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Ytterbium(III) triflate (10mol%) was found to catalyse the selective oxidation of a range of simple benzyl alcohols to benzaldehydes in good to excellent yield using a stoichiometric quantity of 69% nitric acid as the oxidant, and where the only by-products were water and oxides of nitrogen. The catalyst can be readily recovered and recycled by a simple evaporative process.The selective oxidation of benzylic alcohols to benzaldehydes is a transformation of considerable importance in organic synthesis. Whilst numerous reagents have been developed to effect this process in excellent yield, many of them employ greater than stoichiometric quantities of toxic heavy metals or co-oxidants which severely handicap their applicability to large scale industrial processes. 1,2 We have instigated a research program in the area of clean technology whereby we seek to develop atom-economic transformations in which little or no waste is generated, and have recently reported the use of lanthanide(III) salts as recyclable catalysts for the nitration of simple arenes, affording water as the only sideproduct. 3,4 During our attempts to apply this nitration methodology to benzyl alcohol, we noticed a rapid evolution of brown gas as the reaction mixture approached reflux temperature. Within a very short space of time the starting alcohol had undergone almost complete conversion to a less polar product, which upon isolation and analysis proved to benzaldehyde. Whilst oxidation of benzyl alcohols to benzaldehydes using nitric acid has been described previously, the procedure demanded the use of at least 3 equivalents of nitric acid and a resultant neutralization step at the end of the reaction. 5Optimization of the reaction conditions revealed that simply refluxing a solution of benzyl alcohol (1 equivalent), 69% w/w nitric acid (1 equivalent) and ytterbium triflate (10 mole%) in 1,2-dichloroethane effected the formation of benzaldehyde in 91% isolated yield within 0.5h. In the absence of ytterbium triflate, the reaction proceeded to only 17% conversion in 6h. Further studies showed that this oxidation protocol can be applied to a wide range of benzyl alcohols (Table 1).Although the reaction becomes slower as electron withdrawing substituents are added to the ring, the advantages of the catalyst are still clear, with 3,5-dinitrobenzyl alcohol giving a 70% conversion to the aldehyde over 24h, as opposed to the uncatalyzed case, which failed to give any conversion whatsoever. The reaction was unsuccessful for electron rich arenes, which presumably suffered complications due to competing nitration of the aromatic ring.A distinct advantage of the present method is the ease with which the ytterbium triflate can be recycled without detriment to the yields of aldehyde. Simple evaporation of the aqueous phase allows almost quantitative recovery of the catalyst. In the case of 4-bromobenzyl alcohol, 3 consecutive oxidations with the same batch of catalyst effected conversion to the aldehyde in similar timescales and yields (Table 2).In conclusion...

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