Alkenes double bond fission

Fission of carbon-carbon double bonds with the combined use of an osmium catalyst and periodate as a comsumable reagent is an alternative to ozonolysis. In diis process a catalytic amount of osmium is oxidised to osmium(viii) by preiodate and converts the alkene to a glycol which is then cleaved by the periodate. In the electrochemical modification of this process, which uses a divided cell and aque-... 

The mechanism of the reaction of ozone with double bonds (equation 68) is very complex and still subject to arguments. An alkene and ozone may first form a it complex (a), which forms a a complex (b), a molo-zonide (c), or both. The molozonide may change to a dipolar ion (d), which breaks down with the fission of the carbon-carbon bond to a carbonyl compound and another dipolar ion (e). The two species recombine to give the ultimate product, ozonide (f) (1,3,4-trioxolane, also known as 1,3,4-trioxacyclopentane) [76], The temporary presence of the carbonyl com-... 

The oxidative cleavage of unsaturated ketones takes place under the same conditions as that of alkenes or other unsaturated derivatives. The fate of the primary fission product depends on the position of the double bond with respect to the carbonyl group and on the subsequent reactions. Ozonization of A -cholestenone in acetic acid and ethyl acetate, followed by treatment with 30% hydrogen peroxide, gives a keto acid, evidently resulting from the decomposition of the primarily formed diketo acid (equation 444) [1176]. 

This analysis of the simple addition of an electrophilic bromine molecule to a symmetrical alkene or alkyne has highlighted many points. First, there is the induction of a temporary dipole of the soft electrophile by the n electrons of the carbon/carbon double bond. Second, there is the heterolytic fission of the bromine molecule, and the subsequent formation of the cyclic bromonium ion. Third, this cyclic intermediate places certain restrictions on the potential line of attack for the second reagent, and so controls the structural and stereochemical consequences for the product. 

Some examples of cross-metathesis between isotopically labelled alkenes have been given in Chs. 3 and 5. They demonstrate not only the fission of the double bond but also the occurrence of non-productive metathesis. Here we summarize the information on all types of cross-metathesis reaction involving only acyclic compounds. Cross-metathesis between cyclic and acyclic compounds is discussed in Ch, 15. 

The n x interaction can be switched off if two strongly acceptor antiperiplanar a orbitals are available for the lone pair of nitrogen (Figure 6.110b). It was also suggested that the alkene x-system orients itself perpendicular to the lone pair to decrease steric repulsion. " This enamine is so uiueactive that it cannot be isomerized into the trans double bond isomer without fission of the heterocyclic ring. ""... 

This chapter describes the photochemical C-X bond fission in alkene systems that have halogen atoms bonded to the carbon-carbon double bond. The substitution of halogen atoms in alkenes results in a red shift of the n,n absorption band because of the interaction between the lone pair on the halogen atom and the 7t-orbitals. > However, the UV absorption of aliphatic alkenyl halides, except for the iodides, lies in a region of short wavelength less than 254 nm (Table 11.1). Therefore, the majority of the alkenyl halides studied in these photolyses are the iodides. 

To avoid synthetic problems, we prepared aldehyde 47, a substrate in which isomerization of the C-C double bond to the endocyclic position is less likely due to the extended conjugation of the alkene moiety. Compound 47 was obtained as a pure substance and also presents a substitution pattern that, according to the above postulates, should favor the ODPM rearrangement. However, irradiation of 47, using m-methoxyacetophenone as sensitizer, led to formation of the diene 48 (38%), as a result of photodecar-bonylation (Scheme 9). No ODPM product was formed in this process.Irradiation of the corresponding methyl ketone 49, under the same conditions as used for 47, afforded the product of 1,3-acyl migration 50 in 24% yield (Scheme 9). Again, no ODPM product was formed in this instance. The formation of 48 and 50 is reminiscent of Norrish Type 1 processes. However, in these cases, homolytic bond fission does not occur in the carbonyl nit excited state, as is the case in normal Norrish Type 1 reactions. ... 

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