Carbcnium ions

Acetale konnen durch elektrophile Hydride in den meisten Fallen reduziert werden, aus acyclischen Acetalen werden Ather und Alkohole, aus cyclischen tu-Hydroxy-ather erhalten. Der erste Schritt, der durch Lewis-Sauren katalysiert wird, ist auch hier die Bil-dung des entsprechenden Carbcnium-Ions (s.S. 409). Die Methode ermoglicht die einfa-che Herstellung von Athern aus Aldehyden und Ketonen. 

Barton and co-workers have studied30 the reactions of trifluoromethyl hypofluorite with various alkenes and found that (Z)- and (T -stilbene give vicinal fluoro trifluoromclhoxy products and vicinal difluorides in chlorotrifluoromethane or diethyl ether, while vicinal fluoro methoxy adducts predominate when the reaction is carried out in methanol at —78 C. svn Addition predominates in all cases on the basis of the fluorination regioselectivity of many alkenes, the formation of a /1-fluoro carbcnium ion is proposed.30... 

The parameters a do not differ significantly, so that the rates of formation of the various isomers equally deactivate, except the 2,2-di-Me-butazie formation. Where the corresponding carbcnium ion of all other hexane isomers is tertiary, that for 2,2-di-Me-butane is secondary, and thus its formation is kinetically much less favored. This explains the low concentration of 2,2-di-Me-butane in the reaction mixture at the conversions shown in Table 3b, and implies that its formation needs a stronger acid site. Since coke is preferentially formed on the stronger acid sites, the formation of this hexane isomer will be more deactivated by the coke formation. 

Volume E 21 D.l.5.5. Addition of Carbcnium Ions to Olefmic Bonds and Allylic Systems... 

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