Vinyl iodonium salts preparation

Vinyl iodonium salts, prepared by reaction of vinylsilanes with iodosylbenzene and triethyloxonium tetrafluoroborate, have been found to be excellent educts for nucleophilic substitution by a variety of nucleophiles. Reaction with KCu(CN)p in DMF was thus possible and acrylonitrile derivatives prepared accordingly. 

The focus of Chapters 3 and 4 is on vinyl cations. In Chapter 3, T. Muller et al. discuss the preparation, isolation, and characterization of unusually stable vinyl cations whereas Chapter 4 by T. Okuyama and M. Fujita describes the generation and reactions of vinyl cations formed via solvolysis of vinyl iodonium salts. 

Vinyl iodonium salt 40, which is highly effective as an activated species of vinyl iodide, could be synthesized from vinyl silanes 39 by the reaction with iodosyl benzene and triethyloxonium tetrafluoroborate. Thus, from vinyl silane, a,/3-unsaturated ester 41 could be synthesized by Pd-catalyzed carbonylation (Eq. 12). Alkynylphenyliodo-nium tosylates 42 were easily prepared from 1-alkynes with Phl(OH)OTs or by reaction... 

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... 

A hindered vinyl iodide, such as (47), was prepared in high yield by treatment of the iodonium salt (46) with the copper(I) iodide - potassium iodide system. 75... 

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." ... 

More recently, iodonium salts have been widely used as photoinitiators in the polymerization studies of various monomeric precursors, such as copolymerization of butyl vinyl ether and methyl methacrylate by combination of radical and radical promoted cationic mechanisms [22], thermal and photopolymerization of divinyl ethers [23], photopolymerization of vinyl ether networks using an iodonium initiator [24,25], dual photo- and thermally-initiated cationic polymerization of epoxy monomers [26], preparation and properties of elastomers based on a cycloaliphatic diepoxide and poly(tetrahydrofuran) [27], photoinduced crosslinking of divinyl ethers [28], cationic photopolymerization of l,2-epoxy-6-(9-carbazolyl)-4-oxahexane [29], preparation of interpenetrating polymer network hydrogels based on 2-hydroxyethyl methacrylate and N-vinyl-2-pyrrolidone [30], photopolymerization of unsaturated cyclic ethers [31] and many other works. 

Crivello and Lee have described the synthesis and characterization of a series of (4-alkoxyphenyl)phenyliodonium salts 7, which are excellent photo- and thermal-initiators for the cationic polymerization of vinyl and heterocyclic monomers [17]. Iodonium salts 7 are conveniently prepared by the reaction of alkoxyphenols 6 with [hydroxy(tosyloxy)iodo]benzene followed by anion exchange with sodium hexafluoroantimonate (Scheme 7.2). Products 7 have very good solubility and photoresponse characteristics, which make them especially attractive for use in UV curing applications. Compounds 7 with alkoxy chains of eight carbons and longer are essentially nontoxic, compared to diphenyliodonium hexafluoroantimonate, which has an oral LD50 of 40 mg kg (rats) [17]. 

In-plane, backside, vinylic nucleophilic substitution of an appropriate halide leaving group can also be effected. Thus, the ( )-P-alkylvinyl(phenyl)iodonium salt, which can be prepared from the corresponding trimethylsilyl alkene and iodoso-benzene, undergoes both Sn2 substitution to produce the inverted chloroalkene and elimination to the corresponding alkyne (Scheme 7.11). 

The only known alternative procedure for the preparation of alkynyl(phenyl)iodonium arylsulfonates, the latest member of the family of polyvalent Iodine compounds, involves the reaction of [hydroxy(tosyloxy)iodo]benzene. PhlOH-OTs, with terminal alkynes as first reported by Koser and elaborated by us. This procedure has a number of shortcomings. Formation of the desired alkynyliodonium salt is usually accompanied by a related vinyl species, R(TsO)C=CHIPh-OTs, that both decreases the yields and causes purification problems. Furthermore, when the alkyl substituent of the starting alkyne is small, such as CH3, n-Pr, n-Bu, etc., this procedure gives either no product or low yields at best. 

Initial methods of preparation of alkynyl(phenyl)iodonium compounds, as the tosylate salts (11), involved the interaction of terminal alkynes with [hydroxy-(tosyloxy)iodo]benzene (10) (equation 3). This method, however, suffers from lack of generality, low product yields, and separation problems from the concomitantly formed vinyl species (12) (equation 3). Recent modifications and improvements in this procedure have resulted in wider applicability and product yields of 60-90% of iodonium tosylates (11). 

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). ... 

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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