Rules regioselectivity rule

In the case of the 2-dimethylamino- (or 2-amino-)methylene derivatives, the products were at first thought to be pyrimido[4,5-c]isoquinolines (267), but later work with 6-(N-substituted amino)uracils assigned the structures of the products (268) as belonging to the isomeric pyrimido[4,5-f>]quinoline system (74CB2537), in agreement with the regioselection rules above. 

Halofluorinations take place, as a rule, regioselectively (Markovnikov addition), the olefinic carbons can be substituted with a variety of substituents ranging from alkyl or aryl groups to different electron-withdrawing functions see for example refs 31 and 178-180. Bromo-fluorination of 4-/m-butyl-l-methylcyclohexene with /V-bromosuccinimide in 70% hydrogen fluoride/pyridine gave two stereoisomers 1 and 2.181... 

There is no exception to the regioselectivity rule that nucleophile addition proceeds by kinetic control at the unsubstituted internal position of the diene ligand. With 1-methoxy-l,3-cyclohexadiene, the addition/CO insertion/Mel quenching leads to a latent 1,4-dicarbonyl derivative (equation 80). With 2-methoxy-1,3-butadiene, the same conditions (but with protons instead of Mel) lead to a 3-methoxycyclopentanone that is very sensitive to acid, and can be readily converted to the corresponding cyclopentenone (equation 81). 

There is, however, one exception to this regioselectivity rule. 3-Oxaquadricyclane (34) reacts with TCNE in practically quantitative yield in the 1,5-position, i.e. breaking the most strained cyclopropyl bonds which are part of the four-membered ring. ... 

Side-chain bromination of 5,7-dimethyl-6-aryl TPs (e.g., 187, see below) by pyridinium tribromide can be restricted to the 7-positioned methyl (06WOP34848). The same regioselectivity rule governs the reaction of 5,7-dimethyl TP (44) with DMF dimethylacetal which yields enamine 182, and, through oxime 183, nitrile 184 (Scheme 46) (96JHC465). 

Saytzev rule Regioselectivity whereby E2 elimination by a nonhindered base leads to the more stable internal alkenes. 

Mannich reaction Alkylation of enols by electrophilic iminium ions, giving f)-aminocarbonyl compounds. Markovnikov rule Regioselectivity in electrophilic addition to alkenes whereby an electrophile attacks... 

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... 

Dehydrohalogenation of alkyl halides (Sections 5 14-5 16) Strong bases cause a proton and a halide to be lost from adjacent carbons of an alkyl halide to yield an alkene Regioselectivity is in accord with the Zaitsev rule The order of halide reactivity is I > Br > Cl > F A concerted E2 reaction pathway is followed carbocations are not involved and rearrangements do not occur An anti coplanar arrangement of the proton being removed and the halide being lost characterizes the transition state... 

REGIOSELECTIVITY OF HYDROGEN HALIDE ADDITION MARKOVNIKOV S RULE... 

Regioselectivity of Hydrogen Halide Addition Markovnikovs Rule... 

Under free radical conditions in the presence of peroxides addition takes place with a regioselectivity opposite to that of Markovmkov s rule... 

Acid catalyzed hydration converts alkenes to alcohols with regioselectivity according to Markovnikov s rule Frequently however one needs an alcohol having a structure that corresponds to hydration of an alkene with a regioselectivity opposite to that of Markovnikov s rule The conversion of 1 decene to 1 decanol is an example of such a transformation... 

There is a pronounced tendency for boron to become bonded to the less substituted car bon of the double bond Thus the hydrogen atoms of diborane add to C 2 of 1 decene and boron to C 1 This is believed to be mainly a steric effect but the regioselectivity of addition does correspond to Markovmkov s rule m the sense that hydrogen is the neg atively polarized atom m a B—H bond and boron the positively polarized one... 

The regioselectivity of addition follows Markovmkov s rule A proton adds to the car bon that has the greater number of hydrogens and halide adds to the carbon with the fewer hydrogens... 

Electrophilic addition (Section 11 16) An aryl group stabilizes a benzylic carbocation and con trols the regioselectivity of addition to a double bond involving the benzylic carbon Markovni kov s rule is obeyed... 

Hydroboration-oxidation of alkenes (Section 6 11) H and OF add to the double bond with a regioselectivity opposite to that of Markovnikov s rule This is a very good synthetic method addition is syn and no rearrangements are observed... 

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... 

Anti Markovnikov addition (Sections 6 8 6 11) Addition re action for which the regioselectivity is opposite to that pre dieted on the basis of Markovnikov s rule... 

The regioselectivity of the second step is consistent with MarkownikofF s rule because a halogen atom can stabilize a carbocation by resonance. 

More complete interpretations of Diels-Alder regioselectivity have been developed. MO results can be analyzed from an electrostatic perspective by calculating potentials at the various atoms in the diene and dienophile. These results give a more quantitatively accurate estimate of the substituent effects. Diels-Alder regioselectivity can also be accounted for in terms of HSAB theory (see Section 1.2.3). The expectation would be that the most polarizable (softest) atoms would lead to bond formation and that regioselectivity would reflect the best mateh between the diene and dienophile termini. These ideas have been applied using 3-2IG computations. The results are in agreement with the ortho rule for normal-electron-demand Diels-Alder reactions. ... 

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