grade) were stored
under molecular sieves and used without further purification. Ethyl
acetate and hexane (reagent grade) were used for column
chromatography without purification. For oxidation procedures,
reagent grade CH2Cl2 (99.85%) was used. Toluene for synthesis
was degassed in vacuo and dried with molecular sieves. H2O2 was
used as analytical grade 30% aqueous solution. Silica gel 40
(0.063–0.200 mm) for column chromatography was purchased from
Merck. All other chemicals were Aldrich, AlfaAesar, or Acros com-
mercial reagents. 1H and 13C NMR spectra were recorded on a
Bruker DPX 250 spectrometer at 250.13 and 62.87 MHz, respec-
tively, in 5 mm cylindrical tubes. Chemical shifts were referenced
to internal reference TMS, with positive values in the low-field di-
rection. 1H-decoupled 13C NMR measurements: spectral widths,
25000 Hz; spectrum accumulation frequency, 0.2 Hz; number of
scans, 1000; 90° radio-frequency pulse; duration, 6.2 µs.
1H NMR spectroscopic data for the Schiff base precursors of li-
gands 1–3, 5, and 7 can be found in ref.[12c] (Supporting Infor-
mation).
(R,R)-N,NЈ-Bis(3-phenylsalicylidene)-1,2-diphenylethylenediamine:
1H NMR (250 MHz, CCl4, 20 °C): δ = 13.62 (2 H, OH), 8.38 (2
H, CH=N), 7.56, 7.12 (26 H, Ar-H), 4.65 (2 H, C*H) ppm.
(R,R)-N,NЈ-Bis(5-nitro-3-phenylsalicylidene)cyclohexane-1,2-di-
amine and (R,R)-N,NЈ-Bis(salicylidene)cyclohexane-1,2-diamine
were used in imine moiety reduction without separation.
Synthesis of the Salan Ligands: Chiral salan ligands were synthe-
sized as shown in Scheme 2, starting from substituted phenols or
salicylaldehydes.
General Method for the Preparation of Substituted Salicylaldehydes
from the Corresponding Phenols (adopted from ref.[12a], with modifi-
cations): The substituted phenol (10 mmol) was placed in a flask
with dry toluene (20 mL) and a magnetic stirrer under argon, and
General Method for the Reduction of Salens to Salans (adopted
from ref.[12d]): Solid NaBH4 was added portionwise at 0 °C over a
period of 30 min to a solution of the corresponding salen precursor
(1 mmol) in MeOH/THF solution (3:5, 16 mL). After the addition,
Scheme 2. Preparation of salan ligands 1–8.
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Eur. J. Org. Chem. 2008, 3369–3376

Asymmetric Sulfide Oxidation and Sulfoxide Resolution
the mixture was stirred at room temperature overnight (with a
change of the solution color from yellow to colorless, except in the
cases of ligands 2 and 8) and poured into H2O (30 mL). The re-
sulting mixture was extracted with CH2Cl2 (2ϫ30 mL), and the
extract was dried with CaSO4, diluted with hexane, and allowed to
evaporate slowly to yield solid 1–8.
Ti-salan complexes show poorly informative 1H NMR spectra
(broad signals).
General Oxidation Procedure
A: Catalysts Prepared in Situ: The appropriate salan ligand (1.68–
3.0 µmol) and the metal source [Ti(OiPr)4 or VO(acac)2 or
MoO2(acac)2, 2.0 µmol] were combined in CH2Cl2 (2 mL) and
stirred for 30 min. Sulfide (0.1 mmol) was added to the resulting
solution, followed by an appropriate amount of aqueous hydrogen
peroxide (25%), added in one portion. Stirring was continued at
room temperature for 1 h–48 h, the reaction progress being moni-
tored by TLC (eluent: EtOAc/hexane).
(R,R)-N,NЈ-Bis(3,5-di-tert-butylsalicyl)cyclohexane-1,2-diamine (1):
1H NMR (250 MHz, CCl4, 20 °C): δ = 10.17 (s, 2 H, OH), 7.24 (s,
2 H, Ar-H), 6.71 (s, 2 H, Ar-H), 3.98 (d, 2 H, N-CHH-), 3.90 (d,
2 H, N-CHH), 2.44 (2 H, C*H), 2.18, 1.72, 1.62, (6 H, cHex), 1.33,
(s, 18 H, tBu), 1.26 (s, 18 H, tBu) ppm; two cyclohexane protons
are masked by tBu peaks.
B: Preliminarily Prepared Ti Catalysts: The sulfide (0.1 mmol) was
added at the desired temperature to the appropriate Ti-salan cata-
lyst (1 µmol) dissolved in CH2Cl2 (2 mL). The appropriate amount
of aqueous hydrogen peroxide (25%) was then added to the re-
sulting solution in one portion. Stirring was continued at the de-
sired temperature for 1 h–48 h, the reaction progress being moni-
tored by TLC (eluent: EtOAc/hexane). For 0 °C experiments, the
reaction flask was cooled with an ice bath.
(R,R)-N,NЈ-Bis(5-tert-butyl-3-nitrosalicyl)cyclohexane-1,2-diamine
(2): 1H NMR (250 MHz, CCl4, 20 °C): δ = 11.7 (2 H, OH), 8.02
(d, 2 H, Ar-H), 7.72 (d, 2 H, Ar-H), 4.10 (d, 2 H, N-CHH-), 4.04
(d, 2 H, N-CHH-), 2.53 (2 H, C*H), 2.26 (2 H, cHex), 1.39 (s, 18
H, tBu) ppm; some peaks are masked by tBu signals and traces of
THF.
(S,S)-N,NЈ-Bis(3-adamantyl-5-methylsalicyl)cyclohexane-1,2-di-
1
amine (3): H NMR (250 MHz, CCl4, 20 °C): δ = 6.78 (2 H, Ar-
To stop the reaction, the mixture was diluted with water (1 mL).
The organic phase was separated, and volatiles were removed
quickly (in 1–2 min) in a flow of air. The residue was extracted with
CCl4 (8 mL), and the extract was dried with CaSO4 and analyzed
by 1H NMR. The enantiomeric excess values were measured by 1H
NMR with Eu(hfc)3 chiral shift reagent in CCl4 or CCl4/CDCl3.
The absolute configurations were determined by comparison of
Eu(hfc)3-shifted NMR patterns of sulfoxides with those of the sulf-
oxides of known absolute configuration (for details see ref.[7], Sup-
porting Information). The ee measurement uncertainty was Յ1%
(in the range of 10–80% ee) and not higher than 0.5% for Ͼ80% ee.
Conversion and selectivity were calculated from 1H NMR measure-
ments of the sulfide, sulfoxide, and sulfone relative concentrations.
Selected 1H NMR spectroscopic data for the compounds involved,
(250 MHz, CCl4, 20 °C): δ = p-BrPhSCH3 2.45, p-BrPhSOCH3
2.62, p-BrPhSO2CH3 2.94, PhSCH2Ph 4.05, PhSOCH2Ph 3.90 (m),
PhSO2CH2Ph 4.15, p-CH3PhSCH3 2.42, p-CH3PhSCH3 2.31, p-
CH3PhSOCH3 2.57, p-CH3PhSOCH3 2.42, p-CH3PhSO2CH3 2.90,
p-CH3PhSO2CH3 2.46, PhSCH3 2.35, PhSOCH3 2.61, PhSO2CH3
2.92; PhSCH(CH3)2 3.32, PhSOCH(CH3)2 2.64, PhSO2CH(CH3)2
2.92, 2-Naph-SCH3 2.55, 2-Naph-SOCH3 2.68, 2-Naph-SO2CH3
2.99 .
H), 6.53 (2 H, Ar-H), 3.97 (d, 2 H, N-CHH-), 3.83 (d, 2 H, N-
CHH-), 2.39 (2 H, C*H), 2.19 (6 H, Ar-CH3), 2.04 (12 H, Ada-
CH2), 2.00 (6 H, Ada-CH), 1.74 (12 H, Ada-CH2) ppm. Some
peaks masked by intense signals in the range 2.2–2.0 ppm.
(R,R)-N,NЈ-Bis(salicyl)cyclohexane-1,2-diamine (4): 1H NMR
(250 MHz, CCl4, 20 °C): δ = 7.17 (t, 2 H, Ar-H), 6.94 (d, 2 H, Ar-
H), 6.77 (m, 4 H, Ar-H), 4.06 (d, 2 H, N-CHH-), 3.93 (d, 2 H, N-
CHH-), 2.44 (2 H, C*H), 2.20 (2 H, cHex), 1.73 (2 H, cHex), 1.3–
1.1 (4 H, cHex) ppm.
1
(R,R)-N,NЈ-Bis(5-bromosalicyl)cyclohexane-1,2-diamine (5): H
NMR (250 MHz, CDCl3, 20 °C): δ = 7.24 (dd, 2 H, Ar-H), 7.06
(d, 2 H, Ar-H), 6.68 (d, 2 H, Ar-H), 4.03 (d, 2 H, N-CHH-), 3.90
(d, 2 H, N-CHH-), 2.41 (2 H, C*H), 2.17 (2 H, cHex), 1.45 (2 H,
cHex), 1.3–1.1 (4 H, cHex) ppm.
(R,R)-N,NЈ-Bis(3-phenylsalicyl)-1,2-diphenylethylenediamine (6): 1H
NMR (250 MHz, CDCl3, 20 °C): δ = 7.47–6.93 (26 H, Ar-H), 3.91
(d, 2 H, N-CHH-), 3.90 (s, 2 H, C*H), 3.60 (d, 2 H, N-CHH-)
ppm.
(S,S)-N,NЈ-Bis(3-phenylsalicyl)cyclohexane-1,2-diamine (7): 1H
NMR (250 MHz, CDCl3, 20 °C): δ = 7.54, 7.39, 7.25, 6.95, 6.83
(16 H, Ar-H), 4.10 (d, 2 H, N-CHH-), 3.96 (d, 2 H, N-CHH-),
2.50 (2 H, C*H), 2.20 (2 H, cHex), 1.72 (2 H, cHex), 1.25 (4 H,
cHex) ppm.
Kinetic Resolution Procedure: PhSOCH2Ph (0.1 mmol) was added
at room temperature to the Ti-7 catalyst (1 µmol) dissolved in
CH2Cl2 (2 mL). Aqueous hydrogen peroxide (25%, 64 µmol) was
added in one portion to the resulting solution. Stirring was contin-
ued at the desired temperature for 16 h. The reaction products were
separated as described for the general oxidation procedure and
(R,R)-N,NЈ-Bis(5-nitro-3-phenylsalicyl)cyclohexane-1,2-diamine (8):
1H NMR (250 MHz, CDCl3, 20 °C): δ = 8.15 (d, 2 H, Ar-H), 7.88
(d, 2 H, Ar-H), 7.48, 7.36 (10 H, Ar-H), 4.20 (d, 2 H, N-CHH-),
4.04 (d, 2 H, N-CHH-), 2.45 (2 H, C*H), 2.23 (2 H, cHex), 1.78
(2 H, cHex), 1.26 (4 H, cHex) ppm.
1
were analyzed by H NMR spectroscopy.
Supporting Information (see also the footnote on the first page of
this article) includes the mathematical treatment of the model ki-
netic scheme for the tandem sulfoxidation/kinetic resolution pro-
cess.
(R,R)-N,NЈ-Bis(3-phenylsalicylmethyl)cyclohexane-1,2-diamine (7Ј):
This compound was prepared by Mannich condensation of (R,R)-
N,NЈ-dimethylcyclohexane-1,2-diamine with biphenyl-2-ol
(2 equiv.) and purified by column chromatography on SiO2 (eluent:
hexane/ethyl acetate).[12e] 1H NMR (250 MHz, CDCl3, 20 °C): δ =
7.5–6.8 (16 H, Ar-H), 3.90 (d, 2 H, N-CHH-), 3.64 (d, 2 H, N- Acknowledgments
CHH-), 2.72 (2 H, C*H), 2.27 (6 H, N-CH3), 1.8 (2 H, cHex) ppm.
Some signals not found (masked by admixtures of ethyl acetate and
butyl acetate).
The authors thank the Russian Foundation for Basic Research for
financial support, grant 06-03-32214.
Preparation of Bis(µ-oxo) Binuclear Ti-Salan Complexes: Ti-salan
complexes Ti-4 to Ti-8 were prepared by a procedure described in
ref.[6d] and were recrystallized from CH2Cl2/hexane at +4 °C. The
[1] a) G. Solladie, Synthesis 1981, 185–196; b) M. R. Barbachyn,
C. R. Johnson, in: Asymmetric Synthesis (Ed.: J. D. Morrison),
Eur. J. Org. Chem. 2008, 3369–3376
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3375

K. P. Bryliakov, E. P. Talsi
FULL PAPER
Academic Press, Orlando, FL, 1984, vol. 4, pp. 221–261; c)
G. H. Posner, Acc. Chem. Res. 1987, 20, 72–78; d) G. H. Posner,
in: The Chemistry of Sulphones and Sulphoxides (Eds.: S. Patai,
Z. Rappoport, C. J. M. Stirling), John Wiley & Sons, Chiches-
ter, UK, 1988, chapter 16, pp. 823–849; e) K. K. Andersen, in:
The Chemistry of Sulfones and Sulfoxides (Eds.: S. Patai, Z.
Rappoport, C. J. M. Stirling), John Wiley & Sons, Ltd., Chich-
ester, 1988, chapter 3, pp. 55–94; f) M. C. Carreno, Chem. Rev.
1995, 95, 1717–1760; g) H. Pellissier, Tetrahedron 2006, 62,
5559–5601.
J. Am. Chem. Soc. 2007, 129, 8940–8941; d) T. Yamaguchi, K.
Matsumoto, B. Saito, T. Katsuki, Angew. Chem. Int. Ed. 2007,
46, 4729–4731; e) K. Matsumoto, T. Yamaguchi, J. Fujisaki, B.
Saito, T. Katsuki, Chem. Asian J. 2008, 3, 351–358; f) Y. Sa-
wada, K. Matsumoto, S. Kondo, H. Watanabe, T. Ozawa, K.
Suzuki, B. Saito, T. Katsuki, Angew. Chem. Int. Ed. 2006, 45,
3478–3480; g) Y. Sawada, K. Matsumoto, T. Katsuki, Angew.
Chem. Int. Ed. 2007, 46, 4559–4561; h) K. Matsumoto, B.
Saito, T. Katsuki, Chem. Commun. 2007, 3619–3627; i) Y.
Zhang, J. T. Sun, C. Zhu, Chin. Chem. Lett. 2006, 17, 1173–
1176.
[2] a) J. Legros, J. R. Dehli, C. Bolm, Adv. Synth. Catal. 2005, 347,
19–31; b) K. D. Wing, A. H. Glickman, J. E. Casida, Science
1983, 219, 63–65; c) A. H. Glickman, K. D. Wing, J. E. Casida,
Toxicol. Appl. Pharmacol. 1984, 73, 16–22; d) A. Kalir, H. H.
Kalir, in: The Chemistry of Sulfur-Containing Functional
Groups (Eds.: S. Patai, Z., Rappoport), Wiley, New York, 1993,
pp. 957–975; e) A. M. Rouhi, Chem. Eng. News 2003, 81, 56–
61.
[7]
[8]
K. P. Bryliakov, E. P. Talsi, Angew. Chem. Int. Ed. 2004, 43,
5228–5230.
The 5-nitro-3-phenylsalicylidene moiety was introduced by
Skarzewski and was found to be very successful in sulfoxida-
˙
tions catalyzed by vanadium Schiff base systems: J. Skarzewski,
˙
E. Ostrycharz, R. Siedlecka, Tetrahedron: Asymmetry 1999, 10,
3457–3461.
[3] a) I. Fernandez, N. Khiar, Chem. Rev. 2003, 103, 3651–3705;
b) H.-J. Federsel, Chirality 2003, 15, S128–S142; c) H. B. Ka-
gan, in: Catalytic Asymmetric Synthesis 2nd ed. (Ed.: I. Ojima),
Wiley, New York, 2000; chapter 6, pp. 327–356; d) C. Bolm, K.
Muñiz, J. P. Hildebrand, in: Comprehensive Asymmetric Cataly-
sis (Eds.: E. N. Jacobsen, A. Pfaltz, H. Yamamoto), Springer,
Berlin, 1999, p. 697; e) J.-E. Bäckvall, in: Modern Oxidation
Methods (Ed.: J.-E. Bäckvall), Wiley-VCH, Weinheim, 2004,
p. 193; f) K. P. Bryliakov, E. P. Talsi, Curr. Org. Chem. 2008,
12, 386–404.
[4] a) P. Pitchen, H. B. Kagan, Tetrahedron Lett. 1984, 25, 1049–
1052; b) P. Pitchen, M. Desmukh, E. Dunach, H. B. Kagan, J.
Am. Chem. Soc. 1984, 106, 8188; c) F. Di Furia, G. Modena,
R. Seraglia, Synthesis 1984, 325–326; d) H. Cotton, T. Elebr-
ing, M. Larsson, L. Li, H. Sörensen, S. von Unge, Tetrahedron:
Asymmetry 2000, 11, 3819–3825.
[5] a) A. Colombo, G. Marturano, A. Pasini, Gazz. Chim. Ital.
1986, 116, 35–40; b) B. Saito, T. Katsuki, Tetrahedron Lett.
2001, 42, 3873–3876; c) B. Saito, T. Katsuki, Tetrahedron Lett.
2001, 42, 8333–8336; d) T. Tanaka, B. Saito, T. Katsuki, Tetra-
hedron Lett. 2002, 43, 3259–3262; e) S. D. Green, C. Monti,
R. F. W. Jackson, M. S. Anson, S. J. F. Macdonald, Chem.
Commun. 2001, 2594–2595; f) K. P. Bryliakov, E. P. Talsi, J.
Mol. Catal. A J. Mol. Catal. 2007, 264, 280–287; g) K. P. Bryl-
iakov, A. L. Nuzhdin, E. P. Talsi, in: Abstracts of the III Inter-
national Conference Catalysis: Fundamentals and Application,
Novosibirsk, 2007, vol. I, pp. 155–156.
[9]
Recent examples of efficient tandem asymmetric oxidation of
sulfides/kinetic resolution of sulfoxides see ref.[6b] and: a) C.
Drago, L. Caggiano, R. F. W. Jackson, Angew. Chem. Int. Ed.
2005, 44, 7221–7223; b) X. Jia, X. Li, L. Xu, Q. Shi, T. T.-L.
Au-Yeung, C. W. Yip, X. Yao, A. S. C. Chan, Adv. Synth. Ca-
tal. 2004, 346, 723–726; c) A. Lattanzi, S. Piccirillo, A. Scretti,
Adv. Synth. Catal. 2007, 349, 357–363; d) A. Basak, A. U. Bar-
lan, H. Yamamoto, Tetrahedron: Asymmetry 2006, 17, 508–
511; e) F. Naso, C. Cardellicchio, F. Affortunato, M. A. M. Ca-
pozzi, Tetrahedron: Asymmetry 2006, 17, 3226–3229.
[10] H. B. Kagan, J. C. Fiaud, Top. Stereochem. 1988, 18, 249–330.
[11] a) N. N. Karpyshev, O. D. Yakovleva, E. P. Talsi, K. P. Bryl-
iakov, O. V. Tolstikova, A. G. Tolstikov, J. Mol. Catal. A 2000,
157, 91–95; b) K. P. Bryliakov, N. N. Karpyshev, S. A. Fomin-
sky, A. G. Tolstikov, E. P. Talsi, J. Mol. Catal. A 2001, 171, 73–
80; c) K. P. Bryliakov, E. P. Talsi, T. Kühn, C. Bolm, New J.
Chem. 2003, 27, 609–614; d) K. P. Bryliakov, E. P. Talsi, S. N.
Stas’ko, O. A. Kholdeeva, S. A. Popov, A. V. Tkachev, J. Mol.
Catal. A J. Mol. Catal. 2003, 194, 79–88; e) K. P. Bryliakov,
E. P. Talsi, Kinet. Catal. 2003, 44, 334–346.
[12] a) G. Casiraghi, G. Casnati, G. Puglia, G. Sartori, G. Terenghi,
J. Chem. Soc. Perkin Trans. 1 1980, 1862–1865; b) M. Craw-
ford, J. W. Rasburn, J. Chem. Soc. 1956, 2155–2160; c) K. P.
Bryliakov, E. P. Talsi, Chem. Eur. J. 2007, 13, 8045–8050; d) J.
Balsells, P. J. Carroll, P. J. Walsh, Inorg. Chem. 2001, 40, 5568–
5574; e) E. Y. Tshuva, N. Gendeziuk, M. Kol, Tetrahedron Lett.
2001, 42, 6405–6407.
[6] a) C. V. Ward, M. L. Jiang, T. Kee, Tetrahedron Lett. 2000, 41,
6181–6184; b) J. Sun, C. Zhu, Z. Dai, M. Yang, Y. Pan, H. Hu,
J. Org. Chem. 2004, 69, 8500–8503; c) H. Egami, T. Katsuki,
Received: March 13, 2008
Published Online: May 9, 2008
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Eur. J. Org. Chem. 2008, 3369–3376