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loss of H2 from 1, which is known to occur when 2 is used at ele-
vated temperatures.26 To distinguish between these two mecha-
nisms, 1-phenylethanol was oxidized using the optimized
reaction conditions in acetone-d6 in an open system. A 1H NMR
spectrum of the crude reaction mixture showed acetophenone
and isopropanol-d6 in a one-to-one ratio. This result is consistent
with an Oppenauer-type oxidation where 1 equiv of acetone is re-
duced for each equivalent of alcohol that is oxidized (Scheme 1). If
it proceeded through a direct loss of H2, no deuterated isopropanol
would have been observed.
10. (a) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2007, 129, 5816; (b) Casey, C. P.;
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Chem. Soc. 2005, 127, 14062; (c) Casey, C. P.; Johnson, J. B. Can. J. Chem. 2005,
83, 1339. Alternatively, it has been proposed that the ruthenium hydride forms
by an inner-sphere mechanism involving b-hydrogen elimination.; (d) Samec,
J. S. M.; Bäckvall, J.-E.; Andersson, P. G.; Brandt, P. Chem. Soc. Rev. 2006, 35, 237.
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17. Low turnover numbers with similar iron tricarbonyl complexes have been
observed. See Ref. 9a.
In summary, an air-stable, well-defined iron compound was dis-
covered to be an active catalyst for Oppenauer-type alcohol oxida-
tions. By removing one of the carbonyl ligands from an 18-electron
iron species using trimethylamine N-oxide, a coordinatively unsat-
urated species that was catalytically active could be generated.
Secondary benzylic and allylic alcohols are easily oxidized with
this catalyst system. Primary alcohols are oxidized in low yields
and appear to partially inhibit the catalytic activity of the iron spe-
cies. We have provided evidence that the oxidation proceeds
through a transfer dehydrogenation (Oppenauer-type oxidation)
process, which is consistent with the detailed kinetic and spectro-
scopic studies of the reverse reaction.10,11 Current and future work
will be focused on applying this catalytic system to other transfor-
mations and developing new air-stable, user-friendly iron catalysts
by modifying compound 6.
18. Trimethylamine N-oxide has been used as
a stoichiometric oxidant in
connection with iron carbonyl compounds for alcohol oxidations. (a)
Dasgupta, B.; Donaldson, W. A. Tetrahedron Lett. 1998, 39, 343; (b) Pearson,
A. J.; Kwak, Y. Tetrahedron Lett. 2005, 46, 5417.
19. Guan and co-workers used
a
crude iron hydride derivative of
8
in
a
stoichiometric NMR scale reaction with acetone and observed
a
species
suggestive of a diiron bridging hydride similar to 2. They suggested that the
large trimethylsilyl (TMS) groups on 1 disfavor the formation of the bridging
hydride and allow it to remain active. See Ref. 9b.
20. Knölker, H.-J.; Baum, E.; Heber, J. Tetrahedron Lett. 1995, 36, 7647.
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Chem. Soc., Chem. Commun. 1974, 336; (c) Elzinga, J.; Hogeveen, H. J. Chem. Soc.,
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657; (b) Pearson, A. J.; Shively, R. J., Jr. Organometallics 1994, 13, 578. We
gratefully acknowledge a reviewer for bringing this to our attention. Various
attempts to isolate a dicarbonyl/amine variant of 6 were unsuccessful; free
cyclopentadienone was the only species isolated.
Acknowledgment
23. Representative procedure for alcohol oxidation by 6: To a solution of 6 (34 mg,
0.082 mmol, 0.1 equiv) and trimethylamine N-oxide dihydrate (9.2 mg,
The authors thank Gettysburg College for the financial support.
References and notes
0.082 mmol, 0.1 equiv) in 1.6 mL degassed acetone in
a round-bottomed
flask with a condenser connected to a mineral oil bubbler was added 1-
phenylethanol (100 mg, 0.82 mmol, 1 equiv, 0.5 M in acetone), and the solution
was heated to reflux for 18 h. The solvent was removed under reduced
pressure and the remaining residue was purified by flash chromatography on
silica gel (3% ethyl acetate/97% hexanes) to afford 91 mg (92% yield) of
acetophenone.
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3 have been
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alcohols were not reported.
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