Article
Inorganic Chemistry, Vol. 49, No. 18, 2010 8621
Scheme 1. Aminopyridine Ligands Used for the Syntheses of
Manganese(II) Catalysts
implicating metal oxidant complexes as the only intermedi-
ates or as the “second” and “third” active oxidants.4b,8-10 In
this paper, we report the results of a systematic investigation
of enantioselective alkene epoxidations with a broad range of
different terminal oxidants catalyzed by cationic manganese-
(II) complexes with aminopyridine ligands2-4. Importantly,
an unprecedented level of asymmetric induction (up to 79%
ee) has been achieved for manganese aminopyridine cata-
lyzed epoxidations. Moreover, on the basis of the catalytic
and spectroscopic (EPR and NMR) data obtained, one could
reliably discriminate between the alternative oxygen-rebound
and Lewis acid activation mechanisms and identify the new
oxidant (a complex between the terminal oxidant and the
high-valence oxomanganese complex rather similar to the
applied4,5a,5b to monitor the high-valence manganese inter-
mediates in the Katsuki-Jacobsen epoxidations. The MnIII-
(salen) acylperoxo complex was found to be the last detectable
intermediate if one used m-CPBA as the oxidant, while in the
system with PhIO, the OdMnV(salen) complex5c was the
main oxygen-transferring species [stocked up in the form of
μ-oxo-bridged manganese(IV) binuclear complexes].4
“third oxidant” reported by Goldberg and co-workers10a,b
)
as that responsible for enantioselective epoxidation by this
catalyst system.
Results and Discussion
Despite the success of Katsuki-Jacobsen-type oxidations,
manganese salen catalysts bore some intrinsic drawbacks
associated with the ligand structure, in particular, the low
turnover number (5-50 in most cases) due to fast catalyst
degradation. In 2003, Stack and co-workers reported the
aminopyridine [MnII[(R,R)-bpmcn](CF3SO3)2] complex
with ligand 1 (Scheme 1), which efficiently catalyzed the
epoxidation of various alkenes with peracetic acid in high
yields (90-99%) using down to only 0.1 mol % of the
catalyst6a (although the enantioselectivity of these epoxida-
tions was not scrutinized). A total of 19 manganese com-
plexes were screened, revealing [Mn(1)(CF3SO3)2] (Mn-1) as
the most prominent olefin epoxidation catalyst with peracetic
acid.6b In 2007, oxidation of various organic substrates with
peracetic acid and PhIO catalyzed by similar nonheme
manganese complexes was examined.7a Costas and co-
workers synthesized a novel family of pinene-derived amino-
pyridinylmanganese complexes that demonstrated remark-
able performance and moderate stereoselectivities (4-46%
ee) in the epoxidation of various substrates with peracetic
acid.7b
Enantioselective Olefin Epoxidation. New manganese-
(II) complexes with chiral ligands 2-4 were prepared and
tested in the enantioselective epoxidation of various
alkenes with peracetic acid. Some results are presented
in Table 1. Complexes Mn-3 and Mn-4 (bearing either
electron-donating or -withdrawing substituents in the
third position) displayed rather poor activities (Table 1,
entries 1-5); high conversion was observed only for the
epoxidation of styrene over Mn-2. The latter, in con-
trast, led to nearly quantitative formation of epoxides of
styrene and of trans-β-Me-styrene and somewhat lower
conversions for 1,2-dihydronaphthalene and indene
(Table 1, entries 6-9).
Encouraged by the observed enantioselectivity for the
epoxidation of styrene and indene with AcOOH over Mn-
2, we attempted the epoxidation of various alkenes with
other oxidants (Table 2). Both enantiomers of the catalyst
(Mn-2 and Mn-1) demonstrated similar yields and enan-
tioselectivities in the epoxidation of styrene with AcOOH
(entries 1 and 2). A similar result was obtained at 0 °C
using only 0.5 mol % of the catalyst (entry 3). The
Although the importance of understanding the nature of
active intermediates can hardly be overestimated, no direct
experimental evidence for the manganese aminopyridine
systems has been reported so far. It has been supposed [by
analogy with the Mn(salen) chemistry] that high-valence
oxomanganese complexes could be involved in the crucial
oxidation step6a,7b (Nam and co-workers suggested “a
mechanism involving metal-based oxidants”7a). However, in
many cases, the existence of alternative mechanisms for
manganese- and iron-based oxidations has been established,
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