Alkene-oxygen complex

Lewis-acid-catalyzed cycloadditions of dienophiles, such as a,/l-unsaturated carbonyl compounds, with open-chain carbon-dienes, are generally highly ortho-para regioselective because the oxygen complexation increases the difference of LUMO coefficients of the alkene moiety. 

Type 1 intrazeolite photooxygenation of alkenes has been also reported to give mainly allylic hydroperoxides (Scheme 42). In this process, the charge transfer band of the alkene—O2 complex within Na-Y was irradiated to form the alkene radical cation and superoxide ion. The radical ion pair in turn gives the allylic hydroperoxides via an allylic radical intermediate. On the other hand, for the Type II pathway, singlet molecular oxygen ( O2) is produced by energy transfer from the triplet excited state of a photosensitizer to 02. 

Rhodium complexes were generally found to be more effective than iridium, but on the whole they show moderate activity in alkene oxygenation reactions. Significantly, epoxides, a typical product of the oxidation of olefins catalyzed by the middle transition metals, have rarely been evoked as products [18-22]. Although allylic alcohols [23] or ethers [24] have sometimes been described as products, the above cited rhodium and iridiiun complexes are characterized by an excellent selectivity in the oxygenation of terminal alkenes to methyl ketones. 

Carboxidation of linear alkenes is presented in Table 7.8 [173]. The location of the C=C bond and configuration of intermediate oxadiazoline complex are two important parameters with these substrates. For terminal alkenes, the complex may have configuration I and II (Figure 7.4). Complex I has the oxygen bound to the first carbon atom, and its decomposition leads to an aldehyde. Complex II has the oxygen bound to the second carbon atom, and its decomposition leads to a ketone. However, decomposition of the latter complex may occur also in a different way, involving... 

Kubokawa et al. observed that the order of reactivity for various alkenes as regards the initial O2 uptake was pent-l-ene > trans-bni-l-QnQ > propene > buta-1,3-diene > ethene, i.e. it increased with carbon number for the mono-alkenes. All these initial oxygenated complexes showed similar activity for their further oxidation to CO2 and H2O. Although agreeing with Kokes et al. over the initial formation of a peroxy species from propene, Kubokawa et al. concluded that it decomposed to formate and acetate surface species, both from i.r. studies and from an overall O2 propene ratio of —1. Both groups of workers... 

An interesting, but hitherto widely neglected feature of CO2 in this context is its ability to chemically interact with metal-oxygen complexes, leading to transition metal peroxocarbonates [4,82]. These complexes are able to transfer one oxygen atom to oxophiles such as phosphines, alkenes and active methylene... 

Figure 37 Epoxidation of alkene initiated by light irradiation to oxygen complex of Nb porphyrins [367].

As amply indicated in Part V, the Wacker oxidation involves (i) alkene 77-complexation with Pd, (ii) nucleophilic attack by oxygen nucleophiles, such as H2O, and (iii) jS-dehydropaUada-tion (Schemes 2 and 3 in Sect. V.l). In addition to these three critical steps, oxidation of Pd(0) to Pd(II) is necessary for recycling Pd complexes as catalysts, hi this reaction, C—H bond participation occurs in the /3-eUmiiiation or 1,2-elimination of the oxypaUadated intermediate. 

Thus the authors were able to suggest a mechanism for electron transfer within the alkene monooxygenase complex, in which the FAD moiety is reduced to FADH by bound NADH. The electrons are then transported one at a time to the [2Fe-2S] cluster, which in turn reduces the di-iron centre of the epoxygenase. The reduction of the di-iron centre is required for the binding of molecular oxygen and subsequent substrate oxidation. 

Quantitative M.O. models were used in a discussion of the photoelectron spectrum of thiiran 1,1-dioxide. As compared with dimethyl sulphone, all antibonding S—C cr-orbitals increase in energy and bonding S—C a-orbitals decrease in energy because of the relative compression of the S—C bonds in the cyclic sulphone. The chelotropic reactions of thiiran 1,1-dioxide have been compared with the formation of alkene-oxygen Tr-complexes. Activation parameters for the extrusion of sulphur dioxide from thiiran 1,1-dioxide have been obtained. ... 

The 4-hydroxy-1-alkene (homoallylic alcohol) 81 is oxidized to the hetni-acetal 82 of the aldehyde by the participation of the OH group when there is a substituent at C3. In the absence of the substituent, a ketone is obtained. The hemiacetal is converted into butyrolactone 83[117], When Pd nitro complex is used as a catalyst in /-BuOH under oxygen, acetals are obtained from homoallylic alcohols even in the absence of a substituent at C-3[l 18], /-Allylamine is oxidized to the acetal 84 of the aldehyde selectively by participation of the amino group[l 19],... 

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