Trends in alkyl substituent effects on nucleophilic reactions of carbonyl compounds: gas-phase reactions between amines and the methoxy methyl cation

The reactions of various amines (RNH2 : R = H, CH3, C2H5 and i-C3H7) with the methoxy methyl cation (CH2OCH3+) have been investigated using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, and the experimental results are complemented with ab initio calculations. The amines show clear trends in their reactivities with variable degree of: (1) nucleophilic substitution, (2) addition-elimination and (3) hydride abstraction. In all cases, addition-elimination dominates over nucleophilic substitution, and for R not equal H the observed reactions occur at the collisional limit. The potential energy profiles for all three reaction types correlate with the basicities of the amines: i.e. the more basic the amine-the more favorable is the reaction. In other words, nucleophilicity follows basicity in the gas phase.