Anti-Markownikov addition

Other preparative snags also occur in the addition of HHal to alkenes. Thus in solution in H20, or in other hydroxylic solvents, acid-catalysed hydration (p. 187) or solvation may constitute a competing reaction while in less polar solvents radical formation may be encouraged, resulting in anti-Markownikov addition to give 1-bromopropane (MeCH2CH2Br), via the preferentially formed radical intermediate, MeCHCH2Br. This is discussed in detail below (p. 316). 

Anti-Markownikov addition may also result from steric hindrance Addition of iodine azide to the moderately hindered... 

The isomerization reaction of butene-1 has been carried out with D2SO4 in CH3OD [282). The product contained both deuterated and nondeuterated olefins in a ratio consistent with a random scrambling model. The initial step in the reaction involves an insertion of the olefin into the Ni—H bond to form an alkyl. Elimination from the alkyl obtained by Markownikov addition to the olefin can lead to isomerization, whereas elimination from the anti-Markownikov addition product leads to butene-1 being re-formed. The rate of isomerization to deuteration of the olefin is of the order of 170. 

Addition of carbon monoxide and water to an alkene, i.e. hydrocarboxylation, is catalyzed by a variety of transition metal complexes, including [Ni(CO)4], [Co2(CO)s] and [HaPtClg]. Unfortunately this reaction usually leads to mixtures of products due to both metal-catalyzed alkene isomerization and the occurrence of Irath Markownikov and anti-Markownikov addition of the metal hydride intermediate to the alkene. The commercially available zirconium hydride [(C5Hs)2Zr(H)Cl] can be used as a stoichiometric reagent for conversion of alkenes to carboxylic acids under mild conditions (equation 23). In this case the reaction with linear alkenes gives exclusively terminal alkyl complexes even if the alkene double bond is internal. Insertion of CO followed by oxidative hydrolysis then leads to linear carboxylic acids in very good yield. 

Relative rates of anti-Markownikov addition and Markownikov addition, i.e. < (,.roi.an-i oiiM(i.rt)i.an a.oi)-10 A quoted in units of l.mole .atm. min referring to gaseous rather than dissolved olefin. 

The reverse reaction will be discussed in section 8.3. An a-carbon of vinyli-dene ligand, generated from 1,2-hydrogen shift of terminal alkyne upon contact with coordinatively unsaturated metal fragment, is also susceptible to the nucleophilic attack (Scheme 8.17). This attack is believed to constitute a catalytic cycle for anti-Markownikov addition of NuH (Nu = RO, RCOO, etc.) to alkynes [4c]. Schrock carbenes are sensitive to electrophiles (see 8.3.1), but not to nucleophiles. 

With olefins, regioselectivity is governed by the Markownikov rule, but there are examples of anti-Markownikov additions, with hydroboration [18] being the most prominent one. 

---