Aryl fluorides, reduction

Scheme 23 Aryl-fluoride reductive elimination from an isolated Pd(IV) fluoro complex...

Yandulov DV, Tran NT (2007) Aryl-Fluoride reductive elimination from Pd(II) Feasibility assessment from theory and experiment. J Am Chem Soc 129 1342-1358... 

A recently discovered variant of the Wallach technique is the silver ion cata lyzed fluorination of aryl diazo sulfides in hydrogen fluonde-pyridine-toluene solvent [57] (equation 12) Electron withdrawing substituents such as acetyl give higher yields of aryl fluoride (71%) than electron donating groups (butyl 39%, methoxy, 2-14%), reductive dediazoniation competes with fluorination... 

Reduction of aryl fluorides by vanous reagents to hydro denvatives was also treated extensively m reference 12 A httle used method, electrochemical reduction, defluonnates simple aryl fluondes [19, 20] (equations 13 and 14)... 

From the lithium chemistry point of view, the carbon-fluorine bond cannot be considered as a carbon-halogen bond because alkyl or aryl fluorides are not adequate starting materials in lithiation processes, due to the fact that this bond is the strongest that carbon can form . On the other hand, a possible reductive defluorination process could be important from an environmental point of view due to the difficult degradation of fluoro derivatives in nature . 

Electrochemical reduction, though a little-used method, defluorinates aryl fluorides to the corresponding hydrogeno derivatives. 1,3-Difluorobenz.ene is reduced at a mercury cathode to give a mixture of fluorobenzene and benzene,54 while the reduction of perfluorobenzene at an... 

The precursor 130, which was prepared using a combined YNSat-SnAt three-component reaction, has been employed in a route toward the 3-aryloxindole derivative 131 (Equation 36) <2004OL4957>. Similar reductive annulations of intermediates derived by nucleophilic displacement of an immobilized aryl fluoride with dimethyl malonate anions, eventually providing 1,2-dialkoxyindole derivatives, have been described previously <2003OL2935>. 

Several forms of highly reactive Mg metal are available they permit reaction with organic halides that otherwise could not react. Finely divided and highly reactive black Mg powder is prepared by the reduction of anhydr MgCl2 with potassium metal in refluxing THF . This powder forms a reagent even at —78°C or with aryl fluorides, which are otherwise unreactive ... 

Whitfield and Sanford isolated di and monochloro Pd(IV) complexes of 2-phenylpyridine which upon heating in acetic acid gave substantial amounts of the chloroarene (Scheme 22) [55], although the fate of the transition metal remained unclear. More recently, Puruya and Ritter presented the first example of a reductive elimination of aryl fluoride from an isolated Pd(IV) center (Scheme 23) [56]. 

It has been shown that the use of ionic liquids may be beneficial in aromatic fluorinations in protic solvents." Aryl fluorides may also be obtained using a copper-catalysed halide exchange reaction. The evidence suggests a redox Cu(I)/Cu(III) catalytic cycle involving oxidative addition of aryl halide at the copper(I) centre followed by halide exchange and reductive elimination." A mechanistic investigation of the palladium-catalysed conversion of aryl triflates to fluorides has shown that C-F reductive elimination from the palladium—arene complex does not occur when the aryl group is electron rich and requires in situ modification of the catalyst." ... 

Intriguingly, thermolysis of 53 at 80°C for 1 h in nitrobenzene did not lead to significant quantities of aryl fluoride 54 (Eq. 43a). Instead, biaryl 55 was the major organic product (35% yield). Compound 55 could be formed via several different pathways, including (1) transmetalation followed by C-C bond-forming reductive elimination, (2) generation of Ar and subsequent radical coupling, or (3) a bimetallic reductive elimination process. 

Benzimidazoles have been assembled by nucleophilic displacement of aryl fluorides with amines, followed by reduction and condensation with imidates... 

The formation of aryl fluorides from aryl iodides has been achieved using silver fluoride and a cationic copper reagent. The reaction is likely to involve oxidative addition of the aryl iodide to the copper to give an aryl copper(III) iodide, followed by trans-metalation of silver fluoride forming an aryl copper(III) fluoride, (12). The product is formed after reductive elimination. There has been a theoretical study, using DPT, of the gas-phase fluorodenitration of nitrobenzene. ... 

A trialkylsilyl group can be introduced into aryl or alkenyl groups using hexaalkyidisilanes. The Si—Si bond is cleaved with a Pd catalyst, and trans-metallation and reductive elimination afford the silylated products. In this way, 1,2-bis-silylethylene 761 is prepared from 1,2-dichloroethylene (760)[625,626], The facile reaction of (Me3Si)2 to give 762 proceeds at room temperature in the presence of fluoride anion[627]. Alkenyl- and arylsilanes are prepared by the reaction of (Me3Si)3Al (763)[628],... 

Aryl and alkyl trimethylsilyl ethers can often be cleaved by refluxing in aqueous methanol, an advantage for acid- or base-sensitive substrates. The ethers are stable to Grignard and Wittig reactions and to reduction with lithium aluminum hydride at —15°. Aryl -butyldimethylsilyl ethers and other sterically more demanding silyl ethers require acid- or fluoride ion-catalyzed hydrolysis for removal. Increased steric bulk also improves their stability to a much harsher set of conditions. An excellent review of the selective deprotection of alkyl silyl ethers and aryl silyl ethers has been published. ... 

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