Halonium ions ring opening

The ionization of l,3-dihalo-2-methylpropanes with SbF5-S02ClF gave both three-membered and five-membered ring halonium ions and open-chain halocarbe-nium ions. 

Nucleophilic attack of H2O opens the halonium ion ring, forming a new C-X bond. Subsequent loss of a proton forms the neutral halohydrin. 

Because the bridged halonium ion ring is opened by backside attack of H2O, addition of X and OH occurs in an anti fashion and trans products are formed. 

Each addition of X2 involves a two-step process with a bridged halonium ion intermediate, reminiscent of the addition of X2 to alkenes (Section 10.13). A trans dihalide is formed after addition of one equivalent of X2 because the intermediate halonium ion ring is opened upon backside attack of the nucleophile. Mechanism 11.2 illustrates the addition of two equivalents of CI2 to CHaC CCH, to form CH3CCI2CCI2CH3. 

Electrophilic addition of cr in Step [3] forms the bridged halonium ion ring, which is opened with Cr to form the tetrahalide in Step [4]. 

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). 

Unlike a normal carbocation, all the atoms in a halonium ion have filled octets. The three-membered ring has considerable ring strain, however, which, combined with a positive charge on an electronegative halogen atom, makes the halonium ion strongly electrophilic. Attack by a nucleophile, such as a halide ion, opens the halonium ion to give a stable product. 

The cyclization reaction is a typical two-stage electrophilic addition to an alkene (Chapter 20) with attack by the nucleophile at the more substituted end of the intermediate halonium ion. The iodo-nium ring opening is a stercospecific Sjsj2 and, in the simplest cases where stereochemistry can be observed, the stereochemistry of the alkene will be reproduced in the product. 

Nucleophilic attack of X opens the ring of the halonium ion, forming a new C-X bond and relieving the strain in the three-membered ring. 

The ring-opening of bridged halonium ion intermediates resembles the opening of epoxide rings with nucleophiles discussed in Section 9.15. 

This result is reminiscent of the opening of epoxide rings with acids HZ (Z = a nucleophile), which we encountered in Section 9.15B. As in the opening of an epoxide ring, nucleophilic attack occurs at the more substituted carbon end of the bridged halonium ion because that carbon is better able to accommodate a partial positive charge in the transition state. 

Not only ring halonium ions, but also a series of open chain dialkylhalonium ions were obtained in our work with DeMember150 such as the following... 

The diastereoselectivity that arises from the intermediacy of a halonium ion has been exploited in a new type of reaction that produces lactone products. When 322 was treated with iodine and sodium bicarbonate, the initially formed iodonium ion (322) was opened by the carboxylate anion, generated by the reaction of bicarbonate with the acid moiety.Backside displacement of the iodonium ion led to the trans-relationship between the lactone ring and the iodide moiety in iodolactone 324. This reaction is commonly referred to as iodolactonization.251 This reaction can be extended to include bromolactonization and chlorolactonization. 

The mechanisms for addition of CI2 and I2 to alkenes are similar to that for Br2, involving formation and ring opening of their respective halonium ions. 

The first step is the same as that for halogen addition. In the second step, however, the two mechanisms differ. In halohydrin formation, water acts as the nucleophile and attacks one carbon atom of the halonium ion. The three-membered ring opens, and a protonated halohydrin is produced. Loss of a proton then leads to the formation of the halohydrin itself. 

There has also been one report of an enantioselective iodoetherification reaction, which was catalyzed by the Na" " salt of a chiral phosphoric acid organocatalyst (Scheme 13.37) [63]. This transformation was unique in the sense that, in contrast to many other halocyclization reactions, the carbon-Todine bond-forming step was not stereodetermining. Rather, carbon-iodine bond-formation generated meso halonium ions, and it was the ring-opening of these achiral intermediates that was stereodetermining. 

The spiro mode is also favored in the ring opening of cyclic halonium ions. Thus the kinetically controlled products of the reactions of the 3-butenoate ion with bromine and iodine under carefully controlled conditions are j -lactones, which are presumably formed from cyclic bromonium and iodonium ions [Eq. (6)]. ... 

As the accompanying electrostatic potential maps also show, the carbon-halogen bond to the more substituted carbon of the halonium ion is longer than the bond to the less substituted carbon. This difference in bond lengths in the cyclic intermediate state means that the ring-opening transition state can be reached more easily by attack at the more substituted carbon. 

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