Criegee-type intermediate

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. 

Denmark has recently disclosed results that help address the question of whether dioxirane (A) or Criegee-type (B) intermediates are involved in ketone-catalyzed epoxidation reactions (Fig. 14) [98]. [ 0]-Labeling experiments using ketone 40 showed 80% of the expected isotope label was incorporated into the epoxide product, providing compelling evidence that dioxiranes are indeed the active oxidizing species [99]. 

Let us consider as an example the reaction of cyclohexene with ozone in the atmosphere. This reaction has been studied in laboratory chamber experiments by Kalberer et al. (2000). A potential reaction mechanism is depicted in Figure 14.11. The first steps of the reaction are the formation of an initial molozonide M, its transformation to a peroxy radical intermediate, and then to a dioxyrane-type intermediate, and finally to an excited Criegee biradical [CHO(CH2)4CHO O ]. A series of reactions then lead from the Criegee biradical to stable products, some with five and some with six carbon atoms (Figure 14.11). This rather complicated series of reactions leads to the stable gas-phase products listed in Table 14.10. 

The Criegee mechanism, widely accepted for the liquid-phase reaction, does not adequately explain the available gas-phase data. O Neal and Blumstein suggested a biradical structure for the first gas-phase intermediate and proposed three types of unimolecular hydrogen abstraction reactions (Figure 3-10). 

An alternative mechanism involves nucleophilic attack of the C-3 of indole on the distal oxygen atom of the bound O2 coupled with deprotonation. Two pathways to the Al-formylkynureiune product have been proposed, one involves a Criegee rearrangement-type mechaiusm and the other involves an endoperoxide intermediate Further studies are required to estabhsh the exact mechaiusm of product formation. 

Osmic esters of type (I) are intermediates in Hofmann oxidation of olefins and they have been isolated by Criegee.122 They are cleaved to glycols by... 

In many papers, this intermediate is called an azonide in analogy to the similar type of compounds discovered by Criegee (see Criegee and SchrOder, 1960, and Criegee s review, 1975) in the ozonolysis of alkenes. In that reaction a 1,2,3-trioxolane, i.e., an ozonide, is initially formed which, however, rearranges via a carbonyl oxide of type 9.71 and a carbonyl compound into a 1,2,4-trioxolane like 9.77 (see review of Kuczkowski, 1983). 

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