Stereochemistry hydrogen halides

The order of reactivity of the hydrogen halides is HI > HBr > HCl, and reactions of simple alkenes with HCl are quite slow. The studies that have been applied to determining mechanistic details of hydrogen halide addition to alkenes have focused on the kinetics and stereochemistry of the reaction and on the effect of added nucleophiles. The kinetic studies often reveal complex rate expressions which demonstrate that more than one process contributes to the overall reaction rate. For addition of hydrogen bromide or Itydrogen... 

Anti stereochemistry can be explained by a mechanism in which the alkene interacts simultaneously with the proton-donating hydrogen halide and with a source of halide ion, either a second molecule of hydrogen halide or a free halide ion. The anti stereochemistry is consistent with the expectation that the attack of halide ion would be from the opposite... 

Addition of hydrogen halides to alkenes is not stereospecific. In contrast, addition of Br2 proceeds exclusively with anti stereochemistry. 

The stereochemistry of addition of hydrogen halides to alkenes depends on the structure of the alkene and also on the reaction conditions. Addition of hydrogen bromide to cyclohexene and to E- and Z-2-butene is anti.6 The addition of hydrogen chloride to 1 -methylcyclopentene is entirely anti when carried out at 25° C in nitromethane.7... 

The stereochemistry of addition depends on the details of the mechanism. The addition can proceed through an ion pair intermediate formed by an initial protonation step. Most alkenes, however, react via a complex that involves the alkene, hydrogen halide, and a third species that delivers the nucleophilic halide. This termolecular mechanism is generally pictured as a nucleophilic attack on an alkene-hydrogen halide complex. This mechanism bypasses a discrete carbocation and exhibits a preference for anti addition. 

Table 4.1. Stereochemistry of Addition of Hydrogen Halides to Alkenes...

CHAPTER 4 ELECTROPHILIC Alkene Hydrogen halide Stereochemistry Reference... 

The mechanism for the addition of the hydrogen halides to alkenes proceeds through a carbocation intermediate. As was the case in the SN1 reaction, the nucleophile can approach the planar carbocation equally well from either side, so we expect that the products should result from a mixture of syn and anti addition. Indeed, this is often the case. Under some conditions, however, the stereochemisty results from predominant syn addition, whereas anti addition is the favored pathway under other conditions. This occurs because these reactions are often conducted in nonpolar solvents in which ion pair formation is favored. The details of how this may affect the stereochemistry of these reactions are complex. Fortunately, stereochemistry is not an issue in most of the reactions in which hydrogen halides add, including all the examples previously presented, because the carbon to which the proton is adding usually has at least one hydrogen already bonded to it. In such situations, syn addition and anti addition give identical products. Stereochemistry will be more important in some of the other reactions that are discussed later in this chapter. 

As p-hydride elimination is reversible, hydropalladation with the opposite regiochemistry provides a mechanism for forming regioisomers of the alkene. This allows the most stable alkene that is accessible by the hydropalladation-dehydropalladation sequence to dominate. The only restriction is that all of these processes are syn. The migration can be prevented by the addition of bases like silver carbonate, which effectively removes the hydrogen halide from the palladium complex as soon as it is formed. This synthesis of a complex trans dihydrofuran involves the Heck reaction followed by alkene isomerization and then a Heck reaction without migration to preserve the stereochemistry. 

The stereochemistry of addition of hydrogen halides to unconjugated alkenes is usually anti. This is true for addition of HBr to l,2-dimethylcyclohexene, cyclohexene, 1,2-dimethylcyclopentene, cyclopentene, Z- and -2-butene, and 3-hexene, among others. Anti stereochemistry is also dominant for addition of hydrogen chloride to 1,2-dimethylcyclohexene and l-methylcyclopentene. Temperature and solvent can modify the stereochemistry, however. For example, although the addition of HCl to 1,2-dimethylcyclohexene is anti near room temperature, syn addition dominates at —78°C. °... 

From these examples we see that the mechanistic and stereochemical details of hydrogen halide addition depend on the reactant structure. Alkenes that form relatively unstable carbocations are likely to react via a termolecular complex and exhibit anti stereospecificity. Alkenes that can form more stable cations can react via rate-determining protonation and the structure and stereochemistry of the product are determined by the specific properties of the cation. 

The elimination reaction, given in Equation 7e, is accompanied by a formal reduction in the oxidation state of the rhodium from (III) to (I). The stereochemistry of this reaction has also attracted substantial interest. If /8-hydrogens are absent, the alkyl halide is eliminated with retention of configuration at the carbon. If /8-hydrogens are present, /8-hydride elimination occurs as shown in Equation 7f, giving an alkene and hydrogen halide. Investigations into the /8-hydride elimination have shown... 

The stereochemistry of the addition of hydrogen halides to a variety of alkenes has been investigated. The addition of hydrogen chloride to 1-methylcyclopentene is... 

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