Iodine cations

Cyclopentene derivatives with carboxylic acid side-chains can be stereoselectively hydroxy-lated by the iodolactonization procedure (E.J. Corey, 1969, 1970). To the trisubstituted cyclopentene described on p. 210 a large iodine cation is added stereoselectively to the less hindered -side of the 9,10 double bond. Lactone formation occurs on the intermediate iod-onium ion specifically at C-9ot. Later the iodine is reductively removed with tri-n-butyltin hydride. The cyclopentane ring now bears all oxygen and carbon substituents in the right stereochemistry, and the carbon chains can be built starting from the C-8 and C-12 substit""" ... 

The complex that is formed can dissociate to form a cation (n-tr-complex) and an iodide anion, with the iodide ion reacting with the excess iodine molecules that are present. In addition the decomposition of the n-cr-complex can lead to the formation of highly reactive iodine cations, which can initiate further reactions — e.g. oxidations or electrophilic substitutions of aromatic systems [11, 13]. 

In acidic media the /i-eriodide anions or periodide complexes these — like the iodide anion — are appreciably less reactive than the iodine cation [13]. 

Tertiary nitrogen and iodine initially form a /i-o-complex, from which a strongly reactive iodine cation is produced this cation can bring about electrophilic substitutions on aromatic systems or cause oxidations [2]. 

The iodine cation forms iodine monochloride (IC1) in a medium having sufficiently high concentration of HC1 and the latter is subsequently stabilized by complex ion formation. Thus, we have ... 

Solution of the blue iodine cation in oleum has been reinvestigated779 by conductometric, spectrophotometric, and cryoscopic methods confirming the formation of I2+. In 65% oleum, iodine is oxidized to I2+ [Eq. (4.192)]. 

Besida and O Donnell791 have studied the existence and stabilization of iodine cations in hydrogen fluoride using Lewis acids (NbF5, TaF5, SbF5) and NaF to control acidity and basicity. Excess F causes disproportionation of the cations I5+, I3+, and I2+ to I2 and IF5. When these products are dissolved in HF and the acidity level is adjusted appropriately, the individual cations can be generated. 

If cyclopentadienyliron dicarbonyl halides are allowed to react with themselves in the presence of Lewis acids, cations are formed in which the new substituent is the cyclopentadienyliron dicarbonyl halide itself, for example, [CjH6Fe(CO)i-X-(CO)2-FeC8H8]+ (X = Cl, Br, I). All three cations can be prepared best by treatment of the corresponding halides with boron trifluoride diethyl etherate all are isolated as tetrafluoro-borates.18 The bromine complex can also be obtained by a more complicated procedure by the reaction between C8H8Fe-(CO)2Br and AlBr3 in liquid sulfur dioxide 16 the iodine cation can be isolated from a melt of cyclopentadienyliron dicarbonyl iodide and aluminum chloride.17 In the latter two cases the hexafluorophosphate salts can be obtained. These binuclear cations are of special interest, because they are cleaved by electron donors,16-17 e.g., aniline, pyridine, benzonitrile, acetonitrile, acrylonitrile, with the formation of the corresponding [C8H8Fe(CO)2L]+ cations and the parent halide. Equations for preparation of the tetrafluoroborate are ... 

Preliminary step Formation of the electrophile, I+ (the iodine cation). 

Iodination probably involves an electrophilic aromatic substitution with the iodine cation (I+) acting as the electrophile. The iodine cation results from oxidation of iodine by nitric acid. 

Dependence of formation of iodine cations on the level of acidity of hydrogen fluoride"... 

The usual reversibility of disproportionation reactions with increase in acidity provides an effective general route to synthesis of cations in low or fractional oxidation states, as was shown for preparation of Au"+ in Sec. 11.3.4,3 and for iodine cations in Sec. 11.3.4.4. 

The existence of Is" " and Ig was deduced over 30 years ago by Masson (I) from his studies of aromatic iodination reactions, but it is only recently that his conclusions have been confirmed by physical measurements. The controversy over the nature of the blue solutions of iodine in various highly acidic media has now been resolved, and it has been shown conclusively that these solutions contain Ig" (2-4) and not I+ as suggested earlier (5). There is, moreover, no convincing evidence for the existence of Cl or Br as stable species in solution or in the solid state. There is, however, evidence for polyatomic cations of chlorine and bromine analogous to the iodine cations, i.e., Clg", Brg, and Brg. ... 

The first evidence for the existence of a stable iodine cation was obtained by Masson (2) in 1938. He postulated the presence of Ig" and Ig in solutions of iodine and iodic acid in sulfuric acid in order to explain the stoichiometry of the reaction of such solutions with chlorobenzene to form both iodo and iodoso derivatives. Later, Symons and co-workers (6) gave conductometric evidence for I3 formed from iodic acid and iodine in 100% sulfuric acid and suggested that Ig" " may be formed on the basis of changes in the UV and visible spectra when iodine is added to la solutions. Gillespie and co-workers (7) on the basis of detailed conductometric and cryoscopic measurements confirmed that Ig is formed from HlOg and Ig in 100% sulfuric acid according to Eq. (1). The Ig cation may also be prepared in fluoro-... 

An analogous process has been developed for unbranched aldehydes which can be transformed into a-amino ketones when oxidized in the presence of an secondary amine and iodine, as the mediator, in aqueous terf-butanol. The actual reactive species is probably the enamine which is attacked by iodine cations and subsequently by water. Carbonyl transposition reaction releases iodine anions which can be anodically reoxidized [197]. 

Abel (36) has proposed a series of electron transfer reactions involving free radicals similar in principle to those suggested for the iodide-iodine kinetics. He suggests reaction through iodine cations formed for example in the equilibrium ... 

Cationic bis(carbene) adducts of the hypervalent iodine (132 and 133) may be prepared by reaction of the imidazolium salts (128) and (130) with carbene (equation 16). The H and C NMR spectra of (132) and (133) are identical, indicating that the anions do not play an important role in the bonding of the hypervalent iodine cation. X-ray diffraction of (133) showed that the central C-I-C unit (177.5°) is almost linear with slightly longer C-I bond distances than those observed in (128-130) and (133, C-I 2.286A and 2.363 A). The structure of (133) suggests that the C-I-C interaction in (133) is stronger than the C-T-T interactions in (128) and (129). 

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