were added to a solution of 1 with a syringe-pump over a 1 hour
period, selective oxidation (≈85%) of cyclohexane into cyclo-
hexanol occurred in 37% yield. Under the same conditions but
in non-degassed solution no selectivity was observed. A select-
ive oxidation of alkane to alcohol could be assigned to a metal
centered oxidation reaction expected from a genuine mono-
oxygenase mimic. However, MacFaul and co-workers have
clearly shown by using the 2-methyl-1-phenylprop-2-yl hydro-
peroxide (MPPH) that alcohol oxidation selectivity can be due
to freely diffusing alkoxyl radicals.6 The tert-alkoxyl radical
formed after homolysis of the MPPH O–O bond undergoes
via the well-disguised free radical chemistry recently evidenced
by MacFaul et al.6 for the iron–TPA catalysts developed by
Que and co-workers.3 We are currently testing the ability of the
diiron() complex to perform hydrocarbon oxidations in the
presence of other oxidants such as hydrogen peroxide.
Acknowledgements
We are indebted to the CNRS and the Bordeaux 1 University
for financial support. We thank Dr. J.-M. Bassat for providing
the ESR spectrum of the peroxo intermediate.
β-scission (kβ ≈ 2 × 108 Ϫ1) too quickly for it to abstract a
s
hydrogen atom from a saturated hydrocarbon. When MPPH
(10 equiv. added with a syringe-pump and diluted in 2 ml
Me3CN) is used, no oxidation products are detected, showing
that the hydrogen abstracting species with TBHP [eqn. (2)] is
the tert-butoxyl radical produced from the homolysis of the
FeO–OBut bond [eqn. (1)].
Notes and references
† Analytical and spectroscopic data for complex 1: Found: C, 41.73; H,
5.21; N, 10.60; Fe, 9.51; B, 3.22. Calc. for C38H59N8F12Fe2O6B3ؒ2CH3-
OHؒH2O: C, 41.65; H, 5.38; N, 10.22; Fe, 10.19; B, 2.96%. λmax/nm
(Me3CN) 365 (ε/dm3 molϪ1 cmϪ1 1230). 1H NMR (250 MHz
in CD3CN) : the spectrum of complex 1 displays broad resonances
ranging from δ Ϫ40 to 150 in agreement with high spin iron() atoms.
By comparison with the diiron()–TPA complex described by Que et
al.,5 the resonances observed at δ 41 and 43 are tentatively attributed to
the β-protons of the pyridine ring. A minor species (<10%) is also
detected in solution and is assigned to OH ligand exchange by residual
water molecules.
t
t
ؒ
ؒ
FeOOBu → FeO ϩ Bu O
(1)
(2)
t
t
ؒ
ؒ
Bu O ϩ CyH → Bu OH ϩ Cy
ؒ
ؒ
Cy ϩ O2 → CyOO → alcohol and ketone (3)
ؒ
ؒ
Cy ϩ FeO → FeOCy → alcohol
(4)
1 B. J. Wallar and J. D. Lipscomb, Chem. Rev., 1996, 96, 2625;
A. C. Rosenweig, P. Nordlund, P. Takahara, C. A. Frederick and
S. J. Lippard, J. Chem. Biol., 1995, 2, 409.
Cy = cyclohexyl
2 J. B. Vincent, J. C. Huffman, G. Christou, M. A. Nanny, D. N.
Hendrickson, R. H. Fong and R. H. Fish, J. Am. Chem. Soc., 1988,
110, 6898; R. A. Leising, J. Kim, M. A. Pérez and L. Que, jun., J. Am.
Chem. Soc., 1993, 115, 9524; S. Ménage, J.-M. Vincent, C. Lambeaux,
G. Chottard, A. Grand and M. Fontecave, Inorg. Chem., 1993, 32,
4766; J.-M Vincent, S. Ménage, C. Lambeaux and M. Fontecave,
Tetrahedron Lett., 1994, 35, 6287; A. Rabion, S. Chen, J. Wang, R. M.
Buchanan, J.-L. Séris and R. H. Fish, J. Am. Chem. Soc., 1995, 117,
12356.
3 J. Kim, R. G. Harrison, C. Kim and L. Que, jun., J. Am. Chem. Soc.,
1996, 118, 4373.
4 D. Tétard, A. Rabion, J.-B. Verlhac and J. J. Guilhem, J. Chem. Soc.,
Chem. Commun., 1995, 531.
Preliminary, low temperature UV-visible and ESR studies
have shown that a transient iron() alkylperoxo species is
formed in the early stages of the reaction. A blue intermediate,
stabilized at Ϫ40 ЊC and generated by the addition of 50 equiv.
TBHP in an acetonitrile solution of 1, displays a broad and
intense absorption band at 600 nm. This species has a rhombic
ESR signal centered at g = 2 (2.15, 1.94), characteristic of low
spin iron() complexes. This strongly suggests the participation
of an iron() alkylperoxo intermediate as previously found
with the iron–bpy and iron–TPA catalysts.7
Addition of a small amount of CCl3Br (50 equiv., 250 µmol)
to a cyclohexane oxidation reaction gave mainly cyclohexyl
bromide demonstrating that freely diffusing cycloalkyl radicals
are formed during the reaction. These radicals can either: (i) be
trapped by O2 when a large excess of TBHP is used, to produce
cyclohexyl peroxy radicals [eqn. (3)] leading to mixtures of
5 S. Ménage, Y. Zang, M. P. Hendrich and L. Que, jun., J. Am. Chem.
Soc., 1992, 114, 7786.
6 P. A. MacFaul, K. U. Ingold, D. M. Wayner and L. Que, jun., J. Am.
Chem. Soc., 1997, 119, 10594; P. A. MacFaul, I. W. C. Arends, K. U.
Ingold and D. M. Wayner, J. Chem. Soc., Perkin Trans. 2, 1997, 135.
7 S. Ménage, E. C. Wilkinson, L. Que, jun. and M. Fontecave,
Angew. Chem., Int. Ed. Engl., 1995, 34, 203; J. Kim, E. Larka, E. C.
Wilkinson and L. Que, jun., Angew. Chem., Int. Ed. Engl., 1995, 34,
2048.
ؒ
alcohol and ketone or (ii) react with FeO when the TBHP
concentration is very low to produce an iron alkoxy species. The
latter pathway leads to alcohol selectively [eqn. (4)].
Complex 1 represents one of the few examples of a MMO
mimic able to selectively oxidize cyclohexane to cyclohexanol
Communication 9/02225B
1914
J. Chem. Soc., Dalton Trans., 1999, 1913–1914