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McLafferty‐type rearrangement of protonated N ‐[nicotinoyl]phenylethyl amines and consequent elimination of styrene
Author(s) -
Paulose Justin,
Achuthan Revi P.,
Mathai George,
Chander Purna,
Srinivas Ragampeta
Publication year - 2015
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.7104
Subject(s) - chemistry , protonation , ion , mass spectrum , mass spectrometry , molecule , fragmentation (computing) , dissociation (chemistry) , polyatomic ion , medicinal chemistry , organic chemistry , chromatography , computer science , operating system
Rationale McLafferty rearrangements occur in radical cations of molecules containing a carbonyl group and a γ hydrogen atom but are not common in the [M+H] + ions of carbonyl compounds. We propose to investigate the collision‐induced dissociation (CID) of the [M+H] + ions of nicotinoyl and picolinoyl amides of 1‐ and 2‐phenylethylamines to explore the possibility of McLafferty‐type rearrangement. Methods The compounds for study were synthesized by the reaction of methyl nicotinate or methyl picolinate with 1‐ and 2‐phenylethylamines. The CID mass spectra of electrospray ionization (ESI)‐generated protonated molecules were obtained using a QSTAR XL quadrupole time‐of‐flight (QTOF) mass spectrometer, and density functional theory (DFT) calculations using the B3LYP method were employed to elucidate the fragmentation mechanisms. The total electronic and thermal energies of intermediate transition states (TSs) and product ions are reported relative to those of the [M+H] + ions. Results CID of the [M+H] + ions of N ‐[nicotinoyl]‐2‐phenylethylamine (1) yielded product ions of m/z 105 (1‐phenylethyl cation) and 123 ([M+H–styrene] + cation). The competitive formation of the ions of m/z 123 and 105 is proposed to involve a McLafferty‐type rearrangement. Similarly, the [M+H] + ions of the isomeric compound 2 and the N ‐[picolinoyl] phenylethyl amines (3 and 4) dissociate to yield ions of m/z 123 and 105. Conclusions A molecule of styrene was eliminated from the ESI‐generated [M+H] + ions of N ‐[nicotinoyl]phenylethylamines and the isomeric N ‐[picolinoyl]phenylethylamines, through a mechanism involving a McLafferty‐type 1,5‐H shift. The transition state energy for the 1,5‐H shift is less for the amides of 1‐phenylethylamine than for the amides of 2‐phenylethylamine. The process occurs as a charge remote process and the presence of the pyridine ring is essential for the process. Copyright © 2015 John Wiley & Sons, Ltd.