Reactivity vinyl iodonium salts

General reactivities of vinyl iodonium salts are summarized, and reactions of cyclohexenyl, 1-alkenyl, styryl, and 2,2-disubstituted vinyl iodonium salts are discussed in relation to possible formation of vinyl cation intermediates. Primary vinyl cation cannot be generated thermally but rearrangement via neighboring group participation often occurs. Photosolvolysis to give primary vinyl cation is also discussed. 

Vinyl iodonium salts 1 are reactive due to the high leaving ability of the positive iodine group, and the stability is inversely dependent on the stability of the carbocation 2 formed upon departure of the iodonio group (eq 1). ... 

In conclusion, vinyl iodonium salts are excellent pregenitors for vinylic cations due to high nucleofugality of the iodonio group (about 106-fold of triflate), although they are in equilibrium with the less reactive hypervalent... 

Furthermore, arylthiophenes have been prepared using the Stille coupling of hypervalent iodonium salts [96] or organolead compounds [97, 98] as electrophiles in place of aryl or vinyl halides and triflates. Hypervalent iodonium salts are sufficiently reactive to undergo coupling at room temperature. 

Primary vinyl iodonium compounds (1, R3 = H) are readily prepared as stable salts, but secondary analogs (1, R3 = alkyl) cannot be isolated without the presence of electron-accepting moieties. Cyclopent-l-enyl iodonium salt is very stable and poorly reactive toward bases and nucleophiles, while cyclohept-l-enyl... 

The styryl iodonium salt (7 a) reacts slowly with acetic acid to give the E and Z isomeric products (8a) and (9a) in the ratio 85 15.3 The decenyl system (7b) is much more reactive and gives only the inversion product, the Z isomer (9b). It was suggested that the styryl system reacts in a two-step mechanism via the vinylene phenonium ion (10), whereas the decenyl system follows a one-step vinylic. S n2 mechanism, hi contrast, 2-bromo-l-decenyliodonium salt reacts with bromide ion in MeCN with complete retention.4 The observed rate constants show an unexpected dependence on [Br ], in accordance with a mechanism involving ligand coupling within a bromoiodane intermediate. 

Alkenyl(phenyl)iodonium salts are highly reactive in vinylic nucleophilic substitution reactions because of the excellent leaving group ability of the phenyliodonium moiety. Only a few examples of non-catalytic alkenylation of carbon nucleophiles are known [50,51]. In most cases these reactions proceed with predominant retention of configuration via the addition-elimination mechanism or ligand coupling on the iodine [42,50]. 

In systems 1-4 radical polymerization takes place. Decomposition of the photoinitiator (e.g., benzoin and its derivatives) leads to formation of free radicals which react with the carbon-carbon double bonds. In system 5 acidic reactive species are formed which react with cycloaliphatic epoxy compounds and vinyl ethers to form a cross-linked network. Sulfonium, and iodonium salts are used to initiate this cationic polymerization, a typical example is PhjS PFg. ... 

Palladium(II)-copper(I) cocatalyzed couplings of several (/ ,/ -dialkylvinyl)phenyliodo-nium triflates with 4-isopropoxy-3-tri- -butylstannyl-3-cyclobutene-l,2-dione (71) in DMF at room temperature have recently been described (equation 259)157. As observed in earlier studies of Cu(I)- or Pd(II)-promoted reactions of vinyliodonium salts, the vinyl ligands are introduced with retention of configuration. Since iodobenzene is a byproduct of ligand coupling, the production of 3-phenyl-4-isopropoxy-3-cyclobutene-1,2-dione might also be expected. However, the (/ ,/ -dialkylvinyl)iodonium ions are much more reactive... 

Vinyl ether and epoxy chemistry established more than 50 years ago finally found its way into radiation curing technology. These compounds have been demonstrated to be reactive monomers in curing processes photoinitiated by iodonium and sulfonium salts. In particular, vinyl ethers are known to be among the most reactive monomers in photocuring chemistry. 

Heck-type reactions with enol carboxylates (e.g., vinyl acetate) are generally complex. Most common are reactions in which vinyl acetate is employed as an ethylene equivalent (see Scheme 24). However, an example of a preparatively useful reaction with an intact acetate function is given in entry 44.The reaction of vinyl triflates with vinyl phosphonates affords the corresponding conjugate dienylphosphonates (entry 45).f A new access to reactive nonaflates via a one-pot nonaflation-Heck reaction was recently reported (entry 46). " This reaction sequence starts from silyl enol ethers and provides functionalized 1,3-dienes in a simple manner, lodonium salts can be used as RPd precursors (entry 47). It is notable that the palladium(O) insertion preferentially occurs inbetween the iodonium ion and the vinylic, rather than the arylic sp -hybridized carbon (entry 47). Some years ago, Jeffery used acetylenic halides to achieve (JiJ-enynoates and (Ji)-enynones in fair yields at room temperature (entry 48). ... 

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