Endocyclic versus exocyclic mechanisms for methyl migration in protonated N,N'-dimethylpropane-1,3-diamine

A recent paper has suggested that an endocyclic methyl transfer pathway occurs in competition with methylamine loss for protonated N,N'-dimethylpropane-1,3-diamine under conditions of low-energy collision induced dissociation [X. Zhang, S. Yao and Y. Guo, Int. J. Mass Spectrom. 214, 277 (2008)]. However, such endocyclic methyl transfers appear to be unprecedented in the gas phase. Therefore, in order to gain additional insights into the competition between methylamine loss and methyl transfer in this system, DFT calculations were performed at the B3LYP/6-311+G(d,p) level of theory for a number of competing mechanisms. Three mechanisms were considered for loss of methylamine: (i) a 1,2-elimination reaction to give N-methylallylamine (TS = 276.7 kJ mol(-1)); (ii) a neighbouring group reaction to give N-methylazitidine (TS = 146.4 kJ mol(-1)); and (iii) a 1,3-hydride shift to give N-methyl-1-propylimine (TS = 248.5 kJ mol(-1)). Accordingly, the neighbouring group pathway is expected to be kinetically favoured and dominate under conditions of low-energy collision-induced dissociation. Similarly, three different mechanisms were considered for intramolecular methyl transfer: (i) the previously proposed endocyclic reaction involving backside attack with inversion of configuration (TS = 252.3 kJ mol(-1)); (ii) the previously proposed endocyclic reaction involving frontside attack with retention of configuration (TS = 272.4 kJ mol(-1)); (iii) a multi-step mechanism which combines the neighbouring group pathway for methylamine loss and combinations of S(N)2 and proton transfer reactions within a series of ion-molecule complexes (highest TS = 201.7 kJ mol(-1)). These results suggest that the alternative pathway proposed here for methyl transfer should be preferred under conditions of low energy collision- induced dissociation.