Iodine-carbon ylides

In several instances, transylidation through PhI=NTs leads to stable new ylides. For example, upon reaction with anisole, triphenylphosphine, and DM SO the corresponding sulphonium and phosphonium ylides were formed [51]. Further, the same ylide has been used for the preparation of iodine-carbon ylides by exchange with fi-diketones. The mildness of the method makes it suitable for high purity ylides, especially when they are thermally labile [52]. 

Members of this family of zwitterions are iodine-oxygen or iodine-nitrogen 1,4-dipoles. Some of their reactions bear analogy to those of iodine-carbon ylides, whereas in some others there are differences. 

Whereas iodine-carbon ylides undergo the transylidation reaction normally with copper catalysis, 1,4-dipoles require acid catalysis in this way they react with several nucleophiles through their protonated form, i.e. as iodonium salts, some of which are isolable. Pyridine, nicotinamide, isoquinoline, dimethylsulphide, thiolane... 

Phenyliodonium Zwitterions Preparation. Reactivity of Iodine-Carbon Ylides. Reactivity of Iodine-Nitrogen Ylides. Reactivity of 1,4-dipoles. 

Some ylides 92 among them C-silylated ones have been synthesized in order to compare the stabilization influence on the carbanionic center of various C-substituents (I, SiMej, Ph3P ) [ 113]. It appears from this study that the stabilization due to the electron-withdrawing C-substituents (R or R ) is not so negligible by comparison with the hyperconjugative stabilization between the ylidic carbon and the phosphonium group [114]. This is particularly true for the iodine substituent. 

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. 

The purpose of present review is to summarize the application of different classes of iodine(III) compounds in carbon-carbon bond forming reactions. The first two sections of the review (Sects. 2 and 3) discuss the oxidative transformations induced by [bis(acyloxy)iodo] arenes, while Sects. 4 through 9 summarize the reactions of iodonium salts and ylides. A number of previous reviews and books on the chemistry of polyvalent iodine discuss the C-C bond forming reactions [1 -10]. Most notable is the 1990 review by Moriarty and Vaid devoted to carbon-carbon bond formation via hypervalent iodine oxidation [1]. In particular, this review covers earlier literature on cationic carbocyclizations, allyla-tion of aromatic compounds, coupling of /1-dicarbonyl compounds, and some other reactions of hypervalent iodine reagents. In the present review the emphasis is placed on the post 1990s literature. 

These highly reactive yet stable species are strong electrophiles of tetraphilic character, since nucleophiles may attack three different carbon atoms (a,/ ,a ) and iodine. In most reactions the first step is a Michael addition at fi-C with formation of an alkenyl zwitterionic intermediate (ylide) which normally eliminates iodoben-zene, generating an alkylidene carbene then, a 1,2-shift of the nucleophile ensues. The final result is its combination with the alkynyl moiety which behaves formally as an alkynyl cation. The initial adduct may react with an electrophile, notably a proton, in which case alkenyl iodonium salts are obtained also, cyclopentenes may be formed by intramolecular C-H 1,5-insertion from the alkylidenecarbenes ... 

The thermal reaction, catalysed by Cu(acac)2, of thiobenzophenones with ylides coming from bis arylsulphonyl methane is also likely to proceed by an initial transylidation the main products are here benzo[c]thiophenes [35,36], The car-banionic carbon of iodonium ylides is devoid of nucleophilic character, yet PhI=C(S02Ph)2 gave, with iodomethane, the methylated iododisulphone MeC(I)(S02Ph)2 (68%). This reaction, performed at room temperature without any catalyst, is probably the result of a nucleophilic attack from iodine of iodomethane to iodine of the ylide [37]. 

Tributylarsanc reacts with perfluorocyclobutene in diethyl ether at room temperature to form the corresponding ylide 3 in quantitative yield. Presumably, tributylarsanc attacks the double bond in the cyclobutene followed by fluoride ion elimination. Subsequently, fluoride ion adds to the intermediate at the olefinic carbon (I to the arsenic atom. Bromination proceeds much faster than iodination. 

Ylide can be viewed as a special carbanion in which the negative charge on carbon is stabilized by an adjacent positively charged heteroatom. The most common ylides are phospho-nium ylides, sulfur ylides (sulfonium and sulfoxonium ylides) and certain nitrogen-based ylides (ammonium, azomethine, pyridinium and nitrile ylides). In addition to synthetically important phosphorus, sulfur and nitrogen, ylides of tin (Sn) and iodine (I) have been developed in recent years. 

There is evidence, however, that these reactions may not involve conversion of the iodonium ylide into a carbene which then adds to the relevant substrate, but that this substrate is involved prior to cleavage of the carbon-iodine bond [136]. Other ylide interchange reactions which have been recorded include the conversions of various ylides into an oxosulphonium ylide by reaction with sulphur monoxide, generated in situ [141] ... 

Thus the stabilization effect of an iodine atom attached to an ylidic carbon atom in phosphorus ylides increases in the order 7 < 1 < 2 ... 

For alkynyl transfer, no direct evidence has been presented to determine whether organometallic nucleophiles interact with the electrophilic iodine centre or the jS-carbon although, in the case of transfer to the nitrogen nucleophile of a coordinated cyano ligand, the S-interaction mechanism is clearly implicated in the formation of 68 (Scheme 38, related to the mechanism in Scheme 14). This example indicates, for the synthesis of alkynylmetal species, the attractiveness of RC=C(Ph)I reagents where the R group cannot participate in such reactivity, e.g. R = SiMcs, Ph. Alternatively, this type of reaction, where the metal centre may act as a nucleophile at the -carbon, is worthy of exploration as a mode of organometallic synthesis as also is electrophilic attack at the K-carbon of ylides (illustrated by the mechanism in Scheme 14). 

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