Iodonium structure

No product has as yet been isolated that is believed to have the bromonium ion ring intact, but some closely related ionic compounds are known. The latter compounds could have the bromonium and iodonium structure but their colors and the probable stability of the alternative carbonium, ion make it unlikely.282... 

Unsaturated carboxylic acid 17 possesses the requisite structural features for an iodolactonization reaction.16 A source of electrophilic iodine could conceivably engage either diastereoface of the A20,21 double bond in 17. The diastereomeric iodonium ion inter-... 

Arylethynyl(phenyl)iodonium salts, RC=CI+Ph 4-MeC6H4S03-, react as 1,3-dipolarophiles with nitrile oxides R CNO to afford phenyl(substituted isox-azolyl)iodonium salts 210, which give iodoisoxazoles on reaction with nucleophiles. The crystal structure of 210 (R = Ph, R1 = mesityl) has been determined (369). 

Photosolvolysis of some vinyl iodonium substrates can be compared with thermal solvolysis of the same substrates. Photochemical reaction of (E)- and (Z)-2-phenylprop-l-enyl(phenyl)iodonium tetrafluoroborate (31) in methanol and TFE gave products of heterolytic cleavage of vinylic C-I bond as summarized in Table III (for the product structures see eq 14.17 In contrast to... 

Comparisons of product distributions in thermal and photochemical solvolyses show that the primary vinyl cation is not involved in thermolysis but is formed photochemically. The chirality probe approach using optically active 4-methylcyclohexylidenmethyl(aryl)iodonium tetrafluoroborate 19 was applied to the photosolvolysis.24 The rearranged product 4-methylcycloheptanone retained some optical activity, but the enantiomeric product in slight excess has a different structure depending on the iodoarene leaving group Arl of the substrate. The results indicate that the primary vinyl cation involved is not in a free, dissociated achiral form. 

Levothyroxine is used to treat hypothyroidism (an underactive thyroid gland). Thyroid hormone can be made from beef and pork thyroid, but this lacks standardization and it is difficult to control dosage. The synthetic drug is more desirable. Levothyroxine is one of two important thyroid hormones. It is converted into the second important hormone, liothyronine, in the body. The key step in the synthesis of structures such as levothyroxine is the substitution of an iodonium salt by an iodinated phenol. Siql-like reactions on an aromatic ring are not common, but an iodonium salt provides a good leaving group. 

We may seem to have contradicted ourselves because Equation 10-1 shows a carbocation to be formed in bromine addition, but Equation 10-5 suggests a bromonium ion. Actually, the formulation of intermediates in alkene addition reactions as open ions or as cyclic ions is a controversial matter, even after many years of study. Unfortunately, it is not possible to determine the structure of the intermediate ions by any direct physical method because, under the conditions of the reaction, the ions are so reactive that they form products more rapidly than they can be observed. However, it is possible to generate stable bromonium ions, as well as the corresponding chloronium and iodonium ions. The technique is to use low temperatures in the absence of any strong nucleophiles and to start with a 1,2-dihaloalkane and antimony penta-fluoride in liquid sulfur dioxide ... 

Iodonium tetrafluoroborate, biphenylene-X-ray structures, 1, 566 Iodopropenylation alkyl halides, 1, 469 Ionenes uses, 1, 289 Ion exchange resins pyridine polymers and, 1, 308-309 Ionization potentials pyridines, 2, 135 cts- 3-Ionol synthesis, 3, 666 cis-/3-Ionone... 

Tributylstannanes have also been used in the synthesis of alkynyl(aryl)iodonium salts, including 1,3-diynyl derivatives373 and the parent member of the family, HC=CIPh+TfO, which was characterized by X-ray structure analysis.374 The bisphenyliodonium triflate reagent 152 [Eq. (4.110)]372 and analogs375 were synthesized in a similar way. Alkynyltrimethylsilanes may also serve as similar useful starting materials.376,377 X-ray characterization of a variety of alkynyl(aryl)iodonium ion has been reported.332... 

The pareitropone project began quite by accident after an unexpected observation expanded our thinking about potentially accessible targets for alkynyliodonium salt/alkylidenecarbene chemistry (Scheme 18). Treatment of the tosylamide iodonium salt 125 with base under standard conditions was designed to provide no more than routine confirmation of the aryl C-H insertion capabilities, which were first exposed in indoleforming reactions using tosylanilide anion nucleophiles and propynyl(phenyl)iodonium triflate,5b of the intermediate carbene 126. However, this substrate did not perform as expected, since only trace amounts of the 1,5 C-H insertion product 127 was detected. One major product was formed, and analysis of its spectral data provided yet another surprising lesson in alkynyliodonium salt chemistry for us. The data was only consistent with the unusual cycloheptatriene structure 129. 

Studies on the solution structure of A3-iodanes are relatively limited. In polar solvents, cryoscopic and conductance measurements have shown extensive dissociation of diaryl-A3-iodanes (Ar2IL L = BF4, Cl, Br, OAc) into the solvated iodo-nium ions (Ar2I+S S = polar solvents such as H20, MeOH, and DMSO) [3,210]. Even in dichloromethane, bis(4-methylphenyl)-A3-iodane (Ar2IBF4 Ar=p-MeC6H4) dissociates into the solvated iodonium ions with dissociation constant Kdissoc=4.7xlO-6M[211]. 

Detailed studies on the solution structure of [hydroxy(mesyloxy)iodo]ben-zene and [hydroxy(tosyloxy)iodo]benzene 17 suggest that in aqueous solution iodosylbenzene 18 exists as a monomeric iodonium ion form 136, if the pH is < 2.3, and as a neutral species 137 at pH > 5.3 through mildly alkaline conditions [216]. The monomer 137 is soluble only to the extent of about 3 x 10"3 M. Based on these finding, we propose a structure of 138 as a reactive species in the reaction using a combination of (PhIO)n 18 and BF3-Et20. 

The stable (arylsulfonylmethyl)iodonium salts 49 and 50 can be conveniently prepared in two steps starting from the readily available iodomethyl sulfones 47 (Scheme 23) [41]. Iodonium salts 49 and 50 are not moisture sensitive, can be purified by crystallization from acetonitrile, and can be stored for several months in a refrigerator. The structure of iodonium triflate 50 was unambiguously established by a single crystal X-ray analysis [41]. 

Alkynyl(phenyl)iodonium salts have attracted a significant interest as stable and readily available powerful alkynylating reagents. The preparation, structure, and chemistry of alkynyliodonium salts was extensively covered in a recent review [4]. 

Iodonium salts with a hydroxyl or a carboxyl group are readily converted into their inner salts, i.e. zwitterionic compounds of various types. 2-(Phenyliodonio) benzoate, whose structure may be cyclic, belongs to this category it is prepared from o-iodobenzoic acid upon oxidation and coupling with benzene (or arenes) [51], On strong heating, iodobenzene and carbon dioxide are eliminated, with formation of benzyne ... 

Iodonium ylides coming from precursors with one or two keto groups react thermally or photochemically with alkenes, alkynes, nitriles and some heterocumu-lenes. The products are 5-membered heterocycles with at least one oxygen atom, as illustrated in Table 10.2. Their formation can be described as the result of a formal [3 + 2] cycloaddition from the mesomeric 1,3-dipole structure of ketocarbenes, according to the general scheme (a = b mostly ethylenic double bonds) ... 

The 127I NQR spectra of an extensive series of iodonium compounds, [R2I]+BF, have been reported52,53. In addition to the fact that iodine is the only halogen that forms an extensive series of halonium compounds, the study of 127I has the advantage that, since its spin is equal to 5/2, there are two transitions and therefore both the coupling constant and the asymmetry parameter may be obtained. This is particularly important for a polycoordinated nucleus, since we may anticipate that the asymmetry parameter will be far from negligible. A selection of the NQR parameters for these compounds is shown in Table 9. Also studied were the iodonium halides, whose structures differ from those of the... 

Single-crystal X-ray structures of six alkynyliodonium compounds, all containing oxyanions and including four alkynyl(phenyl)iodonium salts, one alkynylbenziodoxolone and one dialkynyliodonium salt, have been reported. Selected structural data for these compounds are given in Table 3. 

Certainly one of the most unusual nucleophiles to be treated with alkynyliodonium salts is the tetra-ter -butyltetraphosphacubane shown in equation 7791. However, despite its unique structure, this compound reacts with ethynyl(phenyl)- and l-propynyl(phenyl)-iodonium triflates in the now expected way to give good yields of alkynylphosphonium salts91. 

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