C O M M U N I C A T I O N S
expected to exert a weaker ligand field than TPA and thus favor
the non-water-assisted pathway.4b,c Indeed 18O labeling studies for
the oxidation of 1-octene show that the predominant diol product
(64%) is doubly labeled by H2O2, as found for 34a,b (see Table S2
in Supporting Information). Interestingly, unlike for 3, there is also
a significant fraction of diol with an oxygen atom from H2O (33%),
suggesting possible involvement of the water-assisted pathway.
Alternatively, water incorporation via the non-water-assisted path-
way can be rationalized if the active catalyst were considered to
be the 1:1 Fe(Ph-DPAH) complex. Since this complex would have
three labile sites, it would then be possible to accommodate both
a side-on hydroperoxo and a water ligand (Figure 3B). Prior
cleavage of the peroxo O-O bond would form a transient FeV-
(O)(OH)(OH2) oxidant in equilibrium with an FeV(OH)3 isomer that
then transfers two of the three oxygen atoms to the olefin.
Figure 2. Comparison of 1 with the cis-dihydroxylation catalysts 2 and 3
in the oxidation of selected substrates. Note the higher selectivities (and
conversion efficiencies) of 1 for cis-diol: 93% (75%) for cyclooctene, 99%
(77%) for 1-octene, and 94% (85%) for styrene.
In summary, 1 is the first iron catalyst for olefin cis-dihydroxy-
lation with a facial N,N,O-ligand arrangement mimicking that found
for the Rieske dioxygenases. It can oxidize a wide range of olefins
efficiently and has proven to be the most selective thus far for
dihydroxylation. 18O labeling experiments suggest the participation
of an FeV oxidant for this bio-inspired catalyst, which carries
implications for the action of the Rieske dioxygenases.1c,d,9
Acknowledgment. This work was supported by the Department
of Energy (DOE DE-FG02-03ER15455). We are grateful to Mr.
Patrick Landreman for experimental assistance, and Dr. Ken Suzuki
for discussions regarding ligand synthesis.
Supporting Information Available: Procedures for the synthesis
of Ph-DPAH and 1 and for the catalytic reactions, as well as
crystallographic and ESI-MS data for 1. This material is available free
Figure 3. Proposed dihydroxylation mechanisms for 2 and 3 (A) and for
1 (B) (S ) CH3CN). See Supporting Information for details of the isotope
labeling experiments for 1-octene.
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The longevity of catalyst 1 was investigated by varying the
amount of oxidant introduced. As shown in Table 1 for the oxidation
of 1-octene, significant deterioration of catalyst efficacy was
observed with the addition of more than 10 equiv of H2O2. The
ESI-MS spectrum of the catalyst solution at the end of an oxidation
experiment showed the formation of a large amount of free ligand,
together with a small amount of an oxo-bridged diiron(III)
byproduct, suggesting catalyst decomposition (see Figure S2 in
Supporting Information).
Previous studies of 2 and 3 have led to the postulation of cis-
dihydroxylation mechanisms that require metal catalysts with at
least two cis-labile sites for the activation of H2O2 to form an FeV-
(O)(OH) oxidant.4b,8 Such an oxidant can be formed by a water-
assisted pathway on the S ) 1/2 H2O-FeIII-OOH reaction surface
or by a non-water-assisted pathway on the S ) 5/2 FeIII-(η2-OOH)
reaction surface (Figure 3A). Extending this mechanistic framework
to coordinatively saturated 1 requires some extent of ligand
dissociation. Although it can be imagined that each Ph-DPAH ligand
becomes bidentate during catalysis, we favor the complete dis-
sociation of one Ph-DPAH ligand to make three sites available for
exogenous ligand binding (Figure 3B). This notion is supported
by ESI-MS data showing the presence of free ligand and the 1:1
[FeII(Ph-DPAH)]2+ complex ion in solution prior to the start of
catalysis, indicative of a ligand dissociation equilibrium (see Figure
S1 in Supporting Information).
Because of its high selectivity for diol formation, 1 resembles 3
more closely in its catalytic behavior,4a-d both of which would be
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