Diaryl ethers, protonation

When three isomers of hydroxyacetophenone were treated separately with 4-fluoronitrobenzene [156] 369 (Scheme 122) and potassium carbonate (DMF, 120 °C) the para- and meta-isomers gave the respective diaryl ethers (0-arylation products). The ortho-isomer 370 behaves in a different way the C-arylation product 373 was formed exclusively (isolated in 73% yield). 0-Arylation of 370 occurred first to give diaryl ether (protonated 371) and this is followed by the rearrangement through the intermediate 372. At lower temperature (60 °C) 371 (protonated) could be isolated in 21% yield along with unchanged 370. 

Alkylaryl ethers and diaryl ethers undergo protonation on either oxygen or carbon, depending upon the acidity of the medium87 (Scheme 4.3). Both the C-protonated species 28 and the (9-protonated species 29 have been observed. The evidence mainly comes from NMR and UV data. Sommer et al.88 have used para-anisaldehyde as an indicator in acidity measurements in the superacidity range. The barrier of rotation around the Cipso—0 bond upon (9-protonation has been used as a criterion in such studies. The torsional barrier around the phenyl-alkoxy bond in the C-protonated forms of aromatic ethers has also been measured by spin-saturation transfer measurements.89... 

Diaryl amines, ethers or sulphides, or their aryl vinyl analogues, provide another 6-electron system related to stilbene, but for these a pair of electrons is provided by the single heteroatom (ArXAr or ArXC=C). With dtaryl compounds the initial photocydized product is a zwitterion that undergoes a proton shift to give, for example, N-methyl-4a,4b-dihydrocarbazole from methyldiphenylamine, with subsequent oxidation to N-methylcarbazole (3.74). With the aryl vinyl analogues the product after the proton shift can normally be isolated (3.751. An especially useful variation of this reaction employs... 

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