Scheme 1
hexane-d12 gave a low kinetic isotope effect (KIE), k
=
H
/k
1.2. The findings that cumene selectively formed acet-
ophenone, cis-decalin gave trans-decalol as the major product
D
the oxidation reaction profiles observed. In alkane oxidation
non-catalytic and catalytic reactions showed practically identi-
cal selectivity with a preponderance of the ketones as products,
a low KIE, formation of acetophenone from cumene and the
formation of an equilibrium mixture of trans-decalol–cis-
decalol from cis-decalin. This reactivity clearly supports a
9
6 6
and the low KIE in the oxidation of C H12–C D12 all point to an
alkyl radical intermediate in the oxidation of alkanes.
The reaction mechanism was further studied by UV–VIS,
II
13
Fig. 1. The original catalyst, Li12[Mn
2
ZnW(ZnW
O
9 34
)
2
], is
reaction occuring through a free alkyl radical intermediate as
yellow. Upon addition of ozone at 2 °C, the solution within a
opposed to a oxygen-rebound mechanism often invoked in
III
14
minute turns pink, (Mn ) lmax = 560 nm, as is known for
metalloporphyrin oxidation. Epoxidation of alkenes with
III
9–10
[Mn
2
ZnW(ZnW
9
O
34
)
2
].
Further addition of ozone turns
retention of stereochemistry is explainable through reaction of
IV
11
III
+
the solution brown forming a Mn –oxo compound. The later
was inactive in a stoichiometric reaction with alkenes.11 The
brown compound was not stable slowly reverting within an hour
the ozonide, POM–Mn –O–O–O canonical form, as an
electrophile with the nucleophilic alkene with co-formation of
III
molecular oxygen and Mn –POM.
to the pink manganese(iii). In a further experiment, a solution of
This research was supported by the Basic Research Founda-
tion administered by the Israel Academy of Sciences and
Humanities.
II
Q12[Mn
2
ZnW(ZnW
9
O
34
)
2
] (Q = trioctylmethyl ammonium)
in acetone was cooled to 278 °C and ozone was passed through
the solution yielding a green solution, lmax = 486, 580 nm,
Fig. 1(d). Further characterization of the green solution by ESR,
Fig. 1 (inset), showed an anisotropic spectrum with peaks at
Notes and References
g
∑
= 2.09 and g
4
= 2.06.
† E-mail: ronny@vms.huji.ac.il
After purging the solution of excess ozone with N
2
,
stoichiometric amounts of alkenes were added. The solution
1
2
R. Neumann and M. Gara, J. Am. Chem. Soc., 1995, 117, 5066.
D. E. Katsoulis and M. T. Pope, J. Chem. Soc., Chem. Commun., 1986,
was brought to 240 °C and turned pink after a few minutes.
2
,3-Dimethyl-2-butene and cyclooctene, 80 and 63% conver-
1
8
186; A. M. Khenkin and C. L. Hill, J. Am. Chem. Soc., 1993, 115,
178.
sion respectively, reacted selectively to give epoxides as sole
products whereas norbornene (74% conversion) gave 94% exo
epoxide and 6% 2-norbornanone. cis-Stilbene was epoxidized
3
C. L. Hill and R. B. Brown, J. Am. Chem. Soc., 1986, 108, 536;
D. Mansuy, J.-F. Bartoli, P. Battioni, D. K. Lyon and R. G. Finke, J. Am.
Chem. Soc., 1991, 113, 7222.
9
5% stereoselectively with no formation of the cleavage
product, benzaldehyde. An identical experiment carried out
with stoichiometric addition of ethylbenzene gave a darkish
brown solution and yielded (28 mol% based on manganese)
acetophenone–1-phenylethanol, ≈ 20:1.
4 R. Neumann and M. Dahan, Nature, 1997, 388, 353.
5
P. S. Bailey, Ozonation in Organic Chemistry, Academic Press, New
York, 1978.
S. Camperstrini, A. Robert and B. Meunier, J. Org. Chem., 1991, 56,
6
3725; Z. Gross, S. Nimri and L. Simkhovich, J. Mol. Catal. A: Chem.,
1996, 113, 231; Z. Gross and L. Simkhovich, J. Mol. Catal. A: Chem.,
1997, 117, 243.
Our interpretation of the results of the catalytic and
stoichiometric oxidation reactions, and the UV–VIS and ESR
II
spectra is summarized in Scheme 1. The initial yellow Mn –
7
B. Meunier, Chem. Rev., 1992, 92, 1411.
III
POM is first oxidized to the pink Mn –POM. The catalytic
8 J. R. Lindsay Smith and P. R. Sleath, J. Chem. Soc., Perkin Trans. 2,
1983, 1165.
III
cycle begins by reaction of Mn –POM with ozone to give the
stipulated active intermediate, the green manganese species.
Based on the spectra and the reactivity profile, we assign the
green compound as a manganese ozonide complex. The ESR
spectrum is attributable to an anisotropic oxygen centered
9 J. T. Groves and T. E. Nemo, J. Am. Chem. Soc., 1983, 105, 6243.
0 C. M. Tourn e´ , G. F. Tourn e´ and F. Zonnevijlle, J. Chem. Soc., Dalton
Trans., 1991, 143.
1
1
1 X. Zhang, M. T. Pope, M. R. Chance and G. B. Jameson, Polyhedron,
1
995, 14, 1381; X. Zhang, G. B. Jameson, C. J. O’Connor and
radical species12 formulated here as POM–Mn –O–O–O ,
IV
·
M. T. Pope, Polyhedron, 1996, 15, 917; X. Zhang, C. J. O’Connor,
J. B. Jameson and M. T. Pope, Inorg. Chem., 1996, 35, 30; X. Zhang and
M. T. Pope, J. Mol. Catal. A: Chem., 1996, 114, 201.
III
3
formed by reaction of Mn –POM and O . Other canonical
forms, POM–Mn –O–O–O or POM–Mn –O–O–O are
possible. The UV–VIS spectrum is supportive of this assign-
III
+
V
2
12 S. Schlick, J. Chem. Phys., 1972, 56, 654; J. Steffen, W. Hesse,
M. Jansen and D. Reinen, Inorg. Chem., 1991, 30, 1923; W. Hesse,
M. Jansen and W. Schnick, Prog. Solid State Chem., 1989, 19, 47.
ment, since peaks at 450–480 nm are typically observed for
ozonides.12 In the absence of a substrate and/or at higher
1
3 E. T. Denisov and T. G. Denisova, Kinet. Catal. (Engl. Transl.), 1996,
7, 46.
4 J. T. Groves, J. Chem. Educ., 1985, 62, 928.
temperatures the compound quickly decays by reduction or
disproportionation to a brown manganese(iv) oxo or hydroxy
species (a typical ESR spectrum with peaks at g = 2 and 4 was
also observed)11 and then more slowly to Mn –POM. The
formulation of the green species as an ozonide is consistent with
3
1
III
Received in Cambridge, UK, 10th June 1998; 8/04396E
1968
Chem. Commun., 1998