B. Nammalwar et al. / Tetrahedron Letters 54 (2013) 2010–2013
2013
Table 2
6. Wang, H.-Q.; Yang, G.-F.; Li, Q.-Y.; Zhong, X.-X.; Wang, F.-P.; Li, Z.-S.; Li, Y.-H.
New J. Chem. 2011, 35, 469–475.
7. Attenburrow, J.; Cameron, A. F. B.; Chapman, J. H.; Evans, R. M.; Hems, B. A.;
Jansen, A. B. A.; Walker, T. J. Chem. Soc. 1952, 1094–1111.
Oxidation of benzylic ethers and amines using nano-, Attenburrow, and commercial
MnO2
8. The temperature on the microwave oven was set at 105 °C, but due to the small
quantities used and the design of the temperature probe holder assembly, the
sensor did not actually contact the reactants. The temperature measured just
above the reaction mixture, however, never exceeded 90 °C.
9. The TGA experiments were performed on synthetic MnO2 samples prepared
according to the literature procedures followed by drying at 90 °C and 1 atm for
1 week. Commercial MnO2 was also dried at 90 °C and 1 atm for 1 week.
10. This reagent has been previously reported to contain 3–4% of firmly bound
water, see Fieser, L. F.; Fieser, M. Reagents for Organic Synthesis; John Wiley and
Sons: New York, 1967.
11. Khan, Y.; Durrani, S. K.; Mehmood, M.; Khan, M. R. J. Mater. Res. 2011, 26, 2268–
2275.
12. For optimum activity, the water must be adsorbed to the oxidant prior to the
reaction. Control experiments revealed that addition of small quantities of
water to reactions using Attenburrow and commercial MnO2 did not improve
the reaction yields.
13. Oxidations under microwave conditions: To a Greenchem Plus glass microwave
reactor tube were added the arylmethylene substrate (1 or 3) and 25 wt % of
nano-MnO2. The tube was flushed with argon and sealed using a Teflon cap.
The sample was placed inside a CEM (Mars Model 230/60) microwave unit and
irradiated at 400 W and 105 °C for 150–240 s.8 The resulting mixture was
cooled, diluted with 30 mL of ether (ethyl acetate for 2j), filtered, and
concentrated. The crude product was purified on a silica gel column eluted
with increasing concentrations of ethyl acetate in hexanes to give the pure
carbonyl compound. Note: In the oxidations of 1a, 1b, 1e and 3a, 3b, the
compounds were pre-adsorbed onto neutral alumina prior to microwave
heating due to the volatility of the substrates.
a
The product spectra matched those in Ref. 14.
A: nano-MnO2; B: Attenburrow MnO2; C: commercial MnO2.
Yields refer to reactions performed under MW conditions without solvent at
b
c
Oxidations under conventional conditions: To
a stirred solution of the
105 °C for 150–240 s. Yields in parenthesis refer to reactions performed under
conventional conditions in DMF at 80 °C for 6–24 h.
The reactant was adsorbed onto neutral alumina prior to MW reaction.
arylmethylene substrate (1 or 3) in THF, 25 wt % of MnO2 was added and the
reaction mixture was heated under reflux for 6–24 h until TLC indicated
complete consumption of the starting material. The crude reaction mixture
was filtered and washed with ethyl acetate, then dried with MgSO4, filtered,
and concentrated under vacuum. Purification by column chromatography
afforded analytical samples of the aldehyde or ketone products. The products
were identified by comparison with known spectra.14 Note: The same reactions
were performed in DMF at 80 °C for 6–24 h. For these reactions, the MnO2 was
removed by filtration and the filtrate was added to ice-cold water. The
resulting aqueous mixture was extracted with ethyl acetate (3 Â 10 mL), and
the combined organic layers were washed with brine, dried (MgSO4), filtered,
and concentrated under vacuum to give the desired product.
d
Supplementary data
Supplementary data (general experimental details, procedures
for the preparation and regeneration of nano-MnO2, a deactivation
profile for recycled nano-MnO2 and 1H and 13C NMR spectral data
for 2j) associated with this article can be found, in the online ver-
14. Compounds were characterized by comparison with known spectra, see
Pouchert, C. J.; Behnke, J. The Aldrich Library of 13C and 1H FT NMR Spectra, 1st
ed.; Aldrich Chemical Co., 1993. Compound, Volume, Spectra Number: 2a, 2,
932B; 2b, 2, 802A; 2c, 2, 926C; 2d, 2, 884C; 2e, 808C; 2f, 810C; 2g, 2, 903B; 2h,
2, 903C; 2i, 2, 921A 2j, see Supplementary data; 3a, 2, 1240A; 3b, 2, 1306C; 3c,
see Ref. 15; 3d, 3,456B.
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