Criegee adduct formation

Katsuki and coworkers reported that Zr(salen)-based derivatives served as efficient catalysts for asymmetric BV reactions using UHP as terminal oxidant . Complex 117 in chlorobenzene at 0 °C showed the best catalytic activity with enantiomeric excesses higher than 80%, in most cases. Similarly to what was proposed for Ti(salen) derivatives (equation 51), the treatment of Zr(salen) complexes with H2O2 would give a Zr-peroxo (salen) complex 118, prone to ring-opening with formation of a Criegee adduct 119 and evolution to products (equation 81). 

Such catalysis is of interest, since both steps of the Baeyer-Villiger reaction cf. Scheme 9.3) can be influenced. Either the carbonyl compound or the oxidant is activated to lead to a more efficient formation of B (first step), or the rearrangement (second step) is enhanced by supporting the decomposition of the Criegee adduct [311[. 

Since then, there have been a number of studies of this reaction, which have been summarized by Hatakeyama and Akimoto (1994). The mechanism appears to involve formation of an adduct that can either decompose to S02 and an isomerization product of the Criegee intermediate or, alternatively, react with a second S02 molecule to generate other products. For the Criegee intermediate formed in the ethene-ozone reaction, for example, the proposed reaction sequence is the following ... 

The reaction of O3 with MPAN is expected to proceed by O3 addition to the C=C bond followed by decomposition of the 1,2,3-trioxolane adduct into two carbonyls and two Criegee intermediates as for the reaction with unsaturated organics. Grosjean et al. (1993d) have reported CH2O as the major reaction product with a formation yield of 60 10%. 

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