Diaryl iodonium salts

While electron beams can produce cations, they are not effective at producing cationic cure in the absence of suitable photoinitiators. The same cationic photoinitiators used for UV cure are often also e-beam sensitive. Examples are triaryl sulfonium or diaryl iodonium salts [41]. 

Low yields of C-arylated ketones have also been obtained by reaction of pyrrolidine enamines with diaryl iodonium salts (68). 

Crivello, J. V., Lockhart, T. P., and Lee, J. L., Diaryl-iodonium salts as thermal initiators of cationic polymerization,... 

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. 

This chapter includes diaryl iodonium salts and some related heteroaromatic and heterocyclic analogues other categories are discussed separately in Chapter 9. Arylation is the dominating reaction of diaryl iodonium salts. From the applied point of view, they are useful in photopolymerizations and for the chemical amplification in imaging systems. Also, they show interesting biological activity [1,2]. 

A large number of methods are available for the preparation of diaryl iodonium salts (hereafter simply iodonium salts). Among them, there are some of general applicability or improved efficiency. Examples from each major category are given. 

Other unsaturated substrates arylated by various diaryl iodonium salts included butenone, acrylic acid, methyl acrylate and acrylonitrile [46]. Allyl alcohols with diaryliodonium bromides and palladium catalysis were arylated with concomitant oxidation for example, from oc-methylallyl alcohol, aldehydes of the general formula ArCH2CH(Me)CHO were formed [47]. Copper acetylide [48] and phenyl-acetylene [49] were also arylated, with palladium catalysis. 

Diaryliodonium salts. lodosoarenes are useful for the preparation of diaryl-iodonium salts. One method involves Friedel-Crafts type condensation with an aromatic compound in the presence of acids. For example, a cold solution of 5 g. 

Suzuki coupling, e.g., I(OH)Ots [186] I > Br or OTF. Diaryl iodonium salts were also reported to couple with various staimanes at r.t. catalyzed by Cul, and therefore have reactivity similar to other formal iodine(I) species [30]. Several extremely effective catalysts are air stable Pd2dba3-CHCl3, Pd(OAc)2, and Pd(PhCN)2Cl2 in conjunction with AsPh3 [4, 9, 10, 75, 187, 188, 189] other co-catalysts, such as trifurylphosphine PPh3 in the absence of Cu salts, were much less effective [75,... 

A reaction using diaryl iodonium salts is thought to proceed via a Pd(ll)-Pd(lV) cycle (simple halides are not sufficiently reactive to carry out oxidative addition on Pd(ll)). ... 

There are two general ways in which to carry out this process (i) reaction with an aryl halide using a Pd, Cu or Ni catalyst, (ii) reaction with an aryl boronic acid catalysed by Cu(II). Minor methods include reactions with diaryl iodonium salts or high oxidation state aryl metals. 

A method of limited scope but involving mild conditions, introduced by Doskotch [161] and used also by us successfully [161] is the reaction of a phenolate anion with a diaryl iodonium salt (Scheme 59). ... 

Arylphosphonic acid esters are produced in the reaction of diaryl iodonium salts with trialkyl phosphites in the presence of copper salts, but despite the simplicity of the procedure yields are low owing to side reactions. The cyclic bisarylphosphinic acid (66) is conveniently prepared by fusion of the phosphine oxide (67) with sodium hydroxide. A successful... 

Research has also been devoted to cationic two-photon photopolymerization using conventional initiator systems such as isopropylthioxanthone (ITX)/diaryl iodonium salt, with ITX serving as the photo sensitizer [45, 46]. Mode-locked operated Ti sapphire laser systems emitting femtosecond light pulses at 600, 710, or 795 nm were employed in these studies. 

In palladium-catalyzed carbonylations, aryl triflates are used regularly as substrates [145-151], while arene diazonium salts [152-156] and diaryl iodonium salts [157-162] are less commonly applied. 

Three reviews have detailed progress in the formation of biaryl systems using metal-catalysed substitutions of carbon—hydrogen bonds. The preferential arylation at the para-position of phenol and aniline derivatives with diaryl iodonium salts has been achieved using copper catalysis. Under similar reaction conditions, a-arylacetamides are selectively arylated at the meta-position. A mechanistic study, including DFT calculations, suggests that the meta-selectivity in the copper-catalysed arylation of anilides derives from a Heck-like four-membered transition state involving a Cu(III)-phenyl species (47). 

A diaryl iodonium salt can be depicted as Ar2l X in which X is a counterion such as PFg , BF, AsF , etc. Its photolysis proceeds as follows ... 

Other examples include 4-bromosydnones,< ) diaryl iodonium salts and bromonaphthalenes.< > The first wave in the polaro-graphic curve of 4-bromosydnones corresponds to the reduction of the C-Br bond, while the second is practically identical to the wave of the parent sydnone. The first two waves of diaryl iodonium... 

Lithium acyltetracarbonylferrate, Li[(RCO)Fe(CO)4], reacts with diaryl-iodonium salts in THF-pentane to give the corresponding ketone in high yield (80—90%) [equation (13)], The products are readily separated by chromatography, but in the absence of pentane the yields of ketone are much lower. 

Few photoinitiators are available for cationic systems. The most widely used are diaryl iodonium salts such as diaryliodonium hexafluoro-antimonate, triaryl salts such as triphenyl sulfonium hexafluoro-phosphate, and mixed triphenyl sulfonium salts. These photoinitiators are decomposed by UV light by a homolytic cleavage to produce a radical anion and a radical cation. The latter abstracts hydrogen from surrounding molecules and generates a proton, which is the initiating species ... 

When used with diphenyliodonium salts in the presence of a metal and an organocatalyst the enantioselective introduction of a phenyl group at the a-position of aldehydes can be accompUshed [42]. This transformation can be extended to a more general concept of enantioselective a-arylation of aldehydes by deployment of different substituted diaryl-iodonium salts. In these reactions CuBr was used as Lewis acid (Scheme 4.12). Based on these findings the authors were able to develop a rapid asymmetric synthesis of the anti-inflammatory drug (S)-ketoprofen. 

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