Aryl-oxygen bonds, reductive cleavage

The surprising stability of triaryloxonium ions has been demonstrated by the fact that reduction of the tris(4-nitrophenyl)oxonium tetrafluoroborate 35 can be performed to give the tris(4-aminophenyl)oxonium salt 36 without cleavage of aryl oxygen bonds [Eq. (4.21)]. Diazotation of this amino derivative and reaction with iodide leads to the tris(/v/ra-iodophenyl)oxonium salt 37 in excellent yields.104... 

The carbon-oxygen bond is strong, like the normal carbon-fluorine bond, it is difficult to reduce However, some structural features facilitate its reductive cleavage Aryl esters of perfluoroalkanesulfomc acids can be cleaved in good yield by... 

Cyclic alkyl aryl ethers lead also to functionalized organolithium compounds by reductive carbon-oxygen bond cleavage in arene-catalyzed lithiation process. Thus, the treatment of 2,3-dihydrobenzofuran (47) with an excess of lithium in the presence of a catalytic amount of DTBB in THF at 0°C gives the dianion (48) which after reaction with different carbonyl compounds and final hydrolysis with water leads to... 

The cleavage of carbon-oxygen bonds from alkenyl or aryl phosphates can be accomplished under reductive conditions with a low valent metal. As vinyl phosphates can be formed readily from ketones, this procedure provides a method to convert a ketone to an alkene. For example, the alkenyl phosphate 74 was prepared by trapping the enolate formed on reduction of the enone 73 and was converted into the alkene 75 (7.55). The chemistry therefore provides a method to prepare structurally specihc alkenes. Low-valent titanium (prepared for example by reduction of titanium(III) chloride with potassium metal) is a convenient alternative to lithium or sodium in liquid ammonia or an amine for the reductive cleavage of alkenyl or aryl phosphates. 

Tanaka et al. demonstrated that the reaction of o-alkynylphenyl vinyl ether 42 was effectively converted to 2,3-disubstituted furans 43 in the presence of rhodium catalysts via C—O bond cleavage (Scheme 27.18) [22]. The reaction is initiated by the generation of rhodacyclopentene 44. Subsequent p-oxygen elimination, followed by aryl-alkenyl bond rotation in the zwitterionic intermediate 45 and cyclization of the rhodium carbenoid 46, leads to oxarhodacycle 47. Finally, reductive elimination of rhodium catalyst yields benzofuran 43. 

Finally, side-chain functionalization may be obtained through a PET process. Apart from the several arylations discussed in Sect. 2.1.3, oxygenation of benzyl radicals obtained from deprotonation or carbon-carbon bond cleavage is preparatively useful (the method has been recently reviewed [254]), and other functionalization are possible, notably fluorination of benzyl substrates by irradiation in the presence of titanium dioxide and fluoride ions [255], Furthermore, the modification of a substituent, as a typical example the reduction of a nitro group [18, 27, 256-259] or an azo group [266], is in some cases obtained through a PET process. 

Homolytic cleavage of the aryl carbon-halogen bond affords aryl free radicals, which can either arylate a suitable aromatic reaction partner or abstract hydrogen from a hydrogen atom donor (reductive dehalogenation), as first reported over 35 years ago. A recent example is the report by Meunier et al. that photolysis of 1,4-dichlorobenzene in the absence of oxygen yielded 4,4 -dichlorobiphenyl and 2,4, 5-trichlorobiphenyl as reaction products. The general reaction mechanism proceeds as follows ... 

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