Dienes 1,3-, halogenation

Conjugated-Diene Butyl. CDB can be obtained by the controlled dehydrohalogenation of halogenated butyl mbbet (27). This product concept remains in the development stage. 

As with addition of other electrophiles, halogenation of conjugated dienes can give 1,2- or 1,4-addition products. When molecular bromine is used as the brominating agent in chlorinated hydrocarbon solvent, the 1,4-addition product dominates by 7 1 in the case of butadiene. ... 

In some cases, diene polymers (for instance polychloroprene rubbers) can add to the growing polymer chain by 1,2 addition (also called vinyl addition). This creates labile hydrogen or reactive halogen on tertiary carbon atoms. A few percent of this type of structure in the rubber will assist cross-linking reactions. 

A -dien-3-ol ethers gives rise to 6-substituted A" -3-ketones. 6-Hydroxy-A" -3-ketones can be obtained also by autooxidation.Structural changes in the steroid molecule may strongly affect the stability of 3-alkyl-A -ethers. Thus 11 j5-hydroxyl and 9a-fluorine substituents greatly increase the lability of the enol ether/ while halogens at C-6 stabilize this system to autooxidation and acid hydrolysis. 

Halogen-free A/-acyl aldimines and N-acyl ketiimnes tautomenze readily to give enamides [J6] In contrast, perfluonnatedyV-acylimines are stable compounds These electron-deficient itnmes not only exhibit high thermal stability but also show umque properties both as electrophiles and as strongly polanzed hetero-1,3-dienes... 

The most notable chemistry of the biscylopen-tadienyls results from the aromaticity of the cyclopentadienyl rings. This is now far too extensively documented to be described in full but an outline of some of its manifestations is in Fig. 25.14. Ferrocene resists catalytic hydrogenation and does not undergo the typical reactions of conjugated dienes, such as the Diels-Alder reaction. Nor are direct nitration and halogenation possible because of oxidation to the ferricinium ion. However, Friedel-Crafts acylation as well as alkylation and metallation reactions, are readily effected. Indeed, electrophilic substitution of ferrocene occurs with such facility compared to, say, benzene (3 x 10 faster) that some explanation is called for. It has been suggested that. 

For the ordinary Diels-Alder reaction the dienophile preferentially is of the electron-poor type electron-withdrawing substituents have a rate enhancing effect. Ethylene and simple alkenes are less reactive. Substituent Z in 2 can be e.g. CHO, COR, COOH, COOR, CN, Ar, NO2, halogen, C=C. Good dienophiles are for example maleic anhydride, acrolein, acrylonitrile, dehydrobenzene, tetracya-noethylene (TCNE), acetylene dicarboxylic esters. The diene preferentially is of the electron-rich type thus it should not bear an electron-withdrawing substituent. 

The HX compounds are electrophilic reagents, and many polyhalo and polycyano alkenes, (e.g., Cl2C=CHCl) do not react with them at all in the absence of free-radical conditions. When such reactions do occur, however, they take place by a nucleophilic addition mechanism, (i.e., initial attack is by X ). This type of mechanism also occurs with Michael-type substrates C=C—Z, where the orientation is always such that the halogen goes to the carbon that does not bear the Z, so the product is of the form X—C—CH—Z, even in the presence of free-radical initiators. Hydrogen iodide adds 1,4 to conjugated dienes in the gas phase by a pericyclic mechanism ... 

In the case of cyclopentadienes having halogen substituents at 5-positions, syn Jt-facial selectivity is expected since the dienes are classified into the case of... 

The most prevalent approach to achieve long-lasting and nonstaining ozone protection of rubber compounds is to use an inherently ozone-resistant, saturated backbone polymer in blends with a diene rubber. The ozone-resistant polymer must be used in sufficient concentration (minimum 25 phr) and must also be sufficiently dispersed to form domains that effectively block the continuous propagation of an ozone-initiated crack through the diene rubber phase within the compound. Elastomers such as ethylene-propylene-diene terpolymers, halogenated butyl mbbers, or brominated isobutylene-co-para-methylstyrene elastomers have been proposed in combination with NR and/or butadiene rubber. 

The localized-valence halogen-bridged platinum(II)/platinum(IV) complexes (175) are of interest because of their highly anisotropic electrical and optical properties. The complexes are characterized by broad and intense intervalence bands in their electronic spectra.542 Partial oxidation of [PtCl(dien)]Cl with iodine yields the mixed valence species [PtI(dien)][PtI3(dien)]I2,543 but when 4Me-dien is used, the mixed valence compound is only obtained if it is trapped quickly, before isomerization of the meridional platinum(IV) complex to a facial form.544 Although these species nominally have localized valences, the extent of delocalization increases as the bridging halide... 

See Dinitrogen oxide or Halogens, both above Oxygen, below Calcium hypochlorite Acetylene Nitric acid Acetylene, Mercury(II) salts Nitrogen oxide Dienes, Oxygen Ozone Acetylene... 

The reason for the different behavior of dienes like 41 and monoenes 37 or 42 is not yet established. It is hard to believe that simple steric factors should make up for the different orientation of the olefin that approaches a metal carbene intermediate. More likely is stereochemical control by an ylide-type interaction between the halogen atom of the (sterically more flexible) monoenes 37 or 42 and the electrophilic metal carbene. 

The substitution of the exo-methylene hydrogen atoms of MCP with halogens seems to favor the [2 + 2] cycloaddition reaction by stabilizing the intermediate diradical. Indeed, chloromethylenecyclopropane (96) reacts with acrylonitrile (519) to give a diastereomeric mixture of spirohexanes in good yield (Table 41, entry 2) [27], but was unreactive towards styrene and ds-stilbene. Anyway, it reacted with dienes (2,3-dimethylbutadiene, cyclopentadiene, cyc-lohexadiene, furan) exclusively in a [4 + 2] fashion (see Sect. 2.1.1) [27], while its... 

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