- Crystal Structures of Diaryliodonium Fluorides and Their Implications for Fluorination Mechanisms
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The radiofluorination of diaryliodonium salts is of value for producing radiotracers for positron emission tomography. We report crystal structures for two diaryliodonium fluorides. Whereas diphenyliodonium fluoride (1 a) exists as a tetramer bridged by four fluoride ions, 2-methylphenyl(phenyl)iodonium fluoride (2 a) forms a fluoride-bridged dimer that is further halogen bonded to two other monomers. We discuss the topological relationships between the two and their implications for fluorination in solution. Both radiofluorination and NMR spectroscopy show that thermolysis of 2 a gives 2-fluorotoluene and fluorobenzene in a 2 to 1 ratio that is in good agreement with the ratio observed from the radiofluorination of 2-methylphenyl(phenyl)iodonium chloride (2 b). The constancy of the product ratio affirms that the fluorinations occur via the same two rapidly interconverting transition states whose energy difference dictates chemoselectivity. From quantum chemical studies with density functional theory we attribute the “ortho-effect” to the favorable electrostatic interaction between the incoming fluoride and the o-methyl in the transition state. By utilizing the crystal structures of 1 a and 2 a, the mechanisms of fluoroarene formation from diaryliodonium fluorides in their monomeric, homodimeric, heterodimeric, and tetrameric states were also investigated. We propose that oligomerization energy dictates whether the fluorination occurs through a monomeric or an oligomeric pathway.
- Lee, Yong-Sok,Chun, Joong-Hyun,Hodo??ek, Milan,Pike, Victor W.
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- Oxidative iodination of arenes with manganese(IV) oxide or potassium permanganate as the oxidants
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Novel and easy laboratory methods (novel Procedures 1-4) are presented for the oxidative mono- and diiodination of both activated and deactivated arenes, which gave the pure iodinated products in 62-89% yields. The reactions were carried out in the anhydrous, strongly acidic system, I2/activated MnO2/AcOH/Ac2O/concd H2SO4, firstly at r.t. for 2 h, then at 45-55 °C for 2-9 h. The resulting mixtures were poured into excess aq Na2SO3 solution buffered with (NH4)2CO3 to neutralize H2SO4. The following workups are given. Similarly, on carrying out the iodination reactions (at 35 °C, for one hour) in the anhydrous, strongly acidic system, I2/KMnO4/AcOH/Ac2O/concd H2SO4, it was possible to mono- or diiodinate several deactivated arenes in 73-87% yields (improved Procedures 5 and 6).
- Lulinski, Piotr,Skulski, Lech
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p. 115 - 120
(2007/10/03)
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- The direct iodination of arenes with chromium(VI) oxide as the oxidant
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An easy and cheap laboratory method is presented for the direct mono- and diiodination of a number of activated and deactivated arenes. The main iodination reactions occurred at the temperatures not exceeding 65°C for 0.5-12 h in the anhydrous, strongly acidic liquid system, I2/AcOH/Ac2O/H2SO4, in the presence of prior dissolved CrO3 used as the oxidant. The yields of the pure iodinated products varied from 31% (for 3,5- diiodobenzoic acid) up to 90% (for 4-iodoanisole). So far, benzonitrile and some oxidizable aromatics, e.g. naphthalene, fluorene, xanthene, and thiophene, have been found to be unsuitable for the effective iodination. Nevertheless, this novel, simple method of direct iodination is worthy to be extended to other appropriate aromatics.
- Lulinski, Piotr,Skulski, Lech
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p. 1665 - 1669
(2007/10/03)
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- Oxidative Anion Metatheses in Diaryliodonium Iodides and Chlorides
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Oxidative anion metatheses in the crude title iodides and chlorides produced the corresponding pure hydrogensulfates, nitrates, tetrafluoroborates, triflates, tosylates, as well as bromides and chlorides (only from the iodides) in 54-86% yields. These procedures are easier and shorter than earlier methods. By using modified oxidative metatheses in the title iodides (in the presence of HBr or HCl) it was possible either to isolate, or to detect only, the intermediate dihaloiodates(I). [Ar2I]+[IX2]- (X = Br or Cl). A complex Ph2I+Cl- -1/2I2 was also obtained in 56% yield. Tetraethylammonium iodide was similarly converted into pure tetrafluoroborate or dibromoiodate(I) in 75 and 76% yields, respectively.
- Kazmierczak, Pawel,Skulski, Lech
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p. 219 - 224
(2007/10/03)
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