Silyloxyoxirane, Rubottom oxidation

The mechanism initially proposed for the Rubottom oxidation involved epoxidation of the enolsilane to afford intermediate silyloxyoxirane 4. It was suggested that this intermediate undergoes acid-mediated cleavage to afford stabilized carbocation 5, which is transformed to the a-silyloxy ketone 6 via 1,4-silicon migration. Hydrolysis of 6 by aqueous acid in a subsequent step generates the a-hydroxy ketone 7.lb 15 Attempts to provide support for this mechanism via isolation of intermediate silyloxyoxiranes derived from simple ketones proved difficult due to the lability of these compounds. However, Brook demonstrated that the related heterocyclic silyloxyoxirane 8 was isolable and was transformed to ketone 9 upon treatment with /j-TsOH. 

Further support for the mechanism described above was obtained in subsequent studies by several groups. Direct evidence for the initial epoxidation event in the Rubottom oxidation of an acyclic enolsilane was first obtained by Weinreb, who described the isolation of silyloxyoxirane 10 and demonstrated its conversion to a-silyloxy ketone 11 upon treatment with PPTS.4 The isolation of a macrocyclic bis(silyloxyoxirane) has also been reported.5... 

Indirect evidence for silyloxyoxirane intermediates was also provided by Hassner, who described the isolation of 14lc in the Rubottom oxidation of 12. This product could potentially derive from the silyloxyoxirane 13 via an acid-mediated SN1 or Sn2 mechanism.10 The possible formation of silyloxyoxirane intermediates analogous to 4 in Rubottom oxidations of enolsilanes derived from ketones has also been discussed.6... 

Many variations of the Rubottom oxidation employ oxidants other than m-CPBA in order to execute the transformation under mild conditions or to allow for enantioselective synthesis. Use of dimethyl dioxirane (DMDO) for the oxidation of enolsilanes has become a popular alternative to traditional conditions for Rubottom oxidations. This mild oxidant has been used to facilitate the isolation of 2-silyloxyoxiranes, which are stable under the essentially neutral reaction conditions." For example, treatment of 26 with DMDO at -40 °C afforded 27 in 99% yield.1 Ib These compounds can subsequently be converted to 2-hydroxyketones, as described above, or can be used in other transformations.12 Chiral dioxiranes generated in situ from chiral ketones and oxone have also been employed in enantioselective Rubottom oxidations developed independently by Shil3a and Adam.13b As shown above, enolsilane 28 was transformed to a-hydroxyketone 29 in 80% yield and 90% ee.l3a... 

A double hydroxylation of enolsilanes under modified Rubottom oxidation conditions has been developed by Nakamura and Kuwajima.20 As shown below, treatment of enolsilane 40 with w-CPBA in the presence of excess KHCO3 generates doubly oxidized product 41 in 72% yield. The mechanism of these transformations is believed to involve elimination/epoxide opening of the intermediate silyloxyoxirane 42 followed by a second oxidation of the resulting enolsilane 43. 

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See also in sourсe #XX -- [ , ]

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