Halonium ions, from addition

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. 

Thus, fluorination of 1,3-dienes proceeds through an allylic ion, while weakly bridged halonium ions are the intermediates in chlorination and bromination of dienes (vide infra). Furthermore, starting from the experimental evidence that 13 is produced under kinetic conditions and not from subsequent rearrangement of the 1,2- and 1,4-adducts, the authors suggested that 13 arose from rearrangement of the allyl cation intermediate, 17. Consistent with an open ion pair intermediate is also the stereoselective formation of the threo isomer from both 1,3-pentadienes, as well as the preference for the addition to the 1,2-bond observed in the reaction of both isomeric pentadienes. This selectivity may indeed... 

The rate of the return from bromonium ion to CT complex depends on the nature of the solvent and of the substituents bonded to olefin81,84. By using adamantylideneadamantane 33, halonium ions 34 are formed as stable (and isolatable) salts85, because of steric hindrance of the nucleophilic counter ion. Consequently, the products of addition, such as the dibromides 29, cannot be obtained. 

Calculated equilibrium geometries, bond lengths, and charge densities have been compared for halonium ions (1-5 X = Cl or Br) derived from the addition of halogen electrophiles to fluoro-substituted terminal alkenes (R = alkyl or perfluoroalkyl with... 

In the gas phase, addition of an ionic electrophile to a neutral (M) is usually accompanied by elimination of a neutral molecule from the reagent ion. This elimination process stabilizes the reaction products by removing excess energy from the initial ion-molecule adduct. Typically, protonation and alkylation reactions normally used in chemical ionization (Cl) and radiolytic experiments are of this type, as shown in equations 3 (HA+ = CH5+, C2H5+, NH4+, etc.) and 4 (R2X+ = an halonium ion, vide infra), respectively. 

Anti stereochemistry results from the bromonium ion mechanism. When a nucleophile attacks a halonium ion, it must do so from the back side, in a manner similar to the SN2 displacement. This back-side attack assures anti stereochemistry of addition. 

Addition of hydrogen halides involves formation of an open carbocation, not a cyclic halonium ion intermediate. The carbocation, which is sp-hybridized and planar, can be attacked by chloride from either top or bottom, yielding products in which the two methyl groups can be either cis or trans to each other. 

As fas as reaction conditions are concerned, two main approaches are usually taken. Either the nucleophilicity of the R5OH to be added is further enhanced by addition of base (normally R50 M +, or nitrogen bases of low nucleophilicity), i.e., base catalysis, or the electrophilicity of the accepting double bond is further increased by adding, e.g., mercuric salts (alkoxymercu-ration), or sources of halonium ions (formation of / -halohydrins). Clearly, the latter protocol, from now on abbreviated as "onium-methods , necessitates a subsequent step for the removal of the auxiliary electrophile, e.g., reductive demercuration of an intermediate /i-alkoxymercu-rial. Whereas base catalysis has successfully been employed with all varieties of acceptors, application of onium-methods thus far appears to be restricted to a,/ -unsaturated carbonyl compounds. Interestingly, conjugate addition of alcohols to a,/l-enones could also be effected photochemically in a couple of cases. 

The mechanism for halohydrin formation is similar to the mechanism for halogenation addition of the electrophile X (from X2) to form a bridged halonium ion, followed by nucleophilic attack by H2O from the back side on the three-membered ring (Mechanism 10.4). Even though X is formed in Step [1] of the mechanism, its concentration is small compared to H2O (often the solvent), so H2O and not X" is the nucleophile. 

If the substrate contains a nucleophile in addition to a vinylcyclopropane moiety, the halonium ion formed during the addition of bromine or iodine to the double bond, can be trapped which results in the formation of cyclization products. Examples of compounds that react in this way are 1-phosphoryl-substituted 2-vinylcyclopropanes, 2-vinylcyclopropyl-sub-stituted alkanoic acids, and 2-vinylcyclopropyl-substituted alkanols, which afford oxaphosphabicycloalkanes, bi- and polycyclic lactones and polycyclic ethers, respectively. However, 2-vinylcyclopropyl-substituted alkylamines behave differently, Very good yields have been obtained in some cases, e.g. formation of 4 from 3. ... 

Electrophilic additions of alkenes can be considered, in accordance with the Hammond postulate, to involve transition states resembling either the intermediate trivalent carbocations (or, in the case of halogenation, threemembered ring halonium ions) or the starting alkenes. In the first case, a new covalent bond is more fully developed in the transition state than in the latter case. The position of the transition state can vary from reaction to reaction and resemble either the starting alkene or the carbenium ion intermediate. 

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