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

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 beta-fluoroalkyl radical. Ab initio molecular orbital calculations (MP2/6-31G*//HF/6-31G*) were used to determine the relative stability of the beta-fluoroalkyl radical intermediates. The more heavily substituted beta-fluoroalkyl radical is NOT the more stable. The addition step is kinetically, rather than thermodynamically, controlled.