Alkyl iodonium salts

Alkynyliodonium ions, 1 and 2, are hypervalent iodine species in which one or two alkynyl ligands are bound to a positively charged iodine(III) atom. They are sensitive to nucleophiles, especially at the /1-carbon atom(s) of the alkynyl ligand(s), and for that reason, the isolation of stable alkynyliodonium salts generally requires the incorporation of nucleofugic anions. A list of known alkynyliodonium compounds (i.e. as of 4/1/94), containing 134 iodonium salts derived from 103 iodonium ions, and references (5-45) to their preparation and characterization are presented in Table 1. Among these compounds, alkynyl(phenyl)iodonium sulfonates and tetrafluoroborates are the most common, while alkynyl(alkyl)iodonium salts of any kind are unknown. 

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... 

Although formation of primary vinyl cation was disproved by the chirality probe approach, a vinyl cationic intermediate can be generated from a primary substrate via participation if a more stable cation could result. Unsymmetrically substituted 2,2-dialkylvinyl iodonium salt 24 gave mainly rearranged products on solvolysis.15 The products involve those of the 1,2-shift of either of the alkyl groups on the p position (Scheme 4). Those formed from migration of the alkyl... 

The unique reactivity pattern of alkynyl iodonium salts discussed in Sections II,A.2 and II,D,la can also serve as two-carbon conjunctive reagents in the synthesis of pyrroles, dihydropyrroles, and indoles. Feldman et al. found that combination of alkyl or aralkyl tosylamide anions 101 with phenyl(propynyl)iodonium triflate (102) furnishes the corresponding dihydropyrroles 103 (95JOC7722) (Scheme 28). 

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]. 

Simple alkyl(phenyl)iodonium salts are not stable. However, when there is in a-position the bulky, electron-withdrawing phenylsulfonyl group they are stabilized, like their precursors, i.e. ArS02CH2I(00CCF3)2. Iodanes of this type were coupled with benzene in presence of CF3S020Tf to afford isolable ArS02CH2I+Ph TfO [142]. 

The use of hypervalent iodine reagents in carbon-carbon bond forming reactions is summarized with particular emphasis on applications in organic synthesis. The most important recent methods involve the radical decarboxylative alkylation of organic substrates with [bis(acyloxy)iodo]arenes, spirocyclization of para- and ortho-substituted phenols, the intramolecular oxidative coupling of phenol ethers, and the reactions of iodonium salts and ylides. A significant recent research activity is centered in the area of the transition metal-mediated coupling reactions of the alkenyl-, aryl-, and alkynyliodonium salts. 

Iodonium salts 46 have been proposed as the reactive intermediates in several synthetically useful carbon-carbon bond forming reactions [1,40]. Reactions of adducts 46 with various silyl enol ethers selectively afford 1,4-diones, while the reactions with alkenes lead to the products of alkylation at the allylic position (Scheme 22) [40]. 

Iodonium salts 49 and 50 are efficient electrophilic alkylating reagents towards a variety of organic nucleophiles, including silyl enol ethers. The reaction with silyl enol ether 51 proceeds under mild conditions and selectively affords the appropriate product of alkylation 52 along with iodobenzene as the by-product (Scheme 24) [41]. 

An endo-exo intramolecular cyclization of a,P-diynylsulfides is brought about by iodonium salts and leads to fused 2H-1-benzothiopyrans (Scheme 118). The reaction is successful with 1,5-, 1,6-, and 1,7 diynes and a heteroatom can be accommodated in the alkyl chain. Furthermore, a substituent can be present at the other terminal site and the iodine which is perforce introduced into the fused alicyclic ring allows elaboration by Pd-catalyzed coupling with a variety of acetylenes <1998AGE3136>. 

Reductive coupling of iodonium salts catalysed by a palladium-zinc system also produced biaryls in good yield [38]. Also very effective was the palladium-catalysed cross-coupling of iodonium salts with sodium tetraphenylborate in water [39]. The reaction of 3-indolyl phenyliodonium trifluoroacetate with several alkyl and aryl lithium reagents gave 3-substituted indoles [40] ... 

Alkyl (or phenyl) lithium formed with iron pentacarbonyl in situ lithium acyl-tetracarbonyl ferrates, Li[RCOFe(CO)4], which with iodonium salts afforded aryl ketones (48-85%) [41]. 

Alkyl and aryl thiols afforded with perfluoroalkyl iodonium salts in the presence of pyridine the corresponding sulphides in good to excellent yields [25], Several sulphides and sulphoxides upon reaction with trifluoroethyl phenyliodonium triflate were converted into 2,2,2-trifluoroethyl sulphonium salts [26]. 

Appropriately substitued alkynyl iodonium salts afforded with nucleophiles cyclopentene derivatives. This annulation can be either [5 + 0], when all carbon atoms come from the alkyl chain of the alkynyl moiety, or a [2 + 3] process, in which three carbon atoms come from the nucleophile. Competition between [5 + 0] and [2 + 3] annulation may occur in some cases. 

Other pathways, with trimethylsilyl azide and the appropriate iodonium salts, involved the formation of 3-alkyl-1-azido-cyclopentenes (Table 9.3.) or Z-fi-azidoalkenyl iodonium salts [69], Several types of alkynyl iodonium salts afforded with lithium diphenylamide push-pull ynamines overcoming some inherent limitations of other methods. 

Finally, the participating alkyl chain for such insertion reactions need not be attached directly to vinyl carbon. Thus, exposure of the [/2-(f2-butylsulfonyl)vinyl]iodonium salt shown in equation 236 to triethylamine gives a cyclic sulfone generated via carbenic insertion into the -butyl group32. 

As evidenced by the treatment of three (Z)-(/l-arylsulfonyl-l-decenyl)phenyliodonium fluoroborates with triethylamine, the presence of electronically diverse substituents (H,p-N02, / -MeO) in the arylsulfonyl groups appears to have little impact on their migratory aptitudes (equation 239)32. Indeed, among six [(/1-alkyl-/l-arylsulfonyl)vinyl]iodonium salts that have been investigated, the cyclopentene/alkyne ratios show little variability (i.e. 70 30 to 80 20)32. Finally, the presence of an appropriately placed alcohol function in the side chain of a (/l-sulfonylvinyl)iodonium salt has been shown to completely suppress migration in favor of insertion (equation 240)32. 

So far, the most frequently used onium salts are aryldiazonium, diaryl iodonium, triarylsulfonium, and tetra alkyl phosphonium salts with nonnucleophillic counter ion (Chart 11.1). 

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." ... 

As discussed in the previous sections, the thermal solvolysis reactions of alkenyl iodonium salts usually occur via trans j8-phenyl- or trans j8-alkyl-assisted departure of the iodobenzene leaving group, by in-plane vinylic Sn2 substitution or by ligand... 

More complex nucleophiles can be reacted with alkynyl(phenyl)iodonium salts as well. For example, protected aminomalonates (50) and 18 give the corresponding alkynylmalonates 51 in 30-90% isolated yields (equation 19). Likewise, a variety of dicarbonyl enolates 52-54 react with 55 to give alkynyl products 56-58 (equations 20-22). These reactions may be looked upon as alkynylations or, in other words, the triple-bond analogs of the well-established alkylation reactions. Unfortunately, they only work with soft nucleophiles such as 52-54 (OTs, PhC02 PhsP, etc.). Nucleophiles such as RO, or simple enolates, do not seem to work. 

This material is produced via synthesis derived from early work by Berlnger et. al (10.11). The key to solubility of the ionic material lies in the nature of the dodecylbenzene used. Linear alkylate (detergent alkylate) dodecylbenzene is a mixture of at least two dozen isomers of several distinct compositions normally ranging from CgHj Ph to Cj H2gPh. The bis (dodecylphenyl) iodonium salt derived from this mixture therefore includes over 400 separate compounds, so that the catalyst behaves like a supercooled fluid due to the freezing point depression phenomenon, and can therefore be dispersed in relatively nonpolar epoxysilicone media. (This catalyst remains immiscible in non-functional dimethylsilicones, however). 

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