Reactions of Iodonium Ylides

Both IBX (799) and DMP (800) are now extensively employed in organic synthesis as mild and highly selective reagents for the oxidation of alcohols to carbonyl compounds, as well as for various other synthetically useful oxidative transformations [1102, 1104-1106], For optimized experimental procedures for the preparation of IBX (Section 2.2.3.1) and DMP (Section 2.2.3.2), see Chapter 2. 

Kita and coworkers developed a catalytic asymmetric oxidation using iodoxybenzene in a cationic reversed micellar system in the presence of chiral tartaric acid derivatives. Under these conditions, sulfides 824 

HypervaJent Iodine Reagents in Organic Synthesis 287 PU02,V0(acac)2,C6Hg 

di(2-methoxy)benzoyl-L-tartaric acid CTAB, toluene-H20 (60 1), rt 

At = 4-MeOC6H4,4-MeC6H4,4-NO2C6H4,4-CNC6H4, d-BiCsEU, 3-NO2C6H4,2-naphthyl, etc. 

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A similar reaction of ylide 200 can also be carried out under thermal conditions or in the presence of catalytic amounts of Cu(acac)2 [143]. The carbenoid reactions of iodonium ylides can also be effectively catalyzed by rhodium(II) complexes [144, 145]. The product composition in the rhodium(II) catalyzed reactions of iodonium ylides was found to be identical to that of the corresponding diazo compounds, which indicates that the mechanism of both processes is similar and involves metallocarbenes as key intermediates as it has been unequivocally established for the diazo decomposition [144]. 

Oxo-4,5,6/7-tetrahydro-6/6-dimethyl-l, 3-benzoxathiole-2-thione 211 was obtained from the reaction of iodonium ylide 144 with carbon disulfide in... 

A most remarkable compound with N-substituents has been obtained by reaction of iodonium ylides with an Fe porphyrinate. The product is a bis(metallocycle) of type D. The iron(II) ion is high spin. The structure has an in-plane iron(II) with Fe-N(alkyl) = 2.242(5) A and Fe-N = 2.082(5) A. The latter bond distance is slightly larger than that of other six-coordinate high-spin iron(II) species. 

Rh(II)-catalyzed reaction of iodonium yhdes with conjugated compounds leading to efficient synthesis of dihydrofurans, oxazoles and dihydrooxepines [ 1068] synthesis of various heterocycles by Rh(II)-catalyzed reactions of iodonium ylides with vinyl ethers, carbon disulfide, alkynes and nitriles [1055] Rh(II)-catalyzed reaction of iodonium ylides with electron-deficient and conjugated alkynes leading to substituted furans... 

Another use of iodonium ylides is as a synthetic route to heterocyclic compounds (Scheme 9). lodonium ylides can be photochemically converted to oxathiole-2-thiones in the presence of CSj. In the presence of phenyl isothiocyanate or styrenes, the iodonium ylides undergo photochemical conversion to 2-phenyliminooxathioles, dihydrofurans, and dihydrobenzofurans, respectively. Many other interesting reactions of iodonium ylides based on the photochemical approach are also reported in the literature. 

Copper(I) catalyzed decomposition of iodonium ylide 12 in the presence of a large excess of benzaldehyde results in the formation of oxirane 14. The reaction probably occurs via carbonyl ylide 13, followed by the ring closure [92JCS(P1)2837],... 

Cycloaddition reactions involving thermal/photochemical/catalytic decomposition of iodonium ylides are applicable to oxazole derivatives... 

Diazocarbonyl compounds are optimum for these transformations, and they may be readily prepared by a variety of methods. The use of iodonium ylides (17) has also been developed, " but they exhibit no obvious advantage for selectivity in carbene-transfer reactions. Enantioselection is much higher with diazoacetates than with diazoacetoacetates (18). 

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. 

The metal-catalyzed carbenoid decomposition of iodonium ylides can be applied in asymmetric reactions [149-152]. For example, the copper(II)-cat-alyzed intramolecular C-H insertion of phenyliodonium ylide 209 in the presence of several chiral ligands affords product 210 (Scheme 74) [151]. Enantiose-lectivities in this reaction vary in the range of 38-72% for different chiral... 

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 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]. 

Another experimental result, the isolation of vinyliodonium salt 110 in the reaction of alkynyliodonium salt 109 with trimethylsilyl azide in wet CH2CI2 (equation 68), supports the intermediate formation of iodonium ylide 102 (Nu = N3) in the mechanism shown in equation 66. 

Mixed phosphonium-iodonium ylides 432 represent a useful class of reagents that combine in one molecule the synthetic advantages of a phosphonium ylide and an iodonium salt. The preparation of the tetrafluoroborate derivatives 432 by the reaction of phosphonium ylides with (diacetoxyiodo)benzene in the presence of HBF4... 

Imidoiodanes or iodonium imides, ArINR, are the I-N analogues of iodonium ylides and iodosylbenzene. The best known and widely used iodonium imide is Af-tosyliminophenyliodane (PhINTs), which has found synthetic application as a useful nitrene precursor under thermal or catalytic conditions in the aziridination of alkenes and the amidation reactions of various organic substrates. The chemistry of iodonium imides was reviewed by Dauban and Dodd in 2003 [623]. 

A transition-metal-free approach for the efficient synthesis of benzofurans was developed by Liang, Li, and coworkers by the addition of arynes to iodonium ylides (Scheme 34) [57]. The reaction proceeds via the addition of iodonium ylide 79 to arynes generating the alkoxy ylide 80, which undergoes intramolecular cyclization followed by release of iodobenzene affords the product. CsF was used for the generation of arynes and the reaction proceeds under mild conditions, and using this method, the synthesis of various of functionalized benzofurans can be achieved. 

Do we have any information about the slow step of the process Well, since we know that the reaction of iodonium salts 1 with /jora-substituted A-(arenesul-fonyl)benzaldimines is sensitive to the nature of the substituents present on the sulfonyl aromatic ring, the imines must be necessarily involved in the rate-determining step of the reaction. In consequence, if the reaction between the ylides and the imines (second step) is rate-limiting, the formation of the ylides (first step) must be a fast pre-equilibrium. 

Muller et al. have also examined the enantioselectivity and the stereochemical course of copper-catalyzed intramolecular CH insertions of phenyl-iodonium ylides [34]. The decomposition of diazo compounds in the presence of transition metals leads to typical reactions for metal-carbenoid intermediates, such as cyclopropanations, insertions into X - H bonds, and formation of ylides with heteroatoms that have available lone pairs. Since diazo compounds are potentially explosive, toxic, and carcinogenic, the number of industrial applications is limited. Phenyliodonium ylides are potential substitutes for diazo compounds in metal-carbenoid reactions. Their photochemical, thermal, and transition-metal-catalyzed decompositions exhibit some similarities to those of diazo compounds. 

Rh2(OAc)4-catalyzed decomposition of 2-diazocyclohexane-l,3-dione 380a or its 5,5-dimethyl derivate 380b in the presence of an aryl iodide leads to an iodonium ylide 381 355). The mild reaction conditions unique to the rhodium catalyst are essential to the successful isolation of the ylide which rearranges to 382 under the more forcing conditions required upon copper catalysis (copper bronze, Cu(acac)2, CuCl2) 355). 

Besides iodonium ylides, alkynyliodonium salts are also useful in heterocyclic synthesis. These salts are obtained from the reaction of the alkynes with an appropriate organohypervalent iodine reagent (Scheme... 

Other examples of the iodonium ylide-based syntheses of furan derivatives involve cycloaddition reactions with alkenes or alkynes. Although the majority of these syntheses involve stable iodonium ylides (86JOC3453 94T11541) (e.g., Eqs. 16 and 17), in some cases the ylides are unstable and are generated in situ (92JOC2135) (e.g., Eq. 18). In the case of alkenes, dihydrofuran derivatives are obtained (Eqs. 16-18). This synthetic route is especially useful for the synthesis of dihydrobenzofuran derivatives that are related to the neolignan family of natural products (Eq. 18). 

Carbenoid sources other than those derived from diazo precursors for catalytic cyclopropanation reactions are currently limited. Inter- and intramolecular catalytic cyclopropanation using iodonium ylide have been reported. Simple olefins react with iodonium ylides of the type shown in equations 83 and 84, catalysed by copper catalysts, to give cyclopropane adducts in moderate yield127 128. In contrast to the intermolecular cyclopropanation, intramolecular cyclopropanation using iodonium ylides affords high yields of products (equations 85 and 86). The key intermediate 88 for the 3,5-cyclovitamin D ring A synthon 89 was prepared in 80% yield as a diastereomeric mixture (70 30) via intramolecular cyclopropanation from iodonium ylide 87 (equation 87)1 0. 

Reactions of phosphines and phosphites have received some attention but their preparative value is limited. The zwitterion formed from diphenylmethylphosphine and benzyne rearranges to ylide (124) which can be captured by Wittig alkenation, with cyclohexanone, in about 20% yield.159 Some synthetically useful reactions of tellurium and selenium compounds with arynes have been reported. For example, heating diphenyl iodonium carboxylate and bis(p-ethoxyphenyl) ditelluride in dichlorobenzene affords the compound (125).160 The corresponding reactions with diphenyl selenide and diphenyl sulfide... 

Ketocarbenes (1) are usually generated from the corresponding diazo compounds (3).s Other sources which are occasionally used are a,a-dibromo compounds (4),9 sulfur ylides (5)10 and iodonium ylides (6 Scheme 2).11 The thermal or photochemical decomposition of diazo compounds in the presence of ir-systems is often complicated by indiscriminate side reactions, such as Wolff rearrangements,12 C—H insertions and hydride migrations. To avoid such problems, the use of metal-catalyzed decomposition of diazo compounds is generally preferred.1 2... 

Iodonium ylides (136), generated in situ with bisacetoxyiodobenzene, are converted to allyl- or benzyl-substituted oxonium or sulfonium ylides (137) via rhodium- or copper-catalysed carbene transfer.115 Such ylides undergo [1,2]- or [2,3]-rearrangement to the corresponding 2-substituted heterocycles (138). An example of the rhodium-catalysed reaction is reported in Scheme 36. 

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