Iodonium Halide

The 127I NQR spectra of an extensive series of iodonium compounds, [R2I]+BF, have been reported52,53. In addition to the fact that iodine is the only halogen that forms an extensive series of halonium compounds, the study of 127I has the advantage that, since its spin is equal to 5/2, there are two transitions and therefore both the coupling constant and the asymmetry parameter may be obtained. This is particularly important for a polycoordinated nucleus, since we may anticipate that the asymmetry parameter will be far from negligible. A selection of the NQR parameters for these compounds is shown in Table 9. Also studied were the iodonium halides, whose structures differ from those of the... 

So far, this method has been employed for the synthesis of (1) (Z)-(jS-sulfonylvinyl)-iodonium tetrafluoroborates from alkynyliodonium tetrafluoroborates with sulfinic acids in methanol (equation 46)32,84, (2) (Z )-(/ -azidovinyl)iodonium tetrafluoroborates from alkynyliodonium tetrafluoroborates with Me3SiN3, H20 (i.e. HN3) in dichloromethane (equation 110)102 and (3) (Z )-/ -bromo- and (jS-chlorovinyl)iodonium halides from alkynyliodonium tetrafluoroborates with lithium halides in acetic acid or with HX in methanol (equation 111 and 112)103. 

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. 

Other pseudo-halides are used for carbonylation. Phenyl tluorosulfonate (484) can be carbonylated to give benzoate[337]. Aryl(aryl)iodonium salts[338], aryl(alkenyl)iodonium salts (485)[339], and arylialkynyl)iodonium salts (486)[340] are reactive compounds and undergo carbonylation under mild conditions (room temperature, 1 atm) to give aryl, alkenyl, and alkynyl esters. lodoxybenzene (487) is carbonylated under mild conditions in... 

Tributylstannyl)-3-cyclobutene-1,2-diones and 4-methyl-3-(tributylstan-nyl)-3-cyclobutene-l,2-dione 2-ethylene acetals undergo the palladium/copper-catalyzed cross coupling with acyl halides, and palladium-catalyzed carbon-ylative cross coupling with aryl/heteroaryl iodides [45]. The coupling reaction of alkenyl (phenyl )iodonium triflates is also performed by a palladium/copper catalyst [46],... 

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. 

Reactions of (ii)-l-decenyl(phenyl)iodonium salt (6a) with halide ions have been examined under various conditions. The products are those of substitution and elimination, usually (Z)-l-halodec-l-ene (6b) and dec-l-yne (6c), as well as iodobenzene (6d), but F gives exclusively elimination. In kinetic studies of secondary kinetic isotope effects, leaving-group substituent effects, and pressure effects on the rate, the results are compatible with the in-plane vinylic mechanism for substitution with inversion. The reactions of four ( )-jS-alkylvinyl(phenyl)iodonium salts with CP in MeCN and other solvents at 25 °C have been examined. Substitution with inversion is usually in competition with elimination to form the alk-l-yne. 

In order to strengthen evidence in favour of the proposition that concerted inplane 5n2 displacement reactions can occur at vinylic carbon the kinetics of reactions of some /3-alkyl-substituted vinyliodonium salts (17) with chloride ion have been studied. Substitution and elimination reactions with formation of (21) and (22), respectively, compete following initial formation of a chloro-A, -iodane reaction intermediate (18). Both (17) and (18) undergo bimolecular substitution by chloride ion while (18) also undergoes a unimolecular (intramolecular) jS-elimination of iodoben-zene and HCl. The [21]/[22] ratios for reactions of (18a-b) increase with halide ion concentration, and there is no evidence for formation of the -isomer of (Z)-alkene (21) iodonium ion (17d) forms only the products of elimination, (22d) and (23). 

Furo[2,3- ]pyridines can be synthesized from alkynylpyridones and iodonium sources (Scheme 31) <20060L1113>. Iodine proved to be much more effective at promoting the iodocyclization reaction than other iodonium sources (ICl, A -iodosuccinimide (NIS)). The pyridinium triiodide salt, 104, can be converted into the corresponding pyridinone by treatment with an external source of iodide. In a variation of the reaction, a one-pot synthesis of the furopyridine derivatives 105 can be achieved, with overall yields of 79-92%, by treatment with iodine followed by sodium iodide without isolation of the triiodide salt. Another similar one-pot synthesis involves 3-iodo-2-pyridones, terminal alkynes, and organic halides in a series of two palladium cross-coupling reactions (Equation 45) <20030L2441>. This reaction could also be carried out in a two-step sequence, but the overall reaction yields were typically improved for the one-pot method. 

Thienylstannanes were also coupled with a series of reagents other, than conventional aryl halides. 2,2 -Bithiophene was prepared from the thienyliodonium salt shown in 6.34. and 2-tributylstannylthiophene. The activity of the iodonium salt allows for the room temperature coupling of the reagents in the presence of only 0.5 mol% palladium dichloride in a coordinating solvent mixture to give an excellent yield of the desired product.48... 

Silyl ethers of aliphatic alcohols are inert towards strong bases, oxidants (ozone [81], Dess-Martin periodinane [605], iodonium salts [610,611], sulfur trioxide-pyridine complex [398]), and weak acids (e.g., 1 mol/L HC02H in DCM [605]), but can be selectively cleaved by treatment with HF in pyridine or with TBAF (Table 3.32). Phenols can also be linked to insoluble supports as silyl ethers, but these are less stable than alkyl silyl ethers and can even be cleaved by treatment with acyl halides under basic reaction conditions [595], Silyl ether attachment has been successfully used for the solid-phase synthesis of oligosaccharides [600,601,612,613] and peptides [614]. 

Iodonium tetrafluoroborate, biphenylene-X-ray structures, 1, 566 Iodopropenylation alkyl halides, 1, 469 Ionenes uses, 1, 289 Ion exchange resins pyridine polymers and, 1, 308-309 Ionization potentials pyridines, 2, 135 cts- 3-Ionol synthesis, 3, 666 cis-/3-Ionone... 

Detailed studies on the UV absorption spectra of alkenyl-A3-iodane 133 (X = Cl,Br,I) in acetonitrile solutions in the presence of tetrabutylammonium halides show the equilibrium formation of the iodate 134 (Sect. 2.1), in addition to the iodonium ion 132 coordinated by acetonitrile via hypervalent bonding. [Eq. (116)] [213]. The equilibrium constants are summarized in Table 5. Determinations of the equilibrium constants by UV spectra were carried out at a low concentration of the A3-iodane (<10 4 M), without considering the formation of the corresponding dimers. The magnitudes of the equilibrium constant K1 clearly decrease in the order Cl >Br >1, which reflects the differences in the stability of alkenyl-A3-iodane 133. 

Studies of the conversion of ( )-jS-alkylvinyl(phenyl)iodonium tetrafluorob-orates to (Z)-haloalkenes (complete inversion) with tetrabutylammonium halides (Cl", Br", I") in acetonitrile provide convincing evidence for the vinylic... 

In a rare example of the use of iodonium salts for heteroatom-heteroatom bond formation, diaryliodonium halides were employed with sodium 0,0-diethyl phosphoroselenolate for a one-pot synthesis of diaryl diselenides (Scheme 9) [27]. These transformations probably occur via arylation of the phosphoroselenolate salt with the diaryliodonium ions, hydrolysis of the resulting aryl phosphoroselenolates with sodium hydroxide, and air oxidation of the arene-selenide ions thus produced. 

For Pd-catalyzed cross-coupling reactions the organopalladium complex is generated from an organic electrophile RX and a Pd(0) complex in the presence of a carbon nucleophile. Not only organic halides but also sulfonium salts [38], iodonium salts [39], diazonium salts [40], or thiol esters (to yield acylpalladium complexes) [41] can be used as electrophiles. With allylic electrophiles (allyl halides, esters, or carbonates, or strained allylic ethers and related compounds) Pd-i73-jt-allyl complexes are formed these react as soft, electrophilic allylating reagents. 

Iodonium ylides 715 undergo rhodium-catalyzed reactions with acyl, phenyl, or benzyl halides to form 3-halo-coumarins in good yield (Equation 284) <2002J(P1)1309>. 

The side chain R in these enals contained functionalities such as a double bond, or an acetoxy group, which were unaffected. The reaction proceeds again through a vinyl iodonium intermediate which serves as a reactive allyl cation. In this way the umpolung of allylsilanes is achieved and, indirectly, the reactivity of allyl halides is considerably increased. 

Iodonium salts readily transfer one of their aryl groups to a heteroatom substrates successfully arylated range from simple halide anions to complex natural products. The plethora of such reactions leaves no doubt that the use of iodonium salts is the best choice for arylations. 

The thermolysis of iodonium salts in which their counteranion is a halide may be performed in the molten state or in solution the products are an iodoarene and a haloarene. The reaction which is a nucleophilic aromatic substitution is, however, not preparatively useful an exception was 3-indolyl phenyliodonium trifluoroace-tate which on heating with various chlorides and bromides in DMSO afforded variable mixtures of 2- and 3-haloindoles. By contrast, the jV-methyl and N-benzyl analogues gave only 2-chloro derivatives [58], Sometimes useful products may be obtained from the thermolysis of dibenziodolium or other heterocyclic salts, as exemplified in the preparation of l-iodo-2-(2-iodophenyl)naphthalene [59] ... 

The substitution of alkenyl iodonium salts by halides, using tetrabutylammonium salts, has been studied (Table 9.2). Exclusive inversion of configuration occurred in acetonitrile, so that -precursors gave solely Z-haloalkenes in high yield [35]. In marked contrast, complete retention occurred with cuprous and potassium halides in dichloromethane. Retention of configuration was also noted in reactions of / -substituted alkenyl iodonium salts for example, from /J-phenylsulphonyl decenyl phenyliodonium ion, cis products were formed exclusively in high yield [34],... 

In some ylides photolytic conditions were necessary for their transylidation [30]. The conversion of iodonium ylides into a-halogeno derivatives of the parent carbonyl compound (or other precursor) with hydrogen halides is normally effected directly, without isolation of their iodonium salts. A similar reaction with halogens leads to the formation of a,a-bis halogenated products [31]. The reaction of pyridines with the non-isolable PhI=C(CN)2 is of interest, since it permits the ready transfer of the C(CN)2 functionality to the nitrogen of pyridine, quinoline, etc. the yields here were generally moderate but in some cases the products could not be obtained using other dicyanocarbene precursors [32],... 

The photochemistry of diphenyl- and bis(4-methylphenyl)iodonium salts has been investigated481,482. Diphenyliodonium halides (140, X = Cl, Br, I) exist as tight ion pairs in acetonitrile. Their photolysis gives almost exclusively iodobenzene by a homolytic cleavage from a charge-transfer excited state. In aqueous acetonitrile, however, the ion pairs are solvent-separated and substantial amounts of 2-, 3- and 4-iodobiphenyls (141) are formed in addition to iodobenzene (142), benzene (143), acetanilide (144) and biphenyl (145) (equation 126). In this medium the photodecomposition occurs via initial heterolysis of the molecule in its excited state, leading to iodobenzene and phenyl cation. 

Since it is known that halo(phenyl)acetylenes add oxidatively to Vaska s complex to give (7-phenylethynyl iridium(III) halides, 32112, the intervention of phenyliodonium iridium(III) and rhodium(III) intermediates, 33, in the alkynyliodonium reactions seems plausible. In any case, the production of cr-alkynyl complexes with alkynyl(phenyl)-iodonium triflates appears to be both more general and efficient21. 

Vinyliodonium ions, 35 and 36, are hypervalent iodine species in which one or two alkenyl ligands are bound to a positively charged iodine(III) atom. Although they are reactive with nucleophilic reagents, they are less labile than alkynyliodonium ions, and stable halide salts of vinyliodonium ions can be prepared. The first vinyliodonium compounds [i.e. (a, / -dichlorovinyl)iodonium salts] were synthesized by the treatment of silver acetylide-silver chloride complexes with (dichloroiodo)arenes or l-(dichloroiodo)-2-chloroethene in the presence of water (equation 152). The early work was summarized by Willgerodt in 1914115. This is, of course, a limited and rather impractical synthetic method, and some time elapsed before the chemistry of vinyliodonium salts was developed. Contemporary synthetic approaches to vinyliodonium compounds include the treatment of (1) vinylsilanes and vinylstannanes with 23-iodanes, (2) terminal alkynes with x3-iodanes, (3) alkynyliodonium salts with nucleophilic reagents and (4) alkynyliodonium salts with dienes. 

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