Halonium ions rearrangement

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 question of intermediates in thiophene substitutions may not always be so straightforward as portrayed so far. The sulfur atom itself can be the site of attack by an electrophile, i.e. (14) and also Section 3.13.2.4. One can postulate that selectivity in thiophenes could result from attack of an electrophile, for example a halonium ion, at sulfur followed by 1,5-sigmatropic rearrangement and deprotonation as shown in equation (6). A determination of the relative stabilities of (23) and (17) for different substituents X is needed for assessment of the validity of this possibility. Note that the rearrangement type represented by (23) to (17) is known in a photochemical variant (equation 7) (73TL3929). From ab initio calculations on thiophene it also appears that initial attack of an electrophile on sulfur is the pathway of higher electron density (72MI31302). 

Eq. (4.121)]. However, some rearrangement to five-membered-ring halonium ions 174 was also observed. 

The gas-phase base-induced elimination reaction of halonium ions was thoroughly investigated in radiolytic experiments22. Radiolytically generated acids C/JH5+ (n = 1,2) were allowed to react at 760 Torr with selected 2,3-dihalobutanes to form the halonium intermediates which, in the presence of trimethylamine, undergo base-induced bimolecu-lar elimination as shown in Scheme 6. This elimination reaction occurs in competition with unimolecular nucleophilic displacement to the cyclic halonium ion and subsequent rearrangement. Isolation and identification of the neutral haloalkenes formed and kinetic treatment of the experimental results indicated that 3-halo-1 -butene is formed preferentially with respect to the isomeric 2-halo-2-butenes and that the bimolecular elimination process occurs predominantly via a transition state with an anti configuration22. 

The intermediate halonium ion may undergo well-established carbocationic rearrangements, for instance in the halofluorination of norbomadiene [276] (Figure 3.63). 

Bridged halonium ions are formed as intermediates No rearrangements can occur. 

Addition of Bromine and Chlorine (Section 6.3D) Clj or Brj is used to convert an alkene into a vicinal dihalide. The mechanism involves attack by the alkene tt bond on one atom of Xj to give a bridged halonium ion intermediate (a cation) that is, in turn, attacked by X from the backside to give the vicinal dihalide. Rearrangements do not occur.The reaction displays anti addition stereoselectivity because of the halonium ion intermediate.The reaction is stereospecific because Z alkenes give different products than do Ealkenes. 

Halogenation (Section 22.1 A) The electrophile is a halonium ion formed as an ion pair by interaction of chlorine or bromine with a Lewis acid. The mechanism involves an initial reaction between Clj and FeClj to generate a molecular complex that can rearrange to give a C1+, FeCl " ion pair. The C1+ reacts as a very strong electrophile with the weakly nucleophilic aromatic 77 cloud to form a resonance-stabilized cation intermediate that loses a proton to give the aryl chloride product. 

In the halonium-ion-mediated 1,2-Wagner-Meerwein-type rearrangement of a series of benzo-fused bi- and tri-cyclic sulfonamides, the carbon-carbon bond that migrates has been reported (Scheme 99). " " ... 

Subsequently, organocatalytic asymmetric halogenation/semipinacol rearrangement sequences, both leading to chiral quaternary haloketones, have been reported by Tu and coworkers [61] and by Hermecke and coworkers [62]. The mechanism of these reactions probably involves an acid-catalyzed diastereoselective semipina-col rearrangement of a chiral halonium ion intermediate obtained by chiral tertiary amine-promoted asymmetric halogenation of an enone [63]. 

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