Loss of benzaldehyde in the fragmentation of protonated benzoylamines: Benzoyl cation as a hydride acceptor in the gas phase

In electrospray ionization tandem mass spectrometry of protonated 1‐benzoylamines (1‐benzoylpiperadine, 1‐benzoylmorpholine, and 1‐benzoyl‐4‐methylpiperazine), the dominant fragmentation pathway was amide bond cleavage to form benzoyl cation and neutral amine. Meanwhile, in their fragmentations, an interesting loss of benzaldehyde (106 Da) was observed and identified to derive from hydride transfer reaction between the benzoyl cation and amine. A stepwise mechanism for loss of 106 Da (benzene and CO) could be excluded with the aid of deuterium labeling experiment. Theoretical calculations indicated that hydride transfers from amines (piperadine, morpholine, and 1‐methylpiperazine) to benzoyl cation were thermodynamically permitted, and 1‐methylpiperazine was the best hydride donor among the 3 amines. The mass spectrometric experimental results were consistent with the computational results. The relative abundance of the iminium cation (relative to the benzoyl cation) in the fragmentation of protonated 1‐benzoyl‐4‐methylpiperazine was higher than that in the fragmentation of the other 2 protonated 1‐benzoylamines. By comparing the fragmentations of protonated 1‐benzyl‐4‐methylpiperazine and protonated 1‐benzoyl‐4‐methylpiperazine and the energetics of their hydride transfer reactions, this study revealed that benzoyl cation was a hydride acceptor in the gas phase, but which was weaker than benzyl cation.