Ylides iodonium

Preparation, Structure and Properties of Polyvalent Iodine Compounds 101 CH2CI2,20 to 40 °C, 12-24 h 

Preparation, Structure and Properties of Polyvalent Iodine Compounds 103 

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 

NMR measurements indicate that ylides 437 are stable up to -30 °C and they can be conveniently used in subsequent transformations without isolation [576-580]. 

The unstable ylides PhIC(H)N02 [581,582] and PhIC(C02Me)N02 [583,584] can be generated in situ from nitromethane and methyl nitroacetate, respectively and used in rhodium(II) carbenoid reactions without isolation. 

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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). 

Iodonium ylides reacted with electron-deficient alkynes or conjugated Rh-catalyst to form trisubstituted furans in moderate yields as depicted in the... 

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... 

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],... 

Michael-type addition of an enolate anion to an alkynyliodonium salt probably produces the unstable iodonium ylide 34 34a. Loss of Phi from... 

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). 

When these cycloaddition reactions are carried out with alkynes, furan derivatives are formed. lodonium ylide 5, for instance, on photochemical reaction with alkynes 43, gives benzofurans 44 (86JOC3453) (Eq. 19). In a similar way, the iodonium ylide derived from 2-hydroxy-1,4-naphthoquinone undergoes a cycloaddition reaction with phenylacety-lene to yield benzofuran 45 (Scheme 16) (89LA167). 

Lewis acid-catalyzed decomposition of iodonium ylide 49 in the presence of alkenes results in the formation of y-lactones 50 (87IZV2873) (Eq. 20). 

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

JOC4885). For example, decomposition of dimedone iodonium ylide (42) in the presence of acetonitrile and phenyl isocyanate provides 4,5,6,7-tetrahydrobenzoxazoles 155 (87TL4449) and 156 (75JOC1166), respectively (Scheme 43). 

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). 

A very simple synthesis of coumestrol (228) has been described by Kappe and coworkers (Scheme 46) (74ZN(B)292). It is based upon dehydrogenation of 4-hydroxy-3-phenyl-coumarins to coumestans (720PP233). A number of 2 -hydroxy 3-phenylcoumarins were oxidized with lead tetraacetate to the corresponding coumestans 3-(l-acetoxy-4-methoxy-2-oxo-3,5-cyclohexadienyl)coumarins were obtained as by-products (76BCJ1955). Coumes-tan itself (226) has been obtained by photolysis of the phenol ether (232), which is in turn available from 4-hydroxycoumarin (229) and (diacetoxyiodo)benzene (Scheme 47) (78CB3857) via an iodonium ylide (231). 

CuCl-catalyzed decomposition of iodonium ylides prepared from /3-keto esters and diacetoxyiodobenzene, has been developed (equation 151)331. 1-Methylbenzvalene is obtained in a good yield by treating a mixture of lithium cyclopentadienide and 1,1-dichloroethane with butyllithium332. The tandem cyclization substitution in l-selenyl-5-hexenyllithiums derived from corresponding selenacetals via selenium/lithium exchange produces bicyclo[3.1.0]hexane derivatives333. 

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. 

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. 

The standard method for the preparation of many phenyliodonium ylides is reaction of compounds having an active methylene group with an iodine(III) species, usually in aqueous alkali, to give iodonium ylides (Scheme 50) [151]. 

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]. 

The catalytical decomposition of iodonium ylides is especially useful as a method of cyclization via intramolecular cycloaddition or bond insertion [146 -148]. Several representative examples of these cyclizations are shown in Scheme 73. Specifically, the intramolecular cyclopropanation of ylide 203 leading to product 204 was used in the synthesis of the 3,5-cyclovitamin D ring A synthon [146]. The copper(I) catalyzed decomposition of phenyliodonium ylides 205 affords the corresponding substituted tetralones 206 in good preparative yields [147]. Under similar conditions,iodonium ylides 207 undergo regio-and stereoselective intramolecular cyclopropanation to form the key bicy-clo[3.1.0]hexane intermediates 208 for prostaglandin synthesis [148]. 

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... 

The cyclic /J-dicarbonyl iodonium ylides can undergo [3 + 2] cycloaddition reactions with various substrates under catalytic or photochemical conditions, presumably via a stepwise mechanism [153-156]. In a recent example, iodonium ylide 211, derived from dimedone, undergoes dirhodium(II) catalyzed thermal [3+ 2]-cycloaddition with acetylenes 212 to form the corresponding furans 213 (Scheme 75). Under photochemical conditions ylide 211 reacts with various alkenes 214 to form dihydrofuran derivatives 215 [156]. 

One of the most notable recent advances in iodonium ylide chemistry is the first demonstrable generation of iodonium enolates 44 formally derived from unactivated monocarbonyl compounds [197]. This was accomplished by the treatment of (Z)-jS-acetoxyvinyliodonium salts with lithium ethoxide, either in THF or in THF-DMSO (12 1) (Scheme 72). 

An efficient construction of the 1 / -methylcarbapenam nucleus (87) was established by Kume and co-workers utilizing acid- or rhodium (II)-catalyzed cyclization of the iodonium ylide derivatives (174), which were easily prepared from the corresponding jS-ketoester derivatives (173) with PIDA [133] (Scheme 41). 

Generation of triplet carbenes from photolysis of iodonium ylides has been discussed in a short review.9... 

Oxidative methanolysis of azetidinone 176 followed by hydrogenolysis of compound 177 afforded /3-lactam 178, which was protected to obtain the protected amine 179. The best conditions for rearrangement of 179 were found using TFA. Conversion of compound 180 to carbacephem 183 was accomplished by ketone reduction, alcohol protection, and elimination of methanol. Synthesis of carbacephem derivative 186 has been performed by rhodium(n)-catalyzed cycliza-tion of iodonium ylide 185 <1997TL6981> (Scheme 33). The iodonium ylide 185 was easily prepared from the corresponding /3-keto ester 184 and [(diacetoxy)iodo]benzene in good yield. 

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>. 

In most of their reactions these ylides behave formally as carbene precursors. Iodonium ylides in this capacity have a resemblance to diazo compounds with which they often compare favourably. Many of their reactions proceed better under photochemical conditions. 

Intramolecular acid- or rhodium [II]-cataIysed carbene nitrogen-hydrogen insertion from iodonium ylide intermediates was used in an efficient synthesis of 1-/1-methylcarbapenems the cyclization products had different stereoselectivity, depending on the catalyst [11] ... 

Several iodonium ylides, thermally or photochemically, transferred their carbene moiety to alkenes which were converted into cyclopropane derivatives. The thermal decomposition of ylides was usually catalysed by copper or rhodium salts and was most efficient in intramolecular cyclopropanation. Reactions of PhI=C(C02Me)2 with styrenes, allylbenzene and phenylacetylene have established the intermediacy of carbenes in the presence of a chiral catalyst, intramolecular cyclopropanation resulted in the preparation of a product in 67% enantiomeric excess [12]. 

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