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Electron impact mass spectra of 2‐(2‐pyridyl)methylene‐1,3‐dicarbonyl compounds: Unusual abundance of the [M + 1] + ion and the effect of stereochemistry
Author(s) -
Pohjala Esko K.,
Nieminen Antti O. K.
Publication year - 1981
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.1210161203
Subject(s) - methylene , chemistry , malonate , mass spectrum , ion , electron ionization , substituent , medicinal chemistry , ketone , diketone , ionization , kinetic isotope effect , stereochemistry , organic chemistry , physics , deuterium , quantum mechanics
Electron impact mass spectra of 2‐(2‐pyridyl)methylene‐1,3‐dicarbonyl compounds and related heteroaryl species have been investigated. In 3‐(2‐pyridyl)methylene‐2,4‐pentanedione, its 6′‐methyl and 6′‐methoxycarbonyl derivatives and in E ‐ and Z‐ethyl 3‐oxo‐2‐(2‐pyridyl)methylenebutanoates the base peak arises from the loss of methyl radical from the molecular ion to produce a 3‐oxo‐3 H ‐indolizinium ion. A marked difference is observed in the behaviour of the geometric isomers of the keto esters. The diketones and E ‐keto ester carrying a 2‐pyridyl substituent and ketone functionality on the same side of the carbon‐carbon double bond exhibit an unusually high [M + 1] + /[M] +˙ ratio (about 2.5) under normal ionization conditions (pressure 10–100 μPa). This abnormality is a function of pressure only and independent of temperature. In the case of the Z‐keto ester, the corresponding malonate, 3‐ and 4‐(2‐pyridyl)methylene‐2,4‐pentanediones, and 2‐furyl, 2‐thienyl and phenyl diketone analogues, the ratio does not differ much from that due to the natural isotope abundance. Results for 1,1,1,5,5,5‐hexadeuterio‐2‐(2‐pyridyl)‐methylene‐2,4‐pentanedione (strong M + 2 peak) suggest one mass unit transfer as an intermolecular proton shift from a methyl group to give a 3‐hydroxy‐3‐methyl‐3 H ‐indolizinium ion. This real mass spectrometric phenomenon is a unique example of low pressure self‐chemical ionization.