Mechanism of Reactions of N-(Methoxymethyl)-N,N-dimethylanilinium Ions with Nucleophilic Reagents

Article abstract of DOI:10.1021/ja00542a021

The prediction that the oxocarbonium ion derived from formaldehyde should have a "lifetime" of ca. 10^-15 s that gives rise to an enforced preassociation or concerted reaction mechanism has been tested by examining the reactions of N-(methoxymethyl)-N,N-dimethylanilinium ions in water in the presence of added nucleophilic reagents.These compounds undergo well-behaved second-order reactions with nucleophiles and give the amount of substitution product that is expected from the rate increase in the presence of nucleophile.Structure-reactivity correlations exhibit behavior intermediate between that expected for S_N2 (Swain-Scott) and carbonium ion (N⁺) reactions.The small values of s = 0.3 and β_nuc = 0.14 and large values of β_1g = -0.7 to -0.9 indicate a transition state that can be described either as an open transition state for S_N2 displacement or as an oxocarbonium ion that is stabilized by weak interactions with the entering and leaving groups.Secondary α-deuterium isotope effects for the second-order reactions range from (k_H/k_D)/D = 0.99 for fluoride ion to 1.18 for iodide ion.Solvolysis and the second-order reaction with n-propylamine exhibit values of ΔS(ex) = -1.2 and -2.1 cal K^-1 mol^-1, respectively, and display similar changes in rate in mixed water-alcohol solvents.It is suggested that the lifetime of the carbonium ion species in the presence of nucleophiles is so short that the reaction mechanism must be concerted.