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Reactivity in the gas phase. Behaviour of isoxazoles under negative ion chemical ionization conditions
Author(s) -
Guarna Antonio,
Moneti Gloriano
Publication year - 1989
Publication title -
organic mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.475
H-Index - 121
eISSN - 1096-9888
pISSN - 0030-493X
DOI - 10.1002/oms.1210240709
Subject(s) - deprotonation , chemistry , fragmentation (computing) , dissociation (chemistry) , deuterium , ion , reactivity (psychology) , isoxazole , medicinal chemistry , bond cleavage , hydrogen–deuterium exchange , stereochemistry , hydrogen , crystallography , organic chemistry , catalysis , atomic physics , medicine , physics , alternative medicine , pathology , computer science , operating system
[M H + ] − ions of isoxazole (la), 3‐methylisoxazole (1b), 5‐methylisoxazole (1c), 5‐phenylisoxazole (1d) and benzoylacetonitrile (2a) are generated using NICI/OH − or NICI/NH 2 − techniques. Their fragmentation pathways are rationalized on the basis of collision‐induced dissociation and mass‐analysed ion kinetic energy spectra and by deuterium labelling studies. 5‐Substituted isoxazoles 1c and 1d, after selective deprotonation at position 3, mainly undergo N O bond cleavage to the stable α‐cyanoenolate NC CH CR O − (R = Me, Ph) that fragments by loss of RCN, or RH, or H 2 O. The same α‐cyanoenolate anion (R = Ph) is obtained from 2a with OH − , or NH 2 − , confirming the structure assigned to the [M H + ] − ion of 1d, On the contrary, 1b is deprotonated mainly at position 5 leading, via NO and C(3)C(4) bond cleavages, to HC ≡ C O − and CH 3 CN. Isoxazole (1a) undergoes deprotonation at either position and subsequent fragmentations. Deuterium labelling revealed an extensive exchange between the hydrogen atoms in the ortho position of the phenyl group and the deuterium atom in the α‐cyanenolate NC CD = CPh O − .