Photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction, Part 15. Investigations involving fluoride anion as the nucleophile and the effect of fluorine substitution on the relative stability of the reaction intermediates

Abstract

The scope of the photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction has been extended to include fluoride anion as the nucleophile. The 4-cyanophenyl substituted fluoroalkanes were obtained in moderate yields with 2,3-dimethyl-2-butene and 2-methyl-2-butene reacting as the olefin. The yields were lower with 2-methylpropene, and no photo-NOCAS products were detected when conjugated dienes were used as the olefin. The selectivity of the fluoride anion towards alkenes can be explained in terms of the Hard–Soft-Acid–Base (HSAB) principle. The regiochemistry of this reaction can be described as predominantly following the anti-Markovnikov mode of addition. The fluoride anion adds preferentially to the less alkyl-substituted end of an unsymmetrical alkene radical cation to produce the more heavily substituted β-fluoroalkyl radical. Ab initio molecular orbital calculations (MP2/6-31G*//HF/6-31G*) were used to determine the relative stability of the β-fluoroalkyl radical intermediates. The more heavily substituted β-fluoroalkyl radical is NOT the more stable. The addition step is kinetically, rather than thermodynamically, controlled. Keywords: photochemistry, radical ions, electron transfer, alkyl fluorides, nucleophilic addition.