Halo-olefins

A considerable number of examples will be found in the text in which halo-genated and other substituted olefins are produced. Their modes of fonnation do not usually differ in principle from the corresponding unsubstituted case. However, some special methods have been used, for example, the direct preparation of halo olefins from ketones with phosphorous hahdes, or via hydrazones. 

Nitrite ion, an ainbident nucleophile in aprotic solvents, favors nitrogen atom attack on the double bond of various fluoro(halo)olefins. 2-Monohydroperfluoro-nitroalkanes can thus be produced [5] (equation 5)... 

For olefins with Ji-substitucnts, whether electron-withdrawing or electron-donating, both the HOMO and LUMO have the higher coefficient 021 the carbon atom remote from the substituent. A predominance of tail addition is expected as a consequence. However, for non-conjugated substituents, or those with lone pairs (e.g. the halo-olefins), the HOMO and LUMO are polarized in opposite directions. This may result in head addition being preferred in the case of a nucleophilic radical interacting with such an olefin. Thus, the data for attack of alkyl and fluoroalkyl radicals on the fluoro-olefins (Table 1.2) have been rationalized in terms of FMO theory.16 Where the radical and olefin both have near neutral philicity, the situation is less clear.21... 

L3 Elimination of Hydrogen Halide from Halo-olefinic Compounds... 

For those substrates more susceptible to nucleophilic attack, e.g., poly halo olefins and olefins of the type C= —Z, it is better to carry out the reaction in basic solution, where the attacking species is RO, I6K The reactions with C=C—Z are of the Michael type, and OR goes to the side away from the Z. Since triple bonds are more susceptible to nucleophilic... 

It is noteworthy that the use of these strong nucleophiles results in the addition-elimination route for the unactivated halo-olefins. 

The monosubstitution of vinyl bromide (Truce et al., 1956b) probably occurs via elimination-addition owing to the low reactivity of the halo-olefin. Acetylene is the only product formed with alkoxide ions or aniline. 

Routes (IV) and (V) of Scheme 1 therefore operate together for the cyclic halo-olefins. 

The methods described thus far represent only a small subset of the available techniques for the formation of C-furanosides and C-pyranosides. Other useful methods include cyclizations of halo olefins and ene-ynes [224] and the direct modification of pyran ring systems [251]. Although not discussed further here, these examples are mentioned to illustrate the breadth of work accomplished in this area. 

Backbonding in coordinated halo-olefins activates the =C—X bond due to contribution of X-orbitals to the antibonding MO s 202,285)... 

Galimberti and co-workers synthesized ethylene/ propylene/4-iodo-l-butene terpolymers with a V(acac)3/AlEt2X catalyst (X = Cl, I). The copolymers prepared were amorphous, having at least 35 wt % propylene content, and possessed a bimodal molecular weight distribution of high polymer (MW > 500 000) and oligomers. The preparation of crystalline copolymers of propylene with linear ft>-halo-olefins using TiCla/EtsAl has also been reported by Hoechst. ... 

A more direct approach to acetylenes has been reported which utilizes phase-transfer generated tetrabutylammonium hydroxide to dehydrohalogenate a-halo-olefins or bis-dehydrohalogenate vicinal dihalides. An example of the latter is the reaction of 1,2-dibromo-l-phenylethane in pentane with 50% aqueous sodium hydroxide in the presence of tetrabutylammonium bisulfate. The product, phenylacetylene, is produced in 87% yield in less than an hour by this method (see Eq. 9.11) [27]. The advantage of this approach is apparent when one considers that in the traditional method, jS-bromostyrene (from the dehydrohalogenation and decarboxylation of cinnamic acid dibromide) is heated with KOH at over 200° and the phenylacetylene distills as formed in 67% yield [28]. 

Hydrogen halides can be eliminated either from 1,1-, or 1,2-dihalogenated hydrocarbons or from 1-halo olefins to yield acetylenes in good yields. 

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