alcohol 7). In each case, the pure aldehyde product 8 was
obtained in quantitative yield, after 1 h, using oxygen in toluene
as the oxidant at 80 °C, even after 12 recycle runs.10 The catalyst
6 was efficiently used for the oxidation of allylic alcohols to the
corresponding enals, and acetophenone was obtained from
1-phenylethanol. However, neither cyclohexanol nor cyclohex-
enol were converted to the corresponding ketones using this
method. Finally, catalyst 6 was used in the preparation of
aldehyde 9 from alcohol 10 on a 1 g scale, using only 25 mg of
the triethylammonium catalyst 6 following the previously
mentioned toluene/oxygen protocol (Scheme 2). A quantitative
yield of the pure aldehyde 9 was obtained. This aldehyde was
used in a ten-step linear synthesis of the potent analgesic natural
product epibatidine, which employed only solid supported
reagents.11
to incorporate these new solid supported reagents in our future
clean chemistry programmes.15
Notes and references
1 R. Lenz and S. V. Ley, J. Chem. Soc., Perkin Trans. 1, 1997, 3291.
2 I. E. Mark, P. R. Giles, M. Isukazaki, C. J. Urch and S. M. Brown, J. Am.
Chem. Soc., 1997, 119, 12 661.
3 S. V. Ley, J. Norman, W. P. Griffith and S. P. Marsden, Synthesis, 1994,
640.
4 B. Hinzen and S. V. Ley, J. Chem. Soc., Perkin Trans. 1, 1997, 1907; B.
Hinzen, R. Lenz and S. V. Ley, Synthesis, 1998, 977.
5 Several reviews considering various aspects of MCM-41 and other
mesoporous materials have appeared: J. Y. Ying, C. P. Mehnert and
M. S. Wong, Angew. Chem., Int Ed.., 1999, 38, 56; S. Biz and M. L.
Occelli, Catal. Rev. Sci. Eng., 1998, 40, 329; K. Moller and T. Bein,
Chem. Mater., 1998, 10, 2950; A. Corma, Top. Catal., 1997, 4, 249;
G. D. Stucky, Q. Huo, A. Firouzi, B. F. Chmelka, S. Schacht, I. G.
Voigt-Martin and F. Schuth, in Progress in Zeolite and Microporous
Materials, Studies in Surface Science and Catalysis, Vol 105, ed. H.
Chon, S.-K. Ihm, Y. S. Uh, Elsevier, Amsterdam, 1997, 3; C. J. Brinker,
Curr. Opin Solid State Mater. Sci., 1996, 1, 798; N. K. Raman, M. T.
Anderson and C. J. Brinker, Chem. Mater., 1996, 8, 1682; D. M.
Antonelli and J. Y. Ying, Curr. Opin. Colloid Interface Sci., 1996, 1,
523; P. Behrens, Angew. Chem., Int. Ed. Engl., 1996, 35, 515; X. S.
Zhao, G. Q. Lu and G. J. Millar, Ind. Eng. Chem. Res., 1996, 35, 2075;
A. Sayari, Chem. Mater., 1996, 8, 1840.
6 For different methods of tethering chiral catalysts, see D. E. De Vos, S.
De Wildemann, B. F. Sels, P. J. Grobet and P. A. Jacobs, Angew. Chem.,
Int. Ed., 1999, 38, 980; B. F. G. Johnson, S. A. Raynor, D. S. Shephard,
T. Maschmeyer, J. M. Thomas, G. Sankar, S. Bromley, R. D. Oldroyd,
L. Gladden and M. D. Mantle, Chem. Commun., 1999, 1167.
7 D. S. Shephard, W. Zhou, T. Maschmeyer, J. M. Matters, C. L. Roper,
S. Parsons, B. F. G. Johnson and M. J. Duer, Angew. Chem., Int. Ed.,
1998, 37, 2719; T. Maschmeyer, R. D. Oldroyd, G. Sankar, J. M.
Thomas, I. J. Shannon, J. A. Klepetko, A. F. Masters, J. K. Beattie and
C. R. A. Catlow, Angew. Chem., Int. Ed. Engl., 1997, 36, 1639; J. Liu,
X. Feng, G. E. Fryxell, L.-Q. Wang, A. Y. Kim and M. Gong, Adv.
Mater., 1998, 10, 161.
Scheme 2 Reagents and conditions: i, O2, 6 (2.5 wt%), toluene, 80 °C,
12 h.
Instead of oxygen, NMO or trimethylamine N-oxide
(TMAO) may be used as oxidant, and although this protocol
was efficient in generating the desired aldehydes in high yield,
a noticeable contamination by both ruthenium and an organic
impurity was found. These were probably unreduced, involatile
NMO or TMAO, and a product formed by b-elimination
(Hofmann elimination) of the ammonium species. Only in this
experiment was the catalyst found not to be reusable.
Leaching was shown not to take place with 6 under the
conditions employed. Thus, oxidation was allowed to proceed
to 50% completion, and the solid catalyst was removed by
filtration and stored under argon; the filtrate was then heated at
80 °C under an oxygen atmosphere and no further oxidation was
observed by HPLC after 12 h. The catalyst was then re-added to
the reaction mixture, the reaction was re-established and
complete oxidation was observed in less than 1 h under the usual
conditions.
8 10 wt% of the solid catalyst 3 corresponds to 0.3 wt% Ru by ICP
analysis.
9 10 wt% of the solid catalyst 6 corresponds to 1.1 wt% Ru by ICP
analysis.
In separate experiments designed to help elucidate the
catalytically active species we have established that: (i) MCM-
41 alone shows no activity; (ii) diphenylsilyl capped MCM-41
treated with potassium perruthenate (lacking the supporting
tether) shows little activity and complete leaching; (iii)
uncapped MCM-41 treated with potassium perruthenate (lack-
ing the supporting tether) shows little activity and complete
leaching; (iv) colloidal RuO2 absorbed within the MCM-41
mesopores12 shows no activity. These experiments demonstrate
that a perruthenate derived species is responsible for the
catalysis and not RuO2.13 Scanning transmission electron
microscopy (STEM) revealed that coverage of the MCM-41
internal surface was uniform (also the external surface of
Carbosil vide infra). There was no evidence for the formation of
colloidal RuO2.14 It is worth noting that X-ray diffraction
studies of the catalysts 5 and 6 indicate that long range order in
the MCM-41 structure is reduced, whilst STEM showed that
there are residual ‘domains’ of parallel mesopores. An alter-
native synthetic route was used to reproduce 6 without the loss
of long range order by eliminating the water solvated ion
exchange. However no superior performance was observed
under the same reaction conditions. X-Ray diffraction also
revealed that no large inorganic crystallites were formed in the
catalyst preparation. We have also demonstrated that an aerogel
silica tethered triethylammonium perruthenate species (prepara-
tion akin to 6) is a good oxidation catalyst although significantly
longer reaction times are needed (5 h on a 0.1 mmol scale).
In summary, we have developed a new, highly active,
recoverable and re-usable heterogeneous catalytic oxidant 6 for
the oxidation of alcohols to carbonyl derivatives by molecular
oxygen. It may be prepared from the siliceous MCM-41
material following a reliable tethering method and ion exchange
with potassium perruthenate. The immobilisation of other
reagents using the improved MCM-41 support should allow us
10 General procedure for the oxidation of alcohols: To a solution of the
alcohol (30.0 mg) in dry toluene (5 cm3) was added MCMSP 6 (3.0 mg)
and the resulting mixture heated at 80 °C, for 30 min to 3 h in an oxygen
atmosphere. After cooling, filtration of the mixture followed by
evaporation of the solution afforded the pure aldehyde in quantitative
yield (Table 1). The solid catalytic oxidant reagent can be re-used many
times (up to 12 times without loss of activity). It can be easily filtered
through a polypropylene filter or through a bond elut cartridge and then
washed off the surface with toluene. In addition, recovery by
centrifugation was also found to be quantitative.
11 J. Habermann, S. V. Ley and J. S. Scott, J. Chem. Soc., Perkin Trans. 1,
1999, 1253.
12 D. S. Shephard, G. Sankar, J. M. Thomas, D. Ozkaya, B. F. G. Johnson,
R. Raja, R. D. Oldroyd and R. G. Bell, Chem. Eur. J., 1998, 4, 1214.
13 In experiments (i) and (iv), no oxidation was observed after 50 h, in
toluene/oxygen at 80 °C, implying that a perruthenate derived species
was responsible for the catalysis and not RuO2. In experiments (i) and
(iii), approximately 10 and 70% oxidation, respectively, to the aldehyde
was observed after 3 days under the same conditions and complete
leaching of potassium perruthenate was observed in both cases. The
used solid material from experiments (ii) and (iii) showed no catalytic
oxidative activity when reused.
14 D. G. Lee, Z. Wang and W. D. Chandler, J. Org. Chem., 1992, 57,
3276.
15 B. Hinzen and S. V. Ley, J. Chem. Soc., Perkin Trans. 1, 1998, 1; F.
Haunert, M. H. Bolli, B. Hinzen and S. V. Ley, J. Chem. Soc., Perkin
Trans. 1, 1998, 2235; S. V. Ley, M. H. Bolli, B. Hinzen, A.-G. Gervois
and B. J. Hall, J. Chem. Soc., Perkin Trans. 1, 1998, 2239; M. H. Bolli
and S. V. Ley, J. Chem. Soc., Perkin Trans. 1, 1998, 2243; J.
Habermann, S. V. Ley and J. S. Scott, J. Chem. Soc., Perkin Trans. 1,
1998, 3127; M. Caldarelli, J. Habermann and S. V. Ley, J. Chem. Soc.,
Perkin Trans. 1, 1999, 107; S. V. Ley, A. W. Thomas and H. Finch,
J. Chem. Soc., Perkin Trans. 1, 1999, 669; S. V. Ley, O. Schucht, A. W.
Thomas and P. J. Murray, J. Chem. Soc., Perkin Trans. 1, 1999,
1251.
Communication 9/06354D
1908
Chem. Commun., 1999, 1907–1908