Nucleophilic halide additives

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... 

There is usually a competing reaction with solvent when lydrogen halide additions to alkenes are carried out in nucleophilic solvents ... 

It is not difficult to incorporate this result into the general mechanism for hydrogen halide additions. These products are formed as the result of solvent competing with halide ion as the nucleophilic component in the addition. Solvent addition can occur via a concerted mechanism or by capture of a carbocation intermediate. Addition of a halide salt increases the likelihood of capture of a carbocation intermediate by halide ion. The effect of added halide salt can be detected kinetically. For example, the presence of tetramethylammonium... 

Monomeric thiazyl halides can be stabilized by coordination to transition metals and a large number of such complexes are known (Section 7.5). In addition, NSX monomers undergo several types of reactions that can be classified as follows (a) reactions involving the n-system of the N=S bond (b) reactions at the nitrogen centre (c) nucleophilic substitution reactions (d) halide abstraction, and (e) halide addition. Examples of each type of behaviour are illustrated below. 

The reduction of carbonyl compounds by reaction with hydride reagents (H -) and the Grignard addition by reaction with organomagnesium halides (R - +MgBr) are examples of nucleophilic carbonyl addition reactions. What analogous product do you think might result from reaction of cyanide ion with a ketone ... 

The role of the DEAD is to activate the triphenylphosphine toward nucleophilic attack by the alcohol. In the course of the reaction the N=N double bond is reduced. As is discussed later, this method is applicable for activation of alcohols to substitution by other nucleophiles in addition to halide ions. The activation of alcohols to nucleophilic attack by the triphenylphosphine-DEAD combination is called the Mitsunobu reaction.76... 

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. 

Thiazyl halide monomers undergo a variety of reactions that can be classified under the general headings (a) reactions involving the 7i-system of the N = S triple bond, (b) nucleophilic substitution, (c) halide abstraction, and (d) halide addition. The cycloaddition of NSF with hexafluoro-1,3-butadiene provides an example of a type (a) reaction. 

Alternatively, the rhodium dimer 30 may be cleaved by an amine nucleophile to give 34. Since amine-rhodium complexes are known to be stable, this interaction may sequester the catalyst from the productive catalytic cycle. Amine-rhodium complexes are also known to undergo a-oxidation to give hydridorhodium imine complexes 35, which may also be a source of catalyst poisoning. However, in the presence of protic and halide additives, the amine-rhodium complex 34 could react to give the dihalorhodate complex 36. This could occur by associative nucleophilic displacement of the amine by a halide anion. Dihalorhodate 36 could then reform the dimeric complex 30 by reaction with another rhodium monomer, or go on to react directly with another substrate molecule with loss of one of the halide ligands. It is important to note that the dihalorhodate 36 may become a new resting state for the catalyst under these conditions, in addition to or in place of the dimeric complex. 

When this dianion is reacted with one equivalent of an alkyl halide, the more basic site acts as a nucleophile. The addition of acid neutralizes the remaining anion. Overall, this process allows the more basic site to be alkylated preferentially. The following equation shows another example ... 

Among the nucleophiles that add exo to coordinated dienes are aUcoxides, amines, azide, acetates, halides, and stabilized carbon enolates, such as malonates and /3-diketones. The alkoxide addition is reversible if the product is treated with HCl. With a few nucleophiles, double addition reactions are observed. Acetate will react with 1,5-cod in the presence of Pb(OAc)4 and palladium salts to give a bicyclic product from addition of two acetate groups, both exo (equation 43). 

The alkylation of amides by alkyl halides or simple sulfonic acid esters is usually of little importance because the alkylation equilibrium is placed on the side of the starting compounds. This is not the case, however, in either the alkylation of vinylogous amides (which has been achieved even with alkyl iodides ) or if intramolecular alkylation is possible, e.g. in -(2-haloethyl)amides. In the latter case cyclic iminium compounds (81 equation 51) are readily available by replacing the more nucleophilic halide by less nucleophilic complex anions, which can be achieved by addition of Lewis acids or AgBF4. °-2 ... 

In the addition of hydrogen halides to alkenes, it is usually found that the nucleophilic halide ion becomes attached to the more-substituted carbon atom. This general observation is called Markovnikov s rule. The basis for this regioselectivity lies in the relative ability of the carbon atoms to accept positive charge. The addition of hydrogen halide is initiated by protonation of the alkene. The new C—H bond is formed from the 7T electrons of the carbon-carbon double bond. It is easy to see that if a carbo-cation is formed as an intermediate, the halide will be added to the more-substituted carbon, since protonation at the less-substituted carbon atom provides the more stable carbocation intermediate. 

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