Haloniums reactions with

Table 1. Dissociation rate constants and partitioning ratios for reactions of various 6i5(pyridine)-halonium ions with acceptor olefins in CICH2CH2CI,...

Figure 5. Stylized representation of the reaction of a monoamine halonium intermediate with a l,co-alkenolproceeding to R and S halocyclizedproducts.

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). 

The ethyleneiodonium ion 158-1 has also been prepared by the direct iodination of ethylene using ICN-SbF5-S02ClF solution388 [Eq. (4.115)]. However, similar reaction with either BrCN or C1CN did not give the corresponding halonium ions 158-Br and 158-CI. 

In the last reaction, the pi electrons of an alkene attack the bromine molecule, expelling bromide ion. A bromonium ion results, containing a three-membered ring with a positive charge on the bromine atom. This bromonium ion is similar in structure to the mercurinium ion discussed in Section 8-5. Similar reactions with other halogens form other halonium ions. The structures of a chloronium ion, a bromonium ion, and an iodonium ion are shown next. 

The reaction with A -bromosuccinimide or Af-bromoacetamide with or without added acid, on the other hand, leads predominantly to the same epoxide trans-6 as formed in the reaction with peracids. It is proposed that in the formation of cis-6 with acetyl hypobromite, hypobromous acid or A -chlorosuccinimide, nucleophilic attack of the halonium ion is fast. The epoxide ratio therefore is governed by the kinetically favored tran.v-halohydrin. However, in the reaction with A -bromosuccinimide or A -bromoacetamide the nucleophilic attack on the bromonium ion species becomes rate determining, attack of the nucleophile therefore follows the path of lowest activation energy. This is the attack on the c/.v-bromonium ion distant to the bulky tert-butyl group leading to the trans-diaxial bromohydrin, according to the Fiirst-Plattner rule, hence giving trans-6. 

Polymer yields would be expected to depend on side-reactions of the propagating carbocation, and the formation of halonium ions with the solvent, CH3X, must be recognized as an important possibility [see reaction (6)]. Since the stability of these depends on the polarizability of X, CH3I should be most... 

Asymmetric halolactonization is a much used procedure for the stereoselective formation of C — Br and C —I bonds (see also Sections D.4.6. and D.7.2.). This intramolecular reaction is used to transfer chiral information in the molecule over three or four bonds, sometimes with high stereoselectivity. Formation of a halonium complex with the olefin is the first step of the reaction followed by intramolecular lactonization. Induction of asymmetry is achieved in different ways. 

We say more about the regioselectivity of opening of halonium ions in Section 5.8, where we compare halonium ions with other intermediates in electrophilic addition reactions. 

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