MASS SPECTRAL MECHANISMS . N-ALKYL-3-CYANO-1,4-DIHYDROPYRIDINE FRAGMENTATION . KINETIC ISOTOPE EFFECTS FOR EXPULSION OF DEUTERIUM FROM 4 POSITION AND TRANSITION FROM QUANTAL TO PURELY CLASSICAL ISOTOPE EFFECT

The mass spectra of N-alkyl-3-cyano-1,4-dihydropyridines-4-d show elimination of a protium or deuterium atom from the 4 position of the molecular ion as the primary decomposition process. The ratio of rate constants for loss of these hydrogen and deuterium atoms is found to be inversely related to the ionizing voltage (kH/kD = 1.25 – 7.8 at voltages near the appearance potential). Most of the fragments are formed from the pyridinium ions resulting from loss of a hydrogen atom from the 4 position of the dihydropyridine. Rearrangement leads to nicotinonitrilium ion {m/e 105). In order for N-methyl-3-cyanopyridinium ion to form m/e 105, it must expel methylene (CH2) as a neutral fragment; this process is relatively unfavorable (2.8% of base peak). In the N-ethyl analog, expulsion of a thermodynamically stable ethylene molecule gives a m/e 105 peak which is the second most intense in the spectrum (95% of base peak). Deuterium labeling in the N-alkyl compounds methyl, ethyl, and n-butyl established that the hydrogen atom transferred to the ring to form the mle 105 ion is preferentially from the β-carbon atom of the alkyl group, probably via a four-center transition state. The opening of the ring was found to occur by cleavage of the 1,2 and 5,6 bonds. Mass spectra of the N-n-propyl and N-n-pentyl analogs and of 1,4,4-trimethyl-1,4-dihydropyridine are also discussed. © 1968, American Chemical Society. All rights reserved.