Alkenes halogen substituted

It has been shown that halogen-substituted alkenes can participate in the metathesis reaction, e.g. 5-bromo-l-pentene reacts with 2-pentene 11). A very interesting reaction is the conversion of methyl-9-octa-decenoate into 9-octadecene and dimethyl-9-octadecenedioate 12) ... 

With the great commercial interest in chlorofluorocarbons, the addition of HF to unsaturated organic chlorides, particularly vinylic chlorides has received considerable industrial attention.4 3 13-19 While these reactions are more difficult to effect than those of simple alkenes and increasing halogen substitution about the C—C double bond decreases the alkene reactivity, the reactions of simple monochloroalkenes proceed at temperatures from -23 to +120 C and can give good yields. Trichloroethylene or 1,1,2-tri-chloropropene, on the other hand, require temperatures in excess of 200 C, and tetrahaloethylenes fail to react in the absence of a catalyst.3 However, substitution of fluorine for chlorine tends to facilitate HF addition.3... 

Elimination reactions often compete with substitution. They involve elimination of the halogen and a hydrogen from adjacent carbons to form an alkene. Like substitution, they occur by two main mechanisms. The E2 mechanism is a one-step process. The nucleophile acts as a base to remove the adjacent proton. The preferred form of the transition state is planar, with the hydrogen and the leaving group in an anti conformation. The E1 mechanism has the same first step as the SN1 mechanism. The resulting carbocation then loses a proton from a carbon atom adjacent to the positive carbon to form the alkene. 

Many synthetic applications have been reported of the photochemical reactions of halogen-substituted 1,4-naphthoquinones with 1,1-diarylethenes487 90, and with related electron-rich alkenes such as 1-aryl-l-trimethylsilyloxyethenes491 93, 2-trimethylsilyloxy-1-alkenes494, 2-methoxy-l-alkenes495 and allyltributylstannane496,497. The process is exemplified by the reaction of 2-bromo-3-methoxy-1,4-naphthoquinone derivatives (152) with 1,1-diphenylethene487 (equation 129). 

This reaction is similar to the attack of an alkene on a halogen, resulting in addition of the halogen across the double bond. The pi bond of an enol is more reactive toward halogens, however, because the carbocation that results is stabilized by resonance with the enol —OH group. Loss of the enol proton converts the intermediate to the product, an a-haloketone. We can stop the acid-catalyzed reaction at the monohalo (or dihalo) product because the halogen-substituted enol intermediate is less stable than the unsubstituted enol. Therefore, under acid-catalyzed conditions, each successive halogenation becomes slower. 

The N-alkylation reaction represents a bifurcation of the normal alkene epoxidation reaction cycle and, therefore, N-alkylation is a suicide event that leads to catalytic inhibition in the native system. With synthetic tetraarylporphyrins that mimic the N-alkylation reaction, the use of halogen-substituted catalysts that are stable toward oxidative degradation (26, 27) provide the most useful model systems because the heme model remains intact for a significantly greater number of turnovers than the partition number. The partition number is the ratio of epoxidation cycles to N-alkylation cycles, i.e., N-alkyl porphyrins are formed before the heme is oxidatively destroyed. 

Examples of nitrofluorination reactions of alkenes, cycloalkencs, halogenated and other substituted alkenes are shown in Tables 28 and 29. It should be mentioned that the yield of the fluoro nitro product is higher if the starting alkene is substituted by electron-withdrawing substituents. 

Reductive coupling of polyhaloalkanes with carbonyl compounds in the presence of Zn is used in the synthesis of various halogen-substituted alkenes. For example, the reaction of benzaldehyde with the organozinc reagent 4.101 gives the trifluoromethyl-substituted alkene 4.102 (Scheme 4.50). 

The resting state (MMOoxd) is reduced back to the diferrous form to close the cycle. In addition to methane, many other substrates react with MMO. These include long-chain alkanes, alkenes, halogenated and other substituted alkanes, alkynes, sulfides, and others. 

Thus alkenes undergo substitution by halogen in exactly the same way as do alkanes. Furthermore, just as the alkyl groups affect the reactivity of the double bond toward addition, so the double bond affects the reactivity of the alkyl groups toward- substitution. 

Williams and coworkers have expanded the utility of this methodology via the application to highly functionalized substrates. transmetalation with R.R)-ot (S,S)-228 is quantitative and dependable, with a variety of allylic stannanes with C-2 substitution. Carbon chains at C-2 of the allyl component may contain additional functionality, including benzyl and allyl ethers, silyl ethers, esters, alkenes, halogens, or thioacetals, as well as stereogeificity. Asymmetric induction in the condensation with aldehydes is dominated by the chiral auxiliary. In this fashion, the allylation reaction may be designed as a convergent... 

Halogen-substituted carbenes attack olefins from their sterically less hindered face, however, exceptions are known for alkenes with oxygen substituents (vide infra). Two of numerous examples demonstrate the directive power of steric effects. Thus, spiro compound 1 and dichlorocarbene combine to provide the product 2 with the dichloromcthylcnc unit exclusively trans to the phenyl group14. 

Bicyclization. Carbonyl ylides generated via decomposition of diazoketones and internal trapping can be put to good use. Accessibility of oxabridged tricyclic by an intramolecular [3 +2] cycloaddition has profound significance to the elaboration of the core structure of platensimycin, and the possibility has been studied. Initial experimentation showed the preponderant formation of an isomeric skeleton but by halogen substitution (change of HOMO coefficient) on the dipolarophilic alkene the desired intermediate can be prepared as the major product. 

Ester- and amide-substituted radicals bearing an adjacent stereocenter abstract hydrogen with high diastereoselectivity1. The radicals are generated via intra- or intermolecular radical addition to alkenes, halogen abstraction from alkyl halides or reductive cleavage of alkylmercury compounds. Some examples are shown in Table 1. 

Whereas alkanes undergo substitution reactions, alkenes and alkynes undergo addition reactions. The principal addition reactions of the unsaturated hydrocarbons are halogenation, hydration, hydrohalogenation, and hydrogenation. Polymers can be made from alkenes or substituted alkenes. 

For the reaction of OH radicals with vinyl chloride and vinyl bromide the halogen elimination reactions are thermo-chemlcally favorable, the overall reactions being <11 kcal mole and <24 kcal mole exothermic for X = Cl and Br, respectively (203). The elimination of Br atoms from activated chloro-bromoalkyl radicals (206, 207), and of H, Cl, or Br atoms from activated fluoroalkyl radicals (208-213), have been studied using molecular beam techniques, these Intermediate radicals being produced by the reaction of Cl atoms with bromlnated alkenes (206, 207) or of F atoms with alkenes and halogen-substituted alkenes (208-213). For the elimination of Br atoms In the reactions... 

Alkenes also substitute the chlorine atoms the number of halogens substituted is very sensitive to the identity of the counterion and the ion of choice is SbCl " [59]. 

A three-component reaction of halogenotrinitromethanes 192 with alkenes 190 and 191 was carried out for the synthesis of halogen substituted isoxa-... 

The major difference between the addition of halogen to an alkene and substitution by halogen on an aromatic ring is the fate of the cation intermediate formed after the halogen is added to the compound. Recall from Section 5.3C that the addition of chlorine to an alkene is a two-step process, the first and slower step of which... 

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