Oxidation of Intermediate Alkyltin Alkoxides

In 1974, David18 reported that cyclic stannylenes (97), formed by reaction of 1,2-diols (96) with dibutyltin oxide— -Bu2SnO—in refluxing benzene with azeotropic elimination of water, reacted with Br2 in solution at room temperature at titrating speed, leading to a-hydroxyketones (98). 

In 1976, Ueno and Okawara highlighted the fact that no oxidation of primary saturated alcohols to aldehydes via tin alkoxides had been reported in the literature and published a procedure for the selective oxidation of secondary alcohols.25 Interestingly, rather than performing the oxidation on pre-formed tin alkoxides, these researchers subjected a mixture of the diol and (Bu3Sn)20 in CH2C12 to the action of Br2. Regardless of the fact that no complete formation of tin alkoxides is secured and no HBr quencher is added, this method may provide useful yields of hydroxyketones during the selective oxidation of diols.26 

The selective oxidation of the secondary alcohol is performed by dropping a bromine solution on a mixture of (Bu3Sn)20 and the diol in CH2CI2. Although, no complete formation of bis-tin alkoxide is secured and the generated HBr—that may cause the hydrolysis of tin alkoxides—is not quenched, a useful yield of hydroxyketone is obtained. 

Subsequent researchers introduced substantial improvements on the Ueno and Okawara s protocol of selective oxidations via tin alkoxides and broadened considerably the scope of its application.223 24b,c Thus, it was established that good yields in the selective oxidation of diols—and even triols and tetrols can be achieved in two steps i) pre-formation of a tin alkoxide, by reaction with either (Bu3Sn)20 or Bu2SnO with elimination of water by molecular sieves or azeotropic distillation of water ii) treatment of the tin alkoxide with Br2 or NBS in the presence of a HBr quencher. 

In this tetrol, a single secondary alcohol is oxidized with 88% yield thanks to the formation of the most stable cyclic stannylene intermediate by the regioselective reaction of BU2S11O with one of the 1,2-diol moieties in the molecule. 

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