Synthesis of a cofacial schiff-base dimanganese(III) complex for asymmetric catalytic oxidation of sulfides

Article abstract of DOI:10.1246/cl.2005.848

An optically active dimanganese(III) complex was synthesized by a template reaction of 5,5′-(9,9-dimethylxanthene-4,5-diyl)bis(salicylaldehyde), (1R,2R)-1,2-cyclohexanediamine, and MnCl₂, in which two manganese(III) Schiff-base units are cofacially arranged by two 9,9-dimethylxanthenediyl spacers. The dimanganese complex catalyzed the asymmetric oxidation of methyl phenyl sulfide by iodosobenzene. Copyright

Full text of DOI:10.1246/cl.2005.848

848
Chemistry Letters Vol.34, No.6 (2005)
Synthesis of a Cofacial Schiff-Base Dimanganese(III) Complex
for Asymmetric Catalytic Oxidation of Sulfides
Masakazu Hirotsu,ꢀy Naoki Ohno, Takashi Nakajima, and Keiji Uenoꢀ
Department of Chemistry, Faculty of Engineering, Gunma University, Kiryu, Gunma 376-8515
yDepartment of Materials Science, Graduate School of Science, Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585
(Received March 24, 2005; CL-050397)
An optically active dimanganese(III) complex was synthe-
sized by a template reaction of 5,50-(9,9-dimethylxanthene-4,5-
diyl)bis(salicylaldehyde), (1R,2R)-1,2-cyclohexanediamine, and
MnCl2, in which two manganese(III) Schiff-base units are cofa-
cially arranged by two 9,9-dimethylxanthenediyl spacers. The
dimanganese complex catalyzed the asymmetric oxidation of
methyl phenyl sulfide by iodosobenzene.
OH
OH
Br
OH
O
B
B
1) TMEDA / Et2O
2) n-BuLi
O
O
3) B(OMe)3
4) HCl aq.
[Pd(PPh3)4]
K2CO3
DME / H2O
OH
OH
1
OH
O
1) NaOH / EtOH
.
2) MnCl2 4H2O
AgSbF6
EtOH
Asymmetric oxidation of sulfides is a key reaction for syn-
thesis of biologically active compounds with chiral sulfoxide
centers.1 It has been reported that optically active salen-type
Schiff-base manganese(III) complexes are efficient catalysts
for the asymmetric oxidation of sulfides (Eq 1).2 However, the
enantioselectivity is not high enough for synthetic applications.
Design of chiral macrocyclic dinuclear complexes would be a
good approach to efficient asymmetric catalysts, because two
metal centers are expected to behave cooperatively as an
enantioselective Lewis-acid center to catch a sulfide and an
active oxidation site upon sulfoxidation, respectively. Among
macrocyclic Schiff-base complexes, face-to-face salen-type
complex dimers have been limited to a few systems.3 In this
work, we report a new catalyst, optically active Schiff-base
manganese(III) complex dimer 3 (Scheme 1). The two Schiff-
base manganese(III) units in 3 are doubly bridged by two 9,9-
dimethylxanthenediyl spacers to form a cofacial structure.
O
A ([Mn2(L)Cl2])
3) C6H10(NH2)2
reflux, 1 h
OH
O
2
H
O
Et
H
N
H
N
Mn
O
O
OH2
(SbF6)2
O
O
OH2
O
O
Mn
N
H
N
H
O
Et
H
3
O
Catalyst, Oxidant
S
Scheme 1.
(1)
S
*
2
1
R
R
2
1
R
R
Complex 3 was synthesized via 5,50-(9,9-dimethylxanthene-
C24
C8
C25
C9
O4
C28
C29
4,5-diyl)bis(salicylaldehyde) (2) as summarized in Scheme 1.
Twofold lithiation of 9,9-dimethylxanthene followed by the
boronation and hydrolysis afforded 9,9-dimethylxanthene-4,5-
diboronic acid (1) in 50% yield.4,5 Palladium-catalyzed cross-
coupling reaction of 1 with 2 equiv. of 5-bromosalicylaldehyde
gave 2 in 70% yield.6,7 The structure of 2 was determined by
X-ray analysis (Figure 1).7 Two salicylaldehyde moieties are
almost parallel and anchored by 9,9-dimethylxanthene-4,5-diyl
unit to form a U-shaped molecule. The interatomic distance of
C10
C23
C7
O1
C26
O5
C27
C12
C11
C13
C14
C22
C21
O3
C6
C20
C16
C15
C5
ꢀ
two terminal phenol oxygen atoms (O2ꢁꢁꢁO4, 4.780(5) A) is com-
O2
C1
C19
ꢀ
parable to that of the two carbon atoms (C1ꢁꢁꢁC7, 4.675(4) A).
C4
C17
Dihedral angles between the phenyl planes of salicylaldehydes
and the xanthene plane are not perpendicular but 49.7(1) and
42.7(1)ꢂ. The interplane separation of two salicylaldehyde units
C18
C2
C3
Figure 1. ORTEP drawing of 2 (thermal ellipsoids at the 50%
probability level).
ꢀ
(C16ꢁꢁꢁC29, 3.332(5) A) is smaller than the C1ꢁꢁꢁC7 distance
ꢀ
(4.675(4) A) owing to the small dihedral angles.
The dimeric manganese(III) Schiff-base complex was syn-
thesized by a template reaction of 2, (1R,2R)-1,2-cyclohexanedi-
was reacted with (1R,2R)-1,2-cyclohexanediamine in ethanol
to give A as a brown precipitate. The electrospray-ionization
mass spectrum of A in methanol showed two clusters of peaks
amine, and Mn2þ ions. The 1:1 mixture of 2 and MnCl2 4H2O
.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.6 (2005)
849
at m=z 581.4 and 1197.4. The observed molecular mass numbers
and the isotopic distribution patterns of these peaks match well
with those of [Mn2(L)]2þ and [Mn2(L)Cl]þ, respectively. These
data and poor solubility of the product suggest that A contains
the neutral dimanganese complex [Mn2(L)Cl2]. To improve the
solubility, successive reaction of A with AgSbF6 in ethanol was
carried out to give [Mn2(L)(EtOH)2(H2O)2](SbF6)2 (3) as brown
crystals (18% yield based on 2).8 The CD spectral pattern of 3
was similar to that of the corresponding mononuclear complex
[Mn(salchxn)Cl] (4: H2salchxn = (1R,2R)-bis(salicylidene)-
1,2-cyclohexanediamine) without the xanthenediyl spacers.
The electrospray-ionization mass spectrum of 3 in CH3CN
showed two clusters of peaks at m=z 581.4 and 1197.4 which
are assignable to [Mn2(L)]2þ and [Mn2(L)(H2O)(OH)]þ, respec-
tively.
In contrast to the template reaction using Mn2þ ions describ-
ed above, the 1:1 reaction of 2 and (1R,2R)-1,2-cyclohexanedi-
amine gave a complex mixture. The desired 2 þ 2 Schiff-base
condensation product, H4L, was not obtained at all. The presence
of Mn2þ ions is essential to obtain the macrocyclic Schiff-base
complexes, as reported for other systems.3a,3b,9
molecular O–HꢁꢁꢁO hydrogen bonds between the coordinated
water oxygen and phenolato oxygen (O9ꢁꢁꢁO3 = 2.865(4),
ꢀ
O10ꢁꢁꢁO2 = 2.873(4) A).
A preliminary investigation of asymmetric oxidation of
methyl phenyl sulfide by PhIO revealed that complex 3 and
the corresponding mononuclear complex 4 catalyzed the reac-
tion to give methyl phenyl sulfoxide in a moderate chemical
yield (65% and 55%, respectively), but the enantioselectivity
was extremely low (6% ee for 3 and 4). Interestingly, addition
of 4-(dimethylamino)pyridine to the reaction system improved
the enantioselectivity to 22% ee in the case of 3, but no effect
for 4 (4% ee). The details of the catalytic reactions as well as
further ligand designs based on 2 are under investigation.
This work was partially supported by Grants-in-Aid for
Young Scientists (No. 14740360), for Scientific Research
(No. 15350030), and for Scientific Research on Priority Areas
(No. 16033212) from the Ministry of Education, Culture, Sports,
Science, and Technology of Japan.
References and Notes
1
2
a) M. C. Carren˜o, Chem. Rev., 95, 1717 (1995). b) H. B. Kagan,
in ‘‘Catalytic Asymmetric Synthesis,’’ 2nd ed., ed. by I. Ojima,
Wiley-VCH, Inc., New York (2000), Chap. 6C, pp 327–356.
a) M. Palucki, P. Hanson, and E. N. Jacobsen, Tetrahedron Lett.,
33, 7111 (1992). b) C. Kokubo and T. Katsuki, Tetrahedron, 52,
13895 (1996). c) Y. N. Ito and T. Katsuki, Bull. Chem. Soc.
Jpn., 72, 603 (1999).
O8
C47
N3
N4
Mn2
O3
3
a) Z. Li and C. Jablonski, Chem. Commun., 1999, 1531. b) Z.
Li and C. Jablonski, Inorg. Chem., 39, 2456 (2000). c) N. C.
Gianneschi, P. A. Bertin, S. T. Nguyen, C. A. Mirkin, L. N.
Zakharov, and A. L. Rheingold, J. Am. Chem. Soc., 125, 10508
(2003).
O5
C41
C62
O4
O9
O1
O10
O2
O6
Mn1
N2
N1
4
5
a) E. B. Schwartz, C. B. Knobler, and D. J. Cram, J. Am. Chem.
Soc., 114, 10775 (1992). b) M. Kranenburg, Y. E. M. van der
Burgt, P. C. J. Kamer, P. W. N. M. van Leeuwen, K. Goubitz,
and J. Fraanje, Organometallics, 14, 3081 (1995).
C56
O7
.
1 H2O: Anal. Calcd for C15H18B2O6: C, 57.03; H, 5.74%. Found:
C, 57.00; H, 5.56%. 1H NMR (300 MHz, DMSO-d6) ꢀ 8.35 (s,
4H), 7.61 (m, 4H), 7.11 (t, J ¼ 7:5 Hz, 2H), 1.57 (s, 6H). 13C NMR
(75.5 MHz, DMSO-d6) ꢀ 153.6, 134.4, 129.0, 128.7, 122.7, 120.6,
33.4, 32.4.
Figure 2. ORTEP drawing of the cationic part of 3 (thermal
ellipsoids at the 50% probability level). Selected interatomic
ꢀ
distances (A): Mn1ꢁꢁꢁMn2, 5.0934(9); Mn1–O1, 1.885(3);
Mn1–O2, 1.893(3); Mn1–O7, 2.289(3); Mn1–O9, 2.285(3);
Mn1–N1, 1.998(4); Mn1–N2, 1.984(3); Mn2–O3, 1.904(3);
Mn2–O4, 1.888(3); Mn2–O8, 2.252(3); Mn2–O10, 2.283(3);
Mn2–N3, 1.998(3); Mn2–N4, 1.999(3).
6
7
a) N. Miyaura, T. Yanagi, and A. Suzuki, Synth. Commun., 11, 513
(1981). b) G. A. Morris and S. T. Nguyen, Tetrahedron Lett., 42,
2093 (2001).
2: Anal. Calcd for C29H22O5: C, 77.32; H, 4.92%. Found: C,
77.08; H, 5.05%. 1H NMR (300 MHz, CDCl3) ꢀ 10.85 (s, 2H),
9.38 (s, 2H), 7.48–7.15 (m, 10H), 6.82 (d, J ¼ 8:4 Hz, 2H), 1.74
(s, 6H).13C NMR (75.5 MHz, CDCl3) ꢀ 195.5, 160.4, 147.2,
137.8, 135.1, 130.8, 129.2, 128.4, 128.1, 125.5, 123.4, 120.1,
117.2, 34.6, 32.2. Crystal Data: Mr 450.48, monoclinic, P21=c,
The structure of 3 was confirmed by X-ray analysis
(Figure 2).8 Two manganese(III) Schiff-base units in the cationic
dimanganese complex [Mn2(L)(EtOH)2(H2O)2]2þ are connect-
ed by two 9,9-dimethylxanthene-4,5-diyl spacers to form a mac-
rocyclic structure. Each manganese center is surrounded by
N2O2 atoms from the Schiff-base ligand and two oxygen atoms
of ethanol and water molecules at the axial positions. Complex 3
has a pseudo-C2 axis through O5 and O6 atoms, thus the cofacial
two manganese(III) Schiff-base units are aligned as an anti man-
ꢂ
ꢀ
a ¼ 8:668ð2Þ, b ¼ 19:732ð2Þ, c ¼ 13:264ð2Þ A, ꢁ ¼ 91:68ð2Þ ,
V ¼ 2267:6ð7Þ A , Z ¼ 4, Dc ¼ 1:319 g cmꢃ3, T ¼ 295 K, R1
ꢀ 3
ðI > 2:0ꢂðIÞÞ ¼ 0:060, wR2 ¼ 0:184 (all data).
.
Anal. Calcd for 3 EtOH H2O, C76H84F12Mn2N4O12Sb2: C, 49.97;
H, 4.63; N, 3.07%. Found: C, 50.28; H, 4.71; N, 3.17%. Crystal
.
8
9
.
.
Data for 3 1.5EtOH 2H2O: fw 1867.93, monoclinic, P21, a ¼
ꢀ
ner. The two manganese centers are separated by 5.0934(9) A. In
contrast to the parallel arrangement of two salicylaldehyde
planes in 2, the interplane separation demonstrated by the
ꢂ
ꢀ
15:8627ð7Þ, b ¼ 13:0340ð5Þ, c ¼ 19:6598ð9Þ A, ꢁ ¼ 90:244ð2Þ ,
V ¼ 4064:7ð3Þ A , Z ¼ 2, Dc ¼ 1:526 g cmꢃ3, T ¼ 113 K, R1
ꢀ 3
ðI > 2:0ꢂðIÞÞ ¼ 0:051, wR2 ¼ 0:147 (all data).
ꢀ
O1ꢁꢁꢁO3 and O2ꢁꢁꢁO4 distances [3.678(4) and 3.409(4) A, respec-
a) F. C. J. M. van Veggel, M. Bos, S. Harkema, H. van de
Bovenkamp, W. Verboom, J. Reedijk, and D. N. Reinhoudt,
J. Org. Chem., 56, 225 (1991). b) H. Shimakoshi, H. Takemoto,
I. Aritome, and Y. Hisaeda, Tetrahedron Lett., 43, 4809 (2002).
tively] is significantly smaller than the C41ꢁꢁꢁC47 and C56ꢁꢁꢁC62
ꢀ
distances [4.644(6) and 4.635(6) A, respectively] in the xan-
thenediyl spacers. This difference is attributable to the two intra-
Published on the web (Advance View) May 21, 2005; DOI 10.1246/cl.2005.848