Synthesis and structural characterization of molybdenum(VI) and iron(II) coordination compounds with S-alkyl-N-methyl-S-(2-pyridyl)sulfoximines and catalytic epoxidation activity of the molybdenum complexes

Article abstract of DOI:10.1021/ic4020306

Coordination compounds oxido(diperoxido)(S-butyl-N-methyl-S-(2-pyridyl) sulfoximine)molybdenum(VI), [MoO(O₂)₂{SO(NMe) ⁿBu(NC₅H₄)}] (5c), and bis{(dichlorido)(N,S- dimethyl-2-pyridylsulfoximine)iron(II)}, tetrahydrofuran solvate (1:1) (6), [FeCl₂{SO(NMe)Me(NC₅H₄)}]₂· THF are prepared from the free ligand 4 and molybdenum(VI) oxidediperoxide(dihydrate) and iron dichloride, respectively. The crystal structures reveal a trigonal bipyramid with the pyridine ring and the single oxygen on molybdenum in a trans arrangement for 5c and a planar μ²-Cl₂Fe₂ ring with trans-oriented exocyclic Cl atoms for 6 whereas the structures of the N,N-dicoordinated ligands are only little effected by the metals. Coordination compounds (5) efficiently catalyze the epoxidation of cyclooctene or of monosubstituted alkenes by tert-butyl hydroperoxide.

Full text of DOI:10.1021/ic4020306

Article
pubs.acs.org/IC
Synthesis and Structural Characterization of Molybdenum(VI) and
Iron(II) Coordination Compounds with S‑Alkyl‑N‑methyl‑S‑(2-
pyridyl)sulfoximines and Catalytic Epoxidation Activity of the
Molybdenum Complexes
Yvonne Brussaard,† Falk Olbrich,‡ and Ernst Schaumann*,†
†Institute of Organic Chemistry, Clausthal University of Technology, Leibnizstrasse 6, D-38678 Clausthal-Zellerfeld, Germany
‡Institute of Inorganic and Applied Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany
S
* Supporting Information
ABSTRACT: Coordination compounds oxido(diperoxido)(S-butyl-N-methyl-S-(2-pyridyl)-
sulfoximine)molybdenum(VI), [MoO(O2)2{SO(NMe)nBu(NC5H4)}] (5c), and bis-
{(dichlorido)(N,S-dimethyl-2-pyridylsulfoximine)iron(II)}, tetrahydrofuran solvate (1:1) (6),
[FeCl2{SO(NMe)Me(NC5H4)}]2·THF are prepared from the free ligand 4 and molybdenum-
(VI) oxidediperoxide(dihydrate) and iron dichloride, respectively. The crystal structures reveal a
trigonal bipyramid with the pyridine ring and the single oxygen on molybdenum in a trans
arrangement for 5c and a planar μ2-Cl2Fe2 ring with trans-oriented exocyclic Cl atoms for 6
whereas the structures of the N,N-dicoordinated ligands are only little effected by the metals.
Coordination compounds (5) efficiently catalyze the epoxidation of cyclooctene or of monosubstituted alkenes by tert-butyl
hydroperoxide.
complexes were usually generated in situ from an appropriate
metal(II) salt,11,16−24,28,29 or a metal(0) precursor10,26 and the
sulfoximine. With the exception of a copper,15 a palladium,19
and a rhodium complex,12 the isolated N,N-bidentate
sulfoximine complexes adopt a noncrystalline state10,26 and
could only be characterized spectroscopically including the
adduct with a potential cycloaddition partner.27 For a full
assessment of the catalyst structure, our goal was to synthesize
crystalline transition metal−sulfoximine coordination com-
pounds again with a chelate structure, where the role of the
second coordination site is adopted by the nitrogen of a
pyridine ring as substituent on sulfur. As for the central atom,
we were aiming at metals that so far were not probed in
sulfoximine chemistry with the idea to further improve catalytic
efficiency and for a broader scope of reactions, which allow
catalysis. Thus, we chose molybdenum(VI), the complexes of
which are known to efficiently catalyze so divergent reactions as
alkyne metathesis30 for the type (Ar3SiO)3MoX (X = N, CR)
or alkene oxidation to epoxides for the type MoO(O2)2(L
L).31−34 Our second example involves an iron sulfoximine
complex where iron has the virtue to be environmentally
benign beyond any doubt and is known to catalyze carbon−
carbon single bond formation especially well in cross-coupling
reactions.35−37
INTRODUCTION
■
Sulfoximines are tetracoordinate sulfur compounds that
formally result from the exchange of one oxygen for a nitrogen
atom in a sulfone. However, in contrast to the well-known
sulfones,1,2 sulfoximines have emerged only recently as
synthetically useful compounds3 and as promising pharmaco-
phores in medicinal chemistry.4 In particular, if different
organyl substituents on sulfur are present, then sulfoximines are
chiral at the sulfur atom and therefore play an important role in
asymmetric synthesis as auxiliaries5,6 or as catalysts in the
thriving field of enantioselective homogeneous catalysis using
transition metal coordination compounds.7−9 The work of the
Bolm group has especially shown that chiral complexes based
on sulfoximines can be successfully applied as catalysts to give
highly enantioselective addition and cycloaddition reactions
mainly to carbonyl groups.10−12 With few exceptions,13,14
chelate complexes were used in this work. This implies the use
of N,N-bidentate ligands in copper(II),11,15−18 palladium(0),10
palladium(II),19,20 or rhodium(I)12 coordination compounds
where the binding sites are offered by the nitrogen atoms of the
two linked sulfoximine units in C2 symmetric bis-sulfox-
imines12,15,16,20 or where besides the C1 symmetric sulfoximine
the nitrogen of a quinoline,21 of an aniline,11,17,22,23 or of an
oxazoline18 offers a convenient binding site. Also, bidentate
N,O complexation of nickel(II) in (2-hydroxyalkyl)-
sulfoximines and N,P complexation of iridium(0) by 2-
(diphenylphospino)aryl sulfoximines has been shown to allow
efficient catalysis of conjugate addition to enones24,25 and
asymmetric hydrogenation of imines,26 respectively. N,P
complexation has also been shown to be useful in palladium-
catalyzed allylic alkylation.28,29 In these studies, the metal
Received: August 10, 2013
Published: November 1, 2013
© 2013 American Chemical Society
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Decyl Methylamino 2-Pyridyl Sulfonium 2,4,6-Trimethyl-
phenylsulfonate (3d). Analogously to 3a, freshly prepared 2 (4.59 g,
20.0 mmol) and 1d (5.23 g, 20.8 mmol) were reacted to give salt 3d.
Yield 7.93 g (82%).
EXPERIMENTAL SECTION
■
For the preparation of sulfides 1a,c,d38 and 1b,39 literature procedures
were adopted. Compounds 4a−d were prepared following the method
of Franek and Claus.40
N,S-Dimethyl-2-pyridylsulfoximine (4a). The salt 2a, as
obtained in the previous step, was dissolved in CH2Cl2, and the
solution was cooled to −78 °C. At this temperature, a moderate
stream of NH3 was introduced for 5 min. After stirring for 10 min, the
treatment with NH3 was repeated. The mixture was allowed to slowly
warm to 0 °C, and 3-chloroperbenzoic acid (5.2 g, 30.1 mmol) in
CH2Cl2 (30 mL) was added dropwise at a pH value between 7 and 9.
If the mixture became more acidic, again a stream of NH3 was
introduced. Then, the reaction mixture was stirred at 0 °C for 30 min,
and the colorless precipitate was removed by filtration. The filtrate was
concentrated in vacuo using a bath temperature not exceeding 30 °C.
The product was isolated by column chromatography (silica, eluent
CHCl3) and finally purified by recrystallization from Et2O/hexane.
Yield 1.72 g (48%) of 4a as colorless crystals, mp 52−53 °C. 1H NMR
(200 MHz, CDCl3, room temperature): δ 2.68 (s, 3H), 3.25 (s, 3H),
7.52 (dd, 1H, J = 4.6, 7.6 Hz), 7.97 (d, 1H, J = 7.8 Hz), 8.13 (dd, 1H, J
= 7.6, 7.8 Hz), 8.78 (d, 1H, J = 4.6 Hz). 13C NMR (50 MHz, CDCl3,
room temperature): δ 29.4, 40.9, 123.3, 126.4, 137.7, 150.3, 157.1. IR
(KBr, cm−1): ν 3100, 3035, 3006, 2959, 2928, 2877, 2805, 1577, 1453,
1426, 1413, 1322, 1290, 1237, 1156, 1124, 1105, 1076, 1041, 992, 953,
906, 854, 790, 771, 737, 704, 615. Elemental analysis is consistent with
the gross formula C7H10N2OS, Mw = 170.23. Anal. Calcd: C, 49.39; H,
5.92; N, 16.46; S, 11.38. Found: C, 49.37; H, 6.07; N, 16.43; S, 11.04.
S-Isopropyl-N-methyl-2-pyridylsulfoximine (4b). The com-
pound 4b was obtained from the crude salt 3b as obtained from 1b
and, after deprotonation with NH3, the intermediate oxidized with 3-
chloroperbenzoic acid (6.16 g, 35.7 mmol) in CH2Cl2 (30 mL). The
product was isolated by column chromatography (silica, eluent
CHCl3) to give 4b as a yellow oil. Yield 1.44 g (31%, based on the
amount of 1b used). 1H NMR (200 MHz, CDCl3, room temperature):
δ 1.21 and 1.46 (each d, 3H, J = 6.9 Hz), 2.69 (s, 3H), 3.70 (m, 1H),
7.50 (dd, 1H, J = 4.6, 7.6 Hz), 7.95 (d, 1H, J = 8.0 Hz), 8.11 (dd, 1H, J
= 7.6, 8.0 Hz), 8.80 (d, 1H, J = 4.6 Hz). 13C NMR (50 MHz, CDCl3,
room temperature): δ 14.8, 16.0, 29.5, 52.7 125.5, 126.2, 137.5, 150.5,
155.9. IR (NaCl plate, cm−1): ν 3076, 3043, 2962, 2932, 2806, 1558,
1466, 1446, 1365, 1265, 1238, 1144, 1116, 1079, 1050, 1036, 989, 939,
883, 857, 795, 749, 730, 678, 616. Elemental analysis is consistent with
the gross formula C9H14N2OS, Mw = 198.28. Anal. Calcd: C, 54.52; H,
7.12; N, 14.13; S, 16.17. Found: C, 54.42; H, 7.26; N, 13.99; S, 16.21.
S-Butyl-N-methyl-2-pyridylsulfoximine (4c). The crude salt 3b
as obtained in the previous step from 1c (2.01 g, 12.0 mmol), and
freshly prepared 2 (2.48 g, 10.8 mmol) was treated with NH3 as
reported above for 4a and oxidized with 3-chloroperbenzoic acid (2.5
g, 14.5 mmol) in CH2Cl2 (20 mL). The product was isolated by
column chromatography (silica, eluent CHCl3) to give 3c as a yellow
oil. Yield 813 mg (38%, based on the amount of 1c used).
Methyl 2-Pyridyl Sulfide (1a). A mixture of pyridine-2-thiol41 (1
with R = H; 19.40 g, 0.175 mol), MeCN (50 mL), and NEt3 (23 mL)
was cooled to 0 °C. MeI (24.84 g, 10.9 mL, 0.175 mol) was added
dropwise. The reaction mixture was stirred at 0 °C for 1 h and then
overnight at ambient temperature. The solvent was evaporated in
vacuo, and the residue was diluted by addition of EtOAc (50 mL).
After several hours in a refrigerator, the precipitated HNEt3I was
filtered by suction. The residue was concentrated in vacuo, and the
product (1a) was isolated by column chromatography (silica, eluent
petroleum ether/EtOAc 4:1 v/v). For final purification, the material
was distilled (bp 77 °C/14 mbar). Yield 17.71 g (81%; lit.38 67%) of
1a as a colorless oil. 1H NMR (200 MHz, CDCl3, room temperature):
δ 2.56 (s, 3H), 6.97 (dd, 1H, J = 4.8, 7.6 Hz), 7.18 (d, 1H, J = 8.2 Hz),
7.48 (dd, 1H, J = 7.6, 8.2 Hz), 8.44 (d, 1H, J = 4.8 Hz). 13C NMR (50
MHz, CDCl3, room temperature): δ 13.2, 119.1, 121.4, 135.8, 149.4,
159.9. IR (NaCl plate, cm−1): ν 3068, 3045, 2995, 2925, 2854, 1581,
1556, 1455, 1436, 1415, 1315, 1279, 1243, 1167, 1146, 1127, 1090,
1043, 985, 964, 757, 712, 619.
Isopropyl 2-Pyridyl Sulfide (1b). At 0 °C, 2-propanethiol (11.75
mL, 9.7 g, 0.127 mol) was added to a suspension of NaOH (5 g, 0.125
mol) in DMF (100 mL). After 2 h at 20 °C, the NaOH had almost
completely dissolved and 2-chloropyridine (11.25 mL, 13.63 g, 0.120
mol) was added. The mixture was gradually heated to 140 °C and
stirred at this temperature for 6 h. The mixture was allowed to cool
overnight and poured onto ice (100 g). Organic material was extracted
with CH2Cl2 (4 × 50 mL). The combined organic phases were
combined and dried (MgSO4). The solvent was evaporated, and the
residue was distilled to give 13.0 g (70%; lit.38 71%) of product,
colorless liquid, bp 58 °C/2 mbar. 1H NMR (200 MHz, CDCl3, room
temperature): δ 1.41 (d, 6H, J = 6.8 Hz), 3.99 (m, 1H), 6.96 (dd, 1H, J
= 4.8, 7.6 Hz), 7.16 (d, 1H, J = 8.2 Hz), 7.47 (dd, 1H, J = 7.6, 8.2 Hz),
8.44 (d, 1H, J = 4.8 Hz). 13C NMR (50 MHz, CDCl3, room
temperature): δ 23.1, 35.0, 119.3, 122.8, 135.8, 149.5, 159.5. IR (NaCl
plate, cm−1): ν 3069, 3045, 2963, 2927, 2866, 1578, 1556, 1453, 1415,
1383, 1364, 1280, 1143, 1125, 1088, 1054, 985, 757, 725.
Butyl 2-Pyridyl Sulfide (1c). This compound was obtained
analogously to 1a from pyridine-2-thiol41 (8.89 g, 0.080 mol) and 1-
iodobutane (14.72 g, 0.080 mol). Yield 12.77 g (95%) of a colorless
oil. This compound was obtained previously by another route.42
Decyl 2-Pyridyl Sulfide (1d). This compound was obtained
analogously to 1a from pyridine-2-thiol41 (5.0 g, 0.045 mol) and 1-
iododecane (9.65 mL, 12.06 g, 0.045 mol) but was isolated by column
chromatography on silica (eluent petroleum ether/EtOAc 4:1 v/v).
Yield 10.89 g (93%) of a colorless oil.
Methyl Methylamino 2-Pyridyl Sulfonium 2,4,6-Trimethyl-
phenylsulfonate (3a). To a solution of 1a (3.74 g, 30.0 mmol) in
CH2Cl2 (20 mL) was added freshly prepared O-mesitylsulfonyl-N-
methylhydroxylamine43 (2, 6.65 g, 29.0 mmol), and the mixture was
stirred at ambient temperature overnight. Et2O (150 mL) was carefully
added. After 48 h at 3 °C, the colorless precipitate was removed by
filtration and the residue was washed with Et2O. Yield 7.51 g (73%) of
S-Decyl-N-methyl-2-pyridylsulfoximine (4d) was prepared analo-
gously to 4a from salt 3d (7.93 g, 15.5 mmol). Yield 1.94 g (39%).
Yellowish oil.
Oxido(diperoxido)(S-butyl-N-methyl-S-(2-pyridyl)-
sulfoximine)molybdenum(VI) (5c). MoO(O2)2(H2O)2 (24 mL of a
0.1 M aqueous solution, 24 mmol)44 was added to 4a (402 mg, 2.36
mmol) in MeOH (2 mL), and the reaction mixture was stirred at
ambient temperature for 2 h. The resulting citrus yellow precipitate
was removed by filtration and washed with H2O, a little MeOH, and
finally Et2O to remove organic impurities. After it was dried in vacuo,
5c (550 mg, 73%) was isolated as a microcrystalline yellow solid, mp
122 °C (dec). Yellow crystals were obtained by carefully placing
hexane on top of a saturated solution in CH2Cl2. 1H NMR (200 MHz,
CDCl3, room temperature): δ 1.01 (t, 3H, J = 7.2 Hz), 1.55 and 1.94
(each m, 2H), 3.73 (s, 3H), 3.82 (m, 2H), 7.69 (dd, 1H, J = 5.0, 7.8
Hz), 7.96 (d, 1H, J = 8.0 Hz), 8.15 (dd, 1H, J = 7.8, 8.0 Hz), 8.46 (d,
1H, J = 5.0 Hz, H-6). 13C NMR (50 MHz, CDCl3, room temperature):
δ 13.4, 21.6, 24.3, 37.2, 58.9, 122.8, 130.2, 140.6, 148.3, 149.5. IR (KBr,
cm−1): ν 3096, 3065, 2933, 2905, 2875, 1593, 1561, 1456, 1433, 1392,
1303, 1279, 1244, 1199, 1168, 1138, 1098, 1082, 1060, 1046, 1017,
1
salt 3a. H NMR (200 MHz, CDCl3, room temperature): δ 2.23 (s,
3H), 2.69 (s, 6H), 2.74 (d, 3H, J = 4.8 Hz), 3.40 (s, 3H), 6.83 (s, 2H),
7.59 (dd, 1H, J = 4.8, 7.8 Hz), 8.04 (d, 1H, J = 8.0 Hz), 8.41 (broad s
and dd, 2H, J = 7.8, 8.0 Hz), 8.70 (d, 1H, J = 4.8 Hz). 13C NMR (50
MHz, CDCl3, room temperature): δ 20.4, 23.1, 32.2, 34.4, 126.7,
127.8, 131.0, 136.6, 138.7, 138.9, 141.2, 149.5, 150.2.
Isopropyl Methylamino 2-Pyridyl Sulfonium 2,4,6-Trimethylphe-
nylsulfonate (3b) was prepared analogously from 2 (5.73 g, 25.0
mmol) and 1b (4.02 g, 27.0 mmol) in CH2Cl2 (20 mL), and the crude
product (ca. 70% conversion) was used as such in the preparation of
4b.
Butyl Methylamino 2-Pyridyl Sulfonium 2,4,6-Trimethylphe-
nylsulfonate (3c). This salt was prepared analogously to 3a to give a
yellow oil still containing about 25% of unreacted starting materials.
This mixture was used as such in the next step.
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Table 1. Selected Crystallographic Data of [MoO(O2)2{SO(NMe)nBu(NC5H4)}] (5c) and [FeCl2{SO(NMe)Me(NC5H4)}]2·
THF (6)
parameter
5c
6
empirical formula
fw, g mol−1
cryst system
space group
unit cell dimens
a, Å
C10H16MoN2O6S
388.25
C18H28Cl4Fe2N4O3S2
666.08
monoclinic
P21/c
triclinic
P1
̅
8.5057(1)
18.5753(2)
9.4720(1)
8.7334(1)
b, Å
8.9939(2)
c, Å
9.3967(2)
α, deg
96.880(1)
β, deg
100.701(1)
103.671(1)
γ, deg
104.004(1)
cell vol, Å3
1470.51(3)
4
683.49(2)
Z
1
calcd density, g cm−3
abs coeff, mm−1
1.754
1.618
1.058
1.632
cryst size, mm
0.50/0.50/0.40
0.71073
173
0.60/0.40/0.05
λ, Å
0.71073
temperature, K
173
θ range for data collection, deg
index ranges
2.19−27.50
−8 < h <11, −24 < k< 23, −12 < l < 8
2.27−27.50
−11 < h 10, −11 < k 11, −12 < l < 12
reflns collected
9860
4732
independent reflns
obs reflns [I > 2σ(Io)]
no. of parameters
goodness-of-fit on F2
R1 [I > 2σ(Io)], all data
wR2 [I > 2σ(Io)], all data
largest diff peak and hole, e·Å−3
CCDC no.51
3357
3088
3031
2516
187
178
1.000
1.000
0.0222, 0.0259
0.0577, 0.0593
+0.608, −0.637
904588
0.0476, 0.0563
0.1169, 0.1204
+0.968, −1.250
907772
955, 892, 865, 781, 760, 657, 585, 537. Elemental analysis is consistent
with the gross formula C10H14MoN2O6S, Mw = 388.25. Anal. Calcd: C,
30.94; H, 4.15; N, 7.22; S, 8.26. Found: C, 30.68; H, 4.10; N, 7.06; S,
8.31.
Et2O carefully, and the mixture was cooled to 0 °C. This led to the
precipitation of more orange-red solid. Overall yield 173 mg (93%),
mp 186 °C (dec). The solid was dissolved in dry THF, the insoluble
material was removed by filtration, and the solution was carefully
covered with a layer of Et2O. After 15 days, yellow to orange-red
crystals of 6 separated. IR (KBr, cm−1): ν 3085, 3074, 3008, 2977,
2902, 2817, 1590, 1562, 1457, 1435, 1239, 1096, 1080, 1061, 1011,
959, 874, 784, 755, 704. Elemental analysis is consistent with the gross
formula C18H28Cl4Fe2N4O3S2, Mw = 666.08. Anal. Calcd: C, 32.46; H,
4.24; N, 8.41; S, 9.62; Cl, 21.29. Found: C, 32.22; H, 4.31; N, 8.16; S,
9.61; Cl, 21.35.
Analogously, the following were prepared:
Oxido(diperoxido)(S-butyl-N-methyl-S-(2-pyridyl)sulfoximine)-
molybdenum(VI) (5a) from sulfoximine 4a (402 mg, 2.36 mmol) and
the molybdenum peroxide solution (24 mL). Yield 647 mg (80%) of a
microcrystalline yellow solid, mp 128−129 °C (dec). IR (KBr, cm−1):
ν 3099, 3013, 2997, 2913, 1593, 1561, 1457, 1432, 1395, 1321, 1304,
1237, 1169, 1143, 1094, 1042, 1018, 957, 899, 863, 758, 713, 654, 637.
Elemental analysis is consistent with the gross formula
C7H10MoN2O6S, Mw = 346.17. Anal. Calcd: C, 24.29; H, 2.91; N,
8.09; S, 9.26. Found: C, 24.09; H, 2.88; N, 7.90; S, 9.27.
Oxido(diperoxido)(S-isopropyl-N-methyl-S-(2-pyridyl)-
sulfoximine)molybdenum(VI) (5b) from sulfoximine 4b (208 mg,
1.05 mmol) and the molybdenum peroxide solution (11 mL). Yield
270 mg (67%) of a microcrystalline yellow solid, mp 136 °C (dec).
Oxido(diperoxido)(S-decyl-N-methyl-S-(2-pyridyl)sulfoximine)-
molybdenum(VI) (5d) from sulfoximine 4d (203 mg, 0.68 mmol)
and the molybdenum peroxide solution (9 mL). Yield 150 mg (47%)
of a microcrystalline yellow solid, mp 84 °C (dec).
Bis{(dichlorido)(N,S-dimethyl-2-pyridylsulfoximine)iron(II)},
Tetrahydrofuran Solvate (1:1) (6). Dry FeCl2 (80 mg, 0.63 mmol)
in dry THF (50 mL) was stirred at ambient temperature till a clear
solution was obtained (if traces of H2O were excluded, this solution
was almost colorless; otherwise a yellow or brown color developed and
the subsequent reaction with the ligand did not proceed well). Ligand
4a (112 mg, 0.65 mmol) was added, which led to an immediate
orange-red color of the solution. The mixture was stirred overnight
and concentrated to one-third of its volume. After 24 h at −5 °C, the
Fe complex separated as a microcrystalline orange-red solid, which was
removed by filtration and dried in vacuo. To the solution was added
(R)-(+)-Methyl 2-Pyridylsulfoxide (7). Diethyl (2R,3R)-tartrate
(3.26 g, 15.8 mmol), Ti(OiPr)4 (2.33 mL, 2.245 g, 7.9 mmol), CH2Cl2
(60 mL), and water (142 μL, 7.9 mmol) were stirred at room
temperature till a homogeneous mixture was obtained. Sulfide 1a (993
mg, 7.9 mmol) was added, and then at −21 °C tBuOOH (2.22 mL of a
3.7 M solution in PhMe, 8.2 mmol) was added. After stirring at this
temperature for 18 h, the mixture was diluted with water (1.4 mL, 10
equiv) and the temperature was maintained for another 30 min. Then,
the mixture was allowed to warm to room temperature and was stirred
for 1 h for complete hydrolysis. The precipitate was removed by
filtration through Celite, and the solution was concentrated in vacuo.
The crude product was purified by column chromatography (silica,
eluent petroleum ether/EtOAc 1:2). Yield 705 mg (63%) of a colorless
23.5
oil, [α]D +62.5 (c = 0.980 in CHCl3), 64% ee as determined by
GLC on Lipodex E. During chromatography, the first fractions
contained almost enantiopure material (99% ee),45,46 but the optical
purity then went continuously down to finally 46% ee.
(R)-(−)-N,S-Dimethyl-2-pyridylsulfoximine (4a). (R)-(+)-Sulf-
oxide 7 (357 mg, 2.5 mmol, 99% ee) and freshly prepared 2 (580 mg,
2.5 mmol) in CH2Cl2 (10 mL) were stirred at 28−30 °C for 24 h.
Then, the solvent was widely removed in vacuo, and the mixture was
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allowed to stand at ambient temperature for 5 days. According to the
1H NMR spectroscopic evidence, 15−20% of 7 had reacted. The crude
product was dissolved in CH2Cl2 (20 mL), and the mixture was cooled
to 0 °C. EtN(iPr)2 (0.2 mL, 1.15 mmol) was added, and the mixture
was stirred first at 0 °C for 1 h and then at ambient temperature for 2
h. The solvent was evaporated, and the residue was purified by column
chromatography (silica, eluent CHCl3), removing 264 mg of unreacted
7. Yield 52 mg (12% based on the total amount of 7 used, 46% based
Scheme 1. Synthesis Pathways to Coordination Compounds
5 and 6
23.5
on conversion), colorless oil, [α]D −171.9 (c = 0.5 in CHCl3).
General Procedure for Epoxidation Reactions. Alkene 8, 10,
or 12 (10 mmol) and catalyst 5 (0.05 mmol) were dissolved in toluene
t
(20 mL) or CHCl3 (25 mL). BuOOH (2.7 mL, 3.7 M in PhMe, 10
mmol) was added, and the mixture was stirred at 100 °C for 5 h or for
1 h at 55 °C with toluene or CHCl3 as solvent, respectively. Alcohol
12 was epoxidized in CHCl3 at 50 °C for 2 days. Yields were
determined by capillary GLC on Lipodex E.
X-ray Data Collection, Structure Determination, and Refine-
ment. The data sets for the complexes were collected on a Siemens
axs SMART CCD system (Mo Kα radiation, graphite monochroma-
tor, ω-scans). A full hemisphere of the reciprocal space was scanned
with 1271 frames in three sets; each frame covered 0.3° in ω. An
empirical absorption correction was carried out with the program
SADABS47 (5c: Tmin 0.703, Tmax 1.000. 6: Tmin 0.561, Tmax 1.000).
Both structures were solved with direct methods (SHELXS-97)48 and
2
refined with full-matrix least-squares against Fo using the program
SHELXL-97.49 All non-hydrogen atoms were refined with anisotropic
temperature factors. Hydrogen atoms were calculated in idealized
positions using a riding model with isotropic temperature factors
combined in different logical groups. For molecular graphics and
publication materials, the program package SHELXTL-PLUS was
used.50 Special refinement procedures are given in the Supporting
Information. Crystallographic details are provided in Table 1.
Scheme 2. Enantioselective Synthesis of Sulfoxide (R)-(+)-7
and of Sulfoximine (R)-(−)-4a
RESULTS AND DISCUSSION
■
Synthesis. Ligands (4) are synthesized starting from 2-
pyridyl sulfides (1), which can be N-methyliminated using N-
methyl-O-mesitylsulfonyl-hydroxylamine (2) adopting a liter-
ature method.40 The resulting salts (3) are deprotonated with
gaseous ammonia and in situ oxidized with 3-chloroperbenzoic
acid to give racemic sulfoximines (4) (Scheme 1). To obtain an
optically active sulfoxime, a different route had to be taken
(Scheme 2). The crucial intermediate is sulfoxide 7, which
could be obtained in optically pure form using a modified
Sharpless procedure as suggested by Kagan,52 where a (2R,3R)-
tartrate provides the sulfoxide in the shown (R) configuration.52
The subsequent imination is known to proceed without loss of
optical purity.53 However, for the methylimination of optically
active sulfoxide 7 using hydroxylamine derivative (2), rather
mild temperatures had to be applied to get a pure product
(Scheme 2). Under these conditions, the reaction turned out to
be rather inefficient, but unreacted starting material (7) could
be recovered and used again. The final deprotonation step was
arrangement (Figure 1); four independently studied crystals
gave the same lattice parameters from which the consistent
occurrence of the trans configuration in the crystalline state can
be derived.54 Also, the NMR spectroscopic evidence shows that
only one diastereomer is formed to which then mutatis
mutandis the trans configuration should be assigned. In
contrast, the synthesis of similar molybdenum coordination
compounds with N,N-dicoordination gave cis/trans mix-
tures.54,55 However, at room temperature in CDCl3 solution,
a slow partial isomerization of the trans to the cis isomer in 5c
is shown by the appearance of a new triplet at δ 0.89 for the C-
1
methyl group of the butyl residue in the H NMR spectrum.
best carried out using Hunig base, preferably at temperatures
not much above ambient temperature (Scheme 2).
Similarly, a new H-6 signal shows up at δ 8.85. A striking
feature of the structure is the long axial N(1)−Mo bond of
2.404(2) Å as compared to 2.175(2) Å for the equatorial
N(2)−Mo bond. This may reflect the trans influence of the
oxygen O(1) and may contribute to the strong preference of
the trans arrangement (i.e., with the more weakly donating
pyridine nitrogen in this position). The effect is more
pronounced than in previously studied molybdenum complexes
with 2,2′-bipyridines56,57 or 3-(2-pyridyl)pyrazoles.54,58 Sim-
ilarly, only the N(2)−Mo value is close to the median of 2.24 Å
for the Mo−N bond in Mo-(N,N) chelates as derived from 529
entries in the CSD.59 However, it should be noted that the
inherently longer S−C bond in the N−S−C−N unit of the
ligand in 5c as compared to the N−C−C−N unit in previously
̈
Molybdenum Complexes (5). Sulfoximines (4) undergo a
smooth reaction with molybdenum(VI) oxidediperoxide-
(dihydrate) as generated from molybdenum trioxide and 30%
hydrogen peroxide.44 For 5c, crystals of sufficient quality for an
X-ray structure analysis can be precipitated by addition of
hexane to a solution of the complex in dichloromethane.
Complex 5c is mononuclear and crystallizes in the monoclinic
space group P21/c. If the peroxo groups are each considered as
one ligand, the central metal ion Mo6+ is coordinated in a
distorted trigonal-bipyramidal fashion, interestingly with the
single oxygen on the metal and the pyridine nitrogen in the
axial position; this configuration is considered the trans
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Figure 2. Structure of [FeCl2{SO(NMe)Me(NC5H4)}]2·THF (6)
without hydrogen atoms and THF solvent molecule (displacement
ellipsoids are shown at the 50% probability level). Bond lenghts (Å):
Fe−Cl1 2.3033(9), Fe−Cl2 2.3915(8), Fe−Cl2A 2.4796(8), Fe−N1
2.204(3), Fe−N2 2.107(2), S−O 1.449(2), S−N2 1.551(3), S−C5
1.777(3), S−C7 1.770(3), N1−C1 1.338(4), N1−C5 1.341(4), N2−
C6 1.491(4). Symmetry operation: A −x + 1, −y + 1, −z. Angles on S
(deg): C5−S−C7 104.52(15), C7−S−O 107.66(16), C5−S−O
109.74(14), C5−S−N2 102.88(14), C7−S−N2 111.56(15), O−S−
N2 119.43(15).
Figure 1. Structure of [MoO2(O2)2{SO(NMe)nBu(NC5H4)}] (5c)
without hydrogen atoms. (Displacement ellipsoids are shown at the
50% probability level.) Bond lenghts (Å): Mo−O1 1.6869(13), Mo−
O2 1.9353(13), Mo−O3 1.9645(13), Mo−O4 1.9554(13), Mo−O5
1.9255(13), Mo−N1 2.4038(14), Mo−N2 2.1750(14), S−O6
1.4498(12), S−N2 1.5746(15), S−C5 1.7854(17), S−C7
1.7917(17), N1−C1 1.335(2), N1−C5 1.344(2), N2−C6 1.497(2),
O2−O3 1.4782(18), O4−O5 1.4804(19). Angles on S (deg): C5−S−
C7 106.14(8), C5−S−O6 110.37(8), C7−S−O6 108.84(8), C5−S−
N2 102.18(8), C7−S−N2 111.52(8), O6−S−N2 117.11(8).
reflecting the electron transfer from nitrogen to the metal.5,12
So the difference in the SN(2) bond distances between 5c
[1.5746(15) Å] and 6 [1.551(3) Å] demonstrates the special
electron-withdrawing effect of molybdenum. Otherwise, the
structure of ligands 4a,c turns out to be rather similar when
coordinated to the metals of 5c and 6 and also in comparison
to free sulfoximines. This is in line with previously obtained
results giving only “minor structural changes”15 on coordina-
tion to copper,15,62 zinc,62 palladium,19 or rhodium.12
Catalytic Effects. Molybdenum coordination compounds
are known to catalyze the epoxidation of alkenes by
hydroperoxides.31−34 This invited to test the molybdenum/
sulfoximine complexes (5) as catalysts in the epoxidation of
various alkenes. An attractive feature should be that
coordination compounds (5) dissolve well in unpolar organic
solvents such as chloroform or toluene. The optimum reaction
conditions were found using cyclooctene (8) as substrate
(Scheme 3). All complexes (5) turned out to catalyze the
epoxidation of 8 by tert-butyl hydroperoxide efficiently at the
low catalyst concentration of 0.5 mol %. In chloroform at 55 °C
or in toluene at 85 °C, yields in the 90−95% range within 30
studied coordination compounds (ca. 4.10 Å)46−50 gives a
longer distance between the N atoms (4.703 Å), which allows
some more flexibility, and obviously favors the observed trans
arrangement of the sulfoximine ligand. The bond angles around
sulfur correspond closely to those in the free ligand5 with the
O−S−N angle being widest.
Iron Complex 6. Similar to 5, coordination compound 6 is
prepared from a solution of iron(II) dichloride in THF and
ligand 4a, with careful exclusion of water. From the orange-red
solution in THF, well-developed crystals of 6 slowly deposit.
Complex 6 is sensitive against air and moisture but could be
subjected to a single-crystal X-ray investigation. Coordination
compound 6 crystallizes as a dimeric, binuclear complex in the
triclinic space group P1 and was found to contain a dinuclear
̅
structure with one molecule of THF positioned close to the
inversion center (Figure 2). In contrast to 5c, ligands around
the iron(2+) atom are arranged to give a distorted square-
pyramid environment, where the metal atom is 0.635(1) Å
above the best plane through N1, N2, Cl2, and Cl2A. The
central unit of the structure of 6 is a planar μ-Cl2Fe2 four-
membered ring with the inversion center of the molecule as
central point. The bond lengths of the iron to the chelate
nitrogens differ much less than in 5c with Fe−N(1) = 2.204(3)
Å and Fe−N(2) = 2.107(2) Å. In contrast to the trans-oriented
external chlorine atoms in 6, an otherwise closely related μ-
Cl2Fe2 complex with a bithiazole ligand shows the cis
orientation of the terminal chlorine atoms.60 But a μ-Cl2Fe2
complex with a dithiolato chelate was also found to have trans
orientation of the chlorine atoms and reveals very similar bond
lengths to 6.61
Scheme 3. Epoxidation of Alkenes Catalyzed by
Coordination Compounds (5)
Sulfoximino Ligands. In both complexes 5c and 6, the
sulfoximine ligands are coordinated to the metal ions via the
two nitrogen atoms (N1, N2). In view of the oxophilic nature
of the metals, this may be surprising but appears to be the
uniform coordination mode in uncharged sulfoximine com-
plexes.12,15,19,62 Not unexpectedly, the main effect of metal
coordination is on the SN bond, which is significantly longer
than in free sulfoximines with an average value of 1.537 Å
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min are observed. This means that our molybdenum complexes
AUTHOR INFORMATION
Corresponding Author
*E. Schaumann. E-mail: ernst.schaumann@tu-clausthal.de.
Notes
■
(5) are among the best-performing epoxidation catalysts.31
A
comparison of turnover numbers (TON) shows that the
efficiency of the catalyst is improved by longer carbon chains on
the sulfoximine sulfur: in toluene, the TONs increase from
1900 to almost 2700 h−1 on going from the S-methyl (5a) via
the S-butyl (5c) to the S-decyl residue (5d). In contrast, the
branched and so sterically more demanding S-isopropyl
compound 5b gives a TON of only 1460 h−1. The variation
of the substrate revealed that monosubstituted alkenes 10 are
conveniently oxidized to oxiranes 11. In toluene at 100 °C and
employing catalyst 5a, styrene (10a), allylbenzene (10b), and
1-octene (10c) give 52, 53 and 64% of epoxidation products
(11) (Scheme 3). Also, the allylic alcohol 12 is successfully
epoxidized to 13 in 61% yield. In contrast, all attempts to use
complex 5a incorporating the optically active ligand (R)-(−)-4a
in enantioselective epoxidation gave no significant asymmetric
induction. Iron complex 6 was tested as a catalyst in the
cyclodimerization of simple dienes, which is known to proceed
well in the presence of an iron/1,4-diaza-1,3-diene catalyst,63
but turned out to have no catalytic effect.
The authors declare no competing financial interest.
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Both metal ions Mo6+ and Fe2+ undergo a smooth reaction with
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