Activated aromatic substitution by piperidine or [1-2H]piperidine in benzene. Dependence of kinetic deuterium isotope effect on the group displaced

Abstract

Rates of reactions of 1-chloro-4,7-dinitronaphthalene (A), 4-chloro-3-nitrobenzotrifluoride (B,B′), 2,4-dinitrophenyl cyclohexyl ether (C), 1-fluoro-4-nitrobenzene (D), or 1-fluoro-4,7-dinitronaphthalene (E) with piperidine or [1-²H]piperidine to form exclusively N-4,7-dinitronaphthyl-(A,E), N-2-nitro-4-trifluoromethylphenyl-(B,B′), N-2,4-dinitrophenyl-(C), or N-4-nitrophenyl-piperidine (D) have been measured as a function of nucleophile concentration in benzene. Both the extent of rate acceleration by piperidine and the value of the kinetic deuterium isotope effect vary with the leaving group. When the leaving group is chloride, contrary to a previous report, the reactions are found to be only very mildly accelerated, and in a linear fashion, by piperidine. Moreover, substitution of deuteriated amine (90% deuteriated) for piperidine does not appreciably alter the kinetics. When it is cyclohexyloxy-(C) or fluoride (D,E), third-order kinetics (second-order in amine concentration) are obtained. The relative reactivity of the light amine over the deuteriated one is 1·49 ± 0·03 for reaction C, 1·03 ± 0·04 for reaction E, and ca. 1·1 for reaction D. These facts are interpreted within the framework of the intermediate complex mechanism with decay of the addition intermediate into products being fast in reactions A and B and slow in the other reactions.