Ipso Nitration. Solvolytic Behavior of 1,4-Dimethyl-4-nitrohexadienyl Acetate and 1,4-Dimethyl-4-nitrocyclohexadienol

Article abstract of DOI:10.1021/ja00398a031

The solvolytic rearomatizations of the E and Z isomers of 1,4-dimethyl-4-nitrocyclohexadienyl acetate (1) have been examined in aqueous ethanol and in sulfuric acid solutions.Solvolysis of 1 in aqueous ethanol involves the elimination of nitrous acid and the migration of the acetoxyl group to yield 2,5-dimethylphenyl acetate.The kinetic behavior of this solvolytic reaction parallels that of the secondary adduct 4-nitro-3,4,5-trimethylcyclohexadienyl acetate (2).No kinetic isotope effects were detected when 1,4-dimethyl-4-nitrocyclohexadienyl-2,3,5,6-d₄ acetate (1-d₄) or 1,4-dimethyl-4-nitrocyclohexadienyl-Me-d₆ (1-d₆) were used as substrates.The collected data suggest a rate-limiting loss of nitrile ion from 1 followed by migration of the acetoxyl group and proton loss.The behavior of Z and E isomers of 1,4-dimethyl-4-nitrocyclohexadienol (3) in aqueous ethanol differs from that of 1 in several ways.The major product of solvolysis is 2,4-dimethylphenol.The rates of solvolysis of the stereoisomers of 3 do not differ appreciably one from another, and the spectral yield is only 30-40percent of that anticipated.These differences are taken to reflect a rate-limiting migration in the solvolysis of 3 rather than a rate-limiting loss of nitrate ion.Solvolysis of 1 in greater than 77percent sulfuric acid gives 1,4-dimethyl-2-nitrobenzene in quantitative yield.The yield of nitro aromatic falls off smoothly as the acid concentration is lowered toward 50percent.Within the 70-50percent acid range, the yield of side-chain substitution products appears to increase, hold roughly constant, and then decrease.Larger yields of nitro aromatic and smaller yields of side-chain substitution product are found when 1-d₆ is used as a reactant.The solvolyses in strong acids are discussed in terms of re-formation of the ipso ion and subsequent partitioning.