Perfluoroalkyl iodonium salts

Alkyl and aryl thiols afforded with perfluoroalkyl iodonium salts in the presence of pyridine the corresponding sulphides in good to excellent yields [25], Several sulphides and sulphoxides upon reaction with trifluoroethyl phenyliodonium triflate were converted into 2,2,2-trifluoroethyl sulphonium salts [26]. 

Perfluoroalkyl iodonium salts are also available as bivalent perfluoroalkylation reagents enabling the introduction of bridging perfluoroalkylene groups between nucleophilic precursors in one synthetic step [22] (Scheme 2.150). 

A variety of perfluoroalkyl(phenyl)iodonium salts were obtained from precursors such as RFI(OH)OTf, RfI(OOCCF3)2 and others when coupled with benzene or arenes, as reported in a recent review article [143]. Similarly, a trifluo-roethyl group attached to iodine(III) can also be coupled to the phenyl ring upon reaction with PhSiMe3 as outlined in Scheme 47 [144]. 

Apart from the above two major general reaction pathways, there are some further possibilities for instance, [bis(trifluoroacetoxy)iodo]benzene reacts as an ambident electrophile and is attacked by hard nucleophiles at its carbonyl carbon, whereas iodylarenes may react similarly from carbon rather than iodine. Alkynyl iodonium salts are actually tetraphilic electrophiles, whereas iodosylbenzene reacts also as a nucleophile from oxygen. Diaryl iodonium salts serve as arylating reagents, mostly homolytically other iodonium salts transfer groups such as perfluoroalkyl, vinyl, alkynyl or cyano to several nucleophiles in various ways. 

Because of the multitude and diversity of iodonium salts and zwitterions, some of which are labile or are prepared for specific purposes, their preparative methods are discussed in Chapter 8, for diaryl iodonium salts and related compounds, Chapter 9 for perfluoroalkyl, alkenyl and alkynyl phenyliodonium salts and Chapter 10 for zwitterionic iodonium compounds. In addition, the preparation of some lesser known reagents, including 2-iodosylbenzoic acid, is given in Chapter 12. 

Phenyliodonium Salts with an Aliphatic Moiety Preparative Methods. Reactivity of Perfluoroalkyl Phenyliodonium Salts. Reactivity of Other Iodonium Salts. 

Attempts to prepare a (/ -fiuorovinyl)iodonium salt from l-decynyl(phenyl)iodonium tetrafluoroborate by the procedures employed for / -chloro- and (/ -bromovinyl)iodonium halides have been unsuccessful103. Admixture of the decynyliodonium salt and lithium fluoride in acetic acid ultimately (2 days, rt) leads to l-acetoxy-2-decanone103. (Z )-(/ -Fluoro-jS-perfluoroalkylvinyl)iodonium triflates, on the other hand, can be made by the treatment of (1H, li/-perfluoroalkyl)phenyliodonium triflates with sodium hydride (equation 178)136. Apart from 2-fluoro-l-hexadecenyl(phenyl)iodonium chloride (synthesis not described)104, these are the only reported examples of (/ -fluorovinyl)iodonium salts. 

Starting materials for the synthesis of all perfluoroalkyl iodonium reagents are the perfluoroalkyl iodides, which themselves play a central role as building blocks in fluoroorganic chemistry. The iodides are available either by pyrolysis of the silver salts of perfluoroalkyl carboxylic acids in the presence on iodine [14] or - on a more technical scale - by iodofluorination of tetrafluoroethylene [15] with the iodine-IFp system [16] and subsequent radical telomerization of tetrafluoroethylene with the resulting intermediate perfluoroalkyl iodides [17] (Scheme 2.143). 

A third type of derivatives of iodonium salts, the perfluoroalkyl phenyliodonium (dfluoromethane sulfonates (FITS), showed an enhanced reactivity compared to the chloride and the tetrafluoroborate salts. In a series of papers, Umemoto et al have extensively studied the synthesis of these compounds and their reactivity towards a wide range of more or less powerful nucleophiles. 206 Different mechanistic pathways have been demonstrated to occur in these reactions. Ligand coupling appears to be involved in a number of nucleophilic displacements implying various types of carbanions, thiols and other nucleophilic substrates. 

Fluoroalkyl(aryl)iodonium salts are the most stable and practically important class of alkyl(aryl)iodonium derivatives. The application of such salts as electrophilic fiuoroalkylating reagents was reviewed in 1996 by Umemoto [1017]. Perfluoroalkyl(phenyl)iodonium trifiates (FITS reagents) 764 are efficient perfluoroalky-lating reagents toward various nucleophiUe substrates, sueh as arenes, carbanions, alkynes, alkenes, carbonyl compounds, amines, phosphines and sulfides [1017]. Scheme 3.300 shows several representative examples of electrophilic perfluoroalkylations using FITS reagents. 

Electrophilic perfluoroalkylation has been performed with the use of iodonium salts RfI(Ar)X, where the perfluoroalkyl group is bonded with a positively charged heteroatom. The trifluoromethylation of thiophene 1 with CgFi7l(Ph)0S02CF3 in dichloromethane at room temperature proceeds in 73 % yield in the presence of 2,6-di-terf-butyl-4-methylpyiidine as a base [46]. 

Interestingly, the most basic of the iodonium reagents, aryl trifluoromethyl iodo-nium salts, are still unknown. They cannot be prepared by the methods outlined in Scheme 2.144. The presumed reason for this unexpected fact is the low stability of the carbon-iodine bond of their potential synthetic precursors CF3IF2 or CF3IO, compared with their analogs with two or more carbon atoms in the perfluoroalkyl chain [18]. 

Like perfluoroalkylation, electrophilic a,a-dihydroperfluoroalkylation can be efficiently performed by the use of the appropriate (a,a-dihydroperfluoroalkyl)phenyliodonium salts. A wide range of nucleophiles were alkylated under mild conditions. Most of the reactions can occur by a ligand coupling taking place on an intermediate trisubstituted iodane(UI) compound. This is the case of the reactions with carbanions, amines, alkoxides, phenoxides, thiolates and carboxylic acid ammonium salts. Dialkylation of aniline was also obtained by using two molar equivalents of the iodonium reagent.219,220 (Table 5.11)... 

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