Markovnikov reactions

Note Do not confnse the term anti with the term anti-Markovnikov. The term anti describes the stereochemistry, while the term anti-Markovnikov describes the regiochemistry. It is possible for an anh-Markovnikov reaction to be a syn addition. In fact, we will see snch an example very soon. 

There are not many examples of addition of alcohols to a triple bond proceeding through an anti-Markovnikov reaction. With RuCl(tris(pyrazolyl)borate)(pyridine)2 as catalyst, addition of allyl alcohol to phenylacetylene proceeds in 72 % overall yield in toluene under reflux, and provides a 1 1 mixture of allyl /3-styryl ether and 2-phenylpent-4-enal resulting from further Claisen rearrangement (Scheme 7) [17]. 

The change in regiochemistry is a result of a change in the mechanism of the reaction, from an ionic mechanism in the Markovnikov reaction to a radical chain mechanism in the anti-Markovnikov reaction. The radical chain mechanism for the addition of hydrogen bromide to 1-butene is outlined in the following equations ... 

In 1933, M. S. Kharasch and F. W. Mayo found that some additions of HBr (but not HC1 or HI) to alkenes gave products that were opposite to those expected from Markovnikov s rule. These anti-Markovnikov reactions were most likely when the reagents or solvents came from old supplies that had accumulated peroxides from exposure to the air. Peroxides give rise to free radicals that initiate the addition, causing it to occur by a radical mechanism. The oxygen-oxygen bond in peroxides is rather weak, so it can break to give two alkoxy radicals. 

Muzart [39] reviewed examples of such anti-Markovnikov reactions forming aldehydes. But many ofthe examples can be explained by the influence of electron-withdrawing groups, which then are Markovnikov conforming, like the formation of nitroacetaldehyde and cyanoacetaldehyde from nitroethylene and acrylonitrile, respectively (see Eq. (9.4) and Table 9.1). 

The undecene methyl ester is saponified. The acid produced is converted to 11-bromoundecanoic acid in an anti-Markovnikov reaction under the influence of peroxides or light and hydrogen bromide. The bromoacid is converted to the ammonium salt of 11-amino undecanoic acid by ammonia and the free acid is liberated on lowering the pH. The free acid is polycondensed to polyamide (Nylon 11). 

It turns out there is a way to add Br or OH to the fess-substituted carbon of an alkene. These reactions are called anti-Markovnikov additions. We will learn partial mechanisms later, but most students find it helpful to make note cards of both Markovnikov and anti-Markovnikov reactions at this point. 

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