Aliphatic Iodides

3 Aliphatic Iodides. Aliphatic iodides give the strongest molecular ion peak of the aliphatic halides. Since iodine is monoisotopic, there is no distinctive isotope peak. The presence of an iodine atom can sometimes be deduced from isotope peaks that are suspiciously low in relation to the molecular ion peaks, and from several distinctive peaks in polyiodo compounds, the large interval between major peaks is characteristic. 

Iodides cleave much as do chlorides and bromides, but the C4H8I+ ion is not as evident as the corresponding chloride and bromide ions. 

3 Aliphatic Iodides Aliphatic iodides give the strongest molecular ion peak of the aliphatic halides. Since iodine is monoisotopic, there is no distinctive iso- 

---

Apart from some aliphatic iodides, which have been oxidized directly to iodosyl derivatives with ozon or dimethyldioxirane [3], iodoarenes give directly iodylarenes with strong oxidants. From a synthetic point of view, potassium bro-mate in sulfuric acid has been used for the preparation of several members. The parent PhI02 can also be obtained by overoxidation of iodobenzene with peracetic acid, followed by hydrolysis as detailed in Organic Syntheses [36]. Another good method is oxidation of Arl by hypochlorite, at room temperature under phase transfer catalysis (Scheme 7) [37]. 

Aliphatic iodides, and especially secondary and tertiary representatives, are subject to hydride ehmination and are not generally useful substrates in transition metal catalysed coupling reactions. The last reaction in Scheme 14 [31], for instance, cannot be executed using current transition metal-based technology. In contrast, vinyl and aryl iodides, which are superb partners in many classical metal-induced couphng reactions, are very poor substrates in the present radical process because of the high energy of vinyl and aryl radicals. The two methods thus nicely complement each other. 

Electron transfer from a dissolving metal also allows the generation and capture of radical intermediates. The reducing ability of the metal and the nature of the medium determine the type of functional groups that can be reduced. The limited number of reported reactions includes zinc metal, which, upon activation by sonication, is capable of producing radicals from aliphatic iodides, as shown in Scheme 8.20. Even though the reaction may also proceed by way of an organic zinc species, evidence has been provided that at least part of the pathway can be attributed to free radicals.23,24... 

Furthermore, alkyl radicals can be easily obtained from aliphatic iodides or bromides by means of a photoinduced electron transfer reaction with a tertiary amine. The radical formed by fragmentation of the radical anion with concomitant halide ion loss added to a C-C triple bond in the 5-exo-dig cyclization... 

Examples of allylation [39a] and vinylation [39b] are shown in Scheme 24. The dichlorovinylation is especially interesting since the products are immediate precursors of alkynes via the Corey-Fuchs reaction. The process is also applicable to aliphatic iodides, exchange of iodine replacing the exchange of xanthate in the mechanistic manifold in Scheme 22 [39b,c[. 

Although most phosphorus chlorides and bromides are commercially available, it is sometimes necessary to prepare fresh reagents. In the case of phosphorus iodides, the reagents have poor shelf-lives (they are unstable and decompose under mild conditions) and are commonly prepared in situ, or immediately prior to use by reaction of red phosphorus with iodine. Phosphorus bromides can also be prepared this way. Reaction of 1,2,3-propanetriol with red phosphorus and bromine, for example, gave l,3-dibromo-2-propanol. 3 iodides are similarly prepared from red phosphorus and iodine, as in the conversion of cetyl alcohol (155) to cetyl iodide (156) in 85% yield. Using P and I2 is a common method for the conversion of aliphatic alcohols to aliphatic iodides. Another popular method is illustrated by treatment of 157 with triphenylphosphine, iodine, and imidazole,. In this case, taken from Hiemstra s synthesis of roseophilin, 67 the primary alcohol unit was converted to iodide 158 in 96% yield. 

C-I Stretch (strong) in aliphatic iodides occurs at 600-485 cm, out of the range of routine spectroscopy using NaCl plates or cells. The trends indicated for aliphatic chlorides hold for iodides. 

The first catalytic version of the Hofmann rearrangement using aryl iodides as catalysts and mCPBA as terminal oxidant was reported by Ochiai and coworkers in 2012 [72], A study of the catalytic efficiency of substituted iodobenzenes and some aliphatic alkyl iodides in the iodane(III) catalyzed Hofmann rearrangement of benzylic carboxamides has demonstrated that iodobenzene is the best catalyst. The introduction of both electron-donating (4-methyl, 3,5-dimethyl and 2,4,6-trimethyl) and electron-withdrawing groups (4-Cl and 4-CF3) into iodobenzene decreased the yield of rearranged products. Aliphatic iodides such as methyl, trifluoroethyl and 1-adamantyl iodides showed no catalytic efficiency. Under optimized reaction conditions,... 

Aromatic iodides, unsubstituted or bearing substituents in meta and/or para positions, readily react with aliphatic iodides and a terminal alkene in the presence of norbomene, a palladium species such as phenylnorbomylpaUadium chloride (PNP) dimer and K2CO3 in DMF at room temperature yielding alkenylarenes symmetrically alkylated at both ortho positions (Scheme lO). As shown in Table 1, selectivity always is very high except when secondary alkyl iodides are used. Yields can be improved by prolonging the reaction time. 

Selective or/Ao-alkylation and -aiylation of aryl iodides can be achieved by the cooperative catalytic action of palladium and noibomene. The first reported case was the or/Ao-dialkylation of aryl iodides, followed by Heck reaction. Here an aiyl iodide with free o-positions reacts with an aliphatic iodide and a terminal olefin in the presence of palladium/noibomene as catalyst and a base, to give a 2,6-substituted virtylarene. Analogously, an aryl iodide with one substituted o-position leads to a virtylarene containing two different ortho groups. ... 

Aliphatic iodides from other halides and sulfonates. 347-348... 

---