Halonium ions preparation

Because the nature of the halogen atom can be varied, these salts show useful selectivity in their alkylation reaetions. We also prepared other halonium ions and studied their alkylating ability. 

A number of intermediates of halogen participation (halonium ions),88 e g., 19 and 20, have been prepared as stable salts in SbF5-S02 or SbFs-SC ClF solutions.89 Some have even... 

Dialkylhalonium Ions. Dialkylhalonium ions were first observed as stable fluoroantimonates and characterized by Olah and DeMember333 in 1969. Since then, a large number of unsymmetrical and symmetrical halonium ions have been prepared. The alkylating ability as well as intermolecular exchange reactions of dialkylhalonium ions were also studied.107,352 355... 

Alkyl(aryl)halonium Ions. Dence and Roberts366 attempted to prepare the cyclopropylphenyliodonium ion from phenyliodoso chloride and cyclopropyllithium [Eq. (4.104)]. However, they were unable to obtain the corresponding iodonium ion or any cyclopropylbenzene from the reaction mixture. Thus, the iodonium ion was not formed, even as an unstable reaction intermediate. 

Alkyl(aryl)halonium ions (other than perfluorinated derivatives) were first prepared by Olah and Melby.368 When a S02 solution of iodobenzene was added to a S02 solution of the CH3F-SbF5 complex (methyl fluoroantimonate) at —78°C, a clear,... 

Similarly, a series of dialkylphenylenedihalonium ions such as 147-150 have been prepared and characterized.365 Even trihalonium ions such as 151 have been prepared. Many of the mentioned halonium ions are stable only below —20°C and above which they undergo ring alkylation. 

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. 

Similarly, a variety of dimethylethylene, trimethylethylene, and tetramethylethy-lene halonium ions have been prepared and studied by 1H and 13C NMR spectroscopy.99 386 389 Some of the representative examples are ions 163-166. 

The unsymmetrical halonium ions such as 165-Br are found in equilibrium with open-chain [3-bromocarbenium ion. Many of the above-mentioned halonium ions have been prepared by the protonation of the appropriate cyclopropyl halides in superacids.390... 

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)]. 

Heteroaromatic Halophenium Ions. Halophenium ions are a class of halonium ions analogous to thiophene, furan, and pyrrole. To date, no parent halophenium ions 189 are known, but many stabilized tetraphenyl iodophenium, benzoiodophenium, and dibenzoiodophenium ions have been prepared by Beringer and co-workers.419,420 Some of them have been analyzed by X-ray crystal structure investigations. Representative examples are ions 190-192. 

Halonium ions are an important class of onium ions.43 The dialkylchloro, bromo, and iodohalonium ions can be prepared and even isolated as stable salts (i.e., 46), as shown by Olah et al. by reacting an excess of haloalkane with strong Lewis acid halides in solvents of low nucleophilic-ity (eq 14). In superacid solution, dialkylhalonium ions show enhanced alkylating reactivity.44 It is considered that this enhanced reactivity is due to further protolytic (or electrophilic) activation involving the non-bonded... 

The regio- and stereo-chemical consequences of preparing an aziridine via IN3, or INCO, or the above chlorination procedure are, in so far as they have bwn investigated, analogous to those of proceeding via an epoxidation, for the sinqile reason that all these last three reaction types appear to proceed via cyclic halonium ions, isosteric with epoxides. 

Halonium ions have received a great deal of attention in the last decade and a wdiole book has already been devoted to these species The stability of dialkyl-halonium ions decreases in the order R2l > R2Br > R2C (thefluoroniumhomolo-gues have never been observed), and iodonium and bromcmium hexafluoroantimonates have been prepared and characterised in Olah s laboratory Under specific conditions these salts are active initiators of the cationic polymerisation of cdefins, and this still little-explored field will be briefly analysed in Chap. IV. 

The methoxymethyl cation can be obtained as a stable sohd, MeOCHj SbF, Carbocations containing either a, p, or y silicon atom are also stabilized, relative to similar ions without the silicon atom. In super acid solution, ions such as CX (X = Cl, Br, I) have been prepared. Vinyl-stabilized halonium ions are also known. ... 

---