Iodonium symmetrical

Interesting results concerning phenyl group participation were observed with ( )-styryl(phenyl)iodonium tetrafluoroborate (26) using a deuterated substrate (eq 12)16 When 26-ad was heated in trifluoroethanol (TFE) at 60 °C, slow reaction gave die E isomer of substitution product 28 quantitatively, but the deuterium was completely scrambled between the a and p positions. This strongly indicates that a symmetrical intermediate is involved during the reaction and the most reasonable one is vinylenebenzenium ion (27) formed by phenyl participation. This intermediate also explain the exclusive formation of the retained ( )-28. 

Under more basic conditions, a-elimination predominates and insertion of the carbene 40 to the solvent gives racemic 22. Non-basic and poorly nucleophilic conditions allow neighboring group participation to form the rearranged substitution product 23 with complete chirality transfer. The participation can be considered as an intramolecular nucleophilic substitution, and does occur only when it is preferable to the external reactions. Under slightly basic conditions with bases in HFIP, participation is allowed, and the weak base can react with the more electrophilic vinylic cation 21 (but not with the iodonium ion 19). A suitably controlled basicity can result in the formation of cycloalkyne 39, which is symmetrical and leads to racemization. These reactivities are illustrated in Scheme 6. 

The latter results have been explained on the basis of the following reaction scheme. The 1,2-regioisomers derived from butadiene are obtained through a non-symmetrical iodonium ion intermediate. The subsequent nucleophilic attack on the allylic position gives, under kinetic control, 1,2-derivatives. Nevertheless, when poorer nucleophiles such as benzene or acetonitrile are employed, the conversion of the initially formed iodonium ion into the allylic cation has been suggested to give 1,4-products, under thermodynamic control. However, other alternatives like nucleophilic attack involving allylic participation have not been excluded for the formation of 1,4-derivatives. 

Other alkyl hypohalites usually add to carbon-carbon multiple bonds in a free-radical process.155-158 Ionic additions may be promoted by oxygen, BF3, or B(OMe)3.156-160 While the BF3-catalyzed reaction of alkyl hypochlorites and hypo-bromites gives mainly halofluorides,159 haloethers are formed in good yields but nonstereoselectively under other ionic conditions.156-158 160 In contrast, tert-BuOI reacts with alkenes in the presence of a catalytic amount of BF3 to produce 2-iodoethers.161 Since the addition is stereoselective, this suggests the participation of a symmetric iodonium ion intermediate without the involvement of carbocationic intermediates. 

A stereo specific addition of BufOI to /1-methylstyrene was observed in the presence of BF3, yielding Markovnikov products. This result contrasts with the non-stereospecific addition of Bu OCl and Bu OBr. It has been suggested that the bridging in the intermediate chloronium and bromonium ion derived from PhCH=CHMe is not as symmetrical as in the iodonium ion. Consequently, charge develops on the benzylic carbon in the first two cases, and rotation occurs about the C—C bond190. By contrast, a radical mechanism is assumed in the absence of BF3 as anti-Markovnikov products are formed (both in the dark and upon UV irradiation)190. 

Symmetrical iodonium salts. Arenes, including non-activated compounds such as nitrobenzene, react with the powerful electrophile iodosyl fluorosulfate (FS020I0, obtained from iodine, iodine pentoxide and fluorosulfonic acid) to give directly diaryliodonium hydro sulfates. The reaction is performed at low temperature (Scheme 32) [97]. 

Unsymmetrical iodonium salts. The methods for the preparation of unsym-metrical iodonium salts are also suitable for symmetric iodonium salts, since a preformed or in situ generated aryliodine(III) species coming also from deactivated arenes may react with any arene or derivative of it, provided it is not strongly deactivated. In this way, the dipolic intermediate of Scheme 32 i.e. PhI+I0S03, also formed from PhIO and S03, can serve for the preparation of unsymmetrical iodonium salts [96]. 

A large scale synthesis of SK FL-94901 (76), which is a novel, selective, and potent thyromimetic, was established by construction of the hindered diaryl ether moiety via symmetrical iodonium salt 169 [131] (Scheme 39). 

Another approach for the preparation of either symmetrical or unsymmetrical iodonium salts used organolithium or organomercury compounds and (dichloroiodo)arenes [12]. The problem of the formation of unwanted isomers during reactions involving aromatic electrophilic substitution may also be overcome by the condensation of iodosylarenes with iodylarenes [12]. Several iodonium triflates were prepared in high yield from activated or mildly deactivated arenes with iodosylbenzene and triflic anhydride or triflic acid [13,14] or sulphur trioxide [15]. Some of these compounds are shown in Table 8.2. 

These iodonium salts may also be symmetrical or unsymmetrical. The methods for their preparation are essentially the same to those already discussed, with some... 

Apart from copper(I)-mediated reactions, few studies of the treatment of vinyliodonium salts with carbanions have appeared. The vinylations of the 2-phenyl- and 2- -hexyl-l,3-indandionate ions shown in equations 222 and 223 are the only reported examples of vinyliodonium-enolate reactions known to this author26,126. ( ,)-l-Dichloroiodo-2-chloroethene has been employed with aryl- and heteroarvllithium reagents for the synthesis of symmetrical diaryliodonium salts (equation 224)149,150. These transformations are thought to occur via the sequential displacement of both chloride ions with ArLi to give diaryl (/ -chlorovinyl)iodanes which then decompose with loss of acetylene (equation 225). That aryl(/ -chlorovinyl)iodonium chlorides are viable intermediates in such reactions has been shown by the conversion of ( )-(/ chlorovinyl)phenyliodonium chloride to diaryliodonium salts with 2-naphthyl- and 2-thienyllithium (equation 226)149,150. 

The method of iodonium salt synthesis using iodonium bromide was similar to that published by Crivello and co-workers [57-59]. Symmetrical iodonium salts... 

A similar behaviour was observed in the reaction of lithium dialkynylcuprates with alkynylaryl iodonium tosylates leading to conjugated diynes. Unsynunetrical diynes (98) were also prepared by this method. However, the selectivity was moderate, due to the competitive formation of the symmetrical diynes (99).202 This reaction has been recently applied to the synthesis of various liquid-... 

Crystal structures have also been obtained for the corresponding chloronium and iodonium ions and for the bromonium ion with a triflate counterion. Each of these structures is somewhat unsymmetrical, as shown by the dimensions below. The significance of this asymmetry is not entirely clear. It has been suggested that the bromonium ion geometry is affected by the counterion and it can be noted that the triflate salt is more symmetrical than the tribromide. On the other hand, the dimensions of the unsymmetrical chloronium ion, where the difference is considerably larger, has been taken as evidence that the bridging is inherently unsymmetrical. Note that the C- C bond lengthens considerably from the double-bond distance of 1.35 A. 

Likewise, the reaction [42] of three equivalents of 7 with the tris-tin-alkyne 37 in cold dichloromethane gives the symmetrical tris-iodonium salt 38 [Eq. (19)]. 

Numerous methods for the preparation of symmetrical and unsymmetrical diaryl- and hetaryliodo-nium organosulfonates have been developed. A common synthetic approach to unsymmetric diaryl-and hetaryl(aryl)iodonium tosylates (e.g 262, 264, 266 and 268) is based on the reactions of [hydroxy(tosyloxy)iodo]arenes with aryltrimethylsilanes 261 [198], aryltributylstannanes 263 [376], aryl-boronic acids 265 [377], or the appropriate heteroaromatic precursors 267 (Scheme 2.75) [378,379]. The reaction of HTIB with arylstannanes proceeds under milder conditions compared to arylsilanes and is applicable to a wide range of arenes with electron-withdrawing substituents. Arylboronic acids in general have some advantage over arylstannanes in the case of the electron-rich heterocyclic precursors [377]. 

A very mild and general method for the preparation of diaryl- and heteroaryliodonium triflates is based on iodonium transfer reactions of iodine(III) cyanides with the respective aryl- or heteroarylstannanes [146,148, 399-401]. Specifically, (dicyano)iodonium triflate (277), generated in situ from iodosyl triflate and MesSiCN, reacts with tributyltin derivatives of aromatic and heteroaromatic compounds to afford the corresponding symmetrical iodonium salts under very mild conditions (Scheme 2.80) [389,390]. 

The O-arylation of appropriate phenols using symmetrical iodonium salts has been employed in the synthesis of hydroxylated and methoxylated polybrominated diphenyl ethers, some of which are related to natural products [872,873]. For example, several polybrominated diphenyl ethers 680 have been prepared by the reaction of iodonium salt 678 with phenols 679 in iV,iV-dimethylacetamide (DMAC) solution in the presence of base (Scheme 3.272) [872]. 

Hennecke developed enantioselective haloetherification reactions via desymmetrisation of in situ generated meso-iodonium intermediates (Scheme 2.37). Chiral sodium phosphate 58 was used for the cyclisation of symmetrical ene-diol substrates 59 with l-iodopyrrolidin-2-one (60), and the corresponding iodoetherification products 61 were obtained with up to 71% ee. 

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