The origin of primary steric effects in aromatic substitution: reactions by alkoxides or amines as nucleophiles

Abstract

Kinetics of the reactions of potassium or benzyltrimethylammonium t-butoxide with 1-fluoro-2- or 1-fluoro-4-nitrobenzene in t-butyl alcohol and of the reactions of piperidine with 2- or 4-nitrophenyl phenyl ether and 2,4-dinitrophenyl phenyl or 2,4-dinitrophenyl cyclohexyl ether in dimethyl sulphoxide are reported. Less quantitative data of similar scope are also reported for the reactions of potassium t-butoxide with the above halogenonitrobenzenes in dimethyl sulphoxide and in some aprotic dipolar solvents or with 1-chloro-2- or 1-chloro-4-nitrobenzene in t-butyl alcohol and for the piperidinodecyclohexyloxylation of 2- or 4-nitrophenyl cyclohexyl ether in neat piperidine. The ortho:para ratio is ca. unity for reactions with piperidine and greater than unity for reactions with t-butoxide. 2,4-Dinitrophenyl phenyl ether is appreciably more reactive than its cyclohexyl analogue but markedly less than was previously found for reactions carried out in benzene. It is shown that in the transition states of these reactions neither inhibition of the resonance of the o-nitro-group nor repulsions between the entering and leaving group occur to any appreciable extent, while the o-nitro-group is markedly rotated out of the plane of the benzene ring both in the starting substrates (chloro-compounds or ethers) and products. These results confirm and extend previous ideas about the origin of primary steric effects in aromatic nucleophilic substitution and their possible implications are discussed.