Non-Markovnikov addition

One of the features that makes the hydrobora ( ion reaction so useful is the regiochemistry that results when an unsymmetrical alkene is hydroborated. For example, hydroboration/oxidation of 1-methylcyclopentene yields trans-2-methylcydopentanol. Boron and hydrogen both add to the alkene from the same face of the double bond—that is, with syn stereochemistry, the opposite of anti—with boron attaching to the less highly substituted carbon. During the oxidation step, the boron is replaced by an -OH with the same stereochemistry, resulting in an overall syn non-Markovnikov addition of water. This stereochemical result is particularly useful because it is complementary to the Markovnikov regiochemistry observed for oxymercuration. 

Thus, most of the product with 1-butene would be expected to be the 1,2-isomer since the preferred Markovnikov adduct cannot isomerize. Also reaction of the non-Markovnikov adduct without isomerization gives 1,2-isomer. The 1,3-isomer from 1-butene is much less favored than it is with 2-butene, for with 1-butene, non-Markovnikov addition followed by isomerization before decomposition is required. This expectation is confirmed. 1-Butene gives one-sixth as much 1,3-isomer as 2-butene. Of course, 1,4-isomer is even less preferred because it requires non-Markovnikov addition followed by two isomerizations before decomposition, and only traces of 1,4-isomer is detected. 

When predicting the product of a reaction, you have to recall what you know about the kind of reaction being carried out and then apply that knowledge to the specific case you re dealing with. In the present instance, recall that the two methods of hydration—hydroboration/oxidation and oxymercura-tion—give complementary products. Hydroboration/oxidation occurs with syn stereochemi. itiy and gives the non-Markovnikov addition product oxymercuration gives the Markovnikov product. 

Posner [72] in 1905 described the reaction of thiols with olefins to produce monosulfides. In more recent work it was established that two modes of thiol addition to olefins are possible and involve either normal addition or abnormal (non-Markovnikov) addition [73],... 

Marvel and Chambers [80] also demonstrated that the UV, free-radical catalyzed polymerization of hexamethylenedithiol and 1,5-hexadiene (Preparation 3-3) gave a linear product (non-Markovnikov addition) that had the infixed spectrum similar to that of the condensation polymer from hexamethylenedithiol and 1,6-dibromohexane (Preparation 3-4) under alkaline conditions, but not similar to that of the product from the alkali condensation of hexamethylenedithiol and 2,5- dibromohexane (Preparation 3-5). 

Radical Initiated Non-Markovnikov Addition of HBr to Alkenes Chain Initiation tionas)... 

When the addition of hydrogen halides to alkenes was first studied systematically in the 1930s, chemists observed that the addition of HBr sometimes gave Markovnikov addition and sometimes gave non-Markovnikov addition. These two modes of addition of HBr are illustrated for 2-methylpropene (isobutylene). 

Addition of HBr to alkenes in the presence of peroxides can lead to non-Markovnikov addition because under these conditions, a free-radical chain mechanism operates. 

HBr Addition to Alkenes Under Radical Conditions Non-Markovnikov addition of... 

The first allylic alcohols studies were 3-buten-2-ol (8) and 2-buten-l-ol (9). As shown in Scheme 15, 8 gave mainly 10, which resulted from the Markovnikov type of hydroxypalladation. The ratio of 10 to the products, 11 and 12, resulting from the non-Markovnikov addition was about 4 1. On the other hand, the products from the internal olefin, crotyl alcohol (9), were predominantly those (11 and 12) resulting from the hydroxypalladation, which put the palladium next to the alcohol function. The ratio of 11 -I-12 to 13, the non-Markovnikov product, was about 35 1. 

Dufiach et a. successfully added thiolacetic acid to alkenes using either InCls or In(OTf)3 as catalysts [105]. The same system, but with ln(OTf)3, was used for both inter- and intramolecular addition of thiols to alkenes. These reactions proceed with high Markovnikov regioselectivities, with most reactions producing only one regioisomer. Non-Markovnikov additions were observed for vinyl cyclohexane, owing to isomerization of the double bond. (Figure 8.52)... 

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