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rogated. The oxidation rate of 2 toward olefins (k2) increased
as the degree of olefin substitution increased (Figure 2, top).
Thus the rate for the oxidation of 2,3-dimethyl-2-butene with
2 was 975 Æ 16mÀ1 sÀ1, almost seven times higher than that for
2-methyl-2-hexene (k2 = 144 Æ 1mÀ1 sÀ1), which in turn reacted
around two and fourteen times faster than cis- and trans-2-
octene, respectively. The lowest rate was observed for
1-octene (8.8 Æ 0.2mÀ1 sÀ1). Most probably this is related to
the increase in the electron density of the double bond, which
is due to inductive effects of the substituents,[1] which supports
the hypothesis that 2 acts as an electrophile. Interestingly,
these results indicate that electronic effects dominate over
steric factors in determining the reactivity of 2 towards
olefins. It is also particularly interesting that the relative
reactivity was configuration-dependent and that the cis
isomer was more prone to oxidation than the trans analogue,
and reacted up to eight times faster in the case of 2-octene.
The impact of electronic effects in defining the relative
reactivity of olefins with 2 was further studied in a systematic
manner by analyzing the reaction rates for the reaction of 2
with several para-substituted styrenes. The logarithm of the
second-order rate constants of the decay was plotted against
the Hammett para substituent constants, s+, affording a good
correlation (R = 0.95) and a negative slope (1) of À1.4 Æ 0.2
(Figure 2, bottom; see also Table S4). The negative 1 value
confirms the electrophilic character of 2 in OAT reac-
tions,[16,17] which is in agreement with the increase in reactivity
with increased olefin substitution (Figure 2, top).
Figure 3. Intermolecular competitive epoxidation of pairs of olefins
(alkeneA and alkeneB) by the 1/AcOOH catalytic system. EpoxideA/
epoxideB: ratio of the epoxideA and epoxideB products determined by
A
GC analysis. k2 /k2B: ratio of the second-order rate constants for the
A
B
reaction of 2 with alkeneA (k2 ) and alkeneB (k2 ) in acetonitrile/acetone
(1:3) at À608C determined by stopped-flow analysis.
when compared to other iron–oxygen species. It must be kept
in mind that comparison with these compounds is not
straightforward, as the oxidation state of the metal and the
ligands, oxidants, and reaction conditions employed are not
equivalent. Furthermore, a cautious note is warranted; iron-
catalyzed epoxidations using other tetra- or pentadentate
ligands have been studied, and alternative reaction mecha-
nisms have been proposed. In most of these cases, the lack of
accumulation of the OAT species, which is presumably very
reactive, prevents comparison.[3,4,6,15,24] With these limitations
in mind, 2 can be ranked among the different types of iron–
oxygen species for which second-order reaction rates for OAT
reactions have been determined (Table 1, Figure 4). The
Compound 1 acts as a catalyst in the epoxidation of
olefins. The catalytic oxidation of excess 1-octene (100 equiv)
at room temperature with peracetic acid (20 equiv) as the
oxidant and 1 as the catalyst afforded 1-octene oxide as the
major product (12 TON; TON = turnover number) and the
corresponding hydroxyacetate as the minor product (3 TON).
Both compounds are proposed to be formed by the reaction
of in situ generated 2 with the olefin. Comparison of the
catalytic results obtained with the 1/AcOOH system
(1:10:100, 1/AcOOH/substrate, À608C) with the kinetic
data gathered for the reaction of 2 with alkenes demonstrates
the relevance of 2 in catalytic epoxidation reactions. The
selectivities of the 1/AcOOH catalytic reactions could be
extracted from the competitive epoxidation of pairs of olefins
(alkeneA and alkeneB) in acetonitrile/acetone (1:3) at À608C.
Three different competition experiments were performed:
styrene vs. 1-octene, cis-cyclooctene vs. cis-2-octene, and cis-
vs. trans-2-octene (Figure 3). Blank experiments showed that
under these experimental conditions, uncatalyzed epoxida-
tion is negligible (< 3% with respect to the oxidant).
Interestingly, the reactions occurred with stereoretention,
and the relative amounts of the two epoxides formed after
each competition reaction (epoxideA/epoxideB) reasonably
matched the ratios of the corresponding k2 values measured
Table 1: Second-order rate constants (k2) for the reaction of 2 and
selected mononuclear iron–oxygen species with 1-octene, styrene, and
cis-cyclooctene (see Figure 4 for the structures of compounds 3–9).
Catalyst T [8C]
k2 [mÀ1 sÀ1
1-octene styrene
]
Ref.
cyclooctene
2
3
À60
0
8.8Æ0.2 149Æ3
375Æ15
this work
[18]
–
–
0.45
4
20
–
–
0.032
[19]
5
25
25
–
–
5.3
–
–
0.026
–
–
3.3
[20]
[21]
[22]
[17]
6[a]
7
0.043Æ0.003
À40
–
8
9
RT
148Æ8.1
29Æ3.6
À60
0.4
–
[23]
[a] Proton-coupled electron transfer (PCET) in the presence of HOTf.
Compound 6 itself cannot oxidize styrene.
iron(IV) oxo complexes 3[18] and 4,[19] bearing pyridine-
containing macrocyclic ligands, and 5 with a N2S2 thioether
ligand[20] exhibit rates that are three to four orders of
magnitude lower than 2, but these reactions were carried
out at much higher temperatures. The same difference in
reactivity was observed for the iron(IV) oxo complex 6, which
bears a pentadentate N4Py ligand, in the presence of triflic
acid.[21] Compound 2 also shows higher reaction rates than the
more reactive S = 2 iron(IV) oxo complex 7, which was
B
individually for the reaction of 2 with each olefin (k2A/k2 ).
This remarkable agreement provides strong evidence that 2
constitutes the active species responsible for OAT in these
catalytic epoxidations.
Aside from its selectivity, the fast reactivity of 2 is also
consistent with a catalytically competent intermediate. The
reaction rates of 2 towards alkenes are especially impressive
6312
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Angew. Chem. Int. Ed. 2016, 55, 6310 –6314