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Following our observation that Os(VI) oxides bond strongly to P4VP and that the supported oxides
are excellent dihydroxylation catalysts, we concluded that the chemistry of ruthenate on P4VP might be
worth investigating.9 Unsupported ruthenate is a well-known, but not very selective, oxidant of alcohols.
To our knowledge there has been no reported literature of any poly(4-vinylpyridine)-supported ruthenium
complexes for the oxidation of alcohols. However, [RuCl2(CO)3]3 supported on poly(4-vinylpyridine) for
alkene hydrogenation has been reported.10 A ruthenium complex in the form of potassium perruthenate
supported on Amberlyst anion exchange resin (IR27) was also prepared.11 We now report on sodium
ruthenate supported on poly(4-vinylpyridine) cross-linked with 2% divinylbenzene in a series of catalytic
alcohol to carbonyl oxidation reactions.
A 2% metal loading (by weight) was achieved by stirring 45 mg of sodium ruthenate in 3.8 ml of a
1 M NaOH solution with 1 g of poly(4-vinylpyridine) for 2 h. The loaded polymer was then dried under
vacuum and stored under nitrogen. Flame emission spectroscopy was used to determine the ruthenium
loading on the polymer as well as the extent of leaching of the sodium ruthenate from the polymeric
support. Negligible leaching of the sodium ruthenate from the support into the solvent was found to
occur, thereby indicating coordination or an interaction between the ruthenate ion and the nitrogen atom
of the pyridine ring of the polymer as observed for P4VP-supported osmium tetroxide.9
The general procedure for the catalytic oxidation of alcohols is as follows: Activated 4 Å molecular
sieves (180 mg), 6 ml of nitrogen saturated solvent (dichloromethane or toluene) and the alcohol
(4.78×10−4 mol) together with 80 µl (5×10−4 mol) of isobutyl–metharylate as the internal standard were
added to a nitrogen filled Schlenk tube. Then 1.5 equivalents (relative to the substrate) of the appropriate
co-oxidant, together with 200 mg of the supported oxidant were added and the resulting mixture was
stirred at room temperature under a nitrogen atmosphere.
Using the 2% loaded polymer supported oxidising agent, benzylic and allylic alcohols were selectively
oxidised to their corresponding oxidation products without any over-oxidation products being formed or
attack at double bonds or heteroatoms occurring. Thus, for example, the nitro group of 4-nitrobenzyl
alcohol was not subject to oxidative attack by this oxidant. The use of tetrabutylammonium periodate
was found to be superior to iodosylbenzene and this effect was more pronounced during the oxidation
of 4-nitrobenzyl alcohol. Geraniol was oxidised to geranial only, whereas isomerisation to citronellal
can occur using perruthenate catalysts.13 Double bond isomerisation during the oxidation of crotyl
alcohol was also not observed. The quantitative oxidation of cyclobutanol to cyclobutanone suggests
that Ru-P4VP acts as a two electron oxidant.12 The results are shown in Table 1. Preliminary results
indicate that the catalyst is also active with a wide range of other co-oxidants, e.g. trimethylamine-N-
oxide, methylmorpholine-N-oxide, H2O2, tertiary butyl-hydroperoxide and NaOCl. The absence of any
carboxylic acid formation or double bond attack also indicates that supported ruthenate is a significantly
milder oxidant than unsupported ruthenate, since unsupported ruthenate oxidises crotyl alcohol, cinnamyl
alcohol and 4-nitrobenzyl alcohol to carboxylic acids.14
The recycling of the polymer-supported sodium ruthenate was carried out by filtering the supported
oxidant from the reaction mixture. It was then used in another oxidation reaction. A slight decrease in the
yields for each subsequent recycle was observed and this was in accordance to the results obtained for
the polymer-supported perruthenate. The method of recycling the polymer-supported sodium ruthenate
differed to the reported techniques for the P4VP, P2VP-CP and P4VP-DC oxidants as only the polymeric
backbone was recycled for these oxidants and not the supported chromium peroxide and dichromate
species.5,6
In conclusion, the poly(4-vinylpyridine)-supported sodium ruthenate was found to be an easily
synthesised, recyclable polymeric oxidant for the efficient and selective oxidation of benzylic and allylic