Criegee adduct

The results of infrared investigations and 0-labeling experiments support the mechanism depicted in Scheme 165, which includes carbonyl group activation by the catalyst and oxygen transfer via Criegee adduct. Two years later Pillai and Sahle-Demessie could... 

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). 

Figure 4 Chemistry of the oxygenation of cyclohexanone by cyclohexanone monooxygenase—a Baeyer-Villager rearrangement. In the first stage, NADPH binds and reduces the FAD to form E Flpgj-NADP. The NADP remains bound to the enzyme during the ensuing reactions. Oxygen reacts to form a C4a-peroxy-FAD (E FI HOO ). In the next phases, cyclohexanone binds and reacts with the peroxyflavin to form a Criegee adduct, which breaks down to form the lactone product and the C4a-hydroxy-FAD (E FI HOH-P). When the NADP dissociates, the hydroxy-flavin loses H2O to reform the oxidized FAD...

The proposed mechanism of flavin-type Baeyer-Villiger MOs is shown. First reduction of the flavin 95 and attack on O2 provides 96. Peroxo-flavin 96 attacks ketone 93 and forms the Criegee adduct 97. Rearrangement provides lactone 94 along with hydroxylated flavin 98. Dehydration returns to the beginning of the catalytic cycle. 

The reaction mechanism over the tin Lewis acid sites is therefore quite different from that over titanium Lewis acid sites. On the basis of Sn MAS NMR, the catalytic tin sites are thought to be within the framework. In the dehydrated solid the sites are tetrahedrally coordinated, and are able to bind with adsorbed molecules to become five- and six-fold coordinated. The Sn site activates the ketone rather than the hydrogen peroxide. The carbonyl oxygen from the ketone is thought to form a Lewis adduct with the framework tin, which is able to expand its coordination sphere as a so-called Criegee adduct (Scheme 9.10). ... 

Scheme 9.3 Criegee adduct B as an intermediate of the Baeyer-Villiger reaction...

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 ... 

Criegee biradical (H2COO ) CRIG Isoprene OH-adduct IS03... 

This mechanism proceed via a peroxidic Zwitterion what is now largely accepted by all the scientific community. Product 1, an ozone-olefin adduct is a very unstable compound giving rapidly product 3, probably through intermediate 2. The Criegee intermediate 3 can lead to different structures like ... 

A mechanism for extradiol cleavage is proposed in Figure 18 [5,158], Substrate binds first to the iron(II) center, followed by 02, to form a ternary complex akin to the ES—NO complex described earlier. Electron transfer from metal to 02 in the Fe(II)—02 adduct results in a superoxide-like moiety and imparts nucleophilic character to the bound 02. This in turn generates semiquinone character on the bound substrate, which is attacked by the nascent superoxide to form a peroxy intermediate that decomposes by a Criegee-type rearrangement to the observed product. 

Thus, Criegee 11) prepared the oxythallation adduct of styrene in methanol which he decomposed at 130°C. to give products which result from solvolysis of the carbon-thallium bond with (HI) and without (IV)... 

Criegee established that dienes form 1,2-, not 1,4-adducts. 

An example of this cw-hydroxylation had been reported earlier by Criegee and Schroder the conversion of the nickel chloride adduct of tetramethyl-cyclobutadiene (4) into cw-dihydroxytetramethylcyclobutene (5). 

At the same time the Criegee mechanism as presented above does not account for some experimental results. For example cis- and trans- olefins yield ozonides with different cis-ltrans- ratios. This has led to the suggestion by several workers that the Criegee mechanisms provide one group of reaction pathways but that these are not exclusive and that other mechanisms may be operative and involve, for example, the cr adduct (VIII). In this context the introduction of foreign 0-labelled aldehydes into the reaction environment has provided interesting, if sometimes conflicting, data (see Murray, 1972). 

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%. 

---