Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Selective anodic fluorination using fluorinating

Anodic fluorination of a-phenylthioacetate 1 in ionic liquid Et3N 3HF proceeds smoothly without anode passivation under ultrasonication to provide the a-mono-fluorinated product 2 in high yield and with high current efficiency. Notably, anodic difluorination of 1 can be also achieved in the same ionic liquid under ultrasonication as shown in Scheme 8.4 [15]. Thus the current efficincy for the anodic difluorination of 1 was greatly improved as compared with that without ultrasonication. As described above, Et3N 3HF ionic liquid is highly useful for selective anodic fluorination. [Pg.94]

As mentioned above, liquid fluoride salts like EtaN nHF (n = 3-5) and Et4NF-4HF have proved to be highly useful as the electrolytic media and fluoride ion source for selective anodic fluorination. However, this solvent-free method has an atom economy problem because of the use of an excessive amount of liquid salts in place of a solvent. [Pg.96]

In 1953, anodic fluorination of 1,1-diphenylethylene in KH2/ACOH was reported by Schmidt and Schmidt [Eq. (2)] [10]. This seems to be the first example of selective electrochemical fluorination. They also used liquid HF, MeCN, EtOH, or AcOH containing anhydrous HF [11], or even AgF as the fluorine source [12]. However, these electrolytic systems are not always effective and not widely applicable. [Pg.1036]

Generally, anodic benzylic substitution reactions take place readily. However, anodic benzylic fluorination does not always occur. The major competitive reaction is acet-amidation when MeCN is used as a solvent [24-26]. In contrast to these cases, triphenylmethane is selectively monofluorinated in 80% yield [27]. When Et4NF-/7HF n = 3,4,5) is used in the absence of MeCN, anodic fluorination of toluene and mono-fluoromethylbenzene provides mainly mono- and difluoromethylbenzenes, respectively [22]. On the other hand, difluoromethylbenzene affords only products fluorinated in the benzene ring [Eq. (4)] [28]. [Pg.1037]

The diastereoselective anodic fluorination of a-phenylsulfenyl acetates has been studied using various chiral auxiliaries [Eq. (15)] [49]. The diastereoselectivity is affected by supporting fluoride salts, and Et4NF-3HF leads to better selectivity relative to Et3N-3HF or Et3N-2HF. [Pg.1040]

As mentioned earlier, passivation of the anode due to the formation of nonconductive polymers on the anode takes place commonly during anodic oxidation of organic substrates in the presence of fluoride ions. For example, as shown in Eq. (38), anodic oxidative difluorodesulfurization of dithioacetals does occur however, the current efficiencies are low due to this passivation phenomenon [89]. In order to prevent such passivation, Fuchigami and coworkers have developed an indirect electrochemical method using various mediators [91-95]. Thus, Br /Br" and triarylamine redox mediators have been shown to be effective for selective mono- and difluorodesulfurization of dithoacetals, respectively [91]. Furthermore, triarylamine has recently been shown to be a highly effective mediator for monofluorodesulfurization of )0-lactams [Eq. (39)] [95]. In the absence of triarylamine, severe passivation of the anode takes place during anodic fluorination. [Pg.1046]

Radical cations of triaiylamines are known to be single electron transfer oxidizing agents. Very recently, these substances have been used as efficient mediators for gcm-difluorodesulfurization as shown in Scheme 56. ° Furthermore, triarylamines have recently been shown to be highly effective mediators for monofluorodesulfurization of P-lactams. Severe passivation of the anode takes place during anodic fluorination of P-phenylthio- P-lactams, which is quite different from the case of a-phenylthio-P-lactams 60. However, selective anodic fluorodesulfurization proceeds efficiently without passivation when tris(2,4-dibromophenyl)amine is employed as a mediator as shown in Scheme 57. ... [Pg.121]

Solvent-free electrochemical fluorination is an alternative method for preventing anode passivation and acetoamidation [18, 19]. As already mentioned, handling extremely corrosive and poisonous anhydrous HF in a laboratory setting is accompanied by serious hazards and experimental difficulties. Molten salts such as 70 % HF/pyridine (Olah s reagent) and commercially available EtaN-SHF [20] are often used to replace anhydrous HF. Other molten salts with the general formula R4NF-nHF (n > 3.5, R = Me, Et, and n-Pr) are useful in selective electrochemical fluorination. These electrolytes... [Pg.1870]

In a later publication [ 129], using the same equipment, Liu et al. describe process improvements in the electrochemical fluorination of octanoyl chloride in which formation of polymeric tar at the anode surface was limited by addition of a mercaptan (1-methyl-1-propanethiol), and by constant current density operation (7 mA cm-2). Continuous operation was achieved by frequent additions of a solution of reactant in hydrogen fluoride. Conversion of reactant to perfluori-nated products was increased to 80%, with good selectivity. [Pg.225]

Selective Organic Electrofluorination. The electrogeneration of fluorine from the electrolysis of a mixture of KF and HF has been well known for many decades. Organic perfluorination using HF and HF/KF is used widely. Now the use of HF complexes with tetraalkylammonium fluoride has led to the anodic difluorination of dithioacetals. [Pg.99]

Kirste A, Nieger M, Malkowsky IM, Stecker F, Fischer A, Waldvogel SR (2009) ortho-Selective phenol-coupling reaction by anodic treatment on boron-doped diamond electrode using fluorinated alcohols. Chem-Eur J 15 2273-2277... [Pg.28]

Laurent and coworkers found that anodic benzylic mono- and difluorination proceeds selectively when the benzylic position is substituted by electron-withdrawing groups (EWG) [Eq. (5)] [29,30]. In these cases, / -methoxy or />-chloro substituents on the benzene ring are necessary for the operation of efficient fluorination. In their absence, benzylic acetamidation becomes a major reaction. The use of sulfolane as a solvent instead of MeCN leads to higher yields of fluorinated products owing to the absence of competing acetamidation [31]. [Pg.1037]

The electrochemical difluorination of diethyl l-(phenylsulfenyl)methylphosphonate using the powerful fluorine source Et4NF4HF in MeCN takes place smoothly without passivation of the anode to give diethyl l-(phenylsulfenyl)-l,l-difluoromethylphosphonate selectively in 50% yield. -... [Pg.102]

Highly regioselective anodic monofluorination of 2-aryl-4-thia-zolidinones 53 can also be promoted by using pulse electrolysis in EtjN-SHF/MeCN as shown in Scheme 41. In this case, benzylic fluorination does not take place and fluorination occurs at the 5-position selectively. The regioselectivity of this process appears also to be controlled kinetically as shown in Scheme 40. The transformation of the sulfones 55 derived from fluorinated products 54 into monofluoro P-lactams 56 is readily performed in excellent yields by thermolysis (Scheme 41). [Pg.110]


See other pages where Selective anodic fluorination using fluorinating is mentioned: [Pg.1043]    [Pg.103]    [Pg.279]    [Pg.658]    [Pg.279]    [Pg.89]    [Pg.3821]    [Pg.1868]    [Pg.106]    [Pg.112]    [Pg.346]    [Pg.162]    [Pg.112]    [Pg.112]    [Pg.1043]    [Pg.1046]    [Pg.364]    [Pg.93]    [Pg.244]    [Pg.79]    [Pg.136]    [Pg.324]    [Pg.325]    [Pg.264]    [Pg.346]    [Pg.2577]    [Pg.623]    [Pg.33]    [Pg.145]    [Pg.251]    [Pg.145]    [Pg.389]    [Pg.31]   


SEARCH



Fluorination, selective

Selective anodic fluorination

© 2024 chempedia.info