Mechanism of reactions of N-(methoxymethyl)-N,N-dimethylanilinium ions with nucleophilic reagents

The prediction that the oxocarbonium ion derived from formaldehyde should have a lifetime of approximately 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/sub N/2 (Swain-Scott) and carbonium ion (N+) reactions. The small values of s = 0.3 and β/sub nuc/ = 0.14 and large values of β/sub lg/ = -0.7 to -0.9 indicate a transition state that can be described either as an open transition state for S/sub N/2 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/sub H//k/sub 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++ = -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