Relative reactivity of some alkenes in oxymercuration

Other nucleophilic solvents besides water can be captured by the mercurinium 

The synthetic utility of the reaction for hydration of carbon-carbon double bonds depends on reactions that can replace the mercury atom by hydrogen. This is accomplished by various reducing agents, in particular sodium borohydride.  

The mechanism of the reductive replacement of mercury by hydrogen involves a free-radical intermediate generated by decomposition of an intermediate alkylmer-cury hydride  

The evidence for this mechanism includes the fact that the course of the reaction can be diverted by oxygen, an efficient radical scavenger. Also, the stereochemistry of the reduction, as studied by using NaBD4 as the reducing agent, is consistent with a radical intermediate. For example, a 50 50 mixture of erythro- and threo-3-deuterio-2-butanol is obtained by oxymercuration of either cis- or trans-2-butene, followed by reduction with NaBD4  

The overall oxymercuration-demercuration procedure is highly regioselective, with the nucleophile being introduced at the most highly substituted carbon  

The reactivity of alkenes toward mercuration varies over a considerable range and is governed by a combination of steric and electronic factors.Terminal double bonds are more reactive than internal ones. Disubstituted terminal olefins, however, are more reactive than monosubstituted cases, as would be expected for electrophilic attack. The differences in relative reactivities are large enough that selectivity can be achieved in certain dienes. Relative reactivity data for some pentene derivatives are given in Table 4.2. 

The addition of chlorine and bromine to alkenes is a very general reaction. Considerable insight has been gained into the mechanism of halogen addition 

CHAPTER 4 ELECTROPHILIC ADDITIONS TO CARBON-CARBON MULTIPLE BONDS 

Substantial amounts of syn addition have been observed for c/5-l-phenylpropene (27-80% syn addition), rraM5-l-phenylpropene (17-29% syn addition), and c/5-stilbene (up to 90% syn addition in polar solvents). A common feature of the compounds that give extensive syn addition is the presence of a 

Although chlorination of aliphatic olefins gives largely anti addition, syn addition is often dominant for phenyl-substituted olefins.These results again 

---

Table 4.2. Relative Reactivity of Some Alkenes in Oxymercuration...

---