Regiospecific nucleophilic substitution in 2,3,4,5,6-pentafluorobiphenyl as model compound for supramolecular systems. Theoretical study of transition states and energy profiles, evidence for tetrahedral S_N2 mechanism

摘要

2,3,4,5,6-Pentafluorobiphenyl (PFBi) was modified by the nucleophilic substitution of one fluorine using a series of 0-, S- and N-nucleophiles, viz. alkaline salts of 2,2,2-trifluoro-ethanol, 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctanol, 1,2;3,4-di-O-isopropylidenexylitol, allylsulfane, 3,3,4,4,5,5,6,6,7,7,8,8,8-trideca-fluorooctane-1-thiol, 3-aminopropan-1-ol (7), and tert-butyl N-(3-aminopropyl)carbamate (8). All the substitutions took place exclusively at the position para to the phenyl group. (3-Amino-propyl)amino derivative of PFBi (15) was further modified at the terminal amino group by acylation or fluoroalkylation. The reaction of 8 was applied to meso-5,10,15,20-tetrakis-(pentafluorophenyl)porphyrin (20) to afford tris- (21) and tetrakis-substituted (22) products with complete para-regioselectivity. Theoretical studies of the reaction pathways of PFBi with ammonia, microsolvated lithium fluoride or lithium hydroxide revealed that no Meisenheimer-type intermediates are formed in the course of the simulated reactions: instead, tetrahedral S_N2 mechanism was found. Significant regioselectivity of the nucleophilic aromatic substitution, leading to 4-substituted products, was predicted based on relative transition state energies in agreement with the observed experimental results.