Angewandte
Chemie
DOI: 10.1002/anie.201202835
Oxygen Transfer
Structure and Reactivity of an Iodosylarene Adduct of a
Manganese(IV)–Salen Complex**
Chunlan Wang, Takuya Kurahashi, and Hiroshi Fujii*
Enantioselective transition-metal-catalyzed oxygenation
reactions have received much attention because of their
demand in organic synthesis strategies and their biological
[
1]
relevance with respect to metalloenzymes. Terminal oxi-
dants, such as peroxides, iodosylarenes, and peracids, have
been utilized as oxygen source for these oxygenation reac-
tions. Because the terminal oxidants must be stable for easy
handling, the primary role of the transition-metal catalyst is to
activate the stable oxidant and to generate a transient species
that remains active enough to transfer an oxygen atom to
a substrate. The activation of a terminal oxidant is initiated by
binding to the metal complex to form a terminal-oxidant
adduct of the metal complex. Recently, evidence has been
mounting in support of the proposal that the terminal-oxidant
adduct of a metal complex is not only a precursor to a reactive
high-valent metal–oxo species, but that the adduct itself may
Scheme 1. Synthesis of manganese(IV)–iodosylarene adducts.
complex were prepared by reaction of a dichloromangane-
IV
se(IV)–salen complex, [Mn (salen)(Cl) ] (3), with an excess
2
of solid PhIO and MesIO, respectively, in dichloromethane at
room temperature (Scheme 1). Adducts 1 and 2 were
1
characterized by absorption, H NMR, electron paramagnetic
resonance (EPR), circular dichroism (CD), and infrared (IR)
spectroscopy, and electrospray ionization mass (ESI-MS)
spectrometry (Figure S1–S7). The ligand-to-metal charge
transfer (LMCT) band at 700 nm for 3 is shifted to a shorter
wavelength upon reaction with iodosylarene, thus indicating
binding of a strong electron donor ligand to the manganese
[2]
also serve as a reactive species in an oxygenation reaction.
Although terminal-oxidant adducts of metal complexes are
unstable and reactive compounds in most cases, metal-
complex adducts with hydrogen peroxide, alkylperoxides,
and m-CPBA have been isolated and structurally character-
1
center. H NMR spectra of 1 and 2 are similar to that of 3, thus
[
3]
ized. In contrast to these successful reports, and much to our
surprise, there have been no examples of structural character-
ization of any iodosylarene adducts of metal complexes,
although they have emerged as useful oxidants for various
suggesting the formation of bis-iodosylarene adduct. EPR
spectra clearly indicate high-spin manganese(IV) oxidation
states (S = 3/2) for 1 and 2. Adducts 1 and 2 show IR bands at
À1
À1
598 and 608 cm , which are shifted to 558 and 567 cm ,
[4]
18
organic reactions. The most intensive spectroscopic study
was performed by Hill and co-workers, who thoroughly
investigated an iodosylbenzene adduct of a manganese–
respectively, with O isotope labeling of iodosylarene. This
result is consistent with the assignment of this mode as the IÀ
[
5a]
OÀMn stretching band. ESI-MS spectra of 1 and 2 show
major signals at m/z 1074.11 and 1158.43, and their isotope
1
127
porphyrin complex with H NMR, IR, and I Mçssbauer
[5]
IV
+
spectroscopy. However, the nature of the bonding inter-
action between iodosylbenzene and the metal ion remains
unclear. Here, we report the preparation and X-ray crystal
structure of an iodosylarene adduct of a manganese(IV)–
salen complex that bears a trans-cyclohexane-1,2-diamine
patterns were fully consistent with [Mn (salen)(PhIO) Cl]
2
IV
+
and [Mn (salen)(MesIO) Cl] , respectively. The CD spectra
2
of 1 and 2 show strong bands at 280 nm, while that of 3 does
not show a strong CD band, thus indicating that the structures
of 1 and 2 are chirally distorted in solution. All of these
spectroscopic data are consistent with bis-iodosylarene
adducts of manganese(IV)–salen complex having stepped
conformations.
[
6]
linkage as chiral unit.
We used iodosylbenzene (PhIO) and iodosylmesitylene
MesIO) as iodosylarene compounds. PhIO and MesIO
(
adducts (1 and 2, respectively) of a manganese(IV)–salen
Brown crystals of 2 suitable for X-ray crystal-structure
determination were obtained by slow diffusion of pentane
[7]
[
*] C. Wang, Dr. T. Kurahashi, Prof. Dr. H. Fujii
Institute for Molecular Science and Okazaki Institute for Integrative
Bioscience, National Institutes of Natural Sciences and Department
of Functional Molecular Science, The Graduate University for
Advanced Studies (SOKENDAI)
into a solution of 2 in dichloromethane at À208C. The X-ray
crystal structure of 2 shows a monomeric bis-MesIO adduct of
the manganese(IV)–salen complex (Figure 1). Structure 2
shows a chirally distorted stepped conformation with one of
two salicylidene rings pointing upward and the other pointing
downward as a result of the steric bias from the trans-
cyclohexane-1,2-diamine moiety. Because 3 shows an almost
Myodaiji, Okazaki 444-8787 (Japan)
E-mail: hiro@ims.ac.jp
[
**] This work was supported by grants from the Japan Society for the
Promotion of Science (Grant-in-Aid for Scientific Research, Grant
No. 22350030 and 23550086).
[
8]
flat structure, the stepped conformation must be induced by
axial iodosylmesitylene ligands. In a previous study, the
stepped conformation was observed for manganese(IV)–
[8]
salen complexes with tightly bound axial ligands. In fact, the
Angew. Chem. Int. Ed. 2012, 51, 1 – 4
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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