Fluorinations, with difluoroiodo arenes

To overcome problems associated with the removal of iodobenzene and its derivatives formed upon fluorination of arylalkenes and arylalkynes with (difluoroiodo)arenes, polymer-supported (difluoroiodo)arenes were proposed.139 With these agents, the separation procedures are reduced to filtration of the iodinated polymer. For this purpose popcorn polystyrene is io-dinated and then transformed into the difluoroiodide by treatment with xenon difluoride in the presence of hydrogen fluoride in dichloromelhane at 25 C. The amount of active fluorine bonded to iodine atoms on the polymer support is estimated by iodometric titration. The reactions with phenyl-substituted alkenes result in rearranged gew-difluorides. The procedure provides the same fluorination products as with (difluoroiodo)benzenc (see Section 4.13.) but in much higher yields, e.g. PhCF2CH2Ph (96%), PhCF2CH(Me)Ph (95%). PhCH2CF2H (86%), and l,l-difluoro-2-phenylcyclopentanc (91 %). 

The use of DFIT in Et3N 5HF(CH2Cl2) for vzcmaZ-difluorinations of variously functionalized terminal alkenes has recently been demonstrated (Scheme 32) [98]. Although fluorinative rearrangements of arylalkenes to 2,2-difluoro-l-arylalkanes with (difluoroiodo)arenes were documented decades ago [99-101],... 

Organosulfur compounds are especially useful for C-fluorine bond forming reactions with (difluoroiodo)arenes. For example, dithioketal derivatives of benzophenones are readily converted to diaryldifluoromethanes with two equivalents of DFIT in dichloromethane [105]. This transformation has also been effected with electro chemically prepared p-(difluoroiodo)anisole/Et3N 3HF, and by anodic oxidations of p-iodoanisole in acetonitrile solutions containing Et3N 3HF and dithioketal substrates (Scheme 35) [96]. Under the latter conditions, p-(difluoroiodo)anisole is continuously regenerated, and the iodoarene was employed at catalytic levels for high yield conversions of the dithioketals to diaryldifluoromethanes. 

Simple alkenes do not give with (difluoroiodo)arenes clean reactions phenylated alkenes under ionic conditions undergo addition of fluorine accompanied by rearrangement in presence of acid (HF, CF3COOH). The reaction proceeds through an initial fluoroalkyl phenyliodonium adduct which is transformed to a phenonium cation and eventually to the rearranged product ... 

Ketones cannot be directly fluorinated by (difluoroiodo)arenes however, a-fluoroketones can be prepared by the reaction of silyl enol ethers with difluoroiodotoluene in the presence of BF3 OEt2 and the Et3N-HF complex [22], Some steroidal silyl enol ethers can be converted into the respective a-fluoroketones in a moderate yield [23]. 

Steroidal dienes 36 react with (difluoroiodo)arenes 37 to afford fluorinated product 38 with a high degree of regioselectivity and stereoselectivity (Scheme 3.14) [14]. (Difluoroiodo)arenes react with terminal alkynes with stereo- and regioselective formation of synthetically useful ( )-2-fluoro-l-alkenyliodonium salts [36-39]. A convenient procedure for the preparation of various ( )-2-fluoroalkenyliodonium fluorides 39 is based on the addition of difluoroiodotoluene to terminal acetylenes (Scheme 3.15) [37-39]. Products 39 can be... 

The generation of (difluoroiodo)arenes by anodic oxidations of aryl iodides in the presence of excess Et3N nHF is a notable recent development (Scheme 31) [96,97]. Because the fluoroiodanes are activated by hydrogen-bonding with HF, such reagent combinations are especially useful for electrophilic fluorinations of organic compounds. When oxidation potentials of the aryl iodides are lower than those of the substrates, indirect anodic fluorinations of the latter can be achieved. 

The (difluoroiodo)arene reagents can be synthesized by anodic oxidation of4-substituted iodo-benzenes. l-(Difluoroiodo)-4-methoxyben/ene is used as a fluorinating agent for l.. -dithio-lanes without isolation and purification. The electrolytic solution is simply mixed with a solution of the dithiolane. The electrolytic cell is depicted in Figure 2. 

Anodic oxidation of dithioketals with /tara-iodoanisole in acetonitrile in the presence of EtsN 3HF as an electrolyte forms gem-difluoro derivatives of diarylmethane (94JOC7190) (Scheme 50). (Difluoroiodo)arenes possess a high fluorinating abihty. These substances need not be isolated pure one can employ their solutions in acetonitrile. 

Shreeve and coworkers reported a convenient procedure for preparing (difluoroiodo)arenes by direct fluorination of the respective iodoarenes with the commercially available fluorinating reagent Selectfluor in acetonitrile solution. This procedure has been further improved by using the corresponding arene, elemental iodine and Selectfluor in a one-pot oxidative iodination/fluorination procedure [33]. 

Difluoroiodo)arenes react with aryl-substituted alkenes to afford the rearranged, geminal difluorides, owing to the migration of the aryl group [32,33]. Likewise, the reaction of substituted cyclic alkenes with difluoroiodotoluene and EtsN/SHF results in a fiuorinative ring-contraction with the selective formation of difluoroalkyl substituted cycloalkanes. Thus, the fluorination of 1-methylcyclohexene derivatives 26 affords... 

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