Electron-donating effect stabilization of carbocation

Any electrophiles such as protons (1) [2], chloronium cations (2,3) [3, 4], benzene sele-nenyl (4) [5] and sulfenyl cations (5) [6], nitronium ion (6) [7], acyl cation (7) [8], hydrox-ylmethyl carbocation (8) [9], and even palladium species (9) [10] react at the methylene 

Another question is whether the fluorine-stabilized carbocation formation is preferred even in the electrophilic reaction to difluoroalkylidene compounds. Sulfuric acid catalyzed 

The stabilization of the carbocation intermediate 35 by the isobutylidene group must be a driving force to promote the reaction. The striking feature in the cyclization is a remarkable acceleration effect by the fluorine substituent at the same carbon. The fluorine substituent gives an excellent rate of acceleration and product yield. In contrast, the SnCU-catalyzed cyclization of nonsubstituted polyolefin 34 (R = H) in hexane gives only 30% of the cyclized product in 20 min. 

The same type of rate-acceleration effect by the a-fluorine substituent is known in electrophilic substitution on the aromatic ring. Fluorine accelerates bromination, while chlorine 

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