Haloniums

Because the nature of the halogen atom can be varied, these salts show useful selectivity in their alkylation reaetions. We also prepared other halonium ions and studied their alkylating ability. 

Halonium ions, including hydrido or alkylhalonium ions, are similarly protolytically activated, indicative of protonation of the non-bonded electron pairs of their halogen atoms. 

MECHANISM OF HALOGEN ADDITION TO ALKENES HALONIUM IONS... 

Mechanism of Halogen Addition to Alkenes Halonium Ions... 

Halohydnn formation as depicted m Figure 6 13 is mechanistically related to halo gen addition to alkenes A halonium ion intermediate is formed which is attacked by water m aqueous solution... 

The regioselectivity of addition is established when water attacks one of the car bons of the halonium ion In the following example the structure of the product tells us that water attacks the more highly substituted carbon... 

Addition of halogens (Sections 6 14-6 16) Bromine and chlorine add to alkenes to form vicinal dihalides A cy clic halonium ion is an intermediate Stereospecific anti addition is observed... 

Bromonium Ion (Section 6 16) A halonium ion in which the halogen is bromine (see halonium ion)... 

Halonium ion (Section 6 16) A species that incorporates apos itively charged halogen Bridged halonium 10ns are inter mediates in the addition of halogens to the double bond of an alkene... 

Cations of the type R R X+ are given names derived from the halonium ion, H2X+, by substitution, e.g., diethyliodonium chloride for (C2H5)2UCF. 

Cocatalysts of two types occur (/) proton-donor substances, such as hydroxy compounds and proton acids, and (2) cation-forming substances (other than proton), including alkyl and acyl haUdes which form carbocations and other donor substances leading to oxonium, sulfonium, halonium, etc, complexes. 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

Bromine and chlorine convert the 1- and 2-butenes to compounds containing two atoms of halogens attached to adjacent carbons (vicinal dihahdes). Iodine fails to react. In this two-step addition mechanism the first step involves the formation of a cation. The halonium ion formed (a three-membered ring) requires antiaddition by the anion. 

Addition of chlorine or bromine in the presence of water can yield compounds containing haUde and hydroxyl on adjacent carbon atoms (haloalcohols or halohydrins). The same products can be obtained in the presence of methanol (13) or acetic acid (14). As expected from the halonium ion intermediate, the addition is anti. As expected from Markovnikov s rule, the positive halogen goes to the same carbon that the hydrogen of a protic reagent would. 

Halogenated Butyl Rubber. Halogenation at the isoprene site ia butyl mbber proceeds by a halonium ion mechanism leading to a double-bond shift and formation of an exomethylene alkyl haUde. Both chlorinated and brominated mbber show the predominate stmcture (1) (>80%), by nmr, as described eadier (33,34). Halogenation of the unsaturation has no apparent effect on the isobutylene backbone chains. Cross-linked samples do not crystallize on extension due to the chain irregularities introduced by the halogenated isoprene units. 

The univalent halogen atoms are included because of the need to name rings containing such atoms in dicoordinate form, as halonium cations. 

Interpretation of reaction stereochemistry has focused attention on the role played by bridged halonium ions. 

Reactions of alkynes with electrophiles are generally similar to those of alkenes. Because the HOMO of alkynes (acetylenes) is also of n type, it is not surprising that there IS a good deal of similarity between alkenes and alkynes in their reactivity toward electrophilic reagents. The fundamental questions about additions to alkynes include the following. How reactive are alkynes in comparison with alkenes What is the stereochemistry of additions to alkynes And what is the regiochemistry of additions to alkynes The important role of halonium ions and mercurinium ions in addition reactions of alkenes raises the question of whether similar species can be involved with alkynes, where the ring would have to include a double bond ... 

Mixed halide additions are most satisfactorily interpreted as proceeding via a halonium complex (1), in which the less electronegative halogen approaches from the less hindered a-side. The energetic preference for a pm-planar transition state leading to the diaxial product (2) is sufliciently great that the... 

The anomalous iodoacetamide-fluoride reaction violates this rule, in that a less stable -halonium complex (18) must be involved, which then opens to (19) in the Markownikoff sense. This has been rationalized in the following way estimates of nonbonded destabilizing interactions in the possible products suggest that the actual product (16) is more stable than the alternative 6)5-fluoro-5a-iodo compound, so the reaction may be subject to a measure of thermodynamic control in the final attack of fluoride ion on the iodonium intermediate. To permit this, the a- and -iodonium complexes would have to exist in equilibrium with the original olefin, product formation being determined by a relatively high rate of attack upon the minor proportion of the less stable )9-iodonium ion. 

Halohydrin fonnation, as depicted in Figure 6.13, is mechanistically related to halogen addition to alkenes. A halonium ion intennediate is fonned, which is attacked by water in aqueous solution. 

When treated with bromine or chlorine in aqueous solution, alkenes are converted to vicinal halohydrins. A halonium ion is an intermediate. The halogen adds to the carbon that has the greater number of hydrogens. Addition is anti. 

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