Cyclic halonium ions

Addition of halogens (Sections 6.14-6.16) Bromine and chlorine add to alkenes to form vicinal dihalides. A cyclic halonium ion is an intermediate. Stereospecific anti addition is observed. 

The reactions of the internal alkynes are considered to involve a cyclic halonium ion intermediate, whereas the terminal alkynes seem to react by a rapid collapse of a vinyl cation. 

Addition is initiated by the positively polarised end (the less electronegative halogen atom) of the unsymmetrical molecule, and a cyclic halonium ion intermediate probably results. Addition of I—Cl is particularly stereoselective (ANTI) because of the ease of formation (and relative stability compared with carbocations) of cyclic iodonium ions. With an unsymmetrical alkene, e.g. 2-methylpropene (32), the more heavily alkyl-substituted carbon will be the more carbocationic (i.e. the less bonded to Br in 33), and will therefore be attacked preferentially by the residual nucleophile, Cle. The overall orientation of addition will thus be Markownikov to yield (34) ... 

A detailed spectroscopic examination should settle the question of whether the ion has the open or the cyclic structure. In general halo-chromic salts lose their color when a covalent bond is established to the central carbon atom, but the bromonium ion might resemble the carbonium ion. Compounds of similar color but which are certainly not cyclic halonium ions are also known ... 

Epoxides, like cyclic halonium ions, undergo ring opening through rearside attack of nucleophiles (see Section 6.3.2). Two mechanisms are shown, for both basic and acidic conditions. Under acidic conditions, protonation of the epoxide oxygen occurs first. The epoxidation-nucleophilic attack sequence also adds substituents to the double bond in an anti sense. 

Cyclic aliphatic halonium ions (I, Br, Cl) have been observed directly in superacid solution by NMR spectroscopy (B-75MI11900). Cyclic halonium ions with ring size three, five and six are formed from open chain dihalides by reaction with strong Lewis acids such as SbFs. Although numerous iodonium, bromonium and chloronium ions are known, no fluoronium ion has been directly observed. NMR spectra of a solution of 2,3-difluoro-2,3-dimethyl-butane (12) in SbF5-S02 at — 90 °C provide evidence for a rapid interconversion of the two open-chain, substituted /3-fluoroethyl cations (67JA4744). The open-chain cation is about 48.2 kJ mol-1 more stable than the closed fluoronium ion (74JA2665). 

You have just seen that cyclic halonium ion intermediates are formed when sources of electrophilic halogen attack a double bond. Likewise, three-membered oxygen-containing rings are formed by the reaction of alkenes with sources of electrophilic oxygen. 

The C2H4Br ions produced in this way are relatively stable and have been shown by nmr to have the cyclic halonium ion structure. 

As with alkenes, in general, anti-addition is often the course of reaction, especially when halonium ions are involved109-112. However, as mentioned earlier, syn addition can take place in the bromination of /Tsilylslyrenes. This stereochemistry is explained by stabilization of the open-chain carbocation by the aromatic group, compared to the cyclic bromonium ion. In this case the conformer 83 has the maximum hyperconjugative stabilization, and is formed by the least motion rotation about the carbon-carbon bond. 

With halogen electrophiles both retention and inversion of stereochemistry have been observed. In this case the addition of the electrophile may lead to the -silicon cation, or a cyclic halonium ion. Scheme 5 shows a generalized mechanism for the reaction of vinylsilanes with electrophilic reagents120. 

The transition-state structures for fluorination, chlorination and bromination were obtained by ab initio MO calculation82. Chlorination and bromination were found to proceed via three-centred geometries (cyclic halonium ions) leading to awfi-addition. In contrast, fluorination involves a four-centred transition state which is consistent with the observed yyw-stereoselectivity82. 

Alternatively, six-membered-ring halonium ions were also formed when equimolar amount of 1,5-dihalopentane was added to dihalonium ions [Eq. (4.122)]. The dihalonium ions were prepared from 1,5-dihalopentane and 2 mol of methyl fluor-oantimonate. Furthermore, the dimethylbromonium ion 129-Br is also a sufficiently active methylating agent to form cyclic pentamethylenebromonium ion 180-Br from 1,5-dibromopentane [Eq. (4.123)]. 

Instead of H20, COOH or OH groups of the substrate located at a suitable distance can also open the halonium ion intermediate through a nucleophilic backside reaction. In this way, cyclic halohydrin derivatives are produced (Figure 3.47). They are referred to as halolactones or haloethers. 

The study of gas-phase acid-induced nucleophilic displacement on 2,3-dihalobutanes has provided stereochemical evidence for the occurrence of cyclic chloronium and bromo-nium ions (X = Cl, Br), but not fluoronium ions17. Protonation or methylation of the neutral 2,3-dihalobutane by a suitable acid GA+ produces a halonium intermediate 2, which in the presence of water ultimately leads to the corresponding halohydrin neutral product (Scheme 4). Analysis of these neutral products indicated that the reaction proceeds with retention of configuration when X = Cl, Br and with inversion of configuration when X = F. The results were rationalized by the mechanisms sketched in Scheme 4, namely direct bimolecular nucleophilic displacement by H20 on 2 when X= F and intramolecular nucleophilic displacement to convert 2 into the cyclic halonium ion 3 (with inversion of configuation) followed by bimolecular nucleophilic displacement on 3 (with inversion of configuration) when X = Cl and Br. 

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. 

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. 

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