Collision-induced dissociation of phenethylamides: role of ion-neutral complexes

Abstract

Rationale: Phenethylamides are a large group of naturally occurring molecules found both in the plant and animal kingdoms. In addition, they are used as intermediates for the synthesis of pharmaceutically important dihydro- and tetrahydroisoquinolines. To enable efficient characterization of this class of molecules, a detailed mass spectrometric fragmentation study of a broad series of analogs was carried out.

Methods: The test compounds were synthesized using standard methods for amide bond formation. Low-energy high-resolution tandem mass spectra were acquired on a hybrid quadrupole/time-of-flight mass spectrometer using positive ion electrospray ionization.

Results: A total of 26 analogs were investigated in the study. Fragmentation of phenethylamides was found to proceed via intermediate ion-neutral complexes. The complexes can break down via multiple pathways including dissociation, proton transfer, Friedel-Crafts acylation, and single electron transfer. The relative contribution of each of these pathways strongly depends on the structure of the coupling amine and acid.

Conclusions: A general scheme for the fragmentation of phenethylamides was developed. This study further extends the knowledge base of the ion-neutral complex by discovering Friedel-Crafts acylation as a novel reaction. The strong influence of minor structural modifications on the fragmentation patterns highlights the importance of testing many analogs in order to fully predict a fragmentation pattern of a particular class of molecules.

Figure 1

Chemical structures of natural phenethylamines

Figure 2

Product ion tandem mass spectra of selected N-acetyl phenethylamines. For ion labels see Scheme 1. Note that loss of ammonia (ion c) was observed only for the highly activated phenethylamine ring. Spectra were obtained at 10 eV collision energy.

Figure 3

Product ion tandem mass spectra of selected aromatic phenethylamides of homoveratrylamine. Note how the relative ratios of the acylium ion (ion a) and ion b vary depending on the stability of the a ion. Spectra were obtained at 10 eV collision energy.

Figure 4

Product ion mass spectra of selected phenethylamides analogs in deuterated solvent.

Figure 5

The Hammet plot of abundance ratios of ion a (I_A) and ion c (I_E) versus Brown-Okamoto substituent constant σ_x⁺ for a series of aromatic homoveratrylamides. The abundances of the ion a are adjusted to include secondary loss of CO which was observed in some cases. The values for the constants were taken from the comprehensive review by Hansch et al. ^[54]

Figure 6

Product ion mass spectra of phenethylamides containing 2-methyl-6-nitro benzoic acid. Note a dramatic change in the appearance of the spectra depending on the number of electron-donating substituents on the phenethylamine ring. Mass spectra were obtained at 10 eV collision energy.

Scheme 1

Summary of proposed fragmentation pathways of phenethylamides. A key intermediate in the Path A is ion-neutral complex (INC-1) between the acylium ion and the base. The complex can break down via simple dissociation (Path A1, ion a), proton transfer (Path A2, ion b1) or Friedel-Crafts acylation (Path A3, ion c). For the path A2, the transferred proton originates from a labile position (shown for p-hydroxybenzoyl analog on the right). In the proposed structure of the ion c the position of the acyl group is drawn arbitrarily for illustration; Path B represents inductive cleavage of the Cα-N bond. It could proceed either via 1,2-hydride assisted cleavage or by neighboring group participation in which aromatic ring serves as the nucleophile to produce a phenonium-type structure.

Scheme 2

Proposed mechanisms for H/D scrambling observed during fragmentation of A) aliphatic and aromatic phenethylamides, and B) amides of with cinnamic acid derivatives.

Scheme 3

Proposed fragmentation pathways for compounds 26 (3A) and 12 (3B). For these analogs, the nitro group may serve as a proton acceptor to transfer the ionizing proton to the amido nitrogen. In contrast to other tested analogs, the ion-neutral complex intermediate (INC-2) can undergo a single electron transfer due to favorable combination of electron donating ability of the activated aromatic ring and electron accepting ability of the o-nitro benzoyl ion. In case of 12 an additional hydrogen abstraction step from the hydroxyl group is proposed to occur resulting in the formation of the ion of m/z 152.068.

Scheme 4

Proposed mechanism for the loss of water from protonated phenethylamides. In this mechanism, protonated amide itself acts as an electrophile that attacks activated aromatic ring of the phenethylamine. After proton transfer and rearomatization, water is eliminated to produce a protonated dihydroisoquinoline structure. This pathway is promoted when R is an electron-withdrawing group.