Criegee intermediate mechanisms

The authors propose a working model relying on the commonly accepted mechanism for BV reactions (Fig. 13, a). Thus the sense of asymmetric induction is determined by the conformation of the Criegee intermediate, which is dictated by the chiral environment created by the catalyst. However, an alternative noncova-lent, bifunctional mechanism may be considered (Fig. 13, b) [80], This work... 

This primary ozonide is not stable. One of the two peroxy 0-0 bonds (marked a or b in Eq. 43) and the C-C bond (marked c) cleave simultaneously to give an aldehyde or ketone and an intermediate called the Criegee intermediate, named after the German chemist who originally proposed this mechanism (e.g., see Criegee, 1975) ... 

The mechanism of decomposition of the Criegee intermediates is believed to occur via several reaction channels shown for the [(R,CH2)(R2)CHOO] Criegee intermediate in Fig. 6.4. The oxygen-atom elimination channel for simple alkenes is not believed to be important. However, the ester and hydroperoxide channels are important and explain the production of free radicals such as OH. Theoretical calculations have shed some light on this (e.g., Gutbrod et al., 1996, 1997a ... 

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 ... 

Hatakeyama et al. [126] also determined the product distribution in 03—C2H4—S02 mixtures at air pressures ranging from 10 to 1140 Torr. On the basis of the observed effects of added S02 on the yields of HCHO and HC(0)0H and sulfuric acid, the following mechanism for the reaction of the Criegee intermediate with S02 was proposed. 

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 ... 

It is known, that the B-V reaction follows two-step mechanism with formation of the Criegee intermediate and reaction conditions determine which step is the rate determining.51... 

A variation on this model has recently been proposed by Kelly et al. who suggested that the hydroxy group of the Criegee intermediate could not be immobilized in such a mechanism, and that unreasonable steric constraints would be imposed for many of the substrates transformed reported for these enzymes. A new tautomer of the the flavin hydroperoxide was proposed as part of an alternative scheme (lower cycle, Fig. 16.5-16) in which an intermediate trioxane decomposes to yield the lactone and flavin hydrate. 

The mechanism for these metal-catalyzed, 02 promoted oxidations was proposed to be metal-catalyzed 02 oxidation of the aldehyde 55 to form a peroxy radical species 56 which adds to ketone 60 to provide a radical variant of a Criegee intermediate, 57. This intermediate would then extract a proton from the aldehyde 55 to provide the normal Criegee intermediate 58. Alternatively, the generated peroxy radical intermediate 56 abstracts a proton from an aldehyde molecule providing peracid 59 which attacks the ketone 60 to provide the Criegee intermediate 58. 

The Baeyer-Wlliger oxidation of ketones affords esters (from open-chain ketones) and lactones (from cyclic ketones), respectively. It is typically carried out using nucleophilic oxidants, such as peracids or dihydrogen peroxide in the presence of bases (29). Nucleophilic oxidants are required because the first step of the reaction mechanism is the addition of the peroxidic oxidant to the ketone s carbonyl function, affording the so-called Criegee-intermediate (Scheme 10). The subsequent cleavage of the 0 0 bond in the latter, and the concerted shift of yield the ester (lactone) product. In enzymatic Baeyer-Vlliger oxidations, flavin hydroperoxides such as 26 act as the nucleophilic oxidant (30). 

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