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observed with 1-phenylethanol, an activated benzylic alcohol
(entry 2). This may be explained by steric effects of the methyl
group which hinder the formation of species IV (Fig. 3), crucial
for the C–H abstraction from the alcohol by the coordinated
TEMPO molecule. In addition, in the case of primary alcohols,
the second b-hydrogen atom can be bonded to the oxygen atom
of TEMPOH, stabilising the radical intermediate VI (Fig. 3).
This is obviously not possible with secondary alcohols. These
results are indeed confirmed when octan-1-ol and octan-2-ol are
used (entries 5 and 7). Octan-1-ol was oxidised in 61%
conversion and > 99% selectivity after one day at room temp.
(95% at 40 °C, entry 6) while no octan-2-one formation is
observed with octan-2-ol. Allylic alcohols, crotyl alcohol and
geraniol (entries 3 and 4) were converted to the corresponding
aldehydes in excellent yields (91 and 100%, respectively) and
selectivities as no by-products were detected by gas chromatog-
raphy. These data (entries 1, 3, 4 and 5) clearly demonstrate that
the oxidation of activated alcohols is faster than aliphatic ones,
indicating that the hydrogen abstraction from the a-carbon atom
by TEMPO (I to II, Fig. 2) is the rate-determining step of the
reaction. Finally, a mixture of benzyl alcohol and octan-2-ol
was reacted with air in the presence of the copper(II)–TEMPO–
base catalyst (entry 8). The result shows the specificity of the
catalytic system towards primary alcohols. Thus, 67% of the
benzyl alcohol was converted whereas octan-2-ol remained
unreacted. This may be of great interest in synthetic organic
chemistry when several alcoholic functions (primary and
secondary) are present in the same molecule.
the catalytic copper complex. Chloride and nitrate ions are more
strongly coordinated to copper than bromide or perchlorate
anions, the last even being non-coordinating. Consequently, it is
easier, for the alcoholate, to enter the copper coordination
sphere when perchlorate is the counter-ion rather than chloride.
This is a plausible explanation for the activities observed.
In conclusion, a new and very mild oxidation of primary
alcohols to aldehydes with excellent conversions has been
developed. The reaction is carried out under air at room temp.
and is catalysed by a [CuBr2(Bipy)] catalyst which is very easy
to handle. In the reaction mechanism, TEMPO seems to be
involved as a hydrogen acceptor. The mechanism of the
oxidation is under investigation and further investigations are
aimed at improving the rate of oxidation of aliphatic alcohols.
For this purpose, studies using other N ligands are currently in
progress.
Financial support from COST Action D21/003/2001 and the
Dutch National Research School Combination Catalysis
(HRSMC and NIOK) is gratefully acknowledged.
Notes and references
† Experimental. All alcohols and solvents were used as received without
any further purification. The oxidation of alcohols was carried out under air
in a 50 mL three-necked round-bottom flask equipped with a magnetic
stirrer. Typically, the alcohol (10.0 mmol) and decane (2.0 mmol; GC
internal standard) were dissolved in 15 mL of a CH3CN/H2O (2 : 1) solvent
mixture. 56 mg (0.5 mmol) of t-BuOK were added followed by 112 mg (0.5
mmol) of Cu(II)Br2, resulting in a blue-green suspension. 78 mg (0.5 mmol)
of 2,2A-Bipy were then introduced leading to a dark-blue mixture. Finally,
TEMPO (78 mg; 0.5 mmol) was added and the reaction suspension
immediately turned brown-orange and clear dark-red after 2–3 min.
Samples of the reaction mixture were taken out regularly to monitor the
reaction by GC. The products of the reaction were determined by
comparison with the commercially available carbonyl compounds.
Different copper(II) salts were tested as catalyst precursors
for the oxidation of benzyl alcohol and the results are reported
in Table 3. CuCl2 and Cu(NO3)2 led to less active catalysts with
only 60 and 66% conversion, respectively, after 1.5 h (entries 1
and 2). CuBr2 afforded 83% conversion of benzyl alcohol in the
same reaction time. Cu(ClO4)2 was the best catalyst precursor
with 90% conversion reached after 1.5 h. These differences are
likely related to the propensity for dissociation of the anion from
1 (a) R. A. Sheldon and J. K. Kochi, Metal-Catalysed Oxidations of
Organic Compounds, Academic Press, New York, 1981; (b) A. E. J. de
Nooy, A. C. Basemer and H. van Bekkum, Synthesis, 1996, 1153.
2 (a) R. A. Sheldon, I. W. C. E. Arends and A. Dijksman, Catal. Today,
2000, 57, 157; (b) T. Mallat and A. Baiker, Catal. Today, 1994, 19, 247;
(c) M. Musawir, P. N. Davey, G. Kelly and I. V. Kozhevnikov, Chem.
Commun., 2003, 1414.
3 (a) G. Cainelli and G. Cardillo, Chromium Oxidations in Organic
Chemistry, Springer, Berlin, 1984; (b) W. S. Trahanovsky, in Oxidation
in Organic Chemistry, ed. A. T. Blomquist and H. Wasserman,
Academic Press, New York, 1978.
4 (a) B.-Z. Zhan, M. A. White, T.-K. Sham, J. A. Pincock, R. J. Doucet,
K. V. Ramana Rao, K. N. Robertson and T. Stanley Cameron, J. Am.
Chem. Soc., 2003, 125, 2195; (b) H. Ji, T. Mizugaki, K. Ebitani and K.
Kaneda, Tetrahedron Lett., 2002, 43, 7179 and references therein.
5 P. Gamez, P. G. Aubel, W. L. Driessen and J. Reedijk, Chem. Soc. Rev.,
2001, 30, 376.
6 (a) M. F. Semmelhack, C. R. Schmid, D. A. Cortés and C. S. Chou, J.
Am. Chem. Soc., 1984, 106, 3374; (b) M. F. Semmelhack, C. R. Schmid
and D. A. Cortés, Tetrahedron Lett., 1986, 27, 1119; (c) I. P. Skibida and
A. M. Sakharov, Catal. Today, 1996, 27, 187.
7 (a) I. E. Markó, P. R. Giles, M. Tsukazaki, I. Chellé-Regnaut, A.
Gautier, S. M. Brown and C. J. Urch, J. Org. Chem., 1999, 64, 2433; (b)
I. E. Markó, P. R. Giles, M. Tsukazaki, S. M. Brown and C. J. Urch,
Science, 1996, 274, 2044.
8 Y. Wang, J. L. DuBois, B. Hedman, K. O. Hodgson and T. D. P. Stack,
Science, 1998, 279, 537.
Fig. 3 Possible explanations for the lack of reactivity of secondary alcohols.
a) Steric hindrance due to the methyl group of the secondary alcohol
preventing the formation of species IV; b) stabilisation of the radical species
VI by the second b-hydrogen of the primary alcohol.
9 P. Chaudhuri, M. Hess, U. Flörke and K. Wieghardt, Angew. Chem., Int.
Ed., 1998, 37, 2217.
10 (a) G. Ragagnin, B. Betzemeier, S. Quici and P. Knochel, Tetrahedron,
2002, 58, 3985; (b) B. Betzemeier, M. Cavazzini, S. Quici and P.
Knochel, Tetrahedron Lett., 2000, 41, 4343.
Table 3 Influence of the copper salt on the oxidation of benzyl alcohol to
benzaldehyde
Conversion (%)
11 I. A. Ansari and R. Gree, Org. Lett., 2002, 4, 1507.
12 (a) A. Dijksman, Ph.D. Thesis, Technical University Delft, 2001; (b) A.
Dijksman, I. W. C. E. Arends and R. A. Sheldon, Org. Biomol. Chem.,
2003, DOI: 10.1039/b305941c.
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Chem. Soc., 1988, 110, 2307; (b) J. Laugier, J.-M. Latour, A. Caneschi
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Entry
Copper salt
0.5 h
1.5 h
1
2
3
4
CuCl2
Cu(NO3)2
CuBr2
30
30
38
42
60
66
83
90
Cu(ClO4)2
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