Elimination regioselectivity

Sol 2. (i) In selenoxide, pyrolysis cycloelimination uses a ring of five atoms therefore, the reaction is necessarily yu-selective. Hence, on heating, cyclo-hexylphenyl selenoxide undergoes iyn-elimination regioselectively to produce the less stable Hofmann product. 

We shall only be concerned here with those reactions, which have been used in constructions of the carbon skeletons of complex compounds with concomitant regioselective incorporation of the double bond. 1,2-Eliminations are discussed on p. 137ff. 

There also exists an acidregioselective condensation of the aldol type, namely the Mannich reaction (B. Reichert, 1959 H. Hellmann, 1960 see also p. 291f.). The condensation of secondary amines with aldehydes yields Immonium salts, which react with ketones to give 3-amino ketones (=Mannich bases). Ketones with two enolizable CHj-groupings may form 1,5-diamino-3-pentanones, but monosubstitution products can always be obtained in high yield. Unsymmetrical ketones react preferentially at the most highly substituted carbon atom. Sterical hindrance can reverse this regioselectivity. Thermal elimination of amines leads to the a,)3-unsaturated ketone. Another efficient pathway to vinyl ketones starts with the addition of terminal alkynes to immonium salts. On mercury(ll) catalyzed hydration the product is converted to the Mannich base (H. Smith, 1964). 

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

The preparative method for the Pd(0) catalyst active in these regioselective eliminations under mild conditions is crucial. The very active catalyst is prepared by mixing equimolar amounts of Pd(OAc) or Pd(acac)2 and pure n-... 

The reaction of isoprene with MeOH catalyzed by Pd(acac)2 and Ph3P is not regioselective, giving a mixture of isomers[37]. However, l-methoxy-2,6-dimethyl-2,7-octadiene (35), the head-to-tail dimer, was obtained in 80% yield, accompanied by the tail-to-tail dimer (15%) using 7r-allylpalladium chloride and BU3P. On heating, 35 was converted into 2.6-dimethyl-1,3,7-octatriene (36) by an elimination reaction[38]. 

Hydroxy-THISs add regioselectively to the C=N bonds of isocyanates or isothiocyanates. The initially formed cycloadducts eliminate carbonyl sulfide with formation of 4-hydroxy- or 4-mercaptoimidazolium hydroxide inner salts (21) (Scheme 21). 4-Hydroxyimidazolium hydroxide... 

Dehydration of this alcohol is selective m respect to its direction Elimination occurs m the direction that leads to the double bond between C 2 and C 3 more than between C 2 and C 1 Reactions that can proceed m more than one direction but m which one direction is preferred are said to be regioselective... 

In 1875 Alexander M Zaitsev of the University of Kazan (Russia) set forth a gen erahzation describing the regioselectivity of p eliminations Zaitsev s rule summarizes the results of numerous experiments m which alkene mixtures were produced by p elim matron In its original form Zaitsev s rule stated that the alkene formed in greatest amount is the one that corresponds to removal of the hydrogen from the f3 carbon hav mg the fewest hydrogens... 

The regioselectivity of dehydrohalogenation of alkyl halides follows the Zaitsev rule p elimination predominates m the direction that leads to the more highly substi tuted alkene... 

The regioselectivity of elimination is accommodated m the E2 mechanism by noting that a partial double bond develops at the transition state Because alkyl groups... 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

Elimination is typically regioselective and gives a conjugated diene rather than an isolated or cumulated diene system of double bonds... 

A novel aspect of the Hofmann elimination is its regioselectivity Elimination m alkyltrimethylammonmm hydroxides proceeds m the direction that gives the less substi tuted alkene... 

The least sterically hindered p hydrogen is removed by the base m Hofmann elim matron reactions Methyl groups are deprotonated m preference to methylene groups and methylene groups are deprotonated m preference to methmes The regioselectivity of Hofmann elimination is opposite to that predicted by the Zaitsev rule (Section 5 10) Elimination reactions of alkyltrimethylammonmm hydroxides are said to obey the Hofmann rule, they yield the less substituted alkene... 

With a regioselectivity opposite to that of the Zaitsev rule the Hofmann ehmma tion IS sometimes used in synthesis to prepare alkenes not accessible by dehydrohalo genation of alkyl halides This application decreased in importance once the Wittig reac tion (Section 17 12) became established as a synthetic method Similarly most of the analytical applications of Hofmann elimination have been replaced by spectroscopic methods... 

Quaternary ammonium hydroxides un dergo elimination on being heated It is an anti elimination of the E2 type The regioselectivity of the Hofmann elimina tion IS opposite to that of the Zaitsev rule and leads to the less highly substi tuted alkene... 

The nucleophilic attack of nitrogen bases leads to a variety of products as the result of addition or addition-elimination reactions The regioselectivity resembles that of attack by alcohols and alkoxides an intermediate carbanion is believed to be involved In the absence of protic reagents, the fluorocarbanion generated by the addition of sodium azide to polyfluonnated olefins can be captured by carbon dioxide or esters of fluonnated acids [J 2, 3] (equation I)... 

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