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norbornene, and 1-octene. As we had observed in the
epoxidation of aromatic olefins, 1 disappeared rapidly upon
addition of the aliphatic olefins at a low temperature (e.g.
À608C), even in the reaction with a less reactive olefin (i.e. 1-
octene; see spectral changes in Figure S12). Second-order
rate constants were determined by plotting first-order rates
against the concentrations of the aliphatic olefins (see Fig-
ure S13). When the second-order rate constants were plotted
against the oxidation potentials (Eox) of the aliphatic olefins,
a good linear correlation was observed (see Table S5 and
Figure S14). Furthermore, product analysis revealed that
epoxides were formed as the major product (e.g. cyclooctene
oxide with a yield of 81(5)%; see Table S4). Furthermore, in
the reaction of cyclohexene, cyclohexene oxide (66(5)%) was
obtained as the major product along with the formation of
cyclohex-2-enol (21(2)%) and cyclohex-2-enone (3.0(2)%;
see Table S4). The source of the oxygen atom in the cyclo-
hexene oxide product (71(5)% 18O) was found to be the
iodosylbenzene group in 1 on the basis of an 18O-labeling
experiment performed with [18O]-1 (75(5)% 18O; see Fig-
ure S15).[11] It is of interest to note that the preference of the
Scheme 2.
iodosylarene adducts (Scheme 2, pathway b) and this inter-
mediate is involved as a reactive species in the catalytic
epoxidation reactions. Indeed, high-valent iron(V)-oxo spe-
cies have been frequently proposed as active oxidants in
various oxidation reactions by nonheme iron catalysts.[16]
Since we failed to synthesize the proposed iron(V)-oxo
intermediate bearing the 13-TMC ligand, we synthesized an
iron(III)-iodosylarene complex with a chiral iodosylarene,
[(13-TMC)FeIII-OIPhc]3+ (5), and used it in the olefin
epoxidation, on the assumption that if this intermediate is
involved in the olefin epoxidation, we would observe the
formation of epoxides with a high enantiomeric excess
(Scheme 3, pathway a). If an iron(V)-oxo complex is involved
=
À
C C epoxidation over the allylic C H bond activation in the
oxidation of cyclohexene by the iron(III)-iodosylbenzene
adduct (1) is different from that reported in the oxidation of
cyclohexene by mononuclear nonheme iron(IV)-oxo com-
plexes.[15] In the latter reactions, allylic oxidation products,
such as cyclohex-2-enol, were formed predominantly with
a trace amount of cyclohexene oxide. In addition, a KIE value
of 1.0(1) was obtained for the oxidation of cyclohexene and
[D10]cyclohexene by 1 (see Figure S13c); it is worth noting
that high KIE values (ca. 60) were obtained in the oxidation
of cyclohexene by the nonheme iron(IV)-oxo complexes
[(N4Py)FeIV(O)]2+ and [(Bn-TPEN)FeIV(O)]2+ (N4Py = N,N-
bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine and Bn-
TPEN = N-benzyl-N,N’,N’-tris(2-pyridylmethyl)ethane-1,2-
diamine).[15a] Finally, [(13-TMC)FeIV(O)]2+ (6) did not react
with cyclohexene under the identical reaction conditions.
We then carried out the epoxidation of olefins by the
iron(II) complex [(13-TMC)FeII]2+ and PhIO at low (e.g.
À608C) and room temperatures under the catalytic reaction
conditions (e.g. [(13-TMC)FeII]2+ (1.0 10À3 m), substrate
(2.0 10À1m), and PhIO (3.0 10À2 m); see the Experimental
Section in the Supporting Information for detailed reaction
procedures). In all of the reactions, epoxides were obtained as
major products (see Table S4). Moreover, the product dis-
tributions were the same as those observed in the stoichio-
metric reactions performed with in situ generated iron(III)-
iodosylbenzene adducts (see Table S4), thereby leading us to
propose that the active oxidant that effects the olefin
epoxidation in the catalytic reactions is iron(III)-iodosylarene
adducts. Furthermore, we rule out the involvement of an
Scheme 3.
in the olefin epoxidation, then we would observe the
formation of a racemic mixture (Scheme 3, pathways b and c).
The iodosylarene (cPhIO) with a chiral auxiliary on the
benzene ring was synthesized from an optically active hyper-
valent iodine reagent (see the structure in Scheme 3; see also
Table S1).[17] The reaction of [(13-TMC)FeII]2+ with PhIO in
c
À
iron(IV)-oxo species, which is formed by a homolytic O I
acetone/CF3CH2OH (3:1) at À608C afforded a high-spin
iron(III)-iodosylarene complex, [(13-TMC)FeIII-OIPhc]3+ (5;
see Figures S2–S4). Interestingly, the reaction of 5 with
chalcone afforded chalcone oxide as the major product with
a good enantioselectivity (i.e. ca. 76% ee; see Table 1, entry 1
and Figure S16). Similarly, moderate enantiomeric excess (ee)
values were obtained in the epoxidation of trisubstituted
aromatic a,b-enones, such as 2-(4-methylbenzylidene)-1-tet-
bond cleavage of the iron(III)-iodosylarene adducts
(Scheme 2, pathway a), as a reactive species in the catalytic
epoxidation reactions, since we have shown in the stoichio-
metric kinetic studies that the iron(IV)-oxo intermediate does
not react with olefins (see above). However, we cannot rule
out the possibility that an iron(V)-oxo species is formed by
À
heterolytic cleavage of an O I bond of the iron(III)-
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Angew. Chem. Int. Ed. 2015, 54, 11740 –11744