Alkenes Other elimination reactions

Except for terpene chemistry, the Wagner-Meerwein rearrangement is of limited synthetic importance. It is rather found as an undesired side-reaction with other reactions, for example in the synthesis of alkenes by elimination reactions. 

The intramolecular elimination just illustrated is under kinetic control, rather than thermodynamic control, and the transition state leading to removal of a P-proton from the less substituted P-carbon is lower in energy, so reaction occurs faster. This is the fiindamental difference between E2 reactions and intramolecular elimination reactions. The transformation of amine 42 to alkene 43 is called Hofmann elimination, named after August Hofmann (Germany 1818-1892). Other elimination reactions proceed by an intramolecular elimination mechanism, but Hofmann ehmination is the only one that will be discussed. 

The Hofmann elimination has also been used in the synthesis of alkenes not readily available by other elimination reactions. A recent example is the synthesis of trimethylenecyclopropane (equation 6)... 

Other Elimination Reactions.—The Norrish Type II dealkoxylation reaction of a-alkoxy-ketones is well known.A closely related process has been developed to convert alcohols into alkenes. Thus irradiation of the O-cholesteryl thio-benzoate (191) gave cholesta-3,5-diene (192) in high yield (Scheme 53). 

Three oxidative reactions of benzene with Pd(OAc)2 via reactive rr-aryl-Pd complexes are known. The insertion of alkenes and elimination afford arylalk-enes. The oxidative functionalization of alkenes with aromatics is treated in Section 2.8. Two other reactions, oxidative homocoupling[324,325] and the acetoxylation[326], are treated in this section. The palladation of aromatic compounds is possible only with Pd(OAc)2. No reaction takes place with PdCl2. 

Direct elimination of a carboxylic acid to an alkene has been accomplished by heating in the presence of palladium catalysts.Carboxylic esters in which the alkyl group has a P hydrogen can be pyrolyzed, most often in the gas phase, to give the corresponding acid and an alkene. No solvent is required. Since rearrangement and other side reactions are few, the reaction is synthetically very useful and is often carried out as an indirect method of accomplishing 17-1. The yields are excellent and the work up is easy. Many alkenes have been prepared in this manner. 

Treatment of 51 with an excess of sodium benzoate in DMF resulted in substitution and elimination, to yield the cyclohexene derivative (228, 36%). The yield was low, but 228 was later shown to be a useful compound for synthesis of carba-oligosaccharides. <9-Deacylation of228 and successive benzylidenation and acetylation gave the alkene 229, which was oxidized with a peroxy acid to give a single epoxide (230) in 60% yield. Treatment of 230 with sodium azide and ammonium chloride in aqueous 2-methoxyeth-anol gave the azide (231,55%) as the major product this was converted into a hydroxyvalidamine derivative in the usual manner. On the other hand, an elimination reaction of the methanesulfonate of 231 with DBU in toluene gave the protected precursor (232, 87%) of 203. 

Both phase transfer and crown ether catalysis have been used to promote a-elimination reactions of chloroform and other haloalkanes.153 The carbene can be trapped by alkenes to form dichlorocyclopropanes. 

Alkyl halides with (3-hydrogens generally undergo only elimination reactions under the conditions of the vinyl substitution (100 C in the presence of an amine or other base). Exceptions are known only in cases where intramolecular reactions are favorable. Even alkyl halides without (3-hydrogens appear not to participate in the intermolecular alkene substitution since no examples have been reported, with the exception of reactions with benzyl chloride and perfluoroalkyl iodides. 

The reaction has been extensively used for the determination of the structure of naturally occurring bases (e.g. the alkaloids). However it has rather limited preparative value, even though the elimination reaction occurs without any rearrangement of the carbon skeleton, and the regioisomer which predominates in the product is the less highly substituted alkene (Hofmann rule contrast the Saytzeff rule). Such alkenes are now more usually prepared by other procedures noted below. 

When the alkene is neither terminal nor conjugated, addition appears to occur the other way round - to give a (3-alkoxylithium such as 34 - which eliminates Li20 under the conditions of the reaction.26 Cyclic allylic alcohols similarly give alkenes by elimination, and geminally disubstituted allylic alcohols such as 35 are simply unreactive.25... 

Another problem that occurs with eliminations is the regiochemistry of the reaction. As we saw in Chapter 9, most eliminations follow Zaitsev s rule and produce the more highly substituted alkene as the major product. However, a significant amount of the less highly substituted product is also formed. In addition, mixtures of ds and trans isomers are produced when possible, further complicating the product mixture. Because separating a mixture of such isomers is usually a difficult task, elimination reactions are often not the best way to prepare alkenes. (Other methods will be described in subsequent chapters.) However, if only one product can be formed, or if one is expected to greatly predominate in the reaction mixture, then these elimination reactions can be quite useful. 

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