TEMPO-modified graphite felt electrodes: attempted enantioselective oxidation of rac-1-phenylethanol in the presence of (–)-sparteine

Abstract

At a TEMPO-modified graphite felt electrode (TMGFE) rac-1-phenylethanol 2 is not enantioselectively oxidised in the presence of (–)-sparteine 1, but instead 1 is dehydrogenated to its iminium salt; 2 is oxidised in solution by the oxoammonium salt 6 in the absence of 1, but not on the TMGFE in the range 0–1.0 V vs. Ag/AgNO₃.

References

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8. The preparation of TEMPO-modified graphite felt electrodes (TMGFE) was carried out following a previously described procedure (ref. 2–7). The graphite felt electrodes (Le Carbone Lorraine, VG 4000) were refluxed in MeOH for 4 h, dried and dipped in a 0.25% polyacrylic acid (PAA, Aldrich)–methanol (Merck) solution and then carefully dried. Subsequently, the coated electrodes were treated with 4 equiv. of 4-amino-2,2,6,6-tetramethylpiperidin-1-oxyl [with respect to the amount of carboxylic acid (evaluated by weight) at the electrode] in the presence of 4.8 equiv. of DCC (ACROS) in DMF (ACROS) at room temperature for 80–90 h. Thereafter the electrodes were crosslinked with 3.3 equiv. of 1,6-diaminohexane (Fluka) in the presence of DCC for 80 h and were subsequently alkylated (CH₂N₂ or Me₂SO₄ or Et₂SO₄). Following this procedure, no difference was found between the electrodes alkylated in ethereal Et₂O or Me₂SO₄ or Et₂SO₄. If alkylation is carried out before the amide linkage formation, the concentration, C_NO·, of the active aminoxyl radical decreases but the electrochemical behaviour is similar to that observed with the other electrodes. Increasing PAA thickness did not increase significantly C_NO· and did not improve the activity of the electrode even if the reaction time of each preparative step was increased proportionally. Amide linkage formation was also carried out before the coating procedure but the resulting film was very unstable. Electrochemical measurements and electrolyses were performed in a 0.2 M NaClO₄(ACROS)–MeCN (Merck) solution (dried on molecular sieves 3Å) using a HEKA PG28 generator-potentiostat. Chromato-graphic analyses were carried out using a Shimadzu chromatograph equipped with a capillary column [HP5 (Hewlett Packard) or FS Hydrodex β-PM (Macherey Nagel)]. rac-1-Phenylethanol, acetophe-none, tetraline and (—)-sparteine were purchased from Aldrich. The work-up of the electrolyses solution consisted of evaporating the MeCN, then the residue was dissolved in EtOAc, filtered and subjected to chromatography.
9. Conversely to the results reported by Osa et al. (ref. 2), no large difference was observed between the peak potentials when the PAA on the electrode was crosslinked and methylated or untreated.
10. C_NO^·=Q_P/nFV, where n is the number of electrons, F is the Faraday equivalent and V the volume of the graphite felt electrode.
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15. Structure assignment was performed by comparison of the mass spectra obtained by GC–MS with spectra from the NIST library.
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