Iodine as electrophile

Whereas chlorine or iodine as electrophiles do not provide clean addition products61 , phenylselenenyl chloride opens siloxycyclopropanes forming a-selenenylated Y-oxoesters 81). Usually, however, this process is better conducted in the presence of Lewis acids at low temperature (vide infra, section 4.5.1). 

Cyclization of iodopyiidinyl aUyl ethers derived fom dihalopyridines and sodium aUyUc oxides leads to formation of furo[2,3-fc]pyridines 292, furo[3,2-c]pyiidines 293, and furo[2,3-c]pyridines 294 by a Heck mode of reaction (Scheme 101). In the reaction sequences, the initial step in the preparation of iodopyridine aUyl ethers involves regioselective lithiation of 3-fluoro, 2-fluoro-, and 4-chloropyridines with LDA. Subsequently, the lithiated species are treated with iodine as electrophile. A variety of iodopyiidinyl ethers have been prepared from dihalopyridines and sodium aUyhc oxides and subjected to the Pd-catalyzed cyclization reactions with formation of the furoannulated pyridine products. When the allyl groups carry a 2-substituent as in stracture 295, hydridopaUadium elimination is prevented. Instead, sodium formate reductive elimination of the palladium substituent gives the product 296. The isomeric structures 297 and 298 are available similarly. ... 

Bromine in benzene or ethanol brings about 3,6-disubstitution of 9-vinylcarbazole as well as electrophilic addition to the double bond. A good yield of 3-iodocarbazole results from the use of iodine-iodic acid in acetic acid at iodine chloride in acetic acid caused 3,6-di-... 

The intermediate in reduction with Rcd-Al is functionalized using iodine as an electrophile. 

The difficulty in dealing with solvent influences on reaction rates is that the free energy of activation, AG, depends not only on the free energy of the transition state but also on the free energy of the initial state. It is therefore of considerable interest to dissect solvent influences on AG into initial-state and transition-state contributions. As far as electrophilic substitution at saturated carbon is concerned, the only cases for which such a dissection has been carried out are (a) for the substitution of tetraalkyltins by mercuric chloride in the methanol-water solvent system (see page 79), and (b) for the iododemetallation of tetraalkylleads in a number of solvents (see p. 173). Data on the latter reaction (6) are more useful from the point of view of the correlation of transition-state effects with solvent properties, and in Table 13 are listed values of AG (Tr), the free energy of transfer (on the mole fraction scale) of the tetraalkyllead/iodine transition states from methanol to other solvents. 

Ketones, I COR2, have been converted to their -dihydroperoxides (112) using a green oxidant, aqueous 30% hydrogen peroxide, with iodine as catalyst.337 The iodine may enhance the electrophilic character of the carbonyl carbon and/or the nucleophilicity of the hydrogen peroxide. The reaction has also been extended to... 

The iodination of pyridine, quinoline, and isoquinoline via a-metalation using lithium di-fert-butyltetramethylpi-peridinozincate (TMP-zincate) proceeds smoothly at room temperature using iodine as the electrophile. The chemoselective deprotonative zincation generated 2-iodopyridine 70 and 1-iodoisoquinoline 71 in 76% and 93% yield, respectively. Quinoline metalated preferentially at the 8-position to give 61% yield of the 8-iodo derivative 72 and 26% yield of 2-iodoisoquinoline 73 (Equations 25-27) <1999JA3539>. 

The synthesis of vinyllithiums 116 and 118 from vinyl bromides 115 and 117 with retention of double bond geometry is generally best done using the method of Seebach 102 the vinyl bromide is treated with two equivalents of r-BuLi in Et20 or THF at low temperature (-78 °C, for non-terminal bromides <-110 °C for terminal bromides, which otherwise lithiate a to Br and generate alkynes). Choice of solvent is essential only with alkyl iodides as electrophiles, for which the use of THF leads to attack at iodine instead of at carbon. Clean alkylation in Et20 is obtained on warming slowly to room temperature.24... 

M. Kunishima, Synthesis of Hypervalent Alkynyl Iodine Compounds and Studies on Their Utility as Electrophilic Reagents , Ph.D. Dissertation (Japanese), Institute for Chemical Research, Kyoto University, 1989. 

A total selectivity has been observed in the cyclization6 of l-benzyloxycarbonyl-2-(1-methylethenyl)pyrrolidine (13), a useful intermediate in the synthesis of the pumiliotoxin A alkaloid class. Thus, A -bromosuccinimide in dimethyl sulfoxide/water or iodine in acetonitrile was utilized as electrophile. 

Amino- as well as 6-chloropurine derivatives are also directly iodinated at C8 via lithium diisopropylamide lithiation followed by reaction of the C8 lithiated species with iodine as an electrophile, e.g. formation of 14a and 14b. ... 

Whereas haloalkanes are widely used for the electrophilic alkylation of a broad variety of nucleophiles, perfluoroalkyl bromides or iodides do not act analogously as electrophilic perfluoroalkylation reagents (Figure 2.7). For example, the reaction of perfluoroalkyl iodides with aliphatic alcoholates does not yield the expected alkyl perfluoroalkyl ether (analogous to the Williamson ether synthesis) but mostly the hydrofluorocarbon resulting from the reduction of the iodide [1]. In contrast, perfluoroalkyl iodides and bromides have been used as preparatively useful electrophilic iodination or bromination reagents [2]. 

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