C O M M U N I C A T I O N S
of olefin cis-dihydroxylation by H2O2. This difference is likely
related to the spin state of the FeIII-OOH intermediate generated
in the course of catalysis. The electrophilicity of the oxidant derived
from 1/H2O2 is consistent with the reactivity expected for the
previously proposed high-valent FeV(dO)OH species derived from
a low-spin FeIII-OOH intermediate.1a,b Such a species may be
viewed as related to the high-valent dioxometal species well known
to carry out olefin cis-hydroxylation.10 The observed nucleophilicity
of the oxidant generated from 2/H2O2, on the other hand, has fewer
precedents and requires the consideration of new mechanisms to
rationalize the high conversion efficiency and stereoselectivity
associated with the putative high-spin FeIII-OOH intermediate. This
study thus establishes the mechanistic versatility of iron-peroxo
species in olefin oxidation; it also lays the foundation for
understanding the mechanism of Rieske dioxygenases,11 enzymes
involved in biodegradation that catalyze cis-dihydroxylation of
arenes and olefins.
Figure 1. Competition experiments for the oxidation of olefin pairs by
catalysts 1 (left) and 2 (right): C ) cyclooctene (red), O ) 1-octene
(orange), A ) tert-butyl acrylate (green), F ) dimethyl fumarate (blue).
Conditions as described under Table 1 except that 1050 µmol each of two
olefins was used. Solid blocks represent the fraction of diol formed, while
patterned blocks represent the fraction of epoxide formed.
Scheme 2. Proposed Mechanisms of cis-Dihydroxylation by a
Nucleophilic Oxidant Generated from 2/H2O2
Acknowledgment. This work was supported by the National
Institutes of Health (GM-33162) and the Petroleum Research Fund
administered by the American Chemical Society (38602-AC). M.C.
is grateful to Fundacio La Caixa for a postdoctoral fellowship that
in part supported his stay at the University of Minnesota. We
appreciate the thoughtful input of one reviewer in formulating
mechanisms for Scheme 2.
Supporting Information Available: Table S1 listing results of 18O-
labeling experiments (PDF). This material is available free of charge
I, epoxidation of R,â-unsaturated ketones is initiated by nucleophilic
attack of an (η2-peroxo)iron(III) porphyrin complex,5 followed by
O-O bond heterolysis, analogous to the action of basic H2O2. In
case II, a high-spin FeIII-η1-OOH intermediate is proposed to
undergo O-O bond homolysis to generate a species that prefer-
entially oxidizes dimethyl sulfoxide over dimethyl sulfide.6 To apply
to 2, these mechanisms must be adapted to account for the
unprecedented formation of cis-diol and its high yield.
Scheme 2 shows two proposed mechanisms for cis-dihydroxy-
lation by 2. Mechanism i entails a nucleophilic attack by the
coordinated peroxide on the olefin, like case I, but followed by
reductive O-O bond homolysis. Mechanism ii involves initial O-O
bond homolysis, like case II, to form a tightly associated FeIVd
O/HO• pair, followed by nucleophilic attack of HO• on the
substrate. (The nucleophilicity of HO• has been documented by
Walling and El-Taliawi, who showed that HO• readily adds to R,â-
unsaturated acids to form water addition products (but not diols).7)
In both mechanisms, the available cis site on the iron center is
recruited to facilitate formation of an FeIV-2-hydroxyalkyl radical
species. This species is the key to diol formation, as iron complexes
of related pentadentate ligands do not catalyze cis-dihydroxylation.1,8
The subsequent collapse of this FeIV-radical species to diol is akin
to the oxygen rebound step in iron-catalyzed alkane hydroxylations.9
The rate of oxygen rebound depends on the stability of the transient
alkyl radical, thus affording a high RC value for cis-2-heptene and
a lower value for dimethyl maleate due to the radical-stabilizing
effect of the adjacent -COOMe group.
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In summary, we have found that 1 and 2, respectively, generate
oxidants with electrophilic and nucleophilic character in the catalysis
JA029863D
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