Change of the favored routes of EI MS fragmentation when proceeding from N1, N1-dimethyl-N2-arylformamidines to 1,1,3,3-tetraalkyl-2-arylguanidines: substituent effects.

Although series of N(1), N(1)-dimethyl-N(2)-arylformamidines and of 1,1,3,3-tetraalkyl-2-arylguanidines are structurally analogous and similar electron-ionization mass spectral fragmentation may be expected, they display important differences in the favored routes of fragmentation and consequently in substituent effects on ion abundances. In the case of formamidines, the cyclization-elimination process (initiated by nucleophilic attack of the N-amino atom on the 2-position of the phenyl ring) and formation of the cyclic benzimidazolium [M-H](+) ions dominates, whereas the loss of the NR(2) group is more favored for guanidines. In order to gain information on the most probable structures of the principal fragments, quantum-chemical calculations were performed on a selected set. A good linear relation between log{I[M-H](+)I [M](+*)} and sigma(R)(+) constants of substituent at para position in the phenyl ring occurs solely for formamidines (r = 0.989). In the case of guanidines, this relation is not significant (r = 0.659). A good linear relation is found between log{I[M-NMe(2)](+)/I [M](+*)} and sigma(p)(+) constants (r = 0.993).