A novel platform for modeling oxidative catalysis in non-heme iron oxygenases with unprecedented efficiency

Article abstract of DOI:10.1002/chem.200800724

A study was conducted to demonstrate a number of non-heme iron complexes, based on the methylpyridine derivatized triazacyclononane (TACN) backbone. It was found that these iron complexes show significant efficiency in the stereospecific oxidation of alkanes with H₂O₂, bypassing the latest oxidations catalyzed by TPA and BPMEN complexes. It was also demonstrated that the type of substitution on the N atoms of the triazamacrocyle and on the pyridine ring are key tools, to control the selectivity of the iron complexes in catalytic alkane and alkene oxidation reactions. It was also shown that this structural control of the catalytic selectivity in bioinspired oxidation reactions makes the iron complexes an efficient platform, to mimic iron dependent oxygenase reactivity.

Full text of DOI:10.1002/chem.200800724

COMMUNICATION
DOI: 10.1002/chem.200800724
A Novel Platform for Modeling Oxidative Catalysis in Non-Heme Iron
Oxygenases with Unprecedented Efficiency
[
a]
[a]
[b]
[a]
Anna Company, Laura Gómez, Xavier Fontrodona, Xavi Ribas, and
[
a]
Miquel Costas*
Non-heme monoiron dependent oxygenases are emerging
as a very diverse and versatile group of enzymes involved in
a number of oxidative transformations, which also hold po-
of-the-art oxidations catalyzed by TPA and BPMEN com-
[3]
plexes. We show that the type of substitution on the N
atoms of the triazamacrocycle and on the pyridine ring are
[1]
II
tential technological implications. These biological cata-
lysts constitute a source of inspiration for the development
key tools to control the selectivity of the corresponding Fe
complexes in catalytic alkane and alkene oxidation reac-
tions. This structural control of the catalytic selectivity in
bioinspired oxidation reactions makes this family of com-
plexes a unique and versatile platform to mimic iron depen-
[2]
of environmentally benign oxidation technologies. On the
other hand, bioinspired synthetic catalysts constitute a valu-
able tool to explore the reaction mechanisms by which non-
[3]
heme enzymes perform their chemistry. Iron complexes
derived from tripodal TPA (TPA=tris(2-methylpyridyl)-
amine) and linear BMPEN (BPMEN = N,N’-bis(2-methyl-
pyridyl)-N,N’dimethyldiaminoethane) type of ligands are
particularly exceptional compounds because of their ability
to perform stereoselective enzyme-like transformations such
as alkane hydroxylation and alkene epoxidation and cis-di-
dent oxygenase-like reactivity.
Reaction of tetradentate ligands
R,R’
PyTACN (Scheme 1)
with Fe
pounds [Fe
A
H
R
U
G
A
T
E
N
(CH CN) in THF afforded title com-
3
3
2
3
2
R,R’
3
2
3
3
3
3
white to yellow analytically pure powders that could be ob-
tained in crystalline form after recrystallization from
CH Cl /Et O.
2
2
2
[3]
hydroxylation, with remarkable efficiency. Such tetraden-
tate backbones wrap around an iron(II) center giving rise to
complexes with two cis available coordination sites which
can be occupied by labile ligands like CH CN or CF SO .
3
3
3
Parallel to the development of these two families of com-
[4]
[5]
plexes, several other examples including tri-, tetra- and
[6]
pentadentate ligands have been explored, yet none of
them can compare with TPA and BPMEN families in terms
of selectivity, versatility and efficiency.
In this work we report a novel family of non-heme iron
complexes based on the methylpyridine derivatized triazacy-
clononane (TACN) backbone. This novel family of com-
plexes shows unprecedented efficiency in the stereospecific
oxidation of alkanes and alkenes with H O , bypassing state-
Scheme 1. Complexes described in this work.
Thermal ellipsoid plots corresponding to the X-ray crys-
2
2
tallographic characterization of 1·CF SO
and [Fe-
3
3
Me,H
A
H
R
U
(CH CN) (
PyTACN)](PF ) , [3·CH CN]PF , are shown
A
T
E
N
[
a] A. Company, L. Gómez, Dr. X. Ribas, Dr. M. Costas
Departament de Química, Universitat de Girona
Campus de Montilivi, 17071 Girona (Spain)
Fax : (+34)972-418-150
3
2
6 2 3 6
in Figure 1 to illustrate the structural properties of this
[7]
family of complexes. The complexes contain iron centers
in a distorted octahedral coordination environment. The tri-
podal tetradentate ligands leave two coordination sites ac-
cessible to exogenous ligands (CF SO or CH CN) trans to
E-mail: miquel.costas@udg.edu
[
b] X. Fontrodona
3
3
3
Serveis T›cnics de Recerca, Universitat de Girona
Campus de Montilivi, 17071 Girona (Spain)
non-equivalent aliphatic N atoms (N-CH -py and Nalkyl-R,
2
R=CH or iPr). The pyridine arm binds trans to one of the
two Nalkyl groups of the ligand. Average FeÀN bond lengths
Supporting information for this article is available on the WWW
under http://www.chemistry.org or from the author.
3
Chem. Eur. J. 2008, 14, 5727 – 5731
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5727

of highly selective metal-cen-
tered oxidants. Thus, reactions
catalyzed by 3 and 4 exhibit re-
markably large A/K ratios (12.3
and 10.2, respectively), and
high selectivities as indicated by
the kinetic isotope effects eval-
uated in the oxidation of a 1:3
mixture
of
cyclohexane/
ACHTUNG-
TERUNG
[D ]cyclohexane (KIE ꢀ3–4)
1
2
and the high C
/C
se-
secondary
tertiary
lectivity in the oxidation of ada-
mantane (adam) (38/28 ꢀ20–
3 3 3 6
Figure 1. Thermal ellipsoid plot (50% probability) of 1·CF SO (left) and [3·CH CN]PF (right). Hydrogen
atoms have been omitted for clarity.
3
0). In addition, the oxidation
of cis-1,2-dimethylcyclohexane
DMCH) catalyzed by 3 and 4
(
in 1·CF SO and [3·CH CN]PF are 2.2 and 2.0 Š, respec-
tively, which are indicative of high and low-spin Fe configu-
rations.
exhibits a large degree of stereoretention. However, the
same oxidation catalyzed by 1 and 2 shows some degree of
loss of stereoretention, which indicates the implication of
longer lived carbon centered radicals.
3
3
3
6
II
The catalytic properties of 1·CF SO –4·CF SO in the hy-
3
3
3
3
[8]
droxylation of alkanes with H O are shown in Table 1.
The origin of the oxygen atoms introduced into the cyclo-
hexanol product could be evaluated by means of isotopic la-
beling experiments performing the oxidation of cyclohexane
2
2
Under conditions of large excess of substrate (cyclohexane),
and 4 are excellent catalysts and convert up to 76% of
H O into oxidized products, with a remarkably high A/K
3
1
8
using 10 equiv H2 O in the presence of 1000 equiv H O
2
2
2
2
(
2
alcohol/ketone) ratio. On the other hand, complexes 1 and
exhibit significantly lower yields very likely due to the
and the complementary experiment using non-labeled H O
2 2
1
8
and 1000 equiv H2 O (Table 1). It is clearly observed that
the incorporation of oxygen atoms from water and/or air
into products is highly dependent on the structure of the
complexes, suggesting a rich and complex mechanistic sce-
nario modulated by the catalyst architecture. For example,
the incorporation of labeled water into cyclohexanol de-
creases in the order 3 (45%), 4 (11%), 1 (8%) and 2 (3%).
On the other hand, incorporation of O derived from air is
inversely related to water incorporation; it constitutes more
than 30% of the oxidized alcohol product generated by 1
and 2, but it accounts for less that 10% in the oxidation re-
actions catalyzed by 3 and 4. All these evidences indicate
that alcohol product is produced via a radical-rebound type
weak tertiary CÀH bond of the isopropyl group that renders
the complexes susceptible to self-oxidation. The excellent
catalytic abilities of 3 and 4 are maintained when larger
amounts of peroxide are delivered (see Table 1). When
1
00 equiv of peroxide are added, catalyst 4 produces 52TN
[9]
of alcohol (A) and 12TN of ketone (K).
and selectivity exhibited by 4 surpasses those of [Fe-
The efficiency
[3c]
A
C
H
T
R
E
U
N
G
(CF SO ) (BPMEN)], the prototypical example of a very
A
C
H
T
R
E
U
N
G
3
3 2
active non-heme iron hydroxylation catalyst (48 TN A+K,
A/K = 2.5, under identical conditions).
Mechanistic studies, using large excess of substrate to
avoid overoxidation reactions, demonstrate the involvement
[3c,10]
mechanism.
Competing with this pathway, O trapping
2
of the radical followed by Russell-type termination accounts
for its incorporation into products. The different level of O2
incorporation into products and DMCH stereoselectivity
studies suggest that the lifetimes of the carbon centered rad-
icals are modulated by the particular ligand. On the other
hand, the decrease on the A/K ratio when the alcohol con-
centration increases (compare reactions with 10 and
Table 1. Alkane hydroxylation reactions catalyzed by 1·CF
3
SO
3
–
[
a]
4
3 3
·CF SO .
Cyclohexane
adam DMCH
[
b]
[c]
[d]
2
[e]
[f]
Cat equiv H
2
O
2
TNA+K
KIE
H
2
O
2
/H
2
O/O
38/28
RC [%]
A
C
H
T
R
E
U
N
G
(A/K)
2.4 (3.6) 4.8
0.8 (2.7)
6.5 (12.3) 4.3
39 (2.6)
7.6 (10.2) 3.4
64 (4.3)
1
2
3
10
10
10
59:8:33
30:3:66
47:45:8
–
85:11:4
–
14
21
30
–
20
–
86
78
93
–
94
–
–
1
00 equiv of H O ) indicates that the ketone product is most
2 2
[9]
likely formed via oxidation of the initially formed alcohol.
1
00
–
The catalytic ability of 1–4 in the oxidation of olefins was
also explored and the main results are showed in Table 2. In
this case, reactions were run under argon to avoid autooxi-
dation reactions that can occur with olefin substrates. Under
large excess of substrate (cyclooctene), all the complexes
convert the peroxide into epoxide (E) and diol (D) prod-
ucts, with good to excellent efficiencies expanding from 50
to 81%. Interestingly, the diol/epoxide (D/E) ratio is dra-
matically modified depending on the catalyst employed.
4
10
1
00
–
[
a] 1000 equiv substrate for cyclohexane and DMCH, 10 equiv for ada-
mantane [b] Turnover number (mols of product/mols of catalyst), A=cy-
clohexanol, K=cyclohexanone. [c] Kinetic Isotope Effect of cyclohexanol
formation. [d] Percentage of oxygen incorporation derived from different
sources into the cyclohexanol product. [e] Tertiary/secondary ratio in ada-
mantane (adam) oxidation=3 ” (1-adamantanol)/(2-adamantanol + 2-
adamantanone). [f] Percentage of retention of configuration in the oxida-
tion of the tertirary CÀH bonds of cis-1,2-dimethylcyclohexane (DMCH).
5728
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 5727 – 5731

Modeling Oxidative Catalysis
COMMUNICATION
[
a]
Table 2. Alkene oxidation reactions catalyzed by 1·CF
3
3 3 3
SO –4·CF SO .
Cyclooctene
cis-2-heptene
RC [%]
[
b]
[b]
[c]
[d]
[e]
Cat equiv H
2
O
2
TN
D
TN
E
D/E
Yield [%]
epoxide/diol
1
2
3
10
6.21.9
73
4.5
10
4.1
50
123
6.0
74
3.3
126.0
0.5
4.0
4.0
49
129
1.1
126. 28 6
29
81
85
9.0
2.5
1.0
1.0
0.8
5.5
88:99
–
1
00
10
50
14
81
99
77
71
64:97
–
93:90
–
1
00
10
1
3
00
00
4
10
91:90
Figure 2. Origin of the oxygen atoms incorporated into the cis-diol prod-
1
3
00
00
–
57
uct in the oxidation of cyclooctene catalyzed by complexes 1·CF
·CF SO . White: percentage of cis-diol with the two oxygen atoms origi-
nating from H . Black: percentage of cis-diol with one oxygen from
and the other from H O.
3 3
SO –
141
4.9
4
3
3
[
a] 1000 equiv substrate, [b] Turnover number (mol product/mol catalyst),
D=cis-diol, E=epoxide. [c] D/E=mols of diol/mols of epoxide.
d] Yield based on the oxidant. [e] Percentage of retention of configura-
2
O
2
H
2
O
2
2
[
tion in the oxidation of the C=C bond of cis-2-heptene for epoxide and
cis-diol products.
experiments indicate a complex mechanistic picture depend-
ing on the catalyst. A clear-cut difference arises from cis-di-
hydroxylation reactions catalyzed by 3 and 4 in comparison
with 1 and 2. 90% of the cis-diol product obtained in cata-
lytic reactions of 3 and 4 incorporates one O from water
and one O from H O , but almost exclusive incorporation of
Thus 3 affords a 1:1 mixture of diol/epoxide and introduc-
tion of a methyl group in the pyridine ring (4) increases the
ratio up to 5.5. Replacement of methyl groups by isopropyl
groups in the TACN ring results in better selectivity towards
the diol as indicated by the higher selectivity towards cis-di-
hydroxylation exhibited by catalysts 1 and 2. Interestingly,
compound 2 exhibits one of the best selectivities towards
cis-dihydroxylation among the iron complexes reported in
the literature, and it constitutes a functional model of
2
2
O from H O is observed for 1 and 2. Thus, olefin oxidation
2
2
reactions catalyzed by this family of complexes exhibit a
tuned selectivity pattern towards cis-dihydroxylation, which
[3c]
resembles that found in TPA and BPMEN families, yet
the present complexes exhibit significantly improved effi-
ciency, and the diol/epoxide selectivity is controlled at two
levels: the substitution in the pyridine ring and in the TACN
backbone.
[3c,11]
Rieske dioxygenases.
The use of larger amounts of per-
oxide evidences strong differences in the catalytic ability of
the complexes. When the peroxide concentration increases
Previous studies on related non-heme iron catalysts have
shown that isotopic labeling in the cis-dihydroxylation reac-
tion is a valuable tool to address the active species responsi-
1
0-fold up to 100 equiv, 73, 50 and 74 TN of cis-diol are ob-
tained for 1, 3 and 4, respectively. Furthermore, 252 TN
D/E = 0.8) and 170 TN (D/E = 4.9) of products are ob-
tained with 3 and 4, respectively, when 300 equiv of H O
[3,5f]
(
ble for this type of chemistry (Scheme 2).
2
2
are used. These numbers are much higher than any previ-
[
3–6,11]
ously described iron complex,
and 4 are the most active non-heme iron catalysts for alkene
and indicate that 1, 3
[3c,11,12]
cis-dihydroxylation reported so far.
On the other
hand, 2 loses its catalytic activity when the amount of perox-
ide is increased up to 100 equiv, presumably because of cata-
lyst decomposition. Analogous reactions run under air ex-
hibited smaller D/E ratios mainly because larger amounts of
epoxides are obtained (see Supporting Information).
Moreover, oxidation of cis-2-heptene indicates that both
epoxidation and cis-dihydroxylation occur with a large
degree of stereoretention, yet significant levels of epimeriza-
tion suggest that the oxygen atom(s) transfer to the olefin
both in epoxidation and in cis-dihydroxylation occur in a
non-concerted fashion. The higher epoxide yields obtained
in reactions run under O and the significant levels of epi-
2
merization indicate that epoxides are formed in some extent
[5f]
via O trapping of carbon centered radical intermediates.
2
Labeling studies were performed by running cyclooctene
1
8
oxidation reactions with 10 equiv H2 O in the presence of
2
1
000 equiv H O, and the complementary experiments were
2
1
8
run with 10 equiv H O /1000 equiv H O (Figure 2). These
Scheme 2.
2
2
2
Chem. Eur. J. 2008, 14, 5727 – 5731
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5729

M. Costas et al.
Class A catalysts insert one atom of oxygen from water
and one atom of oxygen from peroxide, and favor epoxida-
tion over cis-dihydroxylation, when reacting with olefins. On
the other hand, class B catalysts insert both O atoms from
peroxide, and favor cis-dihydroxylation. The former is an in-
Experimental Section
Full experimental details for the preparation of the complexes, details for
the crystallographic characterization of 1·CF SO and [3·CH CN]PF and
3
3
3
6
experimental procedures for catalytic oxidation reactions are included as
Supporting Information.
V
dication of the implication of a Fe (O)(OH) species via a
[13]
water assisted O–O lysis, presumably favored by the low-
spin state of the iron center, which weakens the hydroperox-
[
14]
ide OÀO bond (Scheme 2, left).
Olefin interaction with
Acknowledgements
the oxo group leads to epoxide, while the cis-diol originates
from initial attack via the hydroxide ligand. Instead, for
class B catalysts two mechanistic scenarios have arisen
[15]
Financial support by MEC-Spain (Project CTQ2006–05367 to M.C.).
A.C. and L.G. thank the MEC for FPU-PhD grants. We thank Professor
L. Que and Dr. R. Hage for helpful discussions.
(
Scheme 2, center and right); Que et al. have proposed that
a high-spin side-on ferric hydroperoxide or a high valent
species, generated with no assistance of water, are the active
species responsible for epoxidation and cis-dihydroxylation
Keywords: bioinorganic chemistry
model compounds · non-heme oxygenases · oxidation
· enzyme catalysis ·
[3b]
(
Scheme 2, center). On the other hand, Comba et al. have
proposed, on the basis of DFT calculations, that OÀO bond
II
2+
homolysis of [LFe (H O )] species (L stands for a tetra-
A
C
H
T
R
E
U
N
G
2 2
[
[
[
1] a) M. Costas, M. P. Mehn, M. P. Jensen, L. Que, Jr., Chem. Rev.
dentate bispidine ligand) leads to tautomeric species
2
004, 104, 939–986; b) M. M. Abu-Omar, A. Loaiza, N. Hontzeas,
IV
2+
IV
2+
[
(
LFe (OH) ]
(S=1) and [LFe (O)
A
H
R
U
(OH )]
(S=2)
Chem. Rev. 2005, 105, 2227–2252.
2] a) M. Costas, K. Chen, L. Que, Jr., Coord. Chem. Rev. 2000, 200–
2
2
[5f]
Scheme 2, right). The former is responsible for the cis-di-
2
5
02, 517–544; b) S. Tanase, E. Bouwman, Adv. Inorg. Chem. 2006,
8, 29–75.
hydroxylation pathway, while the latter accounts for epoxi-
dation activity. Evaluation of these two possibilities for the
present systems will require computational analyses which
are currently under investigation. In conclusion, isotopic la-
beling experiments in the cis-dihydroxylation reaction led us
to conclude that the nature of the alkyl substitution in the
TACN ring determines the class dichotomy; catalysts 3 and
3] a) K. Chen, M. Costas, J. Kim, A. K. Tipton, L. Que, Jr., J. Am.
Chem. Soc. 2002, 124, 3026–3035; b) K. Chen, M. Costas, L.
Que, Jr., J. Chem. Soc. Dalton Trans. 2002, 672–679; c) K. Chen, L.
Que, Jr., J. Am. Chem. Soc. 2001, 123, 6327–6337.
[
4] a) G. J. P. Britovsek, J. England, S. K. Spitzmesser, A. J. P. White,
D. J. Williams, Dalton Trans. 2005, 945–955; b) P. C. A. Bruijnincx,
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Spek, G. van Koten, R. J. M. Klein Gebbink, Chem. Eur. J. 2008, 14,
4
undergo water assisted O–O lysis and belong to class A,
1
228–1237.
5] a) G. J. P. Britovsek, J. England, A. J. P. White, Dalton Trans. 2006,
399–1408; b) G. J. P. Britovsek, J. England, A. J. P. White, Inorg.
while 1 and 2 belong to class B. Nevertheless, according to
this scenario, the significant selectivity for cis-dihydroxyla-
tion exhibited by 4 is unexpected, and suggests that yet un-
considered factors play a role in the epoxide/diol selectivity.
As the exact nature of the reaction mechanism operating in
class B catalysts remains a matter of debate, at present it is
not clear how does the alkyl substitution in the TACN ring
exerts this drastic class selectivity. Nevertheless, the present
system constitutes a unique platform that supports both cat-
alytic classes, and thus it is well suited for mechanistic stud-
ies on these biologically relevant scenarios.
[
1
Chem. 2005, 44, 8125–8134; c) J. England, G. J. P. Britovsek, N. Ra-
badia, A. J. P. White, Inorg. Chem. 2007, 46, 3752–3767; d) M. Klop-
stra, G. Roelfes, R. Hage, R. M. Kellogg, B. L. Feringa, Eur. J.
Inorg. Chem. 2004, 4, 846–856; e) R. Mas-BallestØ, M. Costas, T.
van der Berg, L. Que, Jr., Chem. Eur. J. 2006, 12, 7489–7500; f) J.
Bautz, P. Comba, C. Lopez de Laorden, M. Menzel, G. Rajaraman,
Angew. Chem. 2007, 119, 8213–8216, Angew. Chem. Int. Ed. 2007,
46, 8067–8070; g) M. C. White, A. G. Doyle, E. N. Jacobsen, J. Am.
Chem. Soc. 2001, 123, 7194–7195.
[
6] a) S. Gosiewska, M. Lutz, A. L. Spek, R. J. M. Klein Gebbink,
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de Boer, W. R. Browne, G. Roelfes, B. L. Feringa, Chem. Commun.
In conclusion, we have discovered a new family of non-
heme iron complexes with unprecedented catalytic activity
in bioinspired oxidation reactions. The high selectivity of
these reactions along with their degree of stereospecificity
suggests the implication of highly selective metal centered
species with relevance to the active species involved in non-
heme iron enzymes such as naphthalene and toluene oxy-
2
004, 2550–2551; e) V. Balland, D. Mathieu, N. Pons-Y-Moll, J. F.
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[1–2]
genases.
Recently, White et al. have reported a structur-
ally related complex to the BPMEN system, as stereoselec-
tive hydroxylation catalyst for complex organic molecules,
[
7] Crystal data for 1·CF
3
SO
3
: C20H
32
F
6
FeN
4
O
6
S
2
, monoclinic, P21/c, a=
8
.994(3) b=20.446(6), c=15.012(5) Š, b=91.735(5)8, V=2759.4
[16]
3
À3
À1
albeit with low TN numbers. The excellent catalytic abili-
ties exhibited by 3 and 4 make them a structurally different
promising alternative that deserves further exploration.
Mechanistic studies and reaction intermediates involved in
the reactions, as well as their biological relevance are also
currently under investigation.
(14) Š , Z=4, 1calcd =1.585 gcm
,
m=0.781 mm
,
T=100(2) K,
crystal size=0.1”0.05”0.01 mm, q range=1.99–28.158, unique re-
flections=6626, parameters=356, GoF=1.120, R1 [I>2s(I)]=
0
.0650, wR2[ I>2s(I)]=0.1302. Crystal data for
A
H
R
U
G
3 6
[3·CH CN]PF :
18 30 6 2
C H F12FeN P
, monoclinic, P21m, a=7.9099(4), b=9.8035(5), c=
3
1
calcd
6.6584(9) Š, b=90.9770(10)8, V=1291.58(12) Š , Z=2, 1 =
À3
À1
1.739 gcm , m=0.816 mm , T=100(2) K, crystal size=0.4”0.2”
5730
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 5727 – 5731

Modeling Oxidative Catalysis
COMMUNICATION
0
.1 mm, q range=2.41–28.338, unique reflections=3386, parame-
ters=235, GoF=1.242, R1 [I> 2s(I)]=0.0930, wR2[ I> 2s(I)]=
.2102. The asymmetric unit contains half of the molecule. Structure
solution and refinement were done using SHELXTL Version 6.14.
CCDC 662658 (1·CF SO ) and CCDC 662659 (3·CF SO ) contain
[11] a) P. D. Oldenburg, A. A. Shteinman, L. Que, Jr., J. Am. Chem. Soc.
2005, 127, 15672–15673; b) J. Y. Ryu, J. Kim, M. Costas, K. Chen,
W. Nam, L. Que, Jr., Chem. Commun. 2002, 1288–1289.
0
[12] For comparison, oxidation of 1000 equiv cyclooctene with 300 equiv
3
3
3
3
H
2
O
2
using [Fe
A
H
R
U
(CF
3
SO
3 2
) (BPMEN)] as the catalyst afforded a 47%
A
H
R
U
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallograph-
ic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
yield with a D/E ratio of 0.23.
[13] D. QuiÇonero, K. Morokuma, D. G. Musaev, R. Mas-BallestØ, L.
Que, Jr., J. Am. Chem. Soc. 2005, 127, 6548–6549.
[
[
8] Reaction mixtures were analyzed 10 min after H
reaction times did not result in significant differences in reactions ef-
ficiencies.
2
O
2
addition; longer
[14] a) R. Y. N. Ho, G. Roelfes, B. L. Feringa, L. Que, Jr. J. Am. Chem.
Soc. 1999, 121, 264–265; b) N. Lehnert, F. Neese, R. Y. N. Ho, L.
Que, Jr., E. I. Solomon, J. Am. Chem. Soc. 2002, 124, 10810–10822.
[15] A. Bassan, M. R. A. Blomberg, P. E. M. Siegbahn, L. Que, Jr.,
Angew. Chem. 2005, 117, 2999–3001; Angew. Chem. Int. Ed. 2005,
44, 2939–2941.
9] Ketone yield increases with the number of H
see Table 1) due to overoxidation of alcohol product that also acts
as substrate. This was validated by oxidation of cyclohexane in the
presence of cyclohexanol (cat 3/H /cyclohexane/cyclohexanol
:100:972:28) that yielded a A/K =1.75, a value significantly lower
than using only 1000 equiv cyclohexane (A/K=2.6).
2 2
O equivalents added
(
2
O
2
[16] M. S. Chen, M. C. White, Science 2007, 318, 783–787.
1
[
10] A. Company, L. Gómez, M. Güell, J. M. Luis, X. Ribas, L. Que, Jr.,
Received: April 15, 2008
M. Costas, J. Am. Chem. Soc. 2007, 129, 15766–15767.
Published online: May 15, 2008
Chem. Eur. J. 2008, 14, 5727 – 5731
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5731