Stabilization vinyl iodonium salts

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). ... 

The kinetic stability of vinyl(phenyl)iodonium salts is largely due to the instability of the corresponding vinyl cations formed on departure of the iodobenzene nucleofuge. Thus, o -phenyl- or a-alkyl-substituted (secondary) vinyl iodonium ions so far could not be prepared as stable salts." ... 

Cyclohexenyliodonium salts are moderately stable due to a compromise between the stability of the secondary vinyl cation and the ring strain of the cyclic structure. Solvolysis of 4- -butyl-l-cyclohexenyl(phenyl)iodonium tetrafluoroborate (30) was investigated in various alcoholic and aqueous solutions. The main products are those expected for a cyclohexenyl cation intermediate (31), the enol ether (32) and/or cyclohexanone as well as iodobenzene (Scheme 25). 

DialkyIvinyliodonium salts have a better chance to afford primary vinyl cations than their /3-phenyl counterparts due to the stabilizing effect of j8-alkyl substituents discussed above. Solvolyis of (E)- and (Z)-2-methy 1-5-phenyl-1-pentenyl(phenyl)-iodonium tetrafluoroborate (50) gave a variety of extensively rearranged products, as illustrated in Scheme 32 and summarized in Table 12. ... 

Other Onium Salts and Organometallic Photoinitiators. The success of the iodonium and sulfonium salts as photoinitiators has led to the investigation of a number of analogous onium salts based on the halides and the Group VIA atoms however, these alternative initiators have not been widely used for various reasons. For example, chloronium and bromonium salts were prepared (57,58) and they were also found to function as cationic photoinitiators, but these salts are difficult to prepare and they have low thermal stability. Similarly, triarylselenon-ium salts have also been investigated and foimd to function as cationic initiators (59) however their preparation has been foimd to be expensive (60). Other onium salts such as phosphonium and arsonium salts, developed by Abu-Abdoun and co-workers for the photopolymerization ofp-methylstyrene and styrene, have also been reported (61-63) as successful cationic photoinitiators. Photopolymerization of carbazolyloxiranes with sulfonium and tropylium salts has been reported (64). Dialkylphenacyl sulfonium photoinitiator (65-67) has been reported with excellent solubility in both polar and nonpolar monomers. Pyridinium and isoquino-linium salts have also been reported and they were found useful for polymerizing both the epoxide and vinyl ether monomers (68). 

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