Iron-catalyzed imidative kinetic resolution of racemic sulfoxides

Article abstract of DOI:10.1002/chem.201303850

Kinetic resolution of racemic sulfoxides requires either custom substrates or shows moderate enantioselectivity, leading to achiral coproducts (such as sulfones) as an intrinsic part of the process. A new strategy is demonstrated that allows the resolution of racemic sulfoxides through catalytic asymmetric nitrene-transfer reactions. This approach gives rise to both optically active sulfoxides and highly enantioenriched sulfoximines. By using a chiral iron catalyst and a readily available iodinane reagent, high selectivity factors have been achieved under very practical reaction conditions. With respect to the substrate scope, it is noteworthy that this unprecedented imidative kinetic resolution of racemic sulfoxides provides access to both aryl-alkyl and dialkyl sulfoximines in highly enantioenriched forms. Copyright

Full text of DOI:10.1002/chem.201303850

DOI: 10.1002/chem.201303850
Communication
&
Kinetic Resolution
Iron-Catalyzed Imidative Kinetic Resolution of Racemic Sulfoxides
Jun Wang,[a, b] Marcus Frings,[a] and Carsten Bolm*[a]
Abstract: Kinetic resolution of racemic sulfoxides requires
either custom substrates or shows moderate enantioselec-
tivity, leading to achiral coproducts (such as sulfones) as
an intrinsic part of the process. A new strategy is demon-
strated that allows the resolution of racemic sulfoxides
through catalytic asymmetric nitrene-transfer reactions.
This approach gives rise to both optically active sulfoxides
and highly enantioenriched sulfoximines. By using a chiral
iron catalyst and a readily available iodinane reagent, high
selectivity factors have been achieved under very practical
reaction conditions. With respect to the substrate scope, it
is noteworthy that this unprecedented imidative kinetic
resolution of racemic sulfoxides provides access to both
aryl–alkyl and dialkyl sulfoximines in highly enantioen-
riched forms.
Scheme 1. Strategies for the kinetic resolution of racemic sulfoxides.
When a racemic compound reacts with an enantiopure re-
agent, the two enantiomers of the former can convert at very
different reaction rates. Such kinetic resolutions can also occur
with achiral reaction partners when enzymes or chiral catalysts
are applied.[1] In both academia and industry, kinetic resolu-
tions have great significance in the preparation of enantiopure
products. With respect to sulfoxides, three strategies have
been reported to date.[2] The first two have in common that
one sulfoxide enantiomer of a racemic mixture is preferentially
transformed into an achiral product. This can either be a sul-
fone[3] or a sulfide,[4] depending on the use of oxidative or re-
ductive methods, respectively (Scheme 1, a and b). Both routes
have carefully been investigated, but often the efficiency of
the reaction is low and the observed enantioselectivities are
moderate. The preferential conversion of one sulfoxide enan-
to kinetic sulfoxide resolutions by using iron-catalyzed asym-
metric nitrene-transfer reactions (Scheme 1, d). As a result,
both sulfoximines and sulfoxides can be obtained in enantio-
merically enriched forms.[6]
Owing to their potential synthetic applications and interest-
ing biological activities, sulfimides[7] and sulfoximines[8] attract
continuous attention. The synthesis of these compounds usual-
ly involves the imidation of sulfides or sulfoxides;[9] however,
only a few of these methods are enantioselective. Recently, we
described a method for the synthesis of sulfimides and sulfoxi-
mines that involves the application of a chiral iron complex,
leading to high enantioselectivities in imidative sulfide-to-sulfi-
mide conversions.[10,11] Encouraged by those results, we won-
dered how this catalyst system would work in a kinetic-resolu-
tion process starting from racemic sulfoxides.
tiomer into
a different sulfoxide is the third strategy
(Scheme 1, c). Unfortunately, this pathway is limited to some
very specific substrates.[5] Herein, we present a new approach
Initially, racemic methylphenylsulfoxide rac-1a was em-
ployed as the model substrate and N-tosyliminophenyliodi-
nane (PhI=NTs) was employed as the nitrene precursor. The re-
actions were conducted with rac-1a (1 equivalent), PhI=NTs
(0.5 equivalents), and 5 mol% of catalyst (with respect to rac-
1a), in acetone at 08C. Preliminary studies revealed that the
catalyst, generated in situ from [Fe(acac)3] (acac=acetylaceto-
nate) (3a) and (R,R)-2,6-bis(4-phenyl-2-oxazolinyl)pyridine, (R,R)-
Ph-PyBOX (4a), showed a good ability to differentiate the two
enantiomers of the sulfoxide. Hence, sulfoximine 2a was isolat-
ed with an enantiomeric ratio (e.r.) of 88:12 (R/S), in 19% yield
after 15 h (Table 1, entry 1).
[a] Dr. J. Wang, M. Frings, Prof. Dr. C. Bolm
Institute of Organic Chemistry
RWTH Aachen University
Landoltweg 1, 52056 Aachen (Germany)
Fax: (+49)241-8092391
E-mail: carsten.bolm@oc.rwth-aachen.de
Homepage: http://bolm.oc.rwth-aachen.de
[b] Dr. J. Wang
Current address: School of Chemistry and Chemical Engineering
Sun Yat-Sen University
Guangzhou 510275 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303850.
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Communication
instead of methyl) in each 1,3-diketonate, diminished both the
reactivity and enantioselectivity (Table 1, entry 4 versus
entry 2).[13] When the reaction was performed in other solvents
(e.g., dichloromethane or acetonitrile), inferior results were ob-
tained (Table 1, entries 5 and 6). Lowering the reaction temper-
ature from 0 to À208C (Table 1, entry 7) increased the selectivi-
ty factor (s=28.8), and sulfoximine 2a was obtained with an
e.r. of 95:5, in 32% yield. When the reaction time was in-
creased from 15 to 22 h, the yield of 2a increased to 37%, but
the e.r. slightly decreased from 95:5 to 94:6 (Table 1, entry 8).
Performing the reaction at À458C was beneficial for the enan-
tioselectivity, but the reaction rate was dramatically reduced.
After 38 h sulfoximine 2a was isolated with an e.r. of 97:3, but
with only 15% yield (Table 1, entry 9).
Table 1. Optimization of the reaction conditions.[a]
Various other PyBOX ligands, as well as an analogue thereof,
have been screened with iPrCN as solvent. Under the same re-
action conditions, PyBOX ligands 4b–d, with aliphatic or ben-
zylic substituents, gave poor enantioselectivities (Table 1, en-
tries 11–13 versus entry 10). Replacing the phenyl groups in
PyBOX 4a by 2-chlorophenyl or 2-naphthyl substituents result-
ed in decreased enantioselectivities (Table 1, entries 14 and 15).
Introducing two methyl groups at the 5-position of each oxa-
zoline moiety also diminished the enantioselectivity (Table 1,
entry 16). PyBOX ligands containing substituents with distinct
electronic properties on the pyridine ring were also tested. The
enantioselectivity slightly decreased when the ligand had
a chloride group at the 4-position of the PyBOX pyridine ring
Entry Iron Ligand Solvent
salt
T
t
Yield of 2a e.r. of 2a[c] s[d]
[oC] [h] [%][b]
1
2
3
4
5
6
7
8
3a 4a
3b 4a
3c 4a
3d 4a
3b 4a
3b 4a
3b 4a
3b 4a
3b 4a
3b 4a
3b 4b
3b 4c
3b 4d
3b 4e
3b 4 f
3b 4g
3b 4h
3b 4i
3b 4j
acetone
acetone
acetone
acetone
CH2Cl2
0
0
0
0
0
0
15 19
15 28
15 29
15 18
15 35
88:12
93:7
92:8
89:11
83:17
90:10
95:5
94:6
97:3
8.7
18.4
16.1
9.6
6.9
MeCN
3
44
17.0
28.8
26.2
38.0
25.1
3.1
2.2
2.1
6.3
12.6
14.2
13.8
18.7
20.5
acetone À20 15 32
acetone À20 22 37
acetone À45 38 15
9
10
11
12
13
14
15
16
17
18
19
iPrCN
iPrCN
iPrCN
iPrCN
iPrCN
iPrCN
iPrCN
iPrCN
iPrCN
iPrCN
À20 15 42
À20 21 36
À20 21 37
À20 21 39
À20 21 48
À20 21 42
À20 21 42
À20 21 41
À20 15 29
À20 21 34
93:7
71:29
65:35
64:36
79:21
88:12
89:11
89:11
93:7
(Table 1, entry 17). However, PyBOX 4i, containing
a 4-
dimethylamino substituent, gave the same enantioselectivity
as 4a, but a lower reactivity was observed (Table 1, entry 18).
Thus, the basicity of the pyridine moiety influenced the catalyt-
ic process. Interestingly, ligand 4j, with two six-membered 4H-
1,3-oxazine rings, gave the same enantioselectivity as PyBOX
4a (Table 1, entry 19). Taking all of our findings into consider-
ation, PyBOX 4a was identified as the best ligand of those
tested.
Other reaction parameters were also varied[14] and the opti-
mal reaction conditions were found to be: [Fe(acacCl)3] (3b,
5 mol%), (R,R)-Ph-PyBOX (4a, 5 mol%), racemic sulfoxide 1a
(1 equivalent), and PhI=NTs (0.5 equivalents), in acetone (0.2m)
at À208C. Neither moisture nor air had to be excluded.
93:7
[a] Reaction conditions: Iron(III) salt (3, 5 mol%), ligand (4, 5 mol%), rac-
1a (1 equivalent), PhI=NTs (0.5 equivalents), solvent. [b] Based on the
amount of rac-1a. [c] Determined by high-performance liquid chromatog-
raphy (HPLC) analysis by using a chiral stationary phase. [d] Selectivity
factors (s), calculated according to the following equation: s=ln[1ÀC-
(1+ee2a)]/ln[1ÀC(1Àee2a)], in which C=conversion (yield of 2a) and
ee2a =enantiomeric excess of 2a.
Subsequently, various iron(III) acetylacetonate derivatives
were examined. Notably, both the steric and electronic effects
of the 1,3-diketonate moiety influenced the outcome of the re-
action. Thus, with iron(III)-3-chloroacetylacetonate, [Fe(acacCl)3]
(3b), the catalytic activity and the enantioselectivity were
greatly enhanced, leading to sulfoximine 2a with an e.r. of
93:7, in 28% yield. In other words, the selectivity factor (s)[12]
had increased from 8.7 to 18.4 (Table 1, entry 2). Assuming that
the increased Lewis acidity of the iron(III) center played a role
in enhancing the enantioselectivity, iron(III)-3-bromoacetylacet-
onate, [Fe(acacBr)3] (3c), was tested; as hypothesized, 3c led
to a result comparable to that of 3b (Table 1, entry 3). In con-
trast, the use of iron(III)-4-chloro-2,6-dimethyl-3,5-heptanedio-
nate, [Fe(dmhdCl)3] (3d), with two bulky R groups (isopropyl
Table 2. Investigation using different amounts of the nitrene precursor.[a]
Entry
PhI=NTs
2a
1a
[equiv]
Yield
[%]
e.r.[b]
Yield
[%]
e.r.[b]
s[c]
1
2
3
4
0.5
0.6
0.75
1.0
37
46
48
49
94:6
93:7
91:9
91:9
56
45
47
47
78:22
84:16
87:13
85:15
27.5 (26.2)
26.9 (29.1)
22.3 (22.9)
21.1 (24.1)
[a] Reaction conditions: [Fe(acacCl)3] (3b, 5 mol%), (R,R)-Ph-PyBOX (4a,
5 mol%), rac-1a (1 equivalent), PhI=NTs (0.5–1 equivalent), in acetone at
À208C, 18 h. [b] Determined by HPLC by using a chiral stationary phase.
[c] As in Table 1, footnote [d], C=(ee1a)/(ee1a+ee2a). In parentheses: yields
of 2a were used as conversions for calculations.
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To investigate the effect of the
sulfoxide conversion on the out-
come of the kinetic resolution,
reactions were performed under
identical conditions, but with
various amounts (0.5–1 equiva-
lent) of the nitrene precursor
(Table 2). As expected, on in-
creasing the amount of PhI=NTs,
the yield of sulfoximine 2a im-
proved (from 37 to 49%), but
the e.r. decreased (from 94:6 to
91:9). For (recovered) sulfoxide
1a, the opposite trend was ob-
served. The selectivity factor
gradually decreased from 27.5 to
21.1 as the conversion increased.
Next, the conversion of vari-
ous racemic sulfoxides, under
Table 3. Substrate scope of the imidative kinetic resolution.
Entry
1
Sulfoxide
t
[h]
Yield of 2
[%]
e.r. of 2[a]
s[c]
22
22
22
24
24
37
39
37
40
35
94:6
94:6
94:6
94:6
94:6
26.2
2
3
4
5
27.6
26.2
28.3
25.0
the
optimized
conditions,
(Table 1, entry 8) was studied.
Pleasingly, the catalyst was capa-
ble of accommodating a number
of aryl alkyl sulfoxides. Extending
the alkyl chain linearly, from C1
(methyl) to C4 (butyl), had no in-
fluence on the e.r. (94:6) of the
resulting sulfoximines 2a–d, and
their yields (ranging from 37 to
44%) remained essentially the
same (Table 3, entries 1–4). To
evaluate the impact of the arene
substitution pattern, reactions
with three methyltolyl sulfoxides
1e–g were examined (Table 3,
entries 5–7); para- and meta-sub-
stituted 1e and 1 f, respectively,
gave similar results to those of
the model substrate 1a, where-
as, ortho-substituted sulfoxide
1g showed poor selectivity (s=
3.9) and very low conversion
(4% yield after 24 h). Thus, the
presence of an ortho-methyl
group seriously hampered the
catalytic process. Raising the
temperature from À20 to 08C
did not positively affect the reac-
tion rate. Electronic effects were
studied by examining aryl–
methyl sulfoxides 1h–j, which
possess para-MeO, para-Br, and
para-NO2 substituents, respec-
tively. The yields (41 and 43%)
6
24
35
93:7
21.0
24
(21)
4
(6)
3.9
(2.6)
7[b]
79:21 (72:28)
8
24
24
41
43
93:7
24.3
25.9
9
93:7
24
(21)
6
(18)
9.5
(6.1)
10[b]
90:10 (84:16)
11
24
35
86:14
9.0
[a] As determined by HPLC by using a chiral stationary phase. The absolute configurations of the major enan-
tiomers of 2a and 2e were determined to be R by comparing the respective optical rotations with reported
data. [b] The data in parentheses were obtained when reactions were conducted at 08C. [c] As in Table 1, foot-
note [d].
and e.r. values (93:7) of the corresponding sulfoximines (2h
and 2i) of 1h and 1i were essentially the same (Table 3, en-
tries 8 and 9), whereas the results achieved with 1j, which has
a strong electron-withdrawing group, NO2, were unsatisfying
(Table 3, entry 10). Even after 24 h (at À208C), only 6% of sul-
foximine 2j (with an e.r. of 90:10) was obtained, even at 08C
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[6] As indicated in Table 2, the kinetic-resolution process also leads to
enantiomerically enriched sulfoxides. In this study, these sulfoxides
were not isolated (except for 1a) owing to difficulties during purifica-
tion of the product by column chromatography (the sulfoxides had
similar Rf values to the PyBOX ligand).
the yield remained low (18%; e.r.=84:16). As exemplified by
the imidation of cyclohexylmethylsulfoxide 1k, nonaromatic
substrates were also tolerated in this reaction (Table 3,
entry 11). Accordingly, sulfoximine 2k was obtained in 35%
yield, with an e.r. of 86:14 (corresponding to a selectivity factor
of 9.0).
In summary, the catalytic imidative kinetic resolution of race-
mic sulfoxides is a new strategy for the synthesis of optically
active sulfoximines. High selectivity factors (up to 38.0) have
been reached by using an easily accessible chiral iron complex,
providing sulfoximines in enantiomerically enriched form (up
to 97:3 e.r.).[6]
Experimental Section
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General procedure for the iron-catalyzed imidation of race-
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[8] For representative overviews on sulfoximine chemistry, see: a) C. R.
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[Fe(acacCl)3] (3b, 5 mol%, 0.01 mmol, 4.6 mg), (R,R)-Ph-PyBOX (4a,
5 mol%, 0.01 mmol, 3.7 mg), and acetone (1 mL) were added to
a test tube. The mixture was stirred for 30 min at room tempera-
ture and then racemic sulfoxide 1 (1 equivalent, 0.2 mmol) was
added. After cooling the mixture to À208C, PhI=NTs (0.5 equiva-
lents, 0.1 mmol, 37.5 mg) was added in one portion. The resulting
mixture was stirred at À208C. After the time indicated in Table 3,
the reaction mixture was directly subjected to silica-gel column
chromatography. The resulting sulfoximine, 2, was isolated and the
e.r. was determined by HPLC by using a chiral stationary phase.
Acknowledgements
J. W. acknowledges support by the Alexander von Humboldt
Foundation. We thank Dr. Jing Hu for her kind assistance
during this research.
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[12] For more information on the concept of the selectivity factor, see
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Keywords: iron catalysis · kinetic resolution · nitrene transfer ·
sulfoxides · sulfoximines
[1] For representative reviews on kinetic resolutions, see: a) H. B. Kagan,
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[13] Interestingly, in the previously studied enantioselective imidation of sul-
fides (ref. [10]), 3d provided a better result than 3b.
[14] For example, different imidation reagents, containing alternative sulfo-
nyl groups were tested and, furthermore, nitrene precursors generated
in situ by combining the corresponding sulfonamides (ArSO2NH2) with
iodosylbenzene (PhI=O) were applied. None of these attempts led to
improved enantioselectivities or better yields. For details, see the Sup-
porting Information.
[2] For a recent review, see: G. E. O’Mahony, P. Kelly, S. E. Lawrence, A. R.
Maguire, ARKIVOC 2011, i, 1.
[3] Most studies relate to sulfide-to-sulfoxide oxidations, in which kinetic
resolutions from over oxidation can be used to increase the enantio-
meric excess (ee) of the remaining sulfoxide. For representative exam-
ples, see: a) F. Toda, K. Mori, Y. Matsuura, H. Akai, J. Chem. Soc. Chem.
Commun. 1990, 1591; b) N. Komatsu, M. Hashizume, T. Sugita, S.
Received: October 2, 2013
Published online on December 16, 2013
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969
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim